T (2D) S E A Kasi... · Sri Kasi Venkata Nageswara Rao Matta B.Sc. (Physics), B.Tech (Chemical...

D

COMPUTATIONAL EXPLORATION OF

TWO-DIMENSIONAL (2D) MATERIALS

FOR SOLAR ENERGY APPLICATIONS

Sri Kasi Venkata Nageswara Rao MattaB.Sc. (Physics), B.Tech (Chemical Eng.), M.Tech. (Chemical Eng.)

Submitted in fulfilment of the requirement for the degree of Doctor of Philosophy

School of Chemistry, Physics and Mechanical Engineering

Science and Engineering Faculty

Queensland University of Technology

2019

of 163

Keywords

Density Functional Theory, Two-dimensional materials, Semiconducting

materials, Mechanical exfoliation, Electronic band structure, Light harvesting,

Band-gap, Electron Transport layer, hole transporting layer, Perovskite Solar Cell,

Photocatalytic water splitting, Exciton binding energy, Bandgap tuning.

of 163

Abstract

Exploration of new materials for optimising solar energy and photovoltaic

applications is crucial for the commercial uptake of renewable energy technology.

Nanomaterials are in vogue owing to their special properties like very high

electrical and thermal conductivity, high specific surface per unit volume good

potential for light-harvesting as well as possessing more active sites for inter

material bonding. The characterisation of new or innovative materials for solar

energy applications viz. photocatalytic water splitting, photovoltaics and as

material components for charge transfer in perovskite solar cells is included in this

current research study.

The development of 2D materials with good light absorption and having

appropriate band-gap energy could boost solar energy conversion. This research

study is based on theoretical physics principles and uses first-principles theory and

calculations, in the methodical exploration of various two-dimensional (2D)

nanomaterials either to envisage new material utilisation or to improve the existing

optoelectronic performance of solar energy conversion applications.

Photocatalytic water splitting is a process of generating hydrogen and oxygen

through the use of light energy. Hydrogen is known as a clean fuel because

combustion of hydrogen doesn’t emit any harmful gases. If such fuel is produced

without the use of any fossil fuels in the process or it doesn’t lead to the release of

harmful substances then we can term it as ‘clean energy’ production. Mimicking

photosynthesis from nature, if we produce Oxygen and Hydrogen from water, using

solar energy, we call it photocatalytic water splitting. This process requires

of 163

semiconductors which can absorb sunlight and use the energy for water splitting.

This is not a new technique, but the exploration of new2D semiconducting material

for this process is undertaken.

The specific properties of semiconductors for use in photovoltaic

applications, water-splitting etc. are excellent visible light absorption, efficient

exciton generation, and effective charge separation for extraction into the external

circuit. The same principles are required when using two-dimensional (2D)

semiconductor materials. In this work, density functional theory (DFT) principles

are applied to studying 2D materials for their visible range wavelength light

absorption and appropriate band edge position for potential use as a photo-electrode

for efficient water-splitting. A systematic study on the physical, electronic and

optical characteristics of three compounds viz. SiP2, SiAs2 and GeAs2 from group

IV-V combination at the 2D scale are conducted. This study depicts the potential

applications for these materials in photocatalytic water splitting and photovoltaic

applications.

The threshold band gap for a semiconductor to be used as photocatalyst for

overall water-splitting is 1.23 eV whereas the maximum should be around 3.00 eV.

In addition, the CBM and VBM positions of the semiconductor material should be

straddling below and above the redox potentials of water, so that a single

semiconductor can produce both Hydrogen and Oxygen evolution in a single

process. The calculated band gap for SiP2 is 2.25 eV and satisfies this requirement

while in addition, this study revealed that the single/monolayer extraction of SiP2

from its bulk counterpart, is in principle experimentally possible by mechanical

cleavage process. The findings revealed that a SiP2 monolayer could be used as

of 163

photocatalyst for overall water splitting into both hydrogen and oxygen because of

its band edge positions.

This study also includes bandgap tuning possibility while applying strain

engineering techniques. It was observed that there is a directional dependence of

expansion or compressive strain in changing the bandgap. In the case of SiP2, in the

direction of lattice ‘a’, the bandgap is slightly increased during compressive strain

by up to 3%. Further compression makes the band gap decrease and that it

transforms into metallic behaviour at a compression of greater than 7%. However,

the same variation of strain results in a gradual increase in the band-gap in the lattice

‘b’ direction i.e. from compressive to expansion strain. The results reveal the

possibility of bandgap tuning so that we can adopt a suitable mechanism to change

its band gap for a specific application.

The single-layered SiAs2 and GeAs2 materials have a predicted band-gap of

1.91 and 1.64 eV respectively, implying that they exhibit good sunlight harvesting

properties. The calculation of exciton binding energy gave values of 0.25 and 0.14

eV respectively, for SiAs2 & GeAs2. The theoretical results also show that layered

extraction may be feasible for SiAs2 and GeAs2 through mechanical cleavage. It is

also found that the 2D SiAs2 and GeAs2 normally possess good light-harvesting

property implying their potential as good light absorbers. The stress and strain

engineering assessment showed similar behaviour to that exhibited by SiP2 and

therefore bandgap tuning is also possible. This tuning of the semiconductor-metal

transformation, suggests many implications for nanoelectronics devices with strain

engineering methods and that these new series of 2D materials have immense

potential for solar energy applications. However, even though the 2D SiAs2 and

GeAs2 materials have shown a bandgap nearing the threshold level required for

of 163

water splitting, due to the exciton binding energy and band edge position location

compared to the water redox potentials, these materials will not be effective for

photo-catalyst water splitting but could be good for other photovoltaic applications.

In the latest experimental studies, the exfoliation technique was successful for

GeAs2, making our prediction a success.

Perovskite solar cells (PSCs) are one of the new generation solar cell

categories. The PSCs based on methylammonium lead triiodide (MAPbI3)

perovskites show very high performance but are commercially not successful due

to instability in the long-term and charge transfer dynamics issues. The Electron

Transport Layer (ETL) and the Hole Transport Layer (HTL) are highly important

components, for effective charge transfer in solar cells to ensure maximum

efficiency. The second aspect of this study investigates inorganic and organic 2D

materials for use as HTL and determines their suitability as effective charge transfer

layers.

A new type of organic 2D material, namely Carbon Quantum dots (C-dots) is

considered for computational analysis as HTLs. A wide range of structural

alternatives of C-Dots (including the functionalized C-dots with –OH and –COOH

groups) are investigated. Results show that smaller hexagonal C-dots, and those

with –OH and –COOH groups, have a suitable valance band maximum (VBM)

position at higher energy levels than the VBM of the perovskite layer, which makes

it suitable for hole transport. Further study on the impact of the functional group’s

position on the C-dot structure indicated that tuning of the band position is possible

and opens up the potential for use in different solar applications, which may require

different band positions for optimal performance. Two possible perovskite surface

/C-Dot interfaces were studied (i.e. MAI and PbI2 as top surfaced perovskites) with

of 163

the C-Dot as the top layer component of the hybrid structure, where it was found

that there is a good probability of charge transport between the perovskite layer and

the C-dots.

Layered crystal structures of inorganic, post-transition metal halides and

oxides (lead iodide (PbI2), lead monoxide (PbO), tin-oxide (SnO)) were also

considered as Hole Transport Layer (HTL) in perovskite solar cells (PSCs). We

found that single-layered PbO and bi-layered SnO materials are potential

candidates for hole transport layers in PSCs.

of 163

(This page is intentionally kept blank)

of 163

Table of Contents

Keywords ............................................................................................................................ 2

Abstract ............................................................................................................................... 3

Table of Contents ................................................................................................................ 9

List of Figures ................................................................................................................... 11

List of Tables .................................................................................................................... 14

List of Research Publications ........................................................................................... 15

Published journal articles .................................................................................................. 16

Conference Poster Presentations ....................................................................................... 17

Abbreviations .................................................................................................................... 18

Statement of Original Authorship ..................................................................................... 20

Acknowledgements ........................................................................................................... 21

Chapter 1: Introduction ...................................................................................... 23

1.1 Background ............................................................................................................. 23

1.2 Research Context .................................................................................................... 28

1.3 Research Problem ................................................................................................... 32

1.4 Research Purpose .................................................................................................... 33

1.5 Significance, Scope and limitations ........................................................................ 35

1.6 Thesis structure ....................................................................................................... 36

Chapter 2: Literature Review ............................................................................ 39

2.1 Historical Background ............................................................................................ 39

2.2 Study of 2D materialS for Photocatalytic water splitting or photovoltaic applications ....................................................................................................................... 42

2.3 Study of 2D material for electron and Hole Transport in Perovskite Solar Cells... 49

2.4 Summary and Impl ica t ions .......................................................................... 53

Chapter 3: Research Design ............................................................................... 55

3.1 Methodology and design ........................................................................................ 55

3.1.1 Methodology ........................................................................................................... 55

3.1.2 Research Design ..................................................................................................... 56

3.1.2.1 The first principles theory ............................................................................ 56

3.1.2.2 Hohenberg-Kohn theorem ............................................................................ 58

3.1.2.3 Kohn-Sham equation .................................................................................... 58

3.1.2.4 Wannier Functions ....................................................................................... 60

3.1.2.5 GW and BSE calculations ............................................................................ 61

3.2 Software used ......................................................................................................... 62

3.2.1 VASP ...................................................................................................................... 62

of 163

3.2.1.1 Analysis ........................................................................................................ 63

3.2.1.2 Completed Computations ............................................................................. 67

3.3 Ethics and Limitations ............................................................................................ 68

Chapter-4: Two-dimensional silicon diphosphide (SiP2) as photocatalyst .... 70

4.1 Abstract ................................................................................................................... 71

4.2 Introduction ............................................................................................................. 72

4.3 Computational Details ............................................................................................ 74

4.4 Results and discussions ........................................................................................... 76

4.5 Conclusions ............................................................................................................. 84

4.6 Supporting Information ........................................................................................... 85

Chapter-5: Two-dimensional SiAs2 and GeAs2 as photovoltaics .................... 89

5.1. Abstract ................................................................................................................... 91

5.2. Introduction ............................................................................................................. 91

5.3. Computational Details ............................................................................................ 93

5.4. Results and Discussion ........................................................................................... 95

5.5. Conclusion ............................................................................................................ 101

5.6. Supporting Information ......................................................................................... 103

Chapter-6: Carbon Dots for Hole‐Transport in Perovskite Solar Cells ...... 104

6.1. Abstract ................................................................................................................. 105

6.2. Introduction ........................................................................................................... 106

6.3. Computational Methods ........................................................................................ 110

6.4. Results and Discussion ......................................................................................... 112

6.5. Conclusions ........................................................................................................... 118


Chapter-7: Inorganic 2D materials for charge transport in Perovskite Solar Cells .................................................................................................................... 125

7.1. Abstract ................................................................................................................. 126

7.2. Introduction ........................................................................................................... 126

7.3. Computational Methods ........................................................................................ 130

7.4. Results and discussion .......................................................................................... 132

7.5. Conclusions ........................................................................................................... 136


Chapter 8: Conclusions .................................................................................... 141

Chapter 9: Future work ................................................................................... 145

9.1 Theoretical ............................................................................................................ 145

9.2 Experimental work ................................................................................................ 145

References .......................................................................................................... 148

of 163

List of Figures

Figure 1.1: Global CO2 emissions ............................................................. 25

Figure 1.2: Typical solid-state solar cell configuration ........................... 29

Figure 1.3: Perovskite Solar Cell structure .............................................. 30

Figure 1.4: Artificial photosynthesis-Photocatalysis schematic ............. 31

Figure 1.5: Thesis outline ........................................................................... 37

Figure 2.1 : Water-splitting exciton generation concept ......................... 44

Figure 3.1: Energy optimisation algorithm in VASP software for DFT analysis solving the Kohn-Sham equations ............................... 63

Figure 4.1 (a) Side view of the SiP2 bulk (2 × 2 supercells). (b) top view of the SiP2 monolayer. The pink and green balls represent Si and P atoms, respectively. ........................................................... 76

Figure 4.2: Phonon band structure of SiP2 monolayer along the high-symmetric points in the Brillouin zone. ...................................... 77

Figure 4.3: Band structure for SiP2 calculated by HSE-Wannier function method (a) Bulk, and (b) a monolayer. The Fermi level is set as zero ................................................................................... 78

Figure 4.4: Band positions of monolayer SiP2 compared to redox potentials of water. The green dashed line indicates the CBM after band alignment considering optical level with GW/BSE after exciton separation overcoming the e-h binding energy ... 80

Figure 4.5: Calculated light absorption spectrum of SiP2 monolayer (blue line) using HSE06 functional superimposed to the incident AM1.5G solar flux ......................................................... 81

Figure 4.6: (a) GW band structure with indirect band gap of 2.63 eV, and (b) BSE-Optical Absorption spectrum with optical band-gap of 2.02 eV ................................................................................ 83

Figure 4.S1: Electrostatic Vacuum potential for SiP2 monolayer .......... 86

Figure 4.S2: Band gap as a function of biaxial strain calculated with the PBE functional for SiP2 monolayer ............................................ 86

Figure 4.S3: GW+RPA optical absorbance spectrum for 2D SiP2. ...... 87

Figure 5.1: Crystal structure side view of the (a) SiAs2 bulk (3x2 super cells) (b) GeAs2 Bulk. Silicon (Red), Germanium (Blue) and Arsenic (Green) ............................................................................ 96

Figure 5.2: Phonon band structure of a monolayer of (a) SiAs2, and (b) GeAs2 along the high-symmetric points in the 1st Brillouin zone. (c) Energy of formation of the 2D material from its bulk counterparts. The green bar indicates experimentally synthesised, Red bar indicates experimentally not synthesised and the Blue bar indicates theoretically proven for the possibility of synthesis. ................................................................. 97

of 163

Figure 5.3: Band structure for SiAs2 and GeAs2 calculated by the HSE-Wannier function method. The Fermi level is set as zero. (a), (b) and (c) Bulk, Bilayer and Monolayer of SiAs2 respectively; (d), (e) and (f) Bulk, Bilayer and Monolayer of GeAs2 respectively ................................................................................... 98

Figure 5.4: Calculated light absorption spectrum of monolayers of SiAs2 (green) and GeAs2 (blue) using HSE functional superimposed to the incident AM1.5G solar flux. .................... 99

Figure 5.5: (a) and (c) GW-Band structures, (b) and (d) BSE-Optical Absorption spectra of SiAs2 and GeAs2 respectively ............. 100

Figure 5.S1: Band gap as a function of biaxial strain calculated with the PBE functional for monolayers of (a) SiAs2 and (b) GeAs2 ... 103

Figure 6.1: The structures of Hexagonal C‐dots (Hex‐1 [a] & Hex‐2[b]) and the functionalised Hexagonal C‐dots (Hex‐1 [c] & [d])) with −OH and −COOH groups. Color Code of atoms: Brown (Carbon), Blue (Hydrogen), Red (Oxygen).............................. 113

Figure 6.2: (a) The top view and the side views of (b) C‐Dot/MAI, and (c) C‐Dot / PbI2interfaces. Color Code of atoms: Brown (Carbon), Blue (Hydrogen), Green (Nitrogen), Black (Lead) and Pink (Iodine)........................................................................ 114

Figure 6.3: The calculated band edge positions (i. e. VBM and CBM) of the C‐dots compared with those of MAPbI3 (literature values of VBM and CBM). Hex1=Hexagonal C‐Dot (type‐1, smaller or base size); Hex2=Hexagonal C‐Dot (type‐2, bigger in size) ... 115

Figure 6.4: The side view of the charge density difference between the two interface systems (a) C‐Dot/PbI2, and (b) C‐Dot/MAI. The Yellow and Cyan iso‐surface profiles represent electron accumulation and depletion in 3D space with an isovalue of 0.001 e/Å−3. Colour Code of atoms: Brown (Carbon), Blue (Hydrogen), Green (Nitrogen), Black (Lead) and Pink (Iodine)....................................................................................................... 118

Figure 6.S1: The structures of various Carbon Quantum dots (C-Dots). Trigonal C-Dots (a –c), Hexagonal C-Dots with two –OH functional groups substituted (d) side by side, and (e) opposite to each other. Color Code of atoms: Brown (Carbon), Blue (Hydrogen), Red (Oxygen). ....................................................... 120

Figure 6.S2: The calculated band edge positions (i.e. VBM and CBM) of further C-Dots considered in this study and compared with those of MAPbI3 (literature values of VBM and CBM) [Tri-1 – Tri-3 correspond to three different Trigonal C-Dots (S1(a-c)); Hex-1-OH-SbS (side by side) and Hex-1-OH-oppo (opposite) correspond to S1(d) and S1(e) of Figure-S1 ............................ 121

Figure 6.S3: The structures of Hexagonal (Type-1) Carbon Quantum dots (C-Dots) with different bonding locations of two Carboxyl and two hydroxyl groups. The represented Code-named C-Dots are: (a) H1-2C2O-1, (b) H1-2C2O-2, (c) H1-2C2O-3 and (d) H1-2C2O-4. .......................................................... 121

of 163

Figure 6.S4: The structures of Hexagonal (Type-1:H1) Carbon Quantum dots (C-Dots) with different bonding locations of two Carboxyl and One Hydroxyl groups. The represented Code-named C-Dots are: (a) H1-2C, (b) H1-2C1O-1, (c) H1-2C1O-2. ...................................................................................................... 122

Figure 6.S5: The structures of Rectangular (Type-1 (R1) and Type-2(R2)) Carbon Quantum dots (C-Dots) with different bonding locations of One or two Carboxyl (1C or 2C), One Hydroxyl (1O) and/or One Hydroxyl & One carboxyl (1O-1C) groups. The represented Code-named C-Dots are: (a) R1-1C, (b) R1-1O, (c) R1-2C, (d) R1- 2O, (e) R2-1C, (f) R2-1O, (g) R2-2C and (h) R2-1C1O. ............................................................................... 122

Figure 7.1: The structures of (a –b) PbI2 monolayer, (c-d) PbO, and (e-f) SnO monolayers top-view and perspective side-views respectively. ................................................................................. 131

Figure 7.2: HSE Band structures of (a) PbI2, (b) PbO, and (c) SnO monolayers .................................................................................. 133

Figure 7.3: HSE06 Band edge positions (calculated CBM and VBM) of PbI2, PbO and SnO compared with that of MAPbI3 Legend: 1L=monolayer, 2L=bi-layer ..................................................... 134

Figure 7.4 : The top and side view hybrid structures of PbO/PbI2 and PbO/MAI terminated hybrid structures respectively. ............ 135

Figure 7.5: The side view of the charge density difference at an iso-value of 0.003 e Å-3, between the two interface systems (a) PbO/PbI2, and (b) PbO/MAI. The Yellow and Cyan iso-surface profiles represent electron accumulation and depletion in 3D space. The atom positions in dotted region i.e. bottom two layers was fixed during geometry optimisation (Inset: Color Code of atoms) ............................................................................. 136

Figure 7.S1. Vacuum potential obtained from HSE06 method for PbO (a) mono, and (b) bi-layers ........................................................ 138

Figure 7.S2. Vacuum potential obtained from HSE06 method for PbI2 (a) mono, (b) bi-, and (c) tri-layers ........................................... 138

Figure 7.S3. Vacuum potential obtained from HSE06 method for SnO (a) mono layer, (b) bi-layer, and (c) Tri layers ........................ 139

Figure 7.S4. Band Structures calculated from HSE06 method for PbO (a) Mono, and (b) Bi layers ........................................................ 139

Figure 7.S5. Band Structures calculated from HSE06 method for PbI2 (a) Mono, (b) Bi-, and (c) Tri layers ......................................... 139

Figure 7.S6. Band Structures calculated from HSE06 method for SnO (a) Mono, (b) Bi-, and (c) Tri-layers ......................................... 140

of 163

List of Tables

Table 1.1: Finite and renewable energy reserves (TW-years) ............... 28

Table 4.1: Calculated structural parameters of SiP2 bulk and monolayer along with the corresponding experimental values 77

Table 5.1: Calculated structural parameters of SiAs2 and GeAs2 compared with the experimental values ........................ 98

Table 5.2: Calculated band gaps of SiAs2 and GeAs2 – for bulk, bilayers, and monolayers. ............................................................ 99

Table 6.1. Calculated CBM and VBM band edge positions few two‐dimensional Carbon‐Dots. ......................................................... 116

Table 6.S1: Calculated CBM and VBM band edge positions few two-dimensional Carbon-Dots .......................................................... 123

Table 7.1: Bandgaps of the materials calculated with HSE06 functional...................................................................................................... 135

Table 7.S1. Bandgaps, CBM and VBM obtained from HSE06 method...................................................................................................... 140

of 163

List of Research Publications

This PhD thesis is written in “Thesis by Publication” model. The main

research results are published as three journal articles and the fourth article is

submitted for publication review. All these scientific articles are presented

here in the form of Chapters and have almost the same content as the

published paper except a little more information may have been added on a

few occasions, whereas the section headings, serial numbers are altered to

maintain the sequence of the thesis. Hence, the chapter retains its own

introduction, results, discussion and conclusions. Therefore, each Chapter 4,

5, 6 and 7 is self-explanatory and encompass the focussed research topic in

its completeness within the scope of the overall thesis.

Note:

1. The author naming convention in each journal would vary. However, a

uniform representation is followed below for my name, to list the

publications, for this Thesis.

2. My ORCID ID: https://orcid.org/0000-0003-4465-640X

of 163

PUBLISHED JOURNAL ARTICLES

First-authored papers

1. Sri Kasi Matta; Zhang, C.; Jiao, Y.; O'Mullane, A.; Du, A., Versatile two-

dimensional silicon diphosphide (SiP2) for photocatalytic water splitting.

Nanoscale 2018, 10 (14), 6369-6374. (Q1; IF= 7.36)1

2. Sri Kasi Matta; Zhang, C.; Jiao, Y.; O'Mullane, A.; Du, A., Computational

exploration of two-dimensional silicon diarsenide and germanium arsenide

for photovoltaic applications. Beilstein Journal of Nanotechnology 2018,

9, 1247-1253. (Q1; IF=3.13)2

3. Sri Kasi Matta; Zhang, C.; O'Mullane, A.; Du, A., Density Functional

Theory Investigation of Carbon dots as hole transport material in

Perovskite solar cells. ChemPhysChem. 2018, 19, 3018-3023. (Q1; IF=

3.07)3

4. Sri Kasi Matta; Zhang, C.; O'Mullane, A.; Du, A., DFT exploration of

inorganic 2D materials for charge transport in Perovskite Solar Cells:

Transition and post-transition metal chalcogenides, halides and oxides -

manuscript submitted for review.

Co-authored Papers

5. Zhang, C.; Jiao, Y.; Ma, F.; Sri Kasi Matta; Bottle, S.; Du, A. Free-radical

gases on two-dimensional transition metal disulphides (XS2, X = Mo/W):

robust half-metallicity for efficient nitrogen oxide sensors. Beilstein J.

Nanotechnol. 2018, 9, 1641–1646. (Q1; IF 3.13)4

6. Tang, Cheng; Zhang, Chunmei; Sri Kasi Matta; Jiao, Yalong; Ostrikov,

Kostya (Ken); Liao, Ting; Kou, Liangzhi; Du, Aijun., Predicting New Two-

Dimensional Pd3(PS4)2 as an Efficient Photo-Catalyst for Water Splitting.

of 163

The Journal of Physical Chemistry C. (Q1; IF= 4.484)5

7. T. He, Sri Kasi Matta, A. Du, Single tungsten atom supported on N-doped

graphyne as a high-performance electrocatalyst for nitrogen fixation under

ambient conditions, PCCP. 2019, 21, 1546-1551. (Q1; IF= 3.906) 6-

8. C. Tang, F. Ma, C. Zhang, Y. Jiao, Sri Kasi Matta, K. Ostrikov, A. Du,

2D boron dichalcogenides from the substitution of Mo with ionic B2 pair

in MoX2 (X = S, Se and Te): high stability, large excitonic effect and high

charge carrier mobility, J. Mater. Chem. C. 2019. (Q1; IF= 5.976) 7-

9. Zhang, Chunmei, He. Tianwei; Sri Kasi Matta, Liao. Ting, Kou. Liangzhi,

Chen. Zhongfang, Du. Aijun, Predicting Novel 2D MB2 (M=Ti, Hf, V, Nb,

Ta) Monolayers with Ultrafast Dirac Transport Channel and Electron-

Orbital Controlled Negative Poisson’s Ratio, J. Phys. Chem. Lett. 2019,

10, 10, 2567-2573. (Q1; IF= 7.329).

10. Lei Zhang, Xin Mao, Sri Kasi Matta, Yuantong Gu and Aijun Du, Two-

dimensional CuTe2X (X=Cl, Br and I): Potential Photocatalysts for Water

Splitting under the Visible/Infrared Light

- manuscript submitted for review.

CONFERENCE POSTER PRESENTATIONS

11. Poster presentation in NanoS-E3 2017 Conference, Brisbane held at QUT,

Gardens Point Campus, during 26-29 September 2017.

12. Poster presentation on Carbon Dots as Hole Transporting Material for

perovskite solar cells at AM & ST 2018, held at Brisbane during 22-25 July

2018.

of 163

Abbreviations

2D Two-dimensional

3D Three-dimensional

AIMD Ab-initio molecular dynamics

BSE Bethe-Salpeter equation

BP British Petroleum

CBM conduction band minimum

-COOH Carboxylic group

DFPT density-functional perturbation theory

DFT density functional theory

ETM electron Transport Material

eV electron Volt

FTO Fluorine Doped Tin Oxide

GGA-PBE The generalized gradient approximation in the Perdew-Burke-Ernzerhof form

GW Single-particle Green's function (G) & screened Coulomb interaction (W)

GW Giga Watts

HER hydrogen evolution reaction

HOMO highest occupied molecular orbital

HSE Heyd-Scuseria-Ernzerhof exchange-correlation functional

HTL Hole Transport Layer

IPCC Intergovernmental Panel on Climate Change

LUMO lowest unoccupied molecular orbital

MAI Methyl Ammonium Iodide

MAPbI3 Methyl Ammonium Lead Iodide

of 163

Mtoe Million Tonne of oil equivalent

-OH Hydroxyl group

PAW Projector-augmented-wave

PbI2 Lead iodide

ppm parts per million

PSCs Perovskite Solar Cells

PV Photovoltaic

QUT Queensland University of Technology

SOC spin-orbital coupling

TJ Tera Joules

TMDC transition metal dichalcogenides

TW Tera Watt

UNFCCC United Nations Framework Convention on Climate Change

VASP Vienna ab-initio simulation package

VBM valence band maximum

of 163

Statement of Original Authorship

The work contained in this thesis has not been previously submitted to meet

requirements for an award at this or any other higher education institution. To the

best of my knowledge and belief, the thesis contains no material previously

published or written by another person except where due reference is made.

Signature: _________________________

Date: 12.09.2019

QUT Verfied Signature

of 163

Acknowledgements

Most importantly, I must express sincere gratefulness to my mother and father

for bringing me to this wonderful world, and all my teachers (direct and indirect

teachings from nature as well) since my childhood for bringing me up to this stage.

I need to find special adjectives to praise my Principal supervisor, Professor

Aijun Du, for his simple and focussed approach of guiding and constant support

throughout my PhD tenure and made it possible for me to finish my research in a

record duration of 1.5 years. I wonder how he could have such high patience and at

the same time quick in deciding a solution to computational problems. His

motivation and vast knowledge of VASP Software/UNIX/LINUX has helped me

to quickly resolve the issues faced during computations. It has been an honour to

work with him and I enjoyed working under his guidance.

I must put on record that I have been awarded a Supervisor Scholarship

(Bridging) for the duration of my PhD study, and I am very much thankful to Prof.

Aijun Du for providing this award.

Scanning into the past a bit, when I was looking for a project guide, my luck

favoured to contact Professor Anthony O’Mullane at QUT for discussion on the

PhD topic and as per my research interests, he suggested and took me to Dr Aijun

Du and got his instantaneous acceptance to take me in his group as a PhD student.

Dr Anthony is my Associate Supervisor, and I am really indebted to him, for his

support and patient reading of my manuscripts, and suggestions, of all my journal

articles. He has also helped me in reviewing the Thesis.

My sincere appreciation goes to my teammates Chunmei Zhang and Dr

Yalong Jiao for their support during learning and implementation of new software

when I was using DFT. Without their valuable help, it could have been difficult to

conduct my study efficiently. I also wish to express my regards to other team

members: Tianwei He, Cheng Tang, Xin Mao, Lei Zhang and Gurpreet Kour. They

are of wonderful support from time to time during my research, and I am happy to

be part of this team.

Though not directly related to this specific PhD research, I convey my kind

regards to Associate Professor Dr Yasuhiro Tachibana, RMIT University,

of 163

Melbourne for introducing me to experimental research in Dye Sensitised Solar

cells and Quantum Dot synthesis, and various characterisation techniques during

my tenure at RMIT.

I also wish to express my sincere gratitude to Professor Kay Latham, RMIT

University, for giving valuable suggestions while discussing the experiments

during my tenure at RMIT University. Sincere thanks to Satoshi Makuta, Manning

Liu, and Samuel Chan for their support in laboratory safety coordination work and

participate in prolonged and fruitful discussions on experimental techniques.

Sincere regards to all the QUT-staff in various departments including

Research Administration, SEF-Scholarships, SEF-HDR, CPME and many others

whom I might not have met in person. They have a fundamental and primary

contribution for sure, at key stages of my candidature, during my journey of

research and helping me to reach this stage of Ph.D degree.

I must put on record about the endurance of my family: my wife for her

patience and support during all hard times, daughter and son for their love and

motivation during my journey of research. I thank my sister and brother-in-law and

nieces for their moral support through taking care of mother and household chores

at my native place back in India.

