Discovery of Perovskite Cosolvency and Undoped FAPbI₃ Single-Crystal Solar Cells Fabricated in Ambient Air

We report the cosolvency effect of formamidinium lead triiodide (FAPbI₃) in a mixture of γ-butyrolactone (GBL) and 2-methoxyethanol (2ME), a phenomenon where FAPbI₃ shows higher solubility in the solvent blend than in either alone. We found that FAPbI₃ exhibits 10× higher solubility in 30% 2ME in GBL than in 2ME alone and 40% higher solubility than in GBL alone at 90 °C. This enhanced solubility is attributed to the disruption of the hydrogen bonding network within 2ME, allowing its hydroxyl and ether groups to interact more freely with the solute. Leveraging this phenomenon, we grew phase-stable α-FAPbI₃ thin single crystals under ambient air conditions with no doping. Compared to conventional cesium-doped FAPbI₃, the undoped FAPbI₃ single-crystal films exhibited lower defect densities and enhanced charge retention and transfer while also avoiding phase segregation linked to cesium incorporation. Solar cells fabricated with these ambient-air-grown single-crystal films achieved an efficiency of 21.56% (17.72% for cesium-doped FAPbI₃), retaining 90% of performance after six months of storage. These findings advance our understanding of perovskite solubility in solvent blends and offer an efficient pathway for producing stable, high-efficiency FAPbI₃ single-crystal solar cells through ambient air fabrication, overcoming the limitations of doping.