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How Many Solar Cells Do I Need. Crystalline silicon photovoltaic cells

How Many Solar Cells Do I Need. Crystalline silicon photovoltaic cells

    Solar Photovoltaic Cell Basics

    When light shines on a photovoltaic (PV) cell – also called a solar cell – that light may be reflected, absorbed, or pass right through the cell. The PV cell is composed of semiconductor material; the “semi” means that it can conduct electricity better than an insulator but not as well as a good conductor like a metal. There are several different semiconductor materials used in PV cells.

    When the semiconductor is exposed to light, it absorbs the light’s energy and transfers it to negatively charged particles in the material called electrons. This extra energy allows the electrons to flow through the material as an electrical current. This current is extracted through conductive metal contacts – the grid-like lines on a solar cells – and can then be used to power your home and the rest of the electric grid.

    The efficiency of a PV cell is simply the amount of electrical power coming out of the cell compared to the energy from the light shining on it, which indicates how effective the cell is at converting energy from one form to the other. The amount of electricity produced from PV cells depends on the characteristics (such as intensity and wavelengths) of the light available and multiple performance attributes of the cell.

    An important property of PV semiconductors is the bandgap, which indicates what wavelengths of light the material can absorb and convert to electrical energy. If the semiconductor’s bandgap matches the wavelengths of light shining on the PV cell, then that cell can efficiently make use of all the available energy.

    Learn more below about the most commonly-used semiconductor materials for PV cells.

    Silicon

    Silicon is, by far, the most common semiconductor material used in solar cells, representing approximately 95% of the modules sold today. It is also the second most abundant material on Earth (after oxygen) and the most common semiconductor used in computer chips. Crystalline silicon cells are made of silicon atoms connected to one another to form a crystal lattice. This lattice provides an organized structure that makes conversion of light into electricity more efficient.

    Solar cells made out of silicon currently provide a combination of high efficiency, low cost, and long lifetime. Modules are expected to last for 25 years or more, still producing more than 80% of their original power after this time.

    Thin-Film Photovoltaics

    A thin-film solar cell is made by depositing one or more thin layers of PV material on a supporting material such as glass, plastic, or metal. There are two main types of thin-film PV semiconductors on the market today: cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS). Both materials can be deposited directly onto either the front or back of the module surface.

    CdTe is the second-most common PV material after silicon, and CdTe cells can be made using low-cost manufacturing processes. While this makes them a cost-effective alternative, their efficiencies still aren’t quite as high as silicon. CIGS cells have optimal properties for a PV material and high efficiencies in the lab, but the complexity involved in combining four elements makes the transition from lab to manufacturing more challenging. Both CdTe and CIGS require more protection than silicon to enable long-lasting operation outdoors.

    Perovskite Photovoltaics

    Perovskite solar cells are a type of thin-film cell and are named after their characteristic crystal structure. Perovskite cells are built with layers of materials that are printed, coated, or vacuum-deposited onto an underlying support layer, known as the substrate. They are typically easy to assemble and can reach efficiencies similar to crystalline silicon. In the lab, perovskite solar cell efficiencies have improved faster than any other PV material, from 3% in 2009 to over 25% in 2020. To be commercially viable, perovskite PV cells have to become stable enough to survive 20 years outdoors, so researchers are working on making them more durable and developing large-scale, low-cost manufacturing techniques.

    Organic Photovoltaics

    Organic PV, or OPV, cells are composed of carbon-rich (organic) compounds and can be tailored to enhance a specific function of the PV cell, such as bandgap, transparency, or color. OPV cells are currently only about half as efficient as crystalline silicon cells and have shorter operating lifetimes, but could be less expensive to manufacture in high volumes. They can also be applied to a variety of supporting materials, such as flexible plastic, making OPV able to serve a wide variety of uses.PV

    Quantum Dots

    Quantum dot solar cells conduct electricity through tiny particles of different semiconductor materials just a few nanometers wide, called quantum dots. Quantum dots provide a new way to process semiconductor materials, but it is difficult to create an electrical connection between them, so they’re currently not very efficient. However, they are easy to make into solar cells. They can be deposited onto a substrate using a spin-coat method, a spray, or roll-to-roll printers like the ones used to print newspapers.

    Quantum dots come in various sizes and their bandgap is customizable, enabling them to collect light that’s difficult to capture and to be paired with other semiconductors, like perovskites, to optimize the performance of a multijunction solar cell (more on those below).

    Multijunction Photovoltaics

    Another strategy to improve PV cell efficiency is layering multiple semiconductors to make multijunction solar cells. These cells are essentially stacks of different semiconductor materials, as opposed to single-junction cells, which have only one semiconductor. Each layer has a different bandgap, so they each absorb a different part of the solar spectrum, making greater use of sunlight than single-junction cells. Multijunction solar cells can reach record efficiency levels because the light that doesn’t get absorbed by the first semiconductor layer is captured by a layer beneath it.

