research article a study of mixed vegetable dyes with...

Research ArticleA Study of Mixed Vegetable Dyes with DifferentExtraction Concentrations for Use as a Sensitizer forDye-Sensitized Solar Cells

Kun-Ching Cho,1 Ho Chang,2,3 Chih-Hao Chen,4,5 Mu-Jung Kao,6 and Xuan-Rong Lai1

1 Green Energy and Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan2Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 10608, Taiwan3Department of Mechanical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan4Department of Thoracic Surgery, Mackay Memorial Hospital, Taipei 10449, Taiwan5Department of Medicine, Mackay Medical College, New Taipei City 25245, Taiwan6Department of Vehicle Engineering, National Taipei University of Technology, Taipei 10608, Taiwan

Correspondence should be addressed to Ho Chang; [email protected] and Chih-Hao Chen; [email protected]

Received 8 May 2014; Revised 21 June 2014; Accepted 22 June 2014; Published 6 July 2014

Academic Editor: Ching-Song Jwo

Copyright © 2014 Kun-Ching Cho et al.This is an open access article distributed under theCreativeCommonsAttribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Two vegetable dyes are used for the study: chlorophyll dye from sweet potato leaf extract and anthocyanin dye from extracts ofblueberry, purple cabbage, and grape. The chlorophyll and anthocyanin dyes are blended in a cocktail in equal proportions, byvolume. This study determines the effect of different extraction concentrations and different vegetable dyes on the photoelectricconversion efficiency of dye-sensitized solar cells. In order to make the electrode for the experiments, P25 TiO

2powder was coated

on the ITO conducting surface, using a medical blade, to form a thin film with a thickness of around 35 𝜇m. The experimentalresults show that the cocktail dye blended using extracts of sweet potato leaf and blueberries, in the volumetric proportion 1 : 1,at a weight concentration of 40%, using an extraction temperature of 50∘C and an extraction heating time of 10min produces thegreatest photoelectric conversion efficiency (𝜂) of up to 1.57%, an open-circuit voltage (𝑉OC) of 0.61 V, and a short-circuit currentdensity (𝐽SC) of 4.75mA/cm2.

1. Introduction

Of the various renewable energy sources, wind power andsolar energy have the greatest potential. Solar energy causesno public harm and can be used by anybody, withoutlimits. Therefore, many countries are actively endeavoring todevelop solar energy [1]. Dye-sensitized solar cells (DSSC)are increasingly used in conventional inorganic solid solarcells. TiO

2is the most promising electrode material for

DSSCs because of its wide band gaps, which are suitable forinterfacial electron transport. In the early 1990s, Gratzel et al.developed a new type of solar cell that absorbs incident solarlight in the visible light wavelength zone of the dyewith a highsurface area of titanium dioxide (TiO

2). The dye-sensitized

TiO2solar 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 Rucompound 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 severaldrawbacks.

Many inorganic, organic, and hybrid dyes have beenemployed as sensitizers [5–7]. Because ruthenium dyes,including N719 and N3, are very expensive and environmen-tally toxic, numerous metal-free organic dyes have been usedin DSSCs [8, 9]. Recently, several natural organic dyes, suchas anthocyanin, chlorophyll, tannin, and carotene, extractedfrom various plants, fruits, flowers, and leaves, have beensuccessfully used as sensitizers in DSSCs [10–16]. Hao etal. extracted photosensitizer from the plants of black rice,capsicum, Erythrina variegata flower, Rosa xanthina, andkelp to serve as natural dyes. Black rice achieves the greatest

Hindawi Publishing CorporationInternational Journal of PhotoenergyVolume 2014, Article ID 492747, 5 pageshttp://dx.doi.org/10.1155/2014/492747

2 International Journal of Photoenergy

photoelectric conversion efficiency of 0.327% [17]. Wongcha-ree et al. extracted natural pigments from rosella and bluepea and blended these two pigment dyes in equal proportionsto determine the performance of the blended dye, whichachieved a maximum photoelectric conversion efficiency of0.37% [14]. In 2008, Calogero used the extracted fluid fromred Sicilian orange and purple eggplant as a photosensitizer.The cell using red Sicilian orange juice as the sensitizerachieved the highest photoelectric conversion efficiency, witha maximum efficiency of 0.66% [18]. Thambidurai et al.,2011, have fabricated Ixora coccinea, Mulberry, and beetrootextract sensitized ZnO nanorod-based solar cells and havereported that the efficiencies are 0.33, 0.41, and 0.28%,respectively [19]. Park et al., 2013, have fabricated yellowGardenia as natural photosensitizer and reported maximumpower conversion efficiency of 0.35% [20]. Senthil et al., 2014,have fabricated DSSCs using natural dyes extracted fromstrawberry, which showed a maximum conversion efficiencyof 0.49% [21].Thedye-sensitized solar cells (DSSC) fabricatedusing the predye treated and pure TiO

2nanoparticles sensi-

tized by natural dye extract of Lawsonia inermis seed showeda promising solar light to electron conversion efficiency of1.47% and 1%, respectively [22].

