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THE STUDY OF INORGANIC

PEROVSKITE SOLAR CELLS

TANG KAI CHI

MPhil

The Hong Kong Polytechnic University

2019

The Hong Kong Polytechnic University

Department of Applied Physics

The Study of Inorganic Perovskite Solar Cells

TANG Kai Chi

A thesis submitted in partial fulfilment of the

requirements for the degree of Master of Philosophy

July 2018

CERTIFICATE OF ORIGINALITY

I hereby declare that this thesis is my own work and that, to the best of my knowledge

and belief, it reproduces no material previously published or written, nor material that

has been accepted for the award of any other degree or diploma, except where due

acknowledgement has been made in the text.

(Signed)

TANG Kai Chi (Name of student)

THE HONG KONG POLYTECHNIC UNIVERSITY Abstract

TANG Kai Chi I

Abstract

Energy harvesting is a highly concerned issue in our energy-demanded

community. Solar energy is one of the energy sources to provide sustainable energy

and a perovskite solar cell (PSC) is a new kind of solar cells emerging in 2009 with a

power conversion efficiency (PCE) of 3.8%. Due to the intrinsic potential of PSCs and

a great amount of effort by researchers, the PCE of PSCs has been greatly enhanced

to 22.7% in 2017 and is already higher than that of some commercial silicon solar

cells. However, the highly efficient hybrid organic-inorganic PSCs suffered from

instability due to the organic component. Therefore, developing inorganic perovskite

solar cells, for example, caesium lead halide, is one of the most promising strategies

to solve the problem.

In this thesis, inorganic caesium lead halide PSCs were studied and our works

were mainly divided into two parts. The first one was to reduce the high annealing

temperature of caesium lead bromide (CsPbBr3) from 250 °C to 160 °C by a pyridine

treatment. A high-temperature annealing process increased the fabrication cost,

reduced the material choices for different parts of the devices and limited the

possibility of flexible devices. The pyridine-treated PSCs demonstrated the highest

PCE of 6.04%, which was comparable to that of the control devices prepared at a high

temperature. The mechanism was that pyridine vapour reacted with a PbBr2 film to

form an intermediate phase PbBr2.(Py)x which reduced the thermal activation energy

for the formation and crystallization of CsPbBr3 perovskite. The pyridine-treated

devices without encapsulation can also exhibit high stability in the ambient air with

relative humidity up to 70%. This work provides a low-temperature technique for the

fabrication of other inorganic PSCs.

THE HONG KONG POLYTECHNIC UNIVERSITY Abstract

TANG Kai Chi II

Another work was to apply a copper(I) thiocyanate (CuSCN) as a hole transport

layer in the CsPbI2Br PSCs to improve the efficiency and stability of devices. The top

10% of photon flux of the sunlight AM 1.5 is on the range of 550 nm to 810 nm.

CsPbI2Br perovskite has a lower bandgap (1.92 eV) than CsPbBr3, so higher short-

circuit current density (Jsc) can be generated. By replacing the hole transport layer of

spiro-OMeTAD with CuSCN, the PCE of the devices were improved from 7.31% to

10.36% and Jsc was enhanced from 13.1 mA/cm2 to 14.1 mA/cm2. The encapsulated

devices with CuSCN can sustain 93% of its initial efficiency after one month. For film

degradation, we have also identified that the degradation started from the grain

boundaries of the films. These findings pave a way for realizing PSCs with better

stability and higher efficiency.

THE HONG KONG POLYTECHNIC UNIVERSITY Publications

III

Publications

Tang, K. C., You, P. and Yan, F. (2018), Highly Stable All‐Inorganic Perovskite Solar

Cells Processed at Low Temperature. Sol. RRL, 2: 1800075. doi:10.1002/solr.201800075

https://doi.org/10.1002/solr.201800075

THE HONG KONG POLYTECHNIC UNIVERSITY Acknowledgements

IV

Acknowledgements

I would like to express my gratitude to many people for their support during my

MPhil study period at the Hong Kong Polytechnic University.

I would like to thank my supervisor, Prof. Yan Feng who gave me an opportunity

to study at PolyU and provide me with an interesting research topic. He taught me how

to conduct research, write research papers and corporate with the colleagues. He

enlightened me with the knowledge of research, provided helpful suggestions on my

research progress and reminded me to conduct every experimental step very carefully.

He revised my paper patiently and taught me to present my data logically.

I would like to express my appreciation to my group mates who supported me to

conduct my research in different aspects, especially discussion on the detail in

experiments. I would appreciate Dr. Tai Qidong, Dr. You Peng, Dr. Liu Shenghua, Dr.

Wang Meng, Dr. Wu Runsheng, Dr. Xie Chao, Dr. Fu Ying, Dr. Fan Xi, Dr. Ling Hai

Feng, Mr. Tang Guanqi, Mr. CAO Jiupeng, Ms. Wang Tianyue, Mr. Wang Naixiang,

Mr. Yang Anneng, Mr. Li Yuanzhe, and Mr. Liu Chun Ki.

Additionally, I would like to thank my colleagues and friends Mr. Lo Tsz Wing,

Mr. Yau Wang Tat, Mr. Chan Hung Lit, Mr. Ho Kwun Hei, Mr. Yeung Pui Hong, Mr.

Chan Ka Ho, Mr. Chan Cheuk Ho, Mr. Lai Wai Kin, Mr. Lau Ka Seng, Mr. Leung

Man Ho, Mr. Ko Tsz Wai, Mr. Yu Kun, Mr. Mak Chun Hin, and Mr. Qarony Md

Wayesh for their company and assistance.

I would like to acknowledge the financial support from the Research Grants

Council of the Hong Kong Special Administrative Region.

Finally, I would like to thank my parents and my sister for their encouragement

THE HONG KONG POLYTECHNIC UNIVERSITY Acknowledgements

V

and unconditional support.

THE HONG KONG POLYTECHNIC UNIVERSITY Table of Content

VI

Table of Content

Abstract ................................................................................................................ I

Publications .............................................................................................................. III

Acknowledgements .................................................................................................... IV

Table of Content ......................................................................................................... VI

List of Figures ............................................................................................................ IX

List of Tables ........................................................................................................ XVIII

List of Abbreviations ............................................................................................... XIX

Introduction ............................................................................................. 1

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

1.2 Objectives of Research ............................................................................ 3

1.3 Outline of Thesis ..................................................................................... 4

Review of Inorganic Perovskite Solar Cells ........................................... 6

2.1 Introduction ............................................................................................. 6

2.2 Background of Photovoltaic Effect ......................................................... 9

2.2.1 A Brief Working Principle of Photovoltaic Effect ........................... 9

2.2.2 Characterization of Photovoltaic Cells ............................................ 9

2.3 Caesium-based PSCs with Different Halide Composition .................... 11

2.3.1 CsPbBr3 PSCs ................................................................................ 11


VII

2.3.2 CsPbI3 PSCs ................................................................................... 20

2.3.3 CsPbIBr2 PSCs ............................................................................... 31

2.3.4 CsPbI2Br PSCs ............................................................................... 36

2.4 Outlooks ................................................................................................ 49

2.5 Conclusion ............................................................................................ 50

Low-Temperature Processing of All-Inorganic Perovskite Solar Cells

with High Stability ..................................................................................................... 52

3.1 Introduction ........................................................................................... 52

3.2 Experimental Section ............................................................................ 53

3.3 Results and Discussion .......................................................................... 56

3.3.1 Optimization of the Normal CsPbBr3 PSCs ................................... 56

3.3.2 Effect of the Pyridine Treatment on CsPbBr3 PSCs ....................... 58

3.4 Conclusion ............................................................................................ 66

Enhancement on the Short-Circuit Current Density of Cesium Lead

Halide Inorganic Perovskite Solar Cells by Increasing the Content of Iodide........... 68

4.1 Introduction ........................................................................................... 68

4.2 Experimental Section ............................................................................ 70

4.3 Results and Discussion .......................................................................... 72

4.4 Conclusion ............................................................................................ 82

Conclusion and Future Outlook ............................................................ 83


VIII

5.1 Conclusion ............................................................................................ 83

5.2 Future Outlook ...................................................................................... 84

References .............................................................................................................. 87

THE HONG KONG POLYTECHNIC UNIVERSITY List of Figures

IX

List of Figures

Figure 1.1 Overview of the efficiency of different types of solar cells. ...................... 2

Figure 2.1 (a) The standard solar spectrum of AM 1.5.[54] (b) Energy-level diagram

of a simple solar cell. (c) J-V curves of a solar cell.[55] .................................... 10

Figure 2.2 (a) UV-visible spectra of CsPb(I1-xBrx)3 perovskite.[60] (b) J-V curves of

CsPbBr3 PSC with PTAA. (c) TGA of the MAPbBr3, CsPbBr3 and their

corresponding precursor materials. (d) Repetitive sequential EBIC responses of

cross sections of a CsPbBr3 device and a MAPbBr3 device.[40]........................ 14

Figure 2.3 (a) Illustration for the influence of morphology of the perovskite layer on

carriers extraction.[67] (b) Energy-level diagram of CsPbBr3 PSCs. (c) J-V curve

of the champion device based on the structure in (b). The long-term stability of

CsPbBr3 and MAPbI3 devices at (d) 25°C and (e) 100°C.[58] ........................... 15

Figure 2.4 (a) J-V curves of carbon-based devices with and without CuPc. (b) Long-

term stability of MAPbI3 and CsPbBr3 PSCs with CuPc. (c) Illustration for the

functions of CuPc.[70] ........................................................................................ 16

Figure 2.5 (a) Illustration, top-view SEM and cross-sectional SEM of multistep

solution process for CsPbBr3 perovskite films. The effect of spin-coating times of

CsBr precursor on (b) atomic ratio of Cs/Pb on the films, (c) J-V curves and (d)

EQE spectra of CsPbBr3 PSCs. (e) The optimized J-V curves of the CsPbBr3 PSCs


X

and that with GQDs layer.[59] ............................................................................ 18

Figure 2.6 Energy-level diagram of CsPbBr3 PSCs with (a) 0, 1, 3, 5, 7 wt% PtNi

doped carbon electrode[73] and (b) different CuInS2/ZnS QDs. (c) Effect of

CuInS2/ZnS QDs on the PCE of CsPbBr3 PSCs.[75] (d) Voltage and (e) current

generated as a function of time for devices with different plasma treatment

duration. (f) Long-term stability of CsPbBr3 device under RH 80%, 25°C.[74] 19

Figure 2.7 (a) Preparation process of CQD/CsPbBr3 IO PSCs. EQE spectra (b) from

300 nm to 600 nm and (c) from 550 nm to 800 nm for different CsPbBr3-based

PSCs. (d) Top-view SEM image of CQD/CsPbBr3 IO. [57] ............................... 20

Figure 2.8 (a) Phase transition flow based on the synchrotron powder diffraction for

the CsPbI3 compound.[86] (b) XRD patterns of CsPbI3 films and (c) J-V curves

of CsPbI3 devices prepared at different temperatures.[87] (d) SEM images of

CsPbI3 films with and without HI.[88] (e) J-V curves of CsPbI3 devices.[89] (f)

XRD patterns and (g) photographs for CsPbI3 films.[90] ................................... 21

Figure 2.9 (a) Schematic diagram and cross-sectional SEM image of CsPbI3 QDs

PSCs. (b) J-V curves of the CsPbI3 QDs PSCs. (c) XRD patterns of CsPbI3

QDs.[94] .............................................................................................................. 25

Figure 2.10 (a) Half lifetime of carriers as a function of initial charge density for

CsPbI3 prepared by vacuum and spin-coating methods. (b) J-V curves of CsPbI3

PSCs prepared by vacuum (black) and spin-coating methods (red).[98] (c)


XI

Illustration of different vacuum deposition strategies. [99] (d) UV-visible spectra

of CsPbI3 prepared at different temperature.[95] (e) UV-visible spectra of CsPbI3

films and (f) J-V curves of CsPbI3 PSCs with and without the SCG method.[52]

............................................................................................................................. 26

Figure 2.11 (a) XRD patterns and photographs of BA2CsPb2I7 films. (b) Normalized

PCE, Voc, Jsc and FF of CsPbI3 devices as a function of storing time.[100] (c)

Schematic structure of EDAPbI4 with (110) layered 2D perovskite films. (d) AFM

image of CsPbI3·xEDAPbI4 perovskite films. (e) J-V curves and (f) long-term

stability of CsPbI3·xEDAPbI4 PSCs.[102].......................................................... 28

Figure 2.12 (a) Mechanism of α-CsPbI3 stabilized by zwitterion. (b) Cross-sectional

SEM image of CsPbI3 film with SB3-10 zwitterion.[101] (c) Top-view SEM

image of PVP induced α CsPbI3 film. (d) 1H and (b) 13C liquid-state NMR spectra

of PVP solution and CsPbI3 precursor solution with PVP. (f) Photographs of

CsPbI3 perovskite film with and without PVP. (g) Normalized PCE as a function

of ageing time for PVP-doped CsPbI3 PSC and MAPbI3 PSC.[103] ................. 29

Figure 2.13 (a) UV-visible spectra of CsPbI3 perovskite films doped with bismuth. (b)

XPS spectra of CsPb0.96Bi0.04I3. (c) J-V curves of 4 mol% Bi-doped CsPbI3 and

control devices.[104] (d) UV-visible spectra of CsPbI3 perovskite films doped

with calcium. (e) J-V curves of CsPb0.95Ca0.05I3 PSCs with and without MgF2 anti-

reflection layer.[105] ........................................................................................... 30


XII

Figure 2.14 Top-view SEM images of CsPbIBr2 films prepared on (a) 20 °C and (b)

75 °C preheated substrate. (b) J-V curve of CsPbIBr2 PSCs prepared through

vacuum deposition.[112] (c) Illustration of preparing CsPbIBr2 films through

spray deposition. (e) PCE as a function of temperature for CsPbIBr2 PSCs. (f) J-

V curve of CsPbIBr2 PSCs.[113] ........................................................................ 32

Figure 2.15 (a) Photographs of CsPbIBr2 film at low-T phase and high-T phase. (b)

Illustration of phase transition for CsPbIBr2 compound. (c) Vacancy density (ρv)

per unit cell as a function of relative humidity (p/po). (d) Average nucleation time

(τ) as a function of vacancy density (ρv) for CsPbIBr2 and CsPbI2Br. (e) Schematic

diagram of semitransparent PSCs. (f) PCE of CsPbIBr2 PSCs as a function of

phase transition cycles.[116] ............................................................................... 33

Figure 2.16 (a) Illustration of CsPbIBr2 films prepared through step annealing. (b) PCE

of CsPbI2Br PSCs as a function of the thickness of MoOx films.[115] (c) A

superposition of CL spectrum mapping for CsPbIBr2 perovskite film. (d) The

corresponding CL spectrum of (c) at point 1 (GI) and point 2 (GB). (e) J-V curve

of planar CsPbIBr2 PSCs with a large hysteresis.[114] ...................................... 34

Figure 2.17 (a) Energy-level diagram and (b) J-V curves of CsPb1-xMnxI1+2xBr2-2x

PSCs.[118] (c) Photographs, (d) EQE spectra and integrated current densities, and

(e) J-V curves of PSCs based on CsPbBr3 (yellow), CsPbIBr2 (brown), and

CsPb0.9Sn0.1IBr2 (black), respectively. (f) Normalized PCE as a function of ageing


XIII

time for CsPb0.9Sn0.1IBr2 and MAPBI3 devices.[117] ......................................... 36

Figure 2.18 (a) Absorbance onset (black) and PL peak (red) as a function of iodide

concentration in CsPb(IxBr1-x)3 films. (b) Absorbance over time for CsPbI3 and

CsPbI2Br films measured at 675 and 625 nm respectively under RH 50%. (c) J-V

curves of the champion CsPbI2Br PSC.[7] (d) Photographs of CsPb(I1-xBrx)3

perovskite films with a different fraction of iodide. (e) PL peak position as a

function of time for CsPb(I1−xBrx)3 films under 1 sun illumination.[60] ............ 38

Figure 2.19 PL spectra of (a) CsPbI2Br and (b) MAPb(I0.5Br0.5)3 perovskite films

measured under 104 mW/cm2 light. (c) Normalized PCE as a function of time for

CsPbI2Br and MAPbI3 PSCs measured with a 420 nm UV filter.[124] (d) PL

spectra of CsPbI2Br film. (e) J-V curves of CsPbI2Br PSCs as a function of light-

soaking time. (f) Normalized PCE as a function of time for CsPbI2Br and MAPbI3

PSCs under illuminated and dark conditions.[122] ............................................ 40

Figure 2.20 (a) Top-view SEM of CsPbI2Br perovskite film prepared at 260 °C for 60

s. (b) J-V curves of CsPbI2Br PSCs prepared by vacuum deposition. (c) Long-

term stability of encapsulated CsPbI2Br and MAPbI3 PSCs.[121] (d) J-V curves

of CsPbI2Br PSC.[123] (e) FWHM of (100) and (200) peaks as a function of

DMSO concentration for CsPbI2Br precursor. (f) Top-view and cross-sectional

SEM of CsPbI2Br films prepared by 1.15 M precursor.[130] ............................. 41

Figure 2.21 (a) Photographs of perovskite films prepared at 100 °C using different


XIV

precursors. (b) EQE spectrum and integrated Jsc of a CsPbI2Br PSC.[125] (c) J-V

curves of as-grown, aged and recovered CsPbI2Br PSCs.[126] ......................... 42

Figure 2.22 (a) PL spectra and (b) PL decay curves of CsPbI2Br film. (c) J-V curve of

CsPbI2Br PSCs.[127] (d) Mott–Schottky fitting to the CV data of CsPbI2Br. (e)

Energy-level diagram of 3D-2D-0D CsPbI2Br PSCs. (f) Top-view SEM image of

3D-2D-0D CsPbI2Br perovskite film. The inset presents the water contact angle

on the film. (g) Long-term stability of 3D-2D-0D CsPbI2Br PSCs.[128] (h)

Illustration of bandgap alignment based on different ETLs. (i) J-V curve of

CsPbI2Br PSCs.[129] .......................................................................................... 43

Figure 2.23 PL decay curves of (a) CsPbI2Br and (b) Cs0.925K0.075PbI2Br film with and

without c-TiO2 layer. (c) Normalized PCE as a function of time for CsPbI2Br and

Cs0.925K0.075PbI2Br PSCs.[131] (d) Top-view SEM images of CsPbI2Br and CsPb1-

xSrxI2Br films. (e) PCE as a function of mole fraction of Sr.[132] ..................... 46

Figure 2.24 (a) Grain size distribution histograms for 2% MnCl2-doped CsPbI2Br films.

(b) Atomic ratio of Mn/Pb and Mn/Cl as a function of depth from the surface of

MnCl2-doped CsPbBrI2 films. (c) Dark I−V measurements of the devices with

VTFL at the kink points. (d) J-V curve and (e) long-term stability of a 2% MnCl2-

doped CsPbI2Br device.[133] .............................................................................. 48

Figure 2.25 (a) Energy-level alignment of a CsPbI2Br/CsPbI3 QDs PSC. (b) J-V curve

of device with the configuration in (a). (c) PCE, Voc, Jsc and FF of PSCs as a


XV

function of thickness of CsPbI3 QDs layer. (d) Atomic ratio of I/Pb and Br/Pb as

a function of etching time (etching speed of 0.8 nm/s).[134] ............................. 48

Figure 3.1 Illustration of the 2-step fabrication process of perovskite layers with and

without pyridine treatment. ................................................................................. 55

Figure 3.2 J-V curves of CsPbBr3 PCSs (a) with and without a preheated substrate,

and (b) prepared by different drying methods..................................................... 57

Figure 3.3 J-V curves of (a) PSCs with different spin-coating speed for PbBr2 film and

(b) the champion PSC. ........................................................................................ 58

Figure 3.4 (a) Photographs of a CsPbBr3 film and a device. (b) Schematic diagram and

(c) energy-level diagram of inorganic CsPbBr3 PSCs with the configuration of

FTO/c-TiO2/c-TiO2/m-TiO2/CsPbBr3/spiro-OMeTAD/Au. (d) UV-visible spectra

of PbBr2 film, PbBr2.(Py)x film and the optimized CsPbBr3 films made from

PbBr2 and PbBr2.(Py)x......................................................................................... 59

Figure 3.5 J-V curves of (a) pyridine-treated and (b) normal PSCs annealed at different

temperatures. (c) The PCEs of PSCs with or without pyridine treatments as

functions of annealing temperatures. (e) EQE and the integrated current density

of a pyridine-treated PSC annealed at 160 ℃. (e) Long-term stability of the

pyridine-treated CsPbBr3 PSCs annealed at 160 ℃ storing under RH 40% - 70%.

