Photovoltaic Effect and Photocarrier Dynamics in Mn-Doped BaTiO 3 Ferroelectric Single Crystals

Chemical doping is an effective and feasible approach to enhancing ferroelectric photovoltaic (PV) effects, providing unique functionalities that are attractive for optoelectronic applications. In this study, we investigate the PV properties of BaTiO 3 ferroelectric single crystals doped with Mn. We found that the sample with a mixed valence of Mn 2+ and Mn 3+ (Mn 3+/2+ ) exhibits a PV response larger than those with Mn 3+ or Mn 4+ alone, owing to the electron transition from Mn 2+ -3d-derived electron-filled gap states. Transient absorption spectroscopy measurements combined with density functional theory calculations reveal that the filled gap states of the Mn 3+/2+ sample act as sources of nonthermalized electrons in the conduction band, giving rise to a ballistic current, while vacant gap states capture thermalized electrons at the conduction band minimum. These experimental and theoretical investigations uncover the roles of gap states in the PV effect and the photocarrier dynamics associated with the redox of transition metal cations in ferroelectric oxides.