All Solar Panels which consist of photovoltaic (PV) cells in turn consist of two or more thin layers of semi-conducting material, most commonly silicon. When the semiconductor is exposed to light, electrical charges are generated and this can be conducted away by metal contacts as direct current (DC). The electrical output from a single cell is small, so multiple cells are connected together to form a 'string', which produces a direct current.

Solar cells and solar panels are both integral, and closely related, parts of a solar energy system. When reading about solar systems, it may seem as if these titles are almost interchangeable. Writers refer to them both when discussing energy production and output, and often do so without explanation of how these parts work. However, each plays a distinct role. Solar cells contain all the parts necessary to convert sunlight to electricity. Solar panels combine and direct all of that energy output.

To increase their utility, a number of individual PV cells are interconnected together in a sealed, weatherproof package called a Panel (Module). For example, a 12 V Panel (Module) will have 36 cells connected in series and a 24 V Panel (Module) will have 72 PV Cells connected in series

To achieve the desired voltage and current, Modules are wired in series and parallel into what is called a PV Array. The flexibility of the modular PV system allows designers to create solar power systems that can meet a wide variety of electrical needs. Figure below shows PV cell, Panel (Module) and Array.

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The photovoltaic system

Solar panel systems are referred to as photovoltaic systems in the solar industry. This differentiates them from other solar technologies, such as solar thermal and concentrated solar power. Photovoltaic systems have several parts and each plays a distinct role. Photovoltaics is the science behind the most popular form of solar energy. It is the process of converting sunlight directly into electricity. The photovoltaic (PV) effect was first observed in 1839. However, it wasn’t until 1954 that scientists were able to discover exactly how it works.

Historically, space programs were the largest supporters of PV technology, since the system was the best energy source for their satellites. The industry has since grown and you have probably seen PV systems used to power electronics, cars, houses, commercial buildings, and to supplement power grids. Due to increased efficiency, decreasing cost and increased environmental concern, photovoltaic installations have increased dramatically in recent years.

How does the PV process work?

A photovoltaic system uses solar panels to capture sunlight’s photons. These solar panels each have many solar cells made up of layers of different materials. An anti-reflective coating on top helps the cell capture as much light as possible. Beneath that is a semiconductor (usually silicone) sandwiched between a negative conductor on top and a positive conductor on bottom. Once the photons are captured by the solar cell, they begin releasing the outer electrons of atoms within the semiconductor. The negative and positive conductors create a pathway for the electrons and an electric current is created. This electric current is sent to wires that capture the DC electricity. These wires lead to a solar inverter, which then transforms it into the AC electricity used in homes. The more solar cells you install, the more electricity is produced.

The entire system begins with the solar cells. Solar cells are originally made of Semiconductor materials, which have weakly bonded electrons occupying a band of energy called the valence band. So when energy exceeding a certain threshold, called the band-gap energy, is applied to a valence electron, the bonds are broken and the electron is somewhat free to move around in a new energy band called he conduction band where it can conduct electricity through the material.

Thus, the free electrons in the conduction band are separated from the valence band by the band-gap (measured in units of electron volts ). This energy needed to free the electron can be supplied by photons, which are particles of the sunlight.These cells are where sunlight is actually used to produce electricity. Solar panels are a combination of multiple solar cells. These cells are arranged in such a way that the solar panels can capture and combine the electrical output of each cell and send it along a specified path. Once the solar panels have captured the electricity, they send it all to an inverter which converts the DC electricity created by the cells to the AC electricity you use in your home. After this, the electricity is sent through your meter and into your home. All of the parts in a photovoltaic system have distinct roles, but each is so dependent on the other parts that it is easy to confuse them.

The role of solar cells

Solar cells produce electricity through a natural reaction called the photovoltaic effect. This is where the system gets its name. The basic premise of the photovoltaic effect is that sunlight creates electricity in certain materials by knocking their outer electrons loose.

There are multiple layers within solar cells. The most important elements are the two semiconductors in the center. The top semiconductor is a negative layer, which means the material’s atoms contain extra electrons. These electrons are energized by sunlight and the extra electrons are knocked loose. The bottom semiconductor is a positive layer. This material’s atoms have space for more electrons. When electrons are knocked off the negative layer they are attracted to the positive layer. A barrier is formed between them. Negative conductors on top of the cell and positive conductors on bottom force the electrons to travel around the cell in a specific direction. This creates the electrical current you will use. The conductors force the current out of the cell and into an electrical load, which captures the energy. The electrons then continue their path until they re-enter the cell and connect with the positive layer. This completes the circuit and allows the process to continue. The entire purpose of the solar cell is to ensure this process flows unimpeded so that you get as much electricity as possible.

How are solar panels installed?