............................................................................................................................. 61

Figure 3.6 Cross-sectional SEM images of pyridine-treated CsPbBr3 films annealed at


XVI

(a) 160 ℃ and (b) 250 ℃, respectively, and the SEM images of normal CsPbBr3

films without a pyridine treatment annealed at (c) 160 ℃ and (d) 250 ℃,

respectively. ........................................................................................................ 62

Figure 3.7 Top-view SEM images of pyridine-treated CsPbBr3 films annealed at (a)

160 ℃ and (b) 250 ℃, respectively, and the SEM images of normal CsPbBr3 films

without a pyridine treatment annealed at (c) 160 ℃ and (d) 250 ℃, respectively.

............................................................................................................................. 63

Figure 3.8 XRD patterns of different films: PbBr2; PbBr2.(Py)x; normal CsPbBr3 films

annealed at 160 °C and 250 °C; pyridine-treated CsPbBr3 films annealed at 160 °C

and 250 °C. [ □ : monoclinic phase of CsPbBr3 (PDF#18-0364), ^ : orthorhombic

phase of phase PbBr2 (PDF#84-1181), # : tetragonal phase of CsPb2Br5 (PDF#25-

0211), and * : rhombohedral phase of Cs4PbBr6 (PDF#73-2478).] .................... 65

Figure 3.9 (a) TGA of pyridine-treated CsPbBr3 perovskite prepared at 160℃. (b)

Reaction coordinate diagram with supposed mechanisms for the formation of

CsPbBr3 perovskite from PbBr2 or PbBr2.(Py)x. EA1, EA2a and EA2b are the

activation energies for the reactions. ................................................................... 66

Figure 4.1 (a) Photographs of CsPbI2Br film freshly prepared, degraded and

reannealed. (b) UV-visible spectrum of a freshly prepared perovskite film and the

degraded film. (c) Tauc plot of CsPbI2Br perovskite film. (d) Absorbance over

time of CsPbI2Br film measured at 625 nm. ....................................................... 73


XVII

Figure 4.2 Cross-sectional SEM images of structure of FTO/TiO2/CsPbI2Br annealed

at (a) 100 °C and (b) 250 °C. .............................................................................. 74

Figure 4.3 Top-view SEM images (a) of m-TiO2 layer, (b,c) with low magnification

and (d,e) with high magnification at dark region and white region for CsPbI2Br

film prepared at 250 °C, (f) with low magnification and (g) with high

magnification for CsPbI2Br film prepared at 100 °C. ......................................... 75

Figure 4.4 Normalized XRD patterns of CsPbI2Br perovskite films prepared at 100 °C,

250 °C, and the degraded yellow film. ............................................................... 77

Figure 4.5 (a) Schematic diagram, (b) energy-level diagram, and (c) photographs of

CsPbI2Br PSCs with encapsulation. (d) UV-vis spectrum of c-TiO2/m-TiO2 layer.

............................................................................................................................. 78

Figure 4.6 (a) J-V curve and (b) EQE spectrum and the integrated current density of

PSCs with CuSCN. (c) J-V curve of PSCs with spiro-OMeTAD. Histograms of

PCE of 20 CsPbI2Br PSCs incorporating with (d) CuSCN and (e) spiro-OMeTAD.

............................................................................................................................. 80

Figure 4.7 (a) Voc, (b) Jsc, (c) PCE and (d) FF as a function of storing time for

encapsulated CsPbI2Br/CuSCN PSCs. ................................................................ 81


encapsulated CsPbI2Br/spiro-OMeTAD PSCs. ................................................... 82

THE HONG KONG POLYTECHNIC UNIVERSIT List of Tables

XVIII

List of Tables

Table 2.1 Summary of CsPbBr3-based PSCs. (Sol: solution deposition; Vac: vacuum

deposition; RS: reverse scan; FS: forward scan) ................................................ 12

Table 2.2 Summary of CsPbI3-based PSCs. ............................................................... 22

Table 2.3 Summary of CsPbIBr2-based PSCs. ........................................................... 31

Table 2.4 Summary of CsPbI2Br-based PSCs. ........................................................... 37

Table 2.5 Summary of degradation tests for CsPbI2Br through XRD and optical

characterization.[126] ......................................................................................... 43

Table 3.1 Summary of annealing temperature and photovoltaic parameters of

published CsPbBr3 PSCs from each research group. ......................................... 61

Table 4.1 FWHM of XRD peaks for CsPbI2Br perovskite films prepared at 100 °C and

250 °C. ................................................................................................................ 77

Table 4.2 Photovoltaic parameters of CsPbI2Br/CuSCN devices prepared by different

room-temperature-crystallization time. ............................................................... 79

THE HONG KONG POLYTECHNIC UNIVERSIT List of Abbreviations

XIX

List of Abbreviations

Short forms Meanings

BA Butylamine

BDA 1,4-Butanediamine

CBP 4,4’-Bis(N-carbazolyl)-1,1’-biphenyl

DMF Dimethylformamide

DMSO Dimethyl sulfoxide

EA Ethylamine

EDBE 2,2’-(ethylenedioxy)bis(ethylammonium)

FA+ Formamidinium, [(NH2)2CH]+

HOMO the highest occupied molecular orbital

LUMO the lowest unoccupied molecular orbital

MA+ Methylammonium, CH3NH3+

P3HT Poly(3-hexylthiophene)

PTAA Poly[bis(4phenyl)(2,4,6-trimethylphenyl)amine]

PVP Polyvinylpyrrolidone

Spiro Spiro-OMeTAD

TAPC 4,4’-cyclohexylidenebis[N,N-bis(4methylphenyl)benzenamine]

THE HONG KONG POLYTECHNIC UNIVERSITY Chapter 1

1

Introduction

1.1 Background

Through the development of urbanization and technology, the electricity

consumption for lighting and computer work is very high and getting increasing.

However, the main source of electric energy is fossil fuel which is very limited and

will be exhausted in the future. The combustion of fossil fuel generates a large amount

of carbon dioxide which is one main kind of greenhouse gas, leading to worsening the

global warming. In order to alleviate the problems of the energy crisis and global

warming, reducing the use of electricity and searching for new kinds of alternative

energy become concerned in the community. The Sun which provides inexhaustible

energy is one of the fundamental energy sources for the earth so the solar cell is a high-

potential type of sustainable energy.

Nowadays, the commercial solar cells are mainly made from silicon (Si) or groups

13-15 (old groups III-IV) gallium arsenide (GaAs) semiconductors. These kinds of

solar cells are highly stable for the normal use in ambient air and can achieve the best

power conversion efficiencies (PCEs) up to about 26% for single junction without

concentrator and about 38% for multijunction without concentrator (Figure 1.1). The

normal commercial Si solar cells can also achieve PCEs about 15-18%.[1] However,

the fabrication of traditional solar cells requires high-temperature annealing to melt

the silicon, causing the increase of fabrication price and equivalent greenhouse gas

emission. As reported by Intergovernmental Panel on Climate Change (IPCC), the

equivalent carbon dioxide emission per unit (kWh) power generation for photovoltaic

(PV) devices is quite high (about 48 gCO2eq/kWh for utility-scale) comparing to the


2

other alternative energy such as wind energy, hydroelectric energy and nuclear

energy.[2] To solve the high-temperature problem, several kinds of solar cells emerge,

for example, dye-sensitized solar cells, organic solar cell, quantum dots solar cells,

and perovskite solar cells.

Figure 1.1 Overview of the efficiency of different types of solar cells.

The perovskite solar cell (PSC) is one of the most promising types of solar cells

since the perovskite materials used for photovoltaic effect can demonstrate many

outstanding properties in photovoltaic applications, including tunable bandgap for

light absorption in a variable wavelength range, high absorption coefficient, small

exciton binding energy, high carrier mobility, long carrier lifetime and long diffusion

length.[3-10] PSCs can also be fabricated by convenient solution processes with much

lower temperature requirement than traditional silicon solar cells, leading to cost-

effective and mass production.[11-14]. The perovskite was first discovered by Gustav

Rose in 1839 and was later named after L. A. Perovski as any material possessing the

same type of crystal structure with calcium titanium oxide (CaTiO3). The structure of

perovskite used in solar cells is ABX3 where A site is a monovalent cation (e.g. MA+,

FA+, Cs+), B site is divalent cation (e.g. Pb2+, Sn2+), and X site is halide anion (e.g. Cl-,


3

Br-, I-). The perovskite materials used for solar cells were the organic

methylammonium lead halide (MAPbX3) which was first reported by Weber in 1978

while the inorganic CsPbX3 had an even longer history.[15] Until 2009, perovskite

was first utilized by Miyasaka as a photoactive material to generate electricity through

the photovoltaic effect.[16] Through the redox reaction, the devices with MAPbBr3

and MAPbI3 can obtain PCEs of 3.13% and 3.81% respectively. Since then, the PSCs

are getting concerned by the researcher. After 2 years, the second PSCs paper was

reported by Park who studied the MAPbI3 quantum dots (QDs) to enhance the PCE to

6.54% by optimizing the annealing temperature of perovskite, the thickness of TiO2

and interface engineering.[17] In the following years, through the effort of the

researcher on the composition engineering, interface engineering, and optimization,

the efficiency of PSCs are increased rapidly. To date, the highest PCE of PSCs is

22.7% which is already higher than some of the commercial Si solar cells.

The highly efficient PSCs are all based on pure organic perovskite or hybrid

organic-inorganic perovskite which is unstable against high-temperature, moisture,

UV light conditions due to the sensitive organic component. Thus, the devices would

degrade quickly under the atmosphere, leading to a large barrier to commercialization.

In recent year, by occupying the A site by an inorganic cation, the inorganic perovskite

solar cells emerged. Inorganic perovskites possess higher stability, especially thermal

stability, so the lifetime of PSCs can be prolonged by employing the inorganic

perovskites.

1.2 Objectives of Research

It is very important for the solar cells to possess long-term stability under ambient

air and normal operating environment. The stability of pure organic PSCs and hybrid

organic-inorganic PSCs is still further away from the requirement for


4

commercialization. The inorganic PSCs may be one of the possible approaches to

solve the unstable problem. Therefore, in this thesis, we aim to study the potential of

using all-inorganic perovskite as the photoactive layer in solar cells.

In recent year, the PCE of inorganic PSCs were enhanced to 16.14% which is a

very fast improvement possibly due to the great potential of inorganic perovskite and

the solid foundation in the hybrid organic-inorganic perovskite solar cells. In order to

have a comprehensive research on the inorganic PSCs, having a full understanding of

the development of inorganic PSCs is very important. The published works related to

inorganic PSCs were summarized in the literature review to show the development of

inorganic PSCs.

Through the literature review, we found that the annealing temperature of

CsPbBr3 inorganic perovskite is still very high about 250 ºC. The high temperature is

not compatible for cost-effective fabrication, flexible devices, and temperature

sensitive materials. Therefore, we have done a work on using a pyridine treatment to

reduce the annealing temperature down to 160 ºC with acceptable PCE.

Apart from the CsPbBr3 inorganic perovskite, we have also fabricated the PSCs

using CsPbI2Br as photoactive layer. Due to a large bandgap of CsPbBr3, the

absorption range, photocurrent density and PCE are limited. By changing the fraction

of Br- ion to I- ion, the light absorption can be largely increased. Employing copper(I)

thiocyanate as a hole transport layer to the PSCs can further enhance the performance

and stability.

1.3 Outline of Thesis

Chapter 1: Introduction. In this chapter, the background of photovoltaic devices and

appearance of perovskite solar cells are introduced. Also, the objective of this research


5

and the outline of this thesis are presented.

Chapter 2: Review of Inorganic Perovskite Solar Cells. In this chapter, through a

large amount of literature review, the fabrication methods, performance and

development of inorganic PSCs are summarized.

Chapter 3: Low-Temperature Processing of All-Inorganic Perovskite Solar Cells with

High Stability. In this chapter, detailed experimental methods including fabrication

and characterization of pyridine-treated PSCs are shown. The performance,

morphology, and mechanism for reducing annealing temperature are discussed.

Chapter 4: Efficiency and Stability Enhancement by using Copper(I) Thiocyanate

as a Hole Transport Layer on All-Inorganic Perovskite Solar Cells. In this chapter, the

stability of CsPbI2Br PSCs is revealed. The improvement on photovoltaic parameters

by using CuSCN is shown and discussed following.

Chapter 5: Conclusion and Future outlook. In this chapter, the two experimental

works are summarized and further improvements on the devices are suggested.


6

Review of Inorganic Perovskite

Solar Cells

2.1 Introduction

To date, the most remarkable PSCs are all pure organic perovskite or hybrid

organic-inorganic perovskite solar cells. References 16-26 show the main

development of perovskite solar cells.[16-26] Initially, the researcher uses the organic

cation MA+ as the main component of A site, demonstrating a rapid enhancement in

PCE over several years. On the way, another organic cation FA+ are found to be able

to improve the solar cells in different aspects, for example, higher light absorbance,

wider photon absorption wavelength, and longer stability.[27] Doping different

elements, such as aluminium cations (Al3+),[28] antimony cations (Sb3+),[29] cobalt

cation (Co2+)[30] and indium cations (In3+)[31] on B site is also an effective approach

to improve film quality, reduce carrier recombination, enhance charge extraction and

provide more favourable energetic alignment. Also, the researcher found that further

doping the inorganic cation Cs+ into the organic perovskite not only can raise the

device efficiency but also extend the stability under ambient air.[25, 32, 33]

However, the organic perovskite still suffers from the poor stability under heat,

oxygen, moisture and even illuminated conditions mainly due to the organic and

iodide components.[32, 34, 35] The instability of the perovskite may be attributed to

the migration of iodide ions through the interstitial defects, vacancies and chemical

instability of the hydrophilic property of polar MA+ cations.[36-41] These kinds of

degradation are attributed as irreversible decomposition so it is necessary to produce

stable PSCs against those problems. It is essential to prolong the light (or UV light)


7

stability so many researcher have found some ways, such as UV filtering, to reduce

the damage from light.[42, 43] To against high humidity, adding hydrophobic layers

or doping elements are some workable approaches while employing a perfect sealing

to the PSCs is the most useful method.[44] However, sealing or encapsulating the

PSCs would also trap the heat, worsening the thermally unstable issue. Although

numerous works have been done to enhance the stability of organic perovskite, they

are still far from the requirement of commercialization.

Fully inorganic perovskite is treated as an efficient way to solve these problems,

especially the thermal instability. Inorganic cations Cs+ is more stable than organic

cations CH3NH3+ (MA+) and [(NH2)2CH]+ (FA+) because only organic components

would undergo the chemical decomposition to produce ammonium (NH4+) and

seriously destroy the perovskite structure.[45] The caesium lead bromide can

thermally stable up to 580ºC while the MAPbI3 can only tolerate a temperature of

about 200ºC.[40] Therefore, due to the highly stable property, inorganic PSCs have

become a popular topic in research of solar cells.

The first two published all-inorganic perovskite solar cells were using lead-free

perovskite CsSnI3 as the photoactive layer, obtaining PCEs of 0.88% in 2012 and

2.02% in 2014.[46, 47] Recently, even though the PCE of inorganic tin-based PSCs

have increased to 4.81%, the efficiency will deteriorate quickly in the ambient air since

the chemically unstable Sn2+ will oxidize into Sn4+.[48] The Sn4+ cations can form an

oxide or hydroxide layers which accelerate the introduction of oxygen and further

decompose the black-phase perovskite film. Also, the Sn4+ cations will create a highly-

conductive p-type perovskite film, resulting in a short-circuit behaviour and a poor

photovoltaic performance.[49, 50] Therefore, up to now tin-based inorganic

perovskite materials seem not to be a candidate to solve the unstable problem.

Another more popular inorganic perovskite material is caesium lead halide


8

perovskite by simply replacing of Sn2+ by Pb2+ cation. Lead-based inorganic

perovskite CsPbBr3 was first studied by Kulbak in June of 2015, demonstrating PCE

of 4.92-5.95%.[51] In the same year, they also reported the excellent thermal stability

of the CsPbBr3 perovskite, demonstrating the potential of inorganic perovskite solar

cells.[40] The CsPbBr3 inorganic PSCs display negligible degradation under ambient

environment over three months, demonstrating the long-term stability of inorganic

perovskite. To date, at least 50 literature related to inorganic perovskite solar cells are

published in this 3 years and the amount of publication per year keeps increasing,

showing the inorganic perovskite materials are getting concerned. This trend is highly

related to their thermally stable property. Until now, the highest PCE of published

inorganic PSCs is identified as 16.14% by CsPb(I0.85Br0.15)3 as a photoactive layer.[52]

However, changing the fraction of halide from bromide to iodide would cause the

perovskite phase to become unstable in ambient air. The stability of CsPbI3 is even

worse than the organic MAPbI3. Researcher has conducted experiments in different

approaches, such as deposition methods, composition engineering, grain size

confinement and interfacial engineering to improve the efficiency and stability of

inorganic PSCs.

In this Chapter, we focus on discussing and summarizing the progress of lead-

based inorganic perovskite solar cells. Firstly, a brief working principle of

photovoltaic effect and the main characterization for the solar cells are presented.

Since the PCE and stability of inorganic PSCs would perform differently along the

different fraction of iodide to bromide in CsPb(IxBr1-x)3 perovskite, we would divide

caesium lead halide into four categories which are CsPbBr3, CsPbI3, CsPbIBr2 and

CsPbI2Br to individually discuss their performance and development in detail. At each

category, we separately summarize the bare caesium lead halide and the relevant

composition engineering by doping element, polymer or 2D materials in the A site or

B site. At the end of this chapter, the future approaches on how to further improve the


9

PCE and stability of inorganic PSCs will be discussed.

2.2 Background of Photovoltaic Effect

2.2.1 A Brief Working Principle of Photovoltaic Effect

The solar cells can convert the solar energy into electric energy. The Sun, an

energy source, is a blackbody with light spectrum at the temperature of about 5800 K.

By eliminating the absorption by the atmosphere of the earth and modifying the optical

path length of the Sunlight to the zenith with an angle (48.2°), AM 1.5 light spectrum

with a light intensity of 100 mW/cm2 are defined as the standard conditions for

characterization of solar cells on the surface of the earth (Figure 2.1a).[53]

For semiconductor-based photovoltaics, photons with enough energy can excite

the electrons from the conduction band to the valence band across the bandgap,

followed by carriers (electrons and holes) extraction due to the built-in electric field

(Figure 2.1b). A semiconductor with low bandgap can absorb more light and generate

higher current but it will also affect the voltage output which is limited by the

difference of quasi-Fermi levels of electrons and holes.[54] On the contrary, a high

bandgap will limit the light absorption and current output. For single-junction solar

cells, the optimal bandgap is between 1.1 and 1.4 eV which can achieve a high

efficiency of about 33% in maximum.[54]

2.2.2 Characterization of Photovoltaic Cells

To characterize the performance of solar cells, the most important parameter is

the power conversion efficiency (PCE). As illustrated in Figure 2.1c, under

illumination, when a positive voltage is applied, a negative current is flowed on the


10

circuit, indicating power generation from the solar cells. The PCE is defined as the

ratio of output electric power (Pout) to the incident light power (Pin) or determined by

open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF):

𝑃𝐶𝐸 =𝑃𝑜𝑢𝑡

𝑃𝑖𝑛=

𝑉𝑜𝑐×𝐽𝑠𝑐×𝐹𝐹

𝑃𝑖𝑛 (2.1)

𝐹𝐹 =𝑉𝑀𝑃×𝐽𝑀𝑃

𝑉𝑜𝑐×𝐽𝑠𝑐 (2.2)

where VMP and JMP are the voltage and the current density of the maximum power.

