Using inexpensive, solution-based processing, bismuth—halide (BiI3) thin films were produced and found to have optical properties that are suitable for solar energy harvesting. While lead (Pb)-based halide perovskites have enormous promise for photovoltaic solar cells because of their low cost and high solar-to-electric power conversion efficiency, they also have poor stability and there are environmental and health concerns related to the high Pb content. BiI3 could be a more environmentally friendly alternative. The researchers discovered that the morphology of BiI3 can be modified by gently heating in an environment containing solvent vapor. Under certain circumstances this leads to growth of crystallographic domains within the BiI3 films. Electronic and optical characterization revealed that this growth results in improved electronic transport and higher power conversion efficiency than what has previously been achieved using this material. Furthermore, BiI3 demonstrated greater stability against heat, air, and moisture compared to Pb-based perovskites. This research demonstrates the potential for new metal halides such as BiI3 to become next-generation photovoltaic materials for inexpensively converting sunlight to electricity.
BiI3 forms molecular complexes in solvents such as tetrahydrofuran or dimethylformamide, enabling solution-based deposition of thin films. Annealing BiI3 thin films in different solvents can dramatically change the morphology. By annealing in dimethylformamide dense, large-grained films were produce that resulted in higher photocurrent and power conversion efficiency in proof-of-concept solar cells.
Solution-Processed BiI3 Thin Films for Photovoltaic Applications: Improved Carrier Collection via Solvent Annealing