For comparison external quantum efficiency for photocurrent is usually expressed as a percentage where the number of electrons flowing per second in the external circuit of a solar cell is divided by the number of photons per second of a specific energy (or wavelength) that enter the solar cell.  So far no other solar cells show external photocurrent quantum efficiencies above 100% at any wavelength from the solar spectrum.

The NREL team has reached an external quantum efficiency peak value of 114%.  At these efficiencies both solar electricity and solar fuels may be competitive with, or perhaps less costly than, energy from fossil or nuclear fuels.

The paper at Science Magazine entitled “Peak External Photocurrent Quantum Efficiency Exceeding 100 percent via MEG in a Quantum Dot Solar Cell,” is co-authored by NREL scientists Octavi E. Semonin, Joseph M. Luther, Sukgeun Choi, Hsiang-Yu Chen, Jianbo Gao, Arthur J. Nozikand Matthew C. Beard.

The mechanism for producing quantum efficiency above 100% with solar photons is based on a process called Multiple Exciton Generation (MEG).  In a MEG a single absorbed photon of appropriately high energy can produce more than one electron-hole pair in the solar cell.

The idea that would be so came from NREL scientist Arthur J. Nozik who first predicted in a 2001 publication that a MEG would be more efficient in semiconductor quantum dots than in bulk semiconductors.

On the technical side quantum dots are tiny crystals of semiconductor, with sizes in the nanometer (nm) range from 1-20 nm.  One nm equals one-billionth of a meter. At these dimensions semiconductors exhibit dramatic activity due to the quantum physics.

Most importantly quantum physics applied to the nano semiconductors forms correlated electron-hole pairs (called excitons) at room temperature, which enhances coupling of electronic particles (electrons and positive holes) through Coulombic forces.  As the quantum dots are reduced in size a rapidly increasing bandgap occurs.

The physics set up the quantum dots to confine the charges and harvest excess energy.

Quantum dots are still semiconductor crystals with tiny volumes.  When an electrical charge is confined in such a space the energy stays electric instead of becoming heat.  That’s how the high efficiency starts. Spilling out more than one electron per photon strike pushes the result even higher.

The NREL team achieved the 114% external quantum efficiency with a layered cell consisting of antireflection-coated glass with a thin layer of a transparent conductor, a nanostructured zinc oxide layer, a quantum dot layer of lead selenide treated with ethanedithol and hydrazine, and a thin layer of gold for the top electrode.  Other than the gold, the raw materials are low cost.  It is looking like fabrication of quantum dot solar cells may apply to inexpensive, high-throughput roll-to-roll manufacturing.

At this stage one has to ask is plus 100% even believable.  MEG was first demonstrated experimentally in colloidal solutions of quantum dots in 2004 by Richard Schaller and Victor Klimov of the DOE’s Los Alamos National Laboratory.  By 2006 NREL scientists Mark Hanna and Arthur J. Nozik showed that ideal MEG in solar cells based on quantum dots could increase the theoretical thermodynamic power conversion efficiency of solar cells by about 35 percent compared to the cells of the day.

Quantum Efficiency of Photon Energy Using NREL Quantum Dot Solar Collector. Click image for more info.

Meanwhile many researchers around the world, including teams at NREL, have confirmed MEG in many different semiconductor quantum dot designs.  But nearly all those experiments used ultrafast time-resolved spectroscopic measurements of isolated quantum dots dispersed as particles in liquid colloidal solutions.  No power out was measured.

The new NREL team result is a MEG built with an external photocurrent quantum yield greater than 100 percent.  The reporting on the study points out the cells showed significant power conversion efficiencies (defined as the total power generated divided by the input power) as high as 4.5 percent with simulated sunlight.

Still these new solar cells are un-optimized and thus exhibit relatively low power conversion efficiency, which is a product of the photocurrent and photovoltage.

This is still stage one.  The MEG demonstration has important implications because it opens new and unexplored approaches to improve solar cell efficiencies.

As well as being a milestone the new results confirm the previous time-resolved spectroscopic measurements of MEG and hence validate those earlier MEG results.  The confirmation improves when the external quantum efficiency is corrected for the number of photons that are actually absorbed in the photoactive regions of the cell.

In actual absorption the determined quantum yield is called the internal quantum efficiency. The internal quantum efficiency is greater than the external quantum efficiency because a significant fraction of the incoming photons are lost through reflection and absorption in non-photocurrent producing regions of the cell. A peak internal quantum yield of 130% was found taking these reflection and absorption losses into account.

