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Renewable technology harnesses electricity from the darkness

Renewable technology harnesses electricity from the darkness

In economies around the globe, solar power is increasing at breakneck velocity and is already cheaper than the average wholesale price of electricity. This is encouraging considering the emergency of our climate.

However, the thing about solar energy is that it operates only when the sun is up. But at night, though in comparatively minute quantities, it is also feasible to draw power.

Researchers show an innovative tool in recent research that harnesses the distinction in temperature between radiative bodies and the night atmosphere. The power was sufficient to switch on a tiny LED light, making it appropriate for distant location apps and just about anywhere that requires some power at night.

“Remarkably, the device can generate electricity at night, when solar cells don’t work,” says lead author Aaswath Raman, who works as an assistant professor of materials science and engineering at the University of California, Los Angeles.

electricity from the cold dark night

So how exactly is all of this possible?

Solar cells produce electricity by absorbing photons through a semiconducting material that releases electrons collected on the back of the cell by electrodes fitted. When there is no sunlight, it is still possible to use solar power by storing it for subsequent use in batteries.

Batteries, however, can be costly, so it doesn’t make sense to employ them in specific applications, such as in very remote neighborhoods where you only need a bit of electricity to control some sensors, antennas, or small lights.

Rather than harnessing photons, the researchers exploited radiative cooling, the process by which a body loses heat by thermal radiation. Any sky-facing surface will lose heat to the atmosphere, shooting thermal radiation into space, eventually approaching a cooler temperature than the surrounding air. This is why, for example, you will see frost form on vegetation during any cold nights, even though the temperature outside is above water’s freezing point. By controlling this temperature difference, it is possible to generate electricity.

Raman and collaborators, including Stanford University scientists, tested a machine under a clear December sky that harnesses radiative cooling on a rooftop. The low-cost device comprises of a polystyrene enclosure covered in very lightweight aluminized mylar that minimizes the quantity of escaping thermal radiation.

The device was then placed on a desk one meter right above the surface, drawing heat from the surrounding air and releasing it into the night’s sky through a black emitter.

When the thermoelectric module was connected to a voltage boost converter, it was effectively capable of turning on a low-power white LED. Over six hours, the researchers estimated the power output of 25 milliwatts per square meter.

For illustration, a typical solar cell will generate about 150 watts per square meter in peak conditions, almost 10,000 more than the thermal radiative cooling device.

Raman says that the amount of electricity that can be generated per unit area during the night can be primarily increased by order of magnitude with some upgrades. And since it is made from elementary components that can be purchased off the shelf, the researchers understand there are many applications for which their device can find practical use. It can, for instance, operate in scorching, dry climates and could also act as a radiative cooling component.

Journal Reference: Joule

light sound waves

Researchers demonstrate storage and release of mechanical waves without loss of energy

In several technologies which are used today, light and sound waves are the fundamentals for transporting energy and signals. However, until now there has been no method to store a wave for a long period of time and then redirect it to a specific location when needed. This would provide the opportunity to manipulate waves for several purposes such as quantum computing, storing information, energy harvesting and many more.

A team of scientists led by Andrea Alù, founding director of Photonics Initiative, Advanced Science Research Center, CUNY and Massimo Ruzzene, Aeronautics Engineering professor at Georgia Tech has demonstrated experimentally that it is possible to capture a wave and store it efficiently while redirecting it later to a specific location. The work appears in Science Advances journal.

Alù said that the experiment demonstrates new opportunities can be unlocked in wave scattering and propagation through unconventional scattering methods. Researchers found ways to change the basic interaction between waves and particles. On striking an obstacle, a light or sound wave can go through two processes, partial absorption or reflection and scattering. In absorption, the wave is immediately converted to different forms of energy including heat. For those who cannot absorb waves, they are reflected and scattered.

In this experiment, the aim of the researchers was to find some technique to mimic the process of absorption in which the wave would not be converted to any other form instead stored in the material. This is known as coherent virtual absorption and it was introduced by ASRC two years ago.

