Solar Cell
Photovoltaic solar cells are thin silicon disks that convert sunlight into electricity. These disks act as energy sources for a wide variety of uses, including: calculators and other small devices; telecommunications; rooftop panels on individual houses; and for lighting, pumping, and medical refrigeration for villages in developing countries. Solar cells in the form of large arrays are used to power satellites and, in rare cases, to provide electricity for power plants.
When research into electricity began and simple batteries were being made and studied, research into solar electricity followed amazingly quickly. As early as 1839, Antoine-Cesar Becquerel exposed a chemical battery to the sun to see it produce voltage. This first conversion of sunlight to electricity was one percent efficient. That is, one percent of the incoming sunlight was converted into electricity. Willoughby Smith in 1873 discovered that selenium was sensitive to light; in 1877 Adams and Day noted that selenium, when exposed to light, produced an electrical current. Charles Fritts, in the 1880s, also used gold-coated selenium to make the first solar cell, again only one percent efficient. Nevertheless, Fritts considered his cells to be revolutionary. He envisioned free solar energy to be a means of decentralization, predicting that solar cells would replace power plants with individually powered residences.
With Albert Einstein’s explanation in 1905 of the photoelectric effect—metal absorbs energy from light and will retain that energy until too much light hits it—hope soared anew that solar electricity at higher efficiencies would become feasible. Little progress was made, however, until research into diodes and transistors yielded the knowledge necessary for Bell scientists Gordon Pearson, Darryl Chapin, and Cal Fuller to produce a silicon solar cell of four percent efficiency in 1954.
Further work brought the cell’s efficiency up to 15 percent. Solar cells were first used in the rural and isolated city of Americus, Georgia as a power source for a telephone relay system, where it was used successfully for many years.
A type of solar cell to fully meet domestic energy needs has not as yet been developed, but solar cells have become successful in providing energy for artificial satellites. Fuel systems and regular batteries were too heavy in a program where every ounce mattered. Solar cells provide more energy per ounce of weight than all other conventional energy sources, and they are cost-effective.
Only a few large scale photovoltaic power systems have been set up. Most efforts lean toward providing solar cell technology to remote places that have no other means of sophisticated power. About 50 megawatts are installed each year, yet solar cells provide only about. 1 percent of all electricity now being produced. Supporters of solar energy claim that the amount of solar radiation reaching the Earth’s surface each year could easily provide all our energy needs several times over, yet solar cells have a long way to go before they fulfill Charles Fritts’s dream of free, fully accessible solar electricity.
Raw Materials
The basic component of a solar cell is pure 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. The products are carbon dioxide and molten silicon. 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. The resulting pure silicon is then doped (treated with) with phosphorous and boron to produce an excess of electrons and a deficiency of electrons respectively to make a semiconductor capable of conducting electricity. The silicon disks are shiny and require an anti-reflective coating, usually titanium dioxide.
The solar module consists of the silicon semiconductor surrounded by protective material in a metal frame. The protective material consists of an encapsulant of transparent silicon rubber or butyryl plastic (commonly used in automobile windshields) bonded around the cells, which are then embedded in ethylene vinyl acetate. A polyester film (such as mylar or tedlar) makes up the backing. A glass cover is found on terrestrial arrays, a lightweight plastic cover on satellite arrays. The electronic parts are standard and consist mostly of copper. The frame is either steel or aluminum. Silicon is used as the cement to put it all together.
The Manufacturing Process
Purifying the silicon
- 1 The silicon dioxide of either quartzite gravel or crushed quartz is placed into an electric arc furnace. A carbon arc is then applied to release the oxygen. The products are carbon dioxide and molten silicon. This simple process yields silicon with one percent impurity, useful in many industries but not the solar cell industry.
- 2 The 99 percent pure silicon is purified even further using the floating zone technique. A rod of impure silicon is passed through a heated zone several times in the same direction. This procedure drags the impurities toward one end with each pass. At a specific point, the silicon is deemed pure, and the impure end is removed.
Making single crystal silicon
- 3 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.
