Tag Archives: solar cell

Why transparent solar cells could replace windows in the near future

No matter how sustainable, eco-friendly, and clean sources of energy they are, conventional solar panels require a large setup area and heavy initial investment. Due to these limitations, it’s hard to introduce them in urban areas (especially neighborhoods with lots of apartment blocks or shops). But thanks to the work of ingenious engineers at the University of Michigan, that may soon no longer be the case.

The researchers have created transparent solar panels which they claim could be used as power generating windows in our homes, buildings, and even rented apartments.

Image credits: Djim Loic/Unsplash

If these transparent panels are indeed capable of generating electricity cost-efficiently, the days of regular windows may be passing as we speak. Soon, we could have access to cheap solar energy regardless of where we live — and to make it even better, we could be rid of those horrific power cuts that happen every once in a while because, with transparent glass-like solar panels, every house and every tall skyscraper will be able to generate its own power independently.

An overview of the transparent solar panels

In order to generate power from sunlight, solar cells embedded on a solar panel are required to absorb radiation from the sun. Therefore, they cannot allow sunlight to completely pass through them (in the way that a glass window can). So at first, the idea of transparent solar panels might seem preposterous and completely illogical because a transparent panel should be unable to absorb radiation. 

But that’s not necessarily the case, researchers have found. In fact, that’s not the case at all.

Professor R. Lunt at MSU showing the transparent luminescent solar concentrator. Image credits: Michigan State University

The solar panels created by engineers at the University of Michigan consist of transparent luminescent solar concentrators (TLSC). Composed of cyanine, the TLSC is capable of selectively absorbing invisible solar radiation including infrared and UV lights, and letting the rest of the visible rays pass through them. So in other words, these devices are transparent to the human eye (very much like a window) but still absorb a fraction of the solar light which they can then convert into electricity. It’s a relatively new technology, only first developed in 2013, but it’s already seeing some impressive developments.

Panels equipped with TLSC can be molded in the form of thin transparent sheets that can be used further to create windows, smartphone screens, car roofs, etc. Unlike, traditional panels, transparent solar panels do not use silicone; instead they consist of a zinc oxide layer covered with a carbon-based IC-SAM layer and a fullerene layer. The IC-SAM and fullerene layers not only increase the efficiency of the panel but also prevent the radiation-absorbing regions of the solar cells from breaking down.

Surprisingly, the researchers at Michigan State University (MSU) also claim that their transparent solar panels can last for 30 years, making them more durable than most regular solar panels. Basically, you could fit your windows with these transparent solar cells and get free electricity without much hassle for decades. Unsurprisingly, this prospect has a lot of people excited.

According to Professor Richard Lunt (who headed the transparent solar cell experiment at MSU), “highly transparent solar cells represent the wave of the future for new solar applications”. He further adds that these devices in the future can provide a similar electricity-generation potential as rooftop solar systems plus, they can also equip our buildings, automobiles, and gadgets with self-charging abilities.

“That is what we are working towards,” he said. “Traditional solar applications have been actively researched for over five decades, yet we have only been working on these highly transparent solar cells for about five years. Ultimately, this technology offers a promising route to inexpensive, widespread solar adoption on small and large surfaces that were previously inaccessible.”

Recent developments in the field of transparent solar cell technology

Apart from the research work conducted by Professor Richard Lunt and his team at MSU, there are some other research groups and companies working on developing advanced solar-powered glass windows. Earlier this year, a team from ITMO University in Russia developed a cheaper method of producing transparent solar cells. The researchers found a way to produce transparent solar panels much cheaper than ever before.

“Regular thin-film solar cells have a non-transparent metal back contact that allows them to trap more light. Transparent solar cells use a light-permeating back electrode. In that case, some of the photons are inevitably lost when passing through, thus reducing the devices’ performance. Besides, producing a back electrode with the right properties can be quite expensive,” says Pavel Voroshilov, a researcher at ITMO University’s Faculty of Physics and Engineering.

“For our experiments, we took a solar cell based on small molecules and attached nanotubes to it. Next, we doped nanotubes using an ion gate. We also processed the transport layer, which is responsible for allowing a charge from the active layer to successfully reach the electrode. We were able to do this without vacuum chambers and working in ambient conditions. All we had to do was dribble some ionic liquid and apply a slight voltage in order to create the necessary properties,” adds co-author Pavel Voroshilov.

Image credits: Kenrick Baksh/Unsplash

PHYSEE, a technology company from the Netherlands has successfully installed their solar energy-based “PowerWindow” in a 300 square feet area of a bank building in The Netherlands. Though at present, the transparent PowerWindows are not efficient enough to meet the energy demands of the whole building, PHYSEE claims that with some more effort, soon they will be able to increase the feasibility and power generation capacity of their solar windows.   

California-based Ubiquitous Energy is also working on a “ClearView Power” system that aims to create a solar coating that can turn the glass used in windows into transparent solar panels. This solar coating will allow transparent glass windows to absorb high-energy infrared radiations, the company claims to have achieved an efficiency of 9.8% with ClearView solar cells during their initial tests.

In September 2021, the Nippon Sheet Glass (NSG) Corporation facility located in Chiba City became Japan’s first solar window-equipped building. The transparent solar panels installed by NSG in their facility are developed by Ubiquitous Energy.  Recently, as a part of their association with Morgan Creek Ventures, Ubiquitous Energy has also installed transparent solar windows on Boulder Commons II, an under-construction commercial building in Colorado.

All these exciting developments indicate that sooner or later, we also might be able to install transparent power-generating solar windows in our homes. Such a small change in the way we produce energy, on a global scale could turn out to be a great step towards living in a more energy-efficient world.

Not there just yet

If this almost sounds too good to be true, well sort of is. The efficiency of these fully transparent solar panels is around 1%, though the technology has the potential to reach around 10% efficiency — this is compared to the 15% we already have for conventional solar panels (some efficient ones can reach 22% or even a bit higher).

So the efficiency isn’t quite there yet to make transparent solar cells efficient yet, but it may get there in the not-too-distant future. Furthermore, the appeal of this system is that it can be deployed on a small scale, in areas where regular solar panels are not possible. They don’t have to replace regular solar panels, they just have to complement them.

When you think about it, solar energy wasn’t regarded as competitive up to about a decade ago — and a recent report found that now, it’s the cheapest form of electricity available so far in human history. Although transparent solar cells haven’t been truly used yet, we’ve seen how fast this type of technology can develop, and the prospects are there for great results.

The mere idea that we may soon be able to power our buildings through our windows shows how far we’ve come. An energy revolution is in sight, and we’d be wise to take it seriously.

The cells change from transparent to orange-red when heated enough. Credit: UC Berkeley.

‘Solar windows’ change from transparent to tinted at high temperatures, blocking the sun while generating electricity

Berkeley chemists devised a new type of photovoltaic out of cesium-doped perovskite that not only provides power but also doubles as a tinted window. At room temperature, the solar cell is transparent but automatically tints once the temperature rises, blocking the sun, thereby cooling the space behind the window, and generating electricity at the same time.

The cells change from transparent to orange-red when heated enough. Credit: UC Berkeley.

The cells change from transparent to orange-red when heated enough. Credit: UC Berkeley.

Peidong Yang and colleagues at Berkeley Lab described their creative solar cell in the journal Nature Materials. Yang thinks the invention could be used for smart windows or displays in the buildings, vehicles, and the handheld devices of the future.

“This class of inorganic halide perovskite has amazing phase transition chemistry,” Yang said. “It can essentially change from one crystal structure to another when we slightly change the temperature or introduce a little water vapor.”

A cheap and versatile photovoltaics material

The perovskite mineral was originally found in the Ural Mountains in 1839, but it was only in 2009 that its ability to transport solar energy and convert it into electricity was discovered. Just a couple of years later, rated efficiency in the lab had soared from 3.8% to 19.3%, a pace of improvement unmatched by any other solar technology. Currently, the leading commercial solar tech employs crystalline silicon solar cells, which convert roughly 25% of incoming photon energy into electricity but have decades worth of research behind them.

What makes this mineral so exciting is its uncanny ability to diffuse photons over a long distance throughout the cell when prepared in a liquid solution. Typically, solar cells convert energy to electricity by exploiting the hole-pair phenomenon. The photon hits the semiconducting material, then if its energy falls into the semiconductor band gap, an electron is knocked off, leaving a gap in the material or a ‘hole’. The electron travels from atom to atom within the material, occupying holes and knocking out other electrons at the same time until it eventually reaches an electrode and has its charge transferred to a circuit. Last step: profit and generate electricity.