For proving the theory, it was necessary to tailor the time evolution of waves so that on contacting non-absorbing materials, they would not be scattered, transmitted or reflected. This would prevent the wave from escaping and store it inside the material efficiently. Then it could be released on demand. In the course of the experiment, two mechanical waves were propagated in opposite directions along a carbon steel waveguide that had a cavity. Time variations of every wave were controlled so that the cavity would retain all the energy. The excitation of one of the waves was stopped which enabled the researchers to control the stored energy and send it towards a specific direction.

The experiment was performed using elastic waves which traveled inside a solid material. It can also be replicated for light and radiowaves thus opening the doors to exciting opportunities such as efficient harvesting of energy, wireless power transfer and greater control on wave propagation.

Research Paper: Coherent virtual absorption of elastodynamic waves

kilopower nasa

Nuclear reactor for crewed outposts on Mars and Moon could be ready by 2022

A new type of nuclear reactor has been designed to power outposts on Moon and Mars and it could be ready for its first in-space trial within a few years. The next step is a flight test for Kilopower experimental fission reactor which completed a series of ground tests from 2007 to 2018. Patrick McClure, Kilopower project lead at the Los Alamos National Laboratory said that Kilopower should be ready by 2022 although no off-earth demonstration has been scheduled yet.

He added that three years is a doable time frame which is not NASA’s stand who has been developing the project in collaboration with DOE. NASA’s Voyager 1 and Voyager 2 probes along with Curiosity Mars rover and New Horizons spacecraft employ radioisotope thermoelectric generators which convert the heat produced by the radioactive decay of plutonium-238 into electricity. RTGs produce relatively low power. It produces nearly 110 watts of electricity in Curiosity and the upcoming Mars 2020 rover.

A crewed outpost on Mars has higher energy demands around 40 kilowatts even for a small research envisioned by NASA for the late 2030s. Electricity will be required for water purification, oxygen generation for carbon-dioxide dominated atmosphere, charging up rovers, etc. Kilopower is a fission reactor and generates heat by splitting atoms into electricity through Sterling engines. The reactor was able to successfully convert 30% of the fission heat into electricity compared to 7% of regular RTG’s in a ground test series known as KRUSTY (Kilopower Reactor Using Sterling Technology). The project started in 2015 but the basic concepts were proved back in 2012 via an experiment called Demonstration Using Flattop Fissions (DUFF). KRUSTY and DUFF are characters in the Simpsons animated universe.

The reactor is designed to produce an output of 1 kilowatt of electrical power and can be scaled up to 10 kilowatts. NASA would need 4 of such reactors including one spare reactor. The 10-kilowatt machines would be just 11 feet in height and weigh close to 2000 kilograms. Without the astronaut shielding, it would weigh close to 1500 kilograms which would mean burying it in the ground.

The reactors are quite safe and there is no threat of radiation exposure if the rocket crashes back on Earth. The reactor is a self-regulating one if it gets too hot, the Stirling engines draw more heat away from uranium core and if temperature drops, the core naturally contracts which trap more neutrons and more splitting collisions. The device will need to dump a lot of heat on Mars due to the conversion efficiency of 30% will mean 70% of the heat will remain. It will come equipped with radiators.

A potential moon lander was the first task given to them as a demonstration mission for Kilopower but the concept will not end up flying as it targeted the lunar North pole and NASA was interested in the South Pole. Kilopower is the first fission reactor concept developed in the US in the last 40 years and is surely a milestone. US had previously launched reactors in experimental satellite, SNAP-10A in April 1965 but had to shut down after just 43 days. Although the Soviet Union was able to launch more than 30 fission reactors aboard satellites from 1967 to late 1980s.

Earth’s Magnetic Field Reversal Took Three Times Longer Than Thought

Earth’s magnetic field reversal takes much longer time than we thought

Human beings could not record the last time reversal of the magnetic poles of Earth occurred. However now with the help of the flow of ancient lava, it is possible for the scientists to estimate the duration of this phenomenon. 

