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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy ARTICLE IN PRESS Solar Energy Materials & Solar Cells 92 (2008) 1341– 1346 Contents lists available at ScienceDirect Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat Red Sicilian orange and purple eggplant fruits as natural sensitizers for dye-sensitized solar cells Giuseppe Calogero , Gaetano Di Marco CNR, Istituto per i Processi Chimico-Fisici (Sede di Messina) Salita Sperone, C. da Papardo, I-98158 Faro Superiore Messina, Italy a r t i c l e in fo abstract Article history: Received 30 October 2007 Received in revised form 23 April 2008 Accepted 13 May 2008 Available online 20 June 2008 Dye-sensitized solar cells (DSSCs) were assembled by using red Sicilian orange juice (Citrus Sinensis) and the purple extract of eggplant peels (Solanum melongena, L.) as natural sensitizers of TiO2 films. Conversion of solar light into electricity was successfully accomplished with both fruit-based solar cells. The best solar energy conversion efficiency (Z ¼ 0.66%) was obtained by red orange juice dye that, under AM 1.5 illumination, achieved up to Jsc ¼ 3.84 mA/cm2, Voc ¼ 0.340 V and fill factor ¼ 0.50. In the case of the extract of eggplant peels, the values determined were up to Jsc ¼ 3.40 mA/cm2, Voc ¼ 0.350 V and fill factor ¼ 0.40. Cyanidine-3-glucoside (cyanine) and delphinidin 3-[4-(p-coumaroyl)-L-rhamnosyl(1–6)glucopyranoside]-5-glucopyranoside (nasunin) are the main pigments of cocktail dyes for red orange and eggplant, respectively. Actually, their application is far below the industrial requirements. Nevertheless, their study is an interesting multidisciplinary exercise useful for dissemination of knowledge and to educate people on renewable energy sources. Here, we report and discuss the role of the structure, the absorption spectra and the sensitization activity of the mentioned compounds. & 2008 Elsevier B.V. All rights reserved. Keywords: Dye-sensitized solar cell Natural photosensitizer Red Sicilian orange Eggplant Anthocyanin I3  þ 2ecb  ðTiO2 Þ ! 3I þ TiO2 1. Introduction Dye-sensitized solar cells (DSSCs) are devices for the conversion of visible light into electricity based on sensitization of wide band-gap semiconductors [1]. Commonly, the photoanode is prepared by adsorbing a dye (S) into a porous TiO2 layer. By this approach, the dye enables the generation of electricity with visible light, extending the semiconductor’s performance to collect photons at lower energy. The principal photophysical and redox reactions for DSSCs are listed below: S þ hn ! S (1) S þ TiO2 ! Sþ þ ecb  ðTiO2 Þ (2a) S ! S (2b) 2Sþ þ 3I ! 2S þ I3  (3a) Sþ þ ecb  ðTiO2 Þ ! S þ TiO2 (3b) I3  þ 2e ðcatalystÞ ! 3I (4a)  Corresponding author. Tel.: +39 090 39762; fax: +39 090 3974130. E-mail address: calogero@me.cnr.it (G. Calogero). 0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.05.007 (4b) The dye is irradiated by light (hn) and excited to the electronically excited state S* (1). This state is chosen to lie energetically above the conduction band (cb) edge of the semiconductor nanoparticles [1,2]. In this condition, the electron injection to the semiconductor (2a) can occur successfully, competing with the deactivation reaction (2b). In order to achieve a high current generation, the oxidation of iodide (3a) and the redox of iodine (4a) must effectively compete with the chargeseparated state recombination reactions (3b, 4b) that decrease the current production. The mixture of I3/I ions in organic solvents commonly serves as charge carriers. Usually, synthetic inorganic compounds such as ruthenium (II) complexes with carboxylated polypyridyl ligands are employed as molecular sensitizers (S) in DSSCs [2–4]. In order to replace the rare and expensive ruthenium compounds, many kinds of organic synthetic dyes have been actively studied and tested as low-cost materials [5–7]; recently, Hara et al. [8] made a remarkable advance in the use of organic dyes for DSSCs. Other groups have obtained good solar electric power conversion, testing natural dyes as cheap and environmentally friendly alternatives to artificial sensitizers for DSSCs [9–13]. In nature, some