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Amine-Free Synthesis of Cesium Lead Halide Perovskite Quantum Dots for Efficient Light-

Emitting Diodes

Emre Yassitepe, Zhenyu Yang, Oleksandr Voznyy, Younghoon Kim, Grant Walters, Juan Andres Castaeda, Pongsakorn Kanjanaboos, Mingjian Yuan, Xiwen Gong,

Fengjia Fan, Jun Pan, Sjoerd Hoogland, Riccardo Comin, Osman Bakr, Lazaro A. Padilha, Ana F. Nogueira, and Edward H. Sargent

Version Post-Print/Accepted Manuscript

Citation (published version)

Yassitepe, E., Yang, Z., Voznyy, O., Kim, Y., Walters, G., Castaeda, J. A., Kanjanaboos, P., Yuan, M., Gong, X., Fan, F., Pan, J., Hoogland, S., Comin, R., Bakr, O. M., Padilha, L. A., Nogueira, A. F. and Sargent, E. H. (2016), Amine-Free Synthesis of Cesium Lead Halide Perovskite Quantum Dots for Efficient Light-Emitting Diodes. Adv. Funct. Mater.. doi:10.1002/adfm.201604580

Publishers Statement This is the peer reviewed version of the following article: Yassitepe, E., Yang, Z., Voznyy, O., Kim, Y., Walters, G., Castaeda, J. A., Kanjanaboos, P., Yuan, M., Gong, X., Fan, F., Pan, J., Hoogland, S., Comin, R., Bakr, O. M., Padilha, L. A., Nogueira, A. F. and Sargent, E. H. (2016), Amine-Free Synthesis of Cesium Lead Halide Perovskite Quantum Dots for Efficient Light-Emitting Diodes. Adv. Funct. Mater.. doi:10.1002/adfm.201604580, which has been published in final form at https://dx.doi.org/10.1002/adfm.201604580. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

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https://dx.doi.org/10.1002/adfm.201604580

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DOI: 10.1002/ ((please add manuscript number))

Article type: Full Paper

Amine-free Synthesis of Cesium Lead Halide Perovskite Quantum Dots for Efficient

Light-Emitting Diodes

Emre Yassitepe,, Zhenyu Yang, Oleksandr Voznyy, Younghoon Kim, Grant Walters, Juan

Andres Castaeda, Pongsakorn Kanjanaboos, Mingjian Yuan, Xiwen Gong, Fengjia Fan, Jun

Pan, Sjoerd Hoogland, Riccardo Comin, Osman Bakr, Lazaro A. Padilha, Ana F. Nogueira,

Edward H. Sargent*

Dr. Emre Yassitepe, Dr. Zhenyu Yang, Dr. Oleksandr Voznyy, Dr. Younghoon Kim, Grant

Walters, Dr. Pongsakorn Kanjanaboos, Dr. Mingjian Yuan, Xiwen Gong, Dr. Fengjia Fan, Dr.

Sjoerd Hoogland, Dr. Riccardo Comin, Prof. Dr. Edward H. Sargent

Department of Electrical and Computer Engineering, University of Toronto, 10 Kings

College Road, Toronto, Ontario M5S 3G4, Canada, E-mail: ted.sargent@utoronto.ca

Dr. Emre Yassitepe, Prof. Dr. Ana F. Nogueira

Instituto de Qumica, Universidade Estadual de Campinas, Caixa Postal: 6154, CEP: 13083-

970, Campinas, So Paulo, Brazil

Juan Andres Castaeda, Prof. Dr. Lazaro A. Padilha,

Instituto de Fisica Gleb Wataghin, UNICAMP, P.O. Box 6165, 13083-859 Campinas, Sao

Paulo, Brazil

Dr. Jun Pan, Prof. Dr. Osman Bakr

Solar and Photovoltaic Engineering Research Center (SPERC), King Abdullah University of

