Разработка новых электрон-транспортных материалов для высокоэффективных и стабильных перовскитных солнечных батарей тема диссертации и автореферата по ВАК РФ 01.04.17, кандидат наук Элнаггар Мохамед Мохамед Рагаб

• Элнаггар Мохамед Мохамед Рагаб
• кандидат наук
• 2021

Введение диссертации (часть автореферата) на тему «Разработка новых электрон-транспортных материалов для высокоэффективных и стабильных перовскитных солнечных батарей»

Abstract

Hybrid perovskite solar cells (PSCs) have shown a rapid increase in the solar light power conversion efficiency (PCE) within the last few years surpassing recently 25.5 % threshold and coming close to crystalline silicon. However, the progress in improving the stability of PSCs is far from being satisfactory, and therefore, short operation lifetimes represent the main obstacle for the successful commercialization of perovskite photovoltaic technology. Decay in the efficiency of completed p-i-n or n-i-p PSC architectures under light exposure can be caused by tens of different factors. Even when the experiments are performed under an inert atmosphere, which excludes the impact of extrinsic factors, such as oxygen and moisture, the device failure can be induced by degradation of the absorber layer, HTL or ETL materials, absorber/HTL and absorber/ETL interfaces, and interfaces between the chargetransport materials and the electrodes. As such, this thesis is dedicated to the development of advanced electron transport layers (ETLs) for p-i-n perovskite solar cells. ETL is an essential component of PSCs and is responsible for the collection of photogenerated electrons. Optimal ETLs should be chemically inert with respect to the complex lead halides, have minimized LUMO energy level offsets with the conduction band of the absorber material, enable efficient and selective extraction of charge carriers, and provide good isolation of perovskite layer, thus preventing its decomposition. Therefore, we tried to fill this gap by exploring a family of new fullerene derivatives, conjugated polymers, and metal oxides as promising ETL materials for p-i-n PSCs.

In the initial part of the thesis, we discuss the growing evidence that the stability of perovskite solar cells (PSCs) is strongly dependent on the interface chemistry between the absorber films and adjacent charge-transport layers, whereas the exact mechanistic pathways remain poorly understood. A straightforward approach is presented for decoupling the degradation effects induced by the top fullerene-based electron transport layer (ETL) and various bottom hole-transport layer (HTL) materials assembled in p-i-n PSCs. It is shown that the chemical interaction of MAPbb absorber with ETL comprised of the fullerene derivative most aggressively affects the device operational stability. However, washing away the degraded fullerene derivative and depositing fresh ETL leads to the restoration of the initial photovoltaic performance when the bottom perovskite/HTL interface is not degraded. Following this approach, it is possible to compare the photostability of stacks with various HTLs. It is shown that poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)

and NiOx induce significant degradation of the adjacent perovskite layer under light exposure, whereas poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) provides the most stable perovskite/HTL interface.

The second part of the thesis is dedicated to the study of a series of functional fullerene derivatives as suitable ETLs for p-i-n PSCs to enable a stable interface and also improve the device ambient stability (i.e. resistance to moisture). We carried out a systematic study of a family of structurally similar fullerene derivatives as electron transport layer (ETL) materials for p-i-n perovskite solar cells. It is shown that even minor modifications of the molecular structure of the fullerene derivatives have a strong impact on their electrical performance and, particularly, ambient stability of the devices. Indeed, an optimally functionalized fullerene derivative applied as an ETL enables stable operation of perovskite solar cells when exposed to air for >800 h, which is manifested in retention of 90% of the original photovoltaic performance. In contrast, the reference devices with phenyl-C61-butyric acid methyl ester as the ETL degraded-almost completely within less than 100 h of air exposure. Most probably, the side chains of the best-performing fullerene ETL materials are filling the gaps between the carbon spheres, thus preventing the diffusion of oxygen and moisture inside the device.

We also present and discuss the results of a systematic study of other series of fullerene derivatives as promising ETL materials for p-i-n perovskite solar cells. The devices fabricated using new fullerene derivatives demonstrated power conversion efficiencies approaching 17.2%, which is higher than the reference cells assembled using PC61BM, which is a benchmark ETL material (15.9%). The improved photovoltaic performance of the devices incorporating new fullerenes derivatives originated from the decreased nonradiative recombination at the MAPbb/ETLs interface, efficient electron extraction, and full coverage of the perovskite absorber layer. These obtained results feature new functionalized fullerene derivatives as promising ETLs for efficient and potentially stable perovskite solar cells.

The third part of the thesis is dedicated to the exploration of a promising strategy to improve the performance and stability of PSCs by blending two materials to form a composite electron transport layer, which has been successfully implemented in p-i-n PSCs. We introduce a novel pyrrolo[3,4-c]pyrrole-1,4-dione based n-type copolymer as an electron transport material for perovskite solar cells. Using a composite of this polymer with the fullerene derivative PC61BM, we achieved PCE of 16.4% for p-i-n perovskite solar cells using methylammonium-free Cs0.12FA0.88PbI3 absorber and demonstrated long-term operational

stability of these devices. Thus, designing composite organic electron transport materials might facilitate the development and commercialization of efficient and stable perovskite photovoltaics.

