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Solar cells, otherwise known as photovoltaics (PVs), are a rapidly growing class of renewable energy devices that change solar rays (photons) into electricity. Solar cells come in many forms, especially when it comes to the active material that changes sunlight into electricity. The most traditional solar cells rely on inorganic materials as the active material, while the newest solar cells use everything from organic materials to printed inks. Here, we’re focusing on the traditional inorganic solar cell because even though the newer classes bring different benefits (printable, flexible), the inorganic solar cells are still the most widely implemented and efficient solar cells in use today.
Solar cells rely upon the principle of the photoelectric effect, which is when a material exhibits a chemical and physical phenomenon that generates voltage and current when exposed to light. Inorganic solar cells have traditionally used silicon, that is doped to create a semiconducting junction. In many cases, designers use silicon on both sides of the junction, where one side is doped with an atom with one less electron than silicon (p-type), and another side is doped with an atom that has an extra electron than silicon (n-type). In most cases, the silicon is doped with boron or gallium and phosphorous to produce p- and n-type semiconducting regions, respectively. In recent years, there has been a shift from using silicon to various nanomaterials, as the nanomaterials often yield greater device efficiencies. While these efficiencies are greater than silicon, silicon-based solar cells are still the most common inorganic solar cells in use.
Regardless of the material used—whether it is two differently doped silicon materials or nanomaterials—a semiconducting junction is formed where the two regions contain holes and electrons separated by a depleted region. How the electrons and holes are formed and aligned in the semiconducting junction is largely due to the chemical properties of the material and the level of chemical doping. When the silicon is doped to become a p-type region, the doped atom can only form three covalent chemical bonds compared to silicon’s four. This leaves a space where a chemical bond should be. This lack of a bond is known as a hole, which is positively charged. On the other hand, when the silicon is doped with an atom to form an n-type region, the extra electron is left over in the lattice because the lattice geometry that silicon adopts can only accommodate four bonds (the same goes for many nanomaterials, as it is often carbon nanomaterials that are used, and they can still only accommodate four covalent bonds if the material is to be stable). Hence, the extra electron becomes delocalized in the lattice. These doped regions then become hole-rich and electron-rich, respectively..
Before the solar cell is exposed to any light, these electrons and holes are separated by the depletion region, which is the interface of the n-region and the p-region and is often coined a p-n junction. This interfacial area is a region where some of the electrons and holes have come together, causing the other positively and negatively charged carriers to be separated by this ‘neutral zone’. As well as being a neutral zone, it also generates an internal electric field that prevents all the holes and electrons in the two regions from combining.
While the charge carriers are initially separated, this changes when sunlight hits the solar cell. Photons of light possess energy, and when the photons strike the solar cell, this energy then transfers to the holes and electrons that are free on either side of the depletion zone. This extra energy also enables the free charge carriers to move into the depletion zone. As this happens, the width of the depletion zone reduces. Eventually, the internal electrical field can’t counteract the energetic motion of the charge carriers, and the electrons move to the opposite side of the junction, where they recombine with the holes. This generates a constant current. Once the charge carriers start to recombine, the depletion zone increases again, but it never returns to its rest state so long as there is energetic input—i.e., light on the solar cell. So, as long as there is sunlight, a continuous current will flow that can be harvested and stored. Once the sunlight is no longer present, the depletion zone will return to its original thickness, and the device is essentially reset until more sunlight hits the cell.
The inorganic solar cell relies on chemistry, chemical principles, and the effects of chemical reactions to efficiently convert sunlight into electricity through a semiconducting p-n junction. The doping of two regions, be it silicon or another inorganic material, creates a region that enables oppositely charged particles to combine and produce an electrical current under solar irradiation. Inorganic cells have much higher efficiencies than other types of solar cells, but the materials used are highly crystalline in nature and have a regular solid-state lattice. So, this means they are often less flexible than other types of solar cells. However, they are still the most common type, and until flexible and printable solar cells can reach similar efficiency levels, it is likely that inorganic solar cells will be the frontrunner for many years.
