How to Make Your Solar Panels Last Longer
The typical lifespan of a solar panel is 25 years or more. The key to extending its life expectancy is a reputable installer and some basic maintenance.
Andrew King is an award-winning journalist and copywriter from Columbus, Ohio. He has covered sports, local news, entertainment and more for The Athletic, The Columbus Dispatch, Major League Soccer, Columbus Monthly and other outlets, and writes about home energy for CNET. He’s a graduate of Capital University, and recently published a non-fiction book called Friday Night Lies: The Bishop Sycamore Story investigating the fraudulent high school football team that became the talk of the nation.
Solar panels are already super durable. But to get the most bang for your buck, you’ll want to make them last as long as possible.
Powering our homes with solar energy once seemed like science fiction. Even in the last decade, it was a strange sight to see a home covered in solar panels. But thanks to Rapid advancements in technology and plummeting prices, that has changed.
Residential solar panel systems can now cost 20,000 or less after a newly expanded federal tax credit. That means the option to switch to clean energy has never been more attainable.
Can solar panels save you money?
Interested in understanding the impact solar can have on your home? Enter some basic information below, and we’ll instantly provide a free estimate of your energy savings.
Since I got started back in 2008, the cost has dropped by something like 90%, Chris Deline, a research engineer at the National Renewable Energy Laboratory, told CNET.
But solar panels are still an expensive investment, and you want to be sure that investment will still be paying off years from now.
Can solar panels save you money?
Interested in understanding the impact solar can have on your home? Enter some basic information below, and we’ll instantly provide a free estimate of your energy savings.
So how long can adopters expect their solar panels to last, and how can they ensure the maximum lifespan of their investment? The list of factors to consider isn’t too long.
How long do solar panels typically last?
With a 20,000 or more cost of installation, you’ll want your solar panels to last longer than a few years. The good news is that they should.
Deline says most solar panels are designed to last decades, and reputable installers should offer warranties of 25 years or longer.
In the entire system, probably some of the most durable and long-lived components are the solar panels themselves, he said. They often come with 25-year warranties. Further, the materials they’re composed of.- aluminum and glass, primarily.- can be durable enough to last much longer, sometimes 30, 40 or 50 years.
Often, if a failure occurs, it happens in the system’s electrical components. Deline said that in many cases, issues like a problem with the system’s power-inverter, which converts DC power to AC power, can simply be replaced without even climbing up to the panels themselves. In other instances, individual components of a panel’s electronics can be fixed or replaced, which allow for a panel to last years into the future.
What affects a solar panel’s lifespan?
Solar panels aren’t typically very fragile, so there isn’t much that can affect their lifespan.
Deline said the elements of a solar panel degrade very slowly, which means they’ll remain highly functioning well into their life cycles. Between the normal wear and tear of electrical components and micro-cracks that develop on the surface of the panels, he said experts typically estimate a degradation of half a percent per year. That means that if a panel sits on a roof for 20 years in normal conditions, it can still be expected to function at 90% of its original capacity.
Of course, natural disasters can lead to an earlier end to a solar system’s lifespan. Events like a lightning strike, a hail storm or a wind storm can cause damage that the most durable panel can’t withstand. But even in those instances, most panels are resilient. They require a lengthy testing process before being sold, which includes being blasted by hail up to 1.5 inches in diameter, alternating between high and low temperatures and baking in heat and humidity for 2,000 hours.
Which solar panels last the longest?
In the current solar panel industry, there isn’t much room for differentiation between different types of solar panels, which simplifies your choices.
I would hesitate to say that any one panel is going to be more likely to survive longer than any other, Deline said. Panels are pretty much going to be the same. The differences are the quality control of the manufacturer and whether they have a good handle on the chemistry and manufacturing technology.
That makes it critical to ensure that you’re getting your system installed by a reputable source. An increase in federal solar incentives, along with solar lease programs, solar loan offers and solar rebates, has flooded the market with less-than-savory outfits. Deline recommends interested buyers do their research, get a few quotes and avoid deals that sound too good to be true.
Should I replace my roof before getting solar panels?
You might wonder whether you need to have a specialized roof before installing solar panels. The good news is that in 2023, solar panel installation requires very little of a typical roof.