That brings us to the questions.  If every incoming photon was to drive an exciton pair the potential efficiency would seem to be 200%, leaving a lot on the cell, so to speak.  Then the matter of a photon driving even more than just a pair comes to mind.  Follow this with the reality, the NREL team has made the measurement from lab built first successful experimental materials leaving a full range of innovation to come – where those levels could get to isn’t offered, yet.

Lastly, the baseline and materials are geared towards the high-energy end of the solar radiation arriving at the surface.  What can be done to improve the range of the useful spectrum is another intriguing question.

The best perspective is today’s solar cell at 10 to 20% efficiency and marketed with some success now could have a 5 fold increase in the future if the science milestone of this week can scale to commercial production.  And the prospects can only get better with more research and engineering.

On a clear bright day you’ll notice the shadows are a dark area with little light.  But on a day with some overcast the shadows are not so dark and as the overcast intensifies, the shadows can seem to disappear.

One might think, the light is down so much the light and the shadows are equalized, but with some thought – that can’t be.  Instead the high clouds do cut back on the total light, but importantly, the light is scattered sending some in indirectly to light up the shadowed area.  The folks at MIT had an “AHA!” moment in a very big way.

The typical solar panel today, actually all of them, are as flat as pancakes and respond best to directly incoming light. Mounted up on a roof they are a paving of dark glass.  Users can increase the efficiency by pouring in an investment of moving parts called solar tracking that aim the flat sheets to match the sun and can get a very good result. It’s a very costly motorized computer controlled mechanism with two axes to handle to keep things lined up over a day and through the year. It’s a maintenance task of considerable magnitude and expense.

So the MIT team came up with a three-dimensional solar panel design called 3DPV (3 Dimensional Photo Voltaic).  It’s such a good idea the peaks in power generation for the 3DPV designs are better with some overcast light scattering.  The sum of the effects yields an increase in the daily energy generation of 3DPV in cloudy weather to more than a flat panel in clear weather.

The Marco Bernardi team at MIT simulated the performance of various shapes and tested several of these on the roof of a building at MIT. Their results indicate that the 3DPV panel structures can increase the amount of energy that can be generated in a given area by as much as 20 times. The structures can also double the number of useful peak hours of generation and reduce the seasonal variation as well.

There is likely a bunch of pending orders being postponed today.

The new mounting structure is simply a cube, open at the top and covered inside and out with photovoltaic cells, like a box with a panel on both sides of the four sides and one in the bottom. That arrangement can generate as much 3.8 times the power of a flat panel with the same footprint. By comparison, a solar tracking mount, control system and activating machinery produces an increase of only up to 1.8 times.

For buyers it’s the money.  While a cube has 9 times the surface area for panels, the panels are cheap.  It’s the installation, energy connections and current conversion that are still the big dollars.  For most buyers the area to work with is the limitation.  Now the calculation has changed – nearly 4 times the energy for the same area with nearly the same cost for the installation, energy connections and conversion.

This whole thing has to be rethought from the engineering of panels to maximize output from the 3D perspective and drive down the watt-hour cost at the panel output on to the buyers rethinking the investment and the payback with a much-accelerated amortization.

Bernardi suggests the 3DPV panel structures could be shipped as flat packages that easily “pop up” into 3D structures when assembled.

MIT has made the entire pdf file of the published report available to download. Its fair to say every panel builder worldwide will be studying the MIT result intensely today. There’s enough information there to replicate and take the engineering concept much further.  New product choices can’t be far off.

With silicon and printed panels getting very cheap, with a small if any increase of investment for the installation and power management the industry could get a big boost this year.

Please note the Bernardi group has done the math to also show there is an increased range for photovoltaic cells as well.

Also this is just round one.  The idea is a revolutionary start on a new photovoltaic future.  Others are going to take this further and the consumer choices are going to be much more attractive.  The calculations may have a given area, a given level of investment, a required payback period or other baseline thresholds for starting the process to adopt the technology.

What the MIT team has done is shift those thresholds much closer to many more people.

The paper is well worth the time for reading and consideration.  More energy by area, beating out sun tracking, increasing by double the number of useful peak hours and reducing the seasonal and latitude variation of solar energy generation, with even higher productivity in case of cloudy weather all adds up a Major Breakthrough.

