Int. J. Electrochem. Sci., 8 (2013) 6734 - 6743 International Journal of
ELECTROCHEMICAL SCIENCE www.electrochemsci.org Natural Dye Extracted from Rhododendron Species Flowers as a Photosensitizer in Dye Sensitized Solar Cell
Hee-Je Kim1, Yeo-Tae Bin1, S.N. Karthick1, K.V. Hemalatha*,1, C. Justin Raj1, S.Venkatesan2, Songyi Park1, G. Vijayakumar3
1
School of Electrical Engineering, Pusan National University, Jangjeon, Geumjeong, Busan 609-735, Republic of Korea. 2 Measurements Laboratory, Gujarat Tea Processors and Packers Ltd., Khokhra, Ahmadabad-380008, Gujarat, India 3 Department of Chemistry, RVS Faculty of Engineering, Affiliated to Anna university, Coimbatore, India * E-mail: hemaac@gmail.com Received: 7 February 2013 / Accepted: 4 April 2013 / Published: 1 May 2013
The natural dye, anthocyanin, was extracted from the flowers of Rhododendron species with three different colors, pink, red and violet, using a simple extraction technique, and was used as the sensitizer in dye-sensitized solar cells. The dyes were treated with nitric and acetic acids to examine their effects on the power conversion efficiency. According to the experimental results, the performance was better with the acetic acid-treated anthocyanin. The Jsc value for the acetic acidtreated pink, red and violet dyes increased to 0.887, 0.932 and 0.737 mA/cm2 compared to nitric acid treated and bare dye. The conversion efficiency of the acetic acid-treated pink, red and violet was 0.35%, 0.36%, 0.28%, respectively, which was higher than the nitric acid-treated and bare dye. This paper briefly discusses the simple extraction technique of these natural dyes and their performance in DSSCs.
1. INTRODUCTION The dye-sensitized solar cell (DSSC) is a device used to convert light energy to electrical energy, and is based on the sensitization of wide band gap semiconductors using dye molecules [1]. DSSCs have attracted considerably more attention in recent years because of their promising solar energy conversion systems. In the assembly of DSSC, the dye plays an important role in harvesting solar energy and converting it to electrical energy with the aid of a semiconducting photoanode.
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Therefore, the cell performance is totally dependent on the type of dye used as a sensitizer. Many metal complexes and organic dyes have been synthesized and used as sensitizers. Ruthenium (Ru) complexes are considered good sensitizers for DSSCs because of their intense charge-transfer absorption over the entire visible range and highly efficient metal-to-ligand charge transfer [2]. The major drawback of Ru is its rarity, high cost and the complicated synthesis of ruthenium complexes. Accordingly, alternative, readily available and efficient photosensitizers at low cost is a scientific challenge for TiO2-based photovoltaic devices. Recently, research has focused on the easily available dyes extracted from natural sources and as a photosensitizer because of its large absorption coefficients, high light-harvesting efficiency, low cost, easy preparation and environment friendliness. Thus far, several dyes extracted from natural sources, such as black rice, capsicum, Erythrina variegata flower, Rosa xanthina [2], curcumin, red-perilla [3], rosella flower, blue pea flower [1], Jathropha curcas, Citrus aurantium [4], red cabbage [5] etc., have been used as sensitizers in DSSC because the application of naturally available pigments, such as anthocyanins, carotenoids and cholorophylls, for DSSCs has several advantages over rare metal complexes. In this context, anthocyanin is one such flavanoid compound present in many fruits, flowers and leaves, and is responsible for the red, violet and blue colors [6-8]. The advantage of anthocyanin (Fig. 1) is the binding of carbonyl and hydroxyl groups to the surface of a porous TiO2 film. This causes electron transfer from the anthocyanin molecule to the conduction band of TiO2 [9]. In this study, DSSCs were assembled using natural dyes extracted from three different colored flowers of Rhododendron species using a very simple extraction technique. Because the performance of the raw dye is better than the commercial or purified analogues, the dye was extracted using a minimal chemical procedure without further purification because the presence of a natural extract like alcohols, organic acids, etc. act as coadsorbates, suppressing recombination with the electrolyte, reducing the level of dye aggregation and favoring charge injection [10]. The extracted dyes were characterized by Fourier transform infrared (FT-IR) and UV–vis absorption spectroscopy. The photoelectrochemical properties of the pure and acidified extracts, which were used as sensitizers in DSSCs, were examined.