fruits, flowers, leaves, bacteria and so on show various colours and contain several pigments that can be easily extracted and then employed in DSSCs for either educational purposes or indoor applications. Therefore, unlike artificial dyes, the natural ones are available, easy to prepare, low in cost, non-toxic, environmentally Author's personal copy ARTICLE IN PRESS 1342 G. Calogero, G.Di. Marco / Solar Energy Materials & Solar Cells 92 (2008) 1341–1346 friendly and fully biodegradable. In most cases, their photoactivity belongs to the anthocyanin family [13–15]. The word anthocyanin is derived from two Greek words meaning plant and blue. Anthocyanins are natural compounds that give colour to fruits, vegetables and plants [16,17] and are also largely responsible for the purple-red colour of autumn leaves and for the red colour of buds and young shoots. This work presents our investigations on two natural photosensitizers, describing and comparing their sensitization activity with other natural dyes employed in DSSCs. We selected for our studies two typical fruits: the red Sicilian orange (Citrus Sinensis) and the eggplant (Solanum melongena L.). Our choice was motivated by their high anthocyanin content. The red Sicilian orange or blood orange is a variety of orange with crimson, blood-coloured flesh. The fruit is smaller than an average orange; its skin is usually pitted, but can be smooth. The ‘‘blood’’ orange is considered the hallmark of Sicilian fruits. Sicily is the largest producer in the world of red oranges and the European Union recognizes the Eastern Sicily area as a Protected Geographical Indication ‘‘Arancia Rossa di Sicilia’’. Three blood orange varieties, Tarocco, Sanguinello and Moro, can be traced to the hilly areas and plains surrounding the Etna volcano [18]. Here, this orange cultivar developed a particular pool of antioxidant compounds that protect the fruit against extreme day–night thermal excursion due to the volcanic soil. Their ruby flesh contains a high concentration of the red cyanidine-3-glucoside pigment, a strong antioxidant, which belongs to anthocyanin’s family. This pigment is the core composition of the natural dyes found in these fruits and its molecular structure is shown in Fig. 1a [19]. In addition, the fruit contains citric acid and other antioxidants, such as delphinidin-3-glucoside, flavones (hesperedin, narirutin) and hydroxycinnamic acids (caffeic, cumaric, ferulic and sinapic). The eggplant, a native of India, has been cultivated in Sicily since ancient times. It was introduced by Arabs and is well adapted to Sicilian climate. It is one of the most important vegetable crops, grown on over 1.7 million ha world-wide. Known as aubergine or brinjal, it is a plant of the family Solanaceae. It bears a fruit of the same name, commonly used in cooking. The extract from eggplant peels, rich in anthocyanins, contains nasunin, a mixture of cis– trans isomers of delphinidin 3-[4-(pcoumaroyl)-L-rhamnosyl(1–6)-glucopyranoside]-5-glucopyranoside (Fig. 1b) [20,21]. Furthermore, we selected blueberry juice to compare it to our dyes and to the literature values. 2. Experimental 2.1. Preparation of dye-sensitizer solutions The synthetic dye cis-[Ru (2,20 -bipyridyl-4,40 -dicarboxylic acid)2(NCS)2], called N3, was synthesized and purified following the procedure reported in literature [2]. An N3 standard solution was prepared dissolving 20 mg of the complex in 50 mL of ethanol. All the fresh fruits were harvested in the Eastern Sicilian countryside. The blood orange juice was prepared by squeezing fresh fruits and the resulting solution was only filtered in order to remove the pulps and some residual fragments. The eggplant’s dyes were extracted from the peels of the fresh fruit using an ethanol solution 1% in acetic acid and 2% in HCl. The blueberry juice was furnished by a local farm. All solutions were protected from direct sunlight exposition and the juices were stored in a refrigerator at about +5 1C. The eggplant extract remains stable for many months at room temperature in acid solution. On the contrary, the red orange and the blueberry juice have shown decomposition after a week, even at +5 1C. 2.2. Preparation of electrodes The conductive glass