Dye-sensitized Solar Cells From Extracted Bracts Bougainvillea Betalain Pigments
A.R. Hernández-Martínez(1), S. Vargas(1), M. Estevez(1), R. Rodríguez(1,2)*
1) Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México, Campus Juriquilla, Boulevard Juriquilla No. 3001, C.P. 76230., Juriquilla, Qro. Mexico. 2) Ciencias de la Salud, UVM, Campus Querétaro, Juriquilla, Qro. CP 76230, Mexico
ABSTRACT
The performance of a new dye-sensitized solar cells (DSSCs) based in a natural dye extracted from Bougainvillea glabra and spectabilis’ bracts, is reported. Therefore there was an extract from each of both plants, which were studied and compared including the mixture of both extracts. The pigment was betalain obtained from Reddish-purple extract. This dye was purified and characterized using FTIR and UV–Vis. Solar cells were assembled using TiO2 thin film on ITO-coated glass; a mesoporous film was sensitized with the Bougainvillea extracts. The films were applied by dip coating. The obtained solar energy conversion efficiency was of 0.48% with a current density Jsc of 2.29 mA/cm2 using an irradiation of 100mW/cm2 at 25 ºC.
Keywords: solar cell, betalaine, natural dye, solar energy, dip coating. * Corresponding author:
[email protected]
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INTRODUCTION
Dye-sensitized solar cells (DSSCs) are the third generation of photovoltaic devices for the conversion of visible light in electric energy. These new types of solar cells are based on the photosensitization produced the dyes on wide band-gap mesoporous metal oxide semiconductors; this sensitization is produced by the dye absorption of part of the visible light spectrum. One aspect particularly attractive of these DSSCs photocells is the low cost solar energy conversion into electricity; this is possible mainly due to the use of inexpensive materials and the relative easiness in the fabrication processes [1-3]. Recent studies have shown that the use of metal oxides as SnO2, ZnO2 Nb2O5, but mainly TiO2, has been successfully used as photo-anode when a dye is absorbed in the interior of the porous layer. The performance of DSSCs can be understood as a competition between two principal redox processes: electrons injection with rate constants of the order of picoseconds (10-15 - 10-12 s) and the regeneration of the oxidized dye with rate constants from 107 to 109s-1 [4-9]. The injected electrons are transported through the TiO2 film to a transparent electrode (ITO), while a redox-active electrolyte of I-/I3- is utilized to reduce the dye cation charge and transport the resulting positive charge to a counterelectrode; however, before this, the photo-induced electron injection from the sensitizer dye to the TiO2 film conduction band, initiates charge separation [10, 11]. In this sense, the sensitized dye acts as the photo-driven electron pump of the device. Several inorganic, organic and hybrid compounds have been investigated as sensitizer including porphyrins, phtalocyanines, platinum complexes, fluorescent dyes, among others. Ru-based complexes sensitizers had better efficiency [ ]. However their advantages are offset by their expense and tendency 2
to undergo degradation on presence of water. The use of natural pigments as sensitizing dye for the conversion of solar energy in electricity is very interesting because, on one hand enhance the economical aspect and, on the other produce significant benefits from the environmental point of view [12, 18]. Natural pigments extracts from fruits, vegetables or plants, such as chlorophyll and anthocyanins have been extensively investigated as DSSCs sensitizer. Early reports focused on the redox chemistry of betalains, have shown that these dyes have achieved maximum monochromatic photon to electron conversions exceeding 60% and solar energy conversion efficiency of 1.7%, about half of that obtained with an equivalent N719 sensitized cell [19]. Betalain pigments are dyes with high potential as sensitizer, therefore purified extracts from commercial sources have been subjected to a detailed photo-electrochemical study [20]. These pigments are present in flowers petals, fruits, leaves, stems and roots of the Caryophyllales plants; they have high molar extinction coefficients in the visible region and pH-dependent redox properties. The betalain pigments derived from betalamic acid are divided into two subgroups; the red betacyanins with maximum absorptivity at ≈ 535 nm, and the yellow betaxanthins with maximum absorptivity at ≈ 480 nm. Betalaines can be easily obtained from Bougainvillea plants whose colour is produced by 90% from betacyanins, which the most important of those is the bethanidine, while the remaining 10% comes from indicaxanthin a common betaxanthin. These plants often grow in temperated climates having small flowers enclosed by large, brilliant red or purple bracts (modified leaves). The schematic structures of betacyanin (red-purple) and indicaxanthin (yellow-orange) are reported in Fig. 1 [21]. As shown in this figure, both dyes contain carboxylic groups that facilitate the TiO2 surface binding. Here
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it is reported the results in the performance of DSSCs assembled using raw extracts of the species: Bougainvillea spectabilis and glabra. EXPERIMENTAL
Preparation of Dye-Sensitizer Solutions
The fresh Bougainvillea glabra and spectabilis flowers were harvested in central Mexico (Queretaro). There were used violet and red bracts from both species. All flowers’ juices were prepared by grinding 12 g of dried flowers in 500 ml of acid water with pH 5.7; the mixtures were filtered and centrifuged to remove any solid residue and stabilized at pH = 5.7 by addition of aqueous HCl. Two different extracts from red Bougainvillea glabra were used: the first one was taken from the half of the extract and purified using an Amberlited packed column and 1 M CH3COONH4 in acid water as the eluting solvent to leave only the yellow and red betalains fractions; the second half was used as prepared to verify whether an efficient sensitization effect can be achieved with minimal chemical procedures and whether the other compounds, in addition to the dye itself, do not interfere with the performed of the dye. The rest of the extracts; red Spectabilis, violet Spectabilis and violet glabra, were used without further purification. The dye solutions in acid conditions (pH = 5.0) are stable with a halftime deactivation of more than 12 months when they are properly stored, protected from direct sunlight and refrigerated at about 4 °C [22]. The betaxanthin content was determined using UV-Vis spectroscopy.