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Solar cells, otherwise known as photovoltaics (PVs), are a class of renewable energy devices that convert the photons from solar rays into electricity. Solar cells come in many forms, with the most common types composed of inorganic materials. But, new developments in technology have yielded a wide range of thin film solar cells. Notable developments range from the ability for solar cells to be printed using inks to flexible solar cells made of organic materials, solar cells that use quantum dots, and dye-sensitized solar cells (DSSCs). Here, we look at the different types of thin film solar cells that can now be created and the chemistry behind these developments.
Organic solar cells are the second biggest class of solar cells. These organic solar cells typically use polymeric materials—but can use other organic materials—to convert the photons into electricity. Organic solar cells are much cheaper to produce and are much more flexible than inorganic solar cells, but their conversion efficiencies are much lower. Unlike inorganic solar cells, organic molecules can be solution-processed, and design engineers can use these formulations to create much thinner devices than their inorganic counterparts.
The chemical makeup of the polymer is crucial for generating an electric current. Chemical methods can be applied to change the bandgap of the polymer, which allows for electronic tunability. Even though the conversion efficiency of organic solar cells is not as high as inorganic materials, organic materials have a very high optical absorption coefficient, which is why designer engineers can fabricate thinner devices without losing the ability to generate electricity. The chemistry also allows polymers to be processed into formulations that can be printed (printable solar cells) and enables organic solar cells to be transparent, after which they can be used in windows and other areas of buildings.
Many might expect the working mechanisms to be roughly the same as inorganic solar cells. However, the chemical and internal structures are fundamentally different. In inorganic solar cells, dopants are used to change the chemical structure of the inorganic material so that electrons and holes are generated, which are then separated by a depletion region where some of the holes and electrons have already recombined, leading to the separation of the remaining charges. The migration of these separated charge carriers to the opposite side of the depletion region under photon absorption causes a current to flow.
However, organic solar cells are different. Organic solar cells use specific donor and acceptor materials to generate the electrons and holes, rather than doped materials. The organic molecules absorb the photons of light, which generate excitons—an electron and its corresponding hole. Light absorption also causes the electrons within the exciton to become excited, whereupon they move from valence band to the conduction band. The exciton then moves to the interface between the donor and acceptor materials, where it separates into an electron and a hole. This charge separation causes a current to flow because the electrons and holes flow to the electrodes.
Dye-Sensitized Solar Cells (DSSCs) are another emerging class of thin film solar cells, which again have a completely different mechanism for generating an electric current under solar irradiation. It is a class of thin-film solar cells that are semi-transparent and semi-flexible.
In DSSCs, it’s all about the anode. The anode in a DSSC is coated with a semiconducting film, followed by a titanium dioxide layer. This is further followed up by being soaked with a photosensitive dye—commonly a ruthenium complex—that bond to the titanium dioxide layer. The cathode is simply a glass plate coated with a platinum film to act as a catalyst. Between the two electrodes is an electrolyte solution.
As the name suggests, the dye—located in the anode—is key to the current generating mechanism. When the light shines on a DSSC, the dye becomes excited, causing its electrons to shift from a ground state to an excited state. This higher energy enables the dye to overcome the semiconductor’s bandgap, whereupon it becomes oxidized, and an electron is released into the conduction band of the semiconductor. This causes the semiconductor to become conductive, and a current is generated. The electronic balance of the cell is resorted by the electrolyte molecules donating an electron to the dye, where the dye transforms back into a non-excited electronic ground state. The electrolyte is regenerated to its normal electronic state through a reduction reaction at the cathode.
Quantum dot solar cells are not as widely developed as other thin film solar cells, but their interest is growing. Quantum dots are 0D materials (electrons are quantumly confined in all three directions), which are only a few nanometers in size. The size and quantum properties of quantum dots mean they have unique optical absorption and emitting properties. One of the key reasons for using quantum dots is their tunable bandgap. Because they are semiconducting in nature, they work like traditional inorganic semiconductors, but due to the small size of each quantum dot, they essentially act as a multi-junction solar cell.
The tunable bandgap means that they can also be tuned to absorb radiation at many different wavelengths of the electromagnetic spectrum. As it stands, their efficiencies are much lower than other solar cells; however, there is great potential for these devices. Quantum dots are the only type of material used in solar cells that can release more than one electron for each photon absorbed. All other materials have a 1:1 ratio, so quantum dots could potentially increase the conversion efficiency significantly by releasing more electrons for each absorbed photon of light.
