The focus of this review is to summarize the cathode buffer layer for bulk heterojunction organic solar cells (OSCs) in the reported literature. The suitable cathode buffer layer favors the collection and the extraction of the charges and helps to improve the performance of the device. Accordingly, it’s crucial to understand the properties of cathode buffer layers and their role in OSCFs. Herein we review a range of materials applied as cathode buffer layers for OSCs. For the development of OSCs with higher efficiency and better stability, the right choice of interface materials is crucial. To obtain the optimum device, it is of vital importance to optimize the interfaces, the structures of the device (conventional or inverted) and the processing methods (wet or dry deposition).