Bimolecular Recombination in Regioregular and Regiorandom Poly(3-Hexylthiophene):PCBM Using Photoinduced Absorption Spectroscopy

We have used photoinduced absorption (PA) spectroscopy and measured the intensity dependence on thin films of 1:1 blends of regioregular-poly(3-hexylthiophene):PCBM before and after annealing as well as on 1:1 blends of regiorandom-poly(3-hexylthiophene):PCBM. We have calculated the bimolecular recombination coefficients from the out-of-phase component of the photoinduced absorption. Annealing shows no change in the bimolecular recombination coefficients of rr-P3HT:PCBM, but the increased PA amplitude suggests that the geminate (monomolecular) recombination is reduced. In rra-P3HT:PCBM, the bimolecular recombination coefficient estimated by the same method (disregarding dispersion) is significantly smaller, indicating together with a low PA amplitude that the recombination is primarily geminate.