Organic Solar Cells Based on Non-Fullerene Small Molecular Acceptor Y6

Xiang Xu; Kun Li; Qingya Wei; Jun Yuan; Yingping Zou

doi:10.7536/PC201016

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (2) : 165-178 DOI: 10.7536/PC201016

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Organic Solar Cells Based on Non-Fullerene Small Molecular Acceptor Y6

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Abstract

With the development of donor and acceptor materials, the power conversion efficiency(PCE) of organic solar cells(OSCs) has continuously made breakthrough in recent years. Particularly, the emergency of non-fullerene acceptor Y6 has enabled the device efficiency of OSCs over 15%. Y6 has been applied in many aspects and greatly improved its photovoltaic properties. This review focuses on the applications of Y6 in binary, ternary and quaternary, layer-by-layer, flexible, tandem and semitransparent organic solar cells, meanwhile, the future structure optimizations and device applications of Y6 have been outlooked.

Contents

1 Introduction

2 Binary organic solar cells based on Y6

2.1 Polymer donors matching with the Y6 acceptor

2.2 Small molecular donors matching with the Y6 acceptor

3 Ternary and quaternary organic solar cells based on Y6

4 LBL-processed OSCs based on Y6

5 Flexible OSCs based on Y6

6 Tandem OSCs based on Y6

7 Semi-transparent OSCs based on Y6

8 Research on OSCs based on Y6 triplet state

9 Conclusion and outlook

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Keywords

Y6 / organic solar cells / non-fullerene small molecular acceptors / efficiency / applications

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Xiang Xu, Kun Li, Qingya Wei, Jun Yuan, Yingping Zou. Organic Solar Cells Based on Non-Fullerene Small Molecular Acceptor Y6. Progress in Chemistry, 2021, 33(2): 165-178 DOI:10.7536/PC201016

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1 Introduction

Organic solar cells have become a new energy technology with broad prospects because of their advantages such as light weight, wide material sources, flexible and translucent devices^{[1⇓⇓⇓⇓⇓~7]}. In 1986, Deng Qingyun et al. Prepared the first double-layer heterojunction organic solar cell with copper phthalocyanine (CuPC) and tetracarboxylic perylene derivatives, and the device efficiency reached 1% (light intensity was 75 mW/cm²)^[8]. In recent years, thanks to the rapid development of donor/acceptor materials and interface materials, as well as the continuous optimization of device structure and preparation process, the efficiency of organic solar cells has made breakthroughs^{[9⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~23]}. In 2015, Zhan's research group synthesized ITIC with indacene dithiophene (IDT) as the intermediate core and dicyanoindanedione (IC) as the end group, and obtained 6. 8% energy conversion efficiency (PCE) by blending with PTB7-Th^[19]. In 2016, Hou's team used PBDB-T with better energy level matching and absorption coefficient to blend with ITIC as the active layer, and the device efficiency increased to 11.21%^[24]. In 2019, Zou's research group reported that Y6, a non-fullerene small molecule receptor of A-DA 'D type, was blended with PM6 to prepare devices with an efficiency of 15.7%, which is the first time that the efficiency of single-junction organic solar cells has exceeded 15%^[23]. Based on Y6 molecules, many researchers have carried out a series of studies to further improve the efficiency of organic photovoltaic devices and promote the marketization of organic photovoltaic devices^{[25⇓⇓⇓⇓⇓~31]}.

