We fabricated P3HT:PCBM bulk hetero-junction solar cells, where a 5 nm layer of NiO sputtered on top of ITO is used as an effective electron blocking layer. All the steps involving the processing of the organic materials were performed in ambient air. Under 1 sun of AM1.5G illumination such NiO cells exhibited a power conversion efficiency of 3.3% compared to 3.1% exhibited by a PEDOT:PSS cell fabricated under similar conditions. We observed that for the NiO cell processing in air was not detrimental to the ulterior performance of the cell, which in ambient air degraded with a time constant of 303 h. On the contrary, the PEDOT:PSS cell degraded very rapidly and the loss in efficiency was shown to be 29 times faster when compared to that of the NiO cell. (C) 2010 Elsevier B.V. All rights reserved.