Liu Ye, Weiyu Ye, and Shiming Zhang,

1Key Laboratory of FlexibleElectronics (KLOFE) & Institute of Advanced Materials (IAM), JiangsuNationalSynergetic InnovationCenter for AdvancedMaterials (SICAM), Nanjing Tech University, Nanjing211816, China

2JiangsuSeenbom FlexibleElectronics InstituteCo. Ltd., Nanjing210043, China

Abstract: Recently, polymer solar cells developed very fast due to the application of non-fullerence acceptors. Substituting asymmetric small molecules for symmetric small molecule acceptors in the photoactive layer is a strategy to improve the performance of polymer solar cells. The asymmetric design of the molecule is very beneficial for exciton dissociation and charge transport and will also fine-tune the molecular energy level to adjust the open-circuit voltage (Voc) further. The influence on the absorption range and absorption intensity will cause the short-circuit current density (Jsc) to change, resulting in higher device performance. The effect on molecular aggregation and molecular stacking of asymmetric structures can directly change the microscopic morphology, phase separation size, and the active layer's crystallinity. Very recently, thanks to the ingenious design of active layer materials and the optimization of devices, asymmetric non-fullerene polymer solar cells (A-NF-PSCs) have achieved remarkable development. In this review, we have summarized the latest developments in asymmetric small molecule acceptors(A-NF-SMAs) with the acceptor–donor–acceptor (A–D–A) and/or acceptor–donor–acceptor–donor–acceptor (A–D–A–D–A) structures, and the advantages of asymmetric small molecules are explored from the aspects of charge transport, molecular energy level and active layer accumulation morphology.

Key words: polymer solar cells; non-fullereneacceptors; smallasymmetric molecules

1.Introduction

Owingto theflexibility,portability, transparency, and low manufacturing cost,polymersolar cells(PSCs) have great potential for commercial production and application[1–6]. The active layer of PSCs mainly adopts a bulk heterojunction structure with a blend of polymer donors and non-fullerene small molecule acceptors (NF-SMAs)[7]. Compared with fullerene acceptors, the advantagesof non-fullerenesmall molecule acceptors include[8−12]:(1)Structuralmodularity facilitatesmolecular tailoring and property regulation; (2) Strong intramolecular charge transfer (ICT) enhances the absorption of visible light; (3) The planar molecular skeleton is conducive to chargetransport; (4)Sidechainspreventmolecules excessive aggregation; (5) Non-spherical configuration is conducive tomorphological stability. Inaddition,PSCs havetheadvantages of good film-forming properties, so their power conversion efficiencies (PCEs) are relatively high, so PSCs are currently the mainstream research direction of organic solar cells(OSCs)[13,14]. Until now,the highestenergyconversion efficiency of PSCs has reached18.22%[13],far exceeding the 10%PCE commercial bottleneck.

From 2011–2014, plenty of NF-SMAs have been designed with various electron-withdrawing groups such as indanedione[1], dicyannovinyl[2−4], benzothiadiazole[5,6], diketopyrolopyrrole[7,15],and arylene diimide[16,17]. With the unremitting efforts of Zhan Xiaowei's group, 2,2'-[[6,6,12,12-tetrakis(4-hexylphenyl)-s-indacenodithieno[3,2-b]thiophene]methylidyne(3-oxo-1H-indene-2,1(3H)-diylidene)]]bis(propanedinitrile)(ITIC)and 2,2'-2,2'-[(4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis[methylidyne(3-oxo-1H-indene-2,1(3H)-diylidene)]]bis[propanedinitrile] (IDIC)(Fig. 1(b)) were reportedin 2015 and 2017 respectively,which opened a new chapter in the research of PSCs[14,18]. A seriesofworks have beenreported,andthe related research results are greatly conducive to the in-depth understanding ofPSCs, thus promoting the rapid developmentofindustrial research. According to structural characteristics, NF-SMAs can divide into symmetrical and asymmetrical structures. Thanks totheresearchers for their continuous exploration and research, many research results and breakthroughs have been made in symmetricnon-fullerenesmall moleculeacceptors(S-NF-SMAs) and asymmetric non-fullerene small molecule acceptors (A-NF-SMAs) inrecent years.

Fig. 1. Chemical structures of (a) early reports of A-NF-SMAs, (b) early reports of small molecule acceptors.

The asymmetric design of the molecule helps increase the dipole moment and dielectric constant and reduces the excitonbinding energy, which is verybeneficialto exciton dissociation and charge transport[19–21]. Simultaneously,comparedwiththe S-NF-SMAs, theasymmetric design of ANF-SMAs will finely regulate the molecular energy level,therebyfurtheradjusting the open-circuitvoltage(Voc).Changes in the absorption range and absorption intensity will cause the short-circuit current density (Jsc) to change accordingly. The influenceof molecularaggregation and molecular accumulation can directly change the microscopic morphology, phase separation, and trap degree of the active layer[22,23].

Generally speaking,VocandJscare two iconic device performance parameters, and the increase of the value ofVoc×Jscgenerally shows the improvement of device performance.Further optimize the value ofVoc×Jscthrough asymmetric moleculardesign toobtainhigher deviceperformance, which provides the latestresearchideasand unique insights for PSCsresearch[22]. So far, the maximum powerconversion efficiency (PCEmax)of PSCs based onasymmetric non-fullerene acceptors hasgraduallyincreased from0.73%in2011 tomore than 17% in 2020. The development of A-NF-SMAs can be traced back to 2011. Paul Meredith's research group published asymmetric donor–acceptor typed small molecule acceptor K12 and John E. Anthony's research group published a series of asymmetric electron-deficient pentacene acceptors(Fig. 1(a)). K12 and these deficient pentacene acceptors are blended with P3HT as the polymerdonor tofabricatebinary PSCs,andthe highest PCE ofPSCs obtainedreaches 0.73%and 1.27%, respectively[25,26]. In 2012, Pei's researchteam designed and synthesizedanA-NF-SMA(FFI-1) (Fig. 1(a))[27],with polymer donor P3HT and fullerene acceptor PCBMto produce a ternary system PSC. This device produced a breakthrough PCE of 4.1%. In 2014, Cao and coworkers developed an asymmetric non-fullerene SMA, Cor-NI, by introducing the electron-withdrawing n-hexylnaphthalimide moiety part into the C5-symmetric corannulene core[28]. PSCs with P3HT:Cor-NI as the active layer material achieved a PCE of 1.03%.However,during the three yearsfrom 2014 to 2017,there was noresearchrelated to A-NF-SMAs.Until 2017, Tanget al.[29]designed and synthesizedthree new A-NF-SMAs (ITBC, ITBR, ITBRC) byincorporating indenothiophene as the central core. InvertedstructurePSCsusing ITBR (Fig. 1(a)) asthe acceptor and PTB7-Th as the donor material (Fig. 4) afford a PCEmaxof 7.49%.

There have been related review articlessummarizing the latest development of A-NF-SMAs, but its research content focused onthe PSCs withafive-memberedindaceno[1,2-b:5,6-b']dithiophene (IDT) series of A-NF-SMAs[28]. The newly reported"Y series"NF-SMAs, especially Y6 (Fig.1(b)),have brought PSCs into a new golden age[30,31]. Fully applying the asymmetric structural designconcept,researchershave studiedmany A-NF-SMAs based on the "Y series" structure to achieve a further breakthroughin PCEmax. Thehighest PCEof binaryblended PSCs based on the acceptor–donor–acceptor–donor–acceptor(A–D–A–D–A) typedA-NF-SMAs has reached 17.06%,and the highest PCE of ternary blended PSCs has reached 17.43%[32]. Therefore,there isno doubt thatthe high-efficiency A-NF-SMAs have had a profound significance in the development of PSCs inthe past ten years[30].

This review summarizes asymmetric non-fullerene polymersolar cells (A-NF-PSCs)-related contentfromthree aspects: the structural advantages of the asymmetric system,the development history, and the research resultsinrecent years. Meanwhile, we have also made a particular outlook on future researchprospects, hoping to provide someguiding insights on the development of PSCs in the future.

