Evonik’s ULTRASIL Tires increase fuel efficiency

Company Profile:

Evonik Industries is one the world’s lead specialty chemical companies. One of the main goals of the company is to provide product that solve problems and provide a maximum benefit to customers and society. Recently, they have developed a more fuel efficient tire that does not compromise on performance through the implementation of a silica-silane system, known as ULTRASIL.

ULTRASIL:

The greenhouse effect is a natural process in which radiant heat from the sun is captured in the lower half of the atmosphere, directly resulting in higher temperatures and thus global warming. In order to reduce this greenhouse effect, most companies are working towards minimizing carbon dioxide emissions from transportation. Carbon emissions from combustion of energy fuels has accounted for 81.5% of total greenhouse gas emissions over the last several years, and global warming is quickly becoming a major problem throughout the world. Transportation contributes to this on a large scale, and it is responsible for 31% of the CO2 emissions from the United States. However, Evonik’s silica-silane system (ULTRASIL) is a unique approach to this problem. ULTRASIL is created in several different forms and is applicable in many different situations, however its primary purpose is to serve as a coating for tires. This advanced tire technology can reduce the rolling resistance of tires, increase traction in wet conditions, and reduce carbon dioxide emissions. In general, tires have been targeted as quick way to reduce carbon dioxide emissions, as simple changes in size and shape can increase fuel efficiency by up to 15%.

ULTRASIL is able to reduce rolling resistance between tires and wet or icy road conditions due to the presence of intermolecular forces (IMFs), which can determine whether a solid will be hydrophobic (resists water) or hydrophilic (attracts water). This is an important concept to the concept of ULTRASIL because it is produced with hydrophobicity in mind. Being hydrophobic, water will adhere to the ULTRASIL coated tires, resulting in increased traction between the tires and the road. The major types of intermolecular forces that impact hydrophobicity include dipole-dipole forces, hydrogen bonding, ionic interactions, and London dispersion forces.

Dipole-dipole forces, hydrogen bonding, and ionic interactions are all known to be hydrophilic interactions. The larger presence of these forces in a molecule, the more the solid will attract water molecules. Dipole moments in a molecule are dictated by the polarity of a molecule. Polarity is the sum of all of the bond polarities in a molecule, resulting in dipole moments. The dipole moment is measured in a vector as the sum of the individual vector movements. For example, CO2, a linear and non-polar molecule, has no dipole moment. Hydrogen bonds are the interactions of a hydrogen atom with a nitrogen, oxygen, or fluorine atom. They are a much more powerful force than dipole-dipole forces, resulting in a larger increase on the hydrophilicity of the molecule. Similarly, the presence of ionic bonds (interactions between positive and negative ions) can have the same effect.

London dispersion forces, the weakest of the intermolecular forces, are the sole forces that can raise the hydrophobicity of a molecule. This force, also called an induced dipole-dipole force, is a temporary attractive force that results when the electrons in two adjacent positions occupy positions that make the atoms form temporary dipoles. These forces occur in all molecules. In the production of ULTRASIL, Evonik has created a silica-silane system, where the hydrophobic regions of the molecule dominate, causing adhesive forces to arise and increase the tension between tires and wet/icy road conditions. More information about intermolecular forces can be found here, or

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Also, the chemical structures of the silica helps contribute to its unique properties. Silicon dioxide can exhibit one of the largest varieties of crystal structures among the compounds commonly available and used. These many different crystalline forms allow silica to be used in a broad range of applications, including ULTRASIL. Precipitated silica, which is key to this product’s functionality, is a specially prepared form that has an amorphous structure, similar to silica gel or glass, both of which are predominantly silicon dioxide, or silica. As already discussed, adding these silicon dioxide granules to the surface of rubber tires has many beneficial effects on vehicle performance, but binding this hydrophilic molecular solid to the long, continuous, and hydrophobic polymer chains that make up vulcanized rubber can be difficult. It is up to sulfur, linking the polymers of vulcanized rubber to make it more resistant to temperature extremes, to act as a coupling agent for silica, since the hydrocarbon polymers will not bond to it by themselves.

