REPUBLIC OF TURKEY ADNAN MENDERES UNIVERSITY FACULTY OF ENGINEERING MECHANICAL ENGINEERING DEPARTMENT 2017 ME405 Special Topics in Mechanical Engineering SOLAR CELL MATERIALS Volkan URAL Supervisor: Assoc. Prof. Dr. Pınar DEMİRCİOĞLU AYDIN iii REPUBLIC OF TURKEY ADNAN MENDERES UNIVERSITY FACULTY OF ENGINEERING MECHANICAL ENGINEERING AYDIN The thesis with the title of “Solar Cell Materials”prepared by the 121802017, Undergraduate Student at the Mechanical Engineering Department at the Faculty of Engineering was accepted by the jury members whose names and titles presented below as a result of thesis defense on 08.06.2017 Title, Name Surname President : ........................................ Member : ........................................ Member : ........................................ Signature This Undergraduate Thesis accepted by the jury members is endorsed by the decision of the Faculty Board Members with <………> Serial Number and <………………..> date. Prof. Dr. Name SURNAME Head Of Department v REPUBLIC OF TURKEY ADNAN MENDERES UNIVERSITY FACULTY OF ENGINEERING MECHANICAL ENGINEERING AYDIN I hereby declare that all information and results reported in this thesis have been obtained by my part as a result of truthful experiments and observations carried out by the scientific methods, and that I referenced appropriately and completely all data, thought, result information which do not belong my part within this study by virtue of scientific ethical codes. 08/06/2017 Signature Volkan Ural vii ÖZET GÜNEŞ PİLİ MALZEMELERİ Volkan URAL ME405 Makine Mühendisliğinde Özel Konular, Makine Mühendisliği Tez Danışmanı: Doç. Dr. Pınar Demircioğlu 2017, 31 sayfa Güneş ışığı, 21. yüzyılda küresel olarak çevre ve enerji çözümlerine katkıda bulunabilecek yenilenebilir enerji kaynaklarından biridir. Güneş ışığını olabildiğince etkili bir şekilde kullanmak için, pratik kullanım için düşük maliyetli ve etkin güneş pilleri yoğun bir şekilde geliştirilmiştir. Genel olarak bilindiği gibi, pratik silikon esaslı güneş pilleri, yüksek üretim maliyetinin yanı sıra diğer inorganik bazlı güneş pillerini de içerir. Güneş pili prensip olarak ışığı elektrik enerjisine dönüştüren basit bir yarı iletken cihazdır. Dönüşüm, ışığı ve iyonlaştırıcı kristal atomlarını absorbe ederek, böylece serbest, negatif yüklü elektronlar ve pozitif yüklü iyonlar oluşturarak gerçekleştirilir. Eğer bu iyonlar temel kristal atomlardan yaratıldıysa, iyonlaşmış halleri başka bir komşuya ve benzerlerine değiştirilebilen bir komşuya kolaylıkla değiştirilebilir; Yani, bu iyonlaşmış durum taşınabilir; Bir elektron gibi davranıyor ve buna delik deniyor. Karşı elektriği olması dışında serbest elektrona benzer özelliklere sahiptir. Anahtar Kelimeler: Güneş ışığı, güneş pili ix ABSTRACT SOLAR CELL MATERIALS ME405 Special Topics in Mechanical Engineering, Mechanical Engineering Supervisor: Assoc. Prof. Dr. Pınar DEMİRCİOĞLU 2017, 31 pages Sunlight is one of the renewable energy sources that can globally contribute to environmental and energy solutions in the 21st century. In order to use sunlight as efficiently as possible, low cost and efficient solar cells have been vigorously developed for practical use. As is generally known, practical silicon-based solar cells involve high manufacturing cost, as well as any other inorganic-based solar cells. A solar cell is, in principle, a simple semiconductor device that converts light into electric energy. The conversion is accomplished by absorbing light and ionizing crystal atoms, thereby creating free, negatively charged electrons and positively charged ions. If these ions are created from the basic crystal atoms, then their ionized state can be exchanged readily to a neighbor from which it can be exchanged to another neighbor and so forth; that is, this ionized state is mobile; it behaves like an electron, and it is called a hole. It has properties similar to a free electron except that it has the opposite charge. Keywords: Sunlight, solar cell xi ACKNOWLEDGEMENTS I sincerely thank my supervisor, Assoc. Prof. Dr. Pınar DEMİRCİOĞLU for giving me the opportunity to this research work, and because, they have offered me an endless number of facilities since the beginning. I would like also thank all people in Engineering Faculty of Adnan Menderes University for their support. Thank you Volkan URAL xiii TABLE OF CONTENTS ÖZET..................................................................................................................... vii ABSTRACT ............................................................................................................ix ACKNOWLEDGEMENTS ....................................................................................xi 1 . INTRODUCTION ............................................................................................... 1 1.1 . Solar Cell Technology.................................................................................. 1 2 . Solar cell materıals .............................................................................................. 3 2.1 Solar cell........................................................................................................ 3 2.2 Applications ................................................................................................... 4 2.2.1 Cells, modules, panels and systems......................................................... 4 2.3 How solar cell works ...................................................................................... 5 2.4 Materials......................................................................................................... 