A group of scientists used volcanic records for studying the last magnetic field reversal that occurred almost 780,000 years ago. They found that this reversal might have taken much longer duration than what was expected by scientists previously. The paper has been published in Science Advances journal

In the last 2.5 million years, the magnetic field of Earth reversed dozens of times, when north turned to the south and vice versa. It is known to the scientists that the last reversal occurred in the time of Stone Age but they have fewer details about the duration of the reversal and when it might occur next. 

Scientists relied on the lava flow sequences which erupted near to the last reversal for estimating its duration. With the help of this method, scientists estimated that it went on for 22000 years which is indeed quite longer than the past estimates of 1000 to 10000 years. 

Bradley Singer, a geoscience professor at the University of Wisconsin-Madison and the study’s lead author said the last reversal was quite complex which started within the outer core of the Earth. When he was performing studies on a Chilean volcano in 1993, he stumbled on a sequence of the lava which had partially recorded the process of reversal. In an effort to date the lava, Singer observed weird magnetic field directions in the lava flow sequences. These are some of the rarest records and Singer is one of the few scientists to date them. So this incident led to his life-long career goal to understand and explain the timings of the reversals of the magnetic field. 

Reversals occur when the iron molecules in the outer spinning core of the Earth start to move in the opposite direction as the surrounding iron molecules around them. With an increase in numbers, the magnetic field present in the core of the Earth is offset by the molecules. As a result, the magnetic field of our planet gets weaker. John Tarduno, a geophysics professor at the University of Rochester said that this kind of duration of the reversal would weaken the protection of Earth from solar radiations.

Additional effects might include genetic mutations or extinctions when the exposure to UV Sunrays increases. Increasing Sun particles entering Earth would disrupt communication systems, GPS and other satellite-based communication. 

Reports of a magnetic field in direction of Canadian Arctic to Siberia has started discussions over if the next field reversal is imminent and what effect would it have on life. Singer said that there is very less evidence that the decrease in field strength or shifting in the north pole’s position would mean a reversal in polarity in the coming 2000 years. Volcanic records are the best type of records of a particular place and time. 

Journal Reference: Science Advances journal

Ultra-thin Layers of Rust Generate Electricity from Flowing Water

Ultra-thin Layers of Rust Generate Electricity from Flowing Water

There are many ways to generate electricity—batteries, solar panels, wind turbines, and hydroelectric dams, to name a few examples. …. And now there’s rust.

New research conducted by scientists at Caltech and Northwestern University shows that thin films of rust—iron oxide—can generate electricity when saltwater flows over them. These films represent an entirely new way of generating electricity and could be used to develop new forms of sustainable power production.

Interactions between metal compounds and saltwater often generate electricity, but this is usually the result of a chemical reaction in which one or more compounds are converted to new compounds. Reactions like these are what is at work inside batteries.

In contrast, the phenomenon discovered by Tom Miller, Caltech professor of chemistry, and Franz Geiger, Dow Professor of Chemistry at Northwestern, does not involve chemical reactions, but rather converts the kinetic energy of flowing saltwater into electricity.

The phenomenon, the electrokinetic effect, has been observed before in thin films of graphene—sheets of carbon atoms arranged in a hexagonal lattice—and it is remarkably efficient. The effect is around 30 percent efficient at converting kinetic energy into electricity. For reference, the best solar panels are only about 20 percent efficient.

“A similar effect has been seen in some other materials. You can take a drop of saltwater and drag it across graphene and see some electricity generated,” Miller says.

However, it is difficult to fabricate graphene films and scale them up to usable sizes. The iron oxide films discovered by Miller and Geiger are relatively easy to produce and scalable to larger sizes, Miller says.

“It’s basically just rust on iron, so it’s pretty easy to make in large areas,” Miller says. “This is a more robust implementation of the thing seen in graphene.”

Though rust will form on iron alloys on its own, the team needed to ensure it formed in a consistently thin layer. To do that, they used a process called physical vapor deposition (PVD), which turns normally solid materials, in this case iron oxide, into a vapor that condenses on a desired surface. PVD allowed them to create an iron oxide layer 10 nanometers thick, about 10 thousand times thinner than a human hair.

