Ambient Synthesis of Low-Defect Formanmidinium-Based Perovskite Solar Cells with Bromine-Enhanced Structural Stability

Using mixed cations and halides is a useful strategy for synthesizing high-efficiency organic-inorganic metal halide perovskite solar cell because it improves the stability of the perovskite structure. Adding cesium (Cs) cations instead of formamidinium [CH(NH₂)₂⁺, FA] cations in FAPbI₃ addresses the phase transition issue of FAPbI₃ under room temperature, resulting in increased power conversion efficiency (PCE) of solar cell. In CsFAPbI₃ synthesized under ambient conditions, substituting high concentration of Cs instead of FA is important because FA is decomposed into sym-triazine and ammonia in the presence of moisture. Additionally, an increase in Cs concentration leads to lower free energy of mixing in CsFAPbI₃, thereby enhancing structural stability. However, when synthesizing CsFAPbI₃ with high concentration of Cs under ambient conditions, phase separation with <i>δ</i>-CsPbI₃ occurs due to moisture, leading to a decrease in PCE.<br/>In this study, we present a method to suppress <i>δ</i>-CsPbI₃ formation in Cs_0.22FA_0.78PbI₃ by incorporating lead bromide (PbBr₂) into the perovskite precursor. The substitution of bromine (Br) directly forms Br-enriched perovskite phase instead of intermediate phase, leading to homogeneous grain growth, thereby suppressing <i>δ</i>-CsPbI₃ and reducing grain boundaries. Moreover, the properties of perovskites with varying concentrations of Br were analyzed. As the concentration of Br increased, there was a preference grain growth for the (110) plane direction over the (100) plane direction of perovskite phase, leading to decrease thickness of perovskite. Additionally, the light soaking effect, which indicates a separation between Br and iodine (I) during illumination, increased with higher Br concentrations. The highest PCE of 15.56% was observed in the perovskite solar cell with a 10% of Br concentration, attributed to the longest carrier lifetime and strongest PL intensity, resulting in reduced non-radiative recombination between the perovskite layer and charge transport layer. This study demonstrates a promising approach for producing low-defect FA-based perovskite films under ambient conditions, potentially advancing the commercial viability of perovskite-based solar cells.