6 2.4.1 Crystalline silicon.................................................................................... 7 2.4.2 Monocrystalline silicon ........................................................................... 7 2.4.3 Polycrystalline silicon ............................................................................. 7 2.4.4 Ribbon silicon ......................................................................................... 8 2.4.5 Mono-like-multi silicon (MLM) ............................................................. 8 2.4.6 Thin ......................................................................................................... 8 2.4.7 Cadmium telluride ................................................................................... 8 2.4.8 Copper indium gallium selenide ............................................................. 9 2.4.9 Silicon thin film....................................................................................... 9 2.4.10 Gallium arsenide thin film .................................................................... 9 2.5 Multijunction cells ....................................................................................... 10 2.6 Cost of feedstock for solar cells .................................................................. 11 3. CONCLUSIONS ............................................................................................... 12 REFERENCES ...................................................................................................... 13 xv TABLE OF FIGURES Figure 1 Photovoltaic efficiencies from various materials over the years [1]. ........ 1 Figure 2. . The resulting circuit of solar cell [2] ...................................................... 2 Figure 3. . The resulting circuit of solar cell [2] ...................................................... 2 Figure 4. A conventional crystalline silicon solar cell [3] ....................................... 4 Figure 5. Possible components of a photovoltaic system [4] ................................. 5 Figure 6. Working mecanism of solar cell .............................................................. 6 Figure 7. Multijunction cells [6] ............................................................................ 10 xvii 1 1. INTRODUCTION In the current energy crisis, the search for aviable alternative to hydrocarbons has taken many paths: nuclear, wind, solar, etc. Solar cells provide an attractive form of limitless alternative energy. The placement of solar cells can be unobtrusive and provide not only a source of thermal energy, but electricity. However, the development and implementation of effective photovoltaic cells is hindered by two primary components: cost and efficiency. Research into cheaper and more efficient solar cells has been underway for several decades. From the development of thin-film solar cells with efficiencies greater than 10% in the 1970s to the most recent developments in new photovoltaic materials achieving greater than 24% efficiency [1]. Unfortunately, the cost of electricity from current solar cells is about one order of magnitude higher than commercial prices. Figure 1 Photovoltaic efficiencies from various materials over the years [1]. 1.1. Solar Cell Technology Nowadays, a lot of energy research companies or institutions study on this topic and new technologies in this area developed very fast. The only problem to prevent this solar cell from largely implemented into daily use is the cost. Researchers use various methods to make cheper solar cell. Such as thin film solar cell, organic/ polymer solar cell, but the energy conversion efficiency will decreased as te cost decreasing. 2 Basically, the principles of all kinds of solar cells are the same, it is photovoltaic effect. To explain the photovoltaic solar panel more simply, photons from sunlight knock electrons into higher state of energy level, create electricity. Photons heat the solar cell, and maket he elcetrons move in the connected line and generate electricity. The conventional silicon can also uses, for example; use p-type silicon and n- type silicon make a p-n junction ( actually it is a doping process dope n- type into the p-type wafer). When the photons hit on the n-tyepe silicon and maket he electrons into activated state, these electrons break the bonding of atoms and travel to the p-type side, meanwhile, holes left on the n-type side. There are two modes of charge carrier seperation. One is drift, which is driven by an electrostatic field. And another is diffusion, charge carrier diffuse from high concentration to low concentration.Then the current will be formed if using the circuit to connect two side of panel. The resulting circuit of solar cell is show below. Figure 2. . The resulting circuit of solar cell [2] Figure 3. . The resulting circuit of solar cell [2] All photovoltaic devices are same type of phptpdiode. Depletion region; the depletion region, also called depletion layer, depletion zone, junction region or the space charge region, is an insulating region within a conductive, doped semiconductor material where the charge carries have diffused away, or have been forced away by an electric static field. The uncompensated ions are positive on the N side and negative on the P side. This creates ana electric field that provides a force opposing the continued of charge carriers. When the electric field is sufficient to arrest further transfer of holes and electrons, the depletion region has reached its equilibrium dimensions. Integrating the electric field across teh depletion region determines what is called the built-in voltage (also called the junction voltage or barrier voltage or contact potential). Much of the solar radiation reaching the earth is composed of photons with energies greater than the band gap of the silicon. So electrons in the silicon just absorb enough activate energy to break the atomic bonding and form current. The 3 difference of energy emitted and absorbed is converted into heat via lattice vibration. Solar cell is usually connected in series to create an addictive voltage. Connecting in parallel will yield a high current. For example, the average power is equal to 20 % of peak power, so that each peak kilowatt of solar array output power corresponds to energy production of 4.8 kWh per day. 2. SOLAR CELL MATERIALS 2.1 Solar cell 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.