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Electrodes Preparation
The conductive glass plates (ITO-coated glass slide (In2O3:SnO2) with sheet resistance of 30-60 Ω/cm2, 84% transmittance nominal at 550 nm, dimensions (L,W,D) of 25 x 25 x 1.1 mm. Titanium oxide (TiO2) nanopowder (mesh 320) and solvents (Aldrich) were analytic grade; they were used as received. ITO substrates were ultrasonically cleaned in ethanol and water for 30 min, and then heated at 450 ºC others 30 min prior to film deposition. The photoanodes were prepared by depositing two TiO2 film on the cleaned ITO glass: for the first film the ITO glass was immersed in a sol-gel solution containing a mixture of titanium isopropoxide, water and isopropanol at concentrations: 2:1:25 vol at constant speed (1.5 cm/min) [23]; the coated glass was heated at 450 C° during 30 min creating a densified TiO2 this thin film isolate the dye-activated porous TiO2 layer from the conductor glass. Subsequently the two edges of the ITO glass plate were covered with adhesive tape (Scotch 3M) to control the thickness of the film; finally a TiO2 paste was spread uniformly on the substrate by sliding a glass rod along the tape spacer. The TiO2 paste was prepared by blending 3.0 g of commercial TiO2 nano-powder, 10 mL 0.1 N nitric acid, 4 ml polyethylene glycol; this suspension was stirred in a closed glass container for 24 hours to obtain of appropriate viscosity. The film was produced and cooked at 450 ºC for 60 minutes resulting in a mesoporous film with a thicknees of around 8–10 μm and opaque. The TiO2 photo-anodes were first soaked for 12 h in HCl and then immersed in the natural dye solutions for one night at room temperature, according to published procedures [24]. Later, the photoanodes were rinsed with distilled water and ethanol and dried. Carbon coated counter electrodes
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were prepared following a procedure reported elsewhere [25].
DSSC Assembling
DSSC assemble, An electrolyte solution was prepared mixing 0.1 M of I2, 0.05 M of LiI, 0.05 M of 3-methoxypropionitrile in 50 mL of C2H3N, stirring for 60 minutes. This electrolyte solution was poured in the mesoporous TiO2 film, which was previously prepared with a parafilm framework to seal cells. The counter electrode was pressed against the impregnated anode and clamped firmly in a sandwich configuration. No leaks (solvent evaporation) were detected.
Characterization
The topology of the TiO2 films was obtained using a scanning electron microscope (SEM) Jeol JSM- 6060LV operated at 20 kV in secondary electron mode with different magnifications; the samples were covered with a gold film. The particle size and the particle size distribution of the TiO2 particles were determined using a light scattering apparatus Brookhaven Instruments model BI200SM, equipped with a high speed digital correlator PCI-BI9000AT, solid-state detector and He-Ne laser of 35 mW Melles Griot 9167EB-1 as the light source. The crystalline structure of the TiO2 thin was determined using a diffractometer Rigaku model Miniflex+ equipped with a radiation source of 1.54 Å and the angle 2Θ was varied from 5° to 80° at a scan of 2 °/min. FTIR and -Ramman spectra of the initial and the modified ITO-glass substrate were analyzed using a FT-IR Sentra Bruker spectrometer and a
6
spectromerter-Ramman Nicolet model 910, respectively. Absortion spectra were recorded using a UV-Vis spectrometer. DSSC were illuminated with incident power of about 100 mW/cm2 fron a 75 W Halogen lamp using an illumination area of 0.16 cm2 and UV and IR filter glass using in front of the sample. Photocurrent and photovoltage were measured used a Keithley 2400 source meter, and the incident light power was measured using a Powermeter Thor Labs S130A (0-300 mw) and Spectrometer Ocean optics HR 400.