2.Photophysical properties of OSCs

Thestructureand mechanismof photovoltaicdevices have been discussed and studied in many excellent studies[10−12]. Here,we willbriefly mentionthe photophysical mechanism of OSCs and discuss the necessary photophysical performance parameters. The structureof the bulk heterojunction OSCs is composed of a metal cathode, an electron transport layer, an active layer, a hole transport layer and a glass substrate as shown in Figs. 2(b) and 2(c). The photovoltaic ef-fect is the theoretical basis for the photoelectric conversion process of OSCs. Thisprocess mainly occurs inthe active layer, including the following processes (Fig. 3)[33]: (1) exciton generation, transport, anddissociation; (2) chargecarrier transport; (3) charge carrier collected by electrodes.

The photophysical characteristics of OSCs are generally representedbycurrentdensity–voltagecurves (J–V)[34].As shown in Fig. 2(a), the main parameters are as follows:

Fig. 2.(a) A typicalcurrent–voltageJ–V characteristics of solar cells. (b)Standardarchitecture of bulk-heterojunction(BHJ)and(c)invertedstructure. Reproduced with thepermission of Ref. [34].

Fig. 3. Simplified schematic of photoconversion in OSCs with the processes of photon absorption, exciton diffusion, exciton dissociation by charge transfer, and charge carrier collection denoted. Reproduced with permission from Ref. [33].

(1)Voc

Under light conditions,Vocisthe voltageat which the positive and negative poles of the OSCs are in an open circuit state, that is,the maximumoutput voltage ofthe OSCs[27,28].In the OSCs composed of donor/acceptor,Vocis directly related to the extreme difference between the donor’s highest occupied molecular orbital (HOMO) energy leveland acceptor’s lowest unoccupied molecular orbital (LUMO) energy.Therefore, the value ofVoccan be increasedeither by reducing theHOMO levelof thedonororincreasingthe LUMO level of the acceptor[34].

(2)Jsc

Jscis the current per unit area when the positive and negative electrodes of the OSCs are in a short-circuit state, that is, the maximumoutputcurrent densityof the OSCs inthe light environment. As the band-gap of the material decreases,the valueofJscincreases andcan be affectedbythe electron and hole transport efficiency of the active material[22,23].

(3) Fill factor (FF)

FFisadimensionless physicalquantity, which is the ratio of the maximum output power (Pmax) of the OSCs device to the product ofVocandJsc, which can be expressed by Eq.(1).

FF is mainly related to the transfer and collection processofcharge carriers. The balanceofhole/electron mobility in the active layer, the degree of recombination during carrier transport, and the collection efficiency of carriers reaching thebufferlayer will all affect theFF valueof OSCs. The FFsuggests how swiftly the charges can be removed from the cells and theideal valueis 1.0. Several factorscan affect the FF of OSCs and they often interact in intricate ways[35].

(4) Power conversion efficiency (PCE)

PCE is thepercentageof incident light energy converted into effective electric energy, which can be expressed by the ratio ofPmaxof OSCs to incident power (Pin). In this case, the current densityandapplied biasvoltageare expressed byJmaxandVmaxrespectively, the calculation formula of PCE is as follow[36]:

Itcan be seen thatthe PCEis jointly determinedbyVoc,Jsc, and FF, and represents the ability of OSCs to convert solar energy intoelectrical energy.

(5) External quantum efficiency (EQE)

The ratio of the number of electrons that can be collected under a certain wavelengthof radiation tothenumberof incident photons at that wavelength can be expressed by the followingEq. (3):

Amongthem,λisthewavelengthof the incident light,andPinis the power of the incident light. EQE is the product oflightabsorption efficiency, exciton diffusionand dissociation efficiency, charge transfer efficiency, and charge collection efficiency in the photoelectric conversion process. The EQE andJsccan be mutually verified,and high EQE is the prerequisite for realizing high-efficiency OSCs[24].

3.Asymmetric non-fullerene acceptors based on A–D–A structures

Previous studies on A-NF-PSCs have been reviewed and published[28]. This review will only introduce some relevant active layer materials with high-efficiency device performance.Fig. 4 shows the related polymer donors.

3.1.A-NF-SMAs with asymmetric cores

The structure of A-D-A is usually composed of one electron-donating unit and twoelectron-withdrawing units. This push–pull electron behavior in the moleculecontributesto ICT so that the molecule has a strong transition dipole and wide spectral absorption range[8]. Generally speaking, molecularasymmetry features mainlyinclude coreunit asymmetry,terminal group asymmetry, and side-chain asymmetry. The asymmetry of all parts will cause changes in the intra-molecular dipole moment and intermolecular binding energy,thereby affecting the charge transport and exciton dissociation[21,28].

In the related research on the asymmetry of the core unit, the dipole moment and arrangement of the molecule can be adjusted by changing the number of asymmetric thiophene unitsand introducing heavy atoms. In the core unitof electron donation,the IDT coreand theheptacyclic indacenodithieno[3,2-b]thiophene (IDTT) core play an important foundational role in the design of high-performance nonfullerene receptors, a newtype of asymmetric ladder-type thiophene-phenylenethieno[3,2-b]thiophene-fused(TPTT)core combines the structural features of IDT and IDTT. At the same time, the π conjugate length of TPTT and the electrondonating capacity are all between IDT and IDTT. TPTT-IC (its structure is the same as T-TT), TPTT-2F, T-TT-4F, T-TT-4Cl,IDT6CN and IDT6CN-M are all molecules designed based on the TPTT core (Fig. 5). They have terminal groups which are different from each other. Their maximum absorption wavelengths range from 693 nm to 808 nm, and their LUMO energy levels range from –3.37 to –4.04 eV[37−40]. Finally, the device based on PBDB-T:IDT6CN-M blended film achieved the highest PCE of 11.23%, with aVocof 0.92 V, aJscof 15.97 mA/cm2, and a FF of 76.1%, by using 0.5% diiodooctane (DIO)as asolvent additiveandthermal annealing at 100℃.Liet al.designedtheasymmetric molecule TTPT-T-2Fby cutting IDTT fragmentsand synthesized symmetricmoleculeIT-2Fand TTPT-T-2F. TTPT-T-2F,IT-2F and T-TPT-T-2F wererespectively blended withthedonor PBT1-C to fabricate PSCs. Thanksto the TTPT-T-2F-basedPSCs with the highestJscvalue (18.50%),apromisingPCEmaxof12.71%is achievedfor TTPT-T-2FbasedPSCs,which outperformsthose of devices based on IT-2F (PCE = 10.54%) and T-TPT-T-2F (PCE = 10.71%)[41]. Gaoet al. realized the optimization of the molecular morphology by introducing the thioenyl side chain, which promoted BHJ active layer to have good characteristics, such as higher and more balanced carrier mobility, tiny bimolecular reorganization, and orderly structure. Finally, the PBDB-T:IDT6CN-Th system achieved a very high FF of 76.72% and a PCE of 10.41%[42,43]. By further adjusting the terminal groups,IDT6CN-TM has a more significant dipole moment. Compared with IDT6CN-Th and IDT6CN-4F, the strong dipole moment contributes more substantial π–π stacking and higher electron mobility of IDT6CN-TM. The devices based on PM6:IDT6CN-TMobtained PCEof12.4% (Voc= 0.95 V,Jsc=17.40mA/cm2,FF = 74.7%), which is currentlythehighest PCE among non-fullerene PSCs with thienyl side chains.

Fig.5. Chemical structures of A–D–A asymmetric non-fullereneacceptors without nitrogen.

In the combination of monothiophene and trithiophene,extending the IDT nuclear conjugation length can expand the spectral absorption, shift the LUMO energylevel up, and improve electron mobility. At the same time, it can also enhance the π–π stacking between molecules. Compared with PSCs containing dithiophene-based molecules, the PSCs containingthese trithiopheneNF-SMAs TPTTT-2F,α-ITand MeIC1(Fig.5) haveimprovedPSCperformance parameters,includingVoc,Jscand FF[38,44,45]. PBDB-T:MeIC1 blended film has an area of 3.8 mm2as the active layer, and its device has obtained the best PCE of12.58%, with aVocof 0.93 V,Jscof 18.32 mA/cm2and FF of 74.1%. These results indicate that extending the conjugation length of the IDT core is an effective way to improve photovoltaic parameters and achieve high PCEs.

