Types of Solar Cells
Abstract
The most efficient silicon solar cells are made from single crystal silicon. Polycrystalline silicon is slightly less efficient, but popular. A third form, amorphous silicon, is much less efficient, but cheap to produce. An alternative crystalline material, gallium arsenide, is even more efficient, but is much more expensive. Solar cells made from thin film materials are the cheapest to manufacture, exploiting techniques such as vapor deposition, sputtering, or even printing. Cadmium telluride is the most important thin film material for solar cells, alongside amorphous silicon. Solar cells with a single layer have a maximum theoretical efficiency of 34%; multilayer cells can provide much higher efficiency, but at higher cost. Concentrating solar cells offer another way of increasing efficiency. Meanwhile, newer organic and dye-based semiconductors offer extremely cheap alternatives.
Keywords
Crystalline silicon; amorphous silicon; cadmium telluride; gallium arsenide; thin film solar cells; concentrating solar cells; organic semiconductors; dye-sensitized solar cells
Solar cells are manufactured using technologies similar to those used to produce microchips and transistors. Today most of these are made using slices of perfect silicon crystals. The slices are then chemically etched and doped to create the complex structures required for computers and other electronic devices. A solar cell, though simpler in structure than a microchip, can be manufactured in a similar way.
Silicon solar cells made from single crystal silicon are the most efficient available, with reliable commercial cell efficiencies of up to 20%. The record laboratory efficiency for a single solar cell under normal solar irradiation conditions is 25%. Even though this is the most expensive form of silicon, it also remains one of the most popular due to its high efficiency and durability. Polycrystalline silicon is cheaper to manufacture, but the penalty is lower efficiency, with the best measured around 18%. This is also very popular. Cheapest of all to produce is amorphous silicon, which can be made using vapor deposition techniques rather than by expensive crystal growing. However, its best efficiency is only 8%. Amorphous silicon also suffers from degradation when first exposed to light, a problem not seen with crystalline material. This can reduce its initial efficiency by up to 20%.
All silicon solar cells require extremely pure silicon. The manufacture of pure silicon is both expensive and energy intensive. The traditional method of production required 90 kWh of electricity for each kilogram of silicon. Newer methods have been able to reduce this to 15 kWh per kilogram. This still means that depending upon its efficiency, a silicon solar cell can take up to 2 years to generate the energy needed to make it. This compares with around 5 months for a solar thermal power plant. Manufacturers of crystalline silicon are concentrating on ways of reducing the cost of crystalline material by cutting it more efficiently, by reducing the amount in each solar cell, or by finding new ways of growing it. This is helping push costs down, and crystalline silicon remains competitive in spite of efforts by thin film manufacturers using much cheaper materials. Meanwhile, manufacturers are also exploring ways of increasing the average lifetime of a solar cell. Modern cells are usually rated for a 25-year life, but it appears possible with suitable encapsulation techniques to increase this to as much as 100 years.
Another crystalline material used for solar cells is gallium arsenide. This has an almost perfect bandgap for a solar cell, and in the laboratory efficiencies of 28% have been recorded. However, practical cells only reach 20%, and the material is both expensive and composed of hazardous materials. It is rarely used, except for special applications.
The main alternative to crystalline silicon for solar cells is some form of thin film. From a manufacturing point of view these are attractive because they can be produced using cheap techniques such as vapor deposition, sputtering, or even printing. Amorphous silicon is one alternative, but it is not as cheap to produce as cadmium telluride (CdTe), and the latter has a much higher efficiency, with the best recorded at 22% (though the efficiency of the best commercial cells is only 15%). This material also has an almost optimum bandgap for a solar cell (1.44 eV), and its potential efficiency could approach 30%. CdTe is also attractive because it is possible to produce solar cells on a variety of substrates, including building components and flexible plastic sheets.
The maximum efficiency possible with a single-layer solar cell of any semiconductor is 33.7%. It is possible to build more efficient solar cells by layering cells one on top of the other. Such multilayer or multi-junction cells place the semiconductor with the largest bandgap at the top. This top layer absorbs high-energy radiation, but lower-energy radiation passes through it to reach the layers below, where further semiconductor layers of lower bandgap are placed. In principle it is possible to create a cell with up to around 50% energy efficiency with multilayer cells, but these cells are much more expensive to manufacture. The best recorded efficiency achieved to date is 43.7%.
