Guo and coworkers synthesized a series of PhI-based polymers by varying the side chains.[43] Incorporating a single alkoxy substituent and optimizing the S···O interactions afforded polymers P3a-P3c with optimized HOMOs and appropriate aggregation properties. The polymer P3b with the longer-but-less-bulky 3,7-dimethyloctyl side-chain showed the highest PCE of 4.7 % by blending with N2200. They synthesized a D−A1−D−A2 type polymer.[126] The nonhalogenated processed DOTFP-PhI:IT-4F-based OSCs exhibited a PCE of 10.65 %. Introducing fluorine atoms into the PhI unit was also an effective method to improve the performance of non-fullerene OSCs. Guoet al. [49] synthesized two PhI-based D-π-A polymers. Owing to the stronger electron-withdrawing ability of imide group and fluorine atoms, ffPhI-BDT exhibited a deeper HOMO/LUMO levels of -5.39/ -3.36 eV than that (−5.25 eV/-3.25 eV) of TPhI-BDT. Blending with IDIC, the TffPhI-BDT showed a higher PCE of 9.48 % with a largerV OC of 0.93 V, a J SC of 15.92 mA cm-2, and an FF of 63.9 %, which significantly out-performed the PCE of 8.31 % for TPhI-BDT. Furthermore, they synthesized two D-A1-D-A2 type polymers ffPhI-ffBT and PhI-ffBT.[127] Pairing with IT-4F, the polymers ffPhI-ffBT and PhI-ffBT realized high PCEs of 12.74 and 13.31 %, respectively. Zhou et al. [124] synthesized two D-π-A copolymers PE80 and PE81, where thiophene and thiophene [3,2-b] -thiophene (TT) were adopted as the π-bridges. The polymer PE81 with TT as a π bridge exhibited much higher PCE of 10.21 % than 4.11 % for PE80.
The ternary and quaternary strategies are effective methods to improve device performance of OSCs.[131-133] Using PhI-Se and PhI-Th as polymer donor guest, Zhan and co-workers[128] fabricated more efficient parallel-like ternary and quaternary devices. The PhI-Se could form individual phase and selectively tune the packing ordering of Y6 phase, resulting in enhanced photocurrent and increased electron mobility. As a result, the PhI-Se:PM6:Y6-based ternary devices exhibited a PCE of 16.4 %, and the PhI-Se:PM6:Y6:PC71BM-based quaternary OSCs obtained the highest PCE of 17.2 %. Through the chlorination, they also synthesized ultra-wide bandgap (2.10 eV) polymer PhI-Cl with a deep HOMO (−5.58 eV) energy level.[129] The PhI-Cl additive enable PM6: Y6 and PM6: Y6: PC71BM based OSCs to exhibit PCEs over 17 % and 18 %, respectively. These works suggest that PhI is promising building block to construct polymer donors.
3.6. Other Imide-Based Polymers
In addition to NTI, TPD, PhI, BTI, TzBI, many other imide-functionalized building blocks were devised for constructing high-performance polymers.
Compared with DPI unit, the naphthodithiophene imide (NDTI) unit has a larger π-conjugated backbone that could enhance intermolecular π-orbital interactions between polymer chains.[37] It was firstly synthesized in 2012 by Chi and co-workers.[134] From starting material 4,5-dibromophthalic acid, 4,5-dibromophthalic imide was prepared by treating with SOCl2 and refluxing with amine in acetic acid. The 4,5-di(thiophen-3-yl) phthalic imide was synthesized by Stille coupling reaction between 4,5-dibromophthalic imide and the tin reagent of thiophene. NDTI was finally obtained by oxidative cyclization reaction of 4,5-di(thiophen-3-yl) phthalic imide with the iron(III) chloride (Scheme 6, Synthetic route 1). He et al. optimized the synthetic route of NDTI by using cyclization method under UV irradiation (Scheme 6, Synthetic route 2).[37] Wu group presented another approach to synthesize NDTI by direct Stille coupling between 4,5-dibromophthalic imide and (3,3’-bis(trimethylstannyl)-5,5’-diyl)bis(trimethyl-silane)-2,2’-bithiophene to form two Csp2-Csp2 bonds in simple one step reaction (Scheme 6, Synthetic route 3).[55]
Scheme 6 Synthetic Route to NDTI.
Isomerism has become an effective method to enhance photovoltaic performance of OSCs by fine tuning the molecular structure. He and co-workers synthesized two polymers based on the NDTI unit, PAB-αand PAB-γ (Figure 12).[37] The orientation isomerism of thiophene rings could greatly affect the planarity of the two polymers. Due to smaller torsion angle, the polymer PAB-αhave superior molecular planarity, resulting in the stronger intermolecular interactions, more red-shifted absorption, better miscibility and nanophase separation morphology. Thus the PAB-α :Y6-based devices achieved a PCE of 15.05 %, while the PAB-γ -based devices exhibited an efficiency of 0.04 %. Wuet al. [55] developed new NDTI-based polymers PNDT1 and PNDT2 by incorporating different side chains at the N-site of the imide. Compared with the polymer PAB-α with branched alkyl chains, PNDT1 and PNDT2 showed higher crystallinity and hole mobility. By pairing with eC9, the PNDT1, and PNDT2-based PSCs showed overall power conversion efficiency of 17.27 % and 18.13 %, respectively. Importantly, the NDTIs have short π-π stacking and abundant short interactions, and their polymers exhibited superior morphological stability (Figure 13). Therefore, the PNDT2-based OSCs exhibited much better device stability than that of PNDT1, PAB-α , and benchmark polymers PM6 and D18.