Figure 1 The chemical structure of various imide-functionalized building units.
3.Imide-functionalized polymer donors
3.1.Naphthalenothiophene Imide-Based Polymers
Scheme 1 Synthetic route of NTI and schematic reactions to form two Csp2 -Csp2 bonds by simple one-step reaction.
Six-membered imide has strong electron-withdrawing ability, and monomers with two six-membered imides, such as NDI and PDI, are widely used to construct n-type semiconducting materials for OFETs or electron acceptors for OSCs.[63-65] However, the electron absorption ability of these diimides is too strong to be used as building units for polymer donors. Compounds with one six-membered will have weaker electron withdrawing ability and may be used as strong electron deficient monomers to construct polymer donors. Wu et al.reported the synthesis of naphthalenothiophene imide (NTI), which bears six-membered imide and a five-membered aromatic ring.[22] The NTI compound was synthesized by Suzuki coupling between 6,7-dibromo-2-(2-hexyldecyl)-1H-benzo[de]-isoquinoline-1,3(2-H)-dione and 3,4-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophene in a very high yield of 82 %. Notably, this is a new and efficient approach to construct five-membered aromatic ring, forming double Csp2-Csp2 bonds in a simple one-step reaction. The NDTI was also synthesized in a similar method from dibromo and ditin compounds. Because there are various dibromo and ditin/diBpin compounds, numerous fused-ring units for polymers could be created by using this method to form double Csp2-Csp2 bonds in a simple one-step reaction. The chemical structures of NTI-based conjugated polymers are illustrated in Figure 2 and the photovoltaic properties are summarized in Table 1.
Compared with other widely used electron-deficient units, such as BDD, TzBI, BTZ, the NTI has stronger electron-withdrawing ability and could significantly lower the HOMO energy levels of the polymers.[15] Thus, the NTI-based polymer PNTB1 has a low-lying HOMO energy level (−5.42 eV) without introducing any F or Cl groups into the polymer backbone. The non-fullerene OSCs fabricated from PNTB1:Y6 exhibited the highest PCE of 15.18 %. They studied the impact of the thiophene π-bridge on photovoltaic performance through designing two NTI-based polymer donors PNTB and PNTB-2T.[22] PNTB-2T has two more thiophene units in the polymer chain than PNTB, and exhibited better long-range coplanarity of polymer conjugated chains, resulting in closer π-π stacking, more ordered polymer chain packing, and higher hole transport of PNTB-2T. The PNTB-2T:Y6-based OSCs exhibited PCE of 16.72 %, while the PCE for PNTB:Y6 was only 3.81 %. The photovoltaic performance of PNTB-2T-based solar cells could be enhanced to 17.35 % by using PC71BM as second acceptor. Importantly, NTI-based polymer donors exhibit excellent batch-to-batch reproducibility because the twisted polymer conjugated backbone make the polymer packed through weak short interactions rather than π-π stacking, which minimizes the impact of molecular weight on phase separation and blend film morphology.
Wu et al. also synthesized two NTI-based polymers PNTB6-Cl and PNTB-Cl by exploring the effect of employing linear or branched alkyl chains in NTI unit on photovoltaic performance.[66] PNTB-Cl exhibited good solubility in chloroform, however, due to stronger intermolecular interactions and shorter π-π stacking distance, the polymer PNTB6-Cl with linear alkyl chains is insoluble in chloroform (Figure 3a, b). They fabricated layer-by-layer polymer solar cells (LBL-PSCs) with PNTB6-Cl as donor and N3 as acceptor by sequential spin-coating method (Figure 3c, d). Thus, PNTB6- Cl:N3 based LBL devices exhibited a remarkable PCE of 17.59 %, which is much higher than 15.24 % for PNTB-Cl:N3 based devices.