Interfacial Charge Dynamics Studies for High-Performance and Stable Perovskite Solar Cells


Student thesis: Doctoral Thesis

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Awarding Institution
Award date16 Sep 2022


Solar energy has been regarded as a significant component of renewable and clean energy, which can help reduce usage of fossil fuels in the context of carbon peaking and carbon neutrality. Solar cells can transform solar energy into electrical power, which represents the main technique to take advantage of solar energy. In contrast to the commercial solar cells such as polycrystalline silicon (p-Si), copper indium gallium selenide (CIGS) and cadmium telluride (CdTe), organic–inorganic hybrid perovskite solar cells (PSCs) attract tremendous attention because of their unique advantages of low cost, light weight, high performance and capability of fabricating thin-film and flexible devices. The power conversion efficiency (PCE) of PSCs over the previous 10 years has certified at 25.7%, indicating considerable potential for commercialization and eventual replacement of silicon solar cells. Prior to commercialization, PSCs’ performance and stability are two main challenges that need to be resolved. In the typical planar heterojunction structure of PSC devices, perovskite which serves as a light absorber is sandwiched by an electron transporting layer (ETL) and a hole transporting layer (HTL). Electrons and holes are created when the perovskite layer absorbs sunlight. These charge carries then travel across the perovskite layer and are collected by ETL and HTL at the corresponding interface. Due to different obstacles including defects, energy barrier, leakage current, et al., which induce non-radiative recombination, the energy stored in mobile charge carriers suffer a loss during charge transportation. Charge carriers can be spatially trapped before extraction by traps across the bulky perovskite layer and at the interfaces. Using compositional and additive engineering, it is possible to effectively manage the perovskite crystallinity and minimize bulky defects. Due to their defect-rich nature created during the solution processing fabrication, unbound surfaces of perovskite are thought to be the main cause of recombination loss. Interfacial treatment has been demonstrated as effective strategies in order to reduce surface defects through charge compensation, secondary growth or forming ultrathin low dimensional layers on the surface. Charge extraction, meanwhile, depends on energy alignment at interfaces between the charge transporting layer (CTL) and the perovskite layer. Incorporating materials that are well-compatible and have aligned energy levels as CTLs enable efficient charge extraction, preventing charge accumulation and recombination. Moreover, these interfacial defects play a significant role at device stability as they are easily excited to induce photochemical degradation of perovskite layer and interface. Defect assisted ion migration seriously degrades the device, during long-term operation with external stimuli of light and electrical bias, which harms the long-term stability. The interfacial charge dynamics in inverted PSCs will be the main topic of the thesis. Novel polymeric and self-assemble monolayer (SAM) HTLs can be used to effectively extract holes which minimizing energy offset. Furthermore, such bottom interface engineering also assists crystallization of perovskite layer with preferable crystal orientation as well as enhances crystallinity. We use inorganic functional materials to passivate surface defects on the perovskite layer, which reduces flaws and stabilizes the interface, enhancing the performance and stability of the device.

First, in order to substitute PTAA for efficient hole extraction and improving perovskite crystallization, we develop innovative dopant-free polymeric HTLs. Two isomeric polymer HTLs (PPE1 and PPE2) with almost identical photophysical properties, hole-transporting ability, and surface wettability display distinctly different device performance for reasons that can be rationally explained. PPE2 is discovered to improve the quality of perovskite films cast on top with larger grain sizes and more oriented crystallization, which leads to a high PCE of 21.3% in single-junction inverted PSCs. These findings contribute to the development of new HTL design guidelines that affect perovskite growth/crystallization and to improve the performance of inverted PSCs.

Second, surface treatment with functionalized inorganic material is demonstrated to be effective on improving device performance and long-term operational stability. We incorporate robust fluoro-terminated TiO2 nanosheets (F-TiO2 NSs) to be applied as an interlayer between perovskite and electron-transporting layer. The flat nanosheet morphology of F-TiO2 NSs enables close contact with perovskite with abundant surface fluoro groups, which can interact with undercoordinated lead and MA/FA ions to prevent the formation of cation vacancies and alleviate surface defects. Therefore, a remarkable PCE of 22.86% with improved Voc and Jsc can be achieved. By suppressing ion motion in devices, the robust contact between fluoro groups and the perovskite surface improves the operational stability of PSCs. The devices with F-TiO2 NSs interlayer could maintain over 90% of their initial PCEs after being monitored at the maximum power point (MPP) for 1000 h under day/night cycles, along with maintaining 80% efficiencies under continuous MPP tracking at 60 ℃.

Third, the considerable thickness required by conventional HTLs causes transporting loss. The issue can be resolved by the employment of SAMs, which let charges to pass through this ultrathin monolayer with the least amount of transportation loss. Herein, we develop a co-assembled monolayer (co-SAM) in order to obtain efficient hole selection and suppressed recombination at the hole selective interface in inverted PSCs. By engineering the position of methoxy substituents, an aligned energy level and favorable dipole moment can be obtained in DC-PA to achieve a well-matched energy alignment with the perovskite layer. An alkyl ammonium containing SAM is co-assembled in order to further optimize the surface functionalization and interaction with perovskite layer on top. A champion device with an excellent PCE of 23.59% and improved device stability are achieved. This work demonstrates the advantage of using co-SAM in improving performance and stability of PSCs.