What is a Solar Cell?
One of the devices that convert light energy into electrical energy is solar cells. The first solar cell was developed in 1954 by Daryl Chapin, Calvin Souther Fuller and Gerald Pearson at Bell Laboratories. Today, solar cells are used to obtain energy in many areas.
Solar cells are used as the main energy source, especially in spacecraft and satellites orbiting the Earth. In addition to having high power / weight ratios, these devices also allow the extension of duty time without the need for any changes in the spacecraft.
How Solar Cell Works?
The solar cell produces electricity with a photovoltaic effect. The photovoltaic effect is called the physical event where the sun’s rays are converted into electricity. Photons are formed when sunlight hits the semiconductor surface, releasing electrons inside the atom. Photons contain a different amount of energy for each wavelength in the solar radiation spectrum. When the photons land on the solar cell, some are reflected exactly, some are absorbed by the solar cell, and some pass through the solar cell. Photons absorbed by the solar cell produce electricity.
Photons in sunlight are absorbed by semiconductor materials in the cell. Then the electrons stimulated by the photons emit heat energy directly or move in the cell until an electro reaches it. During this movement, electric current is created.
It is possible to obtain electric currents that can be used in daily life with the devices formed by the combination of many solar cells. The direct current produced by solar cells can also be converted into alternating current. French physicist Edmond Beckerel experimentally observed the photovoltaic event in 1839. The first solar cell was developed in 1873. The efficiency of the device produced by Charles Fritts by covering the semiconductor selenium with a thin layer of gold was only around 1%.
Solar cells are named according to the semiconductor material they contain. These materials are designed to absorb the light that falls on them most efficiently, depending on where the cell is used. For example, sunglasses designed for use on earth are different from those designed for use in space. Because the Earth’s atmosphere prevents some of the light coming to Earth from reaching the earth. Thanks to the research and development activities that spanned many years, the efficiency of the sunglasses increased over time while the production costs decreased. Today, the efficiency of silicon sunglasses exceeds 25%, and the efficiency of perovskite solar cells exceeds 20%.
What are Solar Cell Types?
Solar cell types consist of 4 main technologies. These can be listed as crystal structure technology, thin film technology, combined technology and nanotechnology. All solar cell types are listed below.
1. Inorganic Solar Cells
Single layer inorganic solar cell consists of inorganic semiconductor, such as silicon, one of which is semiconductor and placed between 2 metal electrodes with different electrochemical potential. The efficiency of the single layer inorganic solar cells is very low.
2. Two Layer Inorganic Solar Cells
The 2-layer inorganic solar cell is made using 2 semiconductors, n-type and p-type. In these cells, charge separation takes place close to the border between n-type and p-type semiconductors. Inorganic cells are very stable solar cells in terms of chemistry and heat. Today, these solar cells can provide up to 30% efficiency.
3. Single Crystal Silicon Solar Cells
Single crystal silicon solar cell is frequently used in solar panel construction. The cost of single crystalline silicon material is quite high. Therefore, the multi-crystalline solar cell is used more intensively. There are many reasons why silicon material is widely used in solar cell making. These are due to the fact that silicon can maintain its electrical, optical and structural properties for a long time.
Pure single crystal silicon technology is quite expensive and difficult. The most silicon element in the world is found after oxygen. Sand and quartz forms of this element are the most common. It is not preferred because the sand purity structure is very low. But about 90% of the quartz substance consists of silicon. Quartz is processed through many processes and silica is obtained with a purity of 99%. Later, silicon is also obtained from silica.
After these stages, silicon is purified to obtain a semiconductor silicon crystalline silicon. The processes up to the stage of obtaining multi-crystalline silicon are quite costly.
To obtain pure polycrystalline silicon with semiconductor properties, the polycrystalline silicon is melted and enlarged again. The cores are drawn from the molten silicon bath at a very low speed. Thus, the growth of thin single crystal layers is provided.
Most commercially used silicon (Si) solar cells are produced from boron doped single crystal slices (400 micron thick) by the Czochralski (CZ) process. Cage defects do not occur in solar cells produced with CZ process.
Crystalline silicon cells make up about 80% of the solar cell market. The yield of this solar cell varies from 15% to 23%.
4. Multi Crystalline Silicon Solar Cells
The multi-crystalline silicon solar cell material is electrically, optically and structurally the same. The dimensions of the veins are directly proportional to their quality. The discontinuity between the vessels plays a preventive role in the transmission of electrical charge carriers.