From what we know, however, ULTRASIL production takes a rather different approach to solving the problem of coupling silica to rubber: the silica-silane system. By treating the original rubber  material with various organosilanes, a surface that silica particles can easily bond to is created, making it possible to form the desired composite with more cross-links to the silica granules and a higher overall thermal stability than without the treatment. Organosilanes usually have both a nonpolar and polar end and can not only bond with vulcanized rubber, but also with the silicon dioxide particles, through dehydration synthesis of their hydroxyl groups with the hydroxyl groups that cover the surface of the silica particles.

While many of the specific details of the ULTRASIL manufacturing process are trade secrets of Evonik, the company does share the basic concept of how it obtains the very pure amorphous silica needed for its products: precipitation from solution. Precipitated silica is widely used in industrial processes around the world, and Evonik Industries is its largest producer. Just like in ULTRASIL, these fine silica grains are often used in rubber products like tires and shoe soles for benefits similar to those of Evonik’s products. Generally, an aqueous silicate salt is reacted with an inorganic acid (like H2SO4) to form insoluble silica in the following reaction:

Na2SiO3(aq) + H2SO4(aq) → SiO2(s) + Na2SO4(aq) + H2O(l)

    After the silica precipitate has been dried, it still contains no more than 88% silicon dioxide according to Evonik, with most of the rest being water. The ensuing treatment to purify the product varies depending on the desired size and quality of the particles obtained, but eventually a fine powder consisting of 99% silica can be obtained. The precipitated silica used in the ULTRASIL product line consists of miniscule, porous granules often of nanoparticle size to allow a high surface area to volume ratio, with the 7000 GR variant having a surface area of 170 m2 per gram. It is this kind of fine silica that allows for the reinforced rubber of the emerging “Green Tire” that advances in silica rubber have created.

Further Reading:

If you are interested in the chemistry behind Evonik’s ULTASIL, there is a lot of in depth reading available in scientific journals. A thorough account of this technology and the chemistry that drives it can be found

  • In this study by Brinke, Debnath, Reuvkamp, and Noordermeer
  • And this article by Park and Cho
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The Manufacturing Processes of OLEDs

At the recent CES convention in Las Vegas, many new products were unveiled and demonstrated.  Both the public and professional communities were captivated by the new OLED technologies.  However, as the prices were announced, it soon became clear to the techno-savvy consumers at home that it would be many years before OLEDs were a common sight in the average household.  Although the prices seemed daunting, the manufacturing methods used to produce these organic screens have been steadily improving.  As a revolutionary advance in display technology, OLED displays are expected to overtake the LCD market in the near future.  Thus, many researchers have worked tirelessly to develop a more efficient and quicker method in order to make OLEDs a more accessible technology.

Patternable OLEDs operate through a light or heat activated electroactive layer.  This is a polymer that exhibits a physical change when stimulated with an electric field.  A popular polymer for this layer is PEDOT-TMA, which becomes a highly efficiently hole injection layer.  It is a p-type conducting polymer and a modification of the PEDOT monomer, which is based off the EDOT monomer and can be seen below.  PEDOT-TMA is a popular material for this layer because it is noncorrosive and dispersible in organic solvents.  This is due to the methacrylate groups at both ends of the polymer chain.  These caps limit the length of the polymer, making it more soluble in organic solvents and ideal for usage in OLEDs.  These OLEDs are conventionally produced through the use of two main patterning technologies, organic vapor phase deposition (OVPD) and inkjet etching (IJE).

The PEDOT-TMA chain.  The two methacrylate caps at the end are clearly visible.

Organic Vapor Phase Deposition 

The organic vapor phase deposition manufacturing process works by transporting organic molecules to a cold substrate through the use of a hot inert carrier gas.  The final film product can be fine-tuned by altering the gas flow rate and source temperature.  Uniform films are produced by lessening the carrier gas pressure, which creates a larger velocity and mean free path of the gas.  This leads a decrease in the thickness of the boundary layer.  OVPD is also popular for OLED production because the process naturally prevents issues related to contaminants from the walls of the chamber.  This is because the walls are naturally warm, which prevents molecules from sticking to and producing a film.