The key is to have electrons moving for as long as possible, and thanks to its diffusing capabilities, perovskite can theoretically generate more electricity than silicon. Perovskite is also dirt cheap which is highly important if we’re ever to cover a significant portion of the planet’s surface with solar cells for a 100% sustainable energy future. Organometal halide perovskites can also be used the other way around — namely turning electricity into light with high-brightness LEDs, manufactured at low cost and more easily than current commercially-available options.

Credit: UC Berkeley.

Credit: UC Berkeley.

Pull the shades

Yang and colleagues showed that perovskite can be extremely versatile. By tweaking the chemicals in the materials (cesium, lead, iodine, and bromine), the researchers were able to change the material’s transparency.

For the last couple of years, the team has been working on a perovskite solar cell that changes from transparent to opaque when heated. Last year’s version could tint when heated but the cell’s conversion efficiency dropped drastically after several cycles. The new cell retains its conversion efficiency after many cycles between transparent and reddish-tint.

One major downside is that the warmed perovskite transforms only up to 7% of its energy into electricity, which is well below conventional solar cells. Another drawback is that the cells won’t tint unless they’re heated to more than 100°C. However, the researchers claim they’ve already come up with a variation that switches between 50°C–60°C. With a bit more tweaking, they might just find the right composition for their perovskite.

“The solar cell shows fully reversible performance and excellent device stability over repeated phase transition cycles without any color fade or performance degradation,” said Minliang Lai, a graduate student in Yang’s group. “With a device like this, a building or car can harvest solar energy through the smart photovoltaic window.”

Credit: Michigan State University.

Transparent solar technology could provide 40% of US power if deployed across all glass surfaces

Solar energy is growing at breakneck speed all over the world, driven by year-to-year slashed prices for photovoltaic (PV) cells. Solar could grow even faster if we were to incorporate transparent solar harvesting devices into windows. One recent study performed at Michigan State University (MSU) found transparent solar cells that convert invisible wavelengths of light could deliver up to 40 percent of the United States’ power.

Credit: Michigan State University.

Credit: Michigan State University.

“Highly transparent solar cells represent the wave of the future for new solar applications,” Richard Lunt, Associate Professor of Chemical Engineering and Materials Science at MSU, said in a public statement. “We analyzed their potential and show that by harvesting only invisible light, these devices can provide a similar electricity-generation potential as rooftop solar while providing additional functionality to enhance the efficiency of buildings, automobiles and mobile electronics.”

Previously, Lunt and colleagues devised a transparent luminescent solar concentrator comprised of small organic molecules that absorb specific nonvisible wavelengths of sunlight, such as those in the infrared and ultraviolet range. Other efforts developed elsewhere have rendered rather poor results since the solar cells were only semi-transparent, with the material tinted in a colored hue and the cells yielding poor efficiency. The cells made at MSU are totally transparent to our eyes and, when incorporated into windows, do not disrupt the view. Moreover, it’s not only buildings that can employ transparent cells but also car windows, cell phones or other devices with a clear surface.

At the moment, only 1.5 percent of the country’s electricity demand is met by solar power. Completely moving away from fossil fuels requires concentrated efforts using a mix of technologies, some established like hydro and wind power, others more innovative like these transparent solar cells.

At a first glance, harvesting the solar energy that hits American windows might not seem that much. However, after crunching the numbers, the authors found that there are an estimated 5 billion to 7 billion square meters of glass surface in the United States. If deployed across all of these surfaces, transparent solar cells could supply 40 percent of the country’s electricity demand or about the same potential as rooftop solar, as reported in Nature Energy. Together, the two technologies  “could get us close to 100 percent of our demand if we also improve energy storage,” Lund said.

Credit: University of Michigan.

Credit: University of Michigan.

Traditional solar panels currently average a rated efficiency in the 18 percent range. The bulk of solar energy lies in the visible range and hence transparent solar cells don’t come anywhere near rooftop solar, sitting in the 1-2 percent efficiency range. Lund says reaching 5% is possible with 7% being the maximum limit. What transparent solar concentrators lack in efficiency, they make up in surface area availability. It’s enough to take a 360 degree look around any busy business center in America to instantly get a feel of this yet untapped potential. What’s more, transparent solar cells can be totally oblivious to bystanders. For all intents and purposes, these are just windows rather than power generators.

“Traditional solar applications have been actively researched for over five decades, yet we have only been working on these highly transparent solar cells for about five years. Ultimately, this technology offers a promising route to inexpensive, widespread solar adoption on small and large surfaces that were previously inaccessible,” Lund said.

Silicon solar cell with 26.3% efficiency. Credit: Kaneka Corporation.

Record-breaking silicon solar cell efficiency of 26.6% demonstrated by Japanese researchers, very close to the theoretical limit

Improving the efficiency of silicon-based solar cells has proven a challenge in the past couple of years as all the low-hanging fruits had been picked dry. That didn’t stop a team of Japanese researchers from Kaneka Corporation to push the envelope. They report a record-breaking efficiency of 26.3% beating the previous record of 25.6%. Most commercial-grade solar cells operate in the low-20-percent range. There are millions of solar panels in the world and by 2050 there could be billions. Even a fraction of a percent can add up to massive amounts of renewable electricity which is why such work is extremely important.

Silicon solar cell with 26.3% efficiency. Credit: Kaneka Corporation.

Silicon solar cell with 26.3% efficiency. Credit: Kaneka Corporation.

Close to an ideal solar cell

The theoretical limit for silicon solar cells is 29 to 30 percent silicon cells primarily capture the light waves from the red spectrum of sunlight while the rest of the spectrum is not utilized. The most efficient solar cells demonstrated thus far report efficiencies of 34.5% for unconcentrated sunlight and 46% in the case of multi-junction concentrator solar cells. These solar cells, however, aren’t based on silicon but exotic and sometimes toxic materials like indium-gallium-phosphide, indium-gallium-arsenide or germanium. Moreover, these cells can be very expensive which is why most utility scale as well as rooftop solar is made of silicon — an abundant material that has been used by the semiconductor industry for decades and can be cheaply sourced. It’s partly thanks to silicon that solar panels have become increasingly economically feasible. So much so that solar energy is now cheaper than fossil fuels in many parts of the globe, even without subsidies. 

At the same time, upping silicon efficiency can be challenging nowadays. The most recent record-breaking cell is a thin-film heterojunction (HJ) design where multiple bands of silicon are layered within the cell to minimize band gaps. To make the cells, a manufacturing process called plasma-enhanced chemical vapor deposition (PECVD) was used where thin films of silicon are deposited on a wafer from a gas state. It’s the same technique that Panasonic uses to manufacture panels for Tesla at the Solar City plant in Buffalo, says Megan Geuss from Ars Technica.

The team from Kaneka also placed low-resistance electrodes at the rear of the cells which maximize the number of captured photons collected by the cell from the front. Additionally, the top layer of the cell is coated with a layer of amorphous silicon as well as an anti-reflective layer that not only reduces maintenance but also helps collect more photons.

Tests which have been recognized by the National Renewable Energy Lab (NREL) suggest the Kaneka cell are 26.3 percent efficient. After publishing their paper in Nature Energy, Kaneka claims it managed to reach 26.6 percent efficiency.

Kaneka also quantified the energy losses that kept the cell from reaching its full 29 percent efficiency. The researchers conclude overall efficiency was reduced by 0.5% due to resistive losses, 1% from optical losses, and 1.2% to recombination losses. Nevertheless, we’re talking about a 2.7 percent increase in efficiency which can add up considerably.

 

The startup's solar panel is of the same size and can be as easily installed as any other typical residential-grade panel. Credit: EPFL/Alain Herzog

Swiss startup demoes residential solar panels twice as efficient than what the market has to offer

The startup's solar panel is of the same size and can be as easily installed as any other typical residential-grade panel. Credit: EPFL/Alain Herzog

The startup’s solar panel is of typical size and can be as easily installed as any other typical residential-grade panel. Credit: EPFL/Alain Herzog

Solar is growing exponentially fast all around the world. But while prices per kW-hour drop year after year, the same can’t be said about solar efficiency — here, progress is incremental. A Swiss startup backed by the prestigious Swiss Federal Institute of Technology in Lausanne (EPFL) might change all that by helping the industry take a big leap, instead of small steps. Their solar panels are about twice as efficient as most residential solar panels, but only marginally more expensive.

