Worldwide growth of photovoltaics has been fitting an exponential curve for more than two decades. During this period of time, photovoltaics (PV), also known as solar PV, has evolved from a pure niche market of small scale applications towards becoming a mainstream electricity source. The Sun blankets the Earth with enough photons every hour to meet the entire world’s energy needs for a year. The question is how to efficiently convert them into electricity. Solar panels on the market today consist of cells made from a single semiconducting material, usually silicon. Since the material absorbs only a narrow band of the solar spectrum, much of sunlight’s energy is lost as heat: these panels typically convert less than 20 percent of that energy into electricity. Even under small-scale laboratory conditions, the world’s best single-junction solar cells—the kind found in most solar panels—still max out at capturing 29 percent of the sun’s energy.
Simply put, solar panel efficiency (expressed as a percentage) quantifies a solar panel’s ability to convert sunlight into electricity. Given the same amount of sunlight shining for the same duration of time on two solar panels with different efficiency ratings, the more efficient panel will produce more electricity than the less efficient panel. In practical terms, for two solar panels of the same physical size, if one has a 21% efficiency rating and the other has a 14% efficiency rating, the 21% efficient panel will produce 50% more kilowatt hours (kWh) of electricity under the same conditions as the 14% efficient panel. Thus, maximizing energy use and bill savings is heavily reliant on having top-tier solar panel efficiency.
There is a need for cheaper, more efficient solar cells than the traditional silicon solar cells so that more people may have access to this technology. The prices for solar panels have plummeted over the past few years, so continuing to focus on making them less expensive would have little impact on the overall cost of a solar power system; expenses related to things like wiring, land, permitting, and labor now make up the vast majority of that cost. Making modules more efficient would mean that fewer panels would be needed to produce the same amount of power, so the costs of hardware and installation could be greatly reduced.
Reserachers have been developing new architectures , techniques, materials including nanomaterials, stacking multiple layers , developing hybrid technologies and manufacturing processes to enhance the efficnecy and also reduce the cost of solar cells. MSU Foundation Professor James McCusker, Department of Chemistry, believes that the future of solar energy lies in abundant, scalable materials designed to mimic and improve upon the energy conversion systems found in nature.
Ultra efficient solar cells
The ceiling on solar cell efficiency, known as the Shockley-Queisser limit, is between 29 and 33 percent, depending on how you measure it. It assumes a single-junction cell, meaning it’s made using only one type of semiconductor and is energized by direct sunlight. To nose past the limit, researchers have tried stacking multiple types of semiconductors or using lenses to concentrate light so that the cell receives a blast hundreds of times more powerful than the sun. In 2019, the National Renewable Energy Lab set a world record when it used a six-junction solar cell and a beam 143 times more concentrated than sunlight to achieve a whopping 47.1 percent energy efficiency.
But this technology will never be deployed at scale. The reason, says Marc Baldo, a professor of electrical engineering and computer science at MIT, is that these ultra-high-efficiency, multilayer solar cells are far too complex and expensive to produce as solar panels. To actually get more solar energy on the electric grid requires figuring out how to hit the Shockley-Queisser limit with single-junction, silicon-based solar cells, which are comparatively easy and cheap to produce. Better yet would be finding a way to bump the limit higher. And after a decade of work, Baldo and his colleagues may have finally figured out how.
As detailed in a paper published last week in July 2019, Baldo’s team coated solar cells in a thin layer of tetracene, an organic molecule that effectively splits incoming photons in two. This process is known as exciton fission and means that the solar cell is able to use high energy photons from the blue-green part of the visible spectrum.
MIT researchers develop method for collecting two electrons from each photon could break through theoretical solar-cell efficiency limit.
At MIT, researchers have shown through experiments that a photon in the silicon cell could be ‘turbocharged’ in order “to kick out two electrons instead of one, opening the door for a new kind of solar cell with greater efficiency than was thought possible.” That produces a doubling of the amount of energy produced by a given amount of sunlight in the blue and green part of the spectrum. These ‘turbocharged’ silicon cells could potentially raise the power produced by the solar cell — from a current theoretical maximum of 29.1 percent, up to a maximum of about 35 percent, according to a paper published in 2019 by graduate student Markus Einzinger, professor of chemistry Moungi Bawendi, professor of electrical engineering and computer science Marc Baldo, and eight others at MIT and at Princeton University
Here’s how it works. Silicon solar cells generate an electric current by using incoming photons to knock electrons from the silicon into a circuit. How much energy does that take? It depends on an attribute of the material known as its bandgap. Silicon’s bandgap corresponds to infrared photons, which carry less energy than photons in the visible part of the electromagnetic spectrum. Photons outside silicon’s bandgap essentially go to waste. But here’s where the tetracene comes in: It splits blue-green photons into two “packets” of energy that are each equivalent to an infrared photon. So rather than each infrared photon knocking free one electron, a single photon in the blue-green spectrum can knock free two electrons. It’s essentially getting two photons for the price of one.
