Energy level alignment studies in tin perovskite solar cells through incorporation of inorganic cation and charge transport layer selection

Tin halide perovskites are the front-runner for lead-free perovskite solar cells (PSCs). Currently the highest efficiency is reaching 15% which is the highest among other types of lead-free perovskites. There are several issues with tin halide perovskites that need to be addressed before they can really compete with lead-based PSC. Among the issues are the prone of oxidation of Sn2+ into Sn4+ in air, fast crystallization leading to morphologically uneven surface and the formation of pinholes, large energy mismatch between commonly used charge transport layers (CTLs), and current-voltage hysteresis due to unbalanced charge carrier transport and ion migration. In this experiment, we incorporated Cs+ which is a small A-site cation to reduce the energy mismatch between the perovskite layer and CTLs. Although this was explored previously even in tin halide perovskite solar cells, we showed that Cs cation incorporation alone will not improve the performance, however when used simultaneously with [6,6]-Phenyl-C61 butyric acid butyl ester (PCBM) as the electron transport layer (ETL), we managed to improve the efficiency of the tin PSC to more than 13%. To understand this phenomenon, we used impedance spectroscopy analysis to evaluate the charge transport mechanism. We attribute this enhancement due to better energy level alignment between the perovskite layer and ETL.