Theasymmetric structuresof4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b]thiophene[3,2-b]thiophenealt-[5,6-d]dithieno[3,2-b:2’,3’-d]thiophene (IDT8)core give full play to extended conjugation and asymmetric core advantages. This asymmetric structure can not only expand the spectralabsorptionand shift the LUMOenergy level up butalso increasethe dipole momentand fine-tunes relevant parameters. Gao et al. designed and synthesized two asymmetrical small molecule acceptors (IDT6CN-M and IDT8CN-M)(Fig.5). The pure filmforIDT8CN-M and IDT6CNMcomparableabsorption coefficientof 1.35 × 105and1.29×105cm–1with distinguishing maximum absorption wavelength values of 699 vs. 693 nm. It can be seen that IDT8CNM has a long conjugation length and a large maximum absorption wavelength[43]. After using 0.5%DIOasa solvent additive and thermal annealingat100 °C, thecoherence length and domain purity are improved. In the end, the PCE of the device based on IDT8CN-M was further improved to 12.43%,mainlydueto the increasein theJscvalue from15.97 to 17.11 mA/cm2, and the increase inthe FFvaluefrom76.1%to 78.9%. To further optimize the molecular structure and obtain a better balance betweenVocandJsc, Liet al. synthesized three A-NF-SMAs (TTPTTT-IC, TTPTTT-2F and TTPTTT-4F)(TT-IC, TTPTTT-2F and TTPTTT-4F) (Fig.5) by introducing three-terminalgroups with different electron withdrawing properties and using an asymmetric TTPTTT building block as thecentralcoreunit. For TTPTTT-4F, theadditionoffluorinated 2-(3-oxo-2,3-inden-1-ydenne) malononitrile (IC) terminal groups will cause a red shift in light absorption and reduce the LUMO energy level. TTPTTT-4F also has improved electronmobility and intramolecular solid or intermolecular interactions[46]. PSCs based on fluorinated NF-SMAs showed significantly higherJscand FF than their non-fluorinated TTPTTT-IC counterparts. The device achieved a PCE of 12.05% with PBT1-C as polymer donorand TTPTTT-4Fasacceptor, thanks to more coordinatedperformance parameters,especially the significant improvement inJsc.

Compared with thiophene-based NF-SMA, selenophenebased NF-SMA has receivedmuch lessattention, and the performance of the related device is also poor. To promote the absorption of non-fullerene acceptor small molecules, reduce their bandgap, and increase their LUMO energy level, Sun's research group designed an asymmetric small molecule acceptor SePT-IN byintroducing a single-sided selenium atom into the core unit of the symmetric small molecule acceptor TPT-IN (Fig. 5)[47]. When blended with polymer donor PBT1-C(Fig. 4), the SePT-IN finally obtained 10.20% PCE higher than that of TPT-IN,mainlydue tothe significantlyimprovedJsc. Then,Liet al.designedand synthesized twoselenophene-containing A-NF-SMAs SePTT-2F and SePTTT-2F(Fig. 5) with the same terminal group[48]. Moreover, ComparedwithSePTT-2F, SePTTT-2F hasa largerconjugation area inthe mainchain, and SePTTT-2F exhibitsa higherLUMO energy level and higher electron mobility. Both SePTTT-2F and SePTT-2F have similar band gaps and red-shifted absorption peaks. Pairing it with the polymer donor PBT1-C, the performanceof PSCsbased on SePTTT-2Freachedanimpressive PCE of 12.24% with an outstanding FF of 75.9%, which was much higher than that of PSCs based on SePTT-2F. Since the SePTTT-2F system has a higher and more balanced charge transferthanSePTT-2F,SePTTT-2Fhas more effectiveexciton dissociation and charge collection. Amongselenophenebased NF-SMAs, this is the highest value reported in the literature.

Wanget al. and Gaoet al.designed and synthesized ANF-SMAs withthe thienobenzodithiophene structure as the core TBDB-Na, TBDB-Ph, and a-BTTIC (Fig. 5)[49,50]. The ultraviolet-visible absorption redshift of TBDB-Na relative to TBDBPh made the LUMO energy level decrease from –3.74 to–3.77eV, while theJscof the devicebasedon PTBD-BZ:TBDBNa increase from 17.50 to 19.60 mA/cm2. PTBD-BZ:TBDB-Nabased devices achieved a PCEmaxof 12.47%, which is superior to that of the PTBD-BZ:TBDB-Ph-based devices (11.06%).TheLUMO energylevel ofa-BTTIC is –3.83 eV, whichislower than TBDB-Na and TBDB-Ph, whileitsHOMOenergy levelis–5.45 eV, which is similar to that of TBDB-Na and TBDB-Ph.Therefore, the a-BTTIC has a very narrow bandgap, and theJscof thedevice basedon PBDB-T:a-BTTIC blendfilm reaches upto 20.31 mA/cm2, resulting ina high PCEof13.60%. The high performance of these molecules is due to the appropriate energy level matching the donor, the excellent crystallinity of the active layer, and the balancedmiscibility.

The electronegativity of nitrogenandsulfur atomsare 3.04 and 2.58, respectively. Compared with non-N-functionalized molecules in an asymmetric system, the introduction of nitrogen can further adjust the molecules' dipole moment. As shownin Fig. 6, the change of thesulfur atom to a nitrogen atom in the middle of the trithiophene unit determines the design of the dithieno[3,2-b:2',3'-d]pyrrole (DTP) unit. The ability to take advantage of sp2hybrid nitrogen is the most prominentfeature of theDTP unit. Besides, thelone pair of electrons on thevertical πorbitalcan delocalizealongtheπ orbital of the molecule. Extending the delocalized π-electron system can narrow the optical energy bandgap (Egopt) and strengthen theπ–π stacking between molecules[51,52]. PCEs of the device basedonthoserelevant materials are all above 10%[53−60]. Gaoet al. designed and synthesized N7IT (Fig. 6)by introducing nitrogen into the molecule[44]. It has a red-shiftedabsorption of more than 50 nmand animprovedLUMO energylevelcompared toα-IT. These featuresprovideN7ITbased PSCs higherVoc,Jsc,and PCE (13.8%) thanα-IT. In addition, by combining electron-rich N and molecular conformational features, the molecule for N8IT and N7IT comparable dipolemomentof 12 and5 Debye. The high dipole moment of N8IT is the root cause of excessive crystallization and poor morphology of N8IT molecule. The device based on N8IT has relatively lowJsc, FF, and PCE (11.92%). To fully explore the influence of molecular conformationon molecular packingand aggregation characteristics, Yanget al.designedmolecules with different numbers of thiophene extension rings and used S∙O interactions to fine-tune the molecular conformation[53].

In contrasttoC-type IPTT-2F, S-type IPT-2F and IPTTT-2F(Fig. 6) mixed membranes have fewer traps toavoid recombination, conducive to proper phase separation and formation of better active layer morphology. When using PBDB-T as a polymer donor, the PCEs of PSCs based on IPT-2F and IPTTT-2Fwere14%and 12.3%, respectively, which were higher than those of PSCs based on C-type molecule IPTT-2F. Later,Yanget al. designed and synthesized four molecules with different aggregation characteristics IPT-4F, IPT-4Cl, IPTBO-4F,and IPTBO-4Cl (Fig. 6) by introducingdifferent N-alkyl chains andterminal groups[54]. The inherent characteristicsof larger Cl atoms and longer C–Cl bonds significantly extend the main chain's stacking area, thereby enhancing molecules' aggregation. In contrast,the largerBO chain exerts a moresignificant spatial shieldingeffecton themainchain'saccumulation. Both IPT-4F and IPTBO-4Cl showed outstanding performance when blended with PM6. It is worth noting that the PCEmaxof PM6:IPTBO-4Cl-based PSCs is 15%, which is slightly higher than that of 14.96% of PM6:IPT-4F-based PSCs. Considering the 2-ethylhexyl branch's significance, Maet al. and Liet al. introduced the 2-ethylhexyl branch on the N atom while using different terminal electron-withdrawing groups to carefully adjust theenergy level, absorption,crystallinity, and miscibilityofmolecules[55,56]. Electron-withdrawing ability of TPIC, TPIC-4Cl, TPIC-4F, and TPIC-2Cl (Fig. 6) gradually increased when the atom type was introduced by the terminal group from 1H to 4Clto4 F, leading to a gradualdecrease in LUMOenergy levelandVoc. Inthethin-film state, due to the relatively high ICT intensity and the unique atomic properties of Cl, the absorption of TPIC-4Cl has a significant redshift,and the maximumabsorption peak is at 804 nm. Moreover,whenblended with PM6, PM6:TPIC-4Cl-based filmmaintains the most comprehensive absorption range and the narrowest bandgap. Besides, the most suitable miscibility and the most favorable morphology resulted in balanced charge transport, favorable phase separation, and effective exciton dissociation and extraction, so that the device based on PM6:TPIC-4Cl obtained the highest PCE of 15.31%.

