1 Introduction
Organic solar cells (OSCs) have attracted much attention because of their solution processability, light weight, flexibility, large area of translucent roll-to-roll printing, and colorful colors
[1⇓~3]. Researchers have done a lot of research work on device structure, active layer materials, active layer morphology, interface layer materials, device technology and so on
[4,5]. Among them, the research on the chemical structure optimization of active layer materials is the most in-depth, and the energy conversion efficiency (PCE) of devices has also made rapid progress, with the efficiency of single-junction devices exceeding 19% and that of stacked devices exceeding 20%
[6,7][8]. The active layer materials of organic solar cells mainly include donor materials and acceptor materials. Among them, the donor materials mainly use polymer donors with donor-acceptor (D-A) alternating copolymerization, while the acceptor materials mainly include fullerene acceptors and their derivatives and non-fullerene acceptors, which are widely used in the early stage. From the development trend of acceptor materials, it can be seen that non-fullerene acceptor materials, especially the fused ring acceptor materials with acceptor-donor-acceptor (A-D-A) structure, have injected new vitality into the development of organic solar cells
[9⇓⇓~12].
In 2015, Zhan Xiaowei et al. Of Peking University reported that the A-D-a type fused ring small molecule receptor using the indacene dithieno-thiophene (IDTT) structure as the intermediate electron donor core achieved device efficiency comparable to that of fullerene receptor
[13]. With the deepening of research, in 2019, Zou Yingping of Central South University introduced the electron-deficient benzothiadiazole (BT) unit into the fused ring core, designed and synthesized the small molecule receptor Y6 with A-D-A '-D-A structure, and obtained 15.7% device efficiency. Recently, researchers have increased the device efficiency based on this material system to more than 19%
[14][15]. Although organic solar cells based on fused ring acceptor materials have made breakthrough research progress, acceptor materials with fused ring conjugated skeleton usually have long synthetic routes.In particular, the Friedel-Crafts alkylation ring-closure reaction usually has a certain substrate adaptability, which leads to a low reaction yield, resulting in a relatively high synthesis cost of this kind of materials
[16⇓⇓~19]. Therefore, it is particularly important to develop low-cost and efficient active layer materials. Based on this, researchers have developed a series of non-fused ring electron acceptors (NFREAs) that use C — C single bonds to connect the intermediate core units. This material with non-fused ring structure greatly reduces the complexity of synthesis and production cost. However, the molecular structure connected by a single bond has poor stability, and the adjacent units may have problems such as torsion of the single bond, which has a negative impact on the planarity and rigidity of NFREAs molecules
[20,21]. To this end, researchers introduce atoms with strong electronegativity, such as F, O, S, N, etc., into the system to form intramolecular non-covalent interactions such as F … H, F … S, S … O, S … N to inhibit the rotation of single bonds, so as to construct a rigid coplanar structure, improve molecular stacking and charge transport performance, and achieve the purpose of improving the photovoltaic performance of non-fused ring OSCs. For example, the PTIC molecule designed and synthesized by Chen Hongzheng and Li Changzhi of Zhejiang University has formed a good intramolecular S … O non-covalent conformation lock
[22].
Good molecular planarity is obtained, and the corresponding device achieves a device efficiency of 10.27%. In addition to using intramolecular non-covalent conformational lock to maintain the coplanarity of the molecule, researchers also use large steric side chains to inhibit the torsion of single bonds to achieve excellent conformational stability, thus obtaining excellent molecular planarity and ensuring good charge transport performance. For example, the A4T-16 molecule reported by Hou Jianhui et al. Of the Institute of Chemistry of the Chinese Academy of Sciences has obtained good conformational stability and three-dimensional network structure of charge transport channels by introducing a large sterically hindered 2,4,6-triisopropylphenyl side chain, and has obtained a PCE of more than 15%, which can be comparable to the device efficiency of the corresponding fused ring acceptor materials
[23].
Although the device efficiency of non-fused ring acceptor materials is rising steadily, compared with the photovoltaic performance of fused ring acceptor materials, non-fused ring receptor materials still have a large space for development. Therefore, it is very important to systematically study the structure-property relationship (SAR) of non-fused ring acceptor materials, deeply understand the design strategy of non-fused ring acceptor materials, further improve the photovoltaic performance of non-fused-ring acceptor systems, and obtain low-cost and high-efficiency material systems. Based on this, from the perspective of molecular design, this paper briefly analyzes the influence of the structure of non-fused ring acceptor materials on the basic properties, aggregation structure, molecular packing, morphology of active layer and corresponding photovoltaic performance of the materials, and establishes the relationship between the structure and performance of some materials, which provides a reference for the subsequent design and synthesis of non-fused ring acceptor materials with higher performance. Finally, the development of the field is analyzed and prospected from the aspects of material design, device optimization, morphology optimization and device stability.
