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Solar Energy 86 (2012) 15631575Anthocyanins and betalains as light-harvesting pigmentsfor dye-sensitized solar cells

Giuseppe Calogero a,, Jun-Ho Yumb, Alessandro Sinopoli a, Gaetano Di Marco a,Michael Gratzel b, Mohammad Khaja Nazeeruddin b,

aCNR-IPCF, Istituto per i Processi Chimico-Fisici, Viale F. Stagno DAlcontres 37, I-98158 Messina, ItalybLaboratory of Photonics and Interfaces, EPFL SB ISIC LPI, CH-1015 Lausanne, Switzerland

Received 10 November 2011; received in revised form 24 January 2012; accepted 17 February 2012Available online 19 March 2012

Communicated by: Associate Editor Frank NueschAbstract

We present the photoelectrochemical properties of dye-sensitized solar cells using natural pigments containing betalains and antho-cyanins as sensitizers. The dyes extracted from grape, mulberry, blackberry, red Sicilian orange, Sicilian prickly pear, eggplant and radic-chio have shown a monochromatic incident photon to current efficiency (IPCE) ranging from 40% to 69%. Short circuit photocurrentdensities (Jsc) up to 8.8 mA/cm

2, and open circuit voltage (Voc) ranging from 316 to 419 mV, were obtained from these natural dyesunder 100 mW/cm2 (AM 1.5) simulated sunlight. The best solar conversion efficiency of 2.06% was achieved with Sicilian prickly pearfruits extract. The influence of pH and co-absorbers on natural sensitizers, were investigated and discussed. 2012 Elsevier Ltd. All rights reserved.

Keywords: Dye-sensitized solar cells; Natural dyes; Anthocyanins; Betalains; Solar energy; Titanium oxide1. Introduction

Dye-sensitized solar cells (DSSCs) are devices for theconversion of visible light into electricity, based on sensiti-zation of wide band-gap semiconductors (ORegan andGratzel, 1991). The first high efficient nanocrystallineDSSC was pioneered by Gratzel in the early nineties withefficiency (g) exceeding 10% (Nazeeruddin et al., 1993). Atypical DSSC, as shown in Fig. 1, is assembled placing insuccession a transparent photoanode, an electrolyte solu-tion containing a redox system and a counter-electrode(CE) (Calogero et al., 2010a; Bonaccorso, 2010; Calogeroet al., 2011). Usually the photoanode consists of a film oftitanium dioxide (TiO2) nanoparticles deposited onto a0038-092X/$ - see front matter 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.solener.2012.02.018

Corresponding authors. Fax: +41 21 693 4311 (M.K. Nazeeruddin).E-mail addresses: calogero@me.cnr.it (G. Calogero), mdkhaja.nazeer

uddin@epfl.ch (M.K. Nazeeruddin).transparent conductive oxide (TCO) glass support, sensi-tized with dye (D) molecules. The electrolyte system, placedbetween the two electrodes, is usually based on the iodide/iodine redox couple (I=I3 ), although other redox media-tors have been successfully tested. The main processes, ofa DSSC are listed below:

D hv ! D 1D TiO2 ! D ecbTiO2 2aD ! D 2b2D 3I ! 2D I3 3aD ecbTiO2 ! D TiO2 3bI3 2ecatalyst ! 3I 4aI3 2ecbTiO2 ! 3 I TiO2 4bThe dye, upon absorption of a photon (hv), goes to an elec-tronically excited state D (Eq. (1)) which lies energeticallyabove the conduction band (CB) edge of the semiconductor

http://dx.doi.org/10.1016/j.solener.2012.02.018mailto:calogero@me.cnr.itmailto:mdkhaja.nazeeruddin@epfl.chmailto:mdkhaja.nazeeruddin@epfl.chhttp://dx.doi.org/10.1016/j.solener.2012.02.018

Fig. 1. Scheme of a natural dyes sensitized solar cell.

1564 G. Calogero et al. / Solar Energy 86 (2012) 15631575nanoparticles (Eq. (2a)) and injects an electron into theTiO2 conduction band. The deactivation reaction (Eq.(2b)) is a relaxation of the excited states, which occurs incompetition with the electron injection into the TiO2. Thecollection efficiency of the photo-injected electrons at theanode back contact is hindered by two major recombina-tion processes, which are shown in Eq. (3b) (back electrontransfer) and in Eq. (4b) (the TiO2 conduction band elec-tron capture by the oxidized redox couple). These two pro-cesses are in competition with the oxidation of iodide (Eq.(3a)) and tend to reduce the current production of the cell.In the external circuit, the injected electrons give a currentflow and provide for the reduction of iodine at the CE (Eq.(4a)). One of the most important components of the DSSCis the dye. An efficient sensitizer should absorb light over abroad range from the visible to the near-infrared and, theenergy of its electronic excited state should lie energeticallyabove the CB edge of the TiO2. Thermal and photochem-istry stability are other essential characteristics of dyes usedin DSSCs. Commonly, transition metal coordination com-pounds such as ruthenium (Nazeeruddin et al., 1993, 2001)and osmium (Kuciauskas et al., 2001; Argazzi et al., 2004;Altobello et al., 2005) polypyridil complexes and syntheticorganic dyes (Yum et al., 2007; Campbell et al., 2007), areused as effective sensitizers in DSSCs. Over the last twodecades ruthenium complexes, equipped with appropriateligands and anchoring groups, have been the most widelyused choice of charge transfer sensitizers for mesoscopicsolar cells (Nazeeruddin et al., 1993; Polo et al., 2004).However, the preparation routes for metal complexes arebased on multi step procedures involving tedious andexpensive chromatographic purification procedures(Nazeeruddin et al., 1993). Recently the interest in organicdyes is raised. Their solar to electric power conversion effi-ciencies (g), have been sharply increased reaching 9.5% forthe indoline dye D205 developed by Uchida (Ito et al.,2008) and 9.8% for the dye C217 developed by Zhanget al., 2009, which are promising if compared to 11.1% ob-tained with DSSCs based on ruthenium complexes (Nazee-ruddin et al., 2005). Other ruthenium-free sensitizers, basedon pushpull substituted porphyrins, with high solar-to-electric power conversion efficiency have been reported(Bessho et al., 2010). Recently some of the authors of thiswork has improved the efficiency of the famous Gratzel so-lar cells to 12.3% (Yella et al., 2011).

