A Study of Mixed Vegetable Dyes with Different Extraction Concentrations for Use as a Sensitizer for Dye-Sensitized Solar Cells

1 Green Energy and Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan 2Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 10608, Taiwan 3Department of Mechanical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan 4Department of Thoracic Surgery, Mackay Memorial Hospital, Taipei 10449, Taiwan 5Department of Medicine, Mackay Medical College, New Taipei City 25245, Taiwan 6Department of Vehicle Engineering, National Taipei University of Technology, Taipei 10608, Taiwan


Introduction
Of the various renewable energy sources, wind power and solar energy have the greatest potential.Solar energy causes no public harm and can be used by anybody, without limits.Therefore, many countries are actively endeavoring to develop solar energy [1].Dye-sensitized solar cells (DSSC) are increasingly used in conventional inorganic solid solar cells.TiO 2 is the most promising electrode material for DSSCs because of its wide band gaps, which are suitable for interfacial electron transport.In the early 1990s, Grätzel et al. developed a new type of solar cell that absorbs incident solar light in the visible light wavelength zone of the dye with a high surface area of titanium dioxide (TiO 2 ).The dye-sensitized TiO 2 solar cell can be prepared in an ordinary environment, which significantly reduces its cost [2].Currently, the most efficient dye-sensitized solar cell (DSSC), which uses Ru compound absorbed onto nanoscaleTiO 2 , has an efficiency of 11-12% [3,4].Although this type of DSSC is more efficient, its use of expensive metals, such as N3 and N719, has several drawbacks.
Many inorganic, organic, and hybrid dyes have been employed as sensitizers [5][6][7].Because ruthenium dyes, including N719 and N3, are very expensive and environmentally toxic, numerous metal-free organic dyes have been used in DSSCs [8,9].Recently, several natural organic dyes, such as anthocyanin, chlorophyll, tannin, and carotene, extracted from various plants, fruits, flowers, and leaves, have been successfully used as sensitizers in DSSCs [10][11][12][13][14][15][16].Hao et al. extracted photosensitizer from the plants of black rice, capsicum, Erythrina variegata flower, Rosa xanthina, and kelp to serve as natural dyes.Black rice achieves the greatest photoelectric conversion efficiency of 0.327% [17].Wongcharee et al. extracted natural pigments from rosella and blue pea and blended these two pigment dyes in equal proportions to determine the performance of the blended dye, which achieved a maximum photoelectric conversion efficiency of 0.37% [14].In 2008, Calogero used the extracted fluid from red Sicilian orange and purple eggplant as a photosensitizer.The cell using red Sicilian orange juice as the sensitizer achieved the highest photoelectric conversion efficiency, with a maximum efficiency of 0.66% [18].Thambidurai et al., 2011, have fabricated Ixora coccinea, Mulberry, and beetroot extract sensitized ZnO nanorod-based solar cells and have reported that the efficiencies are 0.33, 0.41, and 0.28%, respectively [19].Park et al., 2013, have fabricated yellow Gardenia as natural photosensitizer and reported maximum power conversion efficiency of 0.35% [20].Senthil et al., 2014, have fabricated DSSCs using natural dyes extracted from strawberry, which showed a maximum conversion efficiency of 0.49% [21].The dye-sensitized solar cells (DSSC) fabricated using the predye treated and pure TiO 2 nanoparticles sensitized by natural dye extract of Lawsonia inermis seed showed a promising solar light to electron conversion efficiency of 1.47% and 1%, respectively [22].
This study uses a cocktail of dyes, which are extracted from four different kinds of vegetable dyes, in DSSCs, in order to reduce the cost of fabrication.The effect of different extraction concentrations and different vegetable dyes on the photoelectric conversion efficiency of dye-sensitized solar cells is also determined.

Experimental Details
Four types of single natural-dye materials were extracted from sweet potato leaf, blueberry, blue cabbage, grape, and eggplant.Using a cocktail blending method, chlorophyll dye and anthocyanin dye were blended in the volumetric proportion 1 : 1 to form four types of cocktail dyes.Anhydrous ethanol was used as the extraction solvent.They were heated by double-container boiling with different extracts, at weight concentrations of 10%, 20%, and 40%.The heating time for each extract was 10 minutes and the temperature used for each extract was 50 ∘ C. 10 g of TiO 2 powder in 67 mL of deionized water was violently stirred at room temperature, for 60 minutes and ultrasonically vibrated for 30 minutes. 3 g of polyoxyethylene (M.W. = 8000) was then added to the solution, which was stirred and heated, until it was completely dissolved.The solution was then ultrasonically vibrated for 30 minutes, to form a white gelatinous dispersion that served as a paste.The electrolyte was produced using 0.1 M of LiI, 0.05 M of  2 , and 0.5 M of 4-tert-butylpyridine.The solvent used was acetonitrile (ACN), which served as the electrolyte for the cell.
The photoelectrodes were soaked in N719 dye for 24 hours.The photoelectrodes and counter electrodes were tightly clamped to complete electrode packaging.The electrolyte was then added by capillary action, to reproduce completely assembled DSSCs.An I-V curve analyzer (Keithley

