The effect of crystallinity in donor groups on the performance of photovoltaic devices based on an oligothiophene-fullerene dyad

Organic photovoltaic devices based on an oligothiophene-fullerene dyad mixed with either an oligothiophene as donor or [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor were fabricated to improve charge carrier transport in the films. The origin of the device performance dependence on the mixing ratios was studied by atomic force microscopy and UV-vis absorption spectra and differential scanning calorimetry measurements. We found that the addition of the oligothiophene molecules improved the crystallinity of the donor groups in the film, resulting in a 20% increase in fill factor, while the addition of PCBM slightly lowered the crystallinity, thereby reducing the fill factor.