Recent Progress on Solar Cell Performance Based on Structural Tailoring on DA'D Units of Nonfullerene Acceptors

Chaolumen Xue; Wanru Liu; Tuya Bai; Mingmei Han; Ren Sha; Chuanlang Zhan

doi:10.7536/PC201143

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (2) : 447-459 DOI: 10.7536/PC201143

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Recent Progress on Solar Cell Performance Based on Structural Tailoring on DA'D Units of Nonfullerene Acceptors

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Abstract

In the recent years, designing and synthesizing high-performance nonfullerene acceptor (NFA) materials have become the mainstream on research of organic solar cells (OSCs). Fused-ring based NFAs show high absorption coefficients, adjustable energy levels and bandgaps, straightforward synthesis, and excellent electrical and photovoltaic properties. The power conversion efficiencies (PCEs) have been increased from 3%~4% to 18% just in 7 years. In 2019, Zou et al. reported a smart acceptor molecule, named Y6, which supplied a very impressive PCE of 15.7% when pairing with PM6. Y6 and its analogs were featured with DA'D type fused-ring as the central electron-donating (D) units. The electron-accepting (A') unit was fused through nitrogen with two D units, which helped to downshift the Frontier molecular orbitals and enhance light absorption. Again, the steric effect between the two N-linked alkyl chains and the decorated chains on the fused thienothiophene-β-positions helped to increase the solubility and tune crystallinity. After the report of the smart Y6, intense investigations have been made on structure cutting of the molecule and tens of new structures have been reported. Among these new acceptors, structural tailoring on the DA'D moiety plays a vital role in improving device efficiency and solar cell performance. In this review, we focus on the recent advances on the structural modifications on A' and D units and side chains. We set up several sets of acceptors, by which we classify the recently reported molecules and correlate their optical, electrochemical, electrical and photovoltaic properties with the precise structural modulations so as to present a comprehensive review on the structure-property relationship.

Contents

1 Introduction

2 Progress in structural modification of A'unit

3 Progress in structural modification of DA'D unitπ-conjugate system

4 Effect of side chain

4.1 Effect of side chain R₁

4.2 Effect of side chain R₂

5 Conclusion and outlook

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Keywords

DA'D moiety / non-fullerene / small molecule / structural modification / organic solar cell

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Chaolumen Xue, Wanru Liu, Tuya Bai, Mingmei Han, Ren Sha, Chuanlang Zhan. Recent Progress on Solar Cell Performance Based on Structural Tailoring on DA'D Units of Nonfullerene Acceptors. Progress in Chemistry, 2022, 34(2): 447-459 DOI:10.7536/PC201143

