Strategic molecular engineering of non-fused non-fullerene acceptors: efficiency advances and mechanistic insight

	Fig. 4 (a) Thermal annealing effects on morphology and PCE of PBDB-T:DF-PCIC and PBDB-T:PC71BM blends. Reproduced with permission.39 Copyright 2017, John Wiley and Sons. (b) Stabilities of encapsulated devices under metal halide lamp illumination (non-UV-filtered). Reproduced with permission.57 Copyright 2019, Springer Nature. (c) Molecular absorption spectra in films vs. chloroform solutions. Reproduced with permission.58 Copyright 2019, Springer Nature. (d) Calculated molecular properties of 2T2CSi-4F and 4T2CSi-4F: geometries, ESP surfaces, and FMO distributions. Reproduced with permission.61 Copyright 2022, American Chemical Society. (e) GIWAXS patterns of PM6:Tz-H and PM6:Tz-Cl blend films and the corresponding scattering profiles in the out-of-plane (solid line) and in-plane (dashed line) directions. Reproduced with permission.62 Copyright 2023, John Wiley and Sons. (f) Molecular conformation and single-crystal packing (top view) of TT-X-2F (X = O, S, Se). Reproduced with permission.63 Copyright 2022, Elsevier.

In 2019, the nonfused-ring acceptor ICTP was developed by Zhang et al.⁵⁴via a three-step synthetic approach. The molecular conformation was stabilized through O⋯H and O⋯S non-covalent interactions, yielding a planar and rigid backbone comparable to FRAs. This structural configuration significantly enhanced intramolecular charge transfer (ICT), resulting in a narrowed bandgap and optimized energy level alignment that well-matched medium-bandgap donor PBDB-T. While the limited conjugation length led to a relatively low charge mobility, with a DIO-optimized device exhibiting a modest PCE of 4.43%, further extension of the conjugated system could potentially improve its photovoltaic performance. Concurrently, Wang et al.⁵⁵ synthesized a new acceptor (DFPCBR) featuring a strong electron-donating group and an extended π-conjugated central core. The F⋯H non-covalent interactions facilitated an ideal planar configuration, which not only improved the morphological compatibility with P3HT donor, but also achieved favorable energy level matching and complementary absorption spectra. The P3HT:DFPCBR-based OSCs demonstrated a PCE of 5.34%, with additional advantages of thickness-insensitive performance and low energy loss. These characteristics render this material system particularly promising for commercial applications. Subsequently, Zheng et al.⁵⁶ reported two naphthalene-based positional isomeric NFRAs, NOC6F-1 and NOC6F-2. The 2,6-linked NOC6F-1 adopted a near-perfect planar conformation due to reduced β-position steric hindrance and intramolecular S⋯O interactions. This superior planarity endowed NOC6F-1 with tighter π–π stacking, more extended conjugation, and a pronounced spectral redshift. When mixed with PBDB-T, NOC6F-1 demonstrated ordered molecular packing in both the in-plane and out-of-plane orientations, yielding devices with a remarkable PCE of 10.62%, significantly outperforming the NOC6F-2-based devices (6.74%). Concurrently, Yu et al.⁵⁷ developed fully NFRAs (PTICH, PTIC, and PTICO) featuring O⋯H NoCL effects that substantially reduced the rotational energy barrier between central cores and end-groups, resulting in more planar molecular conformations. Notably, these materials maintained dynamic rotatable conformations in solution (ensuring good processability) while spontaneously assembling into highly ordered planar stacking structures via intramolecular noncovalent interactions in the solid-state films. Among them, PTIC exhibited a low synthetic complexity index, and the derived OSCs demonstrated exceptional stability under continuous illumination (Fig. 4b), achieving outstanding PCEs of 10.27% (single-junction) and 13.97% (tandem). This study fundamentally elucidated the potential of noncovalent conformational locking engineering in controlling solution-to-solid-state molecular conformation transitions.

The noncovalent conformational locking strategy, enabled by intramolecular interactions such as O⋯H, S⋯O, etc., effectively stabilizes the planar conformations of NFRAs. This approach significantly enhances conjugated backbone ordering, optimizes energy levels, induces an absorption redshift, and improves electron mobility. Furthermore, it permits precise control over solution-to-solid-state conformational transitions to facilitate tight π–π stacking. The resulting synergistic enhancement in both device efficiency and stability establishes an effective pathway for balancing the performance with synthetic complexity.

In 2021, Yao et al.⁵⁹ designed three NFRAs (AOTX, X = 1–3) that were isomers of one another. The AOT3 molecule with the largest number of O⋯S non-covalent bonds exhibited brilliant properties, not only with extended absorption spectra, enhanced rigid planar structure, and strong crystallinity, but also with efficient charge transport and high charge mobility. Hence, the photovoltaic device blended with PCE 10 had the highest efficiency of 6.59%. Subsequently, through precise control of the sulfur/oxygen atomic positioning and stoichiometry, the research team synthesized four simplified unfused-ring electron acceptors (PTS-4F, PDS-4F, PDO-4F, and PTO-4F).⁶⁰ Characterization results demonstrated that increasing oxygen content (from PTS-4F to PTO-4F) significantly enhanced both the quantity and strength of intramolecular O⋯S/O⋯H NoCLs, leading to (i) progressively more planar molecular backbones, (ii) redshifted absorption maxima, and (iii) elevated HOMO energy levels. In contrast, sulfur incorporation weakened these interactions, resulting in increased molecular torsional angles. Consequently, the device efficiencies showed a gradual improvement from 4.06% for PTS-4F (NoCL-deficient) to 6.81% for PTO-4F (NoCL-enriched), unequivocally demonstrating the crucial role of NoCL population in modulating both molecular conformation and device performance.

In 2022, Zhong et al.⁶¹ developed two types of NFRAs, 4T2CSi-4F and 2T2CSi-4F, by using non-covalent conformation locking engineering. Both molecules formed multiple intramolecular conformational locks under O⋯S non-covalent interactions. Compared to the asymmetric 4T2CSi-4F, the multiple NoCLs more significantly affected the symmetrically designed 2T2CSi-4F, manifested by 2T2CSi-4F's more planar structure, longer absorption wavelength, and narrower band gap (Fig. 4d). Moreover, 2T2CSi-4F demonstrated enhanced charge transport properties and superior crystallinity, ultimately enabling its photovoltaic devices to achieve a record PCE of 10.04%.

In 2023, Han et al.⁶² successfully synthesized two NFRAs, Tz-H and Tz-Cl, through the introduction of S⋯N and S⋯Cl NoCLs. Among these, Tz-Cl, containing multiple NoCLs (S⋯N and S⋯Cl), exhibited a flatter molecular configuration compared to Tz-H that possessed only S⋯N noncovalent interactions. As a result, Tz-Cl allowed for tighter and more ordered stacking, which favored the formation of well-structured thin films in the Tz-Cl-based polymer blend (Fig. 4e). Additionally, the excellent morphology in Tz-Cl-based devices contributed to the improved performance in exciton dissociation, charge transfer, and recombination, leading to a high PCE of 11.10%, which represented a substantial enhancement over the 6.41% PCE observed for Tz-H-based OSCs. Subsequently, Zheng et al.⁶³ designed a set of acceptors (TT-X-2F, X = O, S, Se) by incorporating NoCLs. Intramolecular noncovalent interactions (S⋯O and Se⋯O) conferred a relatively planar backbone configuration to TT-Se-2F and TT-S-2F, whereas the absence of such interactions in TT-O-2F resulted in a twisted conformation. Notably, TT-S-2F exhibited superior molecular packing behavior, manifesting as a larger end-group overlap area (Fig. 4f), enhanced exciton diffusion length, and improved blend-film morphology and crystallinity, all of which collectively contributed to the significantly enhanced charge transport properties. Ultimately, the D18:TT-S-2F-based OSCs obtained the highest PCE of 15.29%, substantially outperforming the other systems.

Increasing the number of NoCLs effectively strengthens intramolecular interactions, promotes backbone planarization while simultaneously enhancing molecular packing order and crystallinity. These structural improvements optimize the active layer morphology, thereby boosting device performance. Critically, NoCLs incorporation must maintain compatibility with the target molecule's structural characteristics. Symmetric architectures typically exhibit stronger cooperative effects from multiple NoCLs compared to asymmetric counterparts, necessitating precise molecular engineering to achieve optimal system performance.

The regulatory mechanism of multiple noncovalent interactions in NFRAs operates through a hierarchical framework: intramolecular interactions establish a planar and rigid molecular skeleton, while intermolecular interactions enhance the crystalline order. This multi-level interaction network synergistically optimizes molecular packing, thereby improving charge transport properties.

