## 3 Results and Discussion
### Exam Dyes Behavior
#### 3.2.2 Mixing Different Dyes: Co-sensitization  
The absorption spectrum of a single dye hardly follows the whole solar radiation spectrum (see Figures 2 and 3). Mixtures of several dyes with different absorption spectra are often prepared to obtain the maximum absorption within the visible and infra-red region. For DSSCs applications, these mixtures co-sensitize the device to increase the global absorption using the widest wavelength range possible and thus maximizing the efficiency.  
Works including the study of natural dye mixtures show different approaches [6; 26; 27; 28; 64; 72; 29; 73; 74; 75]. [PERSON] and col. used chlorophyll from bougainvillea leaves and anthocyanins from blueberries [72], finding that the efficiency of the cell improved when using a 1:1 dye mixture (0.62% chlorophyll, 0.64% anthocyanins and 0.75% dye mixture). Something similar was found later by [PERSON] and col.: DSSCs were prepared using anthocyanins dyes (flame three flower), chlorophyll dyes (from paw paw paw leaves) and a 1:1 mixture of both, the latter having a higher conversion efficiency [73]. [PERSON] and co-workers [74] studied four pigments, one of each family, and six 1:1 mixtures. EIS measurements showed higher electron recombination lifetime for co-sensitized cells (i.e., individual dyes gave cell efficiencies of 0.11% and 0.64%, while 1:1 co-sensitized cell achieved 1.14%).  
We have also studied mixtures of different betalain dye solutions and in different proportions: orange and pink bougainvillea extracts (1:1, 1:3 and 3:1) and orange bougainvillea extract with beetroot extract 1:1 both with Solution 1. Figure 7 shows UV-Vis spectrum of the latter mixture. In general, when mixing dyes that belong to the same family, we can conclude that the mixture gives an intermediate spectrum with absorption peaks or bands for the same wavelengths as the individual dyes, but with less absorption, as there is less concentration of each individual component. Regarding cell performance, we do not obtain the synergistic effect of the two dyes, probably because both dyes belong to the same family, and thus compete for a place to attach to the semiconductor. Scarce works report an intermediate efficiency for the co-sensitized cell. For example, when mixing anthocyanins from different sources, an efficiency of the co-sensitized cell below the average of individual cell is obtained [76]. This paper led us to test whether co-sensitized cells mixing dyes from betalains family behaved similar  
Figure 6: Absorption spectrum of TiO\({}_{2}\) film (green line 1) and pink bougainvillea extract adsorbed onto TiO\({}_{2}\) film (pink line 2). Inset: main peak of adsorbed dye onto TiO\({}_{2}\) film after removing film contribution (2-1, blue line) compared to pink bougainvillea extract solution (magenta line).  
to co-sensitized cells with anthocyanins or achieved a higher efficiency. In our case, efficiency of the co-sensitized cells mixing betalain dyes is bit more than the average of the single sensitized cells efficiency for the example in Figure 7. (see Table 4). The publications that report a notable increase in efficiency mix different dye families, i.e., betalain (beetroot) & chlorophyll (spinach) [64], chlorophyll & anthocyanins [72; 73; 75] or chlorophyll & betacyanins [74].