Environmental lead-free Cs3Bi2I9 films deposited by vapor transport deposition for stable all-inorganic solar cells

The non-lead metal halides are of wide interest due to their low toxicity and good stability compared with lead -based perovskites. Among them, Cs3Bi2I9 is of interest because of its excellent electronic and optical properties. In this study, high-quality Cs3Bi2I9 films with large grains and good uniformity were deposited for the first time by a vapor transport deposition (VTD) approach using a mixture of CsI and BiI3 crystal powders as evaporation sources. The Cs3Bi2I9 film was then used as an absorber layer in the solar cell using a superstrate structure with FTO/TiO2/Cs3Bi2I9/CuI/Au. Due to the different melting points of CsI (626 degrees C) and BiI3 (408 degrees C), the CsI to BiI3 molar ratio can significantly affect the stoichiometry, structure, and optoelectronic properties of the prepared Cs3Bi2I9 films, which ultimately determine the performance of the Cs3Bi2I9 solar cells. The all-inorganic Cs3Bi2I9 solar cell can achieve a maximum PCE of 1.52% and remain at 80% of its initial value after one month of storage in the air without encapsulation. This work provides a new route to deposit inorganic perovskite films using evaporation sources with different melting points, which has promising applications in solar cells and photodetectors.