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1 Introduction

With the continuous progress of human society, the demand for energy has increased dramatically, and the limited reserves and extensive use of fossil energy have brought serious environmental pollution and harm to human health, so it is urgent to find new renewable energy. Solar energy, as a clean and renewable form of green energy, has attracted more and more attention. In the past decades, bulk heterojunction organic solar cells (OSCs), which are composed of p-type semiconductor as the donor material and n-type semiconductor as the acceptor material, have been developed rapidly due to their advantages such as light weight, adjustable color, flexibility, foldability and translucency^{[1⇓⇓⇓⇓~6]}. Due to the ultrafast electron transfer between the fullerene acceptor material and the polymer donor material, fullerene has been the mainstream acceptor material for OSCs since 1995^{[7⇓⇓⇓⇓~12]}. However, fullerene derivatives have the disadvantages of small absorption coefficient, difficult structural modification, and poor tunability of optical and electrochemical properties, which greatly limit the further improvement of OSC performance. On the contrary, non-fullerene acceptor materials (NFAs) have the advantages of high light absorption coefficient, easy modification of structure, easy adjustment of energy level and band gap, and wide sources. Since 2013, NFAs have gradually replaced fullerene derivatives and become the mainstream acceptor materials of OSCs, and the efficiency of NFA-based OSCs has recently increased to 18%^{[13⇓⇓⇓⇓⇓⇓⇓~21]}. In the development of highly efficient NFAs, the condensed ring NFAs have attracted much attention in recent years. In 2015, Zhan Xiaowei's team took the lead in reporting ITIC, a small molecule receptor material with fused ring structure^[22]. In the ITIC molecule, the electron donor unit (D, IDTT — indacenobisthieno [3,2-b] thiophene) in the middle and the electron acceptor unit (A, IC — 2- (3-oxo-2,3-dihydroindan-1-ylidene) malononitrile) at both ends of the molecule form a coplanar A-D-A structure with a band gap of 1.59 eV, which is a low-band-gap NFA molecule. Another representative molecule is the Y6 molecule (named A1 here) reported by Zou Yingping Research Group of Central South University in 2019^[23]. In combination with high efficiency polymer donor PM6, Y6 gives a 15.7% conversion efficiency with a V_OC of 0.83 V and a FF of 74.8%. Due to the high external quantum efficiency of more than 80%, the J_SC is increased to 25.3 mA·cm^-2, while the voltage loss is reduced to about 0.5 eV. The structure of Y6 molecule is characterized in that 2,1,3-benzothiadiazole (BT) is used as a core, and two thieno [3,2-b] thiophenes (TT) are connected in parallel through nitrogen atoms on both sides of the benzene ring of BT to form a DA'D-type electron donor unit. The introduction of electron-deficient BT (a '), on the one hand, expands the conjugated system, and at the same time, the N atom on the pyrrole ring adopts sp² hybridization, and the p electrons on the N atom participate in conjugation to form a pyrrole bridged ring structure, which is conducive to enhancing the light absorption ability of the material and reducing the band gap; Second, it is helpful to reduce the frontier molecular orbital energy level of molecules; Thirdly, the fused ring structure of DA'D obtained in parallel has DA 'D symmetry, and the introduction of alkyl side chain at the β position of TT in parallel with BT is conducive to the formation of a unique C-type configuration of Y6 molecules (Fig. 1).

Inspired by the high performance of Y6 molecules, structural modifications based on Y6 molecules are in full swing, and dozens of high-performance series of molecules have emerged in a short period of more than a year.The A-type DA 'D acceptor material consists of an DA' D unit in the middle and an A unit at both ends, while the DA 'D unit as its core part is impressive in structural modifications based on the DA' D fused ring structure and its side chains^{[24⇓⇓⇓⇓⇓⇓⇓~32]}. The structural modifications based on the DA 'D unit were mainly developed from three aspects: the A' and D units, the nitrogen atom, and the side chain at the β position of TT (Figure 2). Generally speaking, the structural modification of the A 'unit is mainly reflected in the benzothiadiazole, such as the substitution of benzothiadiazole with quinoxaline and its derivatives, which can change the electron-withdrawing properties of the A' unit, thus regulating the energy level of the receptor molecule and reducing the voltage loss. The structural modification of D unit is more abundant. At present, the absorption, energy level, band gap and electron transport properties of material molecules are mainly adjusted by changing the size of conjugated system. The modification of the side chain can be carried out at the R₁ connected with the pyrrole ring and the R₂ at the β position of TT, respectively. Because it is easy to synthesize, the structural modification on the side chain has been systematically studied. For example, the results show that the fine control of the length of the side chain and the position of the branched chain can not only improve the solubility of materials in organic solvents,But also can adjust the aggregation structure and the appearance of the receptor molecules in the active layer blending film, thereby being beneficial to improving the charge separation and transmission performance^{[33⇓⇓⇓⇓~38]}. In a word, the structural modification of DA 'D unit is an important way to improve the photoelectric conversion performance of cell devices, so it has received extensive attention^{[39⇓⇓~42]}.

Since the Y6 molecule was reported, there have been many review papers summarizing the research progress of this kind of material from different perspectives. Dai Xuexin et al. Summarized the application of DA'D-type fused ring structure in dye-sensitized solar cells, small molecule acceptor materials, hole transport materials and perovskite solar cells^[43]. Meng Hong et al. First analyzed the key factors affecting the device performance, then preliminarily summarized the structure-performance relationship of A-type DA 'D and A-type acceptor materials from the perspective of core, side chain and end group, and finally summarized the latest progress of ternary batteries^[44]. Zou et al reviewed the synthesis of DA 'D and the relationship between the molecular structure and properties of A-DA' D A^[45]. In This review, more than 10 groups of Y6 receptor molecules were compared and analyzed, and the relationship between molecular structure and their optical, electrochemical, electrical and photovoltaic properties was discussed from three aspects of A 'unit, D unit and side chain.An accurate and comprehensive overview of the structure-property relationship of Y6 series molecules is given, and the latest research progress of this series of molecules is summarized.

