Improvement of power conversion efficiency of Pb-free Mesoscopic Rudorffite solar cells by fine-tuning Ag-Bi-I Rudorffite films through binary solvent engineering

•Synthesis of silver bismuth iodide (SBI) light absorbers via melt-solidification.•A binary solvent engineering approach manipulating the crystallization process.•A 75% DMSO promotes the formation of uniform and dense Rudorffite films.•This method leads to Ag2BiI5-based solar cells with an enhanced PCE of 1.22%.•Binary solvents optimize Rudorffites for high-efficiency, low-toxic solar cells. Ag-Bi-I Rudorffites produced through melt solidification, exhibiting direct band gaps below 2 eV, hold the potential for creating highly efficient and economical Pb-free carbon-based solar cells. Despite their potential, bismuth-based films have limitations due to the interplay between rapid crystallization and restricted BiI3 solubility in common solvents. In this work, we employ a binary solvent engineering approach to manipulate the nucleation and growth processes during film fabrication. Our investigation into AgBiI4, Ag2BiI5, Ag3BiI6, and AgBi2I7 absorber layers utilized a binary solvent-engineering strategy employing dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). Notably, a 75 % DMSO volume ratio facilitated the formation of uniform and dense Rudorffite films through a hypothesized intermediate phase involving BiI3-DMSO-AgI. This approach yielded solar cells based on the Ag2BiI5 absorber with an enhanced power conversion efficiency of 1.22 %. This work offers significant advancements for optimizing melt solidification-processed Rudorffite solar cells, paving the way for developing low-toxicity and high-efficiency devices.