Renewable Energy 36 (2011) 2484e2488 Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene Natural dye (cyanidin 3-O-glucoside) sensitized nanocrystalline TiO2 solar cell fabricated using liquid electrolyte/quasi-solid-state polymer electrolyte T.S. Senthil a, N. Muthukumarasamy b, *, Dhayalan Velauthapillai c, S. Agilan b, M. Thambidurai b, R. Balasundaraprabhu d a Department of Physics, Erode Sengunthar Engineering College, Erode, India Department of Physics, Coimbatore Institute of Technology, Coimbatore, India Faculty of Engineering, University College of Bergen, Bergen, Norway d Department of Physics, PSG College of Technology, Coimbatore, India b c a r t i c l e i n f o a b s t r a c t Article history: Received 17 September 2010 Accepted 30 January 2011 Available online 1 March 2011 TiO2 dye-sensitized solar cells (DSSCs) have been fabricated using TiO2 photoelectrodes sensitized using the extracts of Delonix regia (May flower, locally called Vakai) and Eugenia Jambolana (Indian blackberry, locally called Naval) as natural sensitizers and their characteristics have been studied. Among them Eugenia Jambolana gave the best photosensitization effect and presents the prospect to be used as an environment-friendly, low-cost alternative system. The extracts having anthocyanin pigment (cyanidin 3-O-glucoside), which have carboxylic groups in the molecule can attach effectively to the surface of TiO2 film. The solar cell constructed using the Eugenia Jambolana sensitized TiO2 photo-electrode exhibited a short-circuit photocurrent of 1.49 mA and a power conversion efficiency of 0.5%. Ó 2011 Elsevier Ltd. All rights reserved. Keywords: Solar cell Anthocyanin Natural dye TiO2 1. Introduction Photosensitized wide band gap metal oxide semiconductors are used for the conversion of visible light into electricity [1e3]. In last few years nanocrystalline TiO2 thin films have received much attention for solar energy applications [4,5]. In dye-sensitized solar cells TiO2 nanoparticles are used as a working electrode because of the higher efficiency exhibited by these cells when compared to other metal oxide semiconductor based solar cells. After the discovery of an efficient dye-sensitized photovoltaic cell by O’Regan and Gratzel [2], the dye-sensitized solar cells have received much attention as low-cost photovoltaic cells and have become a rapidly expanding field with potential applications. Nanoporous TiO2 electrodes sensitized with ruthenium complexes have exhibited power conversion efficiencies up to 11%, but due to the high cost of ruthenium complexes and the scarce availability of these noble metals, investigation on low cost, readily available dyes as efficient sensitizers for dye-sensitized solar cells has been expedited but still remains a scientific challenge [6,7]. Natural dyes extracted from fruits, flowers and leaves of plants have several advantages over rare metal complexes and other organic dyes. The natural dyes are readily available, easy to extract, * Corresponding author. E-mail address: vishnukutty2002@yahoo.co.in (N. Muthukumarasamy). 0960-1481/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2011.01.031 no need of further purification, environment-friendly and are of less cost. All over the world there are several research groups working to utilize the natural dyes as sensitizers in dye-sensitized solar cells [8e10]. Natural pigments, including chlorophyll, carotenoids, anthocyanin, [9,11e13] nasunin [14] carotenoids, [15] and crocetin [16] are freely available in plant leaves, flowers, and fruits and have the potential to be used as sensitizers. Among these anthocyanins are a group of naturally occurring phenolic compounds responsible for the colour of many flowers, fruits leaves, stems, roots and vegetables. The most common anthocyanidins (Fig. 1a) found in flowers and fruits are cyanidin (orangered), delphinidin (blue-red), malvidin (blue-red) pelargonidin (orange), peonidin (orange-red) and petunidin (blue-red) [17,18]. The hydroxyl groups present in anthocyanidins chelating with TiO2 is shown in Fig. 1b. Delonix regia used in the present study is having anthocyanin as cyanidin 3-O-glucoside [19] and Eugenia Jambolana has anthocyanins (as cyanidin 3-glucoside equivalents) which occur as diglucosides of five anthocyanidins/aglycons: delphinidin, cyanidin, petunidin, peonidin and malvidin [20,21]. The structure of the cyanidin 3-O-glucoside is shown in the Fig. 1c. The other important anthocyanidins and their substitutions are given in Table 1. The carbonyl and hydroxyl groups present in the anthocyanin molecule can attach itself to the surface of a porous TiO2 film. This helps in electron transfer from the anthocyanin molecule to the conduction band of TiO2. 