Solar cells are electronic systems that can convert sun rays into electricity directly. However, we need to know the definition of photovoltaic in order to better understand the definition of solar cells. Photovoltaic word is derived from Greek word photo that means light and voltaic that means voltage. Other names of solar cells are photovoltaic batteries and photovoltaic cells.
Solar cells can be produced with square, rectangular, pyramid or circular shapes. The areas of the solar cells are usually 100 cm2. Their thickness can vary between 0.2 – 0.4 mm. Solar cells have an efficiency of 5% to 20% depending on the materials used in them.
Solar cells make up the solar panels or photovoltaic modules by being connected to each other in series and parallel. Solar panels make up great energy plants which are called as the solar power plant or solar farm by being connected to each other in series and parallel.
The materials used in the solar cell production are as follows;
- Crystalline silicon,
- Amorphous silicon,
- Gallium arsenic,
- Cadmium telluride,
- Copper indium diselenide,
- Optical condensing cells.
How do Solar Cells Work ?
Solar cells generate electricity with photovoltaic effect. The photovoltaic effect is the physical event where sun rays are converted into electricity.
Photons are formed with the release of electrons in the atom when sunlight strikes to the semiconductor surfaces. Photons contain different amounts of energy for each wavelength in the solar radiation spectrum.
When photons arrive on the solar cell some are mirrored as they are, some are absorbed by the solar cell and some pass through the solar cell. Photons absorbed by the solar cell generates electricity.
How is Electric Current Formed in Solar Cells?
Sunlight occurs in different colors with the combination of low-energy infrared photons with high-energy ultraviolet photons and visible light photons in between. Any photovoltaic material responds to a narrow range of these energies depending on its specific bandwidth.
Bandwidth is the amount of energy required to send an electron from the atomic and electron-rich valence band to empty conduction band where electrons are free. Its unit is electron volt and is indicated by the eV symbol.
If it is connected to atom band in order to create a semiconductor n-type electrical negative material, there will be already few electrons in the conduction band. Conversely, a p-type positive material is connected to release electrons or gaps in the valence band.
The connection between N-type and P-type it forms a voltage supply. When incoming photons are absorbed electrons will move towards the positive side of the joint and gaps moves towards the negative side. As a result of this movement electric current forms.
Free electrons directed to the positive side of the joint formed by photons from a photovoltaic cell forms the electric current.
Energy photons lower than bandwidth will keep away before absorption. Energy photons higher than bandwidth are absorbed. the energy of most of the photons turns into heat. Materials with different bandwidth and types can be lined up in order to capture high-energy photons.
What are The Areas of Use of Solar Cells?
The use of solar cell is very common, the main areas of use are;
- Communication systems,
- Oil pipelines,
- Electrical distribution systems,
- Water distribution systems,
- Meteorological stations,
- Forest surveillance towers,
- Security cameras,
- Alarm systems,
- Agricultural irrigation systems,
- Non-electrical gardens or hobby houses.
What Are the Types of Solar Cells?
Types of solar cells consist of 4 main technologies. These are crystal structure technology, thin film technology, combined technology, and nanotechnology. All types of solar cells are listed below.
1. Inorganic Solar Cells
The single-layered inorganic solar cell consists of inorganic semiconductors, such as silicon, placed between 2 metal electrodes which have different electrochemical potentials and one of the electrons is a semiconductor. The efficiency of single-layered inorganic solar cells is very low.
2. Double-Layered Inorganic Solar Cells
Double-layered inorganic solar cells are made by using 2 semiconductors as n-type and p-type. Load separation in these cells takes place in between n-type and p-type semiconductors. Inorganic cells are quite stable solar cells in terms of chemical and heat. Today, these solar pillars can provide up to 30% efficiency.
3. Single Crystal Silicon Solar Cells
The single crystal silicon solar cell is often used in solar panel production. The cost of a single crystal silicon material is very expensive. Thus, the polycrystalline solar cell is heavily used. There are many reasons for the widespread use of silicon in solar cell production. This is because silicium can keep its electrical, optical and structural features for a long time.
Pure single crystal silicon technology is quite expensive and difficult. Silicon is the most common element in the world after oxygen. Sand and quartz forms are the most common ones of this element. Sand is not preferred because the purity structure is very low. However, quartz is made of 90% silicon. 99% silica is obtained after quartz passes through many processes. Then silicon is obtained from silica.
After these steps, silicon is purified and semiconductive polycrystalline silicon is obtained. Processes up to the stage of obtaining polycrystalline silicon are quite costly.
To obtain semiconductive pure polycrystalline silicon, polycrystalline silicon is melted again and enlarged. The cores are drawn from the molten silicon chamber at very low speed. Thus, the enlargement of fine monocrystalline layers is ensured.
Most commercially available silicon (Si) solar cells are produced from boron doped single crystal slices (400-micron thickness) with Czochralski (CZ) process. Lattice defects do not occur in solar cells produced with CZ process.
Crystal silicon cells make up about 80% of the solar cell market. The efficiency of this solar cell type can range from 15% to 23%.
