Figure 13 (a) Thermal stability of PNDT12:eC9, PAB-α:eC9, PM6:eC9 and D18:eC9 based OSCs; (b) The shelf stability of PNDT2:eC9 and PM6:eC9 based device in N2-filled glovebox under room light; (c) The stability of PNDT2:eC9 and PM6:eC9 based device under one simulated solar illumination; (d) Illustration of blend film that compose both polymer and NFA;(e) The single crystal data of NDTI-C2 and NDTI-C4, including the π-π stacking and short interactions between adjacent molecules. Reproduced with permission.[55] Copyright 2023, American Chemical Society.
Chen et al. [130] used naphthalene imide units as electron withdrawing substituents to develop two difluoro-quinoxaline-based polymer donors, namely PDNB and PDNB-2F (Figure 12). The difluoro-quinoxaline with a naphthalimide substituent not only could downshift copolymer HOMO level, but also the rigid and good planarity backbone could promote intra-molecular charge transfer. Besides, the N-alkyl side chain could guarantee good solubility of copolymers. When blended with an NFA of Y6, the PDNB-2F showed an higher PCE of 12.18 % with a larger V OC of 0.87 V, aJ SC of 22.45mA cm-2, and FF of 62.4 %, whereas the device based on PDNB:Y6 showed a comparatively lower PCE of 8.83 % with a V OC of 0.80 V.
Scheme 7 Synthetic Route to DPI/DTID.
Zhan et al. firstly reported the synthesis of dithienophthalimide (DPI) from starting material of maleic anhydride in 2011.[35] Treating maleic anhydride with amine, followed by Suzuki coupling reaction and oxidative cyclization reaction with the iron(III) chloride, DPI could be synthesized efficient (Scheme 7, Synthetic route 1). The dithieno[2,3‐e:3′,2′‐g]isoindole‐7,9(8H)‐dione (DTID),[135] an isomer of DPI, could also be synthesized using this approach. Zhao et al. reported another synthetic route of DPI from starting material of (3,3’-dibromo-[2,2’-bithiophene]-5,5’-diyl)bis(trimethylsilane). A Cr-mediated cyclization reaction of (3,3’-dibromo-[2,2’-bithiophene]-5,5’-diyl)bis(trimethylsilane) with dimethyl acetylenedicarboxylate yield benzo[2,1-b:3,4-b’]dithiophene-4,5-dicarboxylate. Further alkaline hydrolysis, anhydride and imide formation, DPI was synthesized accordingly (Scheme 7, Synthetic route 2). As a fused-ring building block, the high coplanarity and rigidity, existence of two vacant outer α-positions, and suitable π-electron-deficient ability of DPI make it an ideal electron-deficient building unit for polymer donors. Zhan et al. [35] reported the first DPI-based polymer donor P1, which exhibited a PCE of 0.3 % by matching with PC61BM. The chemical structures of some DPI-based conjugated polymers are illustrated in Figure 14 and the photovoltaic properties are summarized in Table 8.