Zhu and his team have discovered that it’s possible to double the number of electrons harvested from one photon of sunlight using an organic plastic semiconductor material.  The team published their discovery Dec. 16 in Science.

Zhu explains saying, “Plastic semiconductor solar cell production has great advantages, one of which is low cost. Combined with the vast capabilities for molecular design and synthesis, our discovery opens the door to an exciting new approach for solar energy conversion, leading to much higher efficiencies.”

As the technology sits today the maximum theoretical efficiency of the silicon solar cell in use today is holding at approximately 31 percent.  The rest of the sun’s energy hitting the cell is too high to be turned into usable electricity. That energy, in the form of “hot electrons,” is instead lost as heat. Harvesting the hot electrons instead of radiating of the heat could potentially increase the efficiency of solar-to-electric power conversion to as high as 66 percent.

About a year and a half ago Zhu and the team first came into the high efficiency through semiconductor nanocrystals, or quantum dots that are suggested for the cooling of hot electrons and can slow down the electrons. In a 2008 paper in Science, a research group from the University of Chicago showed this to be true unambiguously for colloidal semiconductor nanocrystals.

This Illustration of Solar Driving Quantum Dot Energy From the 2010 Technology.

Zhu’s group discovered that hot electrons could be transferred from photo-excited lead selenide nanocrystals to an electron conductor made of widely used titanium dioxide.

Back in 2010 Zhu knew, “The demonstration of this hot electron transfer establishes that a highly efficient hot carrier solar cell is not just a theoretical concept, but an experimental possibility,” and that their methods will work for quantum dots made of other materials, too.

Getting to 66% isn’t a done deal as Zhu explains, “For one thing, that 66 % efficiency can only be achieved when highly focused sunlight is used, not just the raw sunlight that typically hits a solar panel. This creates problems when considering engineering a new material or device.”

To get to the theoretical goal, Zhu and his team have found an alternative. They discovered that a photon produces a dark quantum “shadow state” from which two electrons can then be efficiently captured to generate more energy in the semiconductor pentacene.

Working out the numbers on the pentacene mechanism shows an increase of solar cell efficiency to 44 percent without the need for focusing a solar beam, which would encourage more widespread use of solar technology.

That takes the efficiency more than a third of the way to 66%.

What’s known is a photon striking a pentacene semiconductor creates an excited electron-hole pair called an exciton.  The exciton is coupled quantum mechanically to a dark “shadow state” called a multiexciton.  This dark shadow state can be the most efficient source of two electrons via transfer to an electron acceptor material, such as fullerene, which Zhu’s group used in the study.  Exploiting the dark shadow state to produce double the electrons should increase solar cell efficiency to 44 percent.

Zhu’s team was led by Wai-lun Chan, a postdoctoral fellow in Zhu’s group, with the help of postdoctoral fellows Manuel Ligges, Askat Jailaubekov, Loren Kaake and Luis Miaja-Avila.

The research shows there is still a huge jump to come in solar photovoltaic.  Just how it will come to market isn’t clear, but the physics and chemistry are on their way to new defining potentials.  Sixty six percent was a nutty idea not long ago, and how long it stands is open for more innovation and research.

The paper published in Nature Communications entitled, ‘Broadband Polarization-Independent Resonant Light Absorption Using Ultrathin Plasmonic Super Absorbers’ says in the abstract, “Our super absorber yields broadband and polarization-independent resonant light absorption over the entire visible spectrum (400–700 nm) with an average measured absorption of 0.71 and simulated absorption of 0.85.”

That’s right – the claim is the entire visible spectrum leaving some infrared and ultraviolet yet to be gathered.

Aydin explains, “The solar spectrum is not like a laser – it’s very broadband, starting with UV and going up to near-infrared. To capture this light most efficiently, a solar cell needs to have a broadband response. This design allows us to achieve that.”

Solar cells are only as efficient as the amount of sunlight they collect.  Collect a small percent and the efficiency is a small percent.  Collect a larger percent and the efficiency rockets up.

Light Absorption Grate. Click image for more info.

