Enhanced performance of organic photovoltaics by TiO2-interlayer with precisely controlled thickness between ZnO electron collecting and active layers

Organic photovoltaics (OPVs) consisting of ZnO ripple structures used as electron-collecting layer were fabricated without and with additional TiO2 layers prepared by atomic layer deposition (ALD). We show that TiO2 layer with a thickness of less than 1 nm can enhance OPV performance, since additional TiO2 can heal surface defects of ZnO, which can act as recombination center of electron and hole created in the active layer by photon-absorption. In contrast, a thicker TiO2 film (>2 nm) showed a lower OPV performance than that of bare sample without TiO2, most likely due to a lower conductivity of TiO2 than that of ZnO. We show that precise control of oxide thin layer using ALD can be beneficial for fabricating high-performing OPV.