Fabrication of organic and lead-free perovskite solar cells using electric field assisted spray deposition technique

FABRICATION OF ORGANIC AND LEAD-FREE PEROVSKITE SOLAR CELLS USING ELECTRIC FIELD ASSISTED

SPRAY DEPOSITION TECHNIQUE

TAUHEED MOHAMMAD

CENTRE FOR ENERGY STUDIES

INDIAN INSTITUTE OF TECHNOLOGY DELHI

OCTOBER 2019

©Indian Institute of Technology Delhi (IITD), New Delhi, 2019

FABRICATION OF ORGANIC AND LEAD-FREE PEROVSKITE SOLAR CELLS USING ELECTRIC FIELD ASSISTED

SPRAY DEPOSITION TECHNIQUE

by

TAUHEED MOHAMMAD Centre for Energy Studies

Submitted

In fulfillment of the requirements of the degree of Doctor of Philosophy to the

INDIAN INSTITUTE OF TECHNOLOGY DELHI

OCTOBER 2019

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CERTIFICATE

This is to certified that the thesis entitled "Fabrication of organic and lead-free perovskite solar cells using electric field assisted spray deposition technique" being submitted byMr.TAUHEED MOHAMMAD to Indian Institute of Technology Delhi is ethical of consideration for the award of the degree of Doctor of Philosophy and is a record of the original and bonafide research work carried out by him under my guidance and supervision. The results obtained in the thesis have not been submitted in part or full to any other University or Institute for award of any degree/diploma.

Prof. Viresh Dutta Centre for Energy Studies Indian Institute of Technology Delhi New Delhi – 110016

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ACKNOWLEDGEMENTS

First and foremost, I would like to express my deep gratitude and heartful thanks to my dissertation advisor, Prof. Viresh Dutta, who guided and supported me with his valuable suggestions and insights through my Ph.D. work. I admire and appreciate for his continuous support and was always available whenever needed in difficult situations. I would like to extend my sincere thanks to university Grant Commission (UGC) for awarding the Maulana Azad National Fellowship (MANF), Government of India, in my Ph.D. tenure. Without this support it was not possible for me to complete this thesis.

I would like to thank Prof. T.C. Kandpal, Head CES for their important role and support during my Ph.D. time. I am also grateful to my SRC members Prof. Vamsi K. Komarala, Dr.

Supravat Karak and Dr. Samaresh Das for their valuable suggestions and evaluating my work. I would like to gratefully acknowledge Dr. Sandeep Pathak for his stimulating advice and valuable suggestions. I thank all the faculty members and staff of IIT Delhi for their contribution to make this work possible.

I express my sincere gratitude to my seniors Dr. Vinod Kumar, Dr. Siva Chandra Sekhar P, Mrs. Sapna Mudgal, Dr. Sourav Mandal, Dr. Charu Dwivedi, Dr. Vishal Bharti, Dr. Neetesh Kumar and Dr. Firoz Alam for their indispensable support and intense discussions to complete my experiments. I am thankful to my colleague Kuldeep Kumar and Mohd Alam for their cooperation during my work. I am grateful to Mr. Nagendra Chaudhary, who has excellent technical skills and always available for me whenever I needed his help. Many thanks to Mr.

Naresh Kumar for providing experimental requisites time to time.

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Again I am very grateful and express my sincere gratitude to Prof. Vamsi K. Komarala for allowing me to use his whole lab all the time during my Ph.D.

This is wonderful opportunity for me to acknowledge the people without whom it would not have been possible to complete my doctoral degree. My sincere thanks to everyone who encouraged and helped me throughout my educational journey beginning from my small village to this level. Truly, words are not enough to express my gratitude to all of them.

Finally, my deepest gratitude goes to my parents and brothers for all their selfless support and unconditional love. I am ever indebted to my father for his great sacrifice and enormous patience, during my all degrees.

TAUHEED MOHAMMAD

~ iv ~ ABSTRACT

Renewable energies are critical and necessary technological development deeply connected to human evolution and even survival. The extraordinary technological development of the past century brought tremendous changes to the planet which is undoubtedly affecting the natural ecosystem of Earth. Human evolution does not mean only advanced technological development, but also deeper consciousness and responsibility for the next generations to come.