Last but not the least I thank the seemingly unknown almighty or supreme

power for continuing the spiritual help at all stages of my life.

of 163 Chapter 1: Introduction

Chapter 1: Introduction

In this chapter, the main intent of the research is delineated. The background

of the research is described in Section 1.1 by exploring the basis of the central

problem and the motivation behind the selected research topic. The focus point

selected out of many possibilities of the proposed subject is spelt out in Section 1.2

as ‘Research Context’. The main research problem is deciphered in Section 1.3 as

‘Research Problem’, while the specific aims and objectives of the emphasised topic

are delineated along with selected questions, that would be answered subsequently

by the completion of the project are described in Section 1.4 as ‘Research Purpose’.

Section 1.5 describes briefly why the research is significant and then sets the extent

of the scope of this research. The chapters in this thesis and their outlines are

described in Section 1.6.

1.1 BACKGROUND

1.1.1 Global Energy scenario, Global warming and renewable energy mix

One of the most important global developmental aspects is that ‘the energy

supply should be sustainable and should be sourced from environmentally clean

production processes’. Fulfilling this standpoint requires us to reduce or eliminate

fossil fuel consumption for energy generation.

While looking at the global primary energy demand, it is projected to be 718

Million TJ by the year 20508, which is an increase from 567 Million TJ in the year

2015, which is a growth of 26% 8. At the moment, the current total global energy

production in 2017 is 14080 Mtoe9.


There is however good news in that there is a significant decline in the cost

of renewable energy which is accelerating and therefore is more competitive than

newly built energy generation plants based on fossil fuels (reference period: the

Year 2005 to 2016)8. The combined solar and wind annual capacity rose from a

mere 13 GW (in 2005) to 119 GW (in 2016)8, of which the solar energy component

outgrew the wind energy component in 2016.

The fuel mix for energy production in future, as projected by ‘BP Outlook

2018’ is fairly positive with renewable sources’ contribution increasing to about

25% by 204010. This is a significant rate of increase from the current share of 7%.

A positive trend with respect to reducing the fossil fuel mix, (coal share reduces

from the current contribution of about 40% to 30% by 2040) is observed in the

projections. Nevertheless, it still means that coal-burning contributes to global

greenhouse gases emissions even in 2040. Hence, the concern on climate change

impacts continues.

1.1.2 Fossil fuels for future energy needs – Carbon emissions

Global CO2 emissions as reported by the U.S. Department of Energy 11 is

shown in Figure 1.1. (Redrawn with the data supplied by the reference). The

emissions from fossil fuel burning have considerably increased since 1900 and

much more from 1970 onwards. The large increase is primarily due to burning fossil

fuels and industrial production processes12. Furthermore, the global yearly

consumption of energy is projected to be at 27 TW in the year 2050 and might reach

50 TW by the end of the century13. As delineated above, even in the year 2040 the

coal, gas and oil combined share will contribute to about 50% of power generation.

The remainder is shared by nuclear, hydro and other renewables, of which the other

renewables are about 20%10. About 85% of the present energy demand is produced


by fossil fuel burning. The fact is that the existing fossil fuel sources are enough to

meet the existing demand, for the next several centuries, at the same rate of

consumption14. However, the continuous consumption of such fossil fuels at that

pace will result in huge amounts of GHG emissions, with an irreversible impact on

the climate. If this is allowed, GHGs would continue to be released to the

atmosphere and is projected to reach about 36 billion tonnes of CO2 from the current

level of about 33 billion tonnes of CO210.

Figure 1.1: Global CO2 emissions


1.1.3 Climate changes – observed

Over a period of time, it is evidenced by numerous research and database

analysis that the climate system is getting increasingly warmer15. The influences of

these global warming result in the following changes in the climate system12.

1. Earth’s surface warming12 the global surface temperature average is

increased by 0.85 ± 0.08°C, over the period 1880 to 201212.

2. Ocean warming16 leads to an increase in climate system energy that is

accounting for higher than 90% of accumulated energy from 1971 to

2010.

3. Cryosphere16 The frozen water part (Cryosphere), where glaciers have lost

mass resulting in a huge loss over the period from 2002 to 2011, more so than

over the period from 1992 to 2011 and have contributed to rising sea levels

during the 20th century.

4. Sea level change 16 The mean global rise in sea level during 1901–2010

was by 0.19 [0.17 to 0.21] m. This rise is higher from the middle of the

19th century compared with previous millennia.

These changes are only a few that have impacted natural and human

systems across the globe. The change in global climate is perennial and

will continue if unaddressed. However, the rate of change depends mainly

on the quantity of GHGs 1 emitted annually and the sensitivity of the

influence on Earth’s climate.

1 GHGs = Generally known as heat trapping gases - carbon dioxide (CO2), nitrous oxide(N2O), methane(CH4), and the chlorofluorocarbons (CFCs)


1.1.4 Global problem-Preventing dangerous climate change

Policymakers and scientists have been facing the question for quite a long

time on ‘how to prevent climate change?’ While finding the answer to this

question, the United Nations Framework Convention on Climate Change

(UNFCCC) has considered the ‘Second Assessment-Climate Change 1995’

report by the Intergovernmental Panel on Climate Change (IPCC), for formal

Framework Convention policy. The focus point is to know a numerical set-point

or a threshold value of a ‘change indicator’ at which the climate change starts

aggravating – i.e. would lead to "dangerous" change. In other words, ‘to what

extent the GHGs (CO2 equivalent) are allowed in the atmosphere to bring about

all the negative consequences’ that we deliberated so far.

The debate on finding such a set-point, at various forums concluded that the

grave implications of global warming might be evaded if the average global

temperature rise is restricted to no more than 2°C above pre-industrial levels17,

based on GHG emissions. However, further deliberations lead to a target level

of GHG emissions have given a more stringent value of 350ppm18.

If we aim to counter these global concerns, we face a scientific and technical

challenge to improve the efficiency of current renewable energy technologies

and/or find new technological options. To realize such scenarios, renewable

energy technologies need to develop at a much faster pace than at present. This

calls for improving the current state of fundamental and applied research in these

renewable energy technologies and improve their efficiency and make them

more cost-effective than current conventional technologies. Only then would the

renewable energy share be increased by new additional capacities and

appropriate infrastructure facilities.


1.2 RESEARCH CONTEXT

1.2.1 Renewable energy resources

The world’s rate of energy consumption is predicted to increase from 13.5

TW to 27 TW from 2001 to 2050. This is predicted to triple in the year 2100, to 43

TW 13, 19. In view of the stringent target of 350 ppm of GHG emissions, considered

in the IPCC report 18, we need to actively aim at Zero-carbon emission-based

technologies for most or all future power generation. The overall energy options

available including renewable energy sources and their potential output are given in

Table 1.1.

Table 1.1: Finite and renewable energy reserves (TW-years)

Global energy reserves, 2009 estimate20

Source

Finite sources TW

Coal 830

Petroleum 335

Natural Gas 220

Uranium 185

Renewables TWy/yr

Solar 23000

Wind 75-130

Waves 0.2-2

Ocean Thermal Energy Conversion (OTEC) 3-11

Biomass 2-6

Hydro 3-4

Geothermal 0.2-3

Tidal 0.3

From Table 1.1, it is obvious that solar energy has a huge potential to provide a

significant amount of energy. However, in parallel, all other possible renewable


sources should be utilised to the greatest extent possible to attain an

environmentally sustainable energy mix for future energy demand.

Figure 1.2: Typical solid-state solar cell configuration

Over the past few decades, a large amount of research has focussed on renewable

energy sources, in particular, solar energy.

1.2.1.1 Solar cells

A solar cell is a combination of material interfaces that harvest light and convert the

energy into electricity and is based on the photovoltaic effect. In essence, some

materials have the capability to absorb incident light and generate electricity by

promoting or releasing ‘electrons’ that move in the material. Having said this, the

basic requirement for a material to harvest light is the electronic ‘band-gap’ that

determines the light harvesting capacity of the material. A semiconductor that

possesses an electronic band-gap can be used as the main component of solar cells.

When light shines on them, the energy is absorbed and ‘electron-hole’ pairs are

generated. These ‘electrons’ are then separated through electrodes at both ends, thus

completing an electric circuit in which the ‘ejected electrons’ flow causing

‘electricity’ production. A schematic showing this principle is given in Figure 1.2.


There are many components in a solar cell configuration that play an important

process function and impact its overall efficiency. Each component should be given

a special focus in order to identify a suitable material to fulfil its function. Compared

to the first-generation Silicon-based solid-state solar cell technology, there are now

third-generation solar cells that include Perovskite Solar Cells (PSCs)21 (See

Figure: 1.3). The HTM and ETM play a crucial role in the charge transport

dynamics. At present, the most efficient HTM is an organic material viz. Spiro-

OMeTAD (2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene)

(Empirical formula C81H68N4O8). However, the production process is cost-

prohibitive and it is not particularly stable. Similarly, there are some effective

inorganic ETLs that were reported22 such as TiO223, SnO2

24, ZnO25 etc. Part of my

research is focused on finding simple materials that have the potential to be easily

produced yet having a suitable bandgap and band edge positions aligned to

perovskite layers to achieve hole transport. The research gaps associated with the

selection of such a study are mentioned in the literature review section.

Figure 1.3: Perovskite Solar Cell structure


1.2.1.2 Photocatalysis

Clean fuel production via artificial photosynthesis (generating H2 and O2 through

water splitting) using a photoelectrochemical (PEC) system is also an effective way

for sustainable energy storage where electricity is stored as a clean fuel, namely

hydrogen. This PEC system requires a suitable semiconductor having high light

harvesting properties that can be used as a photocatalyst to split water.

Figure 1.4: Artificial photosynthesis-Photocatalysis schematic

This study of exploring the band edge positions of new materials is part of my

core study on 2D materials. Finding a material with suitable band edge positions

that are aligned with the redox potential of water while possessing a bandgap

suitable for effective water splitting through the photoelectrochemical process (see

Figure 1.4) is also part of my research. Such a semiconductor should have a

bandgap of around 1.5 to 1.8 eV26 for practical purposes. and therefore this study is

to assess innovative 2D materials for overall water splitting and photovoltaic

applications.


1.3 RESEARCH PROBLEM

The special and remarkable electronic and physical properties27-33 of 2D materials

such as high carrier mobility, good light absorption, mechanical flexibility,

thermal conductivity have given rise to promising research in both fundamental

and applied areas. Although there has been immense research progress in the last

decade on these materials, many challenges remain regarding the number of

materials that are yet to be studied and characterised and compared to their bulk

counterparts in order to identify possible applications in solar cells, water-splitting

and nanoelectronics.

1. It is determined from a literature review that the 2D layered GeAs2, SiAs2 and

SiP2 materials significantly less studied than other 2D materials and there is a

shortfall of theoretical data on their properties. There a few available

experimental results but they have not yet been compared with theoretical

studies. This computational investigation based on DFT is undertaken for

checking their applicability in water splitting and photovoltaic applications.

2. Further study on suitable materials for use as HTMs in Perovskite solar cells is

highly desired for this fast-growing perovskite field. A new class of organic 2D

materials and other inorganic materials are considered using DFT analysis to

delineate their suitability in PSCs and thus in solar energy applications.

The research gap is very clear, and my research contributes to filling this gap

to some extent and will pave the way for selecting new materials for solar cells

or in water splitting studies. The bulk and 2D structures of the materials

considered in this research are given in their corresponding chapters and thus

not given here to avoid repetition.


1.4 RESEARCH PURPOSE

My research focusses on finding a suitable semiconductor, with a specific

emphasis on two-dimensional materials (2D materials) - also known as ‘single-layer

materials’- that are crystalline in nature and composed of a single layer of

atoms/unit cell layer. 2D materials have been put to use in a wide range of

applications, including high-performance gas sensors34, electronics, photovoltaics,

spintronics, catalysis, and Li ion-batteries35, 36. These materials are said to have

special properties such as absorbing much higher incident light compared to the

bulk materials, easy to integrate into waveguides which would help in high

performance light modulation for optoelectronics, easily suited for heterostructures

etc. Thus, my core research area has the benefit of potentially applying the same

material for solar cell/photovoltaic applications and water splitting applications that

can produce green energy fuels such as Hydrogen.

1.4.1 Research Aim

The fundamental aim of the study is to identify and suggest new 2D layered

materials for improving the efficiency of solar-based technologies.

Under the above aim, the specific technologies selected are: -

(1) Photo-catalysis applications for clean solar fuel production –for solar water

splitting to generate hydrogen and oxygen

(2) Perovskite solar cell applications – to find innovative 2D materials for

aiding charge transfer focusing on electron and hole transport layers that are

compatible with the perovskite active layer.


1.4.2 Research Objectives

With the declared aims, few objectives are derived to give further emphasis

on the selected research problem. Each objective is based on a set of basic

and specific parameters that need to be assessed in order to investigate the

central point of the research gap with respect to two-dimensional (2D)

materials.

The basic characterisation parameters such as optical, electronic, phonon

spectra and bandgap etc. and prediction of their suitability for solar cell

applications. Specific parameters such as 2D material formation energy, the

binding energy of the hybrid structures/systems designed for a specific study,

charge density difference in hybrid systems to assess the effectiveness of

charge transfer mechanisms etc. are a few factors that need to be considered.

This is applied to new generation solar cell technology (Perovskite solar

cells), and solar applications like water splitting to generate Hydrogen and

Oxygen.

Objectives:

1. To model and analyze the 2D structures of three very less studied Group

IV-V compounds viz. GeAs2, SiAs2 and SiP2 for their suitability in

photoelectrochemical water splitting to generate clean fuels (Hydrogen) or

for general photovoltaic applications. This work aims to develop an in-depth

understanding of their special properties with respect to their bulk structure

counterparts.

2. To design new 2D organic Carbon-Quantum dots (C-Dots) and analyze

their suitability and select the most appropriate structures for use as ‘hole-

transporting’ material for perovskite solar cells (PSCs).


3. To design 2D inorganic materials (transition and post-transition metal

chalcogenides (InS, InSe, TcS2, TcSe2), post-transition metal halides and

oxides (PbI2, PbO, SnO) and analyze their suitability and select the most

appropriate structures for use as an electron or hole-transporting materials

for perovskite solar cells (PSCs).

1.5 SIGNIFICANCE, SCOPE AND LIMITATIONS

The importance of the current research study is highlighted here in terms of

the selected topic, the gravity of the problem to be solved, and other factors.

1.5.1 Significance of the research

The significance of the research study is described below: -

1. This is fundamental research to theoretically predict the characteristics of

either new materials or those materials that are studied in depth so far.

2. This research is primarily focused to contribute to solving the global

problem of climate change.

3. This theoretical exploration helps in pre-empting the results before

proceeding with any experimental studies, helping in saving time and

money, whereby this research could avoid futile efforts of potentially non-

productive experiments.

4. This study emphasises the theoretical characterisation of less known or

less studied 2D materials (oxides or chalcogenides of elements from

Group IV-V) for finding alternatives for existing materials for perovskite

solar cells and provide scope for further studies.

5. This research aims at identifying simply structured materials for possible

low-cost processing and production.


6. New and simple organic structures for Carbon Quantum dots are designed

and explored for aiding charge transport in solar cells.

7. The study is also engaged in the production of solar fuels (Hydrogen and

Oxygen) from solar water-splitting.

8. This investigation contributes to providing scope for further study on the

emerging photovoltaic technology i.e. Perovskite solar cells.

1.6 THESIS STRUCTURE

The organization of this thesis is described here. In Chapter 2, the literature

review and the research problem are given. The research design, theoretical physics

models or computational methods considered, resources used etc. are given in

Chapter 3. The completed work and the results in the form of published / ‘submitted

manuscript for publication’ of articles are given in Chapter 4 through Chapter 7.

Chapter 8 gives the conclusion. The future scope of the research is briefed in

Chapter 9, and then the final section ‘References’ provides the Bibliography. The

basic thesis snapshot is depicted in the Figure-1.5.


Figure 1.5: Thesis outline

of 163 Chapter 2: Literature Review

Chapter 2: Literature Review

2.1 HISTORICAL BACKGROUND

As proclaimed in the previous section ‘research context’, my research focusses

on Two-dimensional (2D) materials that could possess appropriate characteristics

as that of semiconductors for use in solar energy applications. Identifying new

semiconductor materials involve either of the two ways-

1. Make a guess of a probable material, with some experience on known

materials, and then experimentally synthesise, then characterise to find

its suitability for the intended use

2. Make a material guess, again with some previous experience with similar

materials, and then theoretically predict its properties through first

principles of physics, and if found suitable for the desired use, then

proceed with synthesis and characterisation to compare with the

predicted results, and then use in applications, if the experimental results

match considerably with the predicted theoretical values.

The second method is very useful, in such cases, where we do not know

anything or have very little information about a material. This would save time and

money, as some materials after synthesis and characterisation may not be useful.

The current research uses the second method for predicting the probable

semiconductor compound characteristics using fundamental principles of quantum

physics and chemistry. The computational study is based on quantum mechanical

modelling that uses Density Functional Theory (DFT) with ab initio calculations to

predict the properties such as the light absorption spectrum, electronic bandgap,


conduction band minimum (CBM), valance band maximum (VBM) and other

properties such as electronic band structure, phonon spectrum etc. Using these

analytical data, the use of the material can be extrapolated or perceived.

Once suitable characteristics for the material is found it can be explored

further through future research via synthesis by experimental researchers at lab-

scale and the predicted properties can be experimentally determined and compared.

In general, the elements from group IV of the periodic table or a compound

formed with a group III element and a Group V element, or from group II and group

IV mostly have semiconducting properties. In this study, the compounds formed

from the combination of group IV-V are considered and Two-dimensional (2D)

layered materials. Also, a few other materials viz. Lead and Tin are considered. An

innovative and new class of organic carbon-quantum dots are also explored for

usage in solar energy material applications.

Then ‘Why 2D materials?’ The simple answer is – ‘make use of their

remarkable properties’ identified through the introduction of graphene in 200437

(though the layered materials were predicted theoretically much earlier). This has

since become an interesting and intensely studied research field. The reason being

its special properties of having very high thermal38 and electrical conductivity39,

transparency to visible light40, and greater strength than steel41 by weight.

Nanostructured crystalline materials such as nanowires, nanotubes are a new

class of contenders to replace conventional silicon solar cells due to photo-

conversion improvements42. However, size-dependent synthesis of nanomaterials

and their characterisation is time-consuming with many futile trials before one

achieves the right-sized Nano-structure. Therefore, fundamental principle

calculation methods can help to predict material properties before synthesis. Many


researchers have emphasised the importance of computational studies and reviewed

its progress. Martin O. Steinhauser and Stefan Heirmaier [2009]43 have reviewed

on molecular dynamics (MD) in material science, Anubhav Jain et al. [2016]44 have

reviewed the DFT predictions of materials for energy. Further studies of theoretical

predictions include Shengli Zhang et al. 45 revealed that antimonide oxide has direct

band-gap which ranges from 0.0 to 2.28 eV that is suitable for solar cell materials,

with high carrier mobility. Whereas the non-oxide form of antimonide is an indirect

bandgap semiconductor which is not suitable for light absorption and thus for solar

cells.

Furthermore, the prediction of the bandgap as accurate as possible is crucial

for selecting solar materials. Using DFT and post-DFT approximation methods

which include hybrid functionals to get more accurate results for the bandgap that

is comparable to experimental values were obtained by Kittiphong Amnuyswat et

al. 46 for methylammonium lead iodide (CH3NH3PbI3) which is a material for

perovskite solar cells (PSCs). In 2008, Macho Anani et al.47 have considered

Nitrides of group-IIIA elements for a computational study and found a theoretical

efficiency of about 35% making them worth an experimental study for use as solar

cell materials. They proposed these Nitrides in out-door applications because of

their large energies of formation and comparatively better resistance than III-IV

semiconductors to various temperatures.

Though most of the characteristics of 2D materials are similar exceptions still

exist, and each material calls for a thorough study in order to find a suitable material

with desirable properties. The reduced size of nanomaterials helps exciton charge

separation 48-52, while few two-dimensional materials are relatively insensitive to

thickness 53, 54, however Boron Phosphorous has a thickness-dependent direct


bandgap 55-59. Thus, though my research would focus on similar or some of the

characterisations as discussed above it considers new or existing materials that are

currently not well explored.

The latest developments in computational studies, thus emphasise its

importance especially when there is limited scope for synthesis and characterisation

studies on nanomaterials. These DFT studies include investigation of band-gap type

i.e. direct or indirect, difference in value of bandgap with respect to stress or strain,

electronic band structure, hybrid structure binding energies and charge density

differences in hybrid structures, VBM and CBM etc. and many more characteristics

that are useful for predicting the suitable applications in any industry. In my

research, DFT is used to predict characteristics of materials which can be utilised

in solar energy utilisation.

2.2 STUDY OF 2D MATERIALS FOR PHOTOCATALYTIC WATER

SPLITTING OR PHOTOVOLTAIC APPLICATIONS

The concept of photocatalytic water splitting was introduced by Fujishima

and Honda in 1972. They used TiO2 as the photo-catalyst60 in a photo-

electrochemical (PEC) cell. This cell makes use of light energy (in this case solar

light) and splits water into its chemical constituents i.e. hydrogen (H2) and oxygen

(O2) in gaseous form, where the former is considered a clean fuel, as there is no

carbon source used in this process. To make use of the light, either the anode or

cathode (dipped in an electrolyte connected through the exterior circuit) should be

a light-absorbing semiconductor. Depending on which component is the light-

absorbing semiconductor (anode or cathode), the relevant electrode reaction takes

place and the constituent gas is evolved from the cell at the semiconductor-


electrolyte (in this case, water), the second electrode is typically a metal. However,

if both the anode and cathode can be of light-absorbing type semiconductors, then

it is known as a tandem photoelectrochemical cell. Having mentioned this, if a

single semiconductor has the appropriate CBM and VBM positions in its band

structure, then a single semiconductor is enough to achieve HER and OER in the

same cell. This is also one of the main objectives of my research to identify such

material.

The principle of this cell can be described as follows. When solar light shines

on the photosensitive semiconductor electrode, the part of the sunlight, which has

a higher level of energy than the band-gap energy of the semiconductor, is absorbed

by some electrons in the valence band (VB). Attaining more energy, these electrons

jump to the conduction band (CB). This creates a ‘hole, h+’, i.e. positive charge in

the VB. These electron-hole pairs are called excitons. (See Figure 1.6) If we

achieve separation of these excitons into an electron and a hole, within the field

formed in the space-charge region, then these will cross the electrode-electrolyte

interfaces in the system. This electron flow creates electricity. The pH of the cell

system determine the reactions associated with water splitting. In an acidic

environment (at low pH) this charge transfer process occurs through the ‘holes (h+)’

and generates H+ ions at anode’s surface and generates the ‘oxygen evolution

reaction (OER).

2H2 O( l ) + 4h+ → O2 ( g ) + 4H+ (at low pH) – a t anode

Hydrogen ions diffuse through the electrolyte (water) while electrons in the

anode travel through an external circuit and reach the cathode. When these electrons

combine with the Hydrogen ions at the cathode, it produces Hydrogen (gas), known

as the Hydrogen evolution reaction (HER), also, at low pH.


4H+ + 4e− → 2H2 ( g ) (a t low pH) – a t cathode

In the case of an alkaline electrolyte system (i.e. at high pH), instead of Hydrogen

ions, Hydroxyl ions (OH-) take part in the reaction.

4OH− + 4h+ → 2H2O + O2(g) (at high pH) – at anode

4H2O + 4e− → 2H2(g) + 4OH− (at high pH) – at cathode

The water oxidation and reduction potentials are pH-dependent, but the standard

potential of the PEC doesn’t depend on pH. So, the overall water splitting reaction

is given below (Bozoglan, Midilli, & Hepbasli, 2012)61

2hν + H2O(l) → H2(g) + O2(g) …………………………… (2.1)

The process described in equation (1) takes place at the photo-anode when the

absorbed solar photon energy is equal to or larger than the threshold energy. Using

the formula given below62 we can calculate this minimum energy requirement;

Em i n = ∆G 0( H 2 O ) / 2N

Where ∆G0(H2O) is the standard free enthalpy per mole of H2O in eq (1). i.e.

237.14 kJ/mol; N is Avogadro’s number 6.022 × 1023 mol−1. Thus Emin = 1.23 eV.

So, the bandgap of the semiconductor electrode to be used in the PEC must be at

least 1.23 eV.

Figure 2.1 : Water-splitting exciton generation concept


In a PEC, hydrogen and oxygen evolution reactions are separated in different

regions as they evolve at different electrodes. Where photons have an energy

higher than the bandgap energy of the material the surplus energy, ( photon – band-

gap) is lost as heat during the relaxation of energy to Eband-gap. It is thus clear that the

Bandgap energy should be such that it should be high enough to be able to harvest

a major portion of the solar spectrum. To assess the optimum bandgap value, the

internal energy losses (Eloss) linked to the solar energy conversion should be taken

into consideration. James R. Bolton [1996]63 reported that these losses are in the

range of 0.6 to 1.0 eV. A reasonable practical photocatalyst, therefore, should have

a bandgap in the range of 2.0 to 2.25 eV64.

Although having the desired bandgap value, the band edges positions of material

are also an important prerequisite for attaining oxidation and reduction reactions.

The CBM energy of the material should be higher than the water reduction

potential i.e. H+/H2 (−4.44 eV) and the VBM energy should be lower than the water

oxidation potential i.e. O2/H2O(−5.67 eV)65. The difference between the CBM and

the water reduction potential and the VBM to the water oxidation potentials are

called over-potentials, which drive the water-splitting reaction.

For the aim of reducing the overall cost of PECs, it is better to have a single

semiconductor material that satisfies all the salient characteristics; these being light

harvesting properties, exciton separation, charge transfer and transport and having

CBM and VBM at appropriate positions, thus achieving Hydrogen and Oxygen

evolution.

Semiconducting materials that have been studied include Titanium dioxide

(TiO2), Tungsten trioxide (WO3) and alpha Iron oxide (α-Fe2O3)66, bismuth

vanadate (BiVO4) and tantalum nitrate (Ta2N5)67 are a few. TiO2 nanotube arrays


(TNTs) were studied by Tan et al. (2016)68 to improve hydrogen production rate

under UV light.

Studies have also been undertaken with 2D materials as photocatalysts. In 2008,

Fujishima et al. suggested a 2D model with TiO2 to obtain both gases with a single

electrode69. A review article reported by Lin et al. [2011]70 referred to studies done

with nanocrystalline films of Fe2O3, WO3, BiVO4, and InP for water splitting.

Graphitic-carbon Nitride (g-C3N4) nanosheets were also explored for improved

photocatalytic activities (Ping Niu et al.[2012]53. In 2013, Marco Bernadi et al.

studied three transition metal dichalcogenide monolayers viz. MoS2, MoSe2, and

WS2 and predicted that they could be used for water splitting applications. Also,

group-III single-layered monochalcogenides i.e. (MX, where M = Ga and In, and

X = S, Se, and Te) are predicted to have small formation energies and thus are

suitable for photocatalytic water-splitting71. Finding a single photocatalyst for

overall water-splitting seems difficult and recently, studies are being conducted on

2D heterostructure z-schemes to achieve water-splitting72.

In this context, my research is focused on finding a 2D material that can have

such band edge positions, with the desired bandgap, and having enough driving

force to achieve the HER and OER.

Here I focus on a computational study on three compounds which are much less

studied for their feasibility as either appropriate for solar cell materials

(photovoltaics) or for photoelectrochemical water splitting (artificial

photosynthesis).

Recently, research efforts are geared towards utilising advanced materials at the

micro and nanoscale level by controlling the overall size and cost of the materials73.

This supports the use of the special properties of two-dimensional (2D) materials


for photovoltaic applications and as a catalyst for water splitting techniques which

could add new materials to the existing list of materials to produce the clean and

sustainable energy. Regarding 2D materials, I consider either a single layer or

multi-layered structures that have distinct mechanical and optical properties

compared to that of their bulk counterparts.

Layered materials possess weak van der Waals forces between the neighbouring

layers, which in principle makes it feasible to isolate one or more layers from their

bulk crystal forms by means of exfoliation techniques40. Experimental exfoliation

was successful in single-layer extraction of graphene, Boron Nitride, MoS2, and

black phosphorus. This paves an operative means to derive more 2D layered

materials from their bulk counterparts.

Since the introduction of 2D graphene in 200437, many other single-layered

materials have been predicted and also experimentally realized. However,

considering the desirable characteristics required for solar energy applications,

some 2D materials did not meet the specific application-based requirements. For

instance, a Boron Nitride (BN) layer has a very large bandgap (5.97 eV)74 which

hampers the desired capacity of light harvesting. Phosphorene is also known to be

unstable upon exposure to air75. Though single-layered MoS2 has a relatively

suitable bandgap (1-2 eV)76, it is strongly affected by structural defects77, charged

impurities78, dielectric environment79 etc. The HER efficiency must be improved

as most of the known catalysts have non-favourable Gibbs free energies for

hydrogen adsorption. These examples indicate that such materials may not be

useful for exploiting their properties at large commercial scales. This objective

motivates one to find alternatives, and therefore the search for 2D catalysts for

water splitting has received great attention in recent years71. In this regard, 2D


materials are required which are more stable, have a suitable bandgap and band

edge levels compared with water redox potentials, as well as dynamic stability

which is highly important for highly efficient HER activity.

In 2005, Novoselov et al. reported a stable single or multi-layered atomically

thin sheets of two-dimensional crystals boron nitride, graphite, several

dichalcogenides, and complex oxides, using a micromechanical cleavage

method40. This opens the opportunity to develop other 2D materials in a similar

manner. Therefore, the three compounds that are the focus for this section of

research are Silicon Diphosphide (SiP2), Silicon Arsenide (SiAs2) and Germanium

Arsenide (GeAs2) which have not been studied or explored as much to date.