Fig. 6. Chemical structures of A–D–A asymmetric non-fullerene acceptors with nitrogen.

After introducing the N atom, Caoet al. combined selenophene andDTP building blockstopreparean asymmetric heptacyclic electron-donating coreDTPPSe[57].Compared to thiophene, selenophene hasloweraromaticity,improved planarity, increased conjugation length, and lowerEgopt. Meanwhile, selenide has a largersizeand higherpolarizabilitycompared to sulfur. Therefore,selenene can inducethetendency ofintermolecularSe–Se (chalcogen interaction)or seleniumaromaticinteraction in thecorresponding selenium-based NF-SMA. To systematically study the effect of fluorine-containing terminal groups on thephotoelectric propertiesofselenophene-containing A-NF-SMAs,Caoet al. synthesizedthree new typesof acceptors (DTPPSe-IC, DTPPSe-2F,andDTPPSe-4F) with IC unit, monofluorinated IC unit, or double fluorinated IC unit as showed in Fig. 6. With the increasing fluorine atoms, theEgoptof the three types of A-NF-SMAs gradually narrowed, LUMOs and HOMOs levels decreased, and the ultraviolet-visible absorption red shifted. These changes can promote the role of ICT and help to obtain better device performance. Simultaneously, great nano-fibrous and ordered phaseseparated morphology are also conducive to charge transport and exciton dissociation. Therefore, the optimized devices for PBDB-T:DTPPSe-2F, PBDB-T:DTPPSe-IC and PBDBT:DTPPSe-4F blends yield comparable PCEs of 13.76%, 9.88%and 12.03%. The PCE of the device based on PDBBT:DTPPSe-2F blend film is one of the highest values among A-NF-SMAs containing selenium in the literature.

Guoet al. and Luoet al. synthesized S-type seven-ring IDTP-4F and C-type eight-ring IDTTP-4F based on DTP and realized theadjustment of molecularconformation by changing thenumberof thiopheneson the right side (Fig. 6). IDTP-4F and IDTTP-4F bothhavegoodplanarbackbone conducive to intermolecular π–π packing[58,59].Both IDTP-4Fand IDTTP-4F exhibit redshift absorption fromsolution to film,whichindicates strongintermolecular interactionsin thesolid state. Compared with IDTP-4F,IDTTP-4F exhibitsslight redshiftabsorptionand amorepronouncedshoulder peak. Three highly efficientpolymer donors andtwo NF-SMAsIDTP-4FandIDTTP-4F are blendedone-to-one to fabricate devices. The device performance testresultsshow thatthe performance of the StypeIDTP-4F deviceis significantly better than that of the Ctype IDTTP-4F device. The PCE based on PM7:IDTP-4F device reached 15.2%, a remarkable achievement among all binary systems. Zhanget al. designed a series of IPT-based A-NFSMAs IN-4F, INO-4F, IPT-4F and IPCl-4F (Fig. 6) by adopting a corresponding side-chain of 2-ethylhexyl, 2-ethylhexyloxy, hydrogen or chloro onto thieno[2'',3'':5',6']-s-indaceno[2',1':4,5]dithieno[3,2-b:2’,3’-d]pyrrole (IPT) core. The charge density modulation of the IPT core was to systematically study its influence on the electronic structure, molecular stacking and photovoltaic performance of A-NF-SMAs[60]. According to the order of 2-ethylhexyloxy, 2-ethylhexyl, hydrogen, and chlorine, the electric traction ability of these four molecules gradually increases. TheEgoptgraduallyshrinks and theLUMO energy level is reduced, makingJscandVocachieve a good balance. When they were paired with polymer donor PM6, IPT-4F-based devices showed a high PCE of 14.62%. Compared with IPT-4F, PCEs of devices with INO-4F, IN-4F, or IPCl-4F as acceptors all decreaseto different degrees.

Caoet al. introduced two-dimensional (2D) conjugated side chains into IPT core. They developed a 2D conjugated fused ring core semiconductorIPT2F-TCl (Fig. 6) asan electronacceptorforhigh-efficiencyPSCs. Theextended conjugate length in the ring core can optimize device performance[21]. IPT2F-TCl has a lower HOMO energy level of–5.59 eV and a wider absorption range, whichleads to the asymmetricmolecule IPT2F-TClwith a high PCEvalue of 13.74%. The 2D conjugated fused ring core asymmetric acceptor is expected to enhance interaction between molecules through a larger dipole moment so that PSCs have a higher FF andPCE.

In addition to changing the number of thiophene and introducing nitrogen and selenium atoms, the asymmetric design of the molecules can also be achieved by flipping the configuration for A–D–A-basednon-fullereneacceptors. Luoet al.developeda novelNF-SMA ITCNTC(Fig. 7) with anasymmetric core by adjusting the configuration of thiophene[61].Compared with the symmetrical molecule ITCPTC, the absorption spectrum ofITCNTC isblue-shifted, andtheLUMO energy levelrises. J71:ITCNTC-baseddevices haveaPCEmaxvalue of 8.52%, which is much lower than the 11.63% PCEmaxvalue of J71:ITCPTC-based devices. The blue absorption shift and poor morphology of ITCNTC resulted in a sharp drop in bothJscand FF, which led toa rapid decrease inPCE,althoughVocincreased to some extent due to the rising LUMO level. This discovery confirmed that changing the direction of thiophene units to design asymmetric structures may not succeed sometimes.

Jiaoet al. designedand synthesized anA–D–A typed acceptor CC10 with asymmetric donor units by introducing alkylbenzene units into CC5 (Fig. 7)[62]. The ground state dipole momentsof CC5and CC10are2.48and 2.52Debye, the excited state dipolemoments ofCC5 and CC10 are 2.37and 6.80 Debye, the change in the dipole moment from the ground to the excited state for CC5 and CC10 is 0.11 and 7.97 Debye, respectively. The ground state dipole moment of the asymmetric molecule CC10is slightlylarger thanthat ofCC5, whichis conducive to enhancing the intermolecular interaction and intramolecular charge transfer characteristics. The stacking geometric configurations of CC5 dimer and CC10 dimer are showninFig. 8(b).CC5dimer only exhibits onestacking form, while asymmetric CC10 dimerhas three stacking forms.In addition, compared with the CC5 dimer, the CC10 dimer exhibits stronger intermolecular binding energy and stable geometric configuration, indicating thatCC10 exhibits stronger π–π stackingand higher electronmobilityin density functional theory (DFT). In contrast to symmetric molecule CC5, asymmetric molecule CC10 achieves better π–π stacking with a similarabsorptionrangeand energylevel. Therefore, the PCE of CC10-basedPSCs isas highas 11.78%,which ishigher than that of CC5-based devices (6.91%). Wei Hu et al. synthesized N65-IC and N65-2FIC based on the naphthalene dithiophene core (Fig. 7). N65-2FIC composed of fused eight rings[63]. ComparedwiththeS-NF-SMAsN66-ICand N66-2FICwith twosixring bridges, their asymmetric constitutional isomers N65-IC and N65-2FIC with one six-ring bridge and one five-ring bridge both possessed red-shift absorption, better π–π stacking, andhighercrystallinity. The LUMO energylevelsofN66-IC andN65ICare–3.85and–3.89eV,respectively. The LUMO energy levels of N66-2FIC and N65-2FIC are –4.00 and–4.03 eV, respectively. The significant reduction of LUMO energylevelsresults in a narrow bandgap. PBDB-T-2F:N65-2FICbaseddevices have aPCEmaxvalueof 10.19%, whichisthree times higher than the PCEmaxvalue of PBDB-T-2F:N66-2FICbased devices (3.46%). PBDB-T:N65-IC-based devices have a PCEmaxvalueof 9.03%,whichis higher than the 5.45%PCEmaxvalue of PBDB-T:N66-IC-baseddevices.

