Solar cells are made from a layer of semiconducting material, such as silicon, which has been doped with small traces of other elements in two or more layers. Light enters the cell and gives electrons enough energy to escape from the semiconductor and flow through an electrical circuit. This flow of electrons is electricity.
How do solar cells work?
The Short Answer:
Solar cells convert sunlight directly into electricity with no moving parts. These cells are grouped together in solar modules, also known as solar panels. Solar panels can be used to provide power in remote areas, or to run in parallel with the grid supply.
The Long Answer:
Although there are many different types of solar cell available, almost all of them work by using two layers of semi conductor material with a potential difference across them to separate charge when exposed to light. This is especially the case for commercially available solar cells. While there are many types of experimental solar cells, we will concentrate on commercially available technologies.
Semiconductor materials are those which are neither insulators nor conductors of electric charge but rather can behave as either insulators or conductors depending on the conditions to which they are exposed. This allows them to act as electrically activated switches (ie transistors), the basis of all modern microelectronics.
A semiconductor's properties can be altered by doping it with other substances. This involves adding small amounts of a ‘dopant' material into the semiconductor.
A solar cell, which is essentially a large area diode, can be made by putting two layers of doped semiconductor together. One layer is doped with group 5 atoms, such as Phosphorus, and the other is doped with group 3 atoms, such as Boron.
Group 5 atoms have one more electron in their outer shell than group 4 elements such as silicon. Like all atoms, they have the same number of protons as electrons. This extra electron (which is negatively charged as are all electrons) is not tightly bound into the crystal lattice of the silicon atoms and can be easily separated from its parent atom with the addition of only a little energy. The thermal energy at room temperature is easily enough to do this, meaning that the electron is essentially free to move from the moment the phosphorus atom is introduced into the silicon crystal.
Group 3 atoms have one less electron than silicon so when they are introduced into the crystal, there is a ‘hole' where one extra electron should be. Electrons from neighbouring atoms can move into this hole, leaving a hole where they used to be which is in turn filled by another neighbouring electron. In this way, the hole can move through the crystal lattice. Although the hole carries no charge, the electrons which are moving into it represent a negative charge moving in the opposite direction to the hole. Therefore, we can think of the hole as a moving positive charge.
Click here for an animation of a hole moving through a silicon crystal
When the two layers of semiconductor are in contact with each other, the extra electrons in the phosphorus doped layer move into the boron doped layer to fill the holes. The phosphorus doped layer has now lost its extra electrons to the boron doped layer, but retains its extra protons which are tightly bound to the phosphorus nuclei. This gives the layer a net positive charge. We call this positively charged layer, p-type material. Similarly, the boron doped layer has accepted extra electrons but has gained no new protons. It hence has a net negative charge and is called n-type material. The area where the two layers meet is called the p-n junction , or simply the junction.
Click here for an animation of electrons and holes moving to create a p-n junction
When semiconductor material, regardless of its doping, is exposed to light of sufficient energy, the light will give some electrons in the crystal enough energy to escape from the lattice and move around. This leaves a hole behind that also moves. If this occurs in a uniform piece of material with no electric field, then both the electron and the hole will move randomly until they eventually recombine with each other, cancelling each other out. However, if we were to have a piece of material consisting of an n-type and p-type layer, the positive charge of the p-type layer will attract the negatively charged electrons and the negative charge of the n-type layer will attract the positively charged holes. This is the purpose of having the doped layers in a solar cell, to separate the positive and negative charge.
If kept in the dark, the only energy availble to create electrons and holes is the thermal energy available from the 'background' temperature. However, if we shine light on our device, the energy in the light photons can knock extra electrons free from the crystal. This creates a free electron and a hole where that electron used to be. In undoped semiconductor material, these electon-hole pairs would very quickly recombine, but when there is a p-n junction they are seperated before this can happen, with electrons being swept to the p side and holes swept to the n side. In the n side there are many more holes than electrons and vice versa in the p side. This means that there is no chance for the majority of the electons and holes to recombine.
All that is required now to extract energy from the device is to connect an external circuit from the n-type region to the p-type region. Electrons from the p-type layer, wanting to recombine with holes, will flow through the circuit to the n-type layer where they will recombine. This flow of electrons, which can also be seen as a flow of charge, is what we commonly call electricity.
Click here for an animation of electricty being generated in a solar cell
A solar cell then, is a piece of semiconductor made from a p-type layer and an n-type layer connected by an external circuit. The most common materials used for this purpose are silicon doped with boron and phosphorus, although there are many other combinations that are used. Some of these are mentioned below.
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What is the efficiency of a solar cell?
The 'efficiency' of a solar cell is the ratio of the energy coming out of the cell (as electricity) compared to the energy going into the cell (as sunlight).
The most efficient cells in the world now have efficiencies of approximately 40%. These cells are incredibly expensive to make and therefore are only used in situations where money is not an issue but the available area is, such as space applications and in some concentrator systems.
