Разработка дырочно-транспортных материалов для перовскитных солнечных батарей тема диссертации и автореферата по ВАК РФ 00.00.00, кандидат наук Теплякова Марина Михайловна

• Теплякова Марина Михайловна
• кандидат наук
• 2022

Оглавление диссертации кандидат наук Теплякова Марина Михайловна

РЕФЕРАТ

SYNOPSIS

INTRODUCTION

CHAPTER 1. Literature review

1.1 Perovskite solar cells: a perspective photovoltaic technology

1.2 Hole-transport materials in perovskite solar cells: recent advances

1.2.1 Inorganic hole-transport materials

1.2.2 Spiro and analogs

1.2.3 Organic and organometallic small molecular weight hole-transport materials81

1.2.4 Conjugated polymers as hole-transport materials

1.2.5 Double HTLs: a perspective approach for PSC stability improvement

1.4 Research goal

1.5 Thesis outline

CHAPTER 2. Suzuki polycondensation for the synthesis of polytriarylamines

2.1 Introduction

2.2 Materials and methods

2.2.1 General information

2.2.2 Device fabrication

2.3 Synthetic procedures and material characterization

2.3.1 Synthesis of 2,4,6-trimethyl-N,N-diphenylaniline (1)

2.3.2 Synthesis of N,N-bis(4-bromophenyl)-2,4,6-trimethylaniline (2)

2.3.3 Synthesis of 2,4,6-trimethyl-N,N-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline (3)

2.3.4 Synthesis of PTAA (4b)

2.3.5 Synthesis of PTAA (4a)

2.4 Synthesis of PTAA 4a using Suzuki polycondensation from AA- and BB-type monomers

2.5 Summary

CHAPTER 3. Impact of Synthetic Route on Photovoltaic Properties of Isoindigo-containing Conjugated Polymers

3.1 Introduction

3.2 Materials and instrumentation

3.3 Synthesis of compounds

3.3.1 Synthesis of D2

3.3.2 Synthesis of BODID monomer

3.3.3 Synthesis of polymer P1-B

3.3.4 Synthesis of polymer P1-Sn

3.4 Device fabrication and characterization

3.4.1 Organic solar cells

3.4.2 Perovskite solar cells

3.4.3 Device characterization

3.5 Impact of synthetic route on photovoltaic properties of isoindigo-containing conjugated polymers

3.6 Summary

CHAPTER 4. Pyrene-based Hole Transport Materials with Effective n-n Stacking for Dopant-free Perovskite Solar Cells

4.1 Introduction

4.2 Materials

126

4.3 Device fabrication and material characterization

4.4 Strength of Attraction: Pyrene-Based Hole-Transport Materials with Effective n-n Stacking for Dopant-free Perovskite Solar Cells

4.5 Summary

CHAPTER 5. Incorporation of Vanadium(V) Oxide in Double Hole Transport Layer Enables Long-term Operational Stability of Perovskite Solar Cells

5.1 Introduction

5.2 Device fabrication

5.3 Operational stability of devices

5.4 Thermal stability of glass/metal oxide/CH3NH3PbI3 and glass/metal oxide/PTAA/CHsNHsPbIs stacks

5.5 Double organic-inorganic layer comprising metal oxide and PTAA

5.6 Summary

CHAPTER 6. In search of the perfect match: conjugated polymers for stable and efficient perovskite solar cells

6.1 Introduction

6.2 Materials and methods

6.3 Device fabrication and stability testing

6.4 Conjugated polymers as a part of double hole-transport layer for perovskite solar cells

6.5 Summary

Conclusions

Bibliography

ABBREVIATIONS

LIST OF PUBLICATIONS

LIST OF FIGURES AND SCHEMES

LIST OF TABLES

Appendix. Publications

РЕФЕРАТ

Актуальность темы. Согласно концепции устойчивого развития, человечество нацелено на переход к возобновляемым источникам энергии, таким как энергия солнца. На сегодняшний день доминирующее место на рынке фотовольтаических устройств принадлежит солнечным батареям на основе кристаллического кремния, которые достигают высоких значений эффективности преобразования света. Однако, производство таких устройств является дорогостоящим, [1] поэтому ученые всего мира обращаются к другим развивающимся технологиям.

Среди интересных альтернатив следует обратить внимание на перовскитные солнечные батареи (ПСБ), основным преимуществом которых является возможность нанесения фотоактивного слоя из раствора, что значительно снижает стоимость готового устройства, а также позволяет масштабировать производство с использованием печатных технологий. [2] На сегодняшний день эффективности ПСБ достигли значений сравнимых с эффективностями ячеек на основе кристаллического кремния. Основным препятствием для коммерциализации ПСБ остаётся их недостаточная стабильность в условиях эксплуатации.

Стандартная конфигурация ПСБ включает в себя фотоактивный материал со структурой перовскита, помещенный между электрон-транспортным слоем (ЭТС) снизу и дырочно-транспортным слоем (ДТС) сверху. Эти зарядово-транспортные слои выполняют функцию селективной экстракции и переноса соответствующих зарядов к электродам. Поэтому, например, одним из важных требований к ДТС является обеспечение достаточной дырочной проводимости для эффективного транспорта зарядов (^=>10-4 см2В-1с-1). [3]

Механизм деградации ПСБ является сложным и составным процессом. [4, 5] Одной из его составляющих является обратимая реакция разложения

фотоактивного слоя под действием света и температуры с образованием летучих продуктов разложения. [6] Миграция летучих компонентов перовскита из структуры приводит к необратимому разложению перовскита и, как следствие, снижению эффективности устройств. Поэтому, ключом к повышению стабильности является удержание продуктов разложения внутри устройства, что в стандартной конфигурации может быть достигнуто за счет использования ДТС с низкой газопроницаемостью. Другими важными требованиями к ДТС для обеспечения долгосрочной стабильности ПСБ являются термо- и фотостабильность материала, а также его инертность по отношению к соседним слоям.

В литературе предложены различные органические и неорганические ДТС, а также их комбинации. Кроме того, показано, что характеристики ПСБ могут зависеть от выбранного метода синтеза, степени очистки, метода формирования тонкой пленки материала. Однако в настоящее время отсутствуют стратегии рационального дизайна ДТС с высокой термо- и фотостабильностью, обеспечивающих как эффективную и селективную экстракцию зарядов, так и сохранение продуктов разложения перовскита в фотоактивном слое.

Данная работа посвящена разработке стратегии рационального дизайна ДТС, которая позволит приблизиться к созданию стабильных и эффективных ПСБ, и, как следствие, к коммерциализации перовскитной технологии.

Представленная диссертация посвящена синтезу и исследованию различных видов ДТС для стабильных и эффективных ПСБ. Главной целью работы является формулировка принципов рационального дизайна ДТС.

Для достижения поставленной цели были сформулированы следующие задачи:

1. Оценить влияние выбора пути синтеза на физико-химические свойства органических полимерных ДТС;

2. Определить влияние метода формирования слоя дырочно-транспортного материала на эффективность и стабильность фотовольтаических устройств;

3. Выявить корреляций между структурой используемого ДТС и эффективностью и стабильностью ПСБ, включающих этот материал;

4. Исследовать способность двойных слоев, содержащих различные оксиды металлов повышать стабильность ПСБ.

Для выполнения поставленных задач, были предложены следующие направления исследования, которые соответствуют главам диссертации:

1. Обзор литературы, посвященной новейшим разработкам в сфере ДТС для ПСБ;

2. Синтез популярного полимерного ДТС политриариламина (РТАА) с использованием различных реакций поликонденсации;

3. Синтез сопряженного полимера на основе изоиндиго с использованием поликонденсации Стилле и Сузуки, сравнение физико-химических свойств материалов, а также характеристик устройств на их основе;

4. Определение влияния метода нанесения низкомолекулярных ароматических ДТС на характеристики ПСБ на их основе;

5. Применение двойного ДТС на основе комбинации органического полимера и высших оксидов металла для повышения стабильности ПСБ.

6. Исследование ряда сопряженных полимеров для определения зависимости между структурой ДТС и характеристиками ПСБ на их основе.

Следующие положения выносятся на защиту:

1. Путь синтеза полимерного ДТС значительно влияет на его физико-химические свойства и на характеристики ПСБ на его основе.

В частности, полимер политриариламин (РТАА) был получен по реакции поликонденсации из мономеров АА- и ВВ- типа (где А - галоген, а В - остаток бороновой кислоты). Полученный материал обладает более высокими молекулярно-весовыми характеристиками, а также более узким

распределением по сравнению с РТАА, синтезированным из асимметричного мономера АВ-типа. Кроме того, выходы реакций на промежуточных стадиях выше при использовании предложенного синтетического пути. Эффективности ПСБ на основе полученного РТАА также более высокие главным образом за счет повышения потенциала холостого хода, что свидетельствует об уменьшении плотности дефектов в материале.

Кроме того, сопряженный полимер И^п на основе изоиндиго синтезированный по реакции поликонденсации Стилле позволяет достигать более высоких эффективностей в ПСБ по сравнению с полимером Г1-8, полученным по реакции Сузуки. Два материала одинаковой структуры с одинаковыми физико-химическими свойствами отличаются количеством свободных радикалов, что было установлено при помощи спектроскопии электронного парамагнитного резонанса. Полимер Г1-8 обладает в пять раз большим количеством дефектов, которые в фотовольтаических устройствах выступают в роли ловушкек зарядов и приводят к рекомбинационным процессам и к понижению плотностей токов.

2. Плоские ароматические соединения при организации в пленках обладают высокими зарядово-транспортными характеристиками за счет межмолекулярных взаимодействий посредством п- п стекинга.

Данное положение было продемонстрировано на ряде производных пирена с триариламиновыми заместителями. Кроме того, было показано, что нанесение пиреновых ДТС методом термического напыления в вакууме позволяет получить более высокие характеристики в ПСБ.

3. На основе данных о ПСБ, содержащих 46 различных сопряженных полимеров, сформулирован ряд требований к идеальной структуре ДТС.

4. Использование двойного ДТС на основе комбинации органического полимера и высших оксидов переходных металлов позволяет повысить стабильность ПСБ.

5. Для изучения процессов, происходящих в ПСБ впервые были применены комбинировано техники масс спектрометрии вторичных ионов и сканирующей электронной микроскопии.

6. Показана способность ДТС на основе РТАА/УОх обеспечивать долгосрочную стабильную работу ПСБ в течение более 4500 часов.

Введение диссертации (часть автореферата) на тему «Разработка дырочно-транспортных материалов для перовскитных солнечных батарей»

Научная новизна работы.

1. Впервые был синтезирован сопряженный полимер на основе изоиндиго при помощи двух реакций поликонденсации: Сузуки и Стилле. Было показано, что использование кросс-сочетания по Стилле позволяет получить материал, содержащий меньшее количество дефектов, а значит данный путь оптимален для получения высококачественных ДТС на основе сопряженных полимеров с изоиндиго;

2. Продемонстрирована перспективность использования термического напыления для нанесения пиренов с триариламиновыми заместителями в качестве ДТС в ПСБ;

3. Были сформулированы требования к структуре полимера, входящего в состав двойного ДТС, позволяющего добиться высоких эффективностей и стабильностей фотовольтаических устройств. Для этого в качестве органической составляющей ДТС было исследовано 46 полимеров различной структуры. На основе данных о стабильности и эффективности устройств было показано, что молекулярная;

4. Рекордно высокая стабильность (>4500 ч) была продемонстрирована для ПСБ, содержащих двойной ДТС на основе комбинации органического полимера политриариламина и оксида ванадия (У).

Объектом исследования являются дырочно-транспортные материалы: органические и неорганические материалы полупроводники р-типа.

Предметом исследования являются перовскитные солнечные батареи и многослойные структуры стандартной конфигурации, содержащие различные ДТС.

Область исследования соответствует направлению 1.4.4 физическая химия (химические науки): 1. Экспериментально-теоретическое определение энергетических и структурно-динамических параметров строения молекул и молекулярных соединений, а также их спектральных характеристик. 12. Физико-химические основы процессов химической технологии и синтеза новых материалов.

Теоретическая и практическая значимость работы

Экспериментальные результаты, представленные в данной диссертации, способствуют решению проблем, связанных с разработкой ДТС, необходимых для успешной коммерциализации перовскитной фотовольтаики.

Так было продемонстрировано, что природа ДТС значительно влияет на эффективность и стабильность ПСБ. В данной работе определены взаимоотношения между структурой и свойствами полимерных ДТС и характеристиками ПСБ, что ложится в основу рационального дизайна таких материалов. Кроме того, было продемонстрировано, что комбинация полимерного ДТС и высших оксидов переходного металла позволяет повысить стабильность ПСБ.

Таким образом, результаты, полученные в данной работе, позволяют сделать шаг навстречу коммерциализации высокоэффективных, стабильных и недорогих ПСБ.

Достоверность научных данных. Представленная работа была выполнена на высоком уровне. Все впервые полученные материалы были охарактеризованы набором методов, включающих спектроскопию ядерно-магнитного резонанса (ЯМР) на ядрах и 13С, гель-проникающую хроматографию, высокоэффективную жидкостную хроматографию, циклическую вольтамперометрию, спектроскопию поглощения. Работа в устройствах для каждого материала была проверена для более чем двадцати устройств в каждом эксперименте. Таким образом была показана воспроизводимость результатов, а выводы основаны на большой

статистической выборке. Положения и выводы, представленные в работе, были продемонстрированы на высокорейтинговых научных конференциях и опубликованы в международных рецензируемых научных журналах (Q1-Q2 Scopus, WoS).

Апробация результатов работы. Основные результаты работы представлялись и обсуждались на международных и российских конференциях:

1. Alexander V. Akkuratov, Marina M. Tepliakova, Irina V. Klimovich, Ilya E. Kuznetsov, Keith J. Stevenson. Synergy of Organic and Perovskite Materials: Conjugated Polymers for Stable and Efficient Perovskite Solar Cells. HYBRIDOE21, Spain, online (2021)

2. Marina M. Tepliakova, A.V. Akkuratov, I.V. Klimovich, I.E. Kuznetsov, and P. A. Troshin. Conjugated polymers for stable and efficient perovskite solar cells: in search of the perfect match. HOPE-PV (2020)

3. Marina M. Tepliakova, Igor K. Yakushenko, and Pavel A. Troshin. Strength of Attraction: Pyrene-based Hole-transport Materials with Effective n-n Stacking for Dopant-free Perovskite Solar Cells. IFSOE (2020)

4. Marina M. Tepliakova, Igor K. Yakushenko, Keith J. Stevenson, and Pavel A. Troshin. Strength of attraction: pyrene-based hole-transport materials with effective n-n stacking improve efficiency and stability of perovskite solar cells. StabPero (2020)

5. Marina M. Tepliakova. Conjugated polymers: perspective hole transport materials for perovskite solar cells. 1st International School on Hybrid Organic and Perovskite Photovoltaics. Moscow, Russia (2019)

6. Marina M. Tepliakova. Exploring the impact of polymeric hole transport layer materials on the efficiency and stability of perovskite solar cells. EMRS Fall 2019. Warsaw, Poland (2019)

7. Marina M. Osipova. Suzuki polycondensation of poly(triarylamine) hole transport material for perovskite solar cells with improved efficiency. MIT Next Generation. Moscow, Russia (2018)

8. Марина М. Осипова. Новый подход к синтезу политриариламина материала дырочно-транспортного слоя в перовскитных солнечных батареях. Ломоносов 2019. Москва, Россия (2019)

9. Марина Осипова. Синтез нового сопряженного полимера для органических солнечных батарей, изучение влияния метода получения полимера на фотовольтаические свойства устройств. Ломоносов 2018. Москва, Россия (2018).

