## 2 Materials and Methods  
Nanocrystalline TiO\({}_{2}\) (Aeroxide@P-25, Evonik Degussa GmbH, Essen, Germany), nitric acid (65%, Panreac, Castellar del Valles, Spain), H\({}_{2}\)PtCl\({}_{6}\) (Aldrich, Merck, Darmstadt, Germany), Iodine (99.9%, Aldrich, Merck), Li (99.99%, [PERSON], Merck), Triton X-100 (Merck), 4-tert-butylpyridine (96%, [PERSON], Merck), ethanol absolute (J. T. Baker, Phillipsburg, NJ, USA) and 3-metoxypproionitrile (\(\geq\)98%, Aldrich, Merk) were used as received. H\({}_{2}\)O was purified up to \(<\)3 uS/cm by a Milli-Q system provided by Atapa (Seville, Spain). Conducting glass plates (FTO) (F-doped SnO\({}_{2}\), sheet resistance 11-13 \(\Omega\)/sq, Nippon Sheet Glass, Osaka, Japan) were used as substrates for depositing TiO\({}_{2}\) porous films after being cleaned (sonication in a 70% isopropanol solution for 10 min) and activated (fired on air at 420 \({}^{\circ}\)C for 30 min). A Scotch adhesive tape (50 um thickness) was used as the masking material for doctor-blading and a 50 um Surlyn-film from Dupont was used to seal the cell and fix the gap between both electrodes. The raw vegetable material to extract natural dyes was obtained locally and easily. Anthocyanins dyes from eggplant peels were extracted as described elsewhere [37]. Betalain dyes from the colored leaves (bracts) of a local bougainvillea plant were extracted as follows [38]. The leaves were harvested, selected, dried at 70 \({}^{\circ}\)C, and crushed to obtain a colored powder which was stored in the dark. Afterwards, 2 g of the powder was dispersed in different solutions (Table 1) and stirred for 24 h before the solid residue was eliminated by centrifugation (2 \(\times\) 15 min. 4000 rpm + 50 min. 5000 rpm). The extracted dye was stored at 4 \({}^{\circ}\)C until being used. Betalains obtained from beetroot followed the same procedure, dispersing 2 g of boiled root in the Solutions. Clean pollen (mainly flavonoids) was  
Figure 1: Evolution of the number of publications published per year obtained from literature searching using “natural dye” and “solar cell” topics (data source ISI Web of Science). used as collected, dispersing also 2 g in the solutions. Finally, 100 g of ground coffee in 1L of water was stirred at 80 \({}^{\circ}\)C for 6 h.  
Reflectance, transmittance and absorption spectra from solid materials (bougainvillea bracts) were obtained with a LICOR LI-1800 spectroradiometer. The dye extracts UV-Vis absorption spectra in dilution and onto TiO\({}_{2}\) were recorded using a high-resolution spectrophotometer HR4000 (Ocean Optics), following a systematic procedure for all measurements. Absorbed intensity data in all figures are in arbitrary units. DSSC fabrication followed the method proposed by [PERSON] and collaborators [39; 40]. In summary, commercial TiO\({}_{2}\) nanoparticles (15% wt) were stirred in ethanol (61% wt) and diluted nitric acid (pH 3-4, 24% wt) for 12 h at room temperature to obtain a paste of 150 g/l. Two drops of Triton X-100 were added to 20 ml of paste, which was stirred for a further 12 h to obtain the final paste used in this work. The electrolyte was a solution containing 0.05 M l2, 0.5 M Lil, 0.5 M 4-tert-butylpyridine in 3-metoxypropionitrile [38]. Photovoltaic experiments were performed directly under solar radiation in Almeria (Spain), working with similar light irradiation in all tests: 660 W/m\({}^{2}\) (admitting 6% error tolerance). This light source is natural, available in our geographical area, and quite in tune with the possible final applications of devices based on DSSCs. However, some disadvantages like meteorological changes, have to be taken into account. A calibrated high performance thermopile-based pyranometer (Kipp&Zonnen, CMP 11) from CIESOL (Solar Energy Research Center, UAL-PSA, Almeria, Spain) was used as a reference, measuring continuously the direct solar radiation. Digital source meters were used to measure the I-V curves without any external bias.