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

TL;DR

This paper investigates why high polaron pair dissociation efficiency occurs at relatively low electric fields in polymer-fullerene solar cells, using kinetic Monte Carlo simulations that incorporate charge delocalization.

Contribution

The study introduces a kinetic Monte Carlo model including charge delocalization to explain efficient polaron pair dissociation at low fields in organic solar cells.

Findings

Fast local charge transport explains high dissociation yields.

Delocalized charge carriers reduce the required electric field.

Simulation results match experimental efficiencies.

Abstract

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 $10^7$V/m. In contrast, considering just the Coulomb binding of the polaron pair, electric fields above $10^8$V/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 delocalised 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.