However, maintaining a uniform film thickness and dopant concentrations over the large surface area needed for many applications is a demanding task in the context of vacuum evaporation, which is currently the most prevalent method of depositing organic molecular solids.   In addition, there is a relatively low yield with OVPD production, which explains the massive production costs.  As a remedy, low pressure organic vapor phase deposition has been proposed as a substitute technique.  This process has a notably improves command over doping and offers a potential method to rapid, particle-free, uniform deposition of organics on large-area substrates.  Further information detailing the uses and advantages of OVPD can be found here.

Inkjet Etching

The most basic method of producing OLEDs incorporates old technology originating from 1976, when the first inkjet printer was created.  Inkjet printing is a cheap and efficient alternative to traditional OLED production and is used by several start-up OLED companies such as Kateeva.

The Kateeva system for inkjet etching.

Kateeva’s system of inkjet printing involves a movable platform capable of holding screen substrates of up to 55” diagonal length.  The movable platform is able to transfer a selected substrate under various print heads, which are then able to spray droplets of organic material from different nozzles.  These drops of organic material are placed precisely (picoliter scale) on a selected substrate to form the pixels on the screen.  Additionally, the inkjet printing system is a versatile procedure, as it can be adapted from large television screens to small handheld devices such as smartphones and tablets.

This process is not as accurate or efficient without the use of a shadow mask on the substrate.

Shadow mask used for consistency and quality of the final display.

A shadow mask is a fine metal stencil drawn on the substrate that directs the material spray into its correct place, ensuring the consistency and quality of the final screen.  However usage of the shadow mask, while convenient and economical, also has its downfalls.  The shadow mask may lead to contamination of the organic materials of the OLED screen.  Because the OLED screen is composed of organic products, it is extremely sensitive to outside particles that may cause a drop in the vibrant image quality.  Nevertheless, inkjet printing is still a viable method for producing OLED displays and further improvements will reduce the risks of contamination.

New Developments in the OLED Industry

Recently, researchers have discovered that utilizing a molecule of a certain shape and structure allows anew type of OLED that has the potential to emit as much light as a commercial OLED, but without the need for costly rare metals normally demanded to make the OLEDs effective. Rare earth metals such as iridium or platinum are currently vital to OLED displays because it increases the efficiently of the display to commercial standards.  The removal of these expensive metals from the production of OLEDs has the potential to significantly reduce the costs of OLEDs.  This improvement in OLED structure is vital to advance the technology and reduce prices as the OLED industry of smartphones, televisions, and solid-state lighting continues to grow.

New developments regarding OLED production also includes metal oxide deposition systems.  These have potential to become leading production methods because they are more efficient and consistent in their output than the currently costly production methods.  For example, the AKT-55KS, a plasma enhanced chemical vapor deposition (PECVD) system that handles 8.5-Gen substrates, is a good option to consider based on its defect-free nature and its ability to keep out unwanted gases.

AKS-55KS

Two systems that hold potential for widespread use is the new physical vapor deposition (PVD) deposition systems, which comes in 2 models: SKT PiVot 25K which controls Gen-6 substrates, and the SKT PiVot 55K which controls 8.5-gen substrates.  The advantages of these systems are derived from the utilization of tubular cathodes of donor material, where the cathodes rotate because of the process of deposition.

Conclusion

Constant innovation is a necessity of a demanding market, especially in an industry that is still attempting to establish itself as a viable consumer market.  The production of OLEDs is still in its infancy, as there are several obvious and monumental obstacles that present itself in the first phases of starting such a market.  The most pressing issue is the costly and inefficient manner in which these organic LED’s are created, which has prompted researchers to find ways to reduce these expenses. This had led to the introduction of the inkjet printer formatted specifically for the production of OLEds as well as the new deposition systems offer viable alternatives that hold promise for a market that contains endless possibilities. One can only speculate the boundless paths that the OLED industry can take, whether it be into solid-state lighting, a lucrative smartphone market, or both.  Consumers can only wait to see what the raw potential of this revolutionary technology can manifest into.