A DSSC with a ruthenium-based sensitizer has a dyeuptake (C of ca. 107 mol cm2 (layer thickness d =10 lm), which corresponds to 1 g of dye per m2. The priceof the dye per Wp (Watt peak) can be calculated using thefollowing equation:

P total C Mdye Pmass

IAM1:5 g5

where Ptotal is the price per Wp ( W1), C is the surface

coverage above described, Mdye is molecular mass of thedye (g mol1), Pmass is the price per mass ( g

1) while

G. Calogero et al. / Solar Energy 86 (2012) 15631575 1565IAM1.5 is the light intensity of AM 1.5 (W m2) with g being

the efficiency of the DSSC. For ruthenium based com-pound, the Pmass is strictly dependent on the productionscale. Actually its commercial price is estimated to be300 g1 but, large scale production, could reduce it, inthe future it to 30 g1 resulting in a Ptotal of 0.8 perWp. So the cost of the dye for a DSSC based panel of3 kWp will be 2400 . Even, for organic dyes, the produc-tion costs and the Ptotal are estimated similar to rutheniumdyes. Moreover, the actual production is still far to satisfylarge scale demand. Apart from cost related issues (afore-mentioned dyes affect for the 1015% the total expense ofDSSCs) (Smestad et al., 1994; Kalowekamo and Baker,2009), indirect environmental charges, due to the chemicalbyproducts disposal, need to be considered. Unlike syn-thetic dye-sensitizers, natural ones are easy to prepare,cheap, non-toxic, environmentally friendly and fully biode-gradable. Among natural pigments, three main families ofcompounds have been exploited as sensitizer in DSSCs:chlorophylls (Kay and Gratzel, 1993; Chang et al., 2010),anthocyanins (Cherepy et al., 1997; Polo and MurakamiIha, 2006) and betalains (Calogero et al., 2010b). Rawchlorophylls are not efficient sensitizers for DSSC, thus dif-ferent strategies to enhance their efficiency have been testedsuch as chemical reactions, selective purification by columnchromatography (Wang et al., 2007) and the mixing withother natural dyes (Kumara et al., 2006; Chang et al.,2010). In this context the possibility to obtain interestingphotoconversion efficiencies, using natural dyes, was per-formed (Calogero et al., 2009).

In this article we focused our attention only on anthocya-nins and betalains. Anthocyanins are a very large group ofredblue plant pigments, which naturally occurs in all higherplants. They are responsible for the coloration of flowers,fruits, fruit juice, wines, leaves, stems, bulbs, roots, etc. (Tim-berlake and Bridle, 1975). Themain sources of anthocyaninsare somevegetables such as blackberries, grapes, blueberries,eggplant (aubergine), red cabbage, red radicchio, redoranges, elderberry, red onion, black rice, mango and purplecorn. The production of anthocyanins in nature is estimatedto be 109 tonnes/year (Takeoka and Dao, 2002; Timber-lake and Henry, 1988). Over 500 different types of anthocy-anins havebeen isolated fromplants and all are derivatives ofthe single basic core structure of 2-phenylbenzopyrylium(flavylium) ion, shown in Fig. 2. Betalains are a class of redand yellow indole-derived pigments (see Fig. 3), which arefound in plants of the orderCaryophyllales. To date, the bet-alains comprise a quite modest number of about 55 struc-tures. The most promising source of betalains is the pricklypear fruit, a member of the family Cactaceae, which origi-nates from Mexico and spread widely throughout LatinAmerica, South Africa andMediterranean area. Sicily rankssecond among all countries in the world for producing andexporting prickly pear fruits. Like the anthocyanins, the bet-alains show light absorption in the visible region and areantioxidant compounds (Stintzing and Carle, 2007; Aze-redo, 2009); conversely to the anthocyanins, that presentfunctional groups (AOH), betalains have the requisite func-tional groups (ACOOH) to bind better to the TiO2 nanopar-ticles (Calogero et al., 2010b; Quin and Clark, 2007; Zhanget al., 2008), than the functional groups (AOH) presents inthe formers. Indeed the interaction between TiO2 film andcarboxylic functions should bring to a stronger electroniccoupling and rapid forward and reverse electron transferreactions. In this work we present the results obtained usingnatural photosensitizers such as wild Sicilian prickly pear,blackberry, red Sicilian orange, mulberry, radicchio, egg-plant and giacche grape (see Fig. 4). Their sensitization activ-ities are fully described and the results compared with thatobtained with other natural pigments. Results on stabilitytests, essential for the successfully use of natural dyes inDSSCs, are also presented.

