Figure 2 shows the chemical structures of the ITIC series derivatives containing selenophene as acceptors, and Table 1 summarizes the optical, electrochemical, and photovoltaic properties of these ITIC series derivatives containing selenophene as acceptors. In 2015, Zhong Xiaowei et al. successfully synthesized an A-D-A type non-fullerene electron acceptor ITIC, a breakthrough that broke the traditional limitations on the selection of acceptor materials for organic solar cells^[12]. Since then, non-fullerene electron acceptor materials have attracted widespread attention in the scientific research community both domestically and internationally, opening up new research and application directions. The introduction of selenophene into the central core conjugated part of ITIC series molecules results in a reduction of the optical bandgap and a redshift in the absorption spectrum^[14]. When constructing solar cells with wide-bandgap donor materials, complementary absorption characteristics can be achieved, which helps to increase the short-circuit current density (J_SC)^[15]. Additionally, introducing selenium (Se) at different positions of the central core of ITIC series molecules significantly affects the optical and electrical properties due to differences in intermolecular and intramolecular interactions compared to sulfur. Specifically, when selenophene replaces thiophene at the innermost position, the narrowest bandgap and the highest HOMO energy level can be obtained^[16-17]. When replacing at the outermost position, the non-covalent interaction of selenium…oxygen (Se…O) can enhance molecular planarity, thereby improving device stability and lifetime^[18]. The synergistic effect of selenophene substitution and chlorinated end groups can further narrow the bandgap, causing a redshift in the absorption spectrum, and even if the thickness of the active layer increases, it will not significantly reduce the efficiency of the device^[19]. The synergistic effect of single chlorinated end groups at different sites can modulate the crystallinity and microstructure of the molecule^[20]. The synergistic effect of brominated end groups helps to reduce energy loss, has cost advantages, facilitates subsequent structural modifications, and is particularly suitable for multi-junction or tandem organic solar cells due to its narrow bandgap and high fill factor (FF) characteristics^[21-22]. The synergistic effect of selenophene and hetero-dihalogenated end groups can adjust solubility and crystallinity in the acceptor film to achieve optimal performance balance. Selenophene combined with chlorothiophene substituting the benzene ring in INIC (cyanoindanone) end groups can achieve a wider absorption range, lower frontier molecular orbital energy levels, and reduced energy loss^[23]. The synergistic effect of selenophene and non-halogenated thiophene substituting IC end groups, although leading to a blue shift in absorption, can enhance open-circuit voltage (V_OC)^[24-25]. Introducing selenophene into the π-bridge not only reduces the optical bandgap of the molecule and causes a redshift in absorption but also obtains acceptor molecules with high planarity through non-covalent interactions^[26⇓-28].

Moreover, asymmetric selenium atom substitution not only prompts absorption redshift, elevates the HOMO energy level, enhances electron mobility, but also increases intermolecular π-π stacking interactions^[29-30]. When synergized with fluorinated end groups, a narrower optical bandgap and further absorption redshift can be achieved^[31]. Compared to difluorine substitution, the blend film with monofluorine substitution exhibits better fibril and uniform ordered phase separation morphology^[32]. Based on selenophene substitution, extending the conjugated backbone can elevate the LUMO energy level and electron mobility. Moreover, the blend film shows more balanced charge mobility, as well as more efficient exciton dissociation and charge collection capability^[33].

