## 3 Results and Discussion
### Optical Properties of DVB-T-ID SOM  
The solubility test reveals that the synthesized DVB-T-ID SOM presents excellent solubility in most common laboratory solvents like chlorobenzene, tetrahydrofuran (THF) and chloroform. UV-Vis spectroscopy has been performed to explain the absorption behavior of DVB-T-ID SOMs, as depicted in Figure 2a. In chloroform, the prominent absorption band, at \(\sim\)483 nm, and weak absorption, at \(\sim\)315 nm, are detected by DVB-T-ID, as summarized in Table 1. These absorption bands basically represent \(\pi\)-\(\pi\)* (weak absorption) and n-\(\pi\)* (strong absorption) transitions, which might be beneficial for the charge transfer process [33]. As compared to the solution sample, the significant red shift with the absorption band at \(\sim\)515 nm is observed in the DVB-T-ID solid thin film, suggesting the presence of strong intramolecular interaction, backbone planarity and \(\pi\)-\(\pi\) stacking in a solid state [34; 35]. Using the absorption onset value of the maximum absorption peak in Figure 2a, the optical energy band gap (E\({}_{\text{g}}\)\({}^{\text{opt}}\)) of the DVB-T-ID thin film is estimated as \(\sim\)2.06 eV by considering an equation of E\({}_{\text{g}}\)\({}^{\text{opt}}\) = 1240/\(\lambda_{\text{onset}}\). To understand the charge carriers and luminescence properties of the DVB-T-ID SOM, the PL spectroscopies (Figure 2b) are studied in a chloroform solution and in thin films. The DVB-T-ID SOM displays a strong emission peak at \(\sim\)560 and \(\sim\)620 nm in chloroform solution and thin film, respectively. The red shift in the thin film is related to the inter-chromophoric aggregation or planarization of organic molecules.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Chromophore**} & \(\lambda_{\text{max}}\)1  & \(\lambda_{\text{max}}\)1  & \(\lambda_{\text{max}}\)1 [FOOTNOTE:1]Footnote 1: Found using the following equation: E\({}_{\text{g}}\)\({}^{\text{opt}}\) = 1240/\(\lambda_{\text{edge}}\).  
The donor-acceptor (DVB-T-ID:PC\({}_{61}\)BM) thin film was further characterized by UV-Vis absorption spectra to investigate the outcome of photon harvesting. Figure 3a depicts the UV-Vis spectra of DVB-T-ID:PC\({}_{61}\)BM thin films with different blend ratios of 1:1, 1:2 and 1:3, _w/w_. All blended thin films exhibit a broad absorption which covers the visible region of \(\sim\)400 to \(\sim\)600 nm, suggesting DVB-T-ID as a competent light-harvesting OSM for BHJ-OSCs. The DVB-T-ID:PC\({}_{61}\)BM (1:2, _w/w_) thin film displays the highest absorption intensity, which might be associated with the non-aggregated, uniform and smooth surface of the thin film. Furthermore, the PL spectra of DVB-T-ID:PC\({}_{61}\)BM blend thin films are measured to investigate the charge transfer behavior from donor to acceptor, as shown in Figure 3b. As compared with the intensity of PL emission in the DVB-T-ID thin film, a strong quenching occurs in the DVB-T-ID:PC\({}_{61}\)BM blend thin films, confirming a good exciton dissociation and a fast charge transfer at the interface of DVB-T-ID:PC\({}_{61}\)BM [36]. The improved charge transportation in the DVB-T-ID:PC\({}_{61}\)BM blend thin film might impart to generate a high photocurrent. Hence, DVB-T-ID:PC\({}_{61}\)BM blend thin films are promising for fabricating efficient BHJ-OSCs.