Annealing Effect on Efficiency of Aspilia Africana Flowers Dye Sensitized Solar Cells

Authors: Adenike Boyo¹, Henry Boyo², Olasunkanmi Kesinro¹

Abstract: Energy was generated by using methanol as a solvent to extract dye from Aspilia africana Flowers. The maximum absorption of the extracted dye was observed at different wavelengths (350-1000nm). TiO₂ was annealed at different temperatures and phytochemical screening was done. We observed insignificant presence of anthocyanin compared to flavonoids in the flowers. The solar energy conversion efficiency changes from 0.21% to 0.52%, due to the sintering of the TiO₂ at different temperatures. The increase in solar energy conversion efficiency can be attributed to the changes in the morphology, crystalline quality, and the optical properties caused by the sintering effect.

Keywords: Aspilia Africana, Methanol, Anthocyanin, Flavonoids and Dye Sensitized Solar Cell

1.Introduction

The technology of harvesting solar energy in producing photovoltaic systems has attracted worldwide attention. Several researches have been done on photovoltaic technologies which includes dye sensitized solar cells [1], [5],[8], organic solar cells [4], and multijunction solar cells [9]. Dye sensitized solar cells (DSSCs) have played an important role in the development of photovoltaic technology, although encouraging efficiency of the DSSCs have been presented by many authors, but the use of the DSSCs is still not feasible for commercial applications. These low efficiencies of DSSCs might be due to challenges faced in obtaining suitable materials for its production. Thus, careful importance is placed in harnessing dyes that have a long lifespan, cheaper, and environmental friendly solar cells.

The nano-crystalline dye sensitized solar cell is a photo electrochemical cell. The basic working principle of DSSCs is that photons strike the monolayer of dye which gives up energy. This excites electrons from the dye, instead of generating electron – hole pairs. As shown in ‘Fig.1’ the solar cell consist of two conductive glass slides (ITO or FTO) with nanoparticle TiO₂ sintered on one of the conducting glass slides, stained with the dye and the other conducting glass slide is carbonized with soot. DSSCs are placed in the category of third generation photovoltaics where new trends in the photovoltaic technology are applied. In the first generation PV cells, the electric interface is made between doped n-type and p-type bulk silicon. First generation PV cells provide the highest so far conversion efficiency. The second generation PV cells are based on thin film technology. These cells utilize less material and they thus drop the production cost, however, they are less efficient than the bulk cells. Both first and second generation cells are based on opaque materials and necessitate front-face illumination and moving supports to follow sun’s position. Thus they may be either set up in PV parks or on building roofs. Third generation solar cells, are based on nanostructured (mesoscopic) materials and they are made of purely organic or a mixture of organic and inorganic components, thus allowing for a vast and inexhaustible choice of materials.

A liquid triiodide electrolyte is added in between the glass slide.. The absorption of photons causes dye molecules to be attached to the surface of the nanocrystalline titanium dioxide which produces photoelectrons. The titanium dioxide is the photoanode, photoexcitation of the dye results in the transfer of electrons into the conduction band of the nanocrystalline titanium dioxide.

Annealing nanoparticle titanium dioxide at different temperature plays an important role in the solar energy conversion performance of Dye Sensitized Solar Cells. These affect the physical, chemical and optical characteristic of TiO₂. Titanium dioxide (TiO₂) can crystallize in three different phases namely: rutile (tetragonal), anatase (tetragonal) and brookite (orthorhombic) [10]. The rutile-TiO₂ is thermodynamically stable at all temperatures and can be obtained in most crystal growth processes.

The anatase-TiO₂ and brookite are less dense and less stable in comparism to the rutile-TiO₂.phase [10].

We fabricated DSSCs using Aspilia africana flowers dye extracted from methanol and study the effect of annealing of TiO₂ films at different temperature intervals from 100⁰C, to 500 ⁰C for 30 minutes on the performance of DSSCs.

This study was borne out of the availability of these plants in Nigeria. Another important factor is that Nigeria uses about 3 trillion kWh of energy each year, and with energy requirement growing every year, it is the responsibility of the government and other institutions to develop energy harvesting technologies from renewable energy sources for a sustainable future.

Hence, in order to improve the generation of electricity in our country and beyond, it would be necessary that devices like dye-sensitized solar cells are produced at commercial scale to meet energy demands in Nigeria.

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