When they took that rust-coated iron and flowed saltwater solutions of varying concentrations over it, they found that it generated several tens of millivolts and several microamps per cm-2.

“For perspective, plates having an area of 10 square meters each would generate a few kilowatts per hour—enough for a standard US home,” Miller says. “Of course, less demanding applications, including low-power devices in remote locations, are more promising in the near term.”

The mechanism behind the electricity generation is complex, involving ion adsorption and desorption, but it essentially works like this: The ions present in saltwater attract electrons in the iron beneath the layer of rust. As the saltwater flows, so do those ions, and through that attractive force, they drag the electrons in the iron along with them, generating an electrical current.

Miller says this effect could be useful in specific scenarios where there are moving saline solutions, like in the ocean or the human body.

“For example, tidal energy, or things bobbing in the ocean, like buoys, could be used for passive electrical energy conversion,” he says. “You have saltwater flowing in your veins in periodic pulses. That could be used to generate electricity for powering implants.”

The paper describing their findings, titled “Energy Conversion via Metal Nanolayers,” appears in the July 29 issue of the Proceedings of the National Academy of Sciences. Other co-authors include Mavis D. Boamah, Emilie H. Lozier, Paul E. Ohno, and Catherine E. Walker of Northwestern, and Jeongmin Kim, a graduate student in chemistry at Caltech.

Materials provided by the California Institute of Technology

‘Deforming’ solar cells could be clue to improved efficiency

‘Deforming’ solar cells could be clue to improved efficiency

  • Deformations and defects in structures of photoelectric technologies shown to improve their efficiency
  • University of Warwick physicists demonstrate that strain gradient can prevent recombination of photo-excited carriers in solar energy conversion
  • Increasingly important as devices become miniaturised

Solar panelsSolar cells and light-sensing technologies could be made more efficient by taking advantage of an unusual property due to deformations and defects in their structures.

Researchers from the University of Warwick’s Department of Physics have found that the strain gradient (i.e. inhomogeneous strain) in the solar cells, through physical force or induced during the fabrication process, can prevent photo-excited carriers from recombining, leading to an enhanced solar energy conversion efficiency. The results of their experiments have been published in Nature Communications.

The team of scientists used an epitaxial thin film of BiFeO3 grown on LaAlO3 substrate to determine the impact of inhomogenous deformation on the film’s ability to convert light into electricity by examining how its strain gradient affects its ability to separate photo-excited carriers.

Most commercial solar cells are formed of two layers creating at their boundary a junction between two kinds of semiconductors, p-type with positive charge carriers (electron vacancies) and n-type with negative charge carriers (electrons). When light is absorbed, the junction of the two semiconductors sustains an internal field splitting the photo-excited carriers in opposite directions, generating a current and voltage across the junction. Without such junctions the energy cannot be harvested and the photo-excited carriers will simply quickly recombine eliminating any electrical charge.

They found that the strain gradient can help prevent recombination by separating the light-excited electron-holes, enhancing the conversion efficiency of the solar cells. The BiFeO3/LaAlO3 film also exhibited some interesting photoelectric effects, such as persistent photoconductivity (improved electrical conductivity). It has potential applications in UV light sensors, actuators and transducers.

Dr Mingmin Yang from the University of Warwick said: “This work demonstrated the critical role of the strain gradient in mediating local photoelectric properties, which is largely overlooked previously. By engineering photoelectric technologies to take advantage of strain gradient, we may potentially increase the conversion efficiency of solar cells and enhance the sensitivity of light sensors.

“Another factor to consider is the grain boundaries in polycrystalline solar cells. Generally, defects accumulate at the grain boundaries, which would induce photo-carrier recombination, limiting efficiency. However, in some polycrystalline solar cells, such as CdTe solar cells, the grain boundaries would promote the collection of photo-carriers, where the giant strain gradient might play an important role. Therefore, we need to pay attention to the local strain gradient when we study the structure-properties relations in solar cells and light sensor materials.”