In 1963, F. Hulliger et al80. predicted that SiAs, GeAs and GeAs2 should show

semiconductor behaviour. Referring to this study, Tommy Wadsten in1967

synthesised and analysed the crystals of group IV-V2-type compounds viz.: SiP2,

SiAs2 and GeAs2 having the space group of Pbam for all the three compounds

which are orthorhombic with eight formula units per cell81. He also studied other

derivatives of Si and Ge with As and Phosphorous in the same work, but no in-

depth research was done w.r.t band structure, bandgap etc.

Later, in 2011, F.Bachhuber et al.82 conducted a theoretical study on SiP2, using

a Gaussian type function scheme and predicted that the pyrite type SiP2 to be a

semiconductor with Hartree-Fock (HF) and Becke-Lee-Yang-Parr (B3LYP)

methods, but local density approximation (LDA) and generalised gradient

approximation (GGA) predicted it to be semimetal that is more towards the

semiconductor like behaviour.

In 2015, Ping Wu et al. conducted more theoretical explorations on Si and Ge

arsenides 83, where they confirmed that m-GeAs /SiAs, o-GeAs2/SiAs2 are


semiconductors due to the band structure estimates and was in agreement with

experimental results.

However, most of the above studies are focussed on bulk materials and very few

or no studies have yet been reported for two-dimensional counterparts of these

three compounds as per my knowledge. Therefore, in keeping the view of the

special properties that 2D materials possess, there is a wide gap of knowledge

regarding these compounds at the two-dimensional level.

2.3 STUDY OF 2D MATERIAL FOR ELECTRON AND HOLE

TRANSPORT IN PEROVSKITE SOLAR CELLS

The new class of solar cell being developed based on the crystal structure of

perovskites has attracted considerable interest because of its relatively low cost and

high efficiency 84-86.

Miyasaka and co-workers87 reported the first perovskites-based photovoltaic

application. The efficiency for CH3NH3PbBr3 (MAPbBr3) cells was reported

around 2.2% which was later increased to 3.8% by changing the halide from

bromine to iodine in 2009. At the end of 2013, Seok’s group achieved energy

conversion efficiencies of 16.2% by using a mixed-halide CH3NH3PbI3-xBrx(10–

15% Br) and a poly-triarylamine HTM. By 2016, the efficiency of these solar cells

was reported at 22.1% which is considered as record growth in solar cell

technology. 88.

Electrical and structural properties are reliably predicted by the DFT method.

Umebayashi et al.89 and Mosconi et al.90 evaluated the band structure of

CH3NH3PbI3 with and without intervening methylammonium ions, and indicated

that the conduction and valence bands were comprised of the lead iodide


framework, but that there was a limited effect due to the organic molecule. Giorgi

et al. 91 estimated the effective masses of photo-carriers of CH3NH3PbI3 and found

that the value is comparable to silicon in commercial solar cells, which suggests

their great potential for use in PV cells. The development of perovskite solar cell

technology is moving quickly but still needs improvement in efficiency and

stability. Theoretically, management of the constituting elements could improve

charge transportation and stability and thereby increase efficiency in CH3NH3PbI3

solar cells.

Among other components and parameters, the subject of interest for my research

is about finding suitable new and simple Electron Transport Materials (ETM) and

hole-transport material (HTM) with suitable band alignment. An effective ETM

material should promote electron mobility and have band edge energy positions

aligned with respect to the perovskite layer. This material should allow electron

transport but block hole transport, be optically transparent and exhibit low photo-

degradation 92. Some of the effective inorganic ETLs that have been reported22 are

TiO223, SnO2

24, ZnO25 etc., and NiO93, CuOx94, CuI95, CrOx

96 etc. as HTLs. An

interesting study done by Robert J.E. Westbrook et al.97 on interfacial charge

transfer energetics between perovskite and organic HTMs using time-resolved

transient absorption and photo-luminescence indicated that only a small driving

force energy of ~ 0.07 eV would be enough to promote efficient hole transport

between the perovskite-HTM interfaces. They further emphasised that the open-

circuit voltage can be further improved if this interfacial energy offset is within the

range of 0 < ∆E < 0.18 eV.

Qiang Teng et al.98 demonstrated that there is a difference in the hole transport

behaviour when perovskite surfaces are terminated with either Methyl Ammonium


Iodide (MAI) or Lead Iodide (PbI2) and suggested that the PbI2 surface terminated

perovskite has the advantage of better hole transport to contacting HTMs.

Therefore, I have considered the two-possible surface terminated perovskites for

my research on HTM materials. Moreover, it is reported99 that currently, the HTMs

are very costly which prohibit large-scale applications, in spite of the fact that

many organic and inorganic HTMs were studied at the lab scale and found to

perform at reasonable efficiencies ~20% but long-term thermal and operational

stability is still a major concern, for operation at higher temperatures around 60oC.

Thus, there is an inevitable need to find new materials that could be suitable as an

HTM in PSCs. Therefore, this research considered an innovative organic material

for use as an HTL in PSCs. Carbon quantum dots (CQDs) are a new type of 2D

material. In 2009, Xu et al. discovered CQDs100 and further research on their

synthesis and characterization indicated the potential for many applications.

CQDs101 are relatively simple to synthesise and are also fluorescent nanomaterials.

They have shown stability, conductivity, low toxicity and are considered

environmentally friendly. These CDQs have optical properties similar to that of

conventional semiconductor quantum dots102. Graphene quantum dots (GDQs) are

single-layered nanoscale-sized graphene sheets having only carbon atoms.

Functionalized GQDs were studied earlier for tuning the bandgap for suitable

applications103. If the terminal carbon atoms of the GQDs are functionalised with

hydrogen, then these are known as Carbon quantum dots (simply noted C‐dots)

where they were studied earlier by a few researchers103, 104. Photocatalysts, derived

through C‐dots, have very good catalytic activity as reported104 while a few other

studies revealed that the C‐dot edges have an influence on their light harvesting

properties105, 106. Metal-free g‐C3N4 combined with C‐dots have shown excellent

photocatalytic water splitting activity107.


The inorganic materials Lead Iodide (PbI2), Lead Monoxide (PbO) and Tin

Oxide (SnO) also considered for this study. Lead Iodide (PbI2), as reported, is a

layered material with semiconducting properties which has many applications in

the optics field108-110. The theoretical investigation has proven that the bandgap

changes from direct to indirect when the bulk form is modified to a single-layered

two-dimensional material.

Two-dimensional Lead Monoxide (PbO) is another inorganic semiconducting

material who’s mechanical, and exfoliation feasibility and optical properties were

investigated and reported35, 111-113. Favourable attributes for charge transfer or

charge accumulation, high resistivity and low-cost could make PbO a potential

candidate for an HTM, in addition, PbO is hydrophobic which could further help

in the long-term stability of the HTM and thereby perovskite as well113.

Pure Tin Oxide (SnO), having a tetragonal unit cell with P4/nmm space group

with lattice parameters a=b=3.802 Å and c=4.836 Å is also considered for this

study. It’s reported that the pure phase of SnO might not have high hole mobility,

but would show high mobility when it is doped with controlled residual amounts

of metallic tin114. However, our current study is intended to analyse the band

alignment w.r.t the perovskite’s band positions to elucidate the feasibility of hole

transport for PSC, therefore we restricted the focus on pure SnO monolayers. The

experimental realization of few-layered SnO was reported with p-type property

making it a favourable component for 2D logical devices 115, 116.

Thus, my research through prediction of new materials from first principles is

highly advantageous for the research community to save money and time in

synthesis, by exploring through computational studies and thus attains significance

in materials research.


2.4 SUMMARY AND IMPLICAT IONS

Summarising the literature review above there is a shortfall of information and

characterisation of the proposed subject materials. The theoretical framework for

my study includes systematic analysis for finding the essential properties to check

their suitability for solar energy applications.

Deriving from the history of the research work on the selected compounds, there

is a large gap of knowledge and potential scope for exploring the characteristics

and check the possibility for the desired applications. So the research problems can

be expressed as below: -

1. What are the essential properties such as electronic band structure,

phonon spectrum, optical absorption spectrum, tuning of bandgap

through the tensile strain etc. for the selected compounds viz.: SiP2, SiAs2

and GeAs2.

2. Whether these materials SiP2, SiAs2 and GeAs2 at the 2D level, are

suitable for the desired application or not and declare their potential uses.

3. It is also postulated here that C-dots and PbI2, PbO and SnO are quite

suitable for use as HTLs in PSCs.

The actual outcomes of the study are described in subsequent chapters in the

form of published journal articles or submitted for publication.

of 163 Chapter 3: Research Design

Chapter 3: Research Design

3.1 METHODOLOGY AND DESIGN

3.1.1 METHODOLOGY

The methodology is the implementation of theoretical quantum physics

principles to identify and predict the characteristics of nanoscale materials. In this

process, a computer software-based computational study is performed for all the

topics in the presented research.

A stage-wise or step-wise process is adopted to calculate the final desired

representative parameters for the research problem at hand. The steps include

creating or designing the molecular/compound structure, geometry optimisation

via. minimising the energy of the structure to find the ground state energy, then the

optimised configuration is subjected to the next stages of calculations. Further

stages include electronic band structure, optical absorption spectra, phonon

spectrum and Bader analysis for charge density differences in hybrid structures.

This computational method uses Density Functional Theory (DFT) for such

predictions based on quantum mechanical considerations

In this computational physics investigation, I am using elemental electronic

structural relaxations or ground state energy, and calculating properties such as

magnetic, electronic properties of molecules, materials and hybrid solar cell

configurations. This gives rise to finding other associated properties of the unit

cells of the compounds using DFT parametrizing with “Perdew-Burke-Ernzerhof

(PBE)” for “generalized gradient approximation (GGA)117”, which is implemented

in “Vienna ab initio simulation package (VASP)” code118, 119. Improved band


structure assessments are based on hybrid functional calculations as per the “Heyd-

Scuseria-Ernzerhof (HSE06)” method. These functionals will also be used to

explore the dynamic stability of these systems, phonon spectrum analysis will be

executed using the finite displacement method120 and implemented in phonopy

code121. The phonon spectrum or band structure will be computed using “density

functional perturbation theory (DFPT)”122, 123 as executed in the “Quantum-

ESPRESSO” package124.

3.1.2 RESEARCH DESIGN

This research is a qualitative approach to the defined problem. The problem is to

know how the material would behave or react under the designed configuration of

selected solar energy applications. The qualitative methodology mentioned in the

previous section would result in the behavioural prediction of subject materials

based on quantitative or analytical output from the calculations. The computational

principles and the linked concepts of research design outcomes are briefly

explained in this chapter.

3.1.2.1 THE FIRST PRINCIPLES THEORY

Density functional theory (DFT) is developed from first principles which support

to convert many electron-electron interactions effectively into a “one-electron

potential” which is a “functional” of the electron density125. Note that a

“functional” is ‘a function of a function’. Herein, the “electron density” is

functional, which in turn is a function of ‘space and time’. This ‘electron density’

functional is the fundamental property in a DFT study. However, there is another

theory i.e. Hartree-Fock (HF) theory which considers the many-body wavefunction

directly126.


The VASP code that uses the DFT functionals include “local density

approximations (LDA)”, GGA, hybrid functional mixing with DFT and HF

exchange, ‘many-body perturbation theory’ (GW) for quasiparticle spectra Bethe-

Salpeter equations (BSE) etc. VASP uses iterative methods to DFT Hamiltonian

and then allows one to optimise the energy and structure of ‘systems with

thousands of atoms’ and for molecular dynamics of ‘systems with few hundreds of

atoms’. The Hamiltonian of any system is the sum of the kinetic and the potential

energies of the particles or constituents associated with that system. For different

situations or a number of particles, the Hamiltonian is different127.

The Hamilton for an N-electron system can be represented with a total energy

i.e. potential and kinetic energy component, as:

H r, R = ----------- (3.1)

Where is the mass of the electron, the position coordinates of the

electron, the mass of the Ith nucleus, the position coordinates of the

electron. The other terms refer to as, Te-electron kinetic energy, Vee-Coulomb

interaction between electrons, TN-nucleus kinetic energy, VNN-the interaction

between two nuclei and VNe-interaction between nucleus and electrons

respectively. The complications for the many-particle systems is that the energies

depend on the spatial arrangement of the particles.

And the solution to the following Schrodinger’s equation from first principles,

reveals the physical and chemical properties of the systems:-


) = E ------------------------ (3.2)

3.1.2.2 HOHENBERG-KOHN THEOREM

Hohenberg-Kohn theorem128 proposes a solution to the multi-electron system, in

1964, which is the foundation of ‘Density functional theory (DFT)’ and it comes in

a two-part theorem.

Theorem 1.

“The ground state energy of a system of electrons is a unique function of the

ground state density”

n(r)= (3.3)

Theorem 2.

“The ground state energy can be obtained variationally: the density that

minimises the total energy is the exact ground state density”. In Hohenberg-Kohn

theory, the Hamilton of the system can present as

H= (3.4)

The total energy is:

(3.5)

Where, T(n) and Eint(n) are the Kinetic and the potential energies respectively

of the system, and is the external potential, and is the interaction

between nuclei.

3.1.2.3 KOHN-SHAM EQUATION

The density functional and effective potential can be represented as in equations

(3.6) and (3.7):


n(r)= (3.6)

(3.7)

Where, is the exchange-correlation potential written as,

(3.8)

The Kohn-Sham equation can be written as:

(r)= (r) (3.9)

While the kinetic and electron-electron interactions are exact, the correlation

and exchange functional to the system are not exact. So, two types of

approximations are introduced to interpret certain physical quantities/parameters

quite accurately.

Type-1: ‘Local Density Approximation (LDA)’

This LDA is a very widely used method of approximation, where the functional

is assumed to depend only on the electron density at the coordinate r, at which the

functional is assessed. However, the spin-density is not considered.

(3.10)

The assumption of having the same density everywhere, tend to over-estimate the

exchange-correlation energy in case of LDA.

Type-2: ‘Generalised Gradient Approximation (GGA)’

To correct the overestimation tendency by LDA, the density gradients are used to

expand the terms. This facilitates the error correction due to density variations from

the centre point of the coordinate. These expansion terms are referred to as

GGA129 and have the following form:


(3.11)

Among them, is Infinite-dimensional function and is the exchange

energy of the electron gas. GGA approximations for molecular geometry

optimisation and ground-state energies have yielded good results. The main

common functional of GGA includes PBE130, which is most widely used functional.

3.1.2.4 WANNIER FUNCTIONS

The electronic structure in periodic materials is solved generally in terms of Bloch

states, Ψnk. in the first-principles methods. These states are portrayed by n and k

where n is band index and k is crystal momentum, respectively.

Alternatively, these states can be represented by ‘spatially localised functions’

known as ‘Wannier functions (WF)’. The WF is briefly given here, but full details

are found in reported literature 131-133.

The WFs can be written through Bloch states centred on a lattice site R, ѡnR(r),

as;

(3.12)

where V – the volume of the unit cell, integrated over the 1st Brillouin zone (BZ),

and U(k) - unitary matrix mixes with Bloch states at each ‘k’. The unitary matrix,

U(k) has various choices that would lead to various spatial localised WF.

Wannier90 is a code implemented in VASP compute “maximally-localised

Wannier functions (MLWF)” as per Marzari and Vanderbilt (MV)131 and entangled

energy bands, as per Souza, Marzari and Vanderbilt (SMV) 132, 133. This Wannier90

method calculates the band structure of the compounds selected along with HSE06.


3.1.2.5 GW AND BSE CALCULATIONS

In this quantum mechanical context, the electronic excitations are studied in

different ways in the system of interacting electrons and nuclei in the presence of

time-dependent external fields. The Wave function methods (Hartree-Fock, Mote

Carlo etc.), Greens’s function method, and Time-dependent Density Function

Theory (TDDFT) methods. The GW method is an approximation procedure in

which the self-energy of a many-body system is approximated with a simple term.

In fact, the actual many-body system equation for self-energy contains the sum of

all terms of Green’s function G and the screened Coulomb interaction term W of

the bodies in the system. However, in this approximation, only the first term of the

Tylor series expansion of the series i.e. iGW is considered.

Self-Energy (3.13)

Where, exact Hartree-Fock exchange potential = -iGv,

and iG(W-v) = correlation, screening due to other electrons.

Thus, this GW approximation gives the self-energy of the system after performing

standard DFT calculations. However, the GW approximation doesn’t include

excitonic effects. Then Bethe-Salpeter equations (BSE) are used to predict the

optical response functions including the excitonic effects134. That is the inclusion

of electron-hole interactions. The optical response calculation is synonymous to

frequency-dependent dielectric functions. In other words, it aims at finding the

macroscopic dielectric function135, ɛM(ω). The imaginary part of this function gives

the absorption spectrum. A detailed explanation of the terms of summation in GW

approximation and the Bethe-Salpeter Equations is beyond the scope of this thesis.

The process of BSE calculation is a three-step method involving, (i) standard DFT


calculation, using for example LDA, (ii) GW approximation i.e. self-energy with

screened coloumb interactions, (can plot GW band structures and find a bandgap of

the materials, and finally doing (iii) BSE calculations to estimate e-h+ interactions

and plot absorption spectra using the imaginary part of the dielectric function. The

exciton binding energies were estimated for electron-hole pairs in some of the

materials under this study and are discussed in relevant chapters.

3.2 SOFTWARE USED

3.2.1 VASP

The “Vienna ab initio simulation package (VASP)”136 is a software package

program for materials modelling at the atomic scale e.g. quantum-mechanical

molecular dynamics, electronic structure estimation from first principles and DFT.

The principal algorithm flow chart is given in Figure 3.1 to resolve the Kohn-Sham

equation (K-S Equation). At the start of the iteration, the electron density ρ(r) is

assigned with an initial guess value, to calculate the effective potential Veff(r), then

the diagonalization of the K-S equations, and subsequently ρ(r) is evaluated

including total energy (E(tot)). Before the start of the calculation, a convergence

criterion will be given and will be tested after each iteration. If the criterion (given

as a tag in the input file), is not fulfilled, these iterations will be continued with the

ρ(r) just calculated in the last step. This process continues until the criterion is

satisfied, the output quantities are written to the output file.

In addition, the plane-wave base set is used in the code and periodic boundary

conditions (PBC). Therefore, for lower dimensional molecules and solids there

should be enough separation (i.e. vacuum) between the periodically repeated

crystals/cells. Generally, a space vacuum distance of about 10Å to 15Å or even


more will be added depends the crystal being tested. This will eliminate the

interaction between the periodic crystals to get the features of layered materials.

A brief of input file data to be fed to the software is discussed in the next

section (‘Analysis’)

Figure 3.1: Energy optimisation algorithm in VASP software for DFT analysis solving the Kohn-Sham equations

3.2.1.1 ANALYSIS

All the material models and compounds related data are given as input to the

software. No statistical analysis etc. are required to be done externally. The


software would be given five input files viz. INCAR, POSCAR, POTCAR,

KPOINTS and Linux OS dependent job-script file.

A detailed explanation of the contents of each input file is not a prudent option

here in this report. This is because, despite any written explanation, unless a proper

discussion-based basic training is imparted, it is extremely difficult to use the

software without face-to-face discussion, explanation and work with some

examples. Therefore, a brief conceptual clarification of each input file is discussed

below.

The INCAR file is a set of ‘key-tags’ that are specific to the software and a

numerical value assigned to them based on the desired characteristic related

calculation.

POSCAR file contains the sequence of elements and their atomic Cartesian

coordinate positions of each atom of the subject material within the defined crystal

structure.

POTCAR file consists of ‘pseudopotential’ of each atomic species of the

material used in the calculations. The VASP software supplier would provide the

POTCAR files of all elements in a periodic table so that the users can use them in

simulations/calculations. If there are more than one atom species in the subject

material, one needs to ‘add’ or ‘concatenate’ POTCAR files of each element in the

same sequence as they appear in POSCAR file.

KPOINTS file contains the coordinates of the symmetry points in the first

Brillion zone of the material as per its crystal lattice. A Brillouin zone is a specific

choice of the unit cell of the reciprocal lattice of the crystal of the material.

Job-script file contains a set of commends to invoke VASP software to do the

quantum computing from first principles under the Linux or UNIX Operating


System (OS). As this is OS-dependent shell script file, it should have an associated

file extension such as filename.sh etc.

The names of the first four input files (INCAR, POSCAR, POTCAR,

KPOINTS) should not be changed for any system of material/s, whereas the job-

script file can be named as per conventional computer file names.

Each job-run after successful completion gives an output data file (known as

OUTCAR) in a text file format. All the data which is relevant to the task on hand

for the problem will be gathered through a specific shell script or programming

code written in other programming languages such as Perl, Python etc. These

programmes would either be edited time to time or customised for the said problem

in hand, or a ‘pre-compiled’ version will be used to generate the desired data

analysis.

The convergence test in the VASP means, say for geometry optimisation,

whether the selected parameters of Energy cut-off and k-points mesh is enough to

give the consistent value of lowest possible energy of the system. So, basically,

these two parameters are set in INCAR or (default values in POTCAR) and in

KPOINTS files, respectively. The ENCUT is the tag that sets this cut-off energy in

INCAR file in terms of eV. This value is used in the VASP algorithm to check the

difference between the energy of the current geometry of the system and the

previous iteration. Once the value of this difference is within the set point of

ENCUT value, its iterations stop the final energy of the system is written in the

standard output. Similarly, the other parameters for convergence, i.e. number of k-

points specified in KPOINTS file. To check the convergence, a different number of

k-points shall be specified and run the energy convergence and plot a graph or

collate the results of k-points Vs final energy. This gives an indication for which


and how many numbers of k-points would be enough for conducting the

optimisation for the specific system under study. Care should be taken, only one

parameter i.e. either ENCUT or KPOINTS shall be changed at a time and checked

for convergence. If the ENCUT is not explicitly specified in the INCAR file, the

maximum ENMAX value specified in the POTCAR file of the system is considered

or used as ENCUT value as a default by the VASP. In general, in this research, the

PREC (i.e. precision) tag is set as ‘HIGH’. If this is set, the VASP would consider

1.3 times of the maximum of all ENMAX values of elements in the POTCAR file,

as the ENCUT value for convergence test. This yields good results, but of course

with some additional computational cost.

Here I am briefing an example of one such analysis, considering the VASP

software used. Once the geometry optimisation is done for any material structure

or combinations of structures the optimised structure would be written a file called

CONTCAR. This file contains the coordinates of the various elemental positions of

the subject material. This file then, along with other essential input data files, would

be given as input for the next step for calculations to find say, electronic band

structure. Once this job is submitted under the Linux environment of HPC

computing system of QUT, an OUTCAR file is created as an output, that contains

all necessary results written in it for generating the model for the band structure and

then to find the bandgap of the material. This specific data is picked-up and collated

in text file through an executable file created from a Linux system compiled Perl

programme. This text file could then be opened as an MS-Excel file or any other

application which can be used to draw graphs. The result then graphically shows

the band structure. From that structural figure, we can approximately measure the

electronic bandgap (i.e. difference of energy between the lowest of the conduction

band energies (known as CBM) and the highest of valance band energies (known


as the VBM) for the material. However, the accurate numerical value of the

bandgap can also be obtained from the same Perl programme command as a screen

output. The graph will help in assessing whether it is a direct or indirect bandgap

depend on the CBM and VBM position on k-point symmetry represented by a point

on the x-axis.

This procedure of analysis is similar for finding different desired

characteristics for the material or system of materials. However, the command line

executable files would be different. We can thus, obtain light absorption spectrum,

excitation binding energy of electron-hole pair released in a material upon light

incidence etc. Some other parameters such as binding energy between two interface

materials, the energy of formation of 2D material from its bulk counterpart, band-

gap variation with respect to stress and strain applications on materials etc. can also

be obtained from these methods.

To give another example of analysis the energy of formation can be obtained

from the difference in free energy between 2D material and the lowest energy value

of its bulk counterpart. To obtain this value two different sets of programmes for

2D and 3D are ‘run’ on Linux. Then the output from both jobs is used to calculate

the formation of energy from pre-defined equations.

3.2.1.2 COMPLETED COMPUTATIONS

All the calculations required for the research topics are completed through

supercomputing facilities from National Computational Infrastructure (NCI)

supported by the Australian Government through QUT and using High-

Performance Computing (HPC) at QUT.

The results were published through three peer-reviewed journal articles, and a

fourth article has been submitted for review and publication.


The materials for the study are bulk and two-dimensional (2D) structures of the

selected compounds GeAs2, SiAs2, SiP2, Carbon Quantum Dots with and without

additional functional moieties (i.e. –OH and –COOH), Perovskite surfaces

terminating with MAI and PbI2, Lead Iodide (PbI2), Lead monoxide (PbO) and Tin-

Oxide (SnO). The methods for this study are:-

a) Geometrical optimisation through ground state energy relaxation

b) Phonon structure

c) PBE, HSE06 /Wannier90 band structures and the bandgap estimation

d) Optical properties for estimating the light harvesting properties

e) Band-gap variation with respect to tensile strain and compressive stress

f) Schematic representation of the calculated properties to assess the potential

applications in the field of solar cell and photocatalytic applications.

g) Band edge calculations and alignment correcting with vacuum potential,

after HSE06 band structure predictions

h) Interface charge density difference calculations for perovskite surfaces and

the potential HTM materials – Organic and inorganic

3.3 ETHICS AND LIMITATIONS

All the computational works are done using reliable software. The material

structures and analysis are performed within the scope of the software limitations

if any. However, all the methods and options of the software are well within the

scope and the desired output. All the software used are under proper license by the

University.

of 163

Chapter 3: Research Design


Q


QUT Verified Signature

of 163

Chapter-4: Two-dimensional silicon diphosphide (SiP2) as photocatalyst

Chapter-4: Two-dimensional silicon

diphosphide (SiP2) as photocatalyst

DOI: 10.1039/C7NR07994J (Paper) Nanoscale, 2018, 10, 6369-6374

4.0 Versatile two-dimensional silicon diphosphide (SiP2) for

photocatalytic water splitting†

Sri Kasi Matta , Chunmei Zhang, Yalong Jiao, Anthony

O'Mullane and Aijun Du *

School of Chemistry, Physics and Mechanical Engineering, Queensland University of

Technology, Gardens Point Campus, QLD 4001, Brisbane, Australia. E-

mail: [email protected]

Received 27th October 2017, Accepted 27th February 2018

First published on 28th February 2018

of 163


4.1 ABSTRACT

The development of two-dimensional (2D) photocatalysts with excellent visible

light absorption and favourable band alignment is critical for high-efficient water

splitting. Here we systematically study the structural, electronic and optical

properties for an experimentally unexplored 2D Silicon Diphosphide (SiP2) based

on density functional theory (DFT). We find that the single-layer SiP2 is highly

feasible to be obtained experimentally by mechanical cleavage and it is dynamically

stable by analyzing its vibrational normal mode. Two dimensional SiP2 possesses

a direct bandgap of 2.25 eV, which is much smaller than more widely studied

photocatalysts including titania (3.2 eV) and graphitic carbon nitride (2.7eV), thus

displaying excellent ability for sunlight harvesting. Most interestingly, the position

of the conduction band minimum (CBM) and valence band maximum (VBM) in

2D SiP2 fits perfectly water oxidation and reduction potentials, making it a potential

of 163


new 2D material that is suitable as a nanoscale photocatalyst for photo-

electrochemical water splitting.

4.2 INTRODUCTION

Solar energy derived fuels are sustainable and regarded as being clean fuels.

Hydrogen production through photocatalytic water splitting that uses solar

radiation60 - also known as artificial photosynthesis - is an attractive option

as an addendum to renewable energy production which facilitates a cleaner

and greener production environment by contributing to the global energy

mix137-139. In this process140, a semiconductor having specific electronic

properties is used as a photocatalyst to absorb sunlight. This light energy

produces excitons in the catalyst, whereby after charge separation can split

water into its constituent elements viz. hydrogen and oxygen. Ideally, the

specific properties of the plausible photocatalyst semiconductor should

simultaneously have band edge positions above and below the redox

potentials of the water. Therefore, the bandgap is higher than the threshold

value of 1.23 eV but should be less than 3.00 eV. A gap larger than 3.00 eV

will induce poor light harvesting141. Moreover, if the recombination of

electron-hole pairs is also suppressed then this is a favourable situation for

photoelectrochemical (PEC) water splitting137. Due to these stringent

requirements, the number of qualified photocatalysts which can undertake the

production of both hydrogen and oxygen is limited.

Two-dimensional (2D) catalysts138, 142-146 for water splitting have drawn great

attention in recent years due to their extraordinary properties such as very

high surface to volume ratios and a small charge transport distance. This

reduces the probability of exciton recombination thereby enhancing catalytic

of 163


performance71. Some promising 2D materials for H2 production through

water splitting have been revealed including transition-metal dichalcogenides

such as MoS2 and WS2, phosphorene, nanocarbon and g-C3N4 41, 140, 147-154.

However, there still exists a few drawbacks such as non-stability at ambient

conditions that prevent large scale production155. Significantly, for

nanostructured materials, the reduced size promotes charge separation i.e.

exciton separation156. However, thickness-dependent bandgap variation55, 56,

59 is not a common feature for all 2D materials53, 54 and thus each material

must be thoroughly studied for its stability as well as identifying other

properties which are suitable for water splitting.