Solar panels are made up of many solar cells. The number and type of solar cells used depend on the resulting voltage required of the panel. For example, a common 12 volt panel will contain 36 cells. These panels are then installed in ways that capture the most sunlight.
Solar panel installation begins with you choosing the type of panel. Traditional solar panels are the most efficient; however, they are larger and heavier than the other two types. Thin-film PV panels are lighter and cheaper than any other panel, but are generally less efficient. These are usually residential solar panels. Concentrating PV arrays are new and use lenses and mirrors to concentrate sunlight onto the solar cells. These arrays are the most efficient but can only be used in very sunny areas, such as the American Southwest.
Once you select the panel type, the next step is to choose the best location. Installers will analyze the path of the sun over a particular property and take note of any shade to choose the most efficient location for the panels. It is best when solar panels are oriented toward the south and are not shaded at all. By doing so, the panels capture the most sunlight throughout the day.

What is the difference between photovoltaic and solar thermal systems?

You may ask whether PV systems or solar thermal plants are the better option for harnessing solar energy, and that debate has raged for many years. Solar thermal plants use the sun’s energy to heat a liquid (often water) or gas to high temperatures. The resulting heat energy is then used to power a generator and create electricity. PV systems, on the other hand, convert sunlight directly to electricity. Solar thermal systems are best used in large energy plants while PV systems are usually the best option for homes and businesses. Here are some pros and cons of each method.

Solar thermal pros:

Solar thermal plants are better at distributing energy during off-peak hours or seasons through long-term storage of electricity.
Solar thermal energy is more efficient at harnessing energy from sunlight.
Solar thermal systems use less roof space.

Solar thermal cons:

Solar thermal plants are more expensive to run and the excessive heat may cause safety concerns.
Solar thermal energy is most efficiently used for heating things, like water heaters, and is less efficient as a form of electricity.
Solar thermal systems require the use of a generator to produce electricity.

PV system pros:

PV systems produce the most energy during the summer, when it is most needed to power air conditioning.
There are no moving parts and little to no maintenance needed.
PV technology has been in use much longer, proving its usefulness.
PV energy is much more versatile, since it converts sunlight directly into electricity without generators.

PV system cons:

It takes longer for the energy savings to pay back the cost of installation.
PV systems have a lower capacity for harnessing sunlight.
PV systems collect energy only during sunlight hours.

There are many advantages of solar energy, and photovoltaic systems are a powerful form of clean energy. A properly installed PV system will provide you with plenty of, lower your electric bills, help during power outages and may even earn utility bill credits if excess energy is sent to your local power grid. Through this amazing technology, you can be confident in producing environmentally friendly and sustainable energy for your family.​

The role of solar panels

The solar panel’s role is to amplify, protect and direct electricity. Solar cells can produce only a limited amount of energy. When building a solar system, multiple solar cells are connected in series or parallel circuits to create a solar module. This produces higher currents and more energy. The modules also seal all of the solar cells and wiring in a protective case to guard it from the weather. These modules are then wired together as a solar panel. It is important to note that a solar panel may consist of just one module or multiple, meaning modules and panels are sometimes used interchangeably. These solar panels are pre-wired and ready to be installed on your rooftop.

By connecting all of these parts into solar panels, the resulting electricity has a more precise path to follow. When the electrical current leaves the solar cells to travel through the electrical load, it is captured by the load and sent through the solar panels. The method by which the solar cells are wired together determines how the electrical current will flow. Whichever way it flows, it will direct the electricity from all the solar cells together. All the electricity will then be directed out of the solar panels and toward the inverter, where the rest of the photovoltaic process is completed.

Solar cells and solar panels work together to produce the electricity you need for your home. Although they are closely related, with the panels actually containing the solar cells, each plays its own part. Considered as one, the entire photovoltaic system works like an assembly line. Every part performs its job alone, and then passes its product on to the next part. Each product is necessary for the next section to do its job. Only the solar cells can perform alone, but its products are useless without the rest of the system. Your entire system is a clean and quiet production line working to create electricity for you in a natural and efficient way.

Crystalline Silicon (c-Si)

Almost 90% of the World’s photovoltaics today are based on some variation of silicon. The silicon used in PV takes many forms. The main difference is the purity of the silicon. But what does silicon purity really mean? The more perfectly aligned the silicon molecules are, the better the solar cell will be at converting solar energy (sunlight) into electricity (the photoelectric effect). The efficiency of solar panels goes hand in hand with purity, but the processes used to enhance the purity of silicon are expensive. Efficiency should not be your primary concern. As you will later discover, cost-and space-efficiency are the determining factors for most people.
Crystalline silicon forms the basis of mono- and polycrystalline silicon solar cells: 

Monocrystalline Silicon Solar Cells

Solar cells made of monocrystalline silicon (mono-Si), also called single-crystalline silicon (single-crystal-Si), are quite easily recognizable by an external even coloring and uniform look, indicating high-purity silicon.