Публикации. Основные результаты работы изложены в шести публикациях в международных журналах, индексируемых в WoS и Scopus.

1. [Indexed in WoS and Scopus, Q1, IF: 6.367] Tepliakova M. M., Yakushenko I. K., Stevenson K. J., Troshin P. A. Strength of attraction: pyrene-based hole-transport materials with effective n-n stacking for dopant-free perovskite solar cells // Sust. Energy Fuels - 2021. - V. 5. - P. 283-288.

2. [Indexed in WoS and Scopus, Q2, IF: 3.004] Tepliakova M. M., Mikheeva A. N., Somov P. A., Statnik E. S., Korsunsky A. M., Stevenson K. J. Combination of Metal Oxide and Polytriarylamine: A Design Principle to Improve the Stability of Perovskite Solar Cells // Energies - 2021. - V. 14. - P. 5115.

3. [Indexed in WoS and Scopus, Q2, IF: 2.527] Tepliakova M. M., Kuznetsov I. E., Avilova I. A., Stevenson K. J., Akkuratov A. V. Impact of Synthetic Route on Photovoltaic Properties of Isoindigo-Containing Conjugated Polymers // Macromol. Chem. Phys. - 2021. - V. 222. - P. 2100136.

4. [Indexed in WoS and Scopus, Q1, IF: 6.475] Tepliakova M. M., Mikheeva A. N., Frolova L. A., Boldyreva A. G., Elakshar A., Novikov A. V., Tsarev S. A., Ustinova M. I., Yamilova O. R., Nasibulin A. G., Aldoshin S. M., Stevenson K. J., Troshin P. A. Incorporation of vanadium (V) oxide in hybrid hole transport layer enables long-term operational stability of perovskite solar cells // J. Phys. Chem. Lett. - 2020. -V. 11. - P. 5563.

5. [Indexed in WoS and Scopus, Q1, IF: 9.229] Boldyreva A. G., Zhidkov I. S., Tsarev S. A., Akbulatov A. F., Tepliakova M. M., Fedotov Y. S., Bredikhin S. I., Postnova E. Y., Luchkin S. Y., Kurmaev E. Z., Stevenson K. J., Troshin P. A. Unraveling

the Impact of Hole Transport Materials on Photostability of Perovskite Films and p-i-n Solar Cells // ACS Appl. Mater. Interfaces. - 2020. - V. 12. - P. 19161.

6. [Indexed in WoS and Scopus, Q2, IF: 2.415] Tepliakova M. M., Akkuratov A. V., Tsarev S. A., Troshin P. A. Suzuki polycondensation for the synthesis of polytriarylamines: A method to improve hole-transport material performance in perovskite solar cells // Tetrahedron Lett. - 2020. - V. 61. - P. 252217.

Личный вклад автора состоит в синтезе материалов, характеризации их физико-химических свойств (методами ядерно-магнитного резонанса, хроматографии (ВЭЖХ, ГПХ, ГХ), электрохимии, спектроскопии поглощения, термогравиметрии, сканирующей дифференциальной калориметрии, рентгеновской дифракции), сборке фотовольтаических устройств, изучении эффективности и стабильности (измерение внешней квантовой эффективности и вольтамперных характеристик), анализе данных, написании публикаций, представление работ на международных конференциях. Автором также была создана и опубликована обложка для журнала Macromolecular Chemistry and Physics, Wiley (Volume 222 Issue 15).

Структура и объем диссертации. Диссертация состоит из вступления, шести частей, заключения, списка литературы из 188 источников. Объем диссертации составляет 265 страниц, 52 картинки и 10 таблиц.

В первой главе приведена основная информация о перовскитных солнечных батареях, об истории их развития, а также о различных архитектурах устройств. Далее перечислены важнейшие требования к материалам дырочно-транспортного слоя. В данной главе также приведен обзор литературы, посвященный основным достижениям и направлениям развития для различных классов дырочно-транспортных материалов, таких как неорганические, органические низкомолекулярные,

металлоорганические, полимерные органические, а также двойные ДТС, основанные на различных комбинациях.

Вторая глава посвящена синтезу одного из самых распространенных ДТС политриариламина (РТАА) по реакции поликонденсации Сузуки из мономеров АА- и ВВ- типа, где А — галоген, а В — остаток боронового эфира (Рисунок 1). В отличие от описанного в литературе метода синтеза из несимметричного мономера АВ-типа, [7] предложенный синтетический путь позволяет получить менее дефектный материал главным образом за счет простоты и эффективности очистки промежуточных симметрично-замещенных продуктов.

РИ

к-м

4 снси р|1 Вг

2 (99%) Мономер АА-типа

Рс12(с1Ьа)з (0.5 моль%), Р(о-1о1)3 (1.5 моль%)

2+3

толуол, аликват 336 Рс1(ОАс)2 (0.5 моль%), РРИ3(1.5 моль%)

К2С03, вода, 90°С Рс1(РРИ3)4

В

3 (65%) о Мономер ВВ-типа

4а (45%)

4Ь (24%) ЛГ^Г

РТАА

Рисунок 1. Схема синтеза РТАА 4а и 4Ь.

Соединение 1, полученное по реакции кросс-сочетания Бухвальда-Хартвига, бромировали для получения 2 мономера АА-типа с количественным выходом. Далее 2 литировали н-бутиллитием с последующей обработкой дилитиевых промежуточных соединений 2-изопропокси-4,4,5,5-тетраметил-1,3,2-диоксабороланом, в результате чего получили мономер ВВ-типа чистотой 97% по данным высокоэффективной жидкостной хроматографии (ВЭЖХ). Промежуточные соединения характеризовали методами ЯМР на ядрах 1Н и 13С и ВЭЖХ.

Далее, была подобрана система катализаторов. Синтез с использованием Pd2dba3 и Р(о4о1)3 позволил получить полимер 4а со среднечисолвой молекулярной массой Мп=20,8 кДа и узким молекулярно-весовым распределением, характеризуемым низким индексом

полидисперсности Мте/Мп=1,4. Коммерческий материал имел характеристики Мп=25,2 кДа и Мте/Мп=2,2. А использование системы Pd(OAc)2 и РР^ привело к материалу 4Ь с более низкой массой Мп=10,7 kDa и М^/Мп=1,4.

На следующем этапе три фракции полимера были исследованы как ДТС в ПСБ со стандартной п-ьр конфигурацией, представленной на Рисунке 2. Оптимизированная структура ПСБ включала в себя проводящий оксид индия, легированный оловом (1ТО), покрытый электрон-транспортным слоем SnO2, пассивированным производным фуллерена фенил-С61-бутановой кислотой (РСВА). [8, 9] Далее был нанесен фотоактивный слой СН3КН3РЬ13 (МАРЬ13). Различные фракции РТАА были нанесены из раствора в хлорбензоле, а после покрыты электродом из МоОх (10 нм) и Ag (100 нм), нанесенным методом термического напыления в вакууме.

Характеристики лучших устройств с различными ДТС представлены на Рисунке 2б.

Серебро

Оксид металла ДТС

МАРЫ

Бп02 :РСВА 1ТО

о

25 20 15

< 10

4а чо!

- 4Ь \\ ш

РТАА \\ 1

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 V, В

Рисунок 2. Архитектура ПСБ (а); вольтамперные характеристики лучших устройств, содержащих 4а, 4Ь, РТАА ^ (б).

Устройства, содержащие полимер 4Ь достигли эффективностей (КПД) 8,2% с невысокими факторами заполнения (FF) 43%, что свидетельствует о недостаточных зарядово-транспортных характеритиках материала. Что касается коммерческого PTAA Ref и полимера 4a, устройства на их основе показали более высокие эффективности до 16,7% и 17,6%, соответственно. Факторы заполнения (FF) и плотности токов короткого замыкания (Лс)

сравнимы для обоих видов устройств. Важно отметить, что потенциалы холостого хода (Уос) устройств с 4а достигали 1,06 В, что на 40 мВ выше, чем значения Уос для устройств с коммерческим материалом, и может свидетельствовать о меньшей плотности дефектов на границе перовскит/ДТС, что в свою очередь приводит к подавлению процессов рекомбинации на ловушках зарядов. [10]

В третьей главе представлен синтез сопряженного полимера на основе изоиндиго при помощи различных палладий-катализируемых реакций: поликонденсации Сузуки (Р1-В) и Стилле (Р1^п) (Рисунок 3).

Ос*

Мау

02 (49%)

01 (99%)

ТТВТВТТ

Оес

Пег

ВгСЮЮ

ВСЮЮ (40%)

Оес

02+ВЮ0Ю

01+ВСЮЮ

V

IV

Р1-В

Р1-Зп

Оес

Рисунок 3. Схема синтеза материалов Р1-В и р1^п.

Условия: ¡) ^бромсукцинимид, о-ДХБ; (//) , ТГФ, -78°^ SnMeзCl; (ш) бис(пинаколато)дибор, NaOAc, Pd(PPhз)2Cl2, (¡у) Pd(PPhз)4, Bu4NOH, MeOH,

толуол;(у) Pd(PPh3)4, толуол.

Синтез полимеров Р1-В и Р1^п проводился по паладий-катализируемым реакциям Сузуки и Стилле, соответственно. Для гетерофазной реакции Сузуки были получены бороновое производное изоиндиго BODID и дибромзамещенный семизвенный мономер D1. Для реакции Стилле были синтезированы оловосодержащий мономер D2 и дибромзамещенное производное изоиндиго BrODID.

Молекулярно-весовые характеристики полимеров Р1-В и P1-Sn были исследованы при помощи гель-проникающей хроматографии и составили Мте=76 кДа Мте/Мп =1,8, и Мте=77 кДа Мте/Мп =1,7 соответственно (Рисунок 4а). Оптоэлектронные свойства материалов были рассчитаны из спектров поглощения и циклических вольтамперограмм (ЦВА) (Рисунок 4б-в). Удельная концентрация парамагнитных дефектов в материалах была рассчитана из спектров электронного парамагнитного резонанса (ЭПР) (Рисунок 4г).

Рисунок 4. ГПХ (а), спектры поглощения (б), ЦВА (в) и спектры ЭПР (г) для Р1-В

и Р1-Бп.

Далее, полимеры Р1-В и Р1^п были исследованы как донорные составляющие активного слоя органических солнечных батарей вместе с акцепторным фуллереновым производным PCBM (метиловый эфир фенил-С61-бутановой кислоты), а также как ДТС в ПСБ. Конфигурации и вольтамперные характеристики лучших устройств представлены на Рисунке 5.

V, В V, В

Рисунок 5. Вольтамперные характеристики (обратные сканы) и конфигурации органических солнечных батарей (а) и ПСБ (б).

Органические солнечные батареи, содержащие Р1^п достигли более высоких эффективностей КПД=4,1% по сравнению с устройствами, содержащими Р1-В с КПД= 2,7% в основном за счет значительно более низких Уос и «с. Похожая закономерность наблюдалась для перовскитных солнечных батарей, в которых устройства достигли 15,1% с Р1^п и 12,6% с Р1-В, однако в случае ПСБ основной вклад в снижение эффективности внесло значительное понижение токов для устройств с Р1-В до ,ЛС =17,2 мА/см2. Понижение соответствующих характеристик в обоих случаях означает прохождение процессов рекомбинации и высокую плотность ловушек зарядов в Р1-В, что также подтверждается спектрами ЭПР.

В четвертой части четыре производные пирена исследованы в качестве ДТС в ПСБ (Рисунок 6).

Уровни высших занятых молекулярных орбиталей (ВЗМО) материалов, рассчитанные из потенциала окисления на ЦВА, изменяются в интервале от

-5,5 до -5,4 эВ и сочетаются с уровнем валентной зоны перовскита. Для материалов характерны высокие температуры плавления от 270°С (для Y3) и температуры разложения >450°С, что было обнаружено методами термогравиметрии и дифференциальной сканирующей калориметрии.

У1 У2 УЗ У4

Рисунок 6. Структуры пиреновых производных У1-У4.

Тонкие пленки производных пирена Y1-Y4 можно наносить как растворными методами, так и методом термического напыления в вакууме. Поэтому следующим шагом материалы Y1-Y4, нанесенные двумя различными методами, были исследованы как ДТС в ПСБ. Самые высокие эффективности КПД=17,9% удалось получить для устройств, содержащих напылённое производное Y2, чему способствовали высокие Уос=1,1 В. Самые низкие характеристики продемонстрировали ПСБ, содержащие пирен Y4, нанесенный растворным методом, что объясняется пониженной растворимостью характерной для пиреновых производных с заместителями в положениях 1 и 8. [11]

Конфигурации устройств и диаграмма энергетических уровней представлены на Рисунке 7а-б, а статистика, отражающая распределение эффективностей устройств, представлена на Рисунке 7в.

Рисунок 7. Архитектура ПСБ (а); зонная диаграмма (б); гистограммы с распределением эффективностей ПСБ с пиреновыми производными У1-У4, нанесенными двумя различными методами. (в).

Устройства, содержащие термически напылённые ДТС У1-У4 показывают более высокие эффективности для всех четырех материалов. Главным образом, эффективности повышались за счет вклада Уос, что скорее всего свидетельствует о высоком качестве интерфейса между материалом и фотоактивным слоем.

В пятой части показано успешное применение двойного ДТС, содержащего органический компонент РТАА и оксид ванадия (У), позволяющее повысить стабильность ПСБ.

В литературе ранее было показано, что использование оксида молибдена (VI) в сочетании с органическим низкомолекулярным соединением в качестве ДТС позволяет повысить стабильности ПСБ на

воздухе, [12] однако данная система имела недостаточную стабильность при нагревании. [13]

В этой части работы предложен двойной ДТС, содержащий оксд ванадия (V). На начальном этапе было определено, что оптимальная толщина оксида ванадия, обесечивающая наибольшую эффективность в ПСБ, составляет 30-45 нм. Использования двойного слоя РТАА/УОх в ПСБ с различными архитектурами позволяет достичь эффектинвостей до 20.1%. Стабильность устройств с двойным ДТС на основе РТАА^Ох была исследована при постоянном освещении в инертной атмосфере (Рисунок 8).

