Precisely Manipulating Molecular Packing via Tuning Alkyl Side-Chain Topology Enabling High-Performance Nonfused-Ring Electron Acceptors

Boosting organic solar cell efficiency via tailored end-group modifications of novel non-fused ring electron acceptors

In this study, we designed and synthesized two NFREAs, 2BTh-3F and 2BTh-CN, incorporating distinct substituents to modulate their electron-withdrawing properties. We meticulously explore the distinct impacts of these substituents on NFREA performance. Our investigation revealed that the introduction of 3,5-difluoro-4-cyanophenyl in 2BTh-CN significantly enhanced electron withdrawal and intramolecular charge transfer, leading to a red-shifted absorption spectrum and optimized energy levels. Consequently, organic solar cells (OSCs) utilizing 2BTh-CN demonstrate a notable power conversion efficiency (PCE) of 15.07%, outperforming those employing 2BTh-3F (PCE of 9.34%). Moreover, by incorporating 2BTh-CN into the D18:2BTh-C2 system as a third component, we achieve a PCE exceeding 17% in a high-performing ternary OSC, ranking among the most efficient NFREA-based OSCs reported to date. Overall, our study underscores the potential of deliberate design and optimization of non-fused ring acceptor molecular structures to attain outstanding photovoltaic performance.

Configurational Isomerization-Induced Orientation Switching: Non-Fused Ring Dipodal Phosphonic Acids as Hole-Extraction Layers for Efficient Organic Solar Cells

Phosphonic acid (PA) self-assembled molecules have recently emerged as efficient hole-extraction layers (HELs) for organic solar cells (OSCs). However, the structural effects of PAs on their self-assembly behaviors on indium tin oxide (ITO) and thus photovoltaic performance remain obscure. Herein, we present a novel class of PAs, namely "non-fused ring dipodal phosphonic acids" (NFR-DPAs), featuring simple and malleable non-fused ring backbones and dipodal phosphonic acid anchoring groups. The efficacy of configurational isomerism in modulating the photoelectronic properties and switching molecular orientation of PAs atop electrodes results in distinct substrate surface energy and electronic characteristics. The NFR-DPA with linear (C2h symmetry) and brominated backbone exhibits favorable face-on orientation and enhanced work function modification capability compared to its angular (C2v symmetry) and non-brominated counterparts. This makes it versatile HELs in mitigating interfacial resistance for energy barrier-free hole collection, and affording optimal active layer morphology, which results in an impressive efficiency of 19.11 % with a low voltage loss of 0.52 V for binary OSC devices and an excellent efficiency of 19.66 % for ternary OSC devices. This study presents a new dimension to design PA-based HELs for high-performance OSCs.

Precisely Regulating Intermolecular Interactions and Molecular Packing of Nonfused-Ring Electron Acceptors via Halogen Transposition for High-Performance Organic Solar Cells

The structure of molecular aggregates is crucial for charge transport and photovoltaic performance in organic solar cells (OSCs). Herein, the intermolecular interactions and aggregated structures of nonfused-ring electron acceptors (NFREAs) are precisely regulated through a halogen transposition strategy, resulting in a noteworthy transformation from a 2D-layered structure to a 3D-interconnected packing network. Based on the 3D electron transport pathway, the binary and ternary devices deliver outstanding power conversion efficiencies (PCEs) of 17.46 % and 18.24 %, respectively, marking the highest value for NFREA-based OSCs.

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.