EXTERNAL QUANTUM EFFICENECY OFA CADMIUM TELLURIDE CADMIUM SULFIDE PHOTOVOLTAIC CELL. Amy Ferguson

Transcription

1 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 Signature: Committee Member: Richard Hatt Signature: Committee Member: R. Todd Lines Signature:

2 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

3 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

4 Table of Contents 1. BACKGROUND Introduction Physics of Semiconductors Alternative Energy EXPERIMENT How to solve for EQE Structure of Photovoltaic Cell Set-up of Experiment OBSERVATIONS & RESULTS CONCLUSION References Appendix iv

5 LIST OF FIGURES 1: Represents Band Gap : 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 : Set-up of lab equipment : Graph of the EQE% as a function of wavelength LIST OF TABLES 1: Measured Intensity Data : EQE Data v

6 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

7 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

8 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

9 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 ways that once the coal or oil is burned it can t come back.) Renewable energy sources include the sun, ocean waves and tides, wind, and rivers. Also, there is a big push for green energy which means that the energy is clean, low or nonpolluting energy [7]. In all reality every form of energy has some kind of pollution. In the case of coal when burned carbon dioxide is produced. This is hazardous to human health. Carbon dioxide is also produced from the burn of oil and gas, like in a car. There is technology to limit the emission of carbon dioxide but is too expensive for personal use. The technology is used to have clean coal produced a coal production plants [7]. The world is going green and if energy production goes green the debates about which energy is best can cease. A renewable and green energy source is the sun and so a solar cell can be a solution to the problem of finding better and more efficient alternative energy. The development of the photovoltaic is the way to use solar energy and convert it to electricity. The problem is that the efficiency of the photovoltaic cell is not high enough to make them useful. A problem with using the light of the sun is that it isn t concentrated 4

10 on to just one spot, it is spread out and doesn t always hit the cell orthogonally. If the sun light was able to be focused and hitting the cell at an angle of 90 degrees the efficiency would increase and the surface area of the cell and cost would decrease by a factor of 1000 [7]. The development of the CdTe photovoltaic is improving the efficiency and one step closer to finding the solution. 2. EXPERIMENT 2.1 How to solve for EQE The purpose of the experiment is to find the external quantum efficiency (EQE) of the CdTe photovoltaic. The EQE can be determined by dividing the number of electrons by the number of photons, shown in Eq. (1). % = h This is under the assumption that for every photon of light it should produce one electron through the solar cell. The number of photons is determined by measuring the volts and converting it to amps using the gain which is the ratio of voltage and amps. From amps convert it to watts which are joules per second and so we divide the energy of one photon in joules to get the number of photons as shown in the steps of Eq. (2) below. The energy of a photon is dependent to its wavelength so the equation will be a function of wavelength. 5

11 = = (2) h = 1 The number of electrons per second is found by measuring the current (I) which is in coulombs (C) per second and dividing it by the charge of one electron as shown in Eq. (3) below. The current is a function of wavelength and so the number of electrons will also be a function of wavelength. The charge of one electron is x C x C Structure of Photovoltaic Cell Figure 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. As seen in Figure 2 the different layers of the photovoltaic have different thicknesses. The glass layer is approximately 1-3 mm. TCO stands for a transparent conductive oxide and its thickness is µm. HRT is an unknown substance to me and my advisor, by the picture being drawn close to scale it is about the same thickness as CdS. CdS as stated 6

12 above is Cadmium Sulfide and it is a n-type of the semiconductor and has a thickness of µm. The Cadmium Telluride (CdTe) layer is a p-type side of the semiconductor and is µm thick. The back contact is doped with copper (Cu). The dark region in the top view of Figure 2 is the ground. The ovals represent areas that can produce current. To measure the current an electrometer is used and one tip is connected to the dark gray region and then other is connected to where the light is shined on which is in the center of one of the ovals. 2.3 Set-up of Experiment Below is Figure 3 which is the actual set-up of the experiment. First is a 300W lamp that is in place to act like the sun. Next the light is sent through a monochromator that splits the light and makes it only one wavelength anywhere in the spectrum from 1 ev to 4eV. When it comes out of the monochromator it is sent through a chopper that gives the light a certain frequency. The chopper frequency is set to something different than the frequency of light that comes from the light bulbs in the room. The chopper frequency is set to a lock-in amplifier that makes the detector ignore all other frequencies of light. When the light comes out of the chopper it is reflected off a mirror to collimate the light, then it is reflected off another mirror through a lens focusing the light to a point which is shining on a spot of the solar cell or the detector. 7

13 Detector or Solar Cell monochomator lamp Figure 3: Set-up of lab equipment When the detector is present it measures the voltage created from the light. The detector is a NIST-calibrated Silicon detector. The voltage is read into a privately made computer program written by Dr. Nhan V. Nguyen, NIST semiconductor electronics division. The data is in table format and saved into a text file. The file is then taken and opened up into a table represented by Table 1 in the Appendix. Table 1 s data is the intensity of the light measured in volts at different wavelengths measured in electron-volts. Using that data the number of photons per second as a function of photon energy can be determined. 3. OBSERVATIONS & RESULTS The experiment is run using the computer program that Dr. Nguyen wrote. The program reads in the current for every different photon energy. The different photon energy is created by the program changing the wavelength of light coming out of the 8

