Ultrathin TiO<sub>2</sub> 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, TiO<sub>2</sub> thin films were deposited on ZnO surfaces using atomic layer deposition. Ultrathin TiO<sub>2</sub> layers with a mean thickness of <3 nm could enhance photovoltaic performance of the inverted organic solar cell; in particular, short-circuit current (<i>J</i><sub>sc</sub>) and power conversion efficiency (PCE) were increased by deposition of TiO<sub>2</sub> layers. A higher thickness of TiO<sub>2</sub> resulted in reduced photovoltaic performance. Studies on electronic structure using photoelectron spectroscopy showed that the TiO<sub>2</sub> 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 TiO<sub>2</sub>, 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.