Organic solar cells function by separating electrons and holes at a donor-acceptor interface. It has been a long-standing puzzle just how the two charges separate, often with near-100% efficiency and on a sub-picosecond timescale. In a naive model, with charges localized on adjacent donor and acceptor molecules, the Coulombic binding can exceed 15 kT, which would make it impossible for the charges to separate in a reasonable time. Several mechanisms have been proposed to enable the dissociation—including charge-carrier delocalization and excesses vibrational energy of the "hot" CT states—but they do not tell the whole story. Previous studies found that including an entropic contribution to the free energy could reduce the charge-separation barrier somewhat [1,2], but still not enough to effect charge separation without additional help from delocalization or hot states. However, these studies assumed that only one carrier is mobile and that the materials are perfectly ordered. Here, we compute the entropy of charge separation at a heterojunction, allowing both charges to move and including energetic disorder. For the realistic disorder of 100 meV, the barrier to charge separation is only 2 kT, allowing charge separation during the lifetime of the CT state, and confirming that high-efficiency charge dissociation is possible even if charges are localized and thermally relaxed. Including delocalization reduces the barrier further, to the point where it can disappear altogether, explaining why there is minimal CT-state recombination in high-efficiency blends. [1] T. M. Clarke and J. R. Durrant, Chem. Rev. 110, 6736 (2010). [2] B. A. Gregg, J. Phys. Chem. Lett. 2, 3013 (2011).

Entropy and disorder enable charge separation in organic solar cells

251st National Meeting (2016)

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