SEMINAR TOPIC ON IMPROVING THE PERFORMANCE OF SOLAR CELL MADE OF SEMICONDUCTOR

FEDERAL POLYTECHNIC NEKEDE P.M.B 1036 OWERRI IMO STATE SEMINAR TOPIC ON IMPROVING THE PERFORMANCE OF SOLAR CELL MADE OF SEMICONDUCTOR WRITTEN BY CHINEDU J 10H/1862/EE (POWER OPTION) DEPT: ELECTRICAL ELECTRONICS ENGINEERING LEVEL: HND II (M) A Seminar Presented To Department of Electrical /Electronics Engineering Technology In Partial Fulfillment of the Requirement for the Award of Higher National Diploma (HND) in Electrical Electronics Engineering ENGR. U.S MICHAEL SUPERVISOR JUNE 2012 CERTIFICATION I certify that this research seminar has met the requirement for the award of the Higher National Diploma (HND) in Electrical/Electronic Engineering. ----------------------------- ---------------- Engr. U.S Michael Date Supervisor ----------------------------- ---------------- Engr. Emeka Okorie Date (Seminar Coordinator) ----------------------------- ---------------- Rev. (Engr.) B.C. Agwah Date H.O.D E/E DEDICATION This seminar work is dedicated to Lord God Almighty for his faithfulness and loving kindness. ACKNOWLEDGMENT I wish to express my profound gratitude to my supervisor for who have guiding me this seminar would not have been possible without the immeasurable encouragement from Mr. and Mrs. Alaribe who have been providing for me. TABLE OF CONTENTS SEMINAR TOPIC ON IMPROVING THE PERFORMANCE OF SOLAR CELL MADE OF SEMICONDUCTOR TABLE OF CONTENTS Title page i Certification ii Dedication iii Acknowledgement iv Contents v Abstract vi Table of content Abstract CHAPTER ONE 1.0 INTRODUCTION 1.1 Primary energy sources 1.2 Renewable energy sources Photovoltaic solar energy (solar electricity) 1.4 Introduction to photovoltaic solar energy 1.5 Photovoltaic technologies 1.6 Photovoltaic applications and market CHAPTER TWO 2.0 LITERATURE REVIEW Improving the performance of organic solar cells 2.1 Magnetic nanoparticles enhance performance of solar cells 2.3 Improve Solar PV Panel Efficiency and Output Power Solar Cell Technology Fill factor Solar Cells Glazing Solar Panel Orientation Solar tracker Concentrators Solar Charge Controls MPPT Controller Top of Form 2.4 Blu-Ray Disc Can be Used to Improve Solar Cell Performance Data storage pattern transferred to solar cell increases light absorption 2.5 Improving the efficiency of solar panels Light scattering was promoted in the visible part of sunlight's spectrum CHAPTER THREE 3.0 Materials Crystalline silicon Monocrystalline silicon Ribbon silicon Mono-like-multi silicon (MLM) Thin film Cadmium telluride Copper indium gallium selenide Silicon thin film Gallium arsenide thin film Multijunction cells 3.2 Research in solar cells Perovskite solar cells 3.3 Upconversion and Downconversion 3.4 Light-absorbing dyes 3.4 Quantum dots 3.5 Organic/polymer solar cells Manufacture CHAPTER FOUR Conclusion and recommendation reference Abstract The purpose of this project was to investigate how the design of solar cells could be improved so that they could become a more reliable source of energy. The first design change considered was shape manipulation, in which a normal photovoltaic array would be changed from a flat panel to either a cylindrical, parabolic, or spherical light-capture device. The idea is to change the cell or panel so that as much light is absorbed as possible. The second idea explored was to use a home water heating system in conjunction with the solar cells in order to control temperature fluctuation within the solar array, thus optimizing efficiency. The use of wiper blades, similar to windshield wiper systems in cars, was proposed as our third idea in order to remove any snow or debris from the surface of the panels. Other changes considered included the use of light-manipulation methods, such as Fresnel lenses, to enhance solar flux, prisms, which would be used to redirect light towards an array of solar cells, and one way glass, CHAPTER ONE 1.0 INTRODUCTION A solar cell, or photovoltaic cell, is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.[1] It is a form of photoelectric cell, defined as a device whose electrical characteristics, such as current, voltage, or resistance, vary when exposed to light. Solar cells are the building blocks of photovoltaic modules, otherwise known as solar panels. Solar cells are described as being photovoltaic irrespective of whether the source is sunlight or an artificial light. They are used as a photodetector (for example infrared detectors), detecting light or other electromagnetic radiation near the visible range, or measuring light intensity. The operation of a photovoltaic (PV) cell requires 3 basic attributes: • The absorption of light, generating either electron-hole pairs or excitons. • The separation of charge carriers of opposite types. • The separate extraction of those carriers to an external circuit. In contrast, a solar thermal collector supplies heat by absorbing sunlight, for the purpose of either direct heating or indirect electrical power generation from heat. A "photoelectrolytic cell" (photoelectrochemical cell), on the other hand, refers either to a type of photovoltaic cell (like that developed by Edmond Becquerel and modern dye-sensitized solar cells), or to a device that splits water directly into hydrogen and oxygen using only solar illumination. Any change that takes place in the universe is accompanied by a change in a quantity that we name energy. We do not know what energy exactly is, we use this term to describe a capacity of a physical or biological system for movement or change. Energy comes in many forms, such as electrical energy, chemical energy, or mechanical energy, and it can be used to realize many forms of change, such as movement, heating, or chemical change. Any activity, and human activity as well, requires energy. Human beings need it to move their bodies, to cook, to heat and light houses, or to drive vehicles. Human being is a greedy consumer of energy. An active young man needs about 2500 kcal (2.9 kWh) per day to fulfil his daily energy requirements. This means the energy of about 1060 kWh per year. The present global energy consumption is around 19 000 kWh per inhabitant per year. It means that on average a man consumes about 19 times more energy than is needed for his survival and satisfactory health. The mankind has witnessed an enormous increase in energy consumption during last 100 years. While in 1890 the energy use per inhabitant per year was around 5800 kWh it reached 20200 kWh in 1970. Since 1970 the energy use has dropped to the present level of 19000 kWh per inhabitant per year. The increase in energy use in the 20th century can be related to an evolution process that has started about five centuries ago. The underlying motivation of this process was formulated during the Enlightenment period in the 18th century as the philosophy of human progress. The aim of the process was an examination of the surrounding world and its adaptation to the needs of people whose life would become more secure and comfortable. This process was accompanied by growing industrialization and mass production, which were demanding more and more energy. At the end of the 19th century coal was the main source of energy. In this period electricity was introduced in the industrialized countries as a new and elegant form of energy. This form of energy was quickly applied on a large scale. The widespread growth of electricity use led to construction of hydroelectric plants and hydropower became an important source of energy in the first half of the 20th century. In the period after the World War II much effort was put into the reconstruction of the society. The emphasis was directed on the growth and efficiency of the mass production. New technologies and new materials, such as plastic, were applied in the production. The energy demand was tremendously growing in this period. Oil and gas started to play an important role as energy sources in the second half of the 20th century. Coal, oil, and gas form today dominant sources of energy. These three energy sources, also known as fossil fuels, are called the traditional energy sources. In this period nuclear energy was introduced as a new source of energy. Increasing and more efficient mass production resulted in the low price of many household products. The consumption of the products grew enormously and therefore it is not surprising that we characterise today society as a consumption society. Nevertheless, it has become evident at the end of the 20th century that the philosophy of human progress that has manifested itself in a huge production and consumption of goods has a negative side too. It has been recognized that a massive consumption of fossil fuels in order to fulfil the present energy demands has a negative impact on the environment. The deterioration of environment is a clear warning that the present realization of human progress has its limitations. The emerging international environmental consciousness was formulated in a concept of a sustainable human progress. The sustainable human progress is defined as: “… to ensure that it (sustainable development) meets the needs of the present without compromising the ability of future generations to meet their own needs”1. A new challenge has emerged at the end of the 20th century that represents a search for and a utilization of new and sustainable energy sources. The urge of this challenge is underlined by limited resources of the fossil fuels on the Earth and increasing demand for energy production. This is the reason why the attention is turning to the renewable energy sources. Energy is an essence of any human activity. When we are interested