This study uses a cocktail of dyes, which are extractedfrom four different kinds of vegetable dyes, in DSSCs, inorder to reduce the cost of fabrication. The effect of differentextraction concentrations and different vegetable dyes onthe photoelectric conversion efficiency of dye-sensitized solarcells is also determined.

2. Experimental Details

Four types of single natural-dye materials were extractedfrom sweet potato leaf, blueberry, blue cabbage, grape, andeggplant. Using a cocktail blending method, chlorophylldye and anthocyanin dye were blended in the volumetricproportion 1 : 1 to form four types of cocktail dyes. Anhydrousethanol was used as the extraction solvent. They were heatedby double-container boiling with different extracts, at weightconcentrations of 10%, 20%, and 40%. The heating time foreach extract was 10 minutes and the temperature used foreach extract was 50∘C.

10 g of TiO2powder in 67mL of deionized water was

violently stirred at room temperature, for 60 minutes andultrasonically vibrated for 30 minutes. 3 g of polyoxyethylene(M.W. = 8000) was then added to the solution, which wasstirred and heated, until it was completely dissolved. Thesolution was then ultrasonically vibrated for 30 minutes, toform a white gelatinous dispersion that served as a paste.The electrolyte was produced using 0.1M of LiI, 0.05M of𝐼2, and 0.5M of 4-tert-butylpyridine. The solvent used was

acetonitrile (ACN), which served as the electrolyte for thecell.

The photoelectrodes were soaked in N719 dye for 24hours. The photoelectrodes and counter electrodes weretightly clamped to complete electrode packaging. The elec-trolyte was then added by capillary action, to reproduce com-pletely assembled DSSCs. An I-V curve analyzer (Keithley

Figure 1: The FESEM image of the morphology of the TiO2thin

films.

Figure 2: The FESEM image of the cross-section of the fabricatedTiO2thin film.

2400) was used to measure the performance of the preparedDSSC. The open-circuit voltage 𝑉OC (V), the short-circuitcurrent density 𝐽SC (mA/cm2), the fill factor (FF), and thephotoelectric conversion efficiency (𝜂%) of each DSSC werealso measured.

3. Results and Discussion

Figures 1 and 2 show the morphology and cross-section ofthe fabricated TiO

2thin film.This study uses PEG to increase

the specific area of the formed P25 TiO2film, to increase the

amount of dye absorbed and produce more electric currentand to give a more complete TiO

2thin film.

Figure 3 shows the absorption spectra for four naturaldyes. The figure shows that the maximum absorption peaksfor sweet potato leaf are at 665 and 434 nm, the ranges ofabsorption wavelength for the sweet potato leaf are 420∼500and 640∼700 nm, the maximum absorption peak for grape isat 536 nm, the ranges of the absorption wavelength for grapeare 545 nm and 470∼700 nm, the maximum absorption peakfor blueberry is at 541 nm, and the range of the absorptionwavelength for blueberry is 460∼700 nm. Figure 4 showsthe absorption spectra for three types of cocktail dyes.Figure 4 shows that the cocktail dyes increase the range of theabsorptionwavelength and allows further absorption ofmorevisible light, which increases the photoelectric conversionefficiency of a DSSSC.

International Journal of Photoenergy 3

0

1

2

3

4

5

Abso

rban

ce (a

.u.)

Wavelength (nm)

Sweet potato leavesGrape

BlueberryPurple cabbage

400 500 600 700 800

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

0

1

2

3

4

5

Abso

rban

ce (a

.u.)

Wavelength (nm)

Sweet potato leaf and grapeSweet potato leaf and blueberrySweet potato leaf and purple cabbage

400 450 500 550 600 650 700 750 800

Figure 4: The absorption spectra for extracts of the cocktail dyes.

Table 1 shows the photoelectrical parameters for a DSSCthat is sensitized with different types of chlorophyll, antho-cyanin, and cocktail dyes, at a weight concentration of 10%.The cocktail dye that is blended from extracts of sweet potatoleaf and grape in a volumetric proportion of 1 : 1 produces thegreatest photoelectric conversion efficiency of 0.17% and thehighest short-circuit current of 0.53mA/cm2. Sweet potatoleaf dye produces the highest open-circuit voltage of 0.61 V.The cocktail dye that is blended from extracts of sweet potatoleaf and grape in a volumetric proportion of 1 : 1 has thegreatest fill factor of 60%.

Figure 5 and Table 2 show the photoelectrical param-eters for a DSSC that is sensitized using different typesof chlorophyll, anthocyanin, and cocktail dyes, at a weightconcentration of 20%. Sweet potato leaf dye produces thegreatest photoelectric conversion efficiency of 0.391% and

0.0

0.5

1.0

1.5

Voltage (V)

Sweet potato leafBlueberryPurple cabbageGrape

Sweet potato leaf and blueberrySweet potato leaf and purple cabbageSweet potato leaf and grape

0.0 0.2 0.4 0.6 0.8 1.0

Curr

ent d

ensit

y (m

A/c

m2)

Figure 5: The photocurrent-voltage curves for natural dyes at aweight concentration of 20%.

