Photoinduced carrier generation in P3HT/PCBM bulk heterojunction materials

Using ultrafast spectroscopy, we have investigated the generation of mobile carriers in bulk heterojunction (BHJ) blends of regioregular poly(3-hexylthiophene), rrP3HT and the [6,6]-phenyl-C-61 butyric acid methyl ester fullerene derivative, PCBM. Mobile carriers are generated via a two-step process: initial ultrafast charge separation to an intermediate charge transfer (CT) bound state, followed by the transfer of carriers onto the bicontinuous networks. The yield of mobile carriers is decreased by recombination during the time spent in the CT bound state. After annealing the BHJ blends, the recombination loss of mobile carriers is reduced by a factor of 2.5, consistent with the observed increase in photovoltaic conversion efficiency.