2 Research Progress of Non-fused Ring Electron Acceptor
2.1 Origin of intramolecular noncovalent interactions.
As shown in Figure 1, the researchers first used indacene dithiophene (IDT) as the central electron donor core to construct NFREAs. Because the IDT unit has a controllable rigid coplanar aromatic structure, it can effectively inhibit the disordered rotation of the molecule and reduce the reorganization energy. Therefore, the early non-fused ring acceptors mostly use the fused ring core of IDT to connect with 2- (3-oxo-2,3-dihydro-1H-inden-1-yl) malononitrile (INIC), and through the introduction of different π-conjugated bridging units, the optical, electrochemical and photovoltaic properties of NFREAs can be finely tuned.
In 2017, Bo Zhishan et al. Of Beijing Normal University designed and synthesized acceptor molecules IDT-BC6 and IDT-BOC6 with IDT as the electron donor core and 2,5-dihexylphenylene (IDT-BC6) and 2,5-di (hexyloxy) phenylene (IDT-BOC6) as π-conjugated bridging units, respectively
[24]. Benefiting from the existence of the oxygen atom in the π-bridging unit, the π-bridging unit and the intermediate core unit form an intramolecular S … O non-covalent bond interaction to form a conformational lock to achieve the purpose of locking the conformation. Therefore, compared with IDT-BC6, the IDT-BOC6 molecule exhibits a good planar conformation, a significantly red-shifted spectral absorption, a higher and balanced carrier mobility, and a lower energy loss. When blended with polymer PBDB-T, the open circuit voltage (V
oc), short-circuit current density (J
sc) and fill factor (FF) of the device are much higher than those of the device based on IDT-BC6, reaching 9.6%. Next, in order to optimize the geometry of the molecule, they further used the thiophene unit as a π-bridging unit to synthesize the non-fused ring receptor ITOIC
[25]. According to the results of density functional theory (DFT), the π-conjugated bridging unit can form intramolecular S … O, O … H non-covalent interactions with the thiophene unit in the middle core and the INIC at the end, which enhances the planar conformation of the aromatic backbone. When an electron-deficient F atom is introduced into the terminal group of the ITOIC molecule, the light absorption range of the acceptor molecule can be effectively broadened, the intramolecular interaction can be enhanced, and the charge transport can be promoted. As a result, the device efficiencies based on ITOIC-F and ITOIC-2F are improved to 10.65% and 12.17%, respectively.
In order to further reduce the optical band gap of the receptor material, Hou Jianhui et al., Institute of Chemistry, Chinese Academy of Sciences, introduced an alkoxy side chain into the thiophene bridging unit of the non-fused ring receptor IEIC, and designed and synthesized a new narrow band gap receptor molecule IEICO
[26]. Compared with IEIC, the lowest unoccupied molecular orbital (LUMO) level of IEICO is only 0. 05 eV lower, while the highest occupied molecular orbital (HOMO) level is 0. 15 eV higher, although IEICO shows a significant red-shifted absorption. Therefore, the photovoltaic device based on IEICO shows higher J
sc and V
oc, and the PCE of the device is significantly improved to 8. 4%. Subsequently, they continued to introduce fluorine and chlorine atoms with strong electron-pulling ability into the terminal unit of IEICO to enhance the intramolecular charge transfer (ICT) effect, broaden the absorption of acceptor molecules and promote charge transport. Finally, photovoltaic devices based on PBDTTT-EFT: IEICO-4F and PTB7-Th: IEICO-4Cl achieved PCEs of 10.0% and 10.3%, respectively
[27][28]. The above analysis shows that the formation of conformational locks through the formation of intramolecular S … O, O … H interactions can effectively improve molecular planarity and optimize molecular stacking and charge transport. The introduction of halogen atoms in the terminal group can effectively reduce the molecular band gap, broaden the absorption spectrum, enhance the ICT effect of the molecule, promote charge transport, and then improve the performance of photovoltaic devices. The structural formula of the above NFREAs is shown in Fig. 1, and the corresponding photovoltaic data are summarized in Table 1.
2.2 Application of Intramolecular Noncovalent Interactions in Nonfused Ring Receptors
It is inspired by the early use of intramolecular non-covalent interactions to lock the molecular skeleton structure to form materials with coplanar receptors. In 2017, Chen Hongzheng et al. Of Zhejiang University used 2,5-difluorobenzene (DFB) as the non-fused ring core of the receptor molecule and connected two cyclopentadithiophene (CPDT) units with a single bond to synthesize the non-fused ring receptor DF-PCIC with A-D-D '-D-A structure for the first time (Fig. 2)
[29]. The F atom on the DFB unit and the H atom on the adjacent thiophene unit can form an intramolecular F … H noncovalent conformational lock, so that the non-fused ring electron acceptor DF-PCIC has a coplanar structure similar to the fused ring acceptor. At the same time, the CPDT unit can effectively inhibit the excessive aggregation between molecules, so that the active layer can form a more suitable phase separation. Finally, the DF-PCIC: PBDB-T based device achieves a PCE as high as 10.14%. In order to improve the J
sc of photovoltaic devices based on DF-PCIC, they further introduced halogen atoms with strong electron-withdrawing properties to the terminal groups of DF-PCIC, and synthesized two novel non-fused ring acceptors, HF-PCIC and HC-PCIC
[30][31]. The introduction of the halogen atom further enhances the ICT effect of the acceptor molecule, so that the material shows an obvious red-shifted absorption spectrum, and the J
sc of a corresponding photovoltaic device is obviously improved. The PCE based on HF-PCIC and HC-PCIC photovoltaic devices reached 11.49% and 11.57%, respectively.
