AB INITIO ATOMISTIC INSIGHTS INTO LEAD FREE ......Mengmeng Hao, Dongxu He, Yang Bai, Peng Chen, Paul...

AB INITIO ATOMISTIC INSIGHTS INTO

LEAD-FREE PEROVSKITES FOR

PHOTOVOLTAICS AND

OPTOELECTRONICS

Md Roknuzzaman

B.Sc. (Hons.) in Physics, M.Sc. in Solid State Physics

Submitted in fulfilment of the requirements for the degree of

Doctor of Philosophy

School of Chemistry and Physics

Science and Engineering Faculty

Queensland University of Technology

2020

Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics i

Keywords

Perovskites, Inorganic Perovskites, Organic Perovskites, Organic-Inorganic

Perovskites, Hybrid Perovskites, Cs-based Perovskites, MA-based Perovskites, FA-

based Perovskites, Double Perovskites, Hybrid Double Perovskites, Lead-free

Perovskites, Hybrid Semiconductors, Organic Semiconductors, Structural Properties,

Electronic Properties, Optical Properties, Transport Properties, Elastic Properties,

Mechanical Properties, Optoelectronics.

ii Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics

Abstract

Methylammonium lead iodide (CH3NH3PbI3) and some other hybrid perovskites

have drawn significant attention to the science community because of their high power

conversion efficiency in solar cells. In addition, this group of semiconductors has the

potential to be used in a wide range of optoelectronic devices like light-emitting

diodes, lasers, field-effect transistors, photodetectors, photoluminescent,

electroluminescent devices as well as light-emitting electrochemical cells.

Commercialization of perovskite materials may revolutionize the global energy sector

as these materials are abundant in nature and inexpensive, as a result it would be

cheaper and more efficient than silicon-based technology. However, the insufficient

long-term stability and toxicity of lead (Pb) are two major barriers for Pb-based hybrid

perovskites to be adopted in large-scale industrial applications. Therefore, it is utmost

important to find non-toxic Pb-free stable perovskites for the further development of

perovskites based optoelectronic technology. A detailed atomistic insight of the

fundamental properties of perovskite materials can help to understand the basic

characteristics of the materials and it can guide research to find non-toxic stable

materials for photovoltaics and optoelectronics.

This thesis presents a first-principles Density Functional Theory (DFT)

investigations of the structural, electronic, optical and mechanical properties of

caesium (Cs) based inorganic perovskites CsBX3 (B = Pb, Sn, Ge; X = I, Br, Cl) as

well as methylammonium (MA) and formamidinium (FA) based organic-inorganic

hybrid perovskites MABX3 (MA = CH3NH3, B = Pb, Sn, Ge; X = I, Br, Cl) and FABX3

[FA = CH(NH2)2; B = Pb, Sn, Ge; X = I, Br, Cl]. The results suggest that the considered

perovskites are semiconductors with direct energy band gap and are mechanically

stable. Also, the calculated high absorption coefficient, low reflectivity and high

optical conductivity suggest that the considered Pb-free materials have potential to be

used in solar cells and other optoelectronic energy devices. The optical properties of

the considered inorganic perovskites indicate that germanium (Ge) would be a better

replacement of Pb. Indeed, Ge containing compounds have higher optical absorption

and optical conductivity than that of Pb containing compounds. A complementary

analysis of the electronic, optical and mechanical properties of the Cs-based

compounds reveals that CsGeI3 is the best Pb-free inorganic metal halide

Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics iii

semiconductor for the solar cell application while the solid solution CsGe(I0.7Br0.3)3 is

found to be mechanically more ductile than CsGeI3. However, in case of organic-

inorganic hybrid perovskites, tin (Sn) containing compounds show better

optoelectronic properties compared to the Ge-containing counterparts. More

specifically, MA and FA based Sn containing perovskites such as MASnI3 and FASnI3

have superior properties compared to other Pb-free options as the materials have

excellent electronic, optical and mechanical properties. MASnI3 is found to be one of

the best Pb-free materials considering its promising optoelectronic properties as well

as the unique mechanical property of MASnI3 makes this compound flexible and easy

to be fabricated into thin films. In case of FA based perovskites, FASnI3 would be the

preferred Pb-free material for photovoltaic application because of its low carrier

effective mass and high absorption coefficient along with good material ductility.

Furthermore, the optoelectronic properties of a new group of compounds called

organic-inorganic hybrid double perovskites, ABiCuX6 [A = Cs2, (MA)2, (FA)2,

CsMA, CsFA, MAFA; X = I, Br, Cl] have been investigated using the same

methodology to predict their suitability in photovoltaic and optoelectronic

applications. The considered hybrid double perovskites are found as semiconductors

with a tunable band gap characteristics that are suitable for devices like light emitting

diodes. Moreover, the high dielectric constant, high absorption, high optical

conductivity and low reflectivity suggest that the materials have the potential to be

used in a wide range of optoelectronic applications including solar cells. Furthermore,

the organic-inorganic hybrid double perovskite (FA)2BiCuI6 has been predicted as the

best candidate in photovoltaic and optoelectronic applications as this material has

superior optical and electronic properties.

The findings of this study can help the understanding of structure-property

relationships in perovskite materials. Therefore, it is expected that these results will

benefit the development of Pb-free non-toxic sustainable optoelectronic devices.

iv Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics

List of Publications

The published journal and conference papers during the PhD candidature are

listed below.

Journal Papers:

1. Md Roknuzzaman, Kostya (Ken) Ostrikov, Hongxia Wang, Aijun Du,

Tuquabo Tesfamichael, Towards lead-free perovskite photovoltaics and

optoelectronics by ab-initio simulations, Scientific Reports, 7 (2017) 14025.

IF: 4.122, SJR: 1.41 [Q1]. (Included in the thesis)

2. Md Roknuzzaman, Kostya (Ken) Ostrikov, Kimal Chandula

Wasalathilake, Cheng Yan, Hongxia Wang, Tuquabo Tesfamichael, Insight

into lead-free organic-inorganic hybrid perovskites for photovoltaics and

optoelectronics: A first-principles study, Organic Electronics, 59 (2018) 99-

106. IF: 3.495, SJR: 0.94 [Q1]. (Included in the thesis)

3. Md Roknuzzaman, Jose A. Alarco, Hongxia Wang, Aijun Du, Tuquabo

Tesfamichael, Kostya (Ken) Ostrikov, Ab initio atomistic insights into lead-

free formamidinium based hybrid perovskites for photovoltaics and

optoelectronics, Computational Materials Science, 169 (2019) 109118 . IF:

2.292, SJR: 0.81 [Q1]. (Included in the thesis)

4. Md Roknuzzaman, Chunmei Zhang, Kostya (Ken) Ostrikov, Aijun Du,

Hongxia Wang, Lianzhou Wang, Tuquabo Tesfamichael, Electronic and

optical properties of lead-free hybrid double perovskites for photovoltaic

and optoelectronic applications, Scientific Reports, 9 (2019) 718. IF: 4.122,

SJR: 1.41 [Q1]. (Included in the thesis)

5. M. Roknuzzaman, M.A. Hadi, M.A. Ali, M.M. Hossain, N. Jahan, M.M.

Uddin, J.A. Alarco, K. Ostrikov, First hafnium-based MAX phase in the 312

family, Hf3AlC2: A first principles study, Journal of Alloys and Compounds,

727 (2017) 616-626. IF: 3.779, SJR: 1.07 [Q1].

