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Green grasses as light harvesters in dye sensitized solar cells
Accepted Manuscript Green grasses as light harvesters in dye sensitized solar cells Vinoth Shanmugam, Subbaiah Manoharan, A Sharafali, Sambandam Anandan, Ramaswamy Murugan PII: DOI: Reference:
S1386-1425(14)01177-9 http://dx.doi.org/10.1016/j.saa.2014.07.096 SAA 12516
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
19 May 2014 10 July 2014 29 July 2014
Please cite this article as: V. Shanmugam, S. Manoharan, A. Sharafali, S. Anandan, R. Murugan, Green grasses as light harvesters in dye sensitized solar cells, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.07.096
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Green grasses as light harvesters in dye sensitized solar cells Vinoth Shanmugama, Subbaiah Manoharanb, Sharafali. Aa, Sambandam Anandanb and Ramaswamy Murugana* a
b
Department of Physics, Pondicherry University, Puducherry 605 014, India
Nanomaterials and Solar Energy Conversion Lab, Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India *Email: [email protected]
ABSTRACT Chlorophylls, the major pigments presented in plants are responsible for the process of photosynthesis. The working principle of dye sensitized solar cell (DSSC) is analogous to natural photosynthesis in light-harvesting and charge separation. In a similar way, natural dyes extracted from three types of grasses viz. Hierochloe Odorata (HO), Torulinium Odoratum (TO) and Dactyloctenium Aegyptium (DA) were used as light harvesters in dye sensitized solar cells (DSSCs). The UV-Vis absorption spectroscopy, Fourier transform infrared (FT-IR), and liquid chromatography-mass spectrometry (LC-MS) were used to characterize the dyes. The electron transport mechanism and internal resistance of the DSSCs were investigated by the electrochemical impedance spectroscopy (EIS). The performance of the cells fabricated with the grass extract shows comparable efficiencies with the reported natural dyes. Among the three types of grasses, the DSSC fabricated with the dye extracted from Hierochloe Odorata (HO) exhibited the maximum efficiency. LC-MS investigations indicated that the dominant pigment present in HO dye was pheophytin a (Pheo a).
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Keywords: Natural dyes, grass extract, liquid chromatography-mass spectrometry, pheophytin a, Dye sensitized solar cells *Corresponding Author. Tel.: +91 413 2654782; fax: +91 413 2656988 E-mail address: [email protected] (R. Murugan)
1. Introduction There is a global challenge to capture and utilize solar energy for a large scale sustainable growth. In recent years, dye-sensitized solar cells (DSSCs) have attracted great interest in scientific research and applications owing to their low cost, large flexibility in shape, transparency, easy fabrication and greater variety of colors [1-3]. Dye sensitized solar cells (DSSCs) have been fascinated as a unique candidate for the conversion of light photons into electricity in both diffuse light and direct sunlight, without compromising on its performance. The typical DSSCs comprise of a nanocrystalline TiO2, dye molecule, an electrolyte containing the iodine/triiodine (I-/I3-) redox couple and a counter electrode. The process of converting sunlight into electricity takes place in DSSC based on the sensitization of wide band gap semiconductor by dye sensitizer. Hence, numerous inorganic, organic and hybrid dyes have been used as sensitizers in DSSCs. Among the synthetic inorganic dyes, ruthenium polypyridyl complex was effectively employed as molecular sensitizer in DSSCs with high efficiency up to 11-12% [4-7]. However, ruthenium polypyridyl complexes are expensive and less abundant, hence natural dyes are an immense interest towards the use of sensitizers in DSSCs. Several natural pigments such as chlorophyll [8-15], anthocyanin [16-18], tannin [19], and carotene [20,21] have been successfully used as sensitizers in DSSCs. Unlike synthetic inorganic dyes, natural dyes offer various advantages such as non-toxic, 2
environmentally friendly, fully biodegradable and hence an emphasis on using natural products are increasing day by day [22,23]. A paradigm for the use of natural products in DSSC was reported recently, in which the extracts of marine plant (sea tangle) were used as a dye sensitizer, as well as redox couple and counter electrode and this all natural cell exhibits efficiency of 1.4% [24]. The first report on photosensitization of nanoporous TiO2 by chlorophyll derivatives was reported by Kay and Grätzel [8]. Over the years, considerable efforts have been put forward towards employing chlorophylls as sensitizer in DSSCs. Many reports have showed that chlorophylls, which act as an effective photosensitizer in photosynthesis of green plant, have the potential to be an environment friendly dye source [9-12]. Chlorophylls are metal-complexes of magnesium ion, with high symmetry, consisting of a tetrapyrrolic macrocycle, encompass several pigments with common structural elements [11]. The chlorophylls (Chl a and Chl b) are usually accompanied by their major derivatives, chlorophyllides (chlid a and child b) and pheophytins (Pheo a and Pheo b). The carboxylic acid groups in the photosensitizer established an electronic coupling with the conduction band of TiO2 which is helpful for anchoring the dye molecules and an effective electron injection to the conduction band of TiO2. Thus, the carboxylic acid groups in the dye molecule are essential for dye-sensitized solar cell [10]. Chl-a, Chl-b and pheophytins are not strongly bind to the TiO2 surface due to the weak interaction of the phytyl ester group and keto carbonyl groups with the hydrophilic titania surface [8]. But Chlorophyll c1 (Chl-c1) and Chlorophyll c2 (Chl-c2) have the terminal carboxylic acid group, which is connected through a conjugated double bond of the porphyrin macrocycle. Hence Chlc1 and Chl-c2 are strongly bound on the TiO2 surface which ensures efficient electron injection into the TiO2 conduction band and to prevent gradual leaching by the electrolyte [12].