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

Dye 𝑉oc (V)𝐼sc

(mA/cm2) FF (%) 𝜂 (%)

Sweet potato leaves 0.595 0.577 41 0.141Blueberries 0.61 0.488 58 0.164Purple cabbage 0.555 0.424 38 0.09Grape 0.595 0.15 52 0.046Sweet potato leaves andblueberries 0.53 0.096 39 0.02

Sweet potato leaves andpurple cabbage 0.53 0.288 56 0.085

Sweet potato leaves andgrape 0.537 0.53 60 0.17

the highest short-circuit current of 1.23mA/cm2. Grape dyeproduces the highest open-circuit voltage of 0.71 V andblueberry dye produces the greatest fill factor of 61%.

Figure 6 and Table 3 show the photoelectrical param-eters for a DSSC that is sensitized using different typesof chlorophyll, anthocyanin, and cocktail dyes, at a weightconcentration of 40%. The cocktail dye that is blended usingextracts of sweet potato leaf and blueberry in a volumetricproportion of 1 : 1 produces the highest photoelectric conver-sion efficiency of up to 1.57% and the highest short-circuitcurrent of 4.75mA/cm2. It can be seen that sweet potato leafhas higher absorption atwavelength of 420∼450 nmand650∼690 nm. In addition, the blueberries have higher absorptionat wavelength of 480∼640 nm. Furthermore, the incidentphoton-electron conversion efficiency (IPCE) test shows thatthe cocktail dye blended by extracts of sweet potato leaf andblueberries has higher IPCE value at wavelength of 470∼630 nm and 660∼700 nm. Therefore, the cocktail dye that is

4 International Journal of Photoenergy

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


(mA/cm2) FF (%) 𝜂 (%)

Sweet potato leaf 0.645 1.23 49 0.391Blueberries 0.67 0.532 61 0.218Purple cabbage 0.64 0.642 55 0.226Grape 0.71 0.503 45 0.16Sweet potato leaf andblueberries 0.69 0.42 28 0.08

Sweet potato leaves andpurple cabbage 0.645 0.534 60 0.207

Sweet potato leaf andgrape 0.675 0.508 54 0.185

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


(mA/cm2) FF (%) 𝜂 (%)

Sweet potato leaf 0.595 3.83 41 0.931Blueberries 0.575 0.305 41 0.722Purple cabbage 0.56 1.46 40 0.324Grape 0.495 0.583 46 0.134Sweet potato leaf andblueberries 0.61 4.75 53 1.57

Sweet potato leaf andpurple cabbage 0.615 0.918 46 0.257

Sweet potato leaf andgrape 0.515 1.35 37 0.256

blended using extracts of sweet potato leaf and blueberryproduces the highest photoelectric conversion efficiency. Allof the anthocyanins from purple cabbage produce the highestphotoelectric conversion efficiency of 0.722%. Sweet potatoleaf mixed with purple cabbage dye produces the highestopen-circuit voltage of 0.615V. Sweet potato leaf mixed withblueberry dye produces the greatest fill factor of 53%.

It can be seen from Tables 1–3 that the photoelectricconversion efficiency increases with increasing in the weightconcentration. This is because, after the higher weight con-centration of cocktail dyes extract is adsorbed on the surfaceof TiO

2nanoparticles, the absorption intensity is higher and

the absorption wavelength range is broader than in thoselower weight concentrations of cocktail dyes extract. In addi-tion, there is a higher interaction between TiO

2nanoparticles

and the higher weight concentration of cocktail dyes extract,giving the produced DSSCs better charge transfer perfor-mance, which clearly improved efficiency. Further, due to theaddition 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 diffusionlength and the effective electron diffusion coefficient, as wellas strengthening of the electron transfer performance insidethe DSSC.

0

2

4

6

Voltage (V)

Sweet potato leafBlueberryPurple cabbageGrape

Sweet potato leaf and blueberrySweet potato leaf and purple cabbageSweet potato leaf and grape

0.0 0.2 0.4 0.6 0.8 1.0

Curr

ent d

ensit

y (m

A/c

m2)

Figure 6: The photocurrent-voltage curves for natural dyes at aweight concentration of 40%.

4. Conclusions

The experimental results show that the cocktail dye that isblended using extracts of sweet potato leaf and blueberry ina volumetric proportion of 1 : 1, at a weight concentration of40%, extracted at a temperature of 50∘C and extracted usinga heating time of 10min produces the greatest photoelectricconversion efficiency (𝜂) of up to 1.57%, the greatest open-circuit voltage (𝑉OC) of 0.61 V, and the greatest short-circuitcurrent density (𝐽SC) of 4.75mA/cm2. Chlorophyll dye that isextracted from sweet potato leaf also produces a photoelectricconversion efficiency (𝜂) of 0.931%. Of all the anthocyanindyes, the dye that is extracted from blueberry produces thegreatest photoelectric conversion efficiency of 0.722%.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

Acknowledgment

This study was supported by the National Science Council ofTaiwan, under Project Grant no. NSC 101-2221-E-027-010.

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