Organic Solar Cells - Literature review Example

Organic solar cells Table of Contents Chapter Introduction 3 1Introduction 3 2History of solar cells 4 3Organic Semiconductors 6 4Organic solar cells 9 1.5Motivation 11 References 13 Chapter 1: Introduction 1.1 Introduction Solar cells capture the solar radiation and convert the solar light into energy. According to the photoelectric effect, the intensity of the light source influences the output from the solar cells. Solar radiations are the strongest light present on earth. It in not only free from the green house emissions abut also provides free energy. Thus solar energy is economically as well as environmentally an efficient source of energy. Solar cells directly convert solar radiation to electricity. There is no need of any kind of rotary parts in the process. When a light photon strikes the surface of the solar cells, the cells generates an electron in response that is further used by an electric appliance. Since the electrical energy is Direct current (DC) electricity, Alternate current (AC) appliances require an inverter in between to convert the dc current to Alternating current. The first commercially produced solar cells have lower efficiency as compared to the newer models. At first the solar cells were only 4% efficient. However, some time the solar cells attain an efficiency of 11%. Today, the industrially manufactured solar cells have a mean efficiency of about 19%, while some models of solar cells have efficiency as high as 32% (Boxwell, 2010, p32). At the very initial stage solar cells are for their extremely high cost. However, due to the technological breakthrough, the cost of the solar cells is lower to be in the range. Still the cost of the solar technology is more to be accessed by everyone. Organic semiconductor materials are less costly as compared to the other semiconductor materials. It is better to use the organic semiconductor materials to be manufactured at low cost yet highly efficient organic solar cells. By cutting the cost and improving the efficiency of the solar cells, the technology can be promoted to be utilized as the major source power generation systems. 1.2 History of solar cells In 1873 photoconductivity in selenium was first discovered by Willoughby Smith. During 1876, two scientists “William Grylls Adams” and “Richard Evans” found that when selenium is exposed to light, it generated a small amount of electrical current. However, the current produce at that stage was too weak to power an electrical appliance. During 1883, an American Scientist “Charles Fritts” made the solar cells from selenium. In 1905, photoelectric effect was theoretically described by “Albert Einstein”. In 1918, single crystal silicon was grown by “Jan Czochralski”. In 1954, Bell Labs developed the first solar cell that can power an electrical appliance. At that time, the cells were only 4% efficient (Goetzberger and Luther et al., 2002, pp. 1--11). In 1955, Silicon photovoltaic (PV) technologies were licensed. At that stage, the solar power money changers, punch cards and tapes were sold in the market. In 1957, efficient solar cells were produced by “Hoffman Electronics” that have 8% efficiency. In 1958, the photovoltaic cells with 9% efficiency were produced by Hoffman Electronics. In 1959, the efficiency of the photovoltaic cells was further increase to 10%. In 1960, the efficiency of the photovoltaic cells was further increased to 14%. Hoffman Electronics remained determined to increase the efficiency of the photovoltaic cells. In 1960, the photovoltaic solar cells were industrially manufactures in Wisconsin by Silicon Sensors, Inc., of Dodgeville, Wisconsin. In 1963, Sharp Corporation began producing silicon photovoltaic modules. In 1970s, Exxon Corporation designed solar cells with 80% less cost. Dr. Elliot Berman helped the Exxon Corporation to achieve the cost effective method and materials. The purpose of reducing the cost is to promote the utilization of technology for domestic as well as arid locations (Markvart and Bogus, 1994, p36). In 1972, an educational television is power by cadmium sulfide (CdS) photovoltaic system in Niger. In 1976, first amorphous silicon photovoltaic cells were fabricated by RCA Laboratories in coordination with David Carlson and Christopher Wronski. In1980, 10% efficiency is attained by thin film based solar cells utilizing copper sulphide/cadmium sulphide at the University of Delaware. In 1992, the researchers in the University of South Florida developed a thin film solar cell utilizing cadmium telluride that attained an