The difference in electronic structure of MAPI and MASI perovskites and its effect on the interface alignment to the HTMs spiro-MeOTAD and CuI

Abstract

We have studied the electronic structure of CH₃NH₃PbI₃ (MAPI) and CH₃NH₃SnI₃ (MASI) perovskite films by performing X-ray photoelectron spectroscopy (XPS) measurements on in situ grown perovskite films. For the MASI perovskite we observed a 0.7 eV lower ionization potential (IP) as compared to MAPI. Thus, the band edges of MASI are positioned energetically higher with respect to MAPI. In addition, we found MASI to form p-type and MAPI n-type doping. To investigate the impact of different electronic structures on the line up to hole transport materials (HTMs) we performed detailed interface experiments using two commonly used HTMs: spiro-MeOTAD and CuI. The HTMs were step-by-step deposited onto in situ prepared MAPI and MASI layers and characterized via XPS and ultraviolet photoelectron spectroscopy (UPS) after each deposition step. Our results show that the lower IP of MASI results in blocking barriers at the spiro-MeOTAD interface as well as at the CuI contact. We also propose an estimated band alignment of the two perovskites to the TiO₂ front contact by using the vacuum level as the reference energy. The higher lying conduction band maximum (CBM) of MASI as compared to MAPI would limit the open-circuit voltage (V_OC) of a solar cell. Furthermore, for a doped spiro-MeOTAD layer, a V_OC value of up to 750 mV is expected at the MAPI|doped spiro-MeOTAD interface, while it is limited to 210 mV in the case of MASI. Our results show that MASI specific HTMs and ETMs have to be found in order to improve the efficiency of MASI based solar cells.