Electricity Generation from Different Solar PV Cell Technologies
August 18, 2020
In our previous article, “an introduction to the basics of electricity and solar systems in Mauritius” was made. In this article, a key component of the solar photovoltaic (PV) technology is discussed – Solar PV Cells. Solar PV cells are the basic unit of the common solar PV modules/panels, and they are available in different types. The different technologies of solar PV cells depend on the material used in making the cells. Solar PV technology, simply put, provide means to generate electricity by the action of sunlight on semiconductor materials.
The most common solar PV technology in terms of global deployment is the crystalline silicon based solar panels, which according to the MIT Energy Institute, represented about 90% of solar PV modules as at 2015. Fraunhofer Institute for Solar Energy Systems (ISE) reported in May 2020 that crystalline silicon-based PV technology accounted for about 95% of global PV technology production in 2017. Crystalline silicon is based on the silicon element of the periodic table and is discussed later.
In biology, the basic unit of life is a cell, a combination of cells results in a tissue, and so on. Similarly, in solar PV technology, the basic unit is the solar PV cell, a combination of cells gives a solar module or solar panel. A solar module can be described as an arrangement of cells in a matrix form with serial and/or parallel connection, which are then enclosed for protection from external influence while still exposing the cells to light as necessary. An arrangement of a number of solar modules is referred to as an array. Large scale solar array installations are called solar farms or solar parks or solar power stations as they can be compared to conventional power plants in terms of energy generation.
People tend to confuse silicon with silicone. While silicone is made from silicon, they vary a lot with regards to their use and properties. Silicone, considered a polymer, contains silicon, oxygen and other elements like carbon and are used in various applications as highlighted by Chemical Safety Facts. According to Lenntech BV, (silica) sand is used as a source of commercially produced silicon. Also, silicon is non-toxic both as an element and in its natural forms like silica and silicates. Even the silica gel that comes with products are non-toxic; the label warnings are usually because the silica gel poses a choking hazard, so do not ingest. Select Sands Corp defined silica sand as quartz that has been broken down into small aggregates through the prolonged action of water and wind. Shaw Resources noted that for a sand sample to be considered as silica sand (also called quartz sand, white sand or industrial sand), it must contain at least 95% silicon dioxide (SiO2) and less than 0.6% iron oxide, otherwise it is regarded as “regular sand”. If the sand at the beaches (or elsewhere) in Mauritius meet these criteria, then the country is a step closer to making its own silicon. Common volcanic rocks hardly form silica sand after weathering as the percentages of silicon dioxide in the rocks are mostly below the accepted limit. The chart below from Explore Volcanoes shows the percentage of silica in some common volcanic rocks.
Silicon, oxygen, hydrogen, carbon and a host of other materials in their basic form are collectively referred to as elements and can all be seen in the periodic table pictured below.
An element in its smallest or simplest form is known as an atom. When two or more atoms of the same element or different elements combine chemically, they form a molecule. Molecules that are formed from different elements are called compounds. Therefore, while all compounds are molecules, not all molecules are compounds. More information on these differences can be found at World of Molecules. Polymers are substances formed from repeated simpler molecules called monomers. This repetition of monomers is illustrated in the image below.
According to Encyclopedia Britannica and other sources, pure silicon cannot be found naturally but rather in compounds where they combine with elements like oxygen, aluminum and iron. Compounds containing silicon can be found in rocks, sand, clays and soils which makes the silicon material the second most abundant material in nature after oxygen. However, it is the process of extracting pure silicon for use in silicon-based solar PV cells that can be demanding. The processes of extracting silicon and making crystalline silicon was described in a publication by Eric Williams. The publication also noted that the dominant process for making single crystal silicon ingots is the Czochralski method. A base material called an ingot is cut into wafers and then processed to make a solar PV cell. Apart from solar PV cells, the silicon material is used in computer hardware. The U.S. Department of Energy here noted that silicon was the most common semiconductor used in making computer chips.
Solar PV Materials
Apart from Silicon, there are also other active materials used in making solar cells which can be grouped into first, second and third generations.