In 2017, Zou's team first introduced the electron acceptor unit benzotriazole into the central core of the non-fullerene acceptor fused ring to form a DA 'D fused ring structure, and then synthesized the A-DA' D -a organic small molecule receptor BZIC^[32]. The novel non-fullerene small molecule acceptors can effectively broaden the absorption spectrum and reduce the voltage loss of devices, which provides guidance for the design of new materials. Subsequently, the research group cooperated with Yang Yang's research group at the University of California, Los Angeles (UCLA) to replace thiophene with dithiophene, expand pentacyclic ring to heptacyclic ring, and synthesize electron acceptors Y1 and Y2 with different end groups^[33]. The expansion of the conjugated ring broadens the absorption spectrum of the non-fullerene small molecule acceptor material and further reduces the energy loss of the device. At the same time, Zou's research group used benzothiadiazole instead of benzotriazole as the intermediate core of the fused ring, and adjusted the solubility and conformation of the molecule by introducing an alkyl chain at the β position of benzodithiophene to synthesize a non-fullerene acceptor Y6, which was blended with PBDB-T-2F (PM6), and the efficiency of single-junction organic solar cells reached 15.7%^[23]. Subsequently, Y6 was applied to various aspects of organic solar cells and greatly improved their photovoltaic performance.

Based on this, this paper summarizes the research on binary blending, ternary and quaternary blending, layer-by-layer printing, flexible devices, tandem cells and translucent solar cells based on Y6 acceptor materials in the past two years, in order to provide guidance for the future development of organic solar cells.

2 Binary blend solar cells based on Y6

The emergence of Y6 has led to the rapid development of polymer donor/small molecule acceptor and all-small molecule systems, and has continuously promoted the efficiency breakthrough of organic solar cells. This section will be divided into polymer donors/Y6 and small molecule donors/Y6.

2.1 Polymer donor matched to Y6 acceptor

Domestic and foreign research groups have synthesized polymer donor materials matching the absorption spectrum and energy level of Y6 receptor molecules, and the research shows that Y6 receptor has good compatibility and can be well matched with most polymer donors. Fig. 1 shows the polymer donor material, and Table 1 shows the photovoltaic performance parameters of the corresponding devices. In 2018, Li et al. Reported a wide band gap polymer donor material PTQ10, which is a potential practical donor material because of its low cost and simple synthesis steps^[34]. In 2020, the PCE of the dimer prepared by blending PTQ10 and Y6 by Li Research Group reached 16.53%^[35]. Yan et al. Used PM7 as a donor material. Compared with PM6, PM7 has a lower HOMO energy level, which increases the band gap between the E_HOMO-donor and the E_{LUMO-acceptor} and makes it possible to improve the V_oc.The PCE of the binary device prepared by blending with Y6 reaches 17.04%, the open circuit voltage (V_oc) reaches 0.90 V, the short circuit current density (J_sc) is 25.64 mA/cm², and the fill factor (FF) is 0.74^[36]. Zhang et al. polymerized PM6 and two copolymerized units in PBFTz according to the ratio of 4:1 to obtain PM1 polymer, and the device performance prepared by blending PM1 and Y6 reached 17.6%, in which the V_oc was 0.87 V, the J_sc was 25.9 mA/cm², and the FF reached 0.78^[37]. The PM1 based devices performed better than the PM6/Y6 and PBFTz/Y6 devices (photovoltaic performance is shown in Table 1). In addition to excellent performance, PM1-based devices have good repeatability.

In 2019, Huang et al. Used P2F-EHp of the electron-deficient group of imido-benzotriazole (TzBI) as a polymer donor, and obtained a device efficiency of 16.02% by blending with Y6, in which the V_oc reached a 0.81 V,J_sc of 0.74 26.68 mA/cm²,FF^[38]. They further found that the system can still achieve excellent performance in large area. When the device area reaches 1 cm², the device efficiency reaches 14.39%. When PCBM is added as a ternary blend device, the device efficiency increases to 15.13%. The research shows that the system has outstanding application prospects^[39]. In 2020, they synthesized a new donor polymer PTzBI-dF based on TzBI unit, and blended it with Y6 to achieve an efficiency of 16.8%, with a V_oc of 0.85 V,J_sc and a 26.33 mA/cm²,FF of 0.76^[40]. At the same time, compared with its isomer PTzBI-pF, the performance of this donor is more excellent. The study shows that the different positions of fluorine atoms strongly affect the molecular structure and morphology, and then affect the optical absorption and charge transport properties.

