Benzodithiophenedione(BDD)unit is one of the most attractive electron-withdrawing units in high efficiency polymer donors,mainly due to its planar symmetric fused ring structure and self-assembly characteristics.as a result,BDD-based polymers(such As PBDB-T,PM6,PM7,etc.)have achieved rapid development,and these polymer donor materials Have significantly improved the PCE of organic solar cells.Especially after the non-fullerene small molecule acceptor is reported,the PBDB series polymer donor material has the excellent properties of absorption spectrum and energy level structure matched with the small molecule acceptor,orderly molecular stacking,and obvious face-on orientation,and can form a suitable phase separation morphology with most non-fullerene acceptors.At present,the highest device efficiency of non-fullerene organic solar cells based on PBDB-TF(PM6)has exceeded 19%,making this donor material a star material,and there are endless explorations and designs for the modification of this series of materials.the ternary random copolymerization strategy has played a significant role in promoting the innovation of PBDB series materials.Fig.3 is a representative PBDB series random polymer donor material,and its corresponding photovoltaic performance parameters are shown in Table 1。

For the design of the third unit,the simple structural unit is relatively more concerned.the energy level of the polymer Can be effectively adjusted by introducing electron-withdrawing units with simple structures to replace part of the BDD units,and intramolecular non-covalent bonding(O…H,O…S,etc.)can also be Introduced,thereby improving the coplanarity of the polymer molecules and facilitating the improvement of mobility.Among them,simple structures such as electron-withdrawing units such as substituted thiophenes have become the research focus.In 2019,Hou Jianhui et al.introduced ester substituted monothiophene into the main chain of PM6 and synthesized terpolymer T1,which reduced the molecular energy level,and the efficiency of battery devices based on T1:Y6 exceeded 15%^[14]。 In the same year,Yan He et al.Introduced a fluorine atom into an ester-substituted thiophene as the third unit,and also embedded it into the PM6 skeleton to synthesize a polymer donor S1.After blending with Y6,the efficiency of the prepared device was 16.4%^[15]。 In 2021,Xu Xiaofeng et al.Introduced diester-substituted thiophene and diester-substituted thiophene as the third unit into the main chain of PM6,and synthesized two types of random polymers^[16]。 Due to the addition of the third unit,the structural disorder of the polymer increases,the absorption range broadens,and the HOMO energy level decreases.the device efficiency of PBDB-TF-T10:Y6 is 16.4%.In addition,the substitution of halogen for thiophene as the third unit is also a common simple structural design.In 2020,Guo Xugang et al.Replaced monothiophene with chlorine atom as the third unit,introduced it into the molecular skeleton of PBDB-TCl,and developed a terpolymer S3^[17]。 S3 was added to the PM6:Y6 system,and the efficiency of the optimized ternary device was 17.53%after adding 20%S3.In 2023,Yang et al.Introduced a diester-substituted thiophene(DDT)unit as the third unit into the PM6 skeleton,and developed two terpolymers L1 and L2 with thiophene with alkyl chain as the bridging unit^[18]。 Polymers L1 and L2 have deeper HOMO levels,higher molecular stiffness and planarity than PM6 due to their strong electron-withdrawing ability and S…O non-covalent intramolecular interactions.When mixed with the non-fullerene acceptor L8-BO,the device efficiency based on L1:L8-BO is as high as 18.75%.In addition,the substitution of functional groups on the bridge ring unit and the increase or decrease of the number of bridge ring units are also effective methods.For example,Zhang Maojie and Guoxia of Soochow University used fluorine atom substitution design to replace thiophene with single fluorine as the bridged ring unit,and constructed the third unit^[19]。 Because of the small atomic radius of the fluorine atom,there is no additional steric hindrance and the corresponding reduction of molecular coplanarity,and because of the strong electron-withdrawing ability of the fluorine atom,the HOMO energy level of the random polymer is effectively lowered.When blended with Y6,the corresponding blend films show excellent morphology,with significant nanoscale interpenetrating network structure and increased fiber width.As a result,the PF1:Y6-based organic solar cell device achieved an energy conversion efficiency as high as 17.3%.In 2023,Chen Yiwang and Liao Xunfan first designed and synthesized two new isomeric units TTO and TTI by combining ternary copolymerization and regioisomerization strategies,and obtained a series of D-A-D-A₁ternary polymer donor materials(PM6-TTO and PM6-TTI)through random copolymerization^[20]。 It is found that different Cl substituent positions can significantly change the molecular planarity and surface electrostatic potential.Compared with TTI,TTO unit has more and stronger S…O non-covalent interactions,corrected electrostatic potential(ESP)and less isomer structure.Therefore,compared with PM6-TTI-10,the ternary polymer PM6-TTO-10 shows better molecular coplanarity,stronger crystallinity,more obvious aggregation behavior,and proper phase separation in the blend film,which is conducive to more efficient exciton dissociation and charge transfer.Thus,PM6-TTO-10:BTP-eC9 based PSC achieved a high PCE of 18.37%.In addition,the benzene ring can also be used as the third unit.In 2023,Lee et al.Introduced different numbers of trifluoromethyl（CF₃）substituted benzene units into the main chain of PM6 polymer and synthesized a series of ternary polymer donors(PTF3,PTF5,PTF10,PTF20 and PTF50 )^[21]。 Due to the optimized energy levels,molecular aggregation/miscibility,and the bulk heterojunction morphology of the photosensitive material,all photovoltaic parameters can be significantly improved even with the addition of random polymers with little CF₃content,and thus,a PCE of 18.2%was achieved in the best binary OSC based on PTF5:Y6-BO 。