Benefiting from the intramolecular F … H non-covalent interaction between the F atom on the non-fused ring core and the H atom on the adjacent π-bridge unit, a better rigid coplanar conformation can be obtained by using the single-bond-connected framework structure. However, the more electronegative F atom will reduce the electron donating ability of the central D ′ unit, thus narrowing the absorption spectrum of the acceptor. In 2019, Bo Zhishan et al. Of Beijing Normal University designed and synthesized a series of new non-fused ring electron acceptors (DOC6-IC, DOC8-IC, DOC2C6-IC and DOC 2C6-2F) on the basis of DF-PCIC molecule, using 2,5-bis (alkyloxy) phenyl substituted 2,5-difluorobenzene (DFB) as the core unit
[32]. The O atom on the middle core unit and the S atom on the adjacent CPDT can form an intramolecular S … O non-covalent conformational lock, so that the geometric configuration of the receptor molecule shows a good coplanar conformation. With the enhancement of the electron donating ability of the central D 'unit, the ICT effect of the corresponding material is further enhanced, resulting in an obvious red-shifted absorption spectrum. Compared with the photovoltaic device based on PBDB-T: DF-PCIC, when DOC2C6-2F was blended with polymer donor PBDB-T, the J
sc of the device was significantly improved, up to 21.35 mA·cm
-2, and its PCE reached 13. 24%. At the same time, Chen Yongsheng et al. Of Nankai University designed and synthesized a series of non-fused ring acceptor molecules by carefully adjusting the length and steric hindrance of the alkyl side chain on the intermediate electron donor unit, and studied the influence of the change of the side chain structure on the morphology of the active layer and the performance of the device
[33]. The results show that the active layer morphology based on the isooctyl substituted acceptor molecule UF-EH-2F achieves the effective combination of "high efficiency state" and "stable state", and the photovoltaic device achieves the highest PCE (13. 56%) and the optimal device stability (the efficiency is only reduced by 5% after storage for 1356 H). This study elucidates an important link between the alkyl chain of the acceptor molecule and the aggregation state of the active layer. By optimizing the alkyl side chain of the acceptor molecule, the aggregation state of the morphology of the active layer can be effectively regulated, so that the molecular packing is more compact and orderly, thereby obtaining a photovoltaic device with high efficiency and high stability.
In addition to the use of alkyl chains to regulate the aggregation state and active layer morphology of materials, the cooperation between our research group and Xi'an Institute of Modern Chemistry has also proposed the strategy of synergistic halogenation of the middle core and terminal groups of materials to achieve the regulation of photovoltaic performance of materials
[34]. Based on this, we designed and synthesized a series of non-fused ring electron acceptors, BDTC-4F, F-BDTC-4F, and F-BDTC 4Cl, with similar conjugated backbones. The molecular structure is optimized by the synergistic effect of the main chain and the end group halogenation modification, and the device performance is improved. Among them, F-BDTC-4Cl with F atom replacing the side chain unit and Cl atom replacing the terminal group was matched with the polymer donor PBDB-T, the bimolecular recombination of the device was significantly suppressed, and the electron mobility (μ
e) was significantly improved, reaching 10.28% 0.71×10
-4cm
2V
-1s
-1,PCE. Tang Weihua et al. Of Nanjing University of Science and Technology designed and synthesized DNO15T, DNO26T and DNO27T by using molecular isomerization and asymmetric strategies to adjust the molecular packing, film morphology and photoelectric properties of receptor materials
[35]. All three molecules show excellent coplanarity due to intramolecular non-covalent S … O and O … H interactions. Compared with DNO26T and DNO27T, DNO15T with 1,5-alkoxy substitution exhibited the best phase separation morphology, broadened spectral absorption, enhanced crystallinity and promoted charge transport after blending with polymer donor PBDBT-T. Thus, the device based on PBDB-T: DNO15T obtained a PCE of 10.72%. The structural formulas of the above corresponding molecules are shown in Fig. 2, and the relevant photovoltaic data are summarized in Table 2.
In addition to using the electron-donating unit as the intermediate core, the researchers also used the N atom in the electron-deficient unit to form an intramolecular S … N non-covalent interaction with the S atom in the adjacent electron-donating thiophene bridging unit to optimize the geometry of the acceptor molecule. Therefore, the A ′ unit with electron pulling ability was used as the intermediate core to construct NFREAs. Uch as benzothiadiazole (BT), benzotriazole (BTz), quinoxaline (Qx) and the like with electron-deficient properties
[36][37,38][39,40].