Deline said that unless you have a roof designed for aesthetics rather than load-bearing, or if the design of your home means it can’t withstand any more weight, a typical residential house should be just fine for solar panel installation. Your installer will also check the condition of your roof to make sure it will last.
Generally, your installer should be able to figure that out just by looking at it, he said. But if your roof is totally falling apart, it may not be worth it.
How to make your solar panels last longer
So how can solar system adopters ensure their panels last all the way through their 25-year warranties and beyond? Here are a few ways to maximize the lifespan of your solar system, according to Deline.
Use an installer you trust
Because these panels will stay on top of your home for more than two decades, be sure to be thorough when doing your research on who is installing your system. Deline said finding a reputable installer is far and away the most important step in the process, and mistakes upfront can create huge headaches down the line.
Keep an eye on your usage
It may seem obvious, but Deline warns that those with a solar system should be sure to monitor how much they’re generating. That’s because systems often have some kind of shut-off switch, which can be tripped surprisingly easily, even by an expert. And if you turn your system off without realizing, you can waste days or weeks of generation.
I have kids, and we have a big red shut-off handle, he said. I came home one day and it was off, and I found out that a month before, my kid had been messing around outside and had hit the switch. If you don’t keep tabs on it, it could just be off for extended periods of time.
Keep your panels clean
A little bit of dirt and grime won’t render your panels useless, but it’s still a good idea to keep them clean. Deline said different areas of the country lead to different types of buildup, from dirt and soil to snow. With too much buildup, they won’t work as effectively. But the good news is that it’s as simple as cleaning panels off with a push broom. Just be sure not to smash them.
You can’t walk on them, but otherwise they’re pretty resilient, he said. You can even hose them off.
NREL develops perovskite solar cell efficient 2023
By I Ghon Last updated Dec 9, 2022
Thanks to a new structure, the perovskite solar cell developed by NREL researchers achieves a certified efficiency of 24%. This is the highest reported value of its kind, according to NREL.
The high-efficiency solar cell retained 87% of its original efficiency even after 2,400 hours of operation at 55 degrees Celsius.
Research in the field of perovskite solar cells had recently focused largely on how to increase their stability. “Some people can detect perovskites with high stability, but the efficiency is lower,” says Kai Zhu, a senior scientist at NREL’s Chemistry and Nanoscience Center. “Having high efficiency and high stability at the same time is a challenge.”
NREL perovskite solar cell with inverted architecture
The researchers use an inverted architecture and not the “normal” architecture that has achieved the highest efficiencies to date. The difference between the two types is how the layers are applied to the glass substrate.
The inverted perovskite architecture is known for its high stability and integration in tandem solar cells. The NREL-led team also added the molecule 3-(aminomethyl)pyridine (3-APy) to the surface of the perovskite.
The molecule reacts with the formamidinium group within the perovskite and creates an electric field on the surface of the perovskite layer. “This suddenly gave us a huge boost, not only in terms of efficiency, but also in terms of stability,” said Zhu.
The scientists report that the 3-APy reactive surface technique can improve the efficiency of an inverted cell from less than 23% to more than 25%. They also found that reactive surface engineering is an effective approach to take inverted cell performance “to a new, modern level of efficiency and reliability.”
The work was performed in collaboration with scientists from the University of Toledo, the University of Colorado-Boulder and the University of California-San Diego.
What is Perovskite solar cell ?
Perovskite solar cell (PSC) is a type of solar cell that includes perovskite structure compounds, most commonly hybrid organic-inorganic lead or tin halide materials as an active layer for light capture.
Perovskite materials, such as lead methyl ammonium halide and total inorganic caesium halide, are cheap and easy to manufacture.
What are Advantages of perovskite solar cells ?
It has a unique function, which can be used in solar cells. The raw materials used and possible manufacturing methods (such as various printing technologies) are low-cost.
Their high absorption coefficient enables about 500 nm ultra-thin films to absorb a complete visible solar spectrum.
The combination of these characteristics leads to the possibility of creating low-cost, efficient, thin, lightweight and flexible solar modules. Perovskite solar cells have been used to power low-power radio sub-equipment and for the Internet of Things applications for environmental power supply.
What are Materials of perovskite solar cells ?
