Since an interface barrier between a ZnO layer and an adjacent organic active layer affects the carrier injection efficiency, we investigated the relationship between the annealing temperature for the ZnO layer and the photovoltaic performance of inverted cell. An optimized annealing temperature was ranged from 125 to 200 degrees C due to both the sufficient chemical reaction of ZnO precursor solution and the remained organic component, which were evaluated from the atomic force microscope, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. As a result, a highest photoconversion efficiency of 9.34% was achieved for a simple bulk heterojunction structure.