Making silicon wafers
- 4 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.
After the initial purification, the silicon is further refined in a floating zone process. In this process, a silicon rod is passed through a heated zone several times, which serves to ‘drag the impurities toward one end of the rod. The impure end can then be removed. Next, a silicon seed crystal is put into a Czochralski growth apparatus, where it is dipped into melted polycrystalline silicon. The seed crystal rotates as it is withdrawn, forming a cylindrical ingot of very pure silicon. Wafers are then sliced out of the ingot.
Doping
- 6 The traditional way of doping (adding impurities to) silicon wafers with boron and phosphorous is to introduce a small amount of boron during the Czochralski process in step #3 above. 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. A more recent way of doping silicon with phosphorous is to use a small particle accelerator to shoot phosphorous ions into the ingot. By controlling the speed of the ions, it is possible to control their penetrating depth. This new process, however, has generally not been accepted by commercial manufacturers.
Placing electrical contacts
- 7 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
This illustration shows the makeup of a typical solar cell. The cells are encapsulated in ethylene vinyl acetate and placed in a metal frame that has a mylar backsheet and glass cover.
The anti-reflective coating
- 9 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. The most commonly used coatings are titanium dioxide and silicon oxide, though 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.
Encapsulating the cell
- 10 The finished solar cells are then encapsulated; that is, sealed into silicon rubber or ethylene vinyl acetate. The encapsulated solar cells are then placed into an aluminum frame that has a mylar or tedlar backsheet and a glass or plastic cover.
Quality Control
Quality control is important in solar cell manufacture because discrepancy in the many processes and factors can adversely affect 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.
During the entire silicon disk manufacturing process, the temperature, pressure, speed, and quantities of dopants are continuously monitored. Steps are also taken to ensure that impurities in the air and on working surfaces are kept to a minimum.
The completed semiconductors must then undergo electrical tests to see that the current, voltage, and resistance for each meet appropriate standards. An earlier problem with solar cells was a tendency to stop working when partially shaded. This problem has been alleviated by providing shunt diodes that reduce dangerously high voltages to the cell. Shunt resistance must then be tested using partially shaded junctions.
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. The cells are also exposed to heat and cold and tested against vibration, twisting, and hail.
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.
The Future
Considering the present state of relatively expensive, inefficient solar cells, the future can only improve. Some experts predict it will be a billion-dollar industry by the year 2000. This prediction is supported by evidence of more rooftop photovoltaic systems being developed in such countries as Japan, Germany, and Italy. Plans to begin the manufacture of solar cells have been established in Mexico and China. Likewise, Egypt, Botswana, and the Philippines (all three assisted by American companies) are building plants that will manufacture solar cells.
Most current research aims for reducing solar cell cost or increasing efficiency. Innovations in solar cell technology include developing and manufacturing cheaper alternatives to the expensive crystalline silicon cells. These alternatives include solar Windows that mimic photosynthesis, and smaller cells made from tiny, amorphous silicon balls. Already, amorphous silicon and polycrystalline silicon are gaining popularity at the expense of single crystal silicon. Additional innovations including minimizing shade and focusing sunlight through prismatic lenses. This involves layers of different materials (notably, gallium arsenide and silicon) that absorb light at different frequencies, thereby increasing the amount of sunlight effectively used for electricity production.
A few experts foresee the adaptation of hybrid houses; that is, houses that utilize solar water heaters, passive solar heating, and solar cells for reduced energy needs. Another view concerns the space shuttle placing more and more solar arrays into orbit, a solar power satellite that beams power to Earth solar array farms, and even a space colony that will manufacture solar arrays to be used on Earth.
Where To Learn
Books
Bullock, Charles E. and Peter H. Grambs. Solar Electricity: Making the Sun Work for You. Monegon, Ltd., 1981.
Komp, Richard J. Practical Photovoltaics. Aatec Publications, 1984.
Making and Using Electricity from the Sun. Tab Books, 1979.
Periodicals
Crawford, Mark. DOE’s Born-Again Solar Energy Plan, Science. March 23, 1990, pp. 1403-1404.