Based on the predicted semiconductor behaviour of the group IV-V

compounds80, in 1967, Tommy Wadsten synthesised and studied the crystal

structures of group IV-V2-type compounds including SiP2 with the space

group of Pbam which is orthorhombic with eight formula units per cell 81 but

did not determine its electronic and optical properties. In 1969, SiP2 was

synthesised from a molten tin solution to produce a layer-like structured

single crystal of orthorhombic SiP2 by Sprinthrope et al. which is a p-type

semiconductor with an optical energy gap of 1.89 eV157. It has also been

reported that SiP2 has both cubic (space group Pa-3 (No.205)) and

orthorhombic (Pbam) structures81, 158. In 2014, another orthorhombic type

but with space group Pnma (No. 62) was synthesised and a theoretical study

indicated that the bulk phase has an optical band gap of 1.45 eV and an

indirect bandgap of 1.24 eV158. To the best of our knowledge, very limited

theoretical studies are reported but mostly on pyrite type SiP282, 159 and no

clear experimental or theoretical study has yet been reported on 2D SiP2.

of 163


Our computational study is focussed on the rarely studied 2D SiP2, for

calculating the electronic band structure, phonon vibration frequencies,

optical properties, bandgap modulation behaviour with respect to the tensile

strain, exciton binding energy etc. and predicts the potential application for

photocatalytic water splitting. We found that it is feasible to extract a

monolayer of SiP2 from the bulk and the obtained single layer has a

favourable bandgap and the band positions which are perfectly suited to water

redox potentials. Thus, 2D SiP2 is identified as a competitor for acting as a

single photocatalyst for further experimental studies, to perform both

oxidation and reduction reactions for overall water splitting to produce

optimum yields of hydrogen and oxygen.

4.3 COMPUTATIONAL DETAILS

Density Functional Theory (DFT) using plane-wave Vienna Ab initio

simulation package (VASP) code is used for first-principle calculations for

this study119. The geometry optimisation is done with a generalized gradient

approximation in Perdew-Burke-Ernzerhof form (GGA-PBE) exchange-

correlation functional160. A Monkhorst−Pack k-points grid161 of 10 x 3 x 2

and 8 x 3 x 1 was used for sampling in the first Brillouin zone for bulk and

monolayer, respectively for geometry optimization. Both bulk and monolayer

systems are set for relaxing until residual force and energy were converged

to 0.005eV/ Å and 10-6eV, respectively. The electronic band structure is

predicted through hybrid density functional theory based on the Heyd-

Scuseria-Ernzerhof (HSE) exchange-correlation functional162 and Wannier90

package163 on VASP code. The stability of the photocatalytic material in the

aqueous phase is an important factor and for a 2D material, it is indicated by

of 163


the formation energy as ‘the difference in free energy of 2D and the lowest

value of the bulk equivalent of the same material’, Ef. The equation is given

below;

Ef= E2dn2d

- E3dn3d

----------------------- (4.1)

Where, , are the energies of monolayer and the bulk materials

respectively and are the number of atoms present in the unit cells

considered for the calculation65, 71, 164. The positive sign of formation energy

indicates energy is required for the formation, and the negative sign indicates

energy is given out during the formation of the material165. The more negative

the value of formation the more relatively stable it is166. The optical properties

are evaluated by determining the frequency-dependent dielectric tensor

matrix using density functional perturbation theory. The corrected average

electrostatic potential in the unit cell to the vacuum potential is obtained from

HSE Band calculations and then by using it as a generic reference for aligning

band positions that were obtained from HSE06-Wannier90 calculations. A

zero-damped van der Waals correction was incorporated using Grimme’s

scheme 167, 168 to better describe non-covalent bonding interactions. The

projector augmented wave (PAW) method169 was used to describe the

electron-ion interaction and the plane-wave energy cut-off was set to ~ 255

eV. To study 2D monolayer systems under the periodic boundary condition,

a vacuum layer of at least 15 Å was introduced to minimize the spurious

interaction between neighbouring layers. The phonon spectrum was

computed using the density functional perturbation theory122 as implemented

in the Quantum-ESPRESSO package170. The more often used standard

of 163


oxidation and reduction potentials of water splitting were employed65,

namely = −5.67 eV and = −4.44 eV, as reference for

comparing the predicted band edge positions of the 2D-SiP2. The excitonic

properties are studied using Green’s function, G0W0-Bethe-Salpeter

equation (BSE) approach171-174. The exciton binding energy is calculated as

the difference between quasi particle (QP) band gap (from GW band

structure) and the optical band gap (from BSE light absorbance spectrum) 175,

176. A Monkhorst−Pack k-points grid161 of 13 x 5 x 1 was used for sampling

in the first Brillouin zone.

Figure 4.1 (a) Side view of the SiP2 bulk (2 × 2 supercells). (b) top view of the SiP2 monolayer. The pink and green balls represent Si and P atoms, respectively.

4.4 RESULTS AND DISCUSSIONS

Silicon diphosphide has been fabricated and its crystal structure is assigned

to be an orthorhombic structure with the space group Pbam (no.55) with eight

symmetry operators81. Fig. 4.1(a) shows that the bulk configuration consists

of two layers and that the monolayer interacts with neighbouring layers

through weak van der Waals forces. The SiP2 monolayer (Fig. 4.1(b)) is

composed of “atomic layers” which are connected by P-P covalent bonds.

Detailed structural parameters for bulk and monolayer SiP2 are listed in

of 163


Table 4.1. The calculated lattice constants for the bulk SiP2 are a=3.443 Å,

b=9.890 Å, c= 14.419 Å, which are in good agreement with previous

experimental values81. The lattice constants of monolayer SiP2 are a=3.440

Å, b=10.000 Å.

Figure 4.2: Phonon band structure of SiP2 monolayer along the high-symmetric points in the Brillouin zone.

Table 4.1: Calculated structural parameters of SiP2 bulk and monolayer along with the

corresponding experimental values

Bulk (Cal.) Bulk (Exp.) Monolayer (Cal.)

a(Å) 3.443 3.51# (3.436)* 3.440

b(Å) 9.890 10.08# (10.08)* 10.000

c(Å) 14.419 13.97# (13.97)* -

* 81 # 177

of 163


Figure 4.3: Band structure for SiP2 calculated by HSE-Wannier function method (a) Bulk, and (b) a monolayer. The Fermi level is set as zero

The thermodynamic stability identified through the energy of formation

(equation (1)) determines the strength of van der Waals interactions in the

bulk SiP2. The lower the formation energy the easier it can be extracted from

the bulk material. It is reported that less than 200 meV/atom of formation

energies are considered to be a low enough formation energy that the free-

standing 2D layer can be extracted from the bulk counterpart164. In our

calculations the Ef for SiP2 monolayers is found to be 54 meV/atom thus in

principle it is easy to extract single layers from the bulk form. The dynamic

stability of SiP2 monolayer is evaluated by calculating the phonon band

of 163


spectrum. As shown in Figure 4.2, negligible or no imaginary frequency can

be found at any wave vector, confirming that the single-layer SiP2 is

dynamically stable.

Now, we turn to investigate the electronic properties of SiP2. The band

structures by HSE-Wannier90 calculations demonstrate that SiP2 bulk

(Figure 4.3(a)) is a semiconductor with a direct bandgap of 1.89 eV. The

VBM and CBM are located along the Γ-Z line. The result is consistent with

previous experimental work which reported a similar bandgap determined by

optical transmission measurements157.

When the thickness of SiP2 is decreased, the quantum confinement effect

becomes increasingly significant and the bandgap of the system should rise59.

Thus, when the thickness decreases to one layer, we find that the 2D SiP2

shows an increase in bandgap to 2.25 eV, in which both the VBM and CBM

locate along the Y-Γ line (Figure 4.3(b)). The bandgap and electronic band

structure strongly affect a photocatalyst’s activity and water splitting

performance. As mentioned previously, to realise an efficient light harvesting

photocatalyst for water splitting, the bandgap should be between 1.23 eV and

3.00 eV. Therefore, with a bandgap value of 2.25 eV, single-layer SiP2

perfectly meets this criterion.

The electrostatic vacuum potential of 3.49 eV (Supporting information

Figure 4.S1) obtained from the Distance from Z-direction Vs Energy plot

derived from HSE06 calculations is used to align the band edge positions of

HSE-Wannier calculations. The CBM and VBM thus obtained are located

at -3.76 eV and -6.01 eV respectively.

of 163


Figure 4.4: Band positions of monolayer SiP2 compared to redox potentials of water. The green dashed line indicates the CBM after band alignment considering optical level with GW/BSE after exciton separation overcoming the e-h binding energy

To further identify the suitability of SiP2 monolayer for efficient

photocatalytic activity, we align the band edge positions of the VBM and

CBM with respect to the already mentioned redox potentials of water w.r.t

absolute vacuum. We can see from Figure 4.4 that the band edge positions

of SiP2 perfectly encompass the redox potentials of water, suggesting great

applicability for photocatalytic water splitting. The energy difference

between the hydrogen reduction potential and the CBM is known as the

reducing power which is found to be 0.68 eV for a SiP2 monolayer. The

oxidizing power is the energy difference between the VBM and water

oxidation potential and is found to be 0.34 eV. The values of reducing and

oxidizing power are therefore large enough to trigger high performance for

photocatalytic water splitting178.

The ability to harvest solar light is also required for photocatalytic water

splitting. Therefore, the light absorption spectrum is derived by using the

of 163


frequency-dependent dielectric matrix obtained from density functional

perturbation (DFPT). The spectrum in the visible light region (approx.350 -

800 nm) is plotted and compared to the incident AM1.5G solar spectrum as

illustrated in Figure 4.5. It can be found that the absorption onset of single

layer SiP2 is at about 570 nm and that the layer has excellent light absorption

performance in the short wavelength region (350 - 550 nm).

Figure 4.5: Calculated light absorption spectrum of SiP2 monolayer (blue line) using HSE06 functional superimposed to the incident AM1.5G solar flux

The external strain on semiconductor nanostructures impact electronic

properties and thereby optical properties 179, 180. This external strain can be

imparted in many ways at a laboratory scale such as adlayer-substrate lattice

mismatch, external load, bending or by applying stress on the material 181-184.

We studied the PBE functional band gap variation w.r.t tensile strain

(Supporting information Figure 4.S2) where the bandgap variation is

dependent on the direction of the lattice. The expansion or compressive strain

of 163


in the direction a can transform the semiconductor to metal. However, at

higher percentages of strain (extrapolation not has shown), the same variation

in strain causes a continuous increase in bandgap in the direction b, from

compressive strain to expansion strain. This indicates that bandgap variation

is possible and allows the semiconductor to be tuned to the desired electronic

application. However, this bandgap variation is only indicative as it is studied

using a PBE functional which is known to underestimate the results of the

predicted bandgap, but for the purposes of testing dependence of strain on

bandgap variation, these calculations can give indicative results.

The excitons (the electron-hole pairs, also treated as quasiparticles, QP) that

generate during photoexcitation have binding energy due to mutual

electrostatic force which is one of the key factors to be considered for

designing a catalytic activity system which can be found as the difference

between QP direct bandgap and the optical band gaps. We thus found QP

bandgap from BSE Band structure for SiP2 monolayer as 2.63 eV (Figure 4.6

(a)) and the optical band gap as 2.02 eV, from the first absorption peak of

absorption spectra (Figure 4.6 (b)) calculated from Greens function (GW

method)185, 186. Thus, the exciton binding energy is 0.61 eV185, 187, 188. (The

absorbance spectrum with GW+RPA method is given in Supporting

Information at Figure 4.S3). In water-splitting, the exciton first diffuses to

the photocatalyst /water interface and then dissociates into an unbound

electron and hole 164. It is important to have an exciton binding energy that

keeps the e-h pair intact without dissociation during the drift from its

originating point till it reaches the catalyst/water interface. This would avoid

recombination of the exciton e-h, else the electrons tend to recombine with

of 163


the holes in the conduction bands. The CBM band edge position would,

therefore, be shifted when the electron-hole binding energy is introduced and

the revised CBM is marked in Figure 4.4. It is evident from the figure that

even after considering the effect of e-h binding energy on the shifting of

CBM, the new CBM position still facilitates the desired Hydrogen evaluation.

Figure 4.6: (a) GW band structure with indirect band gap of 2.63 eV, and (b) BSE-Optical Absorption spectrum with optical band-gap of 2.02 eV

of 163


4.5 CONCLUSIONS

In conclusion, we have presented 2D SiP2 as a promising nano-photocatalyst

for water splitting. By using a mechanical cleavage strategy, we find that this

sheet is highly feasible for experimental realisation. Moreover, the calculated

phonon spectrum reveals its high dynamical stability. Additionally, the band

alignment of SiP2 can perfectly encompass the redox potentials of water,

indicating its great potential for photocatalytic water splitting. Most

importantly, our results highlight a new two-dimensional (2D) photocatalyst

for water splitting and it is expected to guide future experiments to produce

H2 with high efficiency based on this material.

Conflicts of interest. There are no conflicts to declare.

Acknowledgements

A.D acknowledges the financial support by Australian Research Council under

Discovery Project (DP170103598) and computer resources provided by high-

performance computer time from computing facility at the Queensland University

of Technology, NCI National Facility, and The Pawsey Supercomputing Centre

through the National Computational Merit Allocation Scheme supported by the

Australian Government and the Government of Western Australia.

Footnote † Electronic supplementary information (ESI) available: Electrostatic potential of SiP2 monolayer and band gap variation with respect to tensile strain figures. See DOI: 10.1039/c7nr07994j

This journal is © The Royal Society of Chemistry 2018

of 163


Electronic Supplementary Material (ESI) for Nanoscale.

This journal is © The Royal Society of Chemistry 2018

4.6 SUPPORTING INFORMATION

Versatile Two-dimensional Silicon Diphosphide (SiP2) for

Photocatalytic Water Splitting

Sri Kasi Mattaa, Chunmei Zhanga, Yalong Jiaoa, Anthony O'Mullanea and Aijun Dua,*

a School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Gardens Point Campus, QLD 4001, Brisbane, Australia.

Corresponding Author

* [email protected]

Computational details for BSE calculation

For the optical property calculations, NBANDS=160 is used, and the plane-

wave energy cut-off was set to 100eV. And the numbers of occupied and

virtual bands for electron-hole treatment were set to 8 each. The electrostatic

vacuum potential obtained from HSE06 functional is given below. The value

of 3.49 eV is used to shift the CBM and VBM values w.r.t vacuum and to

compare with water redox potentials.

of 163


Figure 4.S1: Electrostatic Vacuum potential for SiP2 monolayer

We studied the PBE functional band gap variation w.r.t tensile strain, the

bandgap variation is dependent on the direction of the lattice. The external

strain on semiconductor nanostructures would impact electronic properties

and thereby optical properties 179, 180.

Figure 4.S2: Band gap as a function of biaxial strain calculated with the PBE functional for SiP2 monolayer

of 163


For the calculated absorbance spectrum, it can be seen clearly that the SiP2 starts

absorbing light at about 2.5 eV using GW+RPA method.

Figure 4.S3: GW+RPA optical absorbance spectrum for 2D SiP2.

of 163



of 163

Chapter-5: Two-dimensional SiAs2 and GeAs2 as photovoltaics

Chapter-5: Two-dimensional SiAs2 and

GeAs2 as photovoltaics

Beilstein J Nanotechnol. 2018; 9: 1247–1253.

Published online 2018 Apr 19. DOI: 10.3762/bjnano.9.116 PMCID: PMC5942365 PMID: 29765802

5.0 Computational exploration of two-dimensional silicon

diarsenide and germanium arsenide for photovoltaic

applications

Sri Kasi Matta,1 Chunmei Zhang,1 Yalong Jiao,1 Anthony O'Mullane,1 and Aijun Du*,1

1School of Chemistry, Physics and Mechanical Engineering, Queensland University of

Technology, Gardens Point Campus, QLD 4001, Brisbane, Australia.

* E-mail: [email protected]

Nunzio Motta, Associate Editor

of 163


SiAs2 and GeAs2 for Photovoltaic solar cell Applications

Computational exploration of two-dimensional

silicon diarsenide and germanium arsenide for

photovoltaic applications

Sri Kasi Matta, Chunmei Zhang, Yalong Jiao, Anthony O'Mullane and Aijun Du*

Full Research Paper Address: School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology,

Open

Beilstein J. Nanotechnol. 2018, 9, 1247–1253.

Email: Aijun Du* -

Received: 27 November 2017 Accepted: 29 March 2018 Published: 19 April 2018

* Corresponding author © 2018 Matta et al.; licensee Beilstein-Institut.

Keywords: License and terms: see end of document

of 163


5.1. ABSTRACT

The properties of bulk compounds required to be suitable for photovoltaic

applications, such as excellent visible light absorption, favourable

exciton formation, and charge separation are equally essential for two-

dimensional (2D) materials. Here, we systematically study 2D group IV–

V compounds such as SiAs2 and GeAs2 for their structural, electronic and

optical properties using density functional theory (DFT), hybrid

functional and Bethe–Salpeter equation (BSE) approaches. We find that

the exfoliation of single-layer SiAs2 and GeAs2 is highly feasible and in

principle could be carried out experimentally by mechanical cleavage due

to the dynamic stability of the compounds, which is inferred by analyzing

their vibrational normal mode. SiAs2 and GeAs2 monolayers possess a

bandgap of 1.91 and 1.64 eV, respectively, which is excellent for

sunlight harvesting, while the exciton binding energy is found to be 0.25

and 0.14 eV, respectively. Furthermore, band-gap tuning is also possible

by application of tensile strain. Our results highlight a new family of 2D

materials with great potential for solar cell applications.

5.2.INTRODUCTION

The potential applications of two-dimensional (2D) materials are one of

the key research areas for many researchers since graphene was isolated

and characterized in 200427. The number of applications is vast including

photovoltaics, nanoelectronics, Dirac materials and solar fuels through

water splitting, to name but a few. Although single elemental 2D

materials from groups IV and V, such as like phosphorene and stanine,

have been studied in detail 189, 190 we focus our study on the less well-

of 163


known combination of IV–V compound semiconductor materials at the

two-dimensional scale for their possible electronic applications. The less

extensively studied compounds SiAs2 and GeAs2 are considered here for

a computational study. The synthesis of these compounds 191, 192 was

reported a few decades ago as well as their basic crystal structures and

corresponding phase diagrams193, 194.

In 1963, Hulliger et al.195 predicted that the group IV–V compounds

SiAs, GeAs and GeAs2 would show semiconductor behaviour. The

synthesis and crystal structures of the group IV–V2- type compounds

SiP2, SiAs2 and GeAs2 were reported later. All three compounds exhibit

the orthorhombic space group Pbam with eight formula units per

cell191. However, they did not report the band structure or the bandgap

values of these materials. Later, Wu et al. performed theoretical studies

on silicon and germanium arsenides196 to predict and reaffirm that m-

SiAs/ GeAs and o-SiAs2/GeAs2 are indeed semiconductors. The

studies were based on band-structure calculations and agree with

experimental observations.

A recently reported computational study on 2D GeAs2 was performed to

investigate its thermal conductivity and its suitability for thermoelectric

applications 197. In order to further study GeAs2 and to compare it with a

similar material from the same IV–V group combination, we focus our

study on two-dimensional SiAs2 and GeAs2 and compare them with

their bulk counterparts with regard to electronic band structure, phonon-

vibration frequencies, optical properties, bandgap modulation behaviour

of 163


and predict their potential applications.

5.3.COMPUTATIONAL DETAILS

Density functional theory (DFT) using plane-wave Vienna ab initio

simulation package (VASP) code is used for first-principle calculations

for this study 136, 198. The geometry optimisation is done with generalized

gradient approximation in the Perdew–Burke–Ernzerhof form (GGA-

PBE) exchange-correlation functional 160. A Monkhorst–Pack k-points

grid 161 of 9 × 3 × 2 and 9 × 3 × 1 was used for sampling the first Brillouin

zone for bulk and monolayer for geometry optimizations. Both bulk and

monolayer systems are set for relaxing until residual force and energy

converged to 0.005 eV/Å and 10−7eV, respectively. To study 2D

monolayer systems under periodic boundary conditions, a vacuum layer

of about 15 Å was introduced to minimize the spurious interaction

between neigh- boring layers. The electronic band structure is

predicted through hybrid density functional theory based on the

Heyd–Scuseria–Ernzerhof (HSE) exchange–correlation functional 162, 199

and Wannier90 package200 implemented in the VASP code. The

thermodynamic stability of the material is assessed by the formation

energy for the 2D material and is indicated as “the difference in free energy

of the 2D material and the lowest value of the bulk equivalent of the same

material”, Ef:

(5.1)

where, E2D and E3D are the energies of the monolayer and the bulk

of 163


material, respectively. n2D and n3D are the numbers of atoms present

in the unit cells considered for the calculations 71, 201, 202. The optical

properties are evaluated by determining the frequency-dependent

dielectric tensor matrix through hybrid DFT based on the Heyd–

Scuseria–Ernzerhof (HSE) exchange-correlation functional162, 199,

implemented in VASP 171, 203, 204. The corrected average electrostatic

potential in the unit cell with the vacuum potential is obtained from HSE

band calculations. It was subsequently used as a generic reference for

aligning the band positions that were obtained from HSE06-Wannier

calculations. A zero-damped van der Waals correction was

incorporated using the DFT-D3 method of Grimme’s scheme 167, 168 to

better describe non-covalent bonding interactions. The projector-

augmented-wave (PAW) method169 was used to describe the electron-ion

interaction. Also, the plane-wave energy cut-off was set to ca. 255 eV

and, in addition, a high precision option (PREC = high) is used in the input

file, which would further set cut-off energy and other defaults such as

grid spacing representing the augmentation charges, charge densities

and potentials (NGFX, NGFY, NGFZ) so as to get accurate results. The

phonon spectrum was computed using the density functional

perturbation theory (DFPT) 205 as implemented in the Quantum-

ESPRESSO package 170. The excitonic properties are studied using

Green’s function, GW-Bethe–Salpeter equation (BSE) approach

implemented in VASP code 171-174. The GW calculations were

performed with a 13 × 5 × 1 k-grid, the energy cut-off for response

function is set at 100 both in GW and BSE approach for exact

of 163


compatibility. Over and above the GW results, the BSE method was

adopted to obtain the light absorption spectrum 203, 204 and the optical

bandgap. BSE was solved by using the ten highest valance bands and

ten lowest conduction bands and with a 13 × 5 × 1 k-grid. Using the

band gaps obtained from GW and BSE functional methods the exciton

binding energy was obtained 206.

5.4.RESULTS AND DISCUSSION

The crystal structures of SiAs2 and GeAs2 are orthorhombic with the

space group Pbam (no. 55) having eight symmetry operators or atoms

in the unit cell 191. Figure 5.1 shows the side views of the bulk

configurations of SiAs2 and GeAs2 with atomic layers interacting with

neighbouring layers through weak van der Waals forces. Detailed

structural parameters for bulk and monolayers of SiAs2 and GeAs2 are

listed in Table 5.1. The calculated lattice constants for the bulk

materials are in good agreement with previous experimental values 191.

The calculated lattice constants for single-layered SiAs2 and GeAs2 are

slightly smaller than those calculated for the bulk phases. The lattice

constant c is kept constant and greater than 15 Å by adding a vacuum

layer.

of 163


Figure 5.1: Crystal structure side view of the (a) SiAs2 bulk (3x2 super cells) (b) GeAs2 Bulk. Silicon (Red), Germanium (Blue) and Arsenic (Green)

The dynamical stability of SiAs2 and GeAs2 monolayers is evaluated by

analysing the phonon band spectrum. As shown in Figure 5.2a and Figure 5.2b,

no imaginary frequency can be found at any wave vector, confirming that the

single-layer SiAs2 and GeAs2 are dynamically stable. The thermodynamic

stability identified through the energy of formation, Ef, calculated from Equation

1 determines the strength of van der Waals interactions in the bulk materials of

SiAs2 and GeAs2. Thus, the lower the formation energy the easier it can be extracted

from the bulk material. It has been reported that less than 200 meV/atom is

considered to be sufficiently low formation energy that the free-standing 2D

layer can be extracted from the bulk material202. In our calculations, Ef for SiAs2

and GeAs2 is found to be 66.8 and 76.0 meV/atom, respectively. These values are

smaller than the Ef values of already exfoliated 2D materials. The values are

compared with the Ef values reported for chalcogenides of Sn and Pb 202, 207 and

are depicted in Figure 5.2c. According to this evaluation, the exfoliation of

monolayer for these materials from their bulk forms is highly feasible.

of 163


The band structures determined by HSE-Wannier calculations demonstrate that

bulk, bilayer and monolayer structures of SiAs2 and GeAs2 (Figure 5.3a–d) are

semiconductors with indirect band gaps (the VBM and CBM locations are

marked). The bandgaps are given in Table 5.2. These results are consistent with

previously reported calculated values for both bulk and mono-layers of GeAs2 with

values of 0.99 and 1.64 eV, respectively 197. The decrease in the thickness of both

SiAs2 and GeAs2 leads to a quantum confinement effect 208 and thus the band

gap is increased significantly from the bulk to the monolayer structures in both

materials. In addition, the experimentally reported indirect bandgap of 1.06 eV

by Rau et al. for single crystal orthorhombic GeAs2 is in agreement with our

calculated result for the bulk material 192.

Figure 5.2: Phonon band structure of a monolayer of (a) SiAs2, and (b) GeAs2 along the high-symmetric points in the 1st Brillouin zone. (c) Energy of formation of the 2D material from its bulk counterparts. The green bar indicates experimentally synthesised, Red bar indicates experimentally not synthesised and the Blue bar indicates theoretically proven for the possibility of synthesis.

The ability of a GeAs2 monolayer to harvest solar light in the visible region is

higher, both in absorption intensity and in the range of wavelengths covered than

that of SiAs2. This is shown in Figure 5.4, which shows the light absorption

spectrum calculated from HSE functional in the visible light region (approx.

350–800 nm) compared with the AM1.5G solar spectrum. However, both

materials exhibit an almost equal absorption in the wavelength region of 350–

600 nm. Above this, GeAs2 still has good absorption up to around 700 nm.

of 163


Table 5.1: Calculated structural parameters of SiAs2 and GeAs2 compared with the

experimental values

SiAs2 GeAs2

bulk (calc.)

bulk (exp.)

monolayer (calc.)

bulk (calc.)

bulk (exp.)

Monolayer (calc.

a (Å) 3.691 3.636 191 3.676 3.795 3.728 209 (3.721 210)

3.760

b (Å) 10.124 10.37 191 10.258 10.362 10.16 209, 210 (10.12 210 )

10.397

c (Å) 14.857 14.53 191 — 14.666 14.76 209, 210 (14.74 210)

--

Figure 5.3: Band structure for SiAs2 and GeAs2 calculated by the HSE-Wannier function method. The Fermi level is set as zero. (a), (b) and (c) Bulk, Bilayer and Monolayer of SiAs2 respectively; (d), (e) and (f) Bulk, Bilayer and Monolayer of GeAs2 respectively

of 163


Table 5.2: Calculated band gaps of SiAs2 and GeAs2 – for bulk, bilayers, and monolayers.

SiAs2 GeAs2

Bulk Bilayer Monolayer Bulk Bilayer Monolayer

Bandgap

(eV) 1.34 1.86 1.91 0.99 1.34 1.64

Figure 5.4: Calculated light absorption spectrum of monolayers of SiAs2 (green) and GeAs2 (blue) using HSE functional superimposed to the incident AM1.5G solar flux.

It has been reported that exterior strain on semiconductor nanostructures,

especially at the two-dimensional level, influences the electronic properties and

the corresponding optical properties179, 180. We, therefore, studied the PBE

functional band gap variation as a function of tensile strain (Figure 5.S1). The

bandgap variation depends on the viewing direction along the lattice. In the

of 163


laboratory, an external strain can be imparted by different means such as adlayer–

substrate lattice mismatch, external loading, bending or by applying stress on the

material 181, 182, 184, 211. While the expansion or compressive strain in the direction

of a can transform the semiconductor into a metal 201. Higher strains (deduced by

extrapolation, not shown in the figure) and compression can cause a continuous

increase in the bandgap from compressive strain to expansion strain in both SiAs2

and GeAs2. This indicates that there is the possibility of bandgap tuning to make

Figure 5.5: (a) and (c) GW-Band structures, (b) and (d) BSE-Optical Absorption spectra of SiAs2 and GeAs2 respectively

the semiconductor suitable for a desired electronic application. However, this

bandgap variation was calculated using a PBE functional that is known to

underestimate the value of the bandgap. However, for the purpose of illustrating

of 163


the dependence of the bandgap variation on the strain, these calculations can be

helpful.

The quasi-particle band gaps of SiAs2 and GeAs2 monolayer compounds were

found to be 2.26 and 1.86 eV, respectively (Figure 5.5). These values are

comparable with the band gaps obtained by using the HSE-Wannier method (1.91

and 1.64 eV for SiAs2 and GeAs2, respectively) with similar and negligible variation

amongst the two methods. The corresponding first absorption peaks in the

absorption spectra are the optical band gaps at 2.01 and 1.72 eV, respectively.

Thus, the calculated exciton binding energies212-214 are 0.25 and 0.14 eV for

SiAs2 and GeAs2, respectively. Semiconductors with exciton energies in this

range of a few hundred milli-electron volts are supposed to play a key role in

photovoltaic applications 215.

5.5.CONCLUSION

We have presented 2D monolayer compounds of SiAs2 and GeAs2 as promising

light harvesting semiconductor materials for solar cell applications. The

extraction of a monolayer of these materials is likely to be feasible by mechanical

exfoliation. Moreover, the calculated phonon spectrum reveals its high dynamical

stability for both materials. Additionally, the exciton binding energies are quite low

and are comparable to quantum dot semiconductors. It might be possible that these

semiconductors could be synthesized as quantum dots and studied in further detail.

Bandgap tuning appears also possible and could be used to tailor the compounds

for various electronic applications.

Acknowledgements

We acknowledge generous grants of high-performance computer time from

of 163


computing facility at the Queensland University of Technology, The Pawsey

Supercomputing Centre and Australian National Facility. A.D. greatly

appreciates the financial support of the Australian Research Council under

Discovery Project (DP130102420 and DP170103598).

ORCID® iDs

Sri Kasi Matta - https://orcid.org/0000-0003-4465-640X

License and Terms This is an Open Access article under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The license is subject to the Beilstein Journal of Nanotechnology terms and conditions: (https://www.beilstein-journals.org/bjnano)

The definitive version of this article is the electronic one which can be found at: doi:10.3762/bjnano.9.116

Supporting Information Supporting Information shows the band gap variation as a function of tensile strain.