	Fig. 8 (a) Single-crystal structure, molecular packing, network topology (top view), and 3D interpenetrating architecture of A4T-16. Reproduced with permission.80 Copyright 2021, Springer Nature. (b) Low-temperature time-resolved photoluminescence emission spectra and singlet/triplet energy level parameters of PCT-4Cl. Reproduced with permission.81 Copyright 2024, Elsevier. (c) Electron transport pathways and molecular packing evolution in 3TTS-4F and 3TTB-4F. Reproduced with permission.88 Copyright 2024, John Wiley and Sons. (d) Photographic evidence of photostability testing for 3TTCz and 3TTDPA in solution. Reproduced with permission.91 Copyright 2024, Royal Society of Chemistry. (e) Contact angles and surface free energies of PTVT-BT, A4T-16, A4T-31, and A4T-32 neat films. Reproduced with permission.94 Copyright 2022, John Wiley and Sons.

	Fig. 9 Chemical structures of NFRAs regulated by side-chain number and length.

In 2023, Li et al.⁴⁸ synthesized ATTP-1 and ATTP-2 with different substituent groups to investigate the impact of spatial steric effects on the planarity of NFRAs. ATTP-1, incorporating a 2,4,6-triisopropylphenyl group with pronounced steric hindrance, exhibited superior molecular planarity and conformational stability relative to ATTP-2. These structural advantages facilitated both intramolecular and intermolecular charge transfer, while simultaneously broadening the absorption spectral range. Moreover, the stronger molecular packing effectively suppressed excessive aggregation of ATTP-1, resulting in a favorable blend morphology. Accordingly, the devices employing ATTP-1 attained a remarkable PCE of 11.30%, which is more than three times higher than the ATTP-2-based devices (PCE = 3.70%). Furthermore, it demonstrated lower energy loss, measured at 0.67 eV.

In 2024, Luo et al.⁸¹ developed a novel NFRA, PCT-4Cl, through the strategic incorporation of two-dimensional tert-butylcarbazole side chains. This study elucidated the dual modulation mechanism of side-chain engineering on exciton behavior and crystallization kinetics. The distinctive energy level structure of PCT-4Cl (Fig. 8b) – featuring the narrow singlet–triplet gap (0.25 eV) and elevated triplet energy level (1.54 eV) – enabled not only prolonged singlet exciton lifetime but also facilitated efficient triplet exciton transfer to host acceptors (e.g., eC9 or C8C8-4Cl) in blend systems. Crucially, the sterically hindered carbazole side chains in PCT-4Cl effectively regulated crystallinity in ternary blends, optimizing molecular packing while suppressing excessive aggregation of host acceptors, thereby achieving ideal morphological characteristics. These advantages collectively enhanced J_sc and FF while reducing both radiative and non-radiative recombination losses. Consequently, PM6:eC9:PCT-4Cl and PM6:C8C8-4Cl:PCT-4Cl-based OSCs achieved outstanding PCEs of 18.84% and 15.17%, respectively, which were further improved to 19.25% and 15.53% when employing 2PACz as HTL. Notably, PCT-4Cl demonstrated exceptional compatibility with PM6, yielding favorable blend morphology and a binary device efficiency of 13.02%.

Sterically hindered side chains demonstrate the multi-level synergistic optimization mechanism for regulating NFRA performance. Primarily, their three-dimensional steric effects (e.g., 2,4,6-triisopropylphenyl and tert-butylcarbazole groups) preserve backbone planarity at the molecular level while effectively suppressing excessive molecular aggregation, thereby facilitating ideal π–π stacking formation. Secondarily, these side chains precisely modulate crystallinity, which concurrently optimizes blend morphology and significantly reduces energy losses. Of particular interest, their capacity to enhance molecular conformational stability correlates with improved device operational stability.

Distinct structure–property relationships emerge in NFRA systems when comparing linear and branched chain topologies. In 2021, Luo et al.⁸² designed two acceptors (BDC-4F-CX, where X = 6, 8) featuring the straightforward non-fused skeleton and systematically examined how the variation in alkyl side chain types affects molecular properties. In contrast to the BDC-4F-C6 molecule with linear n-hexyl chains, the BDC-4F-C8 molecule with branched 2-ethylhexyl chains exhibited an elevated LUMO value, which was conducive to increasing V_oc. Therefore, when paired with PM6 donor, the PCE of the BDC-4F-C8-based devices was up to 12.53% with an E_loss of 0.51 eV – the lowest reported value at that time. The excellent efficiency of this acceptor was attributed to strong π–π packing, uniform film morphology, terrific charge mobility, and low charge recombination.

In 2023, Zhou et al.⁸³ synthesized two types of NFRAs, 3T-1 and 3T-2, by adjusting their side chains. The introduction of the 2-ethylhexyl chain effectively controlled the compatibility between 3T-2 and donor D18, resulting in a blend film with good molecular orientation and excellent morphology, thereby facilitating efficient charge transport. Ultimately, the D18:3T-2-based OSCs exhibited a far superior PCE of 11.12% compared to the octyl-chain-containing 3T-1 devices (PCE = 2.43%). Zhu et al.⁸⁴ that same year developed two novel asymmetrical NFRAs, IOEH-4Cl and IOMe-4Cl, differentiated by their alkoxy side chain modifications on the electron-donating component. Alkoxy side chain modifications minimally affected the energy levels and absorption spectra, but significantly influenced blend film morphology, crystallinity, and charge carrier mobility. The 2-ethylhexyloxy-substituted IOEH-4Cl exhibited superior performance in the mentioned parameters compared to its methoxy-substituted analogue, IOMe-4Cl. This translates to a higher PCE of 13.67% for the IOEH-4Cl-based devices, compared to 11.45% for the IOMe-4Cl-based devices. Meanwhile, Shen et al.⁸⁵ synthesized two new non-fullerene acceptors, O-PC-OC and O-PC-EH, by incorporating alkoxy chains into the previously reported HF-PCIC. Consistent with previous findings, these modifications contributed to improving the morphology, crystallinity, and charge migration of the blend film. Therefore, the newly synthesized molecules exhibited better performance in the aforementioned parameters compared to HF-PCIC. Particularly, O-PC-EH with the alkoxy side chain not only showed the best crystallinity and molecular orientation but also demonstrated the most efficient charge transfer process, highest photoelectron response, and lowest energy loss (E_loss = 0.68 eV). Consequently, when paired with donor J52, the J52:O-PC-EH-based OSCs showed the best photovoltaic performance (J_sc = 24.44 mA cm⁻², V_oc = 0.82 V, FF = 72.20%, PCE = 14.43%). Concurrently, Yang et al.⁸⁶ developed two fully NFRAs (TBT-2 and TBT-6) based on ortho-bis(2-ethylhexyloxy) benzene units, differing solely in their thiophene-bridged side chains (branched 2-ethylhexoxy for TBT-2 vs. linear octyloxy for TBT-6). Single-crystal analyses revealed that TBT-2's synergistic intramolecular noncovalent interactions and strong π–π stacking between end-groups collectively established a highly planarized conjugated framework with tight molecular packing, yielding a narrow bandgap of 1.51 eV and deep HOMO level at −5.80 eV. Interestingly, while TBT-6 demonstrated superior crystallinity in neat films, its blend with PB2 suffered from excessive crystallization, resulting in oversized phase domains that severely compromised the film morphology and impeded exciton dissociation/charge transport. In contrast, the steric hindrance from TBT-2's branched chains enabled PB2:TBT-2 blends to achieve well-ordered molecular stacking while preventing over-aggregation, resulting in optimal nanoscale phase separation. This morphological contrast directly dictated device performance: TBT-2-based OSCs achieved a PCE of 13.25%, significantly outperforming TBT-6's 9.53%, unequivocally demonstrating the critical role of crystallinity control in organic photovoltaic material design.