In 2019, Zhou et al synthesized two copolymers with similar structures, J52-FS and PE2^[41]. J52-FS uses thiophene as the pi-bridge and PE2 uses thieno [3,2-b] thiophene as the pi-bridge. The PCE of the binary device prepared by blending PE2 and Y6 reaches 13.5%, wherein the V_oc is 0.83 V,J_sc and the 23.24 mA/cm²,FF is 0.70; The PCE of the binary device prepared by blending J52-FS and Y6 is 10. 58%, which indicates that TT as a π-bridge is a better molecular design strategy for constructing efficient polymer photovoltaic materials. In the same year, Zhou et al. Again used 3-hexylthieno [3,2-b] thiophene as a π-bridge to synthesize the polymer donor material PE4^[42]. The PCE of the binary device prepared by blending PE4 and Y6 reaches 14.02%, and the V_oc reaches the 0.84 V,J_sc of 0.75 of the 23.77 mA/cm²,FF. Compared with thiophene as a π-bridge, 3-hexylthieno [3,2-b] thiophene as a π-bridge changes the main chain conformation from zigzag to linear, thus promoting more orderly layer stacking and smaller phase domains.

In 2019, Peng et al. synthesized a wide band gap polymer donor PSFTZ (as shown in Figure 1)^[43]. The PSFTZ has an absorption complementary to Y6 and a matched energy level. However, due to the strong aggregation effect of PSFTZ, the phase separation of the active layer is obvious, which increases the probability of charge recombination, and the efficiency of the device prepared by blending with Y6 is only 13.07%. By adding a small amount of Pt(Ph)₂(DMSO)₂, the polymer PSFTZ can be complexed with platinum (Ⅱ) in Pt(Ph)₂(DMSO)₂. Because the benzene ring on the platinum (II) complex increases the steric hindrance, inhibits the aggregation of the polymer and regulates the phase separation, the device efficiency can be improved to 16.35%. Ding's group synthesized the polymer donor D16 by substituting 5H-dithio [3,2-b: 2 ', 3' -d] pyrrole-5-one (DTTP) for 5H-dithio [3,2-b: 2 ', 3' -d] pyrrole-5-one (DTP), and blended it with Y6, the device efficiency reached 16.72%^[44]. On the basis of D16, D18 was obtained by replacing DTTP with DTBT. Compared with DTTP, DTBT has a larger molecular plane, so that the DTBT-based polymer has a higher hole mobility. The efficiency of D18: Y6-based solar cell reaches 18.22% (NIM certified efficiency is 17.6%)^[45].

2.2 Small molecule donor matched to Y6 receptor

Compared with polymer donor materials, small molecule donor materials have the advantages of clear molecular structure, high repeatability and high purity^{[46⇓⇓⇓⇓~51]}. In the design and synthesis of small molecule donors, our research group selected fluorodithiophene and benzodithiophene as the core group to construct two types of A-π-D-π-a small molecule donors BTEC-1F and BTEC-2F (Figure 2)^[52]. Compared with BTEC-1F: Y6, the performance parameters of the device based on BTEC-2F: Y6 blend layer are :V_oc=0.854 V,J_sc=21.55 mA/cm²,FF=0.72,PCE=13.34%, which is due to the enhanced phase separation, compact molecular packing and highly ordered face-on molecular orientation. Lu's group prepared BTR-Cl by chlorination of benzodithiophene-trithiophene rhodanine (BTR)^[53]. The BTR-Cl molecules have more ordered liquid crystal properties, deeper molecular energy levels and higher crystallinity, and are blended with Y6 acceptors to obtain a suitable film morphology, and the efficiency of the all-small-molecule organic solar cell device reaches 13.6%. On this basis, by introducing PC₇₁BM as the third component, the optimal device efficiency can reach 15. 34%, and the FF is greatly improved to 0. 77. The study shows that the introduction of PC₇₁BM significantly reduces the bimolecular recombination, which leads to the improvement of FF^[54].