6. M.T. Nasir, M.A. Hadi, M.A. Rayhan, M.A. Ali, M.M. Hossain, M.

Roknuzzaman, S.H. Naqib, A.K.M.A. Islam, M.M. Uddin, K. Ostrikov,

First-Principles Study of Superconducting ScRhP and ScIrP Pnictides,

Physica Status Solidi B, 254 (2017) 1700336. IF: 1.454, SJR: 0.52 [Q2].

Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics v

7. M.A. Hadi, M. Roknuzzaman, A. Chroneos, S.H. Naqib, A.K.M.A. Islam,

R.V. Vovk, K. Ostrikov, Elastic and thermodynamic properties of new

(Zr3−xTix)AlC2 MAX-phase solid solutions, Computational Materials

Science, 137 (2017) 318-326. IF: 2.292, SJR: 0.81 [Q1].

8. Kimal Chandula Wasalathilake, Md Roknuzzaman, Kostya (Ken)

Ostrikov, Godwin A. Ayoko, Cheng Yan, Interaction between

functionalized graphene and sulfur compounds in a lithium-sulfur battery: a

density functional theory investigation, RSC Advances, 8 (2018) 2271. IF:

3.049, SJR: 0.81 [Q1].

9. M.A. Ali, M. Anwar Hossain, M.A. Rayhan, M.M. Hossain, M.M. Uddin,

M. Roknuzzaman, K. Ostrikov, A.K.M.A. Islam, S.H. Naqib, First-

principles study of elastic, electronic, optical and thermoelectric properties

of newly synthesized K2Cu2GeS4 chalcogenide, Journal of Alloys and

Compounds, 781 (2019) 37-46. IF: 3.779, SJR: 1.07 [Q1].

10. Mehri Ghasemi , Miaoqiang Lyu, Md Roknuzzaman, Jung-Ho Yun,

Mengmeng Hao, Dongxu He, Yang Bai, Peng Chen, Paul V. Bernhardt,

Kostya (Ken) Ostrikov, Lianzhou Wang, Phenethylammonium bismuth

halides for low-toxic, stable and solution-processable optoelectronics

beyond lead halide perovskites, Journal of Materials Chemistry A, 7 (2019)

20733. IF: 10.733, SJR: 3.37 [Q1].

Conference Presentations:

1. Md Roknuzzaman, Kostya (Ken) Ostrikov, Hongxia Wang, Tuquabo

Tesfamichael, Towards lead-free inorganic and hybrid perovskites for

photovoltaics and optoelectronics, Oral Presentation, 47th Chemeca

Conference, 30 Sep to 3 Oct 2018, Queenstown, New Zealand.

2. Md Roknuzzaman, Kostya (Ken) Ostrikov, Hongxia Wang, Tuquabo

Tesfamichael, Insight into lead-free hybrid double perovskites for

photovoltaics and optoelectronics, Oral Presentation, 48th Chemeca

Conference, 29 Sep to 2 Oct 2019, Sydney, Australia.

vi Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics

Table of Contents

Keywords .................................................................................................................................. i

Abstract .................................................................................................................................... ii

List of Publications .................................................................................................................. iv

Table of Contents .................................................................................................................... vi

List of Figures ......................................................................................................................... ix

List of Tables .......................................................................................................................... xv

List of Abbreviations ............................................................................................................ xvii

Statement of Original Authorship ......................................................................................... xix

Acknowledgements ................................................................................................................ xx

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

1.1 Background .................................................................................................................... 1

1.2 Research Problem .......................................................................................................... 3

1.3 Aims and Objectives ...................................................................................................... 3

1.4 Thesis Outline ................................................................................................................ 6

1.5 References ...................................................................................................................... 7

Chapter 2: Literature Review ........................................................................... 15

2.1 Concept of Perovskite Materials .................................................................................. 15 2.1.1 Perovskite and Perovskite Structure .................................................................. 15 2.1.2 Crystal Structure of Perovskite Materials .......................................................... 15 2.1.3 Classification of Perovskite Materials ............................................................... 17

2.2 Physical Properties and Critical Parameters ................................................................ 18

2.3 Potential Applications of Perovskites .......................................................................... 18

2.4 Double Perovskites ...................................................................................................... 21 2.4.1 Crystal Structure of Double Perovskites ............................................................ 21 2.4.2 Properties and Potential applications of Double Perovskites ............................. 22

2.5 References .................................................................................................................... 23

Chapter 3: Insight into Cs based Inorganic Perovskites CsBX3 (B = Pb, Sn,

Ge; X = I, Br, Cl) ...................................................................................................... 29

3.1 Statement of Contribution ............................................................................................ 30

3.2 Abstract ........................................................................................................................ 32

3.3 Introduction .................................................................................................................. 32

3.4 Results and Discussion ................................................................................................. 34 3.4.1 Structural Properties .......................................................................................... 34 3.4.2 Electronic Properties .......................................................................................... 35 3.4.3 Optical Properties .............................................................................................. 37 3.4.4 Mechanical Properties ........................................................................................ 39

3.5 Lead Free Perovskites .................................................................................................. 41

Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics vii

3.6 Conclusion ....................................................................................................................43

3.7 Computational Methods................................................................................................43

3.8 References ....................................................................................................................44

3.9 Supplementary Information ..........................................................................................47

Chapter 4: Insight into MA based Hybrid Perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl) ...................................................................................................... 63

4.1 Statement of Contribution .............................................................................................64

4.2 Abstract .........................................................................................................................66 4.2.1 Graphical Abstract ..............................................................................................66

4.3 Introduction ..................................................................................................................67


4.5 Results and Discussion .................................................................................................69 4.5.1 Structural Properties ...........................................................................................69 4.5.2 Electronic and Optical Properties .......................................................................71 4.5.3 Mechanical Properties ........................................................................................76

4.6 Conclusions ..................................................................................................................79

4.7 Acknowledgements.......................................................................................................80

4.8 References ....................................................................................................................80

4.9 Supplementary Information ..........................................................................................85

Chapter 5: Insight into FA based Hybrid Perovskites FABX3 (B = Pb, Sn, Ge; X = I, Br, Cl) ...................................................................................................... 91

5.1 Statement of Contribution .............................................................................................92

5.2 Abstract .........................................................................................................................94 5.2.1 Graphical Abstract ..............................................................................................94

5.3 Introduction ..................................................................................................................95


5.5 Results and Discussion .................................................................................................98 5.5.1 Structural Properties ...........................................................................................98 5.5.2 Electronic and Optical Properties .......................................................................99 5.5.3 Elastic Properties ..............................................................................................105

5.6 Conclusion ..................................................................................................................107

5.7 Acknowledgements.....................................................................................................108

5.8 References ..................................................................................................................108

5.9 Supplementary Information ........................................................................................113

