During the photoirreversible degradation of a polymer solar cell (PSC), it is generally assumed that the photooxidation of the organic materials (P3HT, PCBM) affects the device performance. Photooxidation of the organic materials results in a decrease in absorbance as a result of photodecomposition. However, the effect of initial photooxidation on device performance remains to be investigated. In this study, we try to establish the correlation between the initial photooxidation of the organic material and the photovoltaic performance of the PSC. Towards this aim, we investigated the device characteristics by employing a slightly degraded organic material, which had been prepared by photooxidation of the material in the solution state. UV-vis and FT-IR spectra suggest a decrease in the pi-conjugation length along with the formation of ketone and sulfoxide derivatives. MALDI-TOF mass spectra indicated the partial molecular scission of P3HT even at the start of photooxidation. A deterioration in performance was observed during the device characterization, as was an increase in the resistance of the organic films. These results indicate that the fragments and oxides generated by the partial molecular scission of P3HT increase the resistance of organic films, resulting in a drop in device performance. (C) 2013 Elsevier B.V. All rights reserved.