A new ray of hope for the solar industry

The most efficient solar cells were demonstrated by researchers at Soitec, France in December 2014. Their multi-junction concentrator solar cell can harness 46% of the sunlight’s energy into electricity. These sort of cells, however, are extremely expensive and only make economic sense for a narrow range of applications, like satellites in space or utility-scale solar power plants. Reaching this sort of efficiency for residential applications, like rooftop solar, has proven far more challenging.

In its labs based in EPFL’s Innovation Park, Insolight showed that it is possible to harness 36.4% of the sun’s incoming rays using a novel design developed in-house. Other solutions currently traded in the market can only offer 18 to 20 percent efficiency.

Their panels are made out of two main sub-systems. The first part is made of a very thin optical structure comprised of millimetric lenses which act as a small network of magnifiers. The incoming light is thus amplified and concentrated onto a ‘super solar cell’, which comprises the second system. This cell is a multi-junction one consisting of multiple layers that each act to capture a particular band of energy from sunlight.

The lenses used to concentrate solar power onto the cell. Credit: Credit: EPFL/Alain Herzog

The lenses used to concentrate solar power onto the panel’s cell. Credit: Credit: EPFL/Alain Herzog

Multi-junction cells can be prohibitively expensive, but Insolight made their panels in such a way that only a very small super solar cell is used. Even so, when the light is concentrated onto the tiny cell, it generates twice the electricity for the same surface area as the typical solar panels you can buy anywhere today. An important component that helped the Swiss researchers gain this remarkable performance is a proprietary tracking system that can direct 100% of the sun’s rays into the cell no matter the angle of incidence.

“All the components were designed from the start to be easily mass produced,” says Mathieu Ackermann, the company’s CTO. “Working in industry gave us what we needed to reach our goal, which was to develop solar panels that could be rapidly brought to market at a competitive price.”

The results were validated by an independent lab in Germany, part of the Fraunhofer Institute group. But transitioning this performance from the lab to the real world is a whole different matter. The founders are optimistic, though, and say that even if their design falls short of a couple points it will still be better than what’s currently being offered.

“The price of solar panels has dropped sharply in recent years, but not enough to produce electricity at a competitive cost,” Ackermann  says.

“For residential systems, solar panels accounted for less than 20% of total installation costs in the United States in 2015. Even if the solar panels were free, this would not always offset the system’s cost. Currently, most of the margin earned by solar energy developers comes from subsidies. Yet these subsidies are declining.”

This is where Insolight hopes to step in. It remains to be seen whether the young startup can scale production and deliver on its promises. There’s no word for now regarding the wholesale price.

“Insolight has designed a highly innovative system, and these initial prototypes show an impressive yield in external assessments,” says Christophe Ballif, Director of EPFL’s Photovoltaics Laboratory. “They now need to test the limits of their concept, show how a commercial-sized system can perform, and prove the product’s economic potential.”

Photosynthetic solar cell turns carbon dioxide and sunlight into fuel

A team of researchers from the University of Illinois at Chicago (UIC) has created a photosynthetic solar cell that converts atmospheric carbon dioxide into usable hydrocarbon fuel.

The solar cell that converts atmospheric carbon dioxide directly into fuel. Credit: University of Illinois at Chicago/Jenny Fontaine

The solar cell that converts atmospheric carbon dioxide directly into fuel. Credit: University of Illinois at Chicago/Jenny Fontaine

Conventional solar cells convert sunlight into electricity that must be stored in heavy batteries. The new solar cell is potentially game-changing because it converts atmospheric carbon dioxide into fuel, which could not only remove large amounts of carbon from the atmosphere but also create energy-dense fuel.

“The new solar cell is not photovoltaic – it’s photosynthetic,” said Amin Salehi-Khojin, an assistant professor of mechanical and industrial engineering at UIC and senior author of the study. “Instead of producing energy in an unsustainable one-way route from fossil fuels to greenhouse gas, we can now reverse the process and recycle atmospheric carbon into fuel using sunlight.”

If the new solar cell can be taken advantage of on a global scale, it would render fossil fuels obsolete by giving us the ability to turn carbon dioxide into fuel at a cost similar to a gallon of gasoline.

Past studies have failed to find effective catalysts for the conversion of carbon dioxide into burnable forms of carbon. In the current study, Salehi-Khojin and his team focused on using transition metal dichalcogenides (TMDCs) as catalysts, which they paired with an unconventional ionic liquid as the electrolyte. They were then placed inside a two-compartment, three-electrode electrochemical cell.

Of all of the TMDCs that they tried, nanoflake tungsten diselenide turned out to be the ideal catalyst.

“The new catalyst is more active; more able to break carbon dioxide’s chemical bonds,” said Mohammad Asadi of UIC and first author of the paper.

The final solar cell is an artificial leaf that consists of two silicon triple-junction photovoltaic cells that harvest light. On the cathode side is the tungsten diselenide and ionic liquid co-catalyst system, while the anode side possesses cobalt oxide in potassium phosphate electrolyte.

The team hopes that the technology will be able to be adapted not only to large-scale applications such as solar farms, but also small-scale applications.

Journal Reference: Nanostructured transition metal dichalcogenide electrocatalysts for CO2 reduction in ionic liquid. 29 July 2016. 10.1126/science.aaf4767

Nine bacterial solar cells that combined generate 5.59 microwatts. Image: Seokheun "Sean" Choi

You’ve heard all about solar cells, but what about bacterial solar cells?

On the desk of  Seokheun “Sean” Choi sits a 3×3 array that at first glance looks like a lemon squeezer. It is, in fact, a solar panel but not like any you’ve seen or heard about before. Instead of using semiconductors like silicon crystals to convert sunlight into electricity, the array employs a complex system that nurtures cyanobacteria — beings whose metabolism create free electrons which can be harnessed.

Nine bacterial solar cells that combined generate 5.59 microwatts. Image: Seokheun "Sean" Choi

Nine bacterial solar cells that combined generate 5.59 microwatts. Image: Seokheun “Sean” Choi

Choi and colleagues at Binghamton University have been working on the bacteria solar cell for years now. Last year they significantly improved their design by changing the materials for the anodes and cathodes. They also used a microfluidic system chamber that houses and feeds the bacteria, instead of a dual-chamber reactor. Now, they’ve shown how to stack each cell into an array, proving the design is scalable.

Cyanobacteria, being photosynthetic organisms, use the sun’s energy, H2O and CO2 to synthesize their energy storage components, i.e. carbohydrates, lipids and proteins. Some researchers have proposed that it may be feasible to use cyanobacteria to make biofuels and even hydrogen fuel  by the reversible activity of hydrogenase. The Binghamton University researchers, however, are tapping directly into the photocurrent generated by the bacteria themselves.

“Once a functional bio-solar panel becomes available, it could become a permanent power source for supplying long-term power for small, wireless telemetry systems as well as wireless sensors used at remote sites where frequent battery replacement is impractical,” said Choi, an assistant professor of electrical and computer engineering in Binghamton University’s Thomas J. Watson School of Engineering and Applied Science, and co-author of the paper.

“This research could also enable crucial understanding of the photosynthetic extracellular electron transfer processes in a smaller group of microorganisms with excellent control over the microenvironment, thereby enabling a versatile platform for fundamental bio-solar cell studies,” said Choi.

I know you like numbers, so let’s look at some figures. A typical solar panel configuration of, say, 6×10 cells generates roughly 200 watts of electrical power. The same number of bacterial solar cells generates 0.00003726 watts, researchers report in the journal Sensors and Actuators B: Chemical. Alright, that was disappointing but this is still 1) experimental research and 2) bacterial solar cells aren’t meant to compete with traditional solar cells. It’s also worth noting that the power output was measured at 1.28 V operating voltage under a 200 kΩ external resistor, so there’s room for plenty of juice to power small devices.

The findings open the door for more research into how cyanobacteria could be used to power remote devices, like wireless sensors. Other than that, it’s pretty amazing to see other creatures besides hamsters turning a wheel generate raw electricity — and the cyanobacteria don’t seem to mind at all.

“It is time for breakthroughs that can maximize power-generating capabilities/energy efficiency/sustainability,” Choi said. “The metabolic pathways of cyanobacteria or algae are only partially understood, and their significantly low power density and low energy efficiency make them unsuitable for practical applications. There is a need for additional basic research to clarify bacterial metabolism and energy production potential for bio-solar applications.”