Life on Earth exists because of the Sun. Therefore, solar energy is one of the answers for renewable energy.

In this thesis, the research has been conducted on organic solar cells (OSCs) and lead-free perovskite solar cells (PSCs). In particular, the thesis deals with the extensive study of electric field assisted spray technique on the performance of solution processable photovoltaic devices.

The devices were produced using spray technique at ambient condition and characterized without sealing, which is compatible with an industrial production, essential for commercialization.

The concepts of downshifting and significance of surface morphology are explored to improve the photon concentration and performance of OSCs. The OSCs fabricated by using spray deposited Eu³⁺ doped Poly (3,4 ethylene dioxy thiophene):poly(styrene sulfonate) (PEDOT:PSS) as anode buffer layer (ABL). The doping of Eu³⁺ in PEDOT:PSS causes the down-shifting of the UV light to visible spectrum and enhancing the photon concentration in this region. To enhance the wettability and conductivity of PEDOT:PSS for efficient hole injection continuous spray pyrolyzed (CoSP) synthesized molybdenum trioxide (MoO₃) nanorods mingled in PEDOT:PSS to form hybrid hole transport layer (HTL) for OSCs. The use of electric field during the spray deposition of undoped and doped PEDOT:PSS film improves the surface morphology and the electrical conductivity, which helps in improving the solar cell performance.

To enhance the power conversion efficiency (PCE) of OSCs, absorption spectral range of active layer increased while preserving efficient exciton harvesting. Förster resonance energy transfer (FRET) has been identified as an important process utilized for exciton harvesting,

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transport and dissociation. The spectral absorption range broadened by prepared thin films from ternary blend of donors PCE-10 and PCDTBT with the acceptor PC71BM in chlorobenzene by using novel one-step spray coating method.

Spray technique as a prototype was developed for perovskite solar cells by using lead- free perovskite material. Solution-processed Methylammonium iodo bismuthate (MBI) perovskite solar cell is fabricated by spray technique with application of applied voltages from 0 to 1000 V during the deposition. The morphology and surface roughness of MBI films are influenced significantly by the electric field which is also reflected by photovoltaic performance of devices.

This thesis reports the cost-effective technology for time efficient pore filling. The electric field assisted spray (EFAS) technique employing coupled DC power supply to fill the most common nanostructure architecture namely mesoporous, nanorods or nanotubes. The EFAS deposition technique presents a mechanistic model for controlled pore filling of any material solution into nanostructured surface.

Tin sulfide (SnS) nanostructured films deposited on fluorine-doped tin oxide (FTO) coated glass substrate using a continuous spray pyrolysis (CoSP) technique with the application of electric field. The influence of applied electric field on the structural, morphological and optical properties has been investigated and demonstrated with DSSCs device performance.

The results and conclusions of the present study are useful for design and fabrication of OSCs and PSCs by an easy, cost effective and scalable method using electric field assisted spray deposition and pore infiltration technique for obtaining better efficiencies.

सार

अक्षय ऊर्ाा एक महत्वपूर्ा और आवश्यक तकनीकी ववकास है र्ो मानव ववकास और यहाां तक वक मानव अस्तित्व से र्ुडा हुआ है। वपछली सदी के असाधारर् तकनीकी ववकास ने ग्रह में र्बरदि बदलाव लाए र्ो वनस्सांदेह पृथ्वी के प्राकृवतक पाररस्तथिवतकी तांत्र का एक वहस्सा है। मानव ववकास का अिा केवल उन्नत तकनीकी ववकास नहीां है, बस्ति आने वाली पीव़ियोां के वलए गहरी चेतना और वर्म्मेदारी भी है। पृथ्वी पर र्ीवन सूया के कारर् मौर्ूद है। इसवलए, सौर ऊर्ाा अक्षय ऊर्ाा के र्वाबोां में से एक है।