2485 T.S. Senthil et al. / Renewable Energy 36 (2011) 2484e2488 Fig. 1. a. Basic chemical structure of the anthocyanidins, b. hydroxyl groups present in the anthocyanidins chelating with TiO2 and c. structure of the cyanidin 3-O-glucoside. In this work, we have made an attempt to use natural dyes obtained from the extracts of Delonix regia (May flower, locally called Vakai) and Eugenia Jambolana (Indian blackberry, locally called Naval), which have a high content of anthocyanin pigment having a high absorption coefficient in the visible part of the solar spectrum. So these natural dyes can be used for sensitizing TiO2 films for solar cell applications. Natural dye-sensitized TiO2 solar cells have been observed to exhibit an efficiency of 7.1% with high stability [14]. Efficiency over 8.0% has been obtained using similar synthetic organic dyes [22]. Because of the simple preparation technique, ease of availability, and cheap cost, natural dye is found to emerge as an alternative potential photo sensitizer for dye-sensitized solar cells. The efficiency of the cell is found to depend strongly on the parameters like the absorption spectrum of the dye and anchoring of the dye to the surface of TiO2 [23]. Many researchers have studied about the usefulness of natural dyes as sensitizers for dye-sensitized solar cell applications [24e26]. In this paper we report about the systematic extraction of natural dye from Delonix regia (colour of the flower: red) and Eugenia Jambolana (colour of the fruit: black), and the incorporation of the dye molecule (the core molecular groups of natural dyes) into the TiO2 porous film. Natural dye-sensitized TiO2 solar cells have been fabricated and their characteristics have been studied. 2. Experiment 2.1. Fabrication of natural dye-sensitized TiO2 solar cell To prepare the photo-anode of dye-sensitized solar cells, the ITO conducting glass sheet (Asahi Glass; Indium-doped SnO2, sheet resistance: 15 U/square) was first cleaned in a detergent solution using an ultrasonic bath for 15 min, rinsed with double distilled water and 2 propanol, and then dried. The matrix sol was prepared by mixing titanium tetra isopropoxide (TTIP) with absolute ethanol (Aldrich 99.9%) and acetic acid at room temperature. The prepared sol was deposited on the ITO glass by spin coating method as described in one of our earlier works [27]. The thickness of the prepared films was observed to lie in the range of 1 mm. Natural dye-sensitized TiO2 based solar cells have been fabricated with area ranging from 0.1 to 0.25 cm2, and it was found that the cell efficiency was independent of cell area in this range as reported by Yamazaki et al. [16]. The film was preheated at 100  C for 10 min and then sintered at 450  C for 1 h. For Delonix regia flower extract preparation, well cleaned flowers (2.5 kg) were mixed with 250 ml ethanol and were kept for 12 h at room temperature (25  C). For Eugenia Jambolana extract preparation, the fruits were cut into small pieces. After removing the seeds from the fruits, the chosen fruits (2 kg) were soaked in ethanol (250 ml) and kept for 12 h. Then residual parts were removed by filtration and the filtrate was washed with hexane several times to remove any oil or chlorophyll present. The ethanol fraction was separated and few drops of concentrated HCl were added so that the solution became deep red in colour (pH < 1). This was directly used as dye solution for sensitizing TiO2 electrodes. Lithium iodide, Iodine and Acetonitrile purchased from Sigma Aldrich have been used as received for the preparation of electrolyte. The redox electrolyte with [I3 ]/[I ] ¼ 1:9 was prepared by