Wei's group reported a small molecule donor ZR1 with A-π-D-π-a structure (Figure 2), which is complementary to Y6 absorption, and the highest efficiency of the device is 14.34% (the certified efficiency is 14.1%), including V_oc=0.86 V,J_sc=24.34 mA/cm²,FF=0.68^[55]. In 2020, Li project combined SM1-F small molecule electron donor (Figure 2), and the performance parameters of the device blended with Y6 were :V_oc=0.866 V,J_sc=23.25 mA/cm²,FF=0.70,PCE=14.07%^[56]. Zhang et al. Synthesized a new small molecule donor BTTzR (Figure 2). The efficiency of the all-small molecule system based on BTTzR: Y6 reached 13.9%, including V_oc=0.88 V,J_sc=23.20 mA/cm²,FF=0.68, and the non-radiative recombination energy loss of the system was only 0.18 eV, which was close to that of inorganic solar cells^[57].

3 Ternary and quaternary blend solar cells based on Y6

In recent years, progress has been made in binary organic solar cells^{[58⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~69]}. However, due to the narrow optical absorption and the high energy loss caused by the mismatch between HOMO and LUMO energy levels, the improvement of the efficiency of binary organic solar cells is limited^{[70⇓⇓⇓⇓~75]}. In order to solve the above problems, the strategy of introducing the third component to prepare ternary organic solar cells has been proposed and widely used^{[76⇓⇓⇓⇓⇓~82]}. Y6 can be blended with polymer donors to obtain excellent PCEs, and the research results of many research groups show that the efficiency of the dimer can exceed 16%. For PM6: Y6 system, PCEs can be further enhanced by adding third components such as fullerene derivatives, non-fullerene small molecule acceptors, polymer donors or small molecule donors, and solid additives^{[83⇓⇓~86]}.

In 2019, Hou et al. added fullerene derivative PC₆₁BM as the third component on the basis of PM6: Y6 binary system, which can increase the electron mobility and realize the balanced electron/hole charge transport. At the same time, PC₆₁BM can disperse the self-aggregation of Y6, improve the electroluminescent efficiency and reduce the non-radiative energy loss^[87]. The V_oc, J_sc and FF of the prepared ternary organic solar cell are improved at the same time, and the efficiency reaches 16.5%. Zhan's group introduced non-fullerene acceptor IDIC into PM6: Y6, and the device efficiency was 16.5%^[85]. The higher LUMO level of IDIC is one of the reasons for the improvement of V_oc. At the same time, IDIC provides an additional fast hole transfer channel in this system, which contributes to the generation of carriers. Sun et al. Designed and synthesized a new small molecule acceptor 3TP3T-4F as the third component for PM6: Y6 system, and the device performance parameters obtained were V_oc=0.85 V,J_sc=25.9 mA/cm²,FF=0.75,PCE=16.7%^[88]. In 2020, Bao et al. Added small molecule donor DRTB-T-C4 to PM6: Y6 system, and obtained an efficiency of more than 17%, in which the V_oc was 0.85 V,J_sc, the 24.79 mA/cm²,FF was 0.81, and the final photoelectric conversion efficiency was 17.13%^[89]. The addition of DRTB-T-C4 can improve the hole and electron mobility simultaneously, and make the charge transport more balanced. In addition, DRTB-T-C4 has good compatibility with PM6: Y6 blends.