Advantages

Monocrystalline solar panels have the highest efficiency rates since they are made out of the highest-grade silicon. The efficiency rates of monocrystalline solar panels are typically 15-20%. SunPower produces the highest efficiency solar panels on the U.S. market today. Their E20 series provide panel conversion efficiencies of up to 20.1%.[3]Update (April, 2013): SunPower has now released the X-series at a record-breaking efficiency of 21.5%. [7]
Monocrystalline silicon solar panels are space-efficient. Since these solar panels yield the highest power outputs, they also require the least amount of space compared to any other types. Monocrystalline solar panels produce up to four times the amount of electricity as thin-film solar panels.
Monocrystalline solar panels live the longest. Most solar panel manufacturers put a 25-year warranty on their monocrystalline solar panels.
Tend to perform better than similarly rated polycrystalline solar panels at low-light conditions.

Disadvantages

Monocrystalline solar panels are the most expensive. From a financial standpoint, a solar panel that is made of polycrystalline silicon (and in some cases thin-film) can be a better choice for some homeowners.
If the solar panel is partially covered with shade, dirt or snow, the entire circuit can break down. Consider getting micro-inverters instead of central string inverters if you think coverage will be a problem. Micro-inverters will make sure that not the entire solar array is affected by shading issues with only one of the solar panels.
The Czochralski process is used to produce monocrystalline silicon. It results in large cylindrical ingots. Four sides are cut out of the ingots to make silicon wafers. A significant amount of the original silicon ends up as waste.
Monocrystalline solar panels tend to be more efficient in warm weather. Performance suffers as temperature goes up, but less so than polycrystalline solar panels. For most homeowners temperature is not a concern.

Polycrystalline Silicon Solar Cells

The first solar panels based on polycrystalline silicon, which also is known as polysilicon (p-Si) and multi-crystalline silicon (mc-Si), were introduced to the market in 1981. Unlike monocrystalline-based solar panels, polycrystalline solar panels do not require the Czochralski process. Raw silicon is melted and poured into a square mold, which is cooled and cut into perfectly square wafers.

Advantages

The process used to make polycrystalline silicon is simpler and costs less. The amount of waste silicon is less compared to monocrystalline. Polycrystalline solar panels tend to have slightly lower heat tolerance than monocrystalline solar panels. This technically means that they perform slightly worse than monocrystalline solar panels in high temperatures. Heat can affect the performance of solar panels and shorten their lifespans. However, this effect is minor, and most homeowners do not need to take it into account.

 
Disadvantages

The efficiency of polycrystalline-based solar panels is typically 13-16%. Because of lower silicon purity, polycrystalline solar panels are not quite as efficient as monocrystalline solar panels. They have lower space-efficiency. You generally need to cover a larger surface to output the same electrical power as you would with a solar panel made of monocrystalline silicon. However, this does not mean every monocrystalline solar panel performs better than those based on polycrystalline silicon.
Monocrystalline and thin-film solar panels tend to be more aesthetically pleasing since they have a more uniform look compared to the speckled blue color of polycrystalline silicon.

 

String Ribbon Solar Cells

String Ribbon solar panels are also made out of polycrystalline silicon. String Ribbon is the name of a manufacturing technology that produces a form of polycrystalline silicon. Temperature-resistant wires are pulled through molten silicon, which results in very thin silicon ribbons. Solar panels made with this technology looks similar to traditional polycrystalline solar panels.

**Evergreen Solar was the main manufacturer of solar panels using the String Ribbon technology. The company is now bankrupt, rendering the future for String Ribbon solar panels unclear.

Advantages

The manufacturing of String Ribbon solar panels only uses half the amount silicon as monocrystalline manufacturing. This contributes to lower costs.

 Disadvantages

The manufacturing of String Ribbon solar panels is significantly more energy extensive and more costly.
Efficiency is at best on par with the low-end polycrystalline solar panels at around 13-14%. In research laboratories, researchers have pushed the efficiency of String Ribbon solar cells as high as 18.3%.
String Ribbon solar panels have the lowest space-efficiency of any of the main types of crystalline-based solar panels.

 

Thin-Film Solar Cells (TFSC)

Depositing one or several thin layers of photovoltaic material onto a substrate is the basic gist of how thin-film solar cells are manufactured. They are also known as thin-film photovoltaic cells (TFPV). The different types of thin-film solar cells can be categorized by which photovoltaic material is deposited onto the substrate:

Amorphous silicon (a-Si)
Cadmium telluride (CdTe)
Copper indium gallium selenide (CIS/CIGS)
Organic photovoltaic cells (OPC)

Depending on the technology, thin-film module prototypes have reached efficiencies between 7–13% and production modules operate at about 9%. Future module efficiencies are expected to climb close to the about 10–16%.
The market for thin-film PV grew at a 60% annual rate from 2002 to 2007.[5] In 2011, close to 5% of U.S. photovoltaic module shipments to the residential sector were based on thin-film.