4. Polycrystalline Silicon Solar Cells
Polycrystalline silicon solar cell materials are same in terms of electrical, optical and structural features. The dimensions of the grains are directly proportional to the quality. The discontinuity between grains plays a role in preventing the conduction of electrical load carriers.
Polycrystalline silicon solar cell production is easier and less costly. Casting method is used in polycrystalline silicon material production. The production phase is briefly as follows. First, most of the processes performed to obtain single crystalline silicon will be exactly the same. Molten semiconductor silicon will be poured into the molds and waited to cool down. The blocks obtained from the molds are cut in a square shape. The solar cell produced by this method is less efficient. However, the cost is quite low. Polycrystalline silicon (pc-Si) solar cell efficiency can range from 12% to 15%.
5. Thin Film Solar Cells
Thin film solar cells consist of extremely thin semiconductor layers which are put on each other. Thin film solar cell can be produced from a wide variety of materials. Commercially available thin film solar cells are produced from amorphous silicon. Apart from this, polycrystalline copper indium diselenide and cadmium telluride are also used in the production.
Different sedimentation methods are used in thin-film cell technology. These methods are quite cheap. Also, 2×2 inches solar cell can be obtained with this method. Layers are precipitated on a low-cost glass or plastic based materials.
In general, single crystal silicon is designed individually interconnected in solar module however thin film devices can be produced as a single unit. Non-reflecting coat and conductive oxide layers are added to the semiconductor material and rear electrical contacts.
Thin film solar cell efficiency can range from 8% to 12%.
6. Amorphous Silicon Solar Cells
The atoms of amorphous solid materials such as glass are not aligned in a certain order. Such materials do not fully constitute a crystal structure. They also contain a large number of structural and connection faults.
In the past, the electrical properties of amorphous silicon have been described as conductive. However, in the following years, it is thought that amorphous silicon can also be used in photovoltaic cells. Today, amorphous silicon is widely used in low power devices. Carbon, germanium, nitrogen, tin, and amorphous silicon alloys are used to develop multi-functional devices.
Amorphous silicon solar cells exhibit more than 13% activity in the laboratory environment. Thin film solar cells made with gallium arsenide can provide more than 24% efficiency.
7. Polycrystalline Thin Film Solar Cells
Polycrystalline thin-film solar cells consist of very small crystal particles of semiconducting materials. Materials used in such solar cells have different features than silicon. The electric field is easily created with the interface between the two different semiconductor materials in these cells.
Polycrystalline thin film solar cells have a top layer at a thickness thinner than 0.1 microns which are called as the window. Window layer function is to absorb high-energy radiation energy.
This layer has to be thin enough in order to have sufficient bandwidth gap.
8. Thin Film Calgonit Solar Cells
CuxS-CdS, CuxSe-CdSe, and CuuxTe-CdTe thin film solar cells have been developed in 1960. The production of these solar cells is quite simple. CdS, CdSe and CdTe films are produced with chemical precipitation process.
CuxS, CuxSe, and CuxTe layers are produced with CdS, CdSe and CdTe films by dipping it to CuCl solution for 1-2 minutes. These 3 types of solar cells can provide more than 10% efficiency. However, R&D studies of copper calgonit layers have been terminated due to copper diffusion distortion. Today, these types of solar cells are not produced.
9. Cadmium Telluride Solar Cells (CdTe)
Cadmium telluride (CdTe) has a high coefficient of the absorptance of solar ray and ideal bandwidth. Cadmium telluride is one of the types of photovoltaic materials which is promising in thin film solar cell technology. Cadmium telluride solar cell efficiency is more than 15%. And the solar panel modules made with these cells have more than 9% efficiency.
Cadmium telluride is more suitable for easier storage and larger scale production when compared to other thin film solar cell technologies.
Cadmium telluride (CdTe) is a semiconductor which consists of the second group element of the periodic table, cadmium element (Cd) and sixth group tellurium (Te) element. CdTe has a bandwidth gap of 1.45 eV. This value is a very convenient value to obtain electricity with solar cells. The optical absorb level of CdTe is 10^5/cm which is a very high value.
Because of this feature, it is a very suitable material for photovoltaic applications to provide p-type conductivity. It can be easily developed in stoichiometric form with composite 400 C (centigrade) heat.
10. Copper Indium Diselenide Solar Cells
It is semiconductor which is formed by the combination of three or more elements of the first, third and sixth group of the periodic table. The absorption coefficient of this semiconductor is quite high.
The copper indium diselenide solar cell is produced from composite semiconducting material made of copper, indium, and selenium.
The advantages of this thin film solar cell technology from others are;
- The optical absorption coefficient is high,
- Conductivity and resistivity can be changed,
- High-efficiency cells can also be produced in factories.
CIS solar cell has a very high absorbency feature. The first 1-micron layer of this material can absorb 99% of incoming rays. The stability in outdoor tests is very good. Therefore, CIS photovoltaic solar cell is widely used commercially. Also, higher efficiency can be obtained if Ga (gallium) element is included in CIS solar cells.