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Herein, we report the performance of DSSC fabricated using dyes extracted from three types of grasses viz. Hierochloe Odorata (HO), Torulinium Odoratum (TO) and Dactyloctenium Aegyptium (DA) as sensitizers in order to gain further information on the role of various chlorophyll derivatives as sensitizer in DSSCs. 2. Experimental 2.1. Extraction of dyes A simple extraction technique was employed for the extraction of the dye from the grasses. The three grasses Hierochloe Odorata (HO), Torulinium Odoratum (TO) and Dactyloctenium Aegyptium (DA) were collected from the campus of Pondicherry University, India. Fresh grasses were washed with distilled water and vacuum dried at 60 °C. The dried grasses were crushed into fine powder using a mortar. About 1g of the powdered grass of each type was separately put into a beaker containing 50 ml of absolute ethanol, which was kept in the absence of sunlight for 24 hours. Finally the solid residue was filtered to acquire a pure natural dye solution. 2.2. Fabrication of DSSCs The TiO2 working electrode was prepared by deposition of TiO2 paste on FTO glass (Sheet resistance 10 Ωsq. -1; BHEL, India) with the active area of 1×1 cm2 according to the procedure described elsewhere [25] which was sintered at 450 °C in a tubular furnace. And so, the preheated TiO2 electrodes were cooled down to ~80 °C and soaked overnight in the ethanol extract of each dye obtained from three different grasses. Platinized counter electrodes were prepared by drop casting of 0.05 M Hexachloroplatinic acid (H2PtCl6) in isopropanol. After being rinsed with ethanol, the photoanode was placed on top of the counter electrode and tightly clipped together to form a cell. An electrolyte was then injected into the space between two 4
electrodes. The electrolyte was composed of 0.5 M lithium iodide (LiI), 0.05 M iodine (I2), and 0.5 M 4-tert-butylpyridine (TBP) dissolved in 3-methoxypropionitrile. The standard cell using cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II) (N3 dye) as the sensitizer was fabricated similarly for comparison with the DSSCs fabricated with grass extracts. The fabrication details of the DSSCs have been described previously [18]. 2.3. Measurements and Characterizations The absorption spectra of the extracts obtained from grasses were recorded using a UVvis spectrophotometer (UV-2450, Shimadzu). Fourier Transform Infrared spectra (FTIR, Thermo Nicolet 6700) of the extracts were recorded in the range 500-4000 cm-1 using KBr technique. High resolution liquid chromatography-mass spectrometry (Shimadzu LC-MS-2010) was used to record liquid chromatography-mass spectrometry (LC-MS) spectra. The surface morphology of the deposited TiO2 film was examined using a scanning electron microscope (SEM, Hitachi S3400N). The photovoltaic properties of the fabricated devices were measured under dark and simulated solar light intensity of 100 mWcm-2. Electrochemical impedance spectroscopy (EIS) measurements were carried out under 100 mWcm-2 light illumination in the frequency range of 0.1 Hz to 100 kHz at their open circuit potential (OCP). The photocurrent density-voltage and impedance characteristics of the fabricated cells were recorded with an Autolab potentiostat / galvanostat-84610. 3. Results and discussion 3.1. Absorption Spectra Chlorophylls have two major absorption bands red (Qy) and blue (Soret) in the visible region. The red and blue bands of Chl a were found to be located at 662 and 430 nm, and of Chl b at 646 and 457 nm, respectively [26]. The absorption maximum of the red and blue bands for
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Pheo a were located at around 666 and 410 nm, respectively [26]. The major difference observed between the absorption spectra of chlorophylls and pheophytins are that the absorption maximum in Qy region exhibits shift to longer wavelengths and in contrast absorption maximum in soret region exhibits shift to shorter wavelengths for pheophytins compared to chlorophylls [26]. To identify the compound present in the alcoholic extract of HO, TO, and DA grasses (natural dyes) the absorption study was performed and whose corresponding spectra were presented in Fig. 1. Altogether all the three dyes show two major absorption peaks in the region around 660 nm and 400 nm and in addition, two very weak bands were observed at around 540 nm and 606 nm. The observed blue band at 400 nm, red band at 660 nm and weak bands located at 540 and 606 nm are in accordance with the reported data for Pheo a [26]. High-resolution liquid chromatography-mass spectrometry (LC-MS) investigations for each grass extract indicates signals of molecular ions (M+) at m/z 871.55 as shown in Fig. 2 (a-c), which agrees well with the earlier electrospray-ionization mass spectrometry (ESI-MS/MS) studies of chlorophyll pigments [26]. In the mass spectra of the dyes extracted from the investigated grasses, the observed peak at m/z ~871.55 was designated as a molecular ion-peak of Pheophytin a derivative of chlorophyll with the molecular formula (C55H74N4O5) [26]. The chemical structure of Pheophytin a is shown in Fig. 2(d). The second fragmentation found at m/z 621.20 in dyes TO and DA may be chlorophyllide a/b. The LC-MS studies indicated that Pheophytin a is a dominant constituent in HO dye. 3.2. FT-IR spectroscopic studies The recorded FT-IR spectra of the natural dyes extracted from HO, TO and DA shown in Fig. 3 (a-c) exhibit most of the characteristic peaks of the chlorophyll derivatives [27]. Chlorophylls and their derivatives have characteristic infrared bands corresponding to C=O,
6
