## 3 Results and Discussion
### Optical Properties of H3T-4-FOP  
The optical properties of H3T-4-FOP SOM were investigated by UV-Vis and PL spectroscopy in a chloroform solvent, thin films and blend thin films. The representative spectra of H3T-4-FOP SOM are shown in Figure 2a,b and the obtained data are summarized in Table 1. H3T-4-FOP SOM exhibits good solubility in chloroform, tetrahydrofuran (THF) and chlorobenzene solvents. In the chloroform solvent, H3T-4-FOP SOM displays a broad absorption band in the range of \(\sim\)320 nm to \(\sim\)620 nm with an absorption maximum (\(\lambda\)max) at \(\sim\)495 nm, corresponding to \(\pi\)-\(\pi^{\circ}\) (HOMO to LUMO) transitions of the conjugated \(\pi\)-system, as shown in Figure 2a. However, the H3T-4-FOP thin film poses a red shift (\(\sim\)10 nm) with \(\lambda_{\text{max}}\) at \(\sim\)505 nm which is mainly caused by \(\pi\)-\(\pi\) stacking and an extended terminal hexyl side chain which affects the intermolecular interaction and aggregation in solid state. Such type of absorption properties of small molecules with a methyl, ethyl, hexyl and dodecyl chain is discussed by [PERSON] et al. when moving from solution to a thin film [39]. The optical band gap of \(\sim\)2.01 eV for H3T-4-FOP SOM is calculated from absorption onset equation \(\text{E}_{\text{g}}^{\text{opt}}=1240/\lambda_{\text{onset}}\) where \(\lambda_{\text{onset}}\) is in nm and \(\text{E}_{\text{g}}\) in eV [40]. The photoluminescence spectroscopy of H3T-4-FOP SOM is performed in a chloroform solution and thin film coated ITO substrates. A strong and sharp emission band is observed at \(\sim\)625 nm in the solution which is remarkably red-shifted to \(\sim\)670 nm in the thin film state, as seen from Figure 2b. Furthermore, the red shifted PL spectrum is observed moving from solution to thin film state which might be due to an orientation of the molecule in head-to-tail fashion. Thus, negative coulombic coupling is originated, resulting in J-type aggregation (Bathochromic shift) [41]. The absorption and emission of blend thin films (H3T-4-FOP: PC\({}_{61}\)BM, _w/w_) are further characterized to investigate the effect of photon absorption, as represented in Figure 3a,b. The blend thin film covers the absorption range of \(\sim\)350 nm to \(\sim\)650 nm, representing a good photon harvesting material for BHJ-OSCs. From Figure 3a, the intense absorption signal is observed for H3T-4-FOP: PC\({}_{61}\)BM (1:2, _w/w_), which might be due to uniform, well-mixed and smooth surface morphology of the thin film. It is well known that the concentration of donor and acceptor materials in blend thin film plays a critical role in improving the light absorption and charge extraction for high performance solar devices. In our case, the H3T-4-FOP:PC\({}_{61}\)BM (1:2, _w/w_) ratio presents a high absorption intensity due to the complete dissolution of donor-acceptor materials in the chlorobenzene solvent which might induce the film uniformity and transparency with interconnected domains. To further understand the charge transfer behavior in donor-acceptor blends, Figure 3b exhibits the PL emission spectra of thin films of different blend ratios. Herein, the presence of the 4-fluorobenzylacetonitrile acceptor moiety might be responsible for the fast emission due to the strong electron accepting nature and hydrogen bond interaction. The quenching effect is observed in all blend thin films but nearly complete quenching (quenching of \(\sim\)97%) occurred in H3T-4-FOP: PC\({}_{61}\)BM (1:2, _w/w_). However, slightly lower quenching of \(\sim\)93% was seen for H3T-4-FOP: PC\({}_{61}\)BM (1:3, _w/w_) which clearly reflects the modest emission properties of H3T-4-FOP: PC\({}_{61}\)BM (1:2, _w/w_). Herein, high PL quenching in H3T-4-FOP: PC\({}_{61}\)BM (1:2, _w/w_) ensures a smooth and uniform surface with ordered phase separation of donor and acceptor which might reduce the charge recombination within the exciton diffusion length and, therefore, induces the exciton-dissociation at the donor acceptor interfaces [42; 43].