Dyes extracted from Trigonella seeds as photosensitizers for dye-sensitized solar cells

Abstract

In this paper, the extract of Trigonella seeds was used as sensitizer for dye-sensitized solar cells (DSSCs). The natural dye was extracted from the seeds using water and alcohol as solvents for the raw material. The UV-Vis absorption spectra of Trigonella extract solution and dye adsorbed on TiO2 film were measured. DSSCs sensitized by Trigonella extracted using water as a solvent exhibited better performance with efficiency of 0.215 %. The performance of the fabricated DSSCs was attempted to enhance by acid treatment of the FTO substrates with HNO3, H ₃PO₄, and H2SO₄. Electrochemical impedance spectroscopy of the fabricated cells was also carried out.

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Dyes extracted from Trigonella seeds as photosensitizers for dye-sensitized solar cells

Amal Batniji1,2 Monzir S. Abdel-Latif2,3 Taher M. El-Agez1,2 Sofyan A. Taya1,2

Hatem Ghamri1,2

Received: 18 December 2015 / Accepted: 4 June 2016 / Published online: 15 June 2016 The Author(s) 2016. This article is published with open access at Springerlink.com

Abstract In this paper, the extract of Trigonella seeds was used as sensitizer for dye-sensitized solar cells (DSSCs). The natural dye was extracted from the seeds using water and alcohol as solvents for the raw material. The UVVis absorption spectra of Trigonella extract solution and dye adsorbed on TiO2 lm were measured. DSSCs sensitized by Trigonella extracted using water as a solvent exhibited better performance with efciency of 0.215 %. The performance of the fabricated DSSCs was attempted to enhance by acid treatment of the FTO substrates with HNO3, H3PO4, and H2SO4. Electrochemical impedance spectroscopy of the fabricated cells was also carried out.

Keywords Renewable energy Solar cells

Dye-sensitized solar cells Natural dyes

Introduction

Nowadays, the world is looking forward for obtaining clean renewable alternative energy source which is essential for developing our current lifestyle and economic growth. This was an inevitable result of the imbalance between the production and consumption of fossil fuel. Solar energy was one of the most promising technologies

that have been extensively studied. The outcome of these studies was construction of three generations of solar cells. The bulk crystalline-silicon solar cell was the rst generation which has an acceptable efciency, but its high cost and industrial requirements led to the second generation known as thin lm solar cells. The second generation is cheaper but has low efciency, so the third generation came to keep the benets of easy manufacturing and low cost while looking for better converging efciency. Dyesensitized solar cells (DSSCs) invented by ORegan and Gratzel [1] are famous by their simple preparation process accompanied with a reasonable efciency. The main structure of this kind of solar cells is built of a photoactive electrode consisting of a tin-oxide conducting glass mounted by a nanostructured wide band gap metal-oxide semiconductor thin lm anchored to suitable dye molecules, which have the ability to be excited by absorbing light photons. The excited electrons are injected from the dye lowest unoccupied molecular orbital (LUMO) to the semiconductor conduction band, passing through an external load to a platinum counter electrode. To complete the circuit the photoactive and the counter electrodes are sandwiched with the space between them is lled by an iodidetriiodide electrolyte. The main rule of the electrolyte is to regenerate the oxidized dye. Conversely, the electrolyte will be regenerated from the counter electrode. Each component of this simple structure has been extensively discussed by many researchers seeking for efciency improvement. Many wide band gap semiconductors like (TiO2, ZnO, SnO2) were studied with different morphologies (nanopowder, nanotubes, nanowires) [24]. Furthermore, the vital rule for the dye in the working mechanism of DSSCs in light harvesting was investigated. Many chemical groups like homoleptic dyes (N3, N719, N749) developed by Gratzel [5, 6] heteroleptic sensitizers (Z907,

& Sofyan A. Taya [email protected]