Wan et al^[14] synthesized two chlorinated two-dimensional fused-ring electron acceptors, C8T-BDT4Cl and C8T-BDSe4Cl, using benzodithiophene (BDT) and benzoselenophene (BDSe) substituted with alkylthiophene as the central donor unit. Ultimately, the device based on C8T-BDSe4Cl:PM6 achieved a PCE of 13.5% due to its higher J_SC (21.79 mA/cm²), surpassing the 12.2% PCE of the C8T-BDT4Cl:PM6 device. Li et al^[15] synthesized a multi-fused ring molecule IDTIDSe-IC using indeno[6-b']dithienoindeno[5-b']diselenophene as the backbone. Without any additives, the device based on IDTIDSe-IC and J51 achieved a PCE of 8.02%, V_OC of 0.91 V, J_SC of 15.16 mA/cm², and FF of 58.0%. Liu et al^[16] synthesized three A-D-A structured acceptor materials, SeCT-IC, CSeT-IC, and CTSe-IC, using the same electron-deficient end group A unit and different selenophene-containing central core D units. Ultimately, the device based on J71:CTSe-IC achieved a PCE of 11.59%, higher than that of J71:SeCT-IC (10.89%) and J71:CSeT-IC (8.52%). Lin et al^[17] synthesized isomeric ladder-type non-fullerene acceptors SRID-4F and TRID-4F by replacing sulfur with selenium in π-conjugated backbones. The device based on PBDB-T-2F:SRID-4F achieved a PCE of 13.05%, while devices based on PBDB-T-2F:TRID-4F and PBDB-T-2F:IT-4F achieved PCEs of 12.33% and 12.63%, respectively. Lanbanti et al^[18] synthesized two NFA molecules, SeOut and SeIn, by replacing sulfur atoms at the outer or inner positions of the central core with selenium atoms, and then synthesized two additional NFA molecules, SeOutMe and SeInMe, by attaching methyl chains to the terminal groups. Ultimately, the SeOutMe device exhibited excellent operational stability. After 450 hours of operation, its PCE remained at 95% of the initial value, with a half-life reaching 5600 hours, compared to only 400 hours for the ITIC device. Wang et al^[19] designed a new central core selenopheno[2-b]thiophene and synthesized a narrow bandgap non-fullerene acceptor SeTIC, followed by synthesizing SeTIC4Cl through chlorination of the end group. Ultimately, the device based on PM6:SeTIC4Cl achieved a PCE of 13.32%, higher than the device based on PM6:SeTIC (7.46%). Additionally, it was found that when the active layer thickness increased from 100 nm to 300 nm, the PCE showed good tolerance. Ge et al^[20] used indeno[2',3':4,5]thieno[3,2-b]selenophene as the central core and synthesized two non-fullerene acceptors TSeIC-M-Cl and TSeIC-P-Cl with mono-chlorinated (2,3-dihydro-3-oxo-1H-inden-1-ylidene)propanedinitrile at different positions as end groups. Ultimately, the device based on PBT1-C:TSeIC-P-Cl achieved a PCE of 11.26%, outperforming the PCE of the device based on PBT1-C:TSeIC-M-Cl (9.72%). Wan et al^[21] synthesized three near-infrared absorbing non-fullerene small molecular acceptors BDSeIC, BDSeIC2Br, and BDSeIC4Br, using benzo[5-b']diselenophene as the central core and mono-bromo or di-bromo substituted 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene)propanedinitrile as end groups. Without any treatment, the device based on PM6:BDSeIC2Br achieved a high PCE of 11.9%. After thermal annealing, a PCE of 12.05% and E_loss of 0.64 eV were obtained. Zhang et al^[22] synthesized two non-fullerene small molecular acceptors (NF-SMAs) TSeIC4Cl and TSeIC4Br using indeno[2',3':4,5]thieno[2-b]selenophene as the central core and di-chlorinated and di-brominated end groups. Ultimately, the polymer solar cell based on PM6:TSeIC4Br exhibited higher J_SC, V_oc, and FF, achieving a photoelectric conversion efficiency (PCE) of 12%. Wan et al^[23] used benzo[5-b']diselenophene as the central core and synthesized three FREAs, BDSe-4Cl, BDSe-2(BrCl), and BDSe-4Br, with dichloro IC, hybrid dihalogenated IC (IC-BrCl), and dibromo IC, respectively. Ultimately, PCEs of 13.82%, 13.12%, and 14.54% were achieved based on BDSe-4Cl:PM7, BDSe-4Br:PM7, and BDSe-2(BrCl):PM7, respectively. Wan et al^[24] used benzo[5-b']diselenophene as the central core and synthesized two NFSMAs, BDSeThCl and BDSePhCl, using IC with a 2-chlorothiophenyl end group and IC with a mono-chlorinated phenyl end group. The device based on BDSePhCl:PM7 achieved a PCE of 13.68% with a relatively low energy loss of only 0.49 eV, while the device based on BDSeThCl achieved a PCE of 11.91% and V_OC of 0.97 eV. Liu et al^[25] used indeno[2',3':4,5]thieno[2-b]selenophene as the central core unit and non-halogenated thiophene as the terminal group to synthesize an A-D-A structured non-fullerene small molecular acceptor TSeTIC. Ultimately, the TA-treated PM6:TSeTIC device achieved a PCE of 13.71% with lower photovoltaic cell loss (E_loss) (0.60 eV), surpassing the 12.05% PCE and 0.64 eV E_loss of the PM6:TTTIC device. Liang et al^[26] used selenophene as the π bridge to connect the indenodithiophene (IDT) core with 