In 2024, Li et al.⁸⁷ developed two terthiophene-based non-fullerene acceptors, 3TC8 and 3TEH, through strategic incorporation of linear n-octyl and branched 2-ethylhexyl side chains, respectively. 3TEH displayed a highly planar conjugated backbone by intramolecular S⋯O locking interactions, with its 2-ethylhexyl side chains promoting tight π–π stacking between IC end-groups through steric modulation. In contrast, while 3TC8 showed shorter π–π distances and broader coherence lengths (indicating stronger crystallinity), its molecular orientation proved less favorable for charge transport. Thin-film characterization demonstrated that 3TEH possessed superior electron mobility and morphological characteristics, forming nanoscale phase-separated structures with PBQx-TF, whereas 3TC8's excessive crystallinity led to oversized aggregation domains. The PBQx-TF:3TEH systems yielded a breakthrough efficiency of 14.40% with a remarkable figure of merit (FOM = 0.49), significantly outperforming Y6 (0.21), attributable to accelerated charge separation and suppressed non-radiative recombination. The PBQx-TF:3TC8 systems also attained a notable PCE of 11.10%, with both acceptors setting new performance benchmarks for terthiophene-based NFAs. That same year, Gu et al.⁸⁸ developed two distinct NFRAs—linear alkyl-modified 3TTS-4F and branched alkyl-modified 3TTB-4F—through strategic side-chain engineering. Single-crystal analysis (Fig. 8c) revealed that 3TTS-4F formed lamellar stacking via aligned linear alkyl chains, establishing one-dimensional charge transport channels. In contrast, steric hindrance between the branched alkyl chains and rigid backbone in 3TTB-4F disrupted layered ordering, inducing axial rotation to form a three-dimensional interpenetrating network that created highly interconnected multidimensional charge pathways. The unique intermolecular interactions in 3TTB-4F significantly enhanced the blend film's charge transport properties. Its quasi-isotropic electron transport network achieved balanced carrier mobility (μ_h/μ_e = 1.13) while simultaneously suppressing trap-assisted recombination and promoting exciton dissociation, collectively boosting J_sc (24.83 mA cm⁻²) and FF (73.91%). Furthermore, the D18:3TTB-4F blend developed a dense fibrillar network that optimized film morphology, providing the structural foundation for these enhancements. The elevated LUMO level and reduced E_loss (0.54 eV) synergistically increased V_oc to 0.95 V. Benefiting from these advantages, the 3TTB-4F-based systems attained a champion efficiency of 17.38% (certified 16.59%), substantially outperforming 3TTS-4F's 15.86%.

While linear side chains enhance intermolecular π–π interactions and crystallinity, they frequently cause excessive aggregation that forms oversized phase domains, ultimately degrading device performance. In contrast, branched side chains utilize distinctive steric hindrance to maintain molecular planarity while delivering three key advantages: (i) suppressing over-crystallization to achieve ideal nanoscale phase separation, (ii) optimizing packing configurations for efficient multidimensional charge transport, and (iii) balancing carrier mobility with minimal energy losses. Consequently, branched-chain-modified NFRAs outperform linear-chain analogues, establishing critical design principles for high-efficiency organic photovoltaics.

Building upon these findings, the differential modulation mechanisms of open-chain versus closed-chain architectures on NFRAs optoelectronic properties were systematically investigated. In 2022, Zhu et al.⁸⁹ developed two NFRAs—A3T-2 (featuring “open-mode” dialkoxy substitution) and A3T-5 (with “closed-mode” seven-membered ring structure), through precise side-chain engineering. DFT calculations revealed that A3T-5's cyclic design not only effectively alleviated intramolecular steric strain but also exhibited enhanced conformational planarity and stronger intermolecular interactions. These advantages, synergistically combined with its reduced donor–acceptor miscibility and more efficient charge separation characteristics, enabled A3T-5-based devices to attain a superior J_sc of 15.30 mA cm⁻² and PCE of 7.03%, significantly outperforming A3T-2-based systems (J_sc = 12.50 mA cm⁻², PCE = 6.20%).

In 2024, Cui et al.⁹⁰ compared the optoelectronic performance differences of NFRAs incorporating cyclic side chains (CSO4TIC) and linear side chains (LSO4TIC). The experimental results indicated that cyclic side chains are more capable of exhibiting stable molecular conformation, narrow energy gaps, high maximum absorption peaks, good thermal stability, and effective electron delocalization in materials than linear side chains. Furthermore, the cyclic side chains effectively modulated the spatial hindrance of CSO4TIC, enabling the formation of an ideal 3D network stacking structure that facilitated the efficient transmission of electrons. Finally, the CSO4TIC-based devices attained a notable PCE of 14.22%, while the LSO4TIC-based devices showed a lower efficiency of 12.72%. This research effectively revealed the potential of cyclic side chains in regulating molecular conformation, providing new avenues for further developing excellent NFRAs. Concurrently, Xing et al.⁹¹ synthesized two NFRAs (3TTCz and 3TTDPA) with closed-loop carbazole and open-chain diphenylamine side chains, respectively, and systematically investigated the impact of side-chain architecture on material stability and photovoltaic performance. Their study revealed that the closed-loop carbazole side chains, through weakly electron-donating characteristics and pronounced steric hindrance effects, effectively suppressed resonance structure rearrangement into quinoidal oxidative forms, thereby obstructing the oxidative degradation pathway of carbon–carbon double bonds and significantly enhancing photostability (Fig. 8d). In contrast, the aromatic amine groups in open-chain diphenylamine side chains readily formed oxidized states under photoexcitation and promoted CC bond oxidation via quinoidal resonance rearrangement, leading to substantially reduced stability. Furthermore, the carbazole side chains optimized molecular orientation and crystallinity, facilitating charge transport while suppressing recombination, ultimately enabling 3TTCz-based devices to achieve superior performance (PCE = 14.00%, FF = 78.78%) compared to 3TTDPA-based systems (PCE = 13.85%, FF = 66.93%).

The rigid nature of closed-loop side chains stabilizes molecular planarity and enhances π–π stacking, thereby optimizing charge transport properties, while their steric hindrance simultaneously suppresses excessive donor–acceptor miscibility and blocks quinoidal oxidation pathways, significantly improving both device efficiency and operational stability. In contrast, open-chain analogues suffer from performance limitations due to conformational flexibility and oxidative susceptibility, highlighting the unique advantages of closed-loop architectures in achieving efficiency-stability balance.

Beyond conventional alkyl chains, novel side-chain architectures (e.g., aromatic, ether-linked, or ester-functionalized) can further modulate the photovoltaic performance of NFRAs through their distinctive electronic and steric effects. In 2022, t-PTIC4Cl and p-PTIC4Cl acceptors were synthesized by Liu et al.,⁹² who discovered that modifying non-axisymmetric aromatic chains can affect the device's photovoltaic performance. The t-PTIC4Cl acceptor featured 2-hexylthiophene side chains, while p-PTIC4Cl contained 3-hexylbenzene chains, the latter inducing a higher rotational barrier and larger main-chain twist angle. Moreover, the p-PTIC4Cl-based blend exhibited enhanced photostability, charge mobility, and exciton dissociation. Consequently, the PCE of the PBDB-TF:p-PTIC4Cl photovoltaic devices reached 11.30%, which was better than that of the t-PTIC4Cl-based devices (6.49%). After that, three non-fused ring acceptors, TT-TC8, TT-C8T, and TTC6, were developed by Lu et al.⁹³ The TT-TC8 acceptors with large steric hindrance exhibited good solubility, tighter molecular packing, and shorter π–π packing distance. Moreover, the blend film based on this acceptor showed a uniform morphology, and its associated devices possessed the lowest energy loss (E_loss = 0.65 eV). Therefore, the photovoltaic devices based on D18:TT-TC8 achieved a PCE of 13.13%, significantly outperforming TT-C8T (10.42%) and TTC6 (4.41%). Subsequently, Ma et al.⁹⁴ developed two NFRAs, A4T-31 and A4T-32, by incorporating alkyl ether side chains of different polarities. Among these, A4T-32 exhibited the best photovoltaic performance (V_oc = 0.81 V, J_sc = 24.70 mA cm⁻², FF = 79.60%, PCE = 16.00%). This was attributed to its highly polar side chain, which resulted in broader absorption spectra, reduced energetic disorder, and increased surface energy (Fig. 8e). Simultaneously, this side chain facilitated the formation of a favorable blend film morphology with the donor PTVT-BT, thereby improving the A4T-32 device performance and validating the polar side-chain design strategy.

In 2024, Yang et al.⁹⁵ synthesized three NFRAs by combining alkoxy and ester side chains. Specifically, 4T-BE and 4T-TO were designed by introducing ester side chains and alkoxy side chains, respectively, while 4T-BOE was developed through the simultaneous incorporation of both side chains. The study revealed that incorporating these side chains induced highly interconnected fibrous networks in the blend films of the new compounds. Notably, introducing both ester and alkoxy side chains concurrently improved blend film morphology and crystallinity more substantially than incorporating either chain alone. Furthermore, it generated more regular molecular orientation and stacking, thereby facilitating charge separation and transport, achieving balanced high charge mobility, and ultimately exhibiting excellent photovoltaic characteristics. Therefore, the OSCs utilizing PBDB-T:4T-BOE achieved an outstanding PCE of 9.57%, surpassing that of devices based on 4T-BE (4.84%) and 4T-TO (9.39%).