The key to splitting the energy of one photon into two electrons lies in a class of materials that possess “excited states” called excitons, Baldo says: In these excitonic materials, “these packets of energy propagate around like the electrons in a circuit,” but with quite different properties than electrons. “You can use them to change energy — you can cut them in half, you can combine them.” In this case, they were going through a process called singlet exciton fission, which is how the light’s energy gets split into two separate, independently moving packets of energy. The material first absorbs a photon, forming an exciton that rapidly undergoes fission into two excited states, each with half the energy of the original state.
This new cell represents a fundamentally new approach to a well-known truism in photovoltaics research: If you want to pass the Shockley-Queisser limit, you have to capture energy from a wider range of solar photons. Because this cell doesn’t rely on an expensive stack of materials with different bandgaps to broaden its range, it might ultimately be more practical too. Baldo says that using tetracene could bump the theoretical energy efficiency limit up to 35 percent—higher than was ever thought possible for single-junction cells.
But the tricky part was then coupling that energy over into the silicon, a material that is not excitonic. This coupling had never been accomplished before. The key was in a thin intermediate layer. “It turns out this tiny, tiny strip of material at the interface between these two systems [the silicon solar cell and the tetracene layer with its excitonic properties] ended up defining everything. The layer is only a few atoms thick, or just 8 angstroms (ten-billionths of a meter), but it acted as a “nice bridge” for the excited states, Baldo says. That finally made it possible for the single high-energy photons to trigger the release of two electrons inside the silicon cell.
The researchers have measured one special property of hafnium oxynitride that helps it transfer the excitonic energy. “We know that hafnium oxynitride generates additional charge at the interface, which reduces losses by a process called electric field passivation. If we can establish better control over this phenomenon, efficiencies may climb even higher.” Einzinger says. So far, no other material they’ve tested can match its properties.
Other approaches to improving the efficiency of solar cells tend to involve adding another kind of cell, such as a perovskite layer, over the silicon. Baldo says “they’re building one cell on top of another. Fundamentally, we’re making one cell — we’re kind of turbocharging the silicon cell. We’re adding more current into the silicon, as opposed to making two cells.”
In this sense, what the MIT team demonstrated wasn’t so much a competitive technology but a new tack for going beyond the limits of existing photovoltaics, says Joseph Berry, a senior scientist at the National Renewable Energy Laboratory. “What’s cool here is that this is a fundamentally different approach from traditional photovoltaics,” he says. “It’s an idea that’s been around for a long time, but hadn’t been translated into any kind of functional device.”
Berry and his colleagues at NREL are exploring other ways of advancing solar cell efficiency without the added complexity and cost of multi-junction cells. One of the most promising directions being explored by Berry are perovskite cells, which use synthetic materials that have structural properties similar to the naturally occurring mineral Perovskite. The first perovskite solar cells were only produced a decade ago, but since then they have witnessed the fastest efficiency gains of any type of solar cell to date.
Perovskite cells have a number of advantages over traditional silicon solar cells, says Berry, in particular their tolerance for material defects. Just a few unwanted particles on a silicon solar cell can render it useless, but perovskite materials still function well even if they’re not perfect. They also handle photonic energy more efficiently than silicon. Indeed, one of the main reasons silicon has dominated solar cell technology is not because it’s the best material for the job, but simply because scientists know so much about it due to its widespread use in digital technologies.
Nano technology breakthrough enables conversion of infrared light to energy
Invisible infrared light accounts for half of all solar radiation on the Earth’s surface, yet ordinary solar energy systems have limited ability in converting it to power. A research team led by Hans Ågren, professor in theoretical chemistry at KTH Royal Institute of Technology, has developed a film that can be applied on top of ordinary solar cells, which would enable them to use infrared light in energy conversion and increase efficiency by 10 percent or more.
“We have achieved a 10 percent increase in efficiency without yet optimizing the technology,” Ågren says. “With a little more work, we estimate that a 20 to 25 percent increase in efficiency could be achieved.” Photosensitive materials used in solar cells, such as the mineral perovskite, have a limited ability to respond to infrared light. The solution, developed with KTH researchers Haichun Liu and Qingyun Liu, was to combine nanocrystals with chains of microlenses.