Quaternary organic hybrid solar cells use a mixture of four materials (one donor and three acceptors, two donors and two acceptors, or three donors and one acceptor) as the active layer material. The combination of four materials allows researchers to choose more materials and more mechanisms to improve PCE^{[90⇓⇓⇓⇓~95]}. Figure 3 and Table 3 list the chemical structures and device performance parameters of the third or fourth component in some highly efficient ternary and quaternary organic solar cells, respectively. In 2020, Zhan's research group added fullerene derivative PC₇₁BM to PM6: Y6: IDIC to form a quaternary system, and the performance of the device was :V_oc=0.87 V,J_sc=26.19 mA/cm²,FF=0.75,PCE=17.07%^[96]. The higher LUMO energy level of IDIC and PC₇₁BM is the reason for the improvement of V_oc, and the addition of IDIC and PC₇₁BM improves the mobility of electrons and holes, which is conducive to the improvement of J_sc. In 2020, Zhan's team added polymer donor material PhI-Se as the fourth component in the PM6:Y6:PC₇₁BM system, and the device performance was :V_oc=0.85 V,J_sc=26.3 mA/cm²,FF=0.77,PCE=17.2%^[31]. The results show that the PhI-Se guest can form a separate donor phase, and the presence of each PhI-Se phase enhances the absorption of sunlight, and provides additional charge separation and hole transport paths, which can generate more charges and transport more holes. The addition of PhI-Se can also improve the structural order of the Y6 acceptor phase, thereby improving the electron transport. Liu's research group added polymer donor PM7 and fullerene acceptor PC₇₁BM on the basis of high-efficiency PM6: Y6. Through physical characterization, it was found that a double-cascade carrier transport channel was formed in the quaternary blend, and the HOMO level in the middle of PM7 could realize the ultrafast transfer of holes and increase the output current^[97]. At the same time, the introduction of PC₇₁BM reduces the non-radiative recombination loss and provides a fast transfer channel for electrons. Finally, the device achieved a PCE of 18.07% (NREL certified efficiency of 17.35%) with a V_oc of 0.86 V,J_sc and a 26.55 mA/cm²,FF of 0.79.

4 Layer-by-layer fabrication of solar cells based on Y6

In 1986, Deng Qingyun prepared the first double-layer heterojunction organic solar cell^[8]. In 1995, Yu Gang and Heeger et al. Prepared the first bulk heterojunction (BHJ) organic solar cell, in which the polymer donor and fullerene acceptor formed an interpenetrating network structure, which greatly increased the contact area between the donor and acceptor^[98]. Up to now, with the development of photovoltaic materials, organic solar cells have achieved high PCEs through BHJ structure, but it is difficult to accurately control the morphology of BHJ in the process of large-scale preparation, which is not conducive to promoting the industrialization of organic solar cells. In recent years, organic solar cells fabricated by layer-by-layer printing have received renewed attention from researchers. Compared with the BHJ structure, the layer-by-layer printing method has many advantages, such as controllable morphology of the active layer film, good charge transport and extraction performance, universality, and insensitivity to the synthesis batch of donor/acceptor materials^[99,100]^[101⇓~103]^[104].

In 2019, Yip et al. Used continuous layer-by-layer processing to control the morphology of ternary organic solar cells^[100]. A conversion efficiency of 16.4% can be obtained in the ternary LbL organic solar cell composed of PM6: Y6: FBR. The results show that the LbL film can form a bicontinuous interpenetrating network structure, and the crystallinity of the donor-acceptor is almost unaffected, showing efficient charge generation, transport and collection characteristics, and the PCE of the ternary LbL is not sensitive to the proportion of the third component acceptor, thus providing space for optimizing the device performance. In 2020, Min et al. Used LbL to prepare high-performance large-area organic solar cells and modules^[105]. Based on PM6: Y6 system, 16.35% PCE was obtained. The results show that the LbL method has the advantages of material compatibility, high light absorption, vertical phase separation and practicability.

5 Flexible Organic Solar Cell Based on Y6

Flexibility is a prominent advantage of organic solar cells. PCEs based on flexible organic solar cells are still lower than those based on rigid substrates. Due to the lack of high-performance flexible transparent electrodes, their mechanical stability can not meet the needs of flexible electronic products at present^{[73,106⇓⇓⇓⇓⇓⇓⇓ ~114]}. Indium tin oxide (ITO) is the dominant electrode in the field of organic solar cells.However, when ITO is coated on a plastic substrate, it will crack due to the shrinkage of the substrate material during thermal annealing and mechanical bending, resulting in an increase in sheet resistance and diffuse reflectance, which seriously affects the mechanical stability of the device^[108]. In recent years, researchers have optimized the flexible transparent electrode and achieved many results.