In the ternary random copolymerization strategy,nitrogen heterocycles are particularly common In the construction of the third unit due to their electron-withdrawing ability and strong crystallinity,and the typical structural units are benzothiazole and thiazoline.In 2019,Zhang Maojie et al.Reported a class of ternary random polymer donors.By adding 20%of weak electron-withdrawing thiazole(TTz)units to the PM6 polymer skeleton,the polymer PM1 was prepared,and the device efficiency of the organic solar cell based on PM1:Y6 was 17.6%^[22]。 Importantly,the PM1 polymer has excellent batch reproducibility,and the random polymer PM1 has a lower HOMO energy level,and the active layer formed by blending with Y6 has a better morphology structure.in 2020,Zhang Maojie et al.further reported that two kinds of random polymers PM6-Tz20 and PM6-Tz40 were prepared with 5,5'-dithiazole-2,2'-dithiazole(DTBTz)units as the third unit,In which 20%of DTBTz units were added to successfully achieve the fine regulation of molecular order and aggregation performance,thus Further optimizing the morphology of the active layer blended with Y6^[23]。 the results show that the PCE of the device based on PM6-Tz20:Y6 reaches 17.6%.Subsequently,Lu Shirong et al.Introduced ester-substituted thiazole(E-Tz)units into the PM6 backbone,and found that the incorporation of E-Tz units significantly improved the molecular planarity,energy level,and optical absorption range of the polymer donor^[24]。 the PCE based on OPz1:Y6 device reaches 16.28%.Li Yongfang et al.Introduced another nitrogen heterocyclic 2,5-bis(4-(2-ethylhexyl)thiophen-2-yl)pyrazine(PZ-T)unit into The PM6 skeleton to construct a series of random polymer donor materials PMZ-10,PMZ-20 and PMZ-30^[25]。 Compared with PM6,the random polymer has a lower HOMO energy level,so that the Voc of the prepared organic solar cell is significantly improved.the organic solar cell device based on PMZ-10:Y6 exhibits efficient exciton dissociation,higher and balanced electron/hole mobility,ideal aggregation,and a high PCE of 18.23%.In addition,Zhang Maojie et al.Synthesized a series of ternary polymers PMZ1,PMZ2 and PMZ3 by introducing carboxylate pyrazine(CPz)units into the main chain of PM6 using a ternary copolymerization strategy,which have excellent molecular flatness,and the highest device efficiency of PMZ2 blended with Y6 is 17.8%^[26]。

Designing a structural unit similar to the main chain as the third unit is also common In ternary random copolymerization strategies.the use of similar structures can avoid the risk of poor molecular coplanarity caused by the third unit to a certain extent,and can precisely control the properties of polymers by adding some substituents with specific properties.in 2021,Chen et al.Used a chlorine atom and silane chain substituted BDT unit(BDT-SiCl)as the third unit.BDT-SiCl and BDT-2F have the same skeleton.the similarity of the two structures not only greatly reduces the interference of random copolymerization on the ordered arrangement of molecules,but also enhances the face-on orientation of the active layer,which is conducive to charge transport^[27]。 Compared with PM6,the substitution of silane group and chlorine atom makes the terpolymer have lower HOMO energy level and wider spectral absorption range.the results show that the PSC based on ternary polymer PM6-SiCl-10%has the highest efficiency of 16.22%,and the energy loss is only 0.50 eV.Hou et al.Reported two random polymer donors,PBNB80 and PBNB50,by adjusting the molar ratio of two BDD derivatives,NTD and chlorinated BDD units^[28]。 Especially,the PBNB80:BO-4Cl blend can form a more excellent nanoscale phase separation morphology,resulting In an excellent efficiency of 16.0%,which is much higher than the 12.8%and 10.7%of the binary polymer donor device.in 2021,Yang et al.Designed a new iBDD-Si(TIPS replacement for BDD isomer)unit as the third component to embed the backbone of polymer donor PM6,and synthesized a series of iBDD-Si-based polymer donors.the iBDD-Si unit can fine-tune the optical properties,energy levels,and molecular packing,among which the device efficiency of PM6-5Si(iBDD-Si accounts for 5%):N3 reaches 17.01%^[29]。 In 2023,Hou Jianhui et al.introduced quinoxaline derivatives into the PM6 skeleton,and designed and synthesized three ternary polymers PQB-x.Through ternary copolymerization,the molecular orientation of the polymer chain was changed from face-on and edge-on coexistence to face-on stacking,and the energy disorder and energy loss were greatly reduced.the PCE of the device based on PQB-2:PY-IT was 18.1%^[30]。 This is also one of the highest device efficiencies of current all-polymer solar cells。