Science and Technology (KAUST), 23955-6900 Thuwal, Saudi Arabia

Keywords: Perovskite quantum dots; quantum dot LEDs; quarternary alkylammonium

halides; transient absorption spectroscopy

mailto:ted.sargent@utoronto.ca

2

Abstract

Cesium lead halide perovskite quantum dots (PQDs) have attracted significant interest

for optoelectronic applications in view of their high brightness and narrow emission

linewidth at visible wavelengths. A remaining challenge is the degradation of PQDs during

purification from the synthesis solution. This is attributed to proton transfer between oleic

acid and oleylamine surface capping agents that leads to facile ligand loss. Here we report

a new synthetic method that enhances the colloidal stability of PQDs by capping them

solely using oleic acid (OA). We use quaternary alkylammonium halides as precursors,

eliminating the need for oleylamine. This strategy enhances the colloidal stability of OA

capped PQDs during purification, allowing us to remove excess organic content in thin

films. We fabricate inverted red, green and blue PQD light-emitting diodes (LED) first

time with solution-processed polymer-based hole transport layers due to higher

robustness of OA capped PQDs to solution processing. The blue and green LEDs exhibit

improved performance in EQE being threefold for green and tenfold for the blue LEDs

compared to prior related reports for amine/ammonium capped cross-linked PQDs. We

also report the brightest blue LED based on all inorganic CsPb(Br1-xClx)3 PQDs.

1. Introduction

Solution-processed hybrid and inorganic lead halide perovskites are of interest for

optoelectronic applications because of their excellent photophysical and transport properties.

[13] Methylammonium and formamidinium lead halide (MAPbX3, FAPbX3, X = Cl, Br, I)

perovskites are top contenders in solution-processed single junction solar cells, having recently

passed the 22% certified power conversion efficiency milestone.[1,3] Organic Lead Halide

3

perovskites are also explored in photodetectors,[46] lasing applications,[7,8] and light-emitting

diodes (LEDs).[911]

Rapid advances in bulk perovskite materials have in turn brought significant attention

to quantum dots based on perovskites. Colloidal organic metal halide perovskite nanomaterials

were reported based on MAPbBr3 nanocrystals obtained via a recrystallization process.[12] A

hot injection synthesis method was then developed to generate fully inorganic CsPbX3

perovskite quantum dots (PQDs)[13] that showed narrow photoluminescence (PL) spectra and

high photoluminescence quantum yields (PLQY) that reached 90%. Furthermore, MAPbX3

PQDs were reported with PLQY of 70% and a wide color gamut.[14] CsPbX3 PQDs have

particularly wide-ranging quantum-size-effect tunability in light of their large exciton Bohr

radius and halide compositional tuning.

To date, colloidal perovskite nanocrystal syntheses have adopted lead halides as the

source for lead and halogens, a strategy similar to that used to prepare bulk perovskites

materials. This requires dissolution of PbX2 salts in a polar medium such as dimethylformamide

(DMF), and it also demands the use of a mixture of oleylamine and OA as surfactants.[12,1416]

A non-polar solvent method was reported for CsPbX3 PQDs in which PbX2 salts were

solubilized in oleylamine/OAcapping agents in a non-coordinating solvent, 1-octadecene

(ODE), followed by injection of cesium oleate at elevated temperatures (140 - 200C).[13]

However, these PQDs exhibited poor colloidal stability and readily precipitated from the crude

solution.[13,18,19] Recently, Hens, Kovalenko and co-workers explained the reasons for the

limited stability of PQDs, attributing it to facile proton exchange between the oleate and amine

surfactants. This leads to a dynamic equilibrium between bound and unbound ligands, enabling

facile ligand loss.[20] In addition, oleylamine itself can also accelerate the degradation of the

nanocrystals by coordinating the lead species (Pb-oleate, PbX2) and ultimately dissolving the

lead oleate Z-type ligands from the surface.[21]

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For these reasons, new synthetic strategy is urgently needed to overcome the reliance

on oleylamine and the requirement of multiple competitive ligands.

Moving beyond the use of oleylamine is challenging because this ligand is presently

relied upon to solubilize the lead halide precursor. We thus turned instead to Cs-oleate (CsOA)

and Pb-oleate (PbOA2) and searched for a new halide source. We were inspired by ligand

exchanges in chalcogenide quantum dots. Tetrabutylammonium iodide (TBAI) is used to

provide stable iodide passivation of metal rich QD surfaces through the cleavage of lead-

carboxylate bonds in proton-donating solvents (e.g., methanol).[2225] This exchange mechanism

provides predictive pathway for the reaction between CsOA and PbOA2 with quaternary

alkylammonium halide salts (NR4X, R = alkyl group).

