## 3 Results and Discussion
### Electrochemical Properties of H3T-4-Fop  
Figure 4a shows the electrochemical properties of H3T-4-FOP SOM, as investigated by the cyclic voltammetry (CV) method, using an oxidation onset potential value of +0.25 V. Freshly prepared 0.1 M tetrabutylammonium hexafluoro-phosphate (TBAPF\({}_{6}\)) was incorporated in acetonitrile and CV measurements were performed with a scan rate of 100 mV/s, using \(\mathrm{Fc/Fc^{+}}\) as an external reference. From the CV measurements, HOMO and LUMO energies were estimated as \(-\)5.27 eV and \(-\)3.26 eV, respectively, from oxidation onset and optical band gap value by using the formula for LUMO energy; LUMO = HOMO + \(\mathrm{E_{g}^{opt}}\)[44]. Our newly designed H3T-4-FOP SOM exhibits a suitable frontier orbital energy \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Chromophore** & \(\mathbf{\lambda_{\mathrm{max}}}\)****a (nm)** & \(\mathbf{\lambda_{\mathrm{max}}}\)****b (nm)** & \(\mathbf{\lambda_{\mathrm{max}}}\)****c (nm)** & \(\mathbf{\lambda_{\mathrm{max}}}\)****d (nm)** &
\begin{tabular}{c} **HOMO****e** \\ **(eV)** \\ \end{tabular} & \(\mathbf{\mathrm{LUMO}}\)****f** & \(\mathbf{\mathrm{E_{g}^{opt}}}\)**g (eV)** \\ \hline H3T-4-FOP & 495 & 505 & 625 & 670 & \(-\)5.27 & \(-\)3.26 & 2.01 \\ \hline \hline \end{tabular}  
* Absorption in chloroform solution. b Absorption of thin film on ITO. c Photoluminescence in chloroform solution. d Photoluminescence in thin film on ITO. e Calculated from the oxidation onset value of cyclic voltammogram. f Obtained from equation; LUMO = HOMO + \(\mathrm{E_{g}^{opt}}\), g Optical band gap derived by the equation \(\mathrm{E_{g}^{opt}}=1240/\mathrm{\lambda_{edge}}\).  
\end{table} 
Table 1: Optical and electrochemical values of H3T-4-FOP SOM. Figure 3: (a) UV-Vis absorption and (b) photoluminescence spectra of blend films of H3T-4-FOP: \(\mathrm{PC_{61}BM}\) in 1:1 (_w/w_), 1:2 (_w/w_), and 1:3 (_w/w_) ratios. Figure 2: (a) UV-Vis absorption and (b) photoluminescence spectra of H3T-4-FOP SOM in chloroform solvent and in thin film. level which is possibly due to the presence of three electron-accepting functional groups, i.e., keto (CO), cyano (CN), and fluoro (F) in the 4-fluorobenzoylacrylonitrile acceptor unit (Figure 4b). The HOMO and LUMO energy levels of H3T-4-FOP donor are, respectively, higher than the classical PC\({}_{61}\)BM acceptor material to produce enough driving force for an exciton dissociation. The energy levels of the fabricated BHJ-OSCs with H3T-4-FOP: PC\({}_{61}\)BM active layer are shown in Figure 5a. The electrochemical and optical properties of the newly synthesized H3T-4-FOP match well with the PC\({}_{61}\)BM acceptor energy level. The designed SOM is applied for fabricating BHJ-OSCs as the donor material with the PC\({}_{61}\)BM acceptor. The photovoltaic performance of an active layer with different blend ratios (H3T-4-FOP:PC\({}_{61}\)BM, 1:1, 1:2, 1:3, _w/w_) were tested by measuring the I-V curves under a light illumination at 100 mW/cm\({}^{2}\) (AM 1.5), as shown in Figure 5b. BHJ-OSC device with H3T-4-FOP: PC\({}_{61}\)BM (1:2, _w/w_) active layer reveals a promising PCE of \(\sim\)4.38%, a high Jsc = \(\sim\)11.18 mA/cm\({}^{2}\), Voc = 0.782 V and FF of 50%. It is expected that due to the high absorption and scattering behavior, the fabricated BHJ-OSC exhibits a promising Jsc and Voc. Moreover, the improved Jsc and photovoltaic performance of the (H3T-4-FOP: PC\({}_{61}\)BM, 1:2 _w/w_) active layer might be associated with smooth film morphology (discussed in the next section) and a fast intermolecular charge transfer (ICT) between H3T-4-FOP and PC\({}_{61}\)BM which occurs due to the introduction of multifunctional electron deficient groups in the acceptor unit [45]. In support, the introduction of the hexyl side chain in the donor molecule further reduces the interaction between the donor-acceptor interfaces which results in suppressing the charge recombination and hence, improves the Voc [42].