2. Experimental

2.1. Preparation of dye-sensitizers

The synthetic dye di-tetrabutylammonium cis-bis(isothi-ocyanato)bis(2,20-bipyridyl-4,40-dicarboxylato) ruthe-nium(II), called N719, was prepared following theprocedure described elsewhere (Nazeeruddin et al., 1999).Briefly, a solution of 5.5 104 M of N719 was preparedin ethanol and serves as standard reference. All the naturaldyes except for the eggplant, radicchio and red Sicilianorange, were prepared by the following steps: fresh fruitswere crushed using a mortar, then solid residues were fil-trated out. Eggplant and radicchio dyes were extractedfrom the peels and the leaves, respectively, according thefollowing procedures: first the vegetable materials wereimmersed in ethanol solution to remove the chlorophylland successively in a dilute solution of HCl (0.1 M)(Todaro et al., 2009). The resulting solutions, accordingto a patent reported elsewhere (Calogero and Di Marco,2010), were filtered with the addition of some naturalorganic co-absorbers and then stored at pH < 2.0 at 5 Cwith the exception of the Sicilian Prickly pear juice thatwas stored at pH 5.5 (Castellar et al., 2003). The processfor obtaining extracts of the sensitizing dye compoundsfrom vegetal products, was made with the aim of maximiz-ing the amount of extracted dye and to preserve theirchemical properties. The use of acids and co-adsorbers,such as HCl or carboxylic acids, considerably increasesthe sensitizing activities of the studied natural dyes. Usu-ally we added tartaric acid, in powder form to the juicesolution until the pH of the final solution is less than 2.Alternatively to the method previously described, only toextract the sensitizing dye compounds from the aubergineskins, we used the technique named Calorio bis thathas been developed by Calogero and Di Marco, as ahome-made ecologic method (Calogero and Di Marco,2010). According to this new procedure, the aubergineskins (20 g) were putted in a metallic container, with250 g of distilled water; to the mixture, the juice of a lemonfor each 20 g of skins and about 5 g of sodium bicarbonate

Fig. 2. The flavylium ion, the basic structure of some anthocyanins.

Fig. 3. General structure of betalain (A) and indixanthin (B).

1566 G. Calogero et al. / Solar Energy 86 (2012) 15631575(NaHCO3) is added. Then, it is heated at a temperatureranging between 40 C and 60 C, for a period of timeranging between 30 and 120 min, preferably 60 min, untilthe appearance of a violet coloration. For the case of redSicilian orange juice we used the fresh solution by squeez-ing the fruit, although for some experiments we used acid-ified and/or concentrated extracted. In the following of thepaper, the acid juice solution is indicated with the symbolH+ added to the name. All the investigated dyes were pro-tected from direct sunlight exposition and resulted to bestable for more than 1 year.2.2. Preparation of electrodes

Fluorine-doped tin oxide (FTO) deposited on glass isusually used both at the photoanode, as support for thedye-sensitized oxide and at the CE. Indeed, it allows lighttransmission while providing suitable conductivity for cur-rent collection. Photoelectrodes, used in this work, con-sisted of a multi-layer TiO2 film deposited onto the FTOglass. FTO glass plates (Nippon Sheet Glass, Solar, 4 mmthick) were cleaned in a detergent solution using an ultra-sonic bath for 15 min, rinsed with water and ethanol. The

Fig. 4. Picture of the investigated natural dyes sources.

G. Calogero et al. / Solar Energy 86 (2012) 15631575 1567FTO glass plates were immersed in aqueous 40 mM TiCl4at 70 C for 30 min and washed with water and ethanol.A paste (paste A) was prepared as described in literature(Wang et al., 2003), for the transparent nanocrystalline-TiO2 layer, a FTO glass plates was coated by screen print-ing and then dried for 6 min. at 125 C. This coatingdry-ing procedure was repeated to increase the thickness from2.6 (one time) to 9.3 lm (four times), as required. Thethickness of the nanocrystalline TiO2 layer was separatelymeasured, after sintering without scattering layers, byusing a surface profiler. After drying a nanocrystallineTiO2 layer at 125 C, a paste for the scattering layer con-taining 400 nm sized anatase particles (CCIC, HPW-400,paste B) (Wang et al., 2003), was deposited by screen print-ing. One printing application resulted in a 4 lm thick layerof paste B. The electrodes coated with the TiO2 pastes weregradually heated under an air flow at 325 C for 5 min., at375 C for 5 min, at 450 C for 15 min., and at 500 C for15 min. The TiO2 films were treated again with aqueousTiCl4 and sintered at 500 C for 30 min. All the photoa-nodes were obtained for adsorption of TiO2 films in sensi-tizer solutions, 28 h for natural dyes and 18 h for theartificial one. The CE was prepared depositing nanosizedplatinum metal clusters onto FTO glass using a precursorcomposed of 5 mM solution of hexachloroplatinic (IV)acid hexahydrate in anhydrous isopropanol. About 510 lL/cm2 of the precursor solution was spread ontoFTO glass and then left to dry in air for 15 min atFig. 5. Scheme of a closed dye-sensitized solar cell.400 C. This method produces optically transparent elec-trodes (Papageorgiou et al., 1997). The final DSSC (seeFig. 5) was assembled sealing the photoanode with theCE having a pinhole through the glass, needed for theinjection of the liquid electrolyte. The sealing polymer usedis called Surlyn, a plastic foil of 25 lm cut as an O-ring.The electrolyte was then injected through the small holeof the CE with a pump that created a vacuum in the cell.This technique permits to the electrolyte to fill completelythe gap, created by Surlyn, between the two electrodes.Finally, the pinhole was sealed with another full roundpiece of Surlyn, covered by a small piece of glass toenhance the mechanical strength.

2.3. Preparation of electrolytes

Electrolytes were prepared by dissolving/mixing differentcomponents (see Table 1 for detailed composition) in sol-vents such as acetonitrile (AN), valeronitrile (VN), 3-meth-oxyproprionitrile (MPN). Based on the volatility of thesolvent, electrolytes were classified as volatile (AN:VN sol-vent) or nonvolatile (MPN solvent). Generally, apart fromthe redox couples, Guanidinium thiocyanate (GuNCS)and/or 4-tert-butyl-pyridine (TBP) or pyridine (PY) wasadded with the aim to increase the Voc of the cell.