 

 

1 cm2 monolithic perovskite-silicon tandem solar cell. This image was used for illustrative purposes, and the pictured solar cell was not involved in the present research. Credit: Rongrong Cheacharoen/Stanford University

Charge nano-map could help scientists turn perovskite into THE solar cell material

A crystal known to science for more than a century was only recently recognized for its use in harvesting solar power. Since the first successful usage of perovskite in solar cells in 2009, the advances in the field have grown exponentially over time, making it a potential candidate for revamping the solar industry in favor of silicon.  Despite solar cells made with perovskite recently crossed the 20 percent efficiency mark, researchers say there’s still room to improve if only they knew how charge flows at the nanometer scale. They just had to ask, and the  Department of Energy’s Oak Ridge National Laboratory delivered.

Why you should care about perovskite

1 cm2 monolithic perovskite-silicon tandem solar cell. This image was used for illustrative purposes, and the pictured solar cell was not involved in the present research. Credit: Rongrong Cheacharoen/Stanford University

1 cm2 monolithic perovskite-silicon tandem solar cell. This image was used for illustrative purposes, and the pictured solar cell was not involved in the present research. Credit: Rongrong Cheacharoen/Stanford University

The perovskite mineral was originally found in the Ural Mountains in 1839, but it was only a few years ago that its ability to transport solar energy and convert it into electricity was discovered. Just a couple of years later, rated efficiency in the lab has soared from 3.8% to 19.3%, a pace of improvement unmatched by any other solar technology. Currently, the leading commercial solar tech employs crystalline silicon solar cells, which convert roughly 25% of incoming photon energy into electricity, but have decades worth of research backing them.

What makes this mineral so exciting  is its uncanny ability  to diffuse photons a long distance through the cell when prepared in a liquid solution. Typically, solar cells convert energy to electricity by exploiting the hole-pair phenomenon. The photon hits the semiconducting material, then if its energy falls into the semiconductor bandcamp, an electron is offset, leaving a gap in the atom or hole. The electron travels from atom to atom within the material, occupying holes and offsetting at the same time until it eventually reaches an electrode and has its charge transferred to a circuit. Last step: profit and generate electricity.

The key is to have electron moving for as long as possible, and thanks to its diffusing capabilities, perovskite can theoretically generate more electricity. Perovskite is also dirt cheap which is highly important if we’re ever to cover a significant portion of the planet’s surface with solar cells for a 100% sustainable energy future.

Zooming in on charges

(Top) A phasor plot of the transient absorption data shows the presence of free charges and excitons; a false colored image shows their contributions at different spatial positions. Credit: ORNL

(Top) A phasor plot of the transient absorption data shows the presence of free charges and excitons; a false colored image shows their contributions at different spatial positions. Credit: ORNL

An experimental setup combined microscopy and ultra-short laser pulses that fire every 50 millionths of a billionth of a second. These pulses mimicked the sun’s rays by providing photons that got absorbed by a perovskite solar cell. A second laser measured minute changes in light absorption in the material. Eventually, the researchers drew a pixel-by-pixel map of the material.

“The ability to identify what will be created after the solar cell absorbs a photon, either a pair of free charges or their bound form called an exciton, is crucial from both fundamental and applied perspectives,” said co-author Yingzhong Ma, who led the research team. “We found that both free charges and excitons are present, and the strength of our approach lies in not only identifying where they are but also determining what their relative contributions are when they are both present at a given spatial location.”

“With conventional approaches of studying photovoltaic materials, we are unable to accurately map out electronic processes and how electrons are getting lost,” said  said Benjamin Doughty, one of the authors and a member of ORNL’s Chemical Sciences Division. “Those processes can translate into losses in efficiency.”

Now, it only remains to identify what causes this spatial difference, Ma says. If and when this happens, they might also learn what causes the degradation issues due to moisture many have reported.

Reference: “Separation of Distinct Photoexcitation Species in Femtosecond Transient Absorption Microscopy,http://pubs.acs.org/doi/abs/10.1021/acsphotonics.5b00638.

 

Silicon pillars emerge from nanosize holes in a thin gold film. The pillars funnel 97 percent of incoming light to a silicon substrate, a technology that could significantly boost the performance of conventional solar cells. Credit: Vijay Narasimhan, Stanford University

Finally, the metal wiring in solar cells might stop reflecting light. One up solar efficiency

There’s an inherent flaw in solar cells: the metal wiring that’s quintessential to harnessing the electrons reflects the incoming light, acting like a mirror. Now, must people would brush off this issue and leave it like that. It’s a necessary trade off. But a team at Stanford University devised an elegant chemical technique that basically hides the wiring with silicon, away from the light while preserving energy harnessing. Metal wires cover 5 to 10 percent of a solar cell’s surface. Now, in the same area more light can be absorbed, hence more electricity generated which jumps the efficiency. Of course, this also means cheaper solar panels — if only the chemical technique is covered by the recurring costs of increased efficiency.

Silicon pillars emerge from nanosize holes in a thin gold film. The pillars funnel 97 percent of incoming light to a silicon substrate, a technology that could significantly boost the performance of conventional solar cells. Credit: Vijay Narasimhan, Stanford University

Silicon pillars emerge from nanosize holes in a thin gold film. The pillars funnel 97 percent of incoming light to a silicon substrate, a technology that could significantly boost the performance of conventional solar cells. Credit: Vijay Narasimhan, Stanford University

“Using nanotechnology, we have developed a novel way to make the upper metal contact nearly invisible to incoming light,” said study lead author Vijay Narasimhan, who conducted the work as a at Stanford. “Our new technique could significantly improve the efficiency and thereby lower the cost of solar cells.”

The researchers modeled a typical solar cell with a thin film of gold 16-nanometer-thick atop a flat sheet of silicon. Tiny, itsy bitsy holes were perforated on the whole surface, but to the naked eye the gold layered object still looked like a shiny mirror. Analysis showed the hole-ridden gold film covered 65% of the silicon’s surface, reflecting 50% of the incoming light. So far so good. That was predictable.

solar_panel_and_solar_cell

A typical solar cell – wiring is both on top and back. 

Narasimhan and colleagues then immersed the object  in a solution of hydrofluoric acid and hydrogen peroxide. What happened next was the silicon started popping up through the holes, like pillars. These grew up to 330 nanometers in height, transforming the once golden surface into a dark red. That alone indicated the surface wasn’t reflective anymore. Narasimhan compares the silicon pillars to a colander in your kitchen sink

“When you turn on the faucet, not all of the water makes it through the holes in the colander, ” he said. “But if you were to put a tiny funnel on top of each hole, most of the water would flow straight through with no problem. That’s essentially what our structure does: The nanopillars act as funnels that capture light and guide it into the silicon substrate through the holes in the metal grid.”

The metal contacts still work great. Through trial and error, the Stanford researchers eventually reached an optimal design where nearly two-thirds of the surface can be covered with metal, yet the reflection loss is only 3 percent. This not only means that manufacturers can hide metal contacts – they can include more of it since it helps with efficiency! The researchers estimate a conventional 20% solar panel can up its efficiency to 22%, a huge gain. Multiply that by millions of solar panels and you’ve got a massive energy gain and cost reduction.

What about the gold? Yes, gold is expensive but Narasimhan says the technique works with  silver, platinum, nickel and other metals. “We call them covert contacts, because the metal hides in the shadows of the silicon nanopillars,” co-author Ruby Lai. said. “It doesn’t matter what type of metal you put in there. It will be nearly invisible to incoming light.”

“With most optoelectronic devices, you typically build the semiconductor and the contacts separately,” said Cui, co-director of the Department of Energy’s Bay Area Photovoltaic Consortium (BAPVC). “Our results suggest a new paradigm where these components are designed and fabricated together to create a high-performance interface.”

A model gold sheet is, of course, different from an actual solar cell. Looking forward to seeing this work applied to an actual working cell.

Reference: Vijay K. Narasimhan et al. Hybrid Metal–Semiconductor Nanostructure for Ultrahigh Optical Absorption and Low Electrical Resistance at Optoelectronic Interfaces, ACS Nano (2015). DOI: 10.1021/acsnano.5b04034

transparent solar cell

Finally, a fully transparent solar energy harvester

University of Michigan researchers have devised what looks like the world’s first fully transparent solar cell. Think of all of those tall glass buildings; wouldn’t it be nice if all that incoming solar energy was harvested somehow? Likewise, why not let your smartphone charge up a bit while it’s taking a tan. Of course this isn’t a new idea, but previous attempts are rather unattractive because the compromise makes windows too shady or dark. After all, the purpose of a window is to let light in, not make energy. Ideally, you’d want them harness energy as well, complementary. The new system devised at UM is exciting because it offers exactly this: energy generation, with no compromise in visibility.

transparent solar cell

Image: University of Michigan

Obviously, windows are transparent because they let most of the incoming light through. This eventually bounces off your retina and allows you to see outside or inside. This is why all those solar cells meant to line windows make them look dark and make rooms inside shady. They have to absorb some of those light frequencies. The researchers at UM took an alternate route.