First Generation materials consists of wafer-based cells which is characteristic of crystalline silicon cells. Crystalline silicon cells are available in two major types namely monocrystalline and multicrystalline or polycrystalline silicon cells. Another material that belongs to this category is the gallium arsenide solar cells.
Second Generation includes commercial thin-film solar cells made from materials like Cadmium Telluride (CdTe), amorphous Silicon, Copper Indium Gallium Selenide (CIGS).
Third Generation covers emerging technologies that are mostly still in developmental stage and have not been fully deployed commercially. They include hetero/multi-junction cells, organic cells, perovskites and quantum dot materials.
Electricity generation using solar PV technology in the basic sense involves releasing and receiving of electrons, and is initiated by the action of the sun on a solar cell, called the photovoltaic effect. Penn state University (PSU) Department of Energy and Mineral Engineering summarized the process of generating electricity from the sun into three steps which are: absorption of light by the cell(s), generation of electrons and holes which are the charge carriers (Note: the electrons already existed but are excited and set into motion by the action of light on the cells), and separation of the charge carriers between electrical contacts/terminals which causes current to flow when an electric load is connected. More information on how this occurs is explained later in the text.
Solar PV base semiconductor material like pure silicon by itself cannot generate significant electricity when it absorbs energy which is why additional materials are required to transform the semiconductor material to have the ability to generate electricity through a process known as doping. Doping involves adding impurities to semiconductor materials in order to improve their electrical properties. The structure of pure silicon with atomic number – 14 is shown with a simple configuration below.
The atomic number of an atom is the number of protons in the nucleus of an atom, and for a neutral atom, the number of electrons is the same as the number of protons. The proton(s) and neutron(s) of an atom are contained in the nucleus, the center of the atom, and the sum of the two is called the mass number of the atom. Electrons occupy atomic orbitals contained in arbitrary spherical outlines called shells as shown in Figure 4 above; some describe it at a cloud around the atom. These electrons on absorption of energy from sources like sunlight (photon energy), can escape from their original orbital/shell and contribute to current flow (electricity production).
The protons in the nucleus of the atom are positively charged, the neutrons have no charge and electrons are negatively charged. An atom is electrically neutral when its number of electrons is the same as its number of protons. When the atom gains electron(s), it becomes a negatively charged ion because the number of negatively charged electrons is higher than that of positively charged protons. Similarly, when the atom loses electron(s), it becomes a positively charged ion because the number of negatively charged electrons is less than that of positively charged protons. This is explained further in the following videos:
To reduce the energy required for the electrons in the outermost shell of the silicon atom to break free, doping is used to prepare such material as silicon for electricity generation. The material used in doping determine the “type” of the resulting silicon. Two common materials (impurities) used in doping Silicon are Phosphorus and Boron. From the periodic table in Figure 2, Phosphorous has atomic number 15, with the number of electrons per shell from the closest to the nucleus in the order 2,8,5. Boron on the other hand has atomic number 5, with the number of electrons per shell in the order 2,3. Gallium can also be used as an impurity like Boron is used since it has three electrons in the outermost shell; similar to phosphorus is also Arsenic which has five electrons in the outermost shell. Arsenic however is considered very toxic. The closest form of stability for the silicon atom is attaining the configuration of the Argon – 2,8,8, however, it is not desired for the silicon atom to achieve that stability but rather to pursue it. This is why the Silicon atom is doped with Boron or Phosphorus to give the configuration illustrated below.
A look at the compounds with configurations above shows that outermost shells of the Silicon atom never reached stability i.e. eight electrons. Do note that when doping, not all silicon atoms are bonded to impurities, but a small fraction of the crystalline silicon. In pure crystalline silicon, the four electrons in the outermost cell of a silicon atom bonds to another silicon atom to reach stability – eight electrons in both of their outermost cell. When silicon is doped with boron, there is a shortage of one electron in the outermost shell of the bonded silicon atom in contrast to four electrons shorting for a pure silicon atom (In a pure silicon atom with configuration – 2,8,4, there are four electrons in the outermost shell, and filling up the shell will require four more electrons). When silicon is doped with phosphorus, there is an excess of one electron. These result in the terms P-type and N-type silicon. The boron-doped silicon with the shortage (opening for an electron) of an electron in the outermost shell of doped silicon atoms is termed the P-type silicon while the phosphorus-doped silicon with the excess of an electron in the outermost shell of doped silicon atom is called the N-type silicon.