In 2020, Huang Fei et al. Of South China University of Technology synthesized a new type of non-fused ring receptor BTCIC with A-D-A '-D-A structure by replacing 2,5-difluorobenzene (DFB) with benzothiadiazole (BT) unit with electron-deficient group as the non-fused ring core
[41]. Compared with DF-PCIC, the introduction of BT units can effectively enhance the intramolecular charge transfer of BTCIC materials, resulting in a significant red-shift absorption spectrum and a significant expansion of the spectral response range. In addition, the N atom on the BT unit and the S atom on the adjacent CPDT unit can form an intramolecular S … N noncovalent interaction, so that the BTCIC molecule exhibits good coplanarity. At the same time, the 2-ethylhexyl side chain on the CPDT unit not only provides an effective steric effect to avoid excessive aggregation, but also further ensures sufficient solubility. When Cl atoms are introduced on the terminal groups of BTCIC molecules to optimize the absorption, energy level, and packing properties of the molecules, the devices based on PBDB-T: BTCIC-4 Cl achieve PCEs as high as 10.5%. Subsequently, Bo Zhishan et al. Of Beijing Normal University synthesized two new non-fused ring receptors BT2F-IC4F and BTOR-IC4F by introducing F atom and 2-ethylhexyloxy group into BT unit respectively
[42]. The O and F atoms on the middle core unit can form intramolecular S … O, S … F noncovalent interactions with the S atoms on the bridging unit, optimizing the geometry of the non-fused ring molecule. As a result, the efficiency of the device based on PBDB-T: BTOR-IC4F is improved to 11.48%.
In 2021, Huang Hui of the University of Science and Technology of China designed and synthesized BTzO-4F, a non-fused ring receptor with benzotriazole as the intermediate core
[43]. The alkoxy-substituted BTz unit acts as a non-fused ring intermediate core, which increases the LUMO energy level of the acceptor molecule to 3. 88 eV. Benefiting from the intramolecular S … O and N … H noncovalent conformational locking, the receptor molecular structure shows good planarity. When blended with polymer donor PBDB-T to fabricate photovoltaic devices, a high V
oc of 0.84 V and a PCE of 13.8% were achieved. Subsequently, they introduced flexible alkyl side chains into both sides of the π-bridging unit of the BTzO-4F molecule to lock the geometry of the terminal group and the bridging unit. The introduction of the alkyl chain improves the LUMO energy level of NoCA-5, the crystallinity of the material and the rigid structure of the molecule
[44]. When J52 was used as the polymer donor, a PCE as high as 14.82% was obtained.
In addition, Chen Hongzheng and Shi Minmin of Zhejiang University first used quinoxaline as an intermediate core to design and synthesize a series of receptor molecules QCIC1, QCIC2, QCIC3 and QCIC 4 with the same conjugated backbone
[39]. The geometric configuration, packing behavior and photovoltaic performance of the receptor molecule are regulated by the synergistic effect of introducing different alkyl side chains into the conjugated backbone and introducing halogen atoms into the end. Benefiting from the formation of effective intramolecular S … O and N … S noncovalent interactions, the four receptors exhibit good molecular planarity. Among them, the blend film based on PBDB-T: QCIC3 exhibited more optimized π-π stacking and the most suitable phase separation, thus achieving the highest FF (66.9%) and PCE (10.55%). Subsequently, Chang Meijia et al. Of Luoyang Institute of Technology designed and synthesized UF-Qx-2F and UF-Qx-2Cl with aryl substituted quinoxaline unit as the intermediate core and CPDT as the bridging unit
[40]. Compared with QCIC3, in addition to the formation of intramolecular N … S and S … O conformational locks, UF-Qx-2Cl containing the phenyl side chain can effectively inhibit the torsion between the middle quinoxaline and the CPDT bridging unit, and the molecular planarity is further enhanced. Compared with UF-Qx-2F with fluorine atom substitution, the UF-Qx-2Cl-based blend film exhibits more red-shifted absorption, efficient charge transport, and ordered molecular packing. Thus, the device based on J52: UF-Qx-Cl achieves a PCE up to 10.81% and superior stability.
The above results show that the introduction of an electron-deficient intermediate core or the introduction of heteroatomic fluorine or alkoxy chains to form a conformational lock further improves the planarity and rigid structure of the molecule.It can enhance the intramolecular charge transfer of the material, adjust the light absorption range of the acceptor material, and then optimize the geometric structure and stacking mode of the molecule to obtain a non-fused ring acceptor material system with further improved photovoltaic performance. The structural formulas of the above related materials are shown in Fig. 3, and the corresponding photovoltaic data are summarized in Table 2.
2.3 Completely non-fused ring electron acceptor
In the early structures of non-fused ring electron acceptors, CPDT, DFB, BT and other common building units are often used, which can not only provide enough steric hindrance to inhibit the single bond torsion of adjacent units, but also facilitate the fine control of chemical structure, thereby adjusting the absorption, energy level and stacking properties of molecules. With the deepening of research, in order to further simplify the synthesis and post-processing of materials and reduce the cost of synthesis, researchers have developed electron acceptors with completely non-fused ring structure, that is, to construct small molecule acceptors with completely non-fused ring skeleton structure (Fig. 4).
