Invisibility cloak may enhance efficiency of solar cells

A special invisibility cloak redirects sunlight past solar-cell contacts to the active surface area of the solar cell (credit: Martin Schumann/KIT)

A new approach to increasing solar-cell panel efficiency using an “invisibility cloak” has been developed by scientists at Karlsruhe Institute of Technology (KIT) in Germany.

Up to one tenth of the surface area of solar cells is typically covered by “contact fingers” that extract current generated by solar cells. The fingers block some of the light from the active area of the solar cell, decreasing cell efficiency. By guiding the incident light around the contact fingers, the cloak layer makes the contact fingers nearly completely invisible, according to doctoral student Martin Schumann of the KIT Institute of Applied Physics, who conducted the experiments and simulations.

Coordinate transformations enabling invisible contacts on solar cells. The elongated metal contact can be arbitrarily shaped within the black region to make it invisible. (credit: Martin F. Schumann et al./Optica)

To achieve the cloaking effect, the scientists applied a polymer coating onto the solar cell and added a groove along the contact fingers, both helping to refract incident light away from the contact fingers and toward the active surface area of the solar cell. They expect an efficiency increase of 10 percent in followup tests.

The research was published Sept. 25 in an open-access article in the journal Optica.


Abstract of Cloaked contact grids on solar cells by coordinate transformations: designs and prototypes

Nontransparent contact fingers on the sun-facing side of solar cells represent optically dead regions which reduce the energy conversion per area. We consider two approaches for guiding the incident light around the contacts onto the active area. The first approach uses graded-index metamaterials designed by two-dimensional Schwarz–Christoffel conformal maps, and the second uses freeform surfaces designed by one-dimensional coordinate transformations of a point to an interval. We provide proof-of-principle demonstrators using direct laser writing of polymer structures on silicon wafers with opaque contacts. Freeform surfaces are amenable to mass fabrication and allow for complete recovery of the shadowing effect for all relevant incidence angles.

First optical ‘rectenna’ converts light to DC current

This schematic shows the components of the optical rectenna developed at the Georgia Institute of Technology (credit: Thomas Bougher, Georgia Tech)

Using nanometer-scale components, Georgia Institute of Technology researchers have demonstrated the first optical rectenna, a device that combines the functions of an antenna and a rectifier diode to convert light directly into DC current.

Based on multiwall carbon nanotubes and tiny rectifiers fabricated onto them, the optical rectennas could provide a new technology for energy harvesters, including photodetectors that would operate without the need for cooling, convert waste heat to electricity,  and ultimately, efficiently capture solar energy.

In the new devices, the carbon nanotubes act as antennas to capture light from the Sun or other sources. As the waves of light hit the nanotube antennas, they create an oscillating charge that moves through rectifier devices attached to them. The rectifiers switch on and off at record high petahertz speeds, creating a small direct current.

The efficiency of the devices demonstrated so far remains below one percent, but the researchers hope to boost that output by using billions of rectennas in an array, which could produce significant current. They believe a rectenna with commercial potential may be available within a year.

“We could ultimately make solar cells that are twice as efficient at a cost that is ten times lower, and that is to me an opportunity to change the world in a very big way” said Baratunde Cola, an associate professor in the George W. Woodruff School of Mechanical Engineering at Georgia Tech. “As a robust, high-temperature detector, these rectennas could be a completely disruptive technology if we can get to one percent efficiency. If we can get to higher efficiencies, we could apply it to energy conversion technologies and solar energy capture.”

A carbon nanotube optical rectenna converts green laser light to electricity in the laboratory of Baratunde Cola at the Georgia Institute of Technology (credit: Rob Felt, Georgia Tech)

“A rectenna is basically an antenna coupled to a diode, but when you move into the optical spectrum, that usually means a nanoscale antenna coupled to a metal-insulator-metal diode,” Cola explained. “The closer you can get the antenna to the diode, the more efficient it is. So the ideal structure uses the antenna as one of the metals in the diode – which is the structure we made.”

The rectennas fabricated by Cola’s group are grown on rigid substrates, but the goal is to grow them on a foil or other material that would produce flexible solar cells or photodetectors.