2.4. Measurements

For photovoltaic measurements of the DSSCs, the irradi-ation source was a 450 W xenon light source (Osram XBO450, USA) equipped with a filter (Schott 113) to eliminateUV radiation and to provide a good transmittance in the vis-ible range. The lamp power was regulated to the AM 1.5Gsolar standard by using a reference Si photodiode equippedwith a color matched filter (KG-3, Schott) in order to reducethe mismatch, in the region of 350750 nm, between the sim-ulated light and AM 1.5G to less than 4%. The incident pho-ton-to-current conversion efficiency (IPCE) was plotted as afunction of excitation wavelength by using the incident lightfrom a 300 W xenon lamp (ILC Technology, USA), whichwas focused through a Gemini-180 double monochromator(Jobin Yvon Ltd.). The measurement delay time of photo JV characteristics of DSSCs was fixed to 40 ms. In order toreduce the scattered light from the edge of the dyed TiO2layer, a light shading mask was used onto the DSSC. Unlessotherwise specified A6979 was used as electrolyte for the IVand IPCE experiments. The active area of the DSSC wasfixed to 0.2 cm2.

3. Results and discussion

3.1. Absorption spectra of the natural photosensitizers

As mentioned sensitizers for DSSCs, need to fulfil moststringent requirements such as absorb in the visible andnear-infrared regions of the solar spectrum, and stronglybind to the semiconductor oxide surface (Cherepy et al.,

Table 1Electrolyte composition.a

Electrolyte Solvent Composition Additive

A6979 AN:VN (85:15) BMII 0.6 M, LiI 0.1 M, I2 0.05 M A6141 AN:VN (85:15) BMII 0.6 M, I2 0.05 M, TBP 0.5 M

GuNCS 0.1 MG1 MPN LiI 0.7 M, I2 0.07 M G2 MPN LiI 0.7 M, I2 0.07 M PY 0.25 M

a Where AN is acetonitrile, VN is valeronitrile, MPN 3-methoxyproprionitrile, BMII is methyl_benzimidazolium iodide, TBP is 4-tert-butylpyridine, PYis pyridine.

Fig. 6. Absorption spectra of anthocyanins dyes onto TiO2 film: red mulberry (purple dashed dot line), eggplant (blue dot line), red orange extract (reddashed line), radicchio (black dashed line), blackberry (orange dashed dot line), giacche (green line).

1568 G. Calogero et al. / Solar Energy 86 (2012) 156315751997). Moreover, the lowest unoccupied molecular orbital(LUMO) of the dye should lay at higher energy level thanthe semiconductor so that, upon excitation, the dye shouldinject electrons into the conduction band of the TiO2 withhigh quantum yield. The highest occupied molecular orbi-tal (HOMO) energy level of the dye needs to be more posi-tive than the redox potential of the couple (I=I3 ), indeed,if the difference between HOMO of the dye and redoxpotential of the electrolyte mediator is small, a weak driv-ing force lead to a shorter effective electron-diffusion and toa slow dye reduction rate, resulting in a low current densityproduced by the DSSC (Ardo and Meyer, 2009). In natureit is not easy found a pigment which posses all the abovecited photophysical and photochemical properties. Gener-ally, anthocyanins show a broad absorption band in thevisible region due to charge transfer transitions fromHOMO to LUMO (Cherepy et al., 1997; Liu, 2008). Asreported in the literature (Cherepy et al., 1997; Wongcha-ree et al., 2007; Dai and Rabani, 2001; Dai and Rabani,2002a,b) the absorption band of the anthocyanin is sensi-tive to pH and solvent, showing the red flavylium form inacidic solution and the purple deprotonated quinonodialform as increasing the pH. The visible absorption bandalso shifts to lower energy upon complexation with metalions. Adsorption of anthocyanidins onto the TiO2 surfaceforms strong complexes showing prevalently the quinono-dial form, which arises from AOH (or @O) groups andTi(IV) sites on the semiconductor nanocrystalline layer(Kay and Gratzel, 1993; Cherepy et al., 1997; Smestad,1998; Tennakone et al., 1997). The strong chelation onTiO2 photoanode films is evidenced by shift in the absorp-tion maxima from 510 nm in solution to 530550 nm (seeFig. 6). The betalains, that are water soluble nitrogen-based pigments, absorb in the range 450600 nm and aredivided in betanins (Bn) and bethaxantins (Bx). Fig. 7shows the absorption spectrum of raw Sicilian pricklypears fruits juice, adsorbed onto TiO2 monolayer struc-tured film. The absorption maximum peak is at about450 nm with a tail in the longer wavelength range. Thespectrum has three contributions due to betalamic acid(430 nm), yellow indicaxanthin (470 nm) and red purplebetacyanins (536 nm), respectively. This suggests that, theyellow betaxanthin compounds (such as indicaxanthin)and the betalamic acid, are more adsorbed onto TiO2 film

Fig. 7. Absorption spectra of Sicilian prickly pear dyes onto TiO2.

Table 2IPCE% maximum values, measured at kmax for some selected dyesadsorbed on TiO2 multilayer structures (13.3 lm thick, including ascattering layer).

Dye Electrolyte kmax (nm) IPCE (%)

N719 A6979 550 89N719 A6141 550 86Sicilian prickly pear (H+) A6979 450 69Blackberry (H+) G1 550 46Blackberry (H+) A6979 550 44Red Sicilian orange concentrated juice A6979 550 42Red Sicilian orange concentrated juice G1 550 40

G. Calogero et al. / Solar Energy 86 (2012) 15631575 1569than betanin or betanidin. Using the following formulas(where A is the absorbance at the specified wavelength):

Bx 23:8 A482 7:7A536 6Bn 15:38 A536 7

It is possible to estimate, from the absorption spectrum, thebetaxanthin (Bx) and betanin (Bn) concentration in molar-ity. In the case of Sicilian prickly pear fruits the concentra-tion of [Bx] and of [Bn] are 18.7 lM and 6.9 lMrespectively; which are higher if compared to literature(Zhang et al., 2008) data of similar class of sensitizer.