The name comes from the ABS 3 crystal structure of the absorbent material, which is called perovskite structure, where A and B are cations and X are anions. It is found that cations with radius between 1.60Å and 2.50Å form perovskite structures.
The most commonly studied perovskite absorbent is lead methylammonium trihalide (CH 3 NH 3 PbX 3, where X is a halogen ion, such as iodine ion, bromine ion or chloride ion), with an optical Band gap between ~1.55 and 2.3 eV, depending on the halide content.
Methami lead trihalide (H 2 NCHNH 2 PbX 3) also shows hope, with a Band gap between 1.48 and 2.2 eV. The minimum Band gap is closer to the optimal Band gap of a single-junction battery than lead methyl ammonium trihalide, so it should be more efficient.
It was first used in solid-state solar cells in dye-sensitized batteries using CsSnI 3 as a p-type hole transport layer and absorbent. There is general concern about lead as an integral part of solar cell materials, solar cells based on tin-based perovskite absorbents are reported that NH 3 SnI 3 has low power conversion efficiency.
Multi-junction solar cells can achieve higher power conversion efficiency (PCE), raising the threshold to the thermodynamic xxx value set by the Shockley-Queissier limit of single-junction batteries. By having multiple Band gaps in a single battery, it can prevent loss. Photons above or below the Band gap energy of single-junction solar cells.
Among the series (double) solar cells, 31.1% of PCE has been recorded, the triple knot has increased to 37.9%, and the quadrupled solar cells have increased to 38.8%, which is impressive. However, the (MOCVD) process required by metal organic chemical vapour deposition to synthesize lattice matching and crystal solar cells with multiple crystals is very expensive, which makes it impossible for wide application.
The options provided by semiconductors may be comparable to the efficiency of polyjunction solar cells, but they can be synthesized at a reduced cost of xxx under more common conditions. Compared with the above-mentioned double, three- and four-joint solar cells, the xxxPCE of all perovskite series batteries is 31.9%, the tri-joint solar cell reaches 33.1%, and perovskite-Si trijunction batteries have 35.3%.
In addition to being used for cost-effective synthesis, these poly-conite solar cells can also maintain high PCE under various extreme weather conditions, making them available worldwide.
chiral ligand
If organic chiral ligands are used correctly, it is expected to improve the power conversion efficiency of halide solar cells. The chirality in an inorganic semiconductor can be deformed by the enantiomer near the lattice surface, the electron coupling between the substrate and the chiral ligand, which can be assembled into chiral secondary structure or chiral surface defects.
Chiral inorganic-organic perovskite is formed by connecting chiral ligands to non-chiral perovskite brominated lead nanop.
Two regions were found by inorganic organo perovskite by circular dichromatography (CD) spectroscopy. One represents the charge transfer between the ligand and the nanochip (300-350nm), and the other represents the exciton absorption xxx value of perovskite. Evidence of charge transfer in these systems is expected to increase the power conversion efficiency of PSC.
Is perovskite solar cell stable ?
A major challenge for perovskite solar cells (PSC) is short-term and long-term stability.
The instability of PSC is mainly related to environmental impacts (moisture and oxygen), the thermal stress and inherent stability of perovskite based on methyl ammonium, heating at applied voltage, light effects (ultraviolet) and mechanical brittleness.
Some studies have been carried out on the stability of PSC, and it has been proved that some elements are important for the stability of PSC. However, there is no standard “operation” stability protocol for PSC. However, a method to quantify the inherent chemical stability of impurized halide perovskite has recently been proposed.
The water solubility of the organic components of absorbent materials makes the equipment very easy to degrade rapidly in a humid environment. Water-induced degradation can be reduced by optimizing the composition materials, battery structure, interface and environmental conditions in the manufacturing steps.
Wrapping perovskite absorbents with composites of carbon nanotubes and inert polymer matrix can prevent materials from being immediately degraded by humid air at high temperature.
However, the long-term research and comprehensive packaging technology of perovskite solar cells have not been proved. The 2 layer of equipment with mesoporous TiO and the perovskite absorption sensitization are also UV-unstable, due to the interaction between the photonic holes inside TiO2 and the surface of TiO of oxygen free radicals.
