Photovoltaic cells are solid state devices that convert light directly into electricity. Photovoltaic literally means light electricity. These devices can be commonly found providing power for small scale devices such as calculators, watches, and small radios. However, they are not limited to small scale systems. They are also used to power satellites, communications equipment, houses and many other things, especially in remote locations where a power grid is not readily available. In isolated locations the only power attainable comes from the sun. The sun shines approximately 1000 watts of energy per square meter of the planets surface, which if harnessed could power any city.
The commercial development of the photovoltaic cell took more then a hundred years to begin. A french physicist Edmond Becquerel first described the photovoltaic effect in 1839. At the age of 19 Becquerel found that certain materials when exposed to light produced small measurable currents. Henrich Hertz also studied the effect in solids in the 1870’s and he managed to produce photoelectric cells with an efficiency of about 1%. In the 1940’s the new Czochralski process made generating highly pure crystalline silicon possible and furthermore made commercializing photovoltaic cells an option. Development really started however, in 1954 when Bell Laboratories used the Czochralski process to produce a 4% efficient crystalline silicon cell, which was essentially for application in space. This was the beginning of photovoltaic cells.
Photovoltaic cells generally consist of five layers. Two of these layers are semiconductors and the remaining three are merely for protection and to aid absorption of light. The top layer is generally a glass cover for protection against the elements since photovoltaic cells are used outside. Underneath this is an anti-reflective coating. Since most photovoltaic cells are made of silicon and silicon is reflective this prevents the photons from merely being bounced off the cell and allows the highest amount of absorption possible. These two layers are secured with a transparent adhesive. Transparent, obviously, so the photons can pass through it. Surrounding the entire cell is a metallic grid on top and a metal contact on bottom.
The two layers of semiconductor are the most important. Semiconductors are materials that have electrical conductivity intermediate between the high conductivity of metals and the low conductivity of insulators. Conductivity is decided by how many free electrons are available to carry charge, and in turn how tightly electrons are bound to the parent atom. If they are tightly bound electrons can not move and there is no current and the conductivity is low, and vice versa for loosely bound electrons. Semiconductors, such as silicon are characterized by having four electrons in their outer atomic shell. In the semiconductor crystal each atom forms electron pair bonds with four other atoms. Each of these contributes an additional electron, resulting in a total of eight electrons which completely fills the outer atomic shell. Hence, by sharing each atom can fill its outer shell. However this leaves no free electrons for conduction in a pure semiconductor.
A very small amount of conduction can occur because the outer electrons are not tightly bound and can be freed by thermal energy. However for a photovoltaic cell a high amount of conduction is required. To obtain this a semiconductor can be doped. This means that small amounts of impurities are added to the material, usually by diffusion, which yield extra charge carriers and so, raise the conductivity. These impurities are usually elements with a different number of electrons in the outer atomic shell which when put in the semiconductor will destroy the balance of eight electrons in each atoms outer shell. Either an additional electron will be present or a shortage of one. If there is an additional electron this is called an n-type semiconductor, where n stands for negative since the extra charge carrier is a negatively charged electron. If there is a shortage of an electron an imbalance is created where there is a place in the crystal for an electron but no electron to fill it. This is commonly called a hole. This is a p-type semiconductor, where p stands for positive since the hole acts like a positive charge carrier.
Figure 1: n-type semiconductor (left); p-type semiconductor (right)
These two types of semiconductors are grown side by side creating what is known as a pn-junction. It is this pn -junction that creates the basis for the photovoltaic cell. When a p-type and an n-type semiconductor are placed side by side like this the extra electrons on the n side are attracted to the holes on the p side. However the holes closest to the junction are filled first and a barrier is formed preventing electrons from crossing the junction to fill the holes. The system is now electrically imbalanced and equilibrium has been reached so an electric field is set up. The electric field acts as a diode, allowing only electrons from the p side to the n side, not in the reverse direction. To create a current we need a constant flow of electrons from the p side to the n side. This is where light comes into the picture. Photons hit the cell and are absorbed. The energy of the photons can liberate electrons and in turn create a hole. If this occurs within the range of the electric field the electron will be sent to the n side and the hole to the p side. However, while obeying the electric field, electrical neutrality is upset even further. By providing an external path for the current the electrons can be returned to their original side, where they unite with the holes sent there by the electrical field. This electron flow provides the current and the cells electric field provides the voltage needed to produce power.
Figure 2: Photovoltaic cell circuit.
There are a few problems with energy loss in these cells however. The efficiency of a photovoltaic cells absorption is only about 15-25 % of the light energy incident upon it. This is because the incident light is made up of a spectrum of different wavelengths, and not all these wavelengths contain enough energy to create an electron-hole pair. Photons of these wavelengths simply pass through the cell. Also, some of these photons have too much energy. Only a certain amount of energy is required to create an electron -hole pair. This is called the band gap energy and varies depending on the material. Near infra-red photons with wavelengths greater then 1.13 micrometers have energies less then band gap and can not produce electron -hole pairs. A photon with to much energy will create an electron -hole pair but the surplus energy will be lost unless it is enough to create 2 or more electron hole pairs. Photons in the near infra-red with wavelengths between 0.7 and 1.1 micrometers have energies slightly greater then the band gap and so most of there energy can be converted into electricity. Considering just these losses the upper limit of efficiency is about 45 %. As these energy losses became apparent materials with low band gaps were chosen so that more of the energy of the photons could be used however it as found that the band gap determines the strength of our electric field and hence our voltage and if it is allowed to drop too much what is made up in extra current is lost by having a smaller voltage. The optimum band gap is defined to be about 1.4 eV.
Other losses occur as well. The contact on top of the cell is a grid and so the conduction is not as good as it would be with a full metal contact covering the whole surface. However if a full contact is used the photons can not pass through it to the semiconductor and there is no current. Putting the contacts on the sides of the cell increases the distance the electrons have to travel to reach the contacts and since we are using a semiconductor the internal resistance is fairly high creating more losses. A grid is used to decrease electron travel distance while only covering a small portion of the absorption area.
Efficiency has been improved over the years however. Cells containing two different materials allow for two band gaps. By placing the higher energy band gap on top of the other within the cell lower energy photons that are passing through can be absorbed by the second p-n junction. These have proven to be very efficient, and are called multi-junction cells.
Another reason why photovoltaic cells are not being utilized as much as they could be is due to the fact that these systems are much more expensive to install then conventional power sources for homes and buildings. Also the semi-conductor, generally silicon, is assembled manually , and silicon purification is a difficult process and a lot is wasted during it. In addition. Operating silicon requires a cooling system, because performance decreases as temperature increases. However, analysts perceive a bright future for photovoltaic technology. The world is becoming increasingly aware of the environmental concerns of other power sources.
Future possibilities for photovoltaic cells are unlimited. With mass production to decrease costs use will surely increase. They are being increasingly used in developing countries as a source of low maintenance power and there are proposed schemes of orbiting large modules around the earth to replace nuclear power. The only real obstacle for this technology to overcome is cost. There are already many adaptations for these cells but they are not implemented since they are not priced competitively so the future of these cells depends upon research since that will raise efficiencies and bring down costs. Photovoltaic cells are just beginning to become a recognized power source for society. References:
1. Serway, Ramond A, Physics for Scientists & Engineers with Modern Physics, 4th Edition, Saunders college Publishing, 1996
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6. Darnell.com Copyright (c), Silicon Photovoltaic Cells, Photodiodes and Phototransistors, Darnell group Inc.