Previously, the effect of this strain on efficiency was thought to be negligible. With the increasing miniaturisation of technologies, the effect of strain gradient becomes magnified at smaller sizes. So in reducing the size of a device using one of these films, the magnitude of strain gradient increases dramatically.

Dr Yang adds: “The strain gradient induced effect, such as flexo-photovoltaic effect, ionic migration, etc, would be increasingly important at low dimensions.”

Materials provided by University of Warwick

Drone pro dji inspire

Researchers develop new photovoltaic engine which can make drones fly for several days

Researchers at UC Berkeley have broken another record of photovoltaic efficiency, which could lead to the development of an ultra-light engine that could power drones for days. For the past 15 years, there was 23 percent efficiency in converting heat into electricity with the help of thermovoltaics, but more insight allowed the efficiency to raise to 29 percent. Researchers now aim to reach close to 50 percent efficiency in the near future by applying scientific concepts.  The report has been published in Proceedings of the National Academy of Sciences.

This could be a breakthrough for technologies that rely on heavy batteries for power.  Thermophotovoltaics are an alternate power source which could help power drones and other unmanned vehicles to operate continuously for days. It could also be used for powering deep space probes for several centuries and ultimately power a house with the help of a generator of an envelope’s size.

The thermophotovoltaics are compact and efficient for a wide range of application with as little as 100 watts to 100 megawatts, having the ability to deliver electricity to 36,000 homes.

Eli Yablonovitch, professor of electrical engineering and computer science (EECS) and corresponding author on the paper said that this project builds upon the work which found key to boosting the solar efficiency by not absorbing more photons but by emitting them by adding a mirror at the back of the photovoltaic cell which broke the efficiency record. The mirror creates a dense infrared luminescent photon gas with the solar cells which later adds voltage.

The team believes that the mirror could serve a double purpose as it first solves one of the biggest problems in thermophotovoltaics which is to exploit the thermal photons with too little energy to produce electricity. The mirror can be used to reflect the small photons to reheat the thermal source which creates another chance for the creation of high energy photon to create and generate electricity. They have achieved this record-breaking result simply by adding a gold mirror and now they are planning to add a dielectric layer above the gold which could improve the efficiency level to close to 36 percent as claimed by Luis M. Pazos Outόn, a postdoctoral researcher in EECS and also a lead author of the paper.

There was 36 percent result just by increasing reflectivity, making other tweaks to the cell using proven scientific techniques, we can reach our target of 50 percent efficiency said Zunaid Omair, a graduate student researcher in EECS. Before their project, the efficiency which was stalled at 23 percent was increased to 29 percent which is really a big deal.

Journal Reference: Proceedings of the National Academy of Sciences.

Krafla geothermal power station

Researchers develop technique to exploit geothermal energy in a sustainable manner

The way we fuel our power production has been significantly changed by the demand to limit emissions and ascent of renewables, from wind to solar to biomass. Those technologies are the world’s most appealing, energizing and emerging technologies which aim at producing energy. However, there is a massive, permanent and unused energy resource which is existing literally under our noses. We are referring to geothermal energy. Generation of geothermal energy is possible through the devices to make use of heat inside the Earth’s crust.

Researchers from Tokyo Tech have made major progress in understanding and advancement of sensitized thermal cells (STCs) which is a type of battery that can produce electric power at 100-degree Celsius or less. The study has been published in the Journal of Materials Chemistry A.

Before this, they have proposed the use of STCs as a new method for converting heat directly into electric power using dye-sensitized solar cells. They likewise replaced the dye with a semiconductor to enable the system to work using heat rather than light.

In the new cell, an electron transport layer (ETM), a semiconductor layer (germanium), and a solid electrolyte layer (copper ions) are sandwiched between the electrodes of the battery where electrons get thermally excited while going from a low-energy state to a high-energy state in the semiconductor and finally shifting naturally to the ETM.

