## 3 Results and Discussion
The hole mobility of these polymers was measured by space charge limit current (SCLC) method, as listed in Table S1. PTTVBDT-TPD and PTTVBDT-DPP demonstrated hole mobilities of 6.04 \(\times\) 10\({}^{-4}\) and 4.63 \(\times\) 10\({}^{-4}\) cm\({}^{2}\cdot\)V\({}^{-1}\cdot\)s\({}^{-1}\), respectively, which is about 10 times higher than that (5.88 \(\times\) 10\({}^{-5}\) cm\({}^{2}\cdot\)V\({}^{-1}\cdot\)s\({}^{-1}\)) of PTTVBDT, implying the preferred molecular orientation in PTTVBDT-TPD and PTTVBDT-DPP is beneficial to the direction of charge transport in the SCLC method. The highest occupied molecular orbital (HOMO) energies were estimated from the ionization potentials of spin-coated films determined by UV photoelectron spectroscopy in air using an AC2 photoelectron spectrometer (Riken Keiki Co.) [42]. The photoemission threshold energy, also called the work function, was determined from the crossing point of the background and the yield lines. Figure 5a plots the square root of the yield (cps\({}^{0.5}\)) as a function of the photon energy. Therefore, the work functions of PTTVBDT, PTTVBDT-TPD, and PTTVBDT-DPP were found to be \(-\)4.93 eV, \(-\)5.04 eV, and \(-\)4.99 eV, respectively. For comparison, the HOMO value of P3 HT is \(-\)4.69 eV under identical conditions. These results reveal that our series of conjugated copolymers have lower HOMO energies than rr-P3 HT; this will be beneficial for the fabrication of PSCs with high \(V_{\rm oc}\) values. Furthermore, the lowest unoccupied molecular orbital (LUMO) energies were calculated using the optical band gap (E\({}_{\rm g,\ opt. }\)) and the HOMO energies according to the following equation: LUMO = HOMO + E\({}_{\rm g,\ opt. }\) (eV). Therefore, the energy-level diagrams of polymers derived from AC2 photoelectron spectroscopy and UV-vis absorption data are shown in Figure 5b. Figure 4: 2D-GXRD images of thin films of (**a**) PTTVBDT; (**b**) PTTVBDT-TPD; (**c**) PTTVBDT-DPP which were prepared by drop-casting and then thermal annealing at 120 \({}^{\circ}\)C, 20 min. By comparing with the HOMO of PTTVBDT, the HOMO energies of PTTVBDT-TPD and PTTVBDT-DPP are 0.11 and 0.06 eV, respectively, deeper due to the TTV unit being replaced to the stronger acceptor unit which reduces electron-donating ability. On the other hand, the LUMO level is primarily affected by the electron-deficient units in general donor-acceptor polymers. Hence, the order of the LUMO levels is PTTVBDT-DPP (\(-\)3.40 eV) < PTTVBDT-TPD (\(-\)3.29 eV) < PTTVBDT (\(-\)3.00 eV), which not only is consistent with the acceptor strength of the introduced units but also agrees with the results of UV-vis absorption spectra. Moreover, the differences (>0.5 eV) between the LUMO energies of the polymers and PC\({}_{61}\)BM (ca. 4.20 eV) should allow for efficient charge carrier generation to occur in the devices by overcoming Coulombic binding energy of the exciton, since it is normally accepted that the LUMO energies of the donor should be at least 0.3 eV higher than that of the acceptor in BHJ solar cells [9; 43; 44]. To compare the photovoltaic properties of copolymers, the bulk heterojunction PSCs were fabricated with an conventional device configuration of ITO/PEDOT:PSS/polymer:PC\({}_{61}\)BM/Ca/Al. The current density-voltage (\(J\)-\(V\)) curves of the PSC devices under AM 1.5G illumination with an intensity of 100 mW\(\cdot\)cm\({}^{-2}\) are plotted in Figure 6a and the relevant average photovoltaic characteristics, including open-circuit voltage (\(V_{\rm{oc}}\)), short-circuit current (\(J_{\rm{sc}}\)), fill factor (\(FF\)), power conversion efficiency (PCE), series resistance (\(R_{\rm{s}}\)), and shunt resistance (\(R_{\rm{sh}}\)), are listed in Table 3. The optimum PCEs (%) for each PSCs are 3.10% (PTTVBDT/PC\({}_{61}\)BM 5:4 \(w/w\)), 5.01% (PTTVBDT-TPD/PC\({}_{61}\)BM 1:1 \(w/w\)), and 4.39% (PTTVBDT-DPP/PC\({}_{61}\)BM 5:4 \(w/w\)).