Influence of Phase Segregation on Recombination Dynamics in Organic Bulk-Heterojunction Solar Cells

TL;DR

This study investigates how phase segregation affects charge recombination in organic bulk heterojunction solar cells, revealing that different blend ratios influence recombination dynamics and device performance.

Contribution

It provides new insights into the impact of phase segregation on recombination mechanisms in polymer:fullerene solar cells, using advanced characterization techniques.

Findings

Enhanced geminate recombination in 1:1 blend due to isolated phase regions

Increased short circuit current in 1:4 blend from extensive phase segregation

Bimolecular recombination observed at temperatures below 300 K

Abstract

We studied the recombination dynamics of charge carriers in organic bulk heterojunction solar cells made of the blend system poly(2,5-bis(3-dodecyl thiophen-2-yl) thieno[2,3-b]thiophene) (pBTCT-C12):[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) with a donor--acceptor ratio of 1:1 and 1:4. The techniques of charge carrier extraction by linearly increasing voltage (photo-CELIV) and, as local probe, time-resolved microwave conductivity (TRMC) were used. We observed a difference in the initially extracted charge carrier concentration in the photo-CELIV experiment by one order of magnitude, which we assigned to an enhanced geminate recombination due to a fine interpenetrating network with isolated phase regions in the 1:1 pBTCT-C12:PC61BM bulk heterojunction solar cells. In contrast, extensive phase segregation in 1:4 blend devices leads to an efficient polaron generation resulting in an increased short circuit current density of the solar cell. For both studied ratios a bimolecular recombination of polarons was found using the complementary experiments. The charge carrier decay order of above two for temperatures below 300 K can be explained by a release of trapped charges. This mechanism leads to a delayed bimolecular recombination processes. The experimental findings can be generalized to all polymer:fullerene blend systems allowing for phase segregation.