Waiting for the Sunrise, Economist. May 19, 1990, pp. 95.
Edelson, Edward. Solar Cell Update, Popular Science. June, 1992, p. 95.
Murray, Charles J. Solar Power’s Bright Hope, Design News. March 11, 1991, p. 30.
Silicon photovoltaic cell
Most solar cells which are used today are based on crystalline silicon. The silicon can be mono-crystalline or poly-crystalline. Monocrystalline material is produced by the Czrochalski-process, while polycrystalline material is usually prepared by molding. In both cases the generated material is cut to wafers by wire saws. The wafers serve then as substrate material for the solar cell.
The solar cells consists mainly of silicon and is called therefore thick film solar cell, in contrary to thin film solar cells where the semiconductor layers are deposited on substrate of a different material. The bulk silicon is usually lightly p-doped, and conductive for positive charge carriers or holes. On the front side a thin heavily n-doped layer has to be formed by doping, which is conductive for negative charge carriers or electrodes. This way a p/n-junction is formed, which can separate the charge carrier pairs, generated by the absorption of sunlight. On the front side and on the back side metallic contacts have to be formed in order to drain the solar current. At the backside a holohedral aluminium layer is deposited, while at the front side silver contact fingers are generated, which allow most of the sunlight to pass into the cell. Finally a silicon nitride antireflection coating ARC is attached to the front side in order to increase absorption of the sunlight. The last production step is the assembly of the solar cells to solar modules.
Crystec Technology Trading GmbH, Germany, www.crystec.com, 49 8671 882173, FAX 882177
Diffusion furnaces for doping crystalline silicon solar cells.
The doping of the upper, heavily n-doped layer is done with phosphorous as doping material. Two main procedures are used:
- Doping from the gas phase by using phosphorousoxychloride POCl3.
- Doping with doping paste attached by screen printing.
Tube furnaces for doping solar cells with phosphorousoxychloride.
Horizontal furnaces or diffusion furnaces from JTEKT Thermo Systems (previously Koyo Thermo Systems) ensure cost effective doping with high throughput. The LGO heating elements, used in this furnace have a very low thermal mass and can reduce therefore the process time. They can also save energy and costs for the doping process. All normal sizes of solar wafers can be processed in this type of furnace. Liquid POCl3 is supplied in a bubbler. Nitrogen passes at a well defined temperature through the liquid and is transporting the dopant. Typical doping temperature is 800. 900C.
The latest development of JTEKT allows now also the continous doping of silicon wafers with POCl3 in a tube furnace. The wafers are carried through the tube on quartz cars with a load of 100 wafers each. The design of the cars is critical in order to obtain a good temperature uniformity and the process results of the POCl3 doping is therefore also affected seriously. Gas curtains at both ends of the tube separate the process room from the environment.
- Two tubes installed beside each other allow simple automation.
- The footprint gets lower because there is no need for a conventional loading station.
- Throughput is increase much, because there is no time loss for loading or unloading wafers.
- The furnace is saving energy, because there are no heating and cooling cycles.
- There are also no thermic losses by the move out of a transportation conveyor Band, because such a Band is not installed.
- The gas consumption is lower, because no purge cycles are necessary.
- By the more simple configuration of the system, price is also reduced.
For higher demands to the homogeneity of the doping profile or to the automation level, vertical furnaces are available. The smallest version of a JTEKT Thermo Systems (previously Koyo Thermo Systems) vertical furnace can be very well used especially for the use in research and development of solar cells. The furnace model VF1000 is designed as a mini-batch furnace, has a manual loading and is therefore very flexible regarding sample sizes. This vertical furnace is equipped with a cost saving LGO heating element. The process performance equals the big production versions of vertical furnaces for IC production. The price of this unit is similar to the price of a horizontal tube in a horizontal furnace.
For mass production, JTEKT Thermo Systems developed a special vertical furnace, which gives better process results compared to a horizontal one, but still does not increase much the production costs for solar cells. This twin furnace loads 3-4 boats in one vertical tube and has therefore a capacity of 600-800 solar cells. This is the same capacity that you get on a horizontal furnace. Automation is easier to do on a vertical furnace.