Silicon Solar Cells
While there are a range of solar cells made from different materials, the most common as a proportion of solar cell production are cells made from crystalline silicon. These have accounted for between 80% and 90% of global production since around 1992. Two types dominate—single crystal silicon and polycrystalline silicon—with each accounting for around 40%. The single crystal material is made from high purity silicon using the Czochralski process, in which a single crystal is pulled slowly from a crucible of molten silicon. Polycrystalline silicon is composed of a multitude of tiny single crystals and is much simpler to make, though it too requires high purity silicon at the outset.
Trace amounts of other materials can be added to the molten silicon to change its properties. The two common types are n-type doping in which a small amount of an element such as phosphorous or arsenic (with more outer electrons than silicon) is added to the mixture, creating some additional electrons that can move more freely than those in the undoped silicon. The alternative p-type doping involves adding an element with fewer outer electrons, such as boron or gallium. This leads to “holes” being created in the lower band of electron energy levels; these holes can behave like positive electrons, again moving freely. Both types of doping lead to increased conductivity in the silicon.
The key structure for the solar cell is a p-n junction. This is created by taking a single crystal of silicon that has been doped to produce one of these types and then using a technique to change the type of doping in a part of the crystal so that a junction between the two types of doping is created across adjacent layers of the material; the same effect can be achieved by carefully growing a layer of alternative doping on top of the original. Where the two types meet there is a concentration gradient within the bands of the semiconductor because one region naturally holds more electrons while the other has more holes. The natural tendency in such a situation is for electrons to move into the region where there is a surplus of holes and vice versa. The result is that a charge gradient is set up across the junction, which eventually counterbalances the concentration gradient and prevents more particles from moving, as shown in Fig. 9.1.
It is this in-built charge gradient that makes the solar cell function. When the solar cell material absorbs a photon of light, this promotes an electron from the lower energy band to the upper energy band, across the bandgap in the semiconductor. The electron is now free to move, and since it has a negative charge it travels toward the positively charged side of the junction while the positively charged hole left behind travels in the opposite direction. The moving charges will create a current if the cell is connected to an external circuit.
The conventional structure for a solar cell is planar. The cell is made of a thin layer of semiconductor, the top surface of which is doped with a suitable impurity to create the p-n junction within the surface layer of the material. The junction, with its built-in voltage gradient, will then capture electrons once they are generated by light absorption and sweep them away into an external circuit. In order to collect the current, electrical contacts must be formed onto the semiconductor. The rear of the cell can be covered with a planar collector since light does not have to pass through this surface, but on the front surface, where light is absorbed, the collector area must be minimized or it will interfere with light absorption. The front collector is usually made from narrow fingers of metal that allow the maximum amount of sunlight to strike the semiconductor surface. The semiconductor and its contacts will often be placed on a substrate to give it additional strength, and the top layer will be covered with a transparent protective coating. A schematic of a crystalline solar cell is shown in Fig. 9.2.
More advanced structures are possible. The front contacts can be buried, edgewise, into the surface of the cell to minimize the shadow effect of the contact, which reduces efficiency. More modern cells have also been developed that move both contacts to the back of the cell by allowing the front doping to be carried to the back of the semiconductor. This structure is more complex, but creates a more efficient cell by removing any shadowing from the front contacts.
The overall efficiency of a solar cell depends on effective light absorption. This can be improved by making the front surface of the cell nonreflecting and by making the back reflective so that any light reaching the back layer is reflected back into the semiconductor. All these improvements further increase the overall complexity of the cell. Such developments mean that the number of stages in the fabrication of a solar cell can increase to perhaps 15 from a more typical 6–8 in the simplest devices. However, decreasing production costs and increasing efficiency are making such developments worthwhile.
The first solar cell ever produced was made using n-type silicon as the substrate, but over recent years most silicon solar cells have been made from p-type silicon onto which the n-type layer is added to create the p-n junction. More recently, interest has again risen in n-type silicon as the main substrate upon which the solar cell is built. There appear to be advantages because n-type silicon is often of higher quality and performs better in mass production environments. These n-type solar cells have shown high efficiency both in laboratory tests and in solar cell modules.
While laboratory tests have been able to achieve around 25% efficiency in a silicon solar cell, the theoretical maximum efficiency for this type of cell is 29.4%. Techniques such as better light management to improve absorption may be able to push the efficiency higher. However, to go much higher than today’s best will require multilayer or tandem cells in which layers of different semiconductors are used to absorb more of the solar spectrum.