2. EXPERIMENTAL DETAILS 2.1 Preparation of dye sensitizer solutions The flowers of the Rhododendron species with three different colors (pink, red and violet) were collected fresh in South Korea and placed in a vacuum furnace for ~ 10 h at 65° C to remove moisture. After drying, the flowers were crushed in a mortar and pestle and prepared as a powder. Approximately 1g of the powdered sample was dissolved in 60mL of ethanol and kept overnight at ambient temperature for adequate extraction without exposure to sunlight. After extraction, the solid residues were filtered and the clear dye solutions were used as a sensitizer. The dye solution was also characterized by UV-Vis absorption. For FT-IR, the residue of the dye solution was collected by evaporating the ethanol on a hot plate. The freshly prepared extracts were then treated with nitric
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(0.01N, pH 1) and acetic acid (0.4N, pH 4), and used as a sensitizer in solar cells to evaluate the photoelectrochemical performance.
2.2 Solar cell assembly A fluorine-doped Tin Oxide glass plate with resistance of 13 Ω/sq was first cleaned with acetone, ethanol and water for approximately 10 minutes each using an ultrasonic bath. Scotch tape was used as a spacer to control the film thickness and provide non-coated areas for electrical contact. For the photoanode, a TiO2 (Ti-Nanooxide T/SP, Solaronix) paste was coated on a FTO plate using the doctor blade method. The coated TiO2 films were then air-dried for ~ 5 min to reduce the surface irregularities. The films were annealed at 550oC in air for 30 minutes to remove the organic loads and facilitate the interconnection of TiO2 nanoparticles. The film thickness was ~ 20 µm. The active area of the TiO2 electrodes was 0.27 cm2. After cooling at 75oC, the TiO2 film electrodes were immersed in an anthocyanin dye solution for 24 hrs. After dye adsorption, the film was rinsed with pure ethanol and dried with hot air. A Pt electrode was prepared from Pt paste (platisol T/SP, Solaronix) using the doctor blade method. After coating on a FTO plate, it was heated at 450oC for 30 min. The dyecovered TiO2 photoanode and Pt counter electrode were assembled as a sandwich-type cell using a thermoplastic hot-melt ionomer film (SX 1170, Solaronix). A drop of an Iodolyte AN-50 electrolyte solution was injected into the cell through a hole at the back of the counter electrode. The hole was covered with a cover glass on a hot-melt ionomer film and then sealed. Finally, to achieve good electrical contact for the measurements, the edge of each side of the FTO glass was cleaned and soldered (ultrasonic soldering system, Model-9200) with alloy #143 (Cerasolza).
2.3 Characterization of TiO2 powder The structural characterization of the extracted anthocyanin pigment was characterized by FTIR spectroscopy (Nicolet-380 spectrometer) using KBr pellets. UV-Vis spectroscopy was performed using an Optizen 3220UV. The current density-voltage (J-V) measurements were carried out under one sun illumination (AM1.5G) using a solar simulator (SAN-EI Electric XES-301S 300W Xe Lamp) with an intensity of 100 mWcm-2. A Keithley 2400 was used as the source measurement unit. The photoelectric conversion efficiency (η) was calculated using the following equation:
where η is the global efficiency, and VOC, ISC and FF are the open-circuit voltage, short circuit current density and fill factor, respectively. Pin is the maximum input of light energy.