As you can see in these quite impressive photos, their system is fully transparent. That’s because the glass itself is not a solar cell – it’s a transparent luminescent solar concentrator (TLSC). The TLSC is made of organic salts tuned to only absorb ultra-violet and infrared energy, the kind of light frequencies you can’t see. The salt then luminesce at another infrared frequencies that gets picked up by tiny plastic channels that line the edges of the “glass” and direct the infrared rays to tiny conventional solar cells.

transparent solar cell

Image: University of Michigan

As you might have guessed, these score some terrible efficiencies. According to the paper, the TLSC has a rated efficiency of 1%, but the researchers think 5% should be possible. Non-transparent luminescent concentrators (which bathe the room in colorful light) max out at around 7%. Left alone, this doesn’t seem like much, but multiplied by every window in a building this could add up to power the LED lights in rooms, for instance. Definitely, a new awesome fact about solar energy.

Keep in mind this is an innovation. Don’t expect it to be efficient or affordable, yet. I do find this kind of research inspiring however. A lot of people seem to be against the idea of solar panels because they’re obtrusive and ugly. Sure, there are pros and cons to solar panels. Personally, I enjoy them, but hey that’s just me. By blending energy generation, solar cells might garner a new following.

concentrating-solar

Concentrated photovoltaic, now on your rooftop

The most efficient solar cells are those that convert incoming concentrated solar power via lenses, the sort you see on the International Space Station or  in the sun-soaked Middle East where  Shams 1, a 100 MW CSP plant – the largest in the world –  operates, powering 20,000 United Arab Emirates homes. Because of their complex nature, concentrated solar power arrays have been mostly installed in open areas, but a team of engineers at Penn State are set to shift this paradigm. They’ve designed a translation motion micro-array that can concentrate solar power efficiently even in crowded areas, say on your rooftop.

concentrating-solar

Prototype rooftop CPV solar panel being tested outdoors. The small black squares under each lenslet in the close-up are the solar cells. (Credit: Nature Communications)

Concentrated photovoltaics (CPV) are usually very bulky, so to make a rooftop affordable system the team  combined miniaturized gallium-arsenide photovoltaic cells, 3D-printed plastic lens arrays, and a moveable focusing mechanism. This vastly reduced the size, cost and weight of the overall system, which is only 1cm thick. The tiny solar cells are embedded between a pair of plastic lenslet arrays. The lenslet atop act like a magnifying glass, focusing the solar energy, while the ones at the bottom behave like a concave mirror. This way, the duo lens array focuses solar light up to 200 times.

“The new CPV systems use inexpensive optics to concentrate sunlight,” said Noel C. Giebink, assistant professor of electrical engineering, Penn State. “Current CPV systems are the size of billboards and have to be pointed very accurately to track the sun throughout the day. You can’t put a system like this on your roof.”

“We partnered with colleagues at the University of Illinois because they are experts at making small, very efficient multi-junction solar cells,” said Giebink. “These cells are less than 1 square millimeter, made in large, parallel batches, and then an array of them is transferred onto a thin sheet of glass or plastic.”

[ALSO SEE] New solar power material converts 90% of incoming solar energy into heat

The sun’s position, however, isn’t fixed and constantly glides from east to west. Typically, CPV use a dual-axis system that constantly put the concentrating mirrors in motion, tracking the sun’s position. These are bulky, difficult to maintain and highly expensive. The researchers’ took a different approach by making the optics system fixed and the cells moving. To track the Sun over the course of a day, the middle solar cell sheet slides laterally in between the two lenslet arrays  enabling efficient solar focusing for a full eight hour day.

microtracking-microcell-CPV

Schematic illustration of a microtracking microcell CPV panel array of microcell photovoltaics, transfer-printed on a central acrylic sheet that tracks the Sun by sliding laterally between stationary upper and lower acrylic lenslet arrays (credit: Nature Communications)

“The vision is that such a microtracking CPV panel could be placed on a roof in the same space as a traditional solar panel and generate a lot more power,” said Giebink. “The simplicity of this solution is really what gives it practical value.”

The findings appeared in Nature Communications. [source: KurzweilAI]

An ultra-sensitive needle measures the voltage that is generated while the nanospheres are illuminated. Credit: AMOLF/Tremani - Figure: Artist impression of the plasmo-electric effect. - See more at: http://www.caltech.edu/news/new-technique-could-harvest-more-suns-energy-44923#sthash.IukitP4G.dpuf

A new way to harvest solar energy using metal nanoparticles and plasmon resonance

Solar cell technology has improved dramatically over the past couple of year, yet it will be a long time before multi-junction cells – then kind that can reach efficiency well over 40% – will become affordable to small home owners or even large scale installation. New methods are always explored, however, each with its own angle to harnessing solar energy, benefits and disadvantages included. One of the biggest challenges to improving solar cell efficiency is collecting light frequencies that are outside the visible spectrum, and a new technique developed at Caltech makes use of a most interesting physical effect: plasmon resonance on the surface of metal nanoparticles.

An all metal solar cell

An ultra-sensitive needle measures the voltage that is generated while the nanospheres are illuminated. Credit: AMOLF/Tremani - Figure: Artist impression of the plasmo-electric effect. - See more at: http://www.caltech.edu/news/new-technique-could-harvest-more-suns-energy-44923#sthash.IukitP4G.dpuf

An ultra-sensitive needle measures the voltage that is generated while the nanospheres are illuminated.
Credit: AMOLF/Tremani – Figure: Artist impression of the plasmo-electric effect.

 

The researchers report an all-metal geometry that they used to convert optical power to an electrical potential. The potential is too small to become useful for generating power – not yet at least – but the team believes even at its current state of development it could be used in  new types of sensors.

At their core, solar cells function by exploiting a basic principle called the photovoltaic effect, first discovered  by French physicist A. E. Becquerel in 1839. As photons hit a silicon surface – the defacto material of choice for solar cells – the outermost electrons in the atoms get excited until they reach an energy level that lets them jump free from the silicon. The electron then jumps from silicon atom to silicon atom, leaving what’s called a “hole” behind. It’s this electron-hole pair that allows solar cells to harness energy and transform it into electricity, once the electrons eventually reach a metal contact and complete a circuit.

The problem with conventional solar cells is that only frequencies from the visible spectrum are being absorbed. Infrared light, which is also the most energy intensive, can’t be absorb and thus passes right through and becomes lost as heat. In fact, this heat further causes problems because the increased temperature affects efficiency in an unwanted way.

“The silicon absorbs only a certain fraction of the spectrum, and it’s transparent to the rest. If I put a photovoltaic module on my roof, the silicon absorbs that portion of the spectrum, and some of that light gets converted into power. But the rest of it ends up just heating up my roof,” says Harry A. Atwater, the Howard Hughes Professor of Applied Physics and Materials Science, who led the study.

Atwater and colleagues have now found a way to harness these infrared frequencies using a structure made not of silicon, but entirely out of metal! The all metal structure was chosen to make full use of a fascinating phenomenon called plasmon resonance. Plasmons are coordinated waves, or ripples, of electrons that exist on the surfaces of metals at the point where the metal meets the air. The plasmon resonance is usually used in biotherapeutic and drug discovery research, as well as protein activity and stability analysis in biopharmaceutical production, but recently the effect has been probed for solar cell research as well.

In their natural state, metals have a predetermined plasmon resonance. When the same metals are arranged into various nanostructures, however, these resonances can be tuned to other frequencies. With this in mind, Atwater and colleagues placed gold nanoparticles on an indium-tin-oxide substrate and arrays of 100-nm-diameter holes in 20-nm-thick gold films on a glass substrate.

“Normally in a metal like silver or copper or gold, the density of electrons in that metal is fixed; it’s just a property of the material,” Atwater says. “But in the lab, I can add electrons to the atoms of metal nanostructures and charge them up. And when I do that, the resonance frequency will change.”

Negative and positive surface potentials were registered during monochromatic irradiation at wavelengths below or above the plasmon resonance respectively. These potentials were as large as 100 mV under 100 mW/cm^2 illumination.