Рисунок 8. Конфигурация устройств (а), схематическое изображение деградационной камеры (б), изменение эффективности ПСБ с различными ДТС

при постоянном освещении (в).

Референсные устройства с РТАА/МоОх в качестве ДТС потеряли более 50% от своей первоначальной эффективности, в то время как устройства с VOx сохранили более 80% эффективности после 4500 ч.

Дополнительно была исследована термическая стабильность двуслойных образцов с конфигурацией стекло/перовскит/МоОх или VOx. Было обнаружено, что устройства с оксидом ванадия менее подвержены термической деградации.

В шестой части работы представлено исследование влияния стркутуры органичекого ДТС на эффективность и стабильность ПСБ. В эксперименте

было задействовано 46 полимеров, содержащих различные донорные, акцепторные блоки и солюбилизирующие заместители (Рисунок 9).

Y=S: H1 X=CZ Н2, НЗ X=T-BDT Н4-Н6 X=AlkO-BDT Н7 X=TzTz Н8-Н10 X=DTDOne H11,H12X=II Н13, Н14 X=AlkOBz

Y=0: H15, H16 R1=Alk, R2=H, X=Cz

H17, H18 R1=H. R2=OOct, X=Cz H19 X=CPDT

nT*

Y=0: H20 X=T H21,H22X=Cz H23 X=T-BDT H24X=BO-T-CZ Y=N-Dec: H25 X=Cz

Y=C2H2Alk2: H26 X=Cz H27 X=T-CZ-T H28, H29 X=T-BDT

H35 Y=0, X=FI

Y=S:H36, H37 X=FI H38 X=CZ H39 X=BT-T-AlkOBz

fDPP-X^ fT-BDT-X|n

-[т- BT-CP DT- BT-T-X-j-n

H30 X=FI H31 X=CZ H32 X=DBSi

H40 X =BT-T-CZ-T-BT H41X =TzTz

H42 X=FQX H43 X=DTBTz H44 X=DTBIz

H45 -fcZ-BT-j; H46-¡-T-BO-j-n

X

AllfjMk

cz

Dei Dec

T-BDT

Oct

DBSi

Aik FQX DTDOne DTBTz DTBIz

t DT т^-г^ д||.лг>_ d1- U Allrwl d2— lj с d3- u AIL-w

ВТ BO TzTz AlkOBz

R = H, Alkyl R = H, F R = H, Alkyl

Рисунок 9. Структуры полимеров H1-H46.

В представленном наборе присутствуют различные возможные модификации полимерной цепи, представленные в литературе. Большинство полимеров содержат тиофен, бензооксаодиазол или бензотиадиазол. Некоторые материалы содержат очень сильные акцепторные блоки, такие как дикето-пирроло-пиррол (DPP, H40-41), хиноксалин (FQX, H25 and H42), и изоиндиго (II, H11-12), акцептор средней силы тиазолотиазол (TzTz, H7 and H41), сильный донор бензодитиофен (T-BDT, H2-3) и более слабый донорный блок бензодитиофен-дион (DTDOne H8-10) и др. В качестве

солюбилизирющих заместителей материалы содержат линейные и разветвленные алкил-, тиоалкил-, алкокси- и тиоалкокси- заместители. Некоторые материалы также содержат фтор.

Уровни высших занятых молекулярных орбиталей (ВЗМО) полимеров H1-H46 варьируются от -5,74 эВ (H10) до -5,18 эВ (H28). Для каждого материала предварительно проведены эксперименты по оптимизации скорости нанесения. Обнаружено, что уровни энергии ВЗМО и молекулярно-весовые распределения не коррелируют с характеристиками ПСБ.

Далее было исследовано влияние структуры ДТС на стабильность ПСБ. Для этого были созданы ПСБ стандартной конфигурации, содержащие один из материалов H1-H46 в качестве органической составляющей ДТС в комбинации с оксидом ванадия. За изменением эффективности при постоянном освещении следили путем снятия вольтамперных характеристик через определенные промежутки времени. Тепловая карта, отражающая результаты эксперимента и архитектура устройств приведены на Рисунке 10а-б.

Согласно результатам, наиболее стабильными оказались устройства, содержащие H17, H18, H46, H22 (Рисунок 10в). Внимательное рассмотрение и сравнение структур сопряженных полимеров и стабильности устройств позволило вывести некоторые принципы для рационального дизайна полимерных ДТС, более подробно изложенные в тексте диссертации.

Дополнительно, косые срезы устройства сравнения с двойным ДТС PTAA/VOx были исследованы методом масс-спектроскопии вторичных ионов. Было обнаружено, что двойной слой позволяет сохранить продукты разложения перовскита внутри устройства. С другой стороны, слои PTAA и перовскита взаимно диффундируют. [14]

Рисунок 10. Тепловая карта, иллюстрирующая изменение эффективности ПСБ с Н1-И46 в качестве ДТС (а), конфигурация устройств (б), четыре лучших материала (в), СЭМ среза свежего и деградировавшего устройств с РТАА в

качестве ДТС (г).

При помощи сканирующей электронной микроскопии (СЭМ) были охарактеризованы срезы свежих и деградировавших устройств сравнения с РТАА и устройств с И17. Для менее стабильных устройств было обнаружено, что после 1000 часов постоянного освещения в толще перовскита появляются полости, чего не наблюдается для ПСБ с И17. Повышенная стабильность устройств может быть связана со структурой И17, который содержит кислород и серу, которые в свою очередь могут вступать в более сильные взаимодействия на границе с перовскитом за счет межмолекулярных взаимодействий.

Заключение

В данной диссертации проведен ряд экспериментов направленных на разработку правил для рационального дизайна ДТС для эффективных и стабильных ПСБ. Различные аспекты создания ДТС, такие как синтетический путь, выбор метода нанесения, дизайн структуры материала и

комбинация с другими материалами рассмотрены как ключевые факторы, влияющие на работу перовскитных батарей.

Влияние синтетического пути на качество материалов было изучено на примере двух полимеров полупроводников p-типа. В частности, было показано, что синтез широкоиспользуемого ДТС РТАА из мономеров АА- и ВВ- типа позволяет получить высококачественный материал. ПСБ, содержащие полученный полимер достигают высоких эффективностей более 17,6% без использования дополнительных допантов. Превосходство полученного материала обусловлено высокой степенью очистки промежуточных соединений, что позволило получить материал с низкой плотностью дефектов ловушек зарядов.

Кроме того, удалось значительно увеличить выходы реакции по сравнению с синтезом РТАА из мономера АВ-типа, описанным в литературе.

В качестве другого примера показано получение сопряженного полимера, содержащего блок изоиндиго, с использованием двух различных реакций: поликонденсаций Стилле и Сузуки. Оба полимера обладали схожими оптоэлектронными свойствами и молекулярно-весовыми распределениями. Однако, фотовольтаические устройства на основе полученных полимеров продемонстрировали существенную разницу в характеристиках. В частности, перовскитные и органические солнечные элементы, содержащие материал, полученный с помощью реакции Стилле, показали эффективности 15,1% и 4,1% соответственно, тогда как устройства, содержащие полимер, синтезированный по реакции Сузуки, достигали меньших эффективностей 12,6% и 2,7% в соответствующих устройствах. С помощью ЭПР было обнаружено, что материал, полученный с помощью поликонденсации Сузуки, содержит в пять раз больше свободных радикалов, которые ведут себя как ловушки заряда в электронных устройствах. В этом эксперименте основные условия реакции Сузуки влияют на структуру мономера на основе изоиндиго, что приводит к снижению производительности фотовольтаических устройств. Таким образом, на

основании двух примеров было показано, что выбор пути синтеза является важным и зачастую решающим фактором, который может повлиять на качество полупроводникового материала и, следовательно, на работу фотовольтаических устройств.

На следующем этапе четыре производных пирена с ариламиновыми заместителями, были исследованы как ДТС в ПСБ. Наиболее высокой эффективности до 17,9% удалось достичь для устройств, содержащих Y2 с диариламиновыми заместителями в положениях 1 и 6, где один из арилов представлен а-нафтилом. Дополнительно было продемонстрировано, что термическое напыление в вакууме является предпочтительным методом формирования тонких пленок в случае летучих органических соединений. Тонкие пленки, полученные методом спин-коутинга, могут быть неравномерными по толщине, кроме того, испарение растворителей с поверхности происходит с разной скоростью, что может повлиять на воспроизводимость результатов и эффективность устройств. В пользу этих утверждений говорит тот факт, что устройства на основе термически напылённых производных пирена имеют напряжения холостого хода в среднем на 90 мВ выше, чем устройства с ДТС, нанесенными из растворов. Более высокое напряжение холостого хода, в свою очередь, связано с плотностью взаимодействий между перовскитом и поверхностями ДТС.

Далее, было показано, что использование двойного ДТС на основе комбинации органического полимера и неорганических оксидов металлов р-типа позволяет повысить стабильность ПСБ. Так, при использовании комбинации РТАА и оксида ванадия удалось получить устройства стабильные на протяжении более чем 4500 часов. Такой двойной ДТС является более термически стабильным, чем представленный в литературе ДТС на основе оксида молибдена, который теряет целостность при повышенных температурах, что в свою очередь приводит к снижению характеристик устройств. [15, 16] Другой возможной причиной более низкой стабильности устройств, содержащих оксид молибдена, является протекание

паразитных химических процессов при контакте перовскита и оксида. Прохождение химических реакций на границе интерфейса может привести к её деградации и снижению характеристик устройства. Таким образом, роль неорганического компонента двойного ДТС требует дальнейшего изучения.

Используя двойной ДТС, набор из 46 сопряженных полимеров различной структуры исследовали в качестве органической составляющей двойного слоя в ПСБ. Тщательное сравнение динамики деградации ПСБ со структурой полимеров, использованных в качестве ДТС, позволило вывести некоторые закономерности для рационального дизайна полимерных ДТС. Например, наличие в структуре ДТС сильных доноров таких как бензодитиофен, бензодитиофендион, как и введение в структуру фтора привело к быстрой деградации ПСБ. Важно, что близкое расположение на полимерной цепи объемных солюбилизирующих заместителей в ДТС также отрицательно сказывается на стабильности ПСБ. Это можно объяснить образованием доменов из углеродных цепей с высокой газопроницаемостью, не препятствующих миграции продуктов разложения перовскита из структуры. Интересно отметить, что введение кислородсодержащих гетероциклических структур в состав полимера обеспечивает стабильную работу устройств, что, вероятно, связано с сильными межмолекулярными взаимодействиями между кислородом и свинцом.

Список литературы диссертационного исследования кандидат наук Теплякова Марина Михайловна, 2022 год

Литература

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4. Akbulatov A. F., Luchkin S. Y., Frolova L. A., Dremova N. N., Gerasimov K. L., Zhidkov I. S., Anokhin D. V., Kurmaev E. Z., Stevenson K. J., Troshin P. A. Probing the Intrinsic Thermal and Photochemical Stability of Hybrid and Inorganic Lead Halide Perovskites // The Journal of Physical Chemistry Letters. - 2017. - V. 8, № 6. - P. 1211-1218.

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6. Juarez-Perez E. J., Ono L. K., Maeda M., Jiang Y., Hawash Z., Qi Y. Photodecomposition and thermal decomposition in methylammonium halide lead perovskites and inferred design principles to increase photovoltaic device stability // Journal of Materials Chemistry A. - 2018. - V. 6, № 20. - P. 9604-9612.

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12. Sanehira E. M., Tremolet de Villers B. J., Schulz P., Reese M. O., Ferrere S., Zhu K., Lin L. Y., Berry J. J., Luther J. M. Influence of Electrode Interfaces on the Stability of Perovskite Solar Cells: Reduced Degradation Using MoOx/Al for Hole Collection // ACS Energy Letters. - 2016. - V. 1, № 1. - P. 38-45.

13. Christians J. A., Schulz P., Tinkham J. S., Schloemer T. H., Harvey S. P., Tremolet de Villers B. J., Sellinger A., Berry J. J., Luther J. M. Tailored interfaces of unencapsulated perovskite solar cells for >1,000 hour operational stability // Nature Energy. - 2018. - V. 3, № 1. - P. 68-74.

14. Tepliakova M. M., Mikheeva A. N., Somov P. A., Statnik E. S., Korsunsky A. M., Stevenson K. J. Combination of Metal Oxide and Polytriarylamine: A Design Principle to Improve the Stability of Perovskite Solar Cells // Energies. - 2021. - V. 14, № 16. - P. 5115.

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16. Tracy H. Schloemer J. A. R., Timothy S. Gehan, Sanjini Mamayakkara, Taylor Moot, Steven Harvey, Rosemary C. Bramante, Sean Dunfield, Amy Louks, Annalise E. Maughan, Lyle Bliss, Michael D. McGehee, Maikei F.A.M. van Hest, Matthew O. Reese, Stacey F. Bent, Joseph J. Berry, Joseph M. Luther, Alan Sellinger. Vanadium Oxide Interfacial Layer in Perovskite Solar Cells for High Temperature Operational Stability of Unencapsulated Devices in Air // Book

Vanadium Oxide Interfacial Layer in Perovskite Solar Cells for High Temperature Operational Stability of Unencapsulated Devices in Air / Editor, 2020.

Публикации автора по теме диссертации.

1. Tepliakova M. M., Yakushenko I. K., Stevenson K. J., Troshin P. A. Strength of attraction: pyrene-based hole-transport materials with effective n-n stacking for dopant-free perovskite solar cells // Sust. Energy Fuels - 2021. - V. 5. - P. 283-288.

2. Tepliakova M. M., Mikheeva A. N., Somov P. A., Statnik E. S., Korsunsky A. M., Stevenson K. J. Combination of Metal Oxide and Polytriarylamine: A Design Principle to Improve the Stability of Perovskite Solar Cells // Energies - 2021. - V. 14. -P. 5115.

3. Tepliakova M. M., Kuznetsov I. E., Avilova I. A., Stevenson K. J., Akkuratov A. V. Impact of Synthetic Route on Photovoltaic Properties of Isoindigo-Containing Conjugated Polymers // Macromol. Chem. Phys. - 2021. - V. 222. - P. 2100136.

4. Tepliakova M. M., Mikheeva A. N., Frolova L. A., Boldyreva A. G., Elakshar A., Novikov A. V., Tsarev S. A., Ustinova M. I., Yamilova O. R., Nasibulin A. G., Aldoshin S. M., Stevenson K. J., Troshin P. A. Incorporation of vanadium (V) oxide in hybrid hole transport layer enables long-term operational stability of perovskite solar cells // J. Phys. Chem. Lett. - 2020. - V. 11. - P. 5563.