Even though inorganic solar cells remain the most common, there are many different types of solar cells out there. Many of the other solar cells are not as efficient, but what they lack in efficiency, they make up for in other properties. One key driver of using non-inorganic materials is that they can be made much thinner, are more flexible, can be optically transparent, and in some cases, printable. The ability to use other materials enables solar cells to be implemented on parts of buildings—such as in windows or on curved architectures—that cannot be covered using traditional inorganic solar cells. This greatly expands the capabilities of solar cells as a class of renewable energy devices and makes them much more versatile. And, like many things, the chemistry of the materials used makes this possible.
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The advancement of technology has led to the development of new forms of energy storage options. Energy storage devices such as rechargeable batteries continue to dominate the market, but fuel cells are on the rise. It has been almost a century since the first commercially hydrogen-oxygen fuel cell was introduced. Recent advances in fuel cell energy efficiency1, and power densities, coupled with efforts to combat climate change and the push for green energy generation, have made them more attractive, especially those using green hydrogen. This post will look at the chemistry behind fuel cells and the different fuel cell variations that have been developed to support a wide range of applications.
Fuel cells are an energy-generation technology similar to batteries but with some fundamental differences. One of the key differences between fuel cells and batteries is that fuel cells require a continuous fuel source to function. In contrast, the shuttling of already present ions between electrodes produces energy in batteries. If a continuous fuel source is supplied—the most common choices being hydrogen and oxygen—the fuel cell can continue to produce energy. The fuel cell’s ability to provide continuous power can be advantageous in certain applications—for example, as backup power2 for data centers to move away from fossil-fuel-burning standby diesel generators and as an alternative source of electricity (microgrids) for rural areas that will not only have power but also be able to save significantly on infrastructure costs.
Like the standard battery setup, fuel cells possess an anode, cathode, and electrolyte (between the electrodes) and rely on various electrochemical reactions to produce energy. For fuel cells, it is the reactions that occur at both electrodes that generate electricity, and although there are many different types of fuel cells, the most common working mechanism involves the movement of protons—i.e., positively charged hydrogen ions—between the electrodes. Both electrodes contain a catalyst that helps to facilitate the electrochemical reactions and break down the feedstock ‘fuel’ into their corresponding ionized species. The catalyst does vary from fuel cell to fuel cell, but it needs to be a material that can facilitate oxygen and hydrogen reactions, so common choices include platinum and nickel.
While hydrogen is the basic fuel for a cell, they also require oxygen, and these are the two basic feedstocks that need to be continually supplied for most fuel cells to work. Because the electrolyte separates the two electrodes, there are two distinct electrode-electrolyte interfaces, and these are the areas where the electrochemical reactions take place.
When the hydrogen enters via the anode, the electrochemical reactions at the anode remove their electrons, creating positively charged hydrogen ions— i.e., protons. The released electrons then enter the external circuit, generating a current, while the hydrogen ions pass through the electrolyte and to the cathode. The electrolyte acts as a proton exchange membrane (PEM) that only allows positively charged ions through, excluding any removed electrons so that they only enter the external circuit and don’t try to diffuse to the other electrode, as this would stop the relevant chemical reactions from occurring. Oxygen is also fed into the cathode, and when the catalyst breaks the molecular oxygen down into negatively charged oxygen ions, the electron ejected to the external circuit, recombines with the positively charged hydrogen ions as well as the oxygen ions at the cathodic interface. The result is that water is produced as the end product, which is removed via an exhaust from the fuel cell.
This basic fuel cell mechanism is known as the ‘hydrogen fuel cell’ and is the most common type of fuel cell in existence. Though this is the default fuel cell mechanism, there are variations. Most fuel cells work using similar principles, and all require hydrogen and oxygen as the fuel, but some require others in addition. The main differentiator of all the fuel cells is the type of electrolyte used to transport hydrogen ions to the cathode.
The main types of fuel cells are:
As with anything, the different variations all have their place, and some are only suitable in certain situations.
