Direct and charge transfer state mediated photogeneration in polymer–fullerene bulk heterojunction solar cells

We investigated photogeneration yield and recombination dynamics in blends of poly(3-hexyl thiophene) (P3HT) and poly[2-methoxy-5 -(3$'$⁠,7$'$-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) with [6,6]-phenyl-C₆₁butyric acid methyl ester (PC₆₁BM) by means of temperature dependent time delayed collection field measurements. In MDMO-PPV:PC₆₁BM, we find a strongly field dependent polaron pair dissociation which can be attributed to geminate recombination in the device. Our findings are in good agreement with field dependent photoluminescence measurements published before, supporting a scenario of polaron pair dissociation via an intermediate charge transfer state. In contrast, polaron pair dissociation in P3HT:PC₆₁BM shows only a very weak field dependence, indicating an almost field independent polaron pair dissociation or a direct photogeneration. Furthermore, we found Langevin recombination for MDMO-PPV:PC₆₁BM and strongly reduced Langevin recombination for P3HT:PC₆₁BM.