5. Boldyreva A. G., Zhidkov I. S., Tsarev S. A., Akbulatov A. F., Tepliakova M. M., Fedotov Y S., Bredikhin S. I., Postnova E. Y, Luchkin S. Y., Kurmaev E. Z., Stevenson K. J., Troshin P. A. Unraveling the Impact of Hole Transport Materials on Photostability of Perovskite Films and p-i-n Solar Cells // ACS Appl. Mater. Interfaces. - 2020. - V. 12. - P. 19161.

6. Tepliakova M. M., Akkuratov A. V., Tsarev S. A., Troshin P. A. Suzuki polycondensation for the synthesis of polytriarylamines: A method to improve holetransport material performance in perovskite solar cells // Tetrahedron Lett. - 2020. - V. 61. - P. 252217.

SYNOPSIS

In the present work, a comprehensive study of various aspects related to holetransport layer (HTL) materials for stable and efficient perovskite solar cells is presented. First of all, the influence of the synthetic path on the quality of organic polymeric materials was evaluated. In particular, it was shown, that state-of-the-art polymeric HTL poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) synthesized using Suzuki polycondensation from AA- and BB-type materials enables 17.6% efficiencies in perovskite solar cells due to its improved quality.

Furthermore, it was shown, that the novel conjugated isoindigo-containing polymer (P1-Sn) obtained using Stille polycondensation allows reaching 15.1% efficiency in perovskite solar cells, while devices using material possessing the same structure and physicochemical properties and obtained via Suzuki reaction (P1-B) exhibit lower 12.7% efficiency. Using electron spin resonance spectroscopy it was demonstrated, that material P1-B provides a five times higher density of defects, which may function as charge traps impeding photovoltaic device performance.

Additionally, the effect of the deposition technique applied to the formation of the layer of hole-transport material was investigated. Particularly, four pyrene derivatives bearing diarylamine moieties coupled with teri-butylphenyl (Y1), a-naphthyl (Y2, Y4), and P-naphthyl (Y3) substituents were evaluated as HTLs in perovskite solar cells. All the devices incorporating thermally evaporated HTL exhibited superior efficiencies up to 17.9% (for Y2) over devices including spin-coated HTL.

Finally, it was demonstrated, that the combination of organic polymer and vanadium (V) oxide used as a double HTL boosts the stability of perovskite solar cells up to more than 4500 h. Using this approach a row of 46 conjugated polymers was investigated in perovskite solar cells, which allowed establishing the ground

rules for the rational design of polymeric HTL for stable and efficient perovskite solar cells.

Relevance and background of the research topic. According to the concept of sustainable development, humankind is aimed to transfer to renewable energy sources, such as solar energy. Currently, the prevalent photovoltaic technology is based on crystal silicon. Silicon-based solar cells exhibit high power conversion efficiencies. However, the production of crystal silicon is expensive. [1] Therefore, scientists around the world turn their attention towards alternative technologies.

In this regard, the perovskite solar cells (PSCs) are attracting considerable interest. Their main advantage is the low fabrication cost due to the inexpensive photoactive layer formation using solution-based deposition techniques, which can be easily scaled up using printing and roll-to-roll technologies. [2] The efficiencies achieved by the PSCs are already comparable to that of crystal-silicon photovoltaics. The main obstacle on the way to PSCs commercialization is their insufficient stability under working conditions.

The classical configuration of the PSCs includes a photoactive layer with a perovskite structure sandwiched between the electron transport layer on the bottom and the hole-transport layer (HTL) on the top. Both charge-transport layers selectively extract and carry on the corresponding charges to the electrodes. Therefore, the first requirement for the HTL is sufficient hole mobility for effective charge transport (^h>1x10-4 cm2 V-1 s-1). [3]

It is important to note that the decrease in PSC efficiency under working conditions is a complex process, which includes many various degradation paths. [4, 5] One of the most studied is the reversible decomposition of the photoactive layer under working conditions with the formation of volatile decomposition products. [6] The migration of volatile substances from the structure leads to the irreversibility of perovskite decomposition and consequent device efficiency deterioration. Therefore, a key to improving stability is the preservation of the decomposition products in the photoactive layer, which can be achieved by selecting a low gas permeability HTL. Additionally, to ensure long-term stability,

the chosen HTL should be thermally and photo-stable, as well as inert towards adjacent layers of the device.

Previous studies explored as HTLs a plethora of various organic and inorganic materials or their combinations. According to the literature, PSC characteristics depend on the nature of HTL, the chosen synthetic approach, degree of purification, deposition technique, etc. However, currently, there are no strategies for the rational design of HTLs for both efficient and selective extraction of charges, preservation of perovskite decomposition products in the photoactive layer, and material stability.

The present thesis is focused on the development of HTLs rational design strategy, which is an important and relevant scientific problem, as its solution allows one to get closer to stable and efficient PSCs, which is necessary for the commercialization of the technology.

The aims, objectives, and the research methodology of the thesis.

The thesis is focused on the synthesis and investigation of various types of HTLs for stable and efficient PSCs. The main goal of the work is to derive the principles of the rational design of hole transport materials. It implies the following aims and objectives of the present dissertation work:

1. Evaluation of the impact of the synthetic route on the physicochemical properties of organic polymeric HTLs;

2. Assessing the influence of HTL deposition technique(s) on the device performance;

3. Defining the relationship between the structure of the material and its impact on the stability and efficiency of PSCs;

4. Comparing the ability of double HTLs comprising various metal oxides to improve the stability of perovskite solar cells.

The methodology including the following number of steps was proposed:

1. The review of relevant scientific literature on the hole-transport materials for PSCs

2. Synthesis of polymeric HTL polytriarylamine (PTAA) using various polymerization reactions; comparison of the performance of PSCs incorporating obtained materials

3. Synthesis of isoindigo-based conjugated polymer using various polycondensation reactions, comparison of physicochemical properties of obtained materials, and their performance in PSCs

4. Investigation of low molecular weight aromatic materials as HTLs in PSCs along with evaluating the influence of deposition techniques on the performance of devices

5. Application of double HTL with low gas permeability including PTAA and metal oxides in the highest oxidation state for PSC stability improvement

6. Investigation of a row of conjugated polymers for defining the relationship between the material structure and performance.

The statements to be defended in the thesis can be summarized as follows:

1. The synthetic pathway of polymeric hole-transport materials influences their quality and their performance in photovoltaic devices. Particularly, for polytriarylamine (PTAA) the Suzuki polycondensation from AA- and BB- type monomers (where A is halogen group and B is boronic acid group) leads to PTAA with higher yields, higher molecular weights, and lower polydispersity indexes compared to the materials obtained using Suzuki polycondensation from asymmetrical AB-type monomer or with radical polymerization. Additionally, in the case of isoindigo-containing conjugated p-type polymers using Stille polycondensation allows synthesizing less defective material, than in the case of Suzuki polycondensation.

2. Flat aromatic molecules can provide increased charge transfer mechanisms in thin films due to intermolecular interactions via n-n stacking. This can be applied for HTLs in PSCs to satisfy the requirement of effective charge transport without additional p-doping. In particular, pyrene derivatives with triarylamine substituents are perfect candidates for illustrating this concept. Additionally, such materials can

be deposited using physical vapor deposition, which allows improving coverage of the photoactive layer and therefore avoiding undesirable losses in the open-circuit voltage of the photovoltaic device.

3. The requirements to the structure of perfect conjugated polymer for its application as HTL in PSCs can be derived after a comparative study of a row of conjugated polymers.

4. The double HTL comprising a combination of organic polymers with transition metal oxides in the highest oxidation state allows to boost the stability of PSCs.

The following statements disclose the scientific novelty of the present thesis work.

1. Novel conjugated polymer with isoindigo unit was synthesized for the first time, the optimal synthetic route leading to a high-quality material was determined.

2. A large variety of the conjugated polymers were investigated as HTLs in PSCs allowing the author to define several requirements to the structure of the polymeric HTL for PSCs with improved stability and efficiency.

3. High operational stability (more than 4500 h) was demonstrated for the PSCs with double HTL based on the combination of organic material PTAA and vanadium (V) oxide.

The object of research is an HTL: a set of organic and inorganic materials, p-type semiconductors.

The subject of the study is PSC with standard configuration containing various HTL.

The area of the study corresponds to the scope of 1.4.4 Physical Chemistry (Chemical Sciences): 1. Experimental and theoretical determination of the energy and structural-dynamic parameters of the structure of molecules and molecular compounds, as well as their spectral characteristics. 12. Physical and chemical bases chemical technology processes and synthesis of new materials.

The theoretical and practical value of the dissertation is determined by the fact that its results help to solve the crucial problems in the development of PSCs towards their commercialization.

It is demonstrated that the nature of HTL can significantly affect the efficiency and stability of PSCs. Presented scientific findings emphasize the basic relationships between the structure of polymeric HTL and the characteristics of PSCs, which in turn lay down the foundation for a rational design of polymeric HTLs. Additionally, it is demonstrated that the combination of polymeric materials with transition metal oxides in the highest oxidation state boosts the stability of perovskite solar cells.

Overall, the results obtained in the dissertation bring one closer to the commercialization of highly efficient and inexpensive perovskite solar cells and, therefore, to the use of environmentally friendly and resource-saving energy.

The validity and reliability of the research results and conclusions.

The present work was performed at the high scientific level. All novel obtained materials were characterized by a set of complementary techniques including nuclear magnetic resonance spectroscopy 1H and 13C, gel permeation chromatography, cyclic voltammetry, absorption spectroscopy. The reproducibility of each material performance in PSCs was confirmed by the fabrication of more than twenty devices for each material in every experiment. The statements and conclusions were reported at international scientific conferences and published in peer-reviewed scientific journals.

Approbation of the results of the work. The main results of the work were presented and discussed at international and Russian conferences:

1. Alexander V. Akkuratov, Marina M. Tepliakova, Irina V. Klimovich, Ilya E. Kuznetsov, Keith J. Stevenson. Synergy of Organic and Perovskite Materials: Conjugated Polymers for Stable and Efficient Perovskite Solar Cells. HYBRIDOE21, Spain, online (2021)

2. Marina M. Tepliakova, A.V.Akkuratov, I.V. Klimovich, I.E. Kuznetsov, and P. A. Troshin. Conjugated polymers for stable and efficient perovskite solar cells: in search of the perfect match. HOPE-PV (2020)

3. Marina M. Tepliakova, Igor K. Yakushenko, and Pavel A. Troshin. Strength of Attraction: Pyrene-based Hole-transport Materials with Effective n-n Stacking for Dopant-free Perovskite Solar Cells. IFSOE (2020)

4. Marina M. Tepliakova, Igor K. Yakushenko, Keith J. Stevenson, and Pavel A. Troshin. Strength of attraction: pyrene-based hole-transport materials with effective n-n stacking improve efficiency and stability of perovskite solar cells. StabPero (2020)

5. Marina M. Tepliakova. Conjugated polymers: perspective hole transport materials for perovskite solar cells. 1st International School on Hybrid Organic and Perovskite Photovoltaics. Moscow, Russia (2019)

6. Marina M. Tepliakova. Exploring the impact of polymeric hole transport layer materials on the efficiency and stability of perovskite solar cells. EMRS Fall 2019. Warsaw, Poland (2019)

7. Marina M. Osipova. Suzuki polycondensation of poly(triarylamine) hole transport material for perovskite solar cells with improved efficiency. MIT Next Generation. Moscow, Russia (2018)

8. Марина М. Осипова. Новый подход к синтезу политриариламина материала дырочно-транспортного слоя в перовскитных солнечных батареях. Ломоносов 2019. Москва, Россия (2019)

9. Марина Осипова. Синтез нового сопряженного полимера для органических солнечных батарей, изучение влияния метода получения полимера на фотовольтаические свойства устройств. Ломоносов 2018. Москва, Россия (2018).

Publications. Results of the work are presented in six publications in high ranking international journals indexed in WoS and Scopus.

1. [Indexed in WoS and Scopus, Q1, IF: 6.36 Tepliakova M. M., Yakushenko I. K., Stevenson K. J., Troshin P. A. Strength of attraction: pyrene-

based hole-transport materials with effective n-n stacking for dopant-free perovskite solar cells // Sust. Energy Fuels - 2021. - V. 5. - P. 283-288.

2. [Indexed in WoS and Scopus, Q2, IF: 3.004] Tepliakova M. M., Mikheeva A. N., Somov P. A., Statnik E. S., Korsunsky A. M., Stevenson K. J. Combination of Metal Oxide and Polytriarylamine: A Design Principle to Improve the Stability of Perovskite Solar Cells // Energies - 2021. - V. 14. - P. 5115.

3. [Indexed in WoS and Scopus, Q2, IF: 2.527] Tepliakova M. M., Kuznetsov I. E., Avilova I. A., Stevenson K. J., Akkuratov A. V. Impact of Synthetic Route on Photovoltaic Properties of Isoindigo-Containing Conjugated Polymers // Macromol. Chem. Phys. - 2021. - V. 222. - P. 2100136.

4. [Indexed in WoS and Scopus, Q1, IF: 6.475] Tepliakova M. M., Mikheeva A. N., Frolova L. A., Boldyreva A. G., Elakshar A., Novikov A. V., Tsarev S. A., Ustinova M. I., Yamilova O. R., Nasibulin A. G., Aldoshin S. M., Stevenson K. J., Troshin P. A. Incorporation of vanadium (V) oxide in hybrid hole transport layer enables long-term operational stability of perovskite solar cells // J. Phys. Chem. Lett. - 2020. - V. 11. - P. 5563.

5. [Indexed in WoS and Scopus, Q1, IF: 9.229] Boldyreva A. G., Zhidkov I. S., Tsarev S. A., Akbulatov A. F., Tepliakova M. M., Fedotov Y. S., Bredikhin S. I., Postnova E. Y., Luchkin S. Y., Kurmaev E. Z., Stevenson K. J., Troshin P. A. Unraveling the Impact of Hole Transport Materials on Photostability of Perovskite Films and p-i-n Solar Cells // ACS Appl. Mater. Interfaces. - 2020. - V. 12. - P. 19161.

6. [Indexed in WoS and Scopus, Q2, IF: 2.415] Tepliakova M. M.,

Akkuratov A. V., Tsarev S. A., Troshin P. A. Suzuki polycondensation for the synthesis of polytriarylamines: A method to improve hole-transport material performance in perovskite solar cells // Tetrahedron Lett. - 2020. - V. 61. - P. 252217.

The author's personal contribution consists in the synthesis of materials, characterization of their physicochemical properties (nuclear magnetic resonance, chromatography (HPLC, GPC, GC), electrochemistry, absorption spectroscopy,

thermogravimetry, scanning differential calorimetry, X-ray diffraction), the assembly of photovoltaic devices, evaluating their efficiency and stability (external quantum efficiency and current-voltage characteristics measurements), data analysis, writing publications, presentation of the work at international conferences. The author also created and published a cover for the journal Macromolecular Chemistry and Physics, Wiley (Volume 222 Issue 15).

The thesis includes abstract, six chapters, conclusions, a list of symbols and abbreviations, and a list of 188 references. The dissertation consists of 265 pages, including 52 figures and 10 tables.