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Figure 1. Anthocyanin flavylium chromophore, a carboxonium cation: R= sugar or acylated sugar
Figure 2. FT-IR spectra of the anthocyanin pigment extracted from the three different colored flowers of Rhododendron species
3.1 FTIR Spectra The anthocyanin pigment extracted from the Rhododendron flowers using ethanol was confirmed by FTIR spectroscopy. The FTIR spectra were similar to the spectra reported by Luo et al [11]. Fig. 2 shows the FT-IR spectra of the anthocyanin pigment of three different colors. All three different colored dyes have a similar spectral structure. The two peaks at 2922 and 2850 cm-1 were assigned to the –CH stretching modes. The peak at a wavenumber of 3393 cm-1 corresponds to the OH stretching vibration. The spectral region between 1550 to 1700 cm-1 allows infrared absorption of
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C=C. Consequently, the peak at 1630 cm-1 corresponding to the double bond (C=C) stretching vibration and can be correlated with the stretching of aromatic C=C in anthocyanin. The peaks at 1726cm−1 were assigned to the C=O stretching vibration for all three colors [11,12]. This suggests that the anthocyanin in the dye has a partial quinonoidal form. On the other hand, the intensity of the peak at 1726cm−1 was high with the pink and red dyes compared to the violet dyes. All the stretching vibrational modes coincided with the four isolated anthocyanin pigments extracted from canna indica flower [13]. This confirmed the presence of anthocyanin pigments in the extract of Rhododendron flowers.
Figure 3. UV-Vis absorption spectra of the Rhododendron extract (a) fresh dye solution, (b) acetic acid treated dye solution, (c) nitric acid treated dye solution. The inserted optical photos show the three different colors of the dye solution (from left pink, red and violet).
3.2 UV-Vis absorption spectra Fig. 3a and 4a shows the UV-Vis absorption spectra of the fresh dye solution extracted from Rhododendron species with three different colors (pink, red and violet), and after the dye was adsorbed on TiO2, respectively. A sharp absorption spectrum was obtained for the pink, red and violet dyes at ~ 540 nm but the intensity of the absorption varied; it was higher with the pink and red dyes than with violet. After immersing the TiO2-coated photoanode in the red and pink dye extract, the TiO2 films turned a red and pink color, but turned a light violet for the violet colored dye (see the inset fig.4a). Fig. 4a shows the dye adsorbed on the TiO2 with an absorption maximum at 540 nm. The absorption band of the adsorbed dye was broader than the absorption band of the fresh dye solution (Fig.3a) and the tail absorption extended up to 700 nm.
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Figure 4. UV-Vis absorption spectra of the dye adsorbed on TiO2 photoanode (a) fresh dye solution, (b) acetic acid treated dye solution, (c) nitric acid treated dye solution. The inserted optical photos show the three different colors of the dye adsorbed on TiO2.
This broad absorption band was attributed to the binding of the anthocyanin molecule to the oxide surface of the photoanode [11]. The color of the dye turned dark after the acid treatment because the dye was pH dependant and solvent sensitive [14]. Fig. 1 shows the structure of the positively charged flavylium cation, which is the dominant equilibrium form under acidic conditions. The positive charge was delocalized through the pyrylium moiety, but carbons 2 and 4 were more positively charged atoms. The easy deprotonation of the two OH groups at positions 4’ and 7 contributes to the color changes in anthocyanin [15]. According to the literature, the absorption band of the anthocyanin pigment is pH sensitive so the extracted anthocyanin pigment was acidified with nitric and acetic acids and the absorption intensity was observed for the acidified dye solution and pure dye solution. The absorbance was high in the same order, i.e. pink, red and violet, for both the pure dye and acidified dye solutions (Fig. 3a,b,c). The absorbance intensity of the nitric acid-treated dye were very high compared to the acetic acid treated dye (Fig. 3b,c), because nitric acid is a strong oxidatic acid, whereas the acetic acid is a weak acid. In the case of an untreated fresh dye solution, the intensity was quite low compared to the acid treated dyes. On the other hand, the spectra of all the dye-adsorbed TiO2 films were broadened and less intense compared to the corresponding dye solutions, and the tail of the absorption extended up to 700 nm in the bare and acid-treated TiO2 films (Fig. 4a,b,c). The broadening and lower intensity is due to the adsorption of anthocyanin to the semiconductor TiO2 surface. This chemical adsorption occurs because of the binding of alcoholic-bound protons with the hydroxyl group in the surface of nanostructured TiO2. This confirmed the adsorption of dye molecules on the TiO2 surface. These effects were observed for anthocyanins upon complexation with Ti metal ions [16]. Although the
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absorbance band was broadened for acetic acid and nitric acid-treated TiO2 adsorbed dye, the intensity of dye adsorption was similar for the bare dye and acetic acid-treated dye, but the intensity was low for the nitric acid-treated dye. This might be due to etching of the TiO2 surface by nitric acid. In contrast, the acidic acid-treated dye adsorbed photoanode was not affected because acetic acid is a weak acid. The bare and acidified dye adsorbed TiO2 showed similar UV absorption spectra.