In addition to the above types,the design of other structural units as the third unit has also achieved some results In the preparation of modified devices based on PM6.In 2021,Kim et al.Designed two thiophene structures connected by the alkyl chain of C10 as the third unit to modify the PM6 polymer^[31]。 After blending with small molecule acceptor Y7,the device efficiency based on PM6-C10:Y7 exceeds 17%,and has higher J_SCand FF than PM6.In addition,structural units with strong electron-withdrawing ability can effectively control the energy level change of polymers,so they are often used as the third unit of random strategy to modify polymer donors.In 2022,Peng Qiang et al.Designed and synthesized a series of ternary random polymers by introducing a strong polar third unit TzBI into the main chain of high-performance polymer PM6^[32]。 the introduction of a small amount of TzBI units improves the disorder of the molecular skeleton and increases the interaction force between molecules.In addition,the strong electron-withdrawing ability of TzBI units reduces the molecular energy level of the polymer,which realizes the precise control of the nano-morphology of the film.the device efficiency based on PM6-TzBI-10:L8-BO reaches 18.36%.In 2023,they introduced the BNIDT skeleton structure containing B—N coordination bond into the PM6 main chain,designed a series of wide band gap ternary polymer donor materials PNB-1,PNB-3 and PNB-5 by using the anti-resonance effect of B—N bond,realized the reduction of device non-radiative recombination loss and the improvement of device performance,and optimized the active layer morphology.Finally,the device efficiency based on PNB-5:L8-BO reached 19.02%^[33]。 In addition,the organometallic complex building block is also a good choice as the third unit.In 2020,Min Jie et al.First doped heavy metal iridium complexes into the framework of high-performance polymer donor material PM6,and designed and synthesized a series of random polymers PM6-Irx(X=0.0.5,1,2.5 and 5).It is found that the molecular planarity,crystallinity,optical absorption,energy level of PM6 can be effectively controlled by doping with a small amount of iridium complex,and the miscibility with small molecular acceptor material Y6 can be improved^[34]。 Based on PM6-Ir1:Y6 device efficiency up to 17.24%。

equivalent D-A random copolymerization is introduced above,and non-equivalent random copolymerization has also played an important role In the development of organic photovoltaic polymer donor materials.However,compared with the equivalent D-A copolymerization strategy,the research on non-equivalent D-A copolymerization is less at present,and only the modification of PBDB series polymers is available at present.in 2021,Li Yongfang et al.First proposed the non-equivalent D-A copolymerization strategy and synthesized non-equivalent D-A copolymer donor materials^[35]。 A series of efficient D-A-D-D₁type non-equivalent D-a copolymers PM6-Dn were synthesized by inserting more D units into the long chain of the copolymers.It was found that the HOMO energy level decreased,the absorption spectrum blue-shifted and the optical band gap widened with the addition of D units.The test results show that compared with PM6,the non-equivalent D-A copolymers PM6-Dn(n=1,2,3)have higher carrier mobility,better molecular crystallinity and self-assembly ability,among which PM6-D1 is the best,and its device efficiency reaches 17.71%.The corresponding results show that the PM6-Dn:Y6-based device exhibits more efficient exciton dissociation,less bimolecular recombination,and faster charge transport than the PM6:Y6-based device.In 2022,Liu Jinbiao et al.Used the ester-doped BDT unit(BDT-2EST)as the third unit to introduce into the PM6 skeleton,and developed the D-A-D-D₁polymer donor^[36]。 By adjusting the content of the third component,DM1 containing 5%BDT-2 EST has the appropriate crystallinity and miscibility to match the Y6 receptor,and the device efficiency based on DM1:Y6 can be as high as 17.21%.In 2022,our group designed and synthesized a vinyl-functionalized BDT-based monomer(BDTOA-2Br)and simple BDT-Br as the third unit,which were introduced into the PM6 skeleton to prepare two kinds of polymers,PM6-Ax and PM6-Tx^[37][38]。 it was found that the efficiency of PM6-A5:Y6 and PM6-T15:Y6 based devices was higher than that of PM6:Y6 based reference devices.in 2023,our group again synthesized a simple electron-donating unit 2,6-dibromo-4,8-bis(octyloxy)benzo[1,2-b:4,5-b']dithiophene(BDT-O)as the third unit,which was added to the D-A backbone of PM6 to prepare PM6-Ox by non-equivalent D-A random copolymerization strategy.the effect of BDT-O units on the optoelectronic properties of the polymer was studied,and It was found that the addition of a small amount of BDT-O units to PM6 could effectively adjust the absorption,energy level,and molecular alignment of the polymer,resulting In better sunlight harvesting and enhanced charge transport,with a PCE of up to 17.65%based on the PM6-O5:Y6 device^[39]。 in addition,the PM6-O5:Y6-based device has high thermal stability,storage stability,and excellent photostability.the above studies show that non-equivalent random copolymerization has also made a great contribution to the development of non-fullerene solar cells.Fig.4 shows a representative PBDB series of non-equivalent random polymer donor materials,and the corresponding photovoltaic performance parameters are shown In Table 2。

