Polymer semiconductors have aroused interests from both academic and industry due to 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], and perovskite solar cells (PVSCs)^[12-14]. To date, great efforts have been devoted to developing p-type polymer semiconductors, while the development of n-type polymers lags far behind. In fact, n-type polymers are essential 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 p-type 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, Luscombe et al.^[16] reported a NDI-based A–A homopolymer via Yamamoto coupling, however the polymer showed a low μ_e of 6 × 10^–4 cm²/(V·s) due to high steric hindrance of NDI unit. To reduce steric hindrance of NDI, thiophene-fused NDI derivative, naphtho[2,3-b:6,7-b?]-dithiophenediimide (NDTI), was designed by Takimiya et al. Subsequently, various n-type A–A polymer semiconductors (PNDTI-BBT-DP and PNB-TzDP) from NDTI were developed with high electron mobility and electrical conductivity, and were applied in OTFTs and OTEs (Fig. 1)^[17-19]. Reichmanis et 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 with μ_e of 0.067 cm²/(V·s).






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Figure1.
Chemical structures of the representative n-type polymers with acceptor–acceptor backbone.




Among various electron-deficient units, bithiophene imide (BTI) has been proved to be an eminent unit for building n-type polymer semiconductors. Guo et 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] and thiazolothienyl imide dimer (DTzTI)^[23] electron-deficient units by replacing thiophene with thiazole to further push down the frontier molecular orbitals (FMOs) energy levels, as a result, both A–A type polymers PBTzI and PDTzTI (Fig. 1) showed unipolar n-type character in OTFTs. PDTzTI exhibited a remarkably high μ_e of 1.61 cm²/(V·s). This polymer structure also favors to overcome Coulomb interaction in the doped state for OTEs. The doped PDTzTI presented good charge-generation, giving a remarkable electrical conductivity (σ) of 4.6 S/cm and a power factor (PF) of 7.6 μW/(m·K²), much higher than those of NDI polymers^[24]. Owing to its high electron mobility and well-matched energy levels, PDTzTI as electron-transport layer (ETL) in inverted PVSCs yielded a PCE of 20.8% ^[25]. Driven by the success of A–A type homopolymer PDTzTI, Guo et al. also developed a serious of A–A type homopolymers PBTIn (n = 1–5) (Fig. 1)^[26]. Homopolymer PBTI (M_n 12.7 kDa) had μ_e of 1.53 cm²/(V·s). By using off-center spin-coating, the μ_e further increased to 3.71 cm²/(V·s).



Shi et al.^[27] prepared distannylated bithiophene imide (BTI-Tin), and polymerized it with other electron-deficient units to give various A–A type polymers with high molecular weights. They optimized the synthetic routes to give PBTI with high M_n of 35.5 kDa, which showed a μ_e of 2.6 cm²/(V·s) in OTFTs. They also studied the PBTI with different M_n as acceptor in PSCs. The device with PBTI (M_n 12.7 kDa) and PBTI (M_n 35.5 kDa) as acceptors and PTB7-Th as the donor gave PCEs of 0.14% and 6.67%, respectively. BTI-based A–A type polymer L14^[28] presented good device performance with 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 PCEs of 19.5% and 20.8%, respectively, with negligible hysteresis. To solve the high LUMO issue caused by electron-rich thiophene moiety in BTI, Guo et al. synthesized a novel acceptor building block CNI^[30] by incorporating strong electron-withdrawing cyano onto BTI. A–A type copolymer PCNI-BTI with low LUMO level was obtained, offering a σ of 23.3 S/cm and a PF of 10 mW/(m·K²) in OTE. Huang et al. prepared A–A type copolymers based on isoindigo (IIG) and PDI, and used them as acceptors, yielding a PCE of 2.68%^[31]. Liu et al. designed a series of A–A copolymers based on B←N embedded building blocks, and the all-PSCs with P-BN-IID as the acceptor achieved 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 24.8 μW/(m·K²).



The A–A type polymers show great potential in OTFTs, PSCs, PVSCs and OTEs. The acceptor units are fewer than donor units, so more efforts should be devoted to developing novel electron-deficient building blocks.

Acknowledgements

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