    While all solar cells with more than one bandgap are multijunction solar cells, a solar cell with exactly two bandgaps is called a tandem solar cell. Multijunction solar cells that combine semiconductors from columns III and V in the periodic table are called multijunction III-V solar cells.

    Multijunction solar cells have demonstrated efficiencies higher than 45%, but they’re costly and difficult to manufacture, so they’re reserved for space exploration. The military is using III-V solar cells in drones, and researchers are exploring other uses for them where high efficiency is key.

    Concentration Photovoltaics

    Concentration PV, also known as CPV, focuses sunlight onto a solar cell by using a mirror or lens. By focusing sunlight onto a small area, less PV material is required. PV materials become more efficient as the light becomes more concentrated, so the highest overall efficiencies are obtained with CPV cells and modules. However, more expensive materials, manufacturing techniques, and ability to track the movement of the sun are required, so demonstrating the necessary cost advantage over today’s high-volume silicon modules has become challenging.

    Learn more about photovoltaics research in the Solar Energy Technologies Office, check out these solar energy information resources, and find out more about how solar works.

    How Many Solar Cells Do I Need

    Many individual silicon solar cells tend to have an open-circuit voltage of approximately 0.5 volts and a short-circuit output current limited to approximately 3 amps, therefore it is necessary to combine these individual solar cells together in either series and parallel combinations to obtain higher voltages and currents. But how many solar cells do I need to construct a PV panel.

    A commercially available photovoltaic panel is constructed using between 32 and 48 individual solar cells in series to give a panel capable of charging a 12V DC battery. But how many solar cells are in a solar panel, and how many solar cells do I need? Well, as usual, it depends on your specific application.

    The electrical power generated by a photovoltaic cell, ( PV ) has two components: Voltage ( V ) and Current ( I ). The output power generated by the PV cell is measured in Watts, ( P ) that the cell produces is the product of the cell’s output current times its output voltage. In other words, P = V x I.

    The voltage output of the photovoltaic cell remains fairly constant over a wide range of input light intensities because of the cells photovoltaic effect, just as long as there is some light. The output current, however, varies in direct proportion to the amount of sunlight entering the PV cell. The more light entering the cell, the more current it produces up to its maximum. The solar cell’s output voltage remains fairly stable from low to bright sunlight.

    For the purposes of this tutorial here, we will consider a standard 4″ by 4″ (100mm X 100mm) poly-crystalline silicon photovoltaic cell. Mono-crystalline or amorphous silicon cells are available.

    The absolute value of the voltage information will differ slightly, but their general performance tends to remain the same for all types of silicon PV cells for the amount of sunshine it receives on a sunny day. So how does a solar cell work.

    Photovoltaic Cell Voltage

    A poly-crystalline silicon solar cell has an open circuit voltage of about 0.57 Volts at 25°C. Open circuit voltage means that the cell is not connected to any electrical load and is therefore not generating any current.

    When connected to a load, for example a battery, the output voltage of the individual cell will drop to about 0.46 Volts at 25°C as the generated current flows. It will remain around this 0.46 V level regardless of the sun’s intensity or the amount of current the cell produces.

    This decrease in output voltage is caused by internal resistance losses within the cell’s structure as well as voltage drops across the metallic conductors deposited on the cell’s surface to collect the current. Ambient temperature also has an affect on the PV’s cell’s voltage. The higher the temperature is, the lower the cell’s output voltage becomes as it heats up, which is strange seeing that they spend all day sat in the sun.

    Photovoltaic Cell Current

    While the voltage produced by a silicon photovoltaic cell is fairly constant, its output current on the other hand varies considerably. The amount of usable output current that a cell generates depends on how intense the sunlight is shinning onto the cell’s surface, and also the voltage difference between the cell and the load.

    Under normal operating conditions a poly-crystalline cell is rated at about 2.87 Amperes of current. This value can increase considerably on a very cold, very clear, very bright and very snowy winter’s afternoon. Also altitude is another factor that affects the PV cell’s output current. The higher you are, the less atmospheric conditions there is above and the more sunlight the cell will receive, assuming no clouds or snow. So expect to see current gains if used well above sea level.

    Connecting Individual Solar Cells into Modules

    When individual photovoltaic cells are assembled together into modules or panels they are generally wired in series. That is the positive connection or pole of one PV cell is connected to the negative connection or pole of the next cell, and so on until all the cells in the panel are connected together in what is called a series string.

    When individual photovoltaic cells are assembled together into modules or panels they are generally wired in series. That is the positive connection or pole of one PV cell is connected to the negative connection or pole of the next cell, and so on until all the cells in the panel are connected together in what is called a series string.

    This series wiring is done to raise the voltage of the panel. We said earlier that a single cell has a voltage potential of about 0.46 Volts. This is not enough voltage to do any usable work in a 12 Volt system. But if we add the voltages together of say 36 cells by series wiring them, then we have a working voltage 16.7 Volts, and that’s more than enough to charge a 12 Volt battery.