dissolving 0.5 M LiI and 0.05 M I2 in acetonitrile solvent. Since LiI is extremely hygroscopic, electrolytes were prepared in a dry room maintained at dew point of 60  C. Table 1 Selected anthocyanidins and their substitutions. Anthocyanidin R1 R2 R3 R4 R5 R6 R7 Cyanidin Delphinidin Malvidin Peonidin Petunidin eOH eOH eOCH3 eOCH3 eOH eOH eOH eOH eOH eOH eH eOH eOCH3 eH eOCH3 eOH eOH eOH eOH eOH eOH eOH eOH eOH eOH eH eH eH eH eH eOH eOH eOH eOH eOH 2486 T.S. Senthil et al. / Renewable Energy 36 (2011) 2484e2488 a Absorbance (a.u.) Delonix regia extract 400 Dye Dye+TiO2 500 600 800 700 Wavelength (nm) b Eugenia Jambolana extract Absorbance (a.u.) Quasi-solid-state polymer electrolyte was prepared by adding 0.0383 g of P25 TiO2 powder, 0.1 g of LiI, 0.019 g of I2, 0.264 g of PEO, and 44 ml of 4-tert-butylpyridine in 1:1 acetone/propylene carbonate solution. The prepared quasi-solid-state polymer electrolyte was spin coated over dye-sensitized TiO2 electrode. The counter electrode was prepared using platinus chloride as follows: the H2PtCl6 solution in isopropanol (2 mg/ml) was deposited onto the ITO glass by spin coating method. The film was dried at 80  C for 30 min in air and then sintered at 400  C for 30 min in air. For photosensitization of TiO2 the sintered TiO2 electrode was immersed in the extracted dye solution at room temperature for 24 h in the dark. The electrode was then rinsed with ethanol to remove the excess dye present in the electrode and then the electrode was dried. The catalyst-coated counter electrode was placed on the top of the TiO2 electrode, such that the conductive side of the counter electrode faced the TiO2 film with a spacer separating the two electrodes. The two electrodes were clamped firmly together using a binder clip. Now the prepared liquid electrolyte solution was injected into the space between the clamped electrodes. The electrolytes entered into the cell by capillary action. This resulted in the formation of sandwich type cell. In the case of quasi-solid-state polymer electrolyte the spin coated quasi-solid-state polymer sensitized TiO2 electrode is directly placed on the counter electrode. The cross-sectional view of the DSSC is shown in Fig. 2a and b. UVevisible absorption spectra measurements of extracted pigments in acidified ethanol solution and TiO2 films has been carried out using a UVeVISeNIR spectrophotometer (Jasco V-570). The JeV characteristics of the devices in the dark and under illumination were measured by a semiconductor parameter analyzer (Keithley 4200-SCS). A xenon light source (Oriel, USA) was used to give an irradiance of 100 mW/cm2 (equivalent to AM1.5 irradiation). The photoaction spectrum of the devices was measured using a monochromator (Spex 500 M, USA), and the resulting photocurrent was measured with a Keithley electrometer (model 6514). The cell area was 0.25 cm2. Dye Dye+TiO2 3. Results and discussion Fig. 3a and b shows the absorption spectra of Delonix regia and Eugenia Jambolana sensitized TiO2 thin film. The absorption maximum of Delonix regia is at 538.25 nm and that of Eugenia Jambolana is at 542.86 nm. This result indicates that Eugenia Jambolana extract is the strongest light absorbing dye. This has a maximum absorption coefficient that is about 15 times higher than that of the N-719 dye, which is generally used in high efficiency dye-sensitized solar cells. The absorption band of the dye adsorbed TiO2 semiconductor films is shifted to longer wavelength when compared to the absorption spectra of the dye solution as shown in Fig. 3a and b. The intensity has been observed to be enhanced due to the interfacial TieO coupling between the anthocyanidin molecule and the TiO2 molecules. It is generally accepted that the chemical adsorption of these dyes takes place due to the condensation of alcoholic-bound protons with the hydroxyl groups present on the surface of the nanostructured TiO2 (Fig. 1b) 400 500 600 700 800 Wavelength (nm) Fig. 3. a. Absorption spectra of TiO2 film sensitized with the natural dye Delonix regia. b. Absorption spectra of TiO2 film sensitized with the natural dye Eugenia Jambolana. [28]. This chemical attachment affects the energy levels of the highest occupied molecular level (HOMO) and the lowest unoccupied molecular level (LUMO) of the anthocyanidin molecule [29], which eventually affects the band gap of these materials and this results in a shift in the absorption peak of the absorption spectra. Fig. 2. a. Schematic diagram of ITO/TiO2/dye/liquid electrolyte/ITO device. b. Schematic diagram of ITO/TiO2/dye/quasi-solid-state polymer electrolyte/ITO device. 