Small Electron Polarons Formation and Transport in Tantalum Oxynitride: Low Mobility from Hopping Migration

Oxynitrides are potential photoanodes due to combinatory properties of reasonable stability, like pure oxides with a low band gap in the visible range identical to nitrides. However, low carrier mobility restricts the solar-to-hydrogen conversion efficiency from the theoretical limit. We found that polarization potential created by lattice distortion around Ta generates a driving force to trap electrons and forms small electron polarons in tantalum oxynitride (β-TaON). The localized small electron polaronic state is more favorable than the delocalized state. The donated electron from n-type single donor defects becomes self-trapped and forms a weakly bound state with the defect. The electron polarons show non-adiabatic thermally activated migration via nearest neighbor hopping. However, O substitution at bridging the N site increases the Ta–Ta hopping distance and changes the polaron hopping toward an adiabatic regime. The calculated low polaron mobility because of high migration barriers explains the experimentally observed high carrier lifetime and transport property of the β-TaON photoanode. This study provides a fundamental understanding of the charge trapping and formation of small polarons in TaON. It also prescribes a strategy to search for potential dopants to improve photocurrent generation by boosting polaron transport.