1 Physics Department, Islamic University of Gaza, Gaza,Palestine

2 Renewable Energy Center, Islamic University of Gaza, Gaza, Palestine

3 Chemistry Department, Islamic University of Gaza, Gaza, Palestine

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K19, K77), ruthenium polypyridyl complexes, and organic dyes (triphenylamine, carbazole, phenothiazine) have recorded encouraging efciencies up to 1012 %. The high cost and environmental harms of these dyes turned the attention toward natural pigments like anthocyanins, chlorophylls, and hypericin [79] extracted from plant leaves, roots, owers, seeds, and fruits [1016]. In spite of natural dyes low efciencies compared to ruthenium complexes, they attracted the attention of many researchers working on enhancing their performance by studying the effect of various factors in the extraction process, such as drying, grinding, temperature, anchorage time, and using different compounds of solvents [17, 18]. Natural juglone dye was obtained from the outer shell of walnut fruit by hot extraction method followed by doping using aluminium, copper and iron metals and used as a photosensitizer of DSSCs [19]. Fast production of the ZnO nanorods by bottom up approach using arc discharge method in deionized water was carried out for DSSC applications [20]. The synthesis of copper-doped polyaniline (NPANI-Cu-X) was performed in many solvents [21]. The obtained PANI was employed as counter electrode in a DSSC conguration. DSSC bilayer design was developed using an Fe2?/

Fe3? (ferrocene) liquid electrolyte and natural dyes extracted from Hypericum perforatum, Rubia tinctorum L. and Reseda luteola [22].

Dye-sensitized solar cells have been prepared using natural dyes such as red cabbage, red perilla, rosella, blue pea, and curcumin [23]. The efciency of the fabricated DSSCs was improved by selecting a proper sensitizer and modifying the surface of FTO by chemical treatment. The FTO surface was treated by HCl and TiCl4. Moreover, different organic solvents were used to enhance the extent of sensitization [23].

In this work, the natural dye extracted from Trigonella seeds was studied as a photosensitizer of DSSCs. The absorption spectrum of the extract was determined. The effect of using alcohol and water as solvents of the Trigonella seeds in the dye extraction was investigated. Pre-acidic treatment to the conductive glass substrates was conducted to improve the DSSC photovoltaic performance. Impedance spectroscopy study was provided to understand the transport of electron and hole carriers, interfacial charge transfer and recombination.

Experimental

Dye preparation

The clean dry seeds of Trigonella plant (1st washed with distilled water and dried at 70 C) were grinded by a mortar to form a ne powder. 1 g of the ne powder was

immersed in 5 ml ethyl alcohol at room temperature and left in dark for 24 h, and then the solution was ltered to obtain the nal extract solution. The same procedure was repeated for the same amount of Trigonella ne powder but the solvent was replaced with distilled water. Finally, the obtained extracts were kept in dark cold place.

Photoelectrode preparation

A transparent conducting FTO sheets (resistance 1214 X/ cm2; transmittance: 8284 %; Xinyan Technology Ltd, Hong Kong) with dimensions 1 cm 9 1.5 cm were cleaned using ultrasonic bath lled with detergent solution for 20 min, then rinsed with distilled water and dried.

The FTO conductive glass sheets were divided into four groups. The rst group was not treated by acids. The second, third, and fourth groups were immersed for 10 min in acid solutions of HNO3, H2SO4 and H3PO4, respectively, each of 0.1 M concentration.0.2 grams of TiO2 nanopowder (P25) was added to 0.1 g polyethylene glycol and the mixture was grinded for 30 min until a ne paste is obtained.

The TiO2 paste was spread on all the FTO groups by doctor-blade technique, where a thin lm was obtained on an area of 0.25 cm2. The sintering process started by drying the sheets at 70 C for 20 min, raising temperature to 180 C for 10 min, then 450 C for 40 min. After cooling down to 70 C, the TiO2 thin lm samples were soaked in the Trigonella natural extract for 24 h in the dark.

Assembling of DSSC

The prepared TiO2 photoelectrode (anode) and a conductive glass sheet plated with platinum by electrodeposition (cathode) were sandwiched to assemble the DSSC. Finally, the space between the two electrodes was lled with a liquid electrolyte solution (I-/I3-) composed of 2 ml acetonitrile (ACN), 8 ml propylene carbonate (p-carbonate),0.668 g potassium iodide, KI, and 0.0634 g iodine, I2.

Then, the two electrodes were clipped together to form a solar cell ready for light exposure.

