## 3 Results and Discussion
Since the \(V_{\rm{oc}}\) value of PSCs is primarily proportional to the difference between the HOMO energy level of the donor polymer and the LUMO energy level of the fullerene acceptor, these devices prepared from the blends of PTTVBDT, PTTVBDT-TPD, and PTTVBDT-DPP with PC\({}_{61}\)BM exhibited average \(V_{\rm{oc}}\) of 703, 740, and 723 mV, respectively, are in good agreement with the order of the HOMO energy levels of these polymers. Additionally, the \(J_{\rm{sc}}\) of the devices based on blends of PTTVBDT, PTTVBDT-TPD, and PTTVBDT-DPP with PC\({}_{61}\)BM were 8.10, 11.65, and 9.96 mA\(\cdot\)cm\({}^{-2}\), respectively. In order to investigate the improved \(J_{\rm{sc}}\), the monochromatic incident photon-to-electron conversion efficiency (IPCE) spectra of the optimized polymer/PC\({}_{61}\)BM devices were measured under AM 1.5G illumination with an intensity of 100 mW\(\cdot\)cm\({}^{-2}\) and shown in Figure 6b. It is worth noting that due to the high efficiency of the photon-to-electron conversion for PTTVBDT-TPD/PC\({}_{61}\)BM (>60% over the range 340-600 nm and peaking at ca. 85% at 370 nm), they not only possess the highest \(J_{\rm{sc}}\) in all the solar cell devices but are potential materials for photo-detector devices. In addition, the IPCE plot of PTTVBDT-DPP/PC\({}_{61}\)BM displays the broadest IPCE region extended to 800 nm, indicating that the incorporation of the strongest electron-withdrawing DPP units into the polymer backbone effectively improves the \(J_{\rm{sc}}\) value in solar cell devices by enhancing absorption ability in comparison with PTTVBDT/PC\({}_{61}\)BM. Figure 5: (**a**) Work function of polymer thin films determined by photoelectron emission; (**b**) Energy diagram of PTTVBDT, PTTVBDT-TPD, and PTTVBDT-DPP. For photovoltaic devices, the charge mobility is a crucial factor for achieving highly efficient devices. Herein, the space-charge-limited current (SCLC) method with the device structures of ITO/PEDOT:PSS/blend film/Au for the hole mobility and ITO/SAM-NH\({}_{2}\)/blend film/Ca/Al for the electron mobility was applied to verify the hole and electron mobility of the blend films. As listed in Table S1, the hole/electron mobilities of PTTVBDT-TPD, and PTTVBDT-DPP blend films were determined to be 5.70 \(\times\) 10\({}^{-4}\)/8.37 \(\times\) 10\({}^{-4}\) and 4.23 \(\times\) 10\({}^{-4}\)/5.69 \(\times\) 10\({}^{-4}\) cm\({}^{2}\cdot\)V\({}^{-1}\cdot\)s\({}^{-1}\), respectively, which are one order higher than PTTVBDT blend film of 2.01 \(\times\) 10\({}^{-5}\)/3.65 \(\times\) 10\({}^{-5}\) cm\({}^{2}\cdot\)V\({}^{-1}\cdot\)s\({}^{-1}\). This result corresponds to improved series resistance and fill factor for devices based on PTTVBDT-TPD and PTTVBDT-DPP, in comparison with PTTVBDT. Furthermore, the balanced charge transport of PTTVBDT-DPP blend film is closer to 1 than the others, explaining why it exhibits the highest FF and the lowest \(R_{\rm s}\). Atomic force microscopy (AFM) was applied to verify the morphologies of the spin-coated films of polymers/PC\({}_{61}\)BM on top of ITO/PEDOT:PSS, which were prepared by using the same procedure and parameters for preparing the photoactive layers. As the phase images of the blend films were shown in Figure S21, the polymers/PC\({}_{61}\)BM blend films display very similar morphological properties. The root-mean-square (RMS) surface roughness of PTTVBDT blend films appears to be larger than that of the others, corresponding to the highest \(R_{\rm s}\) due to the increased interfacial contact resistance.