The ‘alkali’ in alkali fuel cells comes from the electrolyte used, which is potassium hydroxide (an alkaline substance) and is the only differentiator from other types of fuel cells, other than being a fuel cell that operates at low temperatures. MCFCs, on the other hand, operate at higher temperatures and have an inlet of carbon dioxide as well as oxygen and hydrogen because the carbonate ions in the electrolyte get used up and needs to be replenished by injecting carbon dioxide). MCFCs use salt carbonates as the electrolyte. Because they operate at higher temperatures, they are not suitable for all applications, particularly home use, as the high temperatures can cause leaks.
PAFCs, another low-temperature fuel cell, use phosphoric acid as the electrolyte. However, PAFCs are an interesting variation as the internal workings can tolerate the formation of carbon monoxide, meaning that gasoline can be used as a fuel, although this is not a green option. Unlike the others, SOFCs and PEM fuel cells use a non-liquid electrolyte, with SOFCs using a metal oxide ceramic compound (such as zirconia) and PEM fuel cells using a thin and permeable polymer sheet. SOFCs operate at very high temperatures and are limited in use again as the electrolyte can crack instead of leak. PEM fuel cells, on the other hand, are a very low-temperature fuel cell but their efficiencies are lower, and the fuel must be purified before use.
In addition to producing energy through the electrochemical reactions, the heat produced can also be harnessed to produce extra electricity, so while some of the higher temperature fuel cells are less stable, more energy can be generated if both the fuel and the heat by-product are harnessed simultaneously.
While there are many different types of fuel cells, they all work via a similar mechanism. All fuel cells require hydrogen and oxygen to function and electrochemical reactions at the electrodes break down these gases to produce water, while subsequently producing electricity in the process. Therefore, the overall electrochemical reaction for fuel cells is: Hydrogen + Oxygen = Electricity + Water Vapor.
Fuel cells are an alternative technology to batteries and are seen as a much greener option because they only produce water, which is not harmful to the environment. They are often trickier to implement than many of the commercially available batteries, but aside from being green, one of the key advantages of fuel cells is that will always produce electricity so long as they are being fed with both hydrogen and oxygen gas.
The slow-moving organic solar cell industry finally achieved a breakthrough in its quest to optimize energy conversion, and thanks to an accidental discovery during experimentation, this breakthrough came by a process of coaxing electrons with layers of fullerene molecules—popularly known as “buckyballs.” University of Michigan scientists made the discovery when experimenting with organic solar cell architecture. The researchers added two layers of fullerene molecules on top of an organic cell's energy-producing layer where photons hit solar cells to dislodge electrons.
What they found was that electrons move more freely and go farther in the fullerene layer and that an “energy well” (technically known as a potential well) was also created where electrons could not escape. The result was that these electrons—when coaxed with the layers of fullerene molecules—could travel up to several centimeters (compared to nanometers), which enabled them to produce a greater amount of electrical current.
Organic cells are known to have a weak electronic conductivity because they have loose bonds between individual molecules. Instead of having an efficient conduit between molecules, electrons often get trapped and can only travel up to a couple hundred nanometers at most. In an organic solar cell, this trapping of the electrons is the main obstacle that limits the distance that these electrons can travel. If they are free to move through the structure without impedance, they can travel much further. This is the same for all solar cells, but the organic network presents a much harder challenge for these electrons to travel through. Because the electrons don’t travel far enough before their entrapment (where they cannot move), they are unable to participate in the electrical circuit. This block in the electrons’ participation lowers the conductivity of the cell, and in turn, the conversion efficiency decreases as fewer free-flowing electrons can circulate. As a result, organic solar cells, which consist of non-metallic semiconductors such as polymers, only offer efficiencies of up to 13.1 percent. This level of efficiency doesn’t compete well with silicon-based inorganic solar cells that offer a 26.6 percent power efficiency and are in wide use in solar panels today.
Several positive characteristics of organic solar cells, however, highlight the need for further research into extending their efficiency. For example, besides the potential of lower costs due to simpler polymer processing technologies, organic solar cells are also thinner, more flexible, and transparent. These characteristics are crucial for efficiently converting sunlight into electricity. Also, where projects aim to build net-zero energy buildings (NZEBs) or retrofit existing structures to become more energy efficient, companies can integrate organic solar cells into the structure itself, as on rooftops and walls, where the heavier, inflexible, silicon-based inorganic solar cells are not practical or feasible. These organic solar cells are also beneficial in that they come in a variety of colors and configurations, offering better aesthetics as well.
