Design of Non-fused Ring Acceptors toward High-Performance, Stable, and Low-Cost Organic Photovoltaics

CONSPECTUS: Toward future commercial applications of organic solar cells (OSCs), organic photovoltaic materials that enable high efficiency, excellent stability, and low cost should be developed. Fused-ring electron acceptors (FREAs) have declared that OSCs are capable of showing efficiencies over 19%, whereas stability and cost are not solved yet. As the counterparts of FREAs, non-fused ring electron acceptors (NFREAs) are more flexible in molecular design. They have better stability because of the reduction of intramolecular tension via breaking fused backbone and have more advantages in cost with the reduction of synthetic complexity. However, the challenge for NFREAs is the relatively lower efficiencies (around 15% at current stage), which require better molecular designs for addressing the issues of conformational unicity and effective molecular packing. In this Account, we comprehensively summarize works about NFREAs carried out in our group from three main frameworks, including molecular design and efficiency optimization, material cost, and stability. First, in the part of molecular design and efficiency optimization, the existing rotatable single bond in NFREAs will bring the problem of conformational uncertainty, but it can be solved through proper molecular design, which also regulates the energy levels, light absorption range, and the packing mode of the molecule for obtaining higher performance. Thus, in this part, we discuss the evolution of NFREAs in three aspects, including molecular skeleton optimization, terminal modification, and side chain engineering. Many strategies are used in the design of a molecular skeleton, such as utilizing the quinoid effect, introducing functional groups with the electron push-pulling effect, and using multiple conformational lock. Furthermore, simplifying the skeleton is also the preferred development tendency. As for the terminal, the main modification strategy is adjusting the conjugation length and halogen atoms. What is more, by adjusting the side chain to induce appropriate steric hindrance, we can fix the orientation of molecules, thus regulating molecular packing modes. Second, regarding material cost, we compare the synthesis complexities between state-of-the-art FREAs and NFREAs. Because the synthesis processes of NFREAs reduce the complex cyclization reactions, the synthesis routes are greatly simplified, and the molecule can be obtained through three minimal steps. Third, regarding stability, we analyze the workable strategies used in NFREAs from the views of intrinsic material stability, photostability, and thermal stability. Finally, we conclude the challenges that should be conquered for NFREAs and propose perspectives that could be performed for NFREAs, with the hope of pushing the development of OSCs toward high performance, stability, and low cost.