    The operational voltage of a typical 12 Volt lead acid battery ranges from between 10.5 volts to 14 volts. The battery’s exact voltage depends on its state of charge, ambient temperature, and whether the battery is being charged or discharged at the time. It is this battery voltage curve that the PV panels are designed to fit and so MUST provide a greater voltage than the battery possesses. If the PV panel cannot do this, then it cannot transfer electrons to the battery and therefore it cannot recharge the battery.

    The output current generated by a solar panel of 36 cells in total remains the same as the current produced by one single cell, about 3 Amperes. The series wiring technique causes the voltages to be added together, but the current remains the same. We could parallel connect all the 36 cells but this would add their currents together rather than their voltages. The result of this would be a solar panel that produces 108 Amperes of electric current, (36 x 3) but at only 0.46 Volts, too low.

    So How Many Cells Do I Need

    Most photovoltaic (PV) panel manufacturers make 12 Volt solar panels for battery charging applications with 32, 36, or 48 cells in the series string. They are all rated at about the same current, being composed of the same basic cell. The difference between these panels is one of voltage. The question for us to answer here is how their output voltages relate to the voltages we require for our 12V charging system.

    32 Photovoltaic Cells in Series

    This size of photovoltaic panel has the lowest voltage rating of only 14.7 Volts (0.46 Volts times 32 cells). This is because it has the fewest number of PV cells in its series string. This panel design closely matches the charging curve of a standard 12 Volt lead acid battery. As the battery charges-up, its terminal voltage rises.

    When this battery is almost full its voltage is about the same as the PV cell’s at around 14.7 volts. The 32 cell module simply hasn’t enough voltage to continue charging the battery when its full so cannot overcharge the average, small, lead acid battery.

    The applications suitable for these small 32 cell solar panels are in RV’s, boats, garden lighting and summer cabins. These applications are characterized by their intermittent use and relatively small battery charging capacity. In these these types of low power applications, a 32 cell panel can be used with or without a charge current regulator as the batteries will not become overcharged if left connect to the panel during long periods of non-use.

    36 Photovoltaic Cells in Series

    This size of photovoltaic panel has an output voltage of about 16.7 Volts (0.46 times 36 cells). This is enough output voltage to be able to continue to charge a lead acid battery even though it may be already fully recharged. The 36 cell panel is suitable for a home based 12 Volt alternative energy system with high battery capacities as it has the higher output voltage necessary to recharge deep cycle lead acid batteries.

    However, a 36 cell solar panel will require some form of charge regulation to prevent overcharging the battery during periods of high solar intensities or when battery usage is at its lowest.

    A 36 cell solar panel tends to be more cost effective in a typical home power application because it can produce a good amount of current or high voltages at elevated temperatures. The higher voltage produced by the 36 series wired cells will more effectively recharges a large deep cycle lead acid batteries.

    High ambient temperatures will cause the voltage of any PV panel to reduce slightly, but the 36 cell panel has more than enough voltage surplus to still be an effective battery charger even at high ambient temperatures.

    48 Photovoltaic Cells in Series

    A 48 cell panel is the big daddy of the PV industry. 48 individual photovoltaic cells connected in series produces an output voltage of about 22 volts. These large PV panels have sufficient output current capacity to charge a 12 Volt system, regardless of the battery’s voltage or high temperature.

    However, these large panels do require some form of charge regulation in just about every application. They have the sufficient voltage necessary to raise a solar system’s voltage, while charging full batteries, to well over 16 volts. This over voltage is high enough to ruin any electronic equipment rated at 12 VDC so some form of protection is needed.

    Generally, a 48 cell solar module has very specific applications where high power and currents are required such as in pumping water or are combined together with other 48 cell panels to produce a photovoltaic array. Solar arrays can combine many panels together in various combinations for increaesd power output.

    Another disadvantage of this PV panel is its physical size and additional cost compared to 32 and 36 PV cell panels. 48 cell panels are larger so take up more roof space.On the plus side, a 48 cell panel will perform better in very hot areas and areas with very low levels of sunlight throughout the year.

    To learn more about “Photovoltaic Cells”, solar panels and solar power, or to learn how to build your own DIY solar panel from individual solar cells to make Solar Power in your home a reality so you can save money on your utility bills, and to help you on your way consider Clicking Here and getting one of the solar books from Amazon about home made solar panel construction ensuring so that you have all the necessary information to get your solar power installation working efficiently and effectively the first time.

    Monocrystalline vs Polycrystalline Solar Panels

    When it comes to solar panels, one of the most asked questions is which solar cell type is better: Monocrystalline or Polycrystalline?

    Well, if you are looking for a detailed answer, then you came to just the right place.

    In this article, we will do a full in-depth comparison between Monocrystalline and Polycrystalline solar panels including:

    • How are they made?
    • What do they look like?
    • How efficient are they?
    • How well do they react to heat?
    • What is their expected lifespan?
    • Are they recyclable?
    • How expensive are they?