Recently, Wei et al. reported a room temperature synthetic method based on carboxylic

acid capped CsPbBr3 PQDs and further applied post-synthetic anion exchange techniques to

form CsPbX3 PQDs (X= Cl, Br, I)[26]. However, PQDs show degraded PLQY properties within

2-3 days and, surprisingly, suffered from lower PLQY for above room temperature reactions.

Here we carry out an extensive study to employ OA capped supported by deep experimental

studies and theoretical investigations. We demonstrate an amine-free synthesis that utilizes

tetraoctylammonium halides (TOAX) for preparation of OA capped CsPbX3 PQDs without the

need of post-anion exchange methods. The PQDs show PLQY of 70%, narrow emission spectra,

and enhanced colloidal stability. We show that these PQDs are more stable upon purification

with polar solvents, an advance that we attribute to stronger ligand attachment. We further

fabricate red, green, and blue (RGB) PQD LED using first time solution-processed polymer-

based hole transport layers in an inverted LED structure for PQDs. Green and blue LEDs show

a threefold increase in performance for green, and tenfold for blue LED efficiencies compared

to prior related reports for amine/ammonium capped cross-linked PQDs.[27] We further explore,

using ultrafast spectroscopy, the effects of photocharging and fast Auger recombination in

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PQDs. We identify these as the key factors that need to be addressed to achieve further

improvements in LED performance.

2. Results and Discussion

To understand the reaction mechanism, its limiting steps and possible ways to control

growth, we carried out density functional theory (DFT) molecular dynamics simulations of the

nucleation and crystal growth kinetics starting from the chosen precursors: CsOA, Pb(OA)2,

TOABr (for simplicity, the acetate molecule is shown in the figure instead of oleic acid). In the

first step of the reaction, we find that Cs and Pb oleates react readily with Br anions, since Cs

and Pb cations are large and prefer higher coordination than OA can provide (Figure 1, step

a). However, the resulting (PbOA2Br) clusters do not react further as they become electronically

saturated, even though the size of the cation could allow for higher coordination. In contrast,

both PbBr2 and (PbOABr2) are more reactive and form polymeric chains thanks to the ability

of Br- to coordinate to more than one lead ion (Figure 1, step b). The exchange of the oleate

ion for a bromide anion therefore becomes the limiting step for further growth of the nuclei. It

is known that the detachment of the oleate anion from the surface of the lead cation is a highly

unfavorable unless a proton participates and assists the detachment.[12] Similarly, detachment

of oleate ligands continues to be the limiting step at every stage of the nanocrystal growth

(Figure 1, step c). Instead of a proton, we use TOA cation to facilitate the desorption of OA

and to control the nanocrystal growth. The use of TOA eliminates formation of reversible

amine-ammonium species due to absence of proton. However, when alkylammonium halides

are used there can be reversible exchange between alkylammonium to alkyamine. Therefore,

TOA eliminates species for amine formation.

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In a typical reaction, Cs-acetate, Pb-acetate, ODE, oleic acid are placed in the flask and

heated to 120C until CsOA and PbOA2 are formed under vacuum. Then flask is placed under

nitrogen and temperature is lowered to 75C to inject TOAX source. Injection of TOABr at

75C into a CsOA and PbOA2 containing mixture resulted in an immediate change in the color

of the solution from clear transparent to green, indicating the formation of CsPbBr3 PQDs.

Moreover, the solution remained clear longer than conventionally prepared PQDs, indicating

that the oleic acid capped dots show enhanced colloidal stability (Figure S1). This was also

confirmed during purification of the PQDs by attempting to crash the crude solution without

any anti-solvents. To our surprise, oleic acid capped dots do not precipitate without the

introduction of an anti-solvent, whereas conventionally prepared PQDs precipitate

spontaneously from solution.[13,18,19] CsPbBr3 and CsPb(Br,Cl)3 PQDs showed high stability;

however, CsPb(Br,I)3 PQDs do become unstable over time. This may be attributed to their

sensitivity to moisture. The synthesis of blue and red photoluminescent PQDs is achieved by

the incorporation of mixed halide anions and carried out in a similar fashion to CsPbBr3

PQDs(e.g., Br/Cl and Br/I) (Figure 2a).