Most commercially available cells have efficiencies of around 15%, although the most efficient commercially available cells are as high as 23%. It is important to note that the efficiency of a solar module will be lower than the efficiency of the cells it is made up of. This is due to two main factors. Firstly, there are areas of the module which do not generate any power. These include the frame and any areas where the back sheet of the module is visible. For more information on the consequences of this, please see the section on mono-crystalline and multi-crystalline silicon modules below. The second reason is that there is additional wiring in a solar module to connect all the cells together and hence additional wiring losses.
Areas of Inefficiency
Unfortunately, when a solar cell is illuminated, some of the electrons and holes generated recombine before they can be separated by the p-n junction. The further they have to travel on average to reach the junction, the more likely they are to recombine. One way to limit this effect is to make the cells thinner so that this distance is decreased. This means however that there is less semiconductor material to absorb the light coming into it and therefore more chance of the light passing through the cell without creating any electron-hole pairs. Also, when manufacturing wafer based cells, making them too thin can lead to a sharp rise in the number of cells broken during production, increasing the overall production costs. Currently typical solar cells have thicknesses of around 300 microns (0.3 of a millimetre) although some manufacturers have managed to decrease this to as low as 180 microns (0.18mm).
Also, in order to extract the electrons from the p-type layer, a grid of metal contacts must be laid on its surface. This covers up part of the surface of the solar cell, blocking some light from entering and thereby reducing the efficiency of the cell. Typically, this ‘front contact' is laid on the cell by screen printing a metal paste onto the surface of the cell and then firing the cell and paste to melt the paste into a solid contact. This gives a moderately good electrical connection between the front surface and the front contact and is a very simple and therefore cheap process to implement. For this reason, it is used in the vast majority of commercial solar cells. The drawback of this method however is that the width of lines which can be reliably screen printed is limited to thicknesses of approximately 100 microns or greater. Since the grid lines can be no more than approximately one millimetre apart, this means that the front contact ends up covering approximately 10% of the cell's surface. This results in 10% less light entering the cell and hence 10% less power being produced .
There have been a number of techniques developed to reduce this loss. These include the laser buried contact cell developed at the University of NSW which used a laser to cut very fine grooves into the surface of the cell which were then filled with the metal contact. This method allowed the contact to maximise its surface area giving a good, low resistance contact, but minimise its area on the surface of the cell. Another method, developed at Stanford University was to move the ‘front contact' from the front of the cell to the back, thereby exposing the entire front surface of the cell to light. This was done by altering the shape of the p-type and n-type layers so that they wrapped around the cell, allowing both contacts to be made on the rear surface. Each of these methods however requires more complex manufacturing processes, which increase the cost of the cells .
Another area of inefficiency is light reflected from the surface of the cell, rather than absorbed within it. The two main methods used to reduce this loss are cell texturing and the use of an anti-reflective coating (ARC).
Cell texturing, as its name implies, creates certain textures on the surface of the cell which can help direct light which has been reflected into the cell. The most common texture is a pyramid shape which can be very effective at capturing reflected light.
When light hits the side of the pyramid it either enters the cell or is reflected. If it is reflected it bounces off at an angle and hits another pyramid where again it can either enter the cell or be reflected. This increases chances that a photon of light will enter the cell. These surface textures can be created by immersing the cell in a chemical solution at rhe beginning of production. By using the right chemical solution, temperature and length of immersion, the wafers can be reliably and repeatably textured to produce optimal pyramids. In the past, this has only been possible on mono-crystalline wafers, but at least one major manufacturer now uses this process on multi-crystalline wafers.
The other main method used for reducing reflection is the use of an ARC. This method uses the wave properties of light, by placing a thin layer of dielectric material over the surface of the cell which causes destructive interference of the reflected light, cancelling it out. This method is used on basically all solar cells. Sometimes multiple layers are even used, however if a single layer ARC is used in conjunction with surface texturing, the amount of reflected light can be reduced to almost zero. This is evidenced by the near black appearance of such cells. ARCs are also used on the lenses of some glasses to reduce reflection and improve visual clarity .
Another major efficiency loss is due to impurities in the cell. These can be foreign atoms or molecules in the crystal lattice (including the dopant atoms) or areas of imperfection in the crystal lattice. These areas of imperfection, known as crystal defects can occur throughout the wafer, but the area with the highest concentration is at the surface of the cell where the crystal terminates. The effect of all these impurities is to provide sites where electrons and holes can recombine more effectively, thereby reducing the number of charged particles available to create an electrical current. One of the main disadvantages of screen printed solar cells is that in order to get a decent electrical contact with this method, the area at the surface of the cell to which the front contact is attached must have a very high doping concentration. This means lots of impurities and hence a lot of losses in this area. This effect is possibly of greater significance than the shading losses associated with screen printed contacts. Defects, both at the surface and within the bulk of the cell, can be ‘passivated.' This is usually done by attaching hydrogen atoms to the defect sites. This can stop the defect from interfering with the electrons and holes and thereby raise the efficiency of the cell. One beneficial side effect of the most common ARC process is that it also bombards the cell with hydrogen atoms which passivate defect sites. Since the hydrogen atoms are very small, they can penetrate deep into the bulk of the cell and passivate defects far from the surface.