The Chemistry of Hydrogen Fuel Cells

In cities like Beijing, citizens have to live every day with a suffocating layer of smog in the atmosphere. Fortunately for most Americans, this is not a very prominent issue. However, with the immense amount of pollution produced from the industries that continuously provide for the needs and luxuries of Americans, it could be.

Pollution has been a problem ever since global warming was coined as a term. There are so many ways to contribute to this problem that is growing at such a fast rate that there seems to be no significant way to stop it.

One small step towards solving this problem is the implementation of fuel cells in vehicles. Fuel cells, specifically hydrogen fuel cells, are one of the most environmentally friendly power sources available. The byproducts of their electricity-making process are solely water and heat! Although frequently compared to the battery, technically speaking, the fuel cell is an electrochemical energy conversion device. It converts hydrogen and oxygen into water, producing energy in the form of electricity in the process. Now, how does energy enter into the equation?

This energy actually comes from the reverse of a relatively familiar process – electrolysis. Electrolysis uses electricity to separate a molecule into its original components. By sending an electric current into a solution through an electrolyte, which ionizes when dissolved in a solvent, the flow of ions is stimulated and allows for the non-spontaneous reaction (the break-up of the molecule) to occur. In 1839, a Welsh scientist named Sir William Robert Grove reversed this process and generated electricity and water from hydrogen. He called his creation a gas voltaic battery, now known today as a hydrogen fuel cell.

There are many types of fuel cells that serve different purposes, but they all have the same general setup. In a hydrogen fuel cell hydrogen atoms enter at the anode, where their electrons are stripped by an oxidation reaction: 2H2 –> 4H++ 4e-. As a result, the hydrogen atoms are ionized and carry a positive charge. The electrons then travel through a wire where they produce a direct electric current (DC) output. In some fuel cells, the positively charged hydrogen ions move through the electrolyte (represented by the proton exchange membrane pictured below) and join with oxygen molecules that enter from the cathode and the electrons returning from the wire. Other fuel cells have the oxygen molecules pick up the electrons first before moving through the electrolyte to the cathode, where the electrons combine with the hydrogen ions to form water, as can be seen in the reduction reaction O2 + 4H+ + 4e- –> 2H2O. In this process, water and heat are formed as a result, meaning that the reaction is exothermic. All these steps can be summarized in the net, or “redox” reaction: 2H2 + O2 => 2H2O + energy. Both products are released from the exhaust and are harmless in terms of pollution.

Graphic credit to Marc Marshall, Schatz Energy Research Center/

Not only are fuel cells eco-friendly, but, in comparison with batteries, they have the ability to last much longer. Unlike batteries, fuel cells can be used as long as there is access to hydrogen and oxygen. Batteries, on the other hand, can’t be refueled. The one exception is rechargeable batteries, but even these will eventually die. Additionally, since fuel cells create electricity chemically, they are not subject to the thermodynamic laws that greatly restrict efficiency. Fuel cells are, thus, more efficient than batteries.

There are six main types of fuel cells: polymer exchange membrane (PEMFC), solid oxide (SOFC), alkaline (AFC), molten-carbonate (MCFC), phosphoric acid (PAFC), and direct methanol (DMFC). They each have advantages and disadvantages and hold different futures in the ever-changing world of technology. For more specific information on each type, check out this link.

Let’s explore one of them. Utilizing the simplest reactions, PEMFC is one of the more promising of the fuel cell family and will most likely be used in homes and transportation. It consists of an anode, a cathode, an electrolyte, and a catalyst, and follows the aforementioned description of how a fuel cell works. Hydrogen gas enters the fuel cell on the anode side and is split into H+ ions and electrons once it comes into contact with the catalyst. The electrons move through the anode to the external electric circuit where they produce an electric current, then return to the cell on the cathode side, where oxygen gas is pumped through. Because of oxygen’s high electronegativity, it attracts and pulls the H+ ions through the exchange membrane and forms, along with the electrons that return to the cell, water molecules. This reaction will produce about 0.7 volts. To increase this voltage to a more useful level, several fuel cells are layered on top of each other and connected by bipolar plates, forming a fuel-cell stack. Try out the simulation here!

