## 3 Results and Discussion  
### Dyes' Optical Properties  
The solutions of the two dyes are easily distinguished: one is red-orange (C106), while the other is yellow (Y123). One of the main requirements for a dye to be used as a sensitizer in DSSCs is to absorb light radiation in the visible region of the solar spectrum. Ethanol was chosen as solvent for both dyes since it is biodegradable and a natural product. The polar nature of ethanol ensures a suitable interaction with carboxyl polar functional groups and, on the other hand, its short alkyl chain favorably interacts also with the peripheral hydrocarbon chains in both compounds, C106 and Y123. In Figure 2a, the absorption spectra of C106 and Y123 dyes in ethanol are shown. Concerning the C106 spectrum, it exhibits in the UV region an intense sharp peak at 311 nm centered on ligand (\(\pi\)-\(\pi\)\(\pi\)), one large band (shoulder) from 349 to 398 nm, and another absorption maximum at 538 nm. The latest absorption bands can be assigned to singlet and triplet MLCT (metal to ligand charge transfer) transitions typical of the electronic charge transfer from the d metal orbitals to the \(\pi\)\(\pi\)\(\pi\) orbitals of the organic binders present in the C106. The organic dye Y123 (Figure 2a) is characterized by different optical properties due to its D-\(\pi\)-A structure: two main absorption peaks are found at 474 and 326 nm, with a hump at 260 nm, corresponding to the internal charge transfer (ICT) between the donor and the acceptor moieties and the \(\pi\)-\(\pi\)\(\pi\)\(\pi\) transition, respectively [38]. Moreover, it has been demonstrated that the flatness of the \(\pi\)-conjugated system, due to the presence of a melted cyclopentadititionene structure, greatly increases the molar extinction coefficient at 532 nm (\(53\times 10^{3}\) M\({}^{-1}\)cm\({}^{-1}\)), which is even greater than that reported in literature for C106 (18.7 \(\times\) 10\({}^{3}\) M\({}^{-1}\)cm\({}^{-1}\) at 550 nm) [26]. The absorbance values reported in the spectra are in arbitrary units (a.u.) and cannot be quantitatively compared. The inset of Figure 2b shows two sensitized TiO\({}_{2}\) photoanodes (semiconductive layer thickness: 6-7 \(\upmu\)m) with the C106 and Y123 dyes. The two photoanodes are visibly and uniformly colored, which indicates the dye has been efficiently adsorbed on the TiO\({}_{2}\) layer. From a visual observation, C106 photoanode appears darker than the sensitized one with Y123, even though the immersion time, the TiO\({}_{2}\) layer thickness, and the dyes solution concentrations were the same for the two different sensitizers. Figure 2b shows the absorption spectra of the sensitized photoanodes with C106 and Y123. In the C106 case, the first absorption bands, whose maximum were at 311 and 349 nm in solution, are hidden by the absorption spectrum of the TiO\({}_{2}\), which absorbs the light at higher energies (3.2 eV). On the other hand, the maxima of the third absorption band undergoes a slight bathochromic shift, from 398 to 401 nm and from 538 nm to 541 nm, respectively. This is probably a consequence of the interaction, between the carboxylic groups of the dye-anchoring moiety and the TiO\({}_{2}\), which stabilizes the dye in both the ground and the excited states. The TiO\({}_{2}\)-anchoring group interaction also generates a red-shift of the ICT absorption band for Y123 from 474 nm to 485 nm when the dye is adsorbed on the semiconducting layer. Despite the highest molar extinction coefficient reported in literature for Y123, the Y123-sensitized photoanode exhibits lower intensity values than those of the corresponding C106 one. The most plausible explanation is that, given the same thickness and area, the density of absorbed C106 molecules on the photoanode is greater than those of Y123.