Origin of the Efficient Polaron-Pair Dissociation in Polymer-Fullerene Blends

The separation of photogenerated polaron pairs in organic bulk heterojunction solar cells is the intermediate but crucial step between exciton dissociation and charge transport to the electrodes. In state-of-the-art devices, above 80% of all polaron pairs are separated at fields of below ${10}^7\mathrm{V}/\mathrm{m}$. In contrast, considering just the Coulomb binding of the polaron pair, electric fields above ${10}^8\mathrm{V}/\mathrm{m}$ would be needed to reach similar yields. In order to resolve this discrepancy, we performed kinetic Monte Carlo simulations of polaron-pair dissociation in donor-acceptor blends, considering delocalized charge carriers along conjugated polymer chain segments. We show that the resulting fast local charge carrier transport can indeed explain the high experimental quantum yields in polymer solar cells.