Relation between absorption and crystallinity of poly(3-hexylthiophene)/fullerene films for plastic solar cells

The photovoltaic action of plastic solar cells based on P3HT/PCBM- (poly[3-hexylthiophene-2,5-diyl]/[6,6]-phenyl C61 butyric acid methyl ester)-composites depends strongly on structure of the active layer. In this work, the impact of P3HT-crystallinity on optical absorption of P3HT/PCBM-films was investigated. We observed, that both P3HT-crystallinity and optical absorption of thermally annealed films are increased in comparison to the not annealed ones. The highest crystallinity was achieved for films annealed at 125 °C. Further increase of annealing temperature leads to decrease of P3HT crystallinity. It is shown that the absorption coefficient of the films at low photon energies is proportional to the area under the X-ray-diffraction peak, which is a measure for the degree of film crystallinity.