Direct and charge transfer mediated photogeneration in polymer-fullerene bulk heterojunction solar cells

TL;DR

This study explores the mechanisms of photogeneration and recombination in polymer-fullerene solar cells, revealing different dissociation behaviors and recombination rates in P3HT and MDMO-PPV based blends through temperature-dependent measurements.

Contribution

It provides new insights into charge transfer and dissociation processes, highlighting the differences between P3HT and MDMO-PPV blends in their charge dynamics.

Findings

MDMO-PPV:PC61BM shows strong field-dependent polaron pair dissociation.

P3HT:PC61BM exhibits almost field-independent dissociation.

Langevin recombination is observed in MDMO-PPV:PC61BM, reduced in P3HT:PC61BM.

Abstract

We investigated photogeneration yield and recombination dynamics in blends of poly(3-hexyl thiophene) (P3HT) and poly[2-methoxy-5-(30,70-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) with [6,6]- phenyl-C61 butyric acid methyl ester (PC61BM) by means of temperature dependent time delayed collection field (TDCF) measurements. In MDMO-PPV:PC61BM we find a strongly field dependent polaron pair dissociation which can be attributed to geminate recombination in the device. Our findings are in good agreement with field dependent photoluminescence measurements published before, supporting a scenario of polaron pair dissociation via an intermediate charge transfer (CT) state. In contrast, polaron pair dissociation in P3HT:PC61BM shows only a very weak field dependence, indicating an almost field independent polaron pair dissociation or a direct photogeneration. Furthermore, we found Langevin recombination for MDMO-PPV:PC61BM and strongly reduced Langevin recombination for P3HT:PC61BM.