The N-type (silicon) material with surplus of electrons is placed next to the P-type material with shortage of electrons thereby creating an electric field. The electric field makes it possible for the excess electrons in the N-type material to try to migrate and fill the openings in the P-type material. However, this is not fully achieved as the junction created by the two materials restricts the migration of electrons and acts as a barrier. When photons hit a solar cell, some electrons acquire energy and move through the barrier to the other side; these electrons cannot return to fill the holes they left through the initial path they took. When electrical terminals (electrodes for the positive and the negative) are inserted on both sides of the arrangement and connected to a DC electrical load, the electrons take the external circuit path to return to their original position/state. This flow of electrons in one direction equates the flow of current in the opposite direction which is used for electrical work without the electrons being consumed. Ed Hitchcock made a great video explaining this difference.
It is worth noting that generally, it is the flow of electrons (or current) that is used to do electrical work and not the depletion of the electrons themselves. This can be simply explained by saying that conversion mechanisms are interfaced with the current flow. Examples: an incandescent lamp has a wire filament in it that heats up and glows when current passes through it; most pressing irons have nichrome wire that heats up when current passes through it and the heat is transferred to the base which is used for ironing; electric motors use the electromagnetic effect caused by current flow to drag the rotating part (rotor) of the motor which is how basic appliances like standing and ceiling fans operate. This defined flow and use of current is the reason why Residual Current Circuit Breakers (RCCB) are useful today. It is a device that the Live (L) and the Neutral (N) of an AC circuit (or the Positive and the Negative of a DC circuit) is passed through and at any moment it is measuring the current passing through both lines. Once there is a mismatch between the two, it trips and breaks (cuts off) the connection, which prevents the escalation of electric shock to a person or a fault developed in an electrical appliance.
What is electricity? – Electricity Explained
Does Electricity REALLY Flow? (Electrodynamics)
In addition you can also watch these videos to get a better grasp of electron flow:
How does an Electric Motor work? (DC Motor)
How does an Induction Motor work?
What is Current?
How Do Solar Panels Work? (Physics of Solar Cells)
Since Silicon has a shiny appearance, to absorb more of the energy from the sun as some can be lost through reflection of the light rays, antireflective coatings are applied on the cells. Also, to be able provide a more useful output, solar cells are connected in series and/or parallel to give the desired level of voltage and current. Losses due to reflection and other internal and external factors are the reasons for the relatively low efficiencies of solar PV technology.
Energy Band Gap
A key property of materials in solar PV electricity generation is the energy band gap of the materials. Penn state University explained the energy band gap as the minimum energy required by an electron of a material to break free from the outermost shell of its parent material. It is this breaking free and movement/flow of electrons that leads to current flow. Conductors generally have zero energy band gap as they have free electrons which readily conduct electricity on the application of an electric potential. Insulators on the other hand have very high energy band gap and thus will require a lot of energy to let electrons loose. Semiconductors are somewhat in the middle, requiring some energy but not as much as insulators to break electrons free. The energy band gap with unit in electron-volts (eV) of some materials are in the table below.
|Material||Band Gap (eV)|
|SiO2||Silicon dioxide (Silica)||9|
A plot of some of these band gap energy against possible efficiency values according to Polman A. et al., 2016, is shown in the image below. The 50% and 75% are percentages of the Shockley-Queisser limit which will be discussed later.
