## 3 Results and Discussion
### Device Performances  
First, the two P3 HT:DPP-PhCN devices were fabricated to evaluate the device performance of the binary devices with the configuration ITO/PEDOT:PSS/active layer/LiF/Al. Table 1 summarizes the photovoltaic performance of the binary cells measured in air under AM 1.5G illumination (100 mW cm\({}^{-2}\)). The D:A blend ratio was adjusted to 1:1, as optimized in our previous report [21], and the same D:A ratio (1:1) was also used in the fabrication of the ternary cells used in the present work. CF and \(o\)-DCB were used as processing solvents. Both binary devices that were prepared using CF exhibited similar PCEs of \(\sim\)0.46%. Compared with the \(V_{\text{OC}}\) of P3 HT:\(p\)-DPP-PhCN (0.56 V), that of the P3 HT:\(o\)-DPP-PhCN was greater (0.94 V), which can be explained by the relatively high-lying LUMO energy level of \(o\)-DPP-PhCN (\(-\)3.48 eV, Figure 2c) than that of \(p\)-DPP-PhCN (\(-\)3.55 eV). The \(V_{\text{OC}}\) of the P3 HT:\(p\)-DPP-PhCN film appeared to be even lower than expected from the LUMO level of \(p\)-DPP-PhCN. The strong molecular aggregation of \(p\)-DPP-PhCN, as confirmed from UV-vis measurements, might better stabilize its LUMO level. The P3 HT:\(p\)-DPP-PhCN film prepared from \(o\)-DCB solution showed no photovoltaic characteristics because a uniform film morphology could not be obtained as a consequence of the poor solubility of \(p\)-DPP-PhCN in \(o\)-DCB. By contrast, the PCE of the P3 HT:\(o\)-DPP-PhCN increased to 0.87% (Table 1) when high-boiling-point \(o\)-DCB was used as a processing solvent. The increase in efficiency mainly arose from the increase in the \(J_{\text{SC}}\) from 1.18 (CF) to 2.15 mA cm\({}^{-2}\) (\(o\)-DCB). As depicted in Figure 2b, the absorption spectrum of the \(o\)-DPP-PhCN film prepared from \(o\)-DCB was much more red-shifted and broadened than that of the \(o\)-DPP-PhCN film prepared from CF, reflecting the increased molecular aggregation in the film prepared from \(o\)-DCB. This phenomenon is consistent with the increased \(J_{\text{SC}}\) value of the device prepared from \(o\)-DCB. The slight decrease in the \(V_{\text{OC}}\) of the device with the P3 HT:\(o\)-DPP-PhCN film prepared from \(o\)-DCB originated from the enhanced molecular packing behavior of \(o\)-DPP-PhCN, which might result in a slight stabilization of its LUMO energy level. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Active Layer** & **Solvent** & \(V_{\text{OC}}\) [V] & \(J_{\text{SC}}\) [mA cm\({}^{-2}\)] & **FF [\%]** & **PCE [\%]** \\ \hline P3 HT:\(o\)-DPP-PhCN & CF 1  & 1.09 & 1.19 & 35 & 0.46 \\  & \(o\)-DCB & 0.83 & 2.15 & 49 & 0.87 \\ P3 HT:\(p\)-DPP-PhCN & CF 1  & 0.56 & 1.64 & 50 & 0.47 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Photovoltaic performances of the P3 HT:DPP-PhCN binary cells. Figure 2: (**a**) Normalized UV absorption spectra of **(a)** the \(o\)-DPP-PhCN (o), \(p\)-DPP-PhCN (p), and P3 HT films prepared from a chloroform (CF) solution and **(b)** the \(o\)-DPP-PhCN (o) and P3 HT films prepared from CF and \(o\)-dichlorobenzene (DCB); **(c)** energy diagram of the materials used in organic solar cell (OSC) fabrication. Figure 3 shows the EQE responses of the binary solar cells (P3 HT:DPP-PhCN), together with the UV-vis spectra of the neat films of each donor and acceptor used in the devices. In Figure 3a, the EQE response of the P3 HT:\(o\)-DPP-PhCN device increased when the film prepared from \(o\)-DCB was used, matching its higher \(I_{\text{SC}}\) value. The EQE profiles matched the UV-vis absorption profiles well. When compared with the device with an \(o\)-DPP-PhCN film prepared from CF, the device with an \(o\)-DPP-PhCN film prepared from \(o\)-DCB showed an additional EQE peak in the longer-wavelength region at 690 nm, corresponding to the red-shifted \(\lambda_{\text{max}}\) peak of the \(o\)-DCB-processed \(o\)-DPP-PhCN film at 670 nm. In Figure 3b, the EQE maximum peak of the P3 HT:\(p\)-DPP-PhCN device also corresponded to the \(\lambda_{\text{max}}\) peak of the \(p\)-DPP-PhCN film (670 nm).