Chapter 6: Insight into Double Perovskites ABiCuX6 [A = Cs2, (MA)2, (FA)2,

CsMA, CsFA, MAFA]............................................................................................ 121

6.1 Statement of Contribution ...........................................................................................122

6.2 Abstract .......................................................................................................................124

6.3 Introduction ................................................................................................................124

6.4 Results and Discussion ...............................................................................................126 6.4.1 Structural Properties .........................................................................................126

viii Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics

6.4.2 Electronic Properties ........................................................................................ 127 6.4.3 Optical Properties ............................................................................................ 131

6.5 Conclusion ................................................................................................................. 133

6.6 Computational Methods ............................................................................................. 133

6.7 Acknowledgements .................................................................................................... 134

6.8 References .................................................................................................................. 134

6.9 Supplementary Information ....................................................................................... 138

Chapter 7: Conclusions.................................................................................... 149

7.1 Conclusions ................................................................................................................ 149

7.2 Limitations and Future Recommendations ................................................................ 151

Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics ix

List of Figures

Figure 1.1. Periodic table showing the possible replacements of Pb (red colour)

and I (yellow colour) in inorganic CsPbI3 and hybrid MAPbI3 and

FAPbI3 perovskites. ................................................................................. 4

Figure 1.2. Periodic table showing the possible trivalent (blue boxes) and

monovalent (red box) elements to form double perovskite structure

because of the replacement of Pb from perovskite by complex

substitution. ............................................................................................. 5

Figure 2.1. Unit cell of cubic ABX3 perovskite structure. ......................................... 15

Figure 2.2. The unit cell of CH3NH3PbI3 material [30]. ............................................ 16

Figure 2.3. Structure of a perovskite with a chemical formula ABX3. The red

spheres are X atoms, the blue spheres are B-atoms and the green

spheres are the A-atoms [4]. .................................................................. 16

Figure 2.4. Octahedron in perovskite crystal structure [7]......................................... 17

Figure 2.5. Variation of efficiency over time in different solar cell technology

[19]. ....................................................................................................... 18

Figure 2.6. The change in Power Conversion Efficiency of perovskite solar cells

compared to other types of photovoltaics over recent years. (Data

collected from “www.ossila.com”) [1].................................................. 19

Figure 2.7. Potential optoelectronic properties and applications of typical

perovskite ABX3 (acronyms: QD: quantum dots, NW: nanowire,

NR: nanorod, NC: nanocrystal, MC: millimeter-scale crystal, NP:

nanoparticle, PL: photoluminescence, EL: electroluminescence,

LEDs: light-emitting diodes, FET: field-effect transistor and LECs:

light-emitting electrochemical cells) [25]. ............................................ 20

Figure 2.8. Polyhedral model of the conventional unit cell of a double

perovskites [43]. .................................................................................... 22

Figure 3.1. Unit cells of the considered cubic metal halide perovskites CsBX3 (B

= Sn, Ge; X = I, Br, Cl) as compared with Pb-based compounds

CsPbX3 (X = I, Br, Cl). Replacement of halogen atoms is showing

from left to right while the replacement of Pb is showing from top

to bottom. The molecular models are optimized by DFT

calculations. ........................................................................................... 34

Figure 3.2. Variation of lattice parameter due to the replacement of atoms by

similar atoms. The lattice parameter is seen to change periodically

depending upon the size of atoms in the unit cell. ................................ 35

Figure 3.3. Variation of the electronic band gap due to the replacement of atoms

by similar atoms. ................................................................................... 36

Figure 3.4. Calculated optical absorption of perovskites CsBX3 (B = Pb, Sn, Ge;

X = I, Br, Cl) as a function of incident photon energies. ...................... 38

x Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics

Figure 3.5. Calculated optical conductivity of perovskites CsBX3 (B = Pb, Sn,

Ge; X = I, Br, Cl) as a function of incident photon energies. ................ 38

Figure 3.6. Variation of the Pugh’s ratio of the perovskite with different

composition. The pink dashed line separates the ductile materials

from brittle. ............................................................................................ 41

Figure 3.7. Variation of Pugh’s ratio for different combination of I and Br in

CsGe(I1-xBrx)3. CsGe(I0.7Br0.3)3, CsGe(I0.1Br0.9)3 and CsGeBr3 are

ductile while the others are brittle and the maximum ductility is

found in CsGe(I0.7Br0.3)3. ....................................................................... 42

Figure S3.1. Variation of the unit cell volume due to the replacement of atoms

by similar atoms. ................................................................................... 48

Figure S3.2. Variation of optical band gap due to the replacement of atoms by

similar atoms. The data of optical band gap are taken from

references [24], [32], [22] and [16]. ...................................................... 50

Figure S3.3. Electronic band structure of perovskites CsBX3 (B = Pb, Sn, Ge; X

= I, Br, Cl). ............................................................................................ 51

Figure S3.4. Total and partial densities of states of perovskites CsBX3 (B = Pb,

Sn, Ge; X = I, Br, Cl). The Fermi level EF is set at 0 eV. ..................... 52

Figure S3.5. Calculated real part of dielectric function of perovskites CsBX3 (B

= Pb, Sn, Ge; X = I, Br, Cl) as a function of incident photon energy.

............................................................................................................... 53

Figure S3.6. Calculated imaginary part of dielectric function of perovskites

CsBX3 (B = Pb, Sn, Ge; X = I, Br, Cl) as a function of incident

photon energy. ....................................................................................... 53

Figure S3.7. Calculated reflectivity of perovskites CsBX3 (B = Pb, Sn, Ge; X =

I, Br, Cl) as a function of incident photon energy. ................................ 54

Figure S3.8. Calculated refractive index of perovskites CsBX3 (B = Pb, Sn, Ge;

X = I, Br, Cl) as a function of incident photon energy. ......................... 54

Figure S3.9. Calculated extinction coefficient of perovskites CsBX3 (B = Pb, Sn,

Ge; X = I, Br, Cl) as a function of incident photon energy. .................. 55

Figure S3.10. Periodic change of optical properties due to replacement of

halogen atoms. (a) Calculated optical absorption of perovskites

CsGeX3 (X = I, Br, Cl) as a function of incident photon energies.