Figure 2.1 (a) The standard solar spectrum of AM 1.5.[54] (b) Energy-level diagram

of a simple solar cell. (c) J-V curves of a solar cell.[55]

Another important parameter for characterization is external quantum efficiency

(EQE) which is also named as an incident photon to converted electron (IPCE) ratio

in solar cells. EQE is defined as the ratio of the numbers of electrons output to the

numbers of the incident photon. EQE is determined by many factors, such as reflection

of a solar cell, exciton generation efficiency, exciton dissociation efficiency and

carriers extraction efficiency. For characterization, EQE can be used to estimate the

Jsc with the equation:


11

𝐽𝑠𝑐 = 𝑒 ∫ Φ(𝜆)𝜆𝑚𝑎𝑥

𝜆𝑚𝑖𝑛× 𝐸𝑄𝐸(𝜆)𝑑𝜆 (2.3)

where e is the charge of electron and Φ(λ) is the spectral photon flux of the incident

light.

2.3 Caesium-based PSCs with Different Halide

Composition

As shown in Table 2.1-2.4, the summaries of CsPbBr3, CsPbI3, CsPbIBr2 and

CsPbI2Br PSCs display the configuration, the fabrication methods, the optimized

photovoltaic parameters and the corresponding scan direction in an ascending order of

paper accepted date or published online date. In these tables, all the “c-TiO2/m-TiO2”

structure is simply replaced by “m-TiO2” for a short representation. Most of the PSCs

reveal the best PCE in the reverse scanning direction of J-V curves.

2.3.1 CsPbBr3 PSCs

Caesium lead bromide perovskite possesses a bandgap of about 2.3 eV which can

absorb light with a wavelength shorter than about 540 nm. There may be a small

variable on the bandgap (2.25 eV to 2.37 eV) of thin film perovskite due to different

fabrication technique.[51, 56, 57] According to the absorption spectrum measured in

UV-visible spectroscopy, a characteristic absorption peak is commonly recorded on

material CsPbBr3 (at about 520 nm),[58, 59] however, this absorption peak will

disappear gradually by replacing Br- ions by I- ions (Figure 2.2a).[60] Since the

emission of CsPbBr3 falls on the green light, it is usually used as illumination devices

such as light emitting diodes and lasers.[61-64] That absorption wavelength is limited

in range from ultraviolet to green light so the ideal photocurrent density of CsPbBr3


12

PSCs under 1.5AM sunlight can only reach about 9 mA/cm2. This property allows the

CsPbBr3 PSCs to work as semi-transparent solar cells or the bottom part of the tandem

solar cells. Normally, CsPbBr3 is a yellow orthorhombic phase (Pbnm or Pnma) in

room temperature and transitions to yellow tetragonal phase (P4/mbm) and yellowish

orange cubic phase (Pm-3m) at 88 ºC and 130 ºC. However, the phase transitions of

thin film normally reveal a not obvious colour change.[65, 66]

Table 2.1 Summary of CsPbBr3-based PSCs. (Sol: solution deposition; Vac: vacuum

deposition; RS: reverse scan; FS: forward scan)

2.3.1.1 Fabrication process

The CsPbBr3 perovskite photoactive layers are commonly fabricated through a 2-

step process because of the limited solubility of Br- ions (both PbBr2 and CsBr) in the

typical organic solvent, such as dimethylformamide (DMF). In 2015, Kulbak first

indicated this limitation of fabricating CsPbBr3 perovskite film and demonstrated their

2-step fabrication process which became a guideline for the later researcher. For the

Material Configuration Methods Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

Scan Direction

Date Ref.

CsPbBr3 FTO/m-TiO2/CsPbBr3/spiro/Au Sol 1 6.75 74 4.98 RS 10/6/2015 [51]

FTO/m-TiO2/CsPbBr3/CBP/Au Sol 1.32 6.91 54 4.92 RS 10/6/2015 [51]

FTO/m-TiO2/CsPbBr3/Au Sol 1.21 6.11 74 5.47 RS 10/6/2015 [51]

FTO/m-TiO2/CsPbBr3/PTAA/Au Sol 1.25 6.7 73 6.2 FS 23/12/2015 [40]

FTO/m-TiO2/CsPbBr3/carbon Sol 1.29 5.7 68 5 RS 22/11/2016 [67]

FTO/m-TiO2/CsPbBr3/carbon Sol 1.24 7.4 73 6.7 RS 26/11/2016 [58]

FTO/m-TiO2/CsPbBr3(+Cl)/spiro/Ag Sol 1.02 8.47 72 6.21 NA 16/5/2017 [68]

FTO/m-TiO2/CQD/CsPbBr3/spiro/Ag Sol 1.06 11.34 69 8.29 NA 10/10/2017 [57]

FTO/c-TiO2/CsPbBr3/spiro/Au Sol 1.42 7.01 53 5.6 RS 25/20/2017 [69]

FTO/m-TiO2/CsPbBr3/CuPc/carbon Sol 1.26 6.62 74 6.21 RS 26/12/2017 [70]

FTO/m-TiO2/GQDs/CsPbBr3 IO/carbon Sol 1.46 8.12 82 9.72 NA 30/1/2018 [59]

FTO/GQDs/CsPbBr3/PQDs/carbon Sol 1.21 5.08 67 4.1 RS 12/2/2018 [71]

FTO/c-TiO2/CsPbBr3/carbon Sol 1.34 6.46 68 5.86 RS 27/2/2018 [72]

FTO/m-TiO2/CsPbBr3/carbon Sol 1.13 6.79 70 5.38 RS 2/3/2018 [56]

FTO/m-TiO2/CsPbBr3/CQDs +PtNi NWs Sol 1.43 6.78 81 7.86 NA 25/3/2018 [73]

FTO/m-TiO2/GQDs/CsPbBr3/CISZ-QDs/carbon

Sol 1.52 7.35 84 9.43 NA 30/3/2018 [74]

FTO/m-TiO2/CsPbBr3/CuInS2/ZnS

QDs/carbon Sol 1.45 7.47 78 8.42 NA 3/5/2018 [75]


13

first-step process, different papers regarded to a similar fabrication way that spin-

coating a preheated PbBr2 precursor on a preheated substrate and followed by

annealing the film at about 70 ºC to 85 ºC in ambient air or inside the glovebox.

Preheating both the precursor and the substrate are used to improve the solubility of

1.0 M PbBr2 in DMF and avoid fast and non-uniform crystallization of PbBr2 on the

cold substrate. For the 2nd step, a solution of 0.07 M CsBr in methanol is used to react

with the PbBr2 film to establish the formula of CsPbBr3 in different ways, for example,

dip-coating,[40, 51] substrate face-up[58] and face-down[72] immersion and

multistep solution process.[59] The typical conversion condition for the PbBr2 film in

CsBr methanol solution is about 70 ºC for 10 min. We believe that, during the 2nd-step

process, a higher temperature (70 ºC) can facilitate reaction rate but the real

temperature of methanol solvent is lower than the temperature set on the hot plate

because the boiling point of methanol is only 64.7 ºC. Heating over this boiling

temperature will generate many gas bubbles which can dramatically damage the film

morphology. The CsPbBr3 film is finally annealed at a high temperature of about 250

ºC for crystallization and obtaining the desired phase. Some of the other methods were

also performed, for example, using chemical vapour deposition (CVD) to replace I-

ions in CsPbI3 by Br- ions[56] and fabricating an inverse opal perovskite film using a

one-step method.[57] The corresponding performance of devices using different

methods will be discussed in the next part. Notice that the fabrication process of bulk

perovskite and thin film perovskite for light-emitting diodes (LED) and photodetectors

could be different from that for the solar cells.[65, 76]

2.3.1.2 Performance of CsPbBr3 PSCs

The CsPbBr3 is the first caesium lead halide perovskite being investigated in PSCs

due to its highly stable property in the ambient air. As reported, the devices FTO/c-


14

TiO2/m-TiO2/CsPbBr3/HTL/Au use different hole transport layers (HTLs), such as

spiro-OMeTAD, PTAA, CBP, obtaining PCEs from 4.92 to 5.95%.[51] The devices

without HTL can still achieve a relatively high PCE of 5.47% with negligible

hysteresis, indicating the high hole conductivity of CsPbBr3. In the same year, as

shown in Figure 2.2b, the PSCs using PTAA as HTL are further optimized and achieve

a PCE of 6.2% which is comparable with that of MAPbBr3.[40] Through the

thermogravimetric analysis (TGA) for organic perovskite and inorganic perovskite

(Figure 2.2c), MAPbBr3 starts losing its weight and decomposes at low temperature

about 200ºC while CsPbBr3 can sustain up to about 580 ºC, exhibiting a much higher

thermal stability. Electron beam-induced current (EBIC) images of cross-sectional

PSC devices (Figure 2.2d) are constructed by mapping of current response against

electron beam. The images for CsPbBr3 sample revealed nearly invisible change on

different scanning times, proving structure of CsPbBr3 is more stable than that of

MAPbBr3 against high-energy electrons.

Figure 2.2 (a) UV-visible spectra of CsPb(I1-xBrx)3 perovskite.[60] (b) J-V curves of

CsPbBr3 PSC with PTAA. (c) TGA of the MAPbBr3, CsPbBr3 and their corresponding

precursor materials. (d) Repetitive sequential EBIC responses of cross sections of a

CsPbBr3 device and a MAPbBr3 device.[40]


15

In order to further stabilize and reduce the cost of the inorganic PSCs, HTL-free

and carbon-based PSCs are fabricated to replace organic HTL/Au. Two similar

research works were published nearly at the same time in November of 2016,

reflecting that the unstable problem of organic HTL draws the researcher attention.

Chang optimized the conditions in the 2nd-step fabrication process, such as the reaction

time and temperature of PbBr2 and CsBr.[67] It is found that when immersing the

PbBr2 substrate into the CsBr solution, the most suitable reaction temperature and time

are 50ºC for 40 min, resulting in a PCE of 5.0%. They suggested that overreaction

would induce poor morphology of perovskite film which suppresses the hole

extraction to the carbon electrode (Figure 2.3a). Based on the smooth perovskite film

and large grain size up to 1 μm, Liang reported that the carbon-based PSCs can obtain

the best PCE of 6.7% for 0.12 cm2 area and 5.0% for a large area of 1.00 cm2 (Figure

2.3b,c).[58] The carbon electrode CsPbBr3 PSCs without any encapsulation show no

degradation in high-humidity ambient environment (RH 90-95%, 25 ºC) for over 3

months and no degradation in high temperature (RH 90-95%, 100 ºC) for about 720

hours, exhibiting the most stable PSCs to date (Figure 2.3d,e).

Figure 2.3 (a) Illustration for the influence of morphology of the perovskite layer on

carriers extraction.[67] (b) Energy-level diagram of CsPbBr3 PSCs. (c) J-V curve of

the champion device based on the structure in (b). The long-term stability of CsPbBr3

and MAPbI3 devices at (d) 25°C and (e) 100°C.[58]


16

Figure 2.3b shows the energy band alignment of structure FTO/TiO2/CsPbBr3/

carbon. The large energy difference in the interfaces of TiO2 (LUMO = -4.0 eV)/

CsPbBr3 (LUMO = -3.3 eV) and CsPbBr3 (HOMO = -5.6 eV)/carbon (WF = -5.0 eV)

may not be the best band alignment for carrier transport and may reduce the PCE.[77,

78] In order to achieve a smoother energy band alignment in the PSCs and facilitate

carrier extraction,[79] inserting a HTL copper phthalocyanine (CuPc) (HOMO = -5.2

eV) in between CsPbBr3/carbon can successfully enhance the PCE from 3.8% to

6.21% (4.72% for large area 2.25 cm2) (Figure 2.4a).[70] Also, a hole transport layer

(e.g. CuPc, spiro-OMeTAD) can establish a Schottky barrier to avoid the direct contact

of carbon electrode and ETL through the pinholes by technological defects so that the

carrier recombination can be suppressed (Figure 2.4c). The CuPc-based PSCs show

no degradation after above 900 hours stored in RH 70-80% at 100 ºC (Figure 2.4b),

demonstrating the high thermal stability of CsPbBr3 and CuPc in the air.[80]

Figure 2.4 (a) J-V curves of carbon-based devices with and without CuPc. (b) Long-

term stability of MAPbI3 and CsPbBr3 PSCs with CuPc. (c) Illustration for the

functions of CuPc.[70]

Recently, Tang’s group also had conducted five works contributing to the high-


17

quality fabrication of CsPbBr3 PSCs and the corresponding interfacial engineering for

better band alignment.[59, 71, 73-75] For the 2nd-step fabrication of perovskite film,

by using multiple solution processes, repeating the spin-coating and annealing

processes for four times can obtain the highest quality and purity of CsPbBr3 (Figure

2.5a).[59] This method is similar to the inter-diffusion technique used in organic

PSCs.[8] Carefully controlling the number of times of spin-coating can avoid

insufficient or excess CsBr in the reaction to form CsPb2Br5 or Cs4PbBr6 respectively

(Figure 2.5b). The J-V curves (Figure 2.5c) consistently show the best PCE of 7.54%

by spin-coating CsBr solution for four times. An about 5 nm graphene quantum dots

layer (GQDs) with an energy level of -3.84 eV was inserted in between m-TiO2 and

CsPbBr3 to reduce the large energy difference, leading to efficient electron extraction

and an enhancement in PCE. The best device exhibited a high open-circuit voltage

(Voc) of 1.46 V, a high short-circuit current density (Jsc) of 8.12 mA/cm2, a high fill

factor (FF) of 82% and a record high PCE of 9.72% in CsPbBr3 PSCs to date (Figure

2.5d,e). Surprisingly, the best device can obtain a maximal EQE value of 93% and

extend the EQE to 550 nm. In another work, the work function (WF) of the carbon

electrode was tuned to suit the HOMO of the perovskite layer by integrating platinum

nickel (PtNi) alloy in carbon paste (Figure 2.6a).[73] An amount of 3 wt% PtNi

nanowires (NWs) can create the most obvious PL quenching in glass/CsPbBr3/carbon

and result in the best PCE of 7.86%. Instead of tuning the WF of the carbon electrode,

adding a hydrophobic layer of CuInS2/ZnS QDs (CISZ-QDs) in the interface of

CsPbBr3/carbon is also able to promote the hole extraction (Figure 2.6b). By carefully

modifying the reaction time and the temperature to synthesize different size of CISZ-

QDs, a bandgap of 1.81 eV QDs is the optimal condition for PSCs with a PCE of

8.42% (Figure 2.6c). With a further plasma treatment on the carbon electrode, the

devices were able to harvest solar energy (PCE: 9.43%) on the perovskite layer and

water-vapour energy (0.158μW) on the back carbon electrode in the same time.[74]


18

The carbon electrode became hydrophilic after plasma treatment and allowed water

vapour to flow through. They suggested a similar mechanism with previous works,

that a negative zeta potential on the carbon layer can repulse the negative hydroxyl

ions (OH-) and attract the positive hydrogen ion (H+) so an electron (CB)/proton

(water vapor) electrical double layer can be established at the carbon/water interface

for electricity generation.[81, 82] Figure 2.6d,e showed the optimal time for plasma

treatment is 5 minutes and higher humidity can generate higher voltage and current

outputs. Similar to previous works, the devices storing under RH 80%, 25 ºC without

encapsulation or 40 days can retain 98% of its initial PCE, exhibiting the high stability

of each layer (Figure 2.6f). They also reported the performance of simple carbon-based

CsPbBr3 devices FTO/GQDs/CsPbBr3/PQDs/carbon.[71] The simplified device

displayed a relatively low PCE of 4.1% and poor stability comparing to the one using

TiO2 and CISZ-QDs layers.

Figure 2.5 (a) Illustration, top-view SEM and cross-sectional SEM of multistep

solution process for CsPbBr3 perovskite films. The effect of spin-coating times of

CsBr precursor on (b) atomic ratio of Cs/Pb on the films, (c) J-V curves and (d) EQE

spectra of CsPbBr3 PSCs. (e) The optimized J-V curves of the CsPbBr3 PSCs and that

with GQDs layer.[59]


19

Figure 2.6 Energy-level diagram of CsPbBr3 PSCs with (a) 0, 1, 3, 5, 7 wt% PtNi

doped carbon electrode[73] and (b) different CuInS2/ZnS QDs. (c) Effect of

CuInS2/ZnS QDs on the PCE of CsPbBr3 PSCs.[75] (d) Voltage and (e) current

generated as a function of time for devices with different plasma treatment duration.

(f) Long-term stability of CsPbBr3 device under RH 80%, 25°C.[74]

Carbon quantum dot-sensitized (CQD) inorganic CsPbBr3 inverse opal perovskite

solar cells were developed.[57] Through first depositing CsPbBr3 (0.6 M in DMSO)

precursor on a double-layered polystyrene (PS) template and then removing the PS

template by toluene with CQDs dispersed, a CsPbBr3 inverse opal layer was obtained

(Figure 2.7a). The CQD/CsPbBr3 IO can slightly reduce the bandgap from about 2.35

eV to 2.25 eV and extent the EQE range to 550 nm so that the photocurrent generated

can be increased (Figure 2.7b). Also, the periodically photonic array structure (i.e.

inverse opal film in Figure 2.7c) creates a photonic bandgap (PBG) which causes a

lower intensity in reflectance and transmittance on the wavelength about 670 nm.

Interestingly, as shown in Figure 2.7d, small EQE peaks were recorded near the

wavelength 670 nm for the IO devices. For the CQD/CsPbBr3 IO PSCs, a high current

density of about 11 mA/cm2 were recorded from J-V curves and integrating the EQE

curve results in a Jsc about 10 mA/cm2 which is higher than the ideal value(assume

100% EQE from 300 nm to 550 nm). This error may probably because of the multi-


20

scattering processes or inaccurate active area used. The IO structure was seldom

applied on perovskite solar cells so there may exist some mysteries requiring to be

unsolved.

Figure 2.7 (a) Preparation process of CQD/CsPbBr3 IO PSCs. EQE spectra (b) from

300 nm to 600 nm and (c) from 550 nm to 800 nm for different CsPbBr3-based PSCs.

(d) Top-view SEM image of CQD/CsPbBr3 IO. [57]

2.3.2 CsPbI3 PSCs

The bandgap of cubic phase caesium lead iodide perovskite is about 1.73 eV,

indicating that the onsets of absorbance and EQE are about 720 nm, the perovskite is

black in colour and the maximum current density is about 22 mA/cm2. The black

perovskite phase of CsPbI3 is more unstable than the black phase of the organic

perovskite because CsPbI3 highly prefers non-perovskite orthorhombic phase (Pnma)

in the room temperature. A Goldschmidt's tolerance factor τ, an indicator initially to

describe the geometric stability of the oxide perovskite, extends to the halide

perovskite recently.[83]

𝜏 =(𝑅𝑎+𝑅𝑥)

√2(𝑅𝑏+𝑅𝑥) (2.4)


21

where Ra is A-site cation radius, Rb is B-site cation radius, Rx is X-site anion radius.

The factor τ of CsPbI3 (0.807) is lower than that of CsPbBr3 (0.824), indicating that

the CsPbI3 perovskite suffers a higher octahedral rotation distortion and transform to

the orthorhombic phase more easily.[84] Therefore, the cubic phase of CsPbI3 can

only sustain in high temperature.

Figure 2.8 (a) Phase transition flow based on the synchrotron powder diffraction for

the CsPbI3 compound.[85] (b) XRD patterns of CsPbI3 films and (c) J-V curves of

CsPbI3 devices prepared at different temperatures.[86] (d) SEM images of CsPbI3

films with and without HI.[87] (e) J-V curves of CsPbI3 devices.[88] (f) XRD patterns

and (g) photographs for CsPbI3 films.[89]

As shown in Figure 2.8a, during cooling, CsPbI3 perovskite will degrade by

decreasing the Pb-I-Pb angle and continuously losing its symmetric structure through

3-phase transition. The structure of CsPbI3 changes from the black α phase (cubic, Pm-

3m) to black β phase (tetragonal, P4/mbm) under 260 ºC, to black γ phase


22

(orthorhombic, Pbnm) under 175 ºC, and finally to yellow δ phase (orthorhombic,

Pnma) after cooling to 25ºC for several days.[85, 90, 91] In order words, the CsPbI3

changes from black perovskite to yellow non-perovskite when the [PbI6]4− octahedral

changes from corner-sharing to edge-sharing. The researcher found that fabricating

CsPbI3 perovskite with lower grain size can increase the stability due to the surface

energy. Ayyub suggested that decreasing the crystal size will make crystal tend to

transition to a structure with high symmetry (i.e. cubic phase).[92] Therefore, a series

of methods such as adding hydroiodic acid (HI), isopropyl alcohol (IPA), polymers,

sulfobetaine Zwitterions, doping elements in B site or using quantum dots are used to

modify the microstrain or reduce the grain size.