Cola sees the rectennas built so far as simple proof of principle. “We think we can reduce the resistance by several orders of magnitude just by improving the fabrication of our device structures,” he said. “Based on what others have done and what the theory is showing us, I believe that these devices could get to greater than 40 percent efficiency.”

The research, supported by the Defense Advanced Research Projects Agency (DARPA), the Space and Naval Warfare (SPAWAR) Systems Center and the Army Research Office (ARO), is reported September 28 in the journal Nature Nanotechnology.

* Developed in the 1960s and 1970s, rectennas have operated at wavelengths as short as ten micrometers, but for more than 40 years researchers have been attempting to make devices at optical wavelengths. There were many challenges: making the antennas small enough to couple optical wavelengths, and fabricating a matching rectifier diode small enough and able to operate fast enough to capture the electromagnetic wave oscillations. But the potential of high efficiency and low cost kept scientists working on the technology.

Fabricating the rectennas begins with growing forests of vertically-aligned carbon nanotubes on a conductive substrate. Using atomic layer chemical vapor deposition, the nanotubes are coated with an aluminum oxide material to insulate them. Finally, physical vapor deposition is used to deposit optically-transparent thin layers of calcium then aluminum metals atop the nanotube forest. The difference of work functions between the nanotubes and the calcium provides a potential of about two electron volts, enough to drive electrons out of the carbon nanotube antennas when they are excited by light.

In operation, oscillating waves of light pass through the transparent calcium-aluminum electrode and interact with the nanotubes. The metal-insulator-metal junctions at the nanotube tips serve as rectifiers switching on and off at femtosecond intervals, allowing electrons generated by the antenna to flow one way into the top electrode. Ultra-low capacitance, on the order of a few attofarads, enables the 10-nanometer diameter diode to operate at these exceptional frequencies.


Georgia Tech | Using nanometer-scale components, researchers have demonstrated the first optical rectenna, a device that combines the functions of an antenna and a rectifier diode to convert light directly into DC current.


Abstract of A carbon nanotube optical rectenna

An optical rectenna—a device that directly converts free-propagating electromagnetic waves at optical frequencies to direct current—was first proposed over 40 years ago, yet this concept has not been demonstrated experimentally due to fabrication challenges at the nanoscale. Realizing an optical rectenna requires that an antenna be coupled to a diode that operates on the order of 1 pHz (switching speed on the order of 1 fs). Diodes operating at these frequencies are feasible if their capacitance is on the order of a few attofarads, but they remain extremely difficult to fabricate and to reliably couple to a nanoscale antenna. Here we demonstrate an optical rectenna by engineering metal–insulator–metal tunnel diodes, with a junction capacitance of ∼2 aF, at the tip of vertically aligned multiwalled carbon nanotubes (∼10 nm in diameter), which act as the antenna. Upon irradiation with visible and infrared light, we measure a d.c. open-circuit voltage and a short-circuit current that appear to be due to a rectification process (we account for a very small but quantifiable contribution from thermal effects). In contrast to recent reports of photodetection based on hot electron decay in a plasmonic nanoscale antenna, a coherent optical antenna field appears to be rectified directly in our devices, consistent with rectenna theory. Finally, power rectification is observed under simulated solar illumination, and there is no detectable change in diode performance after numerous current–voltage scans between 5 and 77 °C, indicating a potential for robust operation.

Transparent photonic coating cools solar cells to boost efficiency

Stanford engineers have invented a transparent material that improves the efficiency of solar cells by radiating thermal energy (heat) into space (credit: Stanford Engineering)

Stanford engineers have developed a transparent material that improves the efficiency of solar cells by radiating thermal energy (heat) into space, even in full sunlight.

The invention may solve a longstanding problem for the solar industry: the hotter solar cells become, the less efficient they are at converting sunlight to electricity. The Stanford solution is based on a thin, patterned silica material laid on top of a traditional solar cell. The material is transparent to the visible sunlight that powers solar cells, but captures and emits thermal radiation, or heat, cooling the solar cell and thus allowing it to convert more photons into electricity.

The work by Shanhui Fan, a professor of electrical engineering at Stanford, research associate Aaswath P. Raman and doctoral candidate Linxiao Zhu is described in the current issue of Proceedings of the National Academy of Sciences.