Aydin and his team used two unconventional materials, metal and silicon oxide – to create thin but complex, trapezoid-shaped metal gratings on the nanoscale that can trap a wider range of visible light. The use of these materials is unusual because on their own, they do not absorb light; however, they worked together on the nanoscale to achieve very high absorption rates

The uniquely shaped grating captured a wide range of wavelengths due to the local optical resonances, causing light to spend more time inside the material until it gets absorbed. This composite metamaterial was also able to collect light from many different angles – a useful quality when dealing with sunlight, which hits solar cells at different angles as sun moves from east to west throughout the day.

The catch is as Aydin explains, is the research is not directly applicable to solar cell technology because metal and silicon oxide cannot convert light to electricity. In fact, the photons are converted to heat and might allow novel ways to control the heat flow at the nanoscale. However, the innovative trapezoid shape could be replicated in semiconducting materials that could be used in solar cells.

But, if the geometry and technique can be applied to semiconducting materials, the technology could lead to thinner, lower-cost, and more efficient solar cells.  Aydin thinks it can be done.

Aydin has taken light physics and materials chemistry together to control light absorption and refraction at the nanoscale with an impressive result.  Collecting all the visible light is a significant achievement with materials that seem to be likely to make the transition to the solar cell research area.

Solar electrical generation is in a low spot with a certain widely publicized bankruptcy dragging down a business that is intermittent in performance and very difficult to make completive with things a simple as thermal solar.  The industry needs a shot of innovation and the Aydin’s team at Northwestern might have the answer for the industry and the energy independence minded consumer.

Professor Ted Sargent, corresponding author on the work and holder of the Canada Research Chair in Nanotechnology at U of T said, “We figured out how to shrink the passivating materials to the smallest imaginable size.”

Collodial Quantum Dots (CQD) are nanoscale semiconductors that capture light and convert it into electrical energy. Because of their small scale, the dots can be sprayed on to flexible surfaces, including plastics. This enables the production of solar cells that are less expensive to produce and more durable than the common silicon-based version.

A crucial challenge in quantum dot cell assembly has been striking a balance between convenience and performance. The ideal design is one that tightly packs the quantum dots together. The greater the distance between quantum dots, the lower the efficiency.  But the quantum dots are usually capped with organic molecules that add a nanometer or two. When working on a nanoscale, that is big and bulky. Yet the organic molecules have been an important ingredient in creating a colloid, which is a substance that is dispersed in another substance like pigment in paint.  This allows the quantum dots to be painted on to other surfaces.

To solve the problem, the researcher team turned to inorganic ligands, which bind the quantum dots together while using less space. The result is the same colloid characteristics but without the bulky organic molecules.

Dr. Jiang Tang, the first author of the paper who conducted the research while a post-doctoral fellow in The Edward S. Rogers Department of Electrical & Computer Engineering at U of T explains, “We wrapped a single layer of atoms around each particle. As a result, they packed the quantum dots into a very dense solid.”

The team showed the highest electrical currents, and the highest overall power conversion efficiency, ever seen in CQD solar cells. The performance results were certified by an external laboratory, Newport that is accredited by the US National Renewable Energy Laboratory.

Professor John Asbury of Penn State, a co-author of the work takes the explanation further, “The team proved that we were able to remove charge traps – locations where electrons get stuck – while still packing the quantum dots closely together.”  The combination of close packing and charge trap elimination enabled electrons to move rapidly and smoothly through the solar cells, thus providing record efficiency.

Professor Dmitri Talapin of The University of Chicago, who is a research leader in the field, considers the results saying, “This finding proves the power of inorganic ligands in building practical devices. This new surface chemistry provides the path toward both efficient and stable quantum dot solar cells. It should also impact other electronic and optoelectronic devices that utilize colloidal nanocrystals. Advantages of the all-inorganic approach include vastly improved electronic transport and a path to long-term stability.”

Professor Aram Amassian of KAUST, a co-author on the work said, “At KAUST we were able to visualize, with incredible resolution on the sub-nanometer lengthscale, the structure and composition of this remarkable new class of materials. We proved that the inorganic passivants were tightly correlated with the location of the quantum dots; and that it was this new approach to chemical passivation, rather than nanocrystal ordering, that led to this record-breaking colloidal quantum dot solar cell performance.”

Solar cells fabricated following the team’s technique show up to 6% solar AM1.5G power-conversion efficiency. The CQD films are deposited at room temperature and under ambient atmosphere, rendering the process amenable to low-cost, roll-by-roll fabrication.