3.2. Incident photon-to-electron efficiencies (IPCEs), and

current voltage characterization of natural dyes

The IPCE corresponds to the number of electrons, mea-sured as photocurrent in the external circuit, divided by themonochromatic photon flux that strikes on the cell. Othernames for IPCE include calibrated spectral response andexternal quantum efficiency (EQE). Commonly the IPCEis obtained from the following formula:

IPCE 12:4V nmphotocurrent densitylA=cm2

wavelenghtnm photon fluxW=m2 8

The IPCE can also be expressed as a function of light har-vesting (LHE(k)) and absorbed photon to current conver-sion efficiency (APCE), by:

IPCEk LHEk APCE 9

Here LHE(k) represents the fraction of photons of wave-length (k) absorbed by the dyes and is given by:LHE 110OD 10

where OD is the optical density or absorbance. The LHEdepend on extinction coefficient of the sensitizers, concen-tration of the adsorbed dyes and thickness of TiO2photoanode.

The APCE or internal quantum efficiency (IQE), can besimply calculated by dividing the IPCE by LHE(k at eachk. The APCE is also the product of the quantum yieldfor electron injection /inj), and the electron collection effi-ciency /c) as reported below:

APCEk /inj /c 11

Concerning the anthocyanins, /inj was calculated as 0.98 ina previous work (Cherepy et al., 1997), while for the beta-lains no data are available in literature. IPCE values, forsome selected dyes, have been measured at a fixed wave-length, (Table 2). The best IPCE value for natural dyes

Table 3LHE, IPCE, APCE and Uc values at the (kmax for some selected natural pigments adsorbed on transparent TiO2 film (2.6 lm thick).

Dye kmax (nm) LHE IPCE APCE Uc

Blackberry (H+) 543 0.42 0.32 0.76 0.77Red Sicilian orange concentrated juice 538 0.74 0.29 0.39 0.40Red mulberry (H+) 538 0.21 0.20 0.96 0.97Blackberry 532 0.85 0.19 0.22 0.23

Fig. 8. Currentvoltage characteristics curves of DSSCs sensitized withred mulberry natural pigments (black line), red Sicilian orange concen-trated juice (blue line), blackberry (green line) and Sicilian prickly pear(red line). The photoanode, characterized by multilayer TiO2 structures, is13.3 lm thick. The CE is Pt/FTO optically transparent. A6979 as electrontransfer mediator and power intensity of 100 mW/cm2 of a Xe lamp assimulated solar source, are used.

1570 G. Calogero et al. / Solar Energy 86 (2012) 15631575was obtained from Sicilian prickly pear (69%). Data re-ported from Table 2 demonstrated that the nature of thedye and its preparation, seems to be the key factor forthe IPCE values. IPCE, LHE, APCE and /c have been cal-culated applying the Eqs. (8)(11) to the experimental datameasured for the natural dyes, as reported in Table 3. Theemployment of a thinner single layer TiO2 nanocrystallinefilm (2.6 lm) as photoanode as been necessary. In fact torespect the limit of LambertBeer law, avoid absorbancevalues out of the reasonable range it is required. As canbeen observed comparing the data reported in Table 3the red Sicilian orange concentrated juice shows low /c (de-spite high value of LHE), when compared to blackberry(H+) or red mulberry (H+). The /c is very high confirmingthat the process reported in the patent (Calogero and DiMarco, 2008), which contemporary employ co-adsorbatesand acid, it is important to block surface trap sites limitingtriiodide contact with the semiconductor surface as well asinhibiting dye aggregation. Table 4 summarizes the photo-electrochemical parameters for the natural dyes, adsorbedonto multi layer TiO2 film (9.3 lm). The analysis of thesedata permits us to select the best four sensitizers, in termsof efficiency. Photoanodes with TiO2 multi layer structure(13.3 lm) with on the top a scattering layer, employingsome selected dyes (Sicilian prickly pear, blackberry, redSicilian orange and red mulberry), have been prepared.The corresponding IV curves, for the selected sensitizers,are shown in Fig. 8. The best g was obtained by Sicilianprickly pear dyes, showing a Voc and Jsc values of0.389 V and 8.82 mA/cm2 respectively. In Fig. 9 are shownthe photoaction spectra of the best selected natural dyes.The IPCE spectra (from the blue region of the spectrumto 630 nm) of the Sicilian prickly pear dye showed clearlya higher IPCE value on the ordinate axis when comparedto the other dyes (based on anthocyanins). On the otherhand, in the red part of the spectrum (>630 nm), theTable 4Photoelectrochemical parameters of DSSCs based on natural dye sensitizers, a

Dye Jsc (mA/cm2) Jmax (mA/cm

2)

Sicilian prickly pear (H+) 7.85 7.0Blackberry (H+) 5.85 9Red Sicilian orange extract 5.13 4.5Red mulberry (H+) 4.45 4.0Radicchio (H+) 5.05 4.3Red Sicilian orange (H+) 4.98 3.8Eggplant (H+) 3.48 2.9Giacche grapes (H+) 3.06 2.4anthocyanins show slightly better IPCE value on the ordi-nate axis in comparison with Sicilian prickly pear. Thiscould be ascribed mainly to their red shifted absorptionproperties. The higher IPCE value, obtained with the Sicil-ian prickly pear sensitization, is also reflected in an overallincrease in Jsc compared to the other dyes. In the followingsections the results obtained with the selected sensitizerswill be discussed in greater detail, as well as how the addi-tion of pyridine and 4-tert-butylpyridine, to the liquid elec-trolyte, will affect the final photo-electrochemicalparameters of the DSSCs.dsorbed on TiO2 multilayer structures (9.3 lm thick).

