Solar cell, also referred to as photovoltaic (PV) cells, are devices that transform sunlight directly into electricity through the photovoltaic effect. This occurs when certain materials such as silicon – semiconductors – are exposed to sunlight and produce an electric current. Solar cells consist of one or more layers of these semiconductors with an electric field which helps separate positive and negative charges, creating electron flows which can be captured as electrical current. Solar cells are commonly used in solar panels for various applications like powering homes, businesses and even satellites.
Solar cells, also known as semiconductor devices, convert solar energy into electrical current. Their development has been unstoppable for almost 40 years due to two primary reasons: the need to replace fossil fuels with renewable ones and increasing environmental awareness about eco-friendly methods of energy production.
Photovoltaic effect
The photovoltaic phenomenon, which forms the base of the cell’s operation, is the result of the generation of a voltage inside the semiconductor due to interactions between the material and electromagnetic radiation. Because of the fact that they use only visible spectrum from the solar spectrum devices are also referred to as solar cells. The photovoltaic phenomenon was first observed during the year 1839 in the work of French science researcher Edmond Becquerel. Becquerel created a cell made of platinum plates that were immersed in electrolyte solution some of them coated in AgCl and, in the event of exposure to light the potential for a difference is evident in the circuit’s outer. 40 years later the first device that utilized the photovoltaic effect of solid phase which was based on selenium was developed. The same effect was observed with gold and selenium as well as copper oxide and copper, Thallium sulfide and lead. The research on photoconductivity led to the creation of photovoltaics. The 1950s saw the advent of silicon-based electronic devices, which led to the creation of silicon-based fabrication technology. Silicon – a type of semiconductor that was in contact with a boron-chloride atmosphere was able to create an outer layer of semiconductor p-type which allowed to create a p-n junction. The efficiency of these junctions was around six times better than metal/semiconductor junctions previously used that triggered the rapid growth of photovoltaics using a diverse spectrum of materials.
How do solar cells work
Solar cells utilize the p-n junction which acts as a separator between electron-hole carriers that are light-excited. The diagram below illustrates the structure of the bands in the P-type (a) and the n-type (b) semiconductor. The formation of an p-n junction causes an increase in the gradient of carriers. This gives them a reason to diffusion. Electron holes move from the p type semiconductor to the n-type semiconductor and electrons diffuse from the n-type semiconductor diffuse to the p type semiconductor. The outcome of this process is the creation of a layer that is depleted of carriers and the resulting electric field blocks the further diffusion. If the movement of current carriers caused by the field of electric energy is counterbalanced with the diffusion charge generated from the gradient in concentration of carriers the equilibrium temperature is achieved. In equilibrium the Fermi levels of the n-type and p-type semiconductors are equivalent.
The differences in the potential of two semiconductors of the n-type type and p-type in thermal equilibrium is known as Vb, which is the term used to describe an embedded voltage. or Vb, and is connected to the different free energy between the p-side and the n-side of the junction. The embedded potential can be described in the following relation:
Where: kB – Bolztman’s constant, T – absolute temperature [K], NA and ND – concentration of acceptors and donors in p-type semiconductor and n-type semiconductor, e – elementary charge, ni – internal concentration of carriers.
The majority charge carriers in the depleted layer include donors and acceptors, making it distinctly different from other electrically neutral areas. The width of the depleted layer W can be described by the relation:
Where: ε – the dielectric constant of the semiconductor.
The width of the layer increases when the concentration of donors or acceptors is reduced. For example, when the concentration of acceptors is much greater than the concentration of donors Na>>Nd, the width of the depleted layer is described by the following relation:
The application of an external voltage on the junction, in such such that its positive pole connected to the semiconductor of the type p and the negative pole connects to the type semiconductor results in the potential difference of the layer that is depleted to be decreased. This polarity is a polarization that occurs in the direction of conduction, and it has the characteristic of reducing the electrons’ movement from the n-semiconductor towards the p-semiconductor-side and to increase the movement of electron holes that are in the other direction. It means that minor carriers – electrons – are injected into the p side and electron holes are injected onto the side of the n. Current density J in the event of positive polarization, can be defined by the equation:
where: VF – polarization in the conduction direction, J0 – saturation current density of the junction.