    But first, let’s see how Solar PV works

    Solar Photovoltaics (PV) is the direct conversion to electric current at the junction of two substances exposed to solar energy. It occurs through a process known as the Photovoltaic Effect which cause photons to be absorbed and electron discharge. Solar energy is composed of photons which are small packets of electromagnetic energy. Materials that exhibit this photovoltaic effect are known as PV or Solar cells.

    Solar cells are composed of semiconductor materials, such as silicon, used in the microelectronics industry. For solar cells, a thin semiconductor wafer is specially treated to form an electric field, positive on one side and negative on the other. When light energy strikes the solar cell, electrons are knocked loose from the atoms in the semiconductor material. If electrical conductors are attached to the positive and negative sides, forming an electrical circuit, the electrons can be captured in the form of an electric current — that is, electricity. This electricity can then be used to power a load, such as a light or a tool.

    The first photovoltaic module was built by Bell Laboratories in 1954.

    So, without further ado, let’s jump right into how solar panels are made.

    A. Manufacture

    In 1918, the Polish scientist Jan Czochralski discovered a brilliant method for monocrystalline silicon production and called it the Czochralski Process, and later in 1941, the first cell was constructed.

    The manufacture of monocrystalline solar cells contains 8 main steps and, in this section, we will quickly go through each one of them.

    many, solar, cells, need, crystalline, silicon

    The main ingredient that makes monocrystalline solar panels is silicon also known as Silica sand, Quartzite, or SiO2.

    The first step in manufacturing monocrystalline cells is to extract pure silicon from quartzite to make metallurgical silicon.

    To make metallurgical silicon, special ovens are used to melt SiO2 and Carbon at temperatures of over 2,552 degrees Fahrenheit leaving behind 98% to 99% pure silicon.

    Although the high purity of metallurgical silicon, it’s not pure enough to be used in PV panels.

    Therefore, further purification needs to be done.

    The next step is to purify this metallurgical silicon using the Siemens process.

    First, we expose the powder of metallurgical silicon Si in a reactor with HCl at elevated temperatures resulting in SiHCl3 gas.

    The gas is then cooled and liquefied for distillation.

    Distillation is the process of evaporating then condensing the liquid to get rid of unwanted impurities.

    For instance, you can boil seawater (salted water), then condense the vapor to get pure water, as the salt will remain at the bottom of the pot.

    Using the same concept, the liquified SiHCl3 is heated then cooled to remove impurities with higher and lower boiling points such as Calcium and Aluminum.

    After distillation, the liquefied SiHCl3 is moved to a different insulated reactor with a hot rod, then mixed with Hydrogen gas and vaporized again at a temperature of up to 2732 degrees Fahrenheit.

    Due to the heat and the presence of H2 gas, the Cl atoms will dissolve leaving around 99.9999% pure silicon behind.

    What differs monocrystalline cells from polycrystalline cells is that monocrystalline panels are made of a single pure silicon ingot.

    Making a single pure silicon ingot was really hard until Czochralski discovered this brilliant way.

    First, you dip a seed crystal, which is a small rod of pure single crystal silicon into the molten silicon.

    After dipping the rod, now it’s time to slowly pull and rotate the seed crystal upward at the same time to minimize the effect of convection in the melt.

    As the seed crystal is pulled up, the liquid silicon will slowly solidify over 4 days creating a big homogeneous cylindrical single crystal silicon also known as silicon ingot.

    The size of the silicon ingot depends on 3 factors: temperature gradient, cooling rate, and rotation speed.

    So far you have a huge single crystal silicon ingot, but how can you make solar panels of it?

    Well, the answer is very simple, wire saw.

    The third step is to slice the silicon ingot into very thin slices using a very sharp wire saw creating 1 mm or 0.0393 inches silicon wafers.

    After cutting the wafers, it’s about time to polish and wash the wafers to clean it from dust, dirt, and scratches.

    Because the wafer surface is very flat, many light rays are reflected away, and obviously, you don’t want that, as it will decrease the efficiency of the solar panel.

    For this reason, manufacturers roughen and etch the wafers’ surface, so the light can refract multiple times, which improves the panel’s efficiency and prevents light reflection as much as possible.

    Silicon wafers are positively charged. In other words, they act as a p-type material.

    To conduct electricity you need a p-n junction and in order to create a p-n junction, a negatively charged layer of phosphorus is added to each wafer, then wafers are moved to special 1652 degrees Fahrenheit ovens to inject the phosphorus with nitrogen.

    The mixture of nitrogen and phosphorus creates a powerful n-type layer resulting in a very effective p-n junction wafer, which of course will increase the efficiency of the panel.

    In order to decrease electricity loss, a highly-conductive silver alloy is pressed onto the wafer front, which ensures the power is perfectly transported and improves the monocrystalline cell conductivity even further.

