Encapsulation strategies and applications of zinc oxide in planar perovskite solar cells

Abstract

The energy crisis requires the development of new energy resources for replacing fossil fuels. Solar energy is regarded as a promising candidate due to renewability, easy accessibility and abundance. The conversion from solar energy to electricity is realized by photovoltaic technology. Recently, perovskite solar cells (PSCs) have attracted increasing attention due to the rapid improvement of efficiency. Besides, PCSs can be fabricated by solution process on flexible substrates, which gives the advantage of low cost and light weight. However, the stability of PSCs is one of the main issues that hinder the commercialization. The thesis introduces fundamentals of PSCs, including historical improvements, basic mechanisms, and recent developments. The device fabrication, architecture and characterization including hysteresis effect are also discussed. The sensitivity of perovskite to ambient environment is one of the major reasons for device instability. Besides, other device components including electron transporting layer (ETL), hole transporting layer (HTL) and electrode also affect the stability of PSCs. Encapsulation is one of the effective approaches for improving the device stability. In this thesis, different encapsulation strategies were investigated including selection of sealing material, utilization of protection layer and desiccant. The best encapsulation method was defined by the comparison of stability obtained for different encapsulation methods. The stability tests were conducted following standard protocols established for organic solar cells. Prolonged stability test and outdoor testing were also conducted to further examine the best encapsulation method. In addition to encapsulation, thin film ZnO fabricated by different methods was also investigated to function as ETL in perovskite solar cells. Comparisons were made for different ZnO deposition methods. Due to the thermal instability induced by ZnO, a 5 nm of TiO2 was employed as an interlayer to improve the device performance and the thermal stability of perovskite layer. With the protection of TiO2, perovskite on ZnO can be annealed at higher temperature, resulting in a larger crystal size. Finally, stability test was conducted for ZnO based perovskite solar cells, and compared with PSCs based on TiO2. (330 words)