“The ability of the microlenses to concentrate light allows the nanoparticles to convert the weak IR light radiation to visibile light useful for solar cells,” Ågren says. The research progress has been patented, and presented in the scientific journal Nanoscale.
Nanocone Arrays for super-efficient, ultra-thin silicon solar cells
One of the current popular topics in photovoltaic technology research centers around the use of organic-inorganic halide perovskites as solar cells because of the high power conversion efficiency and the low-cost fabrication. Despite a surge in solar cell R&D in recent years involving emerging materials such as organics and perovskites, the solar cell industry continues to favor inorganic crystalline silicon photovoltaics.
While thin-film solar cells offer several advantages—including lower manufacturing costs—long-term stability of crystalline silicon solar cells, which are typically thicker, tips the scale in their favor, according to Rana Biswas, a senior scientist at Ames Laboratory, who has been studying solar cell materials and architectures for two decades.
“Crystalline silicon solar cells today account for more than 90 percent of all installations worldwide,” said Biswas, co-author of a new study that used supercomputers at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC), a Department of Energy Office of Science User Facility, to evaluate a novel approach for creating more energy-efficient ultra-thin crystalline silicon solar cells. “The industry is very skeptical that any other material could be as stable as silicon.”
Thin-film solar cells typically fabricated from semiconductor materials such as amorphous silicon are only a micron thick. While this makes them less expensive to manufacture than crystalline silicon solar cells, which are around 180 microns thick, it also makes them less efficient—12 to 14 percent energy conversion, versus nearly 25 percent for silicon solar cells (which translates into 15-21 percent for large area panels, depending on the size). This is because if the wavelength of incoming light is longer than the solar cell is thick, the light won’t be absorbed.
This challenge prompted Biswas and colleagues at Ames to look for ways to improve ultra-thin silicon cell architectures and efficiencies. In a paper published in Nanomaterials, they describe their efforts to develop a highly absorbing ultra-thin crystalline silicon solar cell architecture with enhanced light trapping capabilities. “We were able to design a solar cell with a very thin amount of silicon that could still provide high performance, almost as high performance as the thick silicon being used today,” Biswas said.
The key lies in the wavelength of light that is trapped and the nanocone arrays used to trap it. Their proposed solar architecture comprises thin flat spacer titanium dioxide layers on the front and rear surfaces of silicon, nanocone gratings on both sides with optimized pitch and height and rear cones surrounded by a metallic reflector made of silver. They then set up a scattering matrix code to simulate light passing through the different layers and study how the light is reflected and transmitted at different wavelengths by each layer. “This is a light-trapping approach that keeps the light, especially the red and long-wavelength infrared light, trapped within the crystalline silicon cell,” Biswas explained. “We did something similar to this with our amorphous silicon cells, but crystalline behaves a little differently.”
For example, it is critical not to affect the crystalline silicon wafer—the interface of the wafer—in any way, he emphasized. “You want the interface to be completely flat to begin with, then work around that when building the solar cell,” he said. “If you try to pattern it in some way, it will introduce a lot of defects at the interface, which are not good for solar cells. So our approach ensures we don’t disturb that in any way.”
Looking ahead, given that this research is focused on crystalline silicon solar cells, this new design could make its way into the commercial sector in the not-too-distant future—although manufacturing scalability could pose some initial challenges, Biswas noted. “It is possible to do this in a rather inexpensive way using soft lithography or nanoimprint lithography processes,” he said. “It is not that much work, but you need to set up a template or a master to do that. In terms of real-world applications, these panels are quite large, so that is a challenge to do something like this over such a large area. But we are working with some groups that have the ability to do roll to roll processing, which would be something they could get into more easily.”
Self-Assembled Carbon Nanotube Antennas for Solar Power Revolution
NovaSolix’s carbon nanotube (CNT) antennas are small enough to match the nano-scale wavelengths of sunlight. Antennas can convert electromagnetic spectrum much more efficiently than photovoltaic (PV) cells. When perfected, NovaSolix antennas will capture over four times the energy of current solar panels. They will reach nearly 90% efficiency versus ~20% for todays solar panels.
NovaSolix has invented a self-assembling antenna array solar cell which will be 2-4 times more efficient at a less than one-tenth the cost per watt of existing solar. NovaSolix claims to have demonstrated a proof of concept to third parties that has touched 43% efficiency. That’d suggest a 72 cell solar module near 860 watts, with a 90% solar cell pushing 1700 watts.
