Contents
1. Introduction Page No.2
2. Imide-functionalized building units Page No.2
3. Imide-functionalized polymer donors Page No.2
3.1. NTI-Based Polymers Page No.3
3.2. TPD-based polymers Page No.5
3.3. TzBI-based polymers Page No.8
3.4. BTI-Based Polymers Page No.11
3.5. PhI-based polymers Page No.12
3.6. Other Imide-Based Polymers Page No.13
4. Conclusions and Outlook Page No.17
1. Introduction
In the past few years, the field of organic solar cells (OSCs) has experienced a significant increase in power conversion efficiency (PCE), surpassing 19 %, primarily due to the development of high-performance nonfullerene electron acceptors (NFAs).[1-3] Zhanet al. , Zou et al. and Sun et al. made significant contributions by developing benchmark NFAs ITIC,[4] Y6,[5] and L8-BO.[6] Compared with fullerene derivatives, these NFAs have different molecular geometry, strong absorption in the near-infrared (NIR) range, moderate HOMO energy levels, and high crystallinity.[6-13] Because the photophysical properties and morphology compatibility between NFAs and polymer donors significantly affect photovoltaic performance of OSCs, these new NFAs demand matchable physical properties of polymer donors.[12,14-19] Currently, the most widely used polymer donors are donor-acceptor (D-A) copolymers which were synthesized by coupling between electron-rich (D) and electron-deficient (A) units. Although several D-A polymer donors, such as PM6, D18, have shown a power conversion efficiency (PCE) over 19 %,[1,2,20,21] their high cost and poor batch-to-batch reproducibility highlight the importance to develop alternative types of polymer donors.[22-24]
Among various of D-A copolymers, the imide-functionalized polymers have drawn considerable research attention and their use in organic field-effect transistor (OFET) and fullerene-based OSCs have been extensively explored.[25-28] The imide building units have the following distinctive characteristics: 1) the strong electron-withdrawing ability of imide group could downshift the HOMO energy levels which benefit high V OCvalues;[29-31] 2) good π-conjugation of the planar core and interactions between O and other atoms are beneficial for intramolecular interaction and charge transport;[22,32,33] 3) the N-alkyl side chain can adjust solubility and aggregation tendency of the polymer. Additionally, the N-alkyl chains, distant from the aromatic core, could minimize polymer chain π-π stacking and exert little influence on charge carrier mobility.[34-37] 4) Last but not least, synthetic routes of most imide building units are straightforward from facile accessible materials.[38,39] These advantages make imide-functionalized polymer donors really fascinating in nonfullerene OSCs.
Because the imide-functionalized polymers used in OFET and fullerene-based OSCs have been well summarized in previous comprehensive reviews,[25,26,40] here, we focus especially on the recent advances of imide-functionalized polymer donors for non-fullerene OSCs. This review article first summarizes the latest structural evolution and common synthesis routes of classical imide-containing electron-deficient building blocks, then investigate the effects of polymer structure on their physical and optoelectronic properties, and introduce the application and photovoltaic performances of imide functionalized polymers in nonfullerene OSCs. In this review, the polymers were classified according to the types of imide-containing electron-deficient units, and the emphasis on the relationships between the polymer structures, physical and photovoltaic properties will provide the guidelines on how to rationally design imide-functionalized polymer donors. It is hoped that this article provides ideas for future innovation on imide-functionalized polymers, especially towards high-performance, low-cost, and stable OSCs.
2. Imide-functionalized building units
The chemical structure, number of imide groups, and component of electron-deficient imide-functionalized building units significantly affect the photophysical and film-forming properties of their polymers, such as frontier energy levels, bandgap, absorption coefficient, crystallinity and charge transport.[41-43]Historically speaking, the creation of new imide-functionalized unit played a crucial role in the evolution of imide-functionalized polymers. Till now, many imide building units have been created or developed, including naphthalene diimide (NDI),[44] perylene diimide (PDI),[26,45]thieno[3,4-c]pyrrole-4,6-dione (TPD),[39,46,47] phthalimide (PHI),[48,49] pyrrolo [3,4-f ] benzotriazole-5,7(6H )-dione (TzBI),[50,51]dithienophthalimide (DPI),[35,52]naphthalenothiophene imide (NTI),[22] bithiophene imide (BTI) units,[53,54] naphthodithiophene imide (NDTI),[55] thieno[3,4-f]isoindole-5,7-dione unit (TID),[25,56]5,9-di(thiophen-2-yl)-6H -pyrrolo[3,4-g ]quinoxaline-6,8(7H )-dione (PQD),[57,58]N-alkyl-4,7-di(thien-2-yl)-2,1,3-benzothiadiazole-5,6-dicarboxylic imide (DI),[59,60] pyromellitic diimide (PMDI),[61] tetraazabenzodifluoranthene diimide (BFI).[62] The chemical structure of various imide-functionalized building units are illustrated in Figure 1. Due to strong electron-withdrawing ability, diimides building units are commonly employed in the construction of n-type semiconducting polymers for OFETs, as well as electron acceptors for OSCs, which have been well documented in other review articles.[26,34] In this article, we aim to present the latest advancements in imide-functionalized polymer donors, categorizing them based on the specific types of imide units. The chemical structures of acceptor materials used below are shown in Figure 15.