C=C, C–H and N–H groups. The characteristic C=O band corresponding to the ketone in the cyclopentanone ring of pheophytin a is observed at 1725 cm-1 for all the three extracted dyes. Similarly the C=C vibrations of pheophytin a is observed at around 1618 cm-1. Vibrational bands positioned around 3340 cm-1 correspond to N–H stretching vibrations. The bands observed at around 2924 and 2854 cm-1 represent saturated C–H stretching vibrations. The FT-IR spectral analysis also confirms the presence of Pheo a in all the three grass extracts, which is well matched with the UV-Vis and LC-MS spectral results. 3.3. Surface morphology of TiO2 electrode Fig. 4 shows typical SEM image of TiO2 coated FTO glass plate. The SEM image indicates that the TiO2 particles were spherical in shape and composed with a high degree of porosity which may help to improve adsorption of dye molecules on the surface. Further, the fine adsorption of dye molecules may enhances the electron injection and leads to better photoelectric conversion efficiency of the solar cell. 3.4. Photovoltaic Performance To examine the extracts of green grasses as light harvesters and electron injectors for DSSC application, the dye-sensitized solar cells were fabricated using the three grass dyes HO, TO and DA. Photovoltaic cells were prepared with the effective light exposure area of about 1x1 cm2 and a liquid electrolyte composed of 0.05 M I2/0.5 M LiI/0.5 M tert-butylpyridine in 3methoxypropionitrile solution. The cell performances were tested under AM 1.5 solar illumination. The photoelectrochemical parameters such as short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF) and overall conversion efficiency (η) for the fabricated cells are summarized in Table 1 and the corresponding photocurrent density-voltage (J-V) curves are shown in Fig. 5.
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The conversion efficiency of the DSSCs fabricated with natural dyes extracted from HO, TO and DA dyes are 0.46%, 0.32% and 0.24% respectively. The lower efficiency of the solar cells fabricated using the extracted natural photo sensitizers might be due the presence of 4-tertbutylpyridine (TBP) in the electrolyte. The presence of TBP expected to increases the electrolyte pH, which leads to increase in the quasi Fermi level of TiO2 (the energy conduction band). Thus reducing the electron injection yield of pheophytin a and leads to decrease in the Jsc of the DSSC [28,29]. Due to the absence of specific functional groups, most of the natural dyes used as photo sensitizers in DSSC showed very low efficiencies compared to synthetic dyes [30]. In DSSC the generation of short circuit current (Jsc), depends on the quantity of dye molecules adsorbed on the TiO2 surface, dye structure, light harvesting efficiency and the electron injection ability of the dyes [31]. More adsorption of dye molecules on the TiO2 surface generates large number of photons from sunlight, which in turn leads to faster electron injection. The important parameters which decide the efficiency of the DSSC are open circuit voltage (Voc) and short circuit current (Jsc). The open circuit voltage, i.e. the difference between the Fermi level of TiO2 electrode and the potential of redox electrolyte mainly depends on the recombination rate and adsorption mode of the sensitizer [31,32]. The DSSC fabricated with HO dye shows relatively higher Jsc and enhanced efficiency of 0.46% compared to TO and DA dyes. 3.5. Electrochemical Impedance Spectroscopy The electrochemical impedance spectroscopy (EIS) measurements were performed to understand the electron transport behavior and internal resistance of the DSSCs. EIS analysis is a powerful method to obtain additional information and deeper understanding on the interfacial reactions of photoexcited electrons in DSSCs. It is well known that the impedance spectrum of
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DSSC will exhibit three semicircles [33,34]. The smaller semicircle at the high frequency was assigned to the charge-transfer resistance (R1) at the counter electrode-electrolyte interface, while the larger semicircle at the medium frequency (R2) is related to the electron recombination at the TiO2 electrode-electrolyte interface [35]. The semicircle associated with Warburg diffusion of the redox I-/I3- couple within the electrolyte is usually seen at frequencies less than 0.1 Hz and requires an extended acquisition time [36]. The Nyquist plots of the fabricated devices were represented in Fig. 6 which shows only two semicircles corresponds to R1 and R2 and the electrochemical parameters derived from the Nyquist plots were summarized in Table 2. The radii of the second semicircles (R2) indicate a decreasing order HO>TO>DA. The large R2 value for HO dye can be ascribed to huge charge transfer resistance which leads to the slower electron recombination between TiO2 and electrolyte [37]. In addition, the electron recombination lifetime (τe) could be calculated from the equation, τe = (2πfmax)−1 in which fmax is the frequency at the top of the intermediate-frequency arc [38] in Bode phase plots shown in Fig. 7. The calculated electron life time values are listed in the Table 2. The increasing order of electron life time of the extracted dyes are DA
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In summary, the natural dyes extracted from Hierochloe odorata (HO), Torulinium odoratum (TO) and Dactyloctenium aegyptium (DA) grasses were successfully used as light harvesters in dye sensitized solar cells. The result shows that the maximum efficiency obtained for the cell fabricated with HO grass dye is around 0.46%, whereas the conversion efficiency of the cells fabricated with TO and DA dyes are 0.32% and 0.24% respectively. The obtained UV-vis absorption spectra for the extracted dyes show absorption maxima identical to the Pheophytin a. The FT-IR spectra for the extracted dyes replicate the structural properties of Pheophytin a. In the similar way, LC-MS studies reveal the signals of molecular ion (M+) at m/z ~871.55 responsible for the presence of light harvesting pigment Pheophytin a. The electron transport mechanism and internal resistance of the DSSCs were investigated by the electrochemical impedance spectroscopy (EIS) to clarify the relationships between the performance of DSSCs and their internal resistances. The charge transfer resistance (R2) and electron recombination life time for HO dye is high compared to TO and DA dyes, which is well reflected in the photoelectric conversion efficiency of the cell. Hence the high efficiency of DSSC with HO dye is due to the better binding of the dye molecules with TiO2 layer and huge charge transfer resistance at TiO2-dye-electrolyte interface.
