Nanoparticle Solar Cells ECG653 Project Report submitted by Sandeep Sangaraju (sangaraj@unlv.nevada.edu), Fall 2008 1. Introduction: Solar cells are the most promising product in future. These can be of at most use, when all the fossil fuels are extinct. Usually solar cells have less efficiency and are very expensive and unreachable to a common man to buy. So the purpose is to develop cells which perform much more efficient and mainly less expensive. To develop such solar cells, now comes the concept of Nanoparticles. Let us first discuss about solar energy and its importance: More solar energy is absorbed by the earth every minute than is used in fossil fuels every year. This process is of pollution free and the wastes are easily manageable when compared to fossil fuels. Solar energy is of at most importance in providing electricity to remote locations where other forms of energy are difficult or expensive to get. Solar cells convert sunlight directly into electricity. They are usually made of semiconductor materials mainly Silicon. These were first used in spacecrafts and satellites. Traditional solar cells are made by n-type and p-type silicon sandwiched together. These cells are based on using photon to separate charges and creating a electron-hole pairs. Many new type of Solar cells are mainly in research/production stage. ~ 1 ~
2. Project description: Solar cells respond differently to the different wavelengths or colors of light. As each wavelength corresponds to a frequency and energy, the shorter the wavelength, the higher the frequency and the greater is the energy. This implies that Red light is at the low-energy end of the visible spectrum and violet light is at the high-energy end. Light that is too high or too low in energy is not usable by a cell to produce electricity; rather this energy is transformed into heat. So solar cells mostly observe visible light to produce electricity. The principle behind or the working of Solar cells is when photons with energy equivalent to or higher than band gap, falls on semiconducting material, the electrons are released from their atoms, allowing the electrons to flow through the material to produce electricity. ~ 2 ~
A solar cell consists of n-type silicon and p-type silicon sandwiched together. These types of silicon are then contacted with a front and back contact grids. And on the top, layered with an anti-reflecting coating and glass cover. The front contact grid is connected to a load and then to the back contact grid. This makes a complete circuit and the electricity can be circulated. N-type silicon has free electrons and P-type silicon has free holes. When N-type and p-type silicon come into contact, an electric field forms within the cell. This process of converting light to electricity is called the photovoltaic effect. Solar Cells are classified into three generations which indicates the order of which each became important. At present there is concurrent research into all three generations while the first generation technologies are most highly represented in commercial production. First generation cells consist of large-area, high quality and single junction devices. First Generation technologies involve high energy and labour inputs which prevent any significant progress in reducing production costs. Single junction silicon devices are approaching the theoretical limiting efficiency of 33%. Second generation materials have been developed to address energy requirements and production costs of solar cells. Alternative manufacturing techniques such as vapour deposition and electroplating are advantageous as they reduce high temperature processing significantly. It is commonly accepted that as manufacturing techniques evolve production costs will be dominated by constituent material requirements, whether this be a silicon substrate, or glass cover. The most successful second generation materials have been cadmium telluride (CdTe), copper indium gallium selenide, amorphous silicon and micromorphous silicon. These materials are applied in a thin film to a supporting substrate such as glass or ceramics reducing material mass and therefore costs. These technologies do hold promise of higher conversion efficiencies, particularly CIGS-CIS, DSC and CdTe offers significantly cheaper production costs. In second generation, the production cost reduces but the efficiency also reduces. Third generation technologies aim to enhance poor electrical performance of second generation (thin-film technologies) while maintaining very low production costs. Current research is targeting conversion efficiencies of 30-60% while retaining low cost materials and manufacturing techniques. These generation cells has a theoretical efficiency of 40.8%.High efficiency solar cells are a class of solar cells that can generate electricity at higher efficiencies than conventional solar cells. While high efficiency solar cells are more efficient in terms of electrical output per incident energy, much of the industry is focused on the most cost efficient technologies. Still, many businesses and academics are focused on increasing the electrical efficiency of cells, and much development is focused on high efficiency solar cells. ~ 3 ~
Nanoparticles are of most importance in making solar cells. They are important because nanoparticles have high surface area, which changes the properties of materials. Change in size of nanoparticles makes solar cells to observe most of visible light, resulting in high output voltage. Different size nanoparticles observe different wavelengths of light, this helps in getting high efficiency. Mainly nanoparticles are important to make the solar cells smaller in size and light in weight, so that these may be applicable in different applications. One of the most important solar cells in 3 rd generation solar cells is Dye-sensitized solar cells (DSSC). These solar cells give a good efficiency and reduce the production cost too. When a photon is absorbed by the dye and excites an electron in the dye. The electron is then injected into the wire or particles. These electrons move through the external circuit and then to a platinum wire. The electrons from platinum wire is carried by a electron shuttle (iodine molecules) through aqueous solution to the dye coated TiO2. This makes a complete circuit producing electricity. DSSC when compared to traditional silicon based solar cells have less cost, does not require elaborate or complex apparatus. They can work in low light conditions. DSSC has high Price/Performance ratio. DSSC has a few drawbacks when compared to silicon based solar cells. They are slightly lower efficient. Breakdown of the dye can occur. Bandgap is slightly larger than silicon, so requires high energy light. The liquid electrolyte present in DSSC can leak causing a breakdown of the cell. Silicon nanoparticle solar cells most recently enhanced the performance of solar cells efficiency by 60%. These are developed using thin film deposition of silicon nanoparticle on polycrystalline silicon substrate of a photovoltaic cell. ~ 4 ~
The best research cell efficiencies of different types of solar cells till date are plotted and shown below. The Multijunction solar cells have the high efficiency when compared with other kind of cells. Single junction GaAs solar cells with thin film deposition has next lower efficiency level in the graph. Crystalline Si cells, Thin film technologies have the moderate efficiency levels. The least efficient solar cells are emerging PV(organic cells) kind solar cells. In conclusion most of the solar cells are in research stage. To commercialize the product we need to have low cost and high efficient, aiming the same the research on solar cells is improving day by day. For this development nanoparticles have the main role in solar cells. Developing solar cells using nanoparticles not only improves its efficiency and reduce cost, it also make them smaller in size and compatible to carry in hand. With this reason there are several applications for nanoparticle solar cells from satellite to cell phones. 3. References: http://en.wikipedia.org/wiki/solar_cells http://en.wikipedia.org/wiki/dye-sensitized_solar_cell http://www.nsf.gov/news/news_images.jsp?cntn_id=100411&org=oise ~ 5 ~