Measurement

The UVVis absorption spectra of Trigonella dye extracted by water and alcohol were measured using Thermoline Genesys 6 spectrophotometer with wavelength range extending from 350 to 800 nm. The representative UVVis absorption spectrum of the anchored Trigonella extract on TiO2 thin lm layer was measured by collecting a diffuse reectance spectrum with a V-670, JASCO spectrophotometer then transforming it into absorption spectrum according to the KubelkaMunk relationship. Moreover,

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the cyclic voltammetry was used for the measurement of the HOMO, LUMO, and energy band gap. The current densityvoltage (JV) characteristic curves of the DSSCs under study were measured using National Instruments data acquisition card (USB NI 6251) in combination of a Labview program. An applied voltage in the range between -1 and 1 V was applied to the illuminated solar cell during currentvoltage measurements. The JV curves were measured at 100 mW/cm2 irradiations using high pressure mercury arc lamp. Electrochemical impedance spectroscopy (EIS) was carried out using an SP-200 potentiostat (Biologic, USA), with the frequency ranging from 100 Hz to 200 kHz.

Results and discussion

Absorption spectra

Trigonella extract UVVis absorption spectra in both water and alcohol as solvents are shown in Fig. 1. According to the gure, a clear absorption peak was observed at 400 nm in case of alcohol and another peak at 444 nm for water. A diffuse reectance spectrum was collected for the Trigonella dye adsorbed on TiO2 lm and transformed to the absorption spectrum according to the KubelkaMunk relationship [24], F(R) = (1 R)2/2R = k/s = Ac/s where R is the reectance, k is the absorption coefcient, s is the scattering coefcient, A is the absorbance and c is the concentration of absorbing species. When c and s remain the same, the spectrum obtained for KubelkaMunk function can be considered as absorbance. Figure 2 shows the detected absorption peak at 430 nm for the dye adsorbed on TiO2 which indicates a red shift towards higher wavelength compared to that of the extract in solution.

The cyclic voltammetry was used for the measurement of the HOMO, LUMO, and energy band gap. It was found

that EHOMO = 6.28 eV, ELUMO = 3.75 eV and Eopt =

2.5 eV.

Photovoltaic properties

The JV characteristic curves for DSSCs (fabricated using untreated FTO, 1st group,) sensitized by the natural extract of Trigonella seeds using water and alcohol as solvents are illustrated in Fig. 3. The main photoelectrochemical parameters obtained from Fig. 3 are listed in Table 1. These parameters are short circuit current Jsc, open circuit

voltage Voc, ll-factor FF, and conversion efciency g. It is clear from the gure that the performance of DSSCs sensitized by Trigonella extracted using water as a solvent is much better than that of solar cells sensitized by Trigonella extracted with alcohol. Referring to Table 1, Jsc = 0.371 mA/cm2 with alcohol and Jsc = 0.747 mA/

cm2 with water giving 201 % improvement. On the other hand, Voc = 0.599 V and FF = 48 % with alcohol are almost the same as Voc = 0.601 V and FF = 48 % with

0.6

a

0.4

Absorbance

0.2

0.0

400 600 800 1000

Wavelength (nm)

Fig. 2 a Light absorption spectra of Trigonella extract dye solution using alcohol and b dye adsorbed on TiO2

0.8

a

b

0.00.0 0.1 0.2 0.3 0.4 0.5 0.6

0.6

-0.1

-0.2

Abs. (a.u. )

J (mA/ cm2)

0.4

-0.3

-0.4

0.2

-0.5

0.0

-0.6

-0.7

400 500 600 700 800 900

-0.8

wavelength (nm)

Voltage (V)

Fig. 3 Current densityvoltage curves for the DSSCs sensitized by Trigonella seeds dye using alcohol and water as solvents

Fig. 1 Light absorption spectra of Trigonella extract dye solution using a alcohol and b water

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Table 1 Photoelectrochemical parameters of DSSCs sensitized by Trigonella seeds dye

Solvent Jsc (mA/cm2) Voc (V) FF (%) g (%)

Alcohol 0.371 0.599 48 0.106

Water 0.747 0.601 48 0.215

water, which means that the raise in efciency from g = 0.106 % with alcohol up to g = 0.215 % with water leading to 202.8 % improvement is due to the signicant improvement in the short circuit current. It is worth mentioning that the extracts of water are different from those of alcohol. The current improvement can be attributed to the extracts of water.