Crystec Technology Trading GmbH, Germany, www.crystec.com, 49 8671 882173, FAX 882177
Conveyor furnaces for doping of solar cells using doping paste.
Doping with doping paste works with rather harmless materials and allows the usage of a simple conveyor furnace, which is well suited for mass production and can be intergrated easily in in-line production systems. The dopand is diffusing in the furnace from the doping paste into the silicon material. Temperature and anneal time determine the thickness of the generated n-doped top layer. The JTEKT conveyor furnace model 47-MT with mesh belt transportation system can be used for this application.
For research and development, JTEKT can offer a small type conveyor furnace model 810, which is constructed similar to the large furnaces and can simulate a production environment.
JTEKT Thermo Systems and Crystec will be pleased to engineer a cost effective system to satisfy your most demanding and exacting requirements.
- Overview Furnaces
- Vertical furnaces
- VF-1000/VF-3000 Small vertical furnaces
- VF-5100/VF-5300 150mm and 200mm wafer
- VF-5700/VF-5900 300mm wafer
- University of Uppsala: Reference VF1000
- Fraunhofer Institute IISB: Reference VF1000
- Horizontal furnaces
- Comparison of vertical and horizontal furnaces
- Continous furnaces for phosphorous doping with POCl3
- RTP RTA equipment
- Clean ovens. Equipment for pilot lines
- Clean Oven CLH. Baking of Polyamid
- Inert Gas Oven INH
- Conveyor furnaces
- LGO Heating elements
- Moldatherm ® High temperature heating elements
Silicon solar panels come in varying sizes to suit various applications. Silicon solar panels typically comprise 32, 36, 48, 60, 72 and 96 PV cells
Importance of solar energy
Use of solar energy is a trend that is quickly gaining popularity in India as it is abundantly available in most parts of the country and more importantly it is a renewable source of energy, never to be extinguished. Although solar energy is easily available, harnessing that energy into useful purposes is a tricky affair. However, past research has shown that it can be harnessed into electric energy and current research is aimed at converting all the energy received into useful electric energy.
What are photovoltaic cells? Conversion of solar energy into electric energy is accomplished by the principle of photovoltaic (PV) effect. PV effect is the process of converting the sun’s energy into electric energy through PV cells made up of semiconductors such as silicon. When sunlight falls on a PV cell, its constituents known as photons absorb the light energy received and transfer that energy to the electrons in the semiconductor material which then flows as electric current. This electric current becomes the source of electric power. One of the most popular semiconductor materials used in PV cells is silicon which has properties conducive to produce electric power from solar energy.
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A solar panel made up of silicon semiconductors is called a silicon solar panel. A number of PV cells aggregated in a confined panel act as a collector of solar energy. Silicon semiconductors are popular because they exhibit properties such as low weight volume ratio, extended life cycle, robustness and strength. Silicon semiconductors are easy to produce at low cost.
Also Read
Silicon solar panels come in varying sizes to suit various applications. Silicon solar panels typically comprise 32, 36, 48, 60, 72 and 96 PV cells. Several of these panels can be connected in series or parallel to obtain higher electric power suitable for high power demanding applications.
Types of silicon solar panels Basically, there are three types of silicon solar panels primarily classified on the basis of the type of PV cells in them. They can be categorized as monocrystalline, polycrystalline and amorphous or thin film solar panels. Each of these has different properties that can affect the output of the electric power generated.
Monocrystalline solar panels
Monocrystalline solar panels are made from PV cells sliced out of a silicon ingot grown from a pure single-crystal of silicon. When the cylindrical ingot is sliced its circular shape is squared giving the cell a unique octagonal shape. This shape distinguishes the cells from the cells made of polycrystalline silicon. Further, monocrystalline solar cells have a uniform black colour across all the cells. The PV cells in the panel offer better collection surface because of the pyramid pattern of the crystal. With proper treatment and addition of other materials, these cells are durable for up to 30 years or even more and offer higher efficiency than the other two types of silicon solar panels. The efficiency of monocrystalline solar panels, between 15-20%, is the highest among all silicon-based solar panels. These cells are also efficient in terms of space occupied for the same output among all silicon-based cells.
