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History & Development of Solar cells
- Intrinsic Silicon Crystalline Structure
- Extrinsic Silicon
- Formation of Potential Barrier & Photoelectric effect
What is a solar cell? |
A solar cell (photovoltaic cell or photoelectric cell) is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect. The energy of light is transmitted by photons-small packets or quantums of light. Electrical energy is stored in electromagnetic fields, which in turn can make a current of electrons flow.
Assemblies of solar cells are used to make solar modules which are used to capture energy from sunlight. When multiple modules are assembled together (such as prior to installation on a pole-mounted tracker system), the resulting integrated group of modules all oriented in one plane is referred as a solar panel. The electrical energy generated from solar modules, is an example of solar energy. Photovoltaics is the field of technology and research related to the practical application of photovoltaic cells in producing electricity from light, though it is often used specifically to refer to the generation of electricity from sunlight. Cells are described as photovoltaic cells when the light source is not necessarily sunlight. These are used for detecting light or other electromagnetic radiation near the visible range, for example infrared detectors, or measurement of light intensity.
History and Development of Solar Cell Technology |
The development of solar cell technology began with the 1839 research of French physicist Antoine-César Becquerel. Becquerel observed the photovoltaic effect while experimenting with a solid electrode in an electrolyte solution when he saw a voltage develop when light fell upon the electrode. The major events are discussed briefly below, and other milestones can be accessed by clicking on the image shown below.
- Charles Fritts - First Solar Cell: The first genuine solar cell was built around 1883 by Charles Fritts, who used junctions formed by coating selenium (a semiconductor) with an extremely thin layer of gold. The device was only about 1 percent efficient.
- Albert Einstein - Photoelectric Effect: Albert Einstein explained the photoelectric effect in 1905 for which he received the Nobel Prize in Physics in 1921.
- Russell Ohl - Silicon Solar Cell: Early solar cells, however, had energy conversion efficiencies of under one percent. In 1941, the silicon solar cell was invented by Russell Ohl.
- Gerald Pearson, Calvin Fuller and Daryl Chapin - Efficient Solar Cells: In 1954, three American researchers, Gerald Pearson, Calvin Fuller and Daryl Chapin, designed a silicon solar cell capable of a six percent energy conversion efficiency with direct sunlight. They created the first solar panels. Bell Laboratories in New York announced the prototype manufacture of a new solar battery. Bell had funded the research. The first public service trial of the Bell Solar Battery began with a telephone carrier system (Americus, Georgia) on October 4 1955.
Generations of Solar Cells
First Generation: Crystalline Silicon Solar Cell Technology
First generation solar cells are the larger, silicon-based photovoltaic cells. Silicon's ability to remain a semiconductor at higher temperatures has made it a highly attractive raw material for solar panels. Silicon's abundance, however, does not ease the challenges of harvesting and processing it into a usable material for microchips and silicon panels. Solar cells, use silicon wafers consisting of Silicon or Germanium that are doped with Phosphorus and Boron in a pn-junction. Silicon cells have a quite high efficiency, but very pure silicon is needed, and due to the energy-requiring process, the price is high compared to the power output. Crystalline Silicon Solar Cells dominate 80-90% of solar cell market due to their high efficiency, despite their high manufacturing costs.
Second Generation: Thin Film Solar Cell Technology
Second generation solar cell, also known as thin-film solar cell (TFSC) or thin-film photovoltaic cell (TFPV), is made by depositing one or more thin layers (thin films) of photovoltaic material on a substrate. They are significantly cheaper to produce than first generation cells but have lower efficiencies. The great advantage of thin-film solar cells, along with low cost, is their flexibility and versatility to be used in varied environments. This has led to aesthetically pleasing solar innovations such as solar shingles, solar glass and solar panels that can be rolled out onto a roof or other surface. The most successful second generation materials have been cadmium telluride (CdTe), copper indium gallium selenide(CIGS), amorphous silicon and micro amorphous silicon. The thickness range of such a layer is wide and varies from a few nanometers to tens of micrometers. These materials are applied in a thin film to a supporting substrate such as glass or ceramics reducing material mass and therefore costs. It is predicted that second generation cells will dominate the residential solar market.