In addition to the BDD unit,the design of other novel receptor units has evolved In recent years.In 2020,Ding Liming et al.Developed a symmetrical acceptor unit DTBT and prepared D-A copolymer donor D18^[40]。 D18 has a high hole mobility,and the energy conversion efficiency of the device obtained by blending D18 with Y6 is as high as 18.22%.Subsequently,Wang Jinliang and Anqiaoshi added chlorinated BDT units as the third unit to the D18 skeleton,and synthesized terpolymers D18-20%Cl and D18-40%Cl by adjusting the ratio of fluorinated BDT to chlorinated BDT units^[41]。 the device based on D18-20%Cl has a deeper energy level and a higher open circuit voltage,and the efficiency of the device prepared by blending with Y6 is as high as 18.28%.In 2022,Lu Shirong et al.Introduced ester-substituted thiazole(E-Tz)units into the backbone of polymer D18,and synthesized a series of terpolymer donors OPz11,OPz12,OPz13 and OPz14^[42]。 It is found that the addition of E-Tz units can finely tune molecular aggregation and phase separation,promote exciton dissociation and charge transport,and 5%E-Tz unit content makes the efficiency of the device blended with Y6 reach 18.42%.Bo Zhishan et al.synthesized three ternary polymer donor materials(PL1,PL2 and PL3)by random copolymerization strategy by adjusting the ratio of BDD unit to DTBT unit using the monomers used for the synthesis of two high-performance polymers PM6 and D18,and the highest device efficiency based on PL1:BTP-eC9-4F was 18.14%^[43]。 In 2023,they further developed two new receptor units with C_2Vsymmetry,B2TCl-o and B2TCl-i units,and introduced the B2TCl-o unit into the D18 polymer backbone to prepare ternary polymer PW2,with the highest device efficiency of 17.19%based on PW2:BTP-eC9-4F^[44]。 the structural formula of a representative random polymer donor material is shown in fig.5,and The relevant photovoltaic performance parameters are summarized in Table 3。

Polythiophenes(PTs)are considered as candidates for large-scale industrial preparation of OSCs due to their simple chemical structure,easy synthesis,and cheap raw material source.in recent years,some studies have shown that the random copolymerization strategy is a very effective way to optimize the donor properties of polythiophene polymers.Fig.5 contains the structural formulas of representative polythiophene series random polymer donor materials,and the relevant photovoltaic performance parameters are summarized In Table 3.In 2021,Geng Yanhou et al.introduced thiophene units with siloxane chains into the modified PDCBT structure through random copolymerization strategy,and finely adjusted the miscibility of polythiophene and non-fullerene blends by changing the ratio of siloxane terminal chains and alkyl chains in ester-substituted polythiophene^[45]。 A series of ester-substituted polythiophene derivatives(PDCBT-Cl-Six)were synthesized and blended with the acceptor material ITIC-Th1,and the highest PCE of 12.85%was achieved.In addition,Duan Chunhui et al.introduced different amounts of fluorine atoms into polythiophene polymer donors containing cyano substitution,and the main chain fluorination led to strong interchain interaction,thus improving the crystallinity of the polymer,changing the problem of excessive phase separation of the active layer after blending of polythiophene donors and Y6 series acceptors,and optimizing the morphology structure^[46]。 Finally,the PCE of the binary device based on P5TCN-F25 and Y6 was up to 16.6%,and the PCE of the three components based on P5TCN-F25:Y6:PC₇₁BM was up to 17.2%,which was a major breakthrough in OSCs based on PTs 。