Among various core transformation engineering methods for designing asymmetric core units based on symmetrical analogs, the simplest method is to construct structural asymmetry by cutting offtheside chains of thesymmetriccore.Many studies have been conducted on acceptors (ITOTC,ITUTC, ITUIC, IEPC, IOPC, IETC, IOTC, PhITBD, MeITBD, ITDI and CDTDI) that match to donor PBDB-T, and acceptors (PhITBD,TIDT-BT-R2, TIDT-BT-R6, ITBR, ITBRCand ITBC) match todonor PTB7-Th (Fig.7). Zhanget al.synthesizedthreeA–D–A type A-NF-SMAs (ITOTC, ITUTC and ITUIC)[64]. There are only two side chains in the indenothiophene unit of ITUIC, limiting the solubility andcrystallinity of the corresponding NF-SMAs,whichlead topoormiscibility withPBDB-T.Accordingto the absorption onsets of the acceptors in thin films, optical band gaps of ITOTC, ITUTC and ITUIC are calculated to be 1.61, 1.60 and 1.64 eV, respectively. Under the condition that the polymer donor isPBDB-T, comparedwiththe symmetricmolecule ITIC, the asymmetric molecule ITUIC has less absorption complementarity with the donor, so the band gap between ITUIC and the donor is wider. Therefore, under the samedevicefabricatingconditions,ITUIC-baseddevices showworse deviceperformance than ITIC-baseddevices. The optimized devices for PBDB-T:ITUIC and PBDB-T:ITIC blends yield comparable PCEs of 6.45% and 8.66%[65]. Similar to ITUIC,theITUTC-based PSCs (7.68%)and the ITUTC-based PSCs(8.35%) allshowedreduced PCEsthan that of symmetric ITCPTC-based PSCs (11.92%)[95]. By shearing the side chain of the central core of the symmetrical IEIC molecule, Kanget al. synthesized four different A-NF-SMAs (IOTC, IETC, IOPC,and IEPC)to investigatetheeffect of differentterminal acceptor units on photovoltaic properties[66,67]. Compared with the IOPC and IEPC, IOTC and IETC both exhibit remarkable red-shifted absorption, smallerEgopt, higher hole and electron mobility,which isconsistent with thatthe enhanced electron-withdrawing ability from IC to 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile (TC). Hence, with PBDB-T as the donor, the best performance binary devices basedon IOTC and IETC exhibitedPCEs of 7.94% and7.40%,which are higherthanthat ofbinary devices basedonIOPC and IEPC (4.86% and 5.26%). Kimet al. designed a new type of A-NF-SMAs (PhITBD) with an indole thiophene nucleus[69].Thetwistangle betweenthe linearly connected thiophene and theendgroup of PhITBDis22°, which is shown inFig.8(c). It was found that the asymmetric molecule PhITBD has a twisted structure, while the symmetrical molecule IDT-2BM has a trapezoidal shape structure. As shown in Fig. 8(b), the calculated LUMO energy levels of IDT-2BM and PhITBD are –3.65 and –3.69 eV, and the calculated HOMO energy levels of IDT-2BMand PhITBD are –5.56 and–5.74eV,respectively. The asymmetric molecule PhITBD has a lower HOMO energy level and a larger calculated band gap energy than IDT-2BM. The asymmetricstructure ofPhITBD enhancesinternal morphology and the effective regulation of energy levels. Compared with symmetrical molecule IDT-2BM, the film formed by blendingPhITBD and donor PTB7-Thhashigher molarabsorption,more balanced charge transport and well-defined nanophase separation. Due to effective morphology control and absorptionenhancement, PTB7-Th:PhITBD-based deviceshave a PCEmaxvalue of 6.57%, which is higher than the PCEmaxvalue of PTB7-Th:IDT-2BM-based devices (3.97%). It reveals that the molecular cutting strategy has a specific practical meaning for the design of asymmetric cores. Jeonget al. synthesized the asymmetric molecule MeITBD by introducing a methyl substituent at the 7-position of indenothiophene into the molecule PhITBD[97]. The methyl substitution makes MeITBD have a more twistedstructure than PhITBD,which reduces the self-association capability of whole molecules, resulting in better miscibility between MeITBD and PBDB-T, thus forminga smoother surface morphology.PBDB-T:Me-ITBD-based devices have a PCEmaxvalue of 5.75%, which is higher than the 1.78% PCEmaxvalue of PBDB-T:Ph-ITBD-based devices.Thisisattributed to the side-groupengineeringby placing a methyl group on the bay side of PhITBD, forming an acceptable form that is conducive to the performance of PSCs.

Fig.7.Chemical structures of other asymmetric non-fullereneacceptorsbased onthe A–D–A structureand themolecules that are associated.

Fig.8.(a)Chemicalstructureof CC5and CC10.(b)Optimizedgeometries andthecorrespondingintermolecular bindingenergies by DFT calculations of CC5and CC10dimers.ReproducedwiththepermissionofRef. [62]. (c) The molecularstructures,(d)DFT calculated geometries(side view), and theHOMO/LUMO (top view) structures of IDT-2BM and PhITBD. Reproduced with thepermission of Ref. [69].

The molecules (TIDT-BT-R2and TIDT-BT-R6)with the same molecularcoreanddifferentterminal groupsto PhITBD were reported by Baiet al.[68]. TIDT-BT-R2 and TIDT-BT-R6,as compared to the symmetrical molecules (IDT-BT-R and IDT-BT-R-CN), both havea reducedfused-ringcentral core thiophene-indenothiophene (TIDT)[68,70,71]. Although the large side chains of the sp3hybridization of central core TIDT are reduced, TIDT-BT-R2 and TIDT-BT-R6 (Fig.7)canstillmaintain close molecular packing. On one hand, this TIDT core can obtain a higher molar extinction coefficient, more substantialthermal stability, and better-regulated energy levels than the fully fused-ring central core IDT. On the other hand, TIDTBT-R2has goodcrystallinityandhighcarrier mobilitydueto the short-chain length ofthesubstituentsof terminal groups.Therefore, PTB7-Th:TIDT-BT-R2-based devices have a PCEmaxvalue of 8.7%, which is higher than that of PTB7-Th:IDT-BTR2-baseddevices(8.3%). However, thelonger the chain length of the substituents, the weaker the effect of the asymmetric core in reducing molecular self-association. Therefore,themolecular self-association ability of TIDT-BT-R6 isstronger than that of TIDT-BT-R2. The optimized devices for PTB7-Th:TIDT-BT-R6 and PTB7-Th:IDT-BT-R-CN blends yield comparablePCEsof 5.6% and 5.7%. The above resultsindicate that reducing the fusion of core units is an effective strategy to achieve high-performance functions. The PCEs of other asymmetric core molecules (ITBC 4.26%, ITBRC 6.27%, ITBR 7.49%,ITDI8.0%and CDTDI2.75%) werereportedearlierby Tanget al. andKanget al.[29,72]. The bandgaps and energy levels of these A-NF-SMAs (ITBC, ITBRC,ITBR,ITDI andCDTDI) can be easily adjusted by changing terminal groups' electron-withdrawing ability. Besides, the highest PCE of these acceptorsbased PSCs can achieve up to 8%, indicating that small molecules with asymmetric indenothiophene as the core can becomea new type of non-fullerene acceptor for PSCs.

Fig. 9. The chemical structures of asymmetric non-fullerene acceptors arebased on asymmetric terminal groups and their molecules.

3.2.A-NF-SMAs with asymmetric terminal groups

Different terminal groups can induce the permanent dipole moment of A–D–A typed A-NF-SMAs molecules toregulate intermolecularinteractions, leading to a more diverse aggregation tendency ofthe moleculesaffectingthe π–π stacking, crystallization characteristics, and final photovoltaic performance[73]. Simultaneously, combining two-terminal groups with different polarities in a single molecule can fine-tune the absorption and energy level, making the asymmetric A1–D–A2 strategy for energy leveltuning possible[74,75].