2487 T.S. Senthil et al. / Renewable Energy 36 (2011) 2484e2488 Table 2 Solar cell parameters. a Eugenia Jambolana Extract 0.0015 Current Density (mA/cm2) 0.0010 0.0000 -0.0005 Polymer Electrolyte -0.0010 Liquid Electrolyte -0.0020 -0.1 0.0 0.1 0.2 0.3 0.4 Voltage (V) 0.0015 Delonix regia Extract 0.0010 Current Density (mA/cm2) Electrolyte Jsc (mA/cm2)  10 Delonix regia Delonix regia Eugenia Jambolana Eugenia Jambolana Liquid Polymer Liquid Polymer 1.33 0.84 1.49 1.58 3 Voc (V) FF h (%) 0.30 0.30 0.35 0.30 0.39 0.48 0.48 0.46 0.317 0.245 0.505 0.444 0.0005 -0.0015 b Sensitizer 0.0005 0.0000 Polymer electrolyte -0.0005 where Jsc is the short-circuit photocurrent density (mA cm 2), Voc is the open-circuit voltage (volts), Pin is the intensity of the incident light (W cm 2) and Jm (mA cm 2) and Vm (volts) are the maximum current density and voltage in the JeV curve, respectively, at the point of maximum power output. In principle, the maximum Jsc of a dye-sensitized solar cells is determined by how well the absorption window of the dye overlaps with the solar spectrum. The solar cell parameters are given in Table 2. The stability of the natural dyes used was studied by monitoring some of the indicative parameters such as Voc, ff, Jsc, and h under AM1.5 solar irradiation for 5 h and no significant changes were observed. Our reported results on the stability are similar to the one reported by other research groups [11,12,31]. Liquid electrolyte cell was observed to exhibit better efficiency than the quasi-solid-state polymer electrolyte cell. This may due to the fact that the mobility of ions in liquid is more. The conversion efficiency of Eugenia Jambolana based cell is more than that of the Delonix regia dye cell, one of the reasons for this could be the higher amount of Eugenia Jambolana dye adsorbed by the TiO2 film. This is due to a higher intensity and broader range of the light absorption of the extract (Fig. 2), and the strong interaction between TiO2 and anthocyanin in the Eugenia Jambolana extract and this leads to a better charge transfer. Therefore, the Eugenia Jambolana should be an alternative anthocyanin source for dye-sensitized solar cells in geographical regions where it is widely available. 4. Conclusion -0.0010 liquid electrolyte -0.0015 -0.1 0.0 0.1 0.2 0.3 0.4 Voltage (V) Fig. 4. a. JeV characteristics of TiO2 sensitized using Eugenia Jambolana based solar cell. b. JeV characteristics of TiO2 sensitized using Delonix regia based solar cell. The red-shift is also consistent with the reduced band gap of the adsorbed anthocyanidin dye compared to that of free anthocyanidin dye. On the other hand, the bare TiO2 nanoparticles show no absorption beyond 400 nm, consistent with its large band gap of 3.2 eV [27]. Therefore, the dye sensitizer greatly increases the absorption of light by the TiO2 nanoparticles in the visible range, which dominates the terrestrial solar spectrum. The absorption spectra show that the Delonix regia and Eugenia Jambolana absorbed TiO2 nanoparticles exhibit absorption peaks at 559.33 and 560.27 nm respectively. The JeV curves of the dye-sensitized solar cells fabricated using liquid electrolyte and polymer electrolyte are shown in Fig. 4a and b. The solar cell parameters fill factor (FF) and efficiency (h) have been calculated using the following equations [30] FF ¼ Jm Vm =Jsc Voc h ¼ ðFF  Jsc  Voc Þ=Pin  100 In this work we have reported about the studies carried out on anthocyanin (cyanidin 3-O-glucoside) pigments easily available in Delonix regia and Eugenia Jambolana as natural photosensitizers for TiO2 photoelectrodes. Solar cells have been fabricated using two different electrolytes, a liquid electrolyte and a quasi-solid-state polymer electrolyte. 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