in 2013,Chen et al.Synthesized a narrow band gap polymer donor PTB7-Th(PCE10)by introducing 2-ethylhexyl-thiophene into the side chain of the BDT unit In PTB7^[47]。 The results show that the absorption of PTB7-Th has a significant red shift of 25 nm compared with that of PTB7,and the maximum absorption wavelength is 695 nm,and the PCE of OSCs based on PTB7-Th:PC₇₁BM is further improved to 9.35%.Chen et al.introduced BDT-2F and BDT-2Cl units into the polymer donor PCE10 by random copolymerization,and obtained two series of ternary random polymers PCE10-BDT2F-X and PCE10-BDT2Cl-X.Through this simple ternary polymer design strategy,the matching between Y6 and the narrow band gap polymer donor PCE10 was improved^[48]。 Among them,the device based on PCE10-BDT2F-0.8:Y6 achieves an energy conversion efficiency of 13.80%,which is nearly 40%higher than that of the device based on PCE10:Y6.in addition to BDT units,which are often used to design polymer donor materials,donor materials based on fluoro-substituted benzothiadiazole(ffBT)units have also been a hot research topic In recent years.the introduction of strong electron-withdrawing F atoms into benzothiadiazole(BT)units enhances the planarity and hole mobility of their structures.Yan He et al.Reported a polymer donor PffBT4T-2OD(PCE11b)^[49]。 The molecule has a broad absorption spectrum and high crystallinity,and when matched with the PC₇₁BM,the best PCE of the final device reaches 10.8%.In 2021,Zou Yingping et al.introduced siloxane chains into the polymer PffBT4T-2 OD by random copolymerization to prepare the ternary polymer Si25,which controlled the aggregation strength of the polymer and realized the matching with the fused ring non-fullerene acceptor.In the organic solar cell prepared by blending with the acceptor material Y14,when the thickness of the active layer was 430 nm,the highest device efficiency was 15.39%^[50]。 the structural formula of a representative random polymer donor material is shown in fig.5-3,and The relevant photovoltaic performance parameters are summarized in Table 3。

In 2016,Li Yongfang et al.First reported that polymer donor materials J50 and J51 synthesized by copolymerization of BDT unit and benzotriazole(BTz)unit were blended with star non-fullerene small molecule acceptor ITIC.Thanks to the complementary absorption and matching energy level between the donor and acceptor,the efficiency of the prepared device exceeded 9.0%,which aroused the development wave of J series polymer donor materials^[51]。 the structural formula of the representative J series random polymer donor materials is shown in Figure 5-4,and the relevant photovoltaic performance parameters are summarized in Table 3.Yang Renqiang et al.Used benzodithiophene(BDT)unit and thiophene[2,3-f]benzofuran(TBF)unit as two D units to construct a series of ternary polymer donors by ternary copolymerization strategy.the addition of TBF units with high crystallinity to the polymer with BDT units with low crystallinity can not only improve the order of the microstructure,maintain good face-on orientation,promote the formation of phase separation characteristics,but also produce high exciton dissociation and inhibit charge recombination^[52]。 Finally,the PCEs of TBFCl50-FTAZ:ITIC,TBFCl50-BDD:ITIC,and TBFCl50-BDD:IDIC-C4ph based devices were 11.9%,12.4%,and 12.2%,respectively,by the above ternary copolymerization strategy.In 2019,they further designed and synthesized two polymers,fluorine-containing terpolymer(PBTA-PS-F)and fluorine-free polymer(PBTA-PS)^[53]。 the fluorine-containing terpolymer PBTA-PS-F has a deeper HOMO energy level compared with PBTA-PS,and PBTA-PS-F exhibits ordered molecular packing and excellent face-on orientation.the PCE of the device based on PBTA-PS-F:ITIC is 13.48%,which is higher than that of the device based on polymer PBTA-PS(PCE=11.72%).in addition,In 2018,Zhang Maojie et al.Reported that alkylthio thienyl substituted benzodithiophene(BDTT-S)was used as the D unit,and 5,6-difluoro substituted benzotriazole(FBTz)and thiazolothiazole(TTz)were used as the A unit.A series of wide band gap ternary random copolymer donor materials were designed and synthesized,and the optical,electrochemical,molecular arrangement and charge transport properties of the polymers were effectively optimized by controlling the molar ratio of the two A units^[54]。 Among them,the device PCE based on PSBTZ-60:ITIC processed with non-halogenated solvent was 10.3%。