3-(dicyanomethylene)indan-1-one (IC) or difluorinated IC (2F-IC) end groups, synthesizing two non-fullerene acceptors IDT2Se and IDT2Se-4F. The device based on IDTSe-4F achieved a PCE of 11.19%. Liu et al^[27] used thiophene, selenophene, and thieno[2-b]thiophene as π bridges to connect the IDTO central core unit with electron-withdrawing IC-2F end groups, synthesizing three narrow-band non-fullerene acceptors IDTO-T-4F, IDTO-Se-4F, and IDTO-TT-4F. The device based on PBDB-T:IDTO-T-4F ultimately achieved a PCE of 12.62%, with a high V_OC of 0.864 V, a high J_SC of 20.12 mA/cm², and an FF of 72.7%. Zhang et al^[28] used selenophene to connect the alkylated indeno[6-b']dithiophene (IDT) central core with 1,1-dicyanomethylene-3-indanone (ICs) end groups having 0~2 fluorine substitutions, synthesizing three non-fullerene acceptors IDT2SeC2C4, IDT2Se-C2C4-2F, and IDT2Se-C2C4-4F. The device based on PBDB-T:IDT2Se-C2C4-4F mixture achieved a PCE of 10.57%. Tang et al^[29] designed and synthesized three asymmetric A-D-A type non-fullerene acceptors (MQ3, MQ5, and MQ6). The device based on PM6:MQ6 achieved a PCE of 16.39%, V_OC of 0.88 V, FF of 0.7566, and J_SC of 24.62 mA/cm². Li et al^[30] replaced one thiophene in indodithiophene (TPT) with selenophene, developing a new asymmetric selenium-substituted central core SePT, which was then coupled with 2-(3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1-ylidene)propanedinitrile to synthesize an asymmetric A-D-A structured non-fullerene acceptor SePT-IN. When mixed with PBT1-C, the device based on SePT-IN achieved a PCE of 10.20%, surpassing the device based on TPT-IN (8.91%). Liu et al^[31] synthesized three A-D-A type non-fullerene acceptors T-Se, T-Se-4F, and T-Se-Th using three different end groups based on an asymmetric selenium-containing central core. Solar cells based on PM6:T-Se-4F and PM6:T-Se-Th achieved J_SC of 18.09 and 16.90 mA/cm², V_OC of 0.781 and 0.912 V, FF of 66.6% and 66.8%, and PCE of 9.41% and 10.29%, respectively. Cao et al^[32] designed and synthesized an asymmetric seven-ring electron-donating core (DTPPSe) and synthesized three new non-fullerene electron acceptors DTPPSe-IC, DTPPSe-2F, and DTPPSe-4F with varying numbers of fluorine atoms substituted on the end groups. The OSCs based on PBDB-T:DTPPSe-2F had a PCE of 13.76%, J_SC of 22.16 mA/cm², and FF of 73.70%, outperforming the OSCs based on PBDB-T:DTPPSe-IC and PBDB-T:DTPPSe-4F (9.88% and 12.03%). Li et al^[33] designed and synthesized two asymmetric selenium-substituted non-fullerene electron acceptors SePTT-2F and SePTTT-2F with different conjugated backbones. The OSCs based on PBT1-C:SePTTT-2F achieved a PCE of 12.24%, surpassing the PCE of the OSCs based on SePTT (10.9%).

Figure 3 shows the chemical structures of selenium-containing Y-series derivative acceptors, and Table 2 summarizes the optical, electrochemical, and photovoltaic performance of these selenium-containing Y-series derivative acceptors. As one of the materials with currently high efficiency, the Y-series electron acceptor is the focus of our research. Introducing selenium atoms into the benzothiadiazole central core not only broadens the absorption range of the material and enhances the absorption intensity but also improves the electron mobility and enhances the inherent photo-stability of the material. Crucially, the introduction of selenium helps reduce energy disorder and decrease Urbach energy, thereby optimizing device performance^[34]. Based on the selenium-substituted central core, further modification of the branched chain structure can lead to an increase in LUMO levels^[35]. Combining the synergistic effects of selenium substitution in the central core and terminal group chlorination, we found that dichlorination exhibits stronger intramolecular charge transfer compared to monochlorination, and the blended film demonstrates tighter molecular stacking and preferential face-on orientation^[36]. Applying selenium-containing molecules to the second bulk heterojunction structure and transferring them to the first BHJ structure can create an active layer with two different NFA concentration gradients, which is crucial for improving solar cell performance^[37]. Compared to molecules containing furan or thiophene on the outer side of the central core, selenium-containing molecules exhibit more significant red-shifted absorption while not affecting charge transport characteristics^[38]. Moreover, the outer-side substitution of selenophene plays a crucial role in modulating the molecular geometry and packing mode, both of which are prerequisites for ensuring excellent charge transport^[39]. More importantly, using molecules with outer-side selenophene substitution as guest acceptors can maintain a high dielectric constant while disrupting disordered aggregation^[40]. On the basis of selenium atom