In the case of the reverse situation, the electrostatic potential of the depleted layer increases, the diffusion current decreases, and the diffusion current density of the polarization in the negative direction is described by the relation:
where: VR – polarization in the negative direction.
The illumination of the junction by sunlight leads to the formation of electron-hole pairs because of the absorption of light particles with higher energy than the energy from the excited gap in semiconductors and their quantity is proportional to the brightness of the sunlight. The electric field of the depleted layers causes shift of electrons towards semiconductor n , and electron holes towards semiconductor p . This happens inside the depleted layers. Charge separation creates current flow between the n-side and the p-side if the outer circuit is cut. The carriers that are generated in the diffusion range that originates near the edge of depleted layers are a part of the photocurrent because of the diffusion of carriers that are not needed into the charge region. Charge carriers created by light absorption can be separated by the electric field that is present in the charge region of space. Minority charge carriers which diffuse out to the boundary of the charge region before recommingling can also be a source of photocurrent.
For tiny grains (nanomaterials and thin films) The stretching of the bands is minimal, which means that excited electrons spread out towards the surface, and then recombine with electron holes, or are caught through trap levels. The time required for a charge-carrier to get to the surface from the semiconductor volume is strongly dependent on the size of the grain. This can be described as follows:
where: rz – grain diameter of the semiconductor, JD – electron diffusion coefficient.
Separation of charge carriers, as occurs in a p-n junction, can also occur for the junction of two semiconductors with the same type of conductivity, but differing in the value of the induced gap, which is used in dye-sensitized solar cells.
Solar cell structure
The diagram below illustrates the basic structure of a solar cell. The cell’s interior is comprised of two parts that are p-type that is called the base and an n type area that is known as the emitter. The p-type zone is generally coated with boron, while the n-type zone is doped is doped with the element phosphorus. The regions near the contact contain higher levels of dopants than the areas of p and n. They are classified as the p+ and n+ . A proper amount of doping prevents loss of charge at the contacts. The cell’s two surfaces are made up of metal contacts.
The fundamental principle behind the operation of the operation of a solar cell is the photovoltaic effect. It is the process of generating electromotive force in the course of the illumination of the p-n junction. The majority of photons don’t have enough energy to let an electron escape from the bonds of the semiconductor crystal. The main parameter that determines the effectiveness of photoconversion is the size of the space between the electrons. If a photon that has an energy higher than the the energy gap of silicon is positioned on the p-n junction area the energy is lost due to the formation of an electron-hole charge. Charges that result from the electric field at the junction are split and, after passing through the cell to reach the metal contacts. In the event that the circuit becomes shut the current of electricity flows through it. This is why solar energy is transformed into electricity within the cell in an electron-based method that is free of chemical reactions.
Organic solar cells
Photovoltaic cells are devices where the conversion of sunlight into electricity is accomplished through small-molecule organic molecules and electroactive polymers. In this kind of cell the layer of active is situated within two electrodes, namely a conductive transparent glass and a metal contact. In the most basic scenario the active layer is composed of two different materials that have distinct electrical properties. One of them carries an electric charge (donor) as well as the opposite one carries a negatively charged charge (acceptor). When you look at organic solar cells, solar radiation is attracted through the layer of donor and in turn an excitron molecular (electron-hole pair) forms. Excitrons spread to the interface of two different materials, where the dissociation of free charge carriers occurs. Free electrons pass between the layers of acceptor and donor towards the negative electrode. Similarly, holes (positive charge carriers) travel across the layer of donor and reach positively charged electrodes. What happens as a result of the motion of charges is development of a photovoltage between two electrodes.
Organic solar cells have the potential to be employed in many different ways, such as portable electronics with built-in photovoltaics or protective coatings on windows that generate electricity while letting light pass through. However, their performance and longevity must be improved to be able to compete with conventional silicon-based solar cells.
