INTRODUCTION:

Thin film technology is stretching its hands in all directions and has revolutionized the field of optics, electronics, space science and material science¹ . Photovoltaics is the field of technology and research related to the application of solar cells which are capable of producing electricity with higher efficiency^2. Dye-Sensitized Solar Cells - a third generation photovoltaic devices that mimic natural photosynthesis in which the photons are absorbed by a dye that causes electron transfer generating free energy and the electron transfer rate which is fast enough to compete with the deactivation rate of the photo excited state³.

Dye-Sensitized Solar Cells are photoelectrochemical (PEC) cells that use photo-sensitization of wide-band-gap mesoporous oxide semiconductors. It is based on a semiconductor formed between a photo-sensitized anode and an electrolyte DSSCs are easy to fabricate, less expensive, environment friendly and less sensitive to variation of incident light intensity in comparison to conventional solar cells⁴ .The major advantages of DSSCs are: cell's mechanical robustness and their ability to work under cloudy skies and non-direct sunlight.

Another special advantage of the DSSC with respect to competing technologies is that its performance is remarkably insensitive to temperature change. Thus, raising the temperature has practically no effect on the power conversion efficiency.³A novel system that harnesses solar energy is the nano-crystalline TiO₂ Dye-Sensitized Solar Cell, in conjunction with several new concepts, such as nanotechnology and molecular devices. The production process will enhance light harvesting generates very small quantities of residue, resulting in environment friendly devices with low energy demanding production techniques⁵ An efficient photosensitizer should fulfil some requirements such as an intense absorption in the visible region, strong adsorption onto the semiconductor surface and efficient electron injection into the conduction band of the semiconductor. Moreover, it must be rapidly regenerated by the mediator layer in order to avoid electron recombination processes and be fairly stable, both in the ground and excited states. The ideal sensitizer photovoltaic cell converting standard global AM (air mass) 1.5 sunlight to electricity should absorb all light below a threshold wavelength of about 900 nm⁶

Application of naturally occurring pigments such as anthocyanins, carotenoids and chlorophylls for DSSCs has several advantages over rare metal complexes and other organic dyes, such as readily available, easy extraction into cheap organic solvents, can be applied without further purification, environment-friendly and of low cost⁷

TiO₂ is one of the extensively studied transition metal oxide semiconductor. Titanium dioxide does not absorb light because it’s white, it’s necessary to resort to a dye to sensitize it in order to promote the absorption of solar energy. In this work, the effect of three natural photosensitizers (beetroot, grapes and pomegranate) over TiO₂ thin film is investigated. TiO₂ films prepared by Spray Pyrolysis are taken and coated with the dyes. The optical characterization of the dye-sensitized TiO₂ films is determined to study its application in Dye-Sensitized Solar Cell (DSSC). XRD pattern of the thin films are analyzed to determine its structure.

Spray Pyrolysis is a process in which a thin film is deposited by spraying a solution on a heated surface, where the constituents react to form a chemical compound. The chemical reactants are selected such that the products other than the desired compound are volatile at the temperature of deposition⁸. The Chemical Spray Pyrolysis (CSP) technique is convenient for preparing pinhole free, homogeneous, smoother thin films with the required thickness⁹.

METHODOLOGY:

Preparation of Dye-Sensitized TiO₂ thin films:

The TiO₂ thin films were prepared by Spray Pyrolysis technique using Titanium diisopropoxide as precursor. The prepared thin films were sensitized with anthocyanin dyes –an ethanolic extract of beetroot, grapes and pomegranate juices.

Choice of the sensitizer:

The ideal sensitizer for a single junction photovoltaic cell converting standard global AM (air mass) 1.5 sunlight to electricity should absorb all light below a threshold wavelength of about 920 nm. In addition, it must also carry attachment groups such as carboxylate or phosphonate to firmly graft it to the semiconductor oxide surface. Upon excitation it should inject electrons into the solid with a quantum yield of unity.

Extraction of dyes:

Dyes were extracted from the fruits of Grapes and Pomegranate and also from the vegetable Beetroot by choosing ½ kg of each item. Generally the colouring matter of grapes is found only in the cells of the skin. So the skin was separated from the grape fruits and was washed with water to remove adhering matter. Grape (Vitis vinifera) pigments were extracted from the skin peels of the fresh grape fruits by crushing it. The ethanolic extract is obtained by mixing 50 ml of grape juice with 5 ml of ethanol. The juice extracted from pomegranate fruits contains cyanin (flavylium) pigment. Strong chelation of flavylium with TiO₂ changes it to quinonoidal form as shown in Fig 1

Fig 1- Structure of Flavylium

Pomegranate (Punica granatum) pigments were extracted by squeezing its seeds, after removing the skin. The resulting solution was filtered in order to remove the pulps and some residual fragments. Then it is mixed with 10ml of ethanol.

The betacyanin pigment of Beetroot is normally found in the vacuole of the beet root cells. Beetroot (Beta vulgaris var crassa) juice was prepared by cutting and grinding fresh beetroot with skin. It was then filtered and Mixed with 5-10 ml of ethanol solution. Beetroot contains 2 Betacyanins - Betanin and a derivative. The chemical structure of betacyanin is given in Fig 2.