Now for how they work in cars! Fuel cell vehicles consist of five distinct components: the fuel cell stack, electric motor, high-output battery, hydrogen storage tank, and power control unit. The fuel cell stack converts the highly pressurized hydrogen gas (to increase driving range) stored in the hydrogen storage tank with the oxygen from the air into electricity, which powers the electric motor. Compared to a conventional internal combustion engine, the electric motor is much quieter, more efficient, and more smooth. The high-output battery stores energy that is generated from regenerative braking and provides supplemental power to the electric motor. Lastly, the power control unit controls and oversees the flow of electricity.

It is clear that hydrogen fuel cells have a bright future ahead, Of course, there are disadvantages to every new breakthrough in science, but with time, those are ensured to be addressed. Even now, these issues are being acknowledged and improved. Recently, an article from USA Today detailed Toyota’s advancement in fuel cell technology with an increase in range and shortening of the time it takes to refuel. With improvements already being seen, this technology is sure to make its way into the transportation and home infrastructure in no time. So keep your eyes peeled for any mention of fuel cells!

Looking to the Future

From the earliest, most primitive form of “solar panels,” plants, to the contemporary quantum dot solar cells, the harvesting of energy provided by our greatest resource, the sun, has evolved tremendously. Throughout this blog, we have covered quite a bit. We talked about the photovoltaic effect, the premise for the chemistry behind solar panels, semiconductors, especially silicon the most common material for solar panels, solar cell efficiency (link 3rd post), and even some of the cutting edge solar cell research happening right now including perovskites (link 4th post)and quantum dot solar cells (link 5th post).

 Due to the nature of solar technology, the role of chemists is irreplaceable. Chemists must design new dyes, develop materials for better electron transport, and they must also characterise each new material to ensure that the energy of the electron ‘fits in’ with the other materials – the rest of the solar cell. Just like a car engine, if one part doesn’t fit, the whole thing doesn’t work. In this way, chemists, along with physicists and engineers have to work together to find the best materials which fit together to ensure that the solar cell is working as best as possible.

Can solar power change the world? For some communities in developing countries, it already has. Scientists must continue to work with businesses, economists, architects, designers and a whole host of other professions to make sure that solar cells are practical, cost effective and appealing. Solar cells will continue to make a large contribution to reducing the world’s dependency on fossil fuels, closing the poverty gap and changing the world. In one hour, more sunlight falls on the earth then is used by the entire population of our planet in a year. It is the most readily available renewable source of energy we have access to, and it is up to us to utilize it the best way possible.

 So we leave you with a deeper understanding of the mechanisms of a photovoltaic cell, the variety of solar panels, and this awesome (at least we think it is) TED Talk by Joe Jordan, a NASA Researcher about “The Solar Window of Opportunity.”

Not Just Silicon: Countless Material Alternatives

In our most recent blog post, we focused on explaining the properties of silicon and why silicon is the most commonly used material for photovoltaic cells. However, silicon is only one of millions of materials that can be used. In fact, Harvard recently ranked approximately2.3 million possible replacements, and the list continues to grow as we speak. As you can see, solar cells come in all different materials, shapes, and sizes.

Contemporary solar panels exhibit a relatively low average efficiency of 15-25%. Modern research in the area of photovoltaic cells is driven by the desire to increase efficiency. Solar cell efficiency is generally defined as the percent of solar energy that is converted into useful electrical energy. Here, we will discuss other materials that can exhibit the photovoltaic effect in relation to the factors that affect the efficiency of these photovoltaic cells. There are three basic aspects that influence the performance of different materials: structure, absorption, and bandgap.

 

Structure

The crystallinity of a particular material corresponds to how orderly the atoms are packed in the crystal structure. There are several structures that semiconductor materials can come in including single-crystalline, multicrystalline, polycrystalline, and amorphous. At one end of the spectrum, you have single-crystalline materials that are packed in a very orderly manner with a regular repeating pattern from layer to layer. On the other end, you have a material that is composed of several smaller crystals causing  an irregular pattern due to disrupted  arrangements between atoms.