Advantages

Mass-production is simple. This makes them and potentially cheaper to manufacture than crystalline-based solar cells.
Their homogenous appearance makes them look more appealing.
Can be made flexible, which opens up many new potential applications.
High temperatures and shading have less impact on solar panel performance.
In situations where space is not an issue, thin-film solar panels can make sense.

Disdvantages

Thin-film solar panels are in general not very useful for in most residential situations. They are cheap, but they also require a lot of space. SunPower`s monocrystalline solar panels produce up to four times the amount of electricity as thin-film solar panels for the same amount of space.
Low space-efficiency also means that the costs of PV-equipment (e.g. support structures and cables) will increase.
Thin-film solar panels tend to degrade faster than mono- and polycrystalline solar panels, which is why they typically come with a shorter warranty.

 
Solar panels based on amorphous silicon, cadmium telluride and copper indium gallium selenide are currently the only thin-film technologies that are commercially available on the market:

 
Amorphous Silicon (a-Si) Solar Cells

Because the output of electrical power is low, solar cells based on amorphous silicon have traditionally only been used for small-scale applications such as in pocket calculators. However, recent innovations have made them more attractive for some large-scale applications too.
With a manufacturing technique called “stacking”, several layers of amorphous silicon solar cells can be combined, which results in higher efficiency rates (typically around 6-8%).
Only 1% of the silicon used in crystalline silicon solar cells is required in amorphous silicon solar cells. On the other hand, stacking is expensive.

Cadmium Telluride (CdTe) Solar Cells

Cadmium telluride is the only thin-film solar panel technology that has surpassed the cost-efficiency of crystalline silicon solar panels in a significant portion of the market (multi-kilowatt systems).
The efficiency of solar panels based on cadmium telluride usually operates in the range 9-11%.
First Solar has installed over 5 gigawatts (GW) of cadmium telluride thin-film solar panels worldwide. The same company holds the world record for CdTe PV module efficiency of 14.4%.

 
Copper Indium Gallium Selenide (CIS/CIGS) Solar Cells

Compared to the other thin-film technologies above, CIGS solar cells have showed the most potential in terms of efficiency. These solar cells contain less amounts of the toxic material cadmium that is found in CdTe solar cells. Commercial production of flexible CIGS solar panels was started in Germany in 2011.
The efficiency rates for CIGS solar panels typically operate in the range 10-12 %.
Many thin-film solar cell types are still early in the research and testing stages. Some of them have enormous potential, and we will likely see more of them in the future.

​Cheaper production of solar cells

There is intense competition in the market for photovoltaic systems. With constantly new innovations, the manufacturers are reducing their production costs and are increasing the efficiency of the cells and modules. For this purpose they are improving the production processes along the chain from silicon to the module. Companies and research facilities are working together to produce high-quality silicon crystals and wafers that save as much material as possible. They are also improving the material quality using an innovative solidification process for quasi-monocrystalline silicon. And with a new separation process they are producing more wafers from the same amount of silicon.

Wafers made of monocrystalline or multicrystalline silicon are usually used for silicon-based solar cells. Multicrystalline silicon is produced cost-effectively by means of ingot casting, but does not attain the efficiency of standard monocrystalline silicon, which is grown in a complex process using the Czochralski method. The standard cell efficiencies achieved by this method of more than 21% can also be attained, however, using the newly developed quasi-mono silicon, which researchers from SolarWorld can produce more cost-effectively using a new, crucible-free crystal growing process. This replaces the ingot casting method customarily used for microcrystalline silicon cells, in which the crucible and its coating provide a source for impurities and disruptive foreign nucleating agents. The new process enables them to produce a monocrystalline, dislocation-free and low-oxygen silicon.

In the next step, the crystal is cut into fine slices known as wafers. In order to improve this production step, the researchers are replacing the previously used lapping abrasive-based sawing technology with diamond wire cutting and specially adapted cooling liquid. Diamond saws can cut the crystals more quickly into wafers with less material loss. These are currently about 180 μm thick. Substantial material savings are still possible here, and within the next ten years the researchers want to attain 100 μm. By way of comparison, a sheet of paper is about 80 μm thick.

Before the wafers can be further processed into solar cells, they have to be cleaned. With newly developed processes, residues from the coolant and lubricant are removed along with organic adhesions and particles.