Figure 5. Current-voltage (I-V) characteristics of the photovoltaic devices sensitized with (a) pink, red, violet color dye (b) acetic acid treated dye (c) nitric acid treated dye.
3.3 Influence of different types of acid-treated anthocyanin pigment on photoelectrochemical performance The photoelectric performance of the DSSCs is related to acidified and unacidified anthocyanin pigment. Table 1 lists the comparative performance of three different color dyes treated with nitric and acetic acid and the untreated dye in terms of the short circuit photocurrent (Jsc), open circuit voltage (Voc), fill factor (FF) and energy conversion efficiency (η). Fig. 5 shows the I-V curve of the bare dye and dye treated with nitric and acetic acid. The open-circuit voltage (Voc) of the bare pink, red and violet dye was 0.5516, 0.5439 and 0.5439 V, respectively. The short-circuit current density (JSC) was 0.8492, 0.8503 and 0.6938 mA/cm2, and fill factor (FF) was 0.7201, 0.7231 and 0.7159 and conversion efficiency was 0.33%, 0.33% and 0.27%, respectively. The performance of natural dye was lower than the N719 dye due mainly to the molecular structure of natural dye. The natural dye mainly has -OH and -O ligands and lacks -COOH ligands, whereas the -COOH ligands of the Ru dye will combine with the hydroxyl group of the TiO2 particles to produce an ester and allow easy electron transfer to
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the TiO2 conduction band to acquire a rapid electron-transport rate [17]. Another reason is the steric hindrance in the anthocyanin pigment, which affects the effective bond formation on the oxide surface of TiO2. Therefore, there is lack of electron transfer from the dye molecules to the conduction band of TiO2. Table 1. Photovoltaic performance of the DSSC sensitized with the anthocyanin pigment with three different colours under acidic and non- acidic conditions. Sample Pink Red Violet Pink+ CH3COOH Red+ CH3COOH Violet+ CH3COOH Pink+HNO3 Red+HNO3 Violet+ HNO3 Voc (V) 0.5516 0.5439 0.5439 0.5447 0.5380 0.5417 0.5255 0.5222 0.5238 Jsc (mA/cm2) 0.8492 0.8503 0.6938 0.8879 0.9318 0.7371 0.7800 0.7671 0.7175 Fill Factor 0.7201 0.7231 0.7159 0.7241 0.7291 0.713 0.6902 0.6876 0.682 Efficiency (%) 0.33 0.33 0.27 0.35 0.36 0.28 0.28 0.27 0.25
An acid treatment of the dye molecule was attempted to increase the power conversion efficiency. The fill factor for the pink, red and violet dye treated with nitric acid was 0.6902, 0.6876 and 0.682, respectively. The respective values for the acetic acid treated pink, red and violet dye was 0.7241, 0.7291 and 0.713. The fill factor for the bare and acetic acid-treated dyes was high compared to the nitric acid treated dye. The Jsc value of the nitrated pink, red and violet dye was 0.780, 0.767 and 0.717, respectively, whereas with the acetic acid treated, the Jsc increased to 0.887, 0.932 and 0.737, respectively. Better performance was obtained with the acetic acid treatment than the nitric acid because nitric acid is a strong oxidatic acid compared to acetic acid. According to UV-Vis spectroscopy, although the color intensity of the dye solution increased with an acid treatment, the absorption spectra of dye adsorbed TiO2 films of the bare and acetic acid-treated dye were similar. On the other hand, the cell performance was higher with the acetic acid-treated dye than the bare dye. In both cases, the absorption intensity was the same, which confirms that the acid treatment does not affect the dye molecules for more absorbance, but it modifies the TiO2 