DMSO‐Free Solvent Strategy for Stable and Efficient Methylammonium‐Free Sn–Pb Alloyed Perovskite Solar Cells

Abstract Harmful Sn(IV) vacancies and uncontrolled fast crystallization commonly occur in tin–lead alloyed perovskite films. The typical dimethyl sulfoxide (DMSO) processing solvent is suggested to be the primary source of problems. Here, a DMSO‐free solvent strategy is demonstrated to obtain high‐performance Cs_0.25FA_0.75Pb_0.5Sn_0.5I₃ solar cells. A rational solvent selection process via Hansen solubility parameters and Gutmann's donor number shows that N,N′‐dimethylpropyleneurea (DMPU) has a strong coordinate ability to form complete complexation with organic (formamidinium iodide) and inorganic (CsI, PbI₂, and SnI₂) components. This treatment suppresses the iodoplumbate (PbI_n^2‐n) or iodostannate (SnI_n^2‐n) preformed in precursor solution, thereby promoting pure intermediate complexes and retarding crystallization, realizing enlarged grain size, and improved film crystallinity. Additionally, it is demonstrated that DMPU‐based solvent system can further inhibit the oxidation of Sn(II) and reduced Sn(IV) content by nearly 75% due to its superior thermal and chemical stability. This DMSO‐free strategy generates a record efficiency of 22.41%, with a V_oc of 0.88 V and a FF of 82.72% for the MA‐free Sn–Pb alloyed device. The unencapsulated devices display much‐improved humidity stability at 30 ± 5% relative humidity in air for 240 h, impressive thermal stability at 85 °C for 500 h, and promote continuous operation stability at maximum power point for 150 h.