Electron transfer dynamics at the ZnO (10-10) surface

Recently, ZnO nanoparticle films and nanowire arrays have been used in lieu of TiO₂ in dye- and quantum dot-sensitized solar cells and in hybrid organic/inorganic bulk heterojunction photovoltaic devices. The nonpolar (10-10) surface is the lowest-energy ZnO surface and constitutes the side facets of ZnO nanowires and much of the surface area of sintered ZnO nanoparticle films. Consequently, the majority of charge separation and recombination events in these devices occur at the (10-10) or equivalent surfaces. We are using time-resolved photoemission and surface nonlinear optical spectroscopies to study electron transfer dynamics at this important surface. On the clean (10-10) surface we find a continuum of defect-derived surface states extending into the band gap below the conduction band edge which may play an important role in electron transfer across this surface and in electron transport in devices featuring ZnO photoanodes. Additionally, we observe ultrafast sub-20 femtosecond cooling of hot electrons within the ZnO conduction band, suggesting that hot carrier transport may not be feasible in this material.