The absence of oleylamine also accelerates the growth reaction of PQDs. For example,

the growth of average CsPbBr3 (~8 nm) PQDs requires ~150C reaction temperature, whereas

in the absence of oleylamine, similar-sized PQDs were formed at 75C. We attributed the lower

reaction temperature to the absence of oleylamine, already known by its strong coordination

that reduces the reactivity of PbX2. This inhibiting effect had also been reported for syntheses

of InP quantum dots when oleylamine was utilized.[28] We further studied the effect of

oleylamine on as-prepared CsPbBr3 PQDs by observing them with PL spectroscopy (Figure

S2). We find that addition of increasing quantities of oleylamine causes a blue shift, indicating

surface etching or oxidation over time.

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Fourier transform infrared (FTIR) spectra of as-synthesized PQDs show the presence of

OA (COOH) and oleate species (COO-) (Figure 2b). COO- stretching confirmed the binding

of Pb oleate species on PQD surfaces at 1525 and 1553 cm-1 (Figure S3).[29] These signals

confirm the bond chelating and bridging from PbOA2 on the surfaces of oleic acid capped PbS

nanoparticles.[29] As a result of the subsequent washing step, PQDs maintained surface-bound

COO- groups, and their solutions possessed a lower quantity of free OA. This allowed us to

remove most excess ligands without affecting the colloidal stability. The PQD PL peak

positions are not shifted (Figure 2c). In conventional oleylamine/oleic acid capped PQDs, a

similar degree of purification inevitably led to a red shift of PL, indicating growth or fusion of

those PQDs.[19] X-ray diffraction (XRD) measurements confirm the cubic perovskite crystal

structure of RGB PQDs with peak shifts consistent with the formation of alloys (Figure 2d).

We utilized X-ray photoelectron spectroscopy (XPS) to monitor the presence of amine

and ammonium ligands on the surface of PQDs after one- and two-time washing. Both amine

and ammonium species are present on the surface of conventional PQDs even after two washing

steps. This resonates with the conclusions of Hens, Kovalenko and co-workers regarding ready

and reversible proton transfer between OA and amine which accelerates the desorption of

protonated OA or amine-coordinated PbOA2 Z-type ligands and eventually causes the

degradation of PQDs.[20,21,31] High resolution XPS N1s spectra of our solely-oleic-acid capped

CsPbBr3 PQDs do not show any detectable nitrogen after one and two washing steps (Figure

S4), confirming the absence of amine and ammonium species.

High-resolution transmission electron microscopy (HR-TEM) imaging of CsPbBr3

quantum dots confirms the CsPbBr3 perovskite structure (Figure 3a). The lattice spacing (5.8

) corresponds to the {110} lattice planes. HR-TEM analysis show that average particle size is

between 7-10 nm, confirming the quantum confinement effect (Figure S5). Interestingly, the

8

reaction carried out at 75C does not form platelets, a finding that had previously been reported

for oleylamine-oleic acid based PQDs with reaction temperatures 90 - 130C.[32]

Ligand detachment during washing steps enabled PQDs to attach firmly onto

hydrophilic substrates. An inverted LED device architecture was chosen in which the perovskite

quantum dot layer was sandwiched by a TiO2 electron transport layer and by a poly(9,9-

dioctylfluorence) (F8) hole transport layer. The band diagram of the devices, shown in Figure

3b, indicates the well-aligned band levels between PQDs and CTLs. Cross-section scanning

electron microscope (SEM) imaging revealed that the overall thickness of the device is less than

200 nm (TiO2: 10-15 nm, active perovskite layer: ~10 nm, F8: 15 nm, MoO3/Au anode: ~100

nm) (Figure 3c). Atomic force microscopy (AFM) confirmed the low surface roughness (Rq)

of the active layer (on top of TiO2/ITO substrate) was around 2 nm (Figure 3d).