(b) Calculated optical conductivity of perovskites CsGeX3 (X = I,

Br, Cl) as a function of incident photon energies. ................................. 55

Figure S3.11. Change in Poisson ratio due to the replacement of atoms by similar

atoms...................................................................................................... 56

Figure S3.12. Change in Poisson ratio for the solid solutions CsGe(I1-xBrx)3,

where x = 0-1 at the increment of 0.1. ................................................... 57

Figure 4.1. Crystal structure of MASnI3 as an example of the crystal structure of

organic-inorganic perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br,

Cl). (a) Schematic view of the unit cell of MASnI3. (b) The two

dimensional view of the primitive cell of MASnI3. .............................. 70

Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics xi

Figure 4.2. The uniform variation of the structural properties of the considered

organic-inorganic perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br,

Cl). (a) Variation of the lattice parameter because of the

replacement of atoms by similar atoms. (b) Periodic reduction of

the unit cell volume because of the replacement of atoms by similar

atoms. This change of unit cell volume can affect other properties

of these materials. .................................................................................. 70

Figure 4.3. (a) Calculated electronic band gap of the considered organic-

inorganic perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). The

electronic band gap is seen to increase due to the replacement of I

by Br and Cl. (b) Calculated dielectric constant of the considered

organic perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). The

maximum dielectric constant is observed for MASnI3 among the

considered nine perovskites. The dielectric constant is seen to

decrease due to the replacement of I by Br and Cl. ............................... 73

Figure 4.4. The calculated (a) real part of the dielectric function and (b)

imaginary part of the dielectric function of perovskites MABX3 (B

= Pb, Sn, Ge; X = I, Br, Cl) as a function of incident energies up to

30 eV. .................................................................................................... 74

Figure 4.5. Calculated optical properties of the considered organic-inorganic

perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). (a) The

calculated optical absorption as a function of incident photon

energies. (b) The calculated optical conductivity as a function of

incident photon energies. ....................................................................... 75

Figure 4.6. Graphical representation to separate ductile materials from brittle for

the considered organic-inorganic perovskites MABX3 (B = Pb, Sn,

Ge; X = I, Br, Cl). The red dashed line separates the ductile

materials from brittle. (a) The calculated Pugh’s ratio for the

considered compounds. (b) The calculated Poisson ratio for the

considered perovskites. ......................................................................... 78

Figure S4.1. Calculated electronic band structure along the highly symmetric

directions of the Brillouin zone of perovskites MABX3 (B = Pb, Sn,

Ge; X = I, Br, Cl). Electronic states in the valence band are

indicated by blue colour while the electronic states of the

conduction bands are indicated by green colour. The red dashed line

along the energy of 0 eV indicates the Fermi level, EF. The

considered compounds are direct band gap semiconductors where

the lowest band gap is observed at Z point. ........................................... 86

Figure S4.2. Total and partial densities of states of perovskites MABX3 (B = Pb,

Sn, Ge; X = I, Br, Cl). The black dashed line along the energy of 0

eV indicates the Fermi level, EF. The contribution to the total DOS

of C-2s and N-2s near the Fermi level is not significance, therefore

it is not shown in the figure. The total DOS at the upper part of the

valence band mainly comes from the p orbital of halogen atoms (I-

5p, Br-4p, Cl-3p) indicated by blue colour. Also, the total DOS at

the lower part of the conduction band mainly comes from the p

orbital of B atoms (Pb-6p, Sn-5p, Ge-4p) indicated by green colour.

............................................................................................................... 87

xii Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics

Figure S4.3. The calculated (a) reflectivity (b) refractive index and (c) extinction

coefficient of perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl)

as a function of incident photon energies up to 30 eV. ......................... 88

Figure 5.1. Structural properties for the investigated FA-based hybrid

perovskites FABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). (a) A model

crystal structure of the unit cell of FA-based hybrid perovskites. (b)

Periodic change in the calculated lattice parameter for the

investigated compounds. ....................................................................... 98

Figure 5.2. Variation of the studied electronic parameters for the selected FA-

based perovskites FABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). (a)

Calculated band gap. (b) Calculated dielectric constant. ...................... 99

Figure 5.3. Comparison of the calculated carrier effective masses for the studied

FA-based hybrid perovskites FABX3 (B = Pb, Sn, Ge; X = I, Br,

Cl). ....................................................................................................... 101

Figure 5.4. Calculated dielectric function of the studied FA-based hybrid halide

perovskites FABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). (a) Calculated

real part of the dielectric function. (b) Calculated imaginary part of

the dielectric function. ......................................................................... 102

Figure 5.5. Variation of the calculated optical properties for the studied FA-

based hybrid perovskites FABX3 (B = Pb, Sn, Ge; X = I, Br, Cl).

(a) Calculated absorption spectra. (b) Calculated optical

conductivity. ........................................................................................ 103

Figure 5.6. Identification of ductile and brittle materials for the considered FA-

based hybrid halide perovskites FABX3 (B = Pb, Sn, Ge; X = I, Br,

Cl). (a) Calculated Pugh’s ratio. (b) Calculated Poison ratio. ............. 107

Figure S5.1. Calculated electronic band structure for FA-based Pb-free non-toxic

hybrid halide perovskites FABX3 (B = Sn, Ge; X = I, Br, Cl) as well

as for its Pb-based counterparts FAPbX3 (X = I, Br, Cl) along high

symmetry directions of the Brillouin zone. The considered path in

reciprocal space is X(0.5,0,0)-R(0.5,0.5,0.5)-M(0.5,0.5,0)-(0,0,0)-R(0.5,0.5,0.5). Electronic states corresponding to the valence band

maximum (VBM) are indicated by the green colour line, while the

electronic states in corresponding to the conduction band minimum

(CBM) are indicated by the pink colour line. The red dashed line

along the energy of 0 eV indicates the Fermi level. Both the VBM

and CBM are seen for the same k-vector (at the R point of the

Brillouin zone) indicating that the considered compounds are direct

band gap semiconductors. ................................................................... 114

Figure S5.2. Calculated total and partial densities of states for FA-based Pb-free

non-toxic hybrid halide perovskites FABX3 (B = Sn, Ge; X = I, Br,

Cl) as well as for its Pb-based counterparts FAPbX3 (X = I, Br, Cl).

The partial DOSs from H-1s, C-2s and N-2s are not contributed

significantly to the total DOS near the Fermi level, therefore it is

not shown in the figure. The total DOS at the valence band

maximum (VBM) is primarily contributed by the p states of X

atoms (5p states of I, 4p states of Br, 3p states of Cl) and the

corresponding curves are indicated by green colour. Also, the total

Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics xiii

DOS at the conduction band minimum (CBM) is primarily

contributed by the p states of B atoms (6p states of Pb, 5p states of

Sn, 4p states of Ge) and the corresponding curves are indicated by

the pink colour. .................................................................................... 115

Figure S5.3. Calculated (a) reflectivity (b) refractive index and (c) extinction

coefficient for FA-based Pb-free non-toxic hybrid halide

perovskites FABX3 (B = Sn, Ge; X = I, Br, Cl) as well as for its Pb-

based counterparts FAPbX3 (X = I, Br, Cl) with respect to the

energy of incident photons with energies ranged from 0 to 15 eV. .... 116

Figure 6.1. Unit cell of double perovskites Cs2BiCuI6 as an example of the

crystal structure of the considered double perovskites ABiCuX6 [A

= Cs2, (MA)2, (FA)2, CsMA, CsFA, MAFA; X = I, Br, Cl]. .............. 127

Figure 6.2. Comparison of the electronic band gap and dielectric constant of the

selected group of double perovskites ABiCuX6 [A = Cs2, (MA)2,

(FA)2, CsMA, CsFA, MAFA; X = I, Br, Cl]. (a) Electronic band

gap of the materials determined by using GGA-PBE approach. (b)

Calculated dielectric constant or static dielectric function for the

considered double perovskites. ........................................................... 128