Aromatic 3-hexylphenyl side chains, highly polar ether linkages, and ester-alkoxy hybrid chains can significantly optimize blend film morphology and enhance charge transport in NFRAs, leading to effectively improved device efficiency. These structural modifications demonstrate critical application value for advancing NFRA materials development.

The number of side chains in NFRAs exhibits a clear structure–property relationship. A dual-side-chain design optimally balances solubility and molecular ordered packing, leading to optimized blend morphology and suppressed charge recombination. In contrast, completely side-chain-free or single-side-chain structures tend to induce excessive aggregation or packing defects, while excessive side-chain incorporation, despite improving donor–acceptor compatibility, may significantly compromise charge transport efficiency due to pronounced steric hindrance effects that disrupt intermolecular π–π stacking.

Wu et al.¹⁰¹ synthesized two NFRAs, BDDEH-4F and BDDBO-4F, characterized by varying side chain lengths. Side chain length minimally affected optoelectronic properties but significantly modified the active layer microstructure. The BDDEH-4F molecule with shorter side chains showed uniform morphology and enhanced charge mobility. Additionally, the devices utilizing BDDEH-4F exhibited robust stability and impressive photovoltaic performance, achieving a high PCE of 12.59%. In parallel, Xie et al.¹⁰² reported two NFRAs (TT-BO and TT-EH) differentiated by their thiophene-bridge side chain lengths (2-butyloctyl for TT-BO vs. 2-ethylhexyl for TT-EH). TT-EH with shorter side chains exhibited enhanced molecular ordering in thin films, yielding higher electron mobility. When blended with PBQx-TF, the TT-EH system demonstrated superior morphological characteristics, including: (i) well-defined fibrous microstructures, (ii) optimized bulk heterojunction (BHJ) phase separation, (iii) extended crystal coherence lengths, and (iv) reduced π–π stacking distances. These superior structural characteristics facilitated charge transfer while reducing recombination in TT-EH blend films, yielding improved J_sc and FF. Consequently, TT-EH-based OSCs achieved a champion PCE of 14.20%, significantly outperforming the TT-BO-based devices (13.10%).

In 2023, Wang et al.¹⁰³ constructed a set of NFRAs, BOR-CnPh (n = 3,4,6), by incorporating phenyl-alkyl side chains (CnPh) to achieve high-performance, low-cost OSCs. Compared to conventional branched alkyl chains, this judiciously designed side-chain architecture markedly improved molecular solid-state packing behavior while reducing fabrication complexity. Crucially, minor variations in the alkyl spacer length (methylene units) profoundly impacted material properties: BOR-C3Ph exhibited strong yet disordered aggregation, BOR-C4Ph achieved highly ordered face-on stacking, whereas BOR-C6Ph suffered from reduced crystallinity and packing order due to excessive chain length. These structural disparities directly modulated donor–acceptor (D/A) interfacial interactions (Fig. 10a), with the phenyl termini of BOR-C4Ph and BOR-C6Ph forming additional π–π coupling with the PBDB-T donor backbone, thereby optimizing phase-separated morphology. In chlorobenzene-processed as-cast devices, the PBDB-T:BOR-C4Ph systems delivered a champion PCE of 13.12%, significantly outperforming BOR-C3Ph (4.16%) and BOR-C6Ph (11.61%). This superiority originated from BOR-C4Ph's well-ordered molecular aggregation, which generated fibrous networks for efficient charge transport. Concurrently, its balanced crystallinity, favorable donor–acceptor compatibility, and moderate D/A interfacial interactions synergistically yielded an ideal bulk heterojunction morphology. In stark contrast, the poor PCE of PBDB-T:BOR-C3Ph stemmed from morphological defects induced by excessive phase separation, inadequate solubility, imbalanced solvent evaporation kinetics, and weak D/A coupling. Moreover, the PBDB-T:BOR-C4Ph devices demonstrated enhanced thermal stability, attributed to its robust heterojunction morphology and quasi-thermodynamic/kinetic equilibrium state under as-cast conditions.

	Fig. 10 (a) Optimized molecular conformations, IGHM isosurfaces, and stacking diagrams of three D/A pairs in as-cast blends. Reproduced with permission.103 Copyright 2023, John Wiley and Sons. (b) Molecular packing modes and 3D stacking network of DPA-n-series acceptors. Reproduced with permission.104 Copyright 2024, John Wiley and Sons. (c) Schematic diagram of the film formation process of the PBQx-TF:TBT-26 blend film during spin-coating. Reproduced with permission.111 Copyright 2024, John Wiley and Sons. (d) The halogen bond interaction, and short contact of cross-stacked molecules associated in the tic-tac-toe 3D networks for L1 and L2. Reproduced with permission.112 Copyright 2022, John Wiley and Sons. (e) Molecular conformations and packing structures of FIOTT-4F, HIOTT-4F, and MIOTT-4F in single crystals. Reproduced with permission.114 Copyright 2024, RSC Publishing.

	Fig. 11 Chemical structures of NFRAs regulated by side-chain position and functionalization.

In 2024, Han et al.¹⁰⁴ constructed a series of NFRAs (DPA-3 to DPA-5) through systematic alkyl side-chain length modulation, where DPA-3 and DPA-5 possessed the shortest and longest symmetric alkyl chains, respectively, while DPA-4 featured an asymmetric side-chain structure with intermediate length. Structural analyses demonstrated that all three molecules formed 3D stacking configurations with parallelogram voids (Fig. 10b). However, DPA-4's asymmetric side chains induced tighter molecular packing and higher crystallinity, establishing more efficient charge transport pathways and significantly enhancing charge mobility. Notably, the DPA-4-based blend film exhibited optimized phase separation and uniform morphology, enhancing exciton dissociation and charge transport while suppressing bimolecular recombination. Furthermore, the D18:DPA-4 systems achieved the most favorable electroluminescence EQE, resulting in the lowest energy loss (E_loss = 0.54 eV). These synergistic advantages yielded a 16.67% PCE in D18:DPA-4 devices, surpassing DPA-3 (14.62%) and DPA-5 (15.12%) counterparts.

Collectively, these studies reveal a pronounced nonlinear relationship between side-chain length and the optoelectronic properties of NFRAs. Specifically, excessively short side chains compromise processability due to inadequate solubility, while overly long chains degrade crystalline order through steric hindrance effects. Notably, intermediate-length side chains may reconcile the competing demands of solubility and molecular ordering, offering a promising avenue for device optimization. Consequently, systematic modulation of side-chain length remains a critical yet underexplored strategy for advancing NFRA performance in future research.

In 2022, Lu et al.¹⁰⁶ designed two NFRAs (LW-out-2F and LW-in-2F) through side-chain positional engineering. LW-out-2F, with externally positioned side chains, adopted a more organized molecular orientation and exhibited a wider absorption spectrum. The LW-out-2F-based blend film also demonstrated enhanced electron transport. Consequently, the LW-out-2F-based devices achieved a PCE of 12.83%, significantly superior to the LW-in-2F devices (PCE = 0.43%). Meanwhile, Li et al.¹⁰⁷ developed two NFRAs (A4T-25 and A4T-26) by strategically positioning sterically demanding triisopropylphenyl side chains. Structural analyses demonstrated that γ,γ′- and β,β′-substitutions effectively stabilized planar molecular conformations, with A4T-26 exhibiting tighter π–π stacking (d = 3.52 Å) and lamellar ordering that enhanced its electron mobility (1.44 × 10⁻⁴ cm² (V⁻¹ s⁻¹)) by one order of magnitude over A4T-25. Despite comparable narrow bandgaps and energy levels, the PBDB-TF: A4T-26 devices achieved superior efficiency (12.10%) to that of A4T-25 systems (7.83%). A4T-26's unique cis-conformation and compact packing enabled two-dimensional charge transport pathways, in sharp contrast to A4T-25's unidirectional transport, a structural difference that directly correlated with their performance divergence.

In 2023, Zheng et al.¹⁰⁸ synthesized two NFRAs, o-AT-2Cl and m-AT-2Cl, differing only in side-chain position. This positional isomerism modulated steric hindrance, resulting in variations in photovoltaic performance. The o-AT-2Cl with the side chain positioned ortho to the phenyl ring exhibited stable molecular conformation and good solubility due to its large steric hindrance. This effectively inhibited excessive aggregation of the thin film, optimized blend morphology, accelerated exciton dissociation/charge transport, resulting in an excellent efficiency of 12.80% for the o-AT-2Cl-based devices. Conversely, the meta-substituted analogue, m-AT-2Cl, displayed inferior performance. Its inter-ring side chain resulted in an unstable molecular geometry and poor solubility, which impaired blend phase separation and exciton dissociation, culminating in reduced FF, J_sc and PCE (7.66%).