Materials like Lead sulphide (PbS) and Germanium (Ge) would result in a low efficiency because of their low band gap energy. Lower band gap energy can be translated to higher photon energy loss and increase in temperature of the cell. According to Penn State University, the suitability of a material for solar PV application is dependent on the closeness of the band gap energy to the photon energy from the sun. The photon energy is expected to be slightly higher than the band gap energy to minimize losses while knocking off electrons and creating holes in the process. The energy of a photon is also measured in electron-Volts and the energy level of different photons in a beam of light vary from one to another. ZME Science simply defined a photon as the smallest discrete amount of electromagnetic radiation and the basic unit of light. The energy of a photon is dependent on its frequency and this relationship is shown by the equation:
E is the energy of the photon in Joules,
h is the Planck’s constant
f is the photon frequency,
c is the speed of light
λ is the photon wavelength.
From the equation it can be deduced that
and since the speed of light is constant, as the wavelength reduces, the frequency increases and vice versa. The spectrum of radiation reaching the earth from the sun is shown in the image below.
One can see the Ozone molecules (O3) absorbing some of the ultraviolet (UV) rays, of which about 90% of the molecules residing in the stratosphere are referred to as the Ozone layer.
The electromagnetic spectrum showing electromagnetic waves in the order of their increasing wavelength which corresponds to decreasing frequency and decreasing photon energy is shown in the image below. The sun emits some of these waves or radiation at various intensities/irradiance as shown in the solar radiation spectrum.
The US National Aeronautics and Space Administration made an easy comparison of these radiations using the image below.
Consider a photon of wavelength 750 nanometers (7.5 * 10-5 cm). The energy in Joules and electron-Volts are calculated as follows.
When a photon of much higher energy than the energy band gap of the material hits the solar cell, electron breaks loose as expected, however the excess energy will be lost as heat in the solar cell which will reduce the productivity of the cell and its efficiency of energy conversion. Energy Education, an initiative of the University of Calgary noted that as the photon’s energy becomes too high for the material band gap, the probability of releasing an electron decreases. Photons with less energy than the energy band gap are simply not absorbed by the cell. Multijunction solar cells which have different active materials making up multiple junctions take advantage of the different energy levels of photons to set more electrons free. Multijunction solar cells provide a means to exceed the junction solar cell efficiency limit of 33.7% called the Shockley-Queisser limit. A single pn junction in the basic sense is formed by placing an N-type material together with a P-type material.
When more than two active materials are placed together, a multijunction or heterojunction cell is formed. This is briefly discussed later.
The crystalline silicon solar PV technology is the technology with the least material complexity according to researchers at MIT Energy Institute and the order of increasing material complexity for different technologies as presented by the researchers is shown below. Material complexity here was described as the number of atoms in the molecule or crystal unit forming the building block for the technology.
Description of Solar PV Technologies
The graph of the highest efficiencies of different solar cell recorded over time has been published by the U.S. National Renewable Energy Laboratory (NREL) and included below. These efficiencies represent what percentage of the photon energy that is converted to electricity.
Figure 11: Best research-cell efficiencies | Source: NREL
Descriptions of some of these solar PV technologies are as follows.
Monocrystalline Silicon (Mono-Si)
The Penn State University, Department of Energy and Mineral Engineering described this technology as likely the oldest of the solar PV technology and still very applicable today. As per the report from Fraunhofer Institute for Solar Energy Systems (ISE), monocrystalline silicon technology recorded a lab cell efficiency of 26.7%, which was higher than multicrystalline silicon, Copper Indium Gallium Selenide (CIGS), Cadmium Telluride and Perovskite. Also, its best performing modules had a lab efficiency of 24.4%. These relatively higher efficiencies are as a result of the mono-Si solar cells formation from pure silicon crystals with continuous lattice. Because mono-Si cells are made from continuous crystal lattice (single crystal), they are more complex to manufacture and hence relatively more expensive than the Multi-Si counterpart.
The advantages of this cell technology are their better space utilization as a result of their higher efficiency rate, and their longevity, although these come at a higher cost.
Polycrystalline/Multicrystalline Silicon (Multi-Si)
Solar cells that belong to this category are made from multiple silicon crystals/grains, and according to GreenMatch, they are made from melting raw silicon. This technology recorded a lab cell efficiency of 22.3% according to Fraunhofer ISE which as expected was lower than the Mono-Si cells. The process of their manufacture makes them faster, easier and cheaper to make at the expense of their efficiency.