NDI is considered to be a promising acceptor unit in imide-based polymer acceptors due to its high electron affinity rate and high electron mobility^[55,56]。 Fig.6 shows the corresponding chemical structures of representative random polymer acceptor materials based on NDI,and the corresponding photovoltaic performance parameters are shown in Table 4。

thiophene and its derivatives are the most widely used donor units In D-A copolymer acceptors.in 2016,Chen Zhikuan et al.Reported a new random D-A copolymer acceptor PNDI-TT-TVT,which consists of a naphthalene diimine acceptor unit and two donor units,thiophene[2-b]-thiophene(TT)and vinyl-vinyl(TVT)^[57]。 this material structure makes the polymer main chain have better coplanarity,reduces the stacking distance between polymer chains,and improves the charge carrier mobility of the blend film.Subsequently,the PCE of the device prepared by blending PNDI-TT-TVT as the polymer acceptor and PTB7-Th as the polymer donor was 4.86%.in addition,In order to systematically understand the structure-property relationship of This random polymer,a series of random polymer PNDI-TT-TVTs were developed by adjusting the ratio of TT/TVT units^[58]。 The data show that the absorption spectra of the random polymer in the film and solution state are red-shifted with the increase of the unit amount of TVT.Devices based on random polymer acceptors exhibit higher and more balanced charge mobility than those using the corresponding alternating D-A copolymer acceptors,resulting in significantly improved J_SCand device performance.The device fabricated with Copolymer4:PTB7-Th as the active layer material has the highest PCE of 5.27%.In 2021,Chen Yiwang and Chen Lie introduced 3,3'-difluoro-2,2'-bithiophene(2FT)units into NDI-based polymer N2200 by random copolymerization,and synthesized a series of ternary polymer receptors PNDI-2FT-0.1,PNDI-2FT-0.2 and PNDI-2 FT-0.3^[59]。 The absorption coefficient of the terpolymer can be significantly improved by adding N2200 backbone to the 2FT unit,and the random copolymerization of 2FT in the backbone can not only reduce the strong aggregation of the polymer,but also has better miscibility with the donor.the device efficiency based on PBDB-T:PNDI-2FT-0.1 reaches 9.46%。

in addition to thiophene and its derivatives,other heterocycles,such as furan and selenophene units,have also been widely used as D units for the construction of NDI-based random polymer acceptors.In 2017,our group developed a series of random polymer acceptors based on NDI,x-NDI,which can optimize the crystallinity,morphology and charge transport characteristics of the blend film by adjusting the ratio of T-NDI/Se-NDI In the polymer backbone^[60]。 Among them,the device based on PBDT(T)TPD:0.5-NDI has a higher and more balanced charge mobility with a PCE value of 5.4%.In 2019,Jenekhe et al.Reported the synthesis of a series of new n-type semiconductor random polymers BSSx(X=10,20,50),and found that with the increase of monoselenophene content,the LUMO energy level of the acceptor increased,and the crystallinity decreased.all the All-polymer solar cells based on BSSx:PBDB-T produced efficient photovoltaic performance^[61]。 the device based on BSS10 has The highest PCE of 10.1%and internal quantum efficiency of nearly 100%.Chen et al.Designed and synthesized a group of NDI-based polymer acceptors PNDI-2T-TR(5)and PNDI-2T-TR(10)based on rhodanine dyes^[62]。 with the addition of the rhodanine-based dye fragment,the absorption coefficient of both random polymer acceptors increases and the LUMO energy level increases compared to the control polymer acceptor N2200.Therefore,the highest efficiency of the all-polymer solar cell based on PBDB-T:PNDI-2T-TR(5)is 8.13%without any additive treatment.In order to further improve the photovoltaic performance of NDI-based polymer acceptors doped With rhodannine-based dyes,Chen et al.Combined the advantages of fluorinated dithiophene and rhodanine dye units to obtain two random polymer acceptors,PNDI-2FT-TR10 and PNDI-2FT-TR20^[63]。 with the synergistic effect of fluorine atoms and rhodanine dye molecules,the absorption coefficient of the all-polymer blend film is significantly improved,and the morphology and structure of the blend are optimized.the device based on PM6:PNDI-2FT-TR10 has a PCE of 10.71%.in addition,the introduction of NDI units With different side chain substitutions is also an effective way to construct random polymer acceptors.For example,In 2018,Cao Yong et al.Introduced a certain number of linear oligomeric ethylene oxide(OE)side chains to replace the branched alkyl chains on the NDI unit,and synthesized a series of random polymer acceptors NOEx based on NDI^[64]。 Compared with the reference polymer acceptor NOE0,NOE10 has better miscibility with the polymer donor PBDT-TAZ,and the device has a PCE of up to 8.1%and a FF of up to 0.75,with good long-term stability and thermal stability.These results demonstrate the effectiveness of the random copolymerization strategy in fine-tuning the morphology and nanostructure and obtaining efficient imide-based holopolymer materials。

