HANDS-ON SIMPLE PHOTOCELL DEMONSTRATION TO EXPLORE SOLAR ENERGY CONCEPTS

Abstract

The development of clean and renewable energy is the one of the important challenges that scientist will confront in the near future. To get involved the young generation in learning solar energy seems to happen much more difficult, though. Accessibility of some of the materials through home or school-based experiments could be one of the main reasons why they are not eager to learn. Even the research currently made on semiconductors for solar energy conversion indicated the same result. On the other hand, dye sensitized solar cells (DSC) which are used for the electricity production from solar radiation are both cheap and easily accessed compared with the conventional photovoltaic cells. For these reasons, an experiment has been developed by using ordinary and harmless products for the construction of a dye sensitized solar cell that has the similar construction used in natural photosynthesis. The conversion of solar energy to electrical energy can be displayed through the usage of this simple and easily assembled construction. To check the utility of DSC on teaching solar energy conversion a project was realized. The aim of this project was to supply a cheap and an easily accessed hands-on exercise for high school students. Students were both given a short outline about the basic measurements related to photovoltaic cell performance and shown the characterization of this performance. They were also suggested to utilize these concepts for their demonstrations.

Headnote

Abstract: The development of clean and renewable energy is the one of the important challenges that scientist will confront in the near future. To get involved the young generation in learning solar energy seems to happen much more difficult, though. Accessibility of some of the materials through home or school-based experiments could be one of the main reasons why they are not eager to learn. Even the research currently made on semiconductors for solar energy conversion indicated the same result. On the other hand, dye sensitized solar cells (DSC) which are used for the electricity production from solar radiation are both cheap and easily accessed compared with the conventional photovoltaic cells. For these reasons, an experiment has been developed by using ordinary and harmless products for the construction of a dye sensitized solar cell that has the similar construction used in natural photosynthesis. The conversion of solar energy to electrical energy can be displayed through the usage of this simple and easily assembled construction. To check the utility of DSC on teaching solar energy conversion a project was realized. The aim of this project was to supply a cheap and an easily accessed hands-on exercise for high school students. Students were both given a short outline about the basic measurements related to photovoltaic cell performance and shown the characterization of this performance. They were also suggested to utilize these concepts for their demonstrations.

Keywords: Solar Energy, Hands-On Learning; Collaborative Learning; Dye-sensitized Solar Cells

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I. INTRODUCTION

Considering many disadvantages of using non-renewable fossil fuels, it is important to focus on sources of energy which are known as renewable energy. These sustainable sources generally considered as form of an energy resource that can be replaced rapidly by a natural process such as power generated from the sun or from the geothermal sources. It is necessary to develop clean, renewable and sustainable energy conversion systems which can solve global problems such as clean water scarcity, food shortage, and environmental pollution.

Most of the renewable energy, except geothermal and wave power, come from the sun which releases huge amounts of free and clean energy into space. Some of incident radiation is infrared and ultraviolet light, but most of it is visible light which can be converted directly to electricity by solar cells.

This large amount of free energy can be used for generating clean and sustainable electricity without toxic waste, which can be easy and effective solution to environmental problems [1]. The solar energy which strikes Earth in one hour would be enough to supply almost all the global energy demands for one year [2]. However current solar energy conversion technologies require a large land area due to low solar cell efficiencies. This large area could be potentially minimized by improving solar cell efficiency.

II. GRÄTZEL CELLS

One of the important organic dye-sensitized solar cell (DSSC) process is a developing and cost-effective solar energy harvesting approach that operates on principles similar to the process of natural photosynthesis. What makes the DSSC process so attractive due to its price to performance ratio. The materials and manufacturing processes required to construct a DSSC can be relatively inexpensive and more environmentally friendly, compared to a silicon-based solar cell. Because natural dyes such as chlorophyll [3] or anthocyanins [4] commonly found in leaves, fruits, flowering plants and can be used as light harvesting agents which is a kind of mimics of nature.

Most photovoltaic cells are made by adding two separate thin silicon wafers with different electrical characteristics together, so connecting wires enable electrons to travel between layers. Dye sensitized solar cells which is known also as Grätzel solar cells use a dye for directly converting the sunlight's energy into electrical energy.

It is necessary to develop new efficient dye derivatives from simple well-known organic dyes and experimentally determine the efficiency of DSSC based on titanium dioxide. The function of DSSC should be completely understood and based on that, some of the new organic dye are designed to produce a higher efficiency. Also it is important to discover that several factors that can influence on the performance of DSSC, such as the thickness of layers and temperature and to reproduce dark current phenomena with determining the reason why it happens.

The Grätzel cell diagram in figure-1 [5] shows the cell which is made up of different layers to generate solar energy. The top layer of photocell is made of glass coated with a conductive oxide like fluorine doped indium oxide. This is made by adding fluorine impurities to indium oxide which is called photo-anode. The next layer consists of nanostructured titanium dioxide. A photo-sensitive dye is adsorbed onto the TiO2 layer and covered with an organic electrolyte solution containing I-/I3-, a system which can easily absorb or lose electrons and act as an charge carrier. The photo-cathode can be made by depositing a very thin layer of platinum on the bottom layer of glass or by having a very thin layer of graphite coating [6].

