ABSTRACT Organic solar cells (OSCs) have acquired a performance higher than 20%, exhibiting a very promising future. The aggregated structure of the active layer, which is a three‐phase bulk heterojunction (BHJ) structure including the donor, acceptor, and amorphous mixed phases, is critical for the device performance. Although the tuning of the phase separation structure has been studied for more than two decades, the optimal structure is almost inaccessible due to the complex solution processing conditions. Based on theoretical analysis and current experimental evidence, this perspective provides a personal understanding of what the optimal phase separation structure is and how it can be realized. First, an independent parameter group is proposed to quantitatively describe the phase separation structure. Then, how the above parameters independently influence the performance of OSCs is reviewed and discussed. After that, three possible routes are suggested to quantitatively regulate the phase separation structure. Finally, a brief summary and several current challenges are provided.
Fan et al. (2026) studied this question.