## 3 Results and Discussion  
### Dyes Preparation and Characterization  
In a first approach to natural dyeing, at least three main steps should be followed. (i) Select natural raw vegetable pigment. As mentioned before, there are four main pigment families and several important pigments in each of them. The pigments can be found in leaves, roots, bracts, flowers... and to select one or more pigments we have to consider the geographical area, availability, and extraction methods that should be applied. (ii) Try several and select the simplest and best extraction solution and method. Once we have the raw vegetable material, we choose economical and environmentally friendly solvents; consider different solvent combinations; test time spent in solutions, time and centrifugation velocity, filtering processes, etc. (iii) Characterize the resulting dye solutions. The solutions can be characterized initially by UV-Vis spectroscopy and the spectra could be compared to reference artificial dyes like N719 to check the dye's potential. The spectra are also important to identify pigments or pigment families and to decide whether an extraction solution or method works. To tackle this first phase, we have used different vegetable raw material: pollen, coffee grains, beetroot, bougainvillea bracts with different colors, and eggplant peels, all of them easily available. Solvent influence on natural dye extraction has been dealt with previously [21; 22; 42; 23]. In our case, after a previous sequence of tests with different solvents and procedures, four simple and economical \begin{table}
\begin{tabular}{c c c c c c} \hline
**Extraction Method** & **Acetone (\% vol.)** & **Ethanol (\% vol.)** & **HCl (M)** & **H2O (\% vol.)** & **pH Range** \\ \hline Solution 1 & 32 & — & — & 68 & 5.0–5.6 \\ Solution 2 & — & Solvent & 0.1 & — & 1.6–1.7 \\ Solution 3 & — & — & 0.1 & Solvent & 1.3–1.4 \\ Solution 4 & — & 80 & — & 20 & 6.5–7.0 \\ \hline \end{tabular}
\end{table} 
Table 1: Solutions used for extracting the dye from vegetable raw materials. extraction methods have been selected using HCl, acetone, water and ethanol. As it is summarized in Table 1, the four Solutions prepared and numbered from 1 to 4 are: (1) acetone/water 32/68 (vol/vol); (2) 0.1 M HCl in ethanol; (3) 0.1 M HCl in water; and (4) ethanol/water 80/20 (vol/vol). After previous simple preparation (select, dry, crush), the vegetable material in the solution was stirred (800 rpm) for 24 h and the residue was eliminated by centrifugation. See Refs [37; 38; 41] for details. Different filtering processes and different concentrations have also been tested before determining dye extract spectra. Then, after a previous basic study, some of the materials can be ruled out, and the procedure continues only for the a priori good candidates for DSSC sensitizers, finding finally that Solution 1 works better for betalains and Solutions 2 and 3 (much lower pH) go well for anthocyanins [37]. These results are summarized next. Coffee extract showed a spectrum with increasing absorbance towards lower wavelength, and peaks for 450 nm and below. Similar results have already been shown [42], but in our case the spectra were noisy in the UV values, with several peaks also affected by the presence of a caffeine-band around 275 nm [43]. Recently, [PERSON] and col. [44] extracted anthocyanins from coffee bean peels. The extract was used as the natural dye, and the solid waste after the extraction process was carbonated and used to prepare counter electrodes with different thicknesses. In spite of the fact that some previous results support coffee as a dye with not bad efficiency (i.e., 0.33%, as mentioned by [PERSON] et al. [15]), in this work, it was decided not to continue with it, since it is not a local natural product in our region. Something similar happened to pollen. Figure 2 shows spectra of pollen extracts with Solutions 1 and 2.