Study on Ternary Blend Organic Solar Cells Based on Multiple Non-fullerene Acceptors

. In recent years, non-fullerene acceptor materials for organic solar cells ( OSCs ) have attracted much attention. Among them, ITIC, Y6 and perylene diimide ( PDI ) have been widely studied because of their easy structure regulation. The application and development of ITIC and its derivatives, Y6, PDI and its derivatives in ternary blend organic solar cells are reviewed in detail. This work focuses on the molecular structure optimization method and the selection of third component materials. At present, the efficiency of organic solar cells has exceeded 18%, but the relationship between molecular structure and photovoltaic performance needs further exploration. Optimizing the synthesis route is still an important research direction in this field.


Introduction
In the past few decades, traditional energy sources have been unable to fully burn organic matter due to their low utilization efficiency. Therefore, too many toxic and harmful gases have been discharged into the atmosphere, and the problem of environmental pollution has gradually become prominent [1].In order to improve environmental problems and accelerate energy transformation, China has vigorously developed various new energy sources. In this context, research on organic solar cells ( OSCs ) is also active. At present, the non-fullerene acceptor materials in the active layer of OSCs can be divided into two types: small molecule acceptors ( A-D-A, A-DA'D-A ) and polymer acceptors. Many scholars have also studied the above aspects and tried to adopt different methods to improve the efficiency of OSCs.In 1958, Kearns' group at the University of California, Berkeley first discovered the photovoltaic effect of organic crystals [2]. Since then, research on OSCs has been carried out. At first, the fullerene system was the mainstream acceptor material of OSCs [3].In 2015, Zhan 's research group [4] used PTB7-TH as the donor to blend with the A-D-A type nonfullerene acceptor material ITIC, which was designed and developed for the first time. The device energy conversion efficiency ( PCE ) reached the highest value of 6.80% at that time, opening the non-fullerene era. In 2019, Zou 's group of Central South University [5] designed an A-DA'D-A type non-fullerene acceptor molecule Y6, which not only effectively solved the problem of mutual limitation of short-circuit current and open-circuit voltage performance, but also broke the record of PCE of single OSCs device, reaching 15.70 %. While the efficiency of organic batteries based on small molecule receptors continues to break through, polymers, especially perylene diimide (PDI) and its derivative receptors, are also widely used. Based on this, this paper summarizes the research on ternary blend organic solar cells based on ITIC and its derivatives, Y6, perylene diimide (PDI) and its derivatives in recent years, in order to provide reference for future research on OSCs.

Ternary Blend Organic Solar Cells Based On A-D-A Acceptor Material Itic And Its Derivatives
In the A-D-A type acceptor material, A represents the electron-withdrawing unit and D represents the electrondonating unit. The two units are connected by the conjugated spacer unit π. This structure adopts a conjugated push-pull design, which can not only promote the transfer of intramolecular charges, but also achieve the function of each unit independently through fine modification and regulation, and improve the morphology of the active layer blend membrane. Figure 1 is the molecular formula of ITIC and its derivatives, and the corresponding device photovoltaic performance parameters are shown in table 1.
In 2015, Zhan 's group [4] synthesized the first A-D-A type non-fullerene electron acceptor ITIC based on indenodithienothiophene (Figure 1), where IT is the indenothiophene core part in the middle of the molecule, IC is the molecular end position, each end is connected with a carbonyl group and two cyano groups. The researchers combined the donor materials PTB7-Th and ITIC in different proportions and found that when PTB7-Th:ITIC=1:1.3, the battery efficiency reached a maximum of 6.80% at that time, in which the open circuit voltage ( VOC ) was 0.81 V, the short circuit current density ( JSC ) was 14.21 mA / cm 2 , and the fill factor ( FF ) was 0.591. The synthesis of ITIC receptor materials has opened up a new research direction for OSCs, and the efficiency of OSCs based on ITIC and its derivatives has been continuously improved. This section will be divided into two parts : main side chain regulation and end group regulation.

