Tailoring Defects in Bismuth‐Based Cs ₃ Bi ₂ Br ₉ Perovskite‐Inspired Materials through Cooling‐Rate Modulation for Photo‐Supercapacitors

Abstract Perovskite‐inspired materials are a new class of semiconductors to address several challenges faced by lead‐based halide perovskites. These lead‐free halide perovskites potentially eliminate the lead toxicity and improve the stability under operating conditions. However, a lack of understanding of the photophysical and electronic properties of these materials prevents further progress. Here, the effect of cooling rate on defect formation in lead‐free Cs ₃ Bi ₂ Br ₉ single crystals has been investigated. The crystal synthesized under controlled cooling shows reduced trap density. High‐resolution transmission electron microscopy image analysis of these materials reveals that the perovskite crystals synthesized from the controlled‐cooling method does not show the presence of point defects or dislocations, while naturally cooled perovskite crystals have both point defects and dislocations, increasing grain resistivity by an order of magnitude, as supported by electrochemical impedance spectroscopy. Furthermore, contact angle measurements show that the film obtained via controlled cooling exhibits greater surface hydrophobicity, indicating enhanced stability. Cyclic voltammetry measurements of the fabricated thin film‐based monolithic photo‐supercapacitor under illumination show that devices based on naturally cooled perovskites exhibit only 40% enhancement in photo‐capacitance. In contrast, devices fabricated from controlled‐cooled perovskites exhibit a photo‐capacitance enhancement of over 130% at a scan rate of 140 mV s ⁻¹ .