Third Genaration: Dye-Sensitized Solar Cell Technology
The electrochemical dye solar cell was invented in 1988 by Professor Graetzel of Lausanne Polytechnique, in Switzerland. The "Graetzel" dye cell uses dye molecules adsorbed in nanocrystalline oxide semiconductors, such as TiO2, to collect sunlight. Dye cells employ relatively inexpensive materials such as glass, Titania powder, and carbon powder. Graetzel's cell is composed of a porous layer of titanium dioxide nanoparticles, covered with a molecular dye that absorbs sunlight, like the chlorophyll does in green leaves. Third generation solar cells are the cutting edge of solar technology. These solar cells can exceed the theoretical solar conversion efficiency limit for a single energy threshold material. Current research is targeting conversion efficiencies of 30-60% while retaining low cost materials and manufacturing techniques. Third generation contains a wide range of potential solar innovations including multijunction solar cells, polymer solar cells, nanocrystalline-nanowire cells, quantum dot solar cells and dye sensitized solar cells.
How do Solar Cells Work? |
Solar cells, which largely are made from crystalline silicon work on the principle of Photoelectric Effect that this semiconductor exhibits. Silicon in its purest form- Intrinsic Silicon- is doped with a dopant impurity to yield Extrinsic Silicon of desired characteristic (p-type or n-type Silicon). Working of Solar cells can thus be based on crystalline structure of Intrinsic and Extrinsic Silicon. When p and n type silicon combine they result in formation of potential barrier. These and more are discussed below.
Pure Silicon (Intrinsic) Crystalline Structure
Silicon has some special chemical properties, especially in its crystalline form. An atom of silicon has 14 electrons, arranged in three different shells. The first two shells- which hold two and eight electrons respectively- are completely full. The outer shell, however, is only half full with just four electrons (Valence electrons). A silicon atom will always look for ways to fill up its last shell, and to do this, it will share electrons with four nearby atoms. It's like each atom holds hands with its neighbors, except that in this case, each atom has four hands joined to four neighbors. That's what forms the crystalline structure. The only problem is that pure crystalline silicon is a poor conductor of electricity because none of its electrons are free to move about, unlike the electrons in more optimum conductors like copper.
Impurity Added Silicon (Extrinsic): P-type and N-type Semiconductors
Extrinsic silicon in a solar cell has added impurity atoms purposefully mixed in with the silicon atoms, maybe one for every million silicon atoms. Phosphorous has five electrons in its outer shell. It bonds with its silicon neighbor atoms having valency of 4, but in a sense, the phosphorous has one electron that doesn't have anyone to bond with. It doesn't form part of a bond, but there is a positive proton in the phosphorous nucleus holding it in place. When energy is added to pure silicon, in the form of heat, it causes a few electrons to break free of their bonds and leave their atoms. A hole is left behind in each case. These electrons, called free carriers, then wander randomly around the crystalline lattice looking for another hole to fall into and carry an electrical current. In Phosphorous-doped Silicon, it takes a lot less energy to knock loose one of "extra" phosphorous electrons because they aren't tied up in a bond with any neighboring atoms. As a result, most of these electrons break free, and release a lot more free carriers than in pure silicon. The process of adding impurities on purpose is called doping, and when doped with phosphorous, the resulting silicon is called N-type ("n" for negative) because of the prevalence of free electrons. N-type doped silicon is a much better conductor than pure silicon.The other part of a typical solar cell is doped with the element boron, which has only three electrons in its outer shell instead of four, to become P-type silicon. Instead of having free electrons, P-type ("p" for positive) has free openings and carries the opposite (positive) charge.
Formation of Potential Barrier and Photoelectric Effect
The electric field is formed when the N-type and P-type silicon come into contact. Suddenly, the free electrons on the N side combine the openings on the P side. Right at the junction, they combine and form something of a barrier, making it harder and harder for electrons on the N side to cross over to the P side (called POTENTIAL BARRIER). Eventually, equilibrium is reached, and an electric field separating the two sides is set up. This electric field acts as a diode, allowing (and even pushing) electrons to flow from the P side to the N side, but not the other way around. It's like a hill -- electrons can easily go down the hill (to the N side), but can't climb it (to the P side).