The abstract describes concisely the whole dissertation.

The 1st chapter «Introduction» starts with a brief outline of perovskite solar cells development and structure. Afterward, several requirements for holetransport layer materials (HTL) are established. Finally, an overview of the literature on state-of-the-art research works on different types of HTLs for perovskite solar cells including inorganic HTLs, organic and organometallic HTLs, polymeric HTLs with the focus on the approaches to modifications of their structure, and double organic-inorganic HTLs is presented.

The 2nd chapter «Suzuki polycondensation for the synthesis of polytriarylamines» is devoted to the synthesis of PTAA by Suzuki polycondensation using AA- and BB-type monomers, where A is halogen and B is a boronic ester group (Figure 1). In comparison with the Suzuki polycondensation from AB-type monomer presented in the literature, [7] the proposed synthetic pathway allows to increase the yield of the reaction and obtain a high-quality PTAA for perovskite solar cells.

R / R

R_N NBS r'Y^ n-BuLi,>Ç-B-0 ' ^

Ph

Ph CHC'3 Br-^ ^Sr THF'"78°C X/0^-^

1 2 (99%) yr '° 3 (65%) °

„ )=\ AA-type monomer BB-type monomer

R =™\J—

' Pd2(dba)3 (0.5 mol%), P(o-tol)3 (1.5 mol%) q R

4a (45%) ^^^ N

4b (24%) f| '^f

2+3

toluene, aliquat 336 Pd(OAc)2 (0.5 mol%), PPh3(1.5 mol%)

K2C03, water, 90°C Pd(PPh3)4

Figure 1. Synthetic route for PTAA 4a and 4b.

Compound 1 was synthesized via Buchwald-Hartwig coupling between commercially available 2,4,6-trimethylaniline and bromobenzene. The following bromination of 1 produced the AA-type monomer 2 with a quantitative yield. The monomer 2 was additionally sublimed under reduced pressure leading to 99% purity according to high-performance liquid chromatography (HPLC). The BB-type monomer 3 was obtained by the lithiation of 2 with 4 equivalents of n-butyllithium followed by the addition of 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. The high purity of the monomer 3 (97% by HPLC) was achieved by precipitation from isopropanol and further recrystallization from chloroform upon the addition of methanol. All obtained compounds were characterized using 1H NMR spectroscopy and HPLC.

Finally, the catalytic system for Suzuki polycondensation was optimized, and the system comprised of tris(dibenzylideneacetone)dipalladium and tri(o-tolyl)phosphine led to the polymer 4a with weight average by number Mn=20.8 kDa and polydispersity index Mw/Mn=1.4. The commercial material PTAA Ref possessed higher Mn=25.2 kDa and wider polydispersity Mw/Mn=2.2. Finally, material 4b was characterized with low Mn=10.7 kDa and Mw/Mn=1.4.

Three batches of PTAA including commercial material PTAA Ref were evaluated as HTL materials in perovskite solar cells with n-i-p configuration given in Figure 2a. The optimized perovskite solar cell architecture included indium tin oxide (ITO covered with the electron transport layer SnO2 passivated with a

fullerene derivative phenyl-C6i-butyric acid (PCBA). [8, 9] The perovskite absorber layer CH3NH3PM3 was deposited atop. The PTAA layer was spin-coated from the chlorobenzene solution. Finally, the hole-collecting electrode consisting of Mo0x(10 nm) and Ag (100 nm) was deposited by thermal evaporation in the vacuum.

The J-V characteristics of the best devices fabricated with all three polymers are presented in Figure 2b.

Figure 2. Device architecture (a); J-V characteristics of perovskite solar cells (b).

The cells incorporating polymer 4b demonstrated low power conversion efficiencies (PCE) of 8.2% with a modest fill factor (FF) of 43%. These results confirmed poor electronic quality and, consequently, suboptimal charge transport properties of low molecular weight polymer 4b. On the contrary, the devices with HTL based on PTAA Ref and 4a delivered much higher efficiencies of 16.7% and 17.6%, respectively. The fill factors (FF) and current densities (Jsc) were comparable for devices with both polymer batches. The current density values were additionally reconfirmed with external quantum efficiency measurements. Importantly, the open-circuit voltage (Voc) of the devices with 4a approached 1.06 V, which was notably higher compared to the cells with PTAA Ref yielding Voc of 1.02 V. The higher Voc reached for devices with 4a indicates a decrease in the density of the defects at the perovskite/HTL interface suppressing trap-assisted recombination of charge carriers. [10]

The 3rd chapter «Impact of Synthetic Route on Photovoltaic Properties of Isoindigo-containing Conjugated Polymers» presents the synthesis of

isoindigo-based conjugated polymer using various reactions: Suzuki polycondensation (P1-B) and Stille polycondensation (P1-Sn) (Figure 3).

Oct

Hex

D1 (99%)

Dec

Dec

BrODID

BOD ID (40%)

Dec

D1+BODID

D2+BrODID

IV

P1-B

P1-Sn

Figure 3. Synthetic routes towards P1-B using Suzuki polycondensation and P1-Sn using Stille polycondensation. Conditions: i) NBS, o-DCB; (ii) LDA, THF, -78°C, SnMe3Cl; (Hi) bis(pinacolato)diboron, NaOAc, Pd(PPh3)2Cl2, (iv) Pd(PPh3)4, Bu4NOH, MeOH, toluene;(v) Pd(PPh3)4, toluene.

The synthesis of polymers P1-B and P1-Sn was carried out using palladium-catalyzed Suzuki and Stille polycondensation reactions, respectively. For the heterophase Suzuki reaction, the boronic acid bis(pinacol) ester of isoindigo derivative BODID and dibromosubstituted compound D1 were prepared. For the Stille polycondensation reaction, the organotin compound D2 and dibromosubstituted isioindigo derivative BrODID were synthesized.

The molecular weight characteristics of P1-B and P1-Sn were measured using GPC chromatography and found to be Mw=76 kDa and Mw/Mn =1.8, and Mw=77 kDa and Mw/Mn =1.7 respectively (Figure 4a). The optoelectronic properties of both polymers were characterized using cyclic voltammetry and

absorption spectroscopy (Figure 4 b-c). Additionally, the electron spin resonance signals for polymers P1-B and P1-Sn were recorded, and the concentrations of paramagnetic defects normalized by the sample mass were calculated (Figure 4d).

Figure 4. Gel-permeation chromatogramms (a), absorptions spectra (b), cyclic voltammograms (c), and electron spin resonance spectra (d) for P1-B and

P1-Sn.

It was found out, that concentration of radical impurities in polymer P1-B is more than five times higher compared to that for P1-Sn.

Furthermore, polymers P1-B and P1-Sn were investigated as donor materials in organic solar cells with PCBM (6,6 -Phenyl-C61-butyric acid methyl ester) as acceptors, and as HTLs in perovskite solar cells. The configurations of photovoltaic devices and J-V characteristics of best devices are presented in Figure 5.

Figure 5. J-V characteristics (reverse scans) and configurations for organic solar cells (a) and perovskite solar cells (b) based on polymers obtained using Suzuki (P1-B) and Stille (P1-Sn) polycondensation reactions.

The organic solar cells based on P1-Sn demonstrated a higher PCE=4.1% than Pl-B-based devices PCE= 2.7% due to noticeably lower Voc, and Jsc. Perovskite solar cells exhibited similar trend achiving 15.1% with Pl-Sn and 12.6% with Pl-B. The lower PCE for devices comprising with Pl-B originated from the reduced short circuit density values Jsc =17.2 mA/cm2. Mitigation of particular characteristics for both types of photovoltaic devices correspong to the high density of charge traps leading to parasitic charge recombination.

In the 4th chapter «Pyrene-based Hole Transport Materials with Effective n-n Stacking for Dopant-free Perovskite Solar Cells» four pyrene derivatives were evaluated as HTLs in perovskite solar cells (Figure 6).

The highest occupied energy levels of materials extracted from the onset of the oxidation wave on the cyclic voltammograms vary from -5.5 to -5.4 eV thus being in alignment with perovskite valence band. The bandgap of the materials estimated from the low-energy onset of the absorption band lies in the interval between 2.6 and 2.7 eV. The high melting points with the lowest of 270°C for Y3, and decomposition temperatures >450°C were revealed by thermal gravimetry analysis coupled with differential scanning calorimetry.

Y1 Y2 Y3 Y4

Figure 6. Structures of pyrene derivatives Y1-Y4.

Furthermore, the performance of perovskite solar cells containing one of four pyrene derivatives deposited by spin-coating was compared with that of devices with thermally-evaporated Y1-Y4. The best characteristics were achieved for devices with evaporated Y2 with PCE=17.9% and high Voc=1.1 V. The lowest characteristics were achieved for devices with spin-coated Y4 HTL, which can be explained by the extremely low solubility of pyrene derivatives with substituents in 1 and 8 positions. [11]

The configuration of used devices and energy level diagram is given in Figure 6a-b, and statistical data on the efficiency of devices is summarized in histograms (Figure 6c).

Figure 7. Device configuration (a); energy level alignment (b); histograms for PCEs of perovskite solar cells using evaporated and spin-coated Y1-Y4 as

HTLs (c).

The devices incorporating thermally evaporated Y1-Y4 HTLs outperform PSCs with spin-coated HTLs. The AFM measurements exhibited high uniformity and lower density of defects in thermally evaporated layers compared to spin-coated ones.

The 5th chapter «Incorporation of Vanadium(V) Oxide in Double Hole Transport Layer Enables Long-term Operational Stability of Perovskite Solar Cells» describes a successful approach to boost the stability of perovskite solar cells.

The approach was inspired by the recent work, where the MoOx interlayer in combination with the low molecular weight organic hole-transport material was reported to improve the stability of perovskite solar cells under ambient conditions. [12] However, the system was reported to be thermally unstable. [13]

The vanadium (V) oxide (VOx) layer was proposed as a part of double HTL. The optimal thickness of VOx was estimated to lie in the interval of 30-45 nm. The double-layer was utilized in perovskite solar cells with various electron-transport layers (SnO2, TiO2, ZnO) and different absorbers (CH3NH3PM3, Cs(NH2)2CHPbl3 or CsFAPbh), and devices exhibited high efficiencies up to 20.1%. Furthermore, perovskite solar cells containing PTAA/VOx double HTL demonstrated long-term operational stability for more than 4500h under constant illumination and elevated temperature (Figure 8).

Figure 8. Configuration of devices (a), schematic representation of degradation chamber (b), evolution of the PCE for perovskite solar cells incorporating VOx and MoOx under continuous light soaking (c).

The reference system with PTAA/MoOx layer showed ~50% drop in efficiency under the same conditions.

Investigation of thermal stability of bilayer systems with configuration glass/CH3NH3PbI3/MoOx or VOx revealed the superior stability of VOx-containing system. Incorporating the PTAA layer between perovskite and metal oxide boosted the stability even further.

In chapter 6 «In search of the perfect match: conjugated polymers for stable and efficient perovskite solar cells» the stability of perovskite solar cells with a set of different conjugated polymers (Figure 9) is used as an organic component of a double HTL in combination with VOx.

Y=S: H1 X=CZ H2, H3 X=T-BDT H4-H6 X=AlkO-BDT H7 X=TzTz H8-H10 X=DTDOne H11, H12 X=ll H13, H14 X=AlkOBz

Y=0: H15, H16 R1=Alk, R2=H, X=Cz

H17, H18 R1=H. R2=00ct, X=Cz H19 X=CPDT

Y=0: H20 X=T H21,H22X=Cz H23 X=T-BDT H24X=BO-T-CZ Y=N-Dec: H25 X=Cz

Y=C2H2Alk2: H26 X=Cz H27 X=T-CZ-T H28, H29 X=T-BDT

R & R2

H35 Y=0, X=FI

Y=S:H36, H37 X=FI H38 X=CZ H39 X=BT-T-AlkOBz

fDPP-xf; fT-BDT-Xjn

-[T- BT-CP DT- BT-T-X-|"n

H30 X=FI H31 X=CZ H32 X=DBSi

H40 X =BT-T-CZ-T-BT H41X =TzTz

H42 X=FQX H43 X=DTBTz H 44 X=DTBIz

H45

-fcZ-BTfn H46 -[t-boj-n

x

Allcftlk

cz

Detf Dec

T-BDT

Oct

DBSi

\\ // \_/

jy -S- ~OOC« FQX DTDOne

BT BO TzTz AlkOBz

DTBTz DTBIz

R1= H, Alkyl R2= H, F R3= H, Alkyl

Figure 9. Structures of polymers H1-H46.

The selected set represents most of the possible conjugated polymer structure modifications previously reported for HTLs, allowing to conclude on HTL structure versus device performance relationship. Most of the polymers contain benzothiadiazole or benzooxadiazole, and thiophene units. There are materials containing very strong acceptors like diketopyrrole (DPP, H40-41), and quinoxaline (FQX, H25 and H42), and isoindigo (II, H11-12) units, average acceptors like thiazolothiazole (TzTz, H7 and H41), strong donors like benzodithiophene (T-BDT, H2-3), and average donors like dithiophenedione (DTDOne H8-10) etc. The materials bear different solubilizing chains, such as

alkoxy, thioalkoxy, and alkyl both branched and linear. Some materials incorporate fluorine substituents on different building blocks.

The highest occupied molecular orbital energy levels (HOMO) of the materials were characterized using cyclic voltammetry, and they vary from -5.74 eV (H10) to -5.18 (H28) eV. The optimal deposition conditions were optimized in preliminary experiments for each material in devices with CH3NH3PbI3 photoactive layer and SnO2 electron-transport layer. All materials demonstrated decent efficiencies without additional doping, and the correlation between HOMO energy level and molecular weight distribution of the material and the characteristics of perovskite solar cells was not observed.

The effect of the material structure on the stability of PSCs was evaluated in perovskite solar cells (Figure 10a) with the configuration given in Figure 10b.

Figure 10. The heatmap showing evolution of the efficiency of perovskite solar cells with H1-H46 as HTL (a), device configuration (b), three of the best materials (c), cross-sectional SEM for fresh and degraded PTAA (d).

According to the heatmap, superior stability was observed for devices incorporating materials H17, H18, H46, H22, H27. Additionally, after thorough comparison of conjugated polymer structures and the performance of the corresponding devices, several rules for the rational design of conjugated polymer

for stable and efficient perovskite solar cells were derived and summarized in the main text of the thesis.

Using the time-of-flight secondary ion mass spectroscopy it was observed, that for devices with the double HTL PTAA/VOx perovskite decomposition products do not cross the double HTL during photodegradation, however, both the absorber and PTAA insignificantly blur and diffuse into each other, thus stressing out the need for the optimized composition of the absorber and a new organic HTL enabling more robust interface with perovskite. [14] Furthermore, scanning electron microscopy images of the cross-sections for devices with PTAA/VOx double HTL revealed the formation of voids and phase segregation in the perovskite layer. This phenomenon was not observed for the more stable devices with H17 as an example (Figure 10d), which can be attributed to the chemical structure of H17 material. In contrast to PTAA, H17 incorporates heteroatoms like oxygen and sulfur, which can interact with lead in perovskite, thus forming a stable interface with perovskite.