Acknowledgments The authors (S.V. and R.M.) gratefully acknowledge the financial support of UGC, New Delhi, India [F.No.41-918/2012(SR) dated 23.07.2012].
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Figure captions: Fig. 1. UV-vis Absorption spectra for light harvesters obtained from grasses a) HO, b) TO and c) DA. Fig. 2. LC-MS spectra for dyes a) Hierochloe Odarata (HO), b) Torulinium Odaratum, c) Dactyloctenium Aegyptium d) Structure of Pheophytin a (m/z = 871.55) with empirical formula (C55H74N4O5). Fig. 3. FT-IR spectra of three kinds of grass dyes (a) HO, (b) TO and (c) DA. Fig. 4. SEM image of the prepared TiO2 film. Fig. 5. Photocurrent density-voltage (J-V) curves for the DSSCs sensitized by three kinds of grass extracts viz., a) HO, b) TO, and c) DA under the illumination of 100 mW cm-2. The J-V curve of standard N3 dye is shown as inset for comparison. Fig. 6. Nyquist plots for the fabricated cells measured in the range 0.1 Hz to 100 kHz at 100 mWcm−2. The inset shows the Nyquist plots in high frequency region. Fig. 7. Bode phase plots of DSSCs measured at 100 mWcm−2
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Tables Dyes
Jsc (mA/cm2)
Voc (mV)
FF
η (%)
HO
2.199
593.55
0.3554
0.46
TO
1.004
654.21
0.4832
0.32
DA
0.698
718.91
0.4807
0.24
N3 dye
8.30
782
0.6246
4.05
Table 1 Photoelectrochemical parameters of the cells sensitized with grass dye sensitizers.
S. No
Dyes
R2 (Ω)
fmax (Hz)
τe (ms)
1
HO
105.44
1.048
151
2
TO
73.42
2.682
59
3
DA
55.96
5.428
29
Table 2 The electrochemical parameters derived from the Nyquist and Bode phase plots of DSSCs with three kinds of grass dye sensitizers.
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Figure. 1.
A b so r b a n c e (A .U .)
HO TO DA
400
500
600
Wavelength (nm)
18
700
800
Figure. 2(a).
100 90
Relative Abundance
80 70 60
871.60
50 40 30 20 10
365.10 203.95
0 200
300
613.55
475.35
400
756.50
500
600
700
800
887.60
900 1000
m/z Figure. 2(b).
100 90
Relative Abundance
80 70
621.20
60 50
307.15
40 30 20
221.10
424.30
871.55
10 577.10
0 200
300
400
500
600
738.50
700
800
900 1000
m/z 19
Figure. 2(c).
100 90
Relative Abundance
80 70 60 50 40 30 20
871.55 221.10
621.20
365.00
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Figure. 2(d).
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Figure. 3.
% Transmittance (A.U.)
(a) HO
(b) TO
(c) DA
3500
3000
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Wavenumber (cm-1)
21
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Figure. 4.
22
Figure. 5.
Photocurrent density (mA.cm -2 )
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(a) HO 2.0 1.5
(b) TO 1.0
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Photovoltage (mV)
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Figure. 6.
-140 -6
-120
-5
(c) DA
-Z'' (Ω)
-4
-Z'' (Ω )
-100
-3
(a) HO
-2
-80
(b) TO
-1 25 26 27 28 29 30 31 32
-60
Z' (Ω)
-40 (a) HO
-20 (c) DA
0 0
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80 Z' (Ω )
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(b) TO
100 120 140
Figure. 7.
35 (a) HO
30
Theta (Deg)
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(b) TO (c) DA
20 15 10 5 0 0.1
1
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Graphical Abstract:
26
Highlights: •
Chlorophyll derivatives obtained from grasses were used as sensitizer in DSSCs.
•
Pheophytin a was dominant pigment in Hierochloe Odorata (HO) extract.
•
High current density and efficiency was obtained for HO dye based cell.
•
Electron transport mechanism and internal resistance of the DSSCs investigated by EIS.
