Emergence of Low-Cost and High-Performance Nonfused Ring Electron Acceptors

ConspectusOrganic solar cells (OSCs) have garnered significant attention in academic and industrial circles due to their advantages such as lightweight, excellent bending performance, and the ability to be fabricated into semitransparent devices. Since the proposal of the bulk heterojunction concept by Heeger et al. in 1995, conjugated polymer/fullerene pairs have gradually emerged as the optimal choice for active layer materials in OSCs. Fullerene derivatives were preferred as electron acceptors in OSCs because of their high electron mobility. However, due to limitations such as insufficient light absorption, limited derivative potential, and poor energy level tunability, the power conversion efficiency (PCE) of OSCs based on fullerene derivatives has encountered a bottleneck of approximately 12%, despite the continuous updates in polymer donor materials over nearly two decades of development, leading to a gradual decline in their importance. By contrast, nonfullerene electron acceptors (NFAs) have gradually gained dominance in this field since first appearing in 2015, thanks to their advantages of tunable absorption spectrum, adjustable energy levels, and modifiable chemical structure. Among nonfullerene acceptors, fused-ring electron acceptors (FREAs) such as ITIC and Y6 have achieved significant progress, boosting the PCE of OSCs to 20%. This milestone achievement indicates the potential of their commercial applications. However, the synthesis process of FREA is complex and often constrained by low-yield ring-closure reactions, resulting in high costs.The molecular backbone of nonfused ring electron acceptors (NFREAs) is composed of single bonds, which enables the adoption of modular synthesis mainly via Stille (based on organotin reactant) and/or Suzuki (based on organoboron reactant) coupling or C-H activation (without prefunctionalization) and avoids low-yield ring-closing reactions, thus making them a potential alternative to fused-ring acceptors. To achieve a planar molecular backbone and minimize energy loss due to conformational rotation, our team innovatively used intramolecular noncovalent interactions as a replacement for traditional covalent bonds. Furthermore, to address the issues of poor solubility and excessive aggregation during film formation for NFREAs, we strategically introduced sterically hindered side groups, such as 2,6-bis(alkyloxy)phenyl and diphenylamino, into the molecular design, effectively mitigating these problems. These innovative design concepts have significantly advanced the development of high-performance NFREAs and have garnered increasing attention from the research community. The PCEs of OSCs based on NFREAs have significantly improved from less than 10% to close to 20% since their initial discovery. By optimizing the device fabrication process, we have achieved a PCE of over 19%, which is comparable to that of FREAs. This article will delve into the evolution and latest research progress of NFREAs, aiming to provide valuable insights and guidance for the design of cost-effective and high-performance NFREA materials.