Furthermore, we also investigated alternating oligomeric compounds TBTBT and F4TBTBT ("T" - thiophene, "B" - benzothiadiazole) as electron transport materials for p-i-n PSCs. The fabricated devices based on individual (F4)TBTBT and TBTBT as ETL material demonstrated encouraging PCE of 11.7% and 10.5%, which was limited mostly by low FF due to recombination losses. The device performance was largely improved by inserting a thin PC61BM interlayer between the perovskite absorber layer and TBTBT-based ETL. Such modification resulted in spectacular improvement in the device fill factor (FF) from 52% to 82%, which also boosted PCE from 10.5% to 17.8%. Most importantly, using TBTBT as ETL material improved the operational stability of perovskite solar cells under continuous light illumination as compared to the reference devices assembled with the conventional PC61BM-based ETL.

The last part of the thesis is devoted to the use of tungsten oxide WOx as electron transport material in perovskite solar cells. UsingWOX as ETL in p-i-n PSCs delivered the efficiency of 15.5% without using any interlayers and dopants. Furthermore, we improved the device performance by using hybrid C60/WOX as ETL and WOx/PTAA as HTL (hole transport layer) in the same device configuration, which provided PCE of 17.32 % with significantly improved Voc and FF. Thus, we have shown that WOx can play two roles and perform well as a component of both advanced HTL and ETL in p-i-n PSCs. Furthermore, the device performance was improved by using a hybrid C60/WOx/BPhen triple-layer ETL providing the PCE of 18.4 % with negligible hysteresis. Most importantly, using WO3 ETL enabled long-term operational stability of p-i-n PSCs for 4600 h under continuous illumination, which is the record result for MAPbI3-based solar cells. We show that WOx layer blocks the diffusion of ions from the top electrode to the absorber layer and vice versa. On the contrary, using PC61BM as ETL under the same aging conditions resulted in a complete device degradation within just a few hours. Thus, using the advanced WO3-based ETL materials paves a way to reaching long operational lifetimes of PSCs, which are required for the successful commercialization of this promising PV technology.

In conclusion, we have developed an efficient methodology for the investigation of interface degradation effects in p-i-n PSCs and decoupling the contributions of ETL/perovskite

and perovskite/ HTL junctions to the overall device aging kinetics. Also, we performed a systematic study of a big series of new fullerene derivatives, conjugated polymers, and metal oxides as ETL materials for p-i-n PSCs, leading to substantial improvements in the device efficiency and operational stability. The results presented in this thesis are expected to facilitate the transition of perovskite solar cells towards commercial production.

Заключение диссертации по теме «Химическая физика, в том числе физика горения и взрыва», Элнаггар Мохамед Мохамед Рагаб

The main results:

1. An efficient methodology was developed for the investigation of interface degradation effects in p-i-n perovskite solar cells (PSCs). The proposed approach allows one to decouple the contributions of ETL/perovskite and perovskite/HTL interfaces to the overall device aging kinetics.

2. A series of structurally similar fullerene derivatives were systematically studied as ETL materials in p-i-n PSCs. It was shown that even minor modifications of the molecular structure of the fullerene derivative have a strong impact on their electrical performance and, particularly, the ambient stability of the devices. Indeed, an optimally functionalized fullerene derivative applied as an ETL enables stable operation of perovskite solar cells when exposed to air for >800 h, which is manifested in retention of 90% of the original photovoltaic performance.

3. A novel pyrrolo[3,4-c]pyrrole-1,4-dione-based n-type conjugated polymer was investigated as an electron transport material for perovskite solar cells. By blending this polymer with the fullerene derivative PC61BM we achieved a decent power conversion efficiency of 16.4% in p-i-n perovskite solar cells using methylammonium-free Cs0.12FA0.88PbI3 absorber in combination with the substantially improved operational stability of the devices.

4. Conjugated oligomeric compounds TBTBT and F4TBTBT ("T" - thiophene, "B" -benzothiadiazole) were studied as electron transport materials for p-i-n PSCs. The fabricated devices demonstrated a decent power conversion efficiency (PCE) of 10.5%, which was improved to 17.8% by inserting a thin PC61BM interlayer between the perovskite absorber layer and TBTBT-based ETL. The use of TBTBT as ETL enhanced the operational stability of

perovskite solar cells as compared to the reference devices using the conventional fullerene derivative PC61BM.

5. Tungsten oxide (WOx) was integrated for the first time as electron-transport material for p-i-n PSCs. A high photovoltaic efficiency of up to 18.4% was achieved in combination with the record-breaking operational stability of the devices under continuous light soaking at a high temperature of 60 oC. Indeed, the MAPbb-based PSCs with WOx electron-transport layer retained ~70% of their initial performance after 4600 h of aging under such harsh conditions, whereas the reference cell assembled with PC61BM degraded completely within 50 h.

Список литературы диссертационного исследования кандидат наук Элнаггар Мохамед Мохамед Рагаб, 2021 год

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