Main-side Chain Regulation of ITIC
Small molecule receptors can regulate molecular configuration, enhance intramolecular interaction, affect intermolecular stacking behavior, enhance molecular absorption, improve molecular energy level and determine the solubility and crystallization ability of compounds through the regulation of main and side chains. In 2016, Li 's group [6] designed and synthesized a new small molecule receptor m-ITIC by changing the position of the substituted alkyl group on the side chain of ITIC, and combined it with the polymer J61 ( Figure 1) [7] synthesized by the alkylation of benzenedithiophene unit. The prepared binary component J61:m-ITIC obtained 11.77 % PCE. In December of the same year, the subject [8] introduced a trialkylsilyl substituent on the dithienylbenzenedithiophene to synthesize J71 ( Figure 1) with a further reduced HOMO level, and the device efficiency after blending with ITIC exceeded 11 %. In 2017, they [9] synthesized a new copolymer J91 by fluorination of side chain thiophene ( Figure 1). It was found that after blending with m-ITIC, J91 reduced the carrier recombination in the blend film and inhibited the formation of triplets, and the device PCE reached 11.63 %. In 2018, Martin Heeney 's group [10] also modified the alkylbenzene side chain of ITIC and synthesized C8-ITIC ( Figure 1) by alkylation pathway (first synthesized soluble and easily purified intermediates). After blending with polymer PFBDB-T, 13.20 % device efficiency was achieved. In 2020, Zhan 's group [11] synthesized the IDIC small molecule receptor based on the molecular structure of ITIC ( Figure 1). The π-π stacking of the IDIC molecular end group promotes the electron migration in the active layer. The prepared FTAZ:IDIC:INIC3 threecomponent obtained 12.49 % efficiency.

Terminal Regulation of ITIC
The regulation of terminal groups, especially halogenation strategy, is one of the most commonly used methods in molecular modification. The regulation of small molecule receptor terminal groups can enhance intramolecular charge transfer, thereby reducing band gap, adjusting energy level and improving battery efficiency. In 2018, the Alex team [12] synthesized a small molecule 6TIC-4F with a strong power supply core unit and a strong electron-withdrawing acceptor unit on both sides based on ITIC. The photovoltaic device PCE based on PBDB-T : 6TIC-4F can reach 11.14 %. Studies have shown that such acceptor molecules can not only expand the absorption range of the active layer material to the near-infrared region, but also make the device more effectively absorb solar photons to improve battery efficiency. Hou 's group [13] synthesized a new type of small molecule acceptor IT-4F ( Figure 1) by fluorination of the phenyl hydrogen atom located on the ITIC terminal group. It has higher absorption coefficient and more redshift absorption. After combining with the same fluorinated PBDB-T-SF to prepare PBDB-T-SF : IT-4F device, the PCE can reach 13.00 %. In 2019, Shen 's group [14] synthesized a small molecule receptor IN-4F with a silicon-based side chain based on IT-4F molecule. Compared with the former, IN-4F has a higher LUMO energy level and a lower optical band gap. When IN-4F is blended with PBDB-T-2F (PM6) : IT-4F, the PCE of the photovoltaic device reaches 14.90 %. In 2020, He 's group [15] introduced a single bromine atom into the IC end group to synthesize BTIC-2Br-m (Figure 1), which makes the device have a higher open circuit voltage. The introduction of different numbers of bromine atoms also makes the dipole moment significantly different, which indicates that the bromine atom plays an important role in the polarization of the molecular skeleton. The PBDB-TF : BTIC-2Br-m device based on it exhibits 16.11 % PCE. In 2021, Wang 's group [16] designed and synthesized an asymmetric halide IDTT-Cl-2F ( Figure 1) with IDTT as the core unit. Based on the device prepared by blending it with polymer PBDB-T-2Cl (PM7), the efficiency of 13.30 % was obtained. Studies have shown that asymmetry and halogenation strategies have a synergistic effect on the efficiency of non-fullerene small molecule devices.

Summary
In the past two decades, the A-D-A type small molecule receptor ITIC has attracted much attention due to the advantages of easy modification and synthesis of each structural unit in the molecule, and has become one of the most potential receptors to replace fullerene. The main side chain and terminal group regulation of ITIC molecules are analyzed in detail. It is concluded that fluorination, chlorination and bromination regulation can reduce energy loss and maintain good efficiency of the device. Fluorination effectively improves the thermal stability of the acceptor molecule and increases the open circuit voltage of the device. Chlorination is more effective in reducing molecular energy levels and expanding absorption spectra. Bromination is more conducive to the transfer of charge in the device.