    Finally, the last step in building monocrystalline panels is assembling.

    Each monocrystalline solar panel is made of 32 to 96 pure crystal wafers assembled in rows and columns.

    The number of cells in each panel determines the total power output of the cell.

    Polycrystalline also known as multi-crystalline or many-crystal solar panels are also made from pure silicon.

    However, unlike monocrystalline, they are made from many different silicon fragments instead of a single pure ingot.

    The difference between mono and poly solar cell production is that, after purifying the silicon, instead of pulling the ingot slowly to make a homogeneous cylindrical crystal (Czochralski Process), the molten silicon is left to cool and fragment.

    These fragments are then melted in ovens and poured into cubic-shaped growth crucibles.

    After the molten silicon solidifies, the ingots are cut into thin wafers, then polished, improved, diffused, and assembled just like monocrystalline panels.

    B. Monocrystalline vs Polycrystalline Solar Panels Appearance

    Because the pure silicon ingot is round, slicing them will result in square wafers with rounded edges, which creates small gaps between the cells once assembled.

    And due to the fact that they are made of pure silicon, they appear with a uniform dark look because of how light interacts with pure silicon.

    Therefore, you can easily recognize the monocrystalline solar cells by their uniform dark appearance and the rounded edges squares with small spaces between each cell.

    ٍDon’t worry, although the monocrystalline solar cell is dark, there are plenty of colors and designs for the back sheets and frames that will meet your preferences.

    Unlike the uniform dark look the monocrystalline solar cells have, polycrystalline cells tend to have a blue hue because of how sunlight interacts with the multi-crystalline.

    over, because polycrystalline wafers aren’t cut from cylinders like the monocrystalline ones, they won’t have rounded edges.

    Thus, you can easily recognize them by the bluish hue and the absence of rounded edges.

    Polycrystalline cells also have plenty of colorful back sheets and frame designs that will definitely suit your roof.

    C. Monocrystalline vs Polycrystalline Solar Panels Efficiency

    The solar panel efficiency is an indicator of how good the cell is in converting sunlight into electricity.

    For example, if we brought 2 different solar panels, one with an efficiency of 10% and the other with 20% and we shine the same amount of light for the same duration.

    The latter will produce almost double the electricity generated by the first one.

    Among different solar panel types, monocrystalline cells have the highest efficiency typically in the 15-20% range and it’s expected to get even higher.

    Fun fact: In 2019, the National Renewable Energy Laboratory managed to develop a six-junction solar cell with an efficiency of 47.1% setting 2 new world records.

    Because each polycrystalline cell is made of too many crystals, there is less room for electrons to move resulting in a lower electricity generation efficiency.

    Although monocrystalline have higher efficiency rates, the difference between mono and polycrystalline cells isn’t that big.

    Most polycrystalline PV cells have efficiencies between 13% to 16%, which is still a very good ratio and it’s expected to get only higher in the future.

    D. Mono-Si vs Poly-Si Temperature Coefficient?

    Another great factor that is greatly overlooked is the temperature coefficient.

    The temperature coefficient is a measurement of how well the solar cell functions when the temperature rises.

    In other words, it indicated the efficiency loss for every degree the temperature rises.

    Most monocrystalline solar cells have a temperature coefficient of around.0.3% / C to.0.5% / C.

    So when the temperature rises 1 degree Celsius or 32 degrees Fahrenheit, the monocrystalline solar cell will temporarily lose 0.3% to 0.5% of its efficiency.

    Polycrystalline PV cells have a higher temperature coefficient than the monocrystalline ones.

    This means that polycrystalline panels will lose more of their efficiency when the temperature rises making them not optimal to be used in hot areas.

    E. Expected Lifespan

    The lifespan of the solar cell is indicated by the degradation rate or the yearly energy production loss.

    Most solar panels have a degradation rate of 0.3% to 1%.

    Meaning that every year, the total power output of your system will decrease by 0.3% to 1%.

    Most monocrystalline PV panels have a yearly efficiency loss of 0.3% to 0.8%.

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    Let’s assume we have a monocrystalline solar panel with a degradation rate of 0.5%.

    In 10 years, the system will operate at 95% efficiency, in 20 years, the system will operate at 90% efficiency, and so on till it loses a significant amount of its energy production capability that it becomes inefficient.

    Most monocrystalline solar panels come with 25 or 30 years warranties. However, you can expect your system to last for up to 40 years or more.

    Polycrystalline PV cells have a slightly higher degradation rate than, which causes them to lose their efficiency a little faster than the monocrystalline ones.

    Don’t get me wrong, they still have a lifespan of 20-35 years and sometimes even more.

    F. Recyclability

    The short answer is yes, monocrystalline solar cells can be recycled.

    Monocrystalline solar panels are made of 3 main components:

    • Monocrystalline cells: Around 85% of the silicon wafers are recycled
    • Glass: Almost 95% of the glass can be reused
    • Metal: 100% of the metal parts are recyclable

    Are Polycrystalline Solar Panels Recyclable?