ITIC is atypical NF-SMAwiththe advantages of easysynthesis, strongabsorptionin the visible lightregion,adjustable energy level and good stability. Adjusting terminal group substituents is a recent research hotspot in asymmetric terminal group strategy. Laiet al., Aldrichet al., Liet al.,and Gaoet al. used ITIC as the main body and changed the terminal substituents (chlorine, fluorine, hydrogen,and methoxy)to synthesize four different asymmetric moleculesα-ITIC-2Cl, ITIC-2F (its structure is the same asa-IT-2F), ITIC-3F (its structure is the same as IT-3F), and a-IT-2OM (Fig. 9)[76–79]. The position of the chlorine substitution of the asymmetric moleculeα-ITIC-2Cl was changed, and a completely different crystal structure of the symmetric molecule ITIC-2Cl-β(Fig. 9) was constructed.Comparedwith the linear stackingstructure of the symmetric molecule ITIC-2Cl-β, the three-dimensional interpenetrating network structureofthe asymmetric moleculeα-ITIC-2Cl changes the stacking method of molecules, enhances the interaction between molecules, and shortens the π–π stacking distance. The molecules ITIC-2Cl-βandα-ITIC-2Cl showed comparable electron mobilities of 1.1 × 10–4and 2.9× 10–4cm2/(V∙s) hole mobilities of2.4 × 10–4vs.4.2×10–4cm2/(V∙s),with hole/electron mobility ratiosof 1.4and 2.2, respectively. Higher electron mobilities, hole mobilities,and balanced mobility ratios indicate faster charge transfer in theα-ITIC-2Cl based devices[76]. Higher charge mobility can be realized in theα-ITIC-2Cl compared with ITIC-2Cl-β, leadingtoa higherJscin the PSCs. Finally, ahigh PCE of12.2% is obtained byα-ITIC-2Cl, whichis 10%higher than that ofITIC-2Cl-β. Asymmetric chlorine substitution is beneficial to increase charge mobility, thereby promoting device efficiency.Asymmetric chlorine substitution improves the stacking methodbetweenmolecules, which helps increase the charge mobility,thereby promoting the increaseofthe PCEs of PSCs. Both ITIC-2F andITIC-3F (Fig. 9)are A-NF-SMAs with one fluorinated terminal group and one non-fluorinated terminal group.Compared with molecules with symmetrical A–D–A structures, ITIC-2F and ITIC-3F increase molecular order and electron transport ininformative ways. With PBDB-TF (itsstructureisthesame as PM6)asthe donor, the performancebinary devices based on ITIC-2F and ITIC-3F exhibited PCEs of 10.38% and 11.44%, respectively[77]. a-IT-2F, which is synthesized by Liet al., has the same molecular structure as the ITIC-2F (Fig. 9) synthesized by Aldrichet al.[77,78]. The ITIC-3F has thesame molecularstructureas the IT-3F(Fig.9) synthesized by Gaoet al.[77,79].WhenPBDB-Tor PBDB-TF (itsstructure is the same as PM6) was used as the donor, the performance of binary devices based on a-IT-2F (its structure is the same as ITIC-2F) and IT-3F (its structure is the same as ITIC-3F) exhibited PCEsof 10.28%and13.83%, respectively[78,79]. a-IT-2OM has methoxy substitutionsdifferentfrom theFsubstitutions of a-IT-2F (Fig. 9). The methoxy group causes a stronger terminal dipole moment, effectively regulating the molecular binding energy, crystalline properties, and microscopic morphology of the blended films, promoting close molecular packingandeffective charge transfer. The improvedcharge transfercharacteristics support the improved device performance of a-IT-2OM (PCEmax= 12.07%, FF = 71.52%,Jsc=18.11 mA/cm2,Voc= 0.93 V), which are better than those of a-IT-2F (PCEmax= 10.28%, FF = 68.84 %,Jsc= 0.78 mA/cm2,Voc=0.78 V).

Yeet al.synthesized an A-NF-SMA, named IDTT-2F-Th[80].Laiet al.synthesizedthreechlorine-substituted asymmetric non-fullereneacceptors, namedITIC-Cl-δ-Th, ITIC-Cl-γ-Th, and ITIC-2Cl−Th (Fig. 9)[81]. One terminal group of this kind of molecule is the fluorinated or chlorinated IC group, and the other is the 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile (TIC) group[80,81]. Since IDTT-2F-Th has twodifferent terminal groups, the ultraviolet-visiblemaximum absorption wavelength(731 nm)of thefilm ofthe asymmetric molecule IDTT-2F-Th is red-shifted a bit compared with that of the symmetric molecule ITCPTC (725 nm). Simultaneously, the IDTT-Th HOMO energy level of 2F-Th is higher than that of ITCPTC, and the bandgap (1.69 eV) of IDTT-2F-Th energylevel issmaller thanthat of ITCPTC (1.74eV). Although the asymmetry structurecaused bytwodifferent terminal groups leads toaslightdecrease in theorderof IDTT-2F-Th molecular stacking, the carrier mobility of IDTT-2FTh:PBT1-C-2Cl mixture can still reach 53% the above. Ultimately, the PBT1-C-2Cl:IDTT-2F-Th device can obtain a PCE of 12.01%, higher than the 10.9% of the PBT1-C-2Cl:ITCPTC device. Amongthe threemoleculesofITIC-2Cl-Th,ITIC-Cl-γ-Th andITIC-Cl-δ-Th,ITIC-2Cl-Th has the largestnumber of chlorine atoms, the lowest LUMO, and the lowestVoc, which is the reason for hindering the improvement of PCE. Comparing to the two chlorines ITIC-2Cl-Th and single chlorine at the δ-position of ITIC-Cl-δ-Th, the single chlorine at theγ-position of theITIC-Cl-γ-Th terminalgroup canlead to the best moleculeflatnessand thelowest dimerenergy, whichisconduciveto thecharge transfer betweenmolecules. Simultaneously, the monochlorination ofγ-position helps to improve the mobility of electrons and holes. Simultaneously, the inverse structure device PBDB-TF:ITIC-Cl-γ-Th achieves the most effective exciton dissociation and the weakest bimolecular recombinationsothat theITIC-Cl-γ-Th-based bestdevice obtainsan excellent PCEof 12.25%.

Withdithienocy-clopentaindenoindene(ZIT) asthe core,Zhanget al., through one-pot Knoevenagel reaction, synthesized ZITI-m,amixtureofA-NF-SMAZITI-3FandS-NF-SMA ZITI-4F (Fig.9)[82]. Compared with ZITI-3F and ZITI-4F, the one-pot synthesis of mixed material ZITI-m is easier. In the film, ZITI-m shows a higher molar extinction coefficient and a more red-shifted ultraviolet-visible absorption than ZITI-3F and ZITI-4F, resulting in ZITI-m forming the narrowestEgopt(1.48 eV). Using J71 as the donor, the optimized devices of J71:ZITI-3F and J71:ZITI-4F produced 12.97% and 13.02% efficiencies. In contrast, J71:ZITI-m-based PSC shows a very high PCE of 13.65%, which shows that compositing asymmetric molecular with symmetric molecular has great potential in developing PSCs. Duan et al. combined two different terminal groups, rhodanine-flanked benzo[c][1,2,5]thiadiazole (BR)and 2-(5,6-difluoro-3-oxo-2),3-dihydro-1H-inden-1-ylidene)malononitrile (IM2F),connected to IDT core, constructing a new A-NF-SMA IDTBF (Fig. 9)[83]. The device's hole/electronmobility ratio (1.19) basedon polymer donorPM6and asymmetric molecule IDTBF is close to 1, ensuring effective charge transfer and collection. PM6:IDTBF devicesshow asatisfactory photocurrentresponsein the350 to750 nmrange,anddevices based on polymer donor PM6 and IDTBF show promising PCE of 10.43%. Zhaoet al. designed and synthesized two fullerene/non-fullerene hybrids as small molecule acceptors A1andA2(Fig. 9)for PSCs[84]. Thesetwo hybridsintroduce fused-ringIDT andIDTT asthecore, respectively, andconnect to the linear conjugate unit of C60, aiming to combine thephotoelectric properties of PCBM and ITIC.A1andA2 show UV–Vis absorptionin300–500 and500–800 nm, respectively. The absorption in the high-energy region can be attributedto fullerenes, while theabsorptioninthe low-energy regioncomes from intramolecular interactionbetween theinner core and IDM. Acceptor A2 combines the characteristics of PCBM and ITIC. This combination increased the PCE to 4.52% when the device'sVoc(J71 as donor) iscloseto1.0 V.

These great performances of PSCs based on molecules with asymmetric terminal groups reveal that it is meaningful toadjust themolecular polarityandstackmorphology through asymmetric modification of the terminal groups.

3.3.A-NF-SMAs with asymmetric side chains

Side-chain engineering is crucial for device performance,which canalter crystallinity, miscibility,and intermolecular interactions. A suitableside chain canendow the acceptor molecules with good solubility so that the blended film has an appropriatenanoscale phase separation. It canalso prevent small molecules from forming hydrogen aggregatesand increase the charge transport rate.