The ultra-low thermal conductivity of 0.5 W/(Km) measured at room temperature in CH 3 NH 3 PbI 3 can prevent the Rapid propagation of deposited light and make the battery resistant to thermal stress, thus shortening its service life. The PbI 2 residue in perovskite films has been experimentally proven to have a negative impact on the long-term stability of the device.
It is said that the stability problem can be solved by replacing the organic transport layer with a metal oxide layer, so that the battery can maintain 90% capacity after 60 days. In addition, two instability problems can be solved by using multifunctional fluorinated photosensitive polymer coating, which has luminous and easy-to-clean functions on the front of the device, and forms a strong hydrophobic barrier to environmental moisture on the contact side of the back.
The front coating can prevent the ultraviolet light of the whole incident solar spectrum from negatively interacting with the PSC battery stack by converting the ultraviolet light of the entire incident solar spectrum into visible light, while the rear layer can prevent water from penetrating into the solar cell reactor.
In the 180-day aging test conducted in the laboratory and the actual outdoor condition test for more than 3 months, the obtained device showed excellent stability in power conversion efficiency.
In July 2015, the main obstacle was that xxx’s perovskite solar cells were only nail-sized and could degrade rapidly in humid environments. However, researchers from EPFL published a study in June 2017, which successfully proved that no degradation (short circuit) was observed in large perovskite solar cell components within a year.
Now, the research team, together with other organizations, aims to develop a fully printable perovskite solar cell with an efficiency of 22% and 90% performance after aging testing.
In early 2019, the longest stability test reported so far showed that under at least one sunlight, the xenon-based solar simulator can run for at least 4,000 consecutive hours without ultraviolet filtration under the condition of xxx power point tracking (MPPT).
Power out. It is worth noting that the optical collector used in the stability test is the classic perovskite MAPbI 3 based on methyl ammonium (MA), but the device is constructed without an organic group selective layer and no metal back contact. Under these conditions, thermal stress alone is the main factor that reduces the working stability of the packaging device.
The inherent brittleness of perovskite materials requires external enhancement to protect the key layer from mechanical stress.
The fracture resistance of the composite solar cells caused by the mechanical reinforcement bracket to be directly inserted into the active layer of perovskite solar cells has increased by 30 times, thus repositioning the fracture characteristics of perovskite solar cells into the same area as conventional c-Si and CIGS and CdTe solar cells.
InSPIRE
InSPIRE field-based research objectives
Agrivoltaics is the pairing of solar and agriculture for shared benefits. The InSPIRE project unites field research across the United States with advanced modeling and analysis capabilities to provide foundational and actionable data on agrivoltaics and low-impact solar development, while also highlighting region-specific benefits and tradeoffs to ecosystems, grazing habitat, and crop production.
Research Objectives
By using consistent, peer-reviewed methods across all field research sites, we seek to ensure comparability of findings across geographies and system configurations.
InSPIRE analyzes the ecological and economic implications of:
- Growing native vegetation for habitat and ecosystem services under and around ground-mounted solar installations,
- Growing agricultural crops under innovative solar configurations and irrigation regimes,
- Applying low-impact solar development approaches to improve soil quality, carbon storage, stormwater management, microclimate conditions, and solar efficiencies
- Adopting pollinator-friendly solar practices to host beneficial insects and boost local agricultural yields, and
- Grazing animals under panels to provide vegetation management and ecological services.
Resources
Peer-reviewed InSPIRE publications
Free analysis tool for agrivoltaics planning
Explore an archive of agrivoltaics research
Agrivoltaic success factors in the United States
Advisory Group
The Agriculture and Solar Together: Research Opportunities (ASTRO) advisory group provides feedback to the InSPIRE project on research directions and study designs while also facilitating the dissemination of results to relevant stakeholders.
ASTRO group members come from across the United States and represent leading solar industry partners, state agencies, and other organizations focused on research, food and agriculture, and the environment. They help to ensure InSPIRE project research activities meet the most pressing needs of industry and the agricultural community.
Contact
Jordan Macknick, Principal Investigator
National Renewable Energy Laboratory (NREL)
This project is funded by the U.S. Department of Energy and managed by NREL.
Silicon ink is spot on, NREL experiments show
Ink can cause a mess, but the Silicon Ink developed by Innovalight behaves itself so well that when it is added to a solar cell it doesn’t clump or spill, instead it boosts the cell’s power by a startling, profit-boosting 5 to 7 percent.
