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

Abstract Excess lead(II) iodide (PbI2) has controversial roles in affecting the efficiency of perovskite solar cells (PSCs). Since the photoinstability of PbI2 is now known to largely accelerate perovskite degradation, suppressing and/or eliminating excess PbI2 is key to improving the stability of PSCs. Herein, process‐dependent PbI2 formation on the surfaces of formamidinium lead triiodide (FAPbI3) films is examined. Due to the faster evaporation rate of organic substances, crystalline PbI2 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 FAPbI3. Although both strategies successfully eliminate the PbI2 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‐FAPbI3 is improved due to the elimination of the degradation sites induced by the photoinstability of PbI2. The findings explore the formation process of unwanted PbI2 (≈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.