Polycrystalline solar panelsPolycrystalline solar panels are made from PV cells cut from multiple silicon crystals. Melted silicon is poured into square moulds. After the silicon cools in the moulds it is cut into squares. The perfect square shape distinguishes the polycrystalline cell from the monocrystalline cell (which is octagonal in shape). These have the same properties as monocrystalline solar panels but offer lower efficiencies while converting solar energy into electric energy. These cells are cheaper to make than monocrystalline cells because there is less wastage of silicon.
Thin film or amorphous solar panelsThin film or amorphous solar panels comprise a substrate on which thin layers of amorphous silicon is deposited. These are becoming popular because of its mass production capability and use where the surface area for deploying the panels is not a constraint. Amorphous solar panels have low efficiency and therefore are used in small applications such as calculators. But with the new ‘stacking’ technique, layers can be combined to improve efficiency (6-8%). Because amorphous solar panels are flexible, they can be used in innovative ways on curved surfaces too.
Applications of silicon solar panels
- In power plants, silicon solar panels provide the necessary energy to boil water and produce steam that can rotate turbines to produce electricity. They can replace other forms of energy that are not renewable.
- In households, silicon solar panels placed on rooftops can serve as a stand-alone system used to heat water for home consumption. These panels can also become source of free electricity if they charge a rechargeable battery which can then be used to supply power to the homes.
- In commercial establishments, in much the same way as in homes, silicon solar panels placed on rooftops or backyards can serve as primary or standby power.
- In agricultural fields or homes, stand-alone solar power systems use silicon solar panels to power water pump to pump water to a field or to an overhead tank.
- In swimming pools, silicon solar panels can keep the water in the pool hot during the winter.
- For lighting applications, stand-alone systems using silicon solar panels can power a street, pavement, pathway or a backyard. They can also serve as portable torches.
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Solar Cell Production: from silicon wafer to cell
In our earlier article about the production cycle of solar panels we provided a general outline of the standard procedure for making solar PV modules from the second most abundant mineral on earth – quartz.
In chemical terms, quartz consists of combined silicon-oxygen tetrahedra crystal structures of silicon dioxide (SiO2), the very raw material needed for making solar cells.
The production process from raw quartz to solar cells involves a range of steps, starting with the recovery and purification of silicon, followed by its slicing into utilizable disks – the silicon wafers – that are further processed into ready-to-assemble solar cells.
Only a few manufacturers control the whole value chain from quartz to solar cells. While most solar PV module companies are nothing more than assemblers of ready solar cells bought from various suppliers, some factories have at least however their own solar cell production line in which the raw material in form of silicon wafers is further processed and refined.
In this article, we will explain the detailed process of making a solar cell from a silicon wafer.
Solar Cell production industry structure
In the PV industry, the production chain from quartz to solar cells usually involves 3 major types of companies focusing on all or only parts of the value chain:
1.) Producers of solar cells from quartz, which are companies that basically control the whole value chain.
2.) Producers of silicon wafers from quartz – companies that master the production chain up to the slicing of silicon wafers and then sell these wafers to factories with their own solar cell production equipment.
3.) Producers of solar cells from silicon wafers, which basically refers to the limited quantity of solar PV module manufacturers with their own wafer-to-cell production equipment to control the quality and price of the solar cells.
For the purpose of this article, we will look at 3.) which is the production of quality solar cells from silicon wafers.
How are silicon wafers made?
Before even making a silicon wafer, pure silicon is needed which needs to be recovered by reduction and purification of the impure silicon dioxide in quartz.
In this first step, crushed quartz is put in a special furnace, and then a carbon electrode is applied to generate a high-temperature electric arc between the electrode and the silicon dioxide.
That process, called carbon arc welding (CAW), reduces the oxygen from the silicon dioxide and produces carbon dioxide at the electrode and molten silicon.
