## 3 Results and Discussion
### Device Performances
The dependency of various photovoltaic parameters--\(J_{\text{SC}}\), \(V_{\text{OC}}\), fill factor (FF), and PCE--on the weight ratios of the DPP-PhCNs in the ternary cells are plotted in Figure 5. Among the \(o\)-DPP-PhCN-based cells (Figure 5a), the P3 HT:\(o\)-DPP-PhCN binary cell exhibited a higher \(V_{\text{OC}}\) (0.83 V) than the P3 HT:PCBM cell (0.60 V). All ternary devices that were prepared using both acceptors (\(p\)-DPP-PhCN and PCBM) exhibited \(V_{\text{OC}}\) values intermediate between the values corresponding to the two binary cells. With the increasing \(o\)-DPP-PhCN ratio, the \(V_{\text{OC}}\) values gradually increased. In the case of the \(p\)-DPP-PhCN-based cells, the binary P3 HT:\(p\)-DPP-PhCN cell exhibited a lower \(V_{\text{OC}}\) (0.56 V) than that of the P3 HT:PCBM cell (0.64 V). However, the \(V_{\text{OC}}\) values of the resulting ternary cells were not decreased by the addition of \(p\)-DPP-PhCN into P3 HT:PCBM. The \(V_{\text{OC}}\) values of the ternary cells were approximately the same as that of P3 HT:PCBM cell (slightly greater but within the error range). The difference in the trend of the \(V_{\text{OC}}\) variation according to the blend ratios indicates that the working mechanism of these two ternary systems differs. A \(V_{\text{OC}}\) that can be tuned by varying the ratio of the donors (or acceptors) has been previously observed in ternary solar cells with the alloy-like structure, where the two donors (or two acceptors) are mixed and form alloy-like domains, resulting in HOMOs (or LUMOs) that change in proportion to the blend ratios [20]. In our \(o\)-DPP-PhCN-based ternary cells, as the ratio of \(o\)-DPP-PhCN was increased from 0.0 to 1.0, the \(V_{\text{OC}}\) values proportionally increased from 0.60 to 0.83 V. The addition of \(o\)-DPP-PhCN with a relatively high-lying LUMO level positively affected the \(V_{\text{OC}}\) of the ternary cells. However, the \(V_{\text{OC}}\) of the \(p\)-DPP-PhCN-based ternary devices did not substantially vary upon the addition of \(p\)-DPP-PhCN, whereas the \(J_{\text{SC}}\) values gradually increased. In this case, we can propose that a cascade energy-level alignment might exist among the three materials rather than the alloy-like structure [19]. The cascade energy-level alignment is known to be beneficial for a charge transfer. In addition, because the \(V_{\text{OC}}\) values of the ternary cells are similar to those of P3 HT:PCBM, the charge transfer from P3 HT to PCBM might occur more effectively \begin{table}
\begin{tabular}{c c c c c} \hline
**Active Layer (D:A:A)** & \(V_{\text{OC}}\) [V] & \(J_{\text{SC}}\) [mA cm\({}^{-2}\)] & **FF [\%]** & **PCE [\%]** \\ \hline
4.0:0.0:4.0 & 0.64 & 7.03 & 46 & 2.07 \\
4.0:0.5:3.5 & 0.65 & 7.22 & 47 & 2.21 \\
4.0:1:0.3.0 & 0.66 & 7.30 & 51 & 2.48 \\
4.0:1.5:2.5 & 0.66 & 7.52 & 47 & 2.35 \\
4.0.2:0.2.0 & 0.67 & 7.55 & 48 & 2.42 \\
4.0:4.0.0 & 0.56 & 1.64 & 50 & 0.47 \\ \hline \end{tabular}
\end{table} 
Table 3: Photovoltaic performances the P3 HT:\(p\)-DPP-PhCN:PCBM devices fabricated with films prepared using CF as a solvent with various blend ratios. than the transfer from P3 HT to _p_-DPP-PhCN. Notably, the EQE curves of the ternary P3 HT:_p_-DPP-PhCN:PCBM cells show substantial shoulder peaks at 670 nm, corresponding to the absorption of _p_-DPP-PhCN. In this cascade structure, a photoinduced hole transfer (\"channel II\" charge generation) by the light absorption of the acceptor (here, _p_-DPP-PhCN) contributes to the photocurrent generation of the devices, along with electron transfer induced by the absorption of the P3 HT donor. These effects result in broad EQE responses and high \(\rm{J_{SC}}\) values in the ternary cells. Improvements in device efficiencies by the cascade energy-level structure have been reported to occur as a result of more charge transport channels and complementary absorption [9,10].