## 3 Results and Discussion
### Electrochemical and Photovoltaic Properties of DVB-T-ID Osm  
The electronic properties, in terms of HOMO and LUMO energy levels, were studied by performing cyclic voltammetry (CV) of a DVB-T-ID thin film in freshly prepared 0.1 M TBAPF\({}_{6}\) in acetonitrile at a scan rate of 150 mV/s, using Fc/Fc\({}^{+}\) as an external reference. From Figure 4, the DVB-T-ID thin film records an oxidation onset potential value of +0.30 V. For the calculation of HOMO and LUMO energy levels, the following equations are employed [13]:  
\[\text{HOMO}=-\text{e}\ [\text{E}_{\text{ox}}\ -\text{E}_{1/2}\ (\text{Fc/Fc}^{+})+4. 8]\ \text{eV}\]  
Figure 3: (**a**) UV-Vis absorption and (**b**) PL spectra of DVB-T-ID:PC\({}_{61}\)BM) thin films of different blend ratios (1:1, 1:2, and 1:3 (_w/w_)). Figure 2: (**a**) UV-Vis absorption and (**b**) photoluminescence (PL) spectra of a DVB-T-ID SOM in a chloroform solvent and solid state thin film. \[\text{LUMO}=(\text{HOMO}+\text{E}_{\text{g}}^{\text{opt}})\text{ eV}\]  
where E\({}_{\text{ox}}\) and E\({}_{\text{1/2}}\) are the onset oxidation potential of the donor material and the redox potential of Fc/Fc\({}^{+}\) versus Ag/AgCl, respectively. Using the above equations, the DVB-T-ID SOM possesses an impressive HOMO of \(\sim\)\(-\)5.32 eV and LUMO of \(\sim\)\(-\)3.26 eV. The energy offset between the LUMO and HOMO of the donor DVB-T-ID SOM with a PC\({}_{61}\)BM acceptor is sufficiently high and provides enough of a driving force for charge dissociation. Figure 5a illustrates the band energy levels diagram of BHJ-OSCs fabricated with a DVB-T-ID:PC\({}_{61}\)BM active layer. As seen in the electrochemical and optical studies, the newly designed D-\(\pi\)-A (DVB-T-ID) displays a good optical bandgap and molecular energy levels, which originate from the introduction of a \(\pi\)-spacer in the DVB and ID units. It is seen that the HOMO value of DVB-T-ID SOMs is fairly suitable to the HOMO level (\(-\)6.45 eV) of PC\({}_{61}\)BM. The LUMO counterpart of the donor and the HOMO counterpart of the acceptor are rather large for exciton binding energy, which considerably eases the exciton dissociation at the interface of DVB-T-ID:PC\({}_{61}\)BM. This alignment of HOMO and LUMO energy levels in DVB-T-ID:PC\({}_{61}\)BM might increase the charge transfer at the interface of the donor-acceptor layer. To evaluate photovoltaic behavior, the synthesized DVB-T-ID OSM as a donor material was applied in fabricating the BHJ-OSCs. The photovoltaic performance of fabricated BHJ-OSCs with a DVB-T-ID:PC\({}_{61}\)BM active layer was tested by measuring the J-V characteristics under light illumination at 100 mW/cm\({}^{2}\) (AM1.5). Figure 5b presents the J-V curves of fabricated BHJ-OSCs with different DVB-T-ID:PC\({}_{61}\)BM blend ratios (1:1, 1:2, and 1:3_w/w_). The BHJ-OSC with DVB-T-ID:PC\({}_{61}\)BM (1:2, _w/w_) exhibits a PCE = \(\sim\)3.1%, a high Jsc = \(\sim\)10.63 mA/cm\({}^{2}\), Voc = \(\sim\)0.637 V and a high FF = \(\sim\)0.48%. The obtained photovoltaic parameters are higher than the reported OSCs of similar structures, as shown in Table 2. The high Jsc of BHJ-OSCs is ascribed to the broad wavelength in the visible region and the good light scattering properties of the DVB-T-ID:PC\({}_{61}\)BM thin film. Herein, a low Voc value might attribute to a higher LUMO (\(\sim\)\(-\)3.26 eV) value of the DVB-T-ID donor, as compared with the LUMO energy level of the PC\({}_{61}\)BM (\(\sim\)\(-\)4.37 eV). It is notable that the fabricated BHJ-OSC exhibits a relatively high FF, without the addition of additives or promotors in the active layer. Generally, the FF in organic solar cells (OSCs) relies on the surface properties of semiconducting materials, like charge mobility and the impact of morphology of the active layer [37; 38]. Herein, a considerable advancement in the morphology of the active layer of DVB-T-ID:PC\({}_{61}\)BM (1:2, _w/w_) at the nanoscale level delivers a high FF, as discussed in AFM studies.