## 3 Results and Discussion
### Exam Dyes Behavior
#### 3.2.3 How Stable the Dye Extract is  
Some researchers have published studies of DSSC stability or lifetime [8; 12], showing how cell performance does not significantly change [60] (i.e., betacyanin dye cell was found stable for 12 weeks [63]) or diminishes with time [70; 76; 77; 78]. Our group has also tested cell performance, keeping the cell working continuously for 350 h, with the aim to test not the dye, but the electrolyte. It was found that the cell with volatile electrolytes lasted for about 80 h, and when fresh electrolyte was injected, the cell could recover at 70% of the initial efficiency for almost 100 h. However, for cells with non-volatile electrolytes (i.e., ionic liquids), the efficiency kept constant at the beginning but diminished to 80% after 175 h and to 40% after 250 h [79]. In this work here, we have focused on dye extract stability, not the entire cell, just by recording UV-Vis spectra after a certain time, and the stability is not the same for the different extracts. Regarding betalains pigments, on the one hand, both orange and yellow bougainvillea extracts behave similarly. Figure 8b shows an example of two UV-Vis spectra of a yellow BG extract: recently prepared (yellow plot) and 3 months later (dark green plot). The spectrum after 3 months is essentially the same, still with indicaxanthin peak (absorption around 470-480 nm), but the absorbance is 44% lower. Therefore, it is preferable to make dye extract just before cell fabrication, due to possible degradation with time. On the other hand, it has been found that pink and violet bougainvillea extracts quickly degrade into less efficient compounds (see Figure 9, green line). Absorbance of peaks initially in the range [450, 451, 452, 453, 454, 455, 456, 457, 458, 459, 460, 461, 462, 463, 464, 465, 466, 467, 468, 469, 470, 463, 467, 469, 471, 472, 473, 474, 475, 476, 477, 478, 479, 480] nm diminishes 24% after less than 3 months (this is consistent with the previous results for yellow-orange extracts), and the betanin peak initially around 530 nm is only a shoulder with an intensity fall of 44%. This peak gives the red-violet color, and it is also visually obvious that betanin degrades with time, as the dye solution becomes brown. Some results already published [60] state that bougainvillea solutions are stable for at least 12 months, but in this case, the solutions had been previously purified. The maximum stability of betalains is reached for pH between 5 and 6 and it is known that betanin at neutral pH (534 nm) converts to betanidin (540 nm) by \(\beta\)-glucosidase at acid pH, and in general when decreasing pH, \(\lambda_{\text{max}}\) decreases [84]. Besides, betalains degrade in the presence of light and oxygen and also with increasing temperature [85]. Figure 8: Extracts stability: (**a**) Eggplant extract with Solution 2: fresh, recently prepared (purple line) and 3 months later, preserved from air and light (blue line); (**b**) Yellow bougainvillea extract with Solution 2: recently prepared (yellow line) and 3 months old (dark green line). Figure 9: UV-Vis spectra of pink bougainvillea extract 3 months after preparation (green line). UV-Vis spectra of recently prepared dye extracts made from (i) bracts just after been collected (red data) and (ii) bracts preserved for 6 months (blue data). To compensate for this disadvantage of bougainvillea extract, we tried to preserve dry bracts. We verified that pigments responsible for light absorption can be successfully extracted from bracts that have been kept stored for several months and adequately preserved from air and light exposure. Figure 9 also shows UV-Vis spectra of dye extracts made from a) bracts just after having been collected (pink line) and b) b) bracts preserved for 6 months (blue line).