    Similar to monocrystalline, around 90% of all the material used to manufacture polycrystalline cells are recyclable.

    And by the year 2030, it’s expected that almost 45 million new modules will be made using recycled materials, which is equivalent to 380 million USD.

    G. Cost

    Monocrystalline solar panels have numerous advantages but one of their main disadvantages is the high initial cost.

    Among all types of PV solar panels types, monocrystalline is definitely the most expensive one to produce.

    many, solar, cells, need, crystalline, silicon

    This is due to the fact that the process of manufacturing monocrystalline solar cells is very energy-intensive and produces a big amount of silicon waste.

    Compared to their efficiency, polycrystalline solar panels have less cost per watt making them cheaper than the monocrystalline type.

    The reason for this is that the manufacturing process creates less waste and uses less energy resulting in less production costs.

    Fun fact: Sometimes poly-Si panels are made from the left-over pieces of mono-Si production, which reduces the amount of silicon waste.

    It’s important to mention that although poly-Si cells are cheaper, they occupy more space than monocrystalline to generate the same amount of energy making them less space-efficient.

    Monocrystalline vs Polycrystalline Solar Panels

    Monocrystalline Solar Panels Polycrystalline Solar Panels
    Material: Single Pure Silicon Crystal Different Silicon Fragments Molten Together
    Appearance: Uniform dark squares with rounded edges Blue squares with no rounded edge
    Conversion Efficiency: 15% to 20% 13% to 16%
    Space Efficiency: Efficient Less Efficient
    Temperature Coefficient: -0.3% / c to.0.5% / c -0.3% / c to.1% / c
    Lifespan: Around 40 years Around 35 years
    Recyclability: Yes Yes
    Cost:

    Last Words

    We really hope you enjoyed this article as much as we did.

    Did you find this guide helpful?

    If so, please share this article with your friends and let us know your thoughts in the comment section below.

    Thin-Film Solar Panels: An In-Depth Guide | Types, Pros Cons

    When talking about solar technology, most people think about one type of solar panel which is crystalline silicon (c-Si) technology. While this is the most popular technology, there is another great option with a promising outlook: thin-film solar technology.

    Thin-film solar technology has been around for more than 4 decades and has proved itself by providing many versatile and unique applications that crystalline silicon solar cells cannot achieve. In this article, we provide you with a deep review of this technology, the types of solar panels, applications, and more.

    Overview: What are thin-film solar panels?

    Thin-film solar panels use a 2 nd generation technology varying from the crystalline silicon (c-Si) modules, which is the most popular technology. Thin-film solar cells (TFSC) are manufactured using a single or multiple layers of PV elements over a surface comprised of a variety of glass, plastic, or metal.

    The idea for thin-film solar panels came from Prof. Karl Böer in 1970, who recognized the potential of coupling thin-film photovoltaic cells with thermal collectors, but it was not until 1972 that research for this technology officially started. In 1980, researchers finally achieved a 10% efficiency, and by 1986 ARCO Solar released the G-4000, the first commercial thin-film solar panel.

    Thin-film solar panels require less semiconductor material in the manufacturing process than regular crystalline silicon modules, however, they operate fairly similar under the photovoltaic effect. This effect causes the electrons in the semiconductor of the thin-film PV module to move from their position, creating an electric flow, that can be harnessed into electricity through an external circuit.

    Thin-film solar panels are manufactured using materials that are strong light absorbers, suitable for solar power generation. The most commonly used ones for thin-film solar technology are cadmium telluride (CdTe), copper indium gallium selenide (CIGS), amorphous silicon (a-Si), and gallium arsenide (GaAs). The efficiency, weight, and other aspects may vary between materials, but the generation process is the same.

    What are the different types of thin-film solar technology?

    There are several types of materials used to manufacture thin-film solar cells. In this section, we explain the different types of thin-film solar panels regarding the materials used for the cells.

    Cadmium Telluride (CdTe) Thin-Film Panels

    Cadmium Telluride (CdTe) thin-film solar technology was introduced to the world in 1972 by Bonnet, D. and Rabenhorst, H. when they evaluated a Cadmium sulfide (CdS)/CdTe heterojunction which delivered a 6% efficiency. The technology has been improved to reduce manufacturing costs and increase efficiency.

    CdTe solar cells are manufactured using absorber layers comprising a p–n heterojunction, which combines a p-doped Cadmium Telluride layer and an n-doped CdS layer that can also be made with magnesium zinc oxide (MZO). To depose materials on the substrate, manufacturers use the vapor-transport deposition or the close-spaced sublimation technique.

    On top of the absorber layer, CdTe thin-film solar cells include a Transparent Conductive Oxide (TCO) layer usually made with fluorine-doped tin oxide (SnO2:F) or a similar material. The electrical contact for these cells is made with zinc telluride (ZnTe), and the materials are placed over a metal or carbon-paste substrate.