27
S1386-1425(14)01177-9 http://dx.doi.org/10.1016/j.saa.2014.07.096 SAA 12516
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
19 May 2014 10 July 2014 29 July 2014
Please cite this article as: V. Shanmugam, S. Manoharan, A. Sharafali, S. Anandan, R. Murugan, Green grasses as light harvesters in dye sensitized solar cells, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.07.096
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Green grasses as light harvesters in dye sensitized solar cells Vinoth Shanmugama, Subbaiah Manoharanb, Sharafali. Aa, Sambandam Anandanb and Ramaswamy Murugana* a
b
Department of Physics, Pondicherry University, Puducherry 605 014, India
Nanomaterials and Solar Energy Conversion Lab, Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India *Email: [email protected]
ABSTRACT Chlorophylls, the major pigments presented in plants are responsible for the process of photosynthesis. The working principle of dye sensitized solar cell (DSSC) is analogous to natural photosynthesis in light-harvesting and charge separation. In a similar way, natural dyes extracted from three types of grasses viz. Hierochloe Odorata (HO), Torulinium Odoratum (TO) and Dactyloctenium Aegyptium (DA) were used as light harvesters in dye sensitized solar cells (DSSCs). The UV-Vis absorption spectroscopy, Fourier transform infrared (FT-IR), and liquid chromatography-mass spectrometry (LC-MS) were used to characterize the dyes. The electron transport mechanism and internal resistance of the DSSCs were investigated by the electrochemical impedance spectroscopy (EIS). The performance of the cells fabricated with the grass extract shows comparable efficiencies with the reported natural dyes. Among the three types of grasses, the DSSC fabricated with the dye extracted from Hierochloe Odorata (HO) exhibited the maximum efficiency. LC-MS investigations indicated that the dominant pigment present in HO dye was pheophytin a (Pheo a).
1
Keywords: Natural dyes, grass extract, liquid chromatography-mass spectrometry, pheophytin a, Dye sensitized solar cells *Corresponding Author. Tel.: +91 413 2654782; fax: +91 413 2656988 E-mail address: [email protected] (R. Murugan)
1. Introduction There is a global challenge to capture and utilize solar energy for a large scale sustainable growth. In recent years, dye-sensitized solar cells (DSSCs) have attracted great interest in scientific research and applications owing to their low cost, large flexibility in shape, transparency, easy fabrication and greater variety of colors [1-3]. Dye sensitized solar cells (DSSCs) have been fascinated as a unique candidate for the conversion of light photons into electricity in both diffuse light and direct sunlight, without compromising on its performance. The typical DSSCs comprise of a nanocrystalline TiO2, dye molecule, an electrolyte containing the iodine/triiodine (I-/I3-) redox couple and a counter electrode. The process of converting sunlight into electricity takes place in DSSC based on the sensitization of wide band gap semiconductor by dye sensitizer. Hence, numerous inorganic, organic and hybrid dyes have been used as sensitizers in DSSCs. Among the synthetic inorganic dyes, ruthenium polypyridyl complex was effectively employed as molecular sensitizer in DSSCs with high efficiency up to 11-12% [4-7]. However, ruthenium polypyridyl complexes are expensive and less abundant, hence natural dyes are an immense interest towards the use of sensitizers in DSSCs. Several natural pigments such as chlorophyll [8-15], anthocyanin [16-18], tannin [19], and carotene [20,21] have been successfully used as sensitizers in DSSCs. Unlike synthetic inorganic dyes, natural dyes offer various advantages such as non-toxic, 2
environmentally friendly, fully biodegradable and hence an emphasis on using natural products are increasing day by day [22,23]. A paradigm for the use of natural products in DSSC was reported recently, in which the extracts of marine plant (sea tangle) were used as a dye sensitizer, as well as redox couple and counter electrode and this all natural cell exhibits efficiency of 1.4% [24]. The first report on photosensitization of nanoporous TiO2 by chlorophyll derivatives was reported by Kay and Grätzel [8]. Over the years, considerable efforts have been put forward towards employing chlorophylls as sensitizer in DSSCs. Many reports have showed that chlorophylls, which act as an effective photosensitizer in photosynthesis of green plant, have the potential to be an environment friendly dye source [9-12]. Chlorophylls are metal-complexes of magnesium ion, with high symmetry, consisting of a tetrapyrrolic macrocycle, encompass several pigments with common structural elements [11]. The chlorophylls (Chl a and Chl b) are usually accompanied by their major derivatives, chlorophyllides (chlid a and child b) and pheophytins (Pheo a and Pheo b). The carboxylic acid groups in the photosensitizer established an electronic coupling with the conduction band of TiO2 which is helpful for anchoring the dye molecules and an effective electron injection to the conduction band of TiO2. Thus, the carboxylic acid groups in the dye molecule are essential for dye-sensitized solar cell [10]. Chl-a, Chl-b and pheophytins are not strongly bind to the TiO2 surface due to the weak interaction of the phytyl ester group and keto carbonyl groups with the hydrophilic titania surface [8]. But Chlorophyll c1 (Chl-c1) and Chlorophyll c2 (Chl-c2) have the terminal carboxylic acid group, which is connected through a conjugated double bond of the porphyrin macrocycle. Hence Chlc1 and Chl-c2 are strongly bound on the TiO2 surface which ensures efficient electron injection into the TiO2 conduction band and to prevent gradual leaching by the electrolyte [12].