efficiency of 15.9% (R and All, 2006, pp. 53--78). In 1998, Subhendu Guha developed a solar cell that is flexible in nature that is a roofing material. After the development of such type of solar cells a new era of flexible solar cells are invented (Green, 2009, pp. 183--189). In 1999, National Renewable Energy Laboratory in coordination with Spectrolab, Inc developed a solar cell that has 32.3% efficiency. The cells attained such efficiency due to utilizing different absorption layers. However, it does require a focused and concentrated solar light and a cooling mechanism. Generally it requires, 50 times more focused solar light as compared to normal solar cells. In the same period National Renewable Energy Laboratory developed a thin film solar cell that attained an efficiency of about 18.8%. In 2000, Bp developed thin film solar modules that have 10.8 % light to electricity conversion efficacy for its 0.5-square-meter module and 10.6% efficiency for its 0.9-square-meter. A single module produced about 91 watts that is higher produced by thin film modules at that time (Hagfeldt and Boschloo et al., 2010, pp. 6595--6663). 1.3 Organic Semiconductors During 1950, it is found that organic materials have semi conductive properties. The researchers evaluated that polycyclic aromatic compounds have similar properties as that of semiconductors. The experimentation involved the conductivity of charge carries in presence of different types of slats. In 1977, Shirakawa et al, discovered and developed conductive polymers. The discovery of the conductive polymers opened new dimensions of research to optimize the properties of organic semiconductors. Now, organic semiconductors are extensively used in semiconductor based optical devices (Spanggaard and Krebs, 2004, pp. 125--146). Organic semiconductors are the organic compounds that are abundantly present. Organic LEDs, Organic solar cells and Organic FET are the common examples of organic semiconductor based devices. On the other hand, the process to generate organic semiconductors is easier and consumes less energy and cost as compared to the methods evolved to produce inorganic semiconductors. The conductivity in these kinds of materials is due to the single molecules, short chain of molecules and long chain of polymers. Different types of organic semiconductors have different polymeric structures. Pentacene, Rubrene and anthracene are the typical materials used in organic semiconductors that have small molecule. Larger molecule semiconductors include fullerenes and fullerene derivatives. Traditional silicon based inorganic semiconductors are much expensive to produce. It is not the availability of silicon that makes the inorganic material expensive; it is process to convert the amorphous silicon to crystalline silicon. On the other hand germanium based inorganic semiconductors are more expensive to produce as compared silicon based semiconductor products. On the other hand, organic semiconductors can be produced with a much less expensive process. The difference between the cost of organic and inorganic materials is huge and continuous research is diverted to enhance the efficiency of the inorganic semiconductor materials (Kowalsky and Becker et al., 2000, p. 795). Organic semiconductors are soluble in solvents that enable the process of producing different semiconductor devices with the help of the solution. The process would be much cheaper as compared to the general process for producing semiconductor materials and devices. Organic semiconductor devices require much less annealing temperature, however, inorganic semiconductor require higher annealing temperatures, in some conditions more than 500o C. Some organic semiconductors require room temperature as the annealing temperature. The low annealing temperature allows the high temperature sensitive materials but become substrate of the organic semiconductors. The example of such type of substrate is plastic that can be applied to the organic semiconductors as the substrate. There is wide verity of other low cost substrate materials including resin based substrates (Kowalsky and Becker et al., 2000, pp. 795--797). Typically organic thin films can be deposited directly on the semiconductor substrate. In the image below, an organic thin film transistor is shown. The sources and the drain of the transistor are directly connected to the semiconductor substrate. Figure 1: different layers of organic semiconductors are deposited to form a transistor The function of the transistor is similar to an inorganic semiconductor based MOSFET. In