RGB electroluminescence (EL) was achieved by fabricating devices with the

corresponding types of perovskite quantum dots (red: Br/I mixed; green: pure Br; blue: Br/Cl

mixed). Compared to the PL spectra (Figure 4a), no significant EL wavelength shift was

observed in RGB devices (EL peak at ~650 nm, 510 nm, 495 nm respectively). Blue and green

EL signals are symmetric without noticeable peak broadening (FWHM of RGB devices are 35,

25 and 21 nm, respectively), indicating that the luminescence qualities of the active perovskite

quantum dots were well-preserved during solution-processed device fabrication.

Device turn-on voltages were between 2.6 and 4 V depending on the PQD type (Figure

4d). The corresponding peak EQE values are 0.05%, 0.325% and 0.075% for RGB devices,

respectively (Figure 4c). Green and blue LEDs show a threefold increase in performance for

green, and tenfold for blue LED efficiencies compared to prior related reports for

amine/ammonium capped cross-linked PQDs. The relatively low EQE value of the blue-

emitting device arises from inefficient charge blocking by the F8 layer. The conduction band

9

of perovskite was moved closer to the LUMO of F8, which is also reflected in the device current

density-voltage (J-V) response (Figure 4b). We summarize PQD-LED performance published

to date and device architectures in Table S1. All published PQD-LEDs utilize evaporated

electron or hole transport layers. Here, we report a new device architecture that utilizes a

solution-processed polymer-based hole transport layer due to robustness of OA capped PQDs

to solution processing. CsPbX3 PQDs provide LED device performance comparable with

previous reports (Table S1). The range of the luminance was 30 - 930 Cd/m2, comparable to

previous PQD LEDs (Figure 4d).[27] It was, however, achieved at notably lower voltage (R: 2.6

V, G: 2.8 V, and B: 4.1 V). While almost the same brightness was found from the blue and

green devices, we noticed the red-emitting devices were dimmer: their lower performance may

be attributed to the higher air and moisture sensitivity of CsPb(Br3-xIx) PQDs.

A possible reason for the limited EQE in PQD-based LEDs is the strong influence of

Auger-assisted nonradiative decay, which could be a result of charged PQDs due to

uncontrolled charge injection, and consequent trion decay.[33] To verify the strength of Auger

recombination in these materials we performed a series of transient absorption (TA) studies in

solution and films. Figure 5 shows transient absorption (TA) results for the green-emitting

PQDs with biexction lifetime of 38 5 ps. To verify that the fast decay is indeed due to

biexciton formation, we characterized its amplitude as a function of the pump intensity (see

Figure S6), finding that it follows Poisson statistics expected for biexctions. A slower decay of

about 180 ps also emerges as the pump intensity increases, indicating the presence of ionized

PQDs and trion decay.[34] These values agree with the values recently reported for similar

CsPbBr3 PQDs.[35,36] For the sample investigated in this work, photo-ionization, and consequent

trion formation, accounts for less than 10% of the measured signal. However, when comparing

the TA results for PQDs in solution and in a spin-cast films we observed a stronger influence

of this charged species in films. We expect photo-charging to be a slow and cumulative

10

process.[37] This is suggestive that charging could be one factor that limits the devices' EQE.

Considering the PL and trion lifetimes, we can estimate that, if the device is dominated by

charged species, the maximum EQE would be near 3%. This suggests that it is imperative to

develop engineered PQDs heterostructures that can reduce trion formation.

3. Conclusion

We developed an amine-free synthesis for PQDs stabilized solely by oleic acid, enabled

by the choice of a novel halide precursor. Oleic acid capped PQDs showed improved colloidal

stability and provided high stability against polar solvents during purification. This allows

solution-processed conductive PQD films with low organic content and increased stability. We

report a new LED device architecture by utilizing solution-processed polymer based hole

transport layer with highly comparably RGB LED performances. Further optimization of

synthesis and device fabrication of perovskite quantum dots is the subject of ongoing

investigation, in particular, the core-shell structures to reduce the Auger recombination losses.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the authors.

Acknowledgements

E.Y., Z.Y. and O.V contributed equally to this work. This publication is based in part

on work supported by Award KUS-11-009-21, made by King Abdullah University of Science

and Technology (KAUST), by the Ontario Research Fund - Research Excellence Program,

and by the Natural Sciences and Engineering Research Council of Canada (NSERC). E.Y.

acknowledges support from FAPESP-BEPE (2014/18327-9) and FAPESP (2013/05798-0)

fellowship. J.A.C. and L.A.P. acknowledge the financial support from FAPESP (Project

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number 2013/16911-2). A.F.N acknowledge FAPESP, CNPq and INEO. The authors thank

Dr. Y. Zhao for useful discussions. The authors thank L. Levina, E. Palmiano, R. Wolowiec,

and D. Kopilovic for their technical help over the course of this study.