They could buy used manufacturing hardware and retrofit them in the early stages of growth. The first manufacturing lines could cost $4.1 million, and would initially produce ~45% efficient modules, at a clip of 20MW/year with a proposed price of 10¢/W. At full efficiency, costs are cut in half and volumes per year doubled
Photodetector Uses Ultrathin Materials to Increase Efficiency
A prototype developed using quantum mechanical processes could usher in a novel class of ultra-efficient photodetectors that would enable solar cells to turn the light they receive into multiple electrons. The prototype is based on the efficient multiplication of interlayer electron-hole (e-h) pairs in 2D semiconductor heterostructure photocells.
To build the prototype, researchers at University of California, Riverside, stacked two atomic layers of tungsten diselenide (WSe2) on a single atomic layer of molybdenum diselenide (MoSe2). Such stacking resulted in properties that were vastly different from those of the parent layers. The team observed that when a photon struck the WSe2 layer, it knocked loose an electron, freeing the electron to conduct energy through the WSe2. At the junction between WSe2 and MoSe2, the electron dropped down into MoSe2. The resulting energy knocked a second electron from the WSe2 into the MoSe2, where both electrons became free to move and generate electricity.
Researchers noted that additional electrons could potentially be generated by increasing the temperature of the device. Electron multiplication in conventional photocell devices typically requires applied voltages of 10 to 100 volts. To observe the doubling of electrons, the researchers used only 1.2 volts, the typical voltage supplied by an AA battery. By exploiting the highly efficient interlayer e-h pair multiplication process, they were able to show 350 percent enhancement of the optoelectronic responsivity at microwatt power levels in an NIR optoelectronic device.
The findings, which demonstrate efficient carrier multiplication in transition-metal dichalcogenides (TMD)-based optoelectronic devices, could make 2D semiconductor heterostructures viable for a new class of ultra-efficient photodetectors based on layer-indirect e–h excitations. “Such low voltage operation, and therefore low power consumption, may herald a revolutionary direction in photodetector and solar cell material design,” researcher Max Grossnickle said.
Grossnickle added that the efficiency of a photovoltaic device is governed by a simple competition: light energy is either converted into waste heat or useful electronic power. “Ultrathin materials may tip the balance in this competition by simultaneously limiting heat generation, while increasing electronic power,” he said. Gabor believes that the team’s findings could be used in unforeseen ways.
“These materials, being only an atom thick, are nearly transparent. It’s conceivable that one day we might see them included in paint or in solar cells incorporated into windows. Because these materials are flexible, we can envision their application in wearable photovoltaics, with the materials being integrated into the fabric. We could have, say, a suit that generates power — energy-harvesting technology that would be essentially invisible,” he said.
Multi-junction Solar Cells
By switching up materials, adding more junctions, and performing some impressive feats of engineering in between, you can push past that limit. Some triple-junction solar cells, for example, can surpass 45% under concentrated sunlight.
NASA’s High-Efficiency multi-junction Solar Cell
Innovators at NASA’s Glenn Research Center have patented a high-efficiency multi-junction solar cell that uses a thin interlayer of selenium as the bonding material between wafers. Selenium is a unique semiconductor in that its transparent to light at photon energies below the band gap (infrared), enabling light to pass from the multi-junction top cell to the silicon-based bottom cell.
The innovation allows a multi-junction solar cell to be developed without the constraint of lattice matching, and with a low-cost, robust silicon wafer as the supporting bottom substrate and bottom cell. This approach enables a cell that is simultaneously lower in cost, more rugged, and more efficient than existing space-based photovoltaic cells.
This high-efficiency solar technology takes advantage of inexpensive silicon wafers and provides a more robust design for next-generation solar cells in space. For terrestrial applications, it can provide unprecedented efficiencies for auxiliary power units in vehicles, solar roof tiles, power plants, and smart grid systems.
NASA lists its many advantages as
- High efficiency: Expected conversion efficiencies of 30% to over 40%
- Low-cost materials: Substrate material is a low-cost silicon wafer as compared to materials used for space-based photovoltaic cells
- Easy to manufacture: Photovoltaic cells can be manufactured on a large scale
- Low environmental impact: Zero greenhouse gasses are emitted
- Robust: The cell features a rugged design for space applications
A multi-junction photovoltaic cell differs from a single junction cell in that it has multiple sub-cells (p-n junctions) and can convert more of the sun’s energy into electricity as the light passes through each layer. To further improve the efficiencies, this cell has three junctions, where the top wafer is made from high solar energy absorbing materials that form a two-junction cell made from the III-V semiconductor family, and the bottom substrate remains as a simple silicon wafer.