Supporting Information File 1 Additional computational data. [https://www.beilstein-journals.org/bjnano/content/ supplementary/2190-4286-9-116-S1.pdf]

of 163


5.6.SUPPORTING INFORMATION

Computational exploration of two-dimensional silicon

diarsenide and germanium arsenide for photovoltaic

applications

Sri Kasi Matta1, Chunmei Zhang1, Yalong Jiao1, Anthony O'Mullane1 and Aijun Du1,*

Address: School of Chemistry, Physics and Mechanical Engineering, Queensland University of

Technology, Garden Point Campus, QLD 4001, Brisbane, Australia

* Corresponding author Email: Aijun Du - [email protected]

Additional computational data

We studied the variation of the bandgap as a function of the tensile strain by

using the PBE functional (Figure 5 . S1). The bandgap variation depends on

the viewing direction along the lattice. The external strain on semiconductor

nanostructures influences electronic properties and, thereby, optical properties 179,

180.

Figure 5.S1: Band gap as a function of biaxial strain calculated with the PBE functional for monolayers of (a) SiAs2 and (b) GeAs2.

of 163

Chapter-6: Carbon Dots for Hole‐Transport in Perovskite Solar Cells

Chapter-6: Carbon Dots for Hole‐

Transport in Perovskite Solar Cells

6.0 Density Functional Theory Investigation of Carbon Dots as

Hole‐transport Material in Perovskite Solar Cells

Sri Kasi Mattaa, Chunmei Zhang a, Prof. Anthony P. O'Mullane a, Prof. Aijun Dua,*

a School of Chemistry, Physics and Mechanical Engineering, Queensland University of

Technology, Gardens Point Campus, QLD 4001, Brisbane, Australia. *E-


Version of Record online:08 October 2018

Accepted manuscript online:25 September 2018

Manuscript received:29 August 2018

https://doi.org/10.1002/cphc.201800822

of 163


6.1.ABSTRACT

Charge transfer in solar cells is crucial, and so is the hole transporting layer (HTL)

component in perovskite solar cells (PSCs). Finding a suitable material for this

purpose that is inexpensive – either organic or inorganic – is currently one of the

prime research objectives to improve the performance, through charge transfer

dynamics, of PSCs. One such recent finding is carbon quantum dots (C‐dots), which

is a simple and low‐cost organic material that could be an alternative option to the

currently employed high ‐cost and complex ‐structured hole-transporting materials

(HTMs) utilized in perovskite solar cells. A series of C‐dots functionalized with

hydrogen, hydroxyl (−OH), and carboxyl (−COOH) groups are considered in this

study for their hole‐transporting properties. The results reveal that simple

hexagonal structured C‐dots including −OH and −COOH group substituted C‐dots

have suitable valance band maximum (VBM) positions, which are suitable for hole

transport. It is discovered that the position of the functional moieties on the C‐dots

would impact the band-edge positions of the C-dots. This implies that tuning the

of 163


band position is possible so that these two‐dimensional C‐dots could, in principle,

be used for other solar-cell applications that may require different band positions

for optimal performance. As a representative example, we studied the perovskite/C‐

Dot interface of two different possible surfaces (i. e. MAI and PbI2 terminated

perovskites) combined with a hexagonal C‐Dot layer and found that there is a good

probability of charge transfer between the perovskite layer and the C‐dots, which

promotes hole transfer between the perovskite and the C‐dots.

6.2. INTRODUCTION

Conventional photovoltaic solid‐state solar cells (e. g. crystalline silicon solar cells)

work on the simple principle of charge generation through the use of light energy,

upon which there is charge transport through the material components of the cell

which are collected at counter electrodes without any chemical reaction occurring

in the process216. The solar cell efficiency for such conventional devices has been

stabilized which are now commercially used. Therefore, efficiency is not being

improved, but research is on for low-cost alternative materials. The emphasis now

is on developing cost‐effective production processes to reduce the net product cost.

In addition, there is also a need for finding alternatives to conventional solar cells

which has resulted in the rapid emergence of perovskite solar cells (PSCs). This

technology is unique due to the sweeping improvement in power conversion

efficiency (PCE), from 3.9 %87 when it was first introduced in 2009, to 22.7 %217

in 2017. This is significant progress when compared to other solar cell technologies.

The perovskite structure was first reported by Weber in 1978, as an organometal

halide CH3NH3MX3 (M=Pb, X=Chlorine (Cl), Bromine (Br) or Iodine(I)) with a

cubic structure. Significantly, these structures are stable at ambient temperature

which may have varying unit cell parameters of 5.68, 5.92 and 6.27 Å for chloride,

of 163


bromide and iodide anions respectively 218.In this study, a perovskite with iodide,

in the cubic phase considered having a lattice parameter of a=6.29 Å219.In general,

a PSC consists of a transparent conducting glass layer i. e. Fluorine Doped Tin‐

oxide (FTO) coated glass, a TiO2 layer, a perovskite layer, a hole transporting

material (HTM), and a counter electrode220, 221.,The open-circuit voltage (Voc) of

such a cell is governed by the quasi‐Fermi level difference of electrons and holes

at the contact interfaces97. Thus, the introduction of HTM layer in the interface

leads to an increase in holes and thus Voc would decrease. While many

improvements have been achieved by changing the PSC constituents, development

of superior quality HTMs is relatively little. Spiro‐OMeTAD (2,2′,7,7′‐tetrakis‐

(N,N‐di‐p‐methoxy‐phenylamine)‐9,9′‐spiro‐ bi‐fluorene) is one of the most widely

used HTMs in recently developed PSCs 221-223. Based on its special characteristics

of having a stable amorphous structure, excellent ability to form thin films and good

electrical conductivity upon doping led to its use as an HTM224, 225.,However, there

is an ongoing challenge to find alternative materials to spiro‐OMeTAD due to the

expense involved in its synthesis226 and therefore many endeavours have been

successfully made in this direction227. Examples of simple and low‐priced

alternatives for HTMs are phenylamines228,azine229, carbazole230, 231, fluorine232,

furan233, azomethine234, silolothiophene235 which have either linear or star‐shaped

structures236.

As reported previously, there is an indirect impact of structural geometry on the

charge collection efficiency (hcol) and the rate of interfacial charge

recombination223, 237-239. The existence of an alkyl chain in HTMs changes their

physical and chemical properties and therefore influences the performance of

PSCs240, 241. In addition, hole mobility, thermal and photochemical stability are

of 163


expected characteristics of a good HTM242, 243. The thickness of the HTM layer also

influences the series resistance and therefore needs to be optimal, or else the fill

factor will be affected. An optimum thickness should generally be within the 100–

200 nm range244. The hole transfer layer (HTL) and the electron transport layer

(ETL) play an important role in charge carrying but must be economical, highly

durable and more environmentally friendly to ensure commercial viability. 243 In

addition, the HTM in PSCs must minimize or avoid charge recombination at the

metal‐perovskite interface. An HTM's basic requirement is that its valence band

maximum (VBM) position should be at a higher energy level than the VBM of the

perovskite. If this can be achieved, then it will enhance the Voc of the PSC. Laura

Calio et al. reported that hole capture is influenced by the difference in the HTM's

Fermi level and the holes generated in the perovskite layer after illumination245. It

was recently reported that the Voc is controlled by splitting of the quasi‐Fermi levels

and charge recombination246. As mentioned Spiro‐OMeTAD has drawbacks of

being expensive, negatively impacting on device stability as well as possessing sub‐

optimal charge transport as evidenced by modest hole mobility and conductivity of

1.67×10−5 cm2/V−1 s−1 and 3.54×10−7 S/cm−1 respectively243. Therefore, dopants

are required for better performance, but their presence often reduces device

stability243, 247. It is suggested here that an HTM with a simple structure, to aid

device manufacturing at scale, while also possessing the desired properties for an

HTM is possible. Therefore, the focus was on a theoretical study of carbon quantum

dots (CQDs) and demonstrate their capacity for charge transfer between a

perovskite layer and the carbon dots. This interfacial hole transfer study is crucial

to establishing the viability of CQDs for the desired objective.

of 163


Xu et al. discovered CQDs in 2004100 and subsequently, extensive studies have

been undertaken towards their synthesis, characterization, and applications101.

CQDs are part of a new category of carbon fluorescent nanomaterials having

relatively simple synthesis methods, good stability and conductivity, low toxicity

and thus being environmentally friendly, and having optical properties analogous

to conventional semiconductor quantum dots102.

Graphene quantum dots (GDQs i. e. lateral nanoscale dimensioned graphene

sheets) possess only carbon atoms, and the functionalized GQDs are studied earlier

for bandgap tuning and apply for suitable application103. When the carbon atoms at

the edge of these GQDs, are saturated with hydrogen atoms, are denoted as Carbon‐

quantum dots (or also called as C‐dots) which have been studied earlier103,

104.,Various photocatalysts, based on C‐dots, which have outstanding catalytic

activity have been reported104 while other studies have proved that the edges of C‐

dots have an influence on light harvesting105, 106, 248.-Recently, it has been reported

that metal‐free g‐C3N4 in combination with C‐dots, showed excellent results for

photocatalytic water splitting.249

Here, the study on C‐dots for a new application as a hole transport material in

perovskite solar cells. The exploration study on the interaction between the

perovskite material and C‐dots has divulged a finding that some structures of C‐

dots can be used as an HTM. This study also explored the effect of structural design

changes in C‐dots on the bandgap by functionalizing the surface with electron-

withdrawing oxygenated moieties such as −COOH and −OH groups giving the C‐

dots p‐type conductivity250-252. It is observed that some C‐dots have comparable

band edge positions to Spiro‐OMeTAD and could in principle be a potential

alternative to this material.

of 163


6.3. COMPUTATIONAL METHODS

The concept of Density Functional Theory (DFT) using a plane-wave basis set

based Vienna Ab initio simulation package (VASP) code is used to do first‐

principle calculations for this study119.,Geometry optimization calculations were

done with a generalized gradient approximation in Perdew‐Burke‐Ernzerhof form

(GGA‐PBE) exchange‐correlation functional160. A Monkhorst‐Pack k‐points

grid161 of 5×5×5 was used for sampling in the first Brillouin zone for bulk

perovskite (MAPbI3) for geometry optimization. The system relaxation was done

until the residual force and energy were converged to 0.005 eV/Å and 10−6 eV,

respectively. The individual C‐dots and the C‐dot/MAPbI3 system geometries were

optimized with 1×1×1 k‐point grid, at Gamma point only, for two reasons. Firstly,

because the system considers pure C‐dot clusters, and the hybrid system is very

large having more than 400 atoms, so the computation cost would be very high, and

secondly, the only point of interest in this study is the charge transfer between the

perovskite and carbon‐dot interface, and thus the calculation at Gamma point would

give enough information about the charge transfer. The VBM and Conduction Band

Minimum (CBM) for different C‐dot clusters are estimated with respect to vacuum

potential using hybrid density functional theory based on the Heyd‐Scuseria‐

Ernzerhof (HSE) exchange‐correlation functional162 on VASP code. The

interaction energy between C‐dot and perovskite terminated with either methyl‐

ammonium‐iodide (MAI) (001) or with PbI2 (001) surfaces as top layers (i. e. with

respect to C‐dot position) was calculated by using the following equation;

(6.1)

of 163


Where , and are the total energy of the hybrid structure, and

the individual C‐dot and perovskite (i.e. either MAI or PbI2 terminated MAPbI3

surfaces), respectively253. The characterization of electron coupling at the hybrid

component interface is obtained through charge density difference in a hybrid

system, and is calculated using the following equation;

(6.2)

Where, , and are the total charge density of hybrid structure,

individual C‐dot and perovskite (i.e. either MAI or PbI2 terminated MAPbI3

surfaces), respectively.

The hybrid structure is designed with hexagonal C‐dot and perovskite as follows.

The hexagonal Carbon‐dot structure‐1 (named as CDH‐1) cluster is placed at an

approximate distance of 1.8 Å, on top of a 1×3×3 eight‐layer geometry optimised

cubic perovskite structured MAPbI3 (a=6.29 Å) surface slab (containing Pb=36,

I=108, C=34, N=36, H=216, atoms). The C‐dot cluster is placed in such a way that

there is a considerable distance between any two C‐dot clusters, to avoid any mutual

cluster interactions, had the system been extended in x or y directions. In the x-y

direction, the perovskite super cell’s periodic boundary conditions are used (i.e. 3

x 3 supercell in x-y direction) while a vacuum space of about 20 Å in the c‐direction

is added and is large enough to avoid inter slab interactions between neighbouring

layers. It is expected that the atoms/ions on the surface layer of perovskite, above

which the C-dot is placed would de-orient to some extent and is included within the

optimization of the geometry of the hybrid system so as to minimize the energy

within the cut-off set value. However, as the aim of study is more towards

assessment of charge transfer between perovskite and C-dot, the minor orientation

of 163


effects were not delineated in this study. Two systems were designed to place the

C‐dot on either the top of the PbI2 layer‐side or on the MAI layer‐side. In both

cases, the system is very big, and therefore only the Gamma point was used in

1stBrillouin Zone sampling. The projector augmented wave (PAW) method169 was

used to describe the electron‐ion interaction and the plane‐wave energy cut‐off is

set at 400 eV. It's prudent here to mention that the same cut‐off energy value is

maintained for hybrid C‐dot/MAI or PbI2, and the individual MAI and PbI2

structure optimization, and thus further calculating the interface energies with

equation (6.1). The supercell while subjected to system relaxation and geometry

optimization calculations with the GGA‐PBE method, the atom coordinates of the

two layers at the bottom of the perovskite were marked as fixed at the same position

as that of its bulk state position. That means the bottom two layers of atoms are not

subject to change during ionic relaxation. This is done through a ‘selective

dynamics’ setting during VASP input and a corresponding setting at each layer of

co‐ordinates254. The complete system (C‐dot/MAPbI3 structure) contained a total

of 468 atoms with 1548 valence electrons.

A zero‐damped van der Waals correction was included using Grimme's

scheme167, 168, for effectively expressing non‐covalent bonding interactions. The

spin‐orbital‐coupling (SOC) effect was not considered in our calculations for the

reason that the PBE level calculations could elucidate the electrical properties

comparable to experiments, though it was incidental i. e. the unintended

cancellation of the underestimation by the GGA–PBE method90, 91, 255.

6.4. RESULTS AND DISCUSSION

The C‐dots considered for this study are shown in Figure 6.1. The hybrid system

design used in our calculations is depicted in Figure 6.2. The C‐dots are constructed

of 163


as such that the terminal carbon atoms are saturated with hydrogen atoms. In

addition to these C‐dots functionally modified structures were also considered, i. e.

−OH and −COOH group attached to C‐dots, to study the possibility of band edge

position tuning. The various types and sizes of C‐dots studied here are given with

a code notation outlined in Figure 6.1. Further examples of C‐dots considered in

this study are shown in Figure 6.S1. These C‐dots are considered as individual

entities and therefore are disjoint to each other and thus are regarded as being

clusters. The C‐dot clusters are structured in the unit cell such that the inter-cluster

distance (i. e. between any two C‐dots) is about 5 Å, which is large enough (more

than twice that of the crystallographic Van der Waals Radii of edge hydrogen atoms

(1.2 Å256)), to avoid any interaction between the periodical C‐dots. Geometry

optimisation or relaxation was performed for these C‐dots and the MAPbI3 unit cell

by using the conjugate gradient method (CGA) on VASP.

Figure 6.1: The structures of Hexagonal C‐dots (Hex‐1 [a] & Hex‐2[b]) and the functionalised Hexagonal C‐dots (Hex‐1 [c] & [d])) with −OH and −COOH groups. Color Code of atoms: Brown (Carbon), Blue (Hydrogen), Red (Oxygen).

of 163


Figure 6.2: (a) The top view and the side views of (b) C‐Dot/MAI, and (c) C‐Dot / PbI2interfaces. Color Code of atoms: Brown (Carbon), Blue (Hydrogen), Green (Nitrogen), Black (Lead) and Pink (Iodine).

The study of charge transfer between C‐dots and the perovskite surface atoms

requires determining the band edge positions of each component of the hybrid

system. The band edge positions i. e. VBM and CBM of all C‐dots are calculated

with an HSE06 exchange‐correlation functional162. As the primary focus of the

study is on the assessment of new C‐Dots, the band edge positions of perovskite

(MAPbI3) are taken from literature. The summary of band positions of important

C-dots compared with the literature value257 of the band edge positions of MAPbI3

is depicted in Figure 6.3. The band edge positions for other C‐dots considered in

this study are given in the Supporting Information (Figure 6.S2). The quantum

confinement effect is clear when the bandgap of Hex‐2 (bigger sized C‐dot) is

compared with that of Hex‐1 (smaller sized C‐dot). For the current study, the VBM

positions of Hex‐1, Hex1‐OH, and Hex1‐COOH have reasonable energies for the

purpose of hole transport.

of 163


Figure 6.3: The calculated band edge positions (i. e. VBM and CBM) of the C‐dots compared with those of MAPbI3 (literature values of VBM and CBM). Hex1=Hexagonal C‐Dot (type‐1, smaller or base size); Hex2=Hexagonal C‐Dot (type‐2, bigger in size)

The VBM of Hex 1 of −5.23 eV is comparable to poly [bis (4 phenyl) (2,4,6

trimethylphenyl) amine] (PTAA97),(2,2′,7,7′ tetrakis (N,N di p methoxyphen

ylamine) 9, 9′ spriobifluorene (Spiro OMeTAD)219 and poly[N,N′ bis (4

butylphenyl) N,N′ bisphenyl benzidine] (PTPD97). For Hex 2 the VBM of −4.92

eV is comparable to poly((4,8 bis(octyloxy) benzo(1,2 b:4,5b′) dithiophene 2,6

diyl) (2((dodecyloxy) carbonyl) thieno (3,4b) thiophenediyl) (PTB1)97 and poly(3

hexylthiophene2,5 diyl) (P3HT97).The VBM of Hex1 COOH with a value of

−5.37 eV is comparable to poly 4′,4′′(4 (2 octyldodecyl) N, N diphenylaniline)

alt 2,7 (9,9′ spirobi[fluorene]) (ASFH97). Upon further consideration of more

functionalized C dots through the same analysis, it was found that Hex 1 2COOH

2OH (Code: H1 2 C2O), would give similar band edge positions, for both VBM

and CBM to that of Spiro OMeTAD (Table 6.1). This implies that the

of 163


functionalization of C dots would in principle impact the band positions. Further

investigation done on the effect of changing the bonding position of the functional

groups in the Hex 1 structure. This revealed an interesting feature in that the band

edge positions can be further modified. This indicates that fine tuning of the band

gap and the edge positions is possible. The band edge position diagram for the

additional C dots considered in this study along with their structures is given in the

Supporting information.

Table 6.1. Calculated CBM and VBM band edge positions few two‐dimensional Carbon‐

Dots.

C‐Dot (Code Name) [a] CBM* VBM*

H1‐2C2O −2.16 −5.24

H1‐2C2O‐1 −2.39 −5.39

H1‐2C2O‐2 −2.17 −5.29

H1‐2C2O‐3 −2.49 −5.28

H1‐2C1O −2.20 −5.37

H1‐2C1O‐1 −2.48 −5.44

* CBM and VBM of Spiro‐OMeTAD are −2.2 eV and −5.22 eV respectively

[a] H1 or Hex1=Hexagonal‐1; 2 C=2‐COOH; 2O=2‐OH; The suffix ‘1’,’2’ and ‘3’

represent different positions of functional groups. (Ref: Supporting Information for

figures)

The difference in energy between the VBM of MAPbI3 and the VBM of the above

C‐dots is in the range of 0.06 to 0.51 eV. As per a recent study on the effect of

of 163


interfacial energies between perovskite and organic hole transfer materials, it was

found that a very small driving force, i. e. a driving potential of about 0.07 eV would

be enough to achieve efficient hole transfer, and suggested a range of 0<∆E<0.18

eV as being appropriate97. It's inferred from Transmission Absorption Spectroscopy

(TAS) and Photoluminescence (PL) studies that over and above, ∆E 0.18 eV, the

recombination of holes with electrons in perovskite would be faster than hole

transport from perovskite to HTM97. Considering this range, it is suggested that C‐

dots, Hex‐1 (∆E=0.17 eV) and Hex‐1‐COOH (∆E=0.03 eV) could be used as

potential hole transfer materials for perovskite solar cells.

The interface binding energy is calculated as per equation (1) and is found to be

−2.50 eV for C‐dot/MAI perovskite hybrid, and −3.00 eV for the C‐dot/PbI2

perovskite hybrid, which indicates stronger structural stability with the C‐dot/PbI2

hybrid. However, it was observed that the strong interface between the C‐dots and

MAI and PbI2 led to some structural changes in the different layers. It is apparent

that the interaction is greater at the topmost layers and gradually decreases while

approaching the bottom layers. No such change is observed at the bottom two layers

as the atoms’ position was fixed during optimization. The changes in the C−C and

C−H bond lengths in the optimized hybrid structure compared with that of the

pristine C‐dot reveals a subtle increase but they are insignificant (the maximum

change in bond length observed is 0.001 Å) in C−C bonds whereas no change

observed in C−H bonds.

The charge density difference between hybrid and pristine components of the

system, calculated using equation (2), are shown in the three‐dimensional (3D)

graphic in Figure 6.4. For the C‐dot/PbI2 system charge transfer from the C‐dot to

the PbI2 surface in the ground state is observed. More charge accumulation is

of 163


observed just below the surface of the C‐dot indicating that charge gathering occurs

because of the C‐dots presence on the surface results in strong interface energy.

Moreover, charge accumulation is also observed at each of the MAI layers in C‐

dot/PbI2 system (Figure 6.4(a)) which gradually increases from the bottom to the

top layers. Interestingly, though there is a gradual change in charge accumulation

observed in the MAI layers case of the C‐dot/MAI system, this change in

accumulation is much less when compared with C‐dot/PbI2 system. This could be

attributed to the interface binding energy difference between the two systems.

Figure 6.4: The side view of the charge density difference between the two interface systems (a) C‐Dot/PbI2, and (b) C‐Dot/MAI. The Yellow and Cyan iso‐surface profiles represent electron accumulation and depletion in 3D space with an isovalue of 0.001 e/Å−3. Colour Code of atoms: Brown (Carbon), Blue (Hydrogen), Green (Nitrogen), Black (Lead) and Pink (Iodine).

6.5. CONCLUSIONS

Different structural types of C‐dots including those functionalized with −OH and

−COOH moieties have been identified as potential hole transfer materials for

perovskite solar cells. It is also observed that not only functionalizing the surface

with −OH and −COOH groups but also that their bonding position on the C‐Dot

would impact the bandgap and band edge positions. This reveals an excellent

of 163


method of tuning the band positions of C‐dots to the desired value. The VBM levels

for these selected C‐dots are suggested to be more favourable for the objective of

good hole transfer after considering the minimum driving force that is required for

an effective hole transferring material. Furthermore, the typical charge transfer

study based on charge density difference estimations prove that there is more

efficient charge transfer between the C‐Dot/PbI2 system interfaces compared to that

of C‐Dot/MAI system interface. C‐dots can be effective hole transfer materials that

can be synthesized easily compared to organic materials while being cost‐effective

as well as processable to fabricate thin layers which are required for solar cell

applications. In addition, this would have a future scope for experimental studies

for testing the predictions made here in this study and for their non‐toxicity, to

justify their environmentally friendly characteristics, compared with other

conventional organic HTMs.

Acknowledgements

Acknowledgements Text A.D acknowledges the financial support by Australian

Research Council under Discovery Project (DP170103598) and

computer resources provided by high‐performance computer time from computing

facility at the Queensland University of Technology, NCI National Facility, and

The Pawsey Supercomputing Centre through the National Computational Merit

Allocation Scheme supported by the Australian Government and the Government

of Western Australia.

Conflict of interest

The authors declare no conflict of interest.

of 163


6.6. SUPPORTING INFORMATION

© Copyright Wiley-VCH Verlag GmbH & Co. KGaA, 69451

Weinheim, 2018

Density Functional Theory Investigation of Carbon

Dots as Hole-transport Material in Perovskite Solar

Cells

Sri Kasi Matta, Chunmei Zhang, Anthony P. O’Mullane, and Aijun Du*

Figure 6.S1: The structures of various Carbon Quantum dots (C-Dots). Trigonal C-Dots (a –c), Hexagonal C-Dots with two –OH functional groups substituted (d) side by side, and (e) opposite to each other. Color Code of atoms: Brown (Carbon), Blue (Hydrogen), Red (Oxygen).

of 163


Figure 6.S2: The calculated band edge positions (i.e. VBM and CBM) of further C-Dots considered in this study and compared with those of MAPbI3 (literature values of VBM and CBM) [Tri-1 – Tri-3 correspond to three different Trigonal C-Dots (S1(a-c)); Hex-1-OH-SbS (side by side) and Hex-1-OH-oppo (opposite) correspond to S1(d) and S1(e) of Figure-S1

Figure 6.S3: The structures of Hexagonal (Type-1) Carbon Quantum dots (C-Dots) with different bonding locations of two Carboxyl and two hydroxyl groups. The represented Code-named C-Dots are: (a) H1-2C2O-1, (b) H1-2C2O-2, (c) H1-2C2O-3 and (d) H1-2C2O-4.

of 163


Figure 6.S4: The structures of Hexagonal (Type-1:H1) Carbon Quantum dots (C-Dots) with different bonding locations of two Carboxyl and One Hydroxyl groups. The represented Code-named C-Dots are: (a) H1-2C, (b) H1-2C1O-1, (c) H1-2C1O-2.

Code name notation: H1 or Hex1= Hexagonal-1; 2C = 2-COOH; 2O = 2-OH; The suffix ‘1’ and ’2’ represent different positions of functional groups

Figure 6.S5: The structures of Rectangular (Type-1 (R1) and Type-2(R2)) Carbon Quantum dots (C-Dots) with different bonding locations of One or two Carboxyl (1C or 2C), One Hydroxyl (1O) and/or One Hydroxyl & One carboxyl (1O-1C) groups. The represented Code-named C-Dots are: (a) R1-1C, (b) R1-1O, (c) R1-2C, (d) R1- 2O, (e) R2-1C, (f) R2-1O, (g) R2-2C and (h) R2-1C1O.

Code Name notation: R1 & R2 = Rectangular (1 (small or base size) & 2 (Bigger size)); 1C = 1-COOH; 1O = 1-OH; 2C = 2-COOH; 2O = 2-OH

of 163


Table 6.S1: Calculated CBM and VBM band edge positions few two-dimensional

Carbon-Dots

C-Dot (Code)

CBM* VBM*

H1-2C2O -2.16 -5.24

H1-2C2O-1 -2.39 -5.39

H1-2C2O-2 -2.17 -5.29

H1-2C2O-3 -2.49 -5.28

H1-2C -2.22 -5.59

H1-2C1O-1 -2.20 -5.37

H1-2C1O-2 -2.48 -5.44

R1-1C -3.05 -4.38

R1-1O -3.23 -3.83

R1-2C -3.18 -4.53

R1-2O -2.88 -4.23

R2-1C -3.33 -3.95

R2-1O -3.22 -3.83

R2-2C -3.44 -4.07

R2-1C1O -3.34 -3.95

* CBM and VBM of Spiro-OMeTAD are -2.2 and -5.22 respectively. Code name notation given in Figure captions, above.

of 163



of 163

Chapter-7: Inorganic 2D materials for charge transport in Perovskite Solar Cells

Chapter-7: Inorganic 2D materials for

charge transport in Perovskite Solar Cells

7.0 Band alignment investigation of multi-layered Lead Iodide,

Lead monoxide and Tin monoxide: Inorganic Hole-transporting

materials for Perovskite Solar Cells

Sri Kasi Mattaa, Chunmei Zhang a , Prof. Anthony P. O'Mullane a, Prof. Aijun Du a ,*

a School of Chemistry, Physics and Mechanical Engineering, Queensland University of

Technology, Gardens Point Campus, QLD 4001, Brisbane, Australia. *E-


of 163

Chapter-7: Inorganic 2D materials for charge transport in Perovskite Solar Cells

7.1.ABSTRACT

Exploring innovative two-dimensional (2D) materials for hole transporting layers

(HTL) in Perovskite Solar Cells (PSCs) is emphasized in this study with Density

Functional Theory (DFT) analysis as a study tool. Simple and layered crystal

structures of inorganic, post-transition metal halides and oxides (lead iodide (PbI2),

lead monoxide (PbO), tin-oxide (SnO)) are considered. The band edge alignment

of the materials with respect to the perovskite (MAPbI3) layer, is studied to check

their suitability for HTL. Conventional DFT with more accurate hybrid functionals

(HSE06) is used in this computational study, including variation in band edges and

bandgaps with an increase of 2D layers for PbI2, PbO and SnO. We found that a

single-layered PbO and bi-layered SnO are potential candidates for a hole transport

material. Further representative studies on the charge density difference at

perovskite-HTL material interfaces were conducted for PbO with both Methyl

Ammonium-Iodide (MAI) and Lead Iodide (PbI2) surface terminated perovskite

(MAPbI3), with 3D visualisation techniques to establish hole transport.

