Reduced open-circuit voltage loss for highly efficient low-bandgap perovskite solar cells via suppression of silver diffusion

Abstract

The development of low bandgap perovskite solar cells is important as the current best perovskite solar cells are not based on a light harvesting layer with an ideal bandgap, and low bandgap materials can also be used together with large bandgap materials to construct all-perovskite tandem cells with potentially higher power conversion efficiencies. However, low-bandgap perovskite solar cells based on mixed Pb and Sn generally suffer from inferior performance, hindering the further development of tandem perovskite solar cells. One of the main reasons is the relatively high open-circuit voltage loss in low-bandgap perovskite solar cells, which is generally attributed to the high trap density in perovskite films and energy-level mismatch between different functional layers in the device. In this study, we find another mechanism that can cause high open-circuit voltage loss and degrade device performance, which is silver diffusion towards the perovskite layer. Devices based on a typical inverted structure (indium tin oxide/PEDOT:PSS/FA_0.5MA_0.5Sn_0.5Pb_0.5I₃/PCBM/Ag) suffered from severe silver diffusion that contaminated the whole perovskite layer during the thermal deposition process of the silver electrode. Such contamination increased the trap density and down-shifted the energy level of the FA_0.5MA_0.5Sn_0.5Pb_0.5I₃ layer, resulting in high open-circuit voltage loss in the low-bandgap perovskite solar cells. Accordingly, we inserted a ZnO layer between the PCBM layer and silver electrode to significantly suppress silver diffusion, which dramatically improved the open-circuit voltage from 0.59 V to 0.79 V. Ultimately, a highly efficient low-bandgap Sn–Pb mixed perovskite solar cell with a power conversion efficiency of 18.1% and low open-circuit voltage loss of 0.40 V was achieved, which represents one of the state-of-the-art results in this field.