The same inventors previously developed an ultrathin material, covered on KurzweilAI, that radiated infrared light (in the thermal “long” infrared atmospheric transparency window of ~7 to 30 microns) from the Sun directly back toward space without warming the atmosphere (thus avoiding the greenhouse effect). They presented that work in Nature, describing it as “radiative cooling” because it shunted thermal energy directly into the deep, cold void of space. It was specifically intended for cooling buildings.

A silica photonic crystal radiator

In their new paper, the researchers applied that work to improve solar array performance.

Silica photonic crystal radiates far-infrared light (heat) into space (credit: Linxiao Zhu et al./PNAS)

The Stanford team tested their technology on a custom-made solar absorber — a device that mimics the properties of a solar cell without producing electricity — covered with a silica photonic crystal (a micrometer-scale pattern) designed to maximize the capability to dump heat, in the form of infrared light (in 8 to 30 microns range) into space. Their experiments showed that the overlay allowed visible light to pass through to the solar cells, but that the pattern also cooled the underlying absorber by as much as 23 degrees Fahrenheit.

For a typical crystalline silicon solar cell with an efficiency of 20 percent, 23 F of cooling would improve absolute cell efficiency by more than 1 percent, a figure that represents a significant gain in energy production.

The researchers said the new transparent thermal overlays work best in dry, clear environments, which are also preferred sites for large solar arrays. They believe they can scale things up so that commercial and industrial applications are feasible, perhaps using nanoprint lithography, which is a common technique for producing nanometer-scale patterns.

Cooler cars

Zhu said the technology has significant potential for any outdoor device or system that demands cooling but requires the preservation of the visible spectrum of sunlight for either practical or aesthetic reasons.

“Say you have a car that is bright red,” Zhu said. “You really like that color, but you’d also like to take advantage of anything that could aid in cooling your vehicle during hot days. Thermal overlays can help with passive cooling, but it’s a problem if they’re not fully transparent.” That’s because the perception of color requires objects to reflect visible light, so any overlay would need to be transparent, or else tuned such that it would absorb only light outside the visible spectrum.

“Our photonic crystal thermal overlay optimizes use of the thermal portions of the electromagnetic spectrum without affecting visible light,” Zhu said, “so you can radiate heat efficiently without affecting color.”


Abstract of Radiative cooling of solar absorbers using a visibly transparent photonic crystal thermal blackbody

A solar absorber, under the sun, is heated up by sunlight. In many applications, including solar cells and outdoor structures, the absorption of sunlight is intrinsic for either operational or aesthetic considerations, but the resulting heating is undesirable. Because a solar absorber by necessity faces the sky, it also naturally has radiative access to the coldness of the universe. Therefore, in these applications it would be very attractive to directly use the sky as a heat sink while preserving solar absorption properties. Here we experimentally demonstrate a visibly transparent thermal blackbody, based on a silica photonic crystal. When placed on a silicon absorber under sunlight, such a blackbody preserves or even slightly enhances sunlight absorption, but reduces the temperature of the underlying silicon absorber by as much as 13°C due to radiative cooling. Our work shows that the concept of radiative cooling can be used in combination with the utilization of sunlight, enabling new technological capabilities.

Lightweight solar cells track the sun, providing 40 percent more energy than fixed cells

By borrowing from kirigami, the ancient Japanese art of paper cutting, researchers at the University of Michigan have developed solar cells that can track the sun. A flat plastic sheet backing the solar cells splits into wavy, connected ribbons when stretched. The tilt of each of the cells depends on the stretching, a simple mechanism for tracking the sun across the sky. (credit: Aaron Lamoureux)

University of Michigan engineers have developed an innovative array of solar cells that can capture up to 40 percent more energy than conventional fixed solar cells. The trick: borrowing from kirigami (the ancient Japanese art of paper cutting), the solar cells are aimed at different angles, allowing for part of the array to be always perpendicular to the Sun’s rays.

“The design takes what a large tracking solar panel does and condenses it into something that is essentially flat,” said Aaron Lamoureux, a doctoral student in materials science and engineering and first author on the open-access paper in Nature Communications. Residential rooftops would need significant reinforcing to support the weight of conventional costly sun-tracking systems, he said.