RESULTS AND DISCUSSION
3.1. Absorption Spectra
Fig. 1 shows absorption spectrum of red Bougainvillea spectabilis extract (thick line) and spectrum of red Bougainvillea glabra extract (thin line) both diluted in water at pH 5.7. It were found two absorption peaks, the first near 482 nm which can be associated with the indicaxanthin and the second at 535 nm attributable to the betanin. The indicaxanthin concentration (Icon) in µM is estimated from the absorbance at 481 and 535 nm from [26]: (Icon) = 23.8*A481 – 7.7*A535. The extracted and diluted violet Bougainvillea spectabilis and glabra were scanned through with a spectrophotometer between the wavelengths of 260 to 700 nm. The absorption spectrum showed two major absorption peaks corresponded to 280 and 536 nm (Fig. 2). Absorbance peaks at 300 and 535 nm are characteristic absorptions for red violet betalain group, betacyanin [27].
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Violet Bougainvillea
Red Bougainvillea 1.6
1 0.9
Rel. Absorbance
Rel. Absorbance
1.4
0.567 480.0
0.8 0.7 0.6 0.5 0.4
0.595 481.0
0.3
310.0
1.2
0.948 307.0
1.0
0.716 535.0
0.8 0.6 0.4
0.2
0.520 535
0.1
0.2
0.367 547
0.0
0 300
340
380
420
460
500
540
580
620
660
300
700
Wavelength (nm)
340
380
420
460
500
540
580
620
660
700
Wavelength (nm)
Fig. 1 absorption spectrum of red Bougainvillea spectabilis extract
Fig. 2 absorption spectrum of violet Bougainvillea spectabilis extract
(thick line) and spectrum of red Bougainvillea glabra extract (thin
(thick line) and spectrum of violet Bougainvillea glabra extract (thin
line) both diluted in water and pH 5.7
line) both diluted in water and pH 5.7
The substrates were immersed for 12 hrs in a 1 M HCl solution, to facilitate the absorption of betanidin on betaxanthins, this procedure was reported by other authors, and it is known that betanin absorption is inhibited by the presence of the indicaxina. A dip in HCl results in the protonation of the betalainic carboxyl group from its anionic form and promoting its absorption into the TiO2. In the figure 3 the absortion spectrum of TiO2 films sensitized by red Bougainvillea spectabilis extract freshly prepared after its bath in an acid solution, is presented.
3.2. IR and Raman Spectra
TiO2 film on ITO conductor glass does not show clearly the characteristic sloping peak at 700 cm-1 of the pure TiO2 sample. This anomaly is easily explained because the ITO conductor glass presents two peaks, as shown in Figure 3a appreciate near 770 cm-1 and 990 cm-1 each, which mask the 8
characteristic peak of TiO2. However, Figure 3b shows a significant change in chemical structure as has been evidenced by the shift of the first peak of 770 cm-1 to 757 cm-1 (approaching the TiO2 typical peak around 700 cm-1) and the appearance of a third peak near the 1040 cm-1 that joins the peak of 923 cm-1. The third peak can be associated with waste water in the sample or any possible contamination which is not attributable either with the substrate or the TiO2 layer. Fig 3c show the IR spectrum of a conductor glass with two layers of TiO2, due to the higher concentration of TiO2 present in the sample it was obtained a small peak near 700 cm-1, followed by the two characteristic peaks of conductor glass, and a last peak around 1600 cm-1 already reported by other authors associated mainly to moisture within the TiO2.
3350
2800
2250
1700
1150
770
100
100
90 85 80 1045
90
Transmiittance [%]
95
Transmiittance [%]
100 90 80 70 60 50 40 30 20 10 0
Transmiittance [%]
915
3900
Mesoscopic oxide film
Compact thin underlayer of TiO2
ITO conductor glass
80 70 60 1584
923 757
954
50
770
40
75 3900
600
3350
2800
2250
1700
1150
600
3900
3350
2800
2250
1700
1150
600
Wavelength (cm -1)
Wavelength (cm -1)
Wavelength (cm -1)
Fig. 3a IR Spectra of conductive glass plates
Fig. 3b IR Spectra of TiO2 compact layer
Fig. 3c IR Spectra of ITO glass conductor plates
(ITO-coated glass slide In2O3:SnO2)
overlying conductive glass plates by sol-gel
with two layers of TiO2
3.3. Structure and Surface characteristic.
The TiO2 surface morphology is on Figure 4a and 4b. Figure 4a shows a compact structure of the first layer of TiO2, this proves that with immersing the ITO in a sol-gel precursor of TiO2 can be
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obtained slightly porous films in relation to the screen printing technique (Fig. 4b) where the porosity obtained is clearly much higher, other authors have proved the usefulness of this compact layer to achieve greater efficiency in the DSSC, this thin film acts as an insulator between the ITO and the dyeactivated porous TiO2 layer from the conductor. On the other hand, in figure 4b can be seen that the TiO2 second layer formed by screen printing technique present mesoporous. This mesoporous layer is formed by small spherical particles of TiO2 added one by one and that form nanopores across the surface. These nanoporous are useful because they increase the surface effective area where the Bougainvillea extracts access.
