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

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(0.25)FA(0.75)Pb(0.5)Sn(0.5)I(3) 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 (PbIn2-n) or iodostannate (SnIn2-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 degrees C for 500 h, and promote continuous operation stability at maximum power point for 150 h.