In situ photoluminescence and Raman study of nanoscale morphological changes in organic photovoltaics during solvent vapor annealing

Improvement of the photovoltaic efficiency (from 1.2% to 3%) via exposure of organic poly(3-hexylthiophene) (P3HT) + phenyl-C61-butyric acid methyl ester (PCBM) devices to solvent vapor at room temperature is reported. In situ photoluminescence (PL) and Raman spectroscopies, in conjunction with optical absorption data, have been used to provide insight into the nanoscale morphological changes occurring during solvent vapor annealing. We found that in P3HT:PCBM, suppression of PL and the decrease in line-width of the 1442 cm-1 P3HT Raman peak are accompanied by strong modifications in the optical absorption spectra during solvent vapor annealing, in contrast to measurements on P3HT only films. We attribute these observations to de-mixing of PCBM and subsequent stacking of P3HT in coplanar conjugated segments, similar to what is observed during thermal annealing.