इस िीवसस में, र्ैववक सोलर सेल (OSCs ओएससी) और लेड रवहत पेरोवक्साइट सोलर सेल (PSCs) पर शोध वकया गया है। ववशेष रूप से, िीवसस समाधान प्रविया योग्य फोटोवोस्तिक उपकरर्ोां के प्रदशान पर इलेस्तरिक फील्ड अवसस्टेड स्प्रे तकनीक के व्यापक अध्ययन से सांबांवधत है। उपकरर्ोां को पररवेशी स्तथिवत में स्प्रे तकनीक का उपयोग करके उत्पावदत वकया गया िा और वबना सील की ववशेषता एक औद्योवगक उत्पादन के साि सांगत है, र्ो व्यावसायीकरर् के वलए आवश्यक है।

सतह के आकाररकी के डाउनशस्तटांग और महत्व की अवधारर्ाओां को फोटॉन एकाग्रता और ओएससी के प्रदशान में सुधार करने के वलए पता लगाया गया है। ओएससी स्प्रे का उपयोग करके ग़िे गए ओएससी +3 डॉप्ड पॉली (3,4 एविलीन डाइऑक्सी िायोवफन): पाली (स्टायररन सल्फोनेट) (पेडॉट:

पीएसएस) एनोड बफर परत (एबीएल) के रूप में। PEDOT में Eu³⁺ का डोवपांग: PSS यूवी स्पेरिम के डाउन- वशस्तटांग के कारर् दृश्यमान स्पेरिम और इस क्षेत्र में फोटॉन एकाग्रता को ब़िाता है। PEDOT की वेटेवबवलटी

और चालकता को ब़िाने के वलए: PSS कुशल छेद इांर्ेक्शन के वलए सतत स्प्रे पैरालाईज़ड (CoSP) सांश्लेवषत मोवलब्डेनम टिाईऑक्साइड (MoO3) नैनोरोड PEDOT में घुलवमल: PSS OSCs के वलए हाइविड होल पररवहन परत (HTL) बनाने के वलए। अनडोप और डोप वकए गए PEDOT के स्प्रे वडपोवर्शन के दौरान ववद्युत क्षेत्र का उपयोग: PSS वफल्म सतह आकृवत ववज्ञान और ववद्युत चालकता में सुधार करती है, र्ो सौर सेल प्रदशान को बेहतर बनाने में मदद करती है।

ओएससी के शस्ति रूपाांतरर् दक्षता (PCE) को ब़िाने के वलए, कुशल एक्साइटन हावेस्तस्टांग को

सांरवक्षत करते हुए सविय परत की अवशोषर् वर्ािमीय सीमा में वृस्ति हुई। फॉस्टर रेज़ोनेंस एनर्ी टिाांसफ़र (FRET) की पहचान एक महत्वपूर्ा प्रविया के रूप में की गई है वर्सका उपयोग एस्तक्सटॉन हावेस्तस्टांग, पररवहन और पृिक्करर् के वलए वकया र्ाता है। वर्ािमीय अवशोषर् रेंर् दाताओां PCE-10 और PCDTBT के टेरनरी वमश्रर् से तैयार पतली वफल्मोां द्वारा व्यापक हो गई है, वर्समें क्लोरोबेंर्ीन में स्वीकताा PC71BM के साि नॉवेल वन-स्टेप स्प्रे कोवटांग वववध का उपयोग वकया गया है।

एक प्रोटोटाइप के रूप में स्प्रे तकनीक को पेरोवक्साइट सोलर सेल के वलए लेड-फ्री पेरोवक्साइट सामग्री का उपयोग करके ववकवसत वकया गया िा। समाधान-सांसावधत वमिाइलमोवनयम वबस्मि आयोडाइड (एमबीआई) पेरोवक्साइट सोलर सेल को वडप्रेशन के दौरान 0 से 1000 वोि तक लागू वोिेर् के आवेदन के साि स्प्रे तकनीक द्वारा वनवमात वकया र्ाता है। MBI वफल्मोां की आकृवत ववज्ञान और सतह खुरदरापन ववद्युत क्षेत्र से काफी प्रभाववत होता है र्ो वक उपकरर्ोां के फोटोवोस्तिक प्रदशान से भी पररलवक्षत होता है।