The need to discover ways to bring organic solar cells to their full potential is clear, and this recent breakthrough may do just that. According to the University of Michigan’s article entitled “Semiconductor Breakthrough May Be a Game-Changer for Organic Solar Cells,” its researchers started with the organic cell’s power-producing layer, where photons hit solar cells to dislodge electrons. “Using a common technique called vacuum thermal evaporation, they layered in a thin film of C60 fullerenes—each made of 60 carbon atoms.” What they found was that the electrons moved freely in the fullerene layer, rather than being trapped in the loose bonds between organic molecules.
Interestingly, fullerenes are known to be excellent acceptor molecules because of their variable hybridization states, ability to rehybridize, and curved topology. (However, it is significant to note that since this discovery of use for fullerenes in solar cells, a new highly-efficient class has emerged that is now known as the non-fullerene acceptor (NFA) organic solar cells, which has similar electron accepting properties as fullerenes—but is obviously non-fullerene molecules.) Fullerenes are also electronically confined materials, and they contain potential (i.e., quantum) wells. This means that it is hard to remove an electron once it has entered the potential well of a fullerene molecule. The use of an electron blocking layer that sandwiches the fullerene layer to prevent any electrons from leaving and recombining with the holes creates an additional barricade.
The only way that an electron can influence the realm outside of the potential well is through electron tunneling. However, when you place quantum wells side by side, that is, in a way that the fullerenes molecules can be placed next to each other in a layer, they can form what is known as a “superlattice.” If the distances between quantum wells are less than the reach of an electron’s tunneling wavefunction, the electronic wavelengths can overlap and create a connection between the potential wells, which enables the electrons (and a current) to flow. So, by trapping electrons within the fullerene layers, the close proximity of the potential wells from molecule to molecule enables the electrons to flow unimpeded with no risk of entanglement.
Again, because they are freely moving and cannot recombine with the holes in the energy-producing layer, electrons can travel farther—up to several centimeters, rather than mere nanometers—which enables them to produce a greater amount of electrical current. This is a result of, as mentioned, a greater current that is now possible, not because a single electron carries more energy but because there are more current (i.e., charge) carriers flowing around the electrical circuit. Ultimately, these increases in specific current (and efficiency) within organic solar cells is dependent on how many electrons were flowing around the system before the fullerenes were added compared to after.
The researchers at the University of Michigan acknowledge that this discovery is only a start and that there’s more work to be done to improve solar cell design, in particular in researching what else makes good electron conductors in organic materials. Stephen Forrest, professor of engineering at the University of Michigan, anticipates that it could take as many as 10 years for a major organic solar cell solution to emerge.
This fullerene discovery, though, paves the way for organic materials to be useful to create transparent solar cells that are very efficient over longer distances. Solar cell manufacturers can shrink a solar cell's conductive electrode into an invisible grid, for example, and combined with the organic solar cells’ other characteristics, solar cells can form laminations over any surface without obstruction. With lower polymer processing costs associated with organic solar cells, these solutions can be reasonably inexpensive for a wide range of applications. Perhaps the biggest breakthrough in this discovery involving organic solar cells is that more discoveries leading to more advances are on the horizon.
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As the world looks to phase out fossil fuels in favor of cleaner, renewable energy sources, more effort is being put into improving existing renewable technologies and creating devices that can harness renewable energy sources in new ways. Sunlight is currently one of the most used renewable energy sources—in the form of solar cells—but there are also a number of other ways in which sunlight can be harnessed to produce energy.
Given the amount of sun that hits the Earth each day, there is excellent potential for solar-based energy harvesting devices to become one of the front runners in a cleaner and greener society—and expand on their existing position as one of the leading renewable technologies. Designers are testing and using 2D materials in a number of these solar-harvesting systems, from the widespread use of graphene in solar cells to the use of different 2D materials in photocatalytic and photothermal harvesting approaches.
