Enhancing Stability of Efficient Perovskite Solar Cells (PCE ≈ 24.5%) by Suppressing PbI2 Inclusion Formation

Abstract Excess lead(II) iodide (PbI 2 ) has controversial roles in affecting the efficiency of perovskite solar cells (PSCs). Since the photoinstability of PbI 2 is now known to largely accelerate perovskite degradation, suppressing and/or eliminating excess PbI 2 is key to improving the stability of PSCs. Herein, process‐dependent PbI 2 formation on the surfaces of formamidinium lead triiodide (FAPbI 3 ) films is examined. Due to the faster evaporation rate of organic substances, crystalline PbI 2 as an inclusion is found within the triple junction grain boundaries. With this hypothesis, two strategies are suggested: control of the 1) vapor pressure and 2) stoichiometry of precursor solutions to induce sufficient reaction of FAPbI 3 . Although both strategies successfully eliminate the PbI 2 as inclusions, due to the slower evaporation rate, vapor pressure control films also exhibit a larger grain size (≈1.18 µm) with a good film quality to attain the highest power conversion efficiency (PCE) of 24.5%. Furthermore, the phase stability of α ‐FAPbI 3 is improved due to the elimination of the degradation sites induced by the photoinstability of PbI 2 . The findings explore the formation process of unwanted PbI 2 (≈2.8%) and provide a simple method to effectively suppress its formation. This may further boost the PCE and stability, especially for FA‐based perovskites.