यह िीवसस समय कुशल वछद्र भरने के वलए लागत प्रभावी तकनीक की ररपोटा करती है। इलेस्तरिक फील्ड अवसस्टेड स्प्रे (EFAS) तकनीक ने युस्तित डीसी वबर्ली की आपूवता को सबसे सामान्य नैनोस्टिक्चर आवकाटेक्चर अिाात् मेसोपोरस, नैनोरोड्स या नैनोट्यूब को भरने के वलए वनयोवर्त वकया है। EFAS बयान तकनीक वकसी भी भौवतक समाधान के वनयांवत्रत वछद्र भरने के वलए एक यांत्रवत मॉडल प्रिुत करती है र्ो

वक नैनोस्टिक्चर सतह में होती है।

वटन सल्फाइड (SnS) नैनोस्टिक्चर वफल्मोां को फ़्लोरीन-डॉप्ड वटन ऑक्साइड (FTO) लेवपत ग्लास सब्सटिेट पर एक वनरांतर स्प्रे पायरोवलवसस (CoSP) तकनीक का उपयोग करके ववद्युत क्षेत्र के साि र्मा

वकया र्ाता है। सांरचनात्मक, रूपात्मक और ऑविकल गुर्ोां पर लागू ववद्युत क्षेत्र के प्रभाव की र्ाांच और DSSCs वडवाइस प्रदशान के साि प्रदशान वकया गया है।

वतामान अध्ययन के पररर्ाम और वनष्कषा एक बेहतर, लागत प्रभावी और स्केलेबल वववध द्वारा OSCs और PSCs के वडर्ाइन और वनमाार् के वलए उपयोगी हैं, र्ो बेहतर क्षमता प्राप्त करने के वलए इलेस्तरिक फील्ड अवसस्टेड स्प्रे वडपोवर्शन और वछद्र घुसपैठ तकनीक का उपयोग कर रहे हैं।

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TABLE OF CONTENTS

CERTIFICATE i

ACKNOWLEDGEMENTS ii

ABSTRACT iv

TABLE OF CONTENTS vi

LIST OF FIGURES xiii

LIST OF TABLES xviii

ABBREVATIONS xix

CHAPTER 1 Introduction

1.1 History of solar cells ………...……4

1.2 Thesis motivation……….……6

1.3 Organic photovoltaic………9

1.3.1 Carrier transport in organic photovoltaic……….…………9

1.3.2 The working mechanism of organic photovoltaic……….12

1.4 Perovskite PV technology………..17

1.5 Solution processable solar cell engineering ………..……21

1.6 Thesis overview……….……24

References ……….………26

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CHAPTER 2 Experimental and characterization techniques

2.1 Spray deposition technique………34

2.2 Electric field assisted spray technique………...…40

2.3 Characterization techniques………...……42

2.3.1 Transmission electron microscope……….………42

2.3.2 Scanning electron microscope………...………44

2.3.3 X-ray diffraction (XRD) ………...……46

2.3.4 Atomic force microscopy………...……48

2.4 Optical characterizations……….…...…50

2.4.1 Steady state photoluminescence………50

2.4.2 Transient photoluminescence……….………54

2.4.3 UV-Vis-NIR spectrophotometer………56

2.5 Thickness measurement……….………58

2.6 Photovoltaic measurements………...………59

2.6.1 Current Density-Voltage (J-V) measurements………...………60

2.6.2 Quantum efficiency measurements………63

2.6.3 Electrochemical impedance spectroscopy……….………65

References………..………67

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CHAPTER 3A Spray coated europium doped PEDOT:PSS anode buffer layer for organic solar cell: the role of electric field during deposition