Both solar cells and T-shirts can be enhanced with a screen printer, some ink and a squeegee.
But it takes a real special ink to suspend silicon nanoparticles so uniformly that it can lay down the precise microns-thick lines needed to dope the silicon emitter exactly under the front metal contacts. Those contacts make a solar cell work.
Innovalight, a small start-up from Sunnyvale, Calif., came up with an ingenious way to suspend silicon in a solution without the tiny particles glomming onto one another or sinking to the bottom of the container.
But could that Silicon Ink prove useful for solar cells?
Researchers at the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) proved that the answer is yes. And the winners could be the solar cell industry and the environment, because Silicon Ink, when added to the manufacturing process, can make solar cells more efficient and save a large plant hundreds of millions of dollars each year.
NREL and Innovalight shared a coveted RD 100 award for 2011 for the Silicon Ink technology. Given by RD 100 Magazine, the RD 100 awards are referred to in the industry as the Oscars of Invention. Silicon Ink’s ability to boost efficiency in such a low-cost way prompts some in the industry to label it liquid gold.
Impurities in Silicon Are Key to Making Contacts, Making Electricity
Silicon is the key ingredient in most of the billions of solar cells made each year worldwide.
Dopants or impurities are used to change the conductivity of silicon and to create the internal electric fields that are needed to turn photons into electrons and thus into electricity. One of the great challenges is to distribute the exact concentrations of dopants in precisely the correct locations throughout the device.
Innovalight scored big with Silicon Ink because it found a way to suspend silicon nanoparticles evenly in a solution. Those silicon nanoparticles contain dopant atoms that can be driven into silicon solar cell to form a selective emitter.
What Innovalight’s potential customers and investors wanted to know was whether the ink can deliver high concentrations of dopants to extremely localized regions of the emitter and increase a solar cell’s efficiency.
NREL Senior Scientists Kirstin Alberi and David Young listened to what Innovalight wanted to prove and then suggested some experiments that could help them prove it.
The question was, ‘can you print this ink in very well defined lines and drive in dopants only in the material underneath the lines to create a well-defined selective emitter, said Alberi, who began at NREL three years ago as a post-doctorate researcher. If so, the increased concentration of dopants right under the contacts would lower the resistance at the metal contact, while the rest of the cell contained low-doped silicon and that would mean jumps in efficiency and savings of huge amounts of money.
On some level, you want the emitter to be highly doped so it makes a better contact with the metal, Alberi said. But if it’s too heavily doped elsewhere, that’s bad.
That’s why a selective emitter that is heavily doped only in precise portions of a solar cell is such a promising technology.
The Ink Stays Put, Boosting Efficiency of the Cell
The money question: Can Silicon Ink, using a screen-printing approach, lay down those differently-doped lines without having the ink spread all over the place? If the ink spreads, the spots where silicon is supposed to be lightly doped get the overflow from the spots where silicon is highly doped.
They needed to prove this to their investors to show that their company was the best at doing this, Alberi said. They didn’t know how to go about proving this, and that’s where we were able to help.
There wasn’t a Eureka moment, but the dawning realization that the Silicon Ink was performing exactly as well as Innovalight had hoped was extremely gratifying, Alberi said.
It was nice seeing that the results were exactly what they hoped they would be, Alberi said.
The scorecard: Silicon Ink, used in a low-cost screen-printing process, delivered a 1 percentage point absolute increase in the efficiency of the solar cells.
If that doesn’t sound like much, consider that a typical silicon solar cell array in the field may convert 15 percent of the photons that hit it into useable electricity. That 1 percentage point increase actually represents a 7 percent increase in power output for a typical 15 percent.efficient cell at a cost that is so low that it basically goes unnoticed at large solar-cell manufacturing plants.
That’s a huge impact for almost nothing, Richard Mitchell, NREL’s lead investigator on the project, said.
A Very Special Ink, a Very Special Screen and Squeegee
In the manufacturing process, the Silicon Ink spills onto a screen, a squeegee pushes it one way as the silicon wafers pass through, then pushes the ink the other way as new wafers appear below the screen. The Ink only reaches the cell at the precise points where a tiny slit in the screen’s mask lets it get through. The slits are narrower than a human hair.