Acidic treatment of FTO sheets

The DSSCs used in this treatment were sensitized with the natural extract of Trigonella seeds using water as a solvent. The JV characteristic curves of DSSCs fabricated using untreated FTO (1st group) and those prepared using acid treated FTO (2nd, 3rd, and 4th groups) are plotted in Fig. 4. Their photoelectrochemical parameters are listed in Table 2. According to the JV curves we can notice that the FTO treatment with HNO3 and H3PO4 acids gave better performance than untreated cells. From Table 2, the treatment with HNO3 gave a short circuit current

Jsc = 0.840 mA/cm2, Voc = 0.641 V, and g = 0.259 %, leading to an improvement by 112.5, 106.6, and 120.4 %, respectively, over the untreated cells. On the other hand, the enhancements in case of H3PO4 were only 101.2 % for

Jsc and 100.9 % for Voc. This improvement is due to better adhesion of the TiO2 layer to the FTO, which means reducing the resistivity for high charge transport leading to higher short-circuit current density.

Impedance spectroscopy measurements

The solar cells sensitized with the extract of Trigonella seeds were perturbed by a small AC voltage signal of amplitude 10 mV with varying frequency (100 Hz to 200 kHz), to carry out an EIS study. This EIS study was employed to gure out the charge transport kinetics through the analysis of the Nyquist and Bode plots shown in Figs. 5 and 6, respectively. The main parameters calculated from the curves are the charge recombination resistance RS, the charge transfer resistance RCT, double layer capacitance (Cdl), the constant phase element CPE (a) due to double layer capacitance, and the effective lifetime of electrons s. All these parameters were calculated and summarized in Table 3. As can be seen from the table, RS = 31 X which is small enough for fast electron transport, in the same context a very large RCT = 7,204 X indicates long lifetime (62.5 ms) for the electrons in the lm. The CPE coefcient a = 0.76 illustrates electrode surface roughness accompanied with complicated structure double layer capacitance.

Conclusions

The extract of Trigonella seeds as a natural sensitizer was used in the preparation of DSSCs. Two different solvents (alcohol and water) were used in the extraction process of

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

2.5

-0.2

2.0

-0.4

J (mA/cm2)

-Z'' 103 (ohm)

1.5

-0.6

1.0

0.5

-0.8

0.0 -1 0 1 2 3 4 5 6 7 8

Voltage (V)

Fig. 4 Current densityvoltage curves for the DSSCs sensitized by Trigonella using untreated and acid treated FTO substrates

Table 2 Photoelectrochemical parameters of the DSSCs sensitized by Trigonella using untreated and acid treated FTO substrates

Acid Jsc (mA/cm2) Voc (V) FF (%) g (%)

No-treatment 0.747 0.601 48 0.215

HNO3 0.840 0.641 48 0.259 H3PO4 0.756 0.607 49 0.226 H2SO4 0.638 0.580 48 0.179

Z'103(ohm)

Fig. 5 EIS Nyquist plots of DSSCs sensitized by Trigonella under an illumination of 100 mW/cm2 and applied voltage of -0.7 V

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6

Fig. 6 Bode plots of DSSCs sensitized by Trigonella under an illumination of 100 mW/cm2 and applied voltage of -0.7 V

6

-phase10 ( deg)

3

3

Z 103 (ohm)

0

0

1 10 100

0.01 0.1 1 10

Log (f)103 / Hz

Log (f) 103/ Hz

2

- Z" 10 3 (ohm)

1

0

0.1 1 10

Log (f)104 / Hz)

Table 3 The impedance spectroscopy parameters of the DSSCs sensitized by Trigonella

RS (X) RCT (X) Cdl (lF) a (CPE coefcient) s (ms)

31 7,204 8.676 0.76 62.5

the dye from the raw material. The highest conversion efciency of 0.215 % (with water as a solvent) compared to 0.106 % (with alcohol as a solvent) was obtained for the fabricated DSSC. A pre-treatment process for the FTO glass substrates with acidic solutions led to an efciency improvement in case of HNO3 and H3PO4. Impedance spectroscopy study for the fabricated solar cells was obtained from the analysis of the Nyquist and Bode plots, and the most important parameters like charge recombination resistance RS, charge transfer resistance RCT, constant phase element CPE due to double layer capacitance (Cdl), and effective lifetime of electrons s were determined.

Acknowledgments The authors would like to express gratitude to the ministry of higher education for the nancial support of this work.

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/

Web End =http://creative http://creativecommons.org/licenses/by/4.0/

Web End =commons.org/licenses/by/4.0/ ), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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