Exploring ambient-stable lead-free (CH₃NH₃)₃Bi₂I₉ perovskites for solar cell applications

Hybrid organic-inorganic perovskites, with power conversion efficiencies comparable to those of polycrystalline silicon-based counterparts, as well as tunable band gaps and low-temperature solution processability, represent a promising class of optoelectronic semiconductors suitable for next-generation electronics. In this work, we synthesised high-quality single-crystal and thin films of methylammonium bismuth iodide (MBI) using an antisolvent assisted method. The X-ray diffraction analysis showed a hexagonal crystal structure for the synthesised MBI crystal with space group P6₃/mmc. The bandgap of the MBI film estimated from the absorption spectra is 2.19 eV. The thermogravimetric analysis indicates that the thermal stability of MBI exceeds that of its constituents, thereby offering improved thermal stability. Furthermore, a trap state density comparable to that of vacuum processed condition is achieved by antisolvent treatment. The impedance and dielectric characteristics of MBI are recorded for a frequency range of 100 Hz to 1 MHz and the complex impedance is analysed by fitting the Nyquist plot. A mobility of 2.29 × 10^- 6 cm² V^- 1 s^- 1 is deduced by fitting the space charge limited current (SCLC) region, and a photovoltaic analysis was performed using MBI as an absorber.