2 Advances in Structural Modifications on A' Units

In the new generation of NFAs represented by Y6, the electron-deficient unit A 'and the electron-rich five-membered heterocycle are connected in parallel to form the DA'D-type fused ring structural unit. As shown in Figure 2, we summarize by example the performance changes brought about by the structural modification of the A ′ unit (Figure 3).

Taking A1 ~ A6 as an example, the structural difference of these six NFAs molecules lies in the A 'unit, and A1 takes BT as the A unit^[23]; A2, A3, A4, A5 and A6 have quinoxaline derivatives as A 'units^[46]^[46]^[47]^[47]^[48]. In these five NFAs molecules, the A 'unit is connected in parallel with the TT through the nitrogen atom to form the DA'D fused ring structure, the side chain (DA' D) on the nitrogen atom is 2-ethylhexyl, the R₂ at the β position of the TT unit is undecyl side chain, and the electron acceptor unit at both ends of the molecule is IC-2F (2- (5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile). The band gap of A1 is 1.33 eV, and the band gaps of five molecules with quinoxaline and its derivatives as the A ′ unit are slightly higher (the band gaps of A2 and A3 are the same, which are 1.35 eV, and the band gap of A4, A5 and A6 are 1.35, 1.41 and 1.54 eV, respectively). Compared with A1, the lowest unoccupied molecular orbital (LUMO) energy levels of the five molecules with quinoxaline and its derivatives as the A 'unit are higher, among which A4 and A5 show higher LUMO energy levels than A2 and A3 due to the presence of the electron-donating group thiophene. In the corresponding cell devices, the five molecules with quinoxaline and its derivatives as the A ′ unit all showed high V_OC values of 0.89, 0.86, 0.94, 0.92, and 0.94 V, respectively. When PM6 was used as donor, PM6: A2 had serious charge recombination and low hole mobility (μ_h and μ_e were 0.58×10^-4 and 3.72×10^-4cm²·V^-1·s^-1, respectively), which led to the decrease of FF (67%) and affected the device efficiency. The PCE of the PM6: A2 device is 13.31%, while the optimized PM6: A3 blend film shows a uniform and more flat phase-separated morphology, the hole mobility is improved (μ_h is 1.34×10^-4cm²·V^-1·s^-1), the bimolecular recombination is suppressed, both J_SC(25.38 mA·cm^-2) and FF (76%) are improved, and the device efficiency reaches 16.64% (Table 1).

Compared with PM6: A4, the advantage of PM6: A5 lies in the improvement of J_SC and FF. Due to the introduction of fluorine atoms in the quinoxaline side chain thiophene unit, A5 showed a better blend film morphology than A4.At the same time, the higher electron mobility and effective charge separation led to the increase of J_SC and FF of PM6: A5 device to 22.37 mA·cm^-2(PM6:A4 J_SC of the device was 15.42 mA·cm^-2) and 72.16% (FF of PM6: A4 device was 54.52%), respectively.Finally, PM6: A5 device achieved a PCE of 14.62%, and PM6: A4 device achieved a PCE of 7.75%. Although the P2F-EHp: A6 device has the highest V_OC value, the efficiency is only 12.12% due to the lower J_SC(18.27 mA·cm^-2) and FF (70.53%). In contrast, a PCE of 15.7% was obtained due to the higher J_SC(25.2 mA·cm^-2) and FF (76%) despite the lower V_OC(0.82 V) of PM6: A1.

By comparison, it can be seen that the main function of the A 'unit is to adjust the LUMO energy level of the molecule, thereby adjusting the V_OC of the device. A small molecule acceptor taking quinoxaline as an a 'unit has excellent performance, has remarkable effect on improving the V_OC of a device, and generally does not affect the J_SC and FF of the device, so that a higher PCE value is obtained.

3 Progress in Structure Modification of DA 'D Unit π Conjugated System

The size of the π-conjugated system of the acceptor molecule can be further broadened by extending the conjugated system size of the D unit in parallel with the A' unit in A-type DA 'D NFAs. In this part, two groups of molecules (Fig. 4) with benzotriazole (BTA) or benzothiadiazole (BT) in the A 'unit are selected as examples to summarize the effects of structural modification of the parallel D unit on the photoelectric and battery device performance of the acceptor material.

