Ultrafast Carrier Dynamics in Single-Crystal Two-Dimensional CuInSe$_{2}$ Nanosheets

Recently, two-dimensional (2D) layered materials beyond graphene are being extensively studied. In particular, the excitonic effects due to the decreased dielectric screening in 2D materials contributes significantly to the enhanced optical absorptions, which motivates the explorations of more ``conventional'' semiconductors in the 2D form for solar cell applications. One material of interest is CuInSe$_{2}$, with CuInSe$_{2}$-based solar cells among the most efficient thin-film technologies. Here, we report, for the first time, the synthesis of single-crystal CuInSe$_{2}$ nanosheets with the thickness on the nanoscale by solid-state chemical reaction. Carrier dynamics was studied via the measurements of the transient optical reflectivity using an optical pump-probe technique. Hot carrier cooling was suggested to dominate the carrier dynamics within a few ps following the optical excitation. The hot carrier diffusion coefficient was obtained by spatially resolved pump-probe measurements. The dependence of the hot carrier diffusion coefficient on the nanosheet thickness provides insight into the limiting mechanisms of hot carrier transport, and can be used to gauge the possibility of efficient hot carrier collection in nanostructured CuInSe$_{2}$ solar cells.