Thin Film Solar Cells
The main competition for crystalline silicon solar cells comes from a variety of thin film solar cells. Manufacturing solar cells from thin film materials is much easier than making them from solid semiconductor substrates because they can either be produced by vapor deposition techniques, by sputtering, or in some cases by printing. The film is deposited onto a substrate that is often glass, but may also be plastic. The nature of the manufacturing technique means that much larger area cells can be made than are possible with crystalline silicon. This too has economic advantages because fewer cells have to be interconnected, making module production simpler. In addition, it is possible to produce thin film solar cells on flexible materials, including fabrics that might be used for clothing.
There are several thin film semiconductors that have been developed for solar cells. Amorphous silicon is a form of silicon with no crystal structure that is produced using thin film techniques. Its efficiency is low compared to the crystalline material, but it is much cheaper to manufacture and it continues to command a share of the thin film market, particularly for small solar cells for electronic devices.
The main alternative is CdTe. This too is cheaper than crystalline silicon, and solar cells made from it can be deposited on a variety of substrates. It is also significantly more efficient than amorphous silicon. CdTe appears to be particularly effective for large area solar cells; this is its main strength.
Another material that has been developed for thin film solar cells is copper indium gallium diselenide (CuInGaSe2, sometimes known as CIGS). By varying the amounts of copper, indium, and gallium, the bandgap of this material can be changed, and this can be used to tailor the thin film for a specific application. However, production costs appear to be higher than for CdTe. A variant of CIGS is copper indium diselenide. This is also being developed for thin film applications.
The manufacture of a typical thin film CdTe solar cell starts with a perfectly clean glass substrate.3 Onto this is first sprayed a Transparent Conducting Oxide (often referred to as the TCO) layer. The conducting layer can be made from one of a variety of materials, including zinc oxide, indium oxide, indium tin oxide (ITO), and strontium oxide. The conducting layer allows light passing through the glass to reach the semiconductor below, while also providing a top contact to extract power from the cell. On top of the conducting layer, the next layer to be deposited is one of n-type cadmium sulfide that forms the first part of the p-n junction. This is usually referred to as the window layer because light has to pass through it to reach the semiconductor below. Then a layer of p-type CdTe is added. Finally, a conducting back contact is deposited onto the CdTe layer. This is usually a metallic layer such as nickel–aluminum. Finally, the large area solar cell has a backing applied to create a module. A cross section of a CdTe solar cell is shown in Fig. 9.3.
Although there will be variations in the techniques for depositing the various layers, and in the composition of the layers themselves, the process for manufacturing thin film cells from any suitable semiconductor will involve similar stages. Meanwhile, research continues to optimize the various layers, particularly the window layer, in order to achieve the highest efficiency possible.
One drawback of very large-scale deployment of CdTe or CuInGaSe2 is the availability of the elemental constituents. Tellurium is a by-product of copper refining. While it is readily available today, the U.S. Department of Energy has predicted a shortfall by 2025. Indium is also available in limited quantities via the refining of a variety of metals such as zinc, copper, iron, and lead, and is in demand for liquid crystal displays and a variety of coatings. (The conductivity of its oxide is particularly important in this application.) The main producer is China. Availability is not considered an issue today, but could become so in the future. In contrast, silicon is one of the most abundant elements on the planet.
Multilayer Solar Cells
One way of improving the efficiency of electricity production based on solar cells is to use multiple solar cells, with each one capable of absorbing a part of the solar spectrum. In this way more of the energy from the Sun can be utilized. In order for this to be effective, the different solar cells must operate in tandem; these devices are sometimes called tandem cells. The components of a tandem cell are normally built one on top of the next.
As already noted, the absorption of light by a solar cell is controlled by the bandgap of the semiconductor from which it is made. Each semiconductor absorbs photons of light with energy greater than its bandgap, but remains transparent to those photons with energy lower than its bandwidth. These simply pass through the cell.
A tandem cell is constructed with the semiconductor solar cell with the highest bandgap at the top. When light reaches the cell, the highest energy photons are captured by this solar cell, but lower energy photons pass through. Below this cell is a second one made from a semiconductor with a smaller bandgap. This cell absorbs photons that have passed through the first cell. It is possible to have a third layer to capture even more energy, absorbing photons that have passed through both the first and the second cells.
Typical cells of this type might use gallium indium phosphide to capture the highest energy photons at the blue end of the visible spectrum. Gallium phosphide could then be used to capture light photons from the yellow and green parts of the spectrum while a silicon cell would be used to capture low energy visible light photons and infrared photons.