Table 2.2 Summary of CsPbI3-based PSCs.

2.3.2.1 Fabrication process

The CsPbI3 perovskite layers are normally deposited by two methods which are


Jsc (mA/cm2)

FF (%)

PCE (%)

Scan Direction

Date Ref.

CsPbI3 FTO/c-TiO2/CsPbI3/spiro/Au Sol 0.8 12 NA 2.9 FS 4/9/2015 [87]

FTO/m-TiO2/CsPbI3/P3HT/MoO3/Au Sol 0.74 10.48 61 4.68 RS 25/9/2015 [86]

FTO/c-TiO2/CsPbI3/spiro/Ag Sol 0.66 11.92 52 4.13 RS 29/8/2016 [89]

FTO/m-TiO2/CsPbI3 QDs/spiro/MoOx/Au Sol 1.23 13.47 65 10.77 RS 7/9/2016 [93]

ITO/c-TiO2/CsPbI3/P3HT/Au Vac 1.06 13.8 72 10.5 RS 7/12/2016 [94]

ITO/c-TiO2/CsPbI3/P3HT/Ag Vac 0.71 12.06 67 5.71 FS 11/1/2017 [95]

ITO/PEDOT:PSS/CsPbI3/PCBM/BCP/LiF/Al Sol 0.87 8.17 69 4.88 NA 7/2/2017 [88]

FTO/c-TiO2/CsPbI3/carbon Sol 0.67 14.31 48 4.65 NA 18/7/2017 [96]

FTO/SnO2/CsPbI3/spiro/Ag Vac 1 13 68 8.8 RS 28/7/2017 [97]

ITO/c-TiO2/CsPbI3/P3HT/Ag Vac 0.79 12.06 72 6.79 RS 31/7/2017 [98]

ITO/SnO2/CsPbI3/spiro/Au Sol 1.08 18.41 79 15.71 RS 17/4/2018 [52]

ITO/SnO2/CsPb(I0.85Br0.15)3/spiro/Au sol 1.22 17.13 78 16.14 RS 17/4/2018 [52]

Doping FTO/m-TiO2/CsPbI3+BAPBI4/spiro/Au Sol 0.96 8.88 57 4.84 RS 19/12/2016 [99]

ITO/PTAA/CsPb(I0.98Cl0.02)3+ zwitterions/PCBM/C60/BCP

Sol 1.08 14.9 70 11.4 RS 31/7/2017 [100]

FTO/c-

TiO2/CsPbI3+0.025EDAPBI4/spiro/Ag Sol 1.15 14.53 71 11.86 RS 11/9/2017 [101]

FTO/m-TiO2/CsPbI3+10wt%

PVP/spiro/Au Sol 1.11 14.88 65 10.74 RS 25/1/2018 [102]

FTO/c-TiO2/CsPb0.96Bi0.04I3/CuI/Au Sol 0.97 18.76 73 13.21 RS 31/8/2017 [103]

MgF2/glass/FTO/m-

TiO2/CsPb0.95Ca0.05I3/P3HT/Au Sol 0.945 17.9 80 13.5 RS 26/2/2018 [104]

FTO/m-TiO2/m-

Al2O3/CsPb0.96Sb0.04I3/carbon Sol 0.73 14.65 50 5.31 RS 24/4/2018 [105]


23

solution deposition and vacuum deposition. Unlike bromide ions, iodide ions have

high solubility in the common organic solvents DMF and DMSO so one-step

fabrication process of perovskite is commonly used. The perovskite films can be

commonly obtained by spin-coating a CsPbI3 precursor with concentrations of about

0.48 M - 1.0 M in a different ratio of DMF and DMSO and subsequently annealing at

about 310 ºC. A smaller crystal size can stabilize the black phase of CsPbI3. For

deposition of QDs perovskite film, the QDs precursor (~50 mg/ml in octane) was spin-

coated on the substrate, dipped in methyl acetate (MeOAc) solution to remove the

organic ligands, and dried with a stream of air. The thickness of perovskite can be

controlled by repeating the process for three to four times. Vacuum deposition enables

the researcher to study the perovskite more deeply by controlling the thickness

precisely and producing PSCs with more repeatable PCE because of less hand-made

uncertainty. The vacuum deposition approaches include coevaporation, sequential

evaporation, 2 double layers evaporation, and alternative evaporation of PbI2 and CsI,

followed by an annealing at about 310 ºC.

2.3.2.2 Performance of CsPbI3 PSCs

In 2015, Eperon and Ripolles separately published the CsPbI3 PSCs almost on the

same time. Ripolles reported that the CsPbI3 can only achieve black phase perovskite

at an annealing temperature of 350 ºC.[86] The XRD patterns and J-V curves (Figure

2.8b,c) confirms that even the annealing temperature is up to 250 ºC, the films are still

in the yellow phase. With an 8 nm molybdenum trioxide (MoO3) hole transport layer

in between P3HT and silver electrode, the best PSCs can achieve the PCE of 4.68%.

Eperon successfully used hydroiodic acid (HI) to largely reduce the high annealing

temperature for black CsPbI3 from 310 ºC to 100 ºC and obtained a PCE of 2.9% on

the planar PSCs of regular architecture.[87] By adding 33 μl HI in 1 ml precursor, the


24

crystal size can be largely reduced to about 100 nm (Figure 2.8d), providing more

microstrain to induce crystal phase transition. Through the analysis of the coverage

rate of perovskite film and the PCEs of devices, the optimized annealing temperature

for perovskite film with HI additive was later studied and located at 100 ºC (Figure

2.8e).[88] The PSCs with IPA treatment further enable the fabrication process to

conduct in ambient air and prolong the stability of perovskite films, showing

negligible change after 72 hours by naked eyes and XRD analysis (Figure 2.8f,g).[89]

The IPA treatment can also guide the phase transition of the film from Cs4PbBr6 phase

to cubic phase CsPbI3.

As mentioned above, the black phase CsPbI3 perovskite can be stabilized by

reducing the crystal size of perovskite. By reducing the size of crystal to the nanoscale,

Swarnkar applied the perovskite quantum dots in the PSCs, resulting in a remarkable

PCE of 10.77% and a high Voc of 1.23 V on the device (Figure 2.9a-c).[93] For the

synthesis of stable cubic phase CsPbI3 QDs, the size of QDs was first controlled by

the injection temperature. Then a new extraction solvent MeOAc can effectively

remove the excess unreacted precursor without inducing agglomeration so the QDs do

not reverse back to orthorhombic phase for months (Figure 2.9d). For devices

fabrication, after spin-coating the QDs precursor (in octane) on the substrate, the

MeOAc solvent is used again to remove the organic ligands. The PCE of QDs PSCs

was improved in the 60-day storage in the dry ambient air, demonstrating a longer

stability for QDs film.


25

Figure 2.9 (a) Schematic diagram and cross-sectional SEM image of CsPbI3 QDs

PSCs. (b) J-V curves of the CsPbI3 QDs PSCs. (c) XRD patterns of CsPbI3 QDs.[93]

The optoelectronic parameters such as the charge carrier mobility and the charge

carrier lifetime of CsPbI3 through an alternative layers vacuum deposition and a

solution deposition are compared to find out a better method for obtaining a high-

quality film.[97] By the time-resolved microwave conductivity (TRMC)

measurements, the sum of mobility of hole and electron from the vacuum deposition

and spin-coated deposition are about 25 cm2/Vs and 18 cm2/Vs respectively. The

carrier lifetime by vacuum deposition is excess 10 μs which is 50 times longer than

that by the spin-coated method (Figure 2.10a). Consequently, the vacuum deposition

methods can enhance the PCE from 6.4% to 8.8% (Figure 2.10b). Another study

investigated the repeatability of different vacuum deposition (layer-by-layer)

architecture of perovskite film (Figure 2.10c).[98] A four-double-layer method can

produce the most repeatable PCE while a two-double-layer method is able to achieve

a higher PCE. Interestingly, through thermal coevaporation of PbI2 and CsI, the CsPbI3

film can appear in dark phase without annealing or only a low-temperature annealing

of 60 ºC.[94] The dark film change to yellow and return to black phase after further


26

annealing at 80 ºC and 320 ºC respectively (Figure 2.1d). The room-temperature dark

phase is found to be the combination of black cubic phase and yellow orthorhombic

phase. The high-temperature phase CsPbI3 by thermal vacuum deposition can achieve

a relatively high PCE of 10.5%. Very recently, a solution-controlled growth method

was reported that the caesium lead halide films can gradually turn dark after spin-

coating (Figure 2.10e).[52] After a post-annealing process at high temperature and

incorporating with an ETL of SnO2, the champion CsPbI3 PSC can achieve a high PCE

of 15.7% and the CsPb(I0.85Br0.15)3 PSC can even obtain a record high PCE up to

16.14% in the inorganic PSCs (Figure 2.10f).

Figure 2.10 (a) Half lifetime of carriers as a function of initial charge density for

CsPbI3 prepared by vacuum and spin-coating methods. (b) J-V curves of CsPbI3 PSCs

prepared by vacuum (black) and spin-coating methods (red).[97] (c) Illustration of

different vacuum deposition strategies. [98] (d) UV-visible spectra of CsPbI3 prepared

at different temperature.[94] (e) UV-visible spectra of CsPbI3 films and (f) J-V curves

of CsPbI3 PSCs with and without the SCG method.[52]


27

2.3.2.3 Performance of composition engineering CsPbI3 PSCs

By conducting the compositional engineering, for example, adding 2D materials

or doping elements in B site to confine the crystal size on a small scale, the

performance and stability of PSCs can be largely increased. With incorporation of

butylammonium lead iodide (BA2PbI4) 2D material into CsPbI3 in equimolar ratio,

although the PCE of BA2CsPb2I7 solar cells is not very high at about 4.84%, the

devices without encapsulation can retain 91% of its initial efficiency after storing

under RH 30% for 30 days, exhibiting a very large improvement on the stability

(Figure 2.11a-b).[99] With a similar idea of introducing 2D materials, adding

ethylenediamine lead iodide (EDAPbI4) to CsPbI3 perovskite layer was reported

comprehensively by Zhao.[101] In contrast to materials with a mono-functional group

such as BA+ and EA+, the bication property (two NH3+ groups) and the crosslink of

EDA2+ from EDAPbI4 are able to occupy two A sites and stabilize the perovskite.

Comparing with the other two bications BDA2+ and EDBE2+, only EDA2+ can improve

the stability of devices due to the unique (110) layered 2D perovskite arrangement in

EDAPbI4 (Figure 2.11c) and its ability to reduce the crystal size. With 0.025 mole

EDAPbI4 in CsPbI3, the grain size can reduce from about 300 nm to about 35 nm

(Figure 2.11d), further enhancing the stability of black phase perovskite. The

champion PSC can reach a PCE of 11.86% with a high Voc of 1.15 V and the PCE is

only decreased to 10% after storing in a dark dry box for one month without

encapsulation, demonstrating a high enhancement in the stability (Figure 2.11e, f).


28

Figure 2.11 (a) XRD patterns and photographs of BA2CsPb2I7 films. (b) Normalized

PCE, Voc, Jsc and FF of CsPbI3 devices as a function of storing time.[99] (c) Schematic

structure of EDAPbI4 with (110) layered 2D perovskite films. (d) AFM image of

CsPbI3·xEDAPbI4 perovskite films. (e) J-V curves and (f) long-term stability of

CsPbI3·xEDAPbI4 PSCs.[101]

Apart from adding 2D materials, incorporating 1.5wt% sulfobetaine zwitterion into

the perovskite film can decrease the grain size down to about 30 nm.[100] For the

mechanism (Figure 2.12a), the zwitterion can electrostatically interact with the

PbI2.DMSO colloids, separate the colloids to a smaller size in the precursor solution

and finally isolate the cubic phase perovskite along the grain boundary to establish a

small grain size (Figure 2.12b). By further optimizing the PSCs using 6% Cl- ion, the

device can achieve a PCE of 11.4% and remain 85% of the initial efficiency after

storing in ambient air with RH 30% - 40% for 30 days without encapsulation. PVP

polymer, another material which able to interact with the CsPbI3 precursor and reduce

the grain size of CsPbI3 was studied in detail by Li (Figure 2.12c).[102] The A-site

Cs+ cations interact with the nitrogen atom and the oxygen atom of acylamino groups

in the PVP, inducing shifts of peaks in nuclear magnetic resonance (NMR) (Figure

2.12d,e). That interaction increases the electron cloud density for Cs+ of CsPbI3-PVP,


29

thus decrease the surface tension as well as the surface energy,[106, 107] resulting in

stabilization of cubic phase of CsPbI3 in room temperature (Figure 2.12f). By carefully

adjusting the percentage of PVP as 10 wt% and removing the excess PVP by IPA, the

best PSC can obtain a PCE of 10.74% and reveal high stability in ambient air with RH

45-55% without encapsulation (Figure 2.12g).

Figure 2.12 (a) Mechanism of α-CsPbI3 stabilized by zwitterion. (b) Cross-

sectional SEM image of CsPbI3 film with SB3-10 zwitterion.[100] (c) Top-view SEM

image of PVP induced α CsPbI3 film. (d) 1H and (b) 13C liquid-state NMR spectra of

PVP solution and CsPbI3 precursor solution with PVP. (f) Photographs of CsPbI3

perovskite film with and without PVP. (g) Normalized PCE as a function of ageing

time for PVP-doped CsPbI3 PSC and MAPbI3 PSC.[102]

In addition to making interaction with the A-site cation of perovskite, doping ions

with a charge of 2+ or 3+ on the B site can also modify photovoltaic response and

improve the performance of CsPbI3 PSCs.[84] Similar to the organic perovskite,[108]

by incorporating 4 mol% bismuth (Bi) into CsPbI3 can largely reduce the bandgap

from about 1.73 eV to about 1.56 eV, extending the absorption range to about 800 nm

(Figure 2.13a). The small radius of Bi3+ ion (103 pm) can increase the tolerance factor

τ from 0.81 to 0.84 and structurally induce a little lattice distortion to create

microstrain, resulting in stabilization of the black CsPbI3 in room temperature. In XPS,


30

the absence of Pb0 peaks in the Pb 4f spectrum confirmed that the CsPb0.96Bi0.04I3 is

more resistant to decomposition in ambient air (Figure 2.13b). With utilizing the TiO2

and CuI, the totally inorganic PSC can achieve a PCE of 13.21% and a stabilized

efficiency of 13.17% (Figure 2.13c). Interestingly, another work reported that doping

calcium Ca2+ ion in CsPbI3 can widen the bandgap and enlarge the grain size but they

still get a good performance on the devices (Figure 2.13d).[104] The Ca2+ cations tend

to form high bandgap oxides, such as CaO (~6.3 eV) and CaCO3 (7 eV), on the

perovskite surface, leading to surface passivation and recombination suppression

similar to the effect of excess PbI2 in the perovskite.[109, 110] By carefully controlling

the trade-off of surface passivation and thickness of insulating oxide, the devices with

5 mol% Ca2+ cations and 75 nm MgF2 anti-reflection layer on the glass side can

achieve a high PCE of 13.5% (Figure 2.13e).

Figure 2.13 (a) UV-visible spectra of CsPbI3 perovskite films doped with bismuth. (b)

XPS spectra of CsPb0.96Bi0.04I3. (c) J-V curves of 4 mol% Bi-doped CsPbI3 and control

devices.[103] (d) UV-visible spectra of CsPbI3 perovskite films doped with calcium.

(e) J-V curves of CsPb0.95Ca0.05I3 PSCs with and without MgF2 anti-reflection

layer.[104]


31

2.3.3 CsPbIBr2 PSCs

By tuning the ratio of iodide to bromide as 1:2, the CsPbIBr2 perovskite possesses

a bandgap about 2.05 eV which allows the perovskite to absorb light shorter than about

605 nm and generate a short-circuit current density of about 13.5 mA/cm2 in maximum.

The CsPbIBr2 appears red in colour for cubic phase and nearly transparent for the

orthorhombic phase. Due to the bromide ions, the CsPbIBr2 devices can exhibit much

higher stability than that of pure iodide-based devices, but the PCE would be relatively

low. The fabrication methods of CsPbIBr2 to date included two-step fabrication, one-

step fabrication and vacuum deposition, similar to those of CsPbBr3 and CsPbI3. In

order to achieve the desired phase of the perovskite layer, the annealing temperature

reported varied from 135 ºC to 320 ºC.

Table 2.3 Summary of CsPbIBr2-based PSCs.


Jsc (mA/cm2)

FF (%)

PCE (%)

Scan Direction

Date Ref.

CsPbIBr2 FTO/c-TiO2/CsPbIBr2/Au Vac 0.959 8.7 56 4.7 RS 2/2/2016 [111]

FTO/m-TiO2/CsPbIBr2/spiro/Au Sol 1.13 7.8 72 6.3 RS 17/8/2016 [112]

FTO/m-TiO2/CsPbIBr2/spiro/Au Sol 1.23 9.69 67 8.02 RS 12/7/2017 [113]

FTO/NiOx/ CsPbIBr2/MoOx/Au Sol 0.85 10.56 62 5.52 RS 27/8/2017 [114]

FTO/NiOx/CsPbIBr2/ZnO/Al (or Al-doped ITO)

Sol 1.01 8.65 64 5.57 RS 29/11/2017 [115]

FTO/NiOx/CsPbI1.5Br1.5/ZnO/Al Sol 1.0 10.61 68 7.23 RS 29/11/2017 [115]

Doping FTO/m-TiO2/CsPb0.9Sn0.1IBr2/carbon Sol 1.26 14.3 63 11.33 RS 21/9/2017 [116]

FTO/m-TiO2/CsPb0.995Mn0.005I1.01Br1.99/carbon

Sol 0.99 13.15 57 7.36 NA 19/4/2018 [117]

2.3.3.1 Performance of CsPbIBr2 PSCs

The CsPbIBr2 PSCs were first investigated by Ho-Baillie’s group in the early of

2016 using thermal vacuum deposition to evaporate PbBr2 and CsI precursors.[111]

By keeping the substrate temperature at 75ºC during the deposition and post-annealing

the film at 250ºC, the grain size of the perovskite can increase to about 500- 1000 nm

(Figure 2.14a,b). Without using the hole transporting layer, the devices can achieve


32

the best PCE of 4.7% in reverse scan (Figure 2.14c). Due to the solubility problem of

bromide ion, they also demonstrated a two-step solution method including spin-

coating PbBr2 film and spray-coating CsI precursor to manufacture the perovskite film

(Figure 2.14d).[112] The optimized condition was identified as spraying the CsI

precursor on a room-temperature PbBr2 substrate and then annealing the film at 300ºC

for 10 minutes (Figure 2.14e). By incorporating to a spiro-OMeTAD HTL, the best

device can present a PCE of 6.3% with negligible hysteresis on the reverse scan and

the forward scan (Figure 2.14f).

Figure 2.14 Top-view SEM images of CsPbIBr2 films prepared on (a) 20 °C and (b)

75 °C preheated substrate. (b) J-V curve of CsPbIBr2 PSCs prepared through vacuum

deposition.[111] (c) Illustration of preparing CsPbIBr2 films through spray deposition.

(e) PCE as a function of temperature for CsPbIBr2 PSCs. (f) J-V curve of CsPbIBr2

PSCs.[112]

The CsPbIBr2 PSCs can effectively demonstrate a tunable transparency and

photovoltaic effect by controlling the phase of perovskite, revealing the possibility of

application to photovoltaic windows (Figure 2.15a).[115] The colour (or absorbance)

degradation of inorganic CsPbIBr2 perovskite film under room temperature or ambient

air stems from that the moisture act as a catalyst to increase the vacancies in crystal,

facilitate the nucleation, and induce the edge-sharing structure (low-T phase) (Figure


33

2.15b-d). Unlike the organic perovskite, degradation of CsPbIBr2 perovskite is phase

transition instead of decomposition. Therefore, it can be fully recovered to the red

perovskite (“high-T phase”) from colourless phase (“low-T phase”) by annealing

again at 150 ºC. To measure the reversibility of the inorganic PSCs, cycling tests were

conducted by repeating the processes of humid air flow and annealing. The devices

with CsPbIBr2 perovskite film were found to possesses the best reversibility and were

able to achieve a repeatable PCE of about 5% (Figure 2.15e,f). Towards a semi-

transparent solar cell, the aluminium (Al) electrode was replaced by a transparent Al-

doped ITO electrode, meanwhile, the PSC can still achieve a PCE of 4.29%.

