As an n-type inorganic semiconductor, ZnO has been widely used in organic solar cells (OSCs) and hybrid solar cells (HSCs) due to its salient characteristics such as low cost, easy synthesis, non-toxicity, high stability, and good optoelectronic properties. This article reviews the applications of ZnO in solar cells, including ZnO/organic HSCs, and OSCs with ZnO acting as electrode buffer layers or transparent electrodes. For ZnO/organic HSCs, ZnO serves as the electron acceptor material, while organic semiconductors act as electron donor materials. For the buffer layers or electrode applications, ZnO is used as an electron collection and hole blocking material where its structure plays an important role in the determination of the device performance (e.g., power conversion efficiency, lifetime, stability, etc.). Special emphasis goes to the device performance of OSCs and HSCs, which depends not only on the active materials and the device configurations, but also on the structural characteristics of the ZnO buffer layer. Finally, we briefly give an analysis on the opportunities and challenges for this promising semiconductor in OSCs and HSCs.