Morphology Control in Solution-Processed Bulk-Heterojunction Solar Cell Mixtures

The efficiency of bulk-heterojunction solar cells is very sensitive to the nanoscale structure of the active layer. In the past, the final morphology in solution-processed devices has been controlled by varying the casting solvent and by curing the layer using heat tempering or solvent soaking. A recipe for making the "best-performing" morphology can be achieved using these steps. This article presents a review of several new techniques that have been developed to control the morphology in polymer/fullerene heterojunction mixtures. The techniques fall into two broad categories. First, the morphology can be controlled by preparing nanoparticle suspensions of one component The size and shape of the nanoparticles in solution determine the size and shape of the domain in a mixed layer. Second, the morphology can be controlled by adding a secondary solvent or an additive that more strongly affects one component of the mixture during drying. In both cases, the as-cast efficiency of the solar cell is improved with respect to the single-solvent case, which strongly argues that morphology control is an issue that will receive increasing attention in future research.