Zou Yingping's research group designed and synthesized A7 with BTA as the A 'unit and thiophene as the D unit. The optical band gap of A7 is 1.45 eV, and the HOMO/LUMO energy levels are -5.42 eV/-3.88 eV, respectively. The electron mobility of the donor-acceptor blend film formed by A7 and polymer donor HFQx-T is 1.15×10^-4cm²·V^-1s^-1, and the PCE of the cell device is 59% for 6.30%,J_SC and 12.67 mA·cm^-2,FF^[33]. Because thiophene is less aromatic than benzene, which is conducive to the formation of quinoid structure, thiophene is often used to design and synthesize photovoltaic material molecules. On the basis of A7, Yang Yang et al. Connected BTA and TT in parallel, extended the conjugated system, and synthesized A8 with seven rings^[49]. Compared with A7, the band gap is slightly reduced (1.44 eV) due to the red shift of the absorption caused by the enlarged conjugation. By choosing a suitable polymer donor material — PBDB-T-F combined with it, A8 gave higher electron mobility (3.04×10^-4cm²·V^-1·s^-1), less energy loss (E_loss=0.57 eV), and more efficient charge separation. Compared with the device of HFQx-T: A7, the J_SC of PBDB-T-F: A8 was 69% 22.44 mA·cm^-2,FF, and a PCE value of 13.42% was obtained.

The A 'unit of A1 is BT and the D unit is TT, with the band gap of 1.33 eV and the HOMO/LUMO levels of -5.65 eV/-4.10 eV, respectively. Huang Fei's research group took the wide band gap polymer P2F-EHp as the donor polymer, and the electron mobility (μ_e) of the blend film of P2F-EHp: A1 was 0.83×10^-4cm²·V^-1·s^-1, and the hole mobility (μ_h) was 1.18×10^-4cm²·V^-1·s^-1, showing balanced electron and hole charge mobility and weak bimolecular recombination loss.A high FF (74.11%) is obtained. Meanwhile, in the blend film, the π-π stacking of the donor-acceptor materials in the out-of-plane direction is adopted, which is beneficial to the charge transport, so a high J_SC is obtained, and the cell device achieves a PCE of 16.02%^[50]. Then they shortened the seven rings on the DA 'D unit of A1 to five rings to synthesize A9. Compared with A1, the conjugated system of A9 decreases, the band gap increases from 1. 33 eV to 1. 45 eV, and the HOMO/LUMO energy levels are -5. 73 eV/-4. 00 eV, respectively.When combined with the wide band gap polymer donor P2F-EHp, the V_OC of P2F-EHp: A9 is 0.81 V,J_SC is 3.2 mA·cm^-2,EHp:A9 The stacking orientation of the film in the Edge-on direction is obvious, which is not conducive to vertical charge transport, resulting in a significant reduction in FF (43. 78%).Therefore, the device efficiency of. After that, Ge Ziyi's group used benzodithiophene as D unit to broaden the pentacyclic ring of A1 to pentacyclic ring, and synthesized A10^[51]. In the absorption spectrum, the photoresponse of A10 is broadened to 1000 nm, the band gap of A10 is reduced to 1.25 eV compared with A1, the HOMO energy level and LUMO energy level are both increased, and the μ_e of A10 when blended with PBDB-T is 6.10×10^-4cm²·V^-1·s^-1. However, compared with PM6: A1 device, each parameter of PBDB-T: A10 cell device is reduced, where V_OC is 66.1% for 0.73 V,J_SC and 14.67 mA·cm^-2,FF, and PCE is only 7.09%.

It can be seen from the summary that when BT or BTA is used as the A 'unit, with the increase of the π system in NFAs, the LUMO energy level decreases, the band gap decreases, and the device shows better charge transport performance.

4 Influence of side chain

The structural modification on the DA ′ D unit is not only implemented on the A ′ and D units, but also can optimize the solubility and crystallinity of the molecule by adjusting the length or branching position of the side chain (DA 'D) on the nitrogen atom and the side chain (R₂) on the β position of thieno [3,2-b] thiophene (Figures 5 and 6).