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, detecting light or other electromagnetic radiation near the visible range, or measuring light intensity. The operation of a photovoltaic (PV) cell requires three 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, or to a device that splits water directly into hydrogen and oxygen using only solar illumination. [3] 4 Figure 4. A conventional crystalline silicon solar cell [3] 2.2 Applications Assemblies of solar cells are used to make solar modules that generate electrical power from sunlight, as distinguished from a "solar thermal module" or "solar hot water panel". A solar array generates solar power using solar energy. 2.2.1 Cells, modules, panels and systems Multiple solar cells in an integrated group, all oriented in one plane, constitute a solar photovoltaic panel or solar photovoltaic module. Photovoltaic modules often have a sheet of glass on the sun-facing side, allowing light to pass while protecting the semiconductor wafers. Solar cells are usually connected in series and parallel circuits or series in modules, creating an additive voltage. Connecting cells in parallel yields a higher current; however, problems such as shadow effects can shut down the weaker (less illuminated) parallel string (a number of series connected cells) causing substantial power loss and possible damage because of the reverse bias applied to the shadowed cells by their illuminated partners. Strings of series cells are usually handled independently and not connected in parallel, though as of 2014, individual power boxes are often supplied for each module, and are connected in parallel. Although modules can be interconnected to create an array with the desired peak DC voltage and loading current capacity, using independent MPPTs (maximum power point trackers) is preferable. Otherwise, shunt diodes can reduce shadowing power loss in arrays with series/parallel connected cells. [4] 5 Figure 5. Possible components of a photovoltaic system [4] 2.3 How solar cell works The solar cell works in several steps: Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon. Electrons are excited from their current molecular/atomic orbital. Once excited an electron can either dissipate the energy as heat and return to its orbital or travel through the cell until it reaches an electrode. Current flows through the material to cancel the potential and this electricity is captured. The chemical bonds of the material are vital for this process to work, and usually silicon is used in two layers, one layer being doped with boron, the other phosphorus. These layers have different chemical electric charges and subsequently both drive and direct the current of electrons. An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity. An inverter can convert the power to alternating current (AC). The most commonly known solar cell is configured as a large-area p–n junction made from silicon. The illuminated side of a solar cell must have a transparent conducting film to collect the current produced. Films such as indium tin oxide, conducting polymers or conducting nanowire networks are used. [5] 6 Figure 6. Working mecanism of solar cell 2.4 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 7 invested into these technologies as they promise to achieve the goal of producing low-cost, high-efficiency solar cells. 2.4.1 Crystalline silicon Crystalline silicon PV cells are the most common solar cells used in commercially available solar panels, representing more than 85% of world PV cell market sales in 2011. Crystalline silicon PV cells have laboratory energy conversion efficiencies over 25% for single-crystal cells and over 20% for multicrystalline cells. However, industrially produced solar modules currently achieve efficiencies ranging from 18%–22% under standard test conditions. 2.4.2 Monocrystalline silicon Monocrystalline silicon is the base material for silicon chips used in virtually all electronic equipment today. Mono-Si also serves as photovoltaic, light-absorbing material in the manufacture of solar cells. It consists of silicon in which the crystal lattice of the entire solid is continuous, unbroken to its edges, and free of any grain boundaries. Mono-Si can be prepared intrinsic, consisting only of exceedingly pure silicon, or doped, containing very small quantities of other elements added to change its semiconducting properties. Most silicon monocrystals are grown by the Czochralski process into ingots of up to 2 meters in length and weighing several hundred kilograms. These cylinders are then sliced into thin wafers of a few hundred microns for further processing. 2.4.3 Polycrystalline silicon Polycrystalline silicon, or multicrystalline silicon 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, but also less efficient, than those made from monocrystalline silicon. 