In 2019, Chen Hongzheng et al. Of Zhejiang University designed and synthesized PTICH, PTIC and PTICO, which are completely non-fused ring receptors with simple structures, by using monothiophene as a bridging unit and an alkoxyphenyl intermediate core through a two-step method
[22]. Among them, an effective intramolecular O … H noncovalent conformational locking is formed between the non-fused ring core 2,5-bis (alkyloxy) phenyl group and the adjacent thiophene bridging unit, which can ensure its planar conformation. In addition, by changing the side chain structure on the thiophene bridging unit, not only the solubility of the molecule can be adjusted to avoid excessive aggregation, but also the introduction of flexible side chains at both ends of the bridging unit can further inhibit the torsion of the terminal group and enhance the order of molecular packing. Therefore, the PTIC-based single-junction device achieves a PCE as high as 10.27%, while the device shows good illumination stability under continuous illumination.
In 2021, Bo Zhishan of Beijing Normal University and Song Jinsheng of Henan University developed a series of completely non-fused ring electron acceptors (4T-1, 4T-2, 4T-3, 4T-4) based on the tetrathiophene structure
[45]. The solubility of NFREAs, the morphology of the blend film, and the packing pattern were tuned by changing the alkyl side chain on the thiophene unit. Among them, 4T-3 substituted by 2-ethylhexyl showed the most suitable crystallinity, and showed good compatibility after blending with polymer donor PBDB-TF. The photovoltaic device based on PBDBT: 4 T-3 obtains a PCE of 10.15%. When polymer D18 was used as the polymer donor, both the V
oc and the J
sc of the device were improved, and the device efficiency was increased to 12. 04%.
Medium-bandgap semiconductors have unique applications in indoor photovoltaics and tandem photovoltaics, so low-cost and high-efficiency medium-bandgap NFREAs have broad research prospects. On the basis of previous work, Li Changzhi et al. Of Zhejiang University designed and synthesized two complete non-fused ring electron acceptors TPT4F and TPT4Cl with medium band gap
[46]. Due to the formation of intramolecular F … H, S … O non-covalent interactions, the corresponding materials exhibit a good planar conformation. Compared with TPT4F, TPT4Cl with terminal groups substituted by Cl atoms exhibits more orderly π-π stacking and stronger photoluminescence in the thin film state. Therefore, the PBDB-TF: TPT4Cl-based device shows a more balanced electron/hole transport, and the corresponding photovoltaic device achieves a PCE of 10.16%, which is significantly higher than the efficiency of the TPT4F-based device (7.67%).
The above results indicate that the planarity of the completely non-fused ring receptor molecule can be further improved by adjusting the chemical structure of the material, introducing halogen atoms, alkoxy groups, and different types of side chain units to form an effective intramolecular non-covalent conformational lock, which can effectively adjust the absorption, energy level, molecular stacking, and charge transport properties of the molecule. Through the regulation of the chemical structure of the completely non-fused ring receptor material, the formation of non-covalent conformational locking can reduce the cost, simplify the synthesis route, and obtain higher device performance, which also provides a reference for the design of non-fused ring receptors. The relevant performance data of the above materials are shown in Table 3.
1.4 Application of large sterically hindered side chain in non-fused ring receptor
Although the efficiency of non-fused ring OSCs has made a breakthrough, its efficiency is still lower than that of fused ring devices. The main reason is that the C-C single bond between the non-fused ring molecular skeletons inevitably produces the torsion of the single bond. Although the strategy of introducing heteroatoms to form intramolecular non-covalent interactions has suppressed the single bond torsion to some extent, the conformational stability of non-fused ring receptors is still poor compared with the receptor molecules with fused ring conjugated structures. In order to solve this problem, researchers have tried to introduce large steric hindrance groups into the non-fused ring skeleton to further optimize the geometric structure and packing mode of the molecule, and to achieve conformational stability and improve the photovoltaic performance of non-fused ring receptor materials. The chemical structures of some of the NFREAs employing large sterically hindered phenyl side chains are shown in Fig. 6. The relevant photovoltaic performance is summarized in Table 4.
In 2018, Hou Jianhui et al., Institute of Chemistry, Chinese Academy of Sciences, designed and synthesized two NFREAs (BTCN-O and BTCN-M) with the same conjugated backbone and different types of thiophene substituents introduced into the middle core
[47]. Both materials show similar optical absorption spectra and molecular energy levels in solution, however BTCN-O shows a clear red-shifted absorption in the film state. The steric hindrance of the middle core can be effectively changed by manipulating the position of the thiophene side chain on the middle core. As shown in Fig. 5, from the grazing incidence wide-angle X-ray diffraction (GIWAXS) results, BTCN-O with low steric hindrance in the middle core forms an ordered layered stacking structure and dense π-π stacking in the film state, which can be used as an electron donor in photovoltaic devices. However, BTCN-M molecules with high steric hindrance show only weak π-π stacking effect and prefer to act as electron acceptors. When they were blended with PBDB-T and fullerene acceptor PC
71BM, respectively, to fabricate photovoltaic devices, the device based on BTCN-O:PC
71BM (PCE of 6.68%) exhibited better photovoltaic performance than that based on BTCN-M:PC
71BM (PCE of 0.29%), while the efficiency of the device based on PBDB-T: BTCN-M reached 5.89%. It shows that the effective regulation of the alkyl chain in the middle core can change the steric hindrance of the material, which is very important for adjusting the electron donating and electron deficient ability of the material.