surface. According to Jeong et. al (18), the acetic acid-treated TiO2 showed more efficiency than the untreated TiO2 because the acetic acidmodified TiO2 films had a better morphology than the untreated TiO2 film, resulting in an increase in cell performance due to the transportation of electrons. At the same time, the nitric acid-treated dye adsorbed TiO2 film shows less absorption in the UV-Vis spectrum and poor performance in the power conversion efficiency. The nitric acid present in the dye modifies the surface state of the porous film
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and creates defect centers on the TiO2 surface. These defects prevent the transfer of electrons to the TiO2 grid from the excited state of the sensitized dye molecule, which results in lower electron transportation efficiency and a decrease photovoltaic performance [19]. Therefore, the efficiency was higher with the acetic acid-treated dye than the nitric acid and untreated dye. On comparing the efficiency of three different colored pigments the conversion efficiency of pink and red is on par with each other compared to the efficiency of the violet dye in both acid treated and untreated dyes.
4. CONCLUSION The generation of photovoltaic currents was demonstrated by illuminating FTO electrodes coated with TiO2 nanocrystalline film sensitized with anthocyanin pigments extracted from the three different colored flowers of Rhododendron species using a simple extraction technique. In UV-vis spectroscopy, the broad absorption peak of the pigments in the visible range revealed the adsorption of dye molecules to the TiO2 nanoparticles. In FT-IR study, the peaks at 1726cm−1 were assigned to C=O stretching vibration for all the three colors, indicating the partial appearance of the quinonoidal form of the anthocyanin in dye. The anthocyanin pigment treated with acetic acid provided better performance than the nitric acid-treated and bare dye. The short circuit current for the pink, red and violet acetic acid-treated dye was 0.887, 0.932 and 0.737 mA/cm2, respectively. Among the three different colors, the pink and red dyes treated with acetic acid showed the highest conversion efficiency of 0.35 and 0.36% respectively, compared to the violet colored dye. Acid treated dye adsorption on TiO 2 film was expected to have more influence because of its high color intensity. On the other hand, the UV study revealed no significant changes in the absorbance spectra between the acetic acid-treated dye-adsorbed TiO2 and the bare dye adsorbed TiO2 film. On the other hand, the cell performance was good with the acetic acid-treated dye adsorbed TiO2 compared to the other dye solution due mainly to modification of the TiO2 surface and not to the color intensity of the dye solution. In general, natural dyes are promising alternative sources of sensitizers for DSSCs because of the simple extraction procedure, low cost, wide availability and environmentally-friendly nature.
ACKNOWLEDGEMENT This study was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. 20110001295).
References 1. 2. 3. 4. K. Wongcharee, V. Meeyoo, S. Chavadej, Sol. Energy Mater. Sol. Cells 91 (2007) 566-571. S. Hao, J. Wu, Y. Huang, J. Lin, Sol. Energy 80 (2006) 209-214. S. Furukawa, H. Iino, T. Iwamoto, K. Kukita, S. Yamauchi, Thin Solid Films 518 (2009) 526-529. A. C. Nwanya, P.E. Ugwuoke, P. M. Ejikeme, O.U Oparaku and F.I. Ezema, Int. J. Electrochem. Sci. 7 (2012) 11219 – 11235. 5. Y. Li, S.-H. Ku, S.-M. Chen, M. A. Ali, F.M. A. AlHemaid, Int. J. Electrochem. Sci. 8 (2013) 1237 - 1245.
