Photo-electrical conversion efficiency of dye sensitized solar cells prepared from Mopane leaf chlorophyll extract
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Date
2020
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University of Namibia
Abstract
In times of fossil fuel shortage, increasing crude oil prices, as well as rejection of conventional energy sources (e.g. coal or nuclear power plants), sustainable energy forms become more and more the focus of attention. One of several offered solutions is to develop a photovoltaic device (solar cells), since solar energy is abundant. Especially, Namibia receives solar radiation of 5.8 to 6.4 KWh per square meter per day – one of the highest in the world. With 8 to 11 average hours of sunshine per day throughout the year, this offers fantastic potential for solar energy, both photovoltaics and solar thermal. Dye Sensitized Solar Cell (DSSC) which is presented here is a thin film cell. Manufacturing of DSSCs is simple, mostly low cost, and incorporate environmentally friendly materials. They have a good efficiency (about 10-14 %) even under low flux of sunlight. However, a major drawback is the temperature sensitivity of the liquid electrolyte. Hence, this study presents the ongoing research to improve the electrolyte’s performance by using natural dye. A transparent conductive oxide (TCO) film of Fluorine - doped Tin Oxide (FTO) was used as an anode. In this research, chlorophyll extracted from Colophospermum Mopane leaves using methanol was employed as the dye for TiO2-based DSSC. TiO2 precursor solution prepared by Molecular precursor method (MPM) was spin-coated on top of Fluorine- Doped Tin Oxide (FTO) conductive glass and calcined at 420 ̊ C for 20 min to act as an anode electrode. The cathode electrode was a supportive glass substrate coated with a layer TiO2 paste (Degussa P25) followed by graphene layer obtained from charcoals sold in Namibian markets. The anode electrode was then immersed in the dye for 20 min, to make the TiO2-chlorophyll thin film. An iodide/triiodide solution was used as the electrolyte. Both electrodes were then pressed together and sealed so that the cell does not leak to make TiO2-Graphene-Chlorophyll-TiO2 sandwich structure based DSSC. An external load was supplied when sunlight shine on the anode of the cell. The photovoltaic performance of this assembled DSSC was then measured using a multi-meter employing eDAG Potentiostat R466 and Echem software under direct sunlight illumination. Furthermore, the assembled cell efficiency was determined by plotting an IV-curve. The current and voltage values for the cell were 157.4 μA and 510 mV respectively. The resistance of the assembled DSSC was 0.931 Ω. Open circuit voltage 287.1 mV, short circuit current 170.8 μA, fill factor 33.5 %, with an efficiency of 0.37 % was obtained. The cell was tested under direct sunlight illumination having a power input of 4.5 mW, on the solar cell area of 7.8 cm2. These results were supported by the UV/Vis analysis showing that both chlorophyll and TiO2 do absorb photons in the wide range of 300 nm to 1000nm. A wide absorption range means that the cell can absorb in the visible and IR light which make up about 90% of the solar spectrum. Large active surface area exposure for the TiO2, enabled more dye and electrolyte to be stored inside the material so that photon adsorptions from solar energy became more effective, resulting in higher efficiency. Despite the small efficiency, this work demonstrated the opportunities of Chlorophyll and TiO2 to be applied as a DSSC.
Description
A mini thesis submitted in partial fulfillment of the requirements for the Degree of master of Science (Chemistry)
Keywords
Solar cells, Mopane leaf