3.4. Bithiopheneimide (BTI)-Based Polymers
Scheme 4 Synthetic Route to Bithiopheneimide (BTI)
Bithiophene imide (BTI) unit with seven-membered imide ring was first designed and synthesized by Marks and coworkers in 2008.[36] The reduction reaction of 3,3’,5,5’-tetrabromo-2,2’-bithiophene with zinc powder yields 3,3’-dibromo-2,2’-bithiophene. 2,2′-bithiophene-3,3′-dicarboxylic acid was prepared by adding n-BuLi and then treating with carbon dioxide gas. Through condensation cyclization in acetic anhydride and substitution reaction with amine, BTI was successfully synthesized (Scheme 4). The BTI unit shows a strong electron withdrawing ability to reduce the highest occupied molecular orbital (HOMO) level and a good planarity to reduce the steric hindrance effectively.[53,115]The first BTI-based polymer P2 for OPV application exhibited a PCE of 5.5 % when blend with PC71BM.[116] After that, the development of BTI-polymers for OSCs had gained much attention. The chemical structures of BTI-based polymer donors are illustrated in Figure 10 and the photovoltaic properties are summarized in Table 5.
Yang et al. [117] synthesized polymers PBTIBDTT and PBTIBDTT-S and realized high PCEs of 8.6 % and 9.42 % by blending with PC71BM.
The PCEs could remain above 8.6 % as the active layer thickness further increased to 280 nm. By matching with the ITIC-F, PBTIBDTT-based PSCs showed an outstanding PCE of 11.2 %.[118]Notably, A PCE over 9 % can be maintained when the active layer thickness reached 350 nm, suggesting that PBTIBDTT-based devices are insensitive to the active layer thickness. They also reported a ternary OSC using PDOT as the donor polymer and PC71BM and ITIC as co-acceptors.[119] By finely controlling the morphology via the ratio control of ternary blends, a PCE of 11.21 % was achieved in ternary OSC. Wei et al. reported three BTI-based polymers (PDTBDT, PDTBDT-T, and PDTBDT-T-Cl) by introduced thiophene π-bridges and chlorine atoms to optimized light absorption and the energy level of the polymers.[115] The HOMO and LUMO energy levels were -5.67 and -3.28 eV for PDTBDT, -5.40 and -3.21 eV for PDTBDT-T, and -5.58 and -3.26 eV for PDTBDT-T-Cl, respectively. The optimized PDTBDT-T-Cl:Y6 OSCs achieved a PCE of 15.63 %, which is much higher than 12.71 % for PDTBDT-T:Y6 and 8.22 % for PDTBDT:Y6.
Yan et al. reported four BTI-based polymers, Q4, Q5, Q6, and Q7.[120] Blending with MeIC, the devices based on Q4, Q5, Q6, and Q7 showed PCEs of 10.34, 11.06, 5.26, and 0.48 %, respectively. Due to suitable energy levels and complementary optical absorption with PYIT, Q4: PYIT based all-PSCs achieved the highest PCE of 15.06 %, while the other three polymers (Q5-Q7) exhibited much lower PCEs in the range of 0.12-6.71 %. Recently, Guo et al.synthesized two BTI-based polymers that have the same backbone but isomeric alkyl side-chains on thiophene π-bridge.[121] The isomerization of alkyl chains significantly affected their crystallinity and molecular orientation behaviors. G15 with linear octyl side-chains on thiophene π-bridges features more planar backbone, stronger pre-aggregation character in solution and distinctive temperature-dependent aggregation (TDA) property. As a result, G15-based all-polymer OSCs yielded a high PCE of 15.17 %, which is much higher than 3.82 % for G3:L15-based devices.
3.5. Phthalimide (PhI) based polymers
Scheme 5 Synthetic Route to Phthalimide (PhI).
Watson et al. reported the synthesis of PhI-2Br from starting materials phthalic anhydride, which converted to 4, 7-dibromoisobenofuran-1,3-dione in presence of Br2 in fuming sulfuric acid solution, followed by treating with amines in glacial acetic acid.[122] The synthetic route to Phthalimide (PhI) is depicted in Scheme 5. The PhI unit was firstly introduced into polymers for optoelectronic devices by Guo and co-workers in 2009.[48] The first PhI-based polymerpoly(N-(dodecyl)-3,6-bis(4-dodecyloxy-thiophen-2-yl))phthalimide (PhIBT12) exhibited a relatively low PCE of 1.92 % by blending with PC61BM.[123] The PhI-based polymers have very wide bandgap and the highest PCEs were only 6.3 % by matching with fullerene acceptors.[124,125]However, PhI-based polymers demonstrated much better photovoltaic performance by matching with narrow bandgap NFAs. The chemical structures of PhI-based polymer donors are illustrated in Figure 11 and the photovoltaic properties are summarized in Table 6.