3
Herein, we report the performance of DSSC fabricated using dyes extracted from three types of grasses viz. Hierochloe Odorata (HO), Torulinium Odoratum (TO) and Dactyloctenium Aegyptium (DA) as sensitizers in order to gain further information on the role of various chlorophyll derivatives as sensitizer in DSSCs. 2. Experimental 2.1. Extraction of dyes A simple extraction technique was employed for the extraction of the dye from the grasses. The three grasses Hierochloe Odorata (HO), Torulinium Odoratum (TO) and Dactyloctenium Aegyptium (DA) were collected from the campus of Pondicherry University, India. Fresh grasses were washed with distilled water and vacuum dried at 60 °C. The dried grasses were crushed into fine powder using a mortar. About 1g of the powdered grass of each type was separately put into a beaker containing 50 ml of absolute ethanol, which was kept in the absence of sunlight for 24 hours. Finally the solid residue was filtered to acquire a pure natural dye solution. 2.2. Fabrication of DSSCs The TiO2 working electrode was prepared by deposition of TiO2 paste on FTO glass (Sheet resistance 10 Ωsq. -1; BHEL, India) with the active area of 1×1 cm2 according to the procedure described elsewhere [25] which was sintered at 450 °C in a tubular furnace. And so, the preheated TiO2 electrodes were cooled down to ~80 °C and soaked overnight in the ethanol extract of each dye obtained from three different grasses. Platinized counter electrodes were prepared by drop casting of 0.05 M Hexachloroplatinic acid (H2PtCl6) in isopropanol. After being rinsed with ethanol, the photoanode was placed on top of the counter electrode and tightly clipped together to form a cell. An electrolyte was then injected into the space between two 4
electrodes. The electrolyte was composed of 0.5 M lithium iodide (LiI), 0.05 M iodine (I2), and 0.5 M 4-tert-butylpyridine (TBP) dissolved in 3-methoxypropionitrile. The standard cell using cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II) (N3 dye) as the sensitizer was fabricated similarly for comparison with the DSSCs fabricated with grass extracts. The fabrication details of the DSSCs have been described previously [18]. 2.3. Measurements and Characterizations The absorption spectra of the extracts obtained from grasses were recorded using a UVvis spectrophotometer (UV-2450, Shimadzu). Fourier Transform Infrared spectra (FTIR, Thermo Nicolet 6700) of the extracts were recorded in the range 500-4000 cm-1 using KBr technique. High resolution liquid chromatography-mass spectrometry (Shimadzu LC-MS-2010) was used to record liquid chromatography-mass spectrometry (LC-MS) spectra. The surface morphology of the deposited TiO2 film was examined using a scanning electron microscope (SEM, Hitachi S3400N). The photovoltaic properties of the fabricated devices were measured under dark and simulated solar light intensity of 100 mWcm-2. Electrochemical impedance spectroscopy (EIS) measurements were carried out under 100 mWcm-2 light illumination in the frequency range of 0.1 Hz to 100 kHz at their open circuit potential (OCP). The photocurrent density-voltage and impedance characteristics of the fabricated cells were recorded with an Autolab potentiostat / galvanostat-84610. 3. Results and discussion 3.1. Absorption Spectra Chlorophylls have two major absorption bands red (Qy) and blue (Soret) in the visible region. The red and blue bands of Chl a were found to be located at 662 and 430 nm, and of Chl b at 646 and 457 nm, respectively [26]. The absorption maximum of the red and blue bands for
5
Pheo a were located at around 666 and 410 nm, respectively [26]. The major difference observed between the absorption spectra of chlorophylls and pheophytins are that the absorption maximum in Qy region exhibits shift to longer wavelengths and in contrast absorption maximum in soret region exhibits shift to shorter wavelengths for pheophytins compared to chlorophylls [26]. To identify the compound present in the alcoholic extract of HO, TO, and DA grasses (natural dyes) the absorption study was performed and whose corresponding spectra were presented in Fig. 1. Altogether all the three dyes show two major absorption peaks in the region around 660 nm and 400 nm and in addition, two very weak bands were observed at around 540 nm and 606 nm. The observed blue band at 400 nm, red band at 660 nm and weak bands located at 540 and 606 nm are in accordance with the reported data for Pheo a [26]. High-resolution liquid chromatography-mass spectrometry (LC-MS) investigations for each grass extract indicates signals of molecular ions (M+) at m/z 871.55 as shown in Fig. 2 (a-c), which agrees well with the earlier electrospray-ionization mass spectrometry (ESI-MS/MS) studies of chlorophyll pigments [26]. In the mass spectra of the dyes extracted from the investigated grasses, the observed peak at m/z ~871.55 was designated as a molecular ion-peak of Pheophytin a derivative of chlorophyll with the molecular formula (C55H74N4O5) [26]. The chemical structure of Pheophytin a is shown in Fig. 2(d). The second fragmentation found at m/z 621.20 in dyes TO and DA may be chlorophyllide a/b. The LC-MS studies indicated that Pheophytin a is a dominant constituent in HO dye. 3.2. FT-IR spectroscopic studies The recorded FT-IR spectra of the natural dyes extracted from HO, TO and DA shown in Fig. 3 (a-c) exhibit most of the characteristic peaks of the chlorophyll derivatives [27]. Chlorophylls and their derivatives have characteristic infrared bands corresponding to C=O,
6