Because BTI derivatives Have tunable carrier polarity,reduced steric hindrance,extendedπ-electron delocalization,and ideal backbone coplanarity and interchainπ-πstacking,and compared with NDI and PDI,the thiophene ring in the BTI molecule has higher chemical activity and greatly reduced steric hindrance,and the BTI derivative is considered to be a promising acceptor unit in the construction unit of the high-mobility D-A copolymer acceptor material.Guo Xugang et al.have done a lot of excellent research on the development of BTI-based polymer receptors.in order to improve the light absorption and n-type properties,a series of random polymer acceptors based on BTI were also obtained by random copolymerization.Fig.7 shows the corresponding chemical structures of representative BTI-based random polymer acceptor materials,and the corresponding photovoltaic performance parameters are shown in Table 4。

In 2019,Guo et al.Added different proportions of thienopyrrolidone(TPD)receptor units to the polymer(f-BTI2-FT)to construct a series of random ternary polymer receptors,namely BTI2-xTPD^[65]。 Due to the introduction of TPD units,there are intramolecular non-covalent S…O interactions in the main chain of the random polymer,which is beneficial to improve the planarity and charge transport ability of the main chain.When a certain proportion of TPD units was incorporated,the random polymer acceptor BTI2-30TPD had better molecular orientation and better compatibility with the polymer donor compared with the binary polymer.Therefore,the device based on PTB7-Th:BTI2-30TPD has a higher and balanced electron/hole mobility,a device efficiency of up to 8.28%,and good stability.Due to the good efficiency of the above ternary polymer acceptor,they continued to incorporate a certain amount of o-phenylenediamine(PhI)and benzothiadiazole(BT)units into the f-BTI2-FT polymer acceptor,and further designed and synthesized two new random trimers based on BTI2(BTI2-30BT and BTI2-30PhI)^[66]。 the three random polymer acceptors BTI2-30BT,BTI2-30PhI,and BTI2-30TPD showed different electrochemical properties and crystallization ability due to the different electron-withdrawing ability and solubility of BT,PhI,and TPD units.the device based on BTI2-30TPD shows the best photovoltaic performance because of its good face-on orientation and the synergistic effect of suitable crystals,and also shows that the selection of suitable electron-withdrawing units is very important In the development of efficient random terpolymer acceptors.in 2020,Guo Xugang et al.Reported a series of random polymer acceptors composed of BTI2-based units substituted by different side chains and fluorinated thiophene units,namely PBTI2(0 HD)-FT,PBTI2(30 HD)-FT,PBTI2(50 HD)-FT and PBTI2(70 HD)-FT^[67]。 the morphology of the polymer film is optimized by adjusting the length of the side chain.Among them,the device based on PBTI2(50 HD)-FT:PTB7-Th achieved a high PCE of 8.23%without additives,and had good room temperature stability,retaining 90%of the initial efficiency after 400 H of storage。

In 2017,Zhang Zhiguo and Li Yongfang first discovered that"PSMA"polymer receptor has the advantages of low band gap and strong absorbance,and proposed the strategy of building a new generation of polymer receptor with"PSMA"^[68]。 These types of polymer acceptors not only retain the advantages of small molecular acceptors,such as narrow band gap,strong absorbance and suitable aggregation,but also have the characteristics of good film forming performance,long electron transport channel,adjustable photophysical properties,good thermal stability and photostability,which promote the further development of All-PSCs,and the highest energy conversion efficiency is more than 18%.in recent years,the improvement of photovoltaic performance of"PSMA"polymer receptor has also been achieved by random copolymerization strategy.Fig.8 shows the corresponding chemical structure of the"PSMA"random polymer acceptor material,and the corresponding photovoltaic performance parameters are shown In Table 5。

