Optical absorption study of P3HT:PCBM blend photo-oxidation for bulk heterojunction solar cells

• The P3HT:PCBM photo-oxidation is analyzed by difference absorbance spectra.
• The decrease of absorbance features corresponding to excitonic states was observed.
• The absorbance increase in the sub-band gap region due to gap states was found.
• The solar cell short circuit current is related to absorbance changes.

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 (S0) at ∼605 nm corresponding to a singlet exciton generation. This decrease is accompanied with a decline of the absorption bands at ∼558 nm and ∼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 (SGap) 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 (JSC). The correlation between the absorbance features S0, SGap and the JSC of the P3HT:PCBM solar cell implies that the S0 decrease is too low to explain such a dramatic change of the JSC induced by the photo-oxidation. On the contrary, the experimental dependence of the JSC on the SGap is reasonable explained with the increase of the exciton recombination at defect states visible via the SGap.