Figure 6.3. Calculated electronic properties of the considered double perovskite

(FA)2BiCuI6. (a) Electronic band structure along the high symmetry

direction of the Brillouin zone having path (0,0,0)-F(0,0.5,0)-

Q(0,0.5,0.5)-Z(0,0,0.5)-(0,0,0). The bands calculated by GGA-PBE are indicated by blue color whereas the bands calculated by

HSE06 are indicated by pink color. The valence band maximum

(VBM) is seen at F point whereas the conduction band minimum

(CBM) is observed at Z point of the Brillouin zone indicating that

it is an indirect band gap semiconductor. (b) Calculated total and

partial densities of states. The Cu-3d states (green color curve) and

the I-5p (blue color curve) states are seen as the main contributors

towards VBM whereas the Bi-6p (pink color curve) and I-5p (blue

color curve) states are mostly contributed towards CBM. .................. 130

Figure 6.4. Comparison of the optical properties of double perovskites ABiCuX6

[A = Cs2, (MA)2, (FA)2, CsMA, CsFA, MAFA; X = I, Br, Cl] along

the incident electromagnetic radiation of energy from 0 to 5eV. (a)

Calculated dielectric function (real part). (b) Calculated absorption

coefficient. (c) Calculated optical conductivity. ................................. 132

Figure S6.1. Crystal structure of organic-inorganic double perovskites. The

structure of inorganic double perovskite Cs2BiCuI6 was initially

drawn. Then the Cs atoms were replaced by MA

(methylammonium) or FA (formamidinium) to get the structure of

required organic-inorganic double perovskites. (a) Three and two

dimensional views of the unit cell of inorganic double perovskite

Cs2BiCuI6. (b) Three and two dimensional views of the primitive

cell of inorganic double perovskite Cs2BiCuI6. (c) Molecular

structure of CH3NH3 (or, MA) and CH(NH2)2 (or, FA). The carbon-

nitrogen double bond is also shown in the figure. ............................... 139

xiv Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics

Figure S6.2. Calculated electronic band structure of the considered double

perovskites ABiCuI6 [A = Cs2, (MA)2, (FA)2; X = I, Br, Cl] along

the high symmetry direction of the Brillouin zone having path

(0,0,0)-F(0,0.5,0)-Q(0,0.5,0.5)-Z(0,0,0.5)-(0,0,0). The green band indicates the highest energy valence band in which the

valence band maximum (VBM) is observed. On the other hand, the

pink band indicates the lowest energy conduction band in which the

conduction band minimum (CBM) is observed. ................................. 141

Figure S6.3. Calculated electronic band structure of the considered double

perovskites ABiCuI6 [A = CsMA, CsFA, MAFA; X = I, Br, Cl]

along the high symmetry direction of the Brillouin zone having path

(0,0,0)-F(0,0.5,0)-Q(0,0.5,0.5)-Z(0,0,0.5)-(0,0,0). The green band indicates the highest energy valence band in which the

valence band maximum (VBM) is observed. On the other hand, the

pink band indicates the lowest energy conduction band in which the

conduction band minimum (CBM) is observed. ................................. 142

Figure S6.4. Calculated total and partial densities of states of the considered

double perovskites ABiCuI6 (A = Cs2, (MA)2, (FA)2; X = I, Br, Cl).

The Cu-3d states (blue colour curve) and the p states of halogen (I-

5p, Br-4p, Cl-3p) are seen to contribute towards valence band

maximum (VCM). The p states of halogen is also contributed to

conduction band minimum (CBM). However, the maximum

contribution towards the CBM is mainly come from Bi-6p states

(pink colour curve). ............................................................................. 143

Figure S6.5. Calculated total and partial densities of states of the considered

double perovskites ABiCuI6 (A = CsMA, CsFA, MAFA; X = I, Br,

Cl). The Cu-3d states (blue colour curve) and the p states of halogen

(I-5p, Br-4p, Cl-3p) are seen to contribute towards valence band

maximum (VCM). The p states of halogen are also contributed to

conduction band minimum (CBM). However, the maximum

contribution towards the CBM is mainly come from Bi-6p states

(pink colour curve). ............................................................................. 144

Figure S6.6. Calculated (a) Imaginary part of dielectric function and (b)

Reflectivity of the considered double perovskites ABiCuI6 (A = A

= Cs2, (MA)2, (FA)2, CsMA, CsFA, MAFA; X = I, Br, Cl). The

variation of the imaginary part of the dielectric function is quite

similar for the considered compounds except an intense peak of

(FA)2BiCuI6 and (FA)2BiCuI6 at the energy 3.75 eV and 4.3 eV,

respectively. The reflectivity of the considered compounds is seen

to vary between 12 to 30 % implies that the materials have low

reflectivity for incoming solar radiation. ............................................. 145

Figure S6.7. Calculated (a) Refractive Index and (b) Extinction Coefficient of

the considered double perovskites ABiCuI6 (A = Cs2, (MA)2, (FA)2,

CsMA, CsFA, MAFA; X = I, Br, Cl). The variation of the refractive

index is quite similar for the considered compounds. However, an

intense peak is observed in the extinction coefficient spectrum of

(FA)2BiCuI6 double perovskite, this suggests the potential of the

material to be used in solar cell and other optoelectronic devices. ..... 146

Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics xv

List of Tables

Table 3.1. The calculated and the available theoretical and experimental values

of elastic constants Cij (GPa), bulk modulus B (GPa), shear modulus

G (GPa), Young’s modulus Y (GPa), Pugh’s ratio B/G and Poisson

ratio 𝜐 of cubic perovskites CsBX3 (B = Pb, Sn, Ge; X = I, Br, Cl). .... 40

Table 3.2. Summary of the key properties to predict lead free perovskites. .............. 41

Table S3.1. The calculated and the available theoretical and experimental values

of lattice parameter a (in Å) and the calculated unit cell volume V (in Å3) of perovskites CsBX3 (B = Pb, Sn, Ge; X = I, Br, Cl). .............. 48

Table S3.2. The calculated electronic band gap using GGA-PBE along with

available theoretical results and experimental optical bandgap for

perovskites CsBX3 (B = Pb, Sn, Ge; X = I, Br, Cl) in eV. .................... 49

Table S3.3. The calculated elastic constants Cij (in GPa), bulk modulus B (in

GPa), shear modulus G (in GPa), Young’s modulus Y (in GPa),

Pugh’s ratio B/G and Poisson ratio 𝜐 of perovskites solid solutions CsGe(I1-xBrx)3. ....................................................................................... 56

Table 4.1. The calculated elastic constants Cij (GPa), bulk modulus B (GPa),

shear modulus G (GPa), Young’s modulus Y (GPa), Pugh’s ratio

B/G and Poisson ratio 𝜐 of hybrid perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). .................................................................................. 77

Table S4.1. The calculated and the available theoretical and experimental values

of lattice parameter a (in Å) and the calculated unit cell volume V (in Å3) of organic perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). ......................................................................................................... 85

Table S4.2. The calculated electronic band gap, Eg in eV and dielectric constants,

1(0) of organic perovskites MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). ......................................................................................................... 85