This cell technology has the benefit of lower cost than Multi-Si although they do not utilize space as efficient as the Mono-Si cells do.
Gallium Arsenide (GaAs)
Unlike silicon-based solar cells, this technology is based on two active elements – Gallium (Ga) and Arsenic (As), which can also be seen in the periodic table occupying positions 31 and 33 respectively. Gallium Arsenide compound is formed when these two elements bond. According to Appropedia, Gallium Arsenide as a semiconductor compound has more saturated electron velocity and electron mobility than silicon semiconductors; electron mobility being how fast an electron moves through a semiconductor or metal when pulled by an electric field. PV Magazine described Gallium Arsenide cells as highly expensive and highly efficient, with efficiencies ranging from 27% using the more cost-effective HVPE (Hydride Vapor Phase Epitaxy) manufacturing process to 29.1% using MOVPE (Metalorganic Vapor Phase Epitaxy) production process. These high efficiencies of GaAs are evident in the efficiency-band gap curve of Figure 6. Research efforts are being made by organisations like the NREL to improve the economics of the technology here on earth as the technology has mostly been used in space applications like satellites and spacecraft.
This cell technology has the benefits of flexibility and lightweight due to thin layers, boasts of the best efficiency for single junction cells. Cells of this technology are also more resistant to moisture, temperature and the degrading effects of radiations like ultraviolet UV wave. In addition to these, they also have good performance in low-light conditions. The disadvantages include the very high cost of the cell technology, scarceness of materials like gallium, and toxicity of the Arsenic element.
Amorphous Silicon (a-Si)
GreenMatch described this technology as that seen in pocket calculators. They are made from non-crystalline silicon (silicon film) deposited on substrate materials like glass, metal or plastic. They are easier and cheaper to manufacture than crystalline silicon, however they have low efficiency of less than ten. According to Encyclopedia Britannica, amorphous silicon are the oldest and most mature of thin-film solar PV technology, and has a thickness of about 1 micrometer – a millionth of a meter. Penn State University, Department of Energy and Mineral Engineering noted that the a-Si technology is less prone to overheating unlike some other PV technologies affected negatively by increasing temperature. 0.3% of the 2017 global PV production in 2017 was based on this thin-film cell technology.
The advantages of this technology include their low cost, uniformity of performance over large areas as impurities have little effect on them, ability to be fabricated in different shapes, and resistance to heat. However, they have lower efficiencies and possibly shorter lifetime than Mono-Si and Poly-Si cells.
Cadmium Telluride (CdTe)
This cell technology also belongs to the thin-film solar PV category and the most common of these cells are made by placing p-doped Cadmium Telluride (CdTe) with n-doped Cadmium Sulfide (CdS), according to the Office of Energy Efficiency & Renewable Energy of the US Department of Energy (DOE). This cell technology was also noted to be the second most common PV technology after silicon and allows for cheaper and faster production processes. While they were cost-effective, their efficiencies were not as high as the crystalline silicon until in 2016 when First Solar, a U.S. based solar PV manufacturing company with specialty and dominance in thin film technologies, recorded a lab cell efficiency of 22.1% which is comparable to that of Multi-Si. In August 2019, researchers from the National Renewable Energy Laboratory (NREL) and First Solar made a breakthrough in the fabrication cost and long-term performance of CdTe solar cells by doping with arsenic (As), however resulting in a 20.8% efficiency. Copper (Cu) which had been a key element in CdTe cell manufacture was completely avoided because of the costs its use contributed. Of the estimated 4.5% accounted for by thin film cell technology as per the Fraunhofer ISE 2020 photovoltaics report, Cadmium Telluride accounted for 2.3% of the 2017 global production. The report also highlighted that while the efficiency of average crystalline silicon-based modules increased from around 12% to 17% in the last 10 years, that of cadmium telluride increased from 9% to 19% in the same period.