In 2019, Li et al. Proposed the concept of "welding"^[115]. The team used a network of solution-treated aluminum-doped ZnO (AZO) and silver nanowires (AgNW) to form the upper electrode, and used capillary force and the secondary growth of AZO to reduce the resistance of the AgNW junction. Using PM6: Y6 system as the active layer and Em-Ag/AgNWs: AZO-SG as the flexible substrate, they prepared flexible organic solar cells with a PCE of 15.21% and maintained 75% of the initial efficiency after bending nearly 8000 times. In 2020, Ge et al. Used three economical and environmentally friendly acids (citric acid, tartaric acid and malic acid) and polyhydroxy compounds to treat PEDOT: PSS electrodes to prepare high-efficiency foldable flexible organic solar cells^[116]. Based on the PM6: Y6 binary system, a flexible organic solar cell with an efficiency of 14.17% was prepared, and the results are shown in fig. 4. The acid treatment enables the PEDOT: PSS to have high conductivity, and simultaneously, the doping of the polyhydroxy compound contributes to more excellent bending flexibility and foldability of the substrate. Bao et al doped 5wt% polyarylether (PAE) resin into PM6: Y6 system, and the PCE of the device reached 16.13%^[26]. More importantly, the PCE of the device doped with 30wt% PAE can still reach 15.17%, showing an elongation at break of 4.4 times. The results show that the PAE matrix shows the tunneling effect without changing the charge transport channel, and can also inhibit the movement of the molecular chain. Flexible organic solar cells have advantages in wearability, while improving the efficiency and flexibility of devices is still the direction of continuous efforts.

6 Y6-based tandem organic solar cell

In recent years, researchers have made important progress in the development of new donor-acceptor materials, especially the emergence of new high-efficiency non-fullerene acceptor materials, which has promoted the breakthrough of PCEs in organic solar cells, and the PCEs of devices has exceeded 17%^{[19,23,45,116⇓⇓⇓ ~120]}. However, due to the narrow absorption range of organic photovoltaic materials, organic solar cells can not make full use of the solar spectrum, which makes it a challenge to further improve the PCEs of organic solar cells. Multi-component blending and tandem cells are usually used to solve this problem^{[87,101,121⇓⇓ ~124]}. Compared with the multi-element blending solar cell, the stacked organic solar cell is to stack two or more single junction devices in series or in parallel, which can improve the absorption of sunlight to the greatest extent^{[125⇓⇓⇓⇓⇓~131]}.

Since the first tandem solar cell was prepared by solution method in 2006, the highest efficiency of tandem organic solar cell devices is 17.3% due to the development of near-infrared (NIR) non-fullerene acceptor materials^[132]. Although Y6 has achieved great success, its application in lamination is relatively small. In 2020, Ding et al. Used the synthesized medium bandgap small molecule non-fullerene acceptor IBCT to prepare a tandem cell with L1: IBCT as the front cell and L1: Y6 as the back cell, and the device efficiency reached 15.25%^[133]. Li and Meng et al added a double bond between the seven-membered ring and the end group of Y6 as a π bridge to synthesize the acceptor BTPV-4F with an optical band gap of only 1.21 eV. The efficiency of the stacked device prepared by PTB7-Th:BTPV-4F:PC₇₁BM as the front cell and PM6: m-DTC-2F as the back cell reached 16.4%^[134]. Although the efficiency of tandem cells based on Y6 is not high, if more suitable donors and front cells are found, it is believed that the efficiency of tandem devices can be further broken through^[130].