Within the current project, a complex work leading to the development of ground rules for the rational design of hole-transport materials for perovskite solar cells was performed. As a starting point, the HTL material as a key component to perovskite solar cell long-term stability was considered. Several aspects related to the organic HTL synthesis, its chemical structure, deposition method, and the ability of the organic polymers in combination with inorganic metal oxides to boost perovskite solar cell stability, were addressed.

Firstly, the effect of the synthetic pathway on the quality of materials for organic electronics was revealed. In particular, state-of-the-art polymeric HTL polytriarylamine was synthesized from AA- and BB-type monomers, possessing a high quality observed from the narrow molecular weight distribution. Devices using the obtained polymer achieve splendid 17.6% efficiency without additional p-doping. Importantly, the obtained result revealed, that the choice of the synthetic approach identifies the properties of the organic electronic material. Therefore, the synthesis of the polymeric HTL should be conducted from the type of reagents,

which can be easily purified to a high extent in order to avoid inclusions of impurities. Besides, the high cost of the material synthesis using multistep polycondensation reactions implies the optimization of the reaction yield on each step, which can't be achieved for the AB-type monomer, as was shown in the literature.

Further, the conjugated polymer incorporating isoindigo unit was obtained using two various reactions: Suzuki and Stille polycondensations. Both materials exhibit similar optoelectronic properties and molecular weight distribution. However, the photovoltaic devices with these polymers demonstrate a dramatic difference in performance. In particular, perovskite and organic solar cells which contain the material obtained using Stille polycondensation, demonstrate 15.1% and 4.1% power conversion efficiencies, respectively, while the use of the polymer synthesized by Suzuki reaction resulted in the inferior efficiencies 12.6% and 2.7% of the corresponding devices. Using electron spin resonance it was observed, that the material obtained with Suzuki polycondensation incorporates a five times higher density of radical species, which behave like charge traps in electronic devices. In this experiment, the basic conditions of the Suzuki reaction affect the isoindigo-based monomer structure, which leads to the mitigation of the performance of photovoltaic devices. Therefore, the choice of the synthetic pathway is a crucial factor, which can affect the quality of the semiconductor material, and, consequently, the performance of photovoltaic devices.

Furthermore, four pyrene-derivatives bearing arylamine substituents were evaluated as hole-transport materials in perovskite solar cells. The devices containing the pyrene derivative comprising diarylamine in 1,6 positions with one of the aryls being a-naphthyl delivered 17.9% efficiency, outperforming devices employing materials with teri-butylphenyl or P-naphthyl substituents, so as the material bearing substituents in 1,8 positions. Importantly, it was demonstrated, that thermal evaporation is the more reliable deposition method in the case of volatile organic compounds. There are a lot of disadvantages expected for the

layers obtained using spin-coating. For example, the uneven layer thickness, which varies in the center of the substrate and on the edges, can affect the reproducibility of the results. Additionally, the possible solvent trapping with its further evaporation from the substrate leading to inevitable defect formation can dramatically decrease the device efficiency. This statement is supported by the fact that the devices based on thermally evaporated pyrene derivatives exhibit open-circuit voltages on average 90 mV higher, compared to that of devices using spin-coated materials. The higher open-circuit voltage in turn is related to the density of interactions between perovskite and HTL surfaces. Therefore, in the case of the materials, which can't provide bonding with perovskite after annealing, alternative solvent-free deposition methods should be considered.

Furthermore, a perspective approach of a combination of organic and inorganic hole-transport layers including vanadium oxide and PTAA was demonstrated. Using this combination in perovskite solar cells enables the long-term stability of perovskite solar cells under continuous light soaking. Meanwhile, an approach presented in the literature including the incorporation of molybdenum oxide into the HTL leads to lower device stability. This effect can be attributed to the poor thermal stability of the MoOx layer, resulting in its shrinking and delamination under elevated temperatures. The cracked layer is permeable for the volatile perovskite decomposition products, which results in the mitigation of the device characteristics. [15, 16] Another possible reason is the occurrence of parasitic chemical processes on the interface between perovskite and HTL. The undesirable interfacial chemistry can lead to inevitable degradation of the interface, and consequently to the device performance deterioration. To sum up, the role of the inorganic component of double HTL has to be investigated further.

Using the double HTL approach, a set of 46 conjugated polymers comprising various donors and acceptors, so as solubilizing chains of different nature, was compared as the organic counterparts of the double HTL in perovskite solar cells. Such units, as benzodithiophene, benzodithiophenedione, fluorine were found to be undesirable components of the chain of polymeric HTL. Furthermore, using the

HTLs with closely locating thiophene units bearing solubilizing chains tends to decrease the stability of the device. This can be attributed to the formation of alkyl-based insulating regions with high gas permeability, which doesn't prevent the migration of perovskite decomposition products from the structure. Finally, the incorporation of alkoxy units so as the oxygen-containing heterocycles to the structure of the polymer chain leads to the improved stability of devices, which is probably due to more efficient interactions between perovskite and HTL via oxygen-lead bonding. The research findings of this part pave a way for the further study of the effect of HTL nature on the efficiency and stability of perovskite solar cells. One of the perspective approaches is the application of the rules stressed out for polymeric HTLs to the design of a novel polymer with all the features of the best materials for long-term stable devices. Additionally, the assumption proposed in chapter 6 regarding the ability of sulfur- and oxygen-containing materials to form a robust interface between perovskite and HTL has to be investigated further with the assistance of electronic microscopy methods. Besides, with the observable degradation of the CsFAPbI3 material using ToF-SIMS with the formation of voids, there is an urgent need for the search of the more stabilized perovskite composition probably by the application of some additives or inclusion of the 2D perovskite interlayer.

Finally, it should be noted, that the performed studies bring us closer to the stated goal, and shed a light on some relationships between the structure and properties of hole-transport material and the overall performance of PSC, which is a step towards the anticipated technology commercialization.

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14. Tepliakova M. M., Mikheeva A. N., Somov P. A., Statnik E. S., Korsunsky A. M., Stevenson K. J. Combination of Metal Oxide and Polytriarylamine: A Design Principle to Improve the Stability of Perovskite Solar Cells // Energies. - 2021. - V. 14, № 16. - P. 5115.

15. Schloemer T. H., Gehan T. S., Christians J. A., Mitchell D. G., Dixon A., Li Z., Zhu K., Berry J. J., Luther J. M., Sellinger A. Thermally Stable Perovskite Solar Cells by Systematic Molecular Design of the Hole-Transport Layer // ACS Energy Letters. - 2019. - V. 4, № 2. - P. 473-482.

16. Tracy H. Schloemer J. A. R., Timothy S. Gehan, Sanjini Mamayakkara, Taylor Moot, Steven Harvey, Rosemary C. Bramante, Sean Dunfield, Amy Louks, Annalise E. Maughan, Lyle Bliss, Michael D. McGehee, Maikei F.A.M. van Hest, Matthew O. Reese, Stacey F. Bent, Joseph J. Berry, Joseph M. Luther, Alan Sellinger. Vanadium Oxide Interfacial Layer in Perovskite Solar Cells for High Temperature Operational Stability of Unencapsulated Devices in Air // Book Vanadium Oxide Interfacial Layer in Perovskite Solar Cells for High Temperature Operational Stability of Unencapsulated Devices in Air / Editor, 2020.

Author's publications on the dissertation topic

1. Tepliakova M. M., Yakushenko I. K., Stevenson K. J., Troshin P. A. Strength of attraction: pyrene-based hole-transport materials with effective n-n stacking for dopant-free perovskite solar cells // Sust. Energy Fuels - 2021. - V. 5. - P. 283-288.

2. Tepliakova M. M., Mikheeva A. N., Somov P. A., Statnik E. S., Korsunsky A. M., Stevenson K. J. Combination of Metal Oxide and Polytriarylamine: A Design Principle to Improve the Stability of Perovskite Solar Cells // Energies -2021. - V. 14. - P. 5115.

3. Tepliakova M. M., Kuznetsov I. E., Avilova I. A., Stevenson K. J., Akkuratov A. V. Impact of Synthetic Route on Photovoltaic Properties of Isoindigo-Containing Conjugated Polymers // Macromol. Chem. Phys. - 2021. -V. 222. - P. 2100136.

4. Tepliakova M. M., Mikheeva A. N., Frolova L. A., Boldyreva A. G., Elakshar A., Novikov A. V., Tsarev S. A., Ustinova M. I., Yamilova O. R., Nasibulin A. G., Aldoshin S. M., Stevenson K. J., Troshin P. A. Incorporation of vanadium (V) oxide in hybrid hole transport layer enables long-term operational stability of perovskite solar cells // J. Phys. Chem. Lett. - 2020. - V. 11. - P. 5563.

5. Boldyreva A. G., Zhidkov I. S., Tsarev S. A., Akbulatov A. F., Tepliakova M. M., Fedotov Y. S., Bredikhin S. I., Postnova E. Y., Luchkin S. Y., Kurmaev E. Z., Stevenson K. J., Troshin P. A. Unraveling the Impact of Hole Transport Materials on Photostability of Perovskite Films and p-i-n Solar Cells // ACS Appl. Mater. Interfaces. - 2020. - V. 12. - P. 19161.

6. Tepliakova M. M., Akkuratov A. V., Tsarev S. A., Troshin P. A. Suzuki polycondensation for the synthesis of polytriarylamines: A method to improve hole-transport material performance in perovskite solar cells // Tetrahedron Lett. -2020. - V. 61. - P. 252217.

INTRODUCTION

Relevance. According to the concept of sustainable development, humankind is aimed to transfer to renewable energy sources, such as solar energy. Currently, the prevalent photovoltaic technology is based on crystal silicon. Silicon-based solar cells exhibit high power conversion efficiencies. However, the production of crystal silicon is expensive. [1] Therefore, scientists around the world turn their attention towards alternative technologies.

In this regard, the perovskite solar cells (PSCs) are attracting considerable interest. Their main advantage is the low fabrication cost due to the inexpensive photoactive layer formation using solution-based deposition techniques, which can be easily scaled up using printing and roll-to-roll technologies. [2] The efficiencies achieved by the PSCs are already comparable to that of crystal-silicon photovoltaics. The main obstacle on the way to PSCs commercialization is their insufficient stability under working conditions.

The classical configuration of the PSCs includes a photoactive layer with a perovskite structure sandwiched between the electron transport layer on the bottom and the hole-transport layer (HTL) on the top. Both charge-transport layers selectively extract and carry on the corresponding charges to the electrodes. Therefore, the first requirement for the HTL is sufficient hole mobility for effective charge transport (^h>1x10-4 cm2 V-1 s-1). [3]

It is important to note that the decrease in PSC efficiency under working conditions is a complex process, which includes many various degradation paths. [4, 5] One of the most studied is the reversible decomposition of the photoactive layer under working conditions with the formation of volatile decomposition products. [6] The migration of volatile substances from the structure leads to the irreversibility of perovskite decomposition and consequent device efficiency deterioration. Therefore, a key to improving stability is the preservation of the decomposition products in the photoactive layer, which can be achieved by

selecting a low gas permeability HTL. Additionally, to ensure long-term stability, the chosen HTL should be thermally and photo-stable, as well as inert towards adjacent layers of the device.

Previous studies explored as HTLs a plethora of various organic and inorganic materials or their combinations. According to the literature, PSC characteristics depend on the nature of HTL, the chosen synthetic approach, degree of purification, deposition technique, etc. However, currently, there are no strategies for the rational design of HTLs for both efficient and selective extraction of charges, preservation of perovskite decomposition products in the photoactive layer, and material stability.

The present thesis is focused on the development of HTLs rational design strategy, which is an important and relevant scientific problem, as its solution allows one to get closer to stable and efficient PSCs, which is necessary for the commercialization of the technology.

The aims, objectives, and the research methodology of the thesis.

The thesis is devoted to the synthesis and investigation of various types of HTLs for stable and efficient PSCs. The main goal of the work is to derive the principles of the rational design of hole transport materials. It implies the following aims and objectives of the present dissertation work:

1. Evaluation of the impact of the synthetic route on the physicochemical properties of organic polymeric HTLs;

2. Assessing the influence of HTL deposition technique(s) on the device performance;

3. Defining the relationship between the structure of the material and its impact on the stability and efficiency of PSCs;

4. Comparing the ability of double HTLs comprising various metal oxides to improve the stability of perovskite solar cells.

Methodology. Achieving the aims was provided using the following formulated tasks, which correlate to the chapters of the dissertation.

1. The review of relevant scientific literature on the hole-transport materials for PSCs

2. Synthesis of polymeric HTL polytriarylamine (PTAA) using various polymerization reactions; comparison of the performance of PSCs incorporating obtained materials

3. Synthesis of isoindigo-based conjugated polymer using various polycondensation reactions, comparison of physicochemical properties of obtained materials, and their performance in PSCs

4. Investigation of low molecular weight aromatic materials as HTLs in PSCs along with evaluating the influence of deposition techniques on the performance of devices

5. Application of double HTL with low gas permeability including PTAA and metal oxides in the highest oxidation state for PSC stability improvement

6. Investigation of a row of conjugated polymers for defining the relationship between the material structure and performance.

The following statements will be defended in the thesis:

1. The synthetic pathway of polymeric hole-transport materials influences their quality and their performance in photovoltaic devices. Particularly, for polytriarylamine (PTAA) the Suzuki polycondensation from AA- and BB- type monomers (where A is halogen group and B is boronic acid group) leads to PTAA with higher yields, higher molecular weights, and lower polydispersity indexes compared to the materials obtained using Suzuki polycondensation from asymmetrical AB-type monomer or with radical polymerization. Additionally, in the case of isoindigo-containing conjugated p-type polymers using Stille polycondensation allows synthesizing less defective material, than in the case of Suzuki polycondensation.

2. Flat aromatic molecules can provide increased charge transfer mechanisms in thin films due to intermolecular interactions via n-n stacking. This can be applied for HTLs in PSCs to satisfy the requirement of effective charge transport without

additional p-doping. In particular, pyrene derivatives with triarylamine substituents are perfect candidates for illustrating this concept. Additionally, such materials can be deposited using physical vapor deposition, which allows improving coverage of the photoactive layer and therefore avoiding undesirable losses in the open-circuit voltage of the photovoltaic device.

3. The requirements to the structure of perfect conjugated polymer for its application as HTL in PSCs can be derived after a comparative study of a row of conjugated polymers.

4. The double HTL comprising a combination of organic polymers with transition metal oxides in the highest oxidation state allows to boost the stability of PSCs.