plates (FTO glass, fluorine-doped SnO2, sheet resistance 15 O/cm2), the Pt catalyst T/SP and the Tinanoxide (T) paste were purchased from Solaronix SA and used as supplied. All the solvents and the chemicals employed for the experiments were reagent or spectrophotometric grade. The photoanode was prepared by depositing TiO2 film on the FTO conducting glass. Two edges of the FTO glass plate were covered with four layers of adhesive tape (3M Magic) to control the thickness of the film and to mask electric contact strips; successively the TiO2 paste was spread uniformly on the substrate by sliding a glass rod along the tape spacer. The resulting mesoscopic oxide film was 12–14 mm thick and transparent, presenting negligible light scattering. After drying the TiO2, the covered glass plates were sintered in air for 30 min at 450 1C, cooled to about 80 1C and soaked in N3 dye solution for one night; excess dye was removed by rinsing with ethanol and finally the as-prepared anodes were dipped in 4-tert-butylpyridine (TBP)[2]. Concerning natural dyes, we use the above procedure except for the soaking and cleaning methods. Indeed, the transparent photoelectrodes were immersed into natural coloured solutions, at room temperature for 3 h, cleaned with distilled water and subsequently dried without treatment with TBP. The counter electrodes were prepared according to the two following procedures: according to the first method, the Ptcatalyst T/SP paste was spread on FTO glass and heated at 450 1C for 30 min, while in the second one a Pt mirror (350 nm thick) was obtained by thermal vapour deposition. 2.3. Preparation of electrolyte The electrolyte solution for natural DSSCs was prepared dissolving 2.075 g of KI and 0.19 g of I2 in 25 mL of an ethylene glycol/acetonitrile mixture (4:1 by volume). While for the standard N3 solar cell, the electrolyte solution was obtained by dissolving 1.673 g of LiI and 0.3172 g I2 in 25 mL of acetonitrile. 2.4. Measurements The absorption spectra were performed by a Perkin-Elmer L20 spectrophotometer UV–Vis. Solar energy conversion efficiency was measured by using a digital Keithley 236 multimeter connected to a PC and controlled by a homemade program. The photoanode and the platinum counter-electrode were assembled and clipped in a sandwich-type arrangement with the electrolyte solution placed between. Photoelectrochemical experiments for the red orange and the eggplant dyes were carried out under simulated sunlight conditions provided by a LOT-Oriel solar simulator (Model LS0100-1000, 300W Xe Arc lamp Power Supply LSN251 equipped with AM 1.5 filter, 100 mW/cm2), respectively, with 0.5 and 1 cm2 of illuminated active area. Hermetically sealed cells were used to check the long-term stability under simulated solar light. In this case, the photoanode and the Pt counter electrode were sandwiched with a 160 mm thick (before melting) surlyn polymer foil as spacer. Sealing was done by keeping the structure in a hot-press at 120 1C for a few seconds. The liquid electrolyte was introduced into the cell gap through a predrilled hole on the counter electrode. Finally, the hole was sealed with a small drop of Torr-seal paste. The simulated AM 1.5 light intensity was calibrated with an ORIEL radiant power meter equipped with an ORIEL thermopile detector. Author's personal copy ARTICLE IN PRESS 1343 G. Calogero, G.Di. Marco / Solar Energy Materials & Solar Cells 92 (2008) 1341–1346 OH OH HO O+ O OH O OH HO HO OH OH OH HO O+ OH HO O O O O HO OH OH HO OH O H 3C O O HO OH HO R OH HO HO O R= or Cis Trans O Fig. 1. Chemical structures of (a) cyanine and (b) nasunin. 3. Results and discussion 3.1. Absorption spectra of natural dyes Commonly, anthocyanins and their derivates show a broad absorption band in the range of visible light ascribed to charge transfer transitions from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) [13]. The absorption spectrum of the Red Sicilian orange juice, of the Moro variety, having natural pH 6.72, shows a deep red colour (lmax ¼ 515 nm) typical of cyanidine-3-glucoside (see Fig. 2) [13,15], while the absorption spectrum of extract of eggplant peels exhibits a deep purple tonality (lmax ¼ 522 nm) characteristic of the nasunin acid solution at pH ¼ 2.32 (see Fig. 3) [20]. The UV–Vis absorption bands of the adsorbed dyes onto semiconductor’s film are broadened and the corresponding maxima are red Author's personal copy ARTICLE IN PRESS 1344 Absorbance (a.u) G. Calogero, G.Di. Marco / Solar Energy Materials & Solar Cells 92 (2008) 1341–1346 400 a 450 500 b 550 600 650 700 750 800 Wavelength (nm) Absorbance (a.u) Fig. 2. Absorption spectra of red Sicilian orange juice, Moro variety (a) in water solution (thin line) and (b) adsorbed onto the TiO2 photoanode (bold line). 