Bromide Induced Room-Temperature Formation of Photoactive Formamidinium-Based Perovskite for High-Efficiency, Low-Cost Solar Cells

State-of-the art perovskite solar cells (PSCs) are obtained by using a high-crystalline and uniform morphological perovskite film that usually requires a thermal procedure to induce its crystallization. Room temperature processing photoactive perovskite film represents a feasible approach to break through the technology barrier arising from high temperature annealing procedures; however, the photovoltaic performance of fabricated PSCs varies significantly in most routine laboratories with ambient temperature altering. Herein, the authors report for the first time that highly photoactive black phase FA-based perovskite films with high performance and reproducibility can be room temperature processed via the Br induced crystallization effect. It is found that Br adding content varies as a function of ambient temperature fluctuation from 35–40 to 20–25 °C. The halide Br/I formulation are 0.2/2.8 and 0.5/2.5 for room-temperature perovskite crystallization in ambient temperatures of 35–40 and 20-25 °C, yieldings solar to electric power conversion efficiency of 19.59 and 17.53% with high reproducibility, respectively. This efficiency (19.59%) is comparable to the best-performing PSCs based on thermal processing perovskite films. This work provides a valuable and practical guide to room-temperature fabrication of PSCs with high efficiency and reproducibility by delicate control of halide anions.