Results and Discussion
Figures 1 and 2 show the morphology and cross-section of the fabricated TiO 2 thin film.This study uses PEG to increase the specific area of the formed P25 TiO 2 film, to increase the amount of dye absorbed and produce more electric current and to give a more complete TiO 2 thin film.
Figure 3 shows the absorption spectra for four natural dyes.The figure shows that the maximum absorption peaks for sweet potato leaf are at 665 and 434 nm, the ranges of absorption wavelength for the sweet potato leaf are 420∼500 and 640∼700 nm, the maximum absorption peak for grape is at 536 nm, the ranges of the absorption wavelength for grape are 545 nm and 470∼700 nm, the maximum absorption peak for blueberry is at 541 nm, and the range of the absorption wavelength for blueberry is 460∼700 nm. Figure 4 shows the absorption spectra for three types of cocktail dyes.Figure 4 shows that the cocktail dyes increase the range of the absorption wavelength and allows further absorption of more visible light, which increases the photoelectric conversion efficiency of a DSSSC.Table 1 shows the photoelectrical parameters for a DSSC that is sensitized with different types of chlorophyll, anthocyanin, and cocktail dyes, at a weight concentration of 10%.The cocktail dye that is blended from extracts of sweet potato leaf and grape in a volumetric proportion of 1 : 1 produces the greatest photoelectric conversion efficiency of 0.17% and the highest short-circuit current of 0.53 mA/cm 2 .Sweet potato leaf dye produces the highest open-circuit voltage of 0.61 V.The cocktail dye that is blended from extracts of sweet potato leaf and grape in a volumetric proportion of 1 : 1 has the greatest fill factor of 60%.
Figure 5 and Table 2 show the photoelectrical parameters for a DSSC that is sensitized using different types of chlorophyll, anthocyanin, and cocktail dyes, at a weight concentration of 20%.Sweet potato leaf dye produces the greatest photoelectric conversion efficiency of 0.391% and  the highest short-circuit current of 1.23 mA/cm 2 .Grape dye produces the highest open-circuit voltage of 0.71 V and blueberry dye produces the greatest fill factor of 61%. Figure 6 and Table 3 show the photoelectrical parameters for a DSSC that is sensitized using different types of chlorophyll, anthocyanin, and cocktail dyes, at a weight concentration of 40%.The cocktail dye that is blended using extracts of sweet potato leaf and blueberry in a volumetric proportion of 1 : 1 produces the highest photoelectric conversion efficiency of up to 1.57% and the highest short-circuit current of 4.75 mA/cm 2 .It can be seen that sweet potato leaf has higher absorption at wavelength of 420∼450 nm and 650∼ 690 nm.In addition, the blueberries have higher absorption at wavelength of 480∼640 nm.Furthermore, the incident photon-electron conversion efficiency (IPCE) test shows that the cocktail dye blended by extracts of sweet potato leaf and blueberries has higher IPCE value at wavelength of 470∼ 630 nm and 660∼700 nm.Therefore, the cocktail dye that is It can be seen from Tables 1-3 that the photoelectric conversion efficiency increases with increasing in the weight concentration.This is because, after the higher weight concentration of cocktail dyes extract is adsorbed on the surface of TiO 2 nanoparticles, the absorption intensity is higher and the absorption wavelength range is broader than in those lower weight concentrations of cocktail dyes extract.In addition, there is a higher interaction between TiO 2 nanoparticles and the higher weight concentration of cocktail dyes extract, giving the produced DSSCs better charge transfer performance, which clearly improved efficiency.Further, due to the addition of the weight concentration of cocktail dyes extract, the total internal resistance value in the DSSC is decreased, thus leading to an increase of both the electron diffusion length and the effective electron diffusion coefficient, as well as strengthening of the electron transfer performance inside the DSSC.

Conclusions
The experimental results show that the cocktail dye that is blended using extracts of sweet potato leaf and blueberry in a volumetric proportion of 1 : 1, at a weight concentration of 40%, extracted at a temperature of 50 ∘ C and extracted using a heating time of 10 min produces the greatest photoelectric conversion efficiency () of up to 1.57%, the greatest opencircuit voltage ( OC ) of 0.61 V, and the greatest short-circuit current density ( SC ) of 4.75 mA/cm 2 .Chlorophyll dye that is extracted from sweet potato leaf also produces a photoelectric conversion efficiency () of 0.931%.Of all the anthocyanin dyes, the dye that is extracted from blueberry produces the greatest photoelectric conversion efficiency of 0.722%.

Figure 1 :
Figure 1: The FESEM image of the morphology of the TiO 2 thin films.

Figure 2 :
Figure 2: The FESEM image of the cross-section of the fabricated TiO 2 thin film.

Figure 3 :Figure 4 :
Figure 3: The absorption spectra for extracts of sweet potato leaf, grape, blueberry, and purple cabbage, in ethanol solution.

2 )Figure 5 :
Figure 5: The photocurrent-voltage curves for natural dyes at a weight concentration of 20%.

2 )Figure 6 :
Figure 6: The photocurrent-voltage curves for natural dyes at a weight concentration of 40%.

Table 1 :
The performance of DSSCs using natural dye at a weight concentration of 10%.

Table 2 :
The performance of DSSCs using natural dyes at a weight concentration of 20%.

Table 3 :
The performance of DSSCs using natural dyes at a weight concentration of 40%.