7.2.INTRODUCTION

Since being introduced by Akihiro Kojima et al. in2009 with an efficiency

of 3.8%87, 216, perovskite solar cells’ (PSCs) power conversion efficiency

(PCE) had shown an almost six-fold improvement within a span of eight

years, to around 22%87, 258-264. This shows the promise and potential

feasibility of improving the efficiency further by tuning and optimizing the

parameters involved in their operation. Out of the various parameters that

influence efficiency, electron and hole transfer from the perovskite layer to

the ETL and HTL materials plays a key role in PSC architecture. The PSCs

of 163 Chapter-7: Inorganic 2D materials for charge transport in Perovskite Solar Cells

architecture comprises of FTO glass, TiO2 film, perovskite film with the hole

transporting material (HTM) and electron transporting material (ETM) on

either side of it and a counter electrode after the HTM219-221. For the most

efficient operation, the VBM position of the HTM should be above the

valance band energy of the perovskite. Similarly, the CBM position of the

ETM shall ideally be at lower energy than the conduction band of the

perovskite223. Many organic and inorganic ETMs and HTMs were identified

and tested since its inception. However, there are still issues with cost-

effectiveness and development of simple synthesis methods to produce

effective ETM and HTM at scale. It is well established that the open-circuit

voltage (Voc) is the difference between the quasi-Fermi level of the electrons

in the TiO2 layer and valance band position of the HTM265, 266. Increasing this

difference would thus achieve a higher Voc and is a subject of intense

research226. Recently an inorganic CsMBr3 (M=Sn, Bi, Cu) material used as

an HTL resulted in a Voc value of up to 1.61 V92. The driving energy for hole

capture is the difference between the Fermi level of the hole transporter and

that of the holes in the perovskite under illumination245. It has recently been

reported that the Voc is also controlled by splitting of the quasi-Fermi levels

and recombination246. The organic HTM that currently shows the highest

performance in PSCs is the Spiro-OMeTAD which has drawbacks of being

expensive, process-intensive and with the need for dopants267 to get better

performance, however with the disadvantage that they reduce stability. It is

suggested here that an HTM should have as simple a structure as possible to

manufacture, but at the same time possessing the desired VBM band position

to promote hole transport and have longer stability.


In this study, we focused our attention on inorganic two-dimensional (2D)

materials viz. Lead iodide (PbI2), Lead Monoxide (PbO) and Tin-monoxide

(SnO), to study their band structure, band edge positions using ab initio

calculations and compared with that of a perovskite MAPbI3 to select their

potential suitability as HTMs.

Lead Iodide (PbI2), as reported, is a layered material with semiconducting

properties that has many applications in the optics field 108-110. Theoretical

investigation proves the bandgap changes from direct to indirect when the

bulk form is modified to a single-layered two-dimensional material. The

single crystalline quasi monolayers and few layers have been synthesized and

utilized in photodetectors110. The most stable form of PbI2 crystals, at room

temperature, is of the 2H form polytype 268-270. Single layered PbI2 nanotube

synthesis 108 and a theoretical study on PbI2 nanosheets have been reported.

The PbI2 structure of the 2H form is considered for this study with the space

group P3m1268 and is optimized. The lattice constants of the optimized

structure are a=b=4.67 Å. The vacuum slab to avoid the interaction between

layers is set to 30 Å. The PBE band gap has already been reported as 2.51

eV271. The HSE06 hybrid functional resulted in predicting an indirect

bandgap at 3.32 eV, confirming the reported feature271 which is very close to

the published results110. The VBM and CBM are estimated with HSE06 band

structure and are corrected with vacuum potential as depicted in Figure 7.S2

and Table 7.S1.

Two-dimensional Lead Monoxide (PbO) is another inorganic

semiconducting material who’s mechanical, and exfoliation feasibility and

optical properties were investigated and reported35, 111-113. Favourable


attributes for charge transfer or charge accumulation, high resistivity and

low-cost availability could make PbO a potential candidate for an HTM. In

addition, PbO is hydrophobic which could further help in the long-term

stability of the HTM and thereby the overall perovskite solar cell 113. In this

study, the P4/nmm space group structure of PbO is considered with lattice

parameters a≠c, a=4.02 Å and c= 5.02 Å. The PbO monolayer exhibits n-type

conduction, as reported, the reason being the holes are stagnant and the

conduction happens due to electrons113. Successful exfoliation of α-PbO is

feasible as reported113 due to a very low interlayer interaction energy ~0.016

eV113. α-PbO (ICSD2-94333) belongs to the P4/nmm space group and

photoluminescence decay studies on PbO revealed that these atomic sheets

are more robust compared to many quantum dots113. A positive tensile strain

of the monolayer changes it from a direct bandgap material to an indirect

bandgap material272. However, the unstrained monolayer is non-magnetic,

but when it is doped with a hole, it induces ferromagnetism. The VBM and

CBM are estimated with HSE06 hybrid functional and are shown in Figure

7.3.

Pure Tin Oxide (SnO), having a tetragonal unit cell and with a P4/nmm

space group having lattice parameters a=b=3.802 Å and c=4.836 Å is

considered for this study. It is reported that the pure phase of SnO might not

have high hole mobility but would show high mobility when it is doped with

controlled residual amounts of metallic tin114. However, our current study is

intended to analyse the band alignment w.r.t the perovskite’s band positions

to elucidate the feasibility of hole transport for a PSC, therefore we restricted

2 Inorganic Crystal Structure Database


the focus on pure SnO monolayers. The experimental realization of few-

layered SnO was reported with p-type behaviour making it a favourable

component for 2D logical devices115, 116. The layered SnO structure

comprises, that each Sn atom bonded with four O atoms and vice versa 273, 274

Figure 7.1 (e-f). It is theoretically and experimentally determined that the

direction of high mobility is along the c-axis, that is perpendicular to the SnO

layer/s. The Band structure of SnO monolayer and the band alignment of SnO

bi-layer are shown in Figure 7.2 (c) and Figure 7.3 respectively.

7.3.COMPUTATIONAL METHODS

The Density Functional Theory (DFT) method using a plane-wave basis

set based Vienna Ab initio simulation package (VASP) code is used for first-

principle calculations for this study118. The geometry optimization is done

with a generalized gradient approximation in Perdew-Burke-Ernzerhof form

(GGA-PBE) exchange-correlation functional160. A Monkhorst−Pack k-points

grid of 7 x 7 x 1 was used for sampling in the first Brillouin zone for Lead

Iodide (PbI2) monolayer and 9 x 9 x 1 grid was used for both Lead Monoxide

(PbO) and Tin Oxide (SnO) monolayers for structure optimizations. All these

systems are subjected to geometry relaxation until the convergence criteria

for residual force and energy were achieved at 0.005eV/ Å and 10-6 eV,

respectively. A vacuum space of about 30 Å in the c-direction and is well

enough to avoid inter slab interactions between neighbouring layers. The

projector augmented wave (PAW) method169 was used to articulate the

electron-ion interface and the plane-wave energy cut-off is used at 400 eV.

Grimme’s scheme167, 168 was used to incorporate zero-damped van der Waals

correction for effectively describing non-covalent bond interactions. The


spin–orbital-coupling (SOC) effect was not taken into consideration in this

study as the electrical properties are satisfactorily described at the PBE level

compared with experiments,91.

The Valence Band Maximum (VBM) and Conduction Band Minimum

(CBM) for these materials are estimated and corrected with their

corresponding vacuum potentials using hybrid density functional theory

based on the Heyd-Scuseria-Ernzerhof (HSE) exchange-correlation

functional162 in VASP code. These band positions are compared with

perovskite band positions to check the feasibility and type of charge transport

mechanism in a perovskite solar cell architecture.

Figure 7.1: The structures of (a –b) PbI2 monolayer, (c-d) PbO, and (e-f) SnO monolayers top-view and perspective side-views respectively.

The two structures were designed such that there is the closest possible

lattice match between the materials and the perovskite. Accordingly, an eight-

layered 1 x 2 x 2 MAPbI3 is used as a base, on top of which PbO monolayer

is placed with 3 x 3 x 1 supercell. Herein, the PbO is placed on top of two

possible surfaces of perovskite i.e. the perovskite (MAPbI3) surfaces

terminated with PbI2 (001) and MAI (Methyl Ammonium-iodide). In all the

geometry optimization for these hybrid structures, the atoms’ positions in the


bottom two layers of the perovskite were fixed. That’s their coordinates will

not be shifted during optimization iterations. A ‘selective dynamics’ option

is used which is a feature in VASP input files. The lattice mismatch (Lm) is

calculated as (a1 – a2)/a2 x 100% where a1, a2 are lattice parameters of the

material and the perovskite, respectively. The mismatch is about 4.1%

compressive strain in a-direction which is presumed to be negligible in this

study. The following equation was used to calculate the hybrid interface

energies;

……. (7.1)

Where , and are the total energy of the hybrid structure,

and the individual PbO and MAPbI3, respectively275. The characterization of

electron coupling at the hybrid component interface is obtained through

charge density difference in a hybrid system, and is calculated using the

following equation;

(7.2)

Where , and are the total charge density of hybrid

structure, individual PbO and perovskite, respectively. Furthermore, the

quantitative Bader analysis on charge transfer between the

material/perovskite interface was calculated.

7.4.RESULTS AND DISCUSSION

The crystal structures of the inorganic semiconductors considered for this

study are shown in Figure 7.1. The study of hole transportation between the

perovskite surface and the HTM would need the energy positions of the band

edges for the perovskite and the HTMs. The band edge positions of all the


inorganic semiconductors considered in this study, corrected with the vacuum

potentials, are calculated with HSE06 exchange-correlation functional162.

The HSE06 hybrid functional band structures for the monolayer of the

selected materials are shown in Figure 7.2. The results of vacuum potentials

and the other HSE band structures for different layers are given in electronic

supporting information (ESI).

Figure 7.2: HSE Band structures of (a) PbI2, (b) PbO, and (c) SnO monolayers

It is observed that the PbI2, SnO monolayers resulted in indirect band gaps

(Figure 7.2(a,c)) whereas the PbO monolayer demonstrates a direct bandgap

of 3.35 eV (Figure 7.2(b)). It is also found that the bandgap of SnO is reduced

when the number of layers is increased to two-layers (2L). This feature is in

agreement with the reported results276 and the reason for such a feature is the

interlayer lone pair interactions of divalent Sn2+.

The summary of the band gaps calculated for these materials is given in Table

7.1. The band edge positions thus obtained from the band structures, are then

compared with literature values84 of band positions of MAPbI3, (CBM @ -

3.93 eV and VBM @ -5.43 eV). This comparison is depicted in Figure 7.3.

It is clear from the comparison that a PbO monolayer and a Bi-layered SnO

have their VBM positions at -4.98 eV and -4.81 eV respectively, which are

3.31 eV 3.95 eV 3.35 eV


more positive than the VBM value of the perovskite and are thus feasible to

support hole transport in PSCs. Further examination of the charge transfer

driving forces shows that the VBM energy of PbO and the bi-layered SnO

are at 0.38 eV and 0.62 eV more positive energy, respectively than that of the

VBM of MAPbI3.

As per the recent study on the effect of interfacial energies between

perovskite and organic hole transfer materials, it is found that a very small

driving force of about 0.07 eV would be enough to achieve efficient hole

transfer and the authors have also suggested a range of this offset as

0<∆E<0.18 eV97.

Figure 7.3: HSE06 Band edge positions (calculated CBM and VBM) of PbI2, PbO and SnO compared with that of MAPbI3 Legend: 1L=monolayer, 2L=bi-layer

Considering this range, we can hereby suggest PbO and SnO-bilayers could

easily transport holes in perovskite solar cells and thus can be used as HTMs.

Further exploration of the charge distribution of one of these material hybrids

was conducted. As a reference, PbO/Perovskite interface structures (Figure

7.4) for both possible surfaces of perovskite (i.e. PbI2 and MAI as top

surfaces) are evaluated. The interaction/binding energies between the

PbO/perovskite interfaces were calculated using equation (1).


Figure 7.4 : The top and side view hybrid structures of PbO/PbI2 and PbO/MAI terminated hybrid structures respectively.

The binding energy between PbO/perovskite (i.e. PbI2 and MAI surface

perovskite) interfaces is -3.66 eV and -4.52 eV, respectively. Thus, the

PbO/MAI-terminated perovskite interface is more stable. The charge density

difference between the hybrid and pristine components of the system are

calculated using equation (2). The results are pictorially shown in the three-

dimensional (3D) graphic in Figure 7.5.

Table 7.1: Bandgaps of the materials calculated with HSE06 functional

Band Gap / eV

Monolayer Bilayer Tri-layer

PbI2 3.31 3.19 3.11

PbO 3.35 2.95 -

SnO 3.95 1.62 0.93

It is clear from the 3D graphic that the charge depletion region is observed

in the (Figure 7.5 (a)) PbO layer. Similarly, the other materials can also be

subjected to such studies to prove their potential usage as declared in this

study.


Figure 7.5: The side view of the charge density difference at an iso-value of 0.003 e Å-3, between the two interface systems (a) PbO/PbI2, and (b) PbO/MAI. The Yellow and Cyan iso-surface profiles represent electron accumulation and depletion in 3D space. The atom positions in dotted region i.e. bottom two layers was fixed during geometry optimisation (Inset: Color Code of atoms)

In addition, the Bader analysis277-280 has been conducted on these two

selected systems, to further assess the actual charge transfer that takes place

in the two material interfaces. It is found that there is charge depletion of

0.340e (0.009 e/atom) and of 0.084e (0.0023 e/atom) in two PbO/PbI2-

teriminated perovskite and PbO/MAI-terminated surface interfaces,

respectively, in this system. Thus, it is predicted that PbO can be used as

HTM in PSCs. Similar charge transfer phenomenon is anticipated for the

other materials as per the band edge positions and can be used for the

predicted purposes, as discussed.

7.5.CONCLUSIONS

A density functional theory (DFT) study was conducted on different

inorganic materials PbI2, PbO, SnO for their suitability for hole transport in

Perovskite Solar Cells where the band edge positions of these materials were


calculated. Further examination of the charge density difference between the

interfaces of selected materials and the corresponding quantitative Bader

analysis of charge transfer was also conducted for PbO/perovskite interfaces.

We also determined the potential driving force values for charge transfer

between the perovskite-HTM interfaces. It is found that a PbO monolayer

and Bi-layered SnO could be potential hole transport materials in PSCs.

Bader analysis on the two interfaces systems quantified the charge depletion

of 0.009 e/atom and 0.0023 e/atom in PbO/PbI2 and PbO/MAI interface

structures, respectively. This has indicated a new set of possible materials

that can be used in a PSC configuration and paves the way for further

experimental scrutiny.

Conflicts of interest

There are no conflicts to declare.

Acknowledgments

A.D acknowledges the financial support by Australian Research Council

under Discovery Project (DP170103598) and computer resources provided

by high-performance computer time from computing facility at the

Queensland University of Technology, NCI National Facility, and The

Pawsey Supercomputing Centre through the National Computational Merit

Allocation Scheme supported by the Australian Government and the

Government of Western Australia.


7.6.SUPPORTING INFORMATION

Band alignment investigation of multi-layered Lead Iodide, Lead

monoxide and Tin monoxide: Inorganic Hole-transport materials for

Perovskite Solar Cells

Sri Kasi Mattaa, Chunmei Zhanga, Anthony O’Mullane a and Aijun Du*, a

a School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Gardens Point Campus, QLD 4001, Brisbane, Australia.

Corresponding Author

* [email protected]

Figure 7.S1. Vacuum potential obtained from HSE06 method for PbO (a) mono, and (b) bi-layers

Figure 7.S2. Vacuum potential obtained from HSE06 method for PbI2 (a) mono, (b) bi-, and (c) tri-layers


Figure 7.S3. Vacuum potential obtained from HSE06 method for SnO (a) mono layer, (b) bi-layer, and (c) Tri layers

Figure 7.S4. Band Structures calculated from HSE06 method for PbO (a) Mono, and (b) Bi layers

Figure 7.S5. Band Structures calculated from HSE06 method for PbI2 (a) Mono, (b) Bi-, and (c) Tri layers


Figure 7.S6. Band Structures calculated from HSE06 method for SnO (a) Mono, (b) Bi-, and (c) Tri-layers

Table 7.S1. Bandgaps, CBM and VBM obtained from HSE06 method

PbI2 PbO SnO

Mono layer

Bi layer

Tri layer

Mono layer

Bi layer

Mono layer

Bi layer

Tri layer

Bandgap /eV 3.31 3.19 3.11 3.35 2.95 3.95 1.62 0.93

CBM* /eV -3.62 -3.68 -3.72 -1.64 -1.77 -2.00 -3.19 -3.53

VBM* /eV -6.93 -6.87 -6.83 -4.98 -4.72 -5.95 -4.81 -4.46

* Corrected w.r.t Vacuum potential

Note: Some data are repeated in this Table from the main article text for the sake of uniformity.

of 163 Chapter 8: Conclusions

Chapter 8: Conclusions

The primary aim of the research study i.e. theoretical exploration of various Two-

dimensional (2D) semiconducting materials for use in different solar energy

applications has been successfully completed. Inorganic layered compounds formed

from Group IV-V elements that are less studied and some innovative organic carbon

quantum dots were considered for this investigation. The photocatalytic water

splitting, photovoltaic applications and perovskite solar cells were the main solar

technologies considered in this first-principles Density Functional Theory (DFT)

exploration. The fundamental properties viz. light harvesting capability, bandgap and

band edge positions are among the parameters that were explored here in this study.

Regarding water splitting and photovoltaic applications, the exciton binding energies

were considered for discussion. The type of interfacial charge transfer between

perovskite (MAPbI3) and the materials under investigation was also investigated as

well as the charge density differences in the hybrid structures. The perovskite/carbon

dot and perovskite/inorganic semiconductor hybrid structures were studied to check

the type of transfer i.e. either electron or hole transport.

The photocatalytic analysis on the materials revealed that 2D SiP2 can be obtained

by mechanical cleavage and its phonon spectrum predicts dynamic stability. It has

a direct bandgap of 2.25 eV which is comfortably higher than the threshold bandgap

of 1.23 eV for the material to demonstrate water-splitting capability. Furthermore,

the band edge positions perfectly encompassed the redox potentials of water which

indicates great potential for this material for use in photocatalytic water splitting.

The results highlight a new two-dimensional (2D) photocatalyst for overall water


splitting and it is expected to guide future experiments to produce H2 and O2 with

high-efficiency based on this material.

Photovoltaic application feasibility was explored for two other single-layer

materials, SiAs2 and GeAs2. It was found that exfoliation is highly feasible for these

materials and thus experimental mechanical cleavage is predicted to be possible.

Monolayers of SiAs2 and GeAs2 possess a bandgap of 1.91 and 1.64 eV,

respectively, which is excellent for sunlight harvesting and furthermore band-gap

tuning is possible by application of tensile strain. The SiP2 monolayers also show

bandgap tuning by applying tensile strain. In all three materials HSE06 (Wannier90)

band structures were explored and discussed. The exciton binding energies of these

materials was found to be 0.25 and 0.14 eV, respectively. The results highlight a

new family of 2D materials with great potential for photovoltaic applications.

Materials for facilitating Electron or Hole transport in Perovskite solar cells were

explored. Innovative Carbon quantum dot structures including functional moieties

viz. −OH, and −COOH was designed and investigated. The simple hexagonal

structured C‐dots functionalised with −OH and −COOH groups have suitable

valance band maximum (VBM) positions, compared with that of MAPbI3, so that

hole transport from the perovskite is feasible. More C‐dot structures with

functionalisation of -OH and -COOH groups were also identified as potential hole

transfer materials for perovskite solar cells. It was also determined that the

functional groups’ bonding position on the C‐Dot would impact the bandgap and

band edge positions. This reveals an excellent method of tuning the band positions

of C‐dots to the desired value. The typical charge transfer study through charge

density difference assessments also corroborates the fact of hole transport in the

selected C-dots. A small variation in charge transfer between the C‐Dot/PbI2


system interfaces compared to that of C‐Dot/MAI system interfaces was observed.

Thus, C‐dots can be potential hole transport materials that may be cost-effective to

produce. In addition, this would have a future scope for experimental studies for

testing the predictions made here due to their non‐toxicity and environmentally

friendly characteristics, compared with other conventional organic HTMs.

Further study on inorganic 2D materials for HTLs in PSCs was conducted on

layered crystal structures of post-transition metal halides and oxides (lead iodide

(PbI2), lead monoxide (PbO), tin-oxide (SnO)). The CBM and VBM positions and

HSE06 band structures were analysed and summarised to discuss the type of charge

transport that would be feasible with these materials. Further examination of the

charge density difference in the interfaces of perovskite/ HTL for PbO monolayer

and the corresponding quantitative Bader analysis was also discussed. It was

determined that a PbO monolayer and Bi-layered SnO could be potential hole

transport materials in PSCs. A predicted sample values of charge transfer obtained

through Bader analysis for the charge depletion of 0.009 e/atom and 0.0023 e/atom

in PbO/PbI2 and PbO/MAI interface structures, respectively. This study has

therefore identified a new set of possible charge transport materials for PSCs and

paves the way for further experimental work to validate this prediction.

Finally, the research study reveals a few innovative 2D materials for various solar

energy applications and provides scope for further study both theoretically and

experimentally.

of 163 Chapter 9: Future work

Chapter 9: Future work

Based on the research study presented here, we anticipate more insights for the lab-

scale realisation of these new and innovative 2D materials.

9.1 THEORETICAL

The materials explored in this research were less explored computationally and to

some extent, have limited experimental data or no data. This pave the way for future

theoretical exploration of detailed study on the inorganic charge transport materials

PbO, SnO-bi layer for their hole carrier mobilities. Similarly the newly designed

organic C-dots need additional studies of such carrier mobilities and stress and strain

impacts on bandgap variation. As these organic C-dots are simple structured, it might

be possible to conduct experimental studies in parallel to theoretical investigations.

9.2 EXPERIMENTAL WORK

This might take the form of collaboration with experimental scientists from QUT or

might be with universities in Australia or even abroad. In the light of the fast-growing

emerging Perovskite Solar Cell technology research, the HTM studied here in this

research viz. Carbon dots would surely attract attention from the experimentalists and

theoretical physics researchers for the further construction of new models and

analysis for various applications. Some of the materials studied in this research have

good feasibility of producing mono layered crystals, hence there is a quite good scope

for future experimental work. Also, the band-gap tuning possibility with applied

stress or strain can collaborate with experimental physicists in lab-scale studies. The

possibility of applying stress by bending apparatus as described in H.J. Conley et

al.181, or applying external load as explained in J. Feng et al.182 could be explored.

Another method that was referred to, for inducing the strain in crystals, in the main


research chapters is lattice-mismatch. Experimentally this can be done by carefully

selecting a suitable substrate which has a lattice mismatch with that of layered

material that is to be grown on the substrate. Depending on the lattice-mismatch

between the substrate and the material there would be an intrinsic strain in the crystal

formed on the substrate. An example of this method is growing an epitaxial

monolayer of MoS2 on mica as substrate as reported by Q. Ji et al. 183

of 163 References

References

JGD

1. Matta, S. K.; Zhang, C.; Jiao, Y.; O'Mullane, A.; Du, A., Versatile two-dimensional silicon diphosphide (SiP2) for photocatalytic water splitting. Nanoscale 2018, 10 (14), 6369-6374. 2. Matta, S. K.; Zhang, C.; Jiao, Y.; O'Mullane, A.; Du, A., Computational exploration of two-dimensional silicon diarsenide and germanium arsenide for photovoltaic applications. Beilstein Journal of Nanotechnology 2018, 9, 1247-1253. 3. Kasi Matta, S.; Zhang, C.; O'Mullane, A. P.; Du, A., Density Functional Theory Investigation of Carbon Dots as Hole-transport Material in Perovskite Solar Cells. ChemPhysChem 2018, 19 (22), 3018-3023. 4. Zhang, C.; Jiao, Y.; Ma, F.; Kasi Matta, S.; Bottle, S.; Du, A., Free-radical gases on two-dimensional transition-metal disulfides (XS2, X = Mo/W): robust half-metallicity for efficient nitrogen oxide sensors. Beilstein Journal of Nanotechnology 2018, 9, 1641-1646. 5. Tang, C.; Zhang, C.; Matta, S. K.; Jiao, Y.; Ostrikov, K.; Liao, T.; Kou, L.; Du, A., Predicting New Two-Dimensional Pd3(PS4)2 as an Efficient Photocatalyst for Water Splitting. J. Phys. Chem. C. 2018, 122 (38), 21927-21932. 6. He, T.; Matta, S. K.; Du, A., Single tungsten atom supported on N-doped graphyne as a high-performance electrocatalyst for nitrogen fixation under ambient conditions. PCCP 2019, 21 (3), 1546-1551. 7. Tang, C.; Ma, F.; Zhang, C.; Jiao, Y.; Matta, S. K.; Ostrikov, K.; Du, A., 2D boron dichalcogenides from the substitution of Mo with ionic B2 pair in MoX2 (X = S, Se and Te): high stability, large excitonic effect and high charge carrier mobility. J. Mater. Chem. C 2019. 8. McKinsey Energy Insights; 2017. 9. Enerdata Eneroutlook; 2018. 10. BP Energy Outlook; 2018. 11. Boden, T. A., G. Marland, and R.J. Andres. Global, Regional, and National Fossil-Fuel CO2 Emissions; Carbon Dioxide Information Analysis Center: USA, 2017. 12. Edenhofer, O., R. Pichs-Madruga, Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J. Savolainen, S. Schlömer, C. von Stechow, T. Zwickel and J.C. Minx (eds.) IPCC (2014): Climate Change 2014:Mitigation of Climate Change; 2014. 13. Lewis, N. S.; Nocera, D. G., Powering the planet: Chemical challenges in solar energy utilization. Proceedings of the National Academy of Sciences 2006, 103 (43), 15729-15735. 14. World Energy Assessment: United Nations Development Programme, United Nations Department of Economics and Social Affairs, World Energy Council, World Energy Assessment and the Challenge of Sustainability (2000). In Compendium of Sustainable Energy Laws, Cambridge University Press: pp 1-30. 15. Hartmann, D. L. e. a., Observations: Atmosphere and Surface. In Climate Change 2013 - The Physical Science Basis, Cambridge University Press: 2013; pp 159-254. 16. L.A. Meyer (eds.), R. K. P. IPCC (2014): Climate Change 2014:Synthesis Report; 2014. 17. Rijsberman, F. R. S., R. J. (eds.) Targets and Indicators of Climatic Change. Report of Working Group II of the Advisory Group on Greenhouse Gases. Stockholm

of 163 References

Environment Institute. Draft version.; The Stockholm Environment Institute: Sweden, 1990; p 166p. 18. Rockström, J.; Steffen, W.; Noone, K.; Persson, Å.; Chapin, F. S.; Lambin, E. F.; Lenton, T. M.; Scheffer, M.; Folke, C.; Schellnhuber, H. J.; Nykvist, B.; de Wit, C. A.; Hughes, T.; van der Leeuw, S.; Rodhe, H.; Sörlin, S.; Snyder, P. K.; Costanza, R.; Svedin, U.; Falkenmark, M.; Karlberg, L.; Corell, R. W.; Fabry, V. J.; Hansen, J.; Walker, B.; Liverman, D.; Richardson, K.; Crutzen, P.; Foley, J. A., A safe operating space for humanity. Nature 2009, 461 (7263), 472-475. 19. Hoffert, M. I.; Caldeira, K.; Jain, A. K.; Haites, E. F.; Harvey, L. D. D.; Potter, S. D.; Schlesinger, M. E.; Schneider, S. H.; Watts, R. G.; Wigley, T. M. L.; Wuebbles, D. J., Energy implications of future stabilization of atmospheric CO2 content. Nature 1998, 395 (6705), 881-884. 20. Murphy, P. Solar Update; The IEA Solar Heating and Cooling Executive Committee: USA, 2015; p 20. 21. Manser, J. S.; Christians, J. A.; Kamat, P. V., Intriguing Optoelectronic Properties of Metal Halide Perovskites. Chem. Rev. 2016, 116 (21), 12956-13008. 22. Seo, S.; Jeong, S.; Park, H.; Shin, H.; Park, N.-G., Atomic layer deposition for efficient and stable perovskite solar cells. Chem. Commun. (Cambridge, U. K.) 2019, 55 (17), 2403-2416. 23. Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S. I., Iodide management in formamidinium-lead-halide–based perovskite layers for efficient solar cells. Science 2017, 356 (6345), 1376. 24. Jiang, Q.; Chu, Z.; Wang, P.; Yang, X.; Liu, H.; Wang, Y.; Yin, Z.; Wu, J.; Zhang, X.; You, J., Planar-Structure Perovskite Solar Cells with Efficiency beyond 21%. Adv. Mater. 2017, 29 (46), 1703852. 25. Shin, S. S.; Yang, W. S.; Noh, J. H.; Suk, J. H.; Jeon, N. J.; Park, J. H.; Kim, J. S.; Seong, W. M.; Seok, S. I., High-performance flexible perovskite solar cells exploiting Zn2SnO4 prepared in solution below 100 °C. Nature Communications 2015, 6, 7410. 26. Sivula, K.; van de Krol, R., Semiconducting materials for photoelectrochemical energy conversion. Nat. Rev. Mater. 2016, 1 (2). 27. Novoselov, K. S., Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306 (5696), 666-669. 28. Kane, C. L.; Mele, E. J., Quantum Spin Hall Effect in Graphene. Phys. Rev. Lett. 2005, 95 (22). 29. Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A., Two-dimensional gas of massless Dirac fermions in graphene. Nature 2005, 438 (7065), 197-200. 30. Morozov, S. V.; Novoselov, K. S.; Katsnelson, M. I.; Schedin, F.; Ponomarenko, L. A.; Jiang, D.; Geim, A. K., Strong Suppression of Weak Localization in Graphene. Phys. Rev. Lett. 2006, 97 (1). 31. Peres, N. M. R.; Castro Neto, A. H.; Guinea, F., Erratum: Conductance quantization in mesoscopic graphene [Phys. Rev. B73, 195411 (2006)]. Phys. Rev. B. 2006, 73 (23). 32. Abanin, D. A.; Lee, P. A.; Levitov, L. S., Randomness-InducedXYOrdering in a Graphene Quantum Hall Ferromagnet. Phys. Rev. Lett. 2007, 98 (15). 33. Miao, F.; Wijeratne, S.; Zhang, Y.; Coskun, U. C.; Bao, W.; Lau, C. N., Phase-Coherent Transport in Graphene Quantum Billiards. Science 2007, 317 (5844), 1530-1533. 34. Zhao, S.; Li, Z.; Yang, J., Obtaining Two-Dimensional Electron Gas in Free Space without Resorting to Electron Doping: An Electride Based Design. J. Am. Chem. Soc. 2014, 136 (38), 13313-13318. 35. Miró, P.; Audiffred, M.; Heine, T., An atlas of two-dimensional materials. Chem. Soc. Rev. 2014, 43 (18), 6537-6554.