In theory, a multilayer solar cell with an infinite number of layers could achieve close to 87% energy conversion efficiency. In practice it is unlikely that more than two or three layers will be used. Such cells have shown a laboratory efficiency of 43%. Commercial two-layer cells have shown 30% efficiency under normal sunlight conditions and up to 40% when exposed to concentrated sunlight.
The multilayer technique has also been applied to create amorphous silicon solar cells with improved efficiency. One process involves building a cell that is composed of a mixture of layers of amorphous silicon and microcrystalline silicon. This helps with light absorption and therefore improves efficiency. A more cost-effective alternative is to build a multi-junction solar cell comprising a series of layers of amorphous silicon. Very thin amorphous silicon solar cells appear to suffer less degradation over time, however, the thin layer does not absorb all of the light and a significant part passes through. By using multiple layers, more light can be captured while retaining the resistance to degradation. It is also possible to modify the properties of some of the amorphous silicon layers with doping to adjust the bandgap to capture more infrared light.
Enhanced Light Absorption Techniques
Effective absorption of light is key to the efficiency of a solar cell. If all the light is not absorbed then energy is being wasted. Although this can be an issue with all types of solar cells, it is most often discussed in relation to silicon cells.
One way of ensuring that most of the light is absorbed in a silicon solar cell is to use a thick layer of silicon. However, this has an impact on the cost of the cell because more high purity silicon means higher material costs. The trend is therefore toward much thinner cells. If the layer of silicon is too thin, then light will pass all the way through it without being absorbed.
Designers have adopted several strategies to counter this. One is to make the light bounce backwards and forwards within the layer of silicon until the photons have been absorbed. To achieve this, the back surface of the silicon must be made highly reflective so that any light reaching it is reflected back into the cell. Meanwhile, the top surface of the cell is coated with a layer that causes light to be reflected internally again rather than escaping.
Preventing external reflection before light enters the cell is important too. The top outer surface of a silicon solar cell is normally relatively reflective so that around 20–30% of the light that strikes it from outside is reflected rather than allowed to pass into it. Another advanced technique involves etching the surface in such a way that a structure of tiny needles around 10 microns in height and 1 micron or less in diameter is produced. Known as black silicon, this surface treatment can reduce the reflectivity so that only around 5% of the incident light is lost through reflection.
Another advanced technique that can potentially increase the efficiency of all silicon solar cells further is called upconversion. As with all semiconductors, silicon can only absorb photons that have an energy greater than its bandgap. All those longer wavelength photons with a lower energy cannot be absorbed. In upconversion, some of these lower energy photons are absorbed by a special dye. The dyes that are used for upconversion have the ability to absorb two or more low energy photons and then release one photon at a higher energy. So long as this new photon has an energy larger than the semiconductor bandgap, it can now be absorbed. This technique is still at the laboratory stage, but it promises the ability to utilize a wider range of photons. It could raise the maximum theoretical efficiency of a single layer solar cell from around 30% to 50%. The technique could prove of particular value with amorphous silicon solar cells.
Concentrating Solar Cells
The concentrating solar cell offers a slightly different approach to the capture of solar energy using solar cell technology. While traditional solar cells are large area energy capture devices that utilize sunlight at the intensity at which it arrives from the Sun, as well as capturing some diffuse radiation, the concentrating solar cell adopts the approach of concentrating solar thermal power plants.
Concentrating solar systems rely primarily on direct irradiation from the Sun. They cannot concentrate diffuse radiation. This means that most must employ some form of tracking system if they are to be effective. In addition, they are only cost-effective if they are deployed in regions with high direct insolation levels.
A concentrating solar cell uses a cheap concentrating or focusing device to capture solar energy over a wide area and then focuses it onto a small area solar cell. For this to be effective, it requires first that the large area concentrating system be cheaper than an equivalent large area planar solar cell module. Secondly, it requires a solar cell that can absorb the highly concentrated solar energy effectively. Since this solar cell is tiny compared to normal large area planar cells, it is cost-effective to use more complex and expensive devices. Some of these, such as tandem cells, have a higher efficiency than traditional planar devices.
Concentrating solar cells have a technical advantage over planar cells because the efficiency of a solar cell depends on the intensity of the light that is incident upon it. Even a simple planar cell will show higher efficiency if the solar energy is concentrated. More complex multilayer solar cells are capable of even higher efficiency, as noted above. The best efficiency so far demonstrated with a multilayer solar cell is around 45%, and practical modules are expected to be able to deliver DC current at around 36% efficiency. This is twice the efficiency of the best planar solar cells operating under normal light conditions.