Crystallisation: Directional solidification

Directional solidification is the established process used for the large-scale production of multicrystalline silicon ingots for solar cells. New solidification concepts are aimed at producing quasi-monocrystalline silicon ingots that have higher efficiencies and fewer defects that reduce the life of solar modules. Here magnetic flux fields are also used. They allow the melt currents, which are mainly influenced by magnetic fields generated by the resistance heaters, to be adjusted specifically at the solid-liquid growth front, thereby improving the solidification process. A measurement method for such flow structures developed with the TU Freiberg confirms the results of the numerical simulations carried out in the research project.

The researchers developed the current process based on a previous project in which they melted polycrystalline silicon in the crucible to produce quasi-mono material. For this purpose, they laid monocrystalline silicon on the bottom of the crucible, allowed the crystal to grow from there, and then gradually lowered the temperature from below upwards. The process requires very accurate temperature control. It is also more complicated than the production of polycrystalline material as the monocrystalline crystallisation seeds are only allowed to melt slightly.

Since disturbances in the crystal structure and impurities from the crucible still limit the efficiency that can be achieved with this method, the researchers developed the crucible-free quasi-mono process. In addition to improving the structural quality of the crystals with fewer dislocations and smaller recombination-active grain boundaries, this process also enables them to achieve a considerably lower concentration of impurities such as oxygen, carbon and metals. This in turn improves the efficiency of the processed solar cell.

Within the scope of the experimental work, they used various raw material deposits and seed templates, and selectively varied process parameters such as the heat outputs or the growth rate. In order to further improve the manufacturing process and the furnace, the researchers used a newly developed simulation software for the crucible-free technology as well as a new measuring system. This model enables them to predict dislocations and dislocation clusters caused by thermo-mechanical stresses, as well as the distribution of residual stresses in the cooled crystal.

Diamond wire slices the brick into thin wafers

In the next step, the silicon rods are sliced into many thin silicon disks – the wafers. The industry generally uses thin steel wire for this sawing process combined with a silicon carbide suspension as an abrasive medium in the cooling liquid. Many wire loops are used to divide up the crystal ingot within a sawing step. Whereas smooth wires were previously used for this purpose, the developers are now using an optimised wire covered with diamond grains together with a newly developed lubrication and cooling fluid. Together with the manufacturers, they have adapted the saw wire and auxiliary materials for the new saw technology.

The diamond wire consists of a steel core wire to which the diamond particles are attached with metal or synthetic resin deposits. With a diameter of 100-120 μm, the core wire is approximately as thick as a human hair, and the diamond grains used are 5-25 μm in size. The diamond wire enables the wire saw process to be sped up. Less than 3 hours is now required for the cutting, whereas this previously used to take more than 6 hours. In addition, the researchers are also experimenting with thinner wires less than 70 μm thick. This makes an even narrower cutting gap possible. This would reduce the material loss by about 30-40% and thus achieve a greater wafer yield.

A further research project intends to further improve the diamond wire separation process with the aim of reducing the manufacturing costs in the cut-off process by about one-third.

Optimal adaptation of the wire and lubricant

In order to prepare for industrial use, the researchers investigated the wires and their wear behaviour. They examined how wire parameters such as the size, shape, distribution and density of the diamond grains as well as the type of bonding influence the sawing efficiency of the separation process. A cost-determining factor is how many silicon wafers a diamond wire can cut before the cutting properties diminish and the quality of the wafer is compromised.

During the investigations to optimise the process and prevent damage to the wafer surface, they used a special single-gap saw device and modelling software to illustrate the relationship between the process parameters and damage mechanisms. The cost of producing saw wires covered with diamond grains has already been significantly reduced in recent years.

Compared with the conventional wire saw process with abrasive slurry, the diamond wire process can be made more resource-saving and cost-effective. In cooperation with the manufacturer, the researchers have developed for this purpose special lubricants and coolants from a water-surfactant mixture as well as suitable processing methods. This new coolant is cheaper and more recyclable than glycol-based lubricants. It has to meet a wide range of requirements:

·         Chip transport and cooling,

·         lowest possible surface tension,

·         non-corrosive to the metal parts of the machine,

·         as environmentally friendly as possible.

The physicochemical properties of the surfactants used in coolants and cleaners have a considerable influence on the further wafer processing: a cooling lubricant with a good wetting and rinsing effect helps to increase the surface quality of the wafers. A very clean cutting gap can significantly reduce the breakage rate and significantly simplify downstream process steps.

The researchers have also developed special filtration techniques to enable the cooling lubricant to be recycled. These are necessary to separate the accumulated abrasive particles, the smallest of which are only a few hundred nanometres in size.

