## 3 Results and Discussion
### NDSSC Tests  
After this previous exhaustive study of natural-dye solutions, an example of using is presented. This last step is to prepare the cells and check how the dyes work measuring cell performance varying the dye and with all the other parameters fixed. Under equal preparation and irradiation conditions, the efficiencies of NDSSCs with different extracts are measured. DSSC fabrication and test method is thus a simple and optimized procedure consisting of always the same consecutive steps, from paste preparation to cell assembly and photovoltaic measurements: The only differences are due to the different dye solutions used, as the aim of the experiments is to study the dye influence on cell efficiency. Table 4 summarizes some of our results.  
Efficiency values are similar to those obtained in our previous works [37; 38; 41] and in literature, see for example some tables comparing NDSSC and references therein [3; 7; 8; 9; 13; 14; 15; 60]. Artificial dye N719 gives higher efficiency than natural dyes, but lower than values found in literature. This is due to our cell fabrication methodology, which was selected for its simplicity to test the natural dyes and not to maximize the efficiency [40].  
Among all the natural dyes listed in Table 4, the highest efficiencies are obtained when using beetroot extract with Solution 1, reaching up to 0.47%, among the highest values in literature for this pigment (i.e., raw 0.12%--purified 0.4% [61]; 0.11% [77] or 0.19% [84] and finally [PERSON] and col. obtained 0.68% to 0.89% optimizing electrodes preparation [78]). Regarding the other beetroot extracts, the one with Solution 3, even having a similar spectrum and pigment concentration as the former, gives lower efficiency. This may be due to the very low pH, which limits -COOH anchoring to the semiconductor surface. Solution 2 behaves similarly, and, in addition, the absorbance spectrum shows the lowest pigment concentration, so in this case the efficiency is even lower.  
Eggplant extracts gave low efficiencies compared with already published data (i.e., 0.48% [82]). Other works obtain lower efficiency for eggplant (0.15%) compared to other natural dyes, and attribute it to more pronounced recombination reactions for eggplant dye than for the other dyes [34].  
The different bougainvillea extracts achieved similar efficiencies (0.16% to 0.19%) no matter what bract color, which is due to the presence of different betalain groups and in different proportions. Red-purple bracts extracts gave also efficiencies of the same order or up to (0.29 \(\pm\) 0.02) % in our previous works, and the small differences found were due to other factors like film thickness, surfactant used in paste preparation, and porosity of the semiconductor film [38; 41].  
The fill factor does not change noticeably when varying the dye, although in our experiments the highest FF (0.62) has been found for eggplant extracts, while the lowest FF (0.35) is obtained for cells made with beetroot extract.  
We have also concentrated some of the dyes, but no clear increase or change in efficiencies has been seen. In Table 4, we give as an example the cell efficiency when using orange bougainvillea extract concentrated 10% wt. Efficiency does not change, although the fill factor does increase from 0.43 (initial extract) to 0.52 (concentrated).