    CdTe thin-film solar panels reached a 19% efficiency under Standard Testing Conditions (STC), but single solar cells have achieved efficiencies of 22.1%. This technology currently represents 5.1% of the market share worldwide, falling second only under crystalline silicon solar panels that hold 90.9% of the market. The cost for CdTe thin-film solar panels rounds the 0.40/W.

    Copper Indium Gallium Selenide (CIGS) Thin-Film Panels

    The first progress for Copper Indium Gallium Selenide (CIGS) thin-film solar cells was made in 1981 when the Boeing company created a Copper Indium Selenide (CuInSe2 or CIS) solar cell with a 9.4% efficiency, but the CIS thin-film solar cell was synthesized in 1953 by Hahn, H. In 1995, researchers at the National Renewable Energy Laboratory (NREL) embedded Gallium into the CIS matrix to create the first Copper Indium Gallium Selenide (CIGS) thin-film solar cell with a reported efficiency of 17.1%.

    Manufacturing for Copper Indium Gallium Selenide (CIGS) thin-film solar panels has improved throughout history. Currently, CIGS thin-film solar cells are manufactured by placing a molybdenum (Mo) electrode layer over the substrate through a sputtering process. The substrate is usually manufactured with polyimide or a metal foil.

    The absorbing layer is manufactured by combining a p-n heterojunction. The P-doped layer is made with copper indium gallium selenide (CIGS), placed above the electrode, and the CdS n-doped buffer is formed by chemical-bath deposition.

    To protect the absorbing layer of the CIGS thin-film solar panel, a layer of Intrinsic Zinc Oxide (i-ZnO) is placed above the CdS buffer. The materials are finally covered with a thick AZO compound layer made with Aluminium doped Zinc Oxide (Al: ZnO), acting as the TCO layer to protect the cell.

    The first CIGS thin-film solar panel manufactured by NREL reported a 17.1% efficiency, but the most efficient one ever created reported an efficiency of 23.4% and was made by Solar Frontier in 2019. The CIGS technology could be even more promising in the future since these materials can achieve a theoretical efficiency of 33%.

    CIGS modules are not as popular for regular applications, being mostly used for space applications due to their resistance to low temperatures and great performance under low-intensity light conditions found in space. The cost is relatively more expensive than for other technologies, with a current price slightly above 0.60/W, but future manufacturing generations promise to reduce the cost for these panels.

    While CIGS thin-film solar panels have not become as popular as CdTe panels in the market, CIGS technology still holds 2.0% of the PV market share. Considering that thin-film solar modules only hold around 10% of the market, This is still quite popular as a thin-film solar technology.

    Amorphous Silicon (a-Si) Thin-Film Panels

    The first observation of doping in Amorphous Silicon (a-Si) was achieved in 1975 by Spear and LeComber, a year later in 1976 it was demonstrated that Amorphous Silicon (a-Si) thin-film solar cells could be created. Great expectations have surrounded this technology, but the material represents several challenges like weak bonds, a relatively poor efficiency, and several others.

    Unlike other thin-film solar panels, amorphous silicon (a-Si) modules do not include an n-p heterojunction, but a p-i-n or n-i-p configuration, which differs from the n-p heterojunction by adding an i-type or intrinsic semiconductor. There are two routes to manufacture amorphous silicon (a-Si) thin-film solar panels, by processing glass plates or flexible substrates. Efficiency for a-Si solar cells is currently set at 14.0%.

    Disregarding the route taken to manufacture amorphous silicon (a-Si) thin-film solar panels, the following steps are part of the process:

    First, the substrate is conditioned, the TCO and back reflector are placed under the deposition process, and then thin hydrogenated amorphous silicon (a-Si:H)-based layers are placed onto the electrodes, and the cells are connected in a monolithic series via laser scribing and silicon layers. The module is finally assembled and encapsulated, applying framing and electrical connections.

    While manufacturing amorphous silicon (a-Si) requires an inexpensive material in low quantities, the price is relatively expensive, since the conductive glass for these panels is expensive and the process is slow, making the total cost of the panel to be set at 0.69/W. This technology currently holds 2.0% of the retail market for PV modules.

    Gallium Arsenide (GaAs) Thin-Film Panels

    The first Gallium Arsenide (GaAs) thin-film solar panel was made by Zhores Alferov and his students in 1970. The team persisted to create the gallium arsenide semiconductor, until they made a breakthrough in 1967, three years later they created the first gallium arsenide (GaAs) solar cell. Around 10 years later in 1980, the technology was being researched for specific applications like spaceships and satellites.

    The manufacturing process for GaAs thin-film solar cells is more complex than for regular thin-film solar cells.

    The first step is to grow the material. During this step, GaAs buffers are grown on Si substrates by being submitted to several temperature changes and different chemical processes, to finally create the layers for the cell.