7 Semitransparent Organic Solar Cells Based on Y6

Semitransparent organic solar cells (ST-OSCs) can absorb a large amount of near-infrared light to convert into electricity, while transmitting most of the visible light, which has broad application prospects in the fields of building integration, photovoltaic glass, greenhouse agriculture and wearable devices, and has attracted great interest from academia and industry^{[135⇓⇓~138]}. ST-OSCs can selectively absorb visible light and near infrared (NIR) light by changing the composition of the active layer material. In the energy distribution map of AM1.5G solar spectrum, 47% of the solar energy is distributed in the visible region, while 51% of the solar energy is distributed in the near-infrared region, indicating that the utilization of near-infrared energy plays an important role in improving photoelectric conversion^[139]. With the development of non-fullerene receptor materials and semi-transparent device structures, ST-OSCs have achieved rapid development. Average visible transmittance (AVT) and PCE are considered to be two key performance parameters for evaluating ST-OSCs^[140]. In recent years, the PCEs of ST-OSCs have reached 10% ~ 12%, and the AVTs are about 25%. However, the development of ST-OSCs still lags behind that of opaque organic solar cells because of the trade-off between PCEs and AVTs^[141].

In the early stage of the development of ST-OSCs, the active layer materials were mainly based on medium or narrow band gap polymer donor materials and fullerene derivatives^[142]. In recent years, non-fullerene electron acceptors that absorb infrared light have been developed rapidly. These acceptors can effectively utilize infrared photons and transmit visible photons, so they are ideal active layer acceptor materials for ST-OSCs^{[23,26,94,143⇓ ~145]}. As a star non-fullerene receptor molecule, Y6 has strong absorption in the near infrared region, which can ensure sufficient light absorption and is suitable for the preparation of ST-OSCs. In 2019, Li et al. Prepared SnO₂ particles by chemical precipitation as the electron collection layer of ST-OSCs^[84]. Due to its excellent transparency and light reflectivity, the SnO₂ layer can effectively adjust the light distribution of incident light in each layer of the whole ST-OSCs. Under the guidance of the finite-difference time-domain (FDTD) method and the transfer matrix method (TMF), the contradiction between PCEs and AVTs is effectively solved.Combined with the above advantages, the inverted ST-OSCs prepared with PM6: Y6 has a PCE of 12. 88%, an AVT of 25. 6%, and a color rendering index (CRI) of 97. 6, which meets the requirements of commercial glass applications. In 2020, Li and Yip et al designed a new Fabry-P P Érot (F-P) electrode^[146]. The introduction of F-P electrode is mainly to reduce the bandwidth of the transmission spectrum as much as possible so as to reduce the transmission color distortion of the translucent cell. This kind of electrode is usually designed as a three-layer sandwich structure of metal/insulator/metal, and the wavelength of the transmitted light can be modulated by the interference of the F-P electrode. The semi-transparent device structure is shown in fig. 5. With the aid of optical simulations, the device efficiency can exceed 14% and the maximum transmittance is greater than 31%.

8 Study on Organic Solar Cells Based on Y6 Triplet

In order to improve the PCEs of organic solar cells, it is important to broaden the spectral absorption and reduce the energy loss. There are singlet and triplet excited States in organic semiconductors. The lifetime of the singlet exciton is usually in the range of 10 ~ 100 ps, and the exciton diffusion distance is in the range of 3 ~ 10 nm, which is limited by the thickness and morphology of the active layer. In principle, triplet excitons have longer lifetimes and diffusion distances than singlet excitons because triplet recombination is inhibited. Moreover, when the T₁ energy level is closer to the ³CT, the triplet exciton dissociation becomes easier, and the efficient intersystem crossing also helps to improve the utilization rate of the triplet exciton, thereby helping to improve the performance of the device. Therefore, the development and research of triplet materials are also important for the realization of high-performance OSCs^{[147⇓⇓~150]}.

