Improved photovoltaic properties of ((CH₃NH₃)_1-xCs_x)₃Bi₂I₉: (x = 0-1.0) hybrid perovskite solar cells via a hot immersion method

To date, the lead-free perovskite has shown remarkable progress in solar cell development owing to its outstanding properties. Here, we report on the structural, optical, and photovoltaic properties of methylammonium bismuth iodide ((CH₃NH₃)₃Bi₂I₉, MBI) and cesium bismuth iodide (Cs₃Bi₂I₉, CBI) hybrid perovskite solar cells (HPeSCs) fabricated using the hot immersion method (HIM) with changing the composition x, ((CH₃NH₃)_1-xCs_x)₃Bi₂I₉; from x = 0 to x = 1.0. The compact MBI and CBI films were successfully fabricated on FTO glass substrates at x = 0 and x = 1, respectively. On the other hand, the CBI/MBI mixed structure with a rough surface was obtained in the range from x = 0.2 to x = 0.8. An incorporation of CBI in MBI showed improvement, especially in optical properties, indicating that the absorption region was extended toward a longer wavelength region with increasing x. Interestingly, the open-circuit voltage of the hybrid cell was higher than that of the MBI or CBI cell, whereas the short-circuit current was lower than that of the MBI or CBI cell. This work provides alternative ways to fabricate lead-free PeSCs using a simple and low-cost method in the future.