Table 5.1. The calculated independent elastic constants C11, C12 and C44 in GPa,

bulk modulus B in GPa, shear modulus G in GPa, Young’s modulus

Y in GPa, Pugh’s ratio B/G and Poisson ratio 𝜐 for the considered FA-based hybrid halide perovskites FABX3 (B = Pb, Sn, Ge; X = I,

Br, Cl). ................................................................................................. 106

Table S5.1. Calculated values of lattice parameter a in Å and unit cell volume V in Å3, along with available theoretical and experimental results and enthalpy of formation H in KJ/mol for FA-based Pb-free non-toxic

hybrid halide perovskites FABX3 (B = Sn, Ge; X = I, Br, Cl) as well

as for its Pb-based counterparts FAPbX3 (X = I, Br, Cl). ................... 113

Table S5.2. Electronic band gap Eg in eV, calculated by GGA-PBE and HSE06

approach as well as the available experimental results, dielectric

constants 1(0), carrier effective mass for electrons 𝑚𝑒 ∗ and holes 𝑚ℎ ∗ in electron mass units for FA-based Pb-free non-toxic hybrid

xvi Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics

halide perovskites FABX3 (B = Sn, Ge; X = I, Br, Cl) as well as for

its Pb-based counterparts FAPbX3 (X = I, Br, Cl). ............................. 113

Table S6.1. The calculated enthalpy of formation, H (KJ/mol), electronic band

gap, Eg (eV) for GGA-PBE and hybrid HSE06 potential, and

dielectric constants, 1(0) of inorganic-organic double perovskites

ABiCuX6: [A = Cs2, (MA)2, (FA)2, CsMA, CsFA, MAFA; X = I,

Br, Cl]. ................................................................................................. 140

Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics xvii

List of Abbreviations

Ab initio From the beginning (Latin Term “ab initio”)

AO Atomic Orbital

BF Bloch Function

BFGS Broyden–Fletcher–Goldfarb–Shanno

CASTEP Cambridge Serial Total Energy Package

CO Crystalline Orbital

DFT Density Functional Theory

DOS Density of States

EHTB Extended Huckel Tight-Binding

EL Electroluminescence

ETB Empirical Tight Binding

FET Field Effect Transistor

GGA Generalized Gradient Approximation

GTF Gaussian Type Function

HF Hartree-Fock

HSE Heyd-Scuseria-Ernzerhof

HTM Hole Transporting Material

IR Infra-Red

LDA Local Density Approximation

LEC Light Emitting Electro-Chemical Cell

LED Light Emitting Diode

MC Millimeter-Scale

NC Nanocrystal

NP Nano Particle

NR Nanorod

NW Nanowire

PAW Projector Augmented Wave

PBE Perdew-Burke-Ernzerhof

PCE Power Conversion Efficiency

PL Photoluminescence

xviii Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics

PP-PW Pseudo-Potential Plane Wave

QSGW Quasiparticle Self-consistent GW

UV Ultra Violet

VASP Vienna Ab initio Simulation Package

VCSEL Vertical-Cavity Surface-Emitting laser

Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics xix

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: _________________________

QUT Verified Signature

xx Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics

Acknowledgements

I would like to express my sincere and deepest gratitude to my principal

supervisor Prof. Kostya (Ken) Ostrikov for giving me the opportunity to conduct this

research as well as his continuous support, excellent motivation and constructive

guideline throughout my PhD. He always provided quick feedback and it helped me a

lot to go smoothly in this research journey to reach my goal.

I would like to convey my gratitude to my associate supervisors Prof. Hongxia

Wang and Dr. Tuquabo Tesfamichael for their support and feedback throughout my

PhD candidature. The critical and constructive feedback from Dr. Tuquabo is

appreciated and it helped me a lot for the improvement of my papers.

Furthermore, I would like to thank my collaborators Prof. Aijun Du, Prof. Jose

Alarco, Prof. Cheng Yan, Dr. Kimal Chandula Wasalathilake and Ms. Chunmei Zhang

for their support and feedback to my papers. Special thanks to Prof. Jose Alarco for

providing me the access of Computational Chemistry Lab and access to Materials

Studio simulations software. Also, I wish to express my gratitude to my collaborator

Prof. Lianzhou Wang from University of Queensland for his comments and feedback

on our paper.

I would also like to acknowledge the Queensland University of Technology

(QUT), Science and Engineering faculty (SEF), School of Chemistry, Physics and

Mechanical Engineering (CPME) for their facility and financial support through

QUTPRA scholarship and HDR Tuition Fee sponsorship. The High Performance

Computing Facility at QUT is sincerely acknowledged. In addition, I would like to

acknowledge the Nanoscience Discipline at QUT for their additional financial support

to attend in conferences.

I wish to express my thanks to my colleagues at QUT, Dr. Mohammad Saidul

Islam, Dr. Fawad Ali, Mr. Jickson Joseph and Dr. Karthika Prasad for their helpful

discussion and suggestion. Special thanks to my colleague, Dr. Kimal for his

cooperation, help and support at QUT. Also, the partial proofreading of my thesis by

Dr. Kimal is greatly appreciated. I also acknowledge my friends, colleagues at QUT

who gave me support and encouragement throughout my PhD candidature.

Ab initio Atomistic Insights into Lead-free Perovskites for Photovoltaics and Optoelectronics xxi

I wish to extend my sincere thanks to my family, friends, colleagues and well-

wishers in Bangladesh and in Brisbane for their inspiration. Special thanks to my

parents for their endless support and prayers, not only in this PhD journey, but also in

my whole life. I also wish to express my thanks to my younger brother for his love and

support.

Last but not least, I wish to express my immense love and gratitude to my wife

for her unconditional support, love and care. I wish to express my heartfelt love and

blessing to my son, his love, thinking and activities always give me pleasure and

refresh my mind and it gave be mental support throughout this PhD journey.

Chapter 1: Introduction 1

Chapter 1: Introduction

1.1 BACKGROUND

Inorganic and organic-inorganic hybrid halide perovskites having the general

formula ABX3 (A = inorganic or organic cation, B = metal cation, and X = halogen

anion) have great potential for commercial applications due to natural abundance and

low cost. [1]. Furthermore, the materials can easily be converted into thin films as well

as some other suitable crystalline forms for device applications such as nanocrystals,

nanorods, nanowires and nanoparticles [1-5]. Therefore, it is expected that the

technology based on perovskites would be more cheaper and efficient than silicon-

based technology [6]. On the other hand, halide perovskites are an emerging class of

materials because of their extraordinary physical properties such as direct band gap

with tunable bandgap characteristics, high absorption coefficient with broad

absorption spectrum, small electron and hole effective masses, high charge carrier

mobility with long charge diffusion lengths, low non-radiative recombination rates,

high photoconductivity, and dominant point defects [2-7]. Because of these

exceptional optoelectronic properties, this group of semiconductors has the potential

to be used in a wide range of device applications including solar cells, light-emitting

diodes (LEDs), lasers, catalysts, x-ray detectors, field-effect transistors (FET),

photodetectors, photoluminescence (PL), electroluminescence (EL), light-emitting

electrochemical cells (LECs) and solar-to-fuel conversion devices [2-17].

