Surface photovoltage and open-circuit voltage spectroscopies of oxide heterostructures by atomic force microscopy under variable illumination

Photoelectron excitations in oxide heterostructures routinely elicit both surface photovoltage and open-circuit voltage. Surface photovoltage measurements are hence often used as a proxy to gauge open-circuit voltage response. Here, we present a method for studying photovoltaic effects in oxide heterostructures by concurrently measuring, with an atomic force microscope, surface photovoltage (ΔVSPV) and open-circuit voltage (VOC) while varying illumination wavelength and intensity. This combined method is applied at super- and subband-gap illumination wavelengths on Bi₂FeCrO₆/Nb-SrTiO₃ heterostructures annealed under various oxidative and reductive conditions. The results show that ΔVSPV consistently reflects interfacial photocarrier generation, whereas VOC is more governed by photocarrier transport, sensitive to recombination. It is also shown that deep oxygen-vacancy levels act as recombination centers, whereas shallow cation-vacancy levels impact depletion region width. Experimental observations are consistent with DFT density of states of crystals containing isolated and coupled vacancies. Together, ΔVSPV and VOC spectroscopies---applied across multiple excitation energies and intensities---offer valuable insights into mechanisms governing photon-induced electrical power generation in photovoltaic oxide heterostructures.