These are just some of the main areas of inefficiency that researchers and manufacturers target. For more detailed explanations of the above processes and many others that occur in solar cells, the following books are highly recommended:
Solar Cells by Martin Green gives a solid introduction to the inner workings of a solar cell as well as covering some of the areas in which researchers are focusing for future improvements.
Applied Photovoltaics by Wenham, Green, Watt and Corkish covers the basics of how a solar cell works before looking at applications of solar technology in various situations in detail. It covers the design of different types of solar power systems and the reasons such systems are designed the way they are.
Both books are available from the UNSW bookshop and can be purchased online.
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What are the different types of solar cells?
There are a number of different types of solar products available. With one or two minor exceptions, these are all based on the p-n junction semiconductor concept described above. The key differences lie in the type of semiconductor used, the method of construction and the way the cells are assembled into modules, or panels.
Crystalline Silicon PV
Most solar panels available, approximately 90 % are based on crystalline silicon. There are two main types of crystalline silicon used in the manufacture of solar cells. These are mono-crystalline (mono-Si) and multi-crystalline (multi-Si), sometimes also known as polycrystalline silicon. Both of these types use a silicon wafer with doped p-type and n-type regions. The difference is in the silicon material used.
Mono-Si wafers are cut from one large silicon crystal. This single crystal is grown in an ingot which is extracted from a vat of molten silicon. A seed crystal with the desired crystal orientation is placed in the molten silicon and slowly turned and pulled from the vat. As it is pulled out, the diameter of the ingot grows (as controlled by the speed at which it is rotated). It can take about one day to pull a complete ingot of approximately one meter, with the silicon melt kept at above 1400ºC the whole time. The ingot is then sliced into wafers which can be made into solar cells. Since the entire wafer is a single crystal, there are very few crystal defects and hence high efficiencies can be achieved. Furthermore, until recently, these were the only wafers which could be reliably textured with light trapping pyramids. The process of extracting a single crystal from a vat of molten silicon is both time consuming and energy intensive, meaning that solar cells made from mono-Si cells are more expensive and have a larger embodied energy than those made with multi crystalline wafers.
Multi-crystalline wafers are made by die-casting a block of silicon. The molten silicon is poured into a cast and then cooled slowly until it solidifies. As the silicon cools, crystals start to form in many locations and grow outwards. This means the final block is made up of a large number of separate crystals in one solid block. Where the crystals meet, there are a large number of crystal defects due to the different orientation of the crystal grains. These areas are known as the grain boundaries. This lower quality, lower cost material can be improved markedly however by passivating the grain boundaries with atomic hydrogen as explained above. That combined with the ability of some manufacturers to texture the multi-Si surface means that cell efficiencies of multi-Si are creeping closer to those of mono-Si. One other major advantage of multi-Si is that since it is cast in a rectangular (or square) block, the wafers are also rectangular (or square) in shape. Mono-Si wafers when first cut however are round. In order to fit more cells into a module, the round wafers are trimmed to a ‘pseudo-square' shape . Cutting them to a square shape however is too expensive as too much of the wafer needs to be thrown away. This means that solar panels made from mono-Si cells have the distinctive diamond shape in between the corners of each cell. This area is area which does not convert sunlight to electricity, thereby reducing the module efficiency for the panel. What does all this mean? The end effect is that although mono-Si cells are more efficient, the efficiency of some multi-Si modules now exceeds all but the most efficient mono-Si modules, including the laser groove buried contact cells (also known as Saturn Cells). It is the module efficiency that determines how much power can be extracted from a given amount of roof or array area.
Mono-Si cells generally have a higher efficiency than multi-Si but are also generally more expensive. Recently there have been efficiency improvements in multi-Si cells with some leading manufacturers now producing multi-Si modules with higher efficiencies than most mono-Si modules. Between them, these two technologies make up over 90% of global photovoltaic production. This market domination shows no signs of easing in the near term, despite continued speculation that thin film technologies would take over.
Amorphus Thin Film Silicon PV
Like crystalline silicon, amorphus silicon (a-Si) is another technology that has been in commercial production for several decades. It uses only a very thin layer of low quality silicon and making it a lower cost (per unit area), lower efficiency technology than both types of crystalline silicon. Costs per unit of generation capacity, generally referred to as dollars per watt ($/W), are generally similar to crystalline silicon, however due to its lower efficiency much larger areas, approximately double those required for crystalline silicon, are required. This leads to higher installed costs as more associated equipment is required and installation times are longer. Amorphus silicon's main advantage over crystalline technologies is its thermal performance, making it the best choice for use in many building integrated installations, where the PV modules form an integral part of the building, replacing traditional building materials.
Other Thin Film Technologies
There are a number of other ‘thin film' technologies which use a thin layer of some semi-conductor material in place of the traditional silicon wafers. The aim of all these technologies is to reduce the cost of the module by using less material. Some also have relatively high efficiencies compared to amorphus silicon, however each such technology has its own drawbacks which have thus far prevented large scale use, particularly in Australia. These include the use of the toxic metal cadmium and the use of extremely rare and hence expensive elements such as Indium.
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