Tilting toward the Sun

Cuts in plastic substrate allow for an array of solar cells titled at different angles (credit: University of Michigan)

To explore patterns for the array, the engineers worked with paper artist Matthew Shlian, a lecturer in the U-M School of Art and Design, who showed them how to create the solar array in paper using a plotter cutter. Lamoureux then made more precise patterns in Kapton, a space-grade plastic, using a carbon-dioxide laser.

Although the team tried more complex designs, the simplest pattern worked best. With cuts like rows of dashes, the plastic pulled apart into a basic mesh. The interconnected strips of Kapton tilt at different angles in proportion to how much the mesh is stretched, to an accuracy of about one degree.

The design with the very best solar-tracking promise was impossible to make at U-M because the solar cells would be very long and narrow. Scaling up to a feasible width, the cells became too long to fit into the chambers used to make the prototypes on campus, so the team is looking into other options.

“We think it has significant potential, and we’re actively pursuing realistic applications,” said Max Shtein, associate professor of materials science and engineering. “It could ultimately reduce the cost of solar electricity.”

The study was funded by National Science Foundation and NanoFlex Power Corporation. The university is pursuing patent protection for the intellectual property, and is seeking commercialization partners to help bring the technology to market.

Michigan Engineering | Kirigami for sun-tracking solar cells


Abstract of Dynamic kirigami structures for integrated solar tracking

Optical tracking is often combined with conventional flat panel solar cells to maximize electrical power generation over the course of a day. However, conventional trackers are complex and often require costly and cumbersome structural components to support system weight. Here we use kirigami (the art of paper cutting) to realize novel solar cells where tracking is integral to the structure at the substrate level. Specifically, an elegant cut pattern is made in thin-film gallium arsenide solar cells, which are then stretched to produce an array of tilted surface elements which can be controlled to within ±1°. We analyze the combined optical and mechanical properties of the tracking system, and demonstrate a mechanically robust system with optical tracking efficiencies matching conventional trackers. This design suggests a pathway towards enabling new applications for solar tracking, as well as inspiring a broader range of optoelectronic and mechanical devices.

A high-efficiency, sustainable process using solar and carbon dioxide to produce methane for natural gas

Artificial photosynthesis provides electrical current to produce hydrogen gas from water; the hydrogen then synthesizes carbon dioxide (via microbes) into methane (CH4) (credit: Berkeley Lab)

A team of researchers at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) has developed a hybrid system that produces hydrogen and uses it (via microbes) to synthesize carbon dioxide into methane, the primary constituent of natural gas.

“We can expect an electrical-to-chemical efficiency of better than 50 percent and a solar-to-chemical energy conversion efficiency of 10 percent if our system is coupled with state-of-art solar panel and electrolyzer,” says Peidong Yang, a chemist with Berkeley Lab’s Materials Sciences Division and one of the leaders of this study.

“Natural photosynthesis, a solar-to-chemical energy conversion process that combines light, water, and CO2 to make biomass, operates at less than 1% efficiency,” UC Berkeley prof. Chris Chang explained to KurzweilAI. “We have now done a order of magnitude better than nature in this artificial photosynthesis system, albeit in one prototype system where we make methane,” he said. “The advance is that most artificial photosynthesis systems only use light and water, and operate at lower efficiencies to boot. The ability to incorporate CO2 fixation is also a big advance.”

Yang, who also holds appointments with UC Berkeley and the Kavli Energy NanoScience Institute (Kavli-ENSI) at Berkeley, is one of three corresponding authors of a paper describing this research in the Proceedings of the National Academy of Sciences (PNAS). 

Sustainable, efficient

Solar energy, a sustainable source of energy, is used to generate hydrogen from water via the hydrogen evolution reaction (HER). The HER is catalyzed by earth-abundant nickel sulfide nanoparticles that operate effectively under biologically compatible conditions.

“Water is the most sustainable starting feedstock for hydrogen,” Chang said. In comparison, “most hydrogen now comes from hydrocarbons, which gives off CO2.”

“We selected methane as an initial target owing to the ease of product separation, the potential for integration into existing infrastructures for the delivery and use of natural gas, and the fact that direct conversion of carbon dioxide to methane with synthetic catalysts has proven to be a formidable challenge,” said Chang.

“Since we still get the majority of our methane from natural gas, a fossil fuel, often from fracking, the ability to generate methane from a renewable hydrogen source (solar) is another important advance.”