3.1.1 Introduction ………...……75

3.1.2 Experimental details………...77

3.1.2.1 Materials………77

3.1.2.2 Deposition of PEDOT:PSS films and OPV device fabrication……….…78

3.1.2.3 Thin films and device characterization………..………80

3.1.3 Results and discussion………...……80

3.1.3.1 Optical properties………...………80

3.1.3.2 Film morphology………...………82

3.1.3.3 DC conductivity measurement………...……84

3.1.3.4 Current density-Voltage (J-V) characteristics………...………85

3.1.4 Conclusion……….……88

CHAPTER 3B Role of COSP synthesized MoO3 nanorods in PEDOT:PSS matrix by spray deposition on the performance of organic solar cells: electric field impact during deposition 3.2.1 Introduction ………...………91

3.2.2 Experimental details………...………93

3.2.2.1 Materials………93

3.2.2.2 Deposition of hybrid PEDOT:PSS- MoO₃ films and OPV device fabrication………...………94

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3.2.2.2 Characterization……….………95

3.2.3 Results and discussion………...………95

3.2.4 Conclusion………...…………106

References………107

CHAPTER 4 Electric field assisted spray technique for utilizing polymer–polymer Förster energy transfer in bulk heterojunction organic photovoltaics 4.1 Introduction ……….………116

4.2 Experimental details ………118

4.2.1 Materials………..…………118

4.2.2 Deposition of ternary blend thin films and BHJ-OSCs device fabrication….…119 4.2.3 Thin films and device characterization………....…………119

4.3 Results and discussion……….………120

4.4 Conclusion……….………..128

Reference……….……129

CHAPTER 5 Electric field assisted spray coated lead-free bismuth iodide perovskite thin film for solar cell application 5.1 Introduction………..……135

5.2 Experimental details ………138

5.2.1 Materials ……….……138

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5.2.2 Compact c-TiO₂ and mesoporous m-TiO₂ layer coating……….……138

5.2.3 Fabrication of (CH3NH3)3Bi2I9 films ………..…139

5.2.4 Fabrication of MBI solar devices……….…………140

5.2.5 Characterization………...…………141

5.3 Results and discussion……….…………141

5.4 Conclusion………...………152

References………153

CHAPTER 6A Electric-field assisted spray technique for controlled pore filling of nanostructured films: device applications 6.1.1 Introduction………..………161

6.1.2 Experimental………165

6.1.2.1 Materials………..…………165

6.1.2.2 Fabrication of TiO₂ photoanodes……….……165

6.1.2.3 Pioneer time efficient controlled pore filling spray deposition technique………166

6.1.3 Device fabrication………169

6.1.4 Characterization………...………169

6.1.5 Results and Discussion………170

6.1.6 Conclusion………...……178

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CHAPTER 6B Deposition of tin sulfide nanostructured films using electric field assisted continuous spray pyrolysis technique as a platinum-free counter electrode in dye-sensitized solar cells

6.2.1 Introduction………..………180

6.2.2 Experimental details……….………182

6.2.2.1 Materials………..………182

6.2.2.2 Spray deposition of SnS films……….…………182

6.2.2.3 Device fabrication………183

6.2.2.4 Characterization techniques……….…………184

6.2.3 Results and Discussion………185

6.2.4 Conclusion………...………191

References………192

CHAPTER 7 Summary and scope of future work 7.1 Summary ……….………199

7.1.1 Spray Coated Europium Doped PEDOT:PSS Anode Buffer Layer for Organic Solar Cell: The Role of Electric Field During Deposition………...199

7.1.2 Role of CoSP Synthesized MoO3 Nanorods in PEDOT:PSS Matrix By Spray Deposition on the performance of Organic solar cells: Electric Field Impact During Deposition……….……200

7.1.3 Electric Field Assisted Spray Technique for Utilizing Polymer–Polymer Förster Energy Transfer in Bulk Heterojunction Organic Photovoltaics……….201

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7.1.4 Electric Field Assisted Spray Coated Lead-Free Bismuth Iodide

Perovskite Thin Film for Solar Cell Application………..……...……201

7.1.5 Electric-field Assisted Spray Technique for Controlled Pore Filling of Nanostructured films: Device Applications………202

7.1.6 Deposition of Tin Sulfide Nanostructured Films Using Electric Field Assisted Continuous Spray Pyrolysis Technique as A Platinum-Free Counter Electrode in Dye-Sensitized Solar Cells………202

7.2 Scope for the Future Work………...…………203

List of publications……….……205

Bio-Data………..…208

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Figure No. Caption Page No.