When light, in the form of photons, hits solar cell, its energy breaks apart electron-hole pairs(Photoelectric effect). Each photon with enough energy will normally free exactly one electron, resulting in a free hole as well. If this happens close enough to the electric field, or if free electron and free hole happen to wander into its range of influence, the field will send the electron to the N side and the hole to the P side. This causes further disruption of electrical neutrality, and if an external current path is provided, electrons will flow through the path to the P side to unite with holes that the electric field sent there, doing work for us along the way. The electron flow provides the current, and the cell's electric field causes a voltage.
Silicon is very shiny material, which can send photons bouncing away before energizing the electrons, so an antireflective coating is applied to reduce those losses. The final step is to install something that will protect the cell from the external elements- often a glass cover plate. PV modules are generally made by connecting several individual cells together to achieve useful levels of voltage and current, and putting them in a sturdy frame complete with positive and negative terminals.
Manufacturing Technology and Process |
STEP 1 - PURIFICATION OF SILICON: The basic component of a solar cell is intrinsic silicon, which is not pure in its natural state. To make solar cells, the raw materials—silicon dioxide of either quartzite gravel or crushed quartz—are first placed into an electric arc furnace, where a carbon arc is applied to release the oxygen. A Graphite and Thermal insulator trap the heat and maintain the furnace at required temperature for gangue ( impurity) to form a slag. The products are carbon dioxide and molten silicon. Silicon ingot is pulled down from the molten silicon using seed silicon crystallization and floating zone technique. Passing impure silicon in same direction several times that separates impurities- and impure end is later removed. This process yields silicon with one percent impurity, useful in many industries but not the solar cell industry. At this point, the silicon is still not pure enough to be used for solor cells and requires further purification. Pure silicon is derived from such silicon dioxides as quartzite gravel (the purest silica) or crushed quartz.
STEP 2- INGOT AND WAFER PREPARATION: Solar cells are made from silicon boules, polycrystalline structures that have the atomic structure of a single crystal. The most commonly used process for creating the boule is called the Czochralski method. In this process, a seed crystal of silicon is dipped into melted polycrystalline silicon. As the seed crystal is withdrawn and rotated, a cylindrical ingot or "boule" of silicon is formed. The ingot withdrawn is unusually pure, because impurities tend to remain in the liquid. From the boule, silicon wafers are sliced one at a time using a circular saw whose inner diameter cuts into the rod, or many at once with a multiwire saw. (A diamond saw produces cuts that are as wide as the wafer—. 5 millimeter thick.) Only about one-half of the silicon is lost from the boule to the finished circular wafer—more if the wafer is then cut to be rectangular or hexagonal. Rectangular or hexagonal wafers are sometimes used in solar cells because they can be fitted together perfectly, thereby utilizing all available space on the front surface of the solar cell. The wafers are then polished to remove saw marks.
STEP 3 - DOPING: The traditional way of doping silicon wafers with boron and phosphorous is to introduce a small amount of boron during the Czochralski process. The wafers are then sealed back to back and placed in a furnace to be heated to slightly below the melting point of silicon (2,570 degrees Fahrenheit or 1,410 degrees Celsius) in the presence of phosphorous gas. The phosphorous atoms "burrow" into the silicon, which is more porous because it is close to becoming a liquid. The temperature and time given to the process is carefully controlled to ensure a uniform junction of proper depth. These diffusion processes are usually performed through the use of a batch tube furnace or an in-line continuous furnace.The basic furnace construction and process are very similar to the process steps used by packaging engineers.
STEP 4 - SCREEN PRINTING: Electrical contacts are formed through squeezing a metal paste through mesh screens to create a metal grid. This metal paste (usually Ag or Al) needs to be dried so that subsequent layers can be screen-printed using the same method. As a last step, the wafer is heated in a continuous firing furnace at temperatures ranging from 780 to 900°C. These grid- pattern metal screens act as collector electrodes that carry electrons and complete the electrical continuity in the circuit.
STEP 5 - STRINGING AND TABBING: Electrical contacts connect each solar cell to another and to the receiver of produced current. The contacts must be very thin (at least in the front) so as not to block sunlight to the cell. Metals such as palladium/silver, nickel, or copper are vacuum-evaporated After the contacts are in place, thin strips ("fingers") are placed between cells. The most commonly used strips are tin-coated copper.