This molten silicon is 99% pure which is still insufficient to be used for processing into a solar cell, so further purification is undertaken by applying the floating zone technique (FTZ).
During the FTZ, the 99% pure silicon is repeatedly passed in the same direction through a heated tube. This process pushes the 1% impure parts to one end, with the remaining 100% pure parts remaining on the other side. The impure part can then be easily cut off.
Crystal seeds of silicon are in the so-called Czochralski (CZ) process put into polycrystalline silicon melt of the Czochralski growth apparatus. By extracting the seeds from the melt with the puller, they rotate and form a pure cylindrical silicon ingot cast out from the melt and which is used to make mono-crystalline silicon cells.
In order to make multi-crystalline silicon cells, various methods exist:
1.) heat exchange method (HEM)2.) electro-magneto casting (EMC)3.) directional solidification system (DSS)
DSS is the most common method, spearheaded by machinery from renowned equipment manufacturer GT Advanced. By this method, the silicon is passed through the DSS ingot growth furnace and processed into pure quadratic silicon blocks.
During the casting of the ingots, the silicon is often already pre-doped before slicing and selling the wafer disks to the manufacturers. Doping is basically the process of adding impurities into the crystalline silicon wafer to make it electrically conductive.
These positive (p-type) and negative (n-type) doping materials are mostly boron, which has 3 electrons (3-valent) and is used for p-type doping, and phosphorous, which has 5 electrons (5-valent) and is used for n-type doping. Silicon wafers are often pre-doped with boron.
Once we have our ingots ready, they can then – depending on the geometrical shape requirements, for solar cells usually space-saving hexagonal or rectangular shapes- be sliced into usually 125mm or 156mm silicon wafers by using a multiwire saw.
Processing of silicon wafers into solar cells
The standard process flow of producing solar cells from silicon wafers comprises 9 steps from a first quality check of the silicon wafers to the final testing of the ready solar cell.
Step 1: Pre-check and Pretreatment
The raw silicon wafer disks first undergo a pre-check during which they are inspected on their geometric shape and thickness conformity and on damages such as cracks, breakages, scratches, or other anomalities.
Following this pre-check, the wafers are split and cleaned with industrial soaps to remove any metal residues, liquids or other production remains from the surface that would otherwise impact the efficiency of that wafer.
Light reflection difference between a non-textured flat silicon wafer surface and a silicon wafer surface with a random pyramid texture
Step 2: Texturing
Following the initial pre-check, the front surface of the silicon wafers is textured to reduce reflection losses of the incident light.
For monocrystalline silicon wafers, the most common technique is random pyramid texturing which involves the coverage of the surface with aligned upward-pointing pyramid structures.
This is achieved by etching and pointing upwards from the front surface. The proper alignment of the pyramids etched out is a result of the regular, neat atomic structure of monocrystalline silicon.
The regular, neat atomic structure of monocrystalline silicon also benefits the flow of electrons through the cell as with fewer boundaries electrons flow much better. Therefore, monocrystalline silicon has an electrochemical structural advantage offering more efficiency over the grainy atomic structures of multi-crystalline silicon.
Now, with such a pyramid structure in place, the incident light does not reflect back and gets lost in the surrounding air but bounces back onto the surface.
Another, less common texturing technique is the inverted pyramid texturing. Instead of pointing upwards from the front surface, the pyramids are etched into the wafer’s surface, similarly achieving reflection losses of incident light trapped in the inverted pyramid holes.
The texturing of multi-crystallin silicon wafers requires photolithography – a technique involving the engraving of a geometric shape on a substrate by using light – or mechanical cutting of the surface by laser or special saws.
Step 3: Acid Cleaning
After texturing, the wafers undergo acidic rinsing (or: acid cleaning). In this step, any post-texturing particle remains are removed from the surface.
Using hydrogen fluoride (HF) vapor, oxidized silicon layers on the substrate can be etched away from the wafer surface. The result is a wet surface that can be easily dried.
By using hydrogen chloride (HCl), metallic residues on the surface can be absorbed by the chloride and thus removed from the wafer.
