97.3% Pb-Reduced CsPbGeBr Perovskite with Enhanced Phase Stability and Photovoltaic Performance through Surface Cu Doping.

Ge doping has been regarded as an effective way to explore the low-toxicity inorganic halide perovskite. However, Ge ions are easy to oxidize because the Ge dopant raises the valence band maximum (VBM) over the water oxidization (HO/O) potential. Here we find that surface Cu doping can bend down the band levels and decline the VBM of the CsPbGeBr surface below the HO/O potential, then prevent the Ge from being oxidized into Ge by water because the Cu dopant reduces the perovskite surface electron accumulation. Note that the Cu dopant prefers to locate at the perovskite surface rather than the interior, and it reduces the surface energy and enhances the stability. Consequently, the largest Pb reduction increases to 97.3% for the Cu-doped CsPbGeBr surface. Moreover, the exciton binding energy and optical absorption of CsPbGeBr could be further improved by the surface Cu dopant. This work provides guidance for finding low-toxicity stable inorganic perovskites.