Non-fused ring acceptors for organic solar cells

Organic solar cells (OSCs) have experienced rapid development and achieved significant breakthroughs in power conversion efficiencies owing to the emergence of non-fullerene acceptors (NFAs) with ladder-type multiple fused ring structures. However, the high synthetic complexity and production cost of multiple fused ring NFAs hinder the commercial prospects of OSCs. In this context, the development of non-fused ring acceptors (NFRAs) with simple structures and facile synthesis has been proposed. In this mini review, we summarize the important progress in this field spanning from molecular design strategies to structure-performance relationships. Ultimately, with the aim of realizing the practical application of NFRAs in OSCs, we discuss the current challenges and future directions in terms of achieving high performance and low synthetic complexity simultaneously. These discussions provide valuable insights into the development of new NFRAs.


INTRODUCTION
Organic solar cells (OSCs) have attracted considerable attention in recent years due to their advantages that include solution processability, lightweight, flexibility and semitransparency [1][2][3][4][5][6] . Generally, OSCs consist of a bulk heterojunction active layer, which employs a p-type polymer as the electron donor and an n-type small molecule as the electron acceptor [7] . Over a long period, the electron acceptors become dominated by fullerene derivatives [8,9] . However, the maximum power conversion efficiency (PCE) of fullerene-based OSCs is limited to ~12% [10] due to the inherent defects associated with fullerene derivatives, such as a narrow light-absorbing range, low absorption coefficient and nonadjustable energy levels [8,9] .
As shown in Figure 1A, high-performance NFAs, such as ITIC and Y6, consist of a multiple fused ring framework in their central core. Therefore, they are usually referred to as fused ring acceptors (FRAs). The central fused-ring core in FRAs is multifunctional [12,17,[35][36][37][38][39] . First, it serves as an electron-donating donor unit to generate the intramolecular charge transfer (ICT) effect and thus extends the absorption range of the resulting FRAs to the near-infrared (NIR) waveband. Second, its coplanar backbone can facilitate the delocalization of π-electrons, enhance intermolecular π-π interactions and charge transport. Third, it offers positions for introducing suitable side chains, which provide appropriate solubility and tune the packing structures in the solid state. However, FRAs usually suffer from high synthetic complexity and production costs, thus hampering their large-scale synthesis and limiting the commercialization of OSCs [40][41][42][43][44] .
To overcome the drawbacks associated with FRAs, the strategy of using non-fused rings in place of the central fused rings of FRAs was proposed [45] . The acceptors generated with this strategy are usually known as non-fused ring acceptors (NFRAs), which possess the advantages of simple structures and facile synthesis. As shown in Figure 1B, the most important structural characteristic of NFRAs is that each unit of the central core is connected by a single bond. The synthetic complexity and production costs of this kind of NFAs can be greatly reduced due to the fact that the single-bond connected structures are much easier to build than fused-ring structures [46,47] . The absorption spectra, energy levels and molecular packing of NFRAs could be also finely tuned by choosing appropriate building blocks [48][49][50] . Compared with FRAs, NFRAs have more conformational isomers since the single bonds for connecting adjacent units in NFRAs are rotatable, which is harmful to the ordered molecular packing of NFRAs in the solid state [51] . This leads to enhanced reorganization energy and hampers charge transport [17,[52][53][54] . To solve this problem, noncovalent throughspace intramolecular interactions, including O···S, N···S, H···S and F···S, were explored to lock the conformation of NFRAs. With these interactions, the rotation of single bonds could be effectively restricted and thus rigid and coplanar structures can also be formed in NFRAs [55][56][57][58] . As a result, ordered molecular packing in the solid state and efficient electron transport can be realized in NFRAs. So far, the PCEs afforded by NFRAs have exceeded 15% [59] , which are fully comparable to those achieved by A-D-A-type FRAs and are approaching those offered by Y6-derived acceptors.
In this mini review, we summarize the progress of NFRAs in recent years. According to the electronic structure of the building blocks, NFRAs can be classified as A-D-D'-D-A or A-D-A'-D-A-type structures, where D and A denote electron-donating and electron-accepting units, respectively. Moreover, fully nonfused ring structures have also been developed recently. For each kind of NFRA, the relationships between chemical structure and device performance are discussed. Finally, we discuss the challenges and future directions of NFRAs with the aim of achieving high device performance and low production costs simultaneously.