Chapter 1

1.1 Shares of various technologies in new power capacity additions in India. 3 1.2 Global economic dominance in renewable energy by top three countries. 3

1.3 Alexandre-Edmond Becquerel. 5

1.4 National Renewable Energy Laboratory (NREL) certified world record

solar cell PCE for PV technologies. 8

1.5

Spatial arrangement of the p orbitals of carbon for (a) covalent bonding of saturated hydrocarbon, (b) unsaturated hydrocarbon, (c) delocalized P_z orbitals and (d) forming of energy states.

10

1.6

Concept of exciton in organic photovoltaics represented by energy levels of the vacuum state, conduction and valence bands along with the bandgap of a material. (b) Exciton diffusion in planner and bulk heterojunction organic solar cells.

13

1.7

(a) Perovskite crystal structure with general formula ABX3 (b) mesoporous perovskite device structure (c) planner n-i-p regular perovskite device structure and (d) planner p-i-n inverted perovskite device structure.

18

Chapter 2

2.1 Chemical thin film deposition methods. 36

2.2

Schematic of spray solution atomization and the effect of initial droplet sizes variation (A-D) during the film formation on the heated substrate after the pyrolytic reaction.

36 2.3 Digital photograph of spray deposition system used for this thesis work. 41

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2.4

Schematic of the TEM instrument and representing the main components.

(Source: http://www.hk-hy.org/atomic_world/tem/tem02_e.html)

40

2.5 Schematic of the SEM instrument.

(Source: https://www.nanoimages.com/sem-technology-overview/). 43 2.6 Schematic diagram of the glancing angle X-ray diffractometer. 47 2.7 Schematic diagram of the atomic force microscope. 49 2.8 Different processes that take place in photoluminescence process. 52 2.9 Schematic diagram of arrangement of different optical components of PL

instrument. 53

2.10 Schematic of time-correlated single-photon counting (TCSPC). 55 2.11 Schematic diagram of arrangement of different optical components of

Lambda 1050 UV-VIS-NIR spectrophotometer. 57

2.12 Schematic diagram of typical profilometer. 59

2.13 Equivalent circuit of a solar cell. 60

2.14 I-V measurement setup schematic. 62

2.15

(a) Oriel class 3A solar simulator for measuring J-V characteristics of solar cells, (b) IPCE system used for measuring the spectral response of devices and (c) Electrochemical workstation used for the impedance spectroscopy analysis.

64

2.16 Nyquist plots of a typical DSSC 65

Chapter 3

3.1 Structure of (a) donor polymer PTB7 (b) acceptor polymer PC71BM. 78 3.2 Schematic diagram of electric field assisted spray deposition setup. 79 3.3

Optical characterization of spray deposited undoped and Eu³⁺ doped PEDOT:PSS thin film at different electric field (a) Transmittance spectrum (b) Photoluminescence spectrum.

81

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3.4

Tapping mode AFM image of PEDOT:PSS at scale of 5×5 µm² (a) undoped (b) Eu³⁺ doped at 0 V (c) Eu³⁺ doped at 500 V (d) Eu³⁺ doped at 1000 V.

83

3.5 SEM image of PEDOT:PSS (a) undoped (b) Eu³⁺ doped at 0 V (c) Eu³⁺

doped at 500 V (d) Eu³⁺ doped at 1000 V 84

3.6

(a) Schematic diagram of proposed OSCs device with energy level (b) SEM image of active layer for best devices (c) J-V characteristics of proposed OSCs devices at different conditions

87

3.7 IPCE spectra of OSCs fabricated using undoped and Eu³⁺ doped

PEDOT:PSS with different applied voltages. 88

3.8

Photograph of (a) MoO3 nanorods solution (b) hybrid PEDOT:PSS- MoO₃ solution (c) TEM image of hybrid PEDOT:PSS-MoO₃ spray solution and (d) EDX elements mixed mapping profile: Molybdenum La1(red), Carbon Ka1_2(blue), Oxygen Ka1(green) with spectrum details of all elements.

