## 3 Results and Discussion
### Exam Dyes Behavior
#### 3.2.1 How the Dye Solution Adsorbs onto Semiconductor DSSC Film  
The dye must have at least one functional group that helps the dye molecules to attach on the surface of the semiconductor, so the chemical structure of natural dyes ([7; 16]) must be considered. Anthocyanin's basic structure has -OH groups favoring bonding to TiO\({}_{2}\)[53]. Betalains have also anchor groups, such as carboxyl (-COOH), capable of binding to the atoms of the semiconductor. There are several binding modes and sites which are influenced by several factors like dye's structure, total anchoring groups, energy levels, pH, substrate pre-treatment, etc. [61]  
Some works include absorption spectra of dyes onto TiO\({}_{2}\) film [20; 21; 27; 32; 49; 50; 55; 59; 66; 67; 68; 69; 70; 71]. [PERSON] and col. published UV-Vis absorption spectra of anthocyanins and belatains onto TiO\({}_{2}\)[67]. [PERSON] and col. showed UV-Vis absorption spectra of spinach, beet, red cabbage and other natural dyes, and also the spectra of these dyes adsorbed on TiO\({}_{2}\)[69]. [PERSON] and col. studied the influence of solvent on dye adsorption onto TiO\({}_{2}\). Also, [PERSON] and col. published studies of dye absorption on TiO\({}_{2}\) (in terms of concentration) and how the cell efficiency depends on the amount of adsorbed dye [71]. However, [PERSON] et al. [32] showed that a high adsorption rate does not necessarily indicate a highly efficient solar energy conversion; the electrolyte used was more influential.  
The dye-onto-TiO\({}_{2}\) UV-Vis spectra show enhanced light photon absorption and in addition, when TiO\({}_{2}\) contribution is removed, absorption peaks (or shoulders) identifiable with pigments can be seen. Our group has already published data about eggplant extracts (anthocyanins) adsorbed onto TiO\({}_{2}\), detecting the pigment main peak around 530 nm, after removing the TiO\({}_{2}\) contribution [37]. In general, we find that the spectra of the extracts onto TiO\({}_{2}\) are quite similar to that of the TiO\({}_{2}\) film alone, but  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**DSSC Dye** & \(\mathbf{\eta_{av}}\) **(\%)** & \(\mathbf{\eta_{max}}\) **(\%)** & \(\mathbf{FF_{av}}\) & \(\mathbf{FF_{max}}\) \\ \hline N-719 & \(2.3\pm 0.5\) & 3.235 & \(0.44\pm 0.08\) & 0.553 \\ Eggplant 2 [37] & \(0.090\pm 0.006\) & 0.101 & \(0.51\pm 0.06\) & 0.623 \\ Eggplant 3 [37] & \(0.069\pm 0.011\) & 0.083 & \(0.54\pm 0.02\) & 0.568 \\ Pink bougainville & \(0.19\pm 0.07\) & 0.250 & \(0.51\pm 0.08\) & 0.595 \\ Orange Bougainvillea & \(0.19\pm 0.02\) & 0.215 & \(0.427\pm 0.013\) & 0.448 \\ Concentrated orange BG & \(0.188\pm 0.012\) & 0.202 & \(0.521\pm 0.016\) & 0.542 \\ Yellow Bougainvillea & \(0.162\pm 0.017\) & 0.185 & \(0.39\pm 0.04\) & 0.451 \\ Beetroot 1 & \(0.32\pm 0.12\) & 0.467 & \(0.41\pm 0.08\) & 0.469 \\ Beetroot 2 & \(0.08\pm 0.04\) & 0.112 & \(0.35\pm 0.06\) & 0.414 \\ Beetroot 3 & \(0.17\pm 0.06\) & 0.229 & \(0.35\pm 0.03\) & 0.392 \\
1:1 Orange BG + Beetroot 1 & \(0.269\pm 0.004\) & 0.274 & \(0.55\pm 0.02\) & 0.563 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Performance of DSSC prepared with natural dyes: Average and maximum efficiency and fill factors.  
with a slight increase in absorbance and in some cases a modification of shape, as reported also in other works [68; 69]. In this paper, Figure 6 shows how betalain extract from pink bougainvillea adsorbs onto TiO\({}_{2}\) film. In the inset of Figure 6, the peak corresponding to betanin (\(\sim\)540 nm) is clearly seen when TiO\({}_{2}\) film contribution is removed from the spectra of the dye absorbed onto the semiconductor nanostructure (blue line in the inset). The indicaxantin peak (\(\sim\)480 nm) is not specifically detected, probably due to the higher absorption of the TiO\({}_{2}\) for these lower wavelengths, but a small hump can be seen in its place.