in how the human civilization has been producing and using energy, we can describe it in terms of an energy system. The main characteristics of the energy system are: the population, the total consumption of energy, and the sources and forms of energy that people use. The energy system at the beginning of the 21st century is characterised by six billion people that live on the Earth and the total energy consumption of approximately 1.3 × 1010 kW. 1.1 Primary energy sources Figure 1.1 presents an overview of the present primary energy sources 2. The primary energy sources can be divided in two groups. The first group includes those energy sources that will be exhausted by exploiting them. These energy sources are called the depleting energy sources and they are the fossil fuels and nuclear energy. The fossil fuels and nuclear power are the main source of energy in today’s energy system and they supply 78% of the energy demand. Under the assumption that the population of mankind does not change drastically and it consumes energy at the current level, the fossil fuel reserves will be exhausted within 320 years and the nuclear energy within 260 years. This can seem a very long time for us. However, when we compare this period of time to the time span of existence of the Earth or the human civilisation, it is a negligible fraction of time. We have to be aware that the reserves of fossil fuels on the Earth are limited and will be exhausted. It is expected that the world population will grow and will reach 10 billion in 2050. In order to provide the growing population with high living standards, further economic development is essential. The further economic development requires more energy than we use today. The extra energy has to come from additional sources than only the traditional ones. Furthermore when we want to take the concept of sustainable development into account, we have to look for environmentally friendly energy sources. These sources are known as renewable or sustainable energy sources. The renewable energy sources form the second group of the primary energy sources and today they contribute with 22% to the total energy production. By renewable energy we understand energy that is obtained from the continuing flows of energy occurring in the natural environment, such as solar energy, hydropower and energy from biomass. About one third of the primary energy is used to generate electric power. This form of energy has become very popular and is widely used for industrial and household applications. The electrical energy represents about 12% of all energy consumed worldwide. Since most of the energy production is based on the fossil fuels, these have become a global strategic material. Fossil fuels are not equally distributed over the world and the countries that enjoy huge reserves of fossil fuels can influence the world’s economy. The decisions of these countries concerning production levels and price of the fuels have strong effects on the energy production and can result in social tensions. Further, the energy consumption of primary energy is not equal per inhabitant in the world. About ¼ of the world population uses ¾ of the primary energy. For comparison: an inhabitant of the U.S.A. uses on average 10 kW of power produced from fossil fuels, while an inhabitant of the Central Africa uses 0.1 kW of power produced from wood. This discrepancy is even more pronounced in the use of electricity. There is no electricity available in most of the rural areas in the developing countries. It is estimated that about 2 billion people have no access to electricity. It has been recognized that a massive consumption of fossil fuels has a negative impact on the environment. Gases such as CO2 and SOX and NOX are produced as the byproducts during burning of the fossil fuels. Enormous quantities of these gases are emitted into the atmosphere, where they change the natural concentrations. The ecological problems, such as the greenhouse effect and acid rains, are caused by the increase of these gases in the atmosphere. The greenhouse effect is linked to the increase of CO2 in the atmosphere. The CO2 molecules are transparent to solar radiation but are opaque to heat, which is the radiation in the infrared wavelength region. The concentration of CO2 in the atmosphere has increased in the 20th century from 280 ppm to 350 ppm. Scientists expect that when this trend continues, the temperature will rise from 3 ºC to 5 ºC in 2030-2050. In order to avoid this situation, in which the climate change, known also as the global warning, can lead to undesired ecological changes a reduction in CO2 emission is essential. 1.2 Renewable energy sources The negative aspects of today’s energy system have led to the formulation of sustainable human development. The realization of the sustainable development requires an alternative energy system that is based on: i) policies for efficient energy use and ii) renewable energy sources. The world’s largest oil company Shell has published recently a vision on future energy consumption and potential energy sources3. One of the largest energy producers in the world expects that the restructuring of power industry will take place in near future. The Shell’s scenario that is called the "Sustained Growth" is presented in Figure 1.3. The company has concluded that the fossil fuels are still important, but they reach a plateau by 2020. At this time, renewable energy will become a significant source of energy. At first, renewable energy will grow in niche markets rather than compete with traditional sources of energy. The market will decide a share of different forms of renewable energy. In future, the energy supply will become more diversified and hence more robust. It is interesting to notice that Shell expects the photovoltaic (PV) solar energy to become a major energy source within fifty years. Renewable energy sources are based on the continuing flows of energy that is considered inexhaustible from the point of view of human civilisation. Solar radiation represents such an infinite source of energy for the Earth. The sun delivers 1.2 × 1014 kW energy on the Earth, which is about 10.000 times more than the present energy consumption. The energy that the Earth receives from the sun in just one hour is equal to the total amount of energy consumed by humans in one year. The major advantages of using renewable energy sources over the traditional energy sources are reflected in a cleaner environment, creating employment opportunities, and security of energy supply. The use of renewable energy can reduce the emission of greenhouse gases and other pollutants. The expanded use of renewable sources of energy can have a positive impact on job creation in the technology manufacturing industries and also the agricultural sector, which supplies biomass fuel. Renewable energy can play an important role in increasing security of energy supply by providing domestic resources of energy and avoiding dependence on imported supplies of fossil fuels. Today, the renewable energy sources contribute around 22% to energy production, with traditional biomass and hydroelectric power as the main contributors. In Europe, the renewable energy's contribution to the primary energy is 5.3% (1994) in the European Union and 1.7% in Eastern Europe. Since 1989, the use of renewable energy in Europe has been growing at a rate of 2.7% per year, 50% faster than the 1.8% annual growth of the overall energy market over the same period. It is likely that renewable energy will, by 2020, be one of the three largest sources of energy in Europe, along with gas and nuclear. At present, the level of renewable energy market penetration strongly depends on policies, particularly policies for environment, research and development (R&D), and market support policies. Europe's renewable energy industry is already leading the world in some areas, notably in wind and PV, and is a world pioneer in advanced biomass. Electricity from renewable sources is today still more expensive than electricity produced from traditional sources. Therefore, a large-scale application of renewable energy sources as electricity power sources is not yet economically attractive in the industrialized countries. The environmental benefits of renewable technologies are probably the strongest factor for growing market and national policies to encourage renewable energy sources. However, electricity from renewable energy sources is already today the most effective cost solution for two billion people in many parts of the world who have no access to electricity grid. Photovoltaic solar energy (solar electricity) 1.4 Introduction to photovoltaic solar energy The energy of solar radiation is directly utilised in mainly two forms: i) direct conversion into electricity that takes place in semiconductor devices called solar cells ii) accumulation of heat in solar collectors. Therefore, do not confuse solar cells with solar collectors. The direct conversion of solar radiation into electricity is often described as a photovoltaic (PV) energy conversion because it is based on the photovoltaic effect. In general, the photovoltaic effect means the generation of a potential difference at the junction of two different materials in response to visible or other radiation. The whole field of solar energy conversion into electricity is therefore denoted as the “photovoltaics”. Photovoltaics literally means “light-electricity”, because “photo” is a stem from the Greek word “phõs” meaning light and "Volt” is an abbreviation of Alessandro Volta’s (1745-1827) name who was a pioneer in the study of electricity. Since a layman