Before their further processing to form solar cells, the wafers are cleaned and then undergo an intensive final inspection. The purified wafer surface must be absolutely clean, as firmly adhering residues from the cooling lubricants from the separation process would interfere with the subsequent cell production processes. The researchers have developed a cleaning method that removes not only the residual kerf as a waste product of the separation process, but also metallic and organic contaminants. For this purpose they have also adapted an ultrasonic method to meet these special requirements.

Production of mono- and polycrystalline solar silicon

The production of monocrystalline silicon is complex. It is usually carried out using the so-called Czochralski method. A seed crystal is introduced into the liquid silicon melt. When the seed crystal is slowly rotated and extracted, a single crystal with a homogeneous structure is formed. Monocrystalline silicon can achieve an efficiency of 25% under laboratory conditions and about 21% in industrial production.

Poly- or multicrystalline silicon is generally produced in a controlled melting and cooling process by casting ingots in a coated crucible of high-purity quartz material. Crystals grow from the bottom of the crucible upwards. The size, structure and purity of the crystals influence the performance of the later solar cells. Multicrystalline silicon can be produced more cost-effectively, but the industrial and laboratory efficiencies remain about 2% below that of single-crystal silicon.

The new quasi-mono process takes a middle position between ingot casting and the Czochralski method. It functions without a crucible, so no impurities can diffuse from the crucible during crystallisation. In the furnace, molten silicon drips onto a rotating monocrystalline plate and grows to form a quasi-monocrystalline brick. This makes it possible to achieve a cell efficiency of more than 21%.

Increased efficiency, reduced costs

The price-induced pressure to innovate in the photovoltaic industry is leading to rapid progress in the production and efficiency of PV systems. As a result, the production costs for photovoltaic electricity are declining significantly: they currently amount to 3-8 cents per kilowatt hour for new solar power plants.
Of the total costs required for a PV module, approximately 15% is incurred by the production of the Si wafers and the solar cells, while the module production now requires more than half.

The research project focused – with crystallisation and wafering – on optimising two steps in crystalline silicon technology. In the widespread silicon photovoltaics industry, such innovations can be introduced relatively quickly into existing production lines. This enables the industry to improve the efficiency and cost structure while maintaining proven production processes.

In the dominant field of silicon photovoltaics, research facilities and companies are developing solutions that make it possible to generate solar power more cost-effectively at all stages of the cell and module production. For example, they are working on bilaterally active bifacial modules as well as on new tandem solar cells based on silicon in order to achieve even higher efficiencies.

A new kerfless wafer process developed by Nexwave, a spin-off from Fraunhofer ISE, eliminates the melting, crystallisation and sawing steps required in conventional processes. By means of gas-phase deposition, the monocrystalline silicon wafers instantly grow to the required material strength on a multi-reusable seed wafer.

Efficiency records are soon surpassed by new ones not only in the case of silicon but also with other photovoltaic technologies such as thin-film, perovskite and organic PV. It is important that these research results and innovations are transferred as quickly as possible into industry so that photovoltaic electricity can be produced even more efficiently and economically in the future.​

Efficient, organic photovoltaic cells for indoor and outdoor applications

Organic photovoltaics (OPV) may cost less than their silicon counterparts, but their performance remains off-putting to this day. A consortium of European research groups and industries recently demonstrated free-form organic solar modules for three specific, indoor and outdoor applications that should help put such concerns to bed.

Over its three years of intensive research, the ARTESUN consortium had a single objective: the development of high performance materials enabling cost-effective, non-vacuum production of OPV modules boasting an efficiency of over 15%. These modules had to allow for arbitrary size and shape to make their use possible across a large panel of applications.

In a press release published in late December 2016, project partners announced that they successfully realized several types of organic solar modules using newly-developed roll-to-roll (R2R) additive non-vacuum coating and printing techniques.

Thanks to the combination of novel active layer and electrode materials with coating and module interconnection techniques, small and large area demonstrators of various shapes and size could be demonstrated, targeting three different sectors and applications.

Of one these three use cases consisted in the production of RFID tags where the battery pack is replaced by an organic solar module with a size comparable to that of a credit card. The module powers all wireless communications between the RFID tag itself and its reader, along with the integrated sensor device. Cars and buildings are two applications of choice for this novel device.

‘Auxiliary electronics including energy storage in form of a supercapacitor and overvoltage protection are integrated to the RFID tag to secure the operation up to one day during poor light conditions,’ the press release reads. ‘The tag can sense the indoor surrounding temperature, which is monitored wirelessly with a handheld reader. Outdoors, a vehicle can be identified wirelessly with a fixed reader from a reading distance increased by a factor of 10 when utilizing solar power compared to passive mode operation.’