The benefits of this technology are their quick and low-cost manufacturing which translate to low-cost products, possession of close to ideal band-gap of 1.5 eV, and tolerance to electronic defects. However, the use of copper in the cell as had been the norm caused defect in the cell. Also, the limited availability/supply of Tellurium impacts this cell technology.
Copper Indium Gallium Selenide (CIGS)
According to the US DOE, the journey to this thin-film cell technology started off with copper indium diselenide (CIS). The indium in the compound was later fully replaced with gallium to give copper gallium diselenide (CGS) and thus increasing band gap energy from 1.04 eV to 1.68 eV. Better performing cells with more desirable band gap energy and higher efficiency were then reached by partially replacing gallium for indium, leading to what is known as copper indium gallium diselenide (CIGS). The US DOE also noted that even though lab cell efficiencies have surpassed 20%, commercial modules based on this technology operate within the efficiency range of 12% to 14%. This solar cell technology is made by depositing materials on a substrate through either the co-evaporation process or the precursor reaction processes, both of which are described by the US DOE. CI(G)S accounted for 1.9% of the 2017 global PV production according to Fraunhofer.
The absorption of a wide range of the solar spectrum by cells belonging to this cell technology makes this technology attractive. Also, different processes have been employed successfully to manufacture solar PV products based on this technology. Additionally, cells of this technology are more resistant to heat and can be manufactured to be flexible. This cell technology is limited by the cost and complexity of fabrication as well as lesser efficiency which hinders its competition with other major solar PV technologies.
According to BBC, perovskites are the fastest improving solar technology, which is evident in NREL best efficiencies chart of Figure 11. The US DOE described this cell technology as thin-film cells having different layers of materials. These materials are mostly hybrid organic-inorganic metal (lead or tin) halides which absorb the photons. An example of such material is Methylammonium lead halides and a key advantage of this cell technology is its ability to absorb light/photons across a wide range of wavelengths (from Figure 7), as noted by Perovskite Info. The US DOE has noted that a challenge hindering the commercializing of this cell technology is the significant mismatch between the miniature cell and large module efficiencies. This challenge can be linked to their poor cell stability due to the effect of moisture, high temperature and duration of exposure to light on them, when compared to leading PV technologies. Several researches are being made to overcome these challenges as perovskites cell technology promise higher efficiencies which recorded 28% in 2018. In January 2020, it was reported that scientists at the Helmholtz Zentrum Berlin (HZB) produced a perovskite/silicon tandem cell which recorded a 29.15% efficiency, shown in the NREL best efficiencies chart. A perovskite/silicon tandem cell is, in the basic sense, made by stacking both materials one on top of the other, in order to increase the efficiency of the cell by capturing a wider range of photons with different wave lengths. The 23.26% record efficiency of perovskite/CIGS tandem cell is also held by the same group at HZB that recorded the 29.15% efficiency for perovskite/silicon tandem cell.
The merits of this emerging cell technology include their ability to be engineered to absorb energy across almost all wavelengths of visible light, the ease and low-cost means of manufacturing perovskite materials like methylammonium lead halides, and the possibility of making flexible, lightweight and/or semi-transparent cells. However, the cost of fabrication of long-lasting perovskite cells is still high as perovskite cells can degrade in the presence of moisture. Adding better enclosure for the modules adds to the cost and weight of the product. There are also the disadvantages of lead being a pollutant, and the high toxicity of lead iodide (PbI), a breakdown product of perovskite.
Organic Photovoltaic (OPV) Technology
As the name implies, this cell technology uses organic (carbon-based) compounds to convert the energy from the sun. This use of carbon-based compounds is the reason why this cell technology is also referred to as polymer solar cells or plastic solar cells. The NREL highlighted that the uniqueness of organic photovoltaic cells lies in the variety of organic materials that can be used in making the cells. This variety offers the advantage of having different colours of this cell technology and possibly a transparent design, which makes it possible for the technology to be integrated in a wider range of applications like buildings, clothing, and accessories. The diversity and availability of the organic materials required for this cell technology, together with the ease of fabrication was the reason why the US DOE noted that this PV technology has the potential to provide electricity at a lower cost than first- and second-generation solar technologies. However, in addition to the barrier of long-term unreliability, this cell technology has also been significantly limited by its lower efficiencies reported to have recorded 13.2% by the NREL. The US DOE grouped the cells belonging to the technology into two. The small-molecule OPV cells which are made from materials like phthalocyanines, polyacenes, squareness, perylene dyes and fullerenes. The other, polymer-based OPV cells, are made from long-chained molecular systems and derivatized fullerenes. There is also another technology close to the OPV technology – Dye-sensitized PV technology which is based on a hybrid of organic and inorganic materials.
