A Two-Step Heating Strategy for Nonhalogen Solvent-Processed Organic Solar Cells Based on a Low-Cost Polymer Donor

Fabrication of high-efficiency organic solar cells (OSCs) using nonhalogen green solvents is highly desired for commercialization. However, the morphology control of a nonhalogen solvent-processed active layer remains challenging because of poor solubility of the commonly used photovoltaic materials in nonhalogen solvents. Herein, we propose a two-step heating strategy to optimize the morphology of a photovoltaic active layer based on a low-cost polymer donor X1 and a small molecule acceptor (SMA) BTP-eC9 processed from a mixed nonhalogen solvent. First, heating the blend solution can robustly enhance the solubility of the SMA and thus processability of the blend, producing a uniform active layer. Second, further elevating the substrate temperature can shorten the film-formation time and tune the size of the polymer aggregates and SMA crystallites, to form bicontinuous networks in the active layer. As a result, this twostep heating strategy endows the film with improved charge generation, transport, and collection and thus a much higher device efficiency (ca. 4.6 times of that for the as-cast device) in comparison to those from nonoptimal solution and/or substrate temperatures. This work emphasizes the crucial role of regulating processing conditions in fabricating the nonhalogen solvent processed high-efficiency OSCs.