## 3 Results and Discussion
### Photovoltaic Properties and Surface Morphologies of Blend Films  
We investigated the photovoltaic properties of **M1** in BHJ solar cells prepared using different processing conditions. Certain parameters, such as the blend ratio between donor and acceptor, thermal treatment, processing solvent, and the use of additives, were varied. The influence of the **M1**:PC\({}_{71}\)BM blend ratio on photovoltaic performance is reflected in the current-voltage traces in Figure 2a and the results are summarized in Table 1. The devices were fabricated using the following architecture: ITO/PEDOT:PSS/**M1**:PC\({}_{71}\)BM/Al. The active layer was deposited by spin coating from 2% solutions in chloroform. Devices cast from chloroform using a **M1**:PC\({}_{71}\)BM blend ratio of 40:60 showed the best results for power conversion efficiency (PCE) (0.89%) with a short-circuit current (\(J_{\infty}\)) of 1.83 mA/cm\({}^{2}\), open circuit voltage (\(V_{\infty}\)) of 0.86 V, and fill factor (_FF_) of 0.57. By substituting chlorobenzene for chloroform (also 2%solutions with the same **M1**:PC\({}_{71}\)BM ratio), the PCE improved to 1.23% (\(J_{\infty}\) = 2.35 mA/cm\({}^{2}\), \(V_{\infty}\) = 0.86 V, _FF_ = 61%) [63]. The PCE values upon varying the blending ratio are a little higher than some devices fabricated with Poly(3-hexylthiophene) (P3 HT) and PC\({}_{60}\)BM in chloroform without post-treatment or processing additives [64]. Figure 1: **(a)** UV-vis absorption spectra of **M1** in the chloroform solution (black line) and of the film cast from the chloroform solution (red line), **(b)** Energy level diagram of all components in **M1**:PC\({}_{70}\)BM solar cells and **(c)** Differential scanning calorimetry (DSC) profile of **M1**. We conducted a dark current analysis to analyze the charge recombination property of our synthesized organic photovoltaic materials in functioning devices, as shown in Figure 2b. From dark current analysis, series resistances were varying from 70 to 940 ohms and shunt resistances were calculated from 0.14 \(\times\) 10\({}^{6}\) to 4.75 \(\times\) 10\({}^{6}\)\(\Omega\)-cm\({}^{2}\). From the previous research, series and shunt resistors are associated with the recombination confirming that conventional organic solar cells (OSCs) show approximately 8.8 \(\Omega\) of series resistance and 0.87 k\(\Omega\) of shunt resistance [65]. However, by comparing the conventional resistance values and those from **M1**-based devices, it can be noticed that the series resistances were 10-100 times higher than the typical OSC device systems. Therefore, we can expect that **M1**-based devices could have a relatively high recombination and all device characteristics were not optimized yet upon varying the blending ratio. First, devices fabricated from chlorobenzene solutions represented improved PCEs relative to those obtained from chloroform solutions. Hence, it could be suggested that the solubility of **M1** in chlorobenzene would be better than that in chloroform with an enhanced interface area between the D and A phases. Furthermore, the effect of solvent additives diiodooctane (DIO) and chloronaphthalene (CN) on the performances of SM-BHJ solar cells based on **M1**:PC\({}_{71}\)BM (40:60) active layers cast from 2% chlorobenzene solution were also studied, and these were reported in Figure 3 and Table 2. The PCEs with optimized devices attained for SM-BHJ solar cells in each case were 1.36% (0.2% DIO) and 1.86% (0.5% CN), respectively, due to improved phase-separation between SM-donor and PC\({}_{71}\)BM acceptor. Especially, the solvent additive of CN exhibits much more efficient device performance than the DIO due to the better surface morphology, with reduced aggregation and uneven domains, which induced an increased interface area between D/A based on the atomic force microscopy (AFM) results, as shown in Figure 4. This result can also be attributed to the improved solubility of **M1** with CN.