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 (J_sc) 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.