400 b a 450 500 550 600 650 Wavelength (nm) 700 750 800 Fig. 3. Absorption spectra of extract of eggplant (a) in solution (thin line) and (b) adsorbed onto the TiO2 photoanode (bold line). shifted with respect to their solution as presented in Figs. 2 and 3 [22]. As reported in the literature [10,12,13,15,24], the absorption band of the anthocyanin is pH and solvent sensitive, showing red flavylium form (TiA+H) in acidic solution and purple deprotonated quinonodial form (TiA) as pH increases. The visible absorption band also shifts to lower energy upon complexation with metal ions. Adsorption of cyanine to the semiconductor TiO2 surface is a quick reaction, forming a very strong complex showing prevalently the quinonodial form. This chemical reaction is the result of alcoholic bound protons which condense with the hydroxyl groups present at the surface of nanostructured TiO2 film [3,13–15,23] with the contribution of the chelating effect due to the two nearest hydroxyl group towards Ti(IV) sites on the semiconductor nanocrystalline layer (see Fig. 4). 3.2. Performance of the natural sensitizers in the photoelectrochemical solar cells In Table 1 are listed the photoelectrical parameters of DSSCs, under AM 1.5 simulated solar spectrum, sensitized by the natural dyes. Some considerations need to be made before discussion of the results. Concerning natural sensitizers prepared by the solvent extraction procedure, we recognized that the best sensitizer was derived from the skin of Jaboticaba (Table 1) [11]. On the contrary, considering the dyes extracted and doped with co-absorber and additives, i.e. using organic acids for the stabilization of the colorants, the best conversion was obtained by the red cabbage extract (Table 1) [9]. Finally, including natural dyes derived from chemical manipulation, some authors achieved the best sensitization activity with modified chlorophyll (Z ¼ 2.6%) [14,25,26]. So comparing the data reported in the Table 1 and in the literature, to our knowledge, we have found that the red Sicilian orange pigment (Moro) shows the highest solar energy conversion efficiency (Z ¼ 0.66%) for fruit juice prepared only by squeezing, without any of the above reported extraction or manipulation procedures (see Fig. 5). We prepared for comparison an N3 standard DSSC, achieving under the same irradiation conditions Jsc ¼ 10.94 mA/cm2, 2 Jmax ¼ 10.00 mA/cm , Vmax ¼ 0.450 V, Voc ¼ 0.660 V and ff ¼ 0.62, with a solar energy conversion efficiency of 4.5%. Furthermore, as an added control, we tested in the same experimental condition the blueberry juice as a dye, obtaining lower values (Z ¼ 0.46%, Author's personal copy ARTICLE IN PRESS 1345 G. Calogero, G.Di. Marco / Solar Energy Materials & Solar Cells 92 (2008) 1341–1346 Voc ¼ 0.325 V, Isc ¼ 2.41 mA/cm2) than those reported for the red orange juice. We achieved a better sensitization activity using red Sicilian orange juice, of the Moro variety, in comparison with strawberry or blueberry juice because of the high concentration of cyanine in the Moro juice and the natural presence of citric acid or hydroxycinnamic acids, acting as co-absorbers [9,18,26]. In fact, these compounds, filling the free space between the dye molecules, partially block the physical contact between iodine solution and TiO2 semiconductor film surface, reducing reaction (4b) and inhibiting dye aggregation [13]. In addition, encouraging results were obtained with the extract of eggplant peels (see Table 1), achieving a photocurrent Jsc of 3.40 mA/cm2 and energy conversion efficiency of 0.48%. The presence of three hydroxyl groups in the nasunin (typical of the delphidine structure) favoured the