Stabilizing formamidinium lead triiodide α-phase with diamine cations for perovskite solar cells

•Completely 3D perovskite films without 2D phase were fabricated.•The BDACl2-based PSCs achieves a maximum PCE from 22.47% to 24.12%.•The BDACl2-based PSCs retained over 80% initial PCE after 1500 h of operation. Bulk alkyl ammonium salt additives can significantly improve the stability of perovskite solar cells (PSCs) by forming 2D perovskite structures. However, these ammonium salts form organic interlayers impairing device efficiency. We doped the perovskite precursor with 1,4-butanediamine dihydrochloride (BDACl2) with a concentration that is low enough (BDACl2:FAI ≤ 1.0 mol %) not to form 2D perovskite. It is found that BDACl2 doping promotes grain growth and reduces the defect density, thus improving the crystalline quality of the perovskite films, resulting in an increase in PSCs device efficiency from 22.47% to 24.12%. More importantly, BDACl2 can effectively inhibit the transition from α-phase to δ-phase of FAPbI3 under operation. Therefore, PSCs retained more than 80% of their initial PCE after nearly 1500 h of operation with maximum power point tracking under simulated white light illumination in a nitrogen glove box, whereas the control cell without BDACl2 modification showed PCE decay of over 58% after aging for the same time.