## 3 Results and Discussion
Herein, the two-dimensional grazing incidence X-ray diffraction (2D-GIXRD) technique was used to understand the influence of the introduction of acceptors on crystallinity, \(\pi\)-\(\pi\) stacking, and polymer-packing orientation relative to the substrate in BDT-based PTTV's system. The neat polymer films were prepared by drop-casting on SiO\({}_{2}\) wafers and then thermal annealing at 120 \({}^{\circ}\)C, 20 min. The 2D-GIXRD experimental results are shown in Figure 4. The 2D-GIXRD pattern of PTTVBDT shows an isotropic ring of intensity at \(q\approx 0.28\) A\({}^{-1}\), which means it possesses randomly oriented lamellar stacks in thin film. In other words, due to absence of a preferential lamellar stacking orientation relative to the substrate, the face-on and edge-on orientation both appear to produce a ring of uniform intensity in 2D-GIXRD pattern. In contrast, the 2D-GIXRD patterns of PTTVBDT-TPD and PTTVBDT-DPP show a partial arc at \(q_{x}\approx 0.30\) and \(0.31\) A\({}^{-1}\), respectively, implying that these two terpolymers have not only a preferential orientation which may be beneficial to charge transport in PSCs but also closer lamellar packing behaviors. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Polymer**} & **Solution,** & **Film,** & **\(\lambda_{\text{edge}}\)** & **E\({}_{\text{g, opt}}\)** & **E\({}_{\text{HOMO,AC2}}\)** & **E\({}_{\text{LUMO}}\)** \\  & **\(\lambda_{\text{max}}\) (nm) a** & **\(\lambda_{\text{max}}\) (nm) b** & **(nm) b** & **(eV) c** & **(eV) d** & **(eV) e** \\ \hline PTTVBDT & 526 & 536 & 635 & 1.95 & \(-\)4.93 & \(-\)2.98 \\ PTTVBDT-TPD & 534 & 537 & 647 & 1.91 & \(-\)5.04 & \(-\)3.13 \\ PTTVBDT-DPP & 530 & 536 & 791 & 1.56 & \(-\)4.99 & \(-\)3.43 \\ \hline \hline \end{tabular}  
* Measured in dilute CB solution (<10\({}^{-3}\) g/L); b Samples were prepared by spin-coating from CB solution (10 g/L) on glass substrate and then thermal annealed at 120 \({}^{\circ}\)C, 20 min; c Estimated from the onset wavelength of the thin-film absorption spectra: E\({}_{\text{g, opt}}=1240/\lambda_{\text{onset}}\); d HOMO energy levels were evaluated by ultraviolet photoelectron spectroscopy (AC2 equipment) from UV–vis thin-film samples; e Calculated according to the equation: LUMO = HOMO + E\({}_{\text{g, opt}}\). (eV).  
\end{table} 
Table 2: Optical and electrochemical properties of the polymers. Figure 3: UV–vis absorption spectra of PTTVBDT, PTTVBDT-TPD, and PTTVBDT-DPP (**a**) in dilute chlorobenzene solution; (**b**) thin film (spin-coated from chlorobenzene solution and annealed for 10 min at 130 \({}^{\circ}\)C). The 2D diffraction patterns are further converted to the out-of-plane X-ray diffraction profile by fit-2D program, as shown in Figure S20. PTTVBDT, PTTVBDT-TPD and PTTVBDT-DPP exhibit (\(d_{100}\)) diffraction peaks in the 20 range of 2\({}^{\circ}\)-5\({}^{\circ}\). For a further precise comparison, PTTVBDT-TPD and PTTVBDT-DPP thin film possess weaker intensity and broader diffraction peaks with FWHM (full-width at half-maximum of the X-ray diffraction peak) = 1.10\({}^{\circ}\) and 1.13\({}^{\circ}\), respectively, corresponding to average crystallite sizes of 6.2 and 6.1 nm, respectively; meanwhile, the lamellar \(d\)-spacing (\(d_{100}\)) for PTTVBDT-TPD is also calculated as 20.9 A and for PTTVBDT-DPP as 20.6 A, indicating PTTVBDT-TPD and PTTVBDT-DPP have reduced lamellar \(d\)-spacing, lower degree of crystallinities and smaller average crystalline grains in (100) plane than PTTVBDT (a lamellar \(d\)-spacing of 22.4 A with a grain size of 12.7 nm). The different size, shape, and position of alkyl chains in the chemical structures of PTTVBDT-derivatives might probably cause the difference in the lamellar \(d\)-spacing. Regardless, these results reveal that the PTTVBDT-based polymer exhibit ordered orientations which is a critical property in conjugated polymers for application in electronic devices, especially PTTVBDT-TPD and PTTVBDT-DPP, displaying a preferred molecular orientation.