4.1 The effect of side chain R₁.

As shown in Figure 5, Zou Yingping's research team synthesized A1 (Y6) by introducing 2-ethylhexyl side chain on the R₁ and undecyl side chain on the R₂ with BT as A 'and thieno [2-b] thiophene as D unit^[52]. Due to the large steric hindrance between the two 2-ethylhexyl side chains, the two 2-ethylhexyl groups face the two sides of the molecular plane respectively, which limits the H-aggregation between molecules to a certain extent, guides the molecules to form three-dimensional network J-aggregates which are beneficial to charge separation and transmission, and also improves the solubility of the molecules. The band gap of A1 is 1. 33 eV, and the absorption coefficient is 1.07×10⁵cm^-1. The blend of A1 and PM6 forms a phase separation morphology with appropriate size, which is beneficial to charge separation and transport, and is beneficial to obtain a high J_SC(25.3 mA·cm^-2) and FF (74. 8%).

After that, the research group cooperated with Yanhe Research Group of Hong Kong University of Science and Technology to replace the 2-ethylhexyl side chain on A1 with 3-ethylheptyl and 4-ethyloctyl, and synthesized A11 and A12 respectively^[52]^[52]. A11 with side chain branching at the third carbon atom has better solubility and crystallinity than A12 with side chain branching at the fourth carbon atom. Compared with PM6: A12, PM6: A11 has more balanced electron and hole charge mobility and better film morphology.Therefore, the J_SC(25.81 mA·cm^-2) and FF (74%) were higher, and a PCE of 16.0% was obtained based on PM6: A11. When PCBM was introduced as the third component to form a ternary cell, an efficiency of 16.74% was obtained due to the further improvement of V_OC and FF. However, the A12 with the branch position at the fourth carbon atom shows too high solubility, resulting in too large domain size and weakened Edge-on orientation in the morphology, and the PM6: A12 device has low mobility and more unbalanced electron and hole mobility, resulting in low FF and a PCE value of only 14. 31%. Subsequently, they synthesized A13 by exchanging the relative positions of the R₁ and R₂ side chains on A1^[52]. Compared with A1, the solubility of A13 in the solvent is poor, which makes it difficult to purify the molecule by column chromatography. At the same time, the formation of oversized nanodomains in the blend film formed with PM6 leads to low J_SC(25.47 mA·cm^-2) and FF (70.6%), so the PCE of PM6: A13 device is only 12.91%. Although the PCE of PM6: A1 exceeds 15%, the solubility of A1 in nonchlorinated and nonaromatic solvents is very limited. Therefore, Hou Jianhui, Institute of Chemistry, Chinese Academy of Sciences, designed and synthesized A14 by introducing a longer 2-butyloctyl as the side chain of R₁ into A1 molecule^[53]. The longer alkyl chain not only increases the solubility, but also improves the molecular packing order. Compared with PM6: A1, PM6: A14 has higher electron mobility and better film morphology, and achieves higher J_SC(25.3 mA·cm^-2) and FF (76%). Therefore, the PCE value of 16. 4% is obtained. After that, Huang Fei's team at South China University of Technology chose a longer alkyl chain, 2-decyltetradecyl, to replace the R₁ side chain of A1 molecule to synthesize A15^[54]. The introduction of long alkyl chains not only ensures the solubility and solution processability of molecules, but also inhibits the excessive aggregation of molecules due to the large steric hindrance effect. Compared with PM6: A1, PM6: A15 has higher electron mobility and lower charge recombination, and the PM6: A15 blend film treated by chloroform and non-halogen solvent o-xylene has a suitable phase separation morphology. The PM6: A15 device treated with o-xylene gave a FF of the V_OC,75.5% of the J_SC,0.856 V of the 24.9 mA·cm^-2 and a PCE of 16.1%. The PM6: A15 device treated with chloroform gave a FF of the V_OC,75.4% of the J_SC,0.858 V of the 25.2 mA·cm^-2 and a PCE of 16.3%.