“We’ve demonstrated that these resonantly excited metal surfaces can produce a potential”—an effect very similar to rubbing a glass rod with a piece of fur: you deposit electrons on the glass rod. “You charge it up, or build up an electrostatic charge that can be discharged as a mild shock,” he says. “So similarly, exciting these metal nanostructures near their resonance charges up those metal structures, producing an electrostatic potential that you can measure.”

This electrostatic potential is a first step in the creation of electricity, Atwater says. “If we can develop a way to produce a steady-state current, this could potentially be a power source”.

The idea is to use the plasmoelectric effect in tandem with traditional silicon solar cells, so that both visible and infrared light are harnessed. It might be a while before we see anything like this demonstrated. For now, the phenomenon could be exploited to make new types of sensors that detect light based on the electrostatic potential, according to the paper published in Science.

“Like all such inventions or discoveries, the path of this technology is unpredictable,” Atwater says. “But any time you can demonstrate a new effect to create a sensor for light, that finding has almost always yielded some kind of new product.”

 

solar cell blu-ray

Solar cells etched with Blu-ray bit patterns absorb 21.8% more energy

Apart from both being shiny, it’s hard to see any connection between a Blu-ray disk and a solar panel. Northwestern University researchers thought outside the box, however, and used the disk’s tiny stamped grooves and pits to make molds for solar panels. Because of the resulting structure’s geometry, the solar cells were able to absorb 21.8% more light. Overall, the conversion efficiency was raised by 12 percent.

Bad movie? Turn it into a solar cell

solar cell blu-ray

Credit: istockphoto

As solid state drives and cloud storage have taken off, people nowadays see little use in optical storage devices like Blu-ray, let alone DVDs. Why buy a movie when you can stream it? Why store media on an optical device when a flash drive is so much more convenient? It’s a doomed market, but optical storage isn’t completely useless yet; not the Blu-rays at least. Apparently, a Blu-ray disk is an excellent medium for imprinting quasi-random nanostructures, which are essential to solar cells.

The most important parameter in a solar cell is how many photons it can absorb. To increase the number of photons that get trapped, modern solar cells employ so-called quasi-random nanostructures –  patterns that are neither too orderly nor too random. To make the molds that print these patterns onto a solar cell is prohibitively expensive, so alternatives have been sought. Some attempts have been made using Blu-ray disks, since these already have a nanostructure built in. If the disk is empty, however, then the resulting patterns are inefficient. However, when the Northerwestern researchers made a mold from a disk filled with the movie Police Story 3: Supercop, a significant surge in photon absorption was reported in Nature Communications.

Optical images of a half-patterned solar cell showing iridescent scattering due to the periodic nature of the Blu-ray pattern. Credit: Northwestern University

Optical images of a half-patterned solar cell showing iridescent scattering due to the periodic nature of the Blu-ray pattern. Credit: Northwestern University

When a Blu-ray is written, compressed binary sequences are printed with an error control modulation in the form of islands and pits (the 0 and 1) such that each element is at least two to seven digits long. Since a single digit is 75 nanometers (nm) wide, a full Blu-ray is comprised of grooves and pits ranging from 50 nm to 525 nm in size. Coincidentally, these are near optimal for photon trapping.

The disk’s surface was first delaminated to expose the bits, then a negative mold was made. A polymer active layer was pressed onto the mold and vapor deposition was then used to transfer semiconducting material onto the new cell’s surface. When tested, the Blu-ray patterned cells absorbed 21.8 percent more light than non-patterned panels. Conversion efficiency jumped by nearly 12 percent.

So, quite unexpected. Personally, I believe this is a perfect example of how old tech can still creep up and surprise us. Why re-invent the wheel, when you can use what’s already around you.

 

perovskite_battery

One single scrap car battery could be turned into solar cells that power 30 homes

perovskite_battery

One single lead-iron battery could be used to build enough solar panels to power 30 homes. Image: MIT

Lead-acid car batteries used to be the norm, but luckily we’re seeing a massive shift towards more efficient and environmentally friendly alternatives like lithium-ion. Still, there are fleets of hundreds of millions of cars that still employ these archaic and toxic batteries. Typically, manufacturers try to have car owners bring their old lead-acid batteries, which are then converted into more environmentally friendly new batteries. The vast majority of them go into landfills, tough. Researchers at MIT present a remarkable alternative: using old car batteries to power a new generation of dirt cheap and efficient solar cells, based on perovskite. One single used battery could be employed as a prime material for solar cells that could power up to 30 homes.

A new found purpose

“Perovskite” is a general term used to describe a group of materials that have a distinctive crystal structure of cuboid and diamond shapes. In the past few years, researchers have demonstrated that solar cells that use this wonder mineral can reach a solar energy conversion efficiency of up to 19%, comparable to silicon tech that’s somewhere between 21% and 25%. There are some significant differences; perovskite efficiency has only recently crossed the single-digit efficiency margin (they’ve only been investigated for their solar energy potential, seriously at least, only in the last decade, while silicon has been around for more than 30 years), and there’s good reason to think this can be increased. Secondly, manufacturing perovskite solar cells is a whole lot easier energy-wise than the silicon-based variety, not requiring nearly the same temperatures and pressure.

A new research by engineers at MIT suggests that perovskite solar cells can be manufactured efficiently using recycled lead, giving old car batteries as an example of a prime feedstock. So, instead of adding more lead to the environment, potentially poisoning the soil and plants, the material could be recycled from one solar cell to another in much the same way that car batteries continue to use it from generation to generation.

Because the solar cells are very thin, only a small fraction of the lead that comprises of an old car battery would be used. In fact, one such battery could funnel enough lead to make solar panel that power 30 homes.

“This isn’t perfect,” said W.M. Keck Professor of Energy at MIT Angela Belcher said. “It’s very exciting, and lots of people are really pushing the edge of the technology, but it’s also new. Silicon has been around for a long time, and it’s known to work. But we’re excited about this, because we think it could be a competitor that’s easy to process, has rapidly increasing efficiency, and can be made in an environmentally friendly way.”

The findings appeared in the journal Energy and Environmental Science. The video below details the MIT process.

An artist's impression of spray-coating glass with the polymer to create a solar cell. Image: Energy and Environmental Science

Spray-coated solar cells bring solar power to every corner

Solar cells don’t necessarily come in bulky, sandwich panels you see hang up most roofs. There are a myriad of solutions at your disposal: thin or ultrathin cells, organic cells, flexible cells, you name it. Perhaps the most versatile option, in theory at least, is a spray-on solar cell. Image having a solar cell active solution in a can. You can then turn virtually any surface fitted with electrodes into a solar power source. It’s really amazing, but what it makes in ingenuity, it lacks in efficiency. Researchers at University of Sheffield  aim to change all this, after they report they’ve for the first time developed perovskite solar cells using a spray-on process. The resulting cells are efficient and affordable.

Solar power in a can

An artist's impression of spray-coating glass with the polymer to create a solar cell. Image: Energy and Environmental Science

An artist’s impression of spray-coating glass with the polymer to create a solar cell. Image: Energy and Environmental Science

A while ago, I wrote a bit about how perovskite might significantly boost the solar power market. Discovered for its solar power conversion capabilities just a couple of years ago, research into perovskite solar cells have so far rendered fantastic results. In just a couple of years, rated efficiency has jumped from 3.8% to over 19%.  Cheap, readily available and easy to make, perovskite is regarded as a viable candidate for complementing or replacing silicon at the helm of solar cell materials, because it takes less energy to make. Currently silicon solar cells, the mainstream type, have a rated efficiency of over 25%.

Paint-on or spray-on solar cells aren’t exactly new. Either using quantum dots or some other kind of solutions, spray-on solar cells have been very interesting to watch, but not that interesting to use. Expensive and with a low efficiency, they showed little practical use apart from show and tell. Experts at University of Sheffield have now developed  spray-painting method to produce solar cells using perovskite.

Lead researcher Professor David Lidzey said: “There is a lot of excitement around perovskite based photovoltaics.”
“Remarkably, this class of material offers the potential to combine the high performance of mature solar cell technologies with the low embedded energy costs of production of organic photovoltaics.”

Previously, the researchers demonstrated a spray-on manufacturing process for organic semiconductors. Adapting the process, the researchers created devices with a perovskite absorber instead of an organic absorber, and reached much higher efficiency – around 11%. This is huge, considering no spray-on method has led to cells topping two figures in efficiency.

Professor Lidzey said: “This study advances existing work where the perovskite layer has been deposited from solution using laboratory scale techniques. It’s a significant step towards efficient, low-cost solar cell devices made using high volume roll-to-roll processing methods.”
Solar power is becoming an incr

Findings appeared in the journal Energy & Environmental Science.