The production of multi-crystalline silicon cells is easier and less costly. Bulk method is used in the production of polycrystalline silicon material. The production phase is briefly as follows. First of all, most of the procedures for obtaining single crystalline silicon are done exactly. Molten semiconductor silicon is poured into molds and expected to cool. The blocks obtained from the molds are cut square. The solar cell produced by this method is less efficient. But its cost is quite low. Multi-crystalline silicon (pc-Si) solar cell efficiency varies between 12-15%.
5. Thin Film Solar Cells
Thin film solar cell consists of super thin semiconductor layers placed on top of each other. Thin film solar cell can be made from a wide variety of materials. The commercially used thin film solar cell is made of amorphous silicon. Apart from that, very crystalline copper indium diseleneid and cadmium telluride are also used in its construction.
Different precipitation methods are used in thin film cell technology. These methods are quite cheap. In addition, with this method, 2 × 2 inch solar cells can be obtained. The layers are deposited on a low cost glass or plastic based material.
Normally single crystalline silicon is designed individually interconnected within the solar module, while thin-film devices can be made as a single unit. Semiconductor material and back electrical contacts are added with non-reflective grabbing and conductive oxide layers.
Thin film solar cell has an efficiency of 8-12%.
6. Amorphous Silicon Solar Cells
Atoms of amorphous solid materials such as glass are not arranged in a certain order. Materials like this do not form a truly crystalline structure. It also contains numerous structural and connection errors.
The electrical property of amorphous silicon was previously described as an insulator. However, in the following years, it is thought that amorphous silicon can be used in photovoltaic batteries. Today, amorphous silicon is widely used in low power devices. Carbon, germanium, nitrogen and tin and amorphous silicon alloys are used to develop highly functional devices.
Amorphous silicon solar cell shows more than 13% efficiency in the laboratory environment. Thin film solar cell made with gallium arsenite shows more than 24% efficiency.
7. Multi-Crystalline Thin Film Solar Cells
The multi-crystalline thin-film solar cell consists of very small crystal particles of semiconductor materials. Materials used in this type of solar cells have different properties than silicon. In these cells, an electric field is created more easily with the interface between 2 different semiconductor materials.
The multi-crystalline thin solar cell has a top layer thinner than 0.1 micron called the window. The function of the window layer is to absorb high energy radiation energy.
In order for it to have sufficient tape gap, this layer must be thin enough.
8. Thin Film Kalgonit Solar Cells
In 1960, CuxS-CdS, CuxSe-CdSe and CuxTe-CdTe thin film solar cell cells were developed. The production of these solar cells is quite simple. CdS, CdSe and CdTe films are produced by chemical precipitation process.
CuxS, CuxSe and CuxTe layers are produced by dipping in CdC solution for 1-2 minutes together with CdS, CdSe and CdTe films. These 3 types of solar cell cells can yield more than 10%. However, R&D studies have been terminated due to the deterioration of copper calgonite layers with copper diffusion. These solar cell types are no longer produced.
9. Cadmium Telluride Solar Cells (CdTe)
Cadmium telluride (CdTe) has a high absorption coefficient and ideal bandwidth. Cadmium telluride is one of the promising photovoltaic materials in thin film solar cell technology. Cadmium telluride solar cell efficiency is more than 15%. And solar panel modules made with these cells have more than 9% efficiency.
Cadmium telluride is more suitable for easier storage and larger scale production than other thin film solar cell technologies.
Cadmium telluride (CdTe) is a semiconductor formed by the combination of cadmium element (Cd), which is the 2nd group element of the periodic table, and the 6th group Tellür (Te) element. CdTe has a band gap of 1.45 eV. This value is very suitable for obtaining electricity with solar cells. Optical absorption level of CdTe is 10 ^ 5 / cm and it is a very high value.
Because of this feature, photovoltaic applications are also very suitable material to provide p-type conductivity. The compound can be easily developed in stoichiometric form at 400 C (centigrade).
10. Copper Indium Diseleneid Solar Cells
It is a semiconductor formed by combining three or more of the 1st, 3rd and 6th group elements of the periodic table. The absorption coefficient of this semiconductor is quite high.
Copper indium diseleneid solar cell is produced from combined semiconductor material made with copper, indium and selenium.
The advantages of this thin film solar cell technology are among others;
Optical absorbed coefficient is high,
Conductivity and resistivity can be changed,
High-efficiency cells can also be produced in a factory environment.
CIS solar cell has a very high absorption feature. The first 1 micron layer of this material can absorb 99% of the incoming rays. Its stability in outdoor tests is very good. Therefore, the CIS photovoltaic solar cell is widely used commercially. In addition, if Ga (gallium) element is added to CIS solar cell cells, higher efficiency can be obtained.
