Morphology control in co-evaporated bulk heterojunction solar cells

Bulk heterojunction solar cells made by vacuum co-evaporation of polythiophene (PTh) and fullerene (C60) are reported and the blend morphology control through donor–acceptor composition and post-situ annealing demonstrated. Co-deposited heterojunctions are shown to generate about 60% higher photocurrents than their thickness-optimized PTh/C60 planar heterojunction counterparts. Furthermore, by annealing the devices post-situ the power conversion efficiency is improved by as much as 80%. UV–vis, XRD and AFM analysis shows that the enhanced photoresponse is a result of favorable morphological development of PTh upon annealing. Annealing-induced improvement of the donor–acceptor network is demonstrated across a wide range of compositions (20–80% PTh). This illustrates that good morphology control is possible in co-evaporated polymer–fullerene blends through multiple parameters and differentiates this donor–acceptor combination from the standard small-molecule systems.