CuCl₂-modified SnO₂electron transport layer for high efficiency perovskite solar cells

SnO₂film is one of the most widely used electron transport layers (ETL) in perovskite solar cells (PSCs). However, the inherent surface defect states in SnO₂film and mismatch of the energy level alignment with perovskite limit the photovoltaic performance of PSCs. It is of great interesting to modify SnO₂ETL with additive, aiming to decrease the surface defect states and obtain well aligned energy level with perovskite. In this paper, anhydrous copper chloride (CuCl₂) was employed to modify the SnO₂ETL. It is found that the adding of a small amount of CuCl₂into the SnO₂ETL can improve the proportion of Sn⁴⁺in SnO₂, passivate oxygen vacancies at the surface of SnO₂nanocrystals, improve the hydrophobicity and conductivity of ETL, and obtain a good energy level alignment with perovskite. As a result, both the photoelectric conversion efficiency (PCE) and stability of the PSCs based on SnO₂ETLs modified with CuCl₂(SnO₂-CuCl₂) is improved in comparison with that of the PSCs on pristine SnO₂ETLs. The optimal PSC based on SnO₂-CuCl₂ETL exhibits a much higher PCE of 20.31% as compared to the control device (18.15%). The unencapsulated PSCs with CuCl₂modification maintain 89.3% of their initial PCE after exposing for 16 d under ambient conditions with a relative humidity of 35%. Cu(NO₃)₂was also employed to modify the SnO₂ETL and achieved a similar effect as that of CuCl₂, indicating that the cation Cu²⁺plays the main role in SnO₂ETL modification.