Alkyl and alkaryl groups aregenerally usedassidechains ofA-NF-SMAs.Comparedto acceptors with alkylaryl groups,alkyl-substituted acceptor molecules have a shorter π–π stacking distancethanalkylaryl groups. Fenget al. synthesized two A-NF-SMAs IDT-OBand IDTT-OB (Fig. 10) through sidechain engineering[85,86]. The introduction of asymmetric side chains in the asymmetric molecule IDT-OB increases the solubility of the acceptor molecule and forms stereoisomers, effectively reducing the crystallinity. Compared with symmetrical molecules IDT-2O and IDT-2B, IDT-OB (Fig. 10) obtains a closer packingbetween moleculesin a dislocation manner and forms a better phase separation when mixed with PBDBT. As expected, the IDT-OB-based PSCs (10.12%) showed higher PCEthanthose ofIDT-2O-basedPSCs (9.68%) andIDT-2Bbased PSCs (6.42%). When the central core of the IDT-OB molecule extended from the five-heterocyclic IDT core to the seven-heterocyclicindacenodithieno[3,2-b]thiophene(IDTT)core, while the side chain remains unchanged, a new asymmetric molecule IDTT-OB was synthesized. The extended conjugationlengthmakesIDTT-OBhave wider UV–Visabsorption and higher HOMO energy level than IDT-OB. Therefore, when mixed with PBDB-T, the blended film shows strong molecularpacking and highcrystallization behavior, withstrong plane orientation. The optimized devices for PBDB-T:IDTT-OB and PBDB-T:IDTT-OB blends yield comparable PCEs of 11.19%and10.12%.

Leeet al. synthesized A-D-A-type side-chain asymmetric small molecules p-IO1 and o-IO1 (Fig. 10)[87]. By introducing alkoxy groups toform asymmetry to manipulate photoelectron properties and intermolecular organization, o-IO1 and p-IO1 acceptors obtained better photovoltaic performance thanthe symmetrical molecules o-IO2andp-IO2. Simultaneously, the two A-NF-SMAs (o-IO1 and p-IO1) and donor PTB7-Th formed a suitable bandgap and favorable PSC photoactivelayer morphology. Hence, PTB7-Th:o-IO1-based PSCs(26.3 mA/cm2) and PTB7-Th:p-IO1-based PSCs (22.3 mA/cm2)all have highJscand low energy loss of about 0.54 eV. The PSCs based on PTB7-Th:o-IO1 and PTB7-Th:p-IO1 showed PCEs (13.1% and 10.8%) higher than those of PSCs based on PTB7-Th:o-IO2andPTB7-Th:p-IO2 (10.8% and 9.30%).

Fig.10.Chemicalstructureof asymmetric non-fullereneacceptorsbasedon asymmetric branchedchainsand the moleculesassociatedwith them.

Chenet al. designedan A-NF-SMA TOBDT (Fig.10) based onthebenzo[1,2-b:4,5-b']dithiophene (BDT) fused central core with asymmetricalalkoxy and thienyl side chains[88]. The molecular dipole momentgeneratedby the asymmetric side chain and the S–S interaction of the thienyl unit all leads to the enhancement of the intermolecular interaction. The alkoxy group's electron-donating property can help narrow the bandgap by improving the HOMO. Moreover, the other side chain can be used to precisely adjust the energy levels. As a result, TOBDT shows a low optical band gap of 1.41 eV and a suitable energy level, matching the donor PM6 well. The best binary PSCs based on PM6:TOBDT showed PCE of 11.3%(Jsc=18.7 mA/cm2,Voc=0.89 V, FF=0.68).

It can beseen that the introductionof asymmetric side chains can increasethesolubilityof acceptor molecules,make the acceptor moleculesdenselypacked ina dislocationmanner, andform goodphase separationand good device performance.

Table 1 covers the performance parameters of the acceptor–donor–acceptor (A–D–A) typed A-NF-PSCs.

4.Asymmetric non-fullerene acceptors based on A–D–A–D–A structures

A molecule with an A–D–A–D–A structure as the main body has two donor units and three acceptor units, which will cause this type of molecule to have a wider ultraviolet-visible absorption range than the A–D–A type structure.A–D–A–D–A type molecule has more donor units and acceptor units than the A–D–A type molecule, which can provide more frontier molecular orbitals to receive electrons from excited donors[89]. Moreover, as the most popular non-fullerene acceptor moleculeatpresent, Y6has abroad application prospect. Therefore,the following content (1)summarizes a series of adjustments anddesigns ofY6 by scientific research scholarsso far; (2)discussestheadvantages of asymmetric small molecules interms of charge transfer, molecularenergy levels, and active layer accumulation morphology; (3) records the device performance parameters of subsequent optimization-related devices.

4.1.A-NF-SMAs with asymmetric cores

Imitating the design idea of A–D–A molecules, Caiet al.designed andsynthesizedtwonew Y-series non-fullerene acceptorsY21and Y22 (Fig.11),with asymmetriccoresand used them inthestudyof PSCs[93,94]. Bothacceptors have good solubility, strong UV–Vis absorption, and good molecular stacking. Due to the nanofilament morphology of the active layer after mild thermal annealing, PM6:Y21/Y22-based devices' PCEmaxwith invertedstructureisup to 15.4%,with aJscover24 mA/cm2. TwonewA1–D–A2–D–A1 typed Y-series A-NF-SMAs (c-type BDTP-4F and s-type BTDTP-4F), and two new A–D–A typed A-NF-SMAs (c-type IDTTP-4F and s-type IDTP-4F), were designed and synthesized by Luoet al. (Fig.11)[59]. The PCE of BDTP-4F based PSCs with PM6 as the donor was15.24%, significantly higherthanthat of BTDTP-4F based devices (13.12%).FortheA–D–A structure, IDTP-4F with the s-shaped conformationisbetterthanIDTTP-4Fwith the c-shaped conformation. At the same time, the better-performing binary mixture has similar morphological characteristics. Zhanget al. designed and synthesized a new type of asymmetric molecule TB-4Cl (Fig. 11)[92]. Thismoleculeobtains a 1.98 Debye dipole moment by changingtheY6 (dipolemoment= 1.07 Debye)structureto TB-4Cl, furtherenhancing the intermolecular dipole-dipole interaction. At the same time, TB-4Cl has more dipole moments along with the molecular framework. With a more balanced positive and negative potential distribution on the terminal group, a positive potentialis conducive to charge transfer and J-aggregation. As shown inFig.12(a), TB-4Clcontains dihedralanglesof0.74°and 0.61°, so TB-4Cl has a planar structure that is conducive to π–π packing and electron mobility. Due to the asymmetric structure, TB-4Cl has two anti-parallel packing methods (TB-4Cl-1T1, TB-4Cl-2T2) (Figs. 12(c) and 12(d)), while Y6 has only one type.Under DFTcalculations, thestackingdistance of TB-4Cl-2T2 and TB-4Cl-1T1 are 11.03 and13.99Å, respectively,which is longer than that of the Y6 dimer. Compared with the Y6 dimer, TB-4Cl-2T2 and TB-4Cl-1T1 bind stronger, which is conducive to limiting the thermal movement of molecules and improving stability. The binding energy of TB-4Cl and thedonoris greaterthanthe binding energy of Y6 and the donor, so the binding of TB-4Cl andthedonor is tighter, and the active layer is more stable. Its superior thermal stability makes PM6:TB-4Cl-based devices more efficient withoutthermalannealing, reached aPCE of 14.67%.

Table1. Deviceparameters for A–D–A typedA-NF-SMA PSCs.

Continued

4.2.A-NF-SMAs with asymmetric terminal groups

Different terminal groups are applied to the A–D–A–D–A structure molecules, and Liuet al. developed three kinds of A-NF-SMAS by replacing the fluorine atoms on the terminal groupsof Y6with chlorineatoms (Fig. 11), namelySY1(two F atomsandone Cl atom), SY2(twoFatoms and two Cl atoms)and SY3 (threeClatoms). Meanwhile,Y6 with four fluorine atoms substituted terminal groupsandY6-4Clwith fourchlorine atoms substituted terminalgroups were synthesizedas control molecules[90]. Among all acceptors,SY1 hasthe lowest LUMO energy level and theweakestcrystallinity. Theresults showedthat PSCsbased onPM6:SY1blendedfilm yield the champion's PCE was 16.83%, withVoc of 0.871 V,Jscof 25.41 mA/cm2, and FF of 76.0%. Simultaneously, the PCE of devices based on PM6:Y6, PM6:SY2, PM6:SY3, and PM6:Y6-4Cl was 16.19%, 16.01%, 16.23%, and 16.06%, respectively.PM6:SY1-based devices have the best PCEs due to the lowest LUMO energy level of SY1. Besides, more balanced charge transfer,higher charge dissociationandchargecollectionefficiency,andbetter morphological characteristicsof PM6:SY1-based devices alsoplay anessential role in determiningthe highest device performance. This contribution indicates that the systematic structure of A-NF-SMAs with different terminal groups can further improve PSCs' PCE.