Every once in a while, a syringe adds some more Silicon Ink to the screen to ensure the spread is even and the liquid doesn’t run out.
The tests proved that the ink stayed put.
Once the silicon and the dopants are where they should be on the unfinished cell, they are heated not enough to melt them, but just enough to drive the dopants contained within the Silicon Ink into the solar cell.
Kirstin and the others helped Innovalight prove they can actually get the right kind of selective doping in the areas they need, Mitchell said. This is the first technology that showed that exactly where you print is exactly where the cell gets doped to a precision of a micron.
NREL, Innovalight Partnered on RD and Overcoming Barriers
The first NREL/Innovalight partnership was a 2008 cooperative research and development agreement, or CRADA, in which Innovalight paid for the expertise of the scientists at NREL, who in return agreed to keep the proprietary technology secret.
Later, Innovalight won a competitive bid to enroll in NREL’s Photovoltaic Incubator program in which it had to meet stringent deadlines to deliver improvements in its technology in return for the help of NREL scientists in overcoming barriers.
Innovalight eventually worked out the kinks in its process for using Silicon Ink in an ink-jet application. It showed off the technique to potential customers at a demonstration assembly line at its Sunnyvale, Calif., headquarters.
Customers were unfamiliar with ink-jet printing as it applied to solar cells, so manufacturers balked at making the leap. But they all had screen printing on their production lines already, Mitchell said. Adding another screen printer was something their operators would understand.
So, the goals of the Innovalight/NREL partnerships shifted to proving the reliability of Silicon Ink with screen printing.
Chinese Have Signed on to the Technology, United States May Be Next
The manufacturers that have signed contracts for Silicon Ink all in China, including JA Solar, Hanwha SolarOne, and Jinko Solar, are trying the technology on selected assembly lines. If they get the results expected, they’ll likely start using it on all their lines, Mitchell said. If that proves successful, then they might be ready to try Silicon Ink with the ink-jet method, which Innovalight says holds even greater promise to improve cell efficiency and save money.
Last month, DuPont acquired Innovalight, a move that could boost the prospects for American solar-cell manufacturing using the Silicon Ink technology.
The partnership with NREL absolutely was a positive experience from the beginning, said Conrad Burke, who founded Innovalight and is now general manager of DuPont Innovalight, which now has 58 employees.
There are certain capabilities at NREL that companies of our size, or even larger companies, can’t afford to have access to, Burke said. It’s a win-win situation where we have access to those resources when we need them. It’s good to have those resources in the United States.
Provided by National Renewable Energy Laboratory
Sunny superpower: solar cells close in on 50% efficiency
For solar cells, efficiency really matters. This crucial metric determines how much energy can be harvested from rooftops and solar farms, with commercial solar panels made of silicon typically achieving an efficiency of 20%. For satellites, meanwhile, the efficiency defines the size and weight of the solar panels needed to power the spacecraft, which directly affects manufacturing and launch costs.
To make a really efficient device, it is tempting to pick a material that absorbs all the Sun’s radiation – from the high-energy rays in the ultraviolet, through to the visible, and out to the really long wavelengths in the infrared. That approach might lead you to build a cell out of a material like mercury telluride, which converts nearly all of the Sun’s incoming photons into current-generating electrons. But there is an enormous price to pay: each photon absorbed by this material only produces a tiny amount of energy, which means that the power generated by the device would be pitiful.
Hitting the sweet spot
A better tactic is to pick a semiconductor with an absorption profile that optimizes the trade-off between the energy generated by each captured photon and the fraction of sunlight absorbed by the cell. A material at this sweet spot is gallium arsenide (GaAs). Also used in smartphones to amplify radio-frequency signals and create laser-light for facial recognition, GaAs has long been one of the go-to materials for engineering high-efficiency solar cells. These cells are not perfect, however – even after minimizing material defects that degrade performance, the best solar cells made from GaAs still struggle to reach efficiencies beyond 25%.
Further gains come from stacking different semiconductors on top of one another, and carefully selecting a combination that efficiently harvests the Sun’s output. This well-trodden path has seen solar-cell efficiencies climb over several decades, along with the number of light-absorbing layers. Both hit a new high last year when a team from the National Renewable Energy Laboratory (NREL) in Golden, Colorado, unveiled a device with a record-breaking efficiency of 47.1% – tantalizingly close to the 50% milestone (Nature Energy 5 326). Until then, bragging rights had been held by structures with four absorbing layers, but the US researchers found that six is a “natural sweet spot”, according to team leader John Geisz.