14 monochromator. The photon energy was measured in electron-volts (ev), before starting the program the increment change of the photon energy represented by ev was set as seen in Table 1 of the appendix. ev was set to.01ev and for Table 2 of the appendix ev was set to.02ev. First, the experiment is run with the detector in the set-up to measure the intensity in volts. Then, the detector is replaced with the photovoltaic and the current is measured. The program uses the intensity and current data to compute the EQE in percentage. The data is compared with each other in Table 2 in the Appendix. Figure 4 below is the graph representation of that data. Test 1 is a run that was done by Dr. Nguyen before I got to NIST it was the first test to see the EQE was being calculated correctly by predicting the shape of the graph and the result is what was expected. The values are not accurate because it says at the highest point the efficiency will get above 350%. This would mean that we are producing more energy than we are giving into the system. That concept would be ideal but in reality that will never occur. Thinking more about the optics of the light coming into the solar cell we determined that the intensity of light we found that is used to determine the number of photons per second was calculated incorrectly because the intensity would change when the light hits the top layer of glass of the solar cell. The difference in the index of refraction between the air and the glass makes it so not all the light is transmitted through the glass, some is reflected off. So we needed to calculate the intensity of the light that was transmitted. I determined the transmitted intensity by the equations 4, 5, and 6 below. I in equation 4 represents the intensity of light, I incident is what was measured from the detector and that data is in Table 1 in the appendix. In equation 5 n 1 represents the index of refraction of air and n 2 in equation 5 is determined by equation 6. Equation 6 9

15 is the index of refraction for fused silicon. Dr. Nguyen adjusted the computer program for the newly calculated intensity. = 1. 1 = (5) 6 Full Run 1 was done after the corrections were put into the program. The EQE was expected to rise because the intensity would be lower which would make the number of photons per second less having the denominator in Eq. 1 be less which causes a bigger EQE. Our predictions were correct and the EQE jumped up and the data says that there is over 1000% efficiency at some points. This is not correct. The problem was that in the program the conversion was off by a factor of 100 which made the intensity appear smaller than it was. This was fixed in the program for full run 2. Full Run 2 was then run and the data collected. The data was still higher than what we expect. The max EQE for Full Run 2 was recorded as 43.73% at 1.7eV. Previously the highest recorded efficiency measured was to be 24.2% [2]. We were expecting to see an efficiency around 15% and we were 3 times greater than that. 10

16 1.20E+03 CdS-CdTe EQE% EQE % 1.00E E E E E+02 Test 1 Full Run 1 Full Run E Photon Energy (ev) Figure 4: Graph of the EQE% as a function of photon energy 4. CONCLUSION Our experiment produced results that were not close to what we had predicted. The EQE was greater than expected. There is not conclusive evidence that the CdS-CdTe photovoltaic is more efficient than previous photovoltaics cells. There is more work that needs to be done on the experiment. The process of how we collect the data needs to be improved. Also, knowing if all the light that the detector measured is focused on the photovoltaic when measuring the current needs to be looked into more. The detector is bigger and so it detects more light than the solar cell and this is a problem. There is future research still being done by Dr. Nguyen at NIST. Dr. Nguyen is working on modifying the setup to measure 3 D structures of CdTe thin film photovoltaic and triple junction. At this point there is no conclusion if this type of photovoltaic is better or worse. 11

17 Photovoltaic cells can be used to set up residential photovoltaic systems, which would mean a house would run on solar energy. This would be a big market once they are made for residential use. They can also be used to power commercial properties like, office buildings, stores, hospitals, and schools. The transportation industry could also use the cells to power cars, boats, and recreational equipment. Photovoltaic cells will be able to power anything that runs on electricity. There are so many things that can be done with the use of solar energy similar to that which is listed above. The cost for solar energy is relatively low a photovoltaic is just more than 2 cents per kilowatt hour compared to 5 cents per kilowatt hour which is the cost for coal. With the lower cost more electronics are going to move to being powered by solar energy. Imagine an ipod that doesn t have to be recharged by plugging in the cord to an outlet of the compute but that the back of it is a photovoltaic and all that needs to be done is for it to be out in the sun. This is the possibility of a photovoltaic cell. The applications are endless. 12

18 References [1] Nelson, Jenny. The Physics of Solar Cells. (Imperial College Press, London, UK, 2003). [2] SunPower Sets Solar Cell Efficiency Record at 24.2%. [3] Zeghbroeck, B. Van. Principles of Semiconductors. (2007) [4] Stokes, Harold T. Solid State Physics,. (Brigham Young University, Provo, UT, 2007), p [5] Jacobson, Mark Z. Review of Solutions to Global Warming, Air Pollution, and Energy Security. ( 2008). [6] Karpov, V. G.; Shvydka, Diana; Roussillon. Physics of CdTe Photovoltaics: from Front to Back. Invited talk f10.1 MRS Spring Meeting 2005, March28 April 1, San Francisco, CA. [7] Berinstein, Paula. Alternative Energy. (Oryx Press, Westport, CT, 2001). 13

19 Appendix Table 1: Measured Intensity Data SR830 Sensitivity : 1 V/uA; FERTO Preamp: wavelength (ev) wavelength (nm) Intensity (volts)

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26 Table 2: EQE Data ev Test 1 Full Run 1 Full Run E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E+01 21

27 E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E+00 22