    After the GaAs buffer grows, the substrate is processed for the fabrication of the cell. The first step is to deposit a Platinum (Pt)/Gold (Au) layer (10/50 nm) which will serve as the bonding material and electrode for the GaAs solar cell, and then a bonding process is performed on the substrate.

    After the bonding process is completed, the GaAs epitaxial layer that grew on the Si substrate is placed over the new substrate. To complete the assembly process a Pt/Titanium (Ti)/Pt/Au layer of 20/30/20/200 nm is deposited on the top contact layer through electron beam evaporation.

    Since GaAs PV cells are multijunction III-V solar cells composed of graded buffers, they can achieve high efficiencies of up to 39.2%, but the manufacturing time, cost for the materials, and high growth materials, make it a less viable choice for terrestrial applications. The rated efficiency for GaAs thin-film solar cells is recorded at 29.1%.

    The cost for these III-V thin-film solar cells rounds going from 70/W to 170/W, but NREL states that the price can be reduced to 0.50/W in the future. Since this is such an expensive and experimental technology, it is not mass-produced and is mainly destined for space applications, holding the lowest market share.

    Thin-film vs. Crystalline silicon solar panels: What’s the difference?

    Before comparing the different types of thin-film solar panels against crystalline silicon solar panels (c-Si), it is important to remark that there are two main types, monocrystalline silicon (mono c-Si) and polycrystalline silicon (poly c-Si) solar panels.

    In this section, we compare several aspects of both types of crystalline silicon solar panels against the different types of thin-film solar panels.

    Thin-film solar panels have many interesting applications, and they have been growing in the last decade. Below you will find some of the most popular applications for thin-film.

    Building-Integrated Photovoltaics

    One application starting to become widely popular worldwide is the Building-Integrated Photovoltaic (BIPV) highly dependent on thin-film solar technology. There are two main branches of this technology, solar shingles or solar roof tiles, and solar Windows or solar glass.

    The goal for both applications is to provide the means to keep aesthetics for homes and buildings while allowing the possibility of solar power generation. This technology integrates thin-film solar technology to provide a certain generation efficiency, which can be used just like with regular c-Si solar panels.

    Space applications

    One of the most important applications for thin-film solar technology, specifically Copper Indium Gallium Selenide (CIGS) and Gallium Arsenide (GaAs) technology is the space applications. The technology provides many advantages like being extremely lightweight, highly efficient, having a wide temperature of operation range, and even the damage resistance against radiation, making it ideal for these applications.

    Rooftop of vehicles and marine applications

    One common application for thin-film solar panels is the installation of flexible PV modules on vehicle rooftops (commonly RVs or buses) and the decks of boats and other vessels. This application allows the installation of modules on curved surfaces, provides solar power generation while keeping practicality and aesthetics for the vehicles and vessels.

    Portable applications

    An advantage of thin-film solar technology is its portability and size. The technology has been installed for years in calculators, but with much improvement, now there is a possibility of having solar power in remote locations with foldable solar panels, solar power banks, solar-powered laptops, and more.

    Large-scale applications

    Due to its versatility, an important FOCUS of thin-film solar technology is commercial applications. While c-Si solar modules hold the largest market share, efficiency for thin-film solar panels is growing and manufacturing processes are becoming cheaper, which could lead to thin-film solar panels becoming the norm for most installations.

    Another important FOCUS for thin-film solar panels is the industrial level applications, specifically at the utility scale. Since thin-film solar panels degrade at a much slower pace, they offer a potential alternative to the traditional c-Si solar panels, sometimes providing a better investment over time.

    Rounding up: Pros and cons of thin-film solar panels

    Thin-film solar panels have many pros, while only holding a few cons to them. These are the most important pros and cons of this technology.

    Pros

    • Higher resistance to degradation.
    • Lower thermal losses at extreme temperatures due to the low-temperature coefficient.
    • High efficiency for most technologies (CdTe, CIGS, and especially GaAs)
    • Ideal for portable and BIPV applications.
    • Promising research and development with much more ground to cover.
    • Requires less material to create PV modules.
    • Thin-film solar panels are lighter than c-Si PV modules.

    Cons

    • Higher retail cost.
    • Less availability in the market.
    • installation space is required to achieve the same generation capacity as c-Si modules (Except for GaAs PV modules).

    Final Words

    Thin-film solar technology might not be as popular as crystalline silicon, but it has an incredibly promising future. This technology opens possibilities that are not available for c-Si panels, like BIPV applications, portable modules, and even high-efficiency space applications with CIGS and GaAs PV modules.

    While c-Si technology will probably keep having the largest market share due to its currently high rated efficiency, low manufacturing prices, and other pros attached to it, thin-film technology is still a valuable option to consider. As a matter of fact, the market share for thin-film solar has grown in the last decade, and it could keep it up in the following one.

    With further research and breakthroughs for thin-film solar cells, this technology could be adapted to even more applications in the future and potentially increase its market value not only in large-scale applications but also in small commercial and residential sectors in the next 10 years.

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