When it comes to the roll to roll or printing processes the CQD material is the current top efficiency choice.  Producers and consumers still have to wait to get an ideas on the pricing.  One isn’t expecting that CQD is a one time use disposable product – the nirvana of photoelectric solar.  But its getting closer.

If the economy could pick up perhaps this level of sophistication would get better legs into the market.  For now though the flow to industry and products is going to be slowed up.  We don’t need any more massive bankruptcy news from photoelectric.

Biaggio with Exciton Viewing Apparatus. Click image for more info.

The problem for low cost plastic based solar cells the absorption of light creates excitons in the plastic instead of directly inducing a current, as it does in the most commonly used glass covered silicon systems.

Excitons, which are created by the incoming light, play a central role in the harvesting of solar energy using plastic solar cells.  Biaggio explains with an example, “One way to understand the mechanics of excitons is to pour a cup of milk on the floor. The milk spreads out in all directions from the point of impact. How far it goes depends on the type of surface on which it lands. Now imagine that the milk has been replaced with particle-like bundles of energy and the floor with an ordered arrangement of organic molecules.”  An understanding of the exciton diffusion is critical for plastic solar cell technology.

Rubrene is one of a new generation of single-crystal plastic organic semiconductors.  The Lehigh team is using an advanced imaging technique to witness the long-range diffusion of energy-carrying excitons in the rubrene.

The team uses a focused laser beam to create the particles – the excitons – in a crystal made of rubrene. They tracked the movements of the excitons over distances smaller than the size of a human hair by directly taking pictures of the light coming from the laser. Unlike the spilled milk, the excitons spread only in a direction corresponding to a particular arrangement of the molecules.

After the excitons are created in plastic solar cells, they diffuse toward specially designed interfaces where they drive electrons into an external circuit, creating the flow of electrons we know as electric current.

Exciton Diffusion. Click image for more info.

“This is the first time that excitons have been directly viewed in a molecular material at room temperature,” said Biaggio. “We believe the technique we have demonstrated will be exploited by other researchers to develop a better understanding of exciton diffusion and the bottleneck it forms in plastic solar cells.”

The team’s paper entitled “Direct Imaging of Anisotropic Exciton Diffusion and Triplet Diffusion Length in Rubrene Single Crystals” was published July 1 by the journal Physical Review Letters.

The target of research is to improve exciton diffusion lengths until they become as large as the light absorption – that’s the point when sunlight is most efficiently collected and converted into energy.

Irkhin and Biaggio were able to obtain precise measurement of their diffusion length by directly imaging the diffusing excitons.  This length was found to be very large in a particular direction, reaching a value several hundreds of times larger than in the plastic solar cells that are presently in use. The payoff is this is the first time that excitons have been directly viewed in a molecular material at room temperature, and it is believed that the widespread adoption of the technique developed by Irkhin and Biaggio will lead to significantly more progress in the field.

Biaggio sums up with, “It is important that physicists explore the most fundamental phenomena underlying the mechanisms that enable solar energy harvesting with cheap organic materials. Organics have lots of unexplored potential and the very efficient exciton diffusion that we have observed in rubrene may build the basis for new ideas and technologies towards the development of ever more efficient and plastic solar cells.”

For those with a need to build low cost and lightweight solar cells, the Lehigh team has a tool to see what the material candidates will do.  Trial and error can be replaced with visual inspection. This is just the sort of research that speeds things up.  Silicon cells under glass at any appreciable size are expensive, vulnerable to wind and hail and just plain heavy.  Plastics could offer a much larger range and more useful applications for solar collection.  Good work, gentleman.

Hotz believes a novel hybrid system can wring even more useful energy out of the sun’s rays than world sporting photovoltaic solar panels to convert sunlight into electricity.

Hotz compared the hybrid system to three different technologies in an analysis of their exergetic performance. Exergy is a way of describing how much of a given quantity of energy can theoretically be converted to useful work.

“The hybrid system achieved exergetic efficiencies of 28.5 percent in the summer and 18.5 percent in the winter, compared to 5 to 15 percent for the conventional systems in the summer, and 2.5 to 5 percent in the winter,” said Hotz.  Hotz’s comparisons took place during the months of July and February in order to measure each system’s performance during summer and winter months. These are numbers to seize attention.

Hybrid Solar System Schematic from Duke University. Image Credit: Nico Hotz. Click image for the largest view.