chelating effect towards O Ti (IV) O HO titanium (Ti4+). Furthermore, the presence of glycoside groups, positioned in 3 and in 5 (see Fig. 1b), results in a strong steric hindrance for an anthocyanin to form a bond with oxide surface in 7 and avoids the possibility to attach in 5. These circumstances facilitate the attachment of the Ti(IV) to the hydroxyphenol moiety where the LUMO electron density is located (Fig. 4) [13]. Hence, this leads to an efficient electron injection from the dye to the semiconductor film [9,12,13]. We think that for this pigment it is possible to improve sensitization activity by changing the extraction technique (decreasing the pH and avoiding the use of acetic acid). The results show that the natural dyes studied, adsorbed onto surface of TiO2, absorb visible light and promote electron transfer across the dye/semiconductor interface. Preliminary tests on the stability of our natural dyes were carried out monitoring some indicative parameters such as Voc, ff, Isc, and Z under AM 1.5 solar irradiation for 3 h and no significant changes were observed. After 3 h, the performance of orange juice dye decreased, while for the eggplant acidic extract we did not observe any appreciable degradation of the dye for several hours (6 h). There is no stability difference for a cell under load and one at Isc. When stability tests were performed measuring current density under a voltage more than +0.6 V, we noted a quick degradation of our dyes (in 2 h), mainly in the irradiated active area. This effect is probably due to the value of the first oxidation potential of anthocyanin, which is around 0.5 V [27]. Our reported results on the stability are similar to data reported by other research groups [11–13]. O 7 4. Conclusion 3 5 O OH O OH HO HO OH Fig. 4. Schematic representation of cyanine (quinonodial form) attachment by chelating effect to Ti(IV) sites. The use of red and purple Sicilian fruits, such as the juice of red Sicilian orange (Citrus Sinensis) and the extract of eggplant peels (Solanum melongena L.), as natural sensitizers in DSSCs was successfully tested. It should be noted that some of the highefficiency values reported in the literature could be affected by some parameters, originating from different experimental conditions, such as the light source spectrum and intensity, the nature of electrolyte salt, solvent, additives, co-absorbers and catalyst, the size of cell active area and the thickness of TiO2 films. For this purpose, we exercised caution when comparing several solar energy conversion yields. After this consideration, we could conclude that, using the blood orange juice without an extraction procedure, we have obtained, to our knowledge, the best sensitization effect, under AM 1.5 illumination, reaching 0.66% of solar energy conversion efficiency. Obviously, natural dyes show sensitization activity lower than synthetic ones and less stability. Actually, their application is far below the industrial requirements. Nevertheless, their study is an interesting multidisciplinary exercise useful for dissemination of knowledge and to Table 1 Photoelectrochemical properties of fruit juice and natural extracts solar cells Dye Jsc (mA/cm2) Voc (V) Pmax (mW/cm2) ff (%) Cathode (catalyst type) Ref. Red Sicilian orange ‘‘Moro’’ Strawberry Blueberry Orange Red cabbage Cochineal Skin of Jaboticaba Rosella California blackberry Skin of eggplant Black rice 3.84 2.86 4.29 1.02 4.70 6.00 2.6 2.72 2.2 3.40 1.14 0.340 0.405 0.360 0.412 0.525 0.397 0.660 0.408 0.4 0.350 0.551 0.66 0.61 0.52 0.13 1.51 1.20 1.10 0.70 0.56 0.48 0.327 0.50 0.53 0.34 0.31 0.61 0.52 0.62 0.63 Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt – [9] [9] [9] [9] [9] [11] [12] [13] – [22] 0.40 0.52 mirror mirror mirror mirror mirror mirror transparent transparent transparent transparent transparent Author's personal copy ARTICLE IN PRESS 1346 G. Calogero, G.Di. Marco / Solar Energy Materials & Solar Cells 92 (2008) 1341–1346 4.50 Photocurrent (mA/cm2) 4.00 3.50 3.00 2.50 Voc = 0.340 V 2.00 Jsc = 3.84 mA/cm2 FF = 50% 1.50 Pmax = 660 µW/cm2 1.00 0.50 0.00 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Voltage (V) Fig. 5. 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