S-Methylthiouronium Improves the Photostability of Methylammonium Lead Iodide Perovskites

Although the efficiency record of perovskite solar cells is already above 25%, long-term stability is a key challenge for their commercialization. Herein, we report a strategy to stabilize perovskite films and solar cells against photodegradation by using an organic cation, S-methylthiouronium (SMT). Replacing 20% of methylammonium (MA) by S-methylthiouronium in mixed lead-halide perovskites enables a fivefold increase in the stability of perovskite solar cells against degradation in ambient air under full solar irradiation. Perovskites containing SMT cations were additionally more resistant to thermal stress (70 degrees C) under continuous illumination, resulting in about four times lower efficiency drop during 300 h compared with pure MA perovskite solar cell. This partial exchange of cations also contributes to the improvement in film formation quality. The investigation results, including high-performance liquid chromatography measurements, suggest that the material stability improvement is related to the structural rather than chemical stabilization of the perovskite lattice.