What is a solar cell and how does it work?

Solar cells are electronic systems that can convert sunlight directly into electricity. However, to understand the definition of a solar cell well, you need to know the definition of photovoltaics. The word photovoltaic is a combination of the Greek words photo, meaning light, and voltaic, meaning voltage. Other names for solar cells are photovoltaic cells, photovoltaic cells and solar cells.

Solar cells can be made in the shape of a square, rectangle, pyramid or circle. The area of solar cells is usually 100 cm2. Their thickness varies between 0.2-0.4 mm. The solar cell has an efficiency of between 5% and 20% depending on the materials used in its construction.

Solar cells are connected in series and parallel to each other to form a solar panel or photovoltaic module. Solar panels are also connected in series and parallel to each other to form large energy facilities called solar power plants or solar farms.

The materials used in solar cell construction are as follows;

• Crystalline silicon,
• Amorphous silicon,
• Gallium arsenic
• Cadmium telluride
• Copper indium diseleneid,
• Optical concentrator cells.

Solar Cell Working Principle

The solar cell generates electricity through the photovoltaic effect. The photovoltaic effect is the physical phenomenon in which sunlight is converted into electricity.

Photons are created when sunlight strikes a semiconductor surface, releasing electrons inside the atom. Photons contain different amounts of energy for each wavelength in the solar radiation spectrum.

When photons arrive on the solar cell, some of them are reflected back, some are absorbed by the solar cell and some pass through the solar cell. Photons absorbed by the solar cell generate electricity.

How is Electric Current Generated in Solar Cells?

Sunlight is made up of a combination of low-energy infrared photons, high-energy ultraviolet photons and intermediate photons of visible light in different colors. 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 valence band, which is bound to the atom and full of electrons, to the empty conduction band, where electrons move freely. Its unit is the electron volt, denoted by the symbol eV.

If a semiconductor is bonded to dopant atoms to form an n-type electrically negative material, it already has a few electrons in the conduction band. Conversely, a p-type positive material is bonded to leave electrons or vacancies in the valence band.

The connection between the N and P type creates a voltage supply. When the incident photons are absorbed, electrons move towards the positive side of the joint and vacancies move towards the negative side. This movement results in an electric current.

In a photovoltaic cell, free electrons traveling towards the positive side of the junction created by incident photons generate an electric current.

  What are Solar Cell Usage Areas?

The uses of solar cells are very widespread, the main areas of use are as follows;

• Communication systems,
• Oil pipelines,
• In electricity distribution systems,
• In water distribution systems,
• Meteorological stations,
• Lighting
• Forest watchtowers,
• Lighthouses,
• Security cameras,
• Alarm systems,
• Agricultural irrigation systems,
• Garden or hobby houses without electricity.

  What are the Types of Solar Cells?

Solar cell types consist of 4 main technologies. These are crystalline structure technology, thin film technology, combined technology and nanotechnology. All solar cell types are listed below.

 1 Inorganic Solar Cells

A monolayer inorganic solar cell consists of an inorganic semiconductor, such as silicon, placed between 2 metal electrodes with different electrochemical potentials, one of the electrons being a semiconductor. The efficiency of single layer inorganic solar cells is quite low.

 2 Bilayer 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 near the boundary between the n-type and p-type semiconductors. Inorganic cells are chemically and thermally stable solar cells. Today, these solar cells can provide up to 30% efficiency.

 3 Monocrystalline Silicon Solar Cells

Monocrystalline silicon solar cells are widely used in solar panel construction. The cost of single crystalline silicon material is quite high. Therefore, multicrystalline solar cells are used more intensively. There are many reasons why silicon is widely used in solar cell construction. These are because silicon can maintain its electrical, optical and structural properties for a long time.

Pure single crystal silicon technology is expensive and difficult. After oxygen, silicon is the most abundant element on Earth. Sand and quartz forms of this element are the most common. Sand is not preferred because its purity is very low. But about 90% of quartz is composed of silicon. Quartz is processed through many processes to obtain silica with a purity of 99%. Silicon is then obtained from silica.

After these steps, silicon is purified to obtain polycrystalline silicon with semiconductor properties. The processes up to the stage of obtaining polycrystalline silicon are quite costly.

To obtain pure polycrystalline silicon with semiconducting properties, the polycrystalline silicon is re-melted and grown. The nuclei are drawn from the molten silicon bath at very low speed. This results in the growth of thin single-crystal layers.

Most commercially available silicon (Si) solar cells are produced from boron-doped single crystal slices (400 microns thick) by the Czochralski (CZ) process. Solar cells produced by the CZ process do not develop lattice defects.

Crystalline silicon batteries account for about 80% of the solar cell market. The efficiency of this type of solar cell ranges from 15% to 23%.