Ternary Blend Organic Solar Cells Based On A-Da'd-A Acceptor Material Y6
Based on the A-D-A type structure, Zou 's group first designed the DA 'D type fused ring skeleton by introducing chromophores and chromophores into the central fused ring, and constructed the A-DA 'D-A type receptor material. On the basis of the benzothiadiazole central unit, Y6 was synthesized by introducing multiple nitrogen-containing electron-deficient nuclear alkyl chains at the β-position of dithiophene. The emergence of Y6 increases the application range of polymer donors, fullerenes and their derivatives acceptors, non-fullerene small molecules and other substances, improves the photovoltaic performance of the battery, and promotes the development of OSCs. This section will be divided into five parts.

Polymer Donor as the Third Component
The flexibility, machinability, aggregation stability, better film quality and better photovoltaic performance of the polymer donor itself [17] have attracted the attention of researchers and continue to try to add it to the active layer to improve the efficiency of organic photovoltaic devices. Figure 2 is the molecular formula of Y6 and the polymer donor material blended with it. Table 2 is the photovoltaic performance parameters of the device corresponding to the polymer donor material. In 2019, Zou 's group [5] constructed a binary system PM6 : Y6 blended with polymer PM6 (Figure 2) based on the receptor molecule Y6 (Figure 2) they designed and synthesized. The battery efficiency exceeded 15 %, which was the world 's highest record reported at that time. In 2020, Liu 's group [18]

Fullerene Derivative Receptor as Third Component
In the past, the fullerene molecule has stood out from many acceptor materials due to its unique threedimensional spherical structure. However, due to the high symmetry of its wave function, the fullerene and its derivative acceptors have weak light absorption ability, and the absorption band gap is difficult to control. Defects limit the improvement of device efficiency. Some studies have tried to combine fullerene and non-fullerene materials for complementary advantages and improve battery efficiency through fine regulation. Figure 3 is the molecular formula of the fullerene derivative receptor, and Table 3 is the photovoltaic performance parameters of the corresponding device. In 2019, Hou 's research group [21] introduced the fullerene derivative PC61BM ( Figure 3) into the binary mixed system PM6 : Y6.The introduction of PC61BM increased the electron mobility of the device, but also dispersed the Y6 aggregates, reduced the non-radiative energy loss, improved VOC, JSC, and FF, and obtained a device efficiency of 16.50 %. Peng 's group [22] added fullerene derivative PC71BM (Figure 3) as the third component on the basis of PM6 : Y6. The addition of PC71BM promoted the dissociation and generation of charge at the donor / acceptor interface. When mixed in a ratio of 1 : 1.0 : 0.2, the PCE reached 16.67 %. Lu 's group [23] also based on this device, and obtained an efficiency of 16.70 % after mixing in a ratio of 1 : 1.2 : 0.2. Comparing the two groups of experimental results, it can be seen that the performance of the device with the same material composition and different mixing ratios will also be different.

Non-fullerene Small Molecules as the Third Component
The demand for fullerene substitute materials in OSCs has led to the rapid development of non-fullerene receptors. Compared with fullerenes, non-fullerene donors and acceptors have the advantages of better regulation of band gaps and energy levels, simpler synthesis methods, more readily available materials, more easily adjusted structures, and more matching molecules [24]. In order to reduce costs, improve the spectral absorption of organic batteries, improve device stability, and improve battery efficiency [25], the method of introducing non-fullerene molecules as the third component has been proposed and widely used. Figure 4 is the non-fullerene molecular formula, and Table 4 is the photovoltaic performance parameters of the corresponding device. In 2019, Zhan 's group [26] used the ITIC derivative IDIC receptor ( Figure  1) as the third component of PM6 : Y6. The structure of IDIC is not only similar to the main body, but also the lowest unoccupied molecular orbital (LUMO) level is higher than that of Y6, thereby increasing the mobility of the device charge and obtaining higher VOC. The PCE also reached 16.51 %. In 2020, Shi 's group [27] designed and synthesized BTP-S2 (Figure 4), and used it to prepare ternary OSCs : PM6 : Y6 : BTP-S2. The device significantly reduces energy loss and promotes charge separation, and the optimal device efficiency reaches 17.43 %. Li and co-workers [28] used highly crystalline molecular donor DRTB-T-C4 (Figure 4) as the third component of PM6 : Y6, DRTB-T-C4 can avoid selfaggregation, enhance and balance charge transport, and optimize electron mobility. The device finally achieved an efficiency of 17.13 %. Lu 's group [29] successfully prepared OSCs based on PM6 : BTR : Y6 by adding liquid crystal small molecule donor BTR (Figure 4). The synergistic effect of BTR and PM6 can finely regulate the morphology of the active layer of the device, making the donor-acceptor binding closer, optimizing the crystallization ability, and finally increasing the PCE of the device to 14.70 %. In 2021, Liu 's group [30] introduced Y6-BO into PM6 : Y6 to maintain the crystalline morphology of the device film through the interaction between the active layer materials, and obtained 17.84 % PCE. In 2022, Zeng and co-workers [31] added ITIC-M (Figure 4) to the PM6 : Y6 blend system, and the prepared three components had a good complementary light absorption effect, and the PCE was as high as 18.13 %.