C=C, C–H and N–H groups. The characteristic C=O band corresponding to the ketone in the cyclopentanone ring of pheophytin a is observed at 1725 cm-1 for all the three extracted dyes. Similarly the C=C vibrations of pheophytin a is observed at around 1618 cm-1. Vibrational bands positioned around 3340 cm-1 correspond to N–H stretching vibrations. The bands observed at around 2924 and 2854 cm-1 represent saturated C–H stretching vibrations. The FT-IR spectral analysis also confirms the presence of Pheo a in all the three grass extracts, which is well matched with the UV-Vis and LC-MS spectral results. 3.3. Surface morphology of TiO2 electrode Fig. 4 shows typical SEM image of TiO2 coated FTO glass plate. The SEM image indicates that the TiO2 particles were spherical in shape and composed with a high degree of porosity which may help to improve adsorption of dye molecules on the surface. Further, the fine adsorption of dye molecules may enhances the electron injection and leads to better photoelectric conversion efficiency of the solar cell. 3.4. Photovoltaic Performance To examine the extracts of green grasses as light harvesters and electron injectors for DSSC application, the dye-sensitized solar cells were fabricated using the three grass dyes HO, TO and DA. Photovoltaic cells were prepared with the effective light exposure area of about 1x1 cm2 and a liquid electrolyte composed of 0.05 M I2/0.5 M LiI/0.5 M tert-butylpyridine in 3methoxypropionitrile solution. The cell performances were tested under AM 1.5 solar illumination. The photoelectrochemical parameters such as short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF) and overall conversion efficiency (η) for the fabricated cells are summarized in Table 1 and the corresponding photocurrent density-voltage (J-V) curves are shown in Fig. 5.
7
The conversion efficiency of the DSSCs fabricated with natural dyes extracted from HO, TO and DA dyes are 0.46%, 0.32% and 0.24% respectively. The lower efficiency of the solar cells fabricated using the extracted natural photo sensitizers might be due the presence of 4-tertbutylpyridine (TBP) in the electrolyte. The presence of TBP expected to increases the electrolyte pH, which leads to increase in the quasi Fermi level of TiO2 (the energy conduction band). Thus reducing the electron injection yield of pheophytin a and leads to decrease in the Jsc of the DSSC [28,29]. Due to the absence of specific functional groups, most of the natural dyes used as photo sensitizers in DSSC showed very low efficiencies compared to synthetic dyes [30]. In DSSC the generation of short circuit current (Jsc), depends on the quantity of dye molecules adsorbed on the TiO2 surface, dye structure, light harvesting efficiency and the electron injection ability of the dyes [31]. More adsorption of dye molecules on the TiO2 surface generates large number of photons from sunlight, which in turn leads to faster electron injection. The important parameters which decide the efficiency of the DSSC are open circuit voltage (Voc) and short circuit current (Jsc). The open circuit voltage, i.e. the difference between the Fermi level of TiO2 electrode and the potential of redox electrolyte mainly depends on the recombination rate and adsorption mode of the sensitizer [31,32]. The DSSC fabricated with HO dye shows relatively higher Jsc and enhanced efficiency of 0.46% compared to TO and DA dyes. 3.5. Electrochemical Impedance Spectroscopy The electrochemical impedance spectroscopy (EIS) measurements were performed to understand the electron transport behavior and internal resistance of the DSSCs. EIS analysis is a powerful method to obtain additional information and deeper understanding on the interfacial reactions of photoexcited electrons in DSSCs. It is well known that the impedance spectrum of
8
DSSC will exhibit three semicircles [33,34]. The smaller semicircle at the high frequency was assigned to the charge-transfer resistance (R1) at the counter electrode-electrolyte interface, while the larger semicircle at the medium frequency (R2) is related to the electron recombination at the TiO2 electrode-electrolyte interface [35]. The semicircle associated with Warburg diffusion of the redox I-/I3- couple within the electrolyte is usually seen at frequencies less than 0.1 Hz and requires an extended acquisition time [36]. The Nyquist plots of the fabricated devices were represented in Fig. 6 which shows only two semicircles corresponds to R1 and R2 and the electrochemical parameters derived from the Nyquist plots were summarized in Table 2. The radii of the second semicircles (R2) indicate a decreasing order HO>TO>DA. The large R2 value for HO dye can be ascribed to huge charge transfer resistance which leads to the slower electron recombination between TiO2 and electrolyte [37]. In addition, the electron recombination lifetime (τe) could be calculated from the equation, τe = (2πfmax)−1 in which fmax is the frequency at the top of the intermediate-frequency arc [38] in Bode phase plots shown in Fig. 7. The calculated electron life time values are listed in the Table 2. The increasing order of electron life time of the extracted dyes are DA
9
In summary, the natural dyes extracted from Hierochloe odorata (HO), Torulinium odoratum (TO) and Dactyloctenium aegyptium (DA) grasses were successfully used as light harvesters in dye sensitized solar cells. The result shows that the maximum efficiency obtained for the cell fabricated with HO grass dye is around 0.46%, whereas the conversion efficiency of the cells fabricated with TO and DA dyes are 0.32% and 0.24% respectively. The obtained UV-vis absorption spectra for the extracted dyes show absorption maxima identical to the Pheophytin a. The FT-IR spectra for the extracted dyes replicate the structural properties of Pheophytin a. In the similar way, LC-MS studies reveal the signals of molecular ion (M+) at m/z ~871.55 responsible for the presence of light harvesting pigment Pheophytin a. The electron transport mechanism and internal resistance of the DSSCs were investigated by the electrochemical impedance spectroscopy (EIS) to clarify the relationships between the performance of DSSCs and their internal resistances. The charge transfer resistance (R2) and electron recombination life time for HO dye is high compared to TO and DA dyes, which is well reflected in the photoelectric conversion efficiency of the cell. Hence the high efficiency of DSSC with HO dye is due to the better binding of the dye molecules with TiO2 layer and huge charge transfer resistance at TiO2-dye-electrolyte interface.
