Solution-Processed BiI3 Thin Films for Photovoltaic Applications: Improved Carrier Collection via Solvent Annealing

We report all-inorganic solar cells based on solution-processed BiI3. Two-electron donor solvents such as tetrahydrofuran and dimethylformamide were found to form adducts with BiI3, which make them highly soluble in these solvents. BiI3 thin films were deposited by spin-coating. Solvent annealing BiI3 thin films at relatively low temperatures (≤100 °C) resulted in increased grain size and crystallographic reorientation of grains within the films. The BiI3 films were stable against oxidation for several months and could withstand several hours of annealing in air at temperatures below 150 °C without degradation. Surface oxidation was found to improve photovoltaic device performance due to the formation of a BiOI layer at the BiI3 surface which facilitated hole extraction. Nonoptimized BiI3 solar cells achieved the highest power conversion efficiencies of 1.0%, demonstrating the potential of BiI3 as a nontoxic, air-stable metal-halide absorber material for photovoltaic applications.