Perovskites have been well known for many years [18, 19], but they have

recently become more popular after the discovery of perovskites solar cells by

Miyasaka et al. in 2009 [20]. Following the discovery, the organic-inorganic hybrid

methylammonium lead trihalides CH3NH3PbX3 (X = Cl, Br, I) have been subjected to

a huge number of studies to demonstrate their potential in photovoltaic application

[21-34]. The perovskite solar cell, also known as the third generation solar cell, has

the potential to bring about a major change in the solar cell technology. Recently, the

halide hybrid perovskites have drawn significant attention from the science community

because of their fastest growing power conversion efficiency (PCE) in solar cells, as

the PCE of the perovskite solar cells has jumped from 3.8 % [20] in 2009 to 23.7%

[35] in 2018. The highest efficiency reported for perovskite solar cells so far have been

2 Chapter 1: Introduction

found mainly with methylammonium lead (Pb) halide materials [32-39]. Efficient

photovoltaic cells using inorganic–organic lead halide perovskite materials appear

particularly promising for next generation solar cell devices due to their high power

conversion efficiency and low production cost [28].

On the other hand, the possibility of inorganic perovskites in device applications

cannot be ignored as it has the similar properties like organic perovskites and the

inorganic perovskites are expected to be more stable than organic one. However, it is

true that inorganic perovskites show a less PCE than its organic counterparts [40].

Research on inorganic metal halide perovskites also going on and a lot of works have

been done in recent years [8-11, 40-47] to reveal its properties and potential to be used

in a wide range of optoelectronic devices beyond solar cells.

In general, a good light absorbing material is also a good emitter of light. So, the

science community is searching other optoelectronic applications of organometallic

perovskite materials and they have already found the potentiality of these materials in

some other applications [48-51]. Perovskite materials may be good candidates for use

in light-emitting diodes (LEDs) because of their high photoluminescence quantum

efficiencies [27]. Moreover, it is possible to get colourful light emitting diodes as the

band gap of the perovskite materials can be tuned to cover almost the entire visible

spectrum by changing the composition of the content of halogens [38]. Also, the mixed

halide perovskite materials show surprisingly clean semiconducting behaviour and can

be used as optically pumped vertical-cavity surface-emitting lasers (VCSELs) [50].

Furthermore, a new class of materials known as double perovskites having

general formula A2BʹBʹʹX6 (A is a relatively large cation, Bʹ and Bʹʹ are either trivalent

or monovalent cations, and X is either oxygen or halogen) have similar

crystallographic structure like halide perovskites [52]. Therefore, it is expected that

double perovskites also possess similar optoelectronic properties like halide perovskite

and have the potential to be used in a wide range of optoelectronic applications

including solar cells. In addition, halide double perovskites have become more popular

in the community of photovoltaic research because of its potential to overcome the

instability and toxicity issues associated with Pb-based hybrid perovskites [52]. As a

result, several works have been reported on double perovskites [53-61], however the

findings are still far from expectations [62, 63].

https://en.wikipedia.org/wiki/Light-emitting_diodehttps://en.wikipedia.org/wiki/Quantum_efficiencyhttps://en.wikipedia.org/wiki/Quantum_efficiency


1.2 RESEARCH PROBLEM

Although a substantial amount of research has been done within relatively short

time, the halide perovskites are under development as some difficulties are still

remaining. For the commercial applications of perovskite materials, low cost, high

efficiency and especially long term stability is needed [64]. The studied literature

suggests that halide hybrid perovskite MAPbI3 and other Pb-containing hybrid

perovskites have been studied more compared to others [21-37] as the materials show

high power conversion efficiencies (PCEs). However, the problem with the materials

is that they do not show sufficient stability [65, 66] as well as they contains lead (Pb)

which is toxic [67] and a potential threat to the environment [68]. Also, it has been

recently reported that self-degradation of iodine may limit the lifetime of iodine-

containing perovskites [69]. Therefore, it is essential to obtain Pb-free perovskites,

which could be used for energy applications. Furthermore, Pb-containing organic-

inorganic perovskites MAPbX3 (X = I, Br, Cl) have a low relative dielectric constant.

This is a significant limitation for solar cell application because the low dielectric

constant increases the charge carrier recombination rate and affects the overall

performance of solar cells [70].

Therefore, the materials still face a huge challenge in large scale

commercialization because of the structural instability against moisture/air and

temperature as well as the toxicity of lead (Pb) [64, 71]. It is of utmost importance to

find stable and nontoxic perovskites for the further development of perovskites solar

cells. The insatiability issue of the halide perovskites can be reduced by using carbon

encapsulation, multi-cation substitution as well as incorporation of hydrophobic

moieties [72]. However, the only way to address the toxicity of perovskite materials is

the substitution of Pb by non-toxic elements [71].

1.3 AIMS AND OBJECTIVES

The main aim of the present study is to develop Pb-free non-toxic perovskites

for photovoltaic and optoelectronic applications. To achieve this goal it is required to

replace Pb from perovskites by other suitable elements. There are two possible routes

of substitution of Pb by non-toxic elements such as simple substitution (or, direct

substitution) and complex substitution.


Route 1 (simple substitution or direct substitution): Pb is a group IVA

element in the periodic table. Therefore, the easiest way is to use other group

IVA elements like tin (Sn) and germanium (Ge) for the replacement of Pb.

Substitution of Pb by Sn and Ge can be done for both inorganic and hybrid

perovskites. Pb and its possible replacements are highlighted by red colour

in the periodic table as shown in Figure 1.1. In addition, the halogen atom

can be changed by other suitable halogens for the tuning of the properties of

perovskites. The potential possible replacement of iodine (I) are bromine

(Br) and chlorine (Cl) and are highlighted by yellow colour in Figure 1.1.

Also, the considered A site occupants inorganic caesium (Cs) and Organic

methylammonium (MA) and formamidinium (FA) are indicated by green

colour in Figure 1.1.

Figure 1.1. Periodic table showing the possible replacements of Pb (red colour) and I

(yellow colour) in inorganic CsPbI3 and hybrid MAPbI3 and FAPbI3

perovskites.

Route 2 (complex substitution): A complex substitution of Pb in perovskites

can be addressed by a combination of a trivalent and a monovalent cations

to form a new structure known as double perovskites which can be

represented by the general formula, A2BʹBʹʹX6, where A is a relatively large

cation (typically Cs+), Bʹ and Bʹʹ are either trivalent or monovalent cations,


and X is either oxygen or halogen. For example, the replacement of Pb from

perovskite CsPbI3 by a combination of a trivalent element Bi (bismuth) and

a monovalent element Cu (copper) form a new structure Cs2BiCuI6 and it is

a double perovskite. The possible trivalent elements can be scandium (Sc),

yttrium (Y), antimony (Sb) and bismuth (Bi), the elements are indicated by

blue colour boxes in the periodic table as shown in Figure 1.2. Also, the

possible monovalent elements can be copper (Cu), silver (Ag) and gold (Au)

and are indicated by red colour box in Figure 1.2.

Figure 1.2. Periodic table showing the possible trivalent (blue boxes) and monovalent

(red box) elements to form double perovskite structure because of the

replacement of Pb from perovskite by complex substitution.