Fig 2- Basic structure of Betacyanins

Preparation of Dye-sensitizer:

A readily prepared TiO₂ thin film deposited using spray pyrolysis method from titanium diisopropoxide is considered. The extracted dyes were mixed with ethanol and 50ml of each solution is taken in a beaker. The TiO₂ coated glass slides were placed in the beaker using tweezers and left for 15 hours.

RESULTS AND DISCUSSION

The thicknesses of the dye-sensitized TiO₂ films were determined using Stylus thickness measuring device and are found to be 4.0 micrometer for each of grape and beetroot dyes and 4.5 micrometer for pomegranate dye.

UV-Visible absorption Analysis:

The dye-sensitized TiO₂ thin films are taken and its optical absorbance and transmittance were determined by UV-visible spectroscopic method. The thin films were analyzed using UV-VIS-NIR spectrophotometer. The absorption spectra of TiO₂ are given in Fig.3, Fig. 4 and Fig.5 for beetroot, grapes and pomegranate dye-sensitized TiO₂ thin films respectively. The spectrum is given for wavelength between 400nm to 1000 nm.

Fiureg3- With Beetroot Dye

Fiureg.4- With Grapes Dye

Figure 5- With Pomegranate Dye

Similarly the transmission spectra of TiO₂ are given in the Fig.6, Fig.7 and Fig.8 for beetroot, grapes and pomegranate dye-sensitized TiO₂ thin films respectively.

Fig 6- With Beetroot Dye

Fig 7- With Grapes Dye

Fig8- With Pomegranate Dye

The optical absorbance and transmittance of TiO₂ thin films with the sensitized TiO₂ thin films, indicates the presence of increased light transmission in the visible spectrum region. This shows that the addition of dyes have increased the light absorption by the TiO₂ thin films. The results are in agreement with the previously reported paper¹⁰

Figure 9- Beetroot Dye

Figure 10- Grapes Dye

Figure 11- Pomegranate Dye

A graph is drawn between energy (hν) and (αhν)² as shown in Fig. 9-11 for the beetroot, grapes and pomegranate dye-sensitized TiO₂ thin films respectively. The linear part of the graph is extrapolated and it cuts at 3.18 eV in the energy axis, where (αhν)²=0, for beetroot dye-sensitized TiO₂ thin film. Similarly, the energy band gap (E_g) for grapes dye-sensitized TiO₂ thin films and pomegranate dye-sensitized TiO₂ thin films were found to be 3.12 eV and 3.14 eV respectively.

XRD Analysis

The X-ray diffraction analysis was carried out for the prepared thin films to determine its structure. The presence of TiO₂ was confirmed from its XRD pattern having peak at the value of 2θ = 31.49. The diffractogram of TiO₂ is exhibited in the Fig 12

Position (2 θ)

Fig 12- XRD pattern of TiO₂

SUMMARY AND CONCLUSION:

The Dye-Sensitized Solar Cell (DSSC) is a third generation photovoltaic device that may be an economic alternative to the classical solid-state semiconductor devices, because of the use of inexpensive materials and a relatively simple fabrication process. TiO₂ films deposited on glass substrates prepared from titanium diisopropoxide using Spray Pyrolysis Deposition (SPD) were readily taken. Natural dyes from beetroot, grapes and pomegranate were extracted and coated over TiO₂thin films. Dye-sensitized TiO₂ films were prepared by keeping TiO₂ deposited glass substrates in each dye extract for 15 hours.

The thicknesses of the TiO₂ films sensitized with the dyes extracted from beetroot, grapes and pomegranate were calculated using Stylus thickness measurement device and are found to be 4µm, each for beetroot and grapes and 4.5µm for pomegranate. The optical absorbance and transmittance data for the TiO₂filmssensitized with the dyes were observed in the spectral region 400-1000 nm. The X-ray diffraction analysis was carried out to confirm the presence of TiO₂. The presence of TiO₂ was confirmed from its XRD pattern having peak at the value of 2θ = 31.49.

The energy band gap (E_g) was found to be 3.18eV, 3.12eV and 3.14eV for beetroot, grapes and pomegranate dye-sensitized TiO₂ thin films respectively. Hence, these dyes or their mixtures may be used for application in Dye-Sensitized Solar Cells as they contribute more to harvest light energy. Dye sensitized solar cells offer the potential to render solar energy as an economically sustainable energy source.

REFERENCES:

1. Joy George, “Preparation of Thin films” 1992, Marcel Dekkar, Inc, New York.

2. Alfred Smee, “Elements of Electro-biology, or The voltaic mechanism of man”, London, 1849,

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4. Rungnapa Tongpool, Suparerk Aukkaravittayapun, Monthon Nakpathom, Somboon Sahasithiwat, Chanchana Thanachayanont, Thanasat Sooksimuang, Sorachon Yoriya, Saowaluk Chaleawlert-umpon and Patcharee Srimaungsong Photoelectrode development for dye-sensitized solar cells,(patented),2005 National Metal and Materials Technology Center, Thailand

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Received on 10.12.2010 Modified on 02.01.2011

Asian J. Research Chem. 4(4): April, 2011; Page 621-625