Oxidation and reduction reaction involving copper ions take place at the interconnection of both electrolytes while electrons travel from the electrode through an external circuit, pass through the counter electrode, and then reach the electrolyte. This completes an electric circuit shifting low-energy electrons to the semiconductor layer. Scientists during the experiment found out that after a certain time, the electricity stopped flowing instead of working as a perpetual machine. This is due to the completion of redox reactions at the electrolyte end owing to the shifting of different types of copper ions.

Existence of heat simply opens up the external circuit for a short time reverting the situation. Dr Sachiko Matsushita, study leader said that heat which is considered as low-grade energy, would become a great renewable energy source with such design. Scientists are excited about the model as it is nature-friendly and has the possibility to solve the global energy crisis.

Moreover, he added that there is no fear of costly oil, radiation or instability of power generation when done with the help of sun or wind. The goal of future research will be the enhancement of battery with the belief of solving mankind energy needs without harming the earth.

Journal Reference: Journal of Materials Chemistry A

Whitelee wind farm

Scotland is generating enough wind energy to power two Scotlands

Motivated from numerous renewable energy records and landmarks achieved, another milestone has been achieved in the first half of 2019. The country Scotland has produced enough energy from wind power which can power its homes twice.

For a nation which is home to 2.6 million people, producing 9.8 million Megawatt-hours of electricity by using turbines in the time period between January and June which is the adequate power supply to 4.47 million homes is a notable achievement. The record high wind energy is capable of providing enough electricity for every home in Scotland and also most part of Northern England for the first six months of the year. The month with the highest production was March with 2,194,981 Megawatt-hours (MWh) of output.

Robin Parker, the Climate & Energy Policy Manager at the World Wide Fund for Nature (WWF) said that everyone is benefitting from cleaner energy and atmosphere throughout the country and also seeing those incredible statistics, Scotland’s wind energy revolution seems to boost up. Moreover, he added that the data shows that utilising Scotland’s abundant coastal wind potential could supply eco-friendly electricity for millions of houses in both Scotland as well as England.

In the field of renewable energy, the United Kingdom (UK) has just achieved its longest span without depending on coal energy since the Industrial Revolution of the 19th century. According to the National Grid in the UK, for seven days and a total of 167 consecutive hours in a row, coal power stations didn’t supply them energy in which gas turbines, nuclear power, solar energy and other renewables played a part.

Scotland could play a big role in the UK government’s aim of producing energy without coal completely by 2025.

With huge funding in the wind and solar, Germany and other nations are benefiting with increased demand and the possibility of renewable energy production. Qinghai Province in northwest China which is home to five million has been operating for weeks on renewable sources like solar, wind and hydropower.

With the increased ability and more productive technology, scientists discover the ways to produce more electricity with existing solar or wind. With coastal farms at a potential of 8,423 Mega-Watt (MW) as of December 2018, Scotland is a groundbreaker in terms of wind power and hopefully, within next 12 months, they will supply all of its energy from renewables. Alex Wilcox Brooke, Weather Energy Project Manager at Severn Wye Energy Agency said that those statistics actually highlight the compatibility of wind energy in Scotland and its significance in the UK energy market.

Experiments show dramatic increase in solar cell output

Experiments show dramatic increase in solar cell output

In any conventional silicon-based solar cell, there is an absolute limit on overall efficiency, based partly on the fact that each photon of light can only knock loose a single electron, even if that photon carried twice the energy needed to do so. But now, researchers have demonstrated a method for getting high-energy photons striking silicon to kick out two electrons instead of one, opening the door for a new kind of solar cell with greater efficiency than was thought possible.

While conventional silicon cells have an absolute theoretical maximum efficiency of about 29.1 percent conversion of solar energy, the new approach, developed over the last several years by researchers at MIT and elsewhere, could bust through that limit, potentially adding several percentage points to that maximum output. The results are described today in the journal Nature, in a paper by graduate student Markus Einzinger, professor of chemistry Moungi Bawendi, professor of electrical engineering and computer science Marc Baldo, and eight others at MIT and at Princeton University.