Fig. 4a IR SEM images for TiO2 compact layer overlying conductive
Fig. 4b SEM images for second TiO2 layer with nanoporous formed by
glass plates by sol-gel
screen printing technique
3.4. Photoelectrochemistry
Bougainvillea five different extracts were investigated, each of them are presented below. The first four correspond to extracts obtained from the Red Bougainvillea glabra (RBG), Violet 10
Bougainvillea glabra (VBG), Red Bougainvillea spectabilis (RBS), and Violet Bougainvillea spectabilis (VBS), were centrifuged and used fresh without further purification as explained above. The fifth one studied was a fresh centrifuged Red Bougainvillea spectabilis extract purified by a packed column to remove any substance that is not betalaine; the bulk of sucrose compounds were removed, this extract was called (PRBS). Table 1 summarizes the results for the five extracts and in all cases negligible current was obtained. In this section, the Short-circuit photocurrent Isc, the open-circuit voltage Voc, current density maximum power JMP, maximum power Voltage VMP, maximum power PM, theoretical power PT, fill factor FF, and the energy conversion efficiency are reported. It was found that regardless of the species of Bougainvillea used, the extract best performance parameters come from the red bracts as shown in Figure 5. This may possibly be due to the fact that these extracts have betaxanthin and betanin, each with different lengths of absorption, which may help the cell to capture photons of two different energies. Zhang had already reported that betaxanthin parameters are better than the betanin parameters. Figure 6 shows the power curves obtained in terms of voltage and once again you can see the improved sensitivity provided by red bracts with relation to the violet ones.
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Photopower - Photovoltage Curves 5.00E-04
2
4.00E-04
Photopower (mW)
Current Density (mA/cm 2)
Photocurrent - Photovoltage Curves 2.5
1.5 1 0.5
3.00E-04 2.00E-04 1.00E-04 0.00E+00
0 0
0.05
0.1
0.15
0.2
0.25
0
0.3
Violet Bougainvillea spectabilis
Red Bougainvillea glabra
Violet Bougainvillea glabra
0.1
0.15
0.2
0.25
0.3
Photovoltage (V)
Photovoltage (V) Red Bougainvillea spectabilis
0.05
Red Bougainvillea spectabilis
Violet Bougainvillea spectabilis
Red Bougainvillea glabra
Violet Bougainvillea glabra
Fig. 5 Photocurrent-photovoltage curves for a Bougainvillea extract
Fig. 6 Power-photovoltage curves for a Bougainvillea extract sensitized
sensitized solar cell.
solar cell.
CONCLUSIONS In this document the Bougainvillea spectabilis and glabra extract were studied as dye sensitized solar energy conversion. The results in Table 1 are consistent with those reported previously by other authors such as Zhang and Giuseppe on Bougainvillea, however, in this research a bigger FF was found because an insulating layer TiO2 compatible was used, as well as an electrolyte with the less volatility. The results shown that a betaxanthin and betanin mixture give a better conversion than betanin alone. However, by a deep study about the betaxanthin-betanin mixture ratio is possible to find the optimal ratio to improve the cell efficiency. Also were found that the use of a purified extract does not represent a significant efficiency increase, but is possible to see in further studies his effect on the cell’s useful life influenced by the presence of the sucrose compounds and betanin acid. 12
In this research were used best proven reported techniques in the DSSC studies, in consequence cells with higher parameters were successful obtain, as seen in Table 1; leaving the doors open to explore extracts from a variety of plants sources as a mixture or independently and to explore variations on the electrolyte composition to make it less volatile. Dye
Jsc
JMP
Source
(mA/cm-2)
(mA/cm-2)
RBG
2.344
VBG
VOC (V)
VMP (V)
PM (µA)
PT (µA)
FF
η (%)
1.966
0.26
0.231
0.45
0.60
0.74
0.45
1.860
1.568
0.23
0.199
0.31
0.43
0.71
0.31
RBS
2.291
2.083
0.28
0.229
0.48
0.63
0.76
0.48
VBS
1.881
1.552
0.25
0.226
0.35
0.48
0.73
0.35
2.327
2.12
0.26
0.228
0.48
0.61
0.79
0.49
PRBS
Table 1 – Results summary.
ACKNOWLEDGMENTS
The authors are in debt with Dr. Domingo Rangel for his valuable help in the V-I experiments, to A. del Real for the SEM micro-images and DGAPA for the supporting for this investigation.
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