Stable α-FAPbI${}_3$ 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 PbI${}_2$ in situ passivation is implemented to passivate defects at grain boundaries. The addition of excess PbI${}_2$ 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. (© 2022 Wiley‐VCH GmbH)