97

3.9 XPS spectra of the MoO3 films showing Mo 3d level 98 3.10 The measured contact angles for the droplets of (a) pristine PEDOT:PSS

(b) composite PEDOT:PSS -MoO3 solutions onto ITO coated substrates. 99 3.11 AFM images of (a) pristine PEDOT:PSS (b) hybrid PMo-0V (c) hybrid

PMo-500V (0.5kV) and (d) hybrid PMo-1000V. 99

3.12

(a) Spectra of transmittance measured for pure PEDOT:PSS and composite PEDOT:PSS-MoO₃ layer at different applied voltage during spray deposition and (b) absorption spectra in visible and NIR.

101

3.13 Measured Raman spectra of PEDOT:PSS and composite PEDOT:PSS-

MoO₃ layer at different voltage. 103

3.15

(a) J-V characteristics (b) EQE spectra of the OSCs prepared with spray deposited pristine PEDOT:PSS, hybrid PMo-0V, PMo-500V and PMo- 1000V.

105

~ xvi ~

Chapter 4

4.1 Chemical structures of (a) PCDTBT, (b) PCE-10, (c) PC71BM and

fabricated device structure. 122

4.2

(a) The normalized absorption spectra of the polymer PCDTBT and PCE-10 and photoluminescence spectra of PCDTBT (b) absorption spectra of binary blend of PCE-10: PCDTBT:: 7:3 at different applied voltages.

122

4.3 Schematic of molecular orbital for Förster resonance energy transfer. 124 4.4 (a) PL emission spectra of plain PCDTBT, PCE-10 and blend of PEC-

10:PCDTBT::7:3 at different applied voltages during spray deposition 124 4.5

(a) J−V curves of ternary organic solar cells with different applied voltages during spray deposition of active layer (b) Corresponding EQE measurement of all fabricated devices.

126

Chapter 5

5.1 Schematic diagram of spray deposition technique with the application of

DC voltage for the fabrication of MBI thin films. 140 5.2 Role of applied voltage during the deposition on the XRD of MBI thin

films. 141

5.3 SEM image of MBI perovskite thin films at different electric field during

the deposition (a) 0 V (b) 500 V (c) 1000 V. 143 5.4

Cross-sectional SEM images of glass/FTO/TiO2+MBI structure for perovskite films synthesis at (a) 0 V (b) 1000 V applied voltage during the deposition.

144

5.5 EDX graph for MBI thin film deposited for applied voltage of (a) 0 V (b)

1000 V. 144

5.6 Two dimensional AFM images of MBI thin film for different applied

voltage during the deposition (a) 0 V (b) 500 V (c) 1000 V. 147 5.7

(a) Absorbance spectra of MBI thin films at different electric field (b) Calculation of the band gap of MBI film by Tauc’s plot method (c) Photoluminescence spectra MBI thin film at applied voltage during the deposition.

149

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5.8

(a) Schematic diagram of MBI based perovskite solar cells with energy level (b) J-V characteristics of solar cells fabricated from MBI thin film at different electric field.

151

Chapter 6

6.1 Schematic illustration of Electric Field Assisted Spray (EFAS) pore

filling deposition setup. 167

6.2 A schematic illustration of the front dye loading under the additional

electrical driving force. 167

6.3 SEM image of spray deposited SnS particles on ZnO nanorods (a)

without electric field (b) with EFAS deposition setup. 172 6.4

UV-Vis absorption spectra of (a) N719 dye loaded TiO₂ electrodes (b) N719 dye desorbed from the TiO2 electrodes using 0.01M NaOH aqueous solution.

172

6.5 (a) J-V Curves and (b) IPCE spectra of DSSCs having different volumes

of dye solution. 174

6.6 (a) Nyquist plots of DSSCs without and with EFAS deposition technique

and (b) schematic of device structure. 174

6.7

Nitrogen adsorption and desorption isotherm analysis and Barrett-Joyner- Halenda (BJH) pore size distribution (inset) of mesoporous TiO2 sintered at 500◦C.