substitution at the conjugated outer side of the central core, changing the straight chain to a branched chain on the outer side not only provides greater steric hindrance, thus achieving a higher dielectric constant and improving exciton dissociation efficiency, ultimately enhancing EQE, but also balances carrier transport to improve FF while obtaining high V_OC and high J_SC^[41]. Additionally, adjusting the length of the N-alkyl chain on the benzotriazole central core can further regulate the morphology of the blend film^[42]. Due to their strong crystallinity and intermolecular interactions, selenium-containing molecules can be used as guest acceptors to optimize the morphology of binary system blend films^[43]. Furthermore, introducing vinyl groups in the π-bridge while substituting selenium atoms not only reduces the optical bandgap but also helps suppress the formation of triplet excitons^[44]. A significant impact of selenium atom substitution position on molecular properties and photovoltaic performance has been observed. Compared to substitutions at the inner conjugated side of the central core, selenium atom substitution at the outer conjugated side of the central core produces a narrower bandgap and stronger absorption, providing a more rigid molecular skeleton^[45]. Selenium substitution at the central core can further expand the absorption range and reduce the bandgap while enhancing the aggregation of small-molecule acceptors^[46]. Although increasing the number of selenium atom substitutions gradually reduces the molecular bandgap and leads to a redshift in the absorption spectrum, the imbalance between J_SC and V_OC still prevents achieving the ideal power conversion efficiency. Asymmetric selenium atom substitution achieves a good balance in absorption characteristics, energy level distribution, and crystallinity^[47]. On this basis, it was also found that compared to central core selenophene substitution and asymmetric selenophene substitution on the outer side, the outermost asymmetric selenophene substitution creates additional Se...O non-covalent interactions, making the molecule more planar and beneficial for forming more effective charge transport channels. Moreover, as the position of selenophene substitution moves from the center to the outer side, the dipole moment, crystallinity, and electron mobility all gradually increase^[48]. Furthermore, based on this asymmetric selenium atom substitution strategy, it was found that varying side-chain lengths significantly affect intermolecular interactions, crystal stacking, and the morphology of the blend film^[49]. In particular, replacing alkyl side chains with alkoxy side chains not only optimizes charge extraction performance and reduces non-radiative energy loss but also raises the LUMO energy level^[50]. Combining asymmetric selenium substitution and halogenation of terminal groups can further modulate molecular properties, where chlorinated end groups are more conducive to enhancing intermolecular aggregation than fluorinated end groups. When combined with wide-bandgap, highly crystalline polymer donors, higher energy conversion efficiencies can be achieved^[51]. On the basis of selenium atom substitution in the benzothiadiazole central core, obtaining a stronger mixture dielectric constant through selenium atom substitution at the conjugated outer side of the central core helps reduce exciton binding energy, increase free charge generation, promote more efficient exciton dissociation, thereby reducing radiative recombination losses (ΔE₂) in OSCs and improving device efficiency. Most importantly, when used as the primary acceptor material in ternary planar mixed heterojunctions, its performance can be further enhanced^[52]. Furthermore, molecules obtained by substituting selenium atoms for the outermost sulfur atoms in the central core can also be added as guest acceptors in all-small-molecule ternary organic solar cells to improve efficiency^[53]. Recently, selenophene has also been applied in phenazine-based small-molecule acceptors. In such molecules, the introduction of selenophene not only reduces energy loss but also enhances photon absorption capability^[54].