Quantum Dot Technology
The cell technology uses nanoparticles of semiconductors which when exposed to photons of light, capture the photon energy and pass electrons from one dot to the other to generate electric current. AltEnergyMag wrote that these nanoparticles called quantum dots have band gaps that can be adjusted by altering the size of the dots. The US DOE further explained that “quantum dots can absorb light at one wavelength, efficiently convert it and re-emit it at another wavelength.” The re-emitted photons can then be guided to PV cells that can convert them more efficiently. In February 2020, scientists at University of Queensland, Australia set a new world efficiency record of 16.6% for a quantum dot solar cell. Interestingly, the cell that recorded this efficiency was made from a perovskite base. This new efficiency shows a 25% improvement from the previous record of 13.4% set by the NREL in 2017. This trend is promising because the reported cell efficiency in 2010 was only 2.7%, according to PV Magazine.
This technology enjoys the benefits of high power to weight ratio, flexibility of fabrication, and versatility of application. However, they are less stable and very reactive as a result of their high surface-area-to-volume ratio. Also, quantum dot cells based on cadmium-selenide are known to be very toxic in nature and would therefore require stable enclosures.
Passivated Emitter and Rear Cell / Passivated Emitter and Rear Contact (PERC)
This technology improves the efficiency of a cell by adding a dielectric passivation layer at the rear part of a cell in order to improve the operation of the cell. This technology is attractive because only little modification to standard manufacturing processes is necessary to incorporate the technology. The three benefits of the PERC technology mentioned by EnergySage which improve efficiency are the reflection of light by the rear positioned passivation layer back into the cell so that more photons are absorbed, the reduction of electron recombination which causes restriction in the flow of electrons and consequently less efficiency, and the reflection of undesirable light wavelengths which would otherwise heat the module’s metal back sheet and then the cells of the module. More information on this technology has been provided by sinovoltaics.
Passivated Emitter Rear Totally Diffused (PERT)
This technology is applicable to both monofacial and bifacial solar cells and involves replacing localised aluminium-alloy which serves as Back Surface Field for standard cells with a totally diffused layer. This offers benefits like reduction or elimination of light-induced degradation (LID). LID occurs due to inevitable prolonged exposure of solar modules to light.
Passivated Emitter Rear Locally Diffused (PERL)
This technology which is applicable to bifacial cells combines the advantages of both PERC and PERT. According to Sinovoltaics, like PERC cells, the front and rear surface are passivated. However, the rear-end is locally diffused at the metal contacts to limit recombination, unlike the totally diffused structure in PERT cells.
Half Cell Technology
As the name implies, this involves cutting cells in half so as to take advantage of the benefits that presents. A standard 60 cell solar module can be equated to a 120 half-cells solar module. The higher number of cells reduces resistance as there are more conduction paths in the module. Also, the smaller size of the cells reduces their susceptibility to mechanical stress. These make half-cell modules to generally have higher output and reliability than standard solar modules.
Heterojunction Technology (HJT)
This technology combines the desirable qualities of crystalline silicon and thin film technologies like amorphous silicon to produce hybrid cells with better efficiency and performance, even better than the PERC technology, according to Solar Power World. Meyer Burger, a leading HJT production line manufacturer, noted that thin layers of doped and intrinsic amorphous silicon are applied on the two sides of n-type silicon wafers in the process of making products based on this technology.