case of no bias voltages, very few charge carriers flow from one junction to the other. The restriction in the charge carrier motion is due to the difference in the Fermi level present in between metal electrodes and organic semiconductor substrate. In the presence of bias voltage, the charge carries are forced to accumulate at the junction that connects the insulator and semiconductor. This lowers the Fermi level and promotes the conduction of charge carriers. A conductive layer that easily allows the charge carries to pass is formed. Multiple layers are used in such devices to intensify the movement of charge carriers across the layers. New research is also focused to eliminate the hurdles in the flow of electrons and holes through the semiconductor materials. It is still required to enhance the efficiency of the organic semiconductor materials (Kowalsky and Becker et al., 2000, p. 808). 1.4 Organic solar cells Organic photovoltaic solar cells make utilization of organic semiconductor materials in the active regions that absorbs the light and convents it to electric current. These organic semiconductors are mostly polymeric organic compounds and small molecules organic compounds. Solar cells are dependent on the band gap between the conduction band and valance band. The band gap should be of appropriate width to maximize the efficiency of the solar cells. When photos present in the light strike the absorption layer of the solar cell, the energy of the photons is absorbed by the electrons present in the valance shall. After absorbing the energy of the photos, the electrons get excited. These excited electrons leave the valance band and move towards the conduction band. The empty places of electrons are filled by holes. In this way, a charge pair is generated. In both the p and n regions, the electrons and holes move is directions typically opposite to each other that create a DC current (Hoppea and Sariciftci, 2004, p. 1925). Electrons flow through the n-type semiconductor material and holes flow through the P-type materials. It is important that both the holes and electrons flow in opposite direction to maximize the efficiency of the solar cells. If any recombination occur of electron and hole occurs, the electrons returns to the valance band maintaining its original position, reducing the concentration of electrons in the conduction band. The whole process reduces the efficiency of the solar cells as electrons become less likely to follow the path (Spanggaard and Krebs, 2004, p 146). Organic solar cells have a number of advantages and disadvantages. The most important aspect of the organic solar cells is that the organic solar cells are manufactured on large scale at a lower cost as compared to the inorganic solar cells. Organic solar cells have a self organized poly crystalline structure that makes the organic solar cells to be arranges in a much easier manner as compared to the inorganic solar cells (Thompson and FrEchet, 2008, pp. 58--77). However, because of this polycrystalline structure of the organic solar cells, the efficiency of conversion of light to current is lower as compared to the mono crystalline semiconductor solar cells. Inorganic semiconductor materials attain higher efficiency due to highly précised mono crystalline structure that restricts the flow of electrons in more than one direction. Other factors that can decrease the efficiency of the organic solar cells with eth course of time are sensitivity to oxidation and sensitivity to temperature fluctuations. However, the efficiency of the organic solar cells can be improved by improving the charge separation, improving the charge mobility, reducing the impurities and improving the crystalline structure (Liang and Xu et al., 2010, pp. 135--138). Till to date most efficient solar cells attained an efficiency of 32% using a multilayer absorption mechanism and with the single layer absorption, the efficiency is enhanced to 18.8%. Such solar cells use the C60 and its other derivates as the donor materials in the absorption regions of the dollar cells. However, the acceptor materials are likely to be highly efficient materials P3HT (poly (3-hexylthiophene)) and MEH-PPV. Hybrid organic and inorganic materials can attain higher efficiency and can have the potential to reduce the cost of typical solar cells (Yoo and Domercq et al., 2004, pp. 5427--5429). Figure 2: Mechanism of a solar cell and its relative parts 1.5 Motivation Today world is dependent on the fossil fuel resources to generate power. Our major sources of energy are coal, oil and natural gas. Most