Of all the solar harvesting methodologies and technologies in existence, solar cells are by and far the most commercialized, most popular, and most efficient choice. There are several bulk solar cells in existence composed of silicon and perovskite materials. In addition, a number of approaches are being developed where 2D materials (and other nanomaterials) are being used to either improve the performance of bulk silicon solar cells or offer a way to create much thinner and/or more flexible solar cells. In the latter, there are already a number of flexible organic solar cells; however, the active materials tend to suffer from a much lower performance than inorganic materials, so 2D materials offer a way of providing thinner solar cells with much-improved performance.
Many devices are in existence where doped graphene has been used as the photoactive material in the semiconducting photovoltaic junction within solar cells, sometimes on its own and other times in conjunction with silicon in bulkier solar cells. Beyond this, graphene and its derivatives are touted as an alternative for the hole transport layer instead of expensive noble metals, such as silver and gold, in an attempt to lower the cost of large-scale solar cells. Graphene is also used in conjunction with silicon in a number of other hybrid devices, including in the creation of thinner, flexible and semi-transparent solar cells.
While graphene is gathering the most interest, graphene and transition metal dichalcogenides (TMDCs) are being integrated into different solar cells—because of their stability and resistance to several potentially degrading stimuli—to improve their long-term stability and ensure that they run optimally for longer. These 2D materials are being integrated into a wide range of bulk solar cells on a commercial level, including in silicon and perovskite solar cells as well as in tandem solar cells.
In recent years, the interest in flexible solar cells has grown significantly, and while graphene was the first 2D material tested for solar cells, the area has since expanded and continues to do so. Recently, TMDCs have been used alongside graphene to create efficient, flexible solar cells and perovskite materials are now being made into 2D sheets to try and emulate the success of the bulkier perovskite solar cells—but in the form of a much smaller and flexible solar cell. 2D perovskite solar cells have not yet been able to reach the heights of their bulkier counterparts, but it is a much newer addition to the 2D family, so there is still plenty of time for them to play a part in 2D material-enhanced solar cells.
Photocatalytic reactions generate new greener fuels and are different from many other energy harvesting technologies where the direct output is electricity. In photocatalytic harvesting, the energy from sunlight is harvested to produce fuel, which can then be used to power devices and electronic systems. Photocatalytic harvesting is currently a promising approach in water splitting technologies to produce hydrogen fuel.
Photocatalytic harvesting occurs when semiconducting materials are used. The photocatalytic harvesting occurs because the photons from the sunlight (with a high energy) generate an electron-hole pair in the semiconducting material once absorbed. However, these photogenerated excited states tend to be unstable, so the charge carriers recombine. During this separation and recombination process, the electrons and holes are transported across the surface of the material, which either reduces or oxidizes the absorbed atoms. It is this process that leads to the generation of hydrogen gas, and there is a similar photocatalytic process that can be used to generate oxygen molecules as well (often at the same time).
Photocatalytic reactions can be initiated using a number of materials, and a number of different semiconducting 2D materials are used for these photocatalytic harvesting reactions. In many cases, the exposed edges on the 2D sheets offer an effective site for the photocatalytic reactions to take place. TMDCs, such as molybdenum disulfide, have gathered a great deal of interest because the sheets actively absorb hydrogen ions. Graphene and its derivatives, especially doped versions to make them semiconducting, are also well-tested materials because the high surface area of both graphene and TMDCs, alongside their absorption and electrical conductivity properties, make them suitable catalytic sites for hydrogen evolution reactions (HER) to produce hydrogen fuel.
Photothermal harvesting approaches once again offer a different approach to many conventional solar harvesting technologies. In these harvesting methods, photothermal materials are used to absorb light energy and convert it into heat energy. 2D materials have a range of optical properties that can be harnessed to absorb light waves—and subsequently convert it into heat—with some of the 2D materials exhibiting optical properties that are unique and not found in other material classes, while others having very beneficial properties in general.
2D materials are known to absorb wavelengths of electromagnetic radiation across a wide spectrum, including in the visible and near-infrared regions. However, these applications are very different in that these approaches are typically used in medical applications to enable different treatments to take place. The absorption of light and generation of heat means that different medical treatments can take place without the need for more invasive approaches. For example, heat is generated in the form of a vibrational energy, and this energy can be used to target and kill cancer cells. It's an approach that is more niche in nature. While it doesn’t power devices, different 2D materials—from graphene derivatives to TMDCs, to MXenes—are being used to harvest light to provide new and efficient medical treatments. Photothermal therapies are becoming an exciting application area for 2D materials alongside the more conventional solar energy harvesting operations.