Abstract of Hybrid bioinorganic approach to solar-to-chemical conversion

Natural photosynthesis harnesses solar energy to convert CO2 and water to value-added chemical products for sustaining life. We present a hybrid bioinorganic approach to solar-to-chemical conversion in which sustainable electrical and/or solar input drives production of hydrogen from water splitting using biocompatible inorganic catalysts. The hydrogen is then used by living cells as a source of reducing equivalents for conversion of CO2 to the value-added chemical product methane. Using platinum or an earth-abundant substitute, α-NiS, as biocompatible hydrogen evolution reaction (HER) electrocatalysts and Methanosarcina barkeri as a biocatalyst for CO2 fixation, we demonstrate robust and efficient electrochemical CO2 to CH4 conversion at up to 86% overall Faradaic efficiency for ≥7 d. Introduction of indium phosphide photocathodes and titanium dioxide photoanodes affords a fully solar-driven system for methane generation from water and CO2, establishing that compatible inorganic and biological components can synergistically couple light-harvesting and catalytic functions for solar-to-chemical conversion.

‘Artificial leaf’ harnesses sunlight for efficient, safe hydrogen fuel production

Illustration of an efficient, robust and integrated solar-driven prototype featuring protected photoelectrochemical assembly coupled with oxygen and hydrogen evolution reaction catalysts (credit: Image, Joint Center for Artificial Photosynthesis; artwork, Darius Siwek)

The first complete, efficient, safe, integrated solar-driven system for splitting water to create hydrogen fuels has been developed by the Joint Center for Artificial Photosynthesis (JCAP) at Caltech, according to Caltech’s Nate Lewis, George L. Argyros Professor and professor of chemistry, and the JCAP scientific director.

The new solar fuel generation system, or “artificial leaf,” is described in the August 27 online issue of the journal Energy and Environmental Science. The work was done by researchers in the laboratories of Lewis and Harry Atwater, director of JCAP and Howard Hughes Professor of Applied Physics and Materials Science.

A highly efficient photoelectrochemical (PEC) device uses the power of the sun to split water into hydrogen and oxygen. The stand-alone prototype includes two chambers separated by a semi-permeable membrane that allows collection of both gas products. (credit: Lance Hayashida/Caltech)

The new system consists of three main components: two electrodes (photoanode and photocathode) and a membrane.

  • The photoanode uses sunlight to oxidize water molecules, generating protons and electrons as well as oxygen gas.
  • The photocathode recombines the protons and electrons to form hydrogen gas.
  • A plastic membrane keeps the oxygen and hydrogen gases separate. (If the two gases are allowed to mix and are accidentally ignited, an explosion can occur; the membrane lets the hydrogen fuel be separately collected under pressure and safely pushed into a pipeline.)

Preventing corrosion 

Semiconductors such as silicon or gallium arsenide absorb light efficiently and are therefore used in solar panels. However, these materials also oxidize (or rust) on the surface when exposed to water, so cannot be used to directly generate fuel. A major advance that allowed the integrated system to be developed was previous work in Lewis’s laboratory, which showed that adding a nanometers-thick layer of titanium dioxide (TiO2) onto the electrodes could prevent them from corroding while still allowing light and electrons to pass through.

The new complete solar fuel generation system developed by Lewis and colleagues uses such a 62.5-nanometer-thick TiO2 layer to effectively prevent corrosion and improve the stability of a gallium arsenide–based photoelectrode.

Inexpensive catalysts

Another key advance is the use of active, inexpensive catalysts for fuel production. The photoanode requires a catalyst to drive the essential water-splitting reaction. Rare and expensive metals such as platinum can serve as effective catalysts, but in its work the team discovered that it could create a much cheaper, active catalyst by adding a 2-nanometer-thick layer of nickel to the surface of the TiO2. This catalyst is among the most active known catalysts for splitting water molecules into oxygen, protons, and electrons and is a key to the high efficiency displayed by the device.

The photoanode was grown onto a photocathode, which also contains a highly active, inexpensive, nickel-molybdenum catalyst, to create a fully integrated single material that serves as a complete solar-driven water-splitting system.

The demonstration system is approximately one square centimeter in area, converts 10 percent of the energy in sunlight into stored energy in the chemical fuel, and can operate for more than 40 hours continuously.