Vmax (mV) Voc (mV) FF g (%)

267 382 0.62 1.87216 320 0.57 1.07224 329 0.59 1.01243 340 0.64 0.99212 322 0.55 0.90207 325 0.48 0.78221 346 0.53 0.64240 333 0.56 0.57

Fig. 9. IPCE spectra of DSSCs using A6979, nanorized Pt/FTO as CEand TiO2 film sensitized with blackberry extract (green line), red Sicilianorange concentrated juice (blue line) and Sicilian prickly pear (red line).

G. Calogero et al. / Solar Energy 86 (2012) 15631575 15713.2.1. Blackberry dye

The ultrafast charge injection processes from blackberryanthocyanins into the conduction band of the TiO2 semi-conductor, has been demonstrated (Cherepy et al., 1997).This behavior is consistent with the strong electronic cou-pling occurring through catechol moiety, which results ina charge transfer interaction between the cyanin and theTi(IV). Despite very fast charge injection, from blackberryanthocyanins the IPCE and the power conversion efficiencyis quite low (19% and 0.5% respectively), due to dye aggre-gation. Table 5 reports the IV parameters of DSSCs basedon photoanode prepared using the methods reported inSection 2.2. In particular the IV parameters, obtainedby DSSCs equipped with photoanodes of two differentthicknesses, were compared with that reported in literature(Cherepy et al., 1997). Clearly, as the photoanode thicknessincrease, a higher Jsc is obtained. Comparing the data ofthis work, obtained using a multilayer photoanode13.3 lm thick with a similar of 12 lm prepared in a recentprevious patent application (Calogero and Di Marco,2010), we found that the presence of the scattering layerand of the blocking layer increase g of 28% For the dyepreparation we used the same procedure developed andpatented (Calogero and Di Marco, 2010). From the dataTable 5Performance of DSSCs based on blackberry sensitizers presenting different an

Dye Jsc (mA/cm2) Voc (mV) FF

Raw Blackberry (H+) 6.52 316 0.55Raw Blackberry (H+) 5.85 320 0.57Raw Blackberry (H+) 4.68 350 0.54Raw Blackberry 1.57 400 0.67Purified Blackberry 2.20 400 0.63

a A scattering layer (4 lm thick) was employed.reported in Table 5 it is clear how the combination of opti-mized natural sensitizers, and high performance nanostruc-tured photoanode, prepared at EPFL, permitted us tosurpass the state of the art performances, both in term ofJsc and g. The obtained results have been ascribed to thecombination of three important factors: (i) the decreaseof the pH dye-solutions increases the stability of the flavy-lium ion present in the anthocyanin (Calogero et al., 2009;Calogero and Di Marco, 2008; Liu, 2008; Dai and Rabani,2002a,b; Wongcharee et al., 2007), (ii) the introduction ofco-adsorbents in the non-purified sensitizer solutions thatprevents dye aggregation on the nanocrystalline film (Cal-ogero and Di Marco, 2010), (iii) the use of a compact thinTiO2 underlayer minimize dark current effect. Concerningthe IPCE measurements, the best value (46%) was reachedfor the blackberry dye (see Fig. 9). Its good to point outhow obtained value is approaching the theoretical limitof 69% using the following equation and considering/c = 1

IPCE LHE Kinj=Kinj Kr Knr Kb 12

where Kinj 1013 s1 is the injection constant,Kr = 3.85 107 s1 is the radiative recombination con-stant, Knr = 3.85 1011 s1 is the non-radiative recombina-tion constant and Kb = 1.9 1012 s1 is the back-electron-transfer constant reported in literature (Cherepy et al.,1997).3.2.2. Red Sicilian orange dye

Although red Sicilian orange juice was already investi-gated (Calogero and Di Marco, 2008) as sensitizer forDSSCs, here we examined the pH effect and concentrationof the dye for the performance of the DSSC. As for the caseof many natural dyes, the acid treatment (pH = 1.0)induces a red shift in the absorption of the anthocyaninson the order of tens of nanometers. Moreover, the acidtreatment stabilizes the flavylium form and increases themolar extinction coefficient e (Fossen et al., 1998). Further-more, the acidified solution containing the dye affect theconduction band edge of the titanium dioxide accordingEq. (14). So the anthocyanins contained in the red Sicilianorange juice when at pH < 2.0 absorb more light andbecause of this the LHE increase, furthermore because ofthe lowering of the band edge (Ecb) of TiO2 the electroninjection (Eq. (2a)), depending from the gap between theEcb of TiO2 and the LUMO level of the dye, become moreode thickness.

g (%) Anode thickness (lm) Reference

1.13 13.3a This work1.07 9.3 This work0.88 12 Calogero and Di Marco (2010)0.42 12 Calogero and Di Marco (2010)0.55 12 Cherepy et al. (1997)

1572 G. Calogero et al. / Solar Energy 86 (2012) 15631575efficient and faster and the sum of all these effects increasethe Jsc of the DSSC. The use of concentrated red Sicilianorange extract instead of red Sicilian orange acidified juiceincrease the g from 0.78% to 1.01% (see Table 6) probablybecause the DSSCs equipped with photoanodes sensitizedwith concentrated juice contain an high quantity of antho-cyanins (1.38% of cyanin, at is natural pH). On the otherhand, the Voc values remain almost the same (see Table 6).We obtained an improvement of the Jsc (12%), by assem-bling DSSCs which make use of scattering layers. How-ever, this improvement is not reflected in a consequentincrease of g. This is essentially due to the reduction inthe fill factor value.3.2.3. Eggplant dye

