EXTERNAL QUANTUM EFFICENECY OFA CADMIUM TELLURIDE CADMIUM SULFIDE PHOTOVOLTAIC CELL by Amy Ferguson Submitted to the Department of Physics in partial fulfillment of graduation requirements for the degree of Bachelor of Science Brigham Young University – Idaho December 2010 Thesis Advisor: David Oliphant Committee Member: Richard Hatt Signature: __________________ Signature: ____________________ Committee Member: R. Todd Lines Signature: __________________ Abstract Finding the external quantum efficiency of a Cadmium Telluride – Cadmium Sulfide photovoltaic cell is determined by knowing the current that the light produces and the intensity of the light that is used. This process is to determine if the p-n junction of the cadmium telluride – cadmium sulfide produces better efficiency than what has been found before from different types of photovoltaic cells. The project has not been completed due to problems that have not yet to be solved. The efficiencies found are not accurate but the set-up is an idea to determine the external quantum efficiency. ii Acknowledgements I would like to thank the Society of Physics Students for funding my internship at the National Institute of Standards and Technology (NIST). Thanks go out to NIST and my advisor Dr. Nhan V. Nguyen for helping me with my research and the data that is included in this paper. I would finally like to thank Brigham Young University – Idaho Physics Department for all the opportunities I have had while at school and for providing me with my advisor David Oliphant who has helped me a ton. iii Table of Contents 1. BACKGROUND ........................................................................................................................ 1 1.1 Introduction ..................................................................................................................... 1 1.2 Physics of Semiconductors ............................................................................................. 1 1.3 Alternative Energy .......................................................................................................... 3 2. EXPERIMENT ........................................................................................................................... 5 2.1 How to solve for EQE ...................................................................................................... 5 2.2 Structure of Photovoltaic Cell .......................................................................................... 6 2.3 Set-up of Experiment ....................................................................................................... 7 3. OBSERVATIONS & RESULTS ................................................................................................ 8 4. CONCLUSION ......................................................................................................................... 11 References ..................................................................................................................................... 13 Appendix ....................................................................................................................................... 14 iv LIST OF FIGURES 1: Represents Band Gap .......................................................................................... 2 2: CdS-CdTe Photovoltaic cell provided by the University of Toledo, Right is the side view enhanced to see the layers. Left, is the view from the top. ..................... 6 3: Set-up of lab equipment ...................................................................................... 8 4: Graph of the EQE% as a function of wavelength ............................................. 11 LIST OF TABLES 1: Measured Intensity Data ................................................................................... 14 2: EQE Data .......................................................................................................... 21 v 1. BACKGROUND 1.1 Introduction A solar cell uses the photovoltaic effect to convert the light from the sun into electrical energy. If solar cells were created to have higher efficiency, then they would be very useful in the world today that is trying to find other ways to produce energy. The photovoltaic effect is described simply as the conversion of light energy into electrical energy. This was first discovered by Edmund Becquerel in 1839 when he observed an electrical current being created when light acted on a silver coated platinum electrode in an electrolyte solution [1]. So this idea has been around for almost two hundred years. The most common solar cells today are silicon solar cells. This is because a solar cell is essentially a semiconductor and most semiconductors are made with silicon. In 1954 the first successful silicon solar cell was made by Chapin, Fuller, and Pearson [1]. The efficiency of this solar cell was 4% [2]. This came about because of the development in the silicon semiconductor. 1.2 Physics of Semiconductors A semiconductor material is not a conductor and not an insulator, it has the properties of both which makes it unique. A conductor allows the flow of electrons freely without added energy. An insulator is a material that would allow the flow of electrons but the band gap shown in figure 1 is a lot higher for an insulator so more energy is required to move the electrons from the valence band to the conduction band. The energy required for an insulator is too great to allow the flow of electrons. A semiconductor is a material that has a band gap that requires a lower amount of energy to move the electrons. A semiconductor is not just any type of material. It is determined by the lattice 1 structure of the atoms that make up the semiconductor material. The lattice structure is like the structure of a crystal [3]. This gives it the properties for the unique flow of electrons. The use of semiconductors in electronic devices like the television, radio and, computers have “revolutionized our way of life” [4]. A semiconductor material either wants more electrons or wants to get rid of electrons which is why a p-n junction can form. A p-n junction is essential to the structure of the solar cell; it is why the solar cell produces electricity. The p-n junction is caused by a p-type semiconductor being in contact with a n-type semiconductor. The n-type means negative which says there is an excess of electrons, it wants to get rid of its electrons. The p-type is the positive side which lacks electrons; it wants to get more electrons. The electrons flow forward bias from the n side to the p side. Energy needs to be given to the electrons for them to flow. The potential energy difference between the valence band of electrons of the p side and the conduction band of the n side is how much energy is required to create a current. This gap is shown in the Figure 1 below. Figure 1: Represents Band Gap This difference is referred to as the band gap. The band gap is different for every material and that is why there is a difference in the types of photovoltaic cells. The smaller the 2 band gap the less energy is required to move the electrons. The more electrons that move from the conduction band to the valence band the more current that is produced. In a photovoltaic the energy given to the electrons is from light, solar energy. The energy from the sun is free compared to many other sources of energy. If a solar cell can become efficient to produce a large current at little cost, for example around 1 cent per kilowatt hour, then they would be economically valuable. 1.3 Alternative Energy Solar Cells are made from different materials which include, monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium selenide [5]. The solar cell that was used in this project was created at the University of Toledo and it is a Cadmium Sulfide (CdS) Cadmium Telluride (CdTe) solar cell. V. G. Karpov, Diana Shvydka, and Yann Roussillon, from the University of Toledo, wrote a paper titled Physics of CdTe Photovoltaics: from Front to Back , explaining the structure of their photovoltaic cell that is similar to the one I used. They expressed their views on using a CdTe photovoltaic as a practical way of improving photovoltaics. Karpov claims that the unique structure of CdTe creates the possibility for a better photovoltaic. Their results are due to their observations for the need for a good back contact, the band gap between CdS and CdTe is small, and the crystalline structure of CdTe produces an effective photovoltaic [6]. Recently, there has been a big push in alternative energy. The conventional ways of producing energy are coal, oil, natural gas, nuclear, and hydroelectric. The United States relies heavily on coal to produce 22% of the total energy consumption [7]. “A large electric plant consumes more than 20,000 tons of coal per day. Each ton generates 3 about 2,000 kilowatt hours of electricity, enough to power the average home for a third of a year” [7]. 20,000 tons of coal is a lot and that is being burned to produce energy. A concern is that eventually the coal will run out. “Approximately 58 billion tons of coal have been produced in the United States since the first commercial mine was established more than 200 years ago” [7]. So a look for alternative energy is a big concern in politics and science. Alternative energy is focused on using non conventional sources. It includes renewable energy which is when the source replenishes themselves (unlike the conventional