In 2020,Li Yongfang ET al.Designed and synthesized a series of"PSMA"random polymer acceptors by adding a certain amount of 3-ester thiophene(ET)units as the third unit in the main chain of polymer PTPBT PTPBT-ET_x（where X is the molar ratio of ET units )^[69]。 The ternary polymer PTPBT-ET_xhas better solubility,higher LUMO energy level,higher and balanced charge mobility and more excellent morphological structure when blended with the corresponding donor.Finally,the all-polymer solar cell based on PBDB-T:PTPBT-ET_0.3achieves an energy conversion efficiency of 12.52%,and has good photostability,which can reach more than 300 H without significant decline.At the same time,Min Jie et al.Also reported a similar study,which used different side alkyl chain substitutions in the TPBT unit^[70]。 the device based on PYE20:PBDB-T has the highest PCE of 13.6%.And,they found that all the random polymer acceptor-based all-polymer cells had good efficiencies of more than 13%,indicating the high tolerance of the original polymer to the addition of et units.In addition,Yang ET al.Used random polymerization to introduce a simple thiophene with an oligomeric ethylene oxide side chain(TOE)as the third unit into the D-A copolymer PYT to obtain a series of novel random polymers PYT-TOE(X)(X represents the molar ratio of TOE units)^[71]。 The electron-rich unit TOE directly replaces the electron-deficient unit Y5,which realizes the improvement of LUMO energy level.More importantly,an appropriate amount of TOE units can not only adjust the solubility and molecular arrangement of polymer acceptors,but also improve the compatibility between polymer donors and acceptors.Optimized crystallinity andπ-πstacking of the blend films were thus obtained,favoring charge transfer and better film morphology.Finally,with the simultaneous improvement of J_sc,V_ocand FF,the device energy conversion efficiency increases from 11.75%for PBDBT:PYT to 12.77%for PBDB-T:PYT-TOE(10).In 2021,Wang Ergang et al.Copolymerized thiophene linked by thioalkyl chain as the third unit with thiophene and Y5 unit to obtain a series of ternary polymers PYTS-x^[72]。 The addition of FCBS(thiophene linked by non-conjugated thioalkyl chain)can significantly improve the solubility and molecular flexibility of the polymer acceptor,resulting In a good temperature-dependent aggregation of the polymer,resulting in a better crystallinity and charge mobility of the acceptor in the film,and the device efficiency based on PYTS-0.3:PBDB-T is as high as 14.68%.in 2022,Chen Yiwang et al.Designed and synthesized a series of ternary polymers PYSe-TClx(X represents the molar ratio of TCl units)by copolymerizing selenophene as the bridging unit and 3-chlorothiophene(TCl)as the third unit with Y5 unit.Due to the ternary copolymerization,the aggregation degree of the polymer PYSe-TClx is significantly reduced,and the energy level is shifted up.the device efficiency of All-PSC based on PYSe-TCl20 is as high as 14.21%^[73]。

the method of introducing another electron-withdrawing unit as the third unit was also successfully used to construct random"PSMA"polymer acceptors.Recently,inspired by the successful research on modifying the branched side chain of Y6 receptor,Sun Yanming et al.Developed a series of random polymer receptors by introducing Y6 derivatives with branched side chain substitution as the third unit^[74]。 and it was found that controlling the molecular ratio of the SMA-DT unit can effectively lead to tunable optical and electrical properties of the resulting random polymer PSMA receptor.Therefore,the device based on PY-82:PM6 exhibits excellent photovoltaic performance with a PCE of 17.15%,which belongs to the higher value among binary all-polymer solar cells.In addition,the PY-82-based device also has extremely low non-radiative energy loss(0.185 V)and excellent photostability.At the same time,Li Yongfang et al.Used this strategy to synthesize PSMA polymer acceptor PA1-by asymmetric copolymerization of two SMA building units and thiophene linker units^[75]。 the device efficiency of the PM6:PA1-based solar cell is up to 16.0%.in 2022,Chen et al.introduced flexible non-conjugated units into the main chain of polymer PYSe to synthesize PYTCl-A,PYTCl-B,PYTH-A and PYTH-B polymer acceptors.the study showed that the introduction of non-conjugated units into the main chain through ternary random copolymerization could effectively inhibit the excessive aggregation behavior of polymer acceptors In the active layer during printing,and the device efficiency based on PM6:PYTCl-a was 16.16%^[76]。 In 2023,Wei et al.Proposed a new type of terpolymer receptor,which was synthesized by random copolymerization of thiophene units and dithieno-thieno-pyrrolo-benzothiadiazole(BTP)core with alkoxy side chain(BTPO-IC)and alkyl side chain(BTP-IC).It was found that the open-circuit voltage of the device based on this terpolymer increased gradually with the increase of the molar ratio of BTPO-IC units^[77]。 the device efficiency based on PM6:PY-0.2OBO is as high as 16.7%.The results show that random copolymerization is a simple and efficient method for The synthesis of high performance"PSMA"polymer acceptors。