Getting this far has not been easy, because it is far from trivial to create layered structures from different materials. High-efficiency solar cells are formed by epitaxy, a process in which material is grown on a crystalline substrate, one atomic layer at a time. Such epitaxial growth can produce the high-quality crystal structures needed for an efficient solar cell, but only if the atomic spacing of each material within the stack is very similar. This condition, known as lattice matching, restricts the palette of suitable materials: silicon cannot be used, for example, because it is not blessed with a family of alloys with similar atomic spacing.
Devices with multiple materials – referred to as multi-junction cells – have traditionally been based on GaAs, the record-breaking material for a single-junction device. A common architecture is a triple-junction cell comprising three compound semiconductors: a low-energy indium gallium arsenide (InGaAs) sub-cell, a medium-energy sub-cell of GaAs and a high-energy sub-cell of indium gallium phosphide (InGaP). In these multi-junction cells, current flows perpendicularly through all the absorbing layers, which are joined in series. With this electrical configuration, the thickness of every sub-cell must be chosen so that all generate exactly the same current – otherwise any excess flow of electrons would be wasted, reducing the overall efficiency.
Bending the rules
Key to the success of NREL’s device are three InGaAs sub-cells that excel at absorbing light in the infrared, which contains a significant proportion of the Sun’s radiation. Achieving strong absorption at these long wavelengths requires InGaAs compositions with a significantly different atomic spacing to that of the substrate. Additionally, their device has been designed with intermediate transparent layers made from InGaP or AlGaInAs to keep material imperfections in check. Grading the composition of these buffer layers enables a steady increase in lattice constant, thereby providing a strong foundation for local lattice-matched growth of sub-cells that are not riddled with strain-induced defects.
The NREL team, which has pioneered this approach, advocates the so-called “inverted variant” structure. With this architecture, the highest energy cell is grown first, followed by those of decreasing energy, so that the cells lattice-matched to the substrate precede the growth of graded layers. This approach improves the quality of the device, while the fabrication process also results in the removal of the substrate – a step that could trim costs by enabling the substrate to be reused.
One other technique that can further boost solar-cell efficiency is to FOCUS sunlight on the cells, either with mirrors or lenses. The intensity of light on a solar cell is usually measured in “suns”, where one sun is roughly equivalent to 1 kW/m 2. Concentrated sunlight increases the ratio of the current produced when the device is illuminated compared to when it is in the dark, thereby boosting the output voltage and increasing the efficiency. The gain is considerable: the NREL device achieves a maximum efficiency of just 39.2% when tweaked to optimize efficiency without any concentration, a long way short of the 47.1% record.
When Geisz and colleagues assessed how the performance of their six-junction cell varies with concentration, they found that peak efficiency occurs at 143 suns. Nevertheless, the device still produces a very impressive 44.9% efficiency at 1116 suns, which would generate a large amount of power from a very small device. As a comparison, a record-breaking cell operating at 500 suns could deliver the same power as a commercial solar panel from just one-thousandth of the chip area. At such high concentrations, however, steps must be taken to prevent the cell from overheating and diminishing performance.
Just over a decade ago, this approach to generating power from high-efficiency cells spawned a concentrating photovoltaic (CPV) industry, with a clutch of start-up firms producing systems that tracked the position of the Sun to maximize the energy that could be harvested from focusing sunlight on triple-junction cells. Unfortunately, this fledgling industry came up against the unforeseeable double whammy of a global financial crisis and a flooding of the market with incredibly cheap silicon panels produced by Chinese suppliers. The result was that so few CPV systems were deployed that even on a sunny day when all operate at their peak, their global output totals less than one-tenth of the power of a typical UK nuclear power station.
