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.