This is used to refer to solar modules that can generate power from both of their sides. Some of the light from the sun hits the front end of the solar module while the light that reaches the surface beneath the module’s rear end is reflected (depending on the surface) to strike the rear end of the module. This increases the overall efficiency of the module as light is captured from both ends.
Building-Integrated Photovoltaics (BIPV)
This is the type or design of solar photovoltaic products (modules) that can be integrated into buildings. Avenston made a good description of BIPV that distinguishes them from Building Attached Photovoltaics (BAPV). Simply put, BIPV are integral part of buildings, can have other functions other than electricity generation like serving as windows, roofs, tiles, etc., and their absence would make it obvious that something was missing. BAPV on the other hand are mounted after a building is complete. This means they are not an integral part of the building and are only there for the sole purpose of generating electricity. Architectural Solar Association provided a number of BIPV applications in pictures.
Metal Wrap Through (MWT)
Itai Suez, the Vice President of Product development at Silfab Solar, in an interview with AltEnergyMag, described MWT as a technology that builds upon crystalline silicon solar cells. He noted that MWT cells differ from standard c-Si cells by having holes from behind and through the cell in which metals are printed and fired into the cell, instead of having the metal connectors running on the surface of the cells. The metals conduct the generated current without causing obstruction/shading on the frontside of the cell, hence improving the efficiency.
Global PV Market
The 2020 photovoltaics report by Fraunhofer Institute for Solar Energy Systems provided some insights on the global PV market. According to the report, the cumulative PV installation by the end of 2019 was 635 GW, and for the year 2018, electricity generation from solar PV was 585 TWh (585,000 GWh) accounting for 2.2% of total generation for the year. The report noted that for the global PV module manufacture in 2017, 70% was from China and Taiwan, 14.8% from the rest of Asia-Pacific and Central Asia, 3.1% from Europe and 3.7% from the United States and Canada. As noted earlier, 95% of the global PV production in 2017 was crystalline silicon based, of which 62% out of the 95% was multi-crystalline technology. Fraunhofer ISE also reported that the Compound Annual Growth Rate (CAGR) of cumulative solar PV installations was 32% from 2010 to 2019. The International Renewable Energy Agency (IRENA), in their November 2019 “Future of Solar Photovoltaic” report, projected that global solar PV capacity is expected to grow six fold by 2030 and with a CAGR of 8.9% by 2050 both with respect to the 2018 capacity.
A list of some companies that manufacture solar panels, according to their countries, are outlined in the table following.
Table 2: Some solar panel producers from around the world
Some of these companies have offices and/or factories outside the countries where they are headquartered, and they employ different means of supplying their products to the markets they serve. Some are open to having end users buying directly from them while others prefer supplying distributors or companies specialized in installing the systems.
Some of the factors to be considered by companies or countries before venturing into solar PV manufacture include availability of the market to absorb its products, investment into infrastructures, technologies and research required, ability to follow standardized procedures in manufacturing certified products, and developing a supply chain structure that aids in presenting competitive products to the market. These factors are also applicable to manufacturing standard grade silicon compounds or solar cells.
Sample Comparison of Prices of Solar Panels
The prices of solar panels vary between companies as well as between regions. A random consideration of different 320Wp monocrystalline solar panel prices reveal the following as at June 2020.
Points to Note When Buying Solar Panels
Energy Matters provided some tips and points to consider when buying solar power system. Those that apply to solar panels are listed below.
- Ask for recommendations and user experiences from people who had used or are using solar PV components.
- Take note of the product warranty which informs on the expected useful life of the product.
- Have realistic price expectations so as not to spend more than is required or much less for a substandard product.
- Check for product certifications which shows the tests that the product has passed.
- Decide on the panel type that best suits the application and location.
- Decide on the mounting structure so that the required cost is accounted for.
- Seek multiple quotes from sellers and installers before settling for one. This helps one make good comparison of products and services before making a decision.
It is advised to work with trusted/certified solar photovoltaic systems installers who have had some experience to help make good decisions as regards to the use of the products in the market.
Electricity Generation from Different Solar PV Cell Technologies by Onyedika Atuchukwu