STEP 6 - ANTIREFLECTIVE COATING: Because pure silicon is shiny, it can reflect up to 35 percent of the sunlight. To reduce the amount of sunlight lost, an anti-reflective coating is put on the silicon wafer- mostly titanium dioxide, silicon oxide and some others are used. The material used for coating is either heated until its molecules boil off and travel to the silicon and condense, or the material undergoes sputtering. In this process, a high voltage knocks molecules off the material and deposits them onto the silicon at the opposite electrode. Yet another method is to allow the silicon itself to react with oxygen- or nitrogen-containing gases to form silicon dioxide or silicon nitride. Commercial solar cell manufacturers use silicon nitride. Another method to make silicon absorb more light is to make its top surface grained, i.e. pyramid shaped nanostructures that yield 70% absorption that reaches the cell surface after passing through anti-reflective coating.
STEP 7 - MODULE MANUFACTURING
The finished solar cells are then encapsulated; that is, sealed into silicon rubber or ethylene vinyl acetate. Solar module assembly usually involves soldering cells together to produce a 36-cell string (or longer) and laminating it between toughened glass on the top and a polymeric backing sheet on the bottom. The encapsulated solar cells are then placed into an aluminum frame that has a mylar or tedlar backsheet and a glass or plastic cover. Frames are usually applied to allow for mounting in the field, or the laminates may be separately integrated into a mounting system for a specific application such as integration into a building.
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DO IT YOURSELF Modules Cell Manufacturing is very machinery-intensive process. But Module manufacturing process involving tabbing, stringing and framing can be done at home. Many Do-It_youself sites guide you stepwise to make modulesat home. Although industry manufactured efficiencies are not ensued in these, trials can be based on smaller scales. It also helps individuals to understand how these work and troubleshoot the problems. More inquisitive people get to know about cost of each item available and can workout reasonable pricing and ensure others don't feel that they are being fooled by module manufacturers. Some of those sites are: SIX AXIS Solar Free Sun Power Build it Solar |
Other Cell Technology Manufacturing Processes
The process discussed above apply to the crystalline silicon solar cell manufacturing process. Manufacturing of Second Geberation Solar cells (Thin Film) and Third Generation Solar Cells (QD, Dye Sensitized etc) differ from largely utilized Crystalline silicon manufacturing.
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Second Generation Solar Cell Manufacturing |
Third Generation Solar Cell Manufacturing |
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Quality Control, Testing and Standards |
Quality control is important in solar cell manufacture because discrepancy in the many processes and factors can adversely effect the overall efficiency of the cells. The primary research goal is to find ways to improve the efficiency of each solar cell over a longer lifetime. The Low Cost Solar Array Project (initiated by the United States Department of Energy in the late 1970s) sponsored private research that aimed to lower the cost of solar cells. The silicon itself is tested for purity, crystal orientation, and resistivity. Manufacturers also test for the presence of oxygen (which affects its strength and resistance to warp) and carbon (which causes defects). Finished silicon disks are inspected for any damage, flaking, or bending that might have occurred during sawing, polishing, and etching. An earlier problem with solar modules was a tendency to stop working when partially shaded. The result was development of hotspots, module failure, overheating and overall system failure. This problem was alleviated by providing shunt diodes (blocking and bypass diodes) that reduce dangerously high voltages to the cell. Shunt resistance must then be tested using partially shaded junctions.
- Electrical Tests: The completed semiconductors must undergo electrical tests to see that the current, voltage, and resistance for each meet appropriate standards.
- Solar Simulation Test: An important test of solar modules involves providing test cells with conditions and intensity of light that they will encounter under normal conditions and then checking to see that they perform well.
- Physical Test: The cells are also exposed to heat and cold and tested against vibration, twisting, and hail.
- On-site Tests: The final test for solar modules is field site testing, in which finished modules are placed where they will actually be used. This provides the researcher with the best data for determining the efficiency of a solar cell under ambient conditions and the solar cell's effective lifetime, the most important factors of all.
Agencies around the world set standards, codes and certificationsfor equipment performance, integrity and safety. Though we note the country of origin, these standards typically apply industry-wide.