Stability is an essential issue in polymer solar cells for achieving commercially applicable devices. In this work, we report on a highly stable polymer solar cell, based on an inverted structure with the active layer sandwiched between a pair of metal-oxide buffer layers and an inherently air-stable conjugated polymer, a-PTPTBT. After interface modification, the unencapsulated device can retain power conversion efficiency without apparent degradation even after six months of storage in ambient conditions. This device's excellent stability is attributed to the air-stable and chemically stable conjugated polymer, as well as the environmentally robust metal oxides used as the interfacial layers. To further improve the device performance, cathode-side interface modification was also carried out. After the insertion of the TiO2 nanorod interlayer between the ZnO thin film and the active layer, the device efficiency can be successfully increased from 3.3% to 4.1%. The future application of this materials system in inverted structures needs efforts devoted to morphology control and interface engineering to improve the device fill factor. The methodology applied here can serve as an effective strategy for fabricating highly air-stable devices with different materials systems. (C) 2011 Elsevier B.V. All rights reserved.