The basic concept behind this new technology has been known for decades, and the first demonstration that the principle could work was carried out by some members of this team six years ago. But actually translating the method into a full, operational silicon solar cell took years of hard work, Baldo says.

That initial demonstration “was a good test platform” to show that the idea could work, explains Daniel Congreve PhD ’15, an alumnus now at the Rowland Institute at Harvard, who was the lead author in that prior report and is a co-author of the new paper. Now, with the new results, “we’ve done what we set out to do” in that project, he says.

The original study demonstrated the production of two electrons from one photon, but it did so in an organic photovoltaic cell, which is less efficient than a silicon solar cell. It turned out that transferring the two electrons from a top collecting layer made of tetracene into the silicon cell “was not straightforward,” Baldo says. Troy Van Voorhis, a professor of chemistry at MIT who was part of that original team, points out that the concept was first proposed back in the 1970s, and says wryly that turning that idea into a practical device “only took 40 years.”

The key to splitting the energy of one photon into two electrons lies in a class of materials that possess “excited states” called excitons, Baldo says: In these excitonic materials, “these packets of energy propagate around like the electrons in a circuit,” but with quite different properties than electrons. “You can use them to change energy — you can cut them in half, you can combine them.” In this case, they were going through a process called singlet exciton fission, which is how the light’s energy gets split into two separate, independently moving packets of energy. The material first absorbs a photon, forming an exciton that rapidly undergoes fission into two excited states, each with half the energy of the original state.

But the tricky part was then coupling that energy over into the silicon, a material that is not excitonic. This coupling had never been accomplished before.

As an intermediate step, the team tried coupling the energy from the excitonic layer into a material called quantum dots. “They’re still excitonic, but they’re inorganic,” Baldo says. “That worked; it worked like a charm,” he says. By understanding the mechanism taking place in that material, he says, “we had no reason to think that silicon wouldn’t work.”

What that work showed, Van Voorhis says, is that the key to these energy transfers lies in the very surface of the material, not in its bulk. “So it was clear that the surface chemistry on silicon was going to be important. That was what was going to determine what kinds of surface states there were.” That focus on the surface chemistry may have been what allowed this team to succeed where others had not, he suggests.

The key was in a thin intermediate layer. “It turns out this tiny, tiny strip of material at the interface between these two systems [the silicon solar cell and the tetracene layer with its excitonic properties] ended up defining everything. It’s why other researchers couldn’t get this process to work, and why we finally did.” It was Einzinger “who finally cracked that nut,” he says, by using a layer of a material called hafnium oxynitride.

The layer is only a few atoms thick, or just 8 angstroms (ten-billionths of a meter), but it acted as a “nice bridge” for the excited states, Baldo says. That finally made it possible for the single high-energy photons to trigger the release of two electrons inside the silicon cell. That produces a doubling of the amount of energy produced by a given amount of sunlight in the blue and green part of the spectrum. Overall, that could produce an increase in the power produced by the solar cell — from a theoretical maximum of 29.1 percent, up to a maximum of about 35 percent.

Actual silicon cells are not yet at their maximum, and neither is the new material, so more development needs to be done, but the crucial step of coupling the two materials efficiently has now been proven. “We still need to optimize the silicon cells for this process,” Baldo says. For one thing, with the new system those cells can be thinner than current versions. Work also needs to be done on stabilizing the materials for durability. Overall, commercial applications are probably still a few years off, the team says.

Other approaches to improving the efficiency of solar cells tend to involve adding another kind of cell, such as a perovskite layer, over the silicon. Baldo says “they’re building one cell on top of another. Fundamentally, we’re making one cell — we’re kind of turbocharging the silicon cell. We’re adding more current into the silicon, as opposed to making two cells.”

The researchers have measured one special property of hafnium oxynitride that helps it transfer the excitonic energy. “We know that hafnium oxynitride generates additional charge at the interface, which reduces losses by a process called electric field passivation. If we can establish better control over this phenomenon, efficiencies may climb even higher.” Einzinger says. So far, no other material they’ve tested can match its properties.

Materials provided by Massachusetts Institute of Technology