## 3 Results and Discussion
The UV-vis spectra of the polymers in dilute chlorobenzene solution and in spin-coated thin films on glass substrates are shown in Figure 3 and the corresponding absorption data of the polymers are summarized in Table 2. In the solution, the copolymer PTTVBDT exhibits two characteristic bands in the absorption spectra: the lower energy band with absorption peaked at ca. 526 nm corresponded to the \(\pi\)-\(\pi^{\ast}\) transition of the conjugated backbone, while the shorter wavelength absorption band peaked at ca. 362 nm originated from the terthiophene units. Compared to the absorption spectra in the solution, the spectra of PTTVBDT in thin film which the wavelength of maximum absorption (\(\lambda_{\text{max}}\)) and absorption edge (\(\lambda_{\text{edge}}\)) were 536 and 635 nm, respectively, displaying broadened and red-shifted absorption attributed to the intermolecular interaction and aggregation in the solid state. Furthermore, by replacing TTV unit with electron acceptor TPD or DPP unit, a slightly broad and red-shift spectra of PTTVBDT-TPD with absorption edge measured at 647 nm was observed due to addition of the intramolecular charge transfer (ICT) interaction between the TPD and BDT. However, the absorption bands of the TPD containing polymers PTTVBDT-TPD are slightly broadened compared to PTTVBDT but are much narrower than for DPP-containing polymers PTTVBDT-DPP because TPD is a weaker acceptor than DPP [39; 40]. In addition, [PERSON] et al. have reported a series of PBDTBTTPD derivatives which display absorption bands from around 300 to 650 nm with \(\lambda_{\text{max}}\) at ca. 516-553 nm, implying that the presence of electron-rich spacer in TPD-based conjugated polymers leads to a weaker intramolecular charge transfer [41]. Therefore, PTTVBDT-TPD displays slighter redshift and broadening of the absorption bands than PTTVBDT. On the other hand, PTTVBDT-DPP possesses the broadest absorption spectra and displays a significant absorption band around 600-800 nm which is derived from ICT interaction between the DPP and BDT, implying that DPP-BDT have stronger ICT interaction than TPD-BDT. As similar to PTTVBDT, the absorption spectra of PTTVBDT-TPD and PTTVBDT-DPP also displayed broadened and red-shifted absorption in the solid state than solution state. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Polymer** & \(M_{\text{w}}\) **a** (kDa) & \(M_{\text{n}}\) **a** (kDa) & **PDI 1**  & \(m/n\) 2  & \(T_{\text{d}}\) **c** (\({}^{\text{c}}\) **C) \\ \hline PTTVBDT & 124.6 & 47.1 & 2.64 & - & 357 \\ PTTVBDT-TPD & 309.4 & 110.3 & 2.81 & 5.9 & 358 \\ PTTVBDT-DPP & 300.6 & 120.8 & 2.49 & 7.0 & 368 \\ \hline \end{tabular}  
* \(M_{\text{w}},M_{\text{n}}\), and PDI of the polymers were estimated by GPC using polystyrene as standards in THF; \({}^{\text{b}}\)\(m/n\) ratio was calculated by elemental analyses; \({}^{\text{c}}\) 5% weight-loss temperature measured by TGA under N\({}_{2}\) atmosphere.  
\end{table} 
Table 1: Polymerization results and thermal properties of copolymers. In addition, the absorption edges of PTTVBDT, PTTVBDT-TPD and PTTVBDT-DPP thin films were located at 635, 647 and 791 nm, corresponding to optical band gaps (E\({}_{\text{g, opt}}\)) of 1.95, 1.91 and 1.73 eV, respectively. It is worthy to mention that the two ternary copolymers PTTVBDT-TPD and PTTVBDT-DPP exhibit higher absorption coefficients than PTTVBDT in thin film due to the introduction of TPD and DPP units and higher molecular weight. These observations suggest that they can have great potential for more effective light-harvesting and achieve a higher short-circuit current in PSCs. The orientation of conjugated polymers has a direct effect on their charge transport, leading to performance of photovoltaic and other organic electronic applications. It is well-known that investigating the relationship between polymer orientation and chemical structure is a critical and fundamental issue by molecular design.