“This new system shatters all of the combined safety, performance, and stability records for artificial leaf technology by factors of 5 to 10 or more,” Lewis says.

“Our work shows that it is indeed possible to produce fuels from sunlight safely and efficiently in an integrated system with inexpensive components,” Lewis adds, “Of course, we still have work to do to extend the lifetime of the system and to develop methods for cost-effectively manufacturing full systems, both of which are in progress.”


Caltech | Solar Fuels Prototype in Operation


Abstract of A monolithically integrated, intrinsically safe, 10% efficient, solar-driven water-splitting system based on active, stable earth-abundant electrocatalysts in conjunction with tandem III–V light absorbers protected by amorphous TiO2 films

A monolithically integrated device consisting of a tandem-junction GaAs/InGaP photoanode coated by an amorphous TiO2 stabilization layer, in conjunction with Ni-based, earth-abundant active electrocatalysts for the hydrogen-evolution and oxygen-evolution reactions, was used to effect unassisted, solar-driven water splitting in 1.0 M KOH(aq). When connected to a Ni–Mo-coated counterelectrode in a two-electrode cell configuration, the TiO2-protected III–V tandem device exhibited a solar-to-hydrogen conversion efficiency, ηSTH, of 10.5% under 1 sun illumination, with stable performance for >40 h of continuous operation at an efficiency of ηSTH > 10%. The protected tandem device also formed the basis for a monolithically integrated, intrinsically safe solar-hydrogen prototype system (1 cm2) driven by a NiMo/GaAs/InGaP/TiO2/Ni structure. The intrinsically safe system exhibited a hydrogen production rate of 0.81 μL s−1 and a solar-to-hydrogen conversion efficiency of 8.6% under 1 sun illumination in 1.0 M KOH(aq), with minimal product gas crossover while allowing for beneficial collection of separate streams of H2(g) and O2(g).

How to capture and convert CO2 from a smokestack in a single step

Using a novel catalyst, a single chemical assembly (UiO-66-P-BF2) could capture CO2 and also transform it and hydrogen into formic acid (HCOOH) via a two-step (yellow arrows) reaction (credit: Ye and Johnson/ACS Catalysis)

University of Pittsburgh researchers have invented (in computations) a cheap, efficient catalyst that would capture carbon dioxide (CO2) from coal-burning power plants before it reaches the atmosphere and converts the CO2  into formic acid — a valuable chemical that would create a revenue return. 

One current method for capturing CO2 uses Metal–organic frameworks (MOFs), which have a porous, cage-like structure that can absorb CO2, but require expensive catalysts, like platinum.

Instead, the researchers looked for lower-cost non-metallic catalysts, and found that a compound known as UiO-66-PBF2 would do the job.

The method is similar to one developed by Rice University using a combination of amine-rich compounds and carbon-60 molecules.

Those methods both differ from the “diamonds from the sky” approach, which turns CO2 from the air into carbon nanofibers, by capturing the CO2 before it leaves a smokestack. It will be interesting to compare these approaches in terms of CO2 capture efficiency, energy cost, and revenue return.

All three of these methods avoid the costs and energy expenditures from transporting and depositing CO2 at a storage site, required in carbon sequestration.


Abstract of Design of Lewis Pair-Functionalized Metal Organic Frameworks for CO2 Hydrogenation

Efficient catalytic reduction of CO2 is critical for the large-scale utilization of this greenhouse gas. We have used density functional electronic structure methods to design a catalyst for producing formic acid from CO2 and H2 via a two-step pathway having low reaction barriers. The catalyst consists of a microporous metal organic framework that is functionalized with Lewis pair moieties. These functional groups are capable of chemically binding CO2 and heterolytically dissociating H2. Our calculations indicate that the porous framework remains stable after functionalization and chemisorption of CO2 and H2. We have identified a low barrier pathway for simultaneous addition of hydridic and protic hydrogens to carbon and oxygen of CO2, respectively, producing a physisorbed HCOOH product in the pore. We find that activating H2 by dissociative adsorption leads to a much lower energy pathway for hydrogenating CO2 than reacting H2 with chemisorbed CO2. Our calculations provide design strategies for efficient catalysts for CO2 reduction.