Acknowledgments The authors (S.V. and R.M.) gratefully acknowledge the financial support of UGC, New Delhi, India [F.No.41-918/2012(SR) dated 23.07.2012].
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Figure captions: Fig. 1. UV-vis Absorption spectra for light harvesters obtained from grasses a) HO, b) TO and c) DA. Fig. 2. LC-MS spectra for dyes a) Hierochloe Odarata (HO), b) Torulinium Odaratum, c) Dactyloctenium Aegyptium d) Structure of Pheophytin a (m/z = 871.55) with empirical formula (C55H74N4O5). Fig. 3. FT-IR spectra of three kinds of grass dyes (a) HO, (b) TO and (c) DA. Fig. 4. SEM image of the prepared TiO2 film. Fig. 5. Photocurrent density-voltage (J-V) curves for the DSSCs sensitized by three kinds of grass extracts viz., a) HO, b) TO, and c) DA under the illumination of 100 mW cm-2. The J-V curve of standard N3 dye is shown as inset for comparison. Fig. 6. Nyquist plots for the fabricated cells measured in the range 0.1 Hz to 100 kHz at 100 mWcm−2. The inset shows the Nyquist plots in high frequency region. Fig. 7. Bode phase plots of DSSCs measured at 100 mWcm−2
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Tables Dyes
Jsc (mA/cm2)
Voc (mV)
FF
η (%)
HO
2.199
593.55
0.3554
0.46
TO
1.004
654.21
0.4832
0.32
DA
0.698
718.91
0.4807
0.24
N3 dye
8.30
782
0.6246
4.05
Table 1 Photoelectrochemical parameters of the cells sensitized with grass dye sensitizers.
S. No
Dyes
R2 (Ω)
fmax (Hz)
τe (ms)
1
HO
105.44
1.048
151
2
TO
73.42
2.682
59
3
DA
55.96
5.428
29
Table 2 The electrochemical parameters derived from the Nyquist and Bode phase plots of DSSCs with three kinds of grass dye sensitizers.
17
Figure. 1.
A b so r b a n c e (A .U .)
HO TO DA
400
500
600
Wavelength (nm)
18
700
800
Figure. 2(a).
100 90
Relative Abundance
80 70 60
871.60
50 40 30 20 10
365.10 203.95
0 200
300
613.55
475.35
400
756.50
500
600
700
800
887.60
900 1000
m/z Figure. 2(b).
100 90
Relative Abundance
80 70
621.20
60 50
307.15
40 30 20
221.10
424.30
871.55
10 577.10
0 200
300
400
500
600
738.50
700
800
900 1000
m/z 19
Figure. 2(c).
100 90
Relative Abundance
80 70 60 50 40 30 20
871.55 221.10
621.20
365.00
10 422.95
0 200
300
400
574.95
500
600
758.50
700
800
887.35
900 1000
m/z
Figure. 2(d).
20
Figure. 3.
% Transmittance (A.U.)
(a) HO
(b) TO
(c) DA
3500
3000
2500
2000
1500
Wavenumber (cm-1)
21
1000
500
Figure. 4.
22
Figure. 5.
Photocurrent density (mA.cm -2 )
2.5
(a) HO 2.0 1.5
(b) TO 1.0
(c) DA 0.5 0.0 0
150
300
450
Photovoltage (mV)
23
600
750
Figure. 6.
-140 -6
-120
-5
(c) DA
-Z'' (Ω)
-4
-Z'' (Ω )
-100
-3
(a) HO
-2
-80
(b) TO
-1 25 26 27 28 29 30 31 32
-60
Z' (Ω)
-40 (a) HO
-20 (c) DA
0 0
20
40
60
80 Z' (Ω )
24
(b) TO
100 120 140
Figure. 7.
35 (a) HO
30
Theta (Deg)
25
(b) TO (c) DA
20 15 10 5 0 0.1
1
10
100
Frequency (Hz)
25
1000
10000
Graphical Abstract:
26
Highlights: •
Chlorophyll derivatives obtained from grasses were used as sensitizer in DSSCs.
•
Pheophytin a was dominant pigment in Hierochloe Odorata (HO) extract.
•
High current density and efficiency was obtained for HO dye based cell.
•
Electron transport mechanism and internal resistance of the DSSCs investigated by EIS.
27