of 163 References

36. Xu, B.; Tian, H.; Bi, D.; Gabrielsson, E.; Johansson, E. M. J.; Boschloo, G.; Hagfeldt, A.; Sun, L., Efficient solid state dye-sensitized solar cells based on an oligomer hole transport material and an organic dye. J. Mater. Chem. A 2013, 1 (46), 14467. 37. Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A., Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306 (5696), 666. 38. Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N., Superior Thermal Conductivity of Single-Layer Graphene. Nano Lett. 2008, 8 (3), 902-907. 39. Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A., Two-dimensional gas of massless Dirac fermions in graphene. Nature (London, U. K.) 2005, 438 (7065), 197-200. 40. Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K., Two-dimensional atomic crystals. Proceedings of the National Academy of Sciences 2005, 102 (30), 10451-10453. 41. Rahman, M. Z.; Kwong, C. W.; Davey, K.; Qiao, S. Z., 2D phosphorene as a water splitting photocatalyst: fundamentals to applications. Energy Environ. Sci. 2016, 9 (3), 709-728. 42. Franceschetti, A., Nanostructured materials for improved photoconversion. MRS Bull. 2011, 36 (3), 192-197. 43. Steinhauser, M. O.; Hiermaier, S., A Review of Computational Methods in Materials Science: Examples from Shock-Wave and Polymer Physics. International Journal of Molecular Sciences 2009, 10 (12), 5135-5216. 44. Jain, A.; Shin, Y.; Persson, K. A., Computational predictions of energy materials using density functional theory. Nat. Rev. Mater. 2016, 1, 15004. 45. Zhang, S.; Zhou, W.; Ma, Y.; Ji, J.; Cai, B.; Yang, S. A.; Zhu, Z.; Chen, Z.; Zeng, H., Antimonene Oxides: Emerging Tunable Direct Bandgap Semiconductor and Novel Topological Insulator. Nano Lett. 2017, 17 (6), 3434-3440. 46. Amnuyswat, K.; Thanomngam, P., Effect of exchange-correlation and GW approximations on electrical property of cubic, tetragonal and orthorhombic CH3NH3PbI3. Integr. Ferroelectr. 2017, 177 (1), 1-9. 47. Anani, M.; Mathieu, C.; Khadraoui, M.; Chama, Z.; Lebid, S.; Amar, Y., High-grade efficiency III-nitrides semiconductor solar cell. Microelectron. J. 2009, 40 (3), 427-434. 48. Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D. W., Understanding TiO2 Photocatalysis: Mechanisms and Materials. Chem. Rev. (Washington, DC, U. S.) 2014, 114 (19), 9919-9986. 49. Fox, M. A.; Dulay, M. T., Heterogeneous photocatalysis. Chem. Rev. 1993, 93 (1), 341-57. 50. Warren, S. C.; Voitchovsky, K.; Dotan, H.; Leroy, C. M.; Cornuz, M.; Stellacci, F.; Hebert, C.; Rothschild, A.; Graetzel, M., Identifying champion nanostructures for solar water-splitting. Nat. Mater. 2013, 12 (9), 842-849. 51. Dotan, H.; Kfir, O.; Sharlin, E.; Blank, O.; Gross, M.; Dumchin, I.; Ankonina, G.; Rothschild, A., Resonant light trapping in ultrathin films for water splitting. Nat. Mater. 2013, 12 (2), 158-164. 52. Thomann, I.; Pinaud, B. A.; Chen, Z.; Clemens, B. M.; Jaramillo, T. F.; Brongersma, M. L., Plasmon Enhanced Solar-to-Fuel Energy Conversion. Nano Lett. 2011, 11 (8), 3440-3446. 53. Niu, P.; Zhang, L.; Liu, G.; Cheng, H.-M., Graphene-like carbon nitride nanosheets for improved photocatalytic activities. Adv. Funct. Mater. 2012, 22 (22), 4763-4770.

of 163 References

54. Schwinghammer, K.; Mesch, M. B.; Duppel, V.; Ziegler, C.; Senker, J.; Lotsch, B. V., Crystalline Carbon Nitride Nanosheets for Improved Visible-Light Hydrogen Evolution. J. Am. Chem. Soc. 2014, 136 (5), 1730-1733. 55. Zhang, S.; Yang, J.; Xu, R.; Wang, F.; Li, W.; Ghufran, M.; Zhang, Y.-W.; Yu, Z.; Zhang, G.; Qin, Q.; Lu, Y., Extraordinary Photoluminescence and Strong Temperature/Angle-Dependent Raman Responses in Few-Layer Phosphorene. ACS Nano 2014, 8 (9), 9590-9596. 56. Liang, L.; Wang, J.; Lin, W.; Sumpter, B. G.; Meunier, V.; Pan, M., Electronic Bandgap and Edge Reconstruction in Phosphorene Materials. Nano Lett. 2014, 14 (11), 6400-6406. 57. Akahama, Y.; Endo, S.; Narita, S., Electrical properties of black phosphorus single crystals. J. Phys. Soc. Jpn. 1983, 52 (6), 2148-55. 58. Tran, V.; Soklaski, R.; Liang, Y.; Yang, L., Layer-controlled band gap and anisotropic excitons in few-layer black phosphorus. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89 (23), 235319/1-235319/6, 6 pp. 59. Cai, Y.; Zhang, G.; Zhang, Y.-W., Layer-dependent Band Alignment and Work Function of Few-Layer Phosphorene. Sci. Rep. 2014, 4, 6677. 60. Fujishima, A.; Honda, K., Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238 (5358), 37-38. 61. Bozoglan, E.; Midilli, A.; Hepbasli, A., Sustainable assessment of solar hydrogen production techniques. Energy 2012, 46 (1), 85-93. 62. Bak, T.; Nowotny, J.; Rekas, M.; Sorrell, C. C., Photo-electrochemical hydrogen generation from water using solar energy. Materials-related aspects. Int. J. Hydrogen Energy 2002, 27 (10), 991-1022. 63. Bolton, J. R., Solar photoproduction of hydrogen: A review. Solar Energy 1996, 57 (1), 37-50. 64. Varghese, O. K.; Grimes, C. A., Appropriate strategies for determining the photoconversion efficiency of water photoelectrolysis cells: A review with examples using titania nanotube array photoanodes. Sol. Energy Mater. Sol. Cells 2008, 92 (4), 374-384. 65. Jiao, Y.; Zhou, L.; Ma, F.; Gao, G.; Kou, L.; Bell, J.; Sanvito, S.; Du, A., Predicting Single-Layer Technetium Dichalcogenides (TcX2, X = S, Se) with Promising Applications in Photovoltaics and Photocatalysis. ACS Appl. Mater. Interfaces 2016, 8 (8), 5385-5392. 66. Li, Y.; Feng, J.; Li, H.; Wei, X.; Wang, R.; Zhou, A., Photoelectrochemical splitting of natural seawater with α-Fe 2 O 3 /WO 3 nanorod arrays. Int. J. Hydrogen Energy 2016, 41 (7), 4096-4105. 67. Shi, Z.; Wen, X.; Guan, Z.; Cao, D.; Luo, W.; Zou, Z., Recent progress in photoelectrochemical water splitting for solar hydrogen production. Annals of Physics 2015, 358, 236-247. 68. Tan, Y.; Zhang, S.; Shi, R.; Wang, W.; Liang, K., Visible light active Ce/Ce2O/CeO2/TiO2 nanotube arrays for efficient hydrogen production by photoelectrochemical water splitting. Int. J. Hydrogen Energy 2016, 41 (12), 5437-5444. 69. Fujishima, A.; Zhang, X.; Tryk, D., TiO2 photocatalysis and related surface phenomena. Surf. Sci. Rep. 2008, 63 (12), 515-582. 70. Lin, Y.; Yuan, G.; Liu, R.; Zhou, S.; Sheehan, S. W.; Wang, D., Semiconductor nanostructure-based photoelectrochemical water splitting: A brief review. Chem. Phys. Lett. 2011, 507 (4-6), 209-215. 71. Zhuang, H. L.; Hennig, R. G., Single-Layer Group-III Monochalcogenide Photocatalysts for Water Splitting. Chem. Mater. 2013, 25 (15), 3232-3238. 72. Zhu, M.; Sun, Z.; Fujitsuka, M.; Majima, T., Z-Scheme Photocatalytic Water Splitting on a 2D Heterostructure of Black Phosphorus/Bismuth Vanadate Using Visible Light. Angew. Chem., Int. Ed. 2018, 57 (8), 2160-2164.

of 163 References

73. Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S., Solar Water Splitting Cells. Chem. Rev. 2010, 110 (11), 6446-6473. 74. Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L.; Hone, J., Boron nitride substrates for high-quality graphene electronics. Nature Nanotechnology 2010, 5 (10), 722-726. 75. Ziletti, A.; Carvalho, A.; Campbell, D. K.; Coker, D. F.; Castro Neto, A. H., Oxygen Defects in Phosphorene. Phys. Rev. Lett. 2015, 114 (4). 76. Kim, S. J.; Kim, D. W.; Lim, J.; Cho, S.-Y.; Kim, S. O.; Jung, H.-T., Large-Area Buckled MoS2 Films on the Graphene Substrate. ACS Appl. Mater. Interfaces 2016, 8 (21), 13512-13519. 77. Sreeprasad, T. S.; Nguyen, P.; Kim, N.; Berry, V., Controlled, Defect-Guided, Metal-Nanoparticle Incorporation onto MoS2 via Chemical and Microwave Routes: Electrical, Thermal, and Structural Properties. Nano Lett. 2013, 13 (9), 4434-4441. 78. Radisavljevic, B.; Kis, A., Mobility engineering and a metal–insulator transition in monolayer MoS2. Nature Materials 2013, 12 (9), 815-820. 79. Sercombe, D.; Schwarz, S.; Pozo-Zamudio, O. D.; Liu, F.; Robinson, B. J.; Chekhovich, E. A.; Tartakovskii, I. I.; Kolosov, O.; Tartakovskii, A. I., Optical investigation of the natural electron doping in thin MoS2 films deposited on dielectric substrates. Scientific Reports 2013, 3 (1). 80. Hulliger, F.; Mooser, E., Electrical properties of anomalously composed daltonides. Phys. Chem. Solids 1963, 24, 283-95. 81. Wadsten, T., Crystal structures of siliconphosphide, silicon arsenide, and germanium phosphide. Acta Chem. Scand. 1967, 21 (2), 593-4. 82. Bachhuber, F.; Rothballer, J.; Pielnhofer, F.; Weihrich, R., First principles calculations on structure, bonding, and vibrational frequencies of SiP2. J. Chem. Phys. 2011, 135 (12), 124508/1-124508/7. 83. Wu, P.; Huang, M., Stability, bonding, and electronic properties of silicon and germanium arsenides. Phys. Status Solidi B 2016, 253 (5), 862-867. 84. Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Grätzel, M.; Park, N.-G., Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Scientific Reports 2012, 2 (1). 85. You, J.; Hong, Z.; Yang, Y.; Chen, Q.; Cai, M.; Song, T.-B.; Chen, C.-C.; Lu, S.; Liu, Y.; Zhou, H.; Yang, Y., Low-Temperature Solution-Processed Perovskite Solar Cells with High Efficiency and Flexibility. ACS Nano 2014, 8 (2), 1674-1680. 86. Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; Losovyj, Y.; Zhang, X.; Dowben, P. A.; Mohammed, O. F.; Sargent, E. H.; Bakr, O. M., Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals. Science 2015, 347 (6221), 519-522. 87. Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T., Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131 (17), 6050-6051. 88. (NREL), N. R. E. L. Best Research-Cell Efficiencies. https://www.nrel.gov/pv/assets/pdfs/pv-efficiencies-07-17-2018.pdf (accessed 16 October 2018). 89. Umebayashi, T.; Asai, K.; Kondo, T.; Nakao, A., Electronic structures of lead iodide based low-dimensional crystals. Phys. Rev. B. 2003, 67 (15). 90. Mosconi, E.; Amat, A.; Nazeeruddin, M. K.; Grätzel, M.; De Angelis, F., First-Principles Modeling of Mixed Halide Organometal Perovskites for Photovoltaic Applications. J. Phys. Chem. C. 2013, 117 (27), 13902-13913.

of 163 References

91. Giorgi, G.; Fujisawa, J.-I.; Segawa, H.; Yamashita, K., Small Photocarrier Effective Masses Featuring Ambipolar Transport in Methylammonium Lead Iodide Perovskite: A Density Functional Analysis. J. Phys. Chem. Lett. 2013, 4 (24), 4213-4216. 92. Zhao, Y.; Duan, J.; Yuan, H.; Wang, Y.; Yang, X.; He, B.; Tang, Q., Using SnO2 QDs and CsMBr3 (M = Sn, Bi, Cu) QDs as Charge-Transporting Materials for 10.6%-Efficiency All-Inorganic CsPbBr3 Perovskite Solar Cells with an Ultrahigh Open-Circuit Voltage of 1.610 V. Sol. RRL 2019, 3 (3), n/a. 93. Chen, W.; Wu, Y.; Yue, Y.; Liu, J.; Zhang, W.; Yang, X.; Chen, H.; Bi, E.; Ashraful, I.; Gratzel, M.; Han, L., Efficient and stable large-area perovskite solar cells with inorganic charge extraction layers. Science 2015, 350 (6263), 944-948. 94. Zuo, C.; Ding, L., Solution-Processed Cu2O and CuO as Hole Transport Materials for Efficient Perovskite Solar Cells. Small 2015, 11 (41), 5528-5532. 95. Ye, S.; Rao, H.; Zhao, Z.; Zhang, L.; Bao, H.; Sun, W.; Li, Y.; Gu, F.; Wang, J.; Liu, Z.; Bian, Z.; Huang, C., A Breakthrough Efficiency of 19.9% Obtained in Inverted Perovskite Solar Cells by Using an Efficient Trap State Passivator Cu(thiourea)I. J. Am. Chem. Soc. 2017, 139 (22), 7504-7512. 96. Qin, P.-L.; Lei, H.-W.; Zheng, X.-L.; Liu, Q.; Tao, H.; Yang, G.; Ke, W.-J.; Xiong, L.-B.; Qin, M.-C.; Zhao, X.-Z.; Fang, G.-J., Copper-Doped Chromium Oxide Hole-Transporting Layer for Perovskite Solar Cells: Interface Engineering and Performance Improvement. Advanced Materials Interfaces 2016, 3 (14), 1500799. 97. Westbrook, R. J. E.; Sanchez-Molina, D. I.; Manuel Marin-Beloqui, D. J.; Bronstein, D. H.; Haque, D. S. A., Effect of Interfacial Energetics on Charge Transfer from Lead Halide Perovskite to Organic Hole Conductors. J. Phys. Chem. C 2018, 122 (2), 1326-1332. 98. Teng, Q.; Shi, T.-T.; Tian, R.-Y.; Yang, X.-B.; Zhao, Y.-J., Role of organic cations on hybrid halide perovskite CH3NH3PbI3 surfaces. J. Solid State Chem. 2018, 258, 488-494. 99. Arora, N.; Dar, M. I.; Hinderhofer, A.; Pellet, N.; Schreiber, F.; Zakeeruddin, S. M.; Grätzel, M., Perovskite solar cells with CuSCN hole extraction layers yield stabilized efficiencies greater than 20%. Science 2017. 100. Xu, X.; Ray, R.; Gu, Y.; Ploehn, H. J.; Gearheart, L.; Raker, K.; Scrivens, W. A., Electrophoretic Analysis and Purification of Fluorescent Single-Walled Carbon Nanotube Fragments. J. Am. Chem. Soc. 2004, 126 (40), 12736-12737. 101. Wang, Y.; Hu, A., Carbon quantum dots: synthesis, properties and applications. J. Mater. Chem. C 2014, 2 (34), 6921. 102. Chan, W. C. W.; Maxwell, D. J.; Gao, X.; Bailey, R. E.; Han, M.; Nie, S., Luminescent quantum dots for multiplexed biological detection and imaging. Curr. Opin. Biotechnol. 2002, 13 (1), 40-46. 103. Yan, Y.; Chen, J.; Li, N.; Tian, J.; Li, K.; Jiang, J.; Liu, J.; Tian, Q.; Chen, P., Systematic Bandgap Engineering of Graphene Quantum Dots and Applications for Photocatalytic Water Splitting and CO2 Reduction. ACS Nano 2018. 104. Gao, G.; Jiao, Y.; Ma, F.; Jiao, Y.; Waclawik, E.; Du, A., Carbon nanodot decorated graphitic carbon nitride: new insights into the enhanced photocatalytic water splitting from ab initio studies. Phys. Chem. Chem. Phys. 2015, 17 (46), 31140-31144. 105. Feng, X.; Qin, Y.; Liu, Y., Size and edge dependence of two-photon absorption in rectangular graphene quantum dots. Opt. Express 2018, 26 (6), 7132-7139. 106. Li, H.; He, X.; Kang, Z.; Huang, H.; Liu, Y.; Liu, J.; Lian, S.; Tsang, C. H. A.; Yang, X.; Lee, S.-T., Water-Soluble Fluorescent Carbon Quantum Dots and Photocatalyst Design. Angew. Chem. Int. Ed. 2010, 49 (26), 4430-4434. 107. Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S.-T.; Zhong, J.; Kang, Z., Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway. Science 2015, 347 (6225), 970-974.

of 163 References

108. Cabana, L.; Ballesteros, B.; Batista, E.; Magén, C.; Arenal, R.; Oró-Solé, J.; Rurali, R.; Tobias, G., Carbon Nanotubes: Synthesis of PbI2Single-Layered Inorganic Nanotubes Encapsulated Within Carbon Nanotubes (Adv. Mater. 13/2014). Adv. Mater. 2014, 26 (13), 2108-2108. 109. Guloy, A. M.; Tang, Z.; Miranda, P. B.; Srdanov, V. I., A New Luminescent Organic–Inorganic Hybrid Compound with Large Optical Nonlinearity. Adv. Mater. 2001, 13 (11), 833-837. 110. Zhong, M.; Zhang, S.; Huang, L.; You, J.; Wei, Z.; Liu, X.; Li, J., Large-scale 2D PbI2 monolayers: experimental realization and their indirect band-gap related properties. Nanoscale 2017, 9 (11), 3736-3741. 111. Trinquier, G.; Hoffmann, R., Lead monoxide. Electronic structure and bonding. The Journal of Physical Chemistry 1984, 88 (26), 6696-6711. 112. Lebègue, S.; Björkman, T.; Klintenberg, M.; Nieminen, R. M.; Eriksson, O., Two-Dimensional Materials from Data Filtering andAb InitioCalculations. Physical Review X 2013, 3 (3). 113. Kumar, P.; Liu, J.; Ranjan, P.; Hu, Y.; Yamijala, S. S. R. K. C.; Pati, S. K.; Irudayaraj, J.; Cheng, G. J., Alpha Lead Oxide (α-PbO): A New 2D Material with Visible Light Sensitivity. Small 2018, 14 (12), n/a. 114. Caraveo-Frescas, J. A.; Nayak, P. K.; Al-Jawhari, H. A.; Granato, D. B.; Schwingenschlögl, U.; Alshareef, H. N., Record Mobility in Transparent p-Type Tin Monoxide Films and Devices by Phase Engineering. ACS Nano 2013, 7 (6), 5160-5167. 115. Saji, K. J.; Tian, K.; Snure, M.; Tiwari, A., 2D Tin Monoxide-An Unexplored p-Type van der Waals Semiconductor: Material Characteristics and Field Effect Transistors. Advanced Electronic Materials 2016, 2 (4), 1500453. 116. Xiao, C.; Wang, F.; Wang, Y.; Jiang, J.; Lu, Y.; Yang, S. A.; Yang, M.; Wang, S.; Feng, Y., Layer-dependent semiconductor-metal transition of SnO/Si(001) heterostructure and device application. Sci Rep 2017, 7 (1), 2570. 117. Zhou, J.; Huang, J.; Sumpter, B. G.; Kent, P. R. C.; Terrones, H.; Smith, S. C., Structures, Energetics, and Electronic Properties of Layered Materials and Nanotubes of Cadmium Chalcogenides. J. Phys. Chem. C 2013, 117 (48), 25817-25825. 118. Kresse, G.; Furthmueller, J., Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B: Condens. Matter 1996, 54 (16), 11169-11186. 119. Kresse, G.; Furthmuller, J., Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6 (1), 15-50. 120. Parlinski, K.; Li, Z. Q.; Kawazoe, Y., First-principles determination of the soft mode in cubic ZrO2. Phys. Rev. Lett. 1997, 78 (21), 4063-4066. 121. Togo, A.; Oba, F.; Tanaka, I., First-principles calculations of the ferroelastic transition between rutile-type and CaCl2-type SiO2 at high pressures. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78 (13), 134106/1-134106/9. 122. Baroni, S.; De Gironcoli, S.; Dal Corso, A.; Giannozzi, P., Phonons and related crystal properties from density-functional perturbation theory. Rev. Mod. Phys. 2001, 73 (2), 515-562. 123. Gonze, X.; Lee, C., Dynamic matrixes, Born effective charges, dielectric permittivity tensors, and interatomic force constants from density-functional perturbation theory. Phys. Rev. B: Condens. Matter 1997, 55 (16), 10355-10368. 124. Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal, C. A.; de, G. S.; Fabris, S.; Fratesi, G.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari, P.; Wentzcovitch, R. M., QUANTUM ESPRESSO: a modular and open-source

of 163 References

software project for quantum simulations of materials. J Phys Condens Matter 2009, 21 (39), 395502. 125. Hafner, J., Ab-initio simulations of materials using VASP: Density-functional theory and beyond. J. Comput. Chem. 2008, 29 (13), 2044-2078. 126. Page, W. Density Functional Theory for Beginners. http://newton.ex.ac.uk/research/qsystems/people/coomer/dft intro.html (accessed 10 May 2018). 127. Hamiltonian (quantum mechanics). https://en.wikipedia.org/wiki/Hamiltonian (quantum mechanics) (accessed 10 May 2018). 128. Hohenberg, P.; Kohn, W., Inhomogeneous Electron Gas. Physical Review 1964, 136 (3B), B864-B871. 129. Zhou, J.; Huang, J.; Sumpter, B. G.; Kent, P. R. C.; Xie, Y.; Terrones, H.; Smith, S. C., Theoretical Predictions of Freestanding Honeycomb Sheets of Cadmium Chalcogenides. J. Phys. Chem. C 2014, 118 (29), 16236-16245. 130. Perdew; Wang, Pair-distribution function and its coupling-constant average for the spin-polarized electron gas. Phys Rev B Condens Matter 1992, 46 (20), 12947-12954. 131. Marzari, N.; Vanderbilt, D., Maximally localized generalized Wannier functions for composite energy bands. Phys. Rev. B: Condens. Matter 1997, 56 (20), 12847-12865. 132. Mostofi, A. A.; Yates, J. R.; Lee, Y.-S.; Souza, I.; Vanderbilt, D.; Marzari, N., : A tool for obtaining maximally-localised Wannier functions. Comput. Phys. Commun. 2008, 178 (9), 685-699. 133. Souza, I.; Marzari, N.; Vanderbilt, D., Maximally localized Wannier functions for entangled energy bands. Phys. Rev. B. 2001, 65 (3). 134. Hedin, L., New Method for Calculating the One-Particle Green's Function with Application to the Electron-Gas Problem. Physical Review 1965, 139 (3A), A796-A823. 135. Salpeter, E. E.; Bethe, H. A., A Relativistic Equation for Bound-State Problems. Physical Review 1951, 84 (6), 1232-1242. 136. Kresse, G.; Furthmüller, J., Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Computational Materials Science 1996, 6 (1), 15-50. 137. Kishore, M. R. A.; Ravindran, P.; Kishore, M. R. A.; Ravindran, P.; Ravindran, P.; Ravindran, P., Enhanced Photocatalytic Water Splitting in a C2 N Monolayer by C-Site Isoelectronic Substitution. Chemphyschem 2017, 18 (12), 1526-1532. 138. Chang Kwon, K.; Choi, S.; Lee, J.; Hong, K.; Sohn, W.; Andoshe, D. M.; Choi, K. S.; Kim, Y.; Han, S.; Kim, S. Y.; Jang, H. W., Drastically enhanced hydrogen evolution activity by 2D to 3D structural transition in anion-engineered molybdenum disulfide thin films for efficient Si-based water splitting photocathodes. J. Mater. Chem. A 2017, 5 (30), 15534-15542. 139. Qiu, S.; Azofra, L. M.; MacFarlane, D. R.; Sun, C., Unraveling the Role of Ligands in the Hydrogen Evolution Mechanism Catalyzed by [NiFe] Hydrogenases. ACS Catalysis 2016, 6 (8), 5541-5548. 140. Maeda, K., Photocatalytic water splitting using semiconductor particles: History and recent developments. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 2011, 12 (4), 237-268. 141. Ni, M.; Leung, M. K. H.; Leung, D. Y. C.; Sumathy, K., A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production. Renewable Sustainable Energy Rev. 2006, 11 (3), 401-425. 142. Chen, X.; Liu, G.; Zheng, W.; Feng, W.; Cao, W.; Hu, W.; Hu, P., Vertical 2D MoO2/MoSe2 Core-Shell Nanosheet Arrays as High-Performance Electrocatalysts for Hydrogen Evolution Reaction. Adv. Funct. Mater. 2016, 26 (46), 8537-8544. 143. Gao, X.; Li, J.; Du, R.; Zhou, J.; Huang, M.-Y.; Liu, R.; Li, J.; Xie, Z.; Wu, L.-Z.; Liu, Z.; Zhang, J., Direct Synthesis of Graphdiyne Nanowalls on Arbitrary Substrates and

of 163 References

Its Application for Photoelectrochemical Water Splitting Cell. Adv. Mater. (Weinheim, Ger.) 2017, 29 (9), n/a. 144. Wang, F.; Yu, Y.; Yin, X.; Tian, P.; Wang, X., Wafer-scale synthesis of ultrathin CoO nanosheets with enhanced electrochemical catalytic properties. J. Mater. Chem. A 2017, 5 (19), 9060-9066. 145. Liu, J.; Li, X.-B.; Wang, D.; Liu, H.; Peng, P.; Liu, L.-M., Single-layer Group-IVB nitride halides as promising photocatalysts. J. Mater. Chem. A 2014, 2 (19), 6755-6761. 146. Zhang, X.; Zhao, X.; Wu, D.; Jing, Y.; Zhou, Z., MnPSe3 Monolayer: A Promising 2D Visible-Light Photohydrolytic Catalyst with High Carrier Mobility. Advanced Science 2016, 3 (10), 1600062-n/a. 147. Zheng, Y.; Jiao, Y.; Chen, J.; Liu, J.; Liang, J.; Du, A.; Zhang, W.; Zhu, Z.; Smith, S. C.; Jaroniec, M.; Lu, G. Q.; Qiao, S. Z., Nanoporous Graphitic-C3N4@Carbon Metal-Free Electrocatalysts for Highly Efficient Oxygen Reduction. J. Am. Chem. Soc. 2011, 133 (50), 20116-20119. 148. Fei, H.; Dong, J.; Arellano-Jiménez, M. J.; Ye, G.; Dong Kim, N.; Samuel, E. L. G.; Peng, Z.; Zhu, Z.; Qin, F.; Bao, J.; Yacaman, M. J.; Ajayan, P. M.; Chen, D.; Tour, J. M., Atomic cobalt on nitrogen-doped graphene for hydrogen generation. Nature Communications 2015, 6, 8668. 149. Lukowski, M. A.; Daniel, A. S.; Meng, F.; Forticaux, A.; Li, L.; Jin, S., Enhanced Hydrogen Evolution Catalysis from Chemically Exfoliated Metallic MoS2Nanosheets. J. Am. Chem. Soc. 2013, 135 (28), 10274-10277. 150. Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I., Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317 (5834), 100-102. 151. Tang, Q.; Jiang, D.-e., Mechanism of Hydrogen Evolution Reaction on 1T-MoS2 from First Principles. ACS Catalysis 2016, 6 (8), 4953-4961. 152. Gao, G.; Jiao, Y.; Ma, F.; Jiao, Y.; Waclawik, E.; Du, A., Metal-free graphitic carbon nitride as mechano-catalyst for hydrogen evolution reaction. J. Catal. 2015, 332, 149-155. 153. Sarkar, S.; Sampath, S., Equiatomic ternary chalcogenide: PdPS and its reduced graphene oxide composite for efficient electrocatalytic hydrogen evolution. Chem. Commun. 2014, 50 (55), 7359-7362. 154. Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M., A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nat. Mater. 2009, 8 (1), 76-80. 155. Gao, G.; Jiao, Y.; Ma, F.; Jiao, Y.; Waclawik, E.; Du, A., Charge Mediated Semiconducting-to-Metallic Phase Transition in Molybdenum Disulfide Monolayer and Hydrogen Evolution Reaction in New 1Tʹ Phase. J. Phys. Chem. C. 2015, 119 (23), 13124-13128. 156. Hu, J.; Guo, Z.; McWilliams, P. E.; Darges, J. E.; Druffel, D. L.; Moran, A. M.; Warren, S. C., Band Gap Engineering in a 2D Material for Solar-to-Chemical Energy Conversion. Nano Lett. 2016, 16 (1), 74-79. 157. Springthorpe, A. J., Preparation of single crystal orthorhombic SiP2. Mater. Res. Bull. 1969, 4 (2), 125-8. 158. Zhang, X.; Wang, S.; Ruan, H.; Zhang, G.; Tao, X., Structure and growth of single crystal SiP2 using flux method. Solid State Sciences 2014, 37, 1-5. 159. Farberovich, O. V.; Domashevskaya, E. P., Band structure and density of states in silicon phosphide (SiP2). Phys. Status Solidi B 1975, 72 (2), 661-5. 160. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77 (18), 3865-3868. 161. Monkhorst, H. J.; Pack, J. D., Special points for Brillouin-zone integrations. Phys. Rev. B. 1976, 13 (12), 5188-5192.

