Defect-State Transition Associated Hole Dynamics in Cuprous Iodide Nanosheets

The cuprous iodide of zinc-blende structure (gamma- CuI) is a p-type semiconductor with a wide band gap of 3.1 eV. It is considered promising as the third generation of semiconductors, in applications of hole transport layer and transparent electrode due to the high hole density and carrier mobility, but its carrier transport characteristics have not yet been understood in detail. We synthesized gamma-CuI nanosheets with large sizes and measured the carrier dynamics by femtosecond transient absorption (TA) spectroscopy. We observed signals of defect trapping photo generated carriers during band-edge transitions, as well the weak band filling and photoinduced absorption directly associated with defect states. The species and lifetimes of defect assisted transition are identified and quantified based on TA measurements with various excitation fluences, and the changes of spectra and kinetics of defect band filling are correlated to the trend of density of Cu and I vacancies in samples with different thicknesses, based on the observations on a single nanosheet. We extract a diffusion coefficient of similar to 71 cm2 s-1 and derive a diffusion length longer than the sample thickness, which possibly makes a remarkable contribution to the generation of a photocurrent. Our findings are expected to offer help for harnessing the defects in gamma-CuI and improving its performances in optoelectronic applications.