Versatile methods for improving the mechanical properties of fullerene and non-fullerene bulk heterojunction layers to enable stretchable organic solar cells

Abstract

We develop and evaluate two methods aimed at enhancing the mechanical resistance and stretchability of bulk heterojunction active layers in organic solar cells while maintaining photovoltaic performance intact. These techniques consist of adding a cross-linker or an elastomer into the active layer. First, we study P3HT:PC₆₁BM blend as a reference polymer:fullerene acceptor system for our conceptual demonstration. The diazido cross-linker selectively reacts with PC₆₁BM molecules and the resulting blend shows an overall softening effect of the mix, while the power conversion efficiency (PCE) increases by up to 20%, which we attribute to increased aggregation of P3HT. Addition of elastomers affects the blend differently: the crystallinity of the donor decreases, which can have a detrimental effect on the PCE of certain materials. However, the elastomer method is found to be considerably more universal by acting as a mechanical softener in modern fullerene-free blend systems as well. In particular, SEBS elastomer displayed a tailored elasticity with various fullerene and non-fullerene blends: P3HT:PC₆₁BM, PCE10:PC₇₁BM, and PCE13:IT-4F. The potential of our SEBS process is demonstrated in PCE13:IT-4F:SEBS solar cells with a PCE of 11.5%. This versatile approach highlights the ease of manufacturing and scalability achieved by the solution casting processes, along with the high compatibility of acceptor and donor blends.