Organic solar cells, also referred to as organic photovoltaic cells, have gained considerable attention in the renewable energy space due to their unique properties and advantages over conventional silicon-based solar cells. Here we’ll examine some of the major advantages that organic solar cells offer over their silicon counterparts.
- Flexibility Flexible: Organic solar cells are typically constructed from lightweight and flexible materials like polymers. This makes them more adaptable and user-friendly compared to traditional silicon-based solar cells which tend to be stiff and difficult to work with. This flexibility makes organic solar cells ideal for applications such as wearable electronics and building-integrated photovoltaics.
- Cost Effective Manufacturing Organic solar cells offer major advantages due to their production through cost-effective techniques like roll-to-roll printing. This suggests that organic solar cells could potentially be less costly to manufacture than silicon-based ones, which require costly and intricate manufacturing procedures.
- Efficiency: Organic solar cells are still not as efficient as traditional silicon-based ones, but their efficiency has seen dramatic improvements over the last several years. From less than 4% to over 17 percent in just two decades, organic solar cells are now more competitive than their traditional counterparts. Furthermore, since organic solar cells can be manufactured in various dimensions and shapes for specific applications, their efficiency could potentially increase even further.
- Eco-Friendly Organic Solar Cells Eco-friendly Organic solar cells are produced using abundant and sustainable ingredients. Unlike traditional silicon-based solar cells, organic solar cells don’t need toxic heavy metals or chemicals in their manufacturing process – making them a greener and sustainable option for the generation of renewable energy sources.
- Increased Durability Enhance Durability: Organic solar cells have long had problems with durability due to the organic materials they use in production being sensitive to moisture and oxygen. But recent advances have enabled the development of novel materials and techniques for encapsulation that have significantly enhanced both durability and performance of organic solar cells.
Multi junction solar cell
Multi-junction solar cells consist of multiple layers of semiconductors with different Eg energy gap values.
To maximize the efficiency of a multijunction cell, the thickness of the cell should be chosen so that the largest portion of the radiation is absorbed. Multi-junction cells are optically an arrangement of layers, each of which absorbs a different part of the solar spectrum. It is therefore reasonable to design successive layers of semiconductors so that, counting from the transparent layer, successive layers have a smaller energy gap than the previous layer. An example is the GaInP / GaAs/Ge system, where the energy gaps are GaInP (1.9 eV), GaAs (1.4 eV) and Ge (0.7 eV), respectively, with an efficiency of approx. 33%, or AlGaInP/AlGaAs/GaInP/GaInAs/GaInNAs, with energy gaps of AlGaInP (2.2eV), AlGaAs (1.6 eV), GaInP (1.7 eV), GaInAs (1.2 eV), GaInNAs (1.0-1.1 eV). Depending on the percentage composition, the above systems achieve yields of 30-40%.
To maximize the flow of charge in a multi-junction cells, two junctions consecutively linked to one another using tunnel junctions (junctions where particles are able to traverse a barrier with a potential that is higher than its energy of motion). It is composed of highly doped layers, with lower absorption, low voltage drop , and high peak current. Its thickness should be approximately 10mm. Without tunnel junctions photovoltaic energy from junctions that have opposite signification could compensate.
With many connectors, high efficiency could be reached, achieving the limit of Shockley-Queisser.
The efficiency of multi-junction cell will continue to increase. Based on the amount of semiconductors in the cell, simulations have shown efficiencies that exceed than 60 percent. The highest efficiency, even with an infinite number of layers, could be 86.8 percent. This is known as the thermodynamic limit.
Multilayer systems are costlier and usually constructed by using gallium arsenide, which toxicity and the possibility of disposal in a safe manner ongoing research are being conducted. According to NREL research Multi-junction solar cells have achieved 47 percent efficiency. In the beginning, they were utilized exclusively in space because of their cost. In the present, efforts are launched to bring them onto the market.