Extra-terrestrial encounters
Far greater commercial success for makers of multi-junction cells has come from powering satellites, most recently buoyed by the rollout of satellite broadband by companies such as OneWeb and Starlink. The key advantage here is that high-efficiency cells can drive down the costs of making and launching each satellite. As well as reducing the number of cells needed to power the spacecraft, higher efficiencies shrink both the size and weight of the solar panels that form the “wings” of the satellite. While launch costs have plummeted over the last few decades, satellite operators can still expect to pay almost 3000 per kilogram to get their spacecraft into orbit – and thousands of satellites are due to be deployed over the next few years.
For a solar cell in space, the crucial metric is the value at the end of its lifetime – after the device has been bombarded by radiation
However, for a solar cell in space, the crucial metric is not the initial efficiency but the value at the end of its intended lifetime after the device has been bombarded by radiation. Compound semiconductors hold up to this battering far better than those made from silicon. Early studies showed that the difference in efficiency of compound semiconductors rises with age from 25% to 40–60%, which ensured the dominance of triple-junction cells for space applications. Even so, the efficiencies of the best commercial cells for satellites remain limited to around 30–33%. This is partly because the solar spectrum beyond our atmosphere has a stronger contribution in the ultraviolet, where it is much harder to make an efficient cell, and partly because there are no concentrating optics to FOCUS sunlight onto the cell.
To drive down the watts-per-kilogram of solar power in space, a US team working on a project known as MOSAIC (micro-scale optimized solar-cell arrays with integrated concentration) has been making a compelling case for CPV in space. The team points out that it should be relatively easy to orientate the solar panels on a satellite to maximize power generation with lenses in front of the cells shielding them from radiation. Concentrations must be limited to no more than around 100 suns, however, because cells in space cannot be cooled by convection, only by heat dissipation through radiation and conduction.
For CPV to have a chance of succeeding in space, the large and heavy solar modules used in early terrestrial systems must be replaced with a significantly slimmed-down successor. Technology pioneered by project partner Semprius, a now defunct CPV system maker, excels in this regard. The firm developed a process that uses a rubber stamp to parallel-print vast arrays of tiny cells, each one subsequently capped by a small lens.
The best results have come from stacking a dual-junction GaAs-based cell on top of an InP-based triple-junction cell separated by a very thin dielectric polymer. Current cannot pass through this polymer film, so separate electrical connections are made to extract the current from each cell independently. While this doubles the number of electrical connections, it eliminates the need for current matching between the two devices. Lifting this restriction gives greater freedom to the design, potentially enabling this approach to challenge the efficiency of NREL’s record-breaking device under high concentrations. Operating at 92 suns under illumination which mimics that in space, the team’s latest device, still to be fully optimized, has an efficiency of 35.5%.
Towards 50%
The NREL researchers know what they need to do to break the 50% barrier. The goal they are chasing is to cut the resistance in their device by a factor of 10 to a value similar to that found in their three- and four-junction cousins. They are also well aware of the need to bring down the cost of producing such complex multi-junction cells.
Also chasing the 50% efficiency milestone is a team led by Mircea Guina from Tampere University of Technology in Finland. Guina and colleagues are pursuing lattice-matched designs with up to eight junctions, including as many as four from an exotic material system known as dilute nitrides – a combination of the traditional mix of indium, gallium, arsenic and antimonide, plus a few per cent of nitrogen.
Dilute nitrides are notoriously difficult to grow. Back in the 1990s, German electronics powerhouse Infineon developed lasers based on this material, but they were never a commercial success. recently, Stanford University spin-off Solar Junction showcased the potential of this material in solar cells. Although the start-up went to the wall when CPV flopped, devices produced by the company grabbed the record for solar efficiency in 2011 and raised it again in 2012 with triple-junction designs. Guina and co-workers are well positioned to take their technology further. They have made progress in producing all four of the dilute nitride sub-cells needed to produce record-breaking devices, and their efforts are now focused on optimizing the high-energy junction. The team’s work has been delayed due to the COVID-19 pandemic, but Guina believes that the approach could break the 50% barrier, possibly raising the bar as high as 54%.
There is still a question of impetus, however. The lack of commercial interest in terrestrial CPV may well encourage Guina to change direction and FOCUS on chasing the record for space cells with no concentration. Much of today’s multi-junction solar-cell research is not focusing on power generation here on Earth, so while that 50% milestone is tantalizingly close, it might not be broken anytime soon.
Richard Stevenson is editor of Compound Semiconductor magazine, e-mail richardstevenson@zoho.com