often does not know the meaning of the word photovoltaics, a popular and common term to refer to PV solar energy is solar electricity. The oil company Shell expects that PV solar energy will become the main energy source for the "post-fossil-era"3. Developing the PV solar energy as a clean and environmentally friendly energy source is considered at present noble mission. In this mission, the sun is consciously given an additional function to the one that it has had: to provide energy for life on the Earth. The sun’s additional function will be to provide the Earth with energy for people’s comfort and well being by producing the solar electricity. The motifs that were behind the development and application of the PV solar energy were in general the same as for all renewable energy sources. The motifs were based on the prevention of climate and environment and providing clean energy for all people. The current motifs can be divided into three categories: energy, ecology and economy. Energy There is a growing need for energy in the world and since the traditional energy sources based on the fossil fuels are limited and will be exhausted in future, PV solar energy is considered a promising energy source candidate. Large-scale application of PV solar energy will also contribute to the diversification of energy sources resulting in more equal distribution of energy sources in the world. Ecology Large-scale use of PV solar energy, which is considered environmentally friendly source of energy, can lead to a substantial decrease in the emission of gases such as CO2 and SOX and NOX that pollute the atmosphere during burning of the fossil fuels. When we closely look at the contribution of the PV solar energy to the total energy production in the world we see that the PV solar energy contribution is only a tiny part of the total energy production. At present, the total energy production is estimated to be 1.6 × 1010 kW compared to 1.0 × 106 kWp that can be delivered by all solar cells installed worldwide. By Wp (Watt peak) we understand a unit of power that is delivered by a solar cell under a standard illumination. When PV starts to make a substantial contribution to the energy production and consequently to the decrease in the gas emissions depends on the growth rate of the PV solar energy production. When the annual growth of PV solar energy production is 15% then in year 2050 solar cells will produce 2.0 × 108 kWp. The annual growth of 25% will result in the solar electricity power production of 7.5 × 109 kWp in 2040 and the annual growth of 40% will lead to power production of 2.4 × 1010 kWp in 2030. This demonstrates that there must be a steady growth in solar cells production so that PV solar energy becomes a significant energy source after a period of 30 years. Economy The solar cells and solar panels are already on the market. An advantage of the PV solar energy is that the solar panels are modular and can be combined and connected together in such a way that they deliver exactly the required power. We refer to this feature as “custommade” energy. The reliability and very small operations and maintenance costs, as well as modularity and expandability, are enormous advantages of PV solar energy in many rural applications. There are two billion people in mostly rural parts of the world who have no access to electricity and solar electricity is already today the most cost effective solution. Bringing solar electricity to these people represents an enormous market. Some companies and people have realised that solar electricity can make money already now and this fact is probably the real driving force to a widespread development and deployment of the PV solar energy. We can roughly estimate how much money is already involved in the production of solar cells. The total production of solar cells has achieved more than 1200 MWp. An average cost-prize of 1 Wp. was approximately 3.5 €. This means that the money involved in production of solar cells reached 4.2 milliard €. Assuming that a complete PV system is roughly two times the cost of the cells, a total money involved PV in 2004 can be estimated to 10 milliard €. The advantages and drawbacks of the PV solar energy, as seen today, are summarized: Advantages: • environmentally friendly • no noise, no moving parts • no emissions • no use of fuels and water • minimal maintenance requirements • long lifetime, up to 30 years • electricity is generated wherever there is light, solar or artificial • PV operates even in cloudy weather conditions • modular or “custom-made” energy, can be designed for any application from watch to a multi-megawatt power plant Drawbacks: • PV cannot operate without light • high initial costs that overshadow the low maintenance costs and lack of fuel costs • large area needed for large scale applications • PV generates direct current: special DC appliances or inverters are needed in off-grid Photovoltaic (PV) system The solar energy conversion into electricity takes place in a semiconductor device that is called a solar cell. A solar cell is a unit that delivers a certain amount of electrical power that is characterised by an output voltage and current. In order to use solar electricity for practical devices, which require a particular voltage or current for their operation, a number of solar cells are connected together to form a solar panel, also called a PV module. For largescale generation of solar electricity the solar panels are connected together into a solar array. The solar panels are part of a complete PV solar system, which, depending on the application, comprises batteries for electricity storage, dc/ac inverters that connect a PV solar system to the electrical grid, and other miscellaneous electrical components or mounting elements. These additional parts of the PV solar system form a second part of the system that is called balance of system (BOS). Finally, the solar system includes products such as household appliances; radio or TV set that use the solar electricity for their operation. We refer to these products as a load. In summary, the PV solar system consists of three parts: i) solar panels or solar arrays, ii) balance of system, iii) load. 1.5 Photovoltaic technologies The first practical use of solar cells was the generation of electricity on the orbiting satellite Vanguard 1 in 1958. These first solar cells were made from single crystal silicon wafers and had efficiency of 6 %. The space application was for some time the only application of solar cells. The energy crisis in the seventies of the 20th century accelerated a search of new energy sources for terrestrial applications. This search resulted in a growing interest for PV solar energy. The major obstacle of using solar cells for terrestrial electricity generation has been a much higher price of the solar electricity when compared to the price of electricity generated from the traditional sources. Therefore, there has been much effort in the field of solar cells to reduce the price of solar electricity to a level that is comparable to the conventional electricity. The single crystal silicon wafer-based solar cells that had been used in space became also the first solar cells to be used for terrestrial generation of electricity. In order to increase the efficiency of single crystal silicon solar cells and to lower their price, the crystalline silicon solar cell technology has improved dramatically in the past twenty years and today it is the dominant solar cell technology. Crystalline silicon solar cell technology represents today not only single crystal silicon wafer-based solar cells, but also multicrystalline silicon solar cells. Both technologies that deal with “ bulk” crystalline silicon are considered the first generation solar cells for terrestrial applications. As this technology has matured, costs have become increasingly dominated by material costs, namely those of the silicon wafer, the glass cover sheet, and encapsulants. In order to decrease the material costs of crystalline silicon solar cells, research has been directed to develop low cost thin-film solar cells, which represent a second generation solar cells for terrestrial application. There are several semiconductor materials that are potential candidates for thin-film solar cells, namely copper indium gallium diselenide (CuInGaSe2=CIGS), cadmium telluride (CdTe), hydrogenated amorphous silicon (a-Si:H), thin-film polycrystalline silicon (f-Si). The titanium oxide nanocrystals covered with organic molecules represent so called dye-sensitized nano-structred solar cells. It is expected that the efficiency of commercial second generation solar modules is likely to reach 15%. Conversion efficiency has to be increased substantially in order to progress further. Calculations based on thermodynamics demonstrate that the limit on the conversion efficiency of sunlight to electricity is 93% as opposed to the upper limit of 33% for a single junction solar cell, such as a silicon wafer and most present thin-film solar cells. This suggests that the performance of solar cells could be improved 2-3 times when different concepts were used to produce a third generation of high efficiency, thin-film solar cells. 