of 163 References

162. Krukau, A. V.; Vydrov, O. A.; Izmaylov, A. F.; Scuseria, G. E., Influence of the exchange screening parameter on the performance of screened hybrid functionals. J. Chem. Phys. 2006, 125 (22), 224106. 163. Mostofi, A. A.; Yates, J. R.; Pizzi, G.; Lee, Y.-S.; Souza, I.; Vanderbilt, D.; Marzari, N., An updated version of wannier90: A tool for obtaining maximally-localised Wannier functions. Comput. Phys. Commun. 2014, 185 (8), 2309-2310. 164. Singh, A. K.; Mathew, K.; Zhuang, H. L.; Hennig, R. G., Computational Screening of 2D Materials for Photocatalysis. J. Phys. Chem. Lett. 2015, 6 (6), 1087-1098. 165. Kirklin, S.; Saal, J. E.; Meredig, B.; Thompson, A.; Doak, J. W.; Aykol, M.; Rühl, S.; Wolverton, C., The Open Quantum Materials Database (OQMD): assessing the accuracy of DFT formation energies. Npj Computational Materials 2015, 1, 15010. 166. Olsson, P.; Abrikosov, I. A.; Vitos, L.; Wallenius, J., Ab initio formation energies of Fe–Cr alloys. J. Nucl. Mater. 2003, 321 (1), 84-90. 167. Grimme, S., Semiempirical GGA-type density functional constructed with a long-range dispersion correction. J. Comput. Chem. 2006, 27 (15), 1787-1799. 168. Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H., A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132 (15), 154104. 169. Blöchl, P. E., Projector augmented-wave method. Phys. Rev. B. 1994, 50 (24), 17953-17979. 170. Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; de Gironcoli, S.; Fabris, S.; Fratesi, G.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari, P.; Wentzcovitch, R. M., QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials. J. Phys.: Condens. Matter 2009, 21 (39), 395502. 171. Albrecht, S.; Reining, L.; Del Sole, R.; Onida, G., Ab InitioCalculation of Excitonic Effects in the Optical Spectra of Semiconductors. Phys. Rev. Lett. 1998, 80 (20), 4510-4513. 172. Benedict, L. X.; Shirley, E. L.; Bohn, R. B., Optical Absorption of Insulators and the Electron-Hole Interaction: AnAb InitioCalculation. Phys. Rev. Lett. 1998, 80 (20), 4514-4517. 173. Onida, G.; Reining, L.; Rubio, A., Electronic excitations: density-functional versus many-body Green’s-function approaches. Reviews of Modern Physics 2002, 74 (2), 601-659. 174. Rohlfing, M.; Louie, S. G., Electron-Hole Excitations in Semiconductors and Insulators. Phys. Rev. Lett. 1998, 81 (11), 2312-2315. 175. Deslippe, J.; Samsonidze, G.; Strubbe, D. A.; Jain, M.; Cohen, M. L.; Louie, S. G., BerkeleyGW: A massively parallel computer package for the calculation of the quasiparticle and optical properties of materials and nanostructures. Comput. Phys. Commun. 2012, 183 (6), 1269-1289. 176. Rohlfing, M.; Louie, S. G., Electron-hole excitations and optical spectra from first principles. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62 (8), 4927-4944. 177. Hulliger, F.; Lévy, F., Structural Chemistry of Layer-Type Phases. Springer Netherlands: 2012. 178. Hu, S.; Wang, B.; Zhu, M.; Ma, Y.; Lv, Z.; Wang, H., High-performance 1D type-II TiO2@ZnO core-shell nanorods arrays photoanodes for photoelectrochemical solar fuel production. Appl. Surf. Sci. 2017, 403, 126-132. 179. Zhou, M.; Duan, W.; Chen, Y.; Du, A., Single layer lead iodide: computational exploration of structural, electronic and optical properties, strain induced band modulation and the role of spin–orbital-coupling. Nanoscale 2015, 7 (37), 15168-15174.

of 163 References

180. Ma, F.; Zhou, M.; Jiao, Y.; Gao, G.; Gu, Y.; Bilic, A.; Chen, Z.; Du, A., Single Layer Bismuth Iodide: Computational Exploration of Structural, Electrical, Mechanical and Optical Properties. Scientific Reports 2015, 5 (1). 181. Conley, H. J.; Wang, B.; Ziegler, J. I.; Haglund, R. F.; Pantelides, S. T.; Bolotin, K. I., Bandgap Engineering of Strained Monolayer and Bilayer MoS2. Nano Lett. 2013, 13 (8), 3626-3630. 182. Feng, J.; Qian, X.; Huang, C.-W.; Li, J., Strain-engineered artificial atom as a broad-spectrum solar energy funnel. Nature Photonics 2012, 6 (12), 866-872. 183. Ji, Q.; Zhang, Y.; Gao, T.; Zhang, Y.; Ma, D.; Liu, M.; Chen, Y.; Qiao, X.; Tan, P.-H.; Kan, M.; Feng, J.; Sun, Q.; Liu, Z., Epitaxial Monolayer MoS2on Mica with Novel Photoluminescence. Nano Lett. 2013, 13 (8), 3870-3877. 184. Wang, Y. G.; Zhang, Q. L.; Wang, T. H.; Han, W.; Zhou, S. X., Improvement of electron transport in a ZnSe nanowire byin situstrain. J. Phys. D: Appl. Phys. 2011, 44 (12), 125301. 185. Choi, J.-H.; Cui, P.; Lan, H.; Zhang, Z., Linear scaling of the exciton binding energy versus the band gap of two-dimensional materials. Phys. Rev. Lett. 2015, 115 (6), 066403/1-066403/5. 186. Zhao, T.; Sun, Y.; Shuai, Z.; Wang, D., GeAs2: a IV-V group two-dimensional semiconductor with ultralow thermal conductivity and high thermoelectric efficiency. Chem. Mater. 2017, 29 (15), 6261-6268. 187. Zhang, M.; Zhang, M.; Huang, L.-Y.; Zhang, X.; Lu, G., Comment on "Linear Scaling of the Exciton Binding Energy versus the Band Gap of Two-Dimensional Materials". Phys. Rev. Lett. 2017, 118 (20), 209701. 188. Jiang, Z.; Duan, W.; Liu, Z.; Li, Y.; Duan, W., Scaling Universality between Band Gap and Exciton Binding Energy of Two-Dimensional Semiconductors. Phys. Rev. Lett. 2017, 118 (26), 266401. 189. Garcia, J. C.; de Lima, D. B.; Assali, L. V. C.; Justo, J. F., Group IV Graphene- and Graphane-Like Nanosheets. J. Phys. Chem. C. 2011, 115 (27), 13242-13246. 190. Carvalho, A.; Wang, M.; Zhu, X.; Rodin, A. S.; Su, H.; Castro Neto, A. H., Phosphorene: from theory to applications. Nat. Rev. Mater. 2016, 1 (11). 191. Wadsten, T.; Vikan, M.; Krohn, C.; Nilsson, Å.; Theorell, H.; Blinc, R.; Paušak, S.; Ehrenberg, L.; Dumanović, J., The Crystal Structures of SiP2, SiAs2, and GeP. Acta Chem. Scand. 1967, 21, 593-594. 192. Rau, J. W.; Kannewurf, C. R., Optical Absorption, Reflectivity, and Electrical Conductivity in GeAs and GeAs2. Phys. Rev. B. 1971, 3 (8), 2581-2587. 193. Panish, M. B., The Ga-As-Si Ternary Phase System. J. Electrochem. Soc. 1966, 113 (11), 1226. 194. Ugai, Y. A.; Popov, A. E.; Goncharov, E. G.; Tolubaev, K. G., Study of phase equilibria in the germanium arsenide (GeAs2)-arsenic system. Zh. Neorg. Khim. 1983, 28 (11), 2944-7. 195. Hulliger, F.; Mooser, E., Electrical properties of anomalously composed daltonides. J. Phys. Chem. Solids 1963, 24 (2), 283-295. 196. Wu, P.; Huang, M., Stability, bonding, and electronic properties of silicon and germanium arsenides. physica status solidi (b) 2016, 253 (5), 862-867. 197. Zhao, T.; Sun, Y.; Shuai, Z.; Wang, D., GeAs2: A IV–V Group Two-Dimensional Semiconductor with Ultralow Thermal Conductivity and High Thermoelectric Efficiency. Chem. Mater. 2017, 29 (15), 6261-6268. 198. Kresse, G.; Furthmüller, J., Efficient iterative schemes forab initiototal-energy calculations using a plane-wave basis set. Phys. Rev. B. 1996, 54 (16), 11169-11186. 199. Heyd, J.; Scuseria, G. E.; Ernzerhof, M., Hybrid functionals based on a screened Coulomb potential. J. Chem. Phys. 2003, 118 (18), 8207-8215.

of 163 References

200. Mostofi, A. A.; Yates, J. R.; Pizzi, G.; Lee, Y.-S.; Souza, I.; Vanderbilt, D.; Marzari, N., An updated version of wannier90: A tool for obtaining maximally-localised Wannier functions. Comput. Phys. Commun. 2014, 185 (8), 2309-2310. 201. Jiao, Y.; Zhou, L.; Ma, F.; Gao, G.; Kou, L.; Bell, J.; Sanvito, S.; Du, A., Predicting Single-Layer Technetium Dichalcogenides (TcX2, X = S, Se) with Promising Applications in Photovoltaics and Photocatalysis. ACS Applied Materials & Interfaces 2016, 8 (8), 5385-5392. 202. Singh, A. K.; Mathew, K.; Zhuang, H. L.; Hennig, R. G., Computational Screening of 2D Materials for Photocatalysis. J. Phys. Chem. Lett. 2015, 6 (6), 1087-1098. 203. Bernardi, M.; Palummo, M.; Grossman, J. C., Extraordinary Sunlight Absorption and One Nanometer Thick Photovoltaics Using Two-Dimensional Monolayer Materials. Nano Lett. 2013, 13 (8), 3664-3670. 204. Yang, L.; Deslippe, J.; Park, C.-H.; Cohen, M. L.; Louie, S. G., Excitonic Effects on the Optical Response of Graphene and Bilayer Graphene. Phys. Rev. Lett. 2009, 103 (18). 205. Baroni, S.; de Gironcoli, S.; Dal Corso, A.; Giannozzi, P., Phonons and related crystal properties from density-functional perturbation theory. Reviews of Modern Physics 2001, 73 (2), 515-562. 206. Rohlfing, M.; Louie, S. G., Electron-hole excitations and optical spectra from first principles. Phys. Rev. B. 2000, 62 (8), 4927-4944. 207. Singh, A. K.; Hennig, R. G., Computational prediction of two-dimensional group-IV mono-chalcogenides. Appl. Phys. Lett. 2014, 105 (4), 042103. 208. Zhang, M.; Cai, F.; Zhang, S.; Zhang, S.; Song, W., Overexpression of ubiquitin carboxyl-terminal hydrolase L1 (UCHL1) delays Alzheimer's progression in vivo. Scientific Reports 2014, 4 (1). 209. Bryden, J. H., The crystal structures of the germanium–arsenic compounds: I. Germanium diarsenide, GeAs2. Acta Crystallographica 1962, 15 (2), 167-171. 210. Hulliger, F., Repartition of the Layer Structures. In Structural Chemistry of Layer-Type Phases, Springer Netherlands: 1976; pp 28-39. 211. Ji, Q.; Zhang, Y.; Gao, T.; Zhang, Y.; Ma, D.; Liu, M.; Chen, Y.; Qiao, X.; Tan, P.-H.; Kan, M.; Feng, J.; Sun, Q.; Liu, Z., Epitaxial Monolayer MoS2 on Mica with Novel Photoluminescence. Nano Lett. 2013, 13 (8), 3870-3877. 212. Zhou, L.; Zhuo, Z.; Kou, L.; Du, A.; Tretiak, S., Computational Dissection of Two-Dimensional Rectangular Titanium Mononitride TiN: Auxetics and Promises for Photocatalysis. Nano Lett. 2017, 17 (7), 4466-4472. 213. Tran, V.; Soklaski, R.; Liang, Y.; Yang, L., Layer-controlled band gap and anisotropic excitons in few-layer black phosphorus. Phys. Rev. B. 2014, 89 (23). 214. Choi, J.-H.; Cui, P.; Lan, H.; Zhang, Z., Linear Scaling of the Exciton Binding Energy versus the Band Gap of Two-Dimensional Materials. Phys. Rev. Lett. 2015, 115 (6), 066403. 215. Omelchenko, S. T.; Tolstova, Y.; Atwater, H. A.; Lewis, N. S., Excitonic effects in photovoltaic materials with large exciton binding energies. In 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), IEEE: 2016. 216. Jung, H. S.; Park, N.-G., Perovskite Solar Cells: From Materials to Devices. Small 2015, 11 (1), 10-25. 217. Cell, N. R. E. L. N. B. R.; Efficiencies Best Research-Cell Efficiencies. https://www.nrel.gov/pv/assets/images/efficiency-chart.png (accessed 13 July 2018). 218. Weber, D., CH3NH3PbX3, a Pb(II)-System with Cubic Perovskite Structure Z. Naturforsch. 1978, 33b, 1443-1445. 219. Zhang, M.-D.; Zheng, B.-H.; Zhuang, Q.-F.; Huang, C.-Y.; Cao, H.; Chen, M.-D.; Wang, B., Two dimethoxyphenylamine-substituted carbazole derivatives as hole-transporting materials for efficient inorganic-organic hybrid perovskite solar cells. Dyes Pigm. 2017, 146, 589-595.

of 163 References

220. Gao, P.; Gratzel, M.; Nazeeruddin, M. K., Organohalide lead perovskites for photovoltaic applications. Energy. Env. Sci. 2014, 7 (8), 2448-2463. 221. Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J., Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338 (6107), 643. 222. Kim, H. S.; Lee, C. R.; Im, J. H.; Lee, K. B.; Moehl, T.; Marchioro, A.; Moon, S. J.; Humphry-Baker, R.; Yum, J. H.; Moser, J. E.; Grätzel, M.; Park, N. G., J. Am. Chem. Soc. 2012, 2, 591-598. 223. Zong, X.; Qiao, W.; Chen, Y.; Wang, H.; Liu, X.; Sun, Z.; Xue, S., New Efficient 1,1'-Bi-2-naphthylamine-Based Hole-Transporting Materials for Perovskite Solar Cells. ChemistrySelect 2017, 2 (16), 4392-4397. 224. Li, H.; Fu, K.; Hagfeldt, A.; Grätzel, M.; Mhaisalkar, S. G.; Grimsdale, A. C., A Simple 3,4-Ethylenedioxythiophene Based Hole-Transporting Material for Perovskite Solar Cells. Angew. Chem. Int. Ed. 2014, 53 (16), 4085-4088. 225. Jeon, N. J.; Lee, J.; Noh, J. H.; Nazeeruddin, M. K.; Grätzel, M.; Seok, S. I., Efficient Inorganic–Organic Hybrid Perovskite Solar Cells Based on Pyrene Arylamine Derivatives as Hole-Transporting Materials. J. Am. Chem. Soc. 2013, 135 (51), 19087-19090. 226. Liang, J.; Ying, L.; Yang, W.; Peng, J.; Cao, Y., Improved efficiency of blue polymer light-emitting diodes using a hole transport material. J. Mater. Chem. C 2017, 5 (21), 5096-5101. 227. Huang, C.; Fu, W.; Li, C.-Z.; Zhang, Z.; Qiu, W.; Shi, M.; Heremans, P.; Jen, A. K. Y.; Chen, H., Dopant-Free Hole-Transporting Material with a C3h Symmetrical Truxene Core for Highly Efficient Perovskite Solar Cells. J. Am. Chem. Soc. 2016, 138 (8), 2528-2531. 228. Park, S.; Heo, J. H.; Cheon, C. H.; Kim, H.; Im, S. H.; Son, H. J., A [2,2]paracyclophane triarylamine-based hole-transporting material for high performance perovskite solar cells. J. Mater. Chem. A 2015, 3 (48), 24215-24220. 229. Cheng, M.; Xu, B.; Chen, C.; Yang, X.; Zhang, F.; Tan, Q.; Hua, Y.; Kloo, L.; Sun, L., Phenoxazine-Based Small Molecule Material for Efficient Perovskite Solar Cells and Bulk Heterojunction Organic Solar Cells. Adv. Energy. Mater. 2015, 5 (8), 1401720. 230. Sung, S. D.; Kang, M. S.; Choi, I. T.; Kim, H. M.; Kim, H.; Hong, M.; Kim, H. K.; Lee, W. I., 14.8% perovskite solar cells employing carbazole derivatives as hole transporting materials. Chem. Commun. 2014, 50 (91), 14161-14163. 231. Gratia, P.; Magomedov, A.; Malinauskas, T.; Daskeviciene, M.; Abate, A.; Ahmad, S.; Grätzel, M.; Getautis, V.; Nazeeruddin, M. K., A Methoxydiphenylamine-Substituted Carbazole Twin Derivative: An Efficient Hole-Transporting Material for Perovskite Solar Cells. Angew. Chem. Int. Ed. 2015, 54 (39), 11409-11413. 232. Hua, Y.; Zhang, J.; Xu, B.; Liu, P.; Cheng, M.; Kloo, L.; Johansson, E. M. J.; Sveinbjörnsson, K.; Aitola, K.; Boschloo, G.; Sun, L., Facile synthesis of fluorene-based hole transport materials for highly efficient perovskite solar cells and solid-state dye-sensitized solar cells. Nano Energy 2016, 26, 108-113. 233. Krishna, A.; Sabba, D.; Yin, J.; Bruno, A.; Boix, P. P.; Gao, Y.; Dewi, H. A.; Gurzadyan, G. G.; Soci, C.; Mhaisalkar, S. G.; Grimsdale, A. C., Facile Synthesis of a Furan-Arylamine Hole-Transporting Material for High-Efficiency, Mesoscopic Perovskite Solar Cells. Chemistry - A European Journal 2015, 21 (43), 15113-15117. 234. Petrus, M. L.; Bein, T.; Dingemans, T. J.; Docampo, P., A low cost azomethine-based hole transporting material for perovskite photovoltaics. J. Mater. Chem. A 2015, 3 (23), 12159-12162. 235. Abate, A.; Paek, S.; Giordano, F.; Correa-Baena, J.-P.; Saliba, M.; Gao, P.; Matsui, T.; Ko, J.; Zakeeruddin, S. M.; Dahmen, K. H.; Hagfeldt, A.; Grätzel, M.; Nazeeruddin, M. K., Silolothiophene-linked triphenylamines as stable hole transporting

of 163 References

materials for high efficiency perovskite solar cells. Energy. Env. Sci. 2015, 8 (10), 2946-2953. 236. Jeon, N. J.; Lee, H. G.; Kim, Y. C.; Seo, J.; Noh, J. H.; Lee, J.; Seok, S. I., o-Methoxy Substituents in Spiro-OMeTAD for Efficient Inorganic–Organic Hybrid Perovskite Solar Cells. J. Am. Chem. Soc. 2014, 136 (22), 7837-7840. 237. Bi, D.; Yang, L.; Boschloo, G.; Hagfeldt, A.; Johansson, E. M. J., Effect of Different Hole Transport Materials on Recombination in CH3NH3PbI3 Perovskite-Sensitized Mesoscopic Solar Cells. J. Phys. Chem. Lett. 2013, 4 (9), 1532-1536. 238. Yu, Z.; Sun, L., Recent Progress on Hole-Transporting Materials for Emerging Organometal Halide Perovskite Solar Cells. Adv. Energy. Mater. 2015, 5 (12), 1500213. 239. Bi, D.; Xu, B.; Gao, P.; Sun, L.; Grätzel, M.; Hagfeldt, A., Facile synthesized organic hole transporting material for perovskite solar cell with efficiency of 19.8%. Nano Energy 2016, 23, 138-144. 240. Back, J. Y.; Yu, H.; Song, I.; Kang, I.; Ahn, H.; Shin, T. J.; Kwon, S.-K.; Oh, J. H.; Kim, Y.-H., Investigation of Structure-Property Relationships in Diketopyrrolopyrrole-Based Polymer Semiconductors via Side-Chain Engineering. Chem. Mater. 2015, 27 (5), 1732-1739. 241. Lei, T.; Dou, J.-H.; Pei, J., Influence of Alkyl Chain Branching Positions on the Hole Mobilities of Polymer Thin-Film Transistors. Adv. Mater. 2012, 24 (48), 6457-6461. 242. Yu, Z.; Sun, L., Recent Progress on Hole-Transporting Materials for Emerging Organometal Halide Perovskite Solar Cells. Adv. Energy. Mater. 2015, 5 (12), 1500213. 243. Vivo, P.; Salunke, J. K.; Priimagi, A., Hole-Transporting Materials for Printable Perovskite Solar Cells. Materials (Basel) 2017, 10 (9), 1087. 244. Dunlap-Shohl, W. A.; Daunis, T. B.; Wang, X.; Wang, J.; Zhang, B.; Barrera, D.; Yan, Y.; Hsu, J. W. P.; Mitzi, D. B., Room-temperature fabrication of a delafossite CuCrO2 hole transport layer for perovskite solar cells. J. Mater. Chem. A 2018, 6 (2), 469-477. 245. Calió, L.; Kazim, S.; Grätzel, M.; Ahmad, S., Hole-Transport Materials for Perovskite Solar Cells. Angew. Chem. Int. Ed. 2016, 55 (47), 14522-14545. 246. Ravishankar, S.; Gharibzadeh, S.; Roldán-Carmona, C.; Grancini, G.; Lee, Y.; Ralaiarisoa, M.; Asiri, A. M.; Koch, N.; Bisquert, J.; Nazeeruddin, M. K., Influence of Charge Transport Layers on Open-Circuit Voltage and Hysteresis in Perovskite Solar Cells. Joule. 247. Bi, D.; Boschloo, G.; Hagfeldt, A., HIGH-EFFICIENT SOLID-STATE PEROVSKITE SOLAR CELL WITHOUT LITHIUM SALT IN THE HOLE TRANSPORT MATERIAL. Nano 2014, 09 (05), 1440001. 248. Zhang, H.; Huang, H.; Ming, H.; Li, H.; Zhang, L.; Liu, Y.; Kang, Z., Carbon quantum dots/Ag3PO4 complex photocatalysts with enhanced photocatalytic activity and stability under visible light. J. Mater. Chem. 2012, 22 (21), 10501-10506. 249. Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S. T.; Zhong, J.; Kang, Z., Water splitting. Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway. Science 2015, 347 (6225), 970-4. 250. Yeh, T. F.; Teng, C. Y.; Chen, S. J.; Teng, H., Nitrogen-doped graphene oxide quantum dots as photocatalysts for overall water-splitting under visible light illumination. Adv. Mater. 2014, 26 (20), 3297-303. 251. Yeh, T.-F.; Chen, S.-J.; Teng, H., Synergistic effect of oxygen and nitrogen functionalities for graphene-based quantum dots used in photocatalytic H2 production from water decomposition. Nano Energy 2015, 12, 476-485. 252. Yu, X.; Cheng, H.; Zhang, M.; Zhao, Y.; Qu, L.; Shi, G., Graphene-based smart materials. Nat. Rev. Mater. 2017, 2, 17046. 253. Wang, Y.; Sumpter, B. G.; Huang, J.; Zhang, H.; Liu, P.; Yang, H.; Zhao, H., Density Functional Studies of Stoichiometric Surfaces of Orthorhombic Hybrid Perovskite CH3NH3PbI3. J. Phys. Chem. C. 2015, 119 (2), 1136-1145.

of 163 References

254. Vienna, U. o. POSCAR file. http://cms.mpi.univie.ac.at/vasp/guide/node59.html (accessed 16 July 2018). 255. Wang, Y.; Gould, T.; Dobson, J. F.; Zhang, H.; Yang, H.; Yao, X.; Zhao, H., Density functional theory analysis of structural and electronic properties of orthorhombic perovskite CH3NH3PbI3. PCCP 2014, 16 (4), 1424-1429. 256. Batsanov, S. S., Van der Waals Radii of Elements. Inorg. Mater. 2001, 37 (9), 871-885. 257. Zhou, Z.; Pang, S.; Liu, Z.; Xu, H.; Cui, G., Interface engineering for high-performance perovskite hybrid solar cells. J. Mater. Chem. A 2015, 3 (38), 19205-19217. 258. Li, X.; Bi, D.; Yi, C.; Decoppet, J. D.; Luo, J.; Zakeeruddin, S. M.; Hagfeldt, A.; Gratzel, M., A vacuum flash-assisted solution process for high-efficiency large-area perovskite solar cells. Science 2016, 353 (6294), 58-62. 259. Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I., High-performance photovoltaic perovskite layers fabricated through intramolecular exchange. Science 2015, 348 (6240), 1234-1237. 260. Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I., Compositional engineering of perovskite materials for high-performance solar cells. Nature 2015, 517 (7535), 476-480. 261. Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T. b.; Duan, H. S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y., Interface engineering of highly efficient perovskite solar cells. Science 2014, 345 (6196), 542-546. 262. Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I., Solvent engineering for high-performance inorganic–organic hybrid perovskite solar cells. Nature Materials 2014, 13 (9), 897-903. 263. Liu, M.; Johnston, M. B.; Snaith, H. J., Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 2013, 501 (7467), 395-398. 264. Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M., Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 2013, 499 (7458), 316-319. 265. Völker, S. F.; Collavini, S.; Delgado, J. L., Organic Charge Carriers for Perovskite Solar Cells. ChemSusChem 2015, 8 (18), 3012-3028. 266. Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H., Dye-Sensitized Solar Cells. Chem. Rev. 2010, 110 (11), 6595-6663. 267. Choi, H.; Park, S.; Paek, S.; Ekanayake, P.; Nazeeruddin, M. K.; Ko, J., Efficient star-shaped hole transporting materials with diphenylethenyl side arms for an efficient perovskite solar cell. J. Mater. Chem. A 2014, 2 (45), 19136-19140. 268. Flahaut, E.; Sloan, J.; Friedrichs, S.; Kirkland, A. I.; Coleman, K. S.; Williams, V. C.; Hanson, N.; Hutchison, J. L.; Green, M. L. H., Crystallization of 2H and 4H PbI2in Carbon Nanotubes of Varying Diameters and Morphologies. Chem. Mater. 2006, 18 (8), 2059-2069. 269. Palosz, B., The structure of PbI2polytypes 2H and 4H: a study of the 2H-4H transition. J. Phys.: Condens. Matter 1990, 2 (24), 5285-5295. 270. Salje, E.; Palosz, B.; Wruck, B., In situ observation of the polytypic phase transition 2H-12R in PbI2: investigations of the thermodynamic structural and dielectric properties. Journal of Physics C: Solid State Physics 1987, 20 (26), 4077-4096. 271. Zhou, M.; Duan, W.; Chen, Y.; Du, A., Single layer lead iodide: computational exploration of structural, electronic and optical properties, strain induced band modulation and the role of spin-orbital-coupling. Nanoscale 2015, 7 (37), 15168-15174. 272. Wang, Y.; Zhang, Q.; Shen, Q.; Cheng, Y.; Schwingenschlogl, U.; Huang, W., Lead monoxide: a two-dimensional ferromagnetic semiconductor induced by hole-doping. J. Mater. Chem. C 2017, 5 (18), 4520-4525. 273. Tao, J.; Guan, L., Tailoring the electronic and magnetic properties of monolayer SnO by B, C, N, O and F adatoms. Sci. Rep. 2017, 7, 44568.

of 163 References

274. Albar, A.; Tahini, H. A.; Schwingenschlogl, U., Metallicity at interphase boundaries due to polar catastrophe induced by charge density discontinuity. NPG Asia Mater. 2018, 10 (2), e469. 275. Wang, S.; Yang, G.; Yang, S. C., J. Phys. Chem. C 2015, 119 (null), 27938. 276. Chen, Q.; Zhou, H.; Fang, Y.; Stieg, A. Z.; Song, T.-B.; Wang, H.-H.; Xu, X.; Liu, Y.; Lu, S.; You, J.; Sun, P.; McKay, J.; Goorsky, M. S.; Yang, Y., The optoelectronic role of chlorine in CH3NH3PbI3(Cl)-based perovskite solar cells. Nat. Commun. 2015, 6, 7269. 277. Henkelman, G.; Arnaldsson, A.; Jónsson, H., A fast and robust algorithm for Bader decomposition of charge density. Computational Materials Science 2006, 36 (3), 354-360. 278. Sanville, E.; Kenny, S. D.; Smith, R.; Henkelman, G., Improved grid-based algorithm for Bader charge allocation. J. Comput. Chem. 2007, 28 (5), 899-908. 279. Tang, W.; Sanville, E.; Henkelman, G., A grid-based Bader analysis algorithm without lattice bias. J. Phys.: Condens. Matter 2009, 21 (8), 084204. 280. Yu, M.; Trinkle, D. R., Accurate and efficient algorithm for Bader charge integration. J. Chem. Phys. 2011, 134 (6), 064111.

T (2D) S E A Kasi... · Sri Kasi Venkata Nageswara Rao Matta B.Sc. (Physics), B.Tech (Chemical...

Documents

Transcript of T (2D) S E A Kasi... · Sri Kasi Venkata Nageswara Rao Matta B.Sc. (Physics), B.Tech (Chemical...