Solar harvesting, in the form of solar cells, is already an established technology, but 2D materials are offering a way of improving the efficiencies and long-term stabilities of solar cells as well as offering new ways of creating thin, flexible, and transparent solar cells. Beyond solar cells, 2D materials offer great potential for creating hydrogen fuel via photocatalytic reactions and harvesting light for advanced cancer therapies. As modern society looks toward more ways of harvesting our natural environment, 2D materials offer a way to harvest solar rays into a usable output efficiently.
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Energy harvesting uses naturally occurring energy to extend the available power beyond the limits of what finite energy sources, such as batteries, can provide. This approach improves the overall efficiency of the energy source, lengthening the time between charges and improving device performance from unreliable renewable energy sources. The movement toward more sustainable solutions motivates design engineers to move beyond these finite energy sources toward a fully renewable solution: solar cells. Also called photovoltaic (PV) cells, these devices convert the sun’s radiative energy into usable electrical energy in one step through the photovoltaic effect. This effect contacts positively and negatively charged silicon to create an electric field.
These independent microgenerators power local loads exclusively to reduce demands on the grid system while also cutting owners’ utility bills. Applications of solar cells include “off-grid” homes and smaller loads such as roadside electronic signage and IoT equipment—including sensors and actuators—that install remotely and connect wirelessly. Because the energy source of solar cells is free, the ideal state of this type of energy is to “install and forget” it, using the sun as an infinite energy source reservoir. However, engineers must overcome several challenges to realize this approach in the IoT.
The most direct challenge is whether the energy balances demand at various use conditions. Short- and long-range IoT sensors could require between 7 and 16.4uA of energy on average. Design engineers optimize PV cells for a given energy source and add capacitors to reduce dependence on auxiliary power or complete bridge dark periods. The capacitors, in turn, are optimized to minimize leakage current. And designs also ask for a sensor interface for sampling data, storing, and transmitting information.
Fortunately, an innovative product solution and a fundamental understanding of the details of the challenges can solve these problems.
The DC-DC converter receives power from the photovoltaic array and produces electricity available to power devices in a small solar micro-generator. Two of the critical performance metrics are high energy efficiency and power quality. These metrics are essential because the user expects a small device that delivers consistent performance. However, the variability of solar energy supplied to the PV cell presents three principal challenges for self-sustaining photovoltaic IoT sensors:
Other challenges are temperature limits, voltages that do not fit, conversion loss, energy discharge at the wrong times, and aging/degradation of the sensor. With these inefficiencies and losses, the power supply is unstable and may contain sag or ripple.
In addition, solar energy is only supplied part of the day and enters the photovoltaic cell at varying intensities. These inconsistencies further reduce the IoT device’s performance quality. Challenges with low or consistent input energy coupled with limited storage and excessive peak current create the need for a backup solution to level the energy load.
A PV energy harvester can provide this critical backup/augmented power using an advanced supercapacitor like the Hybrid Storage 196 HVC ENYCAP™ Capacitor. ENYCAP is a hybrid storage device, meaning it stores both electrostatic and electrochemical energy that can supply backup power. With robust voltage flexibility from 1.4V for single cells up to 8.4V for multiple, the ENYCAP is available in radial (stacked through-hole), surface mount flat, or lay flat orientations.
It is polarized with high capacitance and offers a high 13Ws/g energy density to minimize package space. The capacitor is also durable up to 1000 hours of run time at 85°C without maintenance or service. In addition, the 196 HVC ENYCAP does not require cell balancing, which is a time-saving advantage over traditional supercapacitors, and it contains a non-hazardous electrolyte. It also exhibits a lower self-discharge when compared with existing market supercapacitors.
The 196 HVC ENYCAP is more than a traditional capacitor suitable for energy harvesting. It is a hybrid energy storage capacitor, a backup system for applications such as miniaturized systems, memory controllers, SRAM/DRAM, cache protection, industrial PC/controls, and emergency lights and micro UPS power sources.