The paper describing the results of Hotz’s analysis was named the top paper during the ASME Energy Sustainability Fuel Cell 2011 conference in Washington, D.C. Hotz recently joined the Duke faculty after completing post-graduate work at the University of California-Berkeley, where he analyzed a model of the new system. He is currently constructing one of the systems at Duke to test whether or not the theoretical efficiencies are born out experimentally.

Here’s how the hybrid works – Like other solar-based systems, the hybrid system begins with the collection of sunlight.  While the hybrid device might look like a traditional solar collector from the distance, it is actually a series of copper tubes coated with a thin layer of aluminum and aluminum oxide and partly filled with catalytic nanoparticles. A combination of water and methanol flows through the tubes, which are sealed in a vacuum.

This is important as Holtz explains, “This set-up allows up to 95 percent of the sunlight to be absorbed with very little being lost as heat to the surroundings. This is crucial because it permits us to achieve temperatures of well over 200º C within the tubes. By comparison, a standard solar collector can only heat water between 60º and 70º C.”

“Once the evaporated liquid achieves these higher temperatures, tiny amounts of a catalyst are added, which produces hydrogen. This combination of high temperature and added catalysts produces hydrogen very efficiently,” Hotz said. The resulting hydrogen can then be immediately directed to a fuel cell to provide electricity to a building during the day, or compressed and stored in a tank to provide power later.

The three systems examined in the analysis were the standard photovoltaic cell which converts sunlight directly into electricity to then split water electrolytically into hydrogen and oxygen; a photocatalytic system producing hydrogen similar to Hotz’s system, but simpler and not mature yet; and a system in which photovoltaic cells turn sunlight into electricity which is then stored in different types of batteries (with lithium ion being the most efficient).

“We performed a cost analysis and found that the hybrid solar-methanol is the least expensive solution, considering the total installation costs of $7,900 if designed to fulfill the requirements in summer, although this is still much more expensive than a conventional fossil fuel-fed generator,” Hotz said.

Costs and efficiencies of systems can vary widely depending on location – since the roof-mounted collectors that could provide all the building’s needs in summer might not be enough for winter. A rooftop system large enough to supply all of a winter’s electrical needs would produce more energy than needed in summer, so the owner could decide to shut down portions of the rooftop structure or, if possible, sell excess energy back to the grid
.
“The installation costs per year including the fuel costs, and the price per amount of electricity produced, however showed that the (hybrid) solar scenarios can compete with the fossil fuel-based system to some degree,” Hotz said. “In summer, the first and third scenarios, as well as the hybrid system, are cheaper than a propane- or diesel-combusting generator.”

That would be an important consideration, especially if a structure is to be located in a remote area where traditional forms of energy or a grid connection would be too difficult or expensive to obtain.
Here’s an interesting ‘Pro American Research’ point. Hotz’s research was supported by the Swiss National Science Fund. Joining him in the study were UC-Berkeley’s Heng Pan and Costas Grigoropoulos, as well as Seung H. Ko of the Korea Advanced Institute of Science and Technology, Daejon, Korea.

As noted the system is still a lab idea.  But the efficiency and available power and fuel is much more substantial than a simpler photovoltaic installation.  The weak point is the comparison to splitting water to get hydrogen.  Going to batteries is an economic decision. How the system would compare to a system of photovoltaic for electricity and thermal collectors for heat isn’t covered in the press release.  Capital costs are going to matter – and that is where Hotz might shine.

Time will tell, it’s definitely an impressive development that could very well have broad applications. The idea may also simply put waste heat to work as well and that could be an impressive business on its own.

Nanosphere Solar Cell Layer Structure. Click image for the largest view.

A couple of years ago there were chastising comments on solar cell claims, but this team’s work is peer reviewed and published by the Journal of Renewable and Sustainable Energy at the American Institute of Physics.  That tends to lend great credibility to the team’s assertion. Also, the team’s work starts its calculations in a sensible zone at about 10% efficiency.

The DSSC cell with the TiO2 photoanodes improvement is significantly better in the visible and near infrared range of the electromagnetic spectrum.  Moreover, the surface area remains the same.  The improvement comes from enhanced light scattering by porous TiO2 nanospheres.
Titanium dioxide is a photosensitive material that’s less expensive than the more traditional silicon solar cells. Silicon cells are rapidly approaching the theoretical limit of their efficiency. Current DSSC cells based on TiO2 designs, however, are only about 10 percent efficient.  One reason for this low efficiency is that light from the infrared portion of the spectrum is not easily absorbed in the solar cell.