of the power generation units are coal based. These power generation units convert the chemical energy in the coal or other fossil fuel resources to electrical energy. The burning of the fossil fuel resources has certain environmental and economic impacts. When fossil fuels are burnt, a large amount of energy gets wasted. On the other hand, large amount of by-products are produced. The most significant by-product of the conventional energy generation system is the green house gas emission. Because of the green house gas emissions, the temperature of the earth is elevated to about 1 oC averagely (Christensen and Wood et al., 2004, pp. 337--363). Other impacts of green house gas emissions are the loss of biodiversity, health issues, change in climate, change in hydrologic cycles, more frequent insect borne diseases, etc (Jacobson, 2009, pp. 148--173). On the other hand, the prices of fossil fuel resources are going up day by day. Fossil fuel resources are going to end one day and cost of the resources will go higher day by day. It is better to build an energy generation system with more stable economic outcomes. Alternate energy resources are free from green house gas emission and are more economically stable as compared to the fossil fuel resources. Among the alternate energy resources, solar energy has the largest potential. However, the conventional solar technology is expensive and does not allow a common person to invest in the technology. Solar technology is table and it is never going to end. Organic solar cells have comparatively low cost. These solar cells are easy to manufacture and the raw materials are abundantly present on the surface of earth. After studying certain perspectives of organic solar cells, it has motivated me to choose the research topic to enhance my knowledge o the organic solar cells and present a research and new perspective of organic solar cells. References Choy, W. C. H. 2013. Organic solar cells. London: Springer. Goetzberger, A., Luther, J. and Willeke, G. 2002. Solar cells: past, present, future. Solar energy materials and solar cells, 74 (1), pp. 1--11. Kowalsky, W., Becker, E., Benstem, T., Johannes, H., Metzdorf, D., Neuner, H. and Sch"Obel, J. 2000. Organic semiconductors: fundamentals and applications. Springer, pp. 795--808. Potscavage Jr, W. J. 2010. Physics and engineering of organic solar cells. Georgia Institute of Technology. W"Ohrle, D. and Meissner, D. 1991. Organic solar cells. Advanced Materials, 3 (3), pp. 129--138. Boxwell, M. 2010. Solar electricity handbook. Ryton on Dunsmore, Warwickshire, U.K.: Greenstream Pub. Christensen, N. S., Wood, A. W., Voisin, N., Lettenmaier, D. P. and Palmer, R. N. 2004. The effects of climate change on the hydrology and water resources of the Colorado River basin. Climatic Change, 62 (1-3), pp. 337--363. Goetzberger, A., Luther, J. and Willeke, G. 2002. Solar cells: past, present, future. Solar energy materials and solar cells, 74 (1), pp. 1--11. Green, M. A. 2009. The path to 25% silicon solar cell efficiency: history of silicon cell evolution. Progress in Photovoltaics: Research and Applications, 17 (3), pp. 183--189. Hagfeldt, A., Boschloo, G., Sun, L., Kloo, L. and Pettersson, H. 2010. Dye-sensitized solar cells. Chemical reviews, 110 (11), pp. 6595--6663. Hoppea, H. and Sariciftci, N. S. 2004. Organic solar cells: An overview. J. Mater. Res, 19 (7), p. 1925. Jacobson, M. Z. 2009. Review of solutions to global warming, air pollution, and energy security. Energy & Environmental Science, 2 (2), pp. 148--173. Liang, Y., Xu, Z., Xia, J., Tsai, S., Wu, Y., Li, G., Ray, C. and Yu, L. 2010. For the bright future—bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%. Advanced Materials, 22 (20), pp. 135--138. Markvart, T. and Bogus, K. 1994. Solar electricity. Chichester: Wiley. R and All, J. F. 2006. Fundamentals of Solar Cells. Designing Indoor Solar Products: Photovoltaic Technologies for AES, pp. 53--78. Spanggaard, H. and Krebs, F. C. 2004. A brief history of the development of organic and polymeric photovoltaics. Solar Energy Materials and Solar Cells, 83 (2), pp. 125--146. Thompson, B. C. and FrEchet, J. M. 2008. Polymer--fullerene composite solar cells. Angewandte Chemie International Edition, 47 (1), pp. 58--77. Thompson, B. C. and FrEchet, J. M. 2008. Polymer--fullerene composite solar cells. Angewandte Chemie International Edition, 47 (1), pp. 58--77. Yoo, S., Domercq, B. and Kippelen, B. 2004. Efficient thin-film organic solar cells based on pentacene/C 60 heterojunctions. Applied Physics Letters, 85 (22), pp. 5427--5429. Read More