In 2024, Huang et al.¹⁰⁹ explored the influence of hexyloxy-substituted phenyl side chain positioning (2,3-, 2,4-, and 2,5-) on NFRA performance, successfully synthesizing three isomeric analogs (o-/m-/p-2T2Se-F). Substitution-site variations substantially altered molecular conformations. Compared to the 2,6-substituted 2T2Se-F reference, these isomers exhibited reduced planarity due to steric hindrance effects, inducing not only blue-shifted absorption spectra but also disrupted molecular packing order. Furthermore, these structural defects exacerbated unfavorable thin-film aggregation behavior, decreasing blend crystallinity with donor polymers and consequently suppressing exciton dissociation and charge transport – ultimately degrading device J_sc and FF. The o-/m-/p-2T2Se-F-based devices consequently achieved markedly lower PCEs (4.43%/1.77%/1.30%, respectively) compared to the 2T2Se-F reference (12.17%). Subsequently, the team¹¹⁰ further developed three NFRAs (5T-C2C6, 5T-2P-1, and 5T-2P-2) by precisely controlling side-chain sterics and positioning. Specifically, 5T-2P-1 was developed by replacing two 2-ethylhexyl chains in 5T-C2C6 with 2,6-di(hexyloxy)phenyl groups, while 5T-2P-2 was obtained by relocating the 2-ethylhexyl chains from inner to outer positions on the flanking biphenyl units of 5T-2P-1. DFT calculations demonstrated that the steric hindrance effect of 2,6-di(hexyloxy)phenyl side chains effectively maintained backbone coplanarity. Although 5T-C2C6 exhibited the narrowest optical bandgap, most red-shifted absorption, and excellent crystallinity/aggregation properties, its twisted conformation resulted in poor compatibility with donor D18, hindering exciton dissociation and charge transport, ultimately yielding only 5.67% PCE. In contrast, the bulky phenyl side chains in 5T-2P-1/2 suppressed excessive molecular aggregation, enabling optimized molecular packing. Notably, 5T-2P-1 showed superior compatibility with D18, forming bicontinuous interpenetrating networks with optimized fibrous phase-separated morphology. These structural characteristics significantly enhanced exciton dissociation and charge transport, with 5T-2P-1 demonstrating superior PCE (12.40%) over its counterpart 5T-2P-2 (10.79%).

Concurrently, Yang et al.¹¹¹ first replaced the para-bis(2-ethylhexyloxy)benzene unit in TBT-2 with its ortho-configurated analog to obtain TBT-9, then systematically substituted the 2-position branched alkoxy chains on thiophene and benzene units with 1-position branched heptan-3-oxy and heptan-4-oxy groups, ultimately yielding target molecule TBT-26. GIWAXS analysis revealed that TBT-26's face-on orientation (large π–π distance: 0.38 nm, small coherence length: 3.2 nm) demonstrated superior charge transport characteristics compared to TBT-9's edge-on orientation. This advantage stemmed from precise crystallinity control enabled by sterically hindered 1-position branched chains, which not only enhanced intramolecular charge transfer (resulting in a 42 nm redshift) but also optimized film-forming kinetics (Fig. 10c) to produce phase-separated morphology favorable for exciton dissociation and charge transport. Consequently, TBT-26-based cells achieved a record PCE of 17.00%. Remarkably, the material exhibited exceptional scalability – large-area (28.8 cm²) blade-coated modules reached 14.30% efficiency while maintaining >80% initial performance after 1000-hour illumination. This work represents a breakthrough in NFRA design, simultaneously advancing efficiency, stability, and processability without increasing synthetic complexity, thereby providing a viable industrial solution for organic photovoltaics.

The substitution position of side chains on molecular backbones systematically governs NFRA optoelectronic properties through steric hindrance effects. Ortho-substitution optimizes the thin-film morphology by stabilizing molecular conformations and suppressing excessive aggregation, whereas peripheral substitution promotes balanced molecular orientation and charge transport. The strategic positioning (e.g., 1-site or 2,6-sites) further enables synergistic improvement of the phase-separated morphology and interfacial interactions via crystallization kinetics control. These performance variations originate from position-dependent modulation of molecular planarity, packing motifs, and donor–acceptor compatibility, ultimately dictating device efficiency metrics.

In 2024, Liu et al.¹¹³ synthesized three NFRAs TTX-4F (X = O/S/Se) by incorporating chalcogen atoms into side chains. The chalcogen atoms not only fine-tuned energy levels but also enhanced intermolecular interactions, endowing TTS-4F and TTSe-4F with superior crystallinity relative to TTO-4F through S⋯S and Se⋯Se interactions. However, excessively strong Se⋯Se interactions in TTSe-4F caused over-aggregation, reducing its miscibility with PM6 and consequently suppressing charge separation. In contrast, TTS-4F's moderate S⋯S interactions promoted the formation of a well-distributed nanofibrillar network in PM6:TTS-4F blends, thereby enhancing charge separation/transport efficiency. This yielded a champion PCE of 14.74% for the TTS-4F-based OSCs, outperforming TTO-4F (8.76%) and TTSe-4F (11.56%). Concurrently, Ye et al.¹¹⁴ developed three novel NFRAs (FIOTT-4F, HIOTT-4F, MIOTT-4F) by introducing F, H, and –CH₃ groups into rigid aromatic side chains. Despite similar optical absorption and energy levels, their crystal structures differed markedly: FIOTT-4F formed herringbone arrangements via F⋯H interchain interactions, creating a 3D charge transport network (Fig. 10e) with orthogonal bidirectional π–π stacking, whereas other acceptors only showed unidirectional π–π stacking. These unique interactions not only suppressed excessive crystallization but also optimized compatibility with donor D18, producing finely interpenetrated D/A phase separation that enhanced charge transport. Consequently, the D18:FIOTT-4F systems exhibited prolonged charge carrier lifetimes and lower recombination probabilities, achieving a record PCE of 17.5%. Moreover, FIOTT-4F-based devices demonstrated superior stability, retaining over 80% of their initial efficiency after 210 h of continuous illumination. This stability is attributable to the electron-withdrawing effect of fluorine, which lowered the HOMO levels (thus enhancing photo-oxidation stability), and to synergistic F⋯H interactions that promoted dense molecular packing to inhibit moisture/oxygen penetration and morphological degradation.

Precise functionalization of side chains (e.g., asymmetric chlorination, fluorination, or sulfur incorporation) serves as a powerful tool to modulate intermolecular interactions, enabling the construction of highly ordered charge transport networks. This strategy significantly enhances carrier mobility while optimizing the blend film morphology, establishing a novel design paradigm for high-efficiency NFRAs.

In 2019, Li et al.¹³¹ successfully constructed two NFRAs (X-PCIC and X1-PCIC) with broad absorption extending to 900 nm by leveraging the quinoidal resonance effect and strong electron-withdrawing characteristics of benzobisthiazole (BBTz). Although both molecules possessed similar skeletal frameworks and energy level distributions, X-PCIC exhibited more pronounced J-aggregation behavior due to stronger intermolecular interactions induced by its fluorinated end groups, compared to the thieno-fused end groups of X1-PCIC. This characteristic directly resulted in a more substantial bathochromic shift in the thin-film absorption spectrum and the formation of an optimized phase-separated morphology. Charge carrier mobility measurements demonstrated that the X-PCIC blend system achieved a balanced charge transport ratio (μ_h/μ_e = 1.43), significantly superior to that of the X1-PCIC system (μ_h/μ_e = 6.06). This disparity primarily originated from enhanced J-aggregation effects, which facilitated more efficient intermolecular electron transport pathways. Furthermore, the stronger J-aggregation endowed X-PCIC with an improved EQE response and lower energy loss (E_loss = 0.53 eV). Ultimately, the X-PCIC-based OSCs attained a PCE of 11.50%, markedly outperforming the X1-PCIC-based devices (10.17%).

In 2021, Wang et al.¹³² reported four simple NFRAs (SCH₃-2F, OCH₃-2F, CH₃-2F, and H-2F) by adjusting the substituents. The results demonstrated that the electrically conductive diphenyl amino group in the main chain of the tetrathiophene molecule enhanced charge transfer and extended the absorption spectrum into the near-infrared region. The conversion from methylthio to methyl obviously increased the efficiency from 6.67% (SCH₃-2F) to 12.28% (CH₃-2F). The CH₃-2F acceptor exhibited outstanding molecular stacking, which was conducive to obtaining superior charge mobility, and thus achieved excellent photovoltaic performance. These findings reaffirmed that subtle modifications in molecular structure can substantially affect the photoelectric characteristics of NFRAs.

