Photoinduced Ultrafast Electron Transfer and Charge Transport in a PbI₂/C₆₀ Heterojunction

Charge transport and electron transfer in a PbI₂/C₆₀ heterojunction were investigated. The charge carrier mobilities and trap densities of PbI₂ were extracted from current–voltage characteristics of hole- and electron-only devices. Femtosecond transient absorption spectroscopy was used to elucidate the photophysical processes occurring in the heterojunction. For a pristine PbI₂ film, trap-limited bimolecular recombination was the dominant mechanism in the case of low pump fluence. We observed photoinduced ultrafast electron transfer from PbI₂ to C₆₀ with a rate constant of 0.45 ps^–1. Semiclassical Marcus electron transfer theory was used to estimate the electronic coupling between the conduction band edge of PbI₂ and the lowest unoccupied molecular orbital of C₆₀. We also fabricated a solar cell using the PbI₂/C₆₀ heterojunction. Electron transfer and charge extract efficiencies in our cell were also deduced. The performance-limiting factors for solar cells based on PbI₂ were discussed and a strategy to improve the device performance was developed. Our work is useful for ultraviolet-harvesting transparent solar cells and other photophysical and photochemical applications.