NFRAS WITH A-D-D'-D-A STRUCTURE
The chemical structures of representative NFRAs with the A-D-D'-D-A framework are shown in Figure 2. The first NFRA (DF-PCIC) for OSCs was reported by Li et al. [45] and possesses an A-D-D'-D-A structure [ Figure 3A]. The central non-fused ring core of DF-PCIC is 2,5-difluorobenzene (DFB, D'), which is connected to two cyclopentadithiophene (CPDT, D) units by single bonds. The coplanar conformation of DF-PCIC was locked by the F···H noncovalent interaction between the fluorine atoms on DFB and the hydrogen atoms on adjacent CPDT units. Meanwhile, the bulky substituents attached on the sp 3 carbon atoms of the CPDT unit can effectively suppress excessive intermolecular aggregation, thus resulting in appropriate phase separation in bulk heterojunction blends. DF-PCIC shows a similar light absorption range [ Figure 3B] and energy levels as ITIC but also has obvious advantages in synthesis simplicity. After device optimization based on DF-PCIC:PBDB-T, a remarkable PCE of 10.14%, with a short-circuit current density (J SC ) of 15.66 mA cm -2 , an open-circuit voltage (V OC ) of 0.91 V and a fill factor (FF) of 72.00% was obtained. Moreover, the relevant OSCs exhibited excellent morphological and device stability. Notably, the J SC of the solar cell based on DF-PCIC:PBDB-T was only 15.66 mA cm -2 , which is much lower than the values obtained by representative FRAs. Generally, the optimal OSCs based on IT-4F and Y6 can afford J SC values of ~21 and ~26 mA cm -2 , respectively [20] . To overcome the low J SC of DF-PCIC, a new NFRA (HF-PCIC) with stronger electron-withdrawing end groups was developed by the same group. This new molecule exhibited an obvious bathochromic absorption spectrum owing to the enhanced ICT effect [60] . As a result, a higher J SC of 17.81 mA cm -2 and a PCE of 11.49% were realized for OSCs based on HF-PCIC. Furthermore, 5,5-bis(4-hexylphenyl)-5Hdithieno[3,2-b:2',3'-d]pyran (DTP), which features a stronger electron-donating ability than CPDT was also used to construct NFRAs with the aim of extending the light absorption range [61] . Interestingly, the two isomers DTP-out-F and DTP-in-F exhibited similar spectra and energy levels but entirely different stacking orientations in the solid state. Due to the formation of the favorable face-on orientation in DTP-in-F-based donor:acceptor blend films observed from grazing incidence wide-angle X-ray scattering (GIWAXS) patterns [ Figure 3C], DTP-in-F offered a higher PCE (10.66%) than DTP-out-F (3.97%) in OSCs.  [45,62] , the Nature Publishing Group [48] and the American Chemical Society [61] , respectively.
Although favorable coplanar conformations of DF-PCIC and HF-PCIC were obtained by the F···H noncovalent interaction, the F atoms often reduce the electron-donating ability of the central phenylene unit, thereby resulting in a narrower absorption spectra. Alternatively, Huang et al. [48] developed a set of NFRAs (DOC6-IC, DOC8-IC, DOC2C6-IC, and DOC2C6-2F) by employing the O···S noncovalent interaction between the central 2,5-bis(alkyloxy)phenylene unit and two flanking CPDT units. These NFRAs all exhibited a favorable coplanar conformation and wide absorption range with an onset approaching 850 nm in films. Furthermore, the solubility of the NFRAs and the morphology of the blend films could be finely tuned by changing the lateral substituents on the central phenylene unit. Among these NFRAs, DOC2C6-2F offered the highest PCE of 13.24% associated with a J SC of 21.35 mA cm -2 and a very low non-radiative recombination voltage loss of only 0.27 V. According to density functional theory (DFT) calculations [ Figure 3D], the reference molecule DC6-IC without the O···S noncovalent interaction exhibited twisted molecular backbone and thus suffered from imbalanced charge transport and a much poorer PCE (6.87%). Based on the excellent performance of DOC2C6-2F, Zhang et al. [62] further broadened the absorption range of DOC2C6-2F by introducing π-extended end groups to increase J SC , wherein the degree of multi-fluorination was also finely tuned.
