Photovoltaic effect in an indium-tin-oxide/ZnO/BiFeO3/Pt heterostructure

We have studied the photovoltaic effect in a metal/semiconductor/ferroelectric/metal heterostructure of In2O3-SnO2/ZnO/BiFeO3/Pt (ITO/ZnO/BFO/Pt) multilayer thin films. The heterolayered structure shows a short-circuit current density (Jsc) of 340 μA/cm2 and an energy conversion efficiency of up to 0.33% under blue monochromatic illumination. The photovoltaic mechanism, specifically in terms of the major generation site of photo-excited electron-hole (e-h) pairs and the driving forces for the separation of e-h pairs, is clarified. The significant increase in photocurrent of the ITO/ZnO/BFO/Pt compared to that of ITO/BFO/Pt is attributed to the abundant e-h pairs generated from ZnO. Ultraviolet photoelectron spectroscopy reveals the energy band alignment of ITO/ZnO/BFO/Pt, where a Schottky barrier and an n+-n junction are formed at the BFO/Pt and ZnO/BFO interfaces, respectively. Therefore, two built-in fields developed at the two interfaces are constructively responsible for the separation and transport of photo-excited e-h pairs.