Photocurrent of regioregular poly(3-alkylthiophene)/fullerene composites in surface-type photocells

We have studied steady-state photocurrents of regioregular poly(3-alkylthiophene)/C60 composite on surface-type photocells formed with Au electrodes. The photocurrent is observed above 1.4 eV, in agreement with the light-induced ESR measurements on the composite. The photocurrent is small up to 1.8 eV and then increases abruptly at 1.9 eV and keeps increasing up to 3 eV. The action spectra is not symbatic nor antibatic with the optical absorption spectra. The exponent n of the light intensity I dependence of the photocurrent P, P ∝ In, is close to n = 0.5 above 1.9 eV, indicating the involvement of the bimolecular recombination. Below 1.9 eV, n is between 0.5 and 1, indicating the contribution of the monomolecular process. The change in the exponent is understood by a universal curve as a function of the photoexcitation density taking account of the fraction of the incident light absorbed and the optical penetration depth.