Figure 2.15 (a) Photographs of CsPbIBr2 film at low-T phase and high-T phase. (b)

Illustration of phase transition for CsPbIBr2 compound. (c) Vacancy density (ρv) per

unit cell as a function of relative humidity (p/po). (d) Average nucleation time (τ) as a

function of vacancy density (ρv) for CsPbIBr2 and CsPbI2Br. (e) Schematic diagram

of semitransparent PSCs. (f) PCE of CsPbIBr2 PSCs as a function of phase transition

cycles.[115]

Liu introduced a step annealing process to crystallize a CsPbIBr2 perovskite film

with better quality by one-step solution process (Figure 2.16a).[114] To improve the

PCE of inorganic PSCs, inserting a MoOx layer in between FTO/NiOx/CsPbIBr2 and

Au were found to increase the PCE from 1.3% to 5.52% (Figure 2.16b). Interestingly,

MoOx is commonly used as a hole transporting layer, but here 4 nm MoOx layer can


34

also demonstrate the function of electron buffer layer to forbid the transport of hole,

promote the electron extraction, and avoid the Schottky contact between CsPbIBr2 and

Au. Hysteresis of PCE is a quite common problem in organic PSCs while the inorganic

PSCs is also not an exclusion. Li suggested that phase segregation of perovskite film

is one of the factors inducing hysteresis in inorganic PCE.[113] Based on the

characterization of photoluminescence, cathodoluminescence (CL) and TEM, the

CsPbIBr2 perovskite film under light or electron beam illumination will induce iodide-

rich segregation as clusters inside the film and at the grain boundaries (GBs) (Figure

2.16c,d). The phase segregation would promote high density of mobile ions

transporting along the GBs and accumulate at the perovskite/TiO2 interface, leading

to a high injection barrier and a large J-V hysteresis in planar devices.[118] As shown

in Figure 2.16e, a large difference of PCE between reverse scan (8.02%) and forward

scan (4.02%). By employing a m-TiO2 layer between the c-TiO2 layer and perovskite

layer to reduce the interfacial traps, the hysteresis is largely reduced (i.e. PCE of 6.63%

for the reverse scan and 6.2% for the forward scan).

Figure 2.16 (a) Illustration of CsPbIBr2 films prepared through step annealing. (b) PCE

of CsPbI2Br PSCs as a function of the thickness of MoOx films.[114] (c) A

superposition of CL spectrum mapping for CsPbIBr2 perovskite film. (d) The

corresponding CL spectrum of (c) at point 1 (GI) and point 2 (GB). (e) J-V curve of

planar CsPbIBr2 PSCs with a large hysteresis.[113]


35

2.3.3.2 Performance of composition engineering CsPbIBr2 PSCs

As reported by Liang, through doping manganese iodide MnI2 into CsPbIBr2

perovskite, the bandgap will gradually decrease from 1.9 eV to 1.75 eV for 5 mol%

MnI2 doping (Figure 2.17a).[117] A very small amount of Mn2+ cations (0.5 mol%)

doping can optimize the perovskite film (CsPb0.995Mn0.005I1.01Br1.99) to have a better

crystallinity and a smaller bandgap 1.85 eV to absorb more light. However, further

increasing the doping ratio to 5 mol% would seriously affect the film morphology and

coverage. The device can obtain the highest PCE of 7.36%, exhibiting a 19.9%

enhancement in efficiency (Figure 2.17b). Doping tin cations Sn2+ into CsPbIBr2 can

also narrow the bandgap because tin-based perovskite possesses much smaller

bandgap than lead-based perovskite.[119] Another work also reported bandgap

reduction to 1.79 eV by doping 10 mol% Sn2+ cation in the CsPbIBr2 perovskite,

indicating a red shift of EQE onset to about 690 nm and a higher Jsc generation about

14.3 mA/cm2 (Figure 2.17c,d).[116] The devices with configuration FTO/c-TiO2/m-

TiO2/CsPb0.9Sn0.1IBr2/carbon can achieve the best PCE of 11.33% with a high Voc of

1.26 V and also exhibit a better thermal stability at 100ºC than the one of MAPbI3 with

encapsulation (Figure 2.17e-f). Interestingly, for the above two works, the bandgap of

CsPbIBr2 in control devices fabricated by 2-step solution process are characterized as

about 1.9 eV which is much lower than the normal bandgap of CsPbIBr2 about 2.05

eV. As reported by previous work, along with the testing time, some PL red shifts

were observed for CsPb(I1-xBrx)3 perovskite in composition 0.4 < x < 1.[60] This

difference may be attributed to two possible reasons related to hybrid halide perovskite.

Firstly, there may be large-area phase segregation of CsPbI2Br (~1.92 eV) on the film

or on the surface of the film. Secondly, during the 2nd step of dip-coating the PbBr2

substrate in excess CsI solution, some unknown reaction such as anion exchange may

undergo to establish another perovskite CsPbI2Br phase instead of CsPbIBr2 phase.


36

Similar to organic perovskite, there are still many unknown areas in inorganic

perovskite and performance variation by using different fabrication techniques. A full

explanation of the difference in the bandgap of CsPbIBr2 from one-step fabrication

and two-step fabrication of spin-coating and dip-coating may contribute to the

development of inorganic perovskite.

Figure 2.17 (a) Energy-level diagram and (b) J-V curves of CsPb1-xMnxI1+2xBr2-2x

PSCs.[117] (c) Photographs, (d) EQE spectra and integrated current densities, and (e)

J-V curves of PSCs based on CsPbBr3 (yellow), CsPbIBr2 (brown), and

CsPb0.9Sn0.1IBr2 (black), respectively. (f) Normalized PCE as a function of ageing

time for CsPb0.9Sn0.1IBr2 and MAPBI3 devices.[116]

2.3.4 CsPbI2Br PSCs

In order to take a balance between efficiency and stability in inorganic caesium

lead halide PSCs, CsPbI2Br perovskite is the most promising material for solar cells.

For stability, unlike pure CsPbI3 film, the CsPbI2Br film will not degrade quickly from

dark phase to yellow phase when shoring inside dry N2 environment. The bandgap of

CsPbI2Br perovskite about 1.92 eV can absorb light with a wavelength shorter than

650 nm and produce a short-circuit current of about 17 mA/cm2 in maximum.

Although CsPbI2Br perovskite is not as stable as CsPbBr3, the PCE of its devices is


37

comparable with or even higher than that of CsPbI3 PSCs. For fabrication, with the

relatively low ratio of bromide, there is no the problem of limited solubility so the

perovskite film can be simply deposited using one-step fabrication method which is

also the most common method.

Table 2.4 Summary of CsPbI2Br-based PSCs.


Jsc (mA/cm2)

FF (%)

PCE (%)

Scan Direction

Date Ref.

CsPbI2Br FTO/m-TiO2/CsPbI2Br/Spiro/Ag Sol 1.11 11.89 75 9.84 RS 2/2/2016 [7]

ITO/PEDOT:PSS/CsPbI2Br/PCBM/BCP/Al Sol 1.06 10.9 NA 6.8 FS 10/2/2016 [60]

ITO/Ca/C60/CsPbI2Br/TAPC/TAPC:MoO3/Ag Vac 1.13 15.2 68 11.8 RS 20/1/2017 [120]

ITO/c-TiO2/CsPbI2Br/Spiro/Ag Sol 1.1 13.99 67 10.34 NA 13/4/2017 [121]

ITO/c-TiO2/CsPbI2Br/Spiro/Au Sol 1.23 12 73 10.7 RS 13/6/2017 [122]

FTO/c-TiO2/CsPbI2Br/Spiro/Au Sol NA NA NA 10.3 RS 17/8/2017 [123]

FTO/c-TiO2/CsPbI2Br/Spiro/Ag Sol 1.13 13.61 69 10.5 RS 21/12/2017 [124]

ITO/c-TiO2/CsPbI2Br/Spiro/Au Sol 1.05 12.7 68 9.08 RS 4/1/2018 [125]

ITO/c-TiO2/CsPbI2Br/P3HT/Au Sol 1.30 13.13 70 12.02 RS 15/1/2018 [126]

FTO/c-TiO2/CsPbBrI2(3D-2D-0D)/PTAA/Au Sol 1.19 12.93 80.5 12.39 RS 24/1/2018 [127]

FTO/NiOx/CsPbI2Br/ ZnO@C60/Ag Sol 1.14 15.2 77 13.3 RS 8/3/2018 [128]

FTO/NiMgLiO/CsPbI2Br/PCBM/Ag Sol 0.98 14.18 66 9.14 RS 19/3/2018 [129]

Doping ITO/c-TiO2/Cs0.925K0.075PbI2Br/Spiro/Au Sol 1.18 11.6 73 10 RS 7/2/2017 [130]

FTO/m-TiO2/CsPb0.98Sr0.02I2Br/P3HT/Au Sol 1.07 14.9 71 11.3 RS 11/9/2017 [131]

FTO/c-TiO2/CsPbI2Br +2mol% MnCl2/CsPbI2Br QDs/PTAA/Au

Sol 1.17 14.37 80 13.47 RS 21/3/2018 [132]

FTO/c-TiO2/CsPbI2Br/Mn-doped CsPbI3

QDs/PTAA/Au Sol 1.20 15.25 79 14.45 RS 12/4/2018 [133]

2.3.4.1 Performance of CsPbI2Br PSCs

The first published study of CsPbI2Br PSCs has already obtained PCE very close

to 10 % in February of 2016 which is much higher than other inorganic PSCs in the

same period. That study also investigated the performance of CsPb(IxBrx-1)3 PSCs with

a composition of 0.67 < x < 1 using the one-step method rather than the two-step

method because of the poor and rough films.[7] By fitting the absorbance onset or PL

peak against perovskite with the different fraction of iodide using Vegard’s law, a

linear trend was obtained for a further prediction of the other fractional iodide

composition (Figure 2.18a). Under a slow air flow with RH 50%, the time-dependence


38

absorbance confirms that CsPbI2Br perovskite film is much stable than CsPbI3

perovskite film (Figure 2.18b). By incorporating to a typical planar conventional

configuration of c-TiO2/CsPbI2Br (~150 nm)/spiro-OMeTAD/Ag, the devices can

obtain an average PCE of 6.02% and a champion PCE of 9.84% (Figure 2.18c). A

similar study was published nearly on the same time, that the colour of perovskite film

changed from yellow to dark when the halide component changed from bromide to

iodide (Figure 2.18d).[60] For the effect of PL peak position of CsPb(I1-xBrx)3 against

time, as mentioned above, strong red shifts were recorded for the fraction of 0.4 < x <

1, a small red shift for x=0.4 and no change for the rest fraction. The PL peak shift

may be attributed to the phase segregation and charge carriers willing to recombine at

lower-energy phase. It is interesting to know why hybrid halide perovskite with x <

0.4 including CsPbI2Br show nearly no change in PL and to recognize whether the PL

peak will continuously shift to red for longer testing time or return to the original

position after a long-time storing in the dark.

Figure 2.18 (a) Absorbance onset (black) and PL peak (red) as a function of iodide

concentration in CsPb(IxBr1-x)3 films. (b) Absorbance over time for CsPbI3 and

CsPbI2Br films measured at 675 and 625 nm respectively under RH 50%. (c) J-V

curves of the champion CsPbI2Br PSC.[7] (d) Photographs of CsPb(I1-xBrx)3

perovskite films with a different fraction of iodide. (e) PL peak position as a function

of time for CsPb(I1−xBrx)3 films under 1 sun illumination.[60]


39

It is important to know whether light-dependent property appears not only in

organic PSCs but also in the inorganic PSCs.[134] Zhou demonstrated that CsPbI2Br

perovskite is a light-independent material in the aspects of ion migration and halide

segregation.[123] By combining the cryogenic galvanostatic test and voltage-current

measurements, the energy barrier of ion migration of CsPbI2Br perovskite was

calculated to be nearly no change from dark (0.45 eV) to illuminated (0.43 eV)

conditions, however, that of the organic MAPbI3 decreased dramatically from 0.62 eV

to 0.07 eV. Also, unlike MAPb(I0.5Br0.5)3, no PL peak shift revealed on the CsPbI2Br

and CsPbIBr2 perovskite film demonstrates that the absence of halide segregation

(Figure 2.19a,b). The best CsPbI2Br PSCs (PCE: 10.3%) without encapsulation

exhibits a negligible change in PCE for 1500 hours under a continuous maximum

power point tracking test in nitrogen glovebox (RH 20%), confirming its photo-

stability (Figure 2.19c). However, another work showed opposite results in the light-

independent property.[121] The PL peaks of CsPbI2Br film display a red shift, like

dealloying the composition from x = 0.33 to x = 0.2 in CsPb(I1-xBrx)3 (Figure 2.19d).

The CsPbI2Br PSCs reveal a light-soaking effect which the PCE increasing over the

exposure time and the highest PCE is 10.3% (Figure 2.19e). Figure 2.19f shows that

the light-induced dealloying is a reversible process under alternative exposing to the

illuminated and dark condition. By separately incorporate to the perovskite film to

ETL and HTL for PL characterization, the dealloying is found to enhance the hole

extraction ability with spiro-OMeTAD instead of electron extraction ability. We

suspect that since the properties of PSCs are highly dependent on fabrication

techniques, this may induce the discrimination of the above two results. More research

is expected to clarify the photo-dependent property of inorganic perovskite and find

out a better fabrication way.


40

Figure 2.19 PL spectra of (a) CsPbI2Br and (b) MAPb(I0.5Br0.5)3 perovskite films

measured under 104 mW/cm2 light. (c) Normalized PCE as a function of time for

CsPbI2Br and MAPbI3 PSCs measured with a 420 nm UV filter.[123] (d) PL spectra

of CsPbI2Br film. (e) J-V curves of CsPbI2Br PSCs as a function of light-soaking time.

(f) Normalized PCE as a function of time for CsPbI2Br and MAPbI3 PSCs under

illuminated and dark conditions.[121]

In order to obtain high-quality films on PSCs, all-vacuum deposition was reported

to enhance the PCE and Jsc of devices with structure ITO/Ca/C60/CsPbI2Br/

TAPC/TAPC:MoO3/Ag to 11.8% and 15.2 mA/cm2.[120] Unlike the organic halide

with the gas-like property, it is easy to control the co-deposition of CsBr and PbI2 in a

stoichiometric amount. By adjusting the annealing time of perovskite, 60 s was the

optimal duration to enlarge the grain size up to about 3 μm (Figure 2.20a,b).

Comparing to the MAPbI3 and CsPbI3 PSCs, CsPbI2Br PSCs exhibit a long-term

stability with encapsulation (Figure 2.20c). For CsPbI2Br perovskite film fabricating

from one-step solution process, 280ºC was the optimized annealing temperature to

achieve smooth and compact morphology with full coverage (without pinholes).[122]

The compact perovskite film is able to achieve the highest stability by avoiding the

diffusion of humid air. Although the thin perovskite film (85 nm) slightly limits the

Jsc of 12 mA/cm2, the device can still obtain a high PCE of 10.7% and a very high Voc

of 1.23 V due to the high-quality perovskite film reducing the charge recombination


41

(Figure 2.20d). Recently, another research improves the quality of perovskite film in

the approach of the solvent engineering of the CsPBI2Br precursor solution.[129] By

adjusting the ratio of DMF to DMSO in the precursor, 40% DMSO is the optimized

composition since that concentration can eliminate the pinholes on the perovskite

films and reduce the full width at half maximum (FWHM), indicating large crystals

(Figure 2.20e). Considering the optimized ratio of solvent, increasing the

concentration to the solubility limit 1.15 M can raise the thickness of the film to 410

nm (Figure 2.20f), leading to a high Jsc of 14.18 mA/cm2 as well as a PCE of 9.14%.

Figure 2.20 (a) Top-view SEM of CsPbI2Br perovskite film prepared at 260 °C for 60

s. (b) J-V curves of CsPbI2Br PSCs prepared by vacuum deposition. (c) Long-term

stability of encapsulated CsPbI2Br and MAPbI3 PSCs.[120] (d) J-V curves of

CsPbI2Br PSC.[122] (e) FWHM of (100) and (200) peaks as a function of DMSO

concentration for CsPbI2Br precursor. (f) Top-view and cross-sectional SEM of

CsPbI2Br films prepared by 1.15 M precursor.[129]

Since the high annealing temperature (> 250ºC) for dark CsPbI2Br perovskite

increases the cost of fabrication and limit the flexibility of PSCs, a low-temperature

technique is preferred on condition that the PCE can remain. Through pre-establishing

a HPbI3+x compound from mixing PbI2 and excess HI solution and then preparing the

perovskite precursor using 2CsI+ HPbI3+x+PbBr2, the deposited film using the solution


42

is able to turn dark at a low temperature of 100ºC (Figure 2.21a).[124] By further

optimizing the annealing temperature at 130ºC and annealing duration of 6 min, the

device can achieve a PCE of 10.5%. Since the ratio of iodide to bromide in the

precursor is higher than 2:1, the bandgap of perovskite film would decrease and extend

the EQE range to about 700 nm (Figure 2.21b). The champion PSC without

encapsulation can still retain a PCE of about 9 % after storing at a dark box with RH

15-20%.

Understanding the degradation mechanism of perovskite films helps the

researcher to solve the stability problem in a more target-oriented way. Mariotti first

examined the stability of the CsPbI2Br perovskite film against visible/UV light, water

vapour, oxygen (O2), and ozone (O3).[125] By combining the results of XRD, visual

and optical characterization, water vapour is identified as the fatal factor degrading

the perovskite film while long-time (one month) exposure to strong UV can also

induce yellow phase (Table 2.5). Through the TGA and NMR measurement, there is

no water detected in the yellow phase CsPbI2Br degraded by moisture, indicating that

water vapour only initiates the phase transition (degradation) instead of incorporating

with the materials. The average PCE of fresh samples, aged samples and recovered

samples through re-annealing are 5.42% (best PCE: 9.08%), 0.09% and 5.42%,

respectively, demonstrating the high phase reversibility through re-annealing (Figure

2.21c).