8 2.4.4 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 are cheaper to make than multi-Si, due to a great reduction in silicon waste, as this approach does not require sawing from ingots.However, they are also less efficient. 2.4.5 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, while the outer edges are sold as conventional poly. This production method results in mono-like cells at poly-like prices. 2.4.6 Thin 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. 2.4.7 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 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.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. 9 2.4.8 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. Traditional methods of fabrication involve vacuum processes including coevaporation and sputtering. Recent developments at IBM and Nanosolar attempt to lower the cost by using non-vacuum solution processes. 2.4.9 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. 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.[56] Nc-Si has about the same bandgap as cSi 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. 2.4.10 Gallium arsenide thin film The semiconductor material Gallium arsenide (GaAs) is also used for singlecrystalline 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%.GaAs is more commonly used in multijunction photovoltaic cells for concentrated 10 photovoltaics (CPV, HCPV) and for solar panels on spacecrafts, as the industry favours efficiency over cost for space-based solar power. 2.5 Multijunction cells Multi-junction cells consist of multiple thin films, each essentially a solar cell grown on top of another, 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, 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.[59] 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 and germanium, pyrolitic boron nitride (pBN) crucibles for growing crystals, and boron oxide, these products are critical to the entire substrate manufacturing industry. Figure 7. Multijunction cells [6] 11 2.6 Cost of feedstock for solar cells As noted in our reporeedstock costs can vary from $0.30/Wp to $0.40/Wp. The lower end of this range is more representative of the price obtained by high volume manufacturers to achieve reliable supplies with fixed cost contracts. New players or those that are not dealing in high volumes are subject to polysilicon "spot price" variations that can be considerably higher. Long-term contract pricing is believed to be around $70/kg. The use of fixed-price contracts has its upsides and downsides. Average contract pricing is estimated to be below $70/kg from most large volume suppliers, which provides some protection against price increases. However, should the market price drop below this, cell costs would remain artificially higher for large volume fabs with long term polysilicon contracts, while smaller cell fabricators would gain an advantage from lower material costs. On the other hand, if the open market price rises, as a result of tight supply, this could pose some significant cost increases in solar cells over time. As a result of high demand for wafers in 2006 and 2007, there has been a move to invest in building more polysilicon production plants and expansions. Current evaluation of the polysilicon availability shows it is in an oversupply situation for the next few years. However, Techcet's research indicates that the polysilicon and wafer availability to solar and semiconductor manufacturers is tight. This is most likely the result of a steep increase in semiconductor device manufacturing as well as healthy demand from the solar cell industry. 12 3. CONCLUSIONS The sun delivers more energy to the Earth in one hour than is used in a year from all currently available sources, however only 0.1% of the world’s energy is derived from it [7]. There are several options available in the production of solar cells. However, it seems unlikely that photovoltaic technology carry the bulk of the world’s energyneeds in the near future. While thin-films show a great deal of promise, the 24% efficiencies of silicon solar cells has yet to be bested [7]. Unfortunately, silicon solar cell production remain too costly at the commercial level. Nanocrystalline hybrid arrays show a promising future for cheaper and more efficient solar cells, despite the infancy of the field. The DOE remains optimistic regarding the future of nano-scale materials in photovoltaics. While organic polymers offer a similar (or better) option in terms of cost, nanocrystalline quantumdots provide a tunable absorption spectrum and they promise harnessing of multiple exciton generation [8]. Fabrication at the nano-scale provides a remarkable increase in the precision and level of control that can be obtained over solar cell development. This ability to experiment with, understand, and thus simulate the behaviour of photovoltaics at such a fundamental level bring the field into a new and lucrative realm [8] 13 REFERENCES [1] T. Surek. Progress in u.s. photovoltaics: Looking back 30 years and looking ahead 20. National Renewable Energy Laboratory. [2] http://www.nus.edu.sg/nurop/2010/Proceedings/FoE/Solar%20Cell%20Tec hnology [3] Solar cells, Wikipedia https://en.wikipedia.org/wiki/Solar_cell [4] Solar cells, Wikipediahttps://en.wikipedia.org/wiki/Solar_cell [5] http://sces.phys.utk.edu/~dagotto/condensed/HW1_2008/BertolliSolarCell Materials [6] Solar cells, Wikipedia https://en.wikipedia.org/wiki/Solar_cell [7] T. Surek. Progress in u.s. photovoltaics: Looking back 30 years and looking ahead 20. National Renewable Energy Laboratory. [8] DOE Office of Science. Basic Research Needs for Solar Energy Utilization, April 2005, available: http://www.sc.doe.gov/bes/reports/list.html.