The investigation of the eggplant is a crucial pointtowards a future successful use of natural dyes in DSSCs.As previous reported in a recent work (Calogero and DiMarco, 2008), eggplant skin extract containing mainly nas-unin could be potentially employed as sensitizer in DSSCs.In fact, the eggplant skin extract has shown high absorptionproperties in the visible region (kmax 545 nm). Consequently,the DSSCs based on eggplant skin extract dye have a highIPCE maximum value (65%) at 540 nm. Furthermore, thepresence of threeOHgroups in the nasunin promote the che-lating effect towards TiO2, while the presence of glycosidegroups in 3 and 5 resulted in a strong steric hindrance whichreduced the probability to link the nasunin with TiO2 usingOHgroups different from30, 40 and 50.Despite of amaximumIPCE value of 65%, the DSSCs equipped with eggplantextract showa g of0.7%and Jsc of 3.4 mA/cm2, as reportedin Table 4. Similar values, in terms of g and Jsc have beenachieved in literature (Calogero et al., 2009). All the cya-nin-based DSSCs with exception of the Giacche grape haveshown IV parameter higher than the corresponding onesbased onnasunin.This canbe explained taking into accountsthe dye regenerations kinetic (Eq. (3a)) which, depending onthe difference between redox potential of iodide and dye,occurs more quickly for cyanin than for nasunin due to amore efficient antioxidant activity of the latter (Noda et al.,2000).

In general the anthocyanins and in particular the nasun-in, due to the slow kinetic regeneration process (Eq. (3a))under intense irradiation, led to an accumulation of thedye oxidized form (D+) onto TiO2 surface, which acceler-ates the recombination process (Eq. (3b)), increasing therecombination current (Jrec). The latter is expressed bythe following formula:Table 6Performance of DSSCs based on red Sicilian orange sensitizers with different

Dye Jsc (mA/cm2) Voc (mV) FF

Red Sicilian orange extract 5.75 337 0.51Red Sicilian orange extract 5.13 329 0.59Red Sicilian orange (H+) 4.98 325 0.48Red Sicilian orange 3.84 340 0.50

a A scattering layer (4 lm thick) was employed.J rec K1nI3 m K2nD 13

with K1 and K2 being the kinetic constant of Eqs. (4b) and(3b), respectively, while n is the electron concentration inTiO2 and m is the reaction order. An elevated Jrec value,usually determines a corresponding decrease in Jsc andVoc (Koelsch et al., 2004; Lee et al., 2009). Furthermore,the absorption process rate (Eq. (1)) is decreased becausemost of D are in the D+ form and not absorb the lightand this also decreases Jsc. This fully explains the highIPCE results, which are performed with a monochromaticlight at low intensity.

3.2.4. Purple wild Sicilian prickly pear dyeWild Sicilian prickly pear fruits dyes have shown inter-

esting g, when employed as sensitizers in DSSCs. The pres-ence of carboxylic groups and an oxidation potential,similar to that of ruthenium polypyridyl complexes, pre-sents mainly two advantages: a better anchoring betweenthe dye and the TiO2 surface and an efficient dye regenera-tion by iodine/iodide redox couple. For this study havebeen selected a typical wild specie that spontaneously growin Sicily (opuntia vulgaris). This specie contains a high con-centration of betaxanthin (Fernandez-Lopez et al., 2010).Since the betaxantin sensitized activity is higher than beta-cyanin (Zhang et al., 2008), a g over 2% (see Table 7) hasbeen obtained using the wild Sicilian prickly pear specie.To our knowledge, this is the highest value ever achievedwith natural dye sensitizers used without any purificationprocess. Indeed, our result improves the state of art witha corresponding IPCE value of 69% (see Table 2). Prelimin-ary tests demonstrated that our natural dyes are stable forlong period (Calogero et al., 2010b; Calogero and DiMarco, 2010). In particular stability tests performed onsealed cells (active area 1 cm2) have shown no decrease inefficiency under 20 h of illumination at 1 sun. Unfortu-nately, the most important problem was the sealant longev-ity due to solvent evaporation. However these results seemto indicate the wild Sicilian prickly pear as the most prom-ising natural dyes for DSSCs.

3.2.5. Pyridine and 4-tert-butylpyridine case

It is well known that a significant improvement of solarcell performance can be obtained, by the addition of cer-tain pyridine-based compounds to the electrolyte. Forexample, the addition of TBP to a DSSC results inincreases of the Voc on the order of 100260 mV (Boschlooet al., 2006). This effect is principally due to the shift of theconduction band edge of TiO2 to the negative direction, asanode thickness.

g (%) Anode thickness (lm) Reference

1.00 13.3a This work1.01 9.3 This work0.78 9.3 This work0.66 12 Calogero and Di Marco (2008)

Table 7Performance of DSSCs based on Sicilian prickly pear sensitizers different anode thickness were employed.

Dye Jsc (mA/cm2) Voc (mV) FF g (%) Anode Thickness (lm) Reference

Sicilian prickly pear (H+) 8.80 389 0.60 2.06 13.3a This workSicilian prickly pear (H+) 7.85 382 0.62 1.87 9.3 This workSicilian prickly pear (H+) 7.32 400 0.41 1.21 12 Calogero et al. (2010b)

a A scattering layer (4 lm thick) was employed.

Table 8TBP effect on photoelectrochemical parameters related to DSSCs sensitized with artificial dye (N719).

Dye Jsc (mA/cm2) Voc (mV) FF g (%) Anode thickness (lm) Electrolyte

N719 16.2 776 0.73 9.12 13.3a A6141N719 17.8 590 0.61 6.37 13.3a A6979

a A scattering layer (4 lm thick) was employed.