1.6 Photovoltaic applications and market Figure 1.4 presents an overview of the different solar cell technologies that are used or being developed for two main solar cell applications, namely space and terrestrial applications. The conversion efficiency of solar cells used in space applications is the initial efficiency measured before the solar cells are launched into the space. This conversion efficiency is also referred to as the begin-of-life efficiency. Today's commercial PV systems in terrestrial applications convert sunlight into electricity with efficiency ranging from 7% to 17%. They are highly reliable and most producers give at least 20 years guarantee on module performance. In case of the thin-film solar cells the best conversion efficiency that has been achieved in laboratory is shown together with the conversion efficiency that is typical for commercial solar cells. CHAPTER TWO 2.0 LITERATURE REVIEW Improving the performance of organic solar cells Much effort has been devoted to the development of polymer solar cells because they can be made inexpensively and offer possibilities for mechanical flexibility. The properties of polymer solar cells depend critically on the electrical and optical properties of the polymer and, as a result, many researchers are looking for ways to tailor these properties for improved performance. In conventional polymer solar cell architectures, the performance rapidly degrades because the top electrode is readily oxidized. An alternative, inverted structure has been developed that has an air-stable metal as the top contact. In this architecture, the interfacial electron- and hole-transporting materials are key to improving device efficiency. The electron-selective materials used in the devices should have frontier molecular orbitals suited to efficient electron extraction and hole blocking, be solvent resistant (so as not to erode during solution processing), have good conductivity, have small absorption coefficients for visible light, and make smooth films that adhere to the glass and active layers in the device. Inorganic metal oxides are good candidates, but they absorb oxygen when illuminated, which decreases their stability. Organic semiconductors allow the design of better interfaces using synthetic chemistry approaches and they tend to be highly flexible. However, they usually lack solvent resistance, making them difficult to process. Alex K.-Y. Jen and co-workers have recently reported a way to overcome the problems previously limiting the use of polymer semiconductors through the in situ crosslinking and n-doping of semiconducting polymers, which results in high conductivity and increased solvent resistance. In the study, thiophene and naphthalene diimide (NDI) copolymers, which are n-type semiconductors, were crosslinked using bis(perfluorophenyl) azide and doped with varying concentrations of an efficient organic n-dopant. The crosslinking prevents erosion during subsequent solution processing steps and the n-doping increases electrical conductivity. Solar cells made from the n-doped, crosslinked polymer showed increased power conversion efficiency with increasing dopant concentration. Overall the properties of the devices were similar to those of a control device made using zinc oxide. The power conversion efficiency for devices with the highest dopant concentrations even exceeded that of the zinc oxide based device. The electrical conductivity of organic thin-film transistors that incorporated the new material increased as the doping concentration was increased. Together these results offer a new step toward improving the performance of all inverted organic solar cells. Improving efficiency of solar photovoltaic cells takes months, sometimes years. The last few days, however, have been very interesting as three different companies announced record-breaking efficiencies. Two of the cells even have the same technology. Trina Solar, one of the leading solar PV modules manufacturers, announced on April 24 that it had set a new world record for high efficiency p-type multi-crystalline silicon PV modules. Trina Solar’s Honey Plus multi-crystalline silicon module reached a new module efficiency record of 19.14%. The efficiency was independently confirmed by the National Center of Supervision and Inspection on Solar Photovoltaic Product Quality (CPVT) in Wuxi, China. On April 27, Germany-based Manz announced that it had achieved 16% efficiency in copper indium gallium selenide (CIGS) solar modules. This efficiency was achieved in commercially mass-produced solar PV modules. Manz managed to transfer the 21.7% world-record efficiency it had achieved in laboratory cells in September last year. The efficiency of the modules was verified by TUV Rheinland. On April 28, Taiwan-based TSMC Solar announced that it achieved efficiency of 16.5% in commercially produced CIGS modules, bettering the Manz’s record made the day before. TSMC also improved upon its own previous record of 15.7% efficiency, achieved in 2013. The efficiency record for TSMC modules was verified by TUV SUD. All three efficiency records seem to be associated with mass-produced solar PV modules and there remains huge potential to further increase the efficiency. Trina Solar also holds the world record for efficiency for lab-based multi-crystalline silicon PV modules. That record currently stands at 20.8%. 2.1 Magnetic nanoparticles enhance performance of solar cells Study at DESY’s X-ray source PETRA III points the way to higher energy yields Download [2.8 MB, 3648 x 2736] Lightweight, flexible and semi-transparent organic solar cells (here on a glass slide for research purposes) are prepared from solution and at room temperature. Download Crystalline structures within polymer solar cells cause characteristic diffraction patterns in experiments with synchrotron radiation. Magnetic nanoparticles can increase the performance of solar cells made from polymers – provided the mix is right. This is the result of an X-ray study at DESY’s synchrotron radiation source PETRA III. Adding about one per cent of such nanoparticles by weight makes the solar cells more efficient, according to the findings of a team of scientists headed by Prof. Peter Müller-Buschbaum from the Technical University of Munich. They are presenting their study in one of the upcoming issues of the journal Advanced Energy Materials (published online in advance). Polymer, or organic, solar cells offer tremendous potential: They are inexpensive, flexible and extremely versatile. Their drawback compared with established silicon solar cells is their lower efficiency. Typically, they only convert a few per cent of the incident light into electrical power. Nevertheless, organic solar cells are already economically viable in many situations, and scientists are looking for new ways to increase their efficiency. One promising method is the addition of nanoparticles. It has been shown, for example, that gold nanoparticles absorb additional sunlight, which in turn produces additional electrical charge carriers when the energy is released again by the gold particles. Müller-Buschbaum’s team has been pursuing a different approach, however. “The light creates pairs of charge carriers in the solar cell, consisting of a negatively charged electron and a positively charged hole, which is a site where an electron is missing,” explains the main author of the current study, Daniel Moseguí González from Müller-Buschbaum’s group. “The art of making an organic solar cell is to separate this electron-hole pair before they can recombine. If they did, the charge produced would be lost. We were looking for ways of extending the life of the electron-hole pair, which would allow us to separate more of them and direct them to opposite electrodes.” This strategy makes use of a quantum physical principle which states that electrons have a kind of internal rotation, known as spin. According to the laws of quantum physics, this spin has a value of 1/2. The positively charged hole also has a spin of 1/2. The two spins can either add up, if they are in the same direction, or cancel each other out if they are in opposite directions. The electron-hole pair can therefore have an overall spin of 0 or 1. Pairs with a spin of 1 exist for longer than those with an overall spin of 0. The researchers set out to find a material that was able to convert the spin 0 state into a spin 1 state. This required nanoparticles of heavy elements, which flip the spin of the electron or the hole so that the spins of the two particles are aligned in the same direction. The iron oxide magnetite (Fe3O4) is in fact able to do just this. “In our experiment, adding magnetite nanoparticles to the substrate increased the efficiency of the solar cells by up to 11 per cent,” reports Moseguí González. The lifetime of the electron-hole pair is significantly prolonged. Adding nanoparticles is a routine procedure which can easily be carried out in the course of the various methods for manufacturing organic solar cells. It is important, however, not to add too many nanoparticles to the solar cell, because the internal structure of organic solar cells is finely adjusted to optimise the distance between the light-collecting, active materials, so that the pairs of charge carriers can be separated as efficiently as possible. These structures lie in the range of 10 to 100 nanometres. “The X-ray investigation shows that if you mix a large number of nanoparticles into the material used to make the solar cell, you change its structure”, explains co-author Dr. Stephan Roth, head of DESY’s beam line P03 at PETRA III, where the experiments were conducted. “The solar cell we looked at will tolerate magnetite nanoparticle doping levels of up to one per cent by mass without changing their structure.” The scientists observed the largest effect when they doped the substrate with 0.6 per cent nanoparticles by weight. This caused the efficiency of the polymer solar cell examined to increase from 3.05 to 3.37 per cent. “An 11 percent increase in energy yield can be crucial in making a material economically viable for a particular application,” emphasises Müller-Buschbaum. The researchers believe it will also be possible to increase the efficiency of other polymer solar cells by doping them with nanoparticles. “The combination of high-performance polymers with nanoparticles holds the promise of further increases in the efficiency of organic solar cells in the future. However, without a detailed examination, such as that using the X-rays emitted by a synchrotron, it would be impossible to gain a fundamental understanding of the underlying processes involved,” concludes Müller-Buschbaum. 