New material allows ultra-thin, transparent solar cells

Image courtesy of Vienna University of Technology, TU Vienna

Extremely thin, semi-transparent and flexible solar cells are one step closer to becoming a reality. Scientists have managed to create a semiconductor structure consisting of two ultra-thin layers which is excellent for solar panels. The solar cells are also transparent, so they could be used as windows or glass fronts.

Several months ago, the team created the first layer, an ultra-thin layer of the photoactive crystal tungsten diselenide. Now, they have successfully combined it with another layer made of molybdenum disulphide, creating a system that could be used in the future generation of solar cells.

Ultra-thin materials, having only several atoms in thickness are a hot topic in science. The most notable example is graphene, the wonder material consisting of only 1 atom of thickness arranged in a lattice. We’ve already written how graphene could make the internet 100 times faster, how it can make incredibly resistant yarn which could revolutionize the textile industry, how it can give us predator vision, develop new transistors, repair itself naturally, and many more. But the good thing about graphene studies is that they didn’t just show us what graphene can do – it also showed us what other extremely thin materials can do. At the Vienna University of Technology, Thomas Mueller, Marco Furchi and Andreas Pospischil have put that knowledge to good use:

“Quite often, two-dimensional crystals have electronic properties that are completely different from those of thicker layers of the same material,” says Thomas Mueller. His team was the first to combine two different ultra-thin semiconductor layers and study their optoelectronic properties.

Tungsten diselenide was known to be able to transform solar energy into electricity and vice versa. It had a significant problem though – a solar cell made from it would require a huge number of tiny electrodes to properly function. For this reason, it was discarded from studies for a while. However, researchers found an elegant way around that using another layer of molybdenium disulphide, which also consists of three atomic layers. The exact mechanism through which it does this is rather complicated.

When light shines upon an object, photons displace electrons from their original position. Without the electron, which is negatively charged, a positively charged hole remains in place. Both the electron and the hole can move freely in the material, but here’s the thing – they only contribute with energy when they are kept apart, so they cannot recombine. In order to prevent recombination of electrons and holes, metallic electrodes can be used to suck the charge away.

The holes move inside the tungsten diselenide layer, the electrons, on the other hand, migrate into the molybednium disulphide,” says Thomas Mueller. Thus, recombination is suppressed.

Of course, this only works if the energy is tuned just right in both layers – but this can be ensured through electrostatic fields. Florian Libisch and Professor Joachim Burgdörfer (TU Vienna) used computer models to predict what energies changes are in both layers and what voltage leads to optimum energy yields.

“One of the greatest challenges was to stack the two materials, creating an atomically flat structure,” says Thomas Mueller. “If there are any molecules between the two layers, so that there is no direct contact, the solar cell will not work.”

Another advantage of this technology is that while part of the light is absorbed and creates energy, most of it passes right through, so these solar cells could be used as glass fronts.

Scientific Reference:

  1. Marco M. Furchi, Andreas Pospischil, Florian Libisch, Joachim Burgdörfer, Thomas Mueller. Photovoltaic Effect in an Electrically Tunable van der Waals Heterojunction. Nano Letters, 2014; 140728125936002 DOI: 10.1021/nl501962c
photosynthesis

Popeye’s secret: spinach provides key insight that might one day lead to artificial photosynthesis

photosynthesisWhile scientists have been studying and incrementally increasing solar cell efficiency, we’ve yet to reach nature’s magnitude of solar energy conversion through photosynthesis. Artificial photosynthesis is a goal in alternative energy research, yet the process is extremely difficult to mimic since, in nature, the process involves numerous stages and transformation of matter and energy. Purdue University physicists used spinach and applied novel techniques to understand what happens during one of the photosynthesis stages.

Just add water, CO2 and sunlight

Photosynthesis is the process plants employ to convert carbon dioxide using energy from the sun into chemical energy in the form of hydrogen-carrying carbohydrates and oxygen. This process is achieved with massive energy conversion efficiency, which is why an artificial photosynthesis system is so desirable. To recreate the photosynthesis that plants have perfected, such an artificial system needs to perform two key roles: harvest energy and split water molecules. Plants accomplish these tasks using chlorophyll, which captures sunlight, and a collection of proteins and enzymes that use that sunlight to break down H2O molecules into hydrogen, electrons and oxygen (protons). The electrons and hydrogen are then used to turn CO2 into carbohydrates, and the oxygen is expelled.

One step close to the power of photosynthesis

Splitting water isn’t that simple though – there are organic catalysts involved and multi-steps processes, and nature had the benefit of millions of years worth of evolution to tweak photosynthesis to the highest efficiency possible. Attempts so far at mimicking this process have been rather counterproductive. Oxidizing water is a four-electron process, but the catalysts used in artificial photosynthesis processes, like cobalt oxide, have been found to be ineffective in the later stages.  So what can we do? Well, one course of action would be to solve and understand each intermediate step and then fill in the puzzle.

Yulia Pushkar, a Purdue associate professor of physics, and her colleagues  extract a protein complex called Photosystem II from spinach they bought at the supermarket. After the protein was extracted, the researchers fired an ultra fast laser that basically acts like sunlight (the protein needs light to activate) and recorded the electron configuration of their molecules. Photosystem II is involved in the photosynthetic mechanism that splits water molecules into oxygen, protons and electrons.

Using the world’s most powerful laser,  the LCLS atthe  Department of Energy’s SLAC National Accelerator Laboratory, the researchers fired fast femtosecond (one-quadrillionth of a second) laser pulses which allowed them to record snapshots of the PSII crystals before they explode in the X-ray beam, a principle called ‘diffraction before destruction. X-rays are great if you want to see structural changes in a crystal, but you can’t see its electron configuration evolution in time.

The Purdue team mimicked the conditions of the serial femtosecond crystallography experiment, but used electron paramagnetic resonance to reveal the electronic configurations of the molecules, Pushkar said. The Purdue team mimicked the conditions of the serial femtosecond crystallography experiment, but used electron paramagnetic resonance to reveal the electronic configurations of the molecules, Pushkar said.

When PSII splits water,  a portion of the protein complex, called the oxygen-evolving complex, cycles through five states in which four electrons are extracted from it. The international team recently revealed the structure of the first and third states at a resolution of 5 and 5.5 Angstroms using the techniques described above.

“The electronic configurations are used to confirm what stage of the process Photosystem II is in at a given time,” she said. “This information is kind of like a time stamp and without it the team wouldn’t have been able to put the structural changes in context.”

Findings appeared in the journal Nature.

perovskite

Perovskite solar cells might help the solar market grow to new heights

NREL Senior Scientist Kai Zhu prepares a perovskite solar cell in his lab. Photo: NREL

NREL Senior Scientist Kai Zhu prepares a perovskite solar cell in his lab. Photo: NREL

A crystal known to science for more than a century has only in recent years become recognized for its use in harvesting solar power. Since the first successful usage of perovskite in solar cells in 2009, the advances in the field have grown exponentially over time, making it a potential candidate for revamping the solar industry. Indeed, the crystal might just be what mainstream solar power has been waiting for: an easy to grow/manufacture solar harvesting platform that can be scaled.

The perovskite mineral was originally found in the Ural Mountains in 1839, but it was only a few years ago that its ability to transport solar energy and convert it into electricity was discovered. Just a couple of years later, rated efficiency in the lab has soared from 3.8% to 19.3%, a pace of improvement unmatched by any other solar technology. Currently, the leading commercial solar tech employs crystalline silicon solar cells, which convert roughly 25% of incoming photon energy into electricity, but have decades worth of research backing them.

[RELATED] This material can be turned into a solar cell by day and light source by night

A promise, not a guarantee

What makes perovskite so interesting is its uncanny ability  to diffuse photons a long distance through the cell when prepared in a liquid solution. Typically, solar cells convert energy to electricity by exploiting the hole-pair phenomenon. The photon hits the semiconducting material, then if its energy falls into the semiconductor bandcamp, an electron is offset, leaving a gap in the atom or hole. The electron travels from atom to atom within the material, occupying holes and offsetting at the same time until it eventually reaches an electrode and has its charge transferred to a circuit. Last step: profit and generate electricity.

The key is to have electron moving for as long as possible, and thanks to its diffusing capabilities, perovskite can theoretically generate more electricity.  So far, however, what’s impressive about perovskite is how its been growing, not actual performance. Perovskite tech has seen 400 percent growth in solar conversion efficiency in less than five years. By contrast, traditional solar cells only increased their performance by 50 percent in their first five years.

