Micro Energy Systems

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.

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.

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.

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.

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. 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 the beginning of production. By using the right chemical solution, temperature and length of immersion, the wafers can be reliably and repeatedly 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.