The current work bears scientific novelty, as can be concluded from the following statements:

1. Novel conjugated polymer with isoindigo unit was synthesized, the optimal synthetic route leading to a high-quality material was determined.

2. PTAA was synthesized using Suzuki polycondensation from AA-type and BB-type monomers for the first time.

3. A large variety of the conjugated polymers were investigated as HTLs in PSCs allowing to define several requirements to the structure of the polymeric HTL for PSCs with improved stability and efficiency.

4. High operational stability (more than 4500 h) was demonstrated for the PSCs with double HTL based on the combination of organic material PTAA and vanadium (V) oxide.

5. The profound study of solar cell slice using complementary techniques ToF-SIMS and cross SEM was performed for the first time.

The object of research is an HTL: a set of organic and inorganic materials, p-type semiconductors.

The subject of the study is PSC with standard configuration containing various HTL.

The area of the study corresponds to the scope of 1.4.4 Physical Chemistry (Chemical Sciences): 1. Experimental and theoretical determination of the energy and structural-dynamic parameters of the structure of molecules and molecular compounds, as well as their spectral characteristics. 12. Physical and chemical bases chemical technology processes and synthesis of new materials.

The theoretical and practical value of the dissertation is determined by the fact that its results help to solve the crucial problems in the development of PSCs towards their commercialization.

It is demonstrated that the nature of HTL can significantly affect the efficiency and stability of PSCs. Presented scientific findings emphasize the basic relationships between the structure of polymeric HTL and the characteristics of PSCs, which in turn lay down the foundation for a rational design of polymeric HTLs. Additionally, it is demonstrated that the combination of polymeric materials with transition metal oxides in the highest oxidation state boosts the stability of perovskite solar cells.

Overall, the results obtained in the dissertation bring one closer to the commercialization of highly efficient and inexpensive perovskite solar cells and, therefore, to the use of environmentally friendly and resource-saving energy.

The validity and reliability of the research results and conclusions.

The present work was performed at the high scientific level. All novel obtained materials were characterized by a set of complementary techniques including nuclear magnetic resonance spectroscopy 1H and 13C, gel permeation chromatography, cyclic voltammetry, absorption spectroscopy. The reproducibility of each material performance in PSCs was confirmed by the fabrication of more than twenty devices for each material in every experiment. The statements and conclusions were reported at international scientific conferences and published in peer-reviewed scientific journals.

Approbation of the results of the work. The main results of the work were presented and discussed at international and Russian conferences including

NanoGe online events (HYBRIDOE21, StabPero 2020), IFSOE 2020, HOPE-PV 19-20, EMRS 2019, MIT Next-Gen 2018, Ломоносов 2018-2019.

Publications. Results of the work are presented in six publications in high ranking international journals indexed in WoS and Scopus.

The author's personal contribution consists in the synthesis of materials, characterization of their physicochemical properties (nuclear magnetic resonance, chromatography (HPLC, GPC, GC), electrochemistry, absorption spectroscopy, thermogravimetry, scanning differential calorimetry, X-ray diffraction), the assembly of photovoltaic devices, evaluating their efficiency and stability (external quantum efficiency and current-voltage characteristics measurements), data analysis, writing publications, presentation of the work at international conferences. The author also created and published a cover for the journal Macromolecular Chemistry and Physics, Wiley (Volume 222 Issue 15).

The thesis includes abstract, six chapters, conclusions, a list of symbols and abbreviations, and a list of 188 references. The dissertation consists of 265 pages, including 52 figures and 10 tables.

CHAPTER 1. Literature review

1.1 Perovskite solar cells: a perspective photovoltaic technology

In the course of the sustainable development program, the world seeks clean and affordable energy, which is when renewable energy sources step into the spotlight. According to the International Energy Agency, the share of renewable energy in worldwide electricity generation reached 28% in 2020 and is expected to increase up to 32% by 2030. [7] Among all renewable energy sources, photovoltaics (PV) provides the cheapest and easiest way for expanding electricity capacity in most countries. Despite the growth of PV implementation was slowed down due to COVID restrictions, the PV technology still provides more than 60% of the clean renewable energy in the world (Figure 1)

Solar Wi nd Hydropower

— Other -- J

r m ■ ■

w

° 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022

Fig. 1. Shares of various renewable energy technologies from 2000 to 2020 and 1-year prediction. [7] (green - hydropower, indigo - wind, blue - solar PV)

Currently, the PV market is dominated by silicon-based solar cells showing efficiencies up to 26.7% providing long-term stable performance for more than 10 years. [8] The closest rival in terms of efficiency is perovskite PV, the relatively young technology, which was first demonstrated in 2009 [9] with 3.8% efficiency and progressed to the current record of 25.5% in less than a decade. [8] Such rapid development of perovskite solar cells (PSCs) can be attributed to the extremely cheap and simple photoactive layer manufacturing approaches utilizing solution-processing techniques, which can be easily scaled up using printing techniques and roll-to-roll fabrication. [2] Importantly, this feature enables twice lower cost of the produced energy compared to that of crystal silicon solar cells. [1] Besides, the photoactive perovskite layer can be employed to the bendable and stretchable substrates. [10] It should be noted, that atop standard solar cell applications PSCs can be applied in lightweight devices due to the very high power-per-weight values, [11] in the energy-generating windows possessing various colors achieved by the composition variation, [12] and as the energy-generating panels for space-related usage. [13]

As it was mentioned before, PSCs were developed first by the group of professor Miyasaka in 2009 as the novel absorber material for dye synthesized

solar cells (DSSC). The authors proposed to use lead-halide light-absorbing perovskite materials CH3NH3PbBr3 and CH3NH3PM3 as light-harvesters covered with the liquid electrolyte containing lithium halide/halogen redox couple (Figure 2a). Devices demonstrated modest 3.1% and 3.8% efficiency, respectively. [9]

The first significant improvement in PSC efficiency can be attributed to the collaborative work of research groups led by Michael Gratzel and Nam-Gyu Park. [14] They proposed all-solid-state PSCs containing solid TiO2 hole-blocking layer, mesoporous TiO2 paste infiltrated with CH3NH3PbI3 nanoparticles, atop of which the solid hole-transport layer Spiro-O-MeTAD with dopants LiTFSI and TBP was applied (Figure 2b). Such devices demonstrated modest compared to the current record efficiencies of 9.7%, however, the system became a starting point for the following efficiency boost. It should be also noted, that the all-solid system demonstrated stable performance for 500 h (about 20 days), which had to be improved further.

The next important PSC architecture modification was presented in the work of Lee et. al., where the mesoporous TiO2 was substituted by insulating Al2O3. This allowed to achieve similar performance as reported for PSC with mesoporous TiO2, thus demonstrating decent electron-mobility provided by perovskite material itself and unnecessity of mesoporous scaffold formation. [15] The further simplification of the device structure performed by Liu et. al. consisted in the complete removal of the mesoporous counterpart from the device leading to the planar-type devices (Figure 2c). Besides, the authors demonstrated, that the physical vapor deposition is preferable for the fabrication of a uniform perovskite layer. In particular, PSC with the photoactive layer deposited using evaporation technique demonstrated remarkable 15.4% efficiency with open-circuit voltages (Voc) reaching 1.07V, whereas PSC with the solution-processed absorber showed modest performance with lower Voc=0.84V, and PCE=8.6%, which can be attributed to the higher density of defects in the interlayer interfaces.

Fig. 2. Miyasaka cell (a); Solid state Graetzel cell (b); Planar-type cell [16] (c); inverted

structure (d).

Nevertheless, the most popular perovskite deposition method remains the dual-step spin-coating, which was proposed by Jeon et. al. as a solvent engineering method. [17] In this work, the perovskite ink containing MAI, MABr, PbI2, PbBr2 dissolved in the mixture of GBL and DMSO was spread over the substrate surface using spin-coating, and then the solvent, which doesn't dissolve perovskite and is miscible with perovskite solvents (sometimes is referred to as antisolvent) was applied to facilitate evaporation of GBL:DMSO. Such an approach followed by substrate annealing enables a very smooth and uniform crystalline perovskite layer (Figure 3). Additionally, in this research as well as in many subsequent works it is shown that various factors may influence the perovskite crystallization process including the temperature of the annealing, substrate rotation speed, time interval before quenching the surface with antisolvent, the amount of antisolvent, and others, and of course nature of all the components. [18-21] To sum up, accurate choice of deposition conditions is crucial for achieving a high-quality perovskite film with tailored microstructure and granularity. [22]

It should be noted, that the natural evolution of perovskite solar cells from DSSC led to the development of n-i-p structure, which consists of the absorber layer with perovskite structure created on the transparent substrate, highly

transparent electron-collecting electrode, and electron-transport layer (ETL). On the top of the perovskite, there is a hole-transport layer (HTL). Another possible PSC architecture is the inverted or p-i-n, which has ETL formed on the photoactive layer and semitransparent HTL under it (Figure 2d).

Fig. 3. The solvent engineering approach for perovskite layer deposition.

The goal of achieving high PSC efficiency can't be fulfilled without a proper choice of charge transport layers (CTL). Despite perovskite itself provides good electron- and hole- mobility, [15, 23] efficiencies comparable with that of Si-based solar cells can't be reached without selective charge extraction layers. The selective charge extraction implies appropriate energy levels for particular charge extraction and opposite charge blocking, and decent charge mobility. [24] A schematic representation of properly selected CTLs is given in Figure 4. [25]

Fig. 4. Perovskite and CTLs energy level alignment for efficient charge extraction.

As it was stated before, perovskite was used for the first time as a light harvester for DSSC. Therefore, the most popular electron transport layer (ETL) material still remains TiO2, which is a common ETL for DSSC. So far the efficiencies of PSCs containing compact and mesoporous TiO2 have reached up to

22.6% certified values. [26] However, the formation of a compact TiO2 layer requires annealing at high temperature (>450°C), which is destructive towards common electron collecting electrode material (ITO) and not compatible with bendable substrates. Therefore the search for new ETLs remains an important subject.

To maintain selective charge extraction, the conduction band of the ETL should be in alignment with that of perovskite. In this case, the transport of electrons from the absorber to the ETL is facilitated. On the other hand, the hole transport from perovskite to ETL should be prohibited, which is ensured with the deep-lying valence band of the ETL compared to that of perovskite. Taking into consideration these conditions, various materials were reported in the literature as ETLs. The SnO2 compact layer was reported to outperform compact TiO2 ETL due to better energy level alignment. [27]

The ZnO layer provides higher electron mobility than TiO2, which can potentially reduce recombination losses inside the ETL, besides ZnO can be deposited using low-temperature solution processing, which enables using flexible substrates. [28] However, so far the PSC efficiencies with ZnO are modest and reach 17.8%. [29] Nevertheless, ZnO is a perspective ETL and there were many efforts for improving the device efficiency. For example, recently it was shown, that ZnO layer reacts with the organic counterpart of perovskite material, which can be avoided by the modification of ZnO with MAI. [30] The record 20.8% efficiency was shown for ZnO-based PSCs containing boron, fluorine, and polyethylenimine. [25]

Among organic materials, the n-type fullerene derivatives with C60 core are of great interest, as they deliver high transparency and barrier-free energy level alignment with the most popular perovskite material MAPbl3. However, the materials are rarely used in n-i-p perovskite solar cells, because they are soluble in perovskite polar organic solvents (DMF, DMSO, etc.) and can be easily washed away during fabrication. Therefore, fullerene derivatives are usually applied in inverted (p-i-n) perovskite solar cells atop perovskite, or as ETL modifiers in

standard perovskite configuration. [31] For example, modification of SnO2 layer with fullerene derivative PCBA was reported recently. [32] According to the mechanism proposed in this work, acidic groups in PCBA react with hydroxyls on the SnO2 layer thus passivating the ETL surface defects and facilitating electron extraction from the absorber layer.

To sum up, the energy level diagram of the most popular ETL material is presented in Figure 5.

Fig. 5. Energy level diagram of electron transport layer materials and perovskite.

The hole-transport layer (HTL) materials are more widely explored in the literature. State-of-the-art HTLs are represented by triarylamine-based organic materials: 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobiuorene (Spiro-O-MeTAD) and polytriarylamine (Figure 6a.) The efficiency of PSCs utilizing these materials developed very similarly and rapidly, as it is demonstrated in (Figure 6b). [33, 34] However, being organic materials, both PTAA and Spiro-O-MeTAD have insufficient intrinsic charge carrier mobilities[35, 36] (<10-4 cm2V-1s-1) and hence additional doping is required to improve their conductivity and enable their efficient operation as HTLs in PSCs. [34, 37-40] The most popular combination of dopant materials is a mixture of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) with tert-butylpyridine (tBP), the application of which is accompanied by oxidation in the air (Figure 6c). [41, 42] However, the hygroscopic nature of these salts leads to mitigation of the

operational stability along with the increasing cost of the device. [34, 40, 43] What is more, under working conditions severe morphological deformations in the HTL can occur, leading to the deterioration of the overall performance of the devices. [43]

In sum, the most widespread HTLs provide top certified efficiency, but insufficient stability to consider PSC appropriate for industrial-scale implementation. Therefore, the search for new p-type materials for application as HTLs in PSCs is extremely intensive.

The design of the perfect HTL material should be conducted with consideration of several requirements. [3] First of all, the HOMO of the material should be in alignment with the perovskite valence band to facilitate the hole extraction from the photoactive layer. At the same time, the lowest unoccupied molecular orbital (LUMO) energy level should be higher than the conduction band of photoactive materials to avoid electron transferring through the HTL. Secondly, the HTL material should provide decent hole mobility, ideally, it should be higher than 10-4 cm2V-1s-1 for effective charge transfer without adding p-dopants. Third, the material should be thermally stable to endure high temperatures at the fabrication and application stages. It should be noted, that the chemical compatibility of the HTL towards perovskite and other PSC counterparts is highly important. [44] Finally, the solubility of the material in nonpolar solvents is desirable. This feature allows to deposit materials atop perovskite using cheap solution-based deposition methods, thus decreasing the total cost of the device and enabling the incorporation of the material deposition to the overall process of roll-to-roll manufacturing using printing techniques.

Fig. 6. State-of-the-art hole-transport layer materials Spiro-O-MeTAD and PTAA (a); efficiency race between Spiro-O-MeTAD and PTAA HTLs (b); [33] the most popular combination of p-dopant materials LiTFSI and tBP (c).

In the following sections, various types of HTLs for PSC will be described. The main focus is directed to the most successful materials in terms of improving PSC efficiency or stability. 1.2 Hole-transport materials in perovskite solar cells: recent advances

1.2.1 Inorganic hole-transport materials Most of the inorganic hole-transport materials are presented by sulfides and oxides of transition metals, such as V2O5, [45, 46] WO3, [47] MoO3, [45] NiOx, [48] CuI, [49] CuSCN, [50] CuS, MnS, [51] and more complex materials mostly Cu-based, such as CuGaS2, CuNiSnS, CuAlO2, LiCoO2, etc. [52] Recent advances in PSC with inorganic HTMs are summarized in Table 1. Inorganic HTMs provide

hole mobility up to 100 cm2V-1s-1 (for Cu2O) along with excellent thermal stability compared to organics. [53]

Table 1. Performance of PSCs containing inorganic HTMs.