In 2024, Khokhlov et al.¹³³ successfully developed two A–D–D1–D–A-type NFRAs (NFA-4 and NFA-5) featuring distinct terminal acceptor units (cyanoindenone and thiopyrimidinedione). NFA-4, incorporating the strongly electron-withdrawing IC-CN unit, exhibited bathochromically shifted absorption and enhanced ICT, while simultaneously modulating the LUMO level to significantly improve the V_oc in D18:NFA-4 devices. Moreover, its larger excited-state dipole moment (13.15 D for NFA-4 vs. 5.97 D for NFA-5) effectively promoted exciton dissociation, intermolecular charge transport, and tighter π–π stacking, consequently enhancing both the J_sc and FF. The more compact molecular packing further optimized electron mobility, ultimately yielding a PCE of 17.07% for D18:NFA-4 devices, markedly superior to the D18:NFA-5 systems (11.27%). When NFA-5 was incorporated into the D18:NFA-4 systems, the ternary devices achieved a further improved PCE of 18.05%, attributable to: (i) an expanded D/A interfacial area that facilitated both efficient exciton dissociation and FRET from NFA-5 to NFA-4, creating synergy between exciton energy transfer and D18/NFA-4 interfacial charge separation; (ii) optimized molecular packing and balanced charge transport that collectively suppressed bimolecular and trap-assisted recombination; and (iii) low exciton binding energy driven by high excited-state dipole moments that overcame HOMO offset limitations, synergistically enhancing J_sc, FF, and PCE.

The regulatory mechanisms of terminal acceptor units and substituents in NFRAs involve three synergistic aspects: (1) strong electron-withdrawing effects from substituents (e.g., IC-CN, fluorinated end groups) modulate molecular energy levels and absorption spectra; (2) induced intermolecular interactions (e.g., J-aggregation) control molecular packing configurations, altering π–π stacking order and phase-separation morphology to optimize charge transport; and (3) molecular-level synergistic effects further enhance exciton dissociation while suppressing charge recombination. These fundamental insights establish a comprehensive framework for understanding NFRA structure–property relationships.

In 2021, Zhao et al.¹³⁵ designed two NFAs, DTP-out-F and DTP-in-F, using a dual strategy incorporating noncovalent conformational locking and regioisomeric design. The C–H⋯F noncovalent interactions effectively stabilized planar molecular conformations, while the asymmetric connection sites (2- vs. 7-position) of the DTP units significantly modulated molecular packing behavior. DTP-in-F, featuring externally shifted side chains, not only improved solubility through enhanced steric hindrance but also formed favorable face-on stacking with polymer-like charge transport channels. This optimized face-to-face interfacial alignment with the PM6 donor substantially enhanced charge carrier mobility and promoted charge separation/transport. In contrast, DTP-out-F showed less desirable edge-on orientation and phase-separated morphology. These distinct packing and interfacial characteristics resulted in a markedly higher PCE of 10.66% for DTP-in-F-based OSCs compared to DTP-out-F devices (3.97%). In the same year, Zhang et al.¹³⁶ synthesized a series of isomeric NFAs (NOF-X, X = 1,2,3) by varying the naphthalene core linkage positions. The balance between intramolecular O⋯S interactions and steric hindrance critically influenced molecular conformation and subsequent packing behavior. Among these, NOF-1 adopted a twisted configuration arising from pronounced steric repulsion between the α-hydrogens of naphthalene units and CPT groups, resulting in poor crystallinity. In contrast, other acceptors (particularly NOF-3) demonstrated optimized face-on orientation and enhanced crystalline order owing to reduced dihedral angles. Furthermore, NOF-3 formed well-ordered packing structures with PBDB-T in blend films, facilitating efficient charge transport and achieving higher, more balanced charge carrier mobility. Consequently, the NOF-3-based devices delivered the highest PCE of 11.58%, outperforming both the NOF-2 (10.67%) and NOF-1 (6.01%) systems. Concurrently, Bi et al.¹³⁷ constructed three high-performance unfused-ring wide-bandgap acceptors (GS-ISO, GS-OC6, and GS-OEH) through synergistic modulation of side chains and intramolecular noncovalent interactions. Among these, GS-ISO achieved a highly stable planar molecular configuration via cooperative effects between steric hindrance and intramolecular S⋯N noncovalent interactions, resulting in superior charge transport properties, optimized molecular packing behavior, and reduced energetic disorder. Accordingly, GS-ISO-based OSCs delivered a remarkable PCE of 11.62%, significantly outperforming both GS-OEH (8.44%) and GS-OC6 (8.69%) systems. Notably, tandem organic photovoltaic devices employing PBDB-TF:BTP-eC9 and PBDB-TF:GS-ISO as active layers achieved a remarkable efficiency of 19.10%.

In 2022, Wang et al.¹³⁸ employed side-chain engineering combined with non-covalent conformation-locking strategies to develop two non-fused ring acceptors, DTh-OC8-2F and BTh-OC8-2F. The presence of non-covalent S⋯O interactions enabled these molecules to retain planar conformations. Although the BTh-OC8-2F acceptor containing trans-side chains exhibited stronger crystallinity than DTh-OC8-2F with cis-side chains, the latter's blend film demonstrated remarkable phase separation morphology, promoting exciton dissociation, suppressing charge recombination, and facilitating efficient charge transport. Additionally, the devices based on this acceptor had the lowest energy loss (E_loss = 0.61 eV). Hence, the DTh-OC8-2F-based devices achieved an outstanding efficiency of 14.13%, exceeding the performance of BTh-OC8-2F (PCE = 11.95%).

In 2024, Wang et al.¹³⁹ synthesized NFRAs with different diphenylamine side chains (tBu-2F, C4-2F, and Me-2F), which achieved coplanar molecular skeletons through the interactions of S⋯O and S⋯N. Precise side-chain engineering effectively modulated both the blend film morphology and optical properties of the devices. The incorporation of n-butyl side chains in C4-2F simultaneously improved solubility and crystallinity, facilitating the formation of an optimal blend film morphology. Moreover, the devices based on this molecule also demonstrated improved exciton dissociation, reduced charge recombination, and lower energy loss, resulting in enhanced charge transfer characteristics. Ultimately, the OSCs based on PM6:C4-2F demonstrated a maximum efficiency (11.66%).

The noncovalent interactions ensure planarity and electron delocalization of the NFRA backbone, effectively stabilizing the molecular conformation, while side-chain modifications optimize processing properties and packing behavior by regulating solubility and intermolecular interaction distances. Their synergistic effect induces the formation of a relatively more favorable molecular orientation and optimized phase-separated morphology, thereby efficiently modulating the charge transport process in the material.

In 2021, Zhang et al.¹⁴¹ synthesized SiOC2C6-4F and SiOC2C6-4Cl, two non-fused ring acceptors, by using non-covalent conformation locking and halogen substitution synergistic strategies based on the electron-donating dithiolene silicon units (DTS). The intramolecular S⋯O noncovalent interactions effectively enforced backbone coplanarity in these molecular systems while simultaneously extending molecular conjugation and enhancing π–π interactions. Compared to its fluorinated analogue SiOC2C6-4F, the chlorinated acceptor SiOC2C6-4Cl demonstrated broader absorption characteristics and superior π–π stacking order in blend films, leading to significantly improved charge transport properties. Consequently, OSCs based on SiOC2C6-4Cl (PCE = 11.29%, J_sc = 20.35 mA cm⁻²) showed higher PCE and J_sc than SiOC2C6-4F (PCE = 9.68%, J_sc = 17.02 mA cm⁻²), which was the best performance of organic photovoltaic devices using silica-based small molecule acceptors to date. Then, Huang et al.¹⁴² employed the same strategy to develop QOC6-4Cl and QOC6-4H, two new quinoxaline-based non-fused ring non-fullerene acceptors. Multiple non-covalent interactions (N⋯H and S⋯F) ensured excellent molecular planarity in both acceptors. Notably, the terminal chlorine substituents in QOC6-4Cl induced stronger intermolecular interactions, resulting in enhanced light absorption and improved charge transport properties. In addition, the OSCs incorporating QOC6-4Cl exhibited outstanding morphological characteristics. Hence, QOC6-4Cl-based photovoltaic devices had high J_sc (22.91 mA cm⁻²) and FF (69.01%), and its PCE reached 12.32%. However, due to its severe trap-assisted recombination and imbalanced charge mobility, the PCE based on the QOC6-4H devices was lower than 8.00%.