The Minnesota team engineered a new layered design described in the paper. The spheres and the layers work to increase the path of the light through the solar cell and convert more of the electromagnetic spectrum into electricity. The cells consist of micrometer-scale spheres with nanometer pores sandwiched between layers of nanoscale particles. The TiO2 spheres act like tightly packed bumpers on a pinball machine, causing photons to bounce around before eventually making their way through the cell.

Nanosphere Solar Cell Structure. Click image for the largest view.

Each time the photon interacts with one of the spheres, a small charge is produced. The interfaces between the layers also help enhance the efficiency by acting like mirrors and keeping the light inside the solar cell where it can be converted to electricity.

The team says the strategy to increase light-harvesting efficiency can be easily integrated into current commercial DSSC production.  If the team is right, a 26% increase is going to change the market dramatically.

The innovation isn’t likely to stop here.  The sphere sizes could be reduced even more, the layer structure isn’t optimized yet, and other insights may offer more improvement.

The dye based cell industry will need to closely look into this work. Costs are not discussed, but TiO2 an inexpensive product.  Getting the spheres sized and sorted might be a challenge.  But getting to 12 or 13% efficiency is a major improvement, well worth the pursuit now that a solution is known.

Jan Kleissl, a professor of environmental engineering at the UC San Diego Jacobs School of Engineering leading a team of researchers can show roof mounted solar panels aren’t just providing clean power; they are cooling your house, or your workplace during the day and holding in heat at night.

Kleissl and his team’s study “Effects of Solar Photovoltaic Panels on Roof Heat Transfer” has been published in the journal Solar Energy with its thought to be the first peer-reviewed measurements of the cooling benefits provided by solar photovoltaic panels.

Solar Cooling Powell Lab Roof Thermal and Photo. Click image for more info.

Using thermal imaging, the researchers determined that during the day, a building’s interior ceiling was 5º F cooler under solar panels than simply under an exposed roof. At night, the panels help hold heat in, reducing heating costs in the winter.

Kleissl explains as solar panels cover an increasing number of residential and commercial roofs, it becomes more important to consider their impact on buildings’ total energy costs.  The team’s calculations show the amount saved on cooling the building amounted to getting a 5 percent discount on the solar panels’ price, over the panels’ lifetime.  Put another way, for the building the researchers studied, savings in cooling costs amounted to selling 5 percent more solar energy to the grid than the panels are actually producing.

Data for the study was gathered over three days in April on the roof of the Powell Structural Systems Laboratory at the Jacobs School of Engineering in San Diego California with a thermal infrared camera. The building is equipped with tilted solar panels, solar panels that are flush with the roof, and panels do not cover some portions of the roof.

Anthony Dominguez, the graduate student lead on the project takes us through the logic.  Rather than the sun beating down onto the roof, which causes heat to be pushed through the roof and inside the ceiling of the building, photovoltaic panels take the solar beating. Then much of the heat is removed by wind blowing between the panels and the roof. The benefits are greater if there is an open gap where air can circulate between the building and the solar panel, so tilted panels provide more cooling. Kleissl adds the more efficient the solar panels, the bigger the cooling effect.

For the building researchers analyzed, the panels reduced the amount of heat reaching the roof by about 38 percent.

Kleissl said he is confident, although the measurements took place over a limited period of time, his team developed a model that allows them to extrapolate their findings to predict cooling effects throughout the year.  For example, in winter, the panels would keep the sun from heating up the building. But at night, they would also keep in whatever heat accumulated inside. For an area like San Diego, the two effects essentially cancel each other out, Kleissl said.

The idea for the study came about when Kleissl, Dominguez and a group of undergraduate students were preparing for an upcoming conference. They decided the undergraduates should take pictures of the Powell Laboratory roof with a thermal infrared camera. The data confirmed the team’s suspicion that the solar panels were indeed cooling the roof, and the building’s ceiling as well.

This is pretty good reality checking.  Kleissl said, “There are more efficient ways to passively cool buildings, such as reflective roof membranes. But, if you are considering installing solar photovoltaic, depending on your roof thermal properties, you can expect a large reduction in the amount of energy you use to cool your residence or business.”