Let’s take a look at Copper Indium Gallium Selenide Solar Cells (CIGS) as an example to further understand the role that structure plays in solar cell efficiency. CIGS  is a tetrahedrally bonded semiconductor and has a polycrystalline chalcopyrite structure. The uniformity of its structure and its  stacked multilayer structure produces a smoother absorbing surface which results in higher efficiency.

Part of the reason solar panels have low efficiency is that the particles collected from the sun enter the solar cell and spread out in all directions. Getting them all to flow one direction typically requires layers of different channeling material. Each time the particles pass between these layers some get lost, decreasing the energy efficiency of the solar cell. This brings us to absorption.

Absorption

The absorption coefficient of a particular material corresponds to how far a photon of a specific energy can penetrate the material before it is fully absorbed. The higher the absorption coefficient, the more readily a material absorbs light, and hence, the higher the solar cell’s efficiency. This property is influenced by two factors: the energy  of the photon and the material of the cell. Since photovoltaic cells are made of semiconductors, there is an abrupt jump in absorptivity. Photons with energies below the bandgap will have low absorptivity, but as soon as you surpass the bandgap, the absorptivity jumps up. In essence, a higher absorption constant correlates with increased efficiency because more of the light is available to be converted into electrical energy. Structure, as discussed previously, is essential in determining absorptivity since geometry can affect factors such as the angular variability of the light absorbed.

In addition to increasing absorptivity of light through structure, researchers have also developed a thermovoltaic cell(TPV) made of nickel photonic crystals in order to reduce the energy losses associated with the absorption of light in a conventional cell. TPVs convert energy from the photons into thermal energy as well by heating up a receiver and transferring this energy to another absorber that matches the cell’s absorption characteristics. Conventional TPV designs include a filter between the emitter and the cell to reflect back the photons with energy lower than the cell bandgap. This way, their energy is recycled to maintain the temperature of the intermediate. TPVs are becoming increasingly popular due to their maximal usage of the solar energy.

Bandgap

Last, but certainly not least, we have bandgap. Bandgap refers to the amount of energy needed to excite an electron of a particular semiconductor material from its ground state to a free state where the electron can be involved in conduction. Basically, bandgap is the difference between the valence band (the lower energy level) and the conduction band (the higher energy level) where the electron is free to roam. Temperature can also influence bandgap width. A higher temperature increases the energy of the electrons in the material by adding kinetic energy. Therefore, a lower amount of energy is needed to excite the electron from the material. A lower bond energy results in a lower bandgap width. The exact mechanics behind this can be understood with lots of crazy looking equations and a good amount of calculus (which you can see here) but for our purposes, it is only important to understand that a lower bandgap corresponds to a lower solar cell efficiency.

A great example of a solar cell with an ideal bandgap is the Gallium Arsenide orGaAs cell (unit cell shown to the left). The GaAs band gap is 1.43 eV, nearly ideal for single-junction solar cells. Unlike traditional cells, GaAs cells are relatively insensitive to heat. Gallium arsenide also has a very high absorptivity and it only requires a cell of a few microns thick to absorb sunlight. (Crystalline silicon requires a layer 100 microns or more in thickness.) These properties make GaAs especially desirable for applications in extreme climates such as space.

Conclusion

There are several factors that influence the efficiency of solar cells. Professor Bermel of Purdue University (some of his interesting photovoltaic research can be seen here) responded to our question of which factor was the most important by saying “Both optical and electronic structure is absolutely essential. A poorly designed structure in either category can result in very low performance.”Clearly, an efficient solar cell requires a good balance of the three major factors we discussed in this post: structure, absorption, and bandgap.

The question of an ideal solar cell remains. When we asked Professor Bermel what the properties of the ideal material for solar cells would be, he gave us seven musts:

1. Bandgap well-matched to the solar spectrum

2. Strong optical absorption for energies above the bandgap

3. High electronic mobility

4. High minority charge carrier lifetime

5. Durable, reliable (e.g., stable under UV light)

6. Inexpensive, and made from widely abundant elements

7. Eco-friendly and non-toxic

Researchers continue to search for an ideal solar cell material that can satisfy these criteria in order to create a future where solar panels can become the mainstream source of energy.