The selenium interlayer is applied between the top and bottom wafers, then pressure annealed at 221°C (the melting temperature of selenium), then cooled. The selenium interlayer acts as a connective layer between the top cell that absorbs the short-wavelength light and the bottom silicon-based cell that absorbs the longer wavelengths. The three-junction solar cell manufactured using selenium as the transparent interlayer has a higher efficiency, converting more than twice the energy into electricity than traditional cells.
To obtain even higher efficiencies of over 40%, both the top and bottom layers can be multi-junction solar cells with the selenium layer sandwiched in between. The resultant high performance multi-junction photovoltaic cell with the selenium interlayer provides more power per unit area while utilizing a low-cost silicon-based substrate. This unprecedented combination of increased efficiency and cost savings has considerable commercial potential.
In a groundbreaking new study in Nature, McCusker reveals a novel process that allows molecules to tell scientists how they should be modified to better absorb and convert solar energy. The method uses a molecular property known as quantum coherence where different aspects of a molecule are synchronous, like when your car’s turn signal blinks in unison with that of the car in front of you. Scientists believe that quantum coherence may play a role in natural photosynthesis.
“Our work is the first time anyone has tried to actively use information gleaned from quantum coherence as a guide — a roadmap — to suggest what are the most important aspects of a molecule’s structure that contribute to a given property,” McCusker said. “We are using sophisticated science that provides the means for nature to teach us what we need to focus on in the lab.”
Sunlight, although abundant, is a low-density energy source. To collect meaningful amounts of energy you need larger amounts of space. However, the most effective materials in use today for solar energy conversion, such as Ruthenium, are some of the rarest metals on Earth. Future solar technologies must be able to scale up with more efficient and cheaper methods of energy conversion.
NREL solar cell
In April 2020 it was reported that Scientists at the National Renewable Energy Laboratory (NREL) have fabricated a solar cell with an efficiency of nearly 50%. The six-junction solar cell now holds the world record for the highest solar conversion efficiency at 47.1%, which was measured under concentrated illumination. A variation of the same cell also set the efficiency record under one-sun illumination at 39.2%. “This device really demonstrates the extraordinary potential of multijunction solar cells,” said John Geisz, a principal scientist in the High-Efficiency Crystalline Photovoltaics Group at NREL and lead author of a new paper on the record-setting cell.
The paper, “Six-junction III-V solar cells with 47.1% conversion efficiency under 143 suns concentration,” appears in the journal Nature Energy. Geisz’s co-authors are NREL scientists Ryan France, Kevin Schulte, Myles Steiner, Andrew Norman, Harvey Guthrey, Matthew Young, Tao Song, and Thomas Moriarty.
To construct the device, NREL researchers relied on III-V materials—so called because of their position on the periodic table—that have a wide range of light absorption properties. Each of the cell’s six junctions (the photoactive layers) is specially designed to capture light from a specific part of the solar spectrum. The device contains about 140 total layers of various III-V materials to support the performance of these junctions, and yet is three times narrower than a human hair. Due to their highly efficient nature and the cost associated with making them, III-V solar cells are most often used to power satellites, which prize III-V’s unmatched performance. On Earth, however, the six-junction solar cell is well-suited for use in concentrator photovoltaics, said Ryan France, co-author and a scientist in the III-V Multijunctions Group at NREL.
“One way to reduce cost is to reduce the required area,” he said, “and you can do that by using a mirror to capture the light and focus the light down to a point. Then you can get away with a hundredth or even a thousandth of the material, compared to a flat-plate silicon cell. You use a lot less semiconductor material by concentrating the light. An additional advantage is that the efficiency goes up as you concentrate the light.” France described the potential for the solar cell to exceed 50% efficiency as “actually very achievable” but that 100% efficiency cannot be reached due to the fundamental limits imposed by thermodynamics.
Geisz said that currently the main research hurdle to topping 50% efficiency is to reduce the resistive barriers inside the cell that impede the flow of current. Meanwhile, he notes that NREL is also heavily engaged in reducing the cost of III-V solar cells, enabling new markets for these highly efficient devices. The U.S. Department of Energy Solar Energy Technologies Office funded the research. NREL is the U.S. Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC. Through the magic of modern engineering and nanotechnology, the new solar cell consists of about 140 layers of the various materials, but it is narrower than a hair.
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