During the same period, Li et al.¹⁴³ successfully synthesized three novel NFRAs (PhO4T-X, X = 1–3) using this strategy. The S⋯O NoCLs not only stabilized the molecular conformations but also enhanced the intermolecular π–π interactions and crystallinity. Concurrently, substituent engineering precisely modulated the electronic structure, molecular packing, and thin-film morphology. Systematic enhancement of terminal electron-withdrawing strength (PhO4T-1 → PhO4T-3) induced progressive bathochromic shifts in solution absorption (709 → 746 nm) with concomitant increases in molar absorptivity. Solid-state films exhibited additional 40–60 nm redshifts, confirming the formation of π–π stacking. Among them, PhO4T-3 with the strongest electron-withdrawing end-group demonstrated optimal performance: the smallest Stokes shift, balanced charge carrier mobility (μ_h/μ_e = 0.997), 96.5% exciton dissociation efficiency, highly ordered packing, and the lowest energetic disorder. These superior characteristics ultimately yielded an efficiency of 13.76% in the corresponding devices, significantly surpassing PhO4T-2 (11.91%) and PhO4T-1 (9.69%). Subsequently, the team employed a two-step synthesis method to prepare a series of S⋯O NoCL-containing BN-nF (n = 0, 2, 4) NFRAs,¹⁴⁴ achieving precise performance optimization through controlled multi-fluorination of π-conjugated end groups. The study demonstrated that moderate fluorination in BN-2F synergistically enhanced both absorption redshift and electron mobility, increasing J_sc from 21.91 mA cm⁻² (BN-0F) to 25.25 mA cm⁻² while maintaining balanced charge transport through optimized crystallinity, which yielded a FF of 70.78%. Consequently, the J52:BN-2F-based devices achieved a record efficiency of 14.53% (certified 13.8%). However, excessive fluorination (BN-4F) further increased J_sc to 25.76 mA cm⁻² but resulted in reduced PCE (13.24%) due to enhanced non-radiative recombination and imbalanced mobility. These results validated the effectiveness of moderate fluorination in balancing key device parameters.

In 2022, Gu et al.¹⁴⁵ successfully synthesized two innovative polymeric acceptors (PBTzO and PBTzO-2F) through an NFRA copolymerization strategy combining NoCLs and end-group fluorination. The NoCLs effectively enhanced backbone coplanarity, promoting ICT effects and π–π stacking, thereby improving light-harvesting capability, while synergistic fluorination further amplified these optoelectronic properties in PBTzO-2F. This molecular design enabled optimized energy-level alignment and well-ordered fibrillar phase-separated morphology in PBDB-T:PBTzO-2F-based devices, facilitating efficient charge separation/transport and reduced energy loss. Consequently, a PCE of 11.04% was achieved – nearly an order of magnitude higher than that of PBTzO-based devices (1.08%). Notably, PBTzO-2F exhibited 40% lower SC and 25% higher FOM (Fig. 16a), demonstrating exceptional commercialization potential.

	Fig. 16 (a) SC/FOM-cost trade-off in representative FRAs and PBTzO-2F.145 (b) Molecular stacking of PTB4F and PTB4Cl. Reproduced with permission.150 Copyright 2021, John Wiley and Sons. (c) Illustration of intermolecular interactions and 3D molecular packing in 2BTh-2F. Reproduced with permission.151 Copyright 2021, John Wiley and Sons. (d) The mechanism diagram of binary solvent-regulated crystallinity and phase separation in blend films. Reproduced with permission.42 Copyright 2024, Springer Nature. (e) Stability test of encapsulated NFRA-based devices. Reproduced with permission.153 Copyright 2022, American Chemical Society. (f) Visual stacking overlap of A–A Stacking. Reproduced with permission.158 Copyright 2025, John Wiley and Sons.

In 2023, He et al.¹⁴⁶ introduced a novel series of NFRAs, known as FQxOC8-X (where X = H, F, Cl), based on quinoxaline units. These acceptors maintained good planarity, facilitated by non-covalent interactions such as S⋯N and H⋯F. The compounds FQxOC8-F and FQxOC8-Cl, featuring fluorine and chlorine substitutions, respectively, exhibited broader absorption spectra compared to FQxOC8-H. Particularly, FQxOC8-Cl displayed the most pronounced bathochromic shift in absorption and excelled in various performance metrics, including morphology, charge generation and collection, reduction of charge recombination, and charge migration within the blended film. Consequently, the FQxOC8-Cl-based devices achieved an outstanding PCE of 10.52%.

In 2024, Ma et al.¹⁴⁷ reported two NFRAs, PTBFTT-F and PTBFTT-Cl, which maintained good coplanarity attributed to several types of intramolecular non-covalent interactions (S⋯O, O⋯H, and F⋯S) and exhibited excellent intramolecular charge transfer properties. The larger atomic radius of chlorine (compared to fluorine) endowed PTBFTT-Cl with an extended π-conjugated surface area, facilitating both well-ordered molecular stacking and ideal photochemical stability. Therefore, the OSCs based on PTBFTT-Cl obtained a better EQE response, achieving a notable PCE of 10.26%.

The combined strategy of noncovalent conformational locking and substituent engineering achieves synergistic optimization of molecular planarity maintenance and electronic structure modulation. Intramolecular noncovalent interactions effectively stabilize the planar configurations of conjugated backbones, significantly enhancing the ICT effect while promoting intermolecular π–π interactions, thereby facilitating favorable charge transport pathways. Building upon this foundation, substituent engineering precisely tailors molecular energy levels and absorption spectral characteristics through the introduction of specific functional groups (e.g., halogens), optimizing the optoelectronic response properties of the materials. The cooperative effect of these two approaches improves the ordered stacking behavior and thin-film morphology of NFRAs, ultimately enhancing device performance.

	Fig. 17 Chemical structures of NFRAs regulated by other combinatorial strategies.

In 2020, Wang et al.¹⁴⁸ developed three NFRAs (BTOR-X, BT2F-X, and BT-X, X = IC4F) through integrated molecular engineering incorporating noncovalent conformational locking, substituent engineering, and side-chain modification. These molecules adopted stable ladder-type planar configurations stabilized by intramolecular S⋯N, S⋯O, and F⋯S noncovalent interactions. Among them, the alkoxy chain-modified BTOR-IC4F not only exhibited significantly enhanced solubility but also achieved a superior PCE of 11.48% with optimized energy level alignment and morphology, along with low energy loss (0.57 eV) and high V_oc (0.80 V), surpassing the unmodified BT-IC4F systems (9.83%). In contrast, the fluorinated BT2F-IC4F showed the lowest device efficiency (PCE = 8.45%) due to excessive phase separation and poor solubility resulting from its strong crystallinity. This study successfully validated the potential of multi-strategy approaches for synergistically optimizing molecular planarity, solubility, and optoelectronic properties, providing new pathways for developing high-performance NFRAs.

In 2021, Ye et al.¹⁴⁹ reported four simple non-fullerene acceptors (QCICX, X = 1–4) with identical main chains but differing alkyl side chains and halogen atoms, all of which had non-fused ring structures. The N⋯S non-covalent interactions within these acceptors ensured molecular planarity. Compared with fluorinated acceptors, chlorinated acceptors exhibited stronger π–π interaction, deeper LUMO energy levels and wider absorption wavelengths. Meanwhile, longer alkyl side chains were conducive to improving molecular solubility, resulting in favorable blend film morphology and stacking patterns. These properties promoted effective exciton dissociation and smooth charge transport. Therefore, the QCIC3 acceptor with chlorinated end groups and extended side chains exhibited excellent photovoltaic characteristics, achieving a peak PCE of 10.55% in the corresponding devices.

In parallel, Wen et al.¹⁵⁰ successfully synthesized two NFRAs (PTB4Cl and PTB4F) through simultaneous side-chain and end-group modulation. The synergistic effect between two-dimensional phenyl side chains and halogenated end groups enables precise control of molecular orientation and solid-state packing behavior. Chlorinated PTB4Cl formed an ordered A–A/A–D stacking structure, exhibiting stronger intermolecular interactions and tighter molecular packing compared to its fluorinated counterpart PTB4F (Fig. 16b). This optimized molecular organization endowed PTB4Cl with superior exciton diffusion and charge transport properties in bulk heterojunction blends. Consequently, the single-junction devices employing PTB4Cl as the active layer yielded a PCE of 12.76%, representing a 124% enhancement over conventional PTIC-based devices. This work elucidated the regulatory mechanisms of 2D side chains and halogenated end-groups in controlling the multiscale aggregation of NFRAs, providing important design guidelines for high-performance, cost-effective organic materials.