Semiconductors and Their Relevance to Solar Panels

Let’s backtrack for a moment and discuss something mentioned in our earlier post: semiconductors.

What Are Semiconductors & How Do They Work?

semiconductor is a material that has a conductivity between that of insulators (i.e. plastics, rubber, Styrofoam, etc.) and conductors (copper, iron, gold, etc.). These semiconductors exhibit the photoelectric effect. This effect causes the semiconductors to absorb photons of light, and the energy from the photons is then transferred to electrons. With an electron’s new gain in energy, it can escape from its normal position in the atom and integrate into the electrical current, which can be used to produce electricity. However, when the electron escapes from its normal position (the valence band), and moves into the conduction band, it leaves a “hole” in the semiconductor (as depicted in picture to the right).

Semiconductors and Solar Panels: What’s the correlation?

 Due to the holes that are formed within the  semiconductors, they are ideal to use within  photovoltaic cells. “Why?” you may ask. In  order to induce an electric field inside a photovoltaic cell, two semiconductors are put together,  one P-type semiconductor and one N-type semiconductor. The P and N correspond to positive  and negative respectively. In a P-type semiconductor, there is an abundance of holes while an  N-type semiconductor has an excess of electrons. By putting a P and N-type semiconductor  together, a P-N junction is formed where they meet.

The excess electrons from the N-type semiconductor flow to the P-type semiconductor to fill  the holes. Now, holes are present in the N-type semiconductor and electrons from the P-type  semiconductor move to the N-type material. This process/motion of electrons between the two  types of semiconductors occurs back and forth, and as a result of this back and forth motion of  electrons, an electric field is created at the junction. This electric field causes the electrons to  jump out from the semiconductor to the surface of the material, and from there, they can  move to the electrical current. Other electrons fill the vacated holes and then move out to the circuit, and in this way, the current continues. No electrons are technically “used up” throughout this process. The photons merely give their energy to create electron-hole pairs. The electrons transfer this energy to the electrical circuit in the form of electrical power, and then they return to the cell to recombine with the positive holes. Simply put, this process conveys the idea of the conservation of energy.

Silicon & What Makes It a Great Material for Solar Panels                  

                        

In the earliest successful solar panels, silicon was the semiconductor material that was used, and no surprise, it is still a commonly used material for solar panels today.

Silicon is a perfect example of a semiconductor. On an atomic level, silicon has four valence electrons and has the ability to form four covalent bonds. When it does so with four neighboring silicon atoms, the resulting crystalline silicon solid is implemented into solar panels.

However, pure crystalline silicon is a metalloid that looks shiny, but is brittle and a fairly poor conductor since its electrons are not free to move around. In order to solve this issue, the silicon that is found in photovoltaic cells is not just silicon alone–it has impurities. Usually when one hears of “impurities,” it has a negative connotation, but that is not the case here. In a process calleddoping, impurities (other atoms) are purposely mixed with silicon atoms. Typically, silicon is mixed with phosphorus and boron. For approximately every 1 million silicon atoms, there is 1 phosphorus atom or 1 boron atom. Why phosphorus and boron?

Let’s start with phosphorus. Phosphorus has five valence electrons instead of four. It can still bond with neighboring silicon atoms, but what makes it different is that one of its electrons does not bond with another atom. In comparison to if it was pure silicon, it takes much less energy to cause this extra electron to break free. With several phosphorus atoms mixed with the silicon atoms, several free electrons, or free carriers, become available, and the silicon becomes an N-type material. N-type doped silicon serves as a good conductor.

Mixing silicon with boron forms P-type doped silicon. Boron has three valence electrons, and only forms three bonds with nearby silicon atoms, leaving one “incomplete bond.” This incomplete bond is capable of capturing one of the free electrons from the phosphorus. The capture of a free electron leaves a “hole,” so another electron moves to fill this “hole.” This causes the aforementioned motion of electrons, the creation of the electric field, and the flow of electric current.

In the end, if doping is not done, silicon photovoltaic cells would not work.

A visual representation of how silicon P-N junctions work in solar panels can be found below. .

Let’s backtrack for a moment and discuss something mentioned in our earlier post: semiconductors.