11. Copper Indium Gallium Diseleneid Solar Cells (CIGS)
Another of the thin film solar cell types is the copper indium gallium diseleneid. It is called CIGS for short. This solar cell is made on a semiconductor flexible base. CIGS solar cell has higher efficiency than other thin film solar cells. While many thin film solar cells have 8% efficiency, CIGS solar cell has around 10% efficiency.
While CIGS and CdTe solar cells have theoretically 30% efficiency, they reach maximum 25% efficiency under application conditions.
12. Flexible CIGS Solar Cells
The most important advantage in thin film solar cell technology is the cheap production method. these solar modules are electrically interconnected. And it can be produced in one piece. Flexible solar cells in rolls in recent years are very popular. In fact, flexible CIGS solar cells are used especially for solar roof systems. Lightweight and roll-shaped, these CIGS solar cells have a high potential in terms of space technology.
13. Multi Jointed Solar Cells
Solar cells made with one type of material can yield 30% theoretical efficiency and 25% efficiency. Therefore, research on multi-joint solar cell has increased considerably. Multi-joint solar cell is made from more than 2 or 2 semiconductor layers. One of these layers absorbs blue light very well while the other absorbs red light better. Therefore, multi-joint solar cell is more efficient than cells made from a single type of material.
Theoretically, the ideal solar cell can consist of hundreds of layers tuned to different wavelengths located between ultraviolet and infrared. In such a situation, it can reach an unbelievable efficiency of 70%. However, this ideal solar cell cannot be made in terms of application. Because of this, scientists are concentrated on solar cells, which are several layers. Today, multi-joint solar cell efficiency has been increased to such levels as 35-40%.
14. Nanofotovoltaic Solar Cells (NanoPV)
Nanofotovoltaic technology is the solar cell technology of the future. It includes nano-microcrystalline high efficiency solar cells. NanoPV (nano photovoltaic) batteries provide 8-10% efficiency compared to other solar cells with their nano-crystalline a-Si: H (hydrogen amorphous silicon) and permeable conductor (TCLO) technology.
Nanomaterials are quite good in terms of their optical, electrical and chemical properties. Therefore, cell efficiency can be increased. 3 types of materials are used in nanofotovoltaic technology;
crystal semiconductor 3-5 materials,
carbon based nanostructures.
Different solutions can be produced in terms of cost and application in solar cells made using these materials. Zinc (ZnO) and titanium (TiO2) nanowires can be used as conductors in solar cell production. Each of these nanowires can be 1000 times thinner than the hair.
Advantages of Nano Solar Cell Technology
With NanoPv technology, architects will be able to use flexible solar cells. It will allow different designs, It will be able to renew and clean itself. Thus, maintenance-operating costs will be eliminated,
Thanks to NanoPV technology, solar cell efficiency will increase at least 8-10%,
Since the solar panels produced with nano technology will be very light, the static load on the building will be at a point that will be neglected,
It will reduce unemployment and open new jobs.
Disadvantages of Nano Solar Cell Technology
Since it is very difficult to manufacture and observe this scale in nanometric dimensions, special production methods will be required,
The initial investment cost of solar panels produced with this technology is considerably higher than other solar panels,
Many years will be required to train technical staff who can work in this field.
15. Quantum Dot Solar Cells
Quantum dots are crystal semiconductors in nanometer size that can be produced by different methods. The advantages of quantum dots are that the spot diameter is simply chosen, allowing the absorption threshold to be adjusted. Quantum dots are often called artificial atoms. These points allow you to control energy carriers by adjusting 3D constraints.
The quantum point is the nanometer-sized granule of the semiconductor material. These nanocrystals act as 3-dimensional channels for electrons.
The solar cell in the p-i-n design can be placed theoretically in 63% efficiency by placing a one-dimensional point in the sequences inside. Quantum dot materials are at the nanometer level and the bandwidth is adjustable.
The reason for the increase in quantum dot solar cell efficiency is the fusion of dots to absorb lower void energies. When the current is drawn with this method, higher efficiency can be obtained than the efficiency of ordinary multi-articulated cell. The efficiency value is limited not by the energy of the photons, but by the host band gap.
16. Dye Sensitive Solar Cells
Dye-sensitive solar cell cells contain semiconductor like silicon and electrolyte fluid, a conduction solution formed with salt that dissolves in solvent liquid such as water. The semiconductor and electrolyte try to separate the closely connected electron-space pairs produced when solar radiation reaches the cell. The source of the load carriers induced by radiation is the photosensitive dyes that give their name to the solar cell cells.
The commonly used dye is iodide. Also, nanomaterials such as titanium dioxide (TiO2) are used to keep the dye molecules in a skeletal structure.