Solution-Processed BiI₃ Films with 1.1 eV Quasi-Fermi Level Splitting: The Role of Water, Temperature, and Solvent during Processing

We present a mechanistic explanation of the BiI3 film formation process and an analysis of the critical factors in preparing high-quality solution-processed BiI3 films. We find that complexation with Lewis bases, relative humidity, and temperature are important factors during solvent vapor annealing (SVA) of films. During SVA, water vapor and higher temperatures limit the formation of the BiI3–dimethylformamide coordination complex. SVA with an optimized water content and temperature produces films with 300–500 nm grains. Films that formed solvent coordination compounds at lower temperatures showed preferential crystal orientation after solvent removal, and we elucidate its implications for carrier transport. Addition of dimethyl sulfoxide to highly concentrated tetrahydrofuran–BiI3 inks prevents film cracking after spin-coating. We have measured a quasi-Fermi level splitting of 1.1 eV and a diffusion length of 70 nm from films processed with optimal temperature and humidity. The best device produced by optimized SVA has a power conversion efficiency of 0.5%, Isc of ∼4 mA/cm2, and VOC of ∼400 mV. The low photocurrent and voltage we attribute to the low diffusion length and the unfavorable band alignment between the absorber and the adjacent transport layers. The deep understanding of the relationship between morphology/crystal structure and optoelectronic properties gained from this work paves the way for future optimization of BiI3-based solar cells