Annealing methods for controlling the morphology of polymer solar cells

Organic solar cells, particularly polymer/fullerene solar cells, have received widespread attention because of their low fabrication costs and high device flexibility. The key to highly efficient devices is to precisely control the polymer morphology. In this Article, we first introduce the fundamentals of polymer/fullerene solar cells, including the materials, working principles, device structures, and evolution. We then describe the use of two representative annealing methods, "solvent annealing" and "post-annealing," that provide polymer photovoltaic devices with ideal morphologies and high power conversion efficiencies. A variation on the "solvent annealing" process, so-called "co-solvent annealing," involves the use of solvents possessing high boiling points as additives in solvent mixtures. This fabrication procedure is simpler than conventional "solvent annealing" because a solvent-saturated environment is not required. We also discuss recent developments in microwave annealing. Finally, we provide a brief outlook regarding the future use of these annealing approaches.