Figure 2.21 (a) Photographs of perovskite films prepared at 100 °C using different

precursors. (b) EQE spectrum and integrated Jsc of a CsPbI2Br PSC.[124] (c) J-V


43

curves of as-grown, aged and recovered CsPbI2Br PSCs.[125]

Table 2.5 Summary of degradation tests for CsPbI2Br through XRD and optical

characterization.[125]

Figure 2.22 (a) PL spectra and (b) PL decay curves of CsPbI2Br film. (c) J-V curve of

CsPbI2Br PSCs.[126] (d) Mott–Schottky fitting to the CV data of CsPbI2Br. (e)

Energy-level diagram of 3D-2D-0D CsPbI2Br PSCs. (f) Top-view SEM image of 3D-

2D-0D CsPbI2Br perovskite film. The inset presents the water contact angle on the

film. (g) Long-term stability of 3D-2D-0D CsPbI2Br PSCs.[127] (h) Illustration of

bandgap alignment based on different ETLs. (i) J-V curve of CsPbI2Br PSCs.[128]


44

In order to enhance the PCE of PSCs, improving the charge carrier extraction

ability is an effective approach. Therefore, modifying the energy band alignment or

incorporating the high-quality perovskite film to a desirable transport layer are

adopted by many researcher. Through fabricating the CsPbI2Br perovskite film by

QDs precursor and accompanying to 5 nm HTL P3HT, a high-quality perovskite film

with grain size about 250 nm was established and surface defect states were

suppressed.[126] By adding the P3HT with annealing at 200 ºC, a blue shift of steady-

state PL peak reflects the reduction of the Stokes shift from 35 to 29 nm, implying the

passivation of surface traps, meanwhile, a fast quenching of PL lifetimes down to 0.89

ns indicates an improvement in hole extraction (Figure 2.22a,b). Therefore, the energy

loss between energy bandgap and open circuit voltage is reduced, resulting in a record

high Voc of 1.32 V. The best-performance PSC can obtain a PCE of 12.02% and a high

Voc of 1.30 V (Figure 2.22c). For the stability, the devices without encapsulation can

retain 90% of the initial PCE after storing for 960 hours under RH 15-40%. Utilizing

the low-dimension perovskite to establish a bulk-nanosheet-quantum dots (3D-2D-0D)

configuration can also promote the hole extraction due to an enhanced built-in electric

field.[127] Through the Mott-Schottky fitting to the capacitance-voltage

measurements, the 3D-2D-0D structure with a smooth energy alignment is identified

as the highest built-in potential comparing to other 3D, 3D-2D or 3D-0D structures,

resulting in a high Voc of 1.19 V (Figure 2.22d). Shortening the average PL lifetime

from 11.60 ns to 4.30 ns demonstrates the quenching effect and efficient hole

extraction of the multigraded structure with PTAA, therefore, a high PCE (12.39 %)

of PSC is obtained. Through the observation of water contact angle on the perovskite

film, the 3D-2D-0D structure clearly demonstrate a hydrophobic property to prevent

the serious degradation problem of moisture (Figure 2.22e). Consequently, the device

without encapsulation exhibits negligible degradation for 60 days in PCE under RH

25-35% and 25ºC, exhibiting a long-term stability (Figure 2.22f). Adopting ZnO@C60


45

bilayer HTL is another method to improve the hole extraction efficiency and reduce

current leakage.[128] By incorporating the ZnO@C60 bilayer to a 2-step annealed

CsPbI2Br perovskite film, the best device with better band alignment is able to

generate a high Jsc of 15.2 mA/cm2 and a high PCE of 13.3% (Figure 2.22 h,i)

2.3.4.2 Performance of Composition Engineering CsPbI2Br PSCs

Apart from solvent engineering and adjustment of the annealing process, the

quality of perovskite film can also be improved by doping other cations. Partially

doping 7.5% potassium (K) cation to the caesium cation in the A site can improve the

crystallization process to obtain a uniform and fully coverage film.[130] By

incorporating the perovskite films to the c-TiO2 layer (ETL), only the

Cs0.925K0.075PbI2Br film exhibits PL quenching, indicating that K-doped perovskite can

improve in electron extraction (Figure 2.23a,b). Thus, the K-doped devices were able

to enhance the average PCE from 8.2% to 9.1% with the best PCE of 10.0% and

improve the stability under ambient air with RH 20% (Figure 2.23c). On B site,

substituting a small amount of Pb by strontium (Sr) can reduce the charge

recombination due to surface passivation by enrichment of Sr (possibly Sr oxide) on

the film surface.[131] Through fabricating the perovskite film at a low temperature of

100 ºC and incorporating the optimized composition CsPb0.98Sr0.02I2Br to the HTL

P3HT, the PCE of the device can raise from 7.7% to 11.3% (Figure 2.23e).


46

Figure 2.23 PL decay curves of (a) CsPbI2Br and (b) Cs0.925K0.075PbI2Br film with and

without c-TiO2 layer. (c) Normalized PCE as a function of time for CsPbI2Br and

Cs0.925K0.075PbI2Br PSCs.[130] (d) Top-view SEM images of CsPbI2Br and CsPb1-

xSrxI2Br films. (e) PCE as a function of mole fraction of Sr.[131]

Liu’s group introduced a doping method of Mn2+ cations to CsPbI2Br through

interstitial instead of substituting to improve different optoelectronic parameters,

resulting in a higher PCE.[132] For morphology, by a series of characterization of

XRD, top-view SEM, Energy-dispersive X-ray (EDX), the additional 2% Mn2+ doping

showed a small negative shift of XRD peak at 14.6º due to slightly enlargement of the

lattice constant, displayed a larger grain size up to about 1200 nm, and revealed more

Mn2+ ion on the grain boundaries to passivate the surface (Figure 2.24a,b). For the

crystalline grain growth driven by Mn2+ cations, a small amount of Mn2+ cations would

insert into the perovskite, enlarge the lattice constant and slow down the perovskite

crystallization process to improve the crystallinity while the excess Mn2+ cations

would gather on the grain boundaries and passivate the surface. Through the analysis

of steady-state and time-resolved PL, dark I-V characterization, Mott-Schottky fitting

to the CV curves and electrochemical impedance spectroscopy (EIS), the 2% Mn-

doped perovskite film demonstrated better electron extraction due to PL quenching,


47

low trap state density, higher built-in potential and longer carrier recombination

lifetime, reflecting an enhanced photovoltaic effect (Figure 2.24c). By incorporating

to a thin CsPbI2Br QDs layer and a HTL of PTAA, the device can improve the PCE

from 11.88% (0% Mn2+) to 13.47% (2% Mn2+) with negligible hysteresis and retain

97% of initial PCE after storing for 35 days under RH 25-35% without encapsulation

(Figure 2.24d,e). By further optimized the PSCs, they reported that a heterojunction

PSC with configuration of FTO/c-TiO2/CsPbI2Br/Mn2+-doped CsPbI3 QDs/PTAA/Au

can achieve a high PCE of 14.45% (STO: 14.41%) while the STO of PSCs from bare

CsPbI2Br layer or bare Mn-doped CsPbI3 QDs layer are only 13.42% or 10.75%

respectively (Figure 2.25a,b).[133] Through the Mn2+ substitution and SCN- capping,

the CsPbI3 QDs become more stability. With formamidinium iodide and ethyl acetate

(FAI-EA) treatment and optimization with the thickness of 300 nm, the

CsPbI2Br/CsPbI3 QDs interface can achieve a gradual change in Br- and I- content for

better energy band alignment to extract carriers and the PSCs can obtain the highest

PCE by a trade-off of Jsc and FF (Figure 2.25c,d). Due to the use of CsPbI3, the stability

is influenced but the device without encapsulation can still remain 90% of the original

PCE for storing in dry oxygen for 20 hours.


48

Figure 2.24 (a) Grain size distribution histograms for 2% MnCl2-doped CsPbI2Br films.

(b) Atomic ratio of Mn/Pb and Mn/Cl as a function of depth from the surface of

MnCl2-doped CsPbBrI2 films. (c) Dark I−V measurements of the devices with VTFL

at the kink points. (d) J-V curve and (e) long-term stability of a 2% MnCl2-doped

CsPbI2Br device.[132]

Figure 2.25 (a) Energy-level alignment of a CsPbI2Br/CsPbI3 QDs PSC. (b) J-V curve

of device with the configuration in (a). (c) PCE, Voc, Jsc and FF of PSCs as a function

of thickness of CsPbI3 QDs layer. (d) Atomic ratio of I/Pb and Br/Pb as a function of

etching time (etching speed of 0.8 nm/s).[133]


49

2.4 Outlooks

Inorganic perovskite solar cells based on different structures and fabrication

methods are mainly reviewed. The inorganic PSCs have exhibited a large

improvement on the aspects of efficiency and stability. Some important difficulties

and problems are required to address to improve the performance and develop a better

understanding of the PSCs.

1. The main advantage of inorganic perovskite is the high thermal stability. They

can also exhibit long-term stability under the illuminated condition and dry

(RH about 20%) ambient air without encapsulation. However, except the

CsPbBr3, the other structures seem cannot sustain the PCE for a long time in

a humid environment due to the phase transition. In the future, enhancement

of the moisture stability will be a crucial factor for commercializing the

inorganic PSCs. To address this problem, similar to the works done before,

doping a small amount of materials, controlling the crystal size, and

introducing hydrophobic layers to the PSCs may be still the main solution.

2. The PCE of inorganic PSCs has shown a great enhancement to 16.14%.

However, the PCE of pure inorganic PSCs is limited by the short absorption

range due to at least 0.3 eV higher than the ideal bandgap (1.1-1.4 eV) for the

best single-junction PSCs. Widening the absorption range requires

modification on electron overlapping of orbitals, doping materials may be the

only known way based on the previous research. The efficiency can also be

improved by maximizing the open-circuit voltage (minimizing the voltage loss)

so improving the morphology and constructing a smoother energy band

alignment are some successful approaches in the past. Therefore, more efforts

are required for trying different doping materials and modifying the transport


50

layers.

3. The amounts of fabrication methods of inorganic perovskite films are still

fewer than that the hybrid organic-inorganic perovskite films. The fabrication

methods are the crucial factors influencing the crystallization and morphology

of the films. Therefore, more fabrication methods are encouraged to be

explored in the future.

4. Some previous studies reveal an unclear explanation of the characterization,

especially, the PL decay lifetime and resistance shown on the Nyquist plots of

the EIS, leading to confusion to the audiences. For example, some literature

use either long carrier lifetime or fast PL quenching in the time-resolved PL

spectrum and some papers use either small charge transfer resistance or large

recombination resistance in the EIS without stating the frequency range to

explain the results that they want. In order to achieve a comprehensive

understanding of the working principle and characterization of PSCs, more

efforts are required to study the mechanism of the optoelectronic behaviour of

PSCs.

2.5 Conclusion

The literature of inorganic caesium lead halide perovskite solar cells is

systematically reviewed. The CsPbX3 can be mainly categorise into CsPbBr3,

CsPbIBr2, CsPBI2Br and CsPBI3 based on their optoelectronic properties. Basically,

except CsPbI3, caesium lead halide PSCs possess higher stability than organic PSCs

under a dry environment. The degradation process of inorganic perovskite is mainly

identified as phase transition instead of the decomposition in organic perovskite.

Among different categories, CsPbBr3 has the highest stability under thermal, moisture,


51

oxygen and UV light while CsPbI3 can generate a relatively high PCE due to the lowest

bandgap. Until now, the highest PCE of CsPbBr3, CsPbIBr2, CsPBI2Br and CsPBI3

based solar cells are identified as 9.72%, 11.3%, 14.45% and 15.7%, respectively.


52

Low-Temperature Processing of

All-Inorganic Perovskite Solar Cells with

High Stability

3.1 Introduction

The perovskite solar cell is one of most potential types in 3rd generation solar cells

because of its rapid development. To date, the efficiency of perovskite solar cells has

enhanced to 22.7% which has already higher than some of the commercial silicon solar

cells. The materials used for energy-harvesting in highly-efficient PSCs are mainly

pure organic perovskite (MAPbI3 and FAPbI3) or hybrid organic-inorganic perovskite

because they possess suitable and direct bandgaps for light absorption in single-

junction solar cells.[27] However, as mentioned before, the organic perovskite have

poor stability under ambient air, leading to a high barrier to commercialization.

In order to overcome the above difficulty, inorganic perovskites PSCs have been

researched in recent year.[51, 58, 87, 120, 135] One of the promising materials is

caesium lead bromide that possesses long-term stability under humid and high-

temperature conditions.[58] The CsPbBr3 also owns some of the important parameters

for application in solar cells, for example, a high electron mobility about 1000 cm2/Vs

and long carrier lifetimes up to 2.5 µs.[65] CsPbBr3 was commonly applied to

illuminated devices such as LED and lasing because of its strong

photoluminescence.[63, 136-138] Recently, employing CsPbBr3 into solar cells were

widely investigated but the crystallization of CsPbBr3 thin films used in PSCs usually

needs a high annealing temperature about 250 ℃ (Table 3.1), which will cause a high

production cost. Furthermore, high-temperature processing will limit the choices of


53

materials for each layer in the devices and reduce the possibility for flexible devices.

In this study, we have demonstrated using a pyridine (Py) vapour treatment in

two-step solution fabrication of CsPbBr3 films to reduce the high annealing

temperature to 160 ºC. The pyridine-treated PSCs annealed at 160 ºC can achieve a

PCE of 6.05% which is comparable with that of normal PSCs annealed at 250 ºC. For

the mechanism, by treating the PbBr2 film with pyridine vapour, PbBr2.(Py)x

intermediate states are formed to reduce the thermal activation energy for

crystallization of CsPbBr3 perovskite films. This research offers a new way for the

low-temperature processing of inorganic PSCs.

3.2 Experimental Section

Materials: All the materials used are listed below: deionized water (DI water),

acetone, isopropanol (IPA), ethanol (99.9%), hydrochloric acid (HCl, 37%), titanium

isopropoxide (TTIP, 97%), 2-butanol (99%), titania paste (30 NR-D), titanium

tetrachloride (TiCl4, 99%), lead bromide (PbBr2, 99.999%), N,N-dimethylformamide

(DMF, anhydrous, 99.9%), pyridine (Py, 99.8%), cesium bromide (CsBr, 99.999%),

methanol (anhydrous, 99.5%), chlorobenzene (CB, anhydrous, 99.8%), spiro-

OMeTAD, tert-butylpyridine (TBP, 96%), acetonitrile (ACN, anhydrous, 99.8%),

lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI, 99.95%), and gold (Au).

Devices fabrication: The commercial fluorine-doped tin oxide (FTO) glass was

first cleaned using detergent, DI water, acetone, IPA and DI water in a sonicator for

10 minutes per each solution step by step, followed by drying in an oven at 80 °C.

Then the cleaned FTO was exposed to an O2 plasma for 5 minutes so that the FTO

glass can be further cleaned by removing the organic residue. Meanwhile, the surface

energy and wettability of the FTO surface is improved so that, in the deposition step,


54

the solution can be spread over the whole surface easily and the film adhesion is

improved. The precursor of compact titanium dioxide film (c-TiO2) ETL was prepared

by mixing 4 ml of ethanol, 182 μl of TTIP and 26 μl of HCl. After stirring the solution

overnight, the precursor was spin-coated on the FTO glass at 4000 rpm for 30 s and

annealed at 500 °C for 1 hour for oxidation. In order to enhance the contact area

between the TiO2 layer and the perovskite layer for efficient charge extraction, the

mesoporous titanium dioxide (m-TiO2) layer was used for surface modification.[139]

The precursor of m-TiO2 was prepared by mixing TiO2 paste and 2-butanol solution

in a weight ratio of 1:7, and followed by a dispersion process in the sonicator and on

the vortex shaker. The dispersed solution was spin-coated on the c-TiO2 substrate at

4000 rpm for 30 s and annealed at 500 °C for 1 hour. To further improve the charge

extraction by reducing the charge recombination at the TiO2/perovskite interface, the

TiO2 substrate was undergone a TiCl4 treatment. The 0.07 M TiCl4 aqueous solution

is carefully prepared by pouring a concentrated TiCl4 solution to the DI water slowly

to avoid precipitation by a fast oxidation. The treatment was conducted by immersing

the TiO2 substrate into the 0.07 M TiCl4 solution at 70 °C for 30 minutes, cleaning the

residue using DI water and IPA, and annealing at 500 °C for another 1 hour.

As shown in Figure 3.1, the perovskite films were fabricated using the 2-step

solution process. For the 1st step, the precursor of 1.0 M PbBr2 in DMF (70 °C

preheating for 30 minutes before use) was spin-coated on the 75 °C preheated m-TiO2

substrate at 2000 rpm for 30 s inside the N2 glovebox. For the normal devices, the

PbBr2 film was annealed at 70 °C for 30 min. For pyridine treatment, the fresh spin-

coated PbBr2 substrate was covered by a petri dish and 10 µl Py was added at the edge

of the petri dish at 70 °C for 5 min. For the 2nd step, the PbBr2 or the PbBr2.(Py)x

substrates were immersed in the precursor of 0.07 M CsBr in methanol at 70 °C for 10

min, rinsed using IPA, spin-dried at 2000 rpm for 30 s and annealed at x °C (where x=

100, 130, 160, 190, 220, 250 or 280) for 10 minutes inside fume cupboard. Then the


55

CsPbBr3 and pyridine-treated CsPbBr3 perovskite films were established. The spiro-

OMeTAD precursor (1 ml CB, 75 mg spiro-OMeTAD, 25.5 µl TBP and 15.5 µl mixed

solution (520 mg/ml Li-TFSI in ACN)) was spin-coated on the perovskite film at 4000

rpm for 30 s inside the glovebox. In order to improve the hole mobility of Li-TFSI

doped spiro-OMeTAD HTL film and FF of the devices, the substrates were transferred

to a dry box with negligible relative humidity for 10-hour oxidation.[140, 141] Finally,

a gold electrode of about 60 nm was deposited on the top of the HTL by thermal

evaporation.

Figure 3.1 Illustration of the 2-step fabrication process of perovskite layers with and

without pyridine treatment.

Characterization: The UV-visible absorption spectra were characterized using a

spectrophotometer (UV-2550, Shimadzu, Japan) under RH about 60%. The current-

voltage curves were measured using Keithley 2400 source meter with a scan rate of

0.1 V/s using an Oriel solar simulator (Newport: 91160) with AM 1.5 filter at 100

mW/cm2 light intensity under RH about 60%. The light intensity was calibrated using

a standard silicon solar cell. The external quantum efficiencies (EQEs) were recorded

by a system of cornerstone 260 1/4m monochromator (Newport: 74125) and research

arc lamp housing (Newport: 66902) under RH about 60%. The composition and

crystal orientation of CsPbBr3 perovskite layer were characterized using an X-ray

Diffractometer (XRD) (Rigaku SmartLab) under RH about 30%. The perovskite

morphology was characterized using a field emission scanning emission microscopy


56

with a model of JEOL JSM 6335F. The thermogravimetric analysis was examined

using thermogravimetric analyser (Mettler Toledo TGA/DSC3+) by putting the

perovskite powders on an alumina crucible for a constant heating rate of 20 ºC/min

under a steady nitrogen gas flow.

3.3 Results and Discussion

3.3.1 Optimization of the Normal CsPbBr3 PSCs

Before applying the pyridine treatment on the CsPbBr3 devices, some techniques

of two-step fabrication for perovskite films are first studied and optimized. The

morphology of the perovskite film largely depends on the quality of the first-step

PbBr2 film because the growth of the perovskite follows the framework of the PbBr2

film. Due to the low solubility of bromide ions, before spin-coating PbBr2 solution,

the precursor is required to heat at 70°C for 30 minutes for complete dissolution.

Dropping the hot precursor on a cold substrate may also induce precipitation and rough

surface which would later influence the growth of perovskite. Therefore, PSCs with

and without preheating the substrate was first studied for optimization. As shown in

Figure 3.2a, the device with substrate preheated can achieve a higher PCE (4.06%)

comparing to that without a preheated substrate (3.88%). The PCE result is consistent

with the prediction and agrees with the previous study that a smooth film was obtained

by preheating the substrate.[142] In the 2nd step of fabrication, after 10-minutes

immersion of the substrate on the CsBr solution, the reacted CsPbBr3 substrate is

required to remove from the CsBr solution quickly or using a uniform speed because

the surface tension of CsBr solution can destroy the morphology of perovskite film

disastrously. After withdrawing the substrate from CsBr solution and washing by IPA,

the drying time can also affect the morphology of the film. Although the film was


57

washed by IPA, a small amount of CsBr salts would still remain in the solution on film

due to miscibility of methanol and IPA. As a result, the crystallization of CsBr on the

film can hamper the hole extraction. To address this problem, we introduce a spin-

drying to quickly remove the solution on the film. Figure 3.2b reveals that the devices

using spin-drying can obtain a higher efficiency.

Figure 3.2 J-V curves of CsPbBr3 PCSs (a) with and without a preheated substrate,

and (b) prepared by different drying methods.

In addition to modifying the technical problems of the fabrication process, the

spin-coating speed for the PbBr2 film is also optimized. Based on the previous

literature, the spin-coating speed that commonly used is between 1500 rpm and 2500

rpm so the speed within this range was used to analyze the device performance. As

shown in Figure 3.3a, the device prepared by 2500 rpm shows a lower Jsc than that of

the others because photons absorption are limited by the thin perovskite film. The

devices prepared by 1500 rpm and 2000 rpm revealed similar PCEs of about 5.2%,

indicating that the thickness of device prepared by 2000 rpm is thick enough to absorb

light. A faster spin-coating speed can deposit a smoother film, therefore, the speed of

2000 rpm was used to fabricate the PbBr2 film for the rest of the study. By further

optimization and familiarity with the techniques, Figure 3.3b shows that the CsPbBr3

PSCs can achieve the best PCE of 6.44% and an average PCE of 5.29%.


58

Figure 3.3 J-V curves of (a) PSCs with different spin-coating speed for PbBr2 film and

(b) the champion PSC.