Zhang et al^[34] used selenium atoms to replace sulfur atoms in the central core benzothiadiazole and synthesized the molecule Y6Se. Eventually, the D18:Y6Se organic solar cell device without post-treatment achieved a power conversion efficiency of 17.7%. Li et al^[35] introduced selenium atoms into the central core benzothiadiazole based on L8-BO and synthesized the molecule Se46. Ultimately, the OSC based on PM6:Se46 achieved a power conversion efficiency of 18.46%, with a V_OC of 0.879 V, a J_SC of 26.22 mA/cm², and an FF of 80.1%. Feng et al^[36] used benzoselenadiazole as the central core and replaced IC with different numbers and positions of chlorine atoms as the end group to synthesize a series of NFAs TSeIC-2Cl-γ, BTSeIC-2Cl-mix, and BTSeIC-4Cl. In the end, the device based on PM6:BTSeIC-4Cl achieved a PCE of 16.14%. BTSeIC-2Cl-mix obtained a PCE of 14.91%, while BTSeIC-2Cl-γ only achieved a PCE of 14.21%. Cheng et al^[37] used benzoselenadiazole as the central core and chlorinated IC as the end group to synthesize the molecule Y6-Se-4Cl. The double BHJ structure devices of PM6:Y6-Se-4Cl/PM6:Y6 and PM6:Y6-Se-4Cl/PM6:IT-4Cl ultimately achieved PCEs of 16.4% and 15.8%, respectively, both higher than the single BHJ device based on PM6:Y6-Se-4Cl (15.0%). Chai et al^[38] used thiopheno[2-b]furan, thiopheno[2-b]thiophene, and selenopheno[2-b]thiophene as central core conjugates to synthesize a series of NFAs BPF-4F, BPT-4F, and BPS-4F. Devices based on BPT-4F and BPS-4F achieved PCEs of 16.8% and 16.3%, respectively. However, the device based on BPF-4F only achieved a PCE of 12.6%. Lin et al^[39] designed the incorporation of selenium into the Y6 molecule to synthesize the molecule CH1007. The introduction of selenium not only caused a red shift in absorption but also did not affect charge transport. Ultimately, the device based on PM6:CH1007:PC71BM achieved a J_SC of 27.48 mA/cm² and a PCE of 17.08%. Subsequently, He et al^[40], upon discovering that selenium substitution on the central core can effectively change the molecular dielectric constant and exhibit faster charge recombination, introduced the selenium-substituted molecule T9SBO-F as a guest acceptor into PM6:L8-BO. The study found that the introduction of this molecule not only improved hole transport but also suppressed bimolecular recombination, thereby reducing non-radiative recombination by 0.221 eV. Ultimately, the device based on PM6:L8-BO:T9SBO-F achieved a high PCE of 19%. Gao et al^[41] designed and synthesized the molecules Se-EH and Se-EHp by changing the outer straight chains to branches of different lengths based on CH1007. Ultimately, the OSCs based on PM6:Se-EH and PM6:Se-EHp achieved PCEs of 18.58% and 17.96%, respectively, both higher than the PCE of 17.47% for the device based on PM6:CH1007. More importantly, the ternary device based on PM6:Se-EH:CH1007 achieved a higher PCE of 19.03%. Qi et al^[42] introduced selenium atoms into the central core conjugated thiophene and changed the N-alkyl chain length of the central core benzotriazole to synthesize two non-fullerene acceptors mBzS-4F and EHBzS-4F. Ultimately, the organic solar cells based on PM6:mBzS-4F and PM6:EHBzS-4F achieved J_SC values of 28.83 and 27.72 mA/cm², and PCEs of 17.48% and 17.02%, respectively. Later, they again changed the N-alkyl chain length of the central core benzotriazole to synthesize non-fullerene acceptors AN6SBO-4F, PN6SBO-4F, and EHN6SEH-4F. Ultimately, compared to the battery based on PM6:EHN6SEH-4F, the batteries based on PM6:PN6SBO-4F and PM6:AN6SBO-4F only achieved PCEs of 12.73% and 8.32%, respectively. Fan et al^[43] combined selenophene fused rings with naphthalene-containing end groups to synthesize the non-fullerene acceptor Y-SeNF, which was then added as a guest acceptor to the binary PM6:L8-BO host. Ultimately, the ternary mixed organic solar cell achieved a PCE of 19.28%. Jia et al^[44] used selenium to replace the outer thiophene on the central core conjugate and introduced vinyl at the π-bridge position to synthesize the molecule BTPSeV-4F. Subsequently, it was used as the back cell acceptor to form a tandem organic solar cell. The single-junction organic solar cell based on PM6:O1-Br achieved a PCE of 15.5% and a V_OC of 1.04 V. When using BTPSeV-4F and O1-Br as the rear and front sub-cell acceptors, the organic tandem solar cell (TOSC) achieved a PCE of 19%. Zhang et al^[45] used selenium to replace sulfur inside and outside the central core conjugated thiophene to synthesize two small molecule acceptors mPh4F-ST and mPh4F-TS. Ultimately, the device based on PM6:mPh4F-TS achieved a PCE of 18.05%, higher than PM6:mPh4F-TT (16.88%) and PM6:mPh4F-ST (17.57%). Yu et al^[46] used selenium to replace sulfur in the central core benzothiadiazole and the outer thiophene of the central core conjugated thiophene to synthesize two new Y6-type small molecule acceptors Y6-Se and Y6-2Se. Ultimately, the device based on PM6:Y6-Se achieved a PCE of 16%, superior to the devices based on PM6:Y6 (15.67%) and PM6:Y6-2Se (14.94%). Yang et al^[47] used different amounts of selenium to replace the outer sulfur of the central core conjugated thiophene to synthesize symmetric and asymmetric non-fullerene small molecule acceptors A-WSSe-Cl and S-WSeSe-Cl. Ultimately, the symmetric devices PM6:S-YSS-Cl and PM6:S-WSeSe-Cl achieved PCEs of 16.73% and 16.01%, respectively, while the asymmetric device PM6:A-WSSe-Cl achieved a PCE of 17.51%. Yang et al^[48] designed and