Stable α-FAPbI3 in Inverted Perovskite Solar Cells with Efficiency Exceeding 22% via a Self-Passivation Strategy

Formamidinium lead iodide (FAPbI(3)) has endowed power conversion efficiencies (PCEs) up to 25.5% in regular-structured perovskite solar cells (PSCs) because of its optimal bandgap and enhanced thermal stability. However, the performance of FAPbI(3)-based inverted-structured PSCs is unsatisfactory. Herein, four kinds of commonly used hole transport materials (HTMs) are selected, including PEDOT:PSS, PTAA, NiOx, and MeO-2PACz, to study their impact on the methylamine chloride (MACl)-assisted one-step deposition of FAPbI(3) films. It is found that MeO-2PACz is the optimal substrate for stabilizing black-phase FAPbI(3) and the corresponding inverted-structured PSCs show the best photovoltaic performance. Nonetheless, the PCE is restricted by low open-circuit voltage (V-OC) due to non-radiative recombination caused by MACl residues. Therefore, homologous PbI2 in situ passivation is implemented to passivate defects at grain boundaries. The addition of excess PbI2 in precursor solution remarkably decreases charge trap densities and elongates carrier lifetimes. As a result, the optimized device achieves an impressive PCE of 22.13%, which is the highest efficiency of FAPbI(3) based on inverted-structured PSCs. Moreover, the best device exhibits free hysteresis and excellent long-term stability, maintaining 92% of the initial PCEs after 800 h aging under ambient conditions.