The ENYCAP can provide up to 2mA of harvest power with ultra-low leakage in the IoT sensor application described above. This performance level minimizes reverse current to improve performance and efficiency.
In light of the growing market share of renewable solar power across consumer and industrial electronics, device manufacturers must ensure that their products consistently deliver high performance. Moreover, designers need to consider renewable energy's variability as well as its primary benefit of free, near-infinite supply. Incorporating an energy harvesting enabler like the Vishay 196 HVC ENYCAP hybrid storage capacitor provides a high-performing hybrid storage solution and solar cell backup.
Adam Kimmel has nearly 20 years as a practicing engineer, R&D manager, and engineering content writer. He creates white papers, website copy, case studies, and blog posts in vertical markets including automotive, industrial/manufacturing, technology, and electronics. Adam has degrees in chemical and mechanical engineering and is the founder and principal at ASK Consulting Solutions, LLC, an engineering and technology content writing firm.
Drivers on Interstate-85 in rural Georgia may find themselves motoring over a stretch of solar pavement, one of the first to be installed in the United States. Solar roadways use photovoltaic modules—solar panels, like the ones found on rooftops—to capture sunlight and convert it into electricity. With goals of creating clean, sustainable energy to power lighting and signage, monitor road conditions, communicate with autonomous vehicles, and more, solar roadway technologies are indeed gaining momentum.
The solar technology deployed in Georgia comes from French company Wattway, which developed a system of solar panels that can be installed directly on a road's surface. Each panel includes solar cells and sensors inserted into a composite material that’s just a few millimeters thick. The resulting photovoltaic pavers are then installed directly over existing pavement. What’s more, the pavers are skid-resistant, adapt to thermal dilation, and are strong enough to support the weight of continuous traffic, including six-axle trucks. Wattway is conducting dozens of outdoor tests like the one in Georgia and says it hopes to commercialize the technology in 2018.
One of Wattway’s largest installations to date is a test site in the French village of Tourouvre-au-Perche, a small Normandy village. The nearly 2,900m2 (3468 yd2) of solar panels along the one kilometer stretch of road generate 280kW of electricity at peak. By 2021, France plans to extend this project to stretch a whopping 1,000km (6201mi), which could produce enough electricity to power all the public lighting in a town of 5,000 for a year.
Other companies are developing solar road solutions as well. A Dutch consortium constructed SolaRoad, the world’s first bike path made from solar panels. The 72m (79yd) stretch located in Krommenie, near Amsterdam, produced enough electricity in its first month to meet the electricity needs of a nearby residential home. SolaRoad consists of prefabricated panels of roadway topped with a tempered glass surface. Under the glass, silicon solar cells collect solar energy that could power the road's lighting, traffic signals, and signage; send electricity to local households; and—someday—to power electric vehicles.
That’s still in the future, with the broader idea of making solar roadways part of an increasingly connected, digitized, and electrified world. In this world, autonomous electric vehicles communicate with other vehicles and with nearby infrastructure like parking structures, traffic lights, emergency services, and so on. Meanwhile, early deployments are relatively small test projects to learn more about the technology and to learn how it stands up to the punishing effects of everyday traffic and all-season weather. But solar costs continue to drop, and that means photovoltaic panels can be affordably integrated into everyday materials. For example, Tesla Motors recently unveiled roof shingles that double as solar panels. Photovoltaic snowmelt systems, contact-free wireless vehicle charging, and photo-luminescent paint are some additional examples being prototyped and tested for solar roadway use.
Reliability and consistent performance are key requirements if the promise of solar roads is to be achieved. Just as solar roads require reliable components to meet the demanding requirements of solar energy production, transportation vehicles require robust and secure connections. For example, Molex designed and developed the Mini50 Sealed Connectors to support efficient, reliable, and flexible transportation interconnections. The Mini50 family now offers a sealed 4- and 10-circuit option, delivering 25 percent space savings over traditional sealed 0.64mm connectors, with smaller terminals to fit more low-current electrical circuits in sealed transportation-vehicle environments.
Mouser is proud to be a distributor for Molex electronics solutions that are helping to make solar energy a reality. Learn more about Mouser’s commitment to innovation by visiting our Shaping Smarter Cities homepage.
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