A16, A17 and A18 were synthesized by replacing 2-ethylhexyl of A1 with 6-phenylhexyl, 6-phenoxyhexyl and n-octyl^[55]. The three molecules have similar electrochemical properties. Compared with A16 and A17, A18 has stronger intermolecular packing and thus shows poor solubility. However, the presence of phenyl groups in A16 and A17 leads to greater steric hindrance, which is conducive to reducing excessive aggregation. Compared with PM6: A18 devices,PM6: A16 and PM6: A17 devices exhibit high electron mobility due to strong intermolecular π-π stacking and preferential out-of-plane orientation (μ_e of PM6:Is the μ_e of 6.8×10^-4cm²·V^-1·s^-1,PM6:A17 is the μ_e of 7.06×10^-4cm²·V^-1·s^-1,PM6:A18 is 2.26×10^-4cm²·V^-1·s^-1). Among the three devices, the PM6: A16 device showed the most uniform blend film morphology, which was favorable for exciton dissociation, while the weaker charge recombination and faster charge extraction were favorable for achieving higher J_SC(21.73 mA·cm^-2) and FF (67.3%), and a PCE of 11.77% was achieved. The PM6: A17 device has the highest electron mobility, but the photovoltaic performance is weaker than that of the PM6: A16 device due to the strong single-molecule recombination. The PM6: a17 device gives a FF of V_OC,61.6% and a PCE of 11.07% for 21.31 mA·cm^-2 J_SC,0.844 V. The PM6: A18 device has the worst photovoltaic performance among the three devices due to its low electron mobility and serious single molecule recombination, and its J_SC are 19.07 mA·cm^-2,V_OC of 0. 863 V, FF of 63. 1% and PCE of 10. 38%. Later, Ge Ziyi, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, used 6-phenylhexyl and 6-phenylbutyl to replace 2-ethylhexyl on the R₁ side chain of A1 molecule to synthesize A16 and A19 molecules^[56]. The two molecules show similar optical and electrochemical properties, but their photovoltaic properties are quite different. The efficiency of the device composed of A16 and PM6 without annealing was increased to 14.54%. The short alkyl phenyl side chain of A19 leads to poor compatibility with polymer donor PM6. Compared with the PM6: A19 device, the PM6: A16 device exhibits a smaller π-π stacking spacing (3.6161 Å) and a larger coherence length (CCL = 17.77 Å), which is beneficial to charge transport, while the appropriate side chain length allows the crystallinity to be balanced with the compatibility, so the PM6; A16 device gives a FF of V_OC,72.68% and a PCE of 14.54% for J_SC,0.84 V of 23.82 mA·cm^-2. However, PM6: A19 device has poor photovoltaic performance due to poor compatibility between donor and acceptor and serious monomolecular recombination, and its J_SC are 15.74 mA·cm^-2,V_OC of 0. 689 V, FF of 67. 01%, and PCE of 7. 28%.

Hou Jianhui's group designed and synthesized a chlorinated non-fullerene acceptor A20 by replacing the fluorine atom in A1 with chlorine atom. Compared with A1, A20 has a larger optical absorption range and a higher absorption coefficient. When combined with PM6 to form an OSC device, A20 has a higher J_SC(25.4 mA·cm^-2) and a lower non-radiative loss (0.206 eV, PM6: A1 is 0.230 eV), so the V_OC is increased from 0.834 V to 0.867 V^[57]. The PCE of PM6: A20 increased from 15. 6% of PM6: A1 to 16. 5% due to the improvement of J_SC and V_OC. After that, they extended the side chain of the R₁ and replaced the 2-ethylhexyl group of A20 with 2-butyloctyl and 2-hexyldecyl groups to synthesize A21 and A22^[58]. With the extension of alkyl side chain, the absorption coefficient of NFAs increases, but the energy levels change little (the HOMO levels of A20, A21 and A22 are -5.67, -5.66 and -5.68 eV, respectively, and the LUMO levels are -4.11, -4.09 and -4.09 eV, respectively). Compared with A22, the device formed by A21 and polymer donor PM6 has higher carrier mobility and weaker charge recombination loss, so the J_SC and FF are improved simultaneously, and the final efficiency is 17. 0%. Compared with PM6: A21, larger domain size and weaker carrier transport properties were observed in the PM6: A22 blend film, resulting in a decrease in the J_SC and FF of the device, with a PCE of 15. 6%. Lin Haoran's group changed the position of the side chain of R₁ and replaced the 2-ethylhexyl of A20 with 3-ethylheptyl to synthesize A23^[59]. Compared with A20, the HOMO/LUMO energy levels of A23 are both increased, and the V_OC of the device is improved to 0. 852 V. Meanwhile, the optimized film morphology and balanced electron and hole mobility are observed in the PM6: A23 blend film, and high J_SC and FF as well as PCE of 16. 53% are obtained.

