Inhibition of defect-induced α-to-δ phase transition for efficient and stable formamidinium perovskite solar cells

Abstract Defects passivation has been widely devoted to improving the performance of formamidinium lead triiodide (FAPbI3) perovskite solar cells; however, the effect of various defects on the α-phase stability is still unclear. Here, using density functional theory, we first reveal the degradation pathway of the FAPbI3 perovskite from α to δ phase and investigate the effect of various defects on the energy barrier of phase transition. The simulation results predict that iodine vacancies are most likely to trigger the degradation, since they obviously reduce the energy barrier of α-to-δ phase transition and have the lowest formation energies at the perovskite surface. A water-insoluble PbC2O4 compact layer was introduced on the perovskite surface to largely suppress the α-phase collapse through hindering the iodine migration and volatilization. Furthermore, this strategy largely reduced the interfacial nonradiative recombination and boosted the efficiency of the solar cells to 25.39% (certified 24.92%). Unpackaged device can maintain 92% of its initial efficiency after operation at maximum power point under simulated air mass 1.5G irradiation for 550 h.