## 4 Conclusions  
In summary, we have successfully synthesized and characterized a novel p-type conjugated copolymer, PTTVBDT; cooperating BDT and TTV units via Stille polymerization. PTTVBDT shows a low-lying HOMO energy level and ordered molecular-packing behavior. Furthermore, by introducing different electron-withdrawing groups, two terpolymers, PTTVBDT-TPD and PTTVBDT-DPP; display stronger absorption ability, lower-lying HOMO energy level, and preferred molecular orientation, indicating the absorption ability, bandgaps, and molecular energy levels of the polymers can be further  
Figure 6: (**a**) _–V_ curves (**b**) IPCE spectra of optimized polymer/PC\({}_{61}\)BM devices under the illumination of AM 1.5G, 100 mW\(\cdot\)cm\({}^{-2}\). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Polymer/fullerene**} & \multirow{2}{*}{\(V_{\infty}\) (mV)} & \multirow{2}{*}{\(J_{\infty}\) (mA/cm\({}^{2}\))} & \multirow{2}{*}{**FF (\%)**} & \multicolumn{2}{c}{**PCE (\%)**} & \multirow{2}{*}{\(R_{\rm sh}\)**} & \multirow{2}{*}{\(R_{\rm s}\)**} \\ \cline{4-4} \cline{6-9}  & & & & & & **Avg.** & & **Best** (\(\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf\mathbf{}} }}}}}}}}}}}}}\)\) & & \\ \hline PTTVBDT/PC\({}_{61}\)BM  & 703 \(\pm\) 13 & 8.10 \(\pm\) 0.06 & 52.24 \(\pm\) 1.08 & 2.98 & 3.10 & 0.89 & 29.17 \\ PTTVBDT-TPD/CA\({}_{61}\)BM  & 723 \(\pm\) 4 & 9.96 \(\pm\) 0.21 & 59.5 \(\pm\) 0.44 & 4.29 & 4.39 & 0.84 & 8.56 \\ \hline \hline \end{tabular}  
\end{table} 
Table 3: Photovoltaic properties of polymer solar cell devices based on the polymers, PTTVBDT, PTTVBDT-TPD, and PTTVBDT-DPP, with optimized ratios of PC\({}_{61}\)BM under the illumination of AM 1.5G, 100 mW\(\cdot\)cm\({}^{-2}\). tuned by replacing TTV-monomer units with different acceptors. The polymer solar cells based on the blends of PTTVBDT, PTTVBDT-TPD, and PTTVBDT-DPP with PC\({}_{61}\)BM exhibited the best power conversion efficiencies of 3.10%, 5.01%, and 4.39%, respectively. This work demonstrates an effective strategy combining a two-dimensional molecular structure with random copolymerization strikes producing conjugated polymers to achieve highly efficient organic photovoltaic devices. The following are available online at www.mdpi.com/2073-4360/8/11/382/s1. Figure S1: \({}^{1}\)H-NMR spectrum of Compound S2; Figure S2: \({}^{13}\)C-NMR spectrum of Compound S2; Figure S3: \({}^{1}\)H-NMR spectrum of Compound S3; Figure S4: \({}^{13}\)C-NMR spectrum of Compound S3; Figure S5: \({}^{1}\)H-NMR spectrum of Compound S4; Figure S6: \({}^{13}\)C-NMR spectrum of Compound S4; Figure S7: \({}^{1}\)H-NMR spectrum of Compound 1; Figure S8: \({}^{13}\)C-NMR spectrum of Compound 1; Figure S9: \({}^{13}\)C-NMR spectrum of Compound S6; Figure S10: \({}^{13}\)C-NMR spectrum of Compound S7; Figure S11: \({}^{1}\)H-NMR spectrum of Compound 3; Figure S12: \({}^{13}\)C-NMR spectrum of Compound 3; Figure S13: \({}^{1}\)H-NMR spectrum of Compound 4; Figure S14: \({}^{13}\)C-NMR spectrum of Compound 4; Figure S15: \({}^{1}\)H-NMR spectrum of Compound M1; Figure S16: \({}^{13}\)C-NMR spectrum of Compound M1; Figure S17: \({}^{1}\)H-NMR spectrum of Compound M3; Figure S18: \({}^{13}\)C-NMR spectrum of Compound M3; Figure S19: (a) TGA and (b) DSC second heating profiles of PTTVBDT, PTTVBDT-TPD, and PTTVBDT-DPP with a heating rate of 10 \({}^{\circ}\)C/min under N\({}_{2}\) atmosphere and a cooling rate of 10 \({}^{\circ}\)C/min; Figure S20: 1D-GIXRD out-of-plane of PTTVBDT, PTTVBDT-TPD, and PTTVBDT-DPP films which were prepared by drop-casting and then thermal annealing at 120 \({}^{\circ}\)C, 20 mins; Figure S21: AFM (a-c) topography images and (d-f) phase images of spin-coated films of polymer/PC\({}_{61}\)BM blends. The scan sizes for all images are 2 \(\upmu\)m \(\times\) 2 \(\upmu\)m. Table S1: Mobility of PTTVBDT, PTTVBDT-TPD, and PTTVBDT-DPP with/without PC\({}_{61}\)BM by the SCLC method.