 4 Polycrystalline Silicon Solar Cells

Polycrystalline silicon solar cell material is electrically, optically and structurally identical. The size of the cores is directly proportional to their quality. The discontinuity between the cores plays a hindering role in the transfer of electrical charge carriers.

Multicrystalline silicon cells are easier and less costly to produce. The bulk method is used in the production of polycrystalline silicon material. The production process is briefly as follows. First, most of the processes for obtaining single-crystal silicon are done exactly the same. Molten semiconductor silicon is poured into molds and allowed to cool. The blocks obtained from the molds are cut into squares. The solar cell produced by this method is less efficient. But the cost is quite low. The efficiency of polycrystalline silicon (pc-Si) solar cells varies between 12-15%.

 5 Thin Film Solar Cells

A thin-film solar cell consists of extremely thin semiconductor layers stacked on top of each other. A 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. Polycrystalline copper indium diseleneide and cadmium telluride are also used.

Different deposition methods are used in thin film cell technology. These methods are quite cheap. In addition, a 2×2 inch solar cell can be obtained with this method. Layers are deposited on low-cost glass or plastic-based material.

While normally single-crystal silicon is designed to be individually interconnected within a solar module, thin-film devices can be made as a single unit. Non-reflective coatings and conductive oxide layers are added to the semiconductor material and back electrical contacts.

Thin film solar cells have an efficiency between 8-12%.

 6 Amorphous Silicon Solar Cells

The atoms of amorphous solid materials such as glass are not arranged in a certain order. Materials like this do not form a fully crystalline structure. They also contain numerous structural and connection defects.

In the past, the electrical properties of amorphous silicon were characterized as insulators. However, in the following years, it was thought that amorphous silicon could be used in photovoltaic batteries. Today, amorphous silicon is widely used in low-power devices. Amorphous silicon alloys with carbon, germanium, nitrogen and tin are used to develop multifunctional devices.

Amorphous silicon solar cells show more than 13% efficiency in the lab. Thin film solar cells made with gallium arsenide show more than 24% efficiency.

 7 Multicrystalline Thin Film Solar Cells

A multicrystalline thin-film solar cell consists of very small crystalline grains of semiconductor materials. The materials used in this type of solar cell have different properties than silicon. In these cells, an electric field is more easily created with the interface between 2 different semiconductor materials.

The multicrystalline thin solar cell has a top layer with a thickness of less than 0.1 microns, called a window. The function of the window layer is to absorb high energy radiation energy.

This layer needs to be sufficiently thin in order to have enough band gap.

 8 Thin Film Calgonite Solar Cells

In the 1960s, CuxS-CdS, CuxSe-CdSe and CuxTe-CdTe thin film solar cells were developed. The production of these solar cells is quite simple. CdS, CdSe and CdTe films are produced by chemical deposition.

CuxS, CuxSe and CuxTe layers are produced by immersing CdS, CdSe and CdTe films in CuCl solution for 1-2 minutes. These 3 types of solar cells can give more than 10% efficiency. However, due to the deterioration of copper calgonite layers by copper diffusion, R&D studies have been terminated. These solar cell types are no longer produced.

 9 Cadmium Telluride Solar Cells (CdTe)

Cadmium telluride (CdTe) has a high solar absorption coefficient and ideal bandwidth. Cadmium telluride is one of the most 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.

Compared to other thin-film solar cell technologies, cadmium telluride is easier to store and more suitable for large-scale production.

Cadmium telluride (CdTe) is a semiconductor formed by the combination of the group 2 element cadmium (Cd) and the group 6 element tellurium (Te). CdTe has a band gap of 1.45 eV. This value is very suitable for generating electricity with solar cells. The optical absorption level of CdTe is 10^5/cm, which is a very high value.

Due to this property, it is a very suitable material for photovoltaic applications to provide p-type conductivity. The compound can be easily developed in stoichiometric form at 400 C (centigrade).

 10 Copper Indium Diseleneide Solar Cells

It is a semiconductor formed by the combination of three or more of the elements of groups 1, 3 and 6 of the periodic table. This semiconductor has a very high absorption coefficient.

The copper indium diseleneid solar cell is manufactured from a combined semiconductor material made with copper, indium and selenium.

The advantages of this thin film solar cell technology over others are

• Optical absorption coefficient is high,
• Its conductivity and resistivity can be changed,
• High-efficiency cells can also be produced in a factory environment.

The CIS solar cell has very high absorptivity. The first 1 micron thick 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 cells, higher efficiency can be achieved.