177

6.8 Schematic of electric field assisted continuous spray pyrolysis deposition

technique. 183

6.9

(a) X-ray diffraction patterns (b) UV-Vis absorption spectrum plot and (c) Tauc's plot showing direct band gap of SnS layers prepared by CoSP technique with and without electric field.

186

6.10

SEM and AFM images of spray deposited SnS nanostructured films on FTO coated glass at applied voltage during the deposition 0 V (a), (c) and 1 kV (b), (d), respectively.

188

6.11 EDX spectrum of SnS thin films prepared by CoSP technique without (a) and with (b) electric field and (c) Current density–voltage (J–V) characteristics of the DSSCs using these SnS films and Pt as CEs.

189

~ xviii ~ List of Tables

Table No. Caption

Page No.

Chapter 3

3.1 DC resistivity and conductivity of undoped and Eu³⁺ doped PEDOT:PSS thin films.

85 3.2 Photovoltaic performance parameters of undoped/ Eu³⁺ doped

PEDOT:PSS (hole transport buffer layer) based BHJ-OSCs

87 3.3 Conductivity of pristine PEDOT:PSS and composite PEDOT:PSS-

MoO₃ thin films.

104

3.4 Photovoltaic parameters of conventional PTB7:PC71BM OSCs incorporating spray coating PEDOT:PSS hybrid PMo-0V, PMo- 500V and PMo-1000V as HTL layers

106

Chapter 4

4.1 Photovoltaic performance parameters of binary and ternary polymer blend BHJ-OSCs

126 Chapter 5

5.1 Roughness values of MBI thin films deposited at different voltages.

147 5.2 Device performance parameters of lead-free perovskite solar cells 152 Chapter 6

6.1 Photovoltaic parameters DSSCs with different volume spray of dye via EFAS.

175 6.2 Compositional data of both SnS thin films from EDX 189 6.3 Photovoltaic performance parameters of SnS-0V, SnS-1000V and

Pt (counter electrode) based DSSCs.

190

~ xix ~

ABBREVATIONS

PV Photovoltaic

BHJ Bulk heterojunction

OSC Organic solar cell

PSC Perovskite solar cell

ITO Indium tin oxide

FTO Florine doped tin oxide

HTL Hole transporting layer

HTM Hole transporting material

ABL Anode buffer layer

PEDOT:PSS Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate

Eu Europium

MoO3 Molybdenum oxide

ETL Electron transporting layer

ETM Electron transporting material

P3HT Poly(3-hexylthiophene-2,5-diyl)

PCDTBT Poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2- thienyl-2',1',3'-benzothiadiazole)]

PTB7 Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5- b']dithiophene-2,6-diyl][3-fluoro-2-[(2-

ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]

PCE-10 Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2- b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3- fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)]

PC71BM [6,6]-Phenyl-C71-butyric acid methyl ester

MAI Methyl ammonium iodide

MBI Methyl ammonium bismuth iodide

Bi Bismuth

~ xx ~

Al Aluminum

Au Gold

Ag Silver

CVD Chemical vapour deposition

CoSP Continuous spray pyrolysis

DSSC Dye sensitized solar cell

EQE External quantum efficiency

HOMO Highest occupied molecular orbital

LUMO Lowest unoccupied molecular orbital

OPV Organic photovoltaic

PL Photoluminescence

PVD Physical vapour deposition

SEM Scanning electron microscopy

TCO Transparent conducting oxide

Pt Platinum

Pb Lead

Sn Tin

DI Deionized water

DMF Dimethylformamide

DMSO Dimethyl sulfoxide

AFM Atomic force microscopy

XRD X-ray diffraction

I-V Current-voltage

IPCE Incident photon to current conversion efficiency

EIS Electrochemical impedance spectroscopy

FF Fill factor

PCE Power conversion efficiency

V_OC Open circuit voltage

JSC Current density

η Efficiency

eV Electron volt

~ xxi ~

ml Milliliter

µm Micro meter

nm Nano meter

mA Milliampere

EFAS Electric field assisted spray