Step 4: Diffusion
Diffusion is basically the process of adding a dopant to the silicon wafer to make it more electrically conductive. There are basically 2 methods of diffusion: solid-state diffusion and emitter diffusion.
While the former method basically involves the already mentioned uniform doping of the wafers with the p-type and n-type materials, the emitter diffusion refers to the placing of a thin dopant material-containing coating on the wafer bypassing the wafers through a diffusion coating furnace.
Wafers that have already been pre-doped with p-type boron during the casting process are during the diffusion process given a negative (n-type) surface by diffusing them with a phosphorous source at a high temperature, creating the positive-negative (p-n) junction.
Why diffuse the wafers though? This junction of electron deficiency in the p-type and high electron concentration in the n-type allows for excess electrons from the n-type to pass to the p-type, a flow creating an electron field at the junction.
Step 5: Etching Edge Isolation
During diffusion, the n-type phosphorous diffuses not only into the desired wafer surface but also around the edges of the wafer as well as on the backside, creating an electrical path between the front and backside and in this way also preventing electrical isolation between the two sides.
The objective of the etching and edge isolation process is to remove this electrical path around the wafer edge by disk stacking the cells on top of each other and then exposing them to a plasma etching chamber using tetrafluoromethane (CF4) to etch exposed edges.
Step 6: Post-Etching Washing
After the etching, particle residues’ potential remains on the wafer and the wafer edges. Therefore the wafers need to undergo a second washing to remove the remains of the previous etching process.
After this second washing, the wafers can further be processed for the deposition of anti-reflective (AR) coating.
Niclas checking cell production at a PEVCD machine
Step 7: Anti-Reflective Coating Deposition
In addition to surface texturing, AR coating is often applied on the surface to further reduce reflection and increase the amount of light absorbed into the cell.
This anti-reflective coating is very much needed as the reflection of bare silicon solar cells is over 30%. For the thin AR Coating, silicon nitride (Si3N4) or titanium oxide (TiO2) is used. The color of the solar cell can be changed by varying the thickness of the anti-reflection coating.
In the semiconductor industry, there are basically three methods to depose layers on wafers, which are:
1.) Atmospheric Pressure Chemical Vapor Deposition (APCVD), which is used only for a few applications and requires high temperatures.
2.) Low-Pressure Chemical Vapor Deposition (LPCVD), which involves the deposition process to be performed in tube furnaces and like the APCVD method requires high temperatures.
3.) Plasma Enhanced Chemical Vapor Deposition (PECVD), which is the most common method for the deposition of AR coating on the wafer.
In the PECVD process, the thin coating exists in a gaseous state and is through a chemical reaction process solidified onto the wafer.
Step 8: Contact Printing and Drying
As the next step, metal inlines are printed on the wafer with the objective to create ohmic contacts. These metal inlines are printed on the rear side of the wafer, which is called backside printing.
Contact Printing and Drying Machinery
This is achieved by printing the metal pastes with special screen printing devices that place these metal inlines onto the backside. After printing, the wafer undergoes a drying process.
Once dry, this process is followed by the printing of the front side contacts, then the wafer is another time dried.
After all, contacts have been printed on the rear and front sides, the screen-printed wafers are passed through a sintering furnace to solidify the dry metal pastes onto the wafers. Then, the wafers are cooled and can already be called solar cells.
Step 9: Testing and Cell Sorting
In this final process, the now ready-to-assemble solar cells are tested under simulated sunlight conditions and then classified and sorted according to their efficiencies.
This is handled by a solar cell testing device that automatically tests and sorts the cells. The factory workers then only need to withdraw the cells from the respective efficiency repository to which the machine assorted the cells.
TO OUR READERS:
In this article we went through the standard production process from silicon wafer to solar cells. However, there are many quality problems that can potentially be created early on in this process. Which potential quality risks are you aware of when processing silicon wafers into solar cells? If you are aware of potential quality problems or have experience in solar cell production, we kindly invite you to share more insights with the Sinovoltaics Community!