synthesized three isomeric SMAs (SCSeF, A-ISeF, and A-OSeF) by controlling the position of selenium monosubstitution (from the central core to the inner and then to the outer). As selenium moves outward, the most outward asymmetric molecule A-OSeF exhibits tighter π-π stacking, 3-D network stacking, higher crystallinity, and electron mobility. Ultimately, due to more favorable phase separation morphology and charge transport in the binary OSCs of PM1:A-OSeF, the highest PCE of 18.5% was achieved. Subsequently, Zhao et al^[49] synthesized double asymmetric small molecule acceptors AYT11Se9-Cl and AYT9Se11-Cl using different lengths of alkyl side chains based on asymmetric selenium substitution on the outer side of the central core conjugate. Ultimately, the binary OSC based on PM6:AYT9Se11-Cl achieved an astonishing PCE of 18.12%, while the binary OSC based on PM6:AYT11Se9-Cl also reached a PCE of 17.52%. Li et al^[50] synthesized the small molecule acceptor BTPOSe-4F by replacing the alkyl side chain with an alkoxy side chain based on asymmetric selenium substitution on the outer side of the central core conjugate. Subsequently, fullerene PC71BM was added as a second acceptor in the binary blend system of PM6:BTPOS-4F, and ultimately, the ternary device achieved a photoelectric conversion efficiency of 17.33%, a V_OC of 0.890 V, a J_SC of 25.42 mA/cm², and an FF of 76.6%. Shi et al^[51] used selenium to asymmetrically replace the sulfur of the outer thiophene on the central core conjugate, then synthesized the non-fullerene acceptors AsymSSe-2F and AsymSSe-2Cl using fluorinated and chlorinated IC as end groups. Ultimately, the device based on D18:AsymSSe-2F achieved a higher J_SC and an FF of 79.46%, ultimately achieving a PCE of 18.31%. The device based on AsymSSe-2Cl achieved a PCE of 17.68%. Gao et al^[52] used selenium to replace sulfur in the central core benzothiadiazole to synthesize the acceptor S9TBO-F, and further used selenium to replace the outer sulfur of the central core conjugated thiophene to synthesize the asymmetric acceptor BS3TSe-4F. Subsequently, another medium bandgap acceptor Y6-O was added to D18/BS3TSe-4F to constitute a ternary PMHJ organic solar cell. Ultimately, the PMHJ device based on D18/BS3TSe-4F achieved a PCE of 18.48%, with a J_SC as high as 29.4 mA/cm². By adding Y6-O as a ternary additive, the final ternary solar cell achieved a PCE of 19%. Gao et al^[53] used selenium to asymmetrically replace the outer sulfur of the central core conjugated benzothiadiazole to synthesize the small molecule acceptor SSe-NIC, which was then used to construct a ternary all-small-molecule organic solar cell with MPhS-C2:BTP-eC9. When SSe-NIC was mixed with the small molecule donor MPhS-C2, the formed binary device MPhS-C2:SSe-NIC achieved a PCE of 16.83%. When a small amount of SSe-NIC was added to the main binary system of MPhS-C2:BTP-eC9, a ternary device was constructed, achieving a PCE of 18.02%. Li et al^[54] introduced different amounts of selenophene into the phthalazine-based small molecule acceptor to design and synthesize the symmetric small molecule acceptor and asymmetric small molecule acceptor PzIC-SeSe-4F and PzIC-SSe-4F. Ultimately, the device based on PM6:PzIC-SeSe-4F achieved a PCE of 17.69%.

Figure 4 shows the chemical structure of Y-series polymeric derivatives containing selenophene, and Table 3 summarizes the optical, electrochemical, and photovoltaic properties of Y-series polymeric derivatives containing selenophene. In 2017, Li^[55] proposed the concept of "polymerized small molecule acceptors" (PSMAs). They used the small molecule acceptor IDIC as the molecular backbone A unit and thiophene as the D unit for copolymerization, synthesizing the polymer acceptor PZ1 with a narrow bandgap and strong absorption characteristics. The all-polymer solar cells based on PZ1 achieved a PCE of 9.19% at that time. Polymerized small molecule acceptors can maintain the advantages of small molecule acceptors (SMAs), such as a defined molecular structure and ease of purification, while also possessing the properties of polymers, including high thermal stability, good mechanical tensile performance, and a wide range of property tunability, thus attracting increasing attention from researchers^[56-57]. Introducing selenium atoms into the main chain of small molecule acceptors (SMAs) in polymer backbones not only improves the mechanical properties of all-polymer solar cells but also exhibits excellent efficiency without post-treatment^[58]. The synergistic effect of selenium atom substitution and chlorinated end groups helps to improve efficiency^[59]. Additionally, differences in the copolymerization sites of chlorinated end groups affect intermolecular interactions and molecular weight^[60]. Asymmetric selenium-substituted polymer acceptors based on small molecule