Ultrathin TiO2 Films on ZnO Electron-Collecting Layers of Inverted Organic Solar Cell

Nanostructured ZnO thin films were fabricated on indium tin oxide (ITO) as hole-blocking and electron-collecting layers of inverted organic solar cells. In addition, TiO2 thin films were deposited on ZnO surfaces using atomic layer deposition. Ultrathin TiO2 layers with a mean thickness of <3 nm could enhance photovoltaic performance of the inverted organic solar cell; in particular, short-circuit current (Jsc) and power conversion efficiency (PCE) were increased by deposition of TiO2 layers. A higher thickness of TiO2 resulted in reduced photovoltaic performance. Studies on electronic structure using photoelectron spectroscopy showed that the TiO2 layers should act as barrier for the electron-collection on ZnO. However, recombination of electrons and holes on the surface of ZnO can be quenched by TiO2, resulting in an improved efficiency of the IOSCs. It is demonstrated that atomic layer deposition with a precise control of film thickness can be of potential application for fabrication of energy devices.