Received: ((will be filled in by the editorial staff))

Revised: ((will be filled in by the editorial staff))

Published online: ((will be filled in by the editorial staff))

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13

Figure 1. Density functional theory molecular dynamics simulation of CsPbBr3 nucleation and

growth steps (for simplicity, the synthesis is shown with acetate group instead of oleate). a)

Dissociated ammonium bromide ions form complexes with CsOA and PbOA2. b) Further

reaction between these complexes requires one of the OA- being replaced with Br-. c) Br

enriched Cs and Pb complexes polymerize through multi-coordinated Br. d) CsPbBr3

nanocrystal saturated with OA ligands.

14

Figure 2. Optical and structural characterization of CsPbX3 (X = Cl, Br, I) PQDs. a) Absorption

and PL spectra of RGB CsPbX3 PQDs color coded with the corresponding color b) FTIR spectra

of unwashed, once washed and twice washed CsPbBr3 PQDs. c) PL spectra of unwashed and

twice washed CsPbBr3 PQDs showing identical PL spectrum. d) X-ray powder diffraction of

CsPb(Br3-xClx), CsPbBr3 and CsPb(Br3-xIx) PQDs confirming cubic perovskite structure

15

Figure 3. a) High resolution spectrum of CsPbBr3 PQDs, inset showing d-spacing calculation

for single crystal. b) Device band diagram, c) Cross sectional SEM image of PQD-LED devices

and d) AFM image of CsPbBr3 PQD emission layer on top of TiO2.

16

Figure 4. Device characterization of RGB PQD-LEDs. a) RGB photoluminescence (dashed

lines) and electroluminescence spectra (straight lines). b) Current density vs. Voltage (J-V)

curve of PQD LED devices. c) Radiance vs. voltage Diagram for RGB PQD-LED. d) EQE vs.

current density plots for RGB PQD-LED devices.

17

Figure 5 a) Transient absorption spectra for green emitting PQDs in toluene solution for several

pump intensities (inset: shows the multiexciton dynamics at high excitation energy ( =

2.2)). b) Comparison between the transient absorption spectra of PQDs in solution and film.

18

The table of contents entry

We report a new synthetic method that enhances the colloidal stability of perovskite quantum

dots by using solely oleic acid ligands. We use quaternary alkylammonium halides as

precursors, eliminating the need for oleylamine.We fabricate inverted red, green and blue

LEDs using solution-processed polymer-based hole transport layers for the first time, allowed

for by the higher robustness of oleic acid capped perovskite QDs against solution processing.

The LEDs exhibit a threefold and tenfold improvement in EQE for green and blue,

respectively, compared to prior reports for amine/ammonium capped perovskite QDs..

Keywords Perovskite; quantum dots; light-emitting diodes; quaternary alkylammonium

halides; transient absorption spectroscopy

Emre Yassitepe,, Zhenyu Yang, Oleksandr Voznyy, Younghoon Kim, Grant Walters, Juan

Andres Castaeda, Pongsakorn Kanjanaboos, Mingjian Yuan, Xiwen Gong, Fengjia Fan, Jun

Pan, Sjoerd Hoogland, Riccardo Comin, Osman Bakr, Lazaro A. Padilha, Ana F. Nogueira,

Edward H. Sargent*

Amine-free Synthesis of Cesium Lead Halide Perovskite Quantum Dots for Efficient

Light-Emitting Diodes

ToC figure

19

Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2013.