acceptor (SMA) backbones in polymer chains can be used as host binary systems, and when combined with guest acceptors having complementary spectral responses and appropriate energy levels, they optimize absorption spectra, molecular stacking between host and guest acceptors, and the microstructure of blends, while suppressing non-radiative recombination^[61-62]. Using asymmetric selenium-substituted polymer acceptors based on small molecule acceptor (SMA) backbones in polymer chains as the third component to construct all-polymer solar cells helps balance phase separation and material crystallinity. Combined with layer-by-layer (LBL) processing, the ternary system structure not only facilitates charge generation and transport performance but also significantly reduces non-radiative energy loss in LBL-type ternary blend systems^[63]. Using selenophene as a copolymerization linker not only enhances intermolecular interactions and donor-acceptor compatibility but also maintains stable molecular aggregation states^[64]. Based on selenophene substitution, altering the central core not only adjusts absorption and energy levels but also influences aggregation and miscibility with donors, thereby affecting phase separation in blend films^[65]. Introducing varying numbers of selenophene units simultaneously into the small molecule acceptor (SMA) backbone and copolymerized π-bridge in the polymer main chain not only broadens the absorption spectrum of the polymer acceptor but also enhances electron mobility and strengthens intermolecular interactions while maintaining good batch-to-batch reproducibility^[66]. On this basis, introducing vinyl groups into the π-bridge further enhances electron-donating ability and quinoidal resonance characteristics, achieving a wider absorption range^[67]. For the currently popular oligomer-dimer structures, non-covalent intramolecular interactions with selenophene as a copolymerization linker promote planarity in the polymer backbone, which is significant for improving device performance, and importantly, it exhibits good compatibility with other types of acceptors^[68]. Besides, in dimers, increasing the number of selenophenes not only extends absorption into the near-infrared range but also reveals that selenophene-containing dimers exhibit better D/A phase separation morphology and lower energy losses compared to selenophene-containing monomers^[69].

Liao et al^[58] used a selenized Y6 derivative as the backbone core and synthesized the polymer acceptor PYSe-2FT with difluoro-substituted thiophene as the copolymer π-bridge. Without any post-treatment, the PCE of the flexible all-polymer organic solar cell was 13.56%, and more importantly, it maintained 86% of the original PCE efficiency after 1000 bends under a curvature radius of 3 mm. Fan et al^[59] synthesized D-A’-D central cores containing selenophene and respectively synthesized two novel near-infrared absorbing polymer small molecule acceptors PY2Se-F and PY2Se-Cl with fluorinated and chlorinated end groups. Ultimately, all-polymer solar cells based on PM6:PY2Se-Cl achieved a PCE of 16.1%, with a photocurrent density of 24.5 mA/cm² and an FF of 0.743. Fan et al^[60] synthesized three selenium-containing end-group isomeric polymer small molecule acceptors PY2Se-Cl-o, PY2Se-Cl-m, and PY2Se-Cl-ran by adjusting the chlorination position of the end group and the copolymerization position. The all-polymer solar cell based on PM6:PY2Se-Cl-ran obtained a PCE of 16.23%, and the ternary device based on PM6:J71:PY2Se-Cl-ran achieved a PCE of 16.86%. Fu et al^[61] synthesized the polymer acceptor PYT-1S1Se using selenium asymmetrically substituted on the Y6-based polymer acceptor central core conjugation. The all-polymer solar cell based on PYT-1S1Se obtained a J_SC of 24.1 mA/cm² and a V_OC of 0.926 V, with an energy loss of only 0.502 eV, ultimately achieving a power conversion efficiency of 16.3%. Sun et al^[62] designed and synthesized the narrow bandgap chlorinated polymer acceptor PY-2Cl, then introduced it as the third component into the selenium-containing binary system (PM6:PY-1S1Se) to construct a ternary all-polymer solar cell. The ternary device based on PM6:PY-1S1Se:PY-2Cl obtained a PCE of 18.2% (certified value of 17.8%), with an FF of 77.2% and a J_SC of 25.74 mA/cm². Wu et al^[63] synthesized the molecule PYT-1S1Se-4cl using chlorine substitution at the end group based on the polymer acceptor PYT-1S1Se, then introduced PYT-1S1Se-4Cl as the third component into the binary host to construct a ternary all-polymer solar cell combined with a layer-by-layer (LBL) process. Ultimately, the PCE of the LBL-type PM6/(PYT-1S1Se:PYT-1S1Se-4cl) ternary all-polymer solar cell was 17.74%, higher than the corresponding binary system PM6/PYT-1S1Se with a PCE of 16.86%, while PM6/PYT-1S1Se-4cl achieved a