Mechanism of Carrier Formation in P3HT-C₆₀-PCBM Solar Cells

Solar cells convert light energy directly into electricity using semiconductor materials. The ternary system, composed of poly(3-hexylthiophene) (P3HT), fullerene (C₆₀), and phenyl-C₆₁-butyric-acid-methyl-ester (PCBM), expressed as P3HT-C₆₀-PCBM, is one of the most efficient organic solar cells. In the present study, the structures and electronic states of P3HT-C₆₀-PCBM have been investigated by means of the density functional theory (DFT) method to shed light on the mechanism of charge separation in semiconductor materials. The thiophene hexamer was used as a model of P3HT. Five geometrical conformers were obtained as the C₆₀-PCBM binary complexes. In the ternary system, P3HT wrapped around C₆₀ in the stable structure of P3HT-C₆₀-PCBM. The intermolecular distances for P3HT-(C₆₀-PCBM) and (P3HT-C₆₀)-PCBM were 3.255 and 2.885 Å, respectively. The binding energies of P3HT + (C₆₀-PCBM) and (P3HT-C₆₀) + PCBM were 27.2 and 19.1 kcal/mol, respectively. The charge transfer bands were found at the low-lying excited states of P3HT-C₆₀-PCBM. These bands strongly correlated with the carrier separation and electron transfer in solar cells. The electronic states at the ground and excited states of P3HT-C₆₀-PCBM were discussed on the basis of the calculated results.