Excited-state dynamics in CZTS nanocrystals

Ultrafast dynamics of conduction-band electrons in stoichiometric Cu ₂ZnSnS₄ nanocrystals are investigated using pump-probe spectroscopy. The nanocrystal samples minimize nonstoichiometric defects that can serve as electron traps, allowing for investigation of the native dynamics of the initially prepared excited states. These excited states are nonemissive and have short lifetimes, returning to the ground state in an average of ∼20 ps, and they show rapid cooling on time scales of ∼1 ps. Transient absorption spectra of the excited electron are bimodal and agree well with theoretical predictions for the conduction band density of states. With the onset of quantum confinement, the excited electron absorption spectrum shifts and dilates. The results are considered in the context of photovoltaic applications.