In 2021, on the basis of previous work, Chen Hongzheng and Li Changzhi of Zhejiang University introduced a sterically hindered hexylbenzene side chain into the thiophene bridging unit of PTIC molecule, and designed and synthesized non-fused ring electron acceptors PTB4F and PTB4Cl by using double fluorine and double chlorine atom substituted INIC as terminal groups, respectively
[48]. The results show that the existence of the sterically hindered two-dimensional side chain in the material can effectively inhibit the torsion of the terminal group, which is conducive to obtaining excellent stacking morphology of the blend film and improving the molecular stacking order, and is conducive to further prolonging the exciton lifetime in the bulk heterojunction (BHJ) blend film and promoting the rapid transfer of charges. In addition, the introduction of phenyl side chain can also provide sufficient steric hindrance to help regulate the intermolecular packing. Therefore, the device based on PBDB-TF: PTB4Cl can achieve a PCE as high as 12.76%, which is 181% and 124% higher than that based on PTB4F and PTIC, respectively, through the introduction of a simple bulky side chain.
Subsequently, Bo Zhishan et al. Of Beijing Normal University designed and synthesized a series of completely non-fused ring electron acceptors o-4TBC-2F and m-4TBC-2F with tetrathiophene as the conjugated backbone
[49]. They introduced phenyl groups on the two thiophene units of the middle core as bulky side chains, and optimized the geometry of the receptor molecule by changing the position of hexyloxy groups on the phenyl side chains. When the hexoxy chain is in the ortho position of the phenyl side chain, it can provide enough steric hindrance, and the o-4 TBC-2F molecule can form a better planar conjugated skeleton and obtain a significant red shift in the absorption spectrum. When blended with polymer PBDB-T, the device based on o-4 TBC-2F shows a significantly improved photovoltaic performance (10.26%), which is much higher than that of the device based on m-4 TBC-2 F (2.63%). Recently, Hou Jianhui et al., Institute of Chemistry, Chinese Academy of Sciences, also reported a series of non-fused ring electron acceptors (A4T-16, A4T-21, A4T-23) based on tetrathiophene as the conjugated backbone
[23]. The geometry of the receptor molecule was optimized by changing the side chain steric hindrance of the two thiophene groups in the middle core. According to the theoretical calculation and single crystal data, A4T-16 with 2,4,6-triisopropylphenyl side chain group has a sterically hindered side chain almost perpendicular to the middle conjugated skeleton, which further improves the molecular coplanarity and rigid skeleton structure. In addition, the material forms a three-dimensional interpenetrating network of electron transport channels, which further improves the charge transport. Therefore, the FF of the PBDB-TF: A4T-16 based device was improved to 79.80%, and its PCE reached 15.20%, which was the highest efficiency of the non-fused ring system based device at that time.
Then, Bo Zhishan et al. Of Beijing Normal University designed and synthesized a series of non-fused ring electron acceptors (CH
3-2F, OCH
3-2F, SCH
3-2F, 2TH-2F) with diphenylamino substituents introduced into the middle core
[50]. The sterically hindered diphenylamine substituent and the slightly distorted molecular skeleton can improve the solubility of the non-fused ring acceptor, while the introduction of the electron-donating nitrogen atom can further enhance the ICT effect of the molecule, resulting in a red-shifted absorption spectrum. Among them, the CH
3-2F molecules show a face-on stacking orientation, and the molecular packing is more orderly, and the PBDB-T:CH
3-2F based blend film has the strongest crystallinity and the most effective charge transport. Thus, the PCE of the CH
3-2F based device reaches 12.28%. Subsequently, they designed and synthesized a series of non-fused ring electron acceptors (2Th-2F, BTh-Th-2F, and 2BTh-2F) with large steric hindrance groups by further extending the length of the conjugated bridging unit based on CH
3-2F
[51]. The molar extinction coefficient (ε) of the acceptor and the electron mobility (μe) of the blend film increase with the increase of the π conjugation length. The PCE of the corresponding devices is more than 10%. It can be seen from the single crystal X-ray diffraction results (Fig. 7 a, B) that the molecular packing changes from the 2D packing mode of 2Th-2F molecules to the 3D network packing structure of 2BTh-2F, which is conducive to charge transport and reduces the energy loss of the device. Finally, the PCE based on PBDB-T: 2BTh-2 F was as high as 14.53%.
