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.

Figure 1

Polaron-pair dissociation yield in $1:1$ donor-acceptor blends at 300 K. The yellow rectangle denotes the typical internal field under short-circuit conditions of organic solar cells. (a) Variation of the effective lifetime ${\tau }_{\mathrm{eff}}$ at a constant donor CL of 1. The efficient experimental dissociation at low electric fields cannot be reached (dielectric constant $\epsilon =3.0$). (b) Variation of the conjugation length CL, with ${\tau }_{\mathrm{eff}}$ fixed at $1\mu \mathrm{s}$ (dielectric constant $\epsilon =3.0$). (c) Simultaneous variation of ${\tau }_{\mathrm{eff}}$ and CL, with the dielectric constant chosen as 3.7. For long polymer chain segments, the high experimentally observed yields are replicated.

Figure 2

Polaron-pair dissociation yield with Monte Carlo simulation (solid lines) and the OB model (dashed lines) in $1:1$ donor-acceptor blends at 300 K (dielectric constant $\epsilon =3.0$). The contours are denoted with the yield values. The yellow rectangle denotes again the typical internal field under short-circuit conditions.