Yongqiang Shi and Liming Ding

1College of Chemistry and Materials Science, AnhuiNormal University, Wuhu 241002, China

2Center for Excellencein Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for

Nanoscience and Technology,Beijing100190, China

Polymersemiconductors havearoused interests from both academicandindustry dueto their wide applications in electronic devices, such as organic thin-film transistors (OTFTs)[1], polymer solar cells(PSCs)[2−6], organic thermoelectrics(OTEs)[7−11],andperovskite solarcells(PVSCs)[12−14]. To date,great efforts have beendevotedto developing p-typepolymer semiconductors,whilethedevelopment ofn-type polymers lags farbehind.Infact, n-type polymers areessential for organic electronic devices.

Currently, lots of n-type polymers are donor–acceptor(D–A) copolymers[15], however, the electron-rich donor units can lift both the lowest unoccupied molecular orbital (LUMO)and highest occupied molecular orbital (HOMO), yielding ptype or ambipolar charge-transport characteristics. To inhibit hole injection and achieve unipolar electron-transport, acceptor–acceptor (A–A) combination could be a good strategy. For instance, Luscombeet al.[16]reported a NDI-based A–A homopolymerviaYamamoto coupling, however the polymer showed a lowμeof 6 × 10–4cm2/(V·s) due to high steric hindrance of NDI unit. To reducesteric hindrance of NDI,thiophene-fusedNDIderivative,naphtho[2,3-b:6,7-bʹ]-dithiophenediimide(NDTI), was designedby Takimiyaet al. Subsequently, variousn-type A–Apolymer semiconductors(PNDTI-BBT-DPandPNB-TzDP) from NDTI weredeveloped withhigh electronmobility and electrical conductivity, and were applied in OTFTs and OTEs (Fig.1)[17−19]. Reichmaniset al. developed a thiazole-based A–A type polymer PDPP5DH-4Tz[20]by replacing the flanked thiophene units of DPP with thiazoles, and it presented unipolar charge-transport properties withμeof 0.067 cm2/(V·s).

Among various electron-deficient units, bithiophene imide (BTI) has been proved to be an eminent unit for building n-type polymer semiconductors. Guoet al.developed a serious of highly electron-deficient semi-ladder BTI derivatives(BTIn) with up to 5 imide groups and 15 rings in a row, offering a remarkable platform for developing n-type polymer semiconductors[21]. Subsequently, they also synthesized bithiazole imide (BTzI)[22]andthiazolothienyl imide dimer(DTzTI)[23]electron-deficientunits byreplacing thiophene withthiazole to furtherpushdown the frontiermolecularorbitals(FMOs) energy levels, asa result, both A–A typepolymersPBTzIand PDTzTI(Fig. 1) showed unipolar n-typecharacter in OTFTs.PDTzTI exhibited aremarkably highμeof 1.61 cm2/(V·s). This polymer structure also favors to overcome Coulombinteractionin the doped state forOTEs. Thedoped PDTzTI presented good charge-generation, giving a remarkableelectrical conductivity(σ) of 4.6 S/cm andapower factor(PF) of 7.6μW/(m·K2), much higher than those of NDI polymers[24].Owing toits highelectronmobility andwellmatched energy levels, PDTzTI as electron-transport layer(ETL) ininvertedPVSCs yieldeda PCE of 20.8%[25]. Driven by the success of A–A type homopolymer PDTzTI, Guoet al.also developed aserious of A–Atype homopolymers PBTIn (n=1–5) (Fig. 1)[26]. Homopolymer PBTI (Mn12.7 kDa) hadμeof 1.53cm2/(V·s). By usingoff-centerspin-coating, theμefurther increased to 3.71 cm2/(V·s).

Fig. 1. Chemicalstructures of the representativen-type polymers with acceptor–acceptor backbone.

Shiet al.[27]prepared distannylatedbithiopheneimide(BTI-Tin), and polymerized it with other electron-deficient units to give various A–A type polymers with high molecular weights. They optimized thesynthetic routesto givePBTI with highMnof 35.5 kDa, which showed aμeof 2.6 cm2/(V·s)in OTFTs. They also studied the PBTI with differentMnas acceptorinPSCs. The devicewith PBTI(Mn12.7kDa)andPBTI(Mn35.5 kDa) as acceptors and PTB7-Th as the donor gave PCEs of 0.14%and6.67%, respectively. BTI-based A–A type polymer L14[28]presentedgooddeviceperformancewith a PCE of 14.3%. Polymerizing BTI-Tin with dibrominated naphthalene diimide(NDI-Br)and perylene diimide (PDI-Br) produced two A–A copolymers P(BTI-NDI) and P(BTI-PDI)[29].When applying them as ETLs in planar p–i–n PVSCs, the devices gave PCEsof 19.5%and20.8%,respectively, with negligible hysteresis. To solve the high LUMO issue caused by electron-rich thiophene moiety in BTI, Guoet al.synthesized a novel acceptor buildingblockCNI[30]by incorporating strong electron-withdrawing cyano onto BTI. A–A type copolymer PCNI-BTI with low LUMO level was obtained, offeringaσof 23.3S/cmand aPF of 10mW/(m·K2) inOTE. Huanget al.prepared A–A type copolymers based on isoindigo (IIG)and PDI, and used themas acceptors, yieldingaPCE of 2.68%[31].Liuet al.designed a seriesofA–A copolymers based on B←N embedded building blocks, and the all-PSCs withP-BN-IID as theacceptorachieved a PCE of 5.04%[32]. Recently, they also synthesized A–A type copolymers PBN-18 and PBN-19[33]with a strong electron-deficient BNBP unit.After n-doping,PBN-19 exhibited σ of 7.8 S/cm and PF of 24.8μW/(m·K2).

TheA–Atypepolymers showgreatpotential in OTFTs,PSCs, PVSCs andOTEs. The acceptorunits are fewer than donor units, so more efforts should be devoted to developing novel electron-deficient building blocks.

Acknowledgements

Y.Shi thanks thefinancialsupportfromthe Doctoral Research Initiation Foundation of Anhui Normal University(752091).L.Dingappreciates theNational Key Research and Development Program of China (2017YFA0206600) and the National Natural Science Foundation of China (51773045,21772030, 51922032, 21961160720) for financial support.