When D18 with a deeper HOMO level was used as the polymer donor, the Jsc and FF of the device were improved, and the PCE of the photovoltaic device based on D18: 2BTh-2 F was further improved to 15. 44%, which is also the highest device efficiency based on non-fused ring acceptor materials.
Subsequently, they optimized the geometry of the non-fused ring receptor on the basis of the 2 Th-2 F molecule by changing the position of the side chain group on the π bridging unit and changing the type of the phenyl group on the middle core
[52]. According to the results of DFT theoretical calculation, the two 2,4,6-triethylphenyl substituents on TT-C8T and TT-TC8 are almost perpendicular to the main chain of the molecule, which makes the receptor material have good solubility and can effectively inhibit the excessive aggregation of the material. When the substituent on the π-bridging unit in the TT-C8T molecule is substituted to the No.3 position, the TT-TC8 molecule exhibits a better coplanar conformation, enhanced charge transport, and tight molecular packing. Finally, the photovoltaic device based on D18: TT-TC8 achieved a PCE of 13.1%. Since then, Bo Zhishan of Beijing Normal University has studied the influence of the position of the sterically hindered side chain on the basic properties and photovoltaic properties of materials, and then explained the influence of molecular isomerization
[53]. They also designed and synthesized non-fused ring electron acceptors LW-in-2F (diarylamine in the interior) and LW-out-2F (diarylamine in the exterior) by using diarylamine groups as sterically hindered side chains. As shown in fig. 7c, when the diphenylamine side chain is located on the outside, the acceptor molecule exhibits a preferential face-on stacking orientation, which is conducive to obtaining vertically distributed charge transport, resulting in high and balanced charge mobility. Therefore, the PCE of the LW-out-2F-based device is as high as 12.83%. Subsequently, Hou Jianhui et al., Institute of Chemistry, Chinese Academy of Sciences, designed and synthesized A4T-25 and A4T-26 by introducing 2,4,6-triisopropylphenyl into different positions of two thiophene units in the middle core
[54]. According to the charge transfer integral calculated from the single crystal structure, A4T-26 can achieve effective charge transfer. Therefore, the electron mobility (μ
e) of the A4T-26-based device is 1.44×10
-4cm
2·V
-1·s
-1, which is much higher than the A4T-25-based electron mobility 1.03×10
-5cm
2·V
-1·s
-1. When PBDB-T is used as the polymer donor, the device based on A4T-26 achieves a PCE of 12.1%. The above studies show that the bulky side chain can provide sufficient steric hindrance to the host molecule, improve the planarity and rigid skeleton structure of the receptor molecule, ensure the formation of effective π-π stacking between the receptor molecules, reduce the negative impact caused by single bond torsion, and improve the conformational stability of the receptor molecule. In addition, the basic properties, aggregation state and photovoltaic performance of materials can be effectively regulated by regulating the chemical structure of the large sterically hindered two-dimensional side chain, which provides more possibilities for optimizing the geometric configuration and stacking mode of non-fused ring electron acceptors.
3 Conclusion and prospect
In this paper, the development and design strategies of non-fused ring electron acceptors are briefly introduced, and the latest research progress of materials based on non-fused ring acceptors is emphatically analyzed, which provides a reference for further understanding the structure-property relationship of non-fused ring acceptors. At present, the optimal device performance based on NFREAs is close to 16%, and the main challenge to further improve the photovoltaic performance of devices comes from how to control the geometry and stacking mode of NFREAs molecules. Poor molecular conformation and packing mode may produce undesirable phase separation and film morphology, which in turn affect the exciton dissociation, charge transport of the corresponding devices leading to lower Jsc and FF. Unlike fused ring acceptor materials, NFREAs can produce relatively disordered molecular packing and elevated reorganization energy due to the torsion of single bonds, which affects charge transport and thus limits their photovoltaic performance. In addition, the efficient design of non-fused ring acceptor materials, the matching of suitable donor materials, and the morphology control of active layer will affect the photovoltaic performance of non-fused ring acceptor systems. In addition, the design and construction of low-cost and efficient active layer materials is also an important research topic for large-scale production of organic solar cells in the future, and non-fused ring acceptor materials are the most potential strategy to achieve this goal. Therefore, this paper briefly summarizes the development of NFREAs from the aspects of material design, device optimization and device stability, which has important guiding significance for the design of non-fused ring acceptor materials and the optimization of photovoltaic devices. At the same time, this paper will also look forward to the future development of non-fused ring acceptor materials from the aspects of material design, device optimization, morphology optimization and device stability.
