## 3 Results and Discussion  
The synthetic routes of the monomers and copolymers are shown in Scheme 1. The monomers M2 and M4 were synthesized by reported methods [31; 32; 34]; M1 was synthesized via the Horner-Wadsworth-Emmons reaction of diisopropyl ((5,5\"-dibromo-[2,2':5',2\"-terthiophen]-3'-yl)methyl) phosphate 1 with 4,4\"-bis(2-ethylhexyl)-[2,2':5',2\"-terthiophene]-3'-carbaldehyde 2. The monomer M3 was synthesized from the bromination of 5-octyl-1,3-di(thiophen-2-yl)-4_H_-thieno[3,4-c]pyrrole-4,6(5_H_)-dione 4, which is the microwave-assisted Stille cross-coupling reaction product of 1,3-dichloro-5-octyl-4_H_-thieno[3,4-c]pyrrole-4,6(5_H_)-dione 3 and tributyl(thiophen-2-yl)stannane. These three copolymers PTTVBDT, PTTVBDT-TPD, and PTTVBDT-DPP were performed by Stille coupling polymerization in chlorobenzene (CB) at 110 \({}^{\circ}\)C using Pd\({}_{2}\)dba\({}_{3}\) and P(\(o\)-tol)\({}_{3}\) as the catalyst and ligand, respectively. In the preparation of PTTVBDT-TPD and PTTVBDT-DPP, the feed molar ratio of M1 to the corresponding donor and the acceptor monomer (TPD or DPP units) in the reaction is 4:5:1. In other words, 2,6-bis(trimethyltin)-4,8-bis(2-ethylhexyloxy) benzo[1,2-b:4,5-b']dithiophene (M2) was copolymerized with 80 mol % of the dibromo donors (M1) and 20 mol % of the dibromo acceptor, M3 and M4, to generate the corresponding copolymer PTTVBDT-TPD and PTTVBDT-DPP, respectively. The raw products were precipitated into methanol and collected through a Soxhlet thimble by filtration, which was then subjected to repeated Soxhlet extraction with methanol, acetone, hexane, and methylene chloride to remove the small molecules and oligomers, and finally with chloroform to collect the target compounds. Due to the presence of 2-ethylhexyl or \(n\)-octyl groups in the monomer units, all the copolymers are soluble in common organic solvents such as chloroform, THF, and chlorobenzene. More detailed synthetic procedures and characterization data of monomers and polymers are described in the Experimental Section and Supplementary Materials (Figures S1-S18) [35; 36; 37]. The number-average molecular weights (\(M_{\text{n}}\)) of the resulting polymers, PTTVBDT, PTTVBDT-TPD, and PTTVBDT-DPP, were 47.1, 110.3, and 120.8 kDa, with a polydispersity index (PDI) of 2.29, 2.81, and 2.49, respectively, as determined by gel-permeation chromatography (GPC) using THF as the eluent at 40 \({}^{\circ}\)C (shown in Table 1). Most PTTVBDT was obtained by Soxhlet extractions in dichloromethane; in contrast, trace oligomers of PTTVBDT-TPD and PTTVBDT-DPP were removed by Soxhlet extractions in dichloromethane and most products were obtained in chloroform, which might probably cause the difference in molecular weight. In addition, the actual compositions ofthe copolymers were verified by elemental analyses: the composition (\(m/n\)) for PTTVBDT-TPD and PTTVBDT-DPP was 5.9 and 7.0, respectively. As shown in Figure S19a, thermogravimetric analysis (TGA) curves of polymers, which were recorded under N\({}_{2}\) atmosphere at a heating rate of 10 \({}^{\circ}\)C/min, show that all polymers had good thermal stability with degradation temperatures (\(T_{\text{d}}\)) at 5% weight loss well beyond 350 \({}^{\circ}\)C (the measured data shown in Table 1). However, differential scanning calorimetry (DSC) analysis, shown in Figure S19b, of all the polymers reveals there are no apparent thermal transitions, such as T\({}_{\text{g}}\) and T\({}_{\text{m}}\), before 240 \({}^{\circ}\)C that may be due to the rigid structure of the polymer which is quite common in low-bandgap conjugated polymers [38]. The optical properties of the polymers were examined by ultraviolet-visible (UV-vis) absorption spectroscopy.