Supporting Information

Emre Yassitepe,, Zhenyu Yang, Oleksandr Voznyy, Younghoon Kim, Grant Walters, Juan

Andres Castaeda, Pongsakorn Kanjanaboos, Mingjian Yuan, Xiwen Gong, Fengjia Fan, Jun

Pan, Sjoerd Hoogland, Riccardo Comin, Osman Bakr, Lazaro A. Padilha, Ana F. Nogueira,

Edward H. Sargent*

Chemicals

All chemicals are obtained from Sigma-Aldrich unless noted otherwise. Materials: oleic acid

(OA, tech. 90%), 1-octadecene (ODE), cesium acetate, lead acetate, tetraoctylammonium

bromide (TOABr), tetraoctylammonium chloride (TOACl), octylamine, hydrogen iodide,

toluene, ethanol anhydrous, octane, acetone, distilled in glass (Caledon).

Tetraoctylammonium iodide (TOAI) was purchased from TCI chemicals.

Synthesis of CsPbX3 PQDs

1 mmol of Cs acetate, 2 mmol of Pb acetate is placed in a flask with 2 ml oleic acid and 5 ml

1-ODE mixture. The flask is kept under vacuum and heated to 100C to form CsOA and PbOA.

For CsPbBr3 PQD synthesis, 2 mmol TOABr source is dissolved in anhydrous toluene and

swiftly injected into the reaction flask at 75C. After 5 seconds, the heating source is removed

and allowed to cool by itself. For CsPb(Br,I)3 , 1mmol (TOABr:TOAl, 2:1 in toluene) and

CsPb(Br,Cl)3 (TOABr:TOACl, 2:1 in Toluene) was injected at 75C. However for CsPb(Br,I)3 synthesis the reaction quenched with ice-water bath and 1-butanol is added to prevent rapid

decomposition of PQDs. CsPbBr3 and CsPb(Br,Cl)3 PQDs are purified by hexane (15 ml) and

acetone (30 ml) with centrifuge. An additional wash can be performed using the similar

procedure. For CsPb(Br,I)3 dots the purification is carried out by only 1-butanol.

Electron microscope measurement

High resolution TEM, SEM and STEM images were obtained by Hitachi S-5200 microscope

equipped with cold field emission gun at an accelerating voltage of 300 kV.

XRD and XPS

Powder XRD patterns were collected using a Rigaku MiniFlex 600 diffractometer equipped

with a NaI scintillation counter and using monochromatized Copper K radiation (=1.5406

). XPS analysis was carried out using the Thermo Scientific K-Alpha XPS system.

Fabrication of light-emitting diodes

Pre-patterned ITO-coated glass substrates were cleaned using isopropyl alcohol, dried by N2

flow, and treated by oxygen plasma for 10 minutes immediately prior to use. The TiO2 layer

20

is fabricated by a two-step method: (i) the initial layer was deposited at 150oC using atomic

layer deposition (ALD) (Cambridge Savannah S100 ALD system) using tetrakis

(dimethylamido) titanium (IV) and water as precursors; (ii) the film was further treated by

TiCl4 aqueous solutions (40 mM) at 70oC for 30 minutes, followed by washing with DI water,

drying by N2 flow, and finally annealed at 400oC for 1 hour. After cooling down, substrates

are stored in a nitrogen-filled glovebox for further use. Perovskite quantum dot ink in octane

was spin-coated onto the TiO2/ITO substrate at room temperature at 2000 rpm for 30 seconds.

The F8 chlorobenzene solution (10 mg/mL) is annealed at 75oC under nitrogen atmosphere

and spin-coated onto the perovskite quantum dot layer at a spin speed of 3000 rpm for 60

seconds to form a uniform hole transport layer. The top electrode, consisting of 6 nm of MoO3

and 100 nm of Au, is deposited by thermal evaporation at a pressure of

21

Transient absorption was performed in PQDs solution in 1 mm spectroscopy grade cuvette, or

in films prepared by spin coating. Excitation was chosen at 3.1 eV with 100 fs pulses at 1 kHz

repetition rate. Probe was selected from an optical parametric amplifier, and it was set near the

position of the first absorption peak. The absorption cross section was measured at 3.1 eV by

using a Poisson statistic to fit the saturation of the amplitude at long delay (~200 ps) as shown

in Fig. S4a. For that, we assume that every PQD that has absorbed at least one photon is left

with only one exciton at 200 ps, this way, the amplitude is given by = (1 ),

where Bmax is the maximum amplitude measured and depends on the experimental conditions.

is the average number of photons absorbed per PQD and is given by = 3.13.1,

3.1 is the number of photons at 3.1 eV in the laser pulse per cm2 (experimentally measured

based on the laser power and spot size) and 3.1 is the PQD absorption cross section at 3.1

eV. Using the same cross-section obtained in Fig. S4a, we could fit the amplitude of the fast

component using the Poisson statistic for biexcitons = (1 (1 + )). This

verifies that the 38 ps decay is due to biexciton. We also observe the presence of trions, but

their contribution in solution is small (less than about 10%), but the PQDs becomes more prone

to photo-ionization when deposited in film.

