Charge Carrier Dissociation and Recombination in Polymer Solar Cells

TL;DR

This study investigates charge carrier dissociation and recombination in polymer solar cells, revealing that polaron pair dissociation is high and weakly field-dependent, and that recombination is not the main loss mechanism in annealed devices.

Contribution

It combines experimental photo-induced charge extraction measurements with Monte Carlo simulations to clarify the roles of dissociation and recombination in organic solar cell efficiency.

Findings

Polaron recombination does not limit photocurrent in annealed devices.

Polaron pair dissociation has high yield with weak field dependence.

Charge extraction, dissociation, and recombination are all crucial for understanding losses.

Abstract

In polymer:fullerene solar cells, the origin of the losses in the field-dependent photocurrent is still controversially debated. We contribute to the ongoing discussion by performing photo-induced charge extraction measurements on poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C$_{61}$ butyric acid methyl ester solar cells in order to investigate the processes ruling charge carrier decay. Calculating the drift length of photogenerated charges, we find that polaron recombination is not limiting the photocurrent for annealed devices. Additionally, we applied Monte Carlo simulations on blends of conjugated polymer chain donors with acceptor molecules in order to gain insight into the polaron pair dissociation. The dissociation yield turns out to be rather high, with only a weak field dependence. With this complementary view on dissociation and recombination, we stress the importance of accounting for polaron pair dissociation, polaron recombination as well as charge extraction when considering the loss mechanisms in organic solar cells.