Formation of a Small Electron Polaron in Tantalum Oxynitride: Origin of Low Mobility

Tantalum oxynitride (beta-TaON) is a potential photoanode because of its suitable band gap and band-edge positions for water-splitting. However, low carrier mobility restricts the solar-to-hydrogen conversion efficiency from the theoretical limit. Here, using the DFT + U formalism, we find that the excess electron tends to form a localized small polaron at the Ta-site (Ta+4 species) over delocalized electrons. The polarization potential created by lattice distortion around Ta+5(d(0)) generates a driving force to construct Ta+4(d(1)) by electron capture. The donated electron from n-type single donor defects becomes self-trapped and forms a weakly bound state with the defect. The thermally activated polaronic charge transfer via nearest-neighbor hopping is non-adiabatic using the DFT + U method. 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 polaron mobility because of the high migration barrier for both in pristine (0.31 eV) and in the presence of the O-N defect (0.36 eV) supports the experimentally observed low mobility and high carrier lifetime in a beta-TaON photoanode. This study provides a mechanistic understanding of the factors controlling the formation and transport of electron polarons, which can guide in designing a beta-TaON photoanode with better efficiency.