2.3 Improve Solar PV Panel Efficiency and Output Power There are a number of means available to increase solar panel output and efficiency — some of which may be utilized by the serious experimenter. These are listed as follows: Solar Cell Technology There are a number of technologies being researched and there are continual advancements. Experimental technologies and highest efficiencies include: • Multi-cell gallium arsenide – 44% • Single cell gallium arsenide – 29% • Crystalline silicon - 25% • Thin film copper-indium-gallium-selenide – 20% • Emerging PV technologies (dye-sensitive cells etc.) – 11% (low efficiency, but very inexpensive) Unfortunately, we live in the real world and the highest efficiency technologies are either unaffordable or have not been put into production. As a result, the experimenter is generally stuck with crystalline silicon technology with efficiencies ranging from 15 to 21.5% — this is what I refer to as “practical efficiency.” Fill factor Fill factor is simply a fancy term for utilization of available surface area. Full utilization of fill area is required to obtain highest output for a given surface area. The fill factor ranges from about 70 to 90%. You have seen solar panels that utilize round or moon shaped PV cells — well, these have a lower fill factor than square cells. This is not really that important — all it means is that panels delivering a specific power may vary in dimensions somewhat. On the other hand, if attempting to maximize the amount of solar power out of a specific area, then fill factor is an issue. Grade A, B, C, D When purchasing name brand solar panels, you will be getting perfect grade A cells. If purchasing garage shop solar panels, the quality of its cells is unknown. If purchasing DIY solar cells, all grades are available, but beware — it is easy to get cheated on quality. If purchasing on eBay, check feedback ratings. • Grade A: No imperfections – output = 100% – (name brand panels) • Grade B: Cosmetic imperfections – output > 90% – (good for DIY panels) • Grade C: Contains chips and/or micro-cracks – output = 75 to 90% – (serious experimentation) • Grade D: Fallout – output = 25% to 75% – (just for messing around) Check out this link just for messing around—note extremely low fill factor: Note that micro-cracks effectively reduce the fill factor so that it takes a larger surface area (more cracked cells) to obtain the same power output. Micro-cracks can also cause localized heating and roof fires — beware! Solar Cells Glazing For long life, solar cells must be protected from the elements (rain, snow, hail, bird dropping etc). Polycarbonate or low-iron glass is generally recommended due to high optical transmissivity — perhaps 90%. Surface coating treatments reduce reflections for even higher transmissivity. Ordinary window glass reduces the output by about 40% — not recommended. Note that my knowledge is weak in this area. Solar Panel Orientation For highest output, solar panels must be perpendicular to the sun’s rays. However, it is generally practical and common for roof-top installations to follow the roof pitch and orientation. For other types of fixed installations, the azimuth is oriented to the south and tilt adjusted for the winter sun. Note that solar power is minimized in the winter mostly due to the reduced daylight period; therefore that is the default for fixed orientation. While this is clearly not optimum in the summer, the longer daytime period more than compensates for the compromised tilt angle. Solar tracker Solar tracking is a great way of increasing the output power. It rotates the panel or array of panels so that they always directly face the sun. However, the larger the array, the more difficult will be the mechanics of this task. Some trackers are simply driven by a “clock” motor like a telescope so that it follows the sun (or wherever it is supposed to be in the cloudy sky). Others have active circuitry that adjusts the orientation for maximum power output. Others may be controlled by a shadow feedback signal technique. The optimum tilt angle changes slowly as the earth rotates on its axis, therefore it is not generally required to track this change automatically. The easiest way to handle this seasonable variable, is to go out and manually adjust the angle every month or so — not a difficult task. Concentrators Solar panel output power may be increased via a light concentrator such as a Fresnel lens or mirror. Note that such a lens must be substantially larger than the panel. Also, concentrators may not be practical for a large array, and orientation of the mirror creates an additional tracking problem. Output may be increased by perhaps 50%. Care must be taken to prevent overheating the panel. Solar Charge Controls Since the solar panel does not put out the correct voltage to charge a battery, it must be controlled via a solar charge controller to prevent battery overcharge. The series voltage regulator control wastes the excess power either by turning off the solar panel current or by dissipating the excess power in heat — that is the function of the heatsink in such controls. electroschematics.com has a number of these controls. MPPT Controller MPPT stands for Maximum Power Point Tracking. The MPPT control is different in that it does not turn the excess power into heat — it turns it into additional charge current so that if the solar panel is putting out 10A, the battery may actually be charging at a higher current (perhaps 12A). The control senses both input voltage and current, and then does some math with its microcontroller and makes adjustments accordingly in order to maximize power transfer. It uses switch mode technology. On the other hand, when the battery is fully charged, it still turns off the solar panel. I was toying with the idea of what to do with this unused power, but have not come up with a really practical use other than perhaps to heat water in a hot water tank. 2.4 Blu-Ray Disc Can be Used to Improve Solar Cell Performance Data storage pattern transferred to solar cell increases light absorption An interdisciplinary team from the McCormick School of Engineering and Applied Science discovered that using the data storage pattern from a Blu-ray disc improves solar cell performance and that video content doesn’t matter. And they figured out why it works. Pictured (left to right) are study authors Dongning Guo, Cheng Sun, Chen Wang, Alexander Smith and Jiaxing Huang. Highlights • Content doesn’t matter: “Supercop” or “Family Guy” works equally well • Unexpected source for improving solar cell efficiency • Interdisciplinary team first to utilize information-rich disc for alternative purpose • Discovery suggests “green” second use for unwanted Blu-ray discs EVANSTON, Ill. --- Who knew Blu-ray discs were so useful? Already one of the best ways to store high-definition movies and television shows because of their high-density data storage, Blu-ray discs also improve the performance of solar cells — suggesting a second use for unwanted discs — according to new research from Northwestern University. An interdisciplinary research team has discovered that the pattern of information written on a Blu-ray disc -- and it doesn’t matter if it’s Jackie Chan’s “Police Story 3: Supercop” or the cartoon “Family Guy” -- works very well for improving light absorption across the solar spectrum. And better yet, the researchers know why. “We had a hunch that Blu-ray discs might work for improving solar cells, and, to our delight, we found the existing patterns are already very good,” said Jiaxing Huang, a materials chemist and an associate professor of materials science and engineering in the McCormick School of Engineering and Applied Science. “It’s as if electrical engineers and computer scientists developing the Blu-ray technology have been subconsciously doing our jobs, too.” Blu-ray discs contain a higher density of data than DVDs or CDs, and it is this quasi-random pattern, perfected by engineers over decades for data storage, that, when transferred to the surface of solar cells, provides the right texture to improve the cells’ light absorption and performance. Working with Cheng Sun, an associate professor of mechanical engineering at McCormick, Huang and his team tested a wide range of movies and television shows stored on Blu-ray discs, including action movies, dramas, documentaries, cartoons and black-and-white content, and found the video content did not matter. All worked equally well for enhancing light absorption in solar cells. The findings are published today (Nov. 25) in the journal Nature Communications. In the field of solar cells, it is known that if texture is placed on the surface of a solar cell, light is scattered more effectively, increasing a cell’s efficiency. Scientists have long been searching for the most effective texture with a reasonable manufacturing cost. The Northwestern researchers have demonstrated that a Blu-ray disc’s strings of binary code 0s and 1s, embedded as islands and pits to store video information, give solar cells