  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.