3.1 Material design
The optimization of NFREAs materials is mainly through the following three ways: (1) adjusting the number of π conjugated bridging units and extending the conjugation length of the molecular backbone to adjust the molecular photoelectric properties and film morphology; (2) introducing different kinds of side chains into the bridging unit to adjust the absorption, energy level, stacking and other properties of the receptor molecule; And (3) introducing halogen atoms and other groups with strong electron-withdrawing properties into the terminal group to adjust the optical and photophysical properties of the material, such as band gap, morphology and the like. By introducing appropriate heteroatoms such as O, S, N, F, Cl into the framework structure through a precise molecular design strategy to form intramolecular S.. O, F.. H, S.. N interactions to form conformational locking,Or the introduction of large steric hindrance groups at appropriate positions in the conjugated backbone to further inhibit the torsion of single bonds is the main way to optimize the molecular geometry, achieve a rigid framework structure, and improve the performance of devices. In the design and synthesis of non-fused ring receptor materials in the future, non-fused ring receptor materials with simpler structures and simpler synthetic steps can be used to control the coplanarity and rigid skeleton structure of the materials through the synergistic use of intramolecular non-covalent conformational locking and the introduction of large steric hindrance side chains. In addition, the design direction of future NFREAs materials is to further improve the performance and stability of non-fused ring receptor materials by introducing more diverse intermediate cores, side chain units and bridging units, and to simplify the synthesis steps and reduce the synthesis cost.
3.2 Device optimization
For organic solar cells based on polymer donor/fused ring acceptor system, appropriate post-treatment optimization of photovoltaic devices is an important strategy to improve the performance of photovoltaic devices.The commonly used device optimization strategies mainly include the matching of appropriate donor/acceptor materials, the optimization of donor concentration, different additives, solvent vapor annealing (SVA), thermal annealing (TA), and the addition of a third component to construct ternary devices. The appropriate device optimization process is helpful for the active layer film of the blend system to form a suitable phase separation morphology, obtain a nano-scale interpenetrating network structure, and further improve the exciton dissociation and charge transfer efficiency, which is essential for further improving the device Jsc and FF. At present, most of the optimization strategies for photovoltaic devices with non-fused ring systems follow the strategies of the corresponding fused ring acceptor systems. In addition to the above material design to optimize the molecular geometry and improve the ordered stacking of materials to obtain excellent active layer morphology, the use of appropriate device optimization process, thin film preparation technology and other strategies are conducive to obtaining ideal active layer morphology, thereby improving the device performance of non-fused ring acceptor material system. Developing more simple and efficient post-treatment process, researching efficient green solvents, and selecting suitable third component to construct ternary devices are effective strategies to further improve the performance of photovoltaic devices and obtain low-cost and efficient non-fused ring photovoltaic devices.
3.3 Topography optimization
Active layer morphology is an important factor affecting the device performance, and the construction of blend morphology with excellent nanoscale interpenetrating network structure is essential for the formation of effective exciton dissociation and charge transfer. At present, the technologies of film morphology control mainly include solvent additives, SVA, TA, the combination of SVA and TA, and the addition of third components to construct ternary devices. Although the photovoltaic device based on non-fused ring acceptor materials has achieved a device efficiency of nearly 16%, its photovoltaic device performance still lags behind that of the corresponding fused ring acceptor materials. In the follow-up study, in addition to the above material design to optimize the geometric configuration of the molecule and enhance the ordered stacking of the material to obtain excellent active layer morphology,Appropriate device optimization process, blending film morphology optimization technology and other strategies are conducive to obtaining the ideal active layer morphology, thereby improving the device performance of non-fused ring acceptor material system. In addition, the development of green non-halogenated solvents or additives and other morphology control technologies will also play an important role in promoting the large-scale production of organic solar cells in the future.
3.4 Device stability
At present, most of the research on OSCs focuses on the design and synthesis of new materials, the optimization of active layer morphology, and the optimization of device structure, mainly focusing on improving the performance of photovoltaic devices. However, there are relatively few studies on the stability of devices. However, the stability of the device is directly related to the service life of the corresponding photovoltaic device and the production cost of large-scale production in the future, so it is of far-reaching significance to study the stability of the device. In the future development of organic photovoltaics, in addition to the design and synthesis of low-cost and efficient active layer materials, more research should be focused on the stability of devices. There are many factors affecting the stability of devices, such as oxygen, water, light, stable and mechanical stress, metastable morphology, diffusion of electrode and buffer layer materials. In recent years, with the in-depth research on the stability of OSCs, most of the external effects such as oxygen, water and so on can be overcome through the packaging process. However, there is still a lack of systematic control methods for the internal influencing factors such as the stability of the active layer (the morphology change of the active layer) and the interface stability (the diffusion of the carrier transport layer, electrode and active layer interface). In order to realize the commercial application of OSCs based on non-fused ring acceptors, the stability of photovoltaic devices needs to be further studied through material design, device process, active layer morphology control, new interface layer material design, packaging technology and process exploration, while continuing to study low-cost and efficient active layer materials in the future.
National Natural Science Foundation Research Project of Shaanxi Province(2021JQ-595)
National Natural Science Foundation of China(22109063)
Xi’an Shiyou University Graduate Innovation and Practical Ability Training Program(YCS21112074)
Scientific Research Program Funded by Shaanxi Provincial Education Department(20JK0845)
Key Projects of Henan Provincial Department of Education(21A150034)
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