Synchronizing Crystallization Enables Thermally Stable All-FA Pb-Sn Perovskites for Printable MA-Free All-Perovskite Tandem Solar Cells

Abstract

Monolithic all-perovskite tandem solar cells based on mixed cation lead-tin (Pb-Sn) have advanced rapidly in recent years. However, the presence of a considerable amount of volatile methylammonium (MA) adversely constrains stability of the solar devices. Here, we first quantitatively evaluated the thermal stability of Pb-Sn perovskite films containing different types of A-site cations. In comparison to the all-MA and MA-FA binary counterparts, all-formamidinium (FA) Pb-Sn films exhibit the highest decomposition activation energy of 149.13 kJ mol-1. On this basis, high-quality all-FA Pb-Sn perovskite films are prepared by blade coating with addition of a small amount of hydrazinium dichloride (HDC) to the perovskite precursor. The selectively strong coordination of HDC with Sn2+ ions not only suppresses the oxidation of Sn2+ but, more importantly, balances the nucleation of the Sn- and Pb-based species, resulting in perovskite films with markedly improved homogeneity of Pb-Sn alloyed phase. The prepared single-junction all-FA Pb-Sn PSCs and MA-free tandem devices yield champion efficiencies of 21.81% and 27.40%, respectively. Moreover, the unencapsulated all-FA Pb-Sn devices maintain >80% of their initial efficiencies following 190 h of thermal stress at 85 °C.