Figure S1 Colloidal stability comparision of conventional oleylamine-oleic acid capped

PQDs (a) vs solely oleic acid capped PQDs (b).

b) a)

22

Figure S2- The effect of 0.2 ml oleylamine addition to as prepared oleylamine- oleic acid

CsPbBr3 quantum dots over 1 hr.

475 500 525 550

P

L In

ten

sit

y (

a.u

)

Wavelength (nm)

Increasing time

Figure S3 FTIR spectra (1400-1700 cm-1) of unwashed, once washed and twice washed

CsPbBr3 PQDs

23

Figure S4. High resolution N 1s and Pb 4f spectrum of a) Conventionally synthesized PQDs

after 1 and 2 washing steps b) PQDs synthesized via oleic acid and alkylammonium halides

after 1 and 2 washing steps

24

Figure S5 Particle size distribution of CsPbBr3 PQDs calculated from HR-TEM image. The

average particle size is found to be 7-10 nms.

Figure S6. Long delay amplitude (a) and the amplitude of the fast decay component (b). Data

was fitted using Poisson statistic for single exciton (a) and biexciton (b) obtaining the cross

section at 3.1 eV of 1.4 x 10-14 cm2.

25

Table S1 MAPbX3, CsPbX3 PQD-LED performance chart with device configurations

Reference

Wavelength (nm)

Turn-on

Voltage (V)

Max. Lumina

nace (Cd m-2)

EQE (%)

Luminous Power

Efficiency (lm W-1)

Luminous

Efficiency

(Cd A-1)

Notes

J. Song, et al., Adv.

Mater.,

2015[2]

455, 516, 586

5.1, 4.2, 4,6

946 0.07, 0.12, 0.09

0.07, 0.18, 586

0.14, 0.43, 0.08

CsPbBr3 QD as active material, Architecture:

ITO/PEDOT:PSS/Perovskite/TPBI/LiF/Al

Y. Ling, et al., Adv.

Mater.,

2016[3]

530 3.8 10590 0.48 1.0 \

MAPbBr3 nanoplatelet as active material, Architeture:

ITO/PEDOT:PSS/Perovskite/PVK:PBD/BCP/LiF/Al

W. Deng, et al., Adv.

Funct. Mater.

, 2016[4]

445, 525, 640

7.8 (only green was

reported)

2673, 2398, 986

1.38, 1.06, 0.53

3.05 (only gree

n was

reported)

3.03 (only green was

reported)

MAPbBr3 QD as active material, Architecture:

ITO/PEDOT:PSS/Perovskite/TPBi/LiF/Ai

X. Zhang et al., Nano Lett.,

2016[5]

516 3.5 1377 0.06 \ 0.19

CsPbBr3 QD as active material, Architecture:

ITO/PEDOT:PSS/poly-TPD+PFI/Perovskite/

TPBi/LiF/Al

G. Li, et al., Adv.

Mater.,

2016[6]

480, 523, 698

2.8, 2.0, 1.4

8.7, 2335, 206

0.007,

0.19, 5.7

\ \

CsPbX3 QD as active material, Architecture:

ITO/ZnO/Perovskite/TPB/MoO3/Ag

J. Pan, et al., Adv.

Mater.,

2016[7]

490, 515

3.0, 3.0

35, 330 1.9, 3.0

\ \

CsPbX3 QD as active material, Architecture:

ITO/PEDOT:PSS/Perovskite/TPBi/LiF/Ai

This Work

495, 510, 650

4.1, 2.8, 2.6

750, 934, 28

0.05, 0.32, 0.075

\ \ CsPbX3 QD as active material,

Architecture: ITO/TiO2/Perovskite/F8/MoO3/Au

26

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