Insight into the Raman shifts and optical absorption changes upon annealing polymer/fullerene solar cells

Raman shifts and optical absorption spectra of bulk heterojunction films were measured to elucidate the origin of the optimum annealing parameters. A series of device optimization studies revealed 413K to be the optimum annealing temperature, leading to a power conversion efficiency of 2.95%. The highest power conversion efficiency coincides with the highest peak in the UV-visible absorption and the lowest full width at half maximum of the CC symmetric stretching signal in the Raman spectra. The changes observed in the vibronic shifts could be useful in obtaining information about the optimal performance and processing conditions for polymer optoelectronic devices.