Quantum dot solar cell
Researchers at the University of Toronto in Canada and KAUST in Saudi Arabia have developed a super-efficient solar cell based on colloidal quantum dots, with an efficiency of around 7%. This value is almost 40% higher compared to previously developed devices based on colloidal quantum dots.
Quantum dot solar cells are solar cells that utilize quantum dots to increase efficiency. These minuscule particles, typically only a few nanometers in diameter, are made of semiconductor materials like cadmium selenide, lead sulfide or indium phosphide.
Quantum dot solar cells utilize tiny particles embedded in a thin film of semiconductor material. When light strikes these quantum dots, energy from the light is absorbed and causes electrons to be excited to higher energies – just like in traditional solar cells. Once excited, these protons can be harvested as electrical power for use elsewhere.
One of the advantages of quantum dot solar cells is their ability to selectively absorb different colors of light by altering their size and material. This enables them to be optimized for specific parts of the solar spectrum, potentially improving efficiency in the process.
Quantum dot solar cells are still in their early stages of research and development, but they show promise for improving efficiency within solar cells and making solar power more cost-effective and practical.
Quantum dot solar cells work by converting light energy into electrical energy by using tiny semiconductor particles called quantum dots.
Light particles, or photons, strike the surface of a quantum dot solar cell and are absorbed by its quantum dots, exciting electrons within them and creating electron-hole pairs. These pairs can then be separated and used to generate electric current.
Quantum dots’ size determines which wavelengths of light they absorb. Can be designed specifically to absorb certain wavelengths for efficient solar spectrum usage. Furthermore, multiple quantum dots can be stacked in order to enhance light absorption and boost solar cell efficiency.
Quantum dots create separated electrons and holes which are then collected by an array of electrodes and sent to an external circuit, where they can be utilized for powering electronic devices or stored as energy in a battery.
Quantum dots colloidal (colloidal quantum dots ) are semiconductors a small amount of nanometers made by the condensation of crystals made from semiconductors solution. Due to this method of making these particles, they are easily put on different types of substrates. One possible application for colloidal quantum dots is to utilize to serve as absorbers of light (absorbing) elements in high-efficiency organic solar cells. The primary benefit for using colloidal quantum dots for photovoltaics is their capacity to absorb light across many wavelengths. This is due to their ability to adjust their energy gaps dots by altering their dimensions.
The challenge in creating solar cells based on colloidal quantum dots is mostly due to the existence of barren spot (areas) in their surfaces. These spots act as electron-trapping centers, restricting the effectiveness of the cells. To avoid this unwanted phenomenon the quantum dot’s surface undergoes a passivation procedure. This involves coating their surface with organic compounds or a semiconductor layer that has a larger energetic gap than that of the (so-called colloidal quantum dots that are core-shell ). A team of scientists under the direction of Edward Sargent of the University of Toronto treated the dots’ surface by placing them in a chloride solution. “The chlorine atoms filled all the areas that were bare in the surface quantum dots, and we could improve the surface’s quality,” Sargent explained.
Quantum dots colloidal were put in a transparent glass surface during the process of breaking the dots, and afterwards covered with a clear layer of electrically conductory material. The next step was to ensure that they were “bound together” by using an organic linker. This created an extremely dense agglomerate dots that absorbs more light than an equilateral “looser” layers of dots. The presence of this densely packed structure of dots is confirmed with X-ray scattering experiments carried out by KAUST scientists. “Most solar cells on the market today are composed from heavy crystals,” Sargent explained. “Our study shows that lightweight and flexible materials, like colloidal quantum dots, could be a possible rival, particularly in terms of cost, as compared to conventional solar cells.”
References:
https://efizyka.net.pl/superwydajne-ogniwo-sloneczne-bazujace-na-koloidalnych-kropkach-kwantowych
https://www.cire.pl/pliki/2/technologia_siuzdak_po_red_po_adpo_kor_ks1po_kor.pdf
http://doi.prz.edu.pl/pl/pdf/biis/189
https://home.agh.edu.pl/~jbanas/4.pdf