Material Device architecture Dep Voc V Jsc mA/cm2 FF % PCE % Ref.

MnS FTO/TiO2/FAMAPbIBr/MnS/Au PVD* 1.11 23.4 76 19.9 [51]

CuS ITO/CuS/ CH3NH3Pbl3/C6o/BCP/Ag SP* 1.02 22.3 71 16.2 [54]

V2O5 ITO/V2O5/CH3NH3Pbl3/PCBM/C6o/BCP/AI SP 1.02 20.8 56 11.7 [46]

WO3 ITO/WO3/CH3NH3PbI3/PCBM/Ag PVD 0.93 16.6 64 9.8 [55]

MoO3 ITO/MoO3/CH3NH3PbI3/PCBM/Ag PVD 0.99 18.8 71 13.1 [55]

CU2O FTO/TiO2/CH3NH3PbI3/Cu2O/Au SP 1.13 22.5 67 17.2 [56]

CuI FTO/CuI/CH3NH3PbI3/PCBM/PEI/Ag I 1.04 20.2 68 14.3 [49]

CuSCN FTO/TiO2/CsFAMAPbIBr/CuSCN/Au SP 1.09 22.7 75 19.2 [50]

CuFeO2 FTO/TiO2/CsFAMAPbIBr/CuFeO2/Au SP 1.01 23.6 65 15.6 [57]

Cu2O/CuSCN ITO/SnO2/CsFAMAPbIBr/Cu2O:CuSCN/Au SP 1.05 23.2 78 19.2 [58]

CuGaS2 ITO/SnO2/CsFAMABrCl I/CuGaS2/Au SP 1.05 22.9 71 17.3 [59]

CuNiSnS4 FTO/TiO2/ CH3NH3PbI3/CuNiSnS4/Au SP 0.92 17.7 54 8.9 [60]

CuCoSnS4 FTO/TiO2/ CH3NH3PbI3/CuCoSnS4/Au SP 0.87 16.9 50 7.3 [60]

Sr:NiOx FTO/NiOx/FAMAPbIBr/PCBM/Ag SP 1.12 22.7 79 20.1 [61]

LiCoO2 ITO/LiCoO2/CH3NH3PbI3/C6o/BCP/Ag MS 1.06 22.5 80 19.1 [62]

*PVD - physical vapor deposition, SP- spin-coating, MS- magnetron sputtering, I - iodization.

Various deposition methods can be used for inorganic hole-transport layers (HTLs) including vapor deposition, spin-coating, magnetron sputtering, electron beam evaporation, pulsed laser deposition, atomic layer deposition, chemical bath deposition, spray-pyrolysis, and electrochemical deposition. [48] Inorganic HTMs are less attractive in inverted PSCs, as they can be easily washed away with perovskite solvent. The solution processing atop perovskite layer in the normal configuration of PSCs requires an accurate choice of the solvent, so the perovskite layer is not affected. With all these complications, the most popular deposition methods for inorganic HTMs are PVD and MS. These methods require a low vacuum and high temperatures, which makes them energy-consuming, thus increasing the overall device cost.

The energy levels of the most popular inorganic HTLs are presented in Figure 7. Most of the inorganic materials provide a very high bandgap, which is good for hole extraction selectivity. Additionally, the energy levels of inorganic

HTLs can be tuned using dopants for better matching between material and perovskite valence bands. [61]

>

CT>

(D C

t/5 CM O D o D u z co 0 CuO NiOx c O0 lo c N cs D U

MAPbl3 z) U D u u CO U O o {N D u

-5.1

-5.2

-5.3 -5.3 -5.3

-5.4

.5.4 -5.4 -5.4 -5.4

Fig. 7. Energy levels of the most popular inorganic HTMs. [61]

In sum, inorganic HTLs provide decent efficiency in PSCs, although require high temperatures and low vacuum for deposition. The main disadvantage of inorganic HTLs is their chemical aggressiveness towards perovskite, which results in poor stability in devices.

1.2.2 Spiro and analogs

As it was stated before, state-of-the-art hole-transport materials (HTMs) are organic materials and the most studied of them is a small molecule 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobiuorene (Spiro-O-MeTAD) (Figure 8).

Fig. 8. Structure of Spiro-O-MeTAD.

Perovskite solar cells using Spiro-O-MeTAD as HTL achieve efficiencies of more than 20%. [34] However, in the literature for this material the hole mobility of ^h=10-5 cm2V-1c-1 is reported, which is insufficient for effective charge transport and raises a requirement for additional p-doping. [35]

Another important issue is the huge cost of Spiro-O-MeTAD. The need to use additional doping leads to a further increase in the cost of devices. In this regard, a search for Spiro-O-MeTAD analogs is being carried out, which will make it possible to obtain high-performance perovskite photovoltaics without doping.

All modifications applied to the Spiro-O-MeTAD structure reported in the literature can be divided into three categories: modification of substituents, [63, 64] modification of core, [65-69], and modification of both substituents and core (Figure 9). [70-75]

Fig. 9. Examples of possible modifications of Spiro-O-MeTAD from the literature. [76]

One of the popular modifications of Spiro-O-MeTAD is modification of the methoxy diphenylamino- substituents. In the recent work, two derivatives 2,4-Spiro-O-MeTAD and 3,4-Spiro-O-MeTAD with methoxy substituents in positions of 2 and 4, 3, and 4 were synthesized and characterized. [63] According to photoluminescence spectroscopy, the derivative with additional methoxy group in ortho position provides more efficient hole injection compared to 3,4-Spiro-O-MeTAD and conventional Spiro-O-MeTAD. What is more, devices with HTM based on 2,4-Spiro-O-MeTAD demonstrated superior performance with PCE = 17.2% compared to devices with the Spiro-O-MeTAD-based HTM with efficiencies not surpassing 15.0%. Besides, introducing substituents into metaposition led to decreasing of the HOMO energy level of the molecule to -5.45 eV, thus deteriorating the device performance to PCE = 9.1% mostly impacted by low open-circuit voltage Voc = 0.75 V. The decrease in Voc evidences the mismatching of the HOMO energy level of 3,4-Spiro-O-MeTAD HTM and the perovskite valence band.

Another approach for the modification of substituents consisted in introducing the 3,4-ethylene dioxythiophene unit between the spiro-core and the diphenylamine substituents. [64] The target Spiro-tBuBED demonstrated excellent performance with impressive Voc=1.1 V and 18.6% efficiency without additional oxidation in the air, whereas devices using Spiro-O-MeTAD as HTM required oxidation with the oxygen of the air for 1h and achieved lower efficiency of 17.4%. What is more, the authors estimated the cost of 1mmol of both HTMs. The synthesis of Spiro-tBuBED was more economical compared to the Spiro-O-MeTAD due to the use of direct heteroarylation reaction instead of the classic polycondensation reactions.

Recently the influence of the alkyl substituent nature on the Spiro-derivatives performance in PSCs was evaluated. [77] To affect the molecular packing with different alkyl substituents, new Spiro-derivatives with the different lengths of the alkoxy-chain as Spiro-OEtTAD, Spiro-OPrTAD, Spiro-OiPrTAD, and Spiro-OBuTAD were studied (Figure 9). The glass transition temperature (Tg) of new

derivatives exceeded that of Spiro-O-MeTAD (Tg=167°C). It should be noted, that in the case of small molecular weight materials with a tendency to crystallize the high Tg is a crucial requirement for an HTM, as it allows avoiding undesirable recrystallization of the layer under operational conditions. The performance of all the studied materials was comparable with the slightly superior efficiencies for devices with Spiro-OEtTAD showing PCE = 20.2%. Additionally, it turned out, that stability under ambient conditions depends on the nature of substituents. In particular, the materials with butoxy- and isopropoxy- substituents demonstrated an efficiency loss of 15%, whereas ethoxy- and methoxy- substituted Spiro-derivatives degraded up to 75% and 53% of initial efficiency respectively after 21 days of degradation.

A positive correlation between the number of redox-active sites in the molecule and its encapsulation ability was defined (Figure 10). [76]

Fig. 10. Structures of four Spiro-based derivatives (a), the evolution of absorption spectra (b), and composition evolution according to XRD (c) for devices with configuration glass/perovskite/HTM. [76]

In particular, four materials Spiro-O-MeTAD, Spiro-TAD, Spiro-DPSP, and Spiro-sexiphenyl containing 12, 4, 2, and 0 heteroatoms in the structure

respectively, were studied. Using XRD and absorption spectroscopy it was shown, that Spiro-O-MeTAD incorporating more heteroatoms in the structure allows to mitigate decomposition processes of perovskite thin films. This could be attributed to the high density of contacts between the perovskite surface and the oxygen and nitrogen in the HTL.

In conclusion, approaches to the modification of Spiro-O-MeTAD were summarized. Variation of alkyl substituents along with the alteration of the core unit allows tuning the physicochemical properties. The long-term stability of the perovskite thin-film covered with Spiro-derivatives correlates with the amount of redox-active cites provided by the molecule.

1.2.3 Organic and organometallic small molecular weight hole-transport materials

Besides the most popular small-molecular weight organic HTLs, which are the abovementioned Spiro-based materials, there are plenty of other wonderful p-type small molecules, which can be applied in PSCs. They can be divided by the dominant unit in their structure into several groups: triphenylamines, triazatruxenes, azulenes, pyrenes, and less popular other materials. Separately, the macrocycles incorporating central metal atoms, such as porphyrins, phthalocyanines, and their analogs, are perspective HTL materials due to their chemical and thermal robustness, low-cost synthesis, and ability to self-organize at the supramolecular level. [78]

Triphenylamines (TPAs) are related to the Spiro-based materials, which also include two TPA groups in the structure. TPA's popularity started to grow long before PSC invention when they were first applied as p-type materials in organic light-emitting diodes (OLED). These materials gained popularity due to their ability to transfer positive charge provided with the tertiary nitrogen atom. Additionally, the propeller-like structure provides the non-planar geometry and prevents aggregation between molecules due to the enhanced solubility of these materials in organic solvents. [3]

To evaluate the influence of the linker between two TPA units on the physicochemical properties of the materials and their performance in PSCs, three

materials denoted as H-Bi, H-Fl, and H-Ca containing biphenyl, fluorine, and carbazole linkers, respectively, were synthesized (Figure 11a). [79] The materials provide very low hole mobility, the highest ^h=8x10-6 cm2V-1s-1 for materials with biphenyl and carbazole linkers, and glass transition temperatures (Tg) inferior compared to that of Spiro-O-MeTAD (Tg=122°C). The lowest Tg=54°C is provided by H-Fl, which is attributed to the higher amount of solubilizing chains on the fluorene. Therefore, introducing an excessive amount of alkyl substituents should be avoided in order to obtain materials with higher Tg. The hole-extracting ability of three materials was evaluated using steady-state photoluminescence (PL). According to PL, H-Bi and H-Ca afford more efficient extraction of charges from the perovskite layer (Figure 11b). Besides, all three materials were evaluated as HTLs in PSCs after proper p-doping with the LiTFSI and tBP. The top efficiency of 17.9% was achieved for devices with H-Ca with very impressive Voc=1.12 V, probably due to perfect HOMO energy level alignment with perovskite valence band (Figure 11c).

Wavelength, nm Voltage, V

Fig. 11. Structures of H-Bi, H-Fl, and H-Ca (a); PL intensity of thin films silicon/perovskite/HTM (b); J-V curves of PSCs containing TPA materials (c). [79]

The information about all the aforementioned materials including their HOMO energy level, the configuration of the device, and the device performance is summarized in Table 2 at the end of the chapter.

The structure of another TPA material with the dithienopyrrolop spacer is presented in Figure 12a. [80] According to the calculated electrostatic surface potential (ESP), in the DTP-C6Th positive charge is localized on the TPA units, while the negative charge highlighted in red is concentrated on the dithienopyrrole spacer. Such kind of charge polarization positively influences intermolecular charge transport. The material providing HOMO = -4.87 eV was investigated in PSCs with MA0.7FA0.3PM3 as perovskite material. The devices based on DTP-C6Th demonstrated 18.6% efficiency. To boost the efficiency further to 20.4%, the authors applied perovskite surface passivation using polymethyl methacrylate (PMMA). Furthermore, they adjusted perovskite composition by addition of bromine (Figure 12b) reaching the 21.0% efficiency with an impressive 1.15 V open-circuit voltage.

Hoy —

ITO

Fig. 12. Structure of DTP-C6Th with electrostatic surface potential distribution (a); modification of PSC architecture leading to increased efficiency (b). [81]

As an example of star-shaped TPAs, two molecules TDT-OMeTPA and TTPA-OMeTPA were synthesized using Buchwald-Hartwig cross-coupling reaction and evaluated as HTLs in PSCs with additional p-dopants (Figure 13). [82] The superior 16.4% efficiency delivered by devices based on the fused material is attributed to its 50 times higher hole-mobility and more adjacent HOMO to perovskite VB energy levels. Besides, the stability of devices based on star-

shaped TPAs was investigated. Impressive device stability under 1 sun illumination was observed for both materials during 500 h probably due to their highly hydrophobic nature, as it was shown using the contact angle measurements.

OMe

OMe OMe

MeO— O^O U^OMeMe°

"OMe MeO ^^ ^^ OMe

TTPA-OMeTPA TDT-OMeTPA

Fig. 13. Structures of star-shaped TPAs. [82]

A very interesting family of small-molecular weight p-type materials is triazatruxenes. These materials contain triindole core and provide C3 symmetric conjugated structure. As TPAs the triazatruxenes were first applied in OLEDs, and the first synthesis of triazatruxene from 1-iodo-N-methylindole using Ullman coupling was reported in 1980. [83] The method received several modifications, and currently, triazatruxene core is usually synthesized using trimerization of indole and 2-indolone in the presence of bromine and POCl3, respectively. [84]

Application of triazatruxenes as HTMs in PSCs allowed achieving efficiencies surpassing 20% without additional p-doping. [85] The dominant amount of synthetic modifications concerns the substituents at the position 3, 8, 13. [81, 86] The set of materials presented in Figure 14 has HOMO levels in the range of -5.12 to -5.41 eV. The devices containing dopant-free C1B3 triazatruxene derivative with dicianovinylene N-methyl-rhodanine deliver 17.5% efficiency. Among the derivatives denoted as SGT, the one containing malononitrile substituent enable better performance in PSCs.

N-Hex

Hex

Hex

Hex

C1B1, R=

Vn

°VN C1B3,

PN CN

Hex

SGT-460,

CI B2

SGT-461, R1 =

NC

SGT-462, =

CN