Strategic construction of top electron transport layers toward efficient and stable perovskite solar cells

Abstract

Perovskite solar cells (PSCs) is a new generational photovoltaic device with the perovskite-structured compound as the photoactive layer. During the past decade, the PSCs have been researched intensively from the wide space of the perovskite materials, various device architectures, different deposition process of the perovskite crystal film and the device stability. Great advances in the efficiency of PSCs have been achieved, its certified power conversion efficiency (PCE) has been increasing rapidly from 3.8% in 2009 to 25.5% in 2020 in single-junction architectures, which has been as high as the necessary for commercialization and is continuously rising to date. To realize the ultimate practical applications of PSCs, simultaneously achieving high efficiency, long-term stability, and low-cost fabrication with good reproducibility is essential. However, the poor long-term stability of PSCs makes it challenging for commercialization. In this thesis, three structures of the electron transport layer (ETL) on the top of perovskite photoactive layer were investigated based on their special deposition method and ion blocking capability, and a new one-step deposition method of simultaneously depositing perovskite/CTL bilayer structure was developed for improving the efficiency and stability of PSCs. (1) The mixed C60:PCBM electron transport layer (ETL) with excellent ion blocking capability was developed for improving device stability. The relationship between ions blocking capability of top ETLs and the device stability was investigated. As a result, we demonstrated that in the PSCs devices, the more ions accumulated at the interface of perovskite/ETL or diffused into the ETL films, the poorer operational stability, higher interface charge density and severer J-V curve hysteresis would be exhibit in these PSCs devices, which will accelerate the device degradation process and reduce device performance unavoidably. (2) The TiO2-fullerene ETL with the structure of thick TiO2 as the backbone on the top of perovskite film and then decorated by the fullerene (mixed C60:PCBM) was demonstrated. This new structure of fullerene-decorated TiO2 ETL can effectively suppress the diffusion of ions or molecular from both the perovskite layer and metal electrode into each other. Besides, the well-matched energy level alignment of perovskite/TiO2/C60:PCBM and excellent electrical conductivity of fullerene-decorated TiO2 film remarkably facilitate carrier extraction and transport. These above-mentioned advantages of TiO2-fullerene ETL improved device performance comprehensively from operational stability and PCE. (3) We demonstrate a new one-step deposition method to simultaneously deposit the perovskite photoactive layer and the top electron transport layer. The perovskite/TiO2 bilayer structure with further improved crystal quality of perovskite film, denser and more compact TiO2 film, and more effective interface contact of perovskite/TiO2 heterojunction were achieved during once spin coating, which improved device efficiency remarkably (PCE = 18.2%) than the conventional sequential deposition method fabricated device (PCE = 5.19%). An equilibrium growth kinetics was proposed to present the growing process of the perovskite crystal grains fabricated by this new one-step deposition method. The device fabrication process in this new one-step method, comparing with the conventional sequential deposition method, was greatly simplified with reduced steps and significantly shortened processing time.