A second use case was presented in the form of a flower-inspired flexible organic solar antenna module. Built using gravure printing, the module is able to power a radio and an environmental sensor in a distributed wireless sensor network. It has been optimized to operate under low or varying light intensities which, according to the consortium, makes it suitable for remote, autonomous precise environmental monitoring in agricultural applications.

Finally, the team successfully developed large area modules and assembled them in a glass-based facade element for use in building integration. The BIPV (Building-integrated photovoltaics) element of 1610mm x 380mm can be integrated as a ventilated façade within well-defined structural elements. ‘Potential market acceptability, in terms of overall subjective properties (robustness, colour, design, reflection, etc.), was tested by means of a visual inspection experts’ panel providing scores from 0 to 10. The result shows an overall excellent acceptance rating between 7-8 for this BIPV product,’ the project team writes.

With these three products, the VTT-led consortium hopes to provide participating European SMEs with a competitive edge.

​

Building-Integrated Photovoltaics (BIPV)

Lastly, we`ll briefly touch on the subject of building integrated photovoltaics. Rather than an individual type of solar cell technology, building integrated photovoltaics have several subtypes (or different methods of integration), which can be based on both crystalline-based and thin-film solar cells.
Building integrated photovoltaics can be facades, roofs, windows, walls and many other things that is combined with photovoltaic material. If you have the extra money and want to seemlessly integrate photovoltaics with the rest of your home, you should look up building integrated photovoltaics. For most homeowners it`s simply way too expensive.

Best Solar Panel Type for Home Use

Having your particular situation evaluated by an expert would be the best way to find out what solar panel type would be best for your household. Here are some of the typical scenarios we see: 

Limited Space

For those who don’t have enough space for thin-film solar panels (the majority of us), or if you want to limit the amount of space their PV-system takes up, crystalline-based solar panels are your best choice (and they would likely be the your best choice even if you had the extra space). There are not a whole lot of solar installers and providers that offer thin-film solar panels for homeowners at this point.

You will have a choice of different solar panel sizes. The 180, 200 and 220-watt rated solar panels are usually physically the same size. They are manufactured exactly the same way, but under- or overperform when tested, hence ending up in different categories for power output. If size is important, you should go for the highest rated power output for a particular physical size.

Both mono- and polycrystalline solar panels are good choices and offer similar advantages. Even though polycrystalline solar panels tend to be less space-efficient and monocrystalline solar panels tend to produce more electrical power, this is not always the case. It would be nearly impossible to recommend one or the other by not examining the solar panels and your situation closer.

Monocrystalline solar panels are slightly more expensive, but also slightly more space-efficient. If you had one polycrystalline and one monocrystalline solar panel, both rated 220-watt, they would generate the same amount of electricity, but the one made of monocrystalline silicon would take up less space.

Lowest Costs

If you want the lowest costs per rated power, or in other words, pay as little as possible for a certain amount of electricity, you should investigate if thin-film solar panels could in fact be a better choice than mono- or polycrystalline solar panels.Type your paragraph here.

Trends in Solar Module Manufacturing

From the distance, today’s solar module might not look any different than 20 years ago, but researchers and module manufacturers have been pretty innovative on improving the solar module’s contribution to efficiency and yield, often independent from the cell.

The so-called cell-to-module losses (CTM) value is a clear indication of this progress. According to the 7th edition of the International Technology Roadmap for PV (ITRPV), published by the German Engineering Federation’s (VDMA) PV Chapter, the CTM losses for multicrystalline modules would be zero already by 2017, whereas monocrystalline modules would reach this level in 2019. This difference is mainly due to monocrystalline cells featuring already a superior surface texturing that leads to relatively lower reflection losses, but it also means that they benefit less from the ‘coupling gain’ after encapsulation.

Cutting Cells in Two

There are different means to decrease the CTM losses. One is to reduce the interconnection losses and improved light trapping approaches. Interconnection losses can be minimized with ‘half cells’, in which a fully processed cell is deliberately cut into two pieces. The logic behind slicing a full cell in half is that the series resistance losses, which are a square of the flowing current, are reduced to a quarter for half-cut cells compared to a full cell. Norwegian company REC even announced in March 2016, it would switch all production at its Singapore facility to half-cut PERC cells. Several module manufacturers have added such module products to their portfolio as well, for example, Hanwha Q CELLS presented a 315 W mono 60-half cells based prototype module in September 2016.
Half but powerful: Slicing a full cell in two equal pieces enables to reduce the series resistance losses. Several module manufacturers are offering such modules. Norwegian company REC even announced to switch all production at its Singapore facility to half-cut PERC cells.

Trapping the Light

Another way to improve module performance is to enhance light trapping. The mechanism involves turning the barren areas of the module into passive energy generation sites. The portion of light that hits the non-active area of the module - such as ribbons and gaps between cells - are reflected back into the ambiance, thus they go to waste. However, employing special materials such as grooved or coated ribbons and reflective backsheets pushes back sunlight onto the active PV area by means of total internal reflection.