Just how much is quite a valuable answer.  That awaits more funding, something an association of solar panel makers might want to get on quite soon.  Kleissl notes if additional funding became available, Kleissl said his team could develop a calculator that people could use to predict the cooling effect on his or her own roof and in their own climate-specific area. To further increase the accuracy of their models, researchers also could compare two climate-controlled, identical buildings in the same neighborhood, one with solar panels, and the other without.

Obviously local savings calculations won’t be simple – solar orientation, roof angle, latitude, roof color and other variables are going to have affects – but knowing is a must when a product is still so far from economic mastery of the market for home power generation.  That 38% early lab study heat reduction transferred to annual cooling load is going to have a major impact for locations that are more cooling need dominated.

There doesn’t seem to be any advantage or disadvantage for heating load in San Diego.  Yet to assume that will be true anywhere is premature – and with the potential for great savings with a panel design with heating in mind – the lack of a marketing advantage can’t last long.

Seems obvious – if not thought about until now – those panels are a shade after all.  Now let see where the product designers can take it.  Perhaps the rooftop solar panel can be a much better deal than anyone thought.

A hybrid solar panel combines a photovoltaic solar cell that converts sunlight into electricity with a thermal solar collector to harvest infrared energy for heat.  So far neither the electricity production or heat production efficiency has been especially interesting for consumers.  The research trend instead has been to find ways to use infrared for electricity production.

Basically a hybrid solar collector is a combination of a photovoltaic solar panel and a thermal solar collector. The residual heat from the photovoltaic cells is used to heat a circulating fluid. Customarily the fluid flows through a system of tubing on a copper or aluminum sheet.

A great deal of infrared energy is needed to heat the fluid in the tubes and the supporting structure. The structure is why the solar collector needs to be fitted with a transparent cover that helps to retain the heat.  This design gets quite hot and the materials used in the photovoltaic solar cell degrade quickly under temperatures of around 120º C. When the design heats up the efficiency is reduced.  At 120º C the efficiency loss get to around 20 per cent and the lifespan is cut to between five and ten years.

Roest’s graduation research as part of a Master’s degree in Sustainable Energy Technology developed a new type of hybrid solar collector with increased electrical efficiency and a longer lifespan.

Roest’s design doesn’t require a transparent cover. The fluid flows through a large number of small aluminum channels directly under the solar panel instead of through a built up tubing on a support sheet. Thus, less heat is required to heat the water sufficiently for household use. Roest then chose not to use a crystalline silicon PV solar panel, opting instead for a thin film solar cells because it’s easier to draw heat from this type of solar cell. Getting rid of the cover meant that the heat of the solar panel would be reduced to around 80º C.

Roest Hybrid Solar Panel from TU Delft. Click image for the largest view.

The design reminds one more of a “heat sink” than an “infrared absorber”.

Using thin film photovoltaic cells exploits their performance.  Thin film cells perform relatively well at high temperatures. At an operating temperature of 80 degrees the efficiency loss is down to 10%, instead of the 20% of crystalline silicon photovoltaic cells. This will more extend  Roest’s hybrid solar collector to an estimated lifespan of 15 to 20 years.

Miro Zeman, professor of Photovoltaic Materials and Devices at TU Delft, who oversaw Roest’s work commented saying, “This innovative design could play an important role in the development of affordable and efficient hybrid systems for household use.”

The second half of the story is Roest needed a means to test his new design.  So Roest also built an actual solar simulator that he used to test the efficiency of his prototype hybrid solar panel.  This prompted considerable commercial interest in the solar simulator, which motivated Roest and a partner to start the TU Delft spin-off company Eternal Sun, so they could quickly put the solar simulator on the market. Eternal Sun recently came out on top at the European finals of the BE.Project, a competition for student-entrepreneurs.

Roest Solar Simulator at TU Delft

TU Delft quickly patented the relevant technology and The Eternal Sun team has now grown to include six students and recent graduates, with five solar simulators sold since January.

Roest has been at this awhile. In 2007, he was the team leader of the Nuon Solar Team that won the World Solar Challenge in Australia with the solar car Nuna4.

That’s two new products out of one project.  Interested parties can find contact information for Roest at the end of the TU Delft press release page and the Eternal Sun link.

Roest has taken hybrid panels to a new level, but this isn’t over yet.  More is sure to come.  But the young team at Eternal Sun has a great start with the simulator selling now and the simulator will help every researcher that has one to speed up their work.