Also in 2021, Wang et al.¹⁵¹ successfully developed three NFRAs (2Th-2F, 2BTh-2F, and BTh-Th-2F) by precisely modulating π-conjugation length and intramolecular interactions. 2BTh-2F, which has the longest π-conjugated system, benefited from intramolecular S⋯N/S⋯O interactions, resulting in a highly planar molecular conformation with tight π–π stacking (3.39 Å), excellent molar extinction coefficients, and a unique three-dimensional network packing structure (Fig. 16c). In contrast, 2Th-2F adopted a twisted configuration with two-dimensional molecular packing. This three-dimensional network structure not only effectively promoted electron delocalization but also established multidimensional charge transport channels. Combined with the favorable fibrillar phase-separated morphology in the blend film, the PBDB-TF:2BTh-2F devices achieved an outstanding PCE of 14.53%, significantly surpassing the 2Th-2F (11.00%) and BTh-Th-2F (12.87%) systems. Further optimization of alkyl side chains (ethyl substitution for n-butyl) yielded 2BTh2-2F(C2), which achieved a breakthrough efficiency of 15.44% when combined with the D18 donor in an o-xylene solvent system, setting a new performance record for non-fused-ring acceptor OSCs at that time. Subsequently, Zeng et al.⁴² addressed critical issues of low crystallinity and difficult phase separation control in the 2BTh2-2F(C2) acceptor (denoted as 2BTh-2F-C₂ in literature) by developing an innovative binary solvent co-regulation strategy (Fig. 16d). By precisely designing a chloroform (CF)/o-xylene (OXY) mixed solvent system that leveraged their differences in volatility (boiling points of 61.2 °C and 144 °C, respectively) and solubility (CF being a good solvent for D18 while OXY being a poor solvent), they achieved spatiotemporal control of donor–acceptor phase separation: CF preferentially evaporated to induce fibrous pre-assembly of D18, followed by OXY evaporation driving the formation of high-purity phase domains of 2BTh-2F-C₂, supplemented by 1,4-diiodobenzene (DIB) additive to enhance crystallization. This strategy constructed an ideal bicontinuous interpenetrating network, enabling D18:2BTh-2F-C₂ devices to achieve high efficiencies of 19.02% (0.052 cm²) and 17.28% (1 cm²) while maintaining 92.3% efficiency retention. This breakthrough maintained the cost advantages of NFRAs while resolving their poor crystallinity bottleneck, providing an important technological pathway for OSC industrialization.

In 2022, Zhang et al.¹⁵² synthesized three non-fused ring acceptors isopropyl-2F, isopropyl-0F, and hexyl-0F through synergistic regulation of end groups and side chains. The hexyl-0F molecule, containing two 4-hexylphenyl side chains, could not be used for OSC fabrication due to its extremely low solubility. The other two acceptors containing vertical side chains exhibited good solubility, especially the isopropyl-2F acceptor with fluorine atoms at the end, which showed the narrowest energy gap and broadest absorption spectrum. In addition, the isopropyl-2F-based blend films demonstrated stronger crystallinity and higher charge mobility, resulting in a higher PCE (11.16%) for the PBDB-T:Isopropyl-2F devices compared to the fluorine-free acceptor (PCE = 9.49%). When paired with D18, the device efficiency was further enhanced to 12.55%. Meanwhile, Luo et al.¹⁵³ constructed six non-fused ring acceptors (C8-X, C6C4-X, C8C8-X, X = 4F, 4Cl) by fine-tuning end groups and alkyl chains. Among them, chlorinated molecules exhibited wider light absorption than fluorinated molecules. Adding 1-octylnonyl side chains with appropriate steric hindrance allowed C8C8-X molecules to achieve proper crystallization and molecular aggregation with compact π–π stacking, resulting in excellent blend film morphology. Consequently, C8C8-4Cl, co-optimized by the chlorinated terminal groups and 1-octylnonyl side chains, demonstrated the best photoelectric properties. When PM6 was taken as the donor, C8C8-4Cl had good miscibility with the donor material. Accordingly, devices employing this donor/acceptor combination showed excellent charge transfer ability and suppressed recombination, along with balanced electron/hole mobilities and superior stability (Fig. 16e). These characteristics contributed to a high J_sc (23.80 mA cm⁻²) and FF (72.50%), ultimately yielding a record PCE (14.11%).

In 2023, Huang et al.¹⁵⁴ designed three star-shaped non-fused ring acceptors, H1–H3, through substituent engineering. These acceptors were incorporated as ternary components to the PM6:Y6 binary blends. Regarding optoelectronic characteristics and structural properties, H1–H3 exhibited complementary interactions with both PM6 and Y6, leading to an enhanced PCE in the ternary BHJ. Notably, the non-fluorinated acceptor H1 demonstrated reduced crystallinity and molecular packing, which positively influenced V_oc and mitigated excessive crystallization of Y6. These characteristics facilitated exciton dissociation and charge collection, improving compatibility and optimizing phase separation in the PM6:Y6:H1 ternary systems, ultimately achieving a remarkable PCE of 16.57%. Meanwhile, Zhou et al.¹⁵⁵ incorporated DFTQA-2FIC, a newly synthesized medium-bandgap acceptor, into the PM6:Y6 blends. The resultant ternary blends showed favorable molecular packing, efficient charge transport, and excellent aggregation morphology, while effectively mitigating charge recombination. Consequently, the ternary OSCs reached an ultra-high PCE of 17.29%. Subsequently, Tan et al.¹⁵⁶ investigated two newly synthesized NFRAs, IOEH-4F and IOEH-N2F, featuring distinct terminal acceptor units, by doping them into PM6:Y6 blend films. The resulting ternary devices exhibited enhanced charge transfer, improved morphological stability, and reduced energy losses compared to their binary counterparts. Ultimately, these devices achieved remarkable efficiencies of 17.80% and 18.13%, respectively, further advancing the research on non-fused ring acceptors.

In 2025, He et al.¹⁵⁷ developed a new NFRA with an A–D–π–A–π–D–A architecture, denoted as TBTBIC-F, by combining ring-locking and dimerization strategies. The synergistic effect of the pyran ring-locked π-bridge and dimerized architecture endowed TBTBIC-F with exceptional tolerance to intense light irradiation and nucleophilic reagents, while enabling stable blend morphology formation with PM6. This combination maintained excellent device stability under photothermal conditions even without encapsulation. Furthermore, TBTBIC-F demonstrated complementary absorption with PM6, effectively extending the light-harvesting range. Coupled with its high V_oc of 0.96 V, these characteristics collectively enabled the TBTBIC-F-based devices to achieve a PCE of 10.01%.

Concurrently, Zhang et al.¹⁵⁸ systematically designed three NFRAs (2T-T-EH, 2T-T-2EH, and 2T-TT-2EH) through synergistic modulation of π-bridges and side chains. The investigation revealed that incorporating inwardly oriented alkyl chains increase steric hindrance, inducing slight backbone distortion in both 2T-T-2EH and 2T-TT-2EH. This structural modification elevated their LUMO energy levels while reducing energy loss, leading to higher V_oc values. Single-crystal analysis demonstrated that the π-extended bridge design in 2T-TT-2EH not only alleviates steric conflicts between bilateral side chains but also facilitates superior linear molecular conformation and tighter intermolecular packing while improving end-group overlap effects (Fig. 16f). More importantly, the JD40:2T-TT-2EH blend films formed a dense fibrous network structure that enhanced phase separation and exciton dissociation. These combined characteristics optimized charge transport and mobility in 2T-TT-2EH-based devices, resulting in significantly enhanced J_sc and ultimately achieving a PCE of 14.17%, which showed clear advantages over 2T-T-EH- (13.44%) and 2T-T-2EH-based devices (11.95%). Notably, when introduced as a ternary component into the D18:BTP-eC9-4F systems, 2T-TT-2EH further boosted the ternary device's PCE to 19.07%, benefiting from its elevated LUMO level and improved V_oc. This study confirmed that cooperative regulation of π-bridges and side chains enables precise control over molecular backbone distortion and end-group stacking behavior, thereby effectively balancing the V_oc–J_sc trade-off in NFRAs.

These combinatorial strategies integrate the unique functional advantages of individual approaches—including stabilized molecular planar configurations, modulated energy levels and absorption characteristics, improved solubility, and optimized π–π stacking distances—while effectively overcoming the inherent limitations of single-strategy approaches (e.g., steric hindrance effects and weak crystallinity) through synergistic effects. The cooperative interactions further enable precise control of molecular orientation, solid-state packing patterns, and phase-separated morphology in NFRAs, thereby enhancing exciton dissociation efficiency and charge transport properties. Simultaneously, these combinatorial strategies can effectively reduce energy loss while improving device stability. It is noteworthy that advanced processing techniques (e.g., ternary and solvent engineering) can surpass the limitations of traditional single-component materials in key parameters like photoelectric conversion efficiency, offering innovative pathways for organic photovoltaic development.