The simplest solar cell model consists of both diode and current source connected parallel. Current is directly proportional to the solar radiation. Diode represents pn junction of a solar cell. Equation of ideal solar cell, which represents the ideal solar cell model, is:

... (1)

The incident monochromatic photon-to-current conversion efficiency (IPCE), sometimes referred to also as the "external quantum efficiency" (EQE), is an important characteristic of a solar cell device. Especially, using devices with same architecture, it is possible to compare the lightharvesting performance of sensitizers. It is defined as the number of electrons generated by light in the external circuit divided by the number of incident photons as a function of excitation wavelength as in equation-2 [12]:

... (2)

The conversion efficiency and fill factor of the cells can be characterized by determination of maximum, open and short-circuit voltages, current densities and calculated by the following equations [13]:

... (4)

Which in equation-3 and 4, h is the conversion efficiency of the cell, Im is the maximum current density, Vm is the maximum voltage , Pin the intensity of the incident light, FF is the fill factor, Voc is the opencircuit voltage and Isc is the short- circuit photocurrent density.

III. METHOD

In this study, hands-on teaching experiment applied for high school science students to set up and test their solar cell device consist of common household products. Three 12th grade high school students carried out this hands-on laboratory exercise. This study can be carried out over three 50-min lab sessions typical in schools. Several reports in the literature discuss solar cell construction in more detail [7], cell performance along with characterization techniques and the basics of electron transfer in solar cell systems [8].

In this work, there was a concrete example and hands-on activity illustrating a nature inspired approach which is highly relevant to today's technological challenges. It is showed that how first-hand solar energy can be converted into electrical energy using dye-sensitize solar cells. Moreover, we analysed generation of solar energy, explored possible future trends in organic solar energy, and demonstrated electron transfer by constructing a dye-sensitized solar cell using fruit (raspberries) products. Fruit raspberry, rich in anthocyanins, is a best source of dye in our dye sensitized solar cell. Smestad [9] developed the nano-TiO2/raspberry DSSC in 1998. This kind of DSSC provides very stable photo-voltage [10]. Due to the easy accessibility, low price and stable outcome, the raspberry was among the best choices of the dyes that mimics the architecture used in natural photosynthesis. The solar cell that we produced made of readily available materials: TiO2 paste that absorbs little light, anthocyanin dye from fruit raspberries, electrolyte which is I2-iodine and KI potassium iodide solution, and finally conductive glass that it is transparent, but acts like a metal [11].

A dye-sensitized solar cell consisting of two conducting glass electrodes in a sandwich arrangement was developed in the laboratory. Both electrodes were coated with tin dioxide after which the non-conductive electrode was again coated with titanium dioxide. The titanium dioxide is designed to serve as an absorbent for the dye. The technology adopted in this paper is that the dye molecules absorb light, and produce excited electrons which in turn generate current in the output terminals of the cell. The dye regains its lost electron with the aid of the iodide electrolyte present in the cell.

The voltage-current characteristic of the cells were carried out using an incandescent lamp as a source of power, operating at 220 V ac and 100watts/cm2. The distance between the lamp and the test sample is adjusted to get the required input intensity on the cell.

The solar cell is implemented with the software package Wolfram Mathematica. Input can be given and output can be visualized with a graphical user interface using Mathematica Player.

IV. RESULTS

The experimental observation shows that the efficiency was 0.012 %. The calculated fill factor value of 0.648, open circuit voltage of 201 mV and the short circuit current of 72 µA were observed for the DSSC cell. From the tabulation it is noted that for higher intensities the efficiency was found to be .0019 %.

It is shown that the evaporation of the liquid electrolyte usually caused some practical limitations of sealing and long term operation. It is suggested that the long term stability can be increased by using p-type semiconductor [14] or hole transporting organic material [15] to replace the liquid electrolyte. However the conversion is low compared to the liquid electrolyte. Therefore quasi solid state ionic liquid electrolyte might be a better choice to increase the stability Fill Factor as it has good physiochemical properties such as high thermal stability, negligible vapour pressure, relatively high ionic conductivity and good stability.

The decreasing trend when the TiO2 layer was thick could be explained in one way. As we know that, TiO2 serves as the electron carrier in the dye- sensitized solar cells. According to results of hands-on dye-sensitized solar cell, efficiency is too low. Because, the thickness of TiO2 couldn't prepared in desired thickness. One of the possible reason for decreasing in output and stable result can be from large thickness which is the reason for low efficiency of our DSSC. Because the semiconductor and graphite were on the different electrode, and the dye solution was in between both of the electrodes, the photon, in order to excite the electron from the dye molecule, must pass through the TiO2 layer which received the photons first. Thick TiO2 will obviously absorb or block the photon to make the way to dye molecules. According to this data, the prediction could be made that if the thickness of the TiO2 layer becomes infinity, if it could be achievable, the output power of the solar cell will approach zero because the thickness of TiO2 prevents photon from reaching the dye molecules.

For cells which incorporated raspberry extractions as the organic dye solution, a sharp increase of output was usually observed in the first several data points, which was considered as the increase of temperature facilitating electron excitation by increasing the rate of photon absorption. However, the decrease of output after about three minute was considered as breaking the electron cycle by the evaporation of the electrolyte solution so that no aqueous medium for the redox reaction to take place; thus, electrons could not regenerate in the organic dye.

V. CONCLUSION

In this study, an introduction to hands-on dye-sensitized solar cell characterization and a demonstration of the photovoltaic effect using household products is presented. This laboratory demonstration can be safely done by high school students and is suitable for an array of disciplines. The goal of this demonstration is not simply to motivate students about solar energy, but also to convey the parallel concepts of electron transfer in photosynthesis and energy conversion for power generation. Only through using similar concepts can students understand how a sustainable and renewable environment can be developed.

Considering the size of the solar cell, the voltage obtained is quite low due to thickness of TiO2. According to other researches, working with dye-sensitized solar cells looks attractive and promising and has a lot of potentials because the materials are available and inexpensive, and the construction process is also too cheap. DSSC can work effectively in low lighting like cloudy condition considering the result obtained despite the source of light used for this experiment.

References

Reference Text and Citations

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