Solid Additives as the Third Component
The blending morphology of the OSCs active layer plays a key role in improving the PCE of the battery, and adding solid additives to the device is one of the common methods to adjust the morphology and improve the efficiency of the battery. Figure 5 is the molecular formula of solid additives, and Table 5 is the photovoltaic performance parameters of the corresponding devices. In 2020, Li 's group [32] added a volatile solid additive 1,4diiodotetrafluorobenzene (A3, Figure 5) containing σpores to PM6 : Y6-based OSCs. Due to the interaction between halogens, the addition of A3 not only makes the stacking of molecules in the device more orderly and dense, but also promotes the mobility of carriers, which makes the device have better charge transport characteristics, and the PCE reaches 16.50 %. However, studies have shown that even if the concentration of A3 continues to increase to 50mg/mL, the cell efficiency remains at a high level, which also shows that A3 has great application value. Although traditional fluorinecontaining solid additives can greatly improve battery efficiency, they cannot be produced on a large scale due to their high toxicity. Therefore, Liu 's group [33] introduced non-toxic halogen-free benzyl benzoate (BB, Figure 5) as an additive into PTB7-Th : PC71BM. It was found that the photovoltaic performance of the device was significantly improved, and the PCE increased from 4.83 % to 9.43 % compared with that without additives.

Summary
The emergence of A-DA 'D-A receptor material Y6 has broken the dominant position of ITIC molecules and their derivatives in the field of OSCs. The rigid coplanar alternate push-pull structure of non-fullerene acceptor Y6 enables it to absorb near-infrared light, solve the problem of low near-infrared sensitivity limitation, reduce the band gap and improve the carrier mobility of the device. At the same time, Y6 also has the disadvantages of difficult synthesis, difficult purification and high cost due to the complexity of its central structural skeleton. At present, although the efficiency of Y6-based organic battery devices is close to 19 %, there is no specific requirement for the selection of the third component. In the future, it is still necessary to further optimize the synthesis steps of donor and acceptor materials and explore the working principle of Y6-based batteries.

Ternary Blend Organic Solar Cells Based on Polymer Acceptor Perylene Diimide (PDI) And Its Derivatives
PDI ( Figure 6) has excellent electron affinity and high electron mobility due to its unique π-π conjugated structure. However, due to the large conjugate plane, the introduction of monomer PDI into the active layer will cause serious molecular self-aggregation, which will inhibit charge diffusion and limit the improvement of battery efficiency. The active sites of PDI can be divided into three types : N, N is called imide, 1,6,7,12 is called Bay, 2,5,8,11 is called α. The paper will introduce groups on the active sites to solve this problem by constructing a non-planar twisted structure. Figure 6 is the molecular formula of PDI and its derivatives, and table 6 is the photovoltaic performance parameters of the corresponding devices.

