Efficient large guanidinium mixed perovskite solar cells with enhanced photovoltage and low energy losses.

We present a strategy for suppressing the open-circuit voltage (Voc) loss of perovskite solar cells by incorporating large guanidinium cations (Gua+) into a perovskite lattice, leading to a significantly improved Voc of 1.19 V and an impressive power conversion efficiency of >21%. Our results manifest the critical role of Gua in passivating defects and highlight the importance of compositional engineering in promoting the performance of perovskites.