How a concentrating solar cell is deployed, and the type of cell used, depends on the concentration level being achieved with the optical system. Low concentration systems use simple, cheap, plastic lenses to generate a solar intensity of between 2 and 100 Suns. The concentration of the solar energy focuses the heat energy from the Sun as well as the light energy, which leads to a significant heat buildup in the solar cell onto which the energy is directed. For low concentration devices, normal silicon solar cells can be used without the need for active cooling, although the design of the system may involve passive cooling of the cells. In addition, such low concentration systems can be deployed without the need to track the Sun across the sky, yet they are still capable of up to 35% efficiency.
For higher concentration levels, both cooling and active tracking are needed. This increases the complexity of the system and increases the overall cost, but can result in greater efficiency. Medium concentration systems that concentrate solar energy by 100–300 times can also use silicon solar cells, but they may exploit alternatives such as gallium arsenide cells or multi-junction solar cells. With plastic lenses and efficiency of up to 40%, this type of system could be cost-effective in a range of applications, including rooftops.
High concentration solar photovoltaic devices usually use similar technology to that used in solar thermal plants, particularly the dish-type system. With a parabolic dish concentrator, a concentration ratio of 1000 times can be achieved. The solar cells and electronic systems in these devices need to be cooled actively, and need to be capable of withstanding relatively high temperatures. The solar cells used are likely to be high-efficiency multilayer cells that, while much more expensive than a simple silicon cell, can achieve significantly higher efficiency. Production cells can now reach 44% efficiency.
An advanced form of concentrator that is still at the development stage is a device called a luminescent concentrator. This comprises a transparent plastic plate that incorporates a luminescent dye compound5 or, alternatively, the luminescent material may be contained in a thin film that coats the planar surfaces of the plate. When this plate is exposed to sunlight, the luminescent dye absorbs sunlight and then re-emits its own luminescent photons. The collector is designed in such a way that the re-emitted fluorescent light is channeled within the transparent plate in much the same way as laser light is trapped inside a fiber optic cable. In the same way as light emerges from the ends of a fiber optic cable, so the fluorescent light emerges from the edges of the plate. Solar cells placed around the edges capture this light to produce electricity. The fluorescent plate collector has the advantage over more traditional concentrators in that it can capture diffuse as well as direct radiation.
Third-Generation Solar Cells
When solar cells are divided into generations, silicon and other single crystal-based solar cells are considered to be the first generation. Continuing on from these, thin film solar cells may be thought of as the second generation. There is now a third-generation made up of solar cells that use either organic semiconductors or dye-sensitized semiconducting materials to capture light and convert it into electricity. These materials are nowhere near as efficient as the earlier generations of materials, and probably never will be. Their advantage is that they can be manufactured extremely cheaply and produced on a diverse range of different substrates; this opens up the possibility of their use in niche products from architectural elements to clothing and consumer electronic devices.
There are a number of organic polymer semiconductors that have been developed in recent years. These can be used to produce light-emitting diodes and printed transistors. Development of small electronic devices using these materials has been successful, but for large-scale energy capture there are difficulties to be overcome. The main obstacle is that an organic polymer semiconductor is a poor conductor of electricity. This means that when a photon is absorbed by the material, generating a free electron and a hole, the two do not get swept away from one another as they would in a material such as silicon, but stay in the same place. This leads to rapid recombination of electron and hole, and the energy is lost as heat.
To overcome this, the organic semiconductor must be combined with a second material that preferentially captures the electron or hole (or both) and allows them to separate before they can recombine. In approaching this issue, scientists have been studying photosynthesis, where a similar process is achieved in organic materials. Current efficiency of organic semiconductors is around 5% at best. The near-term target is to achieve 10% by using advanced designs.
Dye-sensitized solar cells also exploit principles found in plants and photosynthesis. In this case it is a dye that absorbs the photon of light. The cell is created by absorbing the dye onto tiny particles of a semiconductor: titanium oxide. A layer of these particles is then coated onto one of the solar cell electrodes. Above the coated electrode is a conducting electrolyte that is capped with a second transparent electrode and a glass cover. This is shown schematically in Fig. 9.4. When a photon is absorbed by the dye, the electron created is captured at the dye–titanium dioxide interface, while the hole travels through the conducting electrolyte, thus ensuring that they separate before they can recombine. Efficiency is generally higher than for organic semiconductors, and can reach 13% in actual devices.
Both organic and dye-sensitized solar cells can be printed, so manufacturing is cheap. They are also capable of operating in low-light situations. However, they are not as stable as more traditional devices, and can be damaged by ultraviolet light.