He Feng's group used bromine atom to replace the fluorine atom on the end group of A1 to synthesize A24^[60]. However, the low solubility of A24 in chloroform resulted in excessive domain size in the blend film morphology, and the PCE of the PM6: A24 device was only 7.51%. When the donor-acceptor blend solution was dissolved at 50 ℃, the solubility increased, and the PCE of the prepared cell device reached 12%. Subsequently, they extended the side chain of the R₁ by replacing the 2-ethylhexyl group of A24 with 2-butyloctyl and changing the number of bromine atoms (4 or 2) in the electron-withdrawing terminal group to synthesize A25 and A26. Compared with A25, A26 has better solubility in chloroform, larger absorption coefficient, and higher HOMO and LUMO energy levels, which are beneficial to obtain higher J_SC(25.03 mA·cm^-2vs 24.06 mA·cm^-2);A26 in the device, and higher V_OC(0.88 V vs 0.86 V) than A25. Meanwhile, due to the more balanced electron/hole mobility, better morphology, higher exciton dissociation efficiency and charge collection efficiency, higher FF (73. 13% vs 67. 84%) and PCE (16. 11% vs 14. 03%) are obtained.

4.2 Changes in the R₂ of the side chain

As shown in Figure 6, the research team led by Zhan Chuanlang of Inner Mongolia Normal University, together with Zhang Jiansheng of Hong Kong University of Science and Technology and Yang Liwei of Chang'an University, used 6-phenylhexyl and 6-hexylphenyl to replace the R₂ side chain on A1 to synthesize A27 and A28^[61]. The optical band gaps of A27 and A28 are similar, but the LUMO energy level of 6-phenylhexyl-substituted A27 is higher than that of 6-hexylphenyl-substituted A28, thus obtaining higher V_OC(0.865 V vs 0.839 V) in the cell. In A27, the benzene ring directly attached to the β-position of benzothieno [3,2-b] thiophene of the DA 'D is beneficial to inhibiting the rotation of the electron-withdrawing end group and the formation of isomers, reducing the degree of crystal disorder, improving the root-mean-square composition deviation, increasing the electron and hole mobility while reducing the loss of monomolecular recombination, and achieving a high FF of 77% and a high PCE of 16.7% (the PCE of PM6: A28 is 15.5%).

Yanhe Research Group of Hong Kong University of Science and Technology designed and synthesized A29 by introducing sulfur atoms into the R₂ side chain of A1 molecule^[62]. The optical band gap of A29 is 1.38 eV, and the HOMO/LUMO levels are -5.73 eV/-3.91 eV. Compared with A1, the LUMO energy level of A29 (the HOMO/LUMO energy level of A1 is-5. 65 eV/-4. 10 eV) is improved due to the introduction of sulfur atom, which is beneficial to obtain higher V_OC. After combination with PM6, its μ_e and μ_h are 2.1×10^-4 and 1.2×10^-4cm²·V^-1·s^-1, respectively. Higher FF and PCE were obtained in the PM6: A29 blend film due to the appearance of higher domain purity and suitable morphology. Compared with PM6: A1, PM6: A29 has a 24.8 mA·cm^-2,V_OC of 0. 82 V, a FF of 76% and a PCE of 16. 2%, which are the main reasons for the improved performance of the device.

Hou Jianhui's research group, together with Gao Feng of Linkoping University in Sweden, synthesized A30 and A31 by replacing n-undecyl of A21 with n-nonyl and n-heptyl^[15]. Compared with A21, A30 has better solubility and more orderly intermolecular packing, and the blend film formed by using PM6 as the polymer donor has better film morphology and higher electron mobility, so the J_SC of the PM6: A30 device is 81. 1% in 26.2 mA·cm^-2,FF, and a high PCE of 17. 8% is obtained. Compared with A30, A31 has lower solubility and stronger crystallinity, which leads to the formation of larger domain size and serious bimolecular recombination loss in the donor-acceptor blend film, and its electron mobility is also low, only 14.9% of the efficiency is obtained.