11. Copper Indium Gallium Diselenide Solar Cells (CIGS)
Another type of thin film solar cells is copper indium gallium diselenide. It is shortly called CIGS. This solar cell is made on a semiconductor flexible base. CIGS solar cells have a higher efficiency than other thin film solar cells. While many of the thin film solar cells have 8% efficiency, CIGS solar cell has an efficiency of around 10%.
While CIGS and CdTe solar cells theoretically have 30% efficiency, they can reach to 25% efficiency in application conditions.
12. Flexible CIGS Solar Cells
The most important advantage in thin film solar cell technology is cheap production. These solar modules are electrically connected internally. And they can be produced in one piece. Recently, flexible solar cells in the form of the roll are very popular. Even flexible CIGS solar cell types are used especially for solar roof systems. These lightweight and rollable CIGS solar cells have quite a high potential in terms of space technology.
13. Multi-Junction Solar Cells
Solar cells made with uniform materials can provide 30% efficiency in theory and 25% efficiency in application. Therefore research on multi-junction solar cells increased. Multi-junction solar cells are made of 2 or more semiconductor layers. While one of these layers can absorb blue light well, other can absorb the red light well. Therefore multi-junction solar cells are more efficient than the cells made from the uniform material.
The theoretically ideal solar cell can consist of hundreds of layers adjusted to different wavelengths between ultraviolet and infrared. In such a case it can reach an unbelievable efficiency of 70%. However, this ideal solar cell is not possible in the application. Thus, scientists focused on solar cells with several layers. Today, the efficiency of multi-junction solar cells raised up to 35-40%.
14. Nano Photovoltaic Solar Cells (NanoPV)
Nano photovoltaic technology is the solar cell technology of the future. It includes nano-microcrystalline high-efficiency solar cells. NanoPV (nano photovoltaic) cells provide efficiency more than 8-10% when compared to other solar cells thanks to nanocrystalline a-Si:H (hydrogen amorphous silicon) in their structure and conducting conductive (TCLO) technology.
Nanomaterials are very good in terms of their optical, electrical and chemical properties. Therefore cell efficiency can be increased.
Three types of materials are used in nano photovoltaic technology;
- Crystal semiconductor 3-5 materials,
- Polymeric materials,
- Carbon-based nanostructures.
Solar cells made using these materials can offer different solutions in terms of cost and application. Zinc (ZnO) and titanium (TiO2) nanowires can be used as a conductor in solar cell production. Each of these nanowires can be 1000 times thinner than hair.
Advantages of Nano Solar Cell Technology
- Architects will be able to use flexible solar with NanoPv technology. It will allow different designs,
- It will be able to renew and clean itself. Thus maintenance-operation cost will be eliminated,
- Solar cell efficiency will increase by at least 8-10% thanks to NanoPV technology,
- Since solar panels produced with nanotechnology will be very light, static load on the building can be almost neglected,
- It will reduce unemployment and create new job positions.
Disadvantages of Nano Solar Cell Technology
- Special production methods will be required, since making production in nanometric dimensions and observing this scale is challenging,
- The initial investment cost of solar panels produced with this technology will be quite high when compared to others,
- Long years will be needed for the training of technical staff who can work in this area.
15. Quantum Dot Solar Cells
Quantum dots are crystal semiconductors of nanometer size that can be produced with different methods. Advantages of quantum dots are they allow adjustment of the absorption threshold by simply selecting the dot diameter. Quantum dots are generally called artificial atoms. These dots provides the ability to control energy carriers by adjusting 3D constraints.
Quantum dot is the granule in nanometer size of the semiconductor material. These nanocrystals function as a 3D channel for electrons.
The p-i-n design of the solar cell can provide theoretically 63% efficiency by placing the one-dimensional dot in inner ordered sequences. Quantum dot materials are at the nanometer level and the bandwidth can be adjusted.
The reason for the increase in quantum dot solar cell efficiency is the fusion of dots for absorbing the lower gap energy. Higher efficiency can be obtained from the efficiency of ordinary multi-junction cells when current is obtained with this method. Efficiency value is limited by the host band gap but not with the energy of the photons.
16. Dye-Sensitized Solar Cells
Dye-sensitized solar cells contain electrolyte fluid, which is a conduction solution formed with a semiconductor such as silicon and soluble salt in the solvent liquid such as water. Semiconductors and electrolytes try to separate pairs of closely connected electron-gaps produced when sun’s radiation reaches the cell. The source of radiation-induced load carriers are the materials that name the dye-sensitized solar cells.
The commonly used dye is iodide. Also, nanomaterials such as titanium dioxide (TiO2) is used to keep the dye molecules in a skeletal structure. The use of dye-sensitized cells in solar cells based on the work of imitating chlorophyll movements in plants. This method is the same as the method of photosynthesis in plants.
The most important development of these cells is the work in 1991. In this study, TiO2 nanoparticles developed with light-absorbing dye by using complex dye formed by sensing ruthenium (Ru) which is more efficient and stable.