It’s important to note that the upper end efficiencies come at a serious drawback – the complex crystalline structures are designed with lead. The resulting crystals readily dissolve in water or even humid air, which means people could be surprise to find dissolved lead is dripping from their rooftops. Not that great of a prospect, really. There’s a solution to this, however: replacing lead with tin, an element that sits right next to its toxic cousin in the periodic table and thus shares a similar electronic configuration.

perovskite

Comparing the rate of increase in perovskite solar cell efficiencies (purple lines and markers) with leading third-generation (i.e., relatively new) solar cells and with amorphous Si (a-Si), green; dye sensitized, blue; organic, gray. The first two perovskite cells (2009 and 2011) refer to liquid junction cells, which were not stable but were important in initiating the subsequent solid-state cells. PHOTO FROM ADAM SMIGIELSKI/ISTOCKPHOTO

Henry Snaith, a physicist at the University of Oxford in the United Kingdom, and colleagues report making tin perovskite solar cells that achieve a maximum efficiency of 6.4% – a very noticeable drop in efficiency, but there’s not reason to think this couldn’t be improved to match lead-based perovskite performance.

“There is no reason this new material can’t reach an efficiency better than 15 percent,” said Mecouri Kanatzidis, one of the researchers who published in Nature Photonics. “Tin and lead are in the same group in the periodic table, so we expect similar results.”

Tin perovskite is also easier to produce and integrate in current manufacturing processes. Also,  the material can also be fine tuned to access different parts of the solar spectrum, so instead of using multiple semiconductors as is the case of >40% high-efficient multi-junction solar cells, you can use various tweaked layers made from the same crystal.

“It doesn’t require high-temperature processing. You can just dip glass into two chemicals and get the material to form on it,” said Joey Luther, a senior scientist at the Energy Department’s National Renewable Energy Laboratory (NREL).

According to N.R.E.L. senior scientist Joey Luther, the theoretical maximum efficiency of a perovskite-based solar cell would be around 31 percent.

“The goal shouldn’t be to stop at 20% efficiency,” Mr. Luther said. “The goal should be to try to get to 28% or higher. In the lab, the best cells need to be almost perfect at small scale. Then the commercial people can stop at whatever efficiency is economical for them to deploy.”

Clearly, perovskite momentum will definitely drop (a 400% growth in efficiency in the next five years is technically impossible), but it will remain to be seen just how far it can be integrated in commercial applications.  Findings appeared in the journal Nature.

 

multiple junction solar cell

New way to make affordable high efficiency stacked solar cells

multiple junction solar cell

A typical multi junction solar cell. Photo: treehugger.ocm

Researchers at the University of Illinois at Urbana-Champaign report they’ve devised a new type of highly efficient solar cell that is potentially easier to manufacture and cheaper than cells of similar performance. The stacked cell allows photon energy to be garnered from across the whole solar spectrum, and this new design makes use of a novel technique which basically electrically insulates each stack of the cell from each other – something previously deemed unpractical.

One, two, three, four cells

Most commercial solar cells today, the amorphous or noncrystalline silicone cells you see hung over most rooftops, have a top rated efficiency of around 20%. These are single-junction cells that have performance constraints defined by their Shockley–Queisser limits. A single junction means that the cell is comprised of only one type of photon absorbing material or only one bandgap. If the energy of a photon is lower than that bandgap, the photons will simply pass through the material, while higher bandgap photons will release their energy as heat, thus becoming wasted.

Multi-junction cells, however, employ a nifty trick to squeeze as much energy from photons as possible. By layering multi bandgap materials one over the other, you can transform more of the incoming energy into displaced electrons, hence more electrical power and considerably improve efficiency. This way high energy photons are captured in a large bandgap layer on top, while lower energy photons that pass through are captured in a small bandgap layer underneath, can push efficiency above 40%. Solar companies with efficient solutions for solar projects commonly employ these sort of cells.

The idea sounds extremely simple and some might wonder how come engineers didn’t thought of this sooner. They have, trust me, since the early ’60s when solar power was an emerging technology with a lot of attention focused on it. The truth is, multiple junction cells are extremely difficult to design. Different currents are generated in the different layers, charge builds up at the edges of layers, opposing the applied electric field and lowering the cell’s efficiency. Therefore, the layers have to be carefully designed so that the currents match.

‘People have worked out how to manage this in two or three junctions,’ explains author John Rogers of the University of Illinois at Urbana-Champaign. ‘When you get to four, five or six, it becomes more and more daunting.’

A new world record

What you can do, alternatively, is to keep each layer electrically insulated from the other and individually collect electrons, but so far there’s no reliable industrial process for scaling such a sort of cell. This may change in the future.

Rogers and his team started with a three-layer design that, until recently, held the world efficiency record at about 42%. They chopped up a germanium wafer into tiny pieces and mounted the individual cells in a printed circuit board arrangement. They covered the germanium wafers with a very thin layer of electrically insulating arsenic triselenide glass and used lithography techniques to print tiny versions of the three-layer cells on top. Just by placing two small electrodes between the cells, electrons were collected from the two cells separately.

‘This is a first demonstration out of the box,’ Rogers explains. ‘It’s not a one-shot demo.’

While the efficiency of their quadruple-junction cell was only improved by 1.8% (43.9% at concentrations exceeding 1,000 suns), the researchers conclude that ‘this technology is the lowest cost solution for climates that offer high levels of direct sunlight, such as solar cells deployed in arid environments […] The cost of energy is projected to be lower than that of coal-fired power plants.’

Since the materials and manufacturing techniques are compatible with large-scale manufacturing, indeed this projects seems very feasible.

The inventors of the multijunction solar cells were also recently award a $1 million prize

Troy Van Voorhis, professor of chemistry (left), and Marc Baldo, professor of electrical engineering (right). Photo: MIT

Excition fission model could vastly improve solar cell efficiency

Troy Van Voorhis, professor of chemistry (left), and Marc Baldo, professor of electrical engineering (right). Photo: MIT

Troy Van Voorhis, professor of chemistry (left), and Marc Baldo, professor of electrical engineering (right). Photo: MIT

The most basic principle of a solar cell is that it works by transferring the energy from an incoming photon (light) to a molecule, which causes one or more electrons to become displaced until an electrical current is formed. That’s the absolute gist of it, only besides electricity, some of the incoming photon energy gets lost as waste heat. Oddly enough, however, there are some organic materials that behave in the opposite way: when extra energy is given, more electrons form.

Weird physics

A team of researchers at MIT used both experiments and theoretical models to explain the mechanics of this phenomenon – called singlet exciton fission – and thus help solar cells become vastly more efficient.

The phenomenon was first observed in the 1960s, yet the exact mechanism involved has become the subject of intense controversy in the field. MIT’s Troy Van Voorhis, professor of chemistry, and Marc Baldo, professor of electrical engineering, led a team which investigated this odd behaviour. They synthesized and gathered materials made of four types of exciton fission molecules decorated with various sorts of “spinach” — bulky side groups of atoms that change the molecular spacing without altering the physics or chemistry. They then subjected these to various experiments to determine their fission rate.

The MIT team turned to experts including Moungi Bawendi, the Lester Wolfe Professor of Chemistry, and special equipment at Brookhaven National Laboratory and the Cavendish Laboratory at Cambridge University, under the direction of Richard Friend.

Experimental data and theoretical models confirm once and for all what was first proposed some 50 years ago: when excess energy is available in these materials, an electron in an excited molecule swaps places with an electron in an unexcited molecule nearby. The result: one photon in, two electrons out. “

“The simple theory proposed decades ago turns out to explain the behavior,” Van Voorhis says. “The controversial, or ‘exotic,’ mechanisms proposed more recently aren’t required to explain what’s being observed here.”

As such, the results provide a solid guideline for designing solar cells with these sort of exotic materials. They show that molecular packing is important in defining the rate of fission — but only to a point. When the molecules are very close together, the electrons move so quickly that the molecules giving and receiving them don’t have time to adjust. Indeed, a far more important factor is choosing a material that has the right inherent energy levels.

David Reichman, a professor of chemistry at Columbia University who was not involved in this research, considers the new findings “a very important contribution to the singlet fission literature. Via a synergistic combination of modeling, crystal engineering, and experiment, the authors have provided the first systematic study of parameters influencing fission rates,” he says. Their findings “should strongly influence design criteria of fission materials away from goals involving molecular packing and toward a focus on the electronic energy levels of selected materials.”

The results are reported in the journal Nature Chemistry.