In 2020, Pang et al. [50] reported an A-D-A'-D-A-type NFRA with BT as an A' core (BTCIC-4Cl). The absorption of BTCIC-4Cl was extended to the NIR waveband with an onset of 946 nm in the solid state, corresponding to an optical band gap (E g opt ) of 1.31 eV in terms of the quinoidal character of the BT unit and enhanced ICT effect [ Figure 5A]. However, as shown in Figure 5B, the introduction of the BT unit significantly down-shifted the lowest unoccupied molecular orbital (LUMO) energy level of BTCIC-4Cl, which deteriorated the V OC . After device optimization, a moderate photovoltaic performance of BTCIC-4Cl was achieved, with a PCE of 10.50%, a V OC of 0.75 V and a J SC of 21.00 mA cm -2 . Based on the abovementioned results, Wang et al. [58] then systematically investigated the effects of H (BT-IC4F), F (BT2F-IC4F) and alkoxy chains (BTOR-IC4F) at the 5,6-positions of the BT core on the molecular configuration and device performance. The ladder-like molecular structures locked by the N···S, F···S and O···S noncovalent interactions were formed for these acceptors, as suggested by DFT calculations. Compared with BT-IC4F and BT2F-IC4F, the higher molar extinction coefficient, better solubility and elevated LUMO energy level of BTOR-IC4F have endowed more superior performance of OSCs. Consequently, the highest PCE of 11.48% was achieved for PBDB-T:BTOR-IC4F-based devices, with a J SC of 20.57 mA cm -2 and a V OC of 0.80 V.
In contrast, BTz with a weaker electron-withdrawing ability is conducive to further increasing the V OC [77] . The first NFRA (NTTI) bearing fluorinated BTz as the A' core was reported by Lv et al. [77] . However, due to the insertion of F atoms, a strong electron-withdrawing element, a similar V OC compared to BTOR-IC4F was presented [77] . Alternatively, with the aim of enhancing the LUMO energy level of NFRAs, a new molecule (BTzO-4F) was reported by Liu et al. [68] , which adopted the alkoxy-substituted BTz unit as the A' core. After device optimization, an outstanding PCE of 13.80% with a high V OC of 0.84 V was achieved for the photovoltaic device based on PBDB-T:BTzO-4F, which demonstrates the excellent potential of NFRAs for high-performance OSCs.  [50] and John Wiley and Sons [69] , respectively.
Later, terminal side chains, referred to the introduction of additional side chains attached to the terminal βposition of fused-ring cores, were adopted in NFRAs to lock the conformation of end groups, which have been widely applied in high-performance NFAs, especially in Y6 derivatives [20,56,69,78,79] . Compared with the counterpart BTzO-4F (herein renamed as NoCA-1), the terminal side chains on NoCA-5 played a significant role in shifting up the LUMO energy level, promoting crystallinity, improving molecular rigidity and reducing the reorganization energy. In particular, the Urbach energy (E u ) is decreased in the blend of NoCA-5:J52 [ Figure 5C], suggesting that the energetic disorder has been effectively reduced in this blend. Moreover, favorable molecular stacking has been formed in the NoCA-5:J52 blend [ Figure 5D]. As a result, NoCA-5:J52-based OSCs delivered a remarkable PCE of 14.82%, whereas the NoCA-1:J52-based devices only gave a PCE of 11.71%.