G. Calogero et al. / Solar Energy 86 (2012) 15631575 1573consequence of TBP addition. Furthermore, the adsorptionof TBP onto TiO2 determines a dark current suppression,due to covering nanoparticles free space. The dark currentis originated by the reduction of I3 at the photoanode, giv-ing rise an electrons flow opposite to the photocurrentdirection. This photovoltaic effect can take place both,due to porosity of TiO2, on the semiconductor surface oron the FTO-glass one.

The dependence of conduction band edge from the pHfits the equation:

EcbpH EcbpH 0 0:059pH 14

The value of Ecb (pH = 0) for TiO2 anatase (vs SCE) is 0.4 V. The CB we estimated in presence of tert-pyridine0.50.6 eV V vs NHE. (Kalyasundaran and Gratzel,1998)Furthermore, according the following equation:

I3 TBP TBP I2 I 15

the TBP in I3 =I electrolyte solution react with I3 decreas-

ing the concentration of iodine of one order of magnitude(Boschloo et al., 2006). This is proved experimentally inDSSCs sensitized by ruthenium compounds (see Table 8).However, the addition of such compounds fails to havean impact on the DSSCs performance when natural dyesare used as sensitizer. Here is demonstrated that the useof PY (0.25 M) as additive to the electrolyte from one handresults in an increase in Voc and Vmax, but on the other onedecreases drastically the Jsc value of 70%. Taking to a lowsolar energy conversion efficiency (see Table 9). This occursbecause the PY, as well as the TBP, is a basic compound;while on the other hand, anthocyanins and betalainsare pH sensitive. The addition of basic compounds toTable 9Pyridine effect on photoelectrochemical parameters related to DSSCs sensitize

Dye Jsc (mA/cm2) Jmax (mA/cm

2)

Blackberry (H+) 7.68 5.5Blackberry (H+) 1.91 1.8Red Sicilian orange extract 5.90 4.4Red Sicilian orange extract 1.83 1.6anthocyanin dyes electrodes determines: (i) a partial de-sorption of the dyes from the TiO2, (ii) a changes of the cat-ion from stable flavilyium form in partially stable quinon-odial form (iii) a shift of the absorption spectra towards thered region, with a consequently decreasing of the molarextinction coefficient e. The effect is very important, whenPY (0.25 M) is added to the electrolyte, the resulting pHon the DSSC is 9.3, inducing a red shift of the cyaninmaximum wavelength peak and reducing more than 50%the molar extinction coefficient e. Consequently, the opticalabsorption cross section r is reduced as well as the LHE,IPCE and Jsc. Concerning the betalains, the use of TBPwas already investigated and to a modest gain in photo-voltage and in fill factor it corresponded to a dramatic dropin photocurrent. This phenomenon is probably determinedby a reducing activity of the indicaxanthin under basic con-ditions. This effect was not documented by Zhang et al.(2008) because they used betanin purified and they removethe bethaxanthin; indeed, betanin it is more robust anddoes not readily decompose following pH changes (Pedren-o and Escribano, 2001).4. Conclusion

We have considered the use of anthocyanins and beta-lains extracts as sensitizers for DSSCs application. In par-ticular, the use of raw betalains, having high concentrationof betaxantins, gives a promising result converting 2.06%of 1 sun of light power into electricity. The presence of car-boxylic groups in the betalains presents, together with thehigher oxidation potential, an advantage for anchoringd with blackberry and red Sicilian orange dyes.

Vmax (mV) Voc (mV) FF g (%) Electrolyte

208 348 0.43 1. 15 (G1)315 419 0.69 0.55 (G2)220 367 0.45 0.97 (G1)296 408 0.65 0.49 (G2)

1574 G. Calogero et al. / Solar Energy 86 (2012) 15631575the dye to the semiconductor and to interact better with theiodine/iodide redox couple. The addition of organic co-absorbers and the pH variation improved the current den-sity value, produced by DSSCs sensitized by natural dyes.The use of pyridine improved the Voc of the DSSCs sensi-tized by antocyanins but, at the same time, resulted in adecrease of Jsc. Future work will be addressed towardsthe increase in concentration of bethaxantin in betalains;new additives need to be investigated with the aim toimprove the Voc, but avoiding the drastic reduction ofJsc. We stress that the achievement of these results derives,also, from the optimization of the photoanodes, realizedemploying TiO2 multilayer structures. Finally, preliminarystability tests demonstrated that DSSCs based on betalainsare stable for 20 h. Successful test over longer time periodwill require a more severe sealing process and the develop-ment of less volatile solvent.

Acknowledgements

We acknowledge Dr. Francesco Bonaccorso for usefuldiscussion. We thank Dr. P. Rapisarda from CRA-Centrodi Ricerca per lAgrumicoltura e le Colture Mediterranee,Acireale, Italy for generously supply of red orange ex-tract. Technical assistance for realization of electrical con-nection for the prototypes was furnished by G. Lupo andG. Spinella (IPCF_CNR). This research was supportedby the CNR Short Term Mobility Program 2009, by PRIN2008 (2008ALLB79) and Dyecells (cod. 121) project fi-nanced by Ministero dellAmbiente del Territorio e dellaTutela del Mare.References

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Anthocyanins and betalains as light-harvesting pigments for dye-sensitized solar cells1 Introduction2 Experimental2.1 Preparation of dye-sensitizers2.2 Preparation of electrodes2.3 Preparation of electrolytes2.4 Measurements

3 Results and discussion3.1 Absorption spectra of the natural photosensitizers3.2 Incident photon-to-electron efficiencies (IPCEs), and current voltage characterization of natural dyes3.2.1 Blackberry dye3.2.2 Red Sicilian orange dye3.2.3 Eggplant dye3.2.4 Purple wild Sicilian prickly pear dye3.2.5 Pyridine and 4-tert-butylpyridine case

4 ConclusionAcknowledgementsReferences