What Are Semiconductors & How Do They Work?

semiconductor is a material that has a conductivity between that of insulators (i.e. plastics, rubber, Styrofoam, etc.) and conductors (copper, iron, gold, etc.). These semiconductors exhibit the photoelectric effect. This effect causes the semiconductors to absorb photons of light, and the energy from the photons is then transferred to electrons. With an electron’s new gain in energy, it can escape from its normal position in the atom and integrate into the electrical current, which can be used to produce electricity. However, when the electron escapes from its normal position (the valence band), and moves into the conduction band, it leaves a “hole” in the semiconductor (as depicted in picture to the right).

Semiconductors and Solar Panels: What’s the correlation?

 Due to the holes that are formed within the  semiconductors, they are ideal to use within  photovoltaic cells. “Why?” you may ask. In  order to induce an electric field inside a photovoltaic cell, two semiconductors are put together,  one P-type semiconductor and one N-type semiconductor. The P and N correspond to positive  and negative respectively. In a P-type semiconductor, there is an abundance of holes while an  N-type semiconductor has an excess of electrons. By putting a P and N-type semiconductor  together, a P-N junction is formed where they meet.

The excess electrons from the N-type semiconductor flow to the P-type semiconductor to fill  the holes. Now, holes are present in the N-type semiconductor and electrons from the P-type  semiconductor move to the N-type material. This process/motion of electrons between the two  types of semiconductors occurs back and forth, and as a result of this back and forth motion of  electrons, an electric field is created at the junction. This electric field causes the electrons to  jump out from the semiconductor to the surface of the material, and from there, they can  move to the electrical current. Other electrons fill the vacated holes and then move out to the circuit, and in this way, the current continues. No electrons are technically “used up” throughout this process. The photons merely give their energy to create electron-hole pairs. The electrons transfer this energy to the electrical circuit in the form of electrical power, and then they return to the cell to recombine with the positive holes. Simply put, this process conveys the idea of the conservation of energy.

Silicon & What Makes It a Great Material for Solar Panels                  

                        

In the earliest successful solar panels, silicon was the semiconductor material that was used, and no surprise, it is still a commonly used material for solar panels today.

Silicon is a perfect example of a semiconductor. On an atomic level, silicon has four valence electrons and has the ability to form four covalent bonds. When it does so with four neighboring silicon atoms, the resulting crystalline silicon solid is implemented into solar panels.

However, pure crystalline silicon is a metalloid that looks shiny, but is brittle and a fairly poor conductor since its electrons are not free to move around. In order to solve this issue, the silicon that is found in photovoltaic cells is not just silicon alone–it has impurities. Usually when one hears of “impurities,” it has a negative connotation, but that is not the case here. In a process calleddoping, impurities (other atoms) are purposely mixed with silicon atoms. Typically, silicon is mixed with phosphorus and boron. For approximately every 1 million silicon atoms, there is 1 phosphorus atom or 1 boron atom. Why phosphorus and boron?

Let’s start with phosphorus. Phosphorus has five valence electrons instead of four. It can still bond with neighboring silicon atoms, but what makes it different is that one of its electrons does not bond with another atom. In comparison to if it was pure silicon, it takes much less energy to cause this extra electron to break free. With several phosphorus atoms mixed with the silicon atoms, several free electrons, or free carriers, become available, and the silicon becomes an N-type material. N-type doped silicon serves as a good conductor.

Mixing silicon with boron forms P-type doped silicon. Boron has three valence electrons, and only forms three bonds with nearby silicon atoms, leaving one “incomplete bond.” This incomplete bond is capable of capturing one of the free electrons from the phosphorus. The capture of a free electron leaves a “hole,” so another electron moves to fill this “hole.” This causes the aforementioned motion of electrons, the creation of the electric field, and the flow of electric current.

In the end, if doping is not done, silicon photovoltaic cells would not work.

A visual representation of how silicon P-N junctions work in solar panels can be found here.

Up Next…

Although silicon is so widely used, it is not the only material that is used for solar panels. Additionally, it does not absorb light as efficiently as other materials do, so in our next post, we will dive deeper into the usage of other materials that enhance the efficiency of the solar panel.