The BDD unit, possessing a weak electron-withdrawing character, has been widely applied to construct high-performance polymer donors, such as PBDB-T [80] and PM6 [81] . Recently, a set of NFRAs based on the BDD unit as the central A' core has also been reported. In 2021, two simple electron acceptors, BDDEH-4F and BDDBO-4F, featuring separated BDD units with different side chains as the A' core, were reported by Wu et al. [82] . These molecules were synthesized by a three-step synthesis via direct heteroarylation without any hazardous organostannanes or ligands involved. The devices based on BDDEH-4F:PM6 exhibited a higher PCE of 12.59% and a larger J SC of 22.57 mA cm -2 due to the more balanced carrier mobilities and suitable phase separation in the active layer compared with that of BDDBO-4F with longer alkyl side chains (PCE of 9.80%). Other modifications on BDD-based NFRAs, including replacement of the D units (BDIC2F), have also been reported, where similar photovoltaic performance was reported [71,72] . A few other electron-deficient building blocks were also used as the central A' units to construct NRFAs [ Figure 4], which played a unique role in adjusting the absorption, energy level, molecule conformation and active layer morphology [69,71,[73][74][75][76][77] . The relevant performance data are listed in Table 2.

NFRAS WITH FULLY NON-FUSED STRUCTURES
Notably, the CPDT unit, serving as the most common D unit in NFRAs, not only possesses outstanding electron-donating ability, but also enables the introduction of bulky substituents for providing sufficient steric hindrance and suppressing excessive intermolecular aggregation. However, the intrinsic fused ring structure of CPDT inevitably raises the production costs of NFRAs. Therefore, NFRAs with fully non-fused structure have emerged [ Figure 6]. The first fully non-fused ring acceptor (PTICH) was reported by Yu et al. [83] in 2019, where thiophene, rather than CPDT, was used as the D unit. The same as other NFRAs, the coplanar conformation of PTICH was locked by the O···H noncovalent interaction between the central 2,5-bis(alkyloxy)phenylene unit and flanking thiophene units [ Figure 7A]. Furthermore, terminal side chains, including alkyl chains (PTIC) and oxyalkyl chains (PTICO), were adopted to lock the conformation of end groups, which significantly reduced the conformational disorder of the molecules. After device optimization, a PCE of 10.27% was obtained for the PTIC:PBDB-TF-based devices, representing a new molecule design strategy with a low synthetic complexity index. Moreover, the unique structural features, including the hindered outward-chain and planar sp 3 carbon-free backbones of PTIC, played important roles in enhancing the intrinsic photostabilities [ Figure 7B] of the molecule, which in turn benefitted the device stability [84] .
liu et al. [84] further investigated the device stabilities of A-D-A-type NFAs with fused (IT-4F), non-fused (HF-PCIC) and fully non-fused (PTIC) backbones under continuous one-sun-equivalent irradiation. As shown in Figure 7C, the PBDB-TF:PTIC-based device maintained ~72% of its initial PCE value for 500 h of illumination, thereby greatly outperforming the PBDB-TF:IT-4F and PBDB-TF:HF-PCIC-based devices. These results demonstrated that intrinsic photostability and device stability are not limitations to NFRAs. It should be noted that PTIC in neat films exhibited strong molecular stacking, high crystallinity and dominant edge-on orientation from the results of GIWAXS. To optimize the solid stacking and orientation of PTIC, a new NFRA (PTB4Cl) with two-dimensional phenyl chains featuring larger steric hindrance was developed by the same group [85] . Compared with PTIC, PTB4Cl exhibited bi-model face-on and edge-on orientations, whereas PTIC exhibited a nearly pure edge-on orientation, showing only an OOP (100) peak. After device optimization, a higher PCE of 12.76% was obtained for PTB4Cl:PBDB-TF-based devices associated with extended exciton lifetimes and increased charge transfer rates.