3.3.2 Effect of the Pyridine Treatment on CsPbBr3 PSCs

The study of pyridine treatment on CsPbBr3 devices is based on the optimized

condition of the normal CsPbBr3 devices. Figure 3.4a shows the image of a CsPbBr3

perovskite film which is yellow in colour and the image of a CsPbBr3 device prepared

by solution process. As shown in Figure 3.4b, the schematic diagram displays

inorganic perovskite solar cells with a configuration of FTO/c-TiO2/m-TiO2/

CsPbBr3/spiro-OMeTAD/Au. The energy band structure of the devices shown in

Figure 3.4c reveals a suitable band alignment for electron and hole extracting to FTO

and gold, respectively.[58] The perovskite films were fabricated by a two-step solution

process. The first step was spin-coating a PbBr2 film on FTO/TiO2 substrate, followed

by a 5-minutes pyridine treatment at 70 ºC inside a petri dish. In the second step, the

treated substrates were immersed into a CsBr precursor for 10 minutes and annealed

at different temperatures for 10 minutes. Figure 3.4d shows the UV-visible spectra of

both lead bromide films and perovskite films prepared by normal process and pyridine

treatment. Both the perovskite films reveal a similar absorbance with sharp absorption

peaks located at a wavelength of about 520 nm which is commonly appearing in

CsPbBr3 perovskite material.[58, 61, 70]


59

Figure 3.4 (a) Photographs of a CsPbBr3 film and a device. (b) Schematic diagram and

(c) energy-level diagram of inorganic CsPbBr3 PSCs with the configuration of FTO/c-

TiO2/c-TiO2/m-TiO2/CsPbBr3/spiro-OMeTAD/Au. (d) UV-visible spectra of PbBr2

film, PbBr2.(Py)x film and the optimized CsPbBr3 films made from PbBr2 and

PbBr2.(Py)x.

In order to investigate the effect of temperature on the perovskite solar cells, both

perovskite PSCs with and without pyridine treatment are annealed at different

temperatures. Figure 3.5a displays the J-V curves of pyridine-treated PSCs fabricated

at different annealing temperatures. Notably, we can obtain a PCE of 4.19% for the

pyridine-treated device when the annealing temperature is only 100 C. The optimum

device was prepared at 160 C, achieving a PCE of 6.05% with a high open circuit

voltage (Voc) of 1.34 V, a short circuit current (Jsc) of 6.52 mA/cm2 and a fill factor

(FF) of 69%. Figure 3.5c can clearly reveal the effect of annealing temperature on the

average PCEs of the pyridine-treated devices. The PCE of the devices increases with

annealing temperature and saturates at the temperature of 160 C. As shown in Figure


60

3.5d, the external quantum efficiency (EQE) displays the photon-to-electron

conversion ability of a pyridine-treated PSC prepared at the optimum annealing

temperature of 160 C. The EQE spectrum is about 70% from 350 nm to 510 nm with

an integrated short-circuit current density of 5.92 mA/cm2, which is acceptable within

10% error to the Jsc obtained from the J-V curve.[28, 143]

As shown in Figure 3.5b,c, by comparing to the normal two-step fabrication

method without a pyridine treatment, the control devices show a much lower PCE

(2.5%) at 160 C and even exhibits a PCE of 0% at 100 C. Their PCEs increase

monotonically with the annealing temperature and are optimized at 250 C, which is

consistent with the results reported before.[51] The PCE quickly declines to lower than

4% below 250 ℃ due to incomplete crystallization of the perovskite films at lower

temperatures. The significant differences in PCEs prove that the pyridine treatment is

able to reduce the annealing temperature of CsPbBr3 perovskite films. Table 3.1

provides a further comparison of our devices with previous works reported by different

groups in the aspects of annealing temperature and photovoltaic parameters. Most of

devices reported were prepared at 250 ℃, while one device prepared at 70 ℃ obtained

a high Jsc of 11.34 mA/cm2. As discussed in pages 19-20, it was strange that the Jsc of

11.34 mA/cm2 is higher than the ideal value. This work provides a new way to reduce

the annealing temperature down to 160 ℃ with acceptable efficiencies for the pure

CsPbBr3 devices. Furthermore, as shown in Figure 3.5e, the devices without

encapsulation can still retain 70% of the PCE for more than 100 days storing under

RH 40% - 70% at room temperature in ambient air, exhibiting that the pyridine-treated

PSCs also possess long-term stability. This result also supports the highly-stable

properties of CsPbBr3 perovskite.


61

Figure 3.5 J-V curves of (a) pyridine-treated and (b) normal PSCs annealed at different

temperatures. (c) The PCEs of PSCs with or without pyridine treatments as functions

of annealing temperatures. (e) EQE and the integrated current density of a pyridine-

treated PSC annealed at 160 ℃. (e) Long-term stability of the pyridine-treated

CsPbBr3 PSCs annealed at 160 ℃ storing under RH 40% - 70%.

Table 3.1 Summary of annealing temperature and photovoltaic parameters of

published CsPbBr3 PSCs from each research group.

Configuration

Temperature

(℃)

Voc

(V)

Jsc

(mA/cm2)

FF

(%)

PCE

(%)

Ref.

FTO/m-TiO2/CsPbBr3 (Py-treated)/spiro/Au 160 1.34 6.52 69 6.05 This work

Control 250 1.34 6.85 70 6.44 This work

FTO/m-TiO2/CsPbBr3/PTAA/Au 250 1.25 6.7 73 6.2 [40]

FTO/m-TiO2/CsPbBr3/carbon 250 1.29 5.7 68 5.0 [67]


FTO/m-TiO2/CQD/CsPbBr3 IO/spiro/Ag 70 1.06 11.34 69 8.29 [57]

FTO/c-TiO2/CsPbBr3/spiro/Au 225 1.42 7.01 53 5.6 [69]

FTO/m-TiO2/CsPbBr3/CuPc/carbon 250 1.26 6.62 74 6.21 [70]

FTO/m-TiO2/GQDs/CsPbBr3/carbon 250 1.46 8.12 82 9.72 [59]

FTO/c-TiO2/CsPbBr3/carbon 250 1.34 6.46 68 5.86 [72]



62

Figure 3.6 Cross-sectional SEM images of pyridine-treated CsPbBr3 films annealed at

(a) 160 ℃ and (b) 250 ℃, respectively, and the SEM images of normal CsPbBr3 films


In order to understand the mechanism of the pyridine-assisted method, the

morphology of perovskite films is investigated using the scanning electron microscope

(SEM). As shown in Figure 3.6a-d, the cross-sectional SEM images reveal the

thickness of each layer of the devices where the perovskite films are about 380 nm.

The thickness of c-TiO2/m-TiO2 is only 140 nm which is relatively thinner than some

of the previous literature.[144, 145] The function of m-TiO2 here is to use to improve

the contact with perovskite instead of being a thick scaffold to support the perovskite

film.[139] Some previous studies with high PCE confirmed that a thin m-TiO2 layer

can also result in a good performance.[13, 26] The thickness of spiro-OMeTAD is

about 190 nm so it is thick enough to fully cover the perovskite films instead of the

control device at 160 °C. We expect that the problem of low coverage rate may occur

in the other normal devices that prepared at low temperature, therefore the PCEs of

them are low.


63

As shown in Figure 3.7a,b, for the pyridine-treated films, the average grain size

of the CsPbBr3 perovskite film annealed at 160 ℃ is around 450 nm, which is as large

as the one annealed at 250 ℃. The similar grain size of the two samples is consistent

with the similar PCEs achieved in the PSCs with pyridine treatment annealed at the

two temperatures. For the samples without the pyridine treatment, as shown in Figure

3.7c,d, although the average grain size of the perovskite film annealed at 250 ℃ is

also around 450 nm, the grains of film annealed at 160 ℃ can only grow to about 200

nm. This result is coincident with the device performance in J-V curves, where a much

lower PCE was measured at 160 ℃ for PSCs without the pyridine treatment. Normally,

a photoactive film with a larger grain size reflects a lower density of traps for charge

recombination and more efficient carrier transfer in the devices.[146-148] Based on

the characterization by SEM, pyridine treatment was found to facilitate the growth of

CsPbBr3 crystal at a low temperature.

Figure 3.7 Top-view SEM images of pyridine-treated CsPbBr3 films annealed at (a)

160 ℃ and (b) 250 ℃, respectively, and the SEM images of normal CsPbBr3 films



64

In order to further study the function of pyridine in perovskite films, the films

prepared at different steps were characterized using X-ray diffraction (XRD) patterns

to analyze their crystal structure. As shown in Figure 3.8, the pure PbBr2 film prepared

at the first step which exhibits the diffraction peaks from 17º to 37º is identified as the

orthorhombic phase. For the pyridine-treated PbBr2 film, the XRD pattern obviously

changes into only two dominant peaks appeared at low angles of 10º - 12º, indicating

the incorporation of pyridine into the PbBr2 structure and the existence of an

intermediate phase of PbBr2.(Py)x.[149-151] The diffraction patterns of perovskite

films with or without a pyridine treatment all match with the monoclinic phase of

CsPbBr3 (PDF#18-0364 with a, b, c are 5.827 Å, 5.827 Å, 5.891 Å and α, β, γ are 90º,

90º, 89.65º). For the perovskite films without the treatment, two dominant peaks

appear at 15.2º and 30.7º which are corresponding to the planes of (100) and (200),

respectively. For the films with the pyridine treatment, the dominant peaks change to

21.6º and 37.8º which are corresponding to the planes of (110) and (211), respectively.

This result implies that the pyridine treatment can change the growth orientation of

grains in CsPbBr3 films at the 2nd fabrication step. There are some small diffraction

peaks appear on the XRD patterns of the normal and pyridine-treated films prepared

at different temperatures. These small peaks are identified as the composition of

CsPb2Br5 and Cs4PbBr6. Some studies suggested that a small amount of CsPb2Br5 and

Cs4PbBr6 can passivate the CsPbBr3 surface and reduce the carrier recombination in

the PSCs.[152, 153]


65

Figure 3.8 XRD patterns of different films: PbBr2; PbBr2.(Py)x; normal CsPbBr3 films

annealed at 160 °C and 250 °C; pyridine-treated CsPbBr3 films annealed at 160 °C

and 250 °C. [ □ : monoclinic phase of CsPbBr3 (PDF#18-0364), ^ : orthorhombic

phase of phase PbBr2 (PDF#84-1181), # : tetragonal phase of CsPb2Br5 (PDF#25-

0211), and * : rhombohedral phase of Cs4PbBr6 (PDF#73-2478).]

For the mechanism of pyridine treatment, reduction in annealing temperature

stems from that a lower thermal activation energy is required for the conversion from

the precursor PbBr2 + CsBr to the CsPbBr3 perovskite phase. Through the pyridine-

vapour treatment, pyridine can act as ligands to react with the PbBr2 film and form an

intermediate phase PbBr2.(Py)x with following reactions:

𝑃𝑏𝐵𝑟2(𝑠) + 𝑥𝑃𝑦(𝑔) → 𝑃𝑏𝐵𝑟2 ∙ (𝑃𝑦)𝑥(s) (1)

𝑃𝑏𝐵𝑟2 ∙ (𝑃𝑦)𝑥(s) + 𝐶𝑠𝐵𝑟(𝑎𝑞) → 𝐶𝑠𝑃𝑏𝐵𝑟3(𝑠) + 𝑥𝑃𝑦(aq) (2)

Previous research has verified that lead bromide is able to form adducts with

pyridine, as given by Equation (1).[154] In the 2nd step of fabrication, the intermediate

phase PbBr2.(Py)x can facilitate the formation of CsPbBr3 perovskite by lowering the

annealing temperature. When immersing the PbBr2.(Py)x substrates into the CsBr

precursor, CsBr can substitute pyridine molecules to form CsPbBr3, as given by

Equation (2). To confirm all the pyridine is removed from the perovskite film, we have

conducted the thermogravimetric analysis. As expected and shown in Figure 3.9a, no


66

weight loss for the pyridine-treated perovskite samples until 550 ºC since the pyridine

with low boiling (about 115 ºC) will all evaporate from the perovskite films at

optimized annealing temperature at 160 ºC. In thermodynamics, activation energy is

used to indicate the energy required for a phase transition or a chemical reaction. As

illustrated in Figure 3.9b, we, therefore, consider that the reduction in the annealing

temperature of CsPbBr3 perovskite films with the pyridine treatment is attributed to a

new reaction pathway with a lower activation energy. The mechanism of the pyridine

treatment is in agreement with the effect of pyridine used to prepare the normal organic

perovskite films at low temperatures.[150, 151]

Figure 3.9 (a) TGA of pyridine-treated CsPbBr3 perovskite prepared at 160℃. (b)

Reaction coordinate diagram with supposed mechanisms for the formation of CsPbBr3

perovskite from PbBr2 or PbBr2.(Py)x. EA1, EA2a and EA2b are the activation energies

for the reactions.

3.4 Conclusion

In conclusion, we have optimized the normal CsPbBr3 perovskite film based on

the technical issues. More importantly, we have demonstrated that pyridine-vapour

treatment can reduce the annealing temperature of inorganic perovskite CsPbBr3 in a

2-step fabrication process. The mechanism is attributed to the generation of the

intermediate phase that can reduce the thermal activation energy for the formation of

the perovskite phase through an alternative reaction pathway. The low-temperature-


67

processed PSCs without encapsulation demonstrate a long-term stability in ambient

air, indicating its great potential for practical applications. This work also provides a

novel approach to prepare other inorganic PSCs at reduced temperatures.


68

Efficiency and Stability

Enhancement by using Copper(I)

Thiocyanate as a Hole Transport Layer on

All-Inorganic Perovskite Solar Cells.

4.1 Introduction

Inorganic caesium lead halide perovskite solar cells have become a popular topic

of research in solar cells. According to the Shockley-Queisser (SQ) limit, the ideal

bandgap of photoactive materials for solar power conversion is between 1.1 to 1.4 eV

which allows a PCE of about 33% in maximum.[54] That limit is a balance between

the amount of photon absorption and the voltage generation for a single-junction

photoactive layer. For the caesium lead halide PSCs, the iodide-based perovskite

possesses the lowest Eg about 1.73 eV which allows a short-circuit current density of

22 mA/cm2 in maximum.[102] Logically, the CsPbI3 is the most suitable caesium-

based material for power conversion, however, the poor stability of CsPbI3 affect its

own performance. Without controlling the crystal size, it is hard for the black

perovskite phase to sustain in room temperature.[93, 100] Although confining the

crystal size down to nanoscale can maintain the perovskite phase in room temperature,

it will still convert to an orthorhombic non-perovskite phase rapidly when exposed to

a medium humidity. The ultra-sensitive property of CsPbI3 may stem from

geometrically unstable crystal structure due to low Goldschmidt's tolerance factor,

causing an intrinsic barrier for commercialization.[39]

The caesium lead bromide is the first inorganic caesium-based perovskite material


69

being utilized as a photoactive layer to harvest solar energy because of its high stability

in the ambient air.[51] The CsPbBr3 based PSCs can show negligible degradation in

performance after storing under a high relative humidity of 90-95% at 25 °C and

100 °C for over three months and one month, respectively, exhibiting the high

tolerance against moisture and high temperature.[58] However, the efficiency and

short-circuit current density of bromide-based PSCs are low only 9.72% and 8.12

mA/cm2 for the best one.[59] The high optical energy bandgap (Eg ~2.3 eV) of

CsPbBr3 limits the absorption range of light. According to the AM 1.5 solar spectrum,

as shown in Figure 2.1a, the top 10% of photon flux is in between 550 nm to 810 nm.

The highest current density can be generated if the photoactive layer can absorb the

light in this range. The CsPbBr3 perovskite which possesses a wide bandgap wastes

the valuable solar spectrum for wavelength beyond 540 nm, resulting in a low short-

circuit current density of 9 mA/cm2 and a low PCE.

The additive-doped spiro-OMeTAD is a general organic hole transport layer for

the conventional solar cells because it is thick enough to tolerate the perovskite film

with different roughness. However, spiro-OMeTAD can also cause problems on the

PSCs in different aspects. Due to the intrinsically low hole mobility of spiro-OMeTAD

(4x10-5 cm2V-1s-1), it is necessary to dope some additive to improve the mobility and

ensure a strong charge extraction.[155] The doping process in precursor includes some

of the acetonitrile which can slightly damage the perovskite film. A half-day oxidation

process of the doped spiro-OMeTAD can also expose the perovskite to risk and is

time-consuming. The low thermal stability of organic property seriously influences

the long-term stability of the solar cells.[156, 157] Also, spiro-OMeTAD is a very

expensive material (~360 USD/g in Sigma Aldrich), leading to a big barrier on

commercialization.

By tuning the ratio of iodide to bromide to 2:1, the CsPbI2Br PSC is a promising


70

structure for the PSCs due to a relative balance between efficiency and stability.

Comparing to CsPbBr3, CsPbI2Br can extend the absorption wavelength to 650 nm

which is in range of top 10% photon flux and generate higher short-circuit current

density when employing to PSCs. By using the inorganic copper(I) thiocyanate

(CuSCN) as hole transport layer, the problems in spiro-OMeTAD are mainly solved

because CuSCN intrinsically possesses a high hole mobility 0.01-0.1 cm2V-1s-1,[155]

a high thermal stability[158] and a low price (~1.8 USD/g in Sigma Aldrich).

In this chapter, we first discuss the stability issue of CsPbI2Br perovskite film and

investigate the film quality at low temperature and high temperature. After that, we

demonstrate that the CsPbI2Br PSCs incorporating with a HTL of either spiro-

OMeTAD or CuSCN can produce a relatively high short-circuit current density of 14.1

mA/cm2. This is the first time employing CuSCN to the inorganic PSCs, exploring the

possibility of a new structure FTO/c-TiO2/m-TiO2/CsPbI2Br/CuSCN/Au of all-

inorganic PSCs.

4.2 Experimental Section

Materials: All the materials used are listed below. Deionized water (DI water),

acetone, isopropanol (IPA), ethanol (99.9%), hydrochloric acid (HCl, 37%), titanium

isopropoxide (TTIP, 97%), 2-butanol (99%), titania paste (30 NR-D), titanium

tetrachloride (TiCl4, 99%), Lead bromide (PbBr2, 99.999%), Lead iodide (PbI2, 99%),

Dimethyl sulfoxide (DMSO, anhydrous, 99.8%), N,N-dimethylformamide (DMF,

anhydrous, 99.9%), cesium iodide (CsI, 99.9%), chlorobenzene (CB, anhydrous,

99.8%), spiro-OMeTAD, tert-butylpyridine (TBP, 96%), acetonitrile (ACN,

anhydrous, 99.8%), lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI, 99.95%),

Copper(I) thiocyanate (CuSCN, 99%), diethyl sulphide (98%) and gold (Au).


71

Device Fabrication: The fabrication steps including preparing a cleaned FTO,

depositing electron transport layers of c-TiO2/m-TiO2 and a hole transport layer of

spiro-OMeTAD were the same as the previous work in Chapter 3. The main

differences are in the fabrication of perovskite films and another hole transport layer

of CuSCN. The CsPbI2Br perovskite precursor was prepared by dissolving 161.4 mg

PbI2, 128.5 mg PbBr2 and 181.9 mg CsI in 1 ml solvent of DMF and DMSO (volume

ratio = 2:1), stirring at 70 °C for 1 hour and then using 0.45 μm filter to remove the

impurity. Here, the perovskite precursor was prepared by 0.5 PbI2+0.5 PbBr2+1.0 CsI

instead of 1.0 PbI2+1.0 CsBr because the CsBr cannot dissolve totally in the 0.7 M

solution. The CsPbI2Br precursor was spin-coated on the TiO2 substrate at first 1000

rpm for 10 s to confirm a certain thickness and immediately followed by 3000 rpm for

30 s to achieve a smooth surface inside the nitrogen (N2) glovebox. Before annealing

at 100 °C and 250 °C, the spin-coated films were placed at room temperature under

N2 gas for x minutes (where x = 10, 20, 30, 40, 50, 60, 70, or 80). The solvents (DMF

and DMSO) would evaporate slowly and the CsPbI2Br film changed to dark gradually

due to a slow crystallization process so that a high-quality film can be obtained. The

precursor of CuSCN was prepared by dissolving 35 mg CuSCN in l ml diethyl

sulphide and then stirring at room temperature for 1 hour. The CuSCN film was

deposited by spin-coating the precursor at 4000 rpm for 30 s and annealing at 90 °C

for 5 minutes. An about 100 nm gold layer was thermally deposited on the top of the

HTL. Finally, the devices were encapsulated with a piece of glass and Araldite epoxy

AB glue.

