Bulk photovoltaic effect in Cu-doped LiNbO₃ single crystals with controlled oxidation state

We investigate the bulk photovoltaic (PV) effect of Cu-doped LiNbO 3 single crystals with various oxidation states of Cu. The Cu-doped samples exhibit the PV response under below-bandgap excitation, and the onset of photocurrent shifts depending on partial oxygen pressures ( p O 2 ) during the annealing treatment. Open-circuit voltages ( V oc ) under simulated sunlight (AM 1.5 G) illumination are changed by p O 2 , and crystals annealed at p O 2 = 1.0 × 10 −10 atm exhibit the highest V oc of 1700 V. Moreover, density functional theory (DFT) calculations for Cu-doped LiNbO 3 cells with Cu 2+ on the Li site and the Nb site indicate that half-filled gap states derived from 3d orbitals of Cu are formed within the bandgap. Based on Glass coefficients obtained by the analyses of polarization angle-dependent photocurrent densities and the DFT calculations, we consider that Cu 2+ on the Li site is the major active site for the generation and separation of electron–hole pairs under visible light at hν = 2.4 eV.