Polymer-based solar cells with aluminum (Al) cathode often suffer from degradation in air. Here the study focuses on the degradation mechanism at the interface between Al and organic active layer. By performing interface modification combined with electrical and chemical characterization, it is demonstrated that the rapid degradation originates from the formation of a charge blocking layer between evaporated Al cathode and organic active layer. Insertion of a thin interfacial layer of thermally evaporated CrO(x) between organic active layer and Al cathode can greatly improve the device stability. It is found that the CrO(x) interfacial layer functions as a protective layer by stopping or minimizing penetration of thermally evaporated Al into the active layer to form a diffused organic Al interface, which will then result in a large oxidized interfacial area upon air exposure. (C) 2011 Elsevier B.V. All rights reserved.