Different from the strategy of noncovalent interactions afforded by PTB4Cl, the use of peripheral aromatic substituents with large steric hindrance is also an effective method for constructing the coplanar conformation of fully non-fused ring acceptors. Two novel tetrathiophene-based fully non-fused ring acceptors (m-4TBC-2F and o-4TBC-2F) were reported by Chen et al. [49] , which feature different substitution positions of alkoxy chains in peripheral phenyl. According to DFT calculations, the huge steric hindrance caused by the alkoxy chains on the peripheral phenyl led to a large dihedral angle (74°) between the molecule backbone and peripheral phenyl, thus resulting in a coplanar backbone of o-4TBC-2F. However, m-4TBC-2F with smaller steric hindrance and smaller dihedral angles between the molecule backbone and peripheral phenyl (46°) displayed a much more twisted backbone conformation [ Figure 7D]. Therefore, the optimized devices based on o-4TBC-2F showed a higher PCE of 10.26% than its counterpart m-4TBC-2F (2.63%), benefitting from the higher carrier mobilities, lower energy loss and extended absorption spectra. It is noteworthy that there still existed a dihedral angle above 10° between the two central thiophene rings for o-4TBC-2F. Later, Ma et al. [59] found that the dihedral angle could be reduced to 0° when alkoxyphenyl was replaced by 2,4,6-tri-isopropylphenyl (A4T-16) and 2,4,6-tri-methylphenyl (A4T-23), respectively. The two bulky 2,4,6tri-isopropylphenyl groups have effectively restricted the rotation of the C-C single bond between the two adjacent thiophene rings, thus giving rise to a coplanar central bithiophene core. Moreover, the peripheral 2,4,6-tri-isopropylphenyl is perpendicular to the bithiophene core, which was evidenced by the results from DFT calculations and single-crystal structural analysis [ Figure 7E]. With the large steric hindrance caused by the 2,4,6-tri-isopropylphenyl group, excessive intermolecular aggregation can be avoided. The singlecrystal analysis also revealed that A4T-16 can form a three-dimensional-interpenetrated network through the close π-π interaction of its end groups, which facilitates electron transport. As a result, a prominent PCE of 15.20% and FF of 79.80% were achieved by A4T-16, which are the highest values for the OSCs based on NFRAs. Notably, NFRAs usually afford inferior FF with respect to FRAs in OSCs [86,87] . This drawback might be related to the existence of multiple conformational isomers due to the inevitable rotation of the C-C single bonds in NFRAs, which is harmful to the ordered molecular packing of NFRAs in the solid state [51] . The unprecedented FF achieved by Ma et al. [59] provides a promising direction for breaking beyond the FF limitations in NFRA-based OSCs.

Table 2. Device performance of organic solar cells based on representative A-D-A'-D-A-type non-fused ring acceptors
In contrast, the solubility and packing structure of tetrathiophene-based fully non-fused ring acceptors were finely tuned by the side-chain engineering on 4T-n [88] . Among them, 4T-3 with four 2-ethylhexyl side chains showed the most suitable crystallinity. The notable PCEs of 10.15% and 12.04% were achieved for PBDB-T:4T-3 and D18:4T-3-based devices, respectively. Furthermore, compared with other high-efficiency FRAs, such as ITIC-4F, Y6, COi8DFIC, 4TIC-4F and W1, the representative fully non-fused ring acceptor 4T-3 exhibited the highest figure-of-merit, which represents the preliminary cost-performance balance point. The relevant performance data of the above NFRAs are listed in Table 3.

CONCLUSION AND OUTLOOK
Overall, the recent development of NFRAs has been rationally summarized with regards to the relationships between chemical structure and device performance. First, by the incorporation of noncovalent interactions and steric hindrance, the rigid and coplanar structures can be realized in NFRAs, in the results of ordered molecular packing and efficient electron transport in the solid state. Second, the chemical structure of NFRAs can be facilely modified based on numerous building blocks. Third, the intrinsic photostability and device stability are not limitations for NFRAs. Moreover, the champion devices based on NFRAs have afforded a remarkable PCE over 15%, which strongly evidenced the excellent potential of NFRAs for highperformance and cost-effective OSCs. In comparison with FRAs, the development of NFRAs is still in its infancy. To compete with FRAs, the J SC and FF of the relevant OSCs need to be greatly enhanced. Therefore, additional investigations focusing on the molecular design are essential to further enhance the performance of NFRA-based OSCs. Combined with the unique merits of low costs, simple synthesis and flexibility in structural modification, we believe the breakthrough of high-performance NFRAs will lead to the large-scale commercial applications of OSCs becoming more accessible in the near future.   [59,83,84] and John Wiley and Sons [85] , respectively.

Authors' contributions
Made substantial contributions to conception and design of the study and performed data analysis and interpretation: Yang M, Duan C Performed data acquisition, as well as proofread manuscript: Wei W, Zhou X, Wang Z

Availability of data and materials
Not applicable.