Fig. 11. Chemical structures of asymmetric non-fullerene acceptors based on A1–D–A2–D–A1 structure and the molecules that are associated with them.

Fig. 12. (a) Molecular conformation of TB-4Cl and Y6. (b) Chemical structures of TB-4Cl and Y6. Models of (c) TB-4Cl-2T2, (d) TB-4Cl-1T1,and (e) Y6-dimer in front view and side view. Reproduced with the permission of Ref. [92].

Luoet al. developed a new asymmetric small molecule with one terminal group of BTP-4F and one terminal group of BTP-2ThCl, namely BTP-2F-ThCl (Fig. 11)[91]. In terms of optics,electrochemistry, and crystallinity, BTP-2F-ThCl has similar properties to BTP-4F. BTP-2F-ThCl fine-tunes the acceptor energy level by forming asymmetric terminal groups, so BTP-2FThCl achieves a minimal HOMO energy offset and sufficient charge separation. Devices based on BTP-2F-ThCl obtained the best performance of devices based on these three molecules (PCE is 17.06%,Voc is 0.869 V,Jscis 25.38 mA/cm2,and FF is 77.4%), which is attributed to the fact that theVoc×Jscvalue (22.06) of devices based on BTP-2F-ThCl is greater than that of devices based on BTP-4F (21.18) and BTP-2ThCl(20.76). In the meantime, the PCEmaxvaluesof BTP-4Fbased devices and BTP-2ThCl-based devices are 16.37% and 14.49%, respectively. Liet al. introduced halogenated indandione (A1), 3-dicyanomethylene-1-indanone (A2) as two different terminal groups and central core of Y6 to design BTP-S1 and BTP-S2(Fig. 11)[32]. When the polymer donor PM6 was respectively blended with BTP-S1 and BTP-S2, the blendedfilms showed excellent photovoltaic performance. Energy loss analysis shows that device corresponding to the asymmetric moleculeBTP-S2 with six chlorineatomsat the endhas better electroluminescence quantum efficiency (2.3 × 10–2%)than the device corresponding to the symmetric molecule Y6(4.4 × 10−3%),whichresults inBTP-S2-baseddevicewith low non-radiationlossof 0.22eV.Besides, asymmetric BTP-S1 and BTP-S2 with multiple halogen atoms at the end can lead to more efficient charge separation. The optimized devices for PM6:Y6:BTP-S2, PM6:BTP-S2andPM6:Y6blends yield comparable PCEs of 17.43%,16.37% and 15.79%,this resultproves that the ternary hybrid strategy can improve the performance of PSCs more than the binary hybrid strategy. Ultimately,thePCEof PSCsbased on PM6:BTP-S2 was16.37%,which was higher than that of PSCs based on PM6:Y6(15.79%), which showed the effectiveness of the asymmetric design.

4.3.A-NF-SMAs with asymmetric side chains

The branchedalkyl chain has asignificant influenceon thesolubility of the molecule andmorphology of themixed film, which will make the final device have different properties. Chenet al. applied asymmetric alkyl and alkoxy substitution strategiesto the mostadvanced y-series non-fullerene acceptorsand obtainedatype ofA-NF-SMA namedY6-1O (Fig.11)[36]. If an alkoxy chain symmetrically modifies Y6, the resulting molecule will show low solubility and excessive aggregationdueto its conformational locking effect. Y6-1O chooses asymmetric alkyl and alkoxy substituents, which can balance the harmful effects of the alkoxy chain while maintaining the beneficial electronic effects ofthe alkoxy group. It can achieve quitea goodsolubility(20mg/mL)and morphology.Y6-1O can maintain the positive effect ofVoc improvement and obtain quite a good solubility, which helps Y6-1O-based devices toobtain highVoc(0.89 V),Jsc(23.2 mA/cm2)andFF(78.3%). Therefore, the best PCE of PM6:Y6-1O-baseddevices is as high as 16.1%. Further addition of PC71BM to the binary blend will result in a higher PCE of the device (17.6%).

The studyof A–D–A–D–Atypedasymmetricnonfullerene small molecules thoroughly explored the critical role of molecular conformation regulation on morphology and device efficiency, which has important guiding significance for the molecular designof NF-SMAs.

Table 2shows the performance parameters of A–D–A–D–A typed A-NF-PSCs.

5.Conclusion and future outlook

For A–D–A typed asymmetric non-fullerene smallmolecules:

a) In the asymmetry of the core, molecules with DTP units show great potential. The device fabricated based on ANF-SMA TPIC-4Cl with DTP unit obtained a PCEmax of 15.31%, which is the highest PCEmaxin the A–D–A-type A-NFPSCs so far. b) The "one-pot synthesis of mixed materials"strategy provides new ideas for molecular design, which can significantlyreduce the devicecomplexity of the traditional ternary strategy. ZITI-mis amixture of A-NF-SMA ZITI-3F and S-NF-SMA ZITI-4Fsynthesizedby the "one-pot Knoevenagel reaction".Compared with ZITI-3Fand ZITI-4F,the PSCof the hybrid materials ZITI-mand J71 shows a very highPCEmax(13.65%). c) Side-chain asymmetric molecules are expected to further improve device performance, and research in this area should receive more attention. So far, reports on sidechain asymmetry only include 6 molecules IDT-OB, IDTT-OB,P-IO1, O-IO1, TOBDT and Y6-1O. Relative to core asymmetry and terminal group asymmetry, the number of studies related to these molecules is very small. However, the PCEmaxvalues of these molecular-related devices are 10.12%, 11.19%,10.80%,13.10%, and 11.30%, whichare allgreater than 10%,showing great potential inthe fieldof PSCs.

For A–D–A–D–A typed asymmetric non-fullerene small molecules:

a) Substitutingthiophene for the benzene ring at the fluorinated IC terminal to form a new terminal T-IC, and using both the fluorinated IC terminal and the T-IC terminal group in the asymmetric A unit in A-NF-SMA has broad application prospects. The BTP-2F-ThCl synthesized by this move and the PSC fabricated after blending with the donor PM6 have reached the best PCEmax(17.06%) of the recent binary device.b) Fabrication of ternary PSCs also is a promising strategy furtherto improve thephotovoltaic performanceof binary PSCs.This articlerefers tothe PCEmaxof PSCs of the ternary blend(PM6:Y6:BTP-S2)was17.43%, which improved thebinary device'sperformance of PSCsbasedon PM6:BTP-S2 and PM6:Y6 (PCEmax= 16.37%, 15.79%).

In short, the asymmetric design of the molecule has the following advantages:

1) It helps increase the dipole moment and dielectric constant of the molecule and reduces the binding energy of excitons, which is very beneficial for exciton dissociation andchargetransport.2) Theasymmetricstructuredesignwillalso fine-tune the molecularenergy level to adjusttheVocfurther.The influence on the absorption range and absorption intensity will cause theJscto change. TheVoc×Jscvalue can be further optimized through the asymmetric molecular design, resulting in higher device performance. 3) The effect on molecular aggregation and molecular stacking can directly change the microscopicmorphology, phase separationsize, andthe activelayer's crystallinity.

However, thesynthesisof A-NF-SMAsis more complicatedand costly, which isabig problem thatscientists will face in the future. The box chart of PCE distribution for A-NFSMAs is shown in Fig. 13, which records the latest developments in A-NF-SMAs based on different structures. The structural modularity of small molecules facilitates molecular tailoring and property regulation. This advantage can enable the continuous development of A-NF-SMAs in interface engineering,shapecontrol anddevicestructure optimization research, which willfurther promotethe development of PSCs.

Acknowledgments

The authors acknowledgefinancialsupportfromthe National Key R&D Programof "StrategicAdvancedElectronic Materials" (No.2016YFB0401100), the National Natural Science Foundation of China (GrantNo.61574077),Major Program of Natural Science Foundation of the Higher Education Institutions of Jiangsu Province, China (No.19KJA460005) and Natural Science Foundation of Jiangsu Province (BK20170961).