In situ photoluminescence probe to study the melting behavior of regioregular poly(3-hexylthiophene)

In situ photoluminescence (PL) measurements for regioregular poly(3-hexylthiophene) (rr-P3HT) at various annealing temperatures were performed. When the annealing temperature is above $230\ ^{\circ}\text{C}$ , the PL spectra exhibit two significant changes: a sudden increase of integrated intensity and the appearance of a grand new peak at higher energy. According to the corresponding absorbance spectra, it was found that these changes in PL spectra are originated from the torsional defects in the P3HT backbone introduced by melting transition. Furthermore, the variation trend of the new peak to whole PL emission intensity ratio almost overlaps with the endothermic peak in the Differential Scanning Calorimeter (DSC) thermogram. It gives direct evidence that the intensity ratio can be used as a semi-quantitative probe to the melting degree of P3HT. Then, the P3HT melting behavior in a blend of P3HT and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was analyzed using this probe. The results proved that the introduction of PCBM into P3HT does not simply make P3HT melt at a lower temperature, but rather make it have a binary eutectic melting behavior. So, these findings can provide an in situ and damage-free method to study the melting process of P3HT