## 3 Results and Discussion
### Device Performances
The two DPP-PhCNs were then introduced into the active layer of P3 HT:PCBM devices to fabricate ternary device cells. On the basis of the results of the binary devices, the ternary devices based on \(o\)-DPP-PhCN and \(p\)-DPP-PhCN were prepared from \(o\)-DCB and CF, respectively, and various blend ratios were investigated to optimize the performance of the devices. As control devices, P3 HT:PCBM binary devices were also fabricated using both \(o\)-DCB and CF as processing solvents; these devices exhibited PCEs of 2.98% and 2.07%, respectively. The D:A blend ratio was adjusted to 1:1. The ternary P3 HT:DPP-PhCN:PCBM film consisted of one donor (P3 HT) and two acceptors (DPP-PhCN and PCBM), resulting in a D:A:A active layer. The total D:A ratio was maintained at 1:1. The ratio of the DPP-PhCN acceptor in the two-acceptor films was therefore varied; however, the total acceptor amount (i.e., DPP-PhCN + PCBM) was adjusted to be equal to the amount of the P3 HT donor. The photovoltaic characteristics of the devices are summarized in Tables 2 and 3. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Active Layer (D:A:A)** & \(\mathbf{V_{\text{OC}}}\) [V] & \(\mathbf{J_{\text{SC}}}\) [mA cm\({}^{-2}\)] & **FF [\%]** & **PCE [\%]** \\ \hline
4.0:0:0.4:0 & 0.60 & 8.03 & 62 & 2.98 \\
4.0:0:4:3.6 & 0.61 & 7.94 & 61 & 2.95 \\
4.0:0.8:3.2 & 0.63 & 7.70 & 59 & 2.86 \\
4.0:1:2.2.8 & 0.64 & 7.20 & 59 & 2.72 \\
4.0:1:6:2.4 & 0.65 & 7.42 & 55 & 2.67 \\
4.0:2:0.2.0 & 0.67 & 6.31 & 45 & 1.92 \\
4.0:4:0:0.0 & 0.83 & 2.15 & 49 & 0.87 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Photovoltaic performances of the P3 HT:\(o\)-DPP-PhCN:PCBM devices fabricated with films prepared using \(o\)-DCB as a solvent with various blend ratios. Figure 3: External quantum efficiency (EQE) responses of the binary solar cells fabricated with (**a**) P3 HT:\(o\)-DPP-PhCN (o) and (**b**) P3 HT:\(p\)-DPP-PhCN (p) films; the UV-vis spectra of the neat films of each donor and acceptor used in the devices are included for comparison. Figure 4 shows the \(J\)-\(V\) and EQE curves of the ternary devices based on P3 HT:DPP-PhCN:PCBM in various blend ratios. In the ternary devices based on \(o\)-DPP-PhCN, as the ratio of \(o\)-DPP-PhCN increased, the device efficiency gradually decreased, mainly because of the decrease in the \(J_{\text{SC}}\) values (Figure 4a and Table 2). As the amount of \(o\)-DPP-PhCN increased, the EQE edge at the long-wavelength region, which included the contribution of light absorption by \(o\)-DPP-PhCN, was slightly red-shifted and broadened; however, the contribution of the light absorption of P3 HT (-500 nm) to the EQE simultaneously decreased more significantly (Figure 4b). Consequently, the \(J_{\text{SC}}\) and PCE values gradually decreased with the increasing \(o\)-DPP-PhCN content. Notably, however, as the ratio of \(o\)-DPP-PhCN increased, the \(V_{\text{OC}}\) tended to gradually increase, as will be discussed later. By contrast, in the ternary device based on \(p\)-DPP-PhCN, the \(J_{\text{SC}}\) increased with the increasing ratio of \(p\)-DPP-PhCN (Figure 4c and Table 3). The best PCE of 2.48% was obtained at the blend ratio of 4:1:3 (P3 HT:\(p\)-DPP-PhCN:PCBM). The increase in the \(J_{\text{SC}}\) can be explained by the pronounced shoulder peaks (670 nm) of the EQE curves (Figure 4d), which match the maximum EQE peak of the binary P3 HT:\(p\)-DPP-PhCN device. Similar to the trend observed in ternary cells based on \(o\)-DPP-PhCN (Figure 4b), the EQE responses at \(\sim\)500 nm gradually decreased with the increasing \(p\)-DPP-PhCN ratio; however, the increase in the EQE shoulder in the longer-wavelength region was more significant, resulting in a higher \(J_{\text{SC}}\) (Figure 4d).