Nearly every backsheet supplier is supplying reflective backsheets today. Germany’s Schlenk and US-based Ulbrich, for example, have been supplying silver grooved solar ribbons, while Finnish company Luvata and Bruker-Spaleck from Germany are offering colored ribbons.

At the 32nd EU PVSEC conference in 2016 in Munich, Fraunhofer ISE presented another innovative ribbon design, called TriCon-Concept, which has a triangular cross section. The researchers found that in an elevation tracked module using TriCon 2.32% more light reaches the cell surface over the year compared to rectangular interconnectors in a 5-busbar configuration; the improvement was 2.02% over round wires. 
 
More or ‘Multi’ Busbars

All module manufacturers have been increasing the number of busbars - and there’s no end in sight. The bulk of the industry already has or is adapting their processes to 4-busbar configuration, while some module makers have been already started moving towards 5-busbars, such as SolarWorld.

This approach is more beneficial at the cell level - because increasing the number of busbars significantly reduces internal electrical resistance as current carried by fingers reduces with increase in busbar count. But the major implications are on module manufacturing. At some point you need to change the string soldering machine. The leading stringing and tabbing equipment makers are now offering 4 and 5 busbars as a standard feature for their tools. For some earlier 3-busbar models also field upgrade packages are available.

Going beyond 5-busbars is still a topic of discussion, especially if it makes economic sense. An alternative are ‘multi-busbar’ solutions, where the typical flat solar ribbons are replaced with more than 10 round copper wires for realizing the series connection. Meyer Burger from Switzerland and Germany’s Schmid are offering such multi-busbar based interconnection production equipment.

Glass-Glass & Bifacial   

Another important change the PV industry seriously working on at the module level is replacing polymer backsheets with glass. These glass-glass modules enable to take the complete benefit of bifacial cells, which are light sensitive on both sides. Germany’s SolarWorld, for example, is offering bifacial glass-glass designs for their Bisun modules, which come with 5 busbars and are based on monocrystalline PERC technology.
Bifacial technology enables further innovations. At EU PVSEC 2016, a research group from the Institute for Solar Energy Research Hameln (ISFH) presented a new interconnection process called “flip-flop” for bifacial cells. Here, instead of interconnections routed from the front of one cell to the rear of an adjacent cell, the cells are placed with alternate orientation, while the interconnection wire runs straight in front-to-front and rear-to-rear fashion. According to ISFH, the approach has the potential to increase the module efficiency by 0.5% absolute on aperture area.

​Unlike traditional solar modules whose cells can only use the sunlight that strikes the front of the module, bifacial modules use highly efficient bifacial, N-type silicon solar cells that convert to energy the light that strikes both the front and the back of the module, leading to 35% or more additional energy harvesting from the sun. With all solar modules, a substantial amount of light scattered from clouds, the ground, surrounding buildings and rooftops reaches the rear side. In traditional modules this additional light is not converted into electricity and adds unwanted heat to a module, thereby reducing the modules efficiency. A bifacial module , in contrast, takes this additional light and converts it into electricity. This is  especially useful when modules are mounted over reflective backgrounds, such as white roofs or light colored ground covering.

To make these bifacial panels more robust and efficient, they are manufactured using tempered glass-on-glass design and are hermetically sealed to protect the cells from external damage. Thus, the effects of PID are negligible due to the use of high quality materials, double glass structure and hermetic sealing of the module.​  The frameless, tempered glass-on-glass design allows the modules to receive the exceptional weight load rating of 5400Pa, when properly mounted. The frameless design also eliminates the need for the modules to be grounded. These modules are adaptable for use with many different styles of racking or mounting. 

Choosing the Right Platform

These are just a few trends we see in module manufacturing, but there is so much to explore to improve the technology. With all these technical developments requiring a strong cooperation between researchers, equipment makers, material vendors and PV manufacturers, the upcoming 33rd European PV Solar Energy Conference & Exhibition (EU PVSEC) in September in Amsterdam offers the perfect platform to present your innovative solutions for solar module making and beyond. 

Monocrystalline solar cells are made out of silicon ingots, which are cylindrical in shape. To optimize performance and lower costs of a single monocrystalline solar cell, four sides are cut out of the cylindrical ingots to make silicon wafers, which is what gives monocrystalline solar panels their characteristic look.

A good way to separate mono- and polycrystalline solar panels is that polycrystalline solar cells look perfectly rectangular with no rounded edges.

Very advanced standard silicon module: German module manufacturer SolarWorld offers glass-glass panels with bifacial cells and 5 busbars, based on monocrystalline PERC technology. The photo shows the module from the front and back.