In the study of the effect of triplet exciton on OSCs, Y6, as a representative non-fullerene receptor molecule, has attracted the attention of researchers. In 2020, Yi et al studied the important role of π-π stacking of terminal groups in reducing the ΔE_ST (the energy gap between singlet and triplet States) through three representative non-fullerene receptor molecules, ITIC, IT-4F and Y6^[151]. The ΔE_ST can be reduced by intramolecular push-pull effects, and appropriate push-pull effects are essential to achieve high oscillator strengths and moderate ΔE_ST. Among the three non-fullerene acceptor systems, PM6: Y6 system has the lowest E_g and ΔE_ST, and the highest PCE. Huang et al. synthesized two A-DA 'D -a non-fullerene electron acceptors H1 and H2 similar to Y6 (the chemical structural formula is shown in Figure 6)^[152]. The device efficiency of PBDB-T: H1 can reach 14.06% (among them, the device efficiency of V_oc=0.76 V,J_sc=25.74 mA/cm²,FF=0.71),PBDB-T:H2 can reach 15.12% (among them, V_oc=0.79 V,J_sc=25.82 mA/cm²,FF=0.73). H1 and H2 were characterized by steady-state and transient photoluminescence, absorption spectra, electron paramagnetic resonance (EPR) and magnetophotocurrent (MPC). Through the study of the mechanism of triplet exciton, it is found that the twisted D-a structure can lead to large spin-orbit coupling (SOC) and small ΔE_ST, thus achieving efficient intersystem crossing. According to the perturbation theory,

K ISC ∝ < 1 ψ | H SO ⌢ | 3 ψ > / exp Δ E ST 2

, where the

< 1 ψ | H SO ⌢ | 3 ψ >

represents the spin-orbit coupling matrix element, the

H SO ⌢

represents the spin-orbit coupling Hamiltonian, the ΔE_ST represents the energy difference between the singlet and triplet States, and the

K ISC

represents the intersystem crossing rate constant. This formula shows that large spin-orbit coupling and small ΔE_ST can increase the intersystem crossing rate. The energy level of the T₁ is closer to the ³CT, which is beneficial for the triplet exciton to convert into free charge.

9 Conclusion and prospect

With the development of non-fullerene acceptor materials, the device efficiency of organic solar cells has made breakthroughs in recent years. The A-type DA 'D and A-type acceptor structure provide new ideas for the design of high-performance organic semiconductor materials^[153]. As a representative star non-fullerene acceptor molecule in recent two years, Y6 has strong absorption, narrow band gap (1.33 eV) and excellent electron mobility. The PCE of binary devices prepared by blending with PM6 has exceeded 16%, and the PCE of ternary devices based on PM6: Y6 has exceeded 17%. At the same time, Y6 also has good development potential in other organic semiconductor applications such as organic photodetectors^[154].

In order to improve the application of Y6 in organic solar cell devices and enhance its commercial value, it is necessary to optimize the synthesis of donor and acceptor materials and the preparation process of devices in the future, so as to further improve the PCE and stability of devices.

(1) Firstly, a donor material more matched with Y6 is synthesized, the absorption spectrum of the mixed film is broadened, and the energy level matching is optimized to further improve the photovoltaic performance. For example, efficient electron acceptor units can be introduced to synthesize photovoltaic donor materials, which can reduce the HOMO energy level of donor materials and provide the possibility of improving V_oc.

(2) Selecting suitable donor/acceptor materials as the third component and using the ternary strategy to further improve the performance parameters of the Y6 system. The introduction of the third component can broaden the absorption spectrum, improve the crystallinity and increase the carrier mobility. And optimize that structure of the device.

And (3) modif that molecular structure of Y6. First of all, most of the A-DA 'D a type molecules are heptacyclic skeletons, so the skeleton structure can be broadened to more than seven rings to improve the electron transport performance and light collection performance. Secondly, Y6 can also be modified by side chain engineering. Through different alkyl chains, the solubility, crystallinity and miscibility of molecules are adjusted, and the carrier mobility is improved. And finally, introduce different end groups. In addition to the end groups of the IC family, other types of end groups can be applied to improve photovoltaic performance.

And (4) optimizing the preparation process of the device. In addition to the spin coating of the active layer, the processing methods of other kinds of active layer solutions are explored to simplify the processing conditions of the active layer. For example, roll-to-roll and spray coating methods are used to prepare large area devices, which makes more exploration for the practical application of organic solar cells.

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