Homocoupling defects in a conjugated polymer limit exciton diffusion

Copolymers such as PCDTBT (poly(N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl- 2’,1’,3’-benzothiadiazole))) are commonly employed as donor material in bulk heterojunction solar cells. Recently, chemical defects such as homocouplings have been shown to form at the material synthesis stage, strongly reducing the short circuit current in organic photovoltaics. Here we show that both, low molecular weight and homocoupling defects reduce the short circuit current of solar cells because of limited exciton diffusion. We propose a model that unites and explains the influence of both chemical parameters with the distribution of conjugation lengths. The connection between limited exciton diffusion and short circuit current is revealed by kinetic Monte Carlo simulation of bulk heterojunctions. Our findings are likely applicable for copolymers in general.