PCE of 15.83%. Wu et al^[64] used Y5-C20 derivatives as the main building unit, respectively selecting furan, thiophene, and selenophene as the copolymer connection units to synthesize three polymer acceptors PY-O, PY-S, and PY-Se. Devices based on PBDB-T:PY-Se, PBDB-T:PY-O, and PBDB-T:PY-S obtained PCEs of 15.48%, 9.80%, and 14.16%, respectively. Du et al^[65] used benzothiadiazole (BS) and benzotriazole (BN) as the central cores and selenophene as the π-bridge connection unit to synthesize two polymer small molecule acceptors PS-Se and PN-Se. Ultimately, the PCE of the PBDB-T:PN-Se all-polymer solar cell was 16.16%, with a V_OC of 0.907 V, a J_SC of 24.82 mA/cm², and an FF of 0.718. Fan et al^[66] designed and developed a polymer small molecule acceptor PFY-3Se containing multiple selenium in the central skeleton and copolymer connection units. The all-polymer solar cell based on PBDB-T:PFY-3Se obtained a PCE of 15.1%. Subsequently, Fan et al^[67] introduced vinyl groups based on PFY-3Se to synthesize an asymmetric polymer small molecule acceptor PY3Se-1V. The binary all-polymer solar cell based on PBDB-T:PY3Se-1V obtained a PCE of 13.2% and a J_SC of 25.9 mA/cm². Wu et al^[68] used selenophene and thiophene to connect Y6 derivatives to synthesize two dimer acceptor materials DIBP3F-Se and DIBP3F-S. The polymer solar cell based on DIBP3F-Se obtained an efficiency of 18.09%, higher than the efficiency of 16.11% for the polymer solar cell based on DIBP3F-S. More importantly, they exhibited good compatibility with donors D18 and PTQ-10, with PCEs exceeding 17%. Bai et al^[69] designed and synthesized dimers DYSe-1 and DYSe-2 using thiophene and selenophene as connecting units based on YSe. Ultimately, OSCs based on DYSe-1 and DYSe-2 achieved PCEs of 18.56% and 18.22%, respectively.

As shown in Figure 5, it illustrates the chemical structures of other types of acceptors containing selenophene, and Table 4 summarizes the optical, electrochemical, and photovoltaic performance of other types of acceptors containing selenophene. In addition to the well-known ITIC and Y6 and their derivatives, selenophene has also been successfully applied to a new family of small molecule NFAs. Within this family, molecules containing selenophene exhibit higher conjugated backbone planarity compared to their thiophene counterparts, leading to a redshift in the absorption spectrum and an improvement in intermolecular electron transfer efficiency. Moreover, the introduction of selenophene increases the dipole moment of the molecule, which helps reduce energy loss, thereby decreasing the exciton binding energy within the active layer. Molecules containing selenophene have larger dipole moments and dielectric constants compared to those with thiophene, resulting in reduced energy losses and smaller exciton binding energy within the active layer. When selenophene-containing acceptors are used as the main acceptor in ternary organic solar cells, significant enhancements in charge transport and energy transfer performance can be observed^[70]. Although the S…O non-covalent interactions in thiophene and Se…O non-covalent interactions in selenophene both contribute to improving the planarity of the conjugated backbone, molecules containing selenophene demonstrate superior performance over thiophene-based devices in terms of thin-film morphology, electron mobility, and overall efficiency^[71⇓-73]. These findings underscore the crucial role of selenophene in optimizing the performance of small molecule NFAs.

Privado et al^[70] synthesized two non-fullerene acceptors, MPU1 and MPU4, by connecting a diketopyrrolopyrrole central core with two rhodanine (Rh) end units using thiophene and selenophene, respectively. Ultimately, after SVA treatment, the PCE of the SM1:MPU4 device was 8.96%, higher than the 7.22% of the SM1:MPU1 device. The ternary SM1:PC71BM:MPU4 device achieved 10.05%. Subsequently, they^[71] replaced the two rhodanine (Rh) end units of PMU2 with dicyano rhodanine to synthesize the non-fullerene small molecule acceptor MPU5. Ultimately, the PSC with MPU5 as the acceptor and PCDTBT as the donor achieved a PCE of 10.10% with an energy loss of 0.43 eV. They^[72] replaced the selenophene π-bridge of MPU5 with thiophene-based ethynyl selenophene to synthesize the non-fullerene small molecule acceptor MPU6. The TDTBTA:MPU6-based device achieved a PCE of 13.14%, while the TDTBTA:PC71BM:MPU6-based device achieved a PCE of 15.60%. They^[73] also synthesized two small molecule acceptors, MPU7 and MPU8, by connecting the central core of diketopyrrolopyrrole and the end group CPTCN using thiophene-based ethynyl selenophene and thiophene-based ethynyl thiophene, respectively. Ultimately, the PSC based on P:MPU7 achieved a PCE of 14.12%, while the P:MPU8 device achieved a PCE of 12.65% with an energy loss of 0.45 eV.