the near-optimal surface texture to improve their absorption over the broad spectrum of sunlight. In their study, the researchers first selected the Jackie Chan movie “Police Story 3: Supercop.” They replicated the pattern on the active layer of a polymer solar cell and found the cell was more efficient than a control solar cell with a random pattern on its surface. “We found a random pattern or texture does work better than no pattern, but a Blu-ray disc pattern is best of all,” Huang said. “Then I wondered, why did it work? If you don’t understand why, it’s not good science.” Huang puzzled over the question of why for some time. One day, his wife, Shaorong Liu, a database engineer at IBM, suggested it likely had something to do with data compression. That was the insight Huang needed. Huang and Sun then turned to McCormick colleague Dongning Guo, an expert in information theory, to investigate this idea. Guo is an associate professor of electrical engineering and computer science. The researchers looked closely at the data processing algorithms in the Blu-ray standard and noted the algorithms serve two major purposes: • Achieving as high a degree of compression as possible by converting the video signals into a seemingly random sequence of 0s and 1s; and • Increasing error tolerance by adding controlled redundancy into the data sequence, which also limits the number of consecutive 0s and 1s. These two purposes, the researchers said, have resulted in a quasi-random array of islands and pits (0s and 1s) with feature sizes between 150 and 525 nanometers. And this range, it turns out, works quite well for light-trapping applications over the entire solar spectrum. The overall broadband absorption enhancement of a Blu-ray patterned solar cell was measured to be 21.8 percent, the researchers report. “In addition to improving polymer solar cells, our simulation suggests the Blu-ray patterns could be broadly applied for light trapping in other kinds of solar cells,” Sun said. “It has been quite unexpected and truly thrilling to see new science coming out of the intersection of information theory, nanophotonics and materials science,” Huang said. The National Science Foundation supported the research. The paper is titled “Repurposing Blu-ray Movie Discs as Quasi-random Nanoimprinting Templates for Photon Management.” In addition to Huang, Guo and Sun, other authors of the paper are Alexander J. Smith (co-first author) and Chen Wang (co-first author), both of Northwestern. 2.5 Improving the efficiency of solar panels Imperial College London / Nicholas Hylton "Rows of aluminium studs help solar panels extract more energy from sunlight than those with flat surfaces." This picture shows a solar panel with rows of aluminium studs and large electrical connections. The studs have been enlarged here but would normally be so small that they are invisible to the naked eye. Light scattering was promoted in the visible part of sunlight's spectrum At the heart of the blooming solar power industry is the semiconductor material, like silicon or gallium arsenide, which absorbs sunlight and forms the basis of solar panels. It converts electromagnetic energy in the form of sunlight to electrical energy. Now, researchers from London have demonstrated a technique to increase the amount of electrical current produced by a solar panel simply by augmenting its light-facing surface with aluminium nanostructures. When photons, particles of light, are absorbed by the semiconductor, they knock out electrons, which are passed through a circuit and then to a battery for storage as electricity. However, scientists now want to find ways of increasing the absorption of light in thin layers of semiconductors, so that solar panels can be made using less raw-material and at a lower cost. Recent research from the Imperial College, London (ICL), has demonstrated one way to increase the electrical current produced by devices in the lab by 22 per cent. By studding the light-receiving surface of gallium-arsenide (Ga-As) devices with aluminium nanocylinders, like the ridges on Lego blocks, the researchers were able to promote the scattering of light in the visible part of the spectrum, which dominates the energy in sunlight. The scattered light then travels a longer path inside the semiconductor, meaning that more photons can be absorbed and converted into current. It is important that the metal nanocylinders do not absorb the light themselves, as that would prevent it from reaching the panel. “The advantage of aluminium structures is that their absorption occurs in the ultraviolet part of the spectrum. That means that the absorption losses are limited to the ultraviolet and scattering from the aluminium particle dominates in both the visible and near infrared,” said Dr. Nicholas Hylton, a Research Associate at the Blackett Laboratory, ICL, in an email. Dr. Hylton was lead author of the research group’s paper, published in Scientific Reports on October 18. This isn’t the first time such nanostructures have been deployed to enhance the performance of solar panels. Earlier, silver and gold nanoparticles have been used because they improved the performance of the devices in the near-infrared part of the electromagnetic spectrum. “We were able to demonstrate that gold and silver scatter light in the near infrared part of the spectrum but absorb visible light strongly,” Dr. Hylton wrote. The significance of Dr.Hylton’s work lies in demonstrating aluminium’s better performance over silver and gold nanostructures. For one, aluminium is more abundant and less costly than silver and gold. For another, the 22 per cent spike that aluminium provides, as their paper notes, makes thinner-film solar panels technically feasible without “compromising power conversion efficiencies, thus reducing material consumption.” Higher efficiency devices could play a significant role in realising energy goals even in India, making them more cost-effective. Already, according to industry trackers, the price of solar power in India has come from Rs. 18/kWh in 2011 to Rs. 7/kWh in 2013, while the price of thermal power is pushing Rs. 4/kWh with subsidies. CHAPTER THREE 3.0 Materials Solar cells are typically named after the semiconducting material they are made of. These materials must have certain characteristics in order to absorb sunlight. Some cells are designed to handle sunlight that reaches the Earth's surface, while others are optimized for use in space. Solar cells can be made of only one single layer of light-absorbing material (single-junction) or use multiple physical configurations (multi-junctions) to take advantage of various absorption and charge separation mechanisms. Solar cells can be classified into first, second and third generation cells. The first generation cells—also called conventional, traditional or wafer-based cells—are made of crystalline silicon, the commercially predominant PV technology, that includes materials such as polysilicon and monocrystalline silicon. Second generation cells are thin film solar cells, that include amorphous silicon, CdTe and CIGS cells and are commercially significant in utility-scale photovoltaic power stations, building integrated photovoltaics or in small stand-alone power system. The third generation of solar cells includes a number of thin-film technologies often described as emerging photovoltaics—most of them have not yet been commercially applied and are still in the research or development phase. Many use organic materials, often organometallic compounds as well as inorganic substances. Despite the fact that their efficiencies had been low and the stability of the absorber material was often too short for commercial applications, there is a lot of research invested into these technologies as they promise to achieve the goal of producing low-cost, high-efficiency solar cells. Crystalline silicon By far, the most prevalent bulk material for solar cells is crystalline silicon (c-Si), also known as "solar grade silicon". Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, ribbon or wafer. These cells are entirely based around the concept of a p-n junction. Solar cells made of c-Si are made from wafers between 160 to 240 micrometers thick. Monocrystalline silicon Monocrystalline silicon (mono-Si) solar cells are more efficient and more expensive than most other types of cells. The corners of the cells look clipped, like an octagon, because the wafer material is cut from cylindrical ingots, that are typically grown by the Czochralski process. Solar panels using mono-Si cells display a distinctive pattern of small white diamonds. Polycrystalline silicon Polycrystalline silicon, or multicrystalline silicon (multi-Si) cells are made from cast square ingots—large blocks of molten silicon carefully cooled and solidified. They consist of small crystals giving the material its typical metal flake effect. Polysilicon cells are the most common type used in photovoltaics and are less expensive, yet less efficient than those made from monocrystalline silicon. Ribbon silicon Ribbon silicon is a type of polycrystalline silicon—it is formed by drawing flat thin films from molten silicon and results in a polycrystalline structure. These cells have lower efficiencies and costs than multi-Si due to a great reduction in silicon waste, as this approach does not require sawing from ingots.