Introduction of Simple Groups such as Alkyl
Chain or Benzene Ring at Imide, Bay or α Position Welch 's group [34] synthesized a receptor molecule N-PDI with good film-forming ability by introducing a cetylpyrrole ring at the Bay position ( Figure 6). With the addition of this material, the reverse current of the device decreases and the charge mobility increases. The efficiency of the device prepared by blending with PM6 : Y6 reaches 11.20 %. Wasielewskki 's group [35] synthesized Phenyl-PDI receptor ( Figure 6) by introducing phenyl on the four α active sites of PDI. After blending with donor PBDTT-FTTE, the device obtained 3.62 % PCE. Zhan 's group [36] used PDI-2DTT ( Figure  6) as a receptor material and mixed with PTB7-Th : IDT-2BR. The prepared three components well balance the performance between VOC and JSC, showing stronger energy level transfer, and the best device efficiency can reach 10.10 %.

PDI was Introduced at Imide, Bay and α
Position to Form Polymers Narayan 's group [37] connected the imide sites of two PDI molecules to synthesize a dimer molecule TP with PDI monomers perpendicular to each other ( Figure 6). The device with PBDTTT-C-T as donor and TP as acceptor achieved an efficiency of 3.20 %. Wang 's group [38] used a single bond to connect the inner Bay sites of two PDI monomers, and used Se atoms to connect the Bay sites on both sides of the dimer to form a five-membered ring to obtain the small molecule A1 (Figure 6). The Se atom has a larger electron cloud. After blending with the wide bandgap polymer donor PBDT-T1, the degree of intermolecular orbital overlap is increased, and higher device performance is obtained, with an efficiency of 8.42 %. Li and co-workers [39] modified the two sides of the dimer A1 with cyano groups to obtain the receptor molecule A2 (Figure 6). Although the LUMO energy levels of A2 and PDPP2Tz2T donor molecules are lower than -4.0 eV, the efficiency of the hybrid device still reaches 1.40 %. Studies have shown that the rational construction of PDI dimers is an effective strategy to obtain efficient non-fullerene small molecule receptors.

Introducing Spacers at Imide, Bay and α Positions
In 2020, Chen 's group [40] synthesized a PDI tetramer FP 4 3T with a fused ring structure by connecting PDI monomers in pairs with thiophene ( Figure 6). Due to the influence of steric hindrance effect, there is a large torsion angle between PDI monomers. Therefore, FP43T has strong light absorption and low LUMO energy level. The photovoltaic performance of the device is significantly enhanced after mixing with PTB7-Th, and the cell efficiency reaches 6.05 %. Wang and co-workers [41] synthesized a bb-2PDI dimer ( Figure 6) by connecting the Bay sites of two PDI monomers with an oxygen ether bond as a bridge. This structure facilitates more balanced transport of carriers and improves the efficiency of exciton dissociation. The battery efficiency based on PTB7-Th : bb-2PDI is 3.06 %. Li and co-workers [42] synthesized a receptor PM-PDI 3 ( Figure 6) containing four PDI molecules by coupling reaction and bromine substitution reaction. Because of its high LUMO level, high exciton dissociation efficiency and good morphology, the PCE reached 7.58 % after blending with PDBT-T1 donor.

Summary
By modifying the imide, Bay and α positions, researchers aim to improve spectral absorption, adjust HOMO / LUMO energy level matching, and promote electron mobility to optimize the performance of PDI and its derivatives and improve device efficiency. Although the synthesis conditions of PDI derivatives are harsh and the repeatability of the synthesis process is low, they are still concerned by researchers because of their excellent flexibility, high electron mobility and aggregation stability. In the long run, PDI molecules have good application prospects and are expected to make great breakthroughs in the research of non-fullerene small molecule acceptor materials.

Summary and Outlook
In this paper, the development status of ternary blend organic solar cells based on ITIC and its derivatives, Y6, perylene diimide ( PDI ) and its derivatives is introduced in detail. ITIC has good regulation ability. The use of halogenation strategy based on the regulation of the main side chain and end group of ITIC molecules is more conducive to device charge transfer, improving the morphology of the blend film and improving device efficiency. The absorption wavelength range of Y6 molecule is large, which can effectively utilize the light in the near-infrared band and improve the photon capture rate of the device. PDI molecules have many active sites, which can improve the light absorption ability by introducing groups, adjust the LUMO energy level, form a good blend film, and promote the improvement of battery efficiency.
So far, the single-cell efficiency of OSCs has reached 18 %, but there is still a certain gap compared with the commercialized silicon solar cells ( 25 % ). Therefore, the relationship between molecular structure and photovoltaic performance needs to be further explored. How to further optimize the synthesis route is still an important research in this field.