Solvent Engineering for Controlled Crystallization and Growth of All-Inorganic Pb-Free Rudorffite Absorbers of Perovskite Solar Cells.

Abstract

Solution-processed Ag-Bi-I rudorffites with direct band gaps of <2 eV show promise for highly efficient and cost-effective Pb-free solar cells. However, relatively fast crystallization rates of Bi-based films and limited solubility of BiI3 in many solvents result in poor film morphologies, inhibiting their device performance. Here, we conduct a solvent-engineering method to adjust the dynamics of nucleation and growth during film formation. We fabricate Ag2BiI5, AgBiI4, and AgBi2I7 absorber layers using a mixed solvent of dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) and find that a volume percentage of 50% DMSO causes highly uniform and dense perovskite films via a BiI3-DMSO-AgI intermediate phase formation, leading to solar cells with an improved power conversion efficiency of 0.62% for the Ag2BiI5 absorber. These results provide valuable insights into the optimization of the solution processing technique to realize low-toxicity and efficient perovskite solar cells.