A32 synthesized by Yang Yang and Zou Yingping et al. Has no R₂ side chain, and its optical band gap is 1.32 eV, and its HOMO/LUMO energy level is -5.56 eV/-4.11 eV. When PBDB-T and PM6 are used as polymer donors, devices based on PBDB-T: A32 give J_SC of 22.7 mA·cm^-2.However, the V_OC(0.74 V) and FF (57%) are both low, which limit the improvement of PCE (9.6%). On the other hand, the fluorinated PM6: A32 film has better crystallinity, which is beneficial to carrier transport. Therefore, the PM6: A32 device gives FF of V_OC,68.5% and PCE of 14.8% with J_SC,0.83 V of 25.3 mA·cm^-2^[63]. The group of Cao Yong and Wu Hongbin introduced undecyl group into the R₂ side chain of A32 to achieve low energy disorder and 0.17 eV non-radiative recombination loss (ΔE_loss=0.43 eV^[64]. Compared with PM6: A32, PM6: A33 has higher electron mobility, larger J_SC(26.74 mA·cm^-2) and FF (74. 3%) and PCE (16. 54%). Li Yongfang and Zou Yingping shortened the side chain of R₂ and synthesized A34 by replacing the n-undecyl group of A33 with n-hexyl group^[65]. Compared with A33, the band gap of A34 is larger, and the electron mobility of PM6: A34 is 5.27×10^-4cm²·V^-1·s^-1. Compared with the PM6: A33 device, the energy loss of the device is larger (0.53 eV), but the V_OC and FF of the device are improved, and the PCE of 16.52% is obtained.

From the above comparison, it is clear that the corresponding devices show higher efficiency when 2-ethylhexyl, 2-butyloctyl, 3-ethylheptyl are attached to the R₁ side chain of NFAs. At the same time, using different halogen elements such as F, Cl or Br or changing the number of halogen substituents can greatly affect the efficiency of the cell device. Compared with fluoro or bromo acceptor molecules, these R₁ side chains combined with chloro end groups to form acceptor materials, and their battery devices all gave PCEs of more than 16%. In addition, compared with other types of R₂ side chains, A27, A30 and A31 molecules with -Ph-C₆H₁₃, -C₉H₁₉ and -C₆H₁₃ side chains all gave more than 16.5% PCEs.

5 Conclusion and prospect

In recent years, with the emergence of Y6-like NFAs, the efficiency of single-junction OSCs has been improved to 18%, which has made great progress in the research of OSCs. In the process of efficiency improvement, the structural innovation of non-fullerene acceptor materials plays a great role in promoting. Compared with the ITIC-type A-D-a molecules, the A-type DA 'D NFA molecules not only change the absorption, energy level and band gap of the molecules, but also change the J-aggregate structure of the molecules in the donor-acceptor blend film due to the structural innovation of the middle DA' D unit, resulting in not only improved charge separation and generation efficiency, but also reduced non-radiative losses, greatly improving the photoelectric conversion performance of the cell. In this paper, the structure-activity relationships between structure and optical, electrochemical, electrical and photovoltaic properties are given through the classification of molecular structures, which provides ideas and references for the further design of high-performance receptor materials.

It can be concluded that: (1) NFAs with quinoxaline and its derivatives as the core have a significant effect on improving the V_OC of the device due to their high LUMO energy level; (2) It is an effective way to improve the short-circuit current of the battery by expanding the conjugated system and reducing the band gap from the parallel D unit, in which the DA 'D unit with seven ring units has a relatively excellent conjugated system; (3) The chemical modification of R₁ and R₂ side chains provides a necessary means to adjust the morphology of the donor-acceptor blend membrane, improve the charge separation and transport performance, and improve the fill factor of the battery.When the R₁ side chain of the acceptor material is 2-ethylhexyl, 2-butyl-octyl, and 3-ethylheptyl (combined with a chloro end group), the PCEs of the device can reach more than 16%; and when the R₂ side chain of the acceptor material is -Ph-C₆H₁₃, -C₉H₁₉, and -C₆H₁₃ side chain, the PCEs of the device can reach more than 16.5%. Therefore, we believe that single-junction OSCs can achieve a PCE of more than 21% in the near future through continuous optimization of acceptor molecules.

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