To achieve the goal of this study, the specific objectives can be summarised as below:

1. Investigations of the structural, electronic, optical and mechanical properties

of potential inorganic perovskites such as CsBX3 (B = Pb, Sn, Ge; X = I, Br,

Cl) to find best possible Pb-free inorganic perovskites for photovoltaics and

optoelectronics.

2. Studying the structural, electronic, optical and mechanical properties of MA

and FA based organic-inorganic hybrid perovskites ABX3 (A = MA, FA; B

= Pb, Sn, Ge; X = I, Br, Cl) to obtain effective alternatives of Pb in

photovoltaic and optoelectronic applications.


3. Calculations of the essential optoelectronic properties of a range of potential

Pb-free double perovskites to predict their suitability in solar cells and other

optoelectronic devices.

1.4 THESIS OUTLINE

This thesis represents a first-principles density functional theory (DFT)

investigations of the structural, electronic, optical, transport and mechanical properties

of different perovskite materials to find Pb-free materials for photovoltaic and

optoelectronic applications. Herein, the essential optoelectronic properties of

inorganic perovskites (Chapter 3), organic-inorganic hybrid perovskites (Chapter 4

and Chapter 5) and inorganic and hybrid double perovskites (Chapter 6) have been

investigated.

Chapter 1 introduces this research including the background of the research

topic, current problems and challenges as well as the aims and objectives of

this study.

Chapters 2 presents a literature review explaining the background and

current advancement in this field of research. The literature review starts

with the structure, classifications and properties of perovskite materials.

Following this, it presents a wide range of possible potential applications of

perovskites beyond solar cells. Furthermore, the advancement in the

discovery of Pb-free suitable perovskites for photovoltaics and

optoelectronics have been discussed here. In addition, the chapter examines

the up to date literature on the properties and potential applications of a new

group of materials known as double perovskites.

Chapter 3 reports the structural, electronic, optical and mechanical

properties of caesium (Cs) based inorganic perovskites, CsBX3 (B = Pb, Sn,

Ge; X = I, Br, Cl). This section is focused on the investigations of the

fundamental properties of perovskite materials to understand the

characteristics of the materials at atomic level. It also reports the possible

effective alternative of Pb in inorganic perovskites for photovoltaic and

optoelectronic applications.

Chapter 4 presents the studied structural, electronic, optical and mechanical

properties of methylammonium (MA) based organic-inorganic hybrid


perovskites, MABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). This section is focused

on finding of the effective Pb-free alternatives of the most commonly

studied potential perovskites MAPbI3 for solar cell application. It also

describes other possible optoelectronic applications of MA based Pb-free

perovskites.

Chapter 5 demonstrates the structural, electronic, optical, transport and

mechanical properties of formamidinium (FA) based hybrid perovskites,

FABX3 (B = Pb, Sn, Ge; X = I, Br, Cl). This chapter explores possible

alternatives of Pb-based perovskites for photovoltaic and optoelectronic

applications. This section also describes transport properties like electron

and hole effective masses of the considered FA based hybrid perovskites.

Chapter 6 presents the optoelectronic properties of a new group of 18 non-

toxic lead-free organic-inorganic halide double perovskites, ABiCuX6 [A =

Cs2, (MA)2, (FA)2, CsMA, CsFA, MAFA; X = I, Br, Cl] to predict their

suitability in photovoltaic and optoelectronic applications. This section also

describes the possible solution of finding Pb-free non-toxic materials for

photovoltaic and optoelectronic applications. The chapter also represents a

new approach based on the investigations of the optoelectronic properties

for a group of new hypothetical compounds to find suitable materials for

specific applications. Particularly, the incorporation of organic MA or FA

into Cs based inorganic double perovskites is described here and this

approach can provide opportunities to tune and enhance the photovoltaic

and optoelectronic properties of the materials.

Finally, chapter 7 summarizes the major findings of the present

investigations. The contribution towards achieving Pb-free non-toxic

perovskites for photovoltaics and optoelectronics are highlighted.

Considering the findings of the study, the important recommendations for

the further advancement of the field are presented.

1.5 REFERENCES

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with promising optoelectronic applications, Journal of Materials Chemistry C, 4

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[3] Y. Yang, J.J.N.N. You, Make perovskite solar cells stable, Nature, 544 (2017) 155-

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Chapter 2: Literature Review 15

Chapter 2: Literature Review

2.1 CONCEPT OF PEROVSKITE MATERIALS

2.1.1 Perovskite and Perovskite Structure

A perovskite is a mineral that was first found in the Ural Mountains of Russia

by Gustav Rose in 1839 and is named after Russian mineralogist von Perovski who

was the founder of the Russian Geographical Society [1, 2]. On the other hand, a

perovskite structure or a perovskite material is a compound that has the same structure

like the perovskite mineral. The first discovered perovskite mineral is calcium titanium

oxide (CaTiO3) [2, 3]. However, a perovskite material is a material that has the same

crystallographic structure as perovskite mineral and having the form ABX3, where 'A'

and 'B' are two cations of very different sizes, and X is an anion that bonds to both [4].

2.1.2 Crystal Structure of Perovskite Materials

The inorganic and organic halide perovskites have been found in different phases

depending on the temperature [5]. However, the phases are equivalent to each other

according to the orientation of atoms in unit cell. At high temperature, these materials

have a cubic perovskite ABX3 structure with space group 𝑝𝑚3̅𝑚 having space group

number 221 [6]. The unit cell contains one formula unit, where the “A” atoms occupy

the corner positions (0, 0, 0) of the cube, “B” atoms occupy the body centred (½, ½,

½) positions and the “X” atoms occupy the face centred (½, ½, 0) positions as shown

in Figure 2.1.

Figure 2.1. Unit cell of cubic ABX3 perovskite structure.

https://en.wikipedia.org/wiki/Calcium_titanium_oxidehttps://en.wikipedia.org/wiki/Calcium_titanium_oxidehttps://en.wikipedia.org/wiki/Cationhttps://en.wikipedia.org/wiki/Anion

16 Chapter 2: Literature Review

Figure 2.2. The unit cell of CH3NH3PbI3 material [30].

The unit cell of CH3NH3PbI3 is shown in Figure 2.2 as an example of organic

perovskite materials. On the other hand, the ideal cubic-symmetry structure has the

“B” cation in 6-fold coordination, surrounded by an octahedron of anions and the “A”

cation in 12-fold cuboctahedral coordination as shown in Figure 2.3 [4].

Figure 2.3. Structure of a perovskite with a chemical formula ABX3. The red spheres

are X atoms, the blue spheres are B-atoms and the green spheres are the

A-atoms [4].

https://en.wikipedia.org/wiki/Cationhttps://en.wikipedia.org/wiki/Octahedronhttps://en.wikipedia.org/wiki/Anionhttps://en.wikipedia.org/wiki/Cuboctahedron

Chapter 2: Literature Review 17

Figure 2.4. Octahedron in perovskite crystal structure [7].

The slight buckling and distortion can produce several lower-symmetry distorted

versions, in which the coordination numbers of A cations, B cations or both are

reduced because the requirements of relat

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