Ambipolar charge transport in polymer:fullerene bulk heterojunctions for different polymer side-chains

Abstract

We use field-effect transistors to investigate electron and hole mobilities in polymer:fullerene blends. Low-band-gap polymers with a common conjugated backbone and differing side-chains are utilized in order to clarify the link between the side-chain molecular structure and grafting position, and the power-conversion efficiency of related bulk heterojunction solar cells. The results show that, at a fixed polymer:fullerene weight ratio, the electron mobility increases by more than four orders of magnitude when changing from linear to branched side-chains. As a consequence, the photovoltaic performances are highest at low fullerene contents for branched chains while the opposite is true for linear chains.