We investigated the photo-oxidation of the poly(3-hexylthiophene) (P3HT):(6,6)-phenyl C-butyric acid methyl ester (PCBM) blend thin films in ambient air by ultraviolet-visible spectrophotometry in the differential mode. The decomposition of the difference absorbance spectra showed with the exposure time a successive decrease of the excitonic absorption (S-0) at similar to 605 nm corresponding to a singlet exciton generation. This decrease is accompanied with a decline of the absorption bands at similar to 558 nm and similar to 509 nm connected with one exciton plus one phonon and one exciton plus two phonons generation, respectively. Simultaneously, there is observed an increase of the sub-bandgap absorbance (S-Gap) in the wavelength interval of 640-800 nm. Though the observed changes were in the order of thousandth of the pristine absorbance, these changes caused a dramatic degradation of the organic solar cell performance mainly due to a drop of the short circuit current (J(SC)). The correlation between the absorbance features S-0, S-Gap and the J(SC) of the P3HT:PCBM solar cell implies that the S-0 decrease is too low to explain such a dramatic change of the J(SC) induced by the photo-oxidation. On the contrary, the experimental dependence of the J(SC) on the S-Gap is reasonable explained with the increase of the exciton recombination at defect states visible via the S-Gap. (C) 2016 Published by Elsevier Ltd.