[33] Mono-like-multi silicon (MLM) This form was developed in the 2000s and introduced commercially around 2009. Also called cast-mono, this design uses polycrystalline casting chambers with small "seeds" of mono material. The result is a bulk mono-like material that is polycrystalline around the outsides. When sliced for processing, the inner sections are high-efficiency mono-like cells (but square instead of "clipped"), while the outer edges are sold as conventional poly. This production method results in mono-like cells at poly-like prices. Thin film Thin-film technologies reduce the amount of active material in a cell. Most designs sandwich active material between two panes of glass. Since silicon solar panels only use one pane of glass, thin film panels are approximately twice as heavy as crystalline silicon panels, although they have a smaller ecological impact (determined from life cycle analysis).[35] The majority of film panels have 2-3 percentage points lower conversion efficiencies than crystalline silicon.[36] Cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and amorphous silicon (a-Si) are three thin-film technologies often used for outdoor applications. As of December 2013, CdTe cost per installed watt was $0.59 as reported by First Solar. CIGS technology laboratory demonstrations reached 20.4% conversion efficiency as of December 2013. The lab efficiency of GaAs thin film technology topped 28%. The quantum efficiency of thin film solar cells is also lower due to reduced number of collected charge carriers per incident photon. Most recently, CZTS solar cell emerge as the less-toxic thin film solar cell technology, which achieved ~12% efficiency.[37] Thin film solar cells are increasing due to it being silent, renewable and solar energy being the most abundant energy source on Earth.[38] Cadmium telluride Cadmium telluride is the only thin film material so far to rival crystalline silicon in cost/watt. However cadmium is highly toxic and tellurium (anion: "telluride") supplies are limited. The cadmium present in the cells would be toxic if released. However, release is impossible during normal operation of the cells and is unlikely during fires in residential roofs.[39] A square meter of CdTe contains approximately the same amount of Cd as a single C cell nickel-cadmium battery, in a more stable and less soluble form.[39] Copper indium gallium selenide Copper indium gallium selenide (CIGS) is a direct band gap material. It has the highest efficiency (~20%) among all commercially significant thin film materials (see CIGS solar cell). Traditional methods of fabrication involve vacuum processes including co-evaporation and sputtering. Recent developments at IBM and Nanosolar attempt to lower the cost by using non-vacuum solution processes.[40] Silicon thin film Silicon thin-film cells are mainly deposited by chemical vapor deposition (typically plasma-enhanced, PE-CVD) from silane gas and hydrogen gas. Depending on the deposition parameters, this can yield amorphous silicon (a-Si or a-Si:H), protocrystalline silicon or nanocrystalline silicon (nc-Si or nc-Si:H), also called microcrystalline silicon.[41] Amorphous silicon is the most well-developed thin film technology to-date. An amorphous silicon (a-Si) solar cell is made of non-crystalline or microcrystalline silicon. Amorphous silicon has a higher bandgap (1.7 eV) than crystalline silicon (c-Si) (1.1 eV), which means it absorbs the visible part of the solar spectrum more strongly than the higher power density infrared portion of the spectrum. The production of a-Si thin film solar cells uses glass as a substrate and deposits a very thin layer of silicon by plasma-enhanced chemical vapor deposition (PECVD). Protocrystalline silicon with a low volume fraction of nanocrystalline silicon is optimal for high open circuit voltage.[42] Nc-Si has about the same bandgap as c-Si and nc-Si and a-Si can advantageously be combined in thin layers, creating a layered cell called a tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nc-Si. Gallium arsenide thin film The semiconductor material Gallium arsenide (GaAs) is also used for single-crystalline thin film solar cells. Although GaAs cells are very expensive, they hold the world's record in efficiency for a single-junction solar cell at 28.8%.[43] GaAs is more commonly used in multijunction photovoltaic cells for concentrated photovoltaics (CPV, HCPV) and for solar panels on spacecrafts, as the industry favours efficiency over cost for space-based solar power. Multijunction cells Multi-junction cells consist of multiple thin films, each essentially a solar cell grown on top of each other, typically using metalorganic vapour phase epitaxy. Each layers has a different band gap energy to allow it to absorb electromagnetic radiation over a different portion of the spectrum. Multi-junction cells were originally developed for special applications such as satellites and space exploration, but are now used increasingly in terrestrial concentrator photovoltaics (CPV), an emerging technology that uses lenses and curved mirrors to concentrate sunlight onto small but highly efficient multi-junction solar cells. By concentrating sunlight up to a thousand times, High concentrated photovoltaics (HCPV) has the potential to outcompete conventional solar PV in the future. Tandem solar cells based on monolithic, series connected, gallium indium phosphide (GaInP), gallium arsenide (GaAs), and germanium (Ge) p–n junctions, are increasing sales, despite cost pressures.[45] Between December 2006 and December 2007, the cost of 4N gallium metal rose from about $350 per kg to $680 per kg. Additionally, germanium metal prices have risen substantially to $1000–1200 per kg this year. Those materials include gallium (4N, 6N and 7N Ga), arsenic (4N, 6N and 7N) and germanium, pyrolitic boron nitride (pBN) crucibles for growing crystals, and boron oxide, these products are critical to the entire substrate manufacturing industry.[citation needed] A triple-junction cell, for example, may consist of the semiconductors: GaAs, Ge, and GaInP 2.[46] Triple-junction GaAs solar cells were used as the power source of the Dutch four-time World Solar Challenge winners Nuna in 2003, 2005 and 2007 and by the Dutch solar cars Solutra (2005), Twente One (2007) and 21Revolution (2009).[citation needed] GaAs based multi-junction devices are the most efficient solar cells to date. On 15 October 2012, triple junction metamorphic cells reached a record high of 44%.[47] 3.2 Research in solar cells Perovskite solar cells Perovskite solar cells are solar cells that include a perovskite-structured material as the active layer. Most commonly, this is a solution-processed hybrid organic-inorganic tin or lead halide based material. Efficiencies have increased from below 10% at their first usage in 2009 to over 20% in 2014, making them a very rapidly advancing technology and a hot topic in the solar cell field.[48] Perovskite solar cells are also forecast to be extremely cheap to scale up, making them a very attractive option for commercialisation. 3.3 Upconversion and Downconversion Photon upconversion is the process of using two low-energy (e.g., infrared) photons to produce one higher energy photon; downconversion is the process of using one high energy photon (e.g.,, ultraviolet) to produce two lower energy photons. Either of these techniques could be used to produce higher efficiency solar cells by allowing solar photons to be more efficiently used. The difficulty, however, is that the conversion efficiency of existing phosphors exhibiting up- or down-conversion is low, and is typically narrow band. One upconversion technique is to incorporate lanthanide-doped materials (Er3+, Yb3+, Ho3+ or a combination), taking advantage of their luminescence to convert infrared radiation to visible light. Upconversion process occurs when two infrared photons are absorbed by rare-earth ions to generate a (high-energy) absorbable photon. As example, the energy transfer upconversion process (ETU), consists in successive transfer processes between excited ions in the near infrared. The upconverter material could be placed below the solar cell to absorb the infrared light that passes through the silicon. Useful ions are most commonly found in the trivalent state. Er+ions have been the most used. Er3+ions absorb solar radiation around 1.54 µm. Two Er3+ions that have absorbed this radiation can interact with each other through an upconversion process. The excited ion emits light above the Si bandgap that is absorbed by the solar cell and creates an additional electron–hole pair that can generate current. However, the increased efficiency was small. In addition, fluoroindate glasses have low phonon energy and have been proposed as suitable matrix doped with Ho3+ ions. 3.4 Light-absorbing dyes Dye-sensitized solar cells (DSSCs) are made of low-cost materials and do not need elaborate manufacturing equipment, so they can be made in a DIY fashion. In bulk it should be significantly less expensive than older solid-state cell designs. DSSC's can be engineered into flexible sheets and although its conversion efficiency is less than the best thin film cells, its price/performance ratio may be high enough to allow them to compete with fossil fuel electrical generation. Typically a ruthenium metalorganic dye (Ru-centered) is used as a monolayer of light-absorbing material. The dye-sensitized solar cell depends on a mesoporous layer of nanoparticulate titanium dioxide to greatly amplify the surface area (200–300 m2/g TiO 2, as compared to approximately 10 m2/g of flat single crystal). The photogenerated electrons from the light absorbing dye are passed on to the n-type TiO2 and the holes are absorbed by an electrolyte on the other side of the dye. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid. This type of cell allows more flexible use of materials and is typically manufactured by screen printing or ultrasonic nozzles, with the potential for lower processing costs than those used for bulk solar cells. However, the dyes in these cells also suffer from degradation under heat and UV light and the cell casing is difficult to seal due to the solvents used in assembly. The first commercial shipment of DSSC solar modules occurred in July 2009 from G24i Innovations. 3.4 Quantum dots Quantum dot solar cells (QDSCs) are based on the Gratzel cell, or dye-sensitized solar cell architecture, but employ low band gap semiconductor nanoparticles, fabricated with crystallite sizes small enough to form quantum dots (such as CdS, CdSe, Sb 2S 3, PbS, etc.), instead of organic or organometallic dyes as light absorbers. QD's size quantization allows for the band gap to be tuned by simply changing particle size. They also have high extinction coefficients and have shown the possibility of multiple exciton generation. In a QDSC, a mesoporous layer of titanium dioxide nanoparticles forms the backbone of the cell, much like in a DSSC. This TiO 2 layer can then be made photoactive by coating with semiconductor quantum dots using chemical bath deposition, electrophoretic deposition or successive ionic layer adsorption and reaction. The electrical circuit is then completed through the use of a liquid or solid redox couple. The efficiency of QDSCs has increased[53] to over 5% shown for both liquid-junction[54] and solid state cells.[55] In an effort to decrease production costs, the Prashant Kamat research group[56] demonstrated a solar paint made with TiO2 and CdSe that can be applied using a one-step method to any conductive surface with efficiencies over 1%. 3.5 Organic/polymer solar cells Organic solar cells and polymer solar cells are built from thin films (typically 100 nm) of organic semiconductors including polymers, such as polyphenylene vinylene and small-molecule compounds like copper phthalocyanine (a blue or green organic pigment) and carbon fullerenes and fullerene derivatives such as PCBM. They can be processed from liquid solution, offering the possibility of a simple roll-to-roll printing process, potentially leading to inexpensive, large scale production. In addition, these cells could be beneficial for some applications where mechanical flexibility and disposability are important. Current cell efficiencies are, however, very low, and practical devices are essentially non-existent. Energy conversion efficiencies achieved to date using conductive polymers are very low compared to inorganic materials. However, Konarka Power Plastic reached efficiency of 8.3% and organic tandem cells in 2012 reached 11.1%. The active region of an organic device consists of two materials, one electron donor and one electron acceptor. When a photon is converted into an electron hole pair, typically in the donor material, the charges tend to remain bound in the form of an exciton, separating when the exciton diffuses to the donor-acceptor interface, unlike most other solar cell types. The short exciton diffusion lengths of most polymer systems tend to limit the efficiency of such devices. Nanostructured interfaces, sometimes in the form of bulk heterojunctions, can improve performance.[59] In 2011, MIT and Michigan State researchers developed solar cells with a power efficiency close to 2% with a transparency to the human eye greater than 65%, achieved by selectively absorbing the ultraviolet and near-infrared parts of the spectrum with small-molecule compounds.[60][61] Researchers at UCLA more recently developed an analogous polymer solar cell, following the same approach, that is 70% transparent and has a 4% power conversion efficiency.[62][63][64] These lightweight, flexible cells can be produced in bulk at a low cost and could be used to create power generating windows. In 2013, researchers announced polymer cells with some 3% efficiency. They used block copolymers, self-assembling organic materials that arrange themselves into distinct layers. The research focused on P3HT-b-PFTBT that separates into bands some 16 nanometers wide. Adaptive cells Adaptive cells change their absorption/reflection characteristics depending to respond to environmental conditions. An adaptive material responds to the intensity and angle of incident light. At the part of the cell where the light is most intense, the cell surface changes from reflective to adaptive, allowing the light to penetrate the cell. The other parts of the cell remain reflective increasing the retention of the absorbed light within the cell.[67] In 2014 a system that combined an adaptive surface with a glass substrate that redirect the absorbed to a light absorber on the edges of the sheet. The system also included an array of fixed lenses/mirrors to concentrate light onto the adaptive surface. As the day continues, the concentrated light moves along the surface of the cell. That surface switches from reflective to adaptive when the light is most concentrated and back to reflective after the light moves along. Manufacture Early solar-powered calculator This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (June 2014) Solar cells share some of the same processing and manufacturing techniques as other semiconductor devices. However, the stringent requirements for cleanliness and quality control of semiconductor fabrication are more relaxed for solar cells, lowering costs. Polycrystalline silicon wafers are made by wire-sawing block-cast silicon ingots into 180 to 350 micrometer wafers. The wafers are usually lightly p-type-doped. A surface diffusion of n-type dopants is performed on the front side of the wafer. This forms a p–n junction a few hundred nanometers below the surface. Anti-reflection coatings are then typically applied to increase the amount of light coupled into the solar cell. Silicon nitride has gradually replaced titanium dioxide as the preferred material, because of its excellent surface passivation qualities. It prevents carrier recombination at the cell surface. A layer several hundred nanometers thick is applied using PECVD. Some solar cells have textured front surfaces that, like anti-reflection coatings, increase the amount of light reaching the wafer. Such surfaces were first applied to single-crystal silicon, followed by multicrystalline silicon somewhat later. A full area metal contact is made on the back surface, and a grid-like metal contact made up of fine "fingers" and larger "bus bars" are screen-printed onto the front surface using a silver paste. This is an evolution of the so-called "wet" process for applying electrodes, first described in a US patent filed in 1981 by Bayer AG.[68] The rear contact is formed by screen-printing a metal paste, typically aluminium. Usually this contact covers the entire rear, though some designs employ a grid pattern. The paste is then fired at several hundred degrees Celsius to form metal electrodes in ohmic contact with the silicon. Some companies use an additional electro-plating step to increase efficiency. After the metal contacts are made, the solar cells are interconnected by flat wires or metal ribbons, and assembled into modules or "solar panels". Solar panels have a sheet of tempered glass on the front, and a polymer encapsulation on the back. CHAPTER FOUR Conclusion and recommendation Solar cells currently used contain p-n junctions. Theoretically, with no reflection of light, perfect absorption and no losses inside the material or at the contacts, the efficiency of a solar cell containing a single junction could reach 33%. The actual maximum efficiencies are around 25% with use of the best materials, which are very expensive. Various alternatives to the expensive devices are being reasearched. The main goal for the future is to further reduce the cost of PV modules, so the PV can be more widely used. Apart from reducing amount of material used, the price is also reduced indirectly by increasing the efficiency of PV devices. The efficiency is most effectively increased by using tandem cells in combination with light concentration systems, where mirrors are used to focus sunlight on to the surface of PV modules. Efficiencies above 30% are possible. The limiting factor to the efficiency is heating of the system exposed to light irradiation with power of several hundreds of suns. In the past 10 years, the price of all known types of PV modules has fallen significantly. For example, the price of electricity produced by c-Si modules has fallen from _0.40 e/kWh to _0.30 e/kWh and for 14 a-Si modules the price has fallen from _0.20 e/kWh to _0.15 e/kWh. The price of electricity produced by CIGS modules was _0.17 e/kWh in 2003 and has fallen to _0.09 e/kWh until 2010. All of the mentioned modules have a lifetime of at least 20 years. During this time the e_ciency falls for about 20%. There are many installation possibilities for PV modules, among the best known being installation on the roofs of the houses. In urban areas, the modules may be connected to the grid system, while at remote locations, they are independent systems and have to be combined with a good charge storage system. There are some ideas for the future, considering installation of PV, for example in large plain deserts, which are extremely sunny locations. The electric energy produced in deserts would have to be transported to other parts of the world, which could be done by using power lines, or transport of liquid hydrogen. Water is split into hydrogen and oxygen by electrolysis and hydrogen can then serve as the energy carrier. References [1] http://en.wikipedia.org/wiki/Electricity generation (12. 5. 2012). 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