Characterization: The UV-visible absorption spectra were characterized using a

spectrophotometer (UV-2550, Shimadzu, Japan) under RH about 60%. The current-

voltage curves were measured using Keithley 2400 source meter with a scan rate of

0.1 V/s using an Oriel solar simulator (Newport: 91160) with AM 1.5 filter at 100

mW/cm2 light intensity under RH about 60%. The light intensity was calibrated using


72

a standard silicon solar cell. The external quantum efficiencies (EQEs) were recorded

by a system of cornerstone 260 1/4m monochromator (Newport: 74125) and research

arc lamp housing (Newport: 66902) under RH about 60%. The composition and

crystal orientation of CsPbBr3 perovskite layer were characterized using an X-ray

Diffractometer (XRD) (Rigaku SmartLab) under RH about 30%. The perovskite

morphology was characterized using a field emission scanning emission microscopy

with a model of JEOL JSM 6335F.

4.3 Results and Discussion

Figure 4.1a shows the photographs of the same CsPbI2Br film after fresh

preparation, degradation and re-annealing at 250 °C under the ambient air. Although

the reannealed film shows slightly rougher than the fresh film, their colours are almost

the same, implying that the reannealed film is also in the perovskite phase. The result

is consistent with the previous literature that the degradation of caesium lead halide is

a phase transition from γ orthorhombic phase (Pbnm) to δ orthorhombic phase (Pnma)

instead of chemical decomposition.[85, 91, 115, 125] In order to identify the

absorption ability of CsPbI2Br film, the UV-visible spectrum was first used for

characterization. Figure 4.1b shows the absorbance of a freshly prepared CsPbI2Br

perovskite film and the degraded film. The perovskite film reveals an absorption onset

at about 650 nm and an absorption peak at about 625 nm while the degraded film

shows an absorption peak at about 400 nm. Since CsPbI2Br perovskite is a direct-

bandgap material, as shown in Figure 4.1c, the optical bandgap can be determined

using a Tauc plot by setting the y-axis as (αhν)2 where α is the absorption coefficient,

h is the Planck constant, ν is the frequency of light.[159-161] The absorption

coefficient, α can be roughly calculated from the absorbance and thickness of

perovskite film with the following equation:


73

𝑎𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒 = 𝑙𝑜𝑔Φ𝑒𝑖

Φ𝑒𝑡 =

1

𝑙𝑛10𝑙𝑛

𝐼𝑜

𝐼𝑡=

𝛼𝑡

2.303 (4.1)

where Φ𝑒𝑖 , Φ𝑒

𝑡 , 𝐼𝑜, 𝐼𝑡, and t are incident radiant flux, transmitted radiant flux, incident

light intensity, the light intensity at the depth t and the thickness of the perovskite film.

The bandgap of CsPbI2Br perovskite film is identified as about 1.9 eV. As shown in

Figure 4.1d, the time-dependent absorbance of a fresh perovskite film decays very

quickly in 60 s under RH about 60%, indicating a fast degradation process of the

perovskite film. This fast degradation can explain the appearance of a peak at 400 nm

in the UV-visible spectrum for the freshly prepared perovskite film.

Figure 4.1 (a) Photographs of CsPbI2Br film freshly prepared, degraded and

reannealed. (b) UV-visible spectrum of a freshly prepared perovskite film and the

degraded film. (c) Tauc plot of CsPbI2Br perovskite film. (d) Absorbance over time of

CsPbI2Br film measured at 625 nm.

For one-step solution process of perovskite film, we found that the freshly spin-

coated film can gradually change to dark colour over time without any annealing inside


74

the nitrogen glovebox. This phenomenon may demonstrate that the dark CsPbI2Br

perovskite phase is partially preferred and can be stable at room temperature during

the evaporation of the precursor solvent and crystallization of film after spin-coating.

The dark films are subsequently annealed to increase the crystallinity. The common

annealing temperature of the CsPbI2Br film varies from 100 °C to 280 °C among the

different published papers.[122, 124, 127] In order to observe the CsPbI2Br perovskite

film quality prepared at different temperatures, scanning electron microscopy was

used to analyze the morphology. Figure 4.2a,b reveals the cross-sectional SEM images

of structure FTO/c-TiO2/m-TiO2/CsPbI2Br where the thickness of the CsPbI2Br film

and TiO2 film are about 250 nm and 100 nm, respectively. The thickness of FTO for

the SEM image at 100 °C is large than that at 250 °C simply due to a deviation of the

commercial products.

Figure 4.2 Cross-sectional SEM images of structure of FTO/TiO2/CsPbI2Br annealed

at (a) 100 °C and (b) 250 °C.


75

Figure 4.3 Top-view SEM images (a) of m-TiO2 layer, (b,c) with low magnification

and (d,e) with high magnification at dark region and white region for CsPbI2Br film

prepared at 250 °C, (f) with low magnification and (g) with high magnification for

CsPbI2Br film prepared at 100 °C.

Figure 4.3b-g shows top-view SEM images of the CsPbI2Br film deposited on the

m-TiO2 layer with different magnification. Before settling the SEM samples inside the

SEM evacuated chamber, the samples are unavoidably exposed to the humid ambient

air (RH 60%) for about one minute. As the fast degradation mentioned in the UV-

visible spectrum, partially degraded perovskite films are clearly observed on the low-

magnification SEM images. As shown in Figure 4.3b,f, the dark regions are the

perovskite phase while the white regions are the degraded phase which is possibly an

orthorhombic non-perovskite phase (Pnma). It is obvious that the film annealed at

250 °C displays much more dark area, indicating that a high-temperature annealing is

able to produce a more stable perovskite film. The variation in the stability is attributed

to the grain size of the film. As shown in Figure 4.3c,e, in the centre of the white circle

region, the film starts degradation by generating cracks along the grain boundaries


76

(GBs). A film with a larger average grain size can reduce the amounts of the grain

boundaries. It is clear that the grain size of the film prepared at 250 °C (about 700 nm)

is much higher than that prepared at 100 °C (about 200 nm) because the high

temperature can assist the grain growth. For the film prepared at 100 °C as shown in

Figure 4.3g, many pinholes are generated on the GBs and inside the grains on the white

region, also leading to deterioration in degradation. The appearance of cracks and

pinholes can enhance the diffusion of water molecules into the film, further accelerate

the degradation process. Therefore, using a relatively high temperature for annealing

can stabilize the perovskite film.

To further study the temperature influence on the CsPbI2Br films, X-ray

diffraction patterns were characterized to analyze the composition and crystallinity.

The relative humidity on the measurement environment of XRD is low about 30%,

therefore the perovskite films are stable enough for the characterization. Previous

literature has reported that the degradation rate of CsPb(I1-xBrx)3 is exponentially

increased with relative humidity.[115] As shown in Figure 4.4, the XRD patterns of

perovskite film prepared at 100 °C displays peaks with a relatively similar intensity

and the reference noise is obvious, indicating a relatively low crystallinity. Comparing

to the perovskite film prepared at 250 °C, XRD pattern reveals two dominant peaks at

14.6 ° and 29.4 ° which are crystal layers of (100) and (200), respectively. The XRD

peaks of the CsPbI2Br film prepared at a high temperature in the work are consistent

with previous literature.[7, 122, 125, 128] Table 4.1 shows the full width at half

maximum (FWHM) of the four labelled XRD peaks for the film prepared at 250 °C

and at 100 °C. All the FWHM of crystal layers of (100), (110), (200), and (211) for

the film prepared at 250 °C is smaller than those of 100 °C. The result is consistent

with a high crystallinity and a large grain size for a film prepared at high

temperature.[162] Similar to previous literature, the degraded CsPbI2Br film shows

totally different XRD peaks comparing with the perovskite film.[115, 125]


77

Figure 4.4 Normalized XRD patterns of CsPbI2Br perovskite films prepared at 100 °C,

250 °C, and the degraded yellow film.

Table 4.1 FWHM of XRD peaks for CsPbI2Br perovskite films prepared at 100 °C

and 250 °C.

(100) (110) (200) (211)

100 °C 0.308 ° 0.325 ° 0.296 ° 0.391 °

250 °C 0.215 ° 0.222 ° 0.222 ° 0.209 °

On the basis of quality and morphology of the perovskite film, we conducted

photovoltaic measurement of the CsPbI2Br PSCs by using annealing temperature at

250 °C. Figure 4.5a shows the configuration of the CsPbI2Br PSCs incorporating with

a HTL of either spiro-OMeTAD or CuSCN. Figure 4.5b reveals the energy-level

diagram of the PSCs in which the energy-band level of CsPbI2Br and CuSCN follow

the previous literature.[133, 163] The HOMO of CuSCN and spiro-OMeTAD are 5.3

eV and 5.1 eV respectively, which show a favourable band alignment for the HOMO

of CsPbI2Br and the work function of gold, indicating no barrier on charge extraction.

Figure 4.5c shows photographs of the devices with encapsulation to avoid fast

degradation of the perovskite films. For a conventional structure of TiO2-based PSC,

the light needs to penetrate the c-TiO2/m-TiO2 layers before reaching the photoactive

layer, therefore, the absorbance and transmittance of the TiO2 layers are characterized.

As shown in Figure 4.5d, only light shorter than 370 nm being blocked by the TiO2 is

matched with the bandgap of TiO2 so it is favourable to be the front ETL.


78

Figure 4.5 (a) Schematic diagram, (b) energy-level diagram, and (c) photographs of

CsPbI2Br PSCs with encapsulation. (d) UV-vis spectrum of c-TiO2/m-TiO2 layer.

The performance of PSCs highly depends on the crystallization process of

perovskite film. The CsPbI2Br film can gradually turn dark without annealing after

spin-coating, reflecting the crystallization of perovskite structure over time. In order

to investigate the effect of the room-temperature-crystallization time, devices prepared

at the time from 10 to 90 minutes are fabricated and the relevant photovoltaic

parameters are shown in Table 4.2. The devices with a CuSCN HTL reveal a gradual

improvement of Voc, Jsc, PCE and FF over time and the optimal condition with the

highest PCE are located at 70 minutes for further study. As shown in Figure 4.6a,c,

the CsPbI2Br/CuSCN PSCs exhibit an average PCE of 7.90% and the best PCE of

10.36% which is much higher than those of CsPbI2Br/spiro-OMeTAD PSCs about

5.52% and 7.31%, respectively, demonstrating the potential of CuSCN. The PSCs with

CuSCN can obtain a higher open-circuit voltage (1.04V) than that of spiro-OMeTAD

(0.96V). The devices with CuSCN and spiro-OMeTAD can achieve high short-circuit

current densities up to 14.1 mA/cm2 and 13.1 mA/cm2, respectively, which are much


79

higher than that of CsPbBr3/spiro-OMeTAD devices in Chapter 3 about 6.85 mA/cm2,

demonstrating a large enhancement on the current density output. This is consistent

that more light is absorbed by CsPbI2Br perovskite due to a lower bandgap. The high

value of fill factor (70%) of CuSCN-based devices may be possibly attributed to that

the CuSCN possesses a high hole mobility of 0.01-0.1 cm2V-1s-1 while that of the

spiro-OMeTAD is only 4.5x10-4 cm2V-1s-1.[155] The higher performance of devices

with CuSCN comparing to that of spiro-OMeTAD in all the photovoltaic parameters

also stems from the unavoidable degradation of the sensitive perovskite film during

the oxidation of spiro-OMeTAD. The corresponding EQE spectrum and the integrated

current density of champion device for CuSCN are characterized and shown in Figure

4.6b. The device reveals an EQE onset on about 650 nm and an EQE end on about 300

nm, which is consistent with the bandgap of CsPbI2Br and the transmittance limit of

the TiO2 layers shown in Figure 4.5d. The average PCE of CsPbI2Br devices with

different HTLs are represented as histograms in Figure 4.6d,e which show a normal

distribution on both devices. Although the performance of the PSCs based on different

hole transport layers is relatively low comparing to the previous literature (Table 2.4),

the PCE results provide a workable configuration for incorporating CsPbI2Br

inorganic perovskite layer with the inorganic CuSCN HTL.

Table 4.2 Photovoltaic parameters of CsPbI2Br/CuSCN devices prepared by different

room-temperature-crystallization time.

Time /min Voc /V Jsc /mA/cm2 PCE/% FF /% 10 0.98 13.67 7.03 52.4 20 0.96 13.46 7.47 57.7 30 0.99 12.43 7.76 63.4 40 1.02 12.19 7.44 59.9 50 1.02 13.19 8.33 62.1 60 1.04 14.40 9.63 64.1 70 1.04 14.12 10.36 70.4 80 1.03 14.55 10.12 67.5


80

Figure 4.6 (a) J-V curve and (b) EQE spectrum and the integrated current density of

PSCs with CuSCN. (c) J-V curve of PSCs with spiro-OMeTAD. Histograms of PCE

of 20 CsPbI2Br PSCs incorporating with (d) CuSCN and (e) spiro-OMeTAD.

In order to reveal the stability of the PSCs, three devices with similar initial

efficiency are chosen for the stability test. All the tested devices are encapsulated using

epoxy AB glue and a piece of glass otherwise they will degrade very quickly on the

ambient air due to the high humidity. After J-V measurement, the devices are stored

in the ambient air with RH of about 20% at room temperature. As shown in Figure 4.7

for devices with CuSCN, the parameters of Voc, Jsc, PCE, and FF are all analyzed to

identify the main factor for degradation. The Voc maintains at a stable value about 1.03

V over the testing time while the Jsc increases from about 11 mA/cm2 to about 14

mA/cm2 for the first week and keep that value afterwards. The increase of Jsc may due

to a slow light-soaking process by a weak indoor light in the laboratory during the

storing period.[164] The fill factor decreases gradually over time. After a period of the

trade-off between Jsc and FF, the best PCE are recorded in the 13th day after fabricated.

Comparing to the devices with spiro-OMeTAD in Figure 4.8, similarly, the Voc almost

keep at a stable value over time while the Jsc decreases linearly with the time. The

main factor that influences the device efficiency is the fill factor which shows almost


81

the same trend with the PCE over the time. The PCE and FF of devices increase to the

maximum value in 2-3 days due to the optimization of hole conductivity of spiro-

OMeTAD layer through oxidation.[140, 141] The PCE of devices with CuSCN can

retain 93% of the initial efficiency after 31 days while the devices with spiro-

OMeTAD can only preserve 72% of the initial efficiency after 11 days, demonstrating

the higher stability of CuSCN. Although all the devices are encapsulated, the water

molecules can still enter the perovskite films through the pinholes and cracks along

grain boundaries (Figure 4.3e,g) from the unencapsulated area (Figure 4.5c).

Therefore, the PCE of both the CsPbI2Br devices with CuSCN and spiro-OMeTAD

decrease with time gradually.


encapsulated CsPbI2Br/CuSCN PSCs.


82


encapsulated CsPbI2Br/spiro-OMeTAD PSCs.

4.4 Conclusion

In conclusion, we have investigated the stability and degradation issues of

CsPbI2Br perovskite film. The perovskite film fabricated would degrade rapidly under

the environment with high humidity through the phase transition. The degradation of

perovskite film will start from the grain boundaries by creating some cracks or

pinholes. A high annealing temperature can enlarge the grain size and thereby stabilize

the perovskite films. By fabricating the CsPbI2Br PSCs using 250 °C, the devices

incorporating with CuSCN can achieve the best PCE of 10.36% with a high

photocurrent density of 14.1 mA/cm2 which are much higher than the one with spiro-

OMeTAD. This work shows a new workable configuration for employing a cheap

HTL of CuSCN to the CsPbI2Br perovskite film, demonstrating an all-inorganic

structure of PSC.


83

Conclusion and Future Outlook

5.1 Conclusion

Perovskite solar cell is a potential device in harvesting the solar energy, showing

a rapid improvement in the power conversion efficiency. By being aware of the

stability problems of organic PSCs, therefore we concentrate on the study of inorganic

PSCs. After reviewing a large amount of literature, we find that the PCEs of inorganic

PSCs have also quickly increased from 4.92% in 2015 to 16.14% in 2018. The

CsPbBr3 PSC is the most stability perovskite device in the ambient air. Due to the

property of large bandgap, it can be applied as a bottom cell in the tandem solar cell

or utilized as a semi-transparent solar cell on the window. The CsPbIBr2 is also a stable

and semi-transparent material. Based on its transmission-tunable property from fully

transparent to brown colour by switching structural phase, CsPbIBr2 PSC is a more

potential candidate to be applied on smart photovoltaic windows. The CsPbI2Br PSC

can achieve a PCE up to 14.45%. However, due to a lower tolerance factor of using

more iodide in the caesium lead halide, CsPbI2Br perovskite is unstable under high

humidity. The CsPbI3 is even more unstable than the others initially. With great efforts

by the researcher, doping different kinds of additive is found as a useful way to

improve the stability and the performance. To date, the best PCE of CsPbI3 PSCs is

already up to 15.71% which is very close to the commercial silicon solar cells.

By realizing the problem of the high-annealing temperature of stable CsPbBr3

perovskite film, therefore we demonstrate a pyridine-vapour treatment ease the

problem. Through the application of the pyridine treatment in the two-step fabrication

process of the perovskite film, we can reduce the annealing temperature from 250 °C


84

to 160 °C with a comparable efficiency of 6.04% to that of the normal device. For the

mechanism, the pyridine vapour reacts with the PbBr2 film and establish an

intermediate phase PbBr2.(Py)x in the 1st step process. This phase afterwards reacts

with CsBr to form the perovskite film through an alternative reaction pathway with a

lower activation energy required. The pyridine-treated devices are confirmed to

possess high stability over 100 days under ambient air with high relative humidity up

to 70%. This work provides a novel approach to the low-temperature fabrication

technique in inorganic PSCs.

We realize efficiency limitation of CsPbBr3 and the very unstable property of

CsPbI3 at the room temperature or in the ambient air. On the other work, we study the

workability of applying CsPbI2Br perovskite film as the photoactive layer in solar cells.

The perovskite film was found very unstable when exposing to the high humidity. The

degradation of perovskite film was identified as the creation of creaks along the grain

boundaries initially, followed by phase transition to δ orthorhombic phase near the

creaks. The film prepared at low temperature degrade more quickly than the one at

high temperature due to the appearance of pinholes. By incorporating the perovskite

films to a HTL of CuSCN, the PSCs can achieve a high Jsc of 14.1 mA/cm2 and a high

PCE of 10.36% which are higher than that of PSCs with the typical spiro-OMeTAD

HTL. The enhancement of photovoltaic performance is originated from the extension

of the light absorption range due to lower bandgap. This is the first time integrating

the CsPbI2Br perovskite film and the cheap CuSCN HTL as an all-inorganic

perovskite solar cell and it helps to take a step further on the development of the

inorganic perovskite solar cells.

5.2 Future Outlook

We have demonstrated a low-temperature method to fabricate CsPbBr3 perovskite


85

films through a two-step process, resulting in a PCE of about 6%. Some researcher

recently suggested a new two-step method for fabrication of CsPbBr3 films at 250 °C

with a remarkable PCE of 9.72% in the devices. As expected, the pyridine method is

able to incorporate with the new two-step method to produce high-performance solar

cells with a low-temperature process. The HTL of spiro-OMeTAD is one of the

degradation factors in the devices so incorporating the perovskite film with an

inorganic HTL of CuSCN or a HTL-free carbon electrode may improve the stability.

Meanwhile, the cost of the solar cells can be largely reduced. Since we have reduced

the temperature for perovskite film, the selection of materials for other layers are now

less limited. In the future, there will be more combinations among different layers,

widening the range of explorations on the use of CsPbBr3.

We have fabricated the CsPbI2Br PSCs with PCE up to 10.36% which is relatively

lower than the other literature reported. This difference may be originated from the

technical or conditional variation. In the future, the condition of perovskite films is

required to optimize so that the device efficiency and stability can be improved. We

have identified that the degradation of perovskite film is initiated at the grain

boundaries. Introducing grain boundary medication methods, such as filling the GBs

with hydrophobic 2-D materials, are expected as a useful strategy to improve the

stability. Doping elements and polymers to the perovskite precursor are also some

effective methods reported to improve the quality and stability of films. Stability

problem not only is an issue in our work, but also becomes one of the most crucial

factors hampering commercialization of other PSCs. Therefore, identifying the

degradation problem here can help the development of PSCs. Comparing to traditional

solar cells, the advantages of PSCs are their time- and cost-effective fabrication

process. The application of CuSCN HTL to PSCs can meet the ideas rightly and result

in a higher PCE than the one with spiro-OMeTAD HTL so the inorganic CuSCN is

expected as a potential candidate in PSCs. Finally, we hope the works in this thesis


86

can pave the way for the development of perovskite solar cells.

THE HONG KONG POLYTECHNIC UNIVERSITY References

87

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