Low-Cost Nonfused-Ring Electron Acceptors Enabled by Noncovalent Conformational Locks

ConspectusSince the first bilayer-structured organic solar cells (OSCs) in 1986, fullerenes and their derivatives have dominated the landscape for two decades due to their unique properties. In recent years, the breakthrough in nonfullerene acceptors (NFAs) was mainly attributed to the development of fused-ring electron acceptors (FREAs), whose photovoltaic performance surpassed that of fullerene derivatives. Through the unremitting efforts of the whole community, the power conversion efficiencies (PCEs) have surpassed 19% in FREA-based OSCs. However, FREAs generally suffered from complex synthetic approaches and high product costs, which hindered large-scale production. Therefore, many researchers are seeking a new type of NFA to achieve cost-effective, highly efficient OSCs.In collaboration with Marks and Facchetti in 2012, Huang et al. (Huang, H. J. Am. Chem. Soc. 2012, 134, 10966-10973, 10.1021/ja303401s) proposed the concept of "noncovalent conformational locks" (NoCLs). In the following years, our group has been focusing on the theoretical and experimental exploration of NoCLs, revealing their fundamental nature, formulating a simple descriptor for quantifying their strength, and employing this approach to achieve high-performance organic/polymeric semiconductors for optoelectronics, such as OSCs, thin-film transistors, room-temperature phosphorescence, and photodetectors. The NoCLs strategy has been proven to be a simple and effective approach for enhancing molecular rigidity and planarity, thus improving the charge transport mobilities of organic/polymeric semiconductors, attributed to reduced reorganization energy and suppressed nonradiative decay.In 2018, Chen et al. (Li, S. Adv. Mater. 2018, 30, 1705208, 10.1002/adma.201705208) reported the first example of nonfused-ring electron acceptors (NFREAs) with intramolecular noncovalent F···H interactions. The NoCLs strategy is essential in NFREAs, as it simplifies the conjugated structures while maintaining high coplanarity comparable to that of FREAs. Due to their simple structures and concise synthesis routes, NFREAs show great potential for achieving cost-effective and highly efficient OSCs. In this Account, we provide an overview of our efforts in developing NFREAs with the NoCLs strategy. We begin with a discussion on the distinct features of NFREAs compared with FREAs, and the structural simplification from FREAs to NFREAs to completely NFREAs. Next, we examine several selected typical examples of NFREAs with remarkable photovoltaic performance, aiming to provide an in-depth exploration of the molecular design principle and structure-property-performance relationships. Then, we discuss how to achieve a balance among efficiency, stability, and cost through a two-in-one strategy of polymerized NFREAs (PNFREAs). Finally, we offer our views on the current challenges and future prospects of NFREAs. We hope this Account will trigger intensive research interest in this field, thus propelling OSCs into a new stage.