Resolving Mixed Intermediate Phases in Methylammonium-Free Sn-Pb Alloyed Perovskites for High-Performance Solar Cells

The complete elimination of methylammonium (MA) cations in Sn-Pb composites can extend their light and thermal stabilities. Unfortunately, MA-free Sn-Pb alloyed perovskite thin films suffer from wrinkled surfaces and poor crystallization, due to the coexistence of mixed intermediate phases. Here, we report an additive strategy for finely regulating the impurities in the intermediate phase of Cs_0.25FA_0.75Pb_0.6Sn_0.4I₃ and, thereby, obtaining high-performance solar cells. We introduced d-homoserine lactone hydrochloride (D-HLH) to form hydrogen bonds and strong Pb-O/Sn-O bonds with perovskite precursors, thereby weakening the incomplete complexation effect between polar aprotic solvents (e.g., DMSO) and organic (FAI) or inorganic (CsI, PbI₂, and SnI₂) components, and balancing their nucleation processes. This treatment completely transformed mixed intermediate phases into pure preformed perovskite nuclei prior to thermal annealing. Besides, this D-HLH substantially inhibited the oxidation of Sn²⁺ species. This strategy generated a record efficiency of 21.61%, with a V_oc of 0.88 V for an MA-free Sn-Pb device, and an efficiency of 23.82% for its tandem device. The unencapsulated devices displayed impressive thermal stability at 85 °C for 300 h and much improved continuous operation stability at MPP for 120 h.