 11 Copper Indium Gallium Diseleneide Solar Cells (CIGS)

Another type of thin film solar cell is copper indium gallium diseleneide. It is called CIGS for short. This solar cell is made on a semiconductor flexible base. The CIGS solar cell has a higher efficiency than other thin film solar cells. While many thin-film solar cells have an efficiency of 8%, the CIGS solar cell has an efficiency of around 10%.

While CIGS and CdTe solar cells have a theoretical efficiency of 30%, they reach a maximum efficiency of 25% under practical conditions.

 12 Flexible CIGS Solar Cells

The most important advantage of thin-film solar cell technology is its cheap production. these solar modules are electrically internally connected. and can be produced in one piece. In recent years, flexible solar cells in rolls have become very popular. In fact, flexible CIGS solar cells are used especially for solar rooftop systems. Lightweight and rollable, these CIGS solar cells have great potential for space technology.

 13 Multijunction Solar Cells

Solar cells made with a single type of material can provide 30% efficiency in theory and 25% in practice. Therefore, research on multi-junction solar cells has increased considerably. A multi-junction solar cell is made of 2 or more than 2 semiconductor layers. One of these layers absorbs blue light very well, while the other absorbs red light better. Therefore, a multi-junction solar cell is more efficient than cells made of a single type of material.

Theoretically, the ideal solar cell could consist of hundreds of layers tuned to different wavelengths between ultraviolet and infrared. In such a case, it could reach an unbelievable efficiency of 70%. But this ideal solar cell is practically unfeasible. Therefore, scientists are concentrating on solar cells with a few layers. Today, the efficiency of a multi-junction solar cell can be as high as 35-40%.

 14 Nanophotovoltaic Solar Cells (NanoPV)

Nanophotovoltaic technology is the solar cell technology of the future. It involves nano-microcrystalline high-efficiency solar cells. NanoPV (nano photovoltaic) cells provide 8-10% higher efficiency than other solar cells with nanocrystalline a-Si:H (hydrogen amorphous silicon) and permeable conductor (TCLO) technology.

Nanomaterials have good optical, electrical and chemical properties. Therefore, cell efficiency can be increased. Three types of materials are used in nanophotovoltaic technology;

• 3-5 crystalline semiconductor materials,
• polymeric materials,
• carbon-based nanostructures.

In solar cells made using these materials, different solutions can be produced in terms of cost and application. 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 a 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 regenerate and clean itself. Thus, maintenance and operation costs will be eliminated,
• NanoPV technology will increase solar cell efficiency by at least 8-10%,
• Since solar panels produced with nano technology will be very light, the static load on the building will be almost negligible,
• It will reduce unemployment and create new jobs.

Disadvantages of nano solar cell technology

• Since it is very difficult to manufacture at nanometric dimensions and observe this scale, specialized production methods will be required,
• The initial investment cost of solar panels produced with this technology is considerably higher than other solar panels,
• It will take many years to train the technical staff who can work in this field.

 15 Quantum Dot Solar Cells

Quantum dots are nanometer-sized crystalline semiconductors that can be fabricated by different methods. The advantages of quantum dots are that the absorption threshold can be adjusted by simply choosing the dot diameter. Quantum dots are often referred to as artificial atoms. These dots allow to control the energy carriers by tuning the 3D constraints.

A quantum dot is a nanometer-sized granule of semiconductor material. These nanocrystals act as a 3D channel for electrons.

The solar cell in the p-i-n design can theoretically achieve an efficiency of 63% by placing one-dimensional dots in the inner part of the array. Quantum dot materials are at the nanometer level and the bandwidth is tunable.

The reason for the increase in the efficiency of a quantum dot solar cell is the fusion of the dots to absorb sub-cavity energies. With this method, higher efficiency can be achieved than the efficiency of an ordinary multi-junction cell when current is drawn. The efficiency value is limited by the host band gap, not the energy of the photons.

 16 Dye Sensitized Solar Cells

Dye-sensitized solar cell cells contain a semiconductor, such as silicon, and an electrolyte liquid, a conduction solution formed with salt dissolved in a solvent liquid, such as water. The semiconductor and electrolyte work to separate the closely bound electron-vacancy pairs produced when solar radiation reaches the cell. The source of the charge carriers induced by the radiation are the photosensitive dyes that give solar cell cells their name.

The commonly used dye is iodide. In addition, nanomaterials such as titanium dioxide (TiO2) are used to hold the dye molecules in a skeletal structure. The use of dye-sensitized cells in solar cell applications dates back to the beginning of efforts to mimic chlorophyll movement in plants. This method is identical to photosynthesis in plants.

The most important development of these cells is the 1991 study. In this study, TiO2 nanoparticles were made by developing a light absorbing dye using a complex dye formed by sensing the more efficient and stable ruthenium (ru).