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A close-up of a photovoltaic solar panel.

The light glinting off the surface of this solar photovoltaic cell signifies lost efficiency. Scientists are looking to nanotechnology to boost solar power, including by reducing the amount of sunlight that silicon wastes through reflection.

Photograph by Martin Bond, Science Source

Patrick J. Kiger

For National Geographic News

Published April 28, 2013

Nearly 60 years after researchers first demonstrated a way to convert sunlight into energy, science is still grappling with a critical limitation of the solar photovoltaic cell.

It just isn't that efficient at turning the tremendous power of the sun into electricity.

And even though commercial solar cells today have double to four times the 6 percent efficiency of the one first unveiled in 1954 by Bell Laboratories in New Jersey, that hasn't been sufficient to push fossil fuel from its preeminent place in the world energy mix.

But now, alternative energy researchers think that something really small—nanotechnology, the engineering of structures a fraction of the width of a human hair—could give a gigantic boost to solar energy. (See related quiz: "What You Don't Know About Solar Power.")

"Advances in nanotechnology will lead to higher efficiencies and lower costs, and these can and likely will be significant," explains Matt Beard, a senior scientist for the U.S. Department of Energy's National Renewable Energy Laboratory (NREL). "In fact, nanotechnology is already having dramatic effects on the science of solar cells."

Of course, the super-expensive solar arrays used in NASA's space program are far more efficient than those installed on rooftops. (See related story: "Beam It Down: A Drive to Launch Space-Based Solar.") And in the laboratory, scientists have achieved record-breaking efficiencies of more than 40 percent. But such contests are a testament to the gap between solar potential and the mass market cells of today.

The power output of the sun that reaches the Earth could provide as much as 10,000 times more energy than the combined output of all the commercial power plants on the planet, according to the National Academy of Engineering. The problem is how to harvest that energy. Today’s commercial solar cells, usually fashioned from silicon, are still relatively expensive to produce (even though prices have come down), and they generally manage to capture only 10 to 20 percent of the sunlight that strikes them. This contributes to the high cost of solar-generated electricity compared to power generated by conventional fossil-fuel-burning plants. By one comparative measure, the U.S. Energy Information Administration estimated the levelized cost of new solar PV as of 2012 was about 56 percent higher than the cost of generation from a conventional coal plant.

Nanotechnology may provide an answer to the efficiency problem, by tinkering with solar power cells at a fundamental level to boost their ability to convert sunlight into power, and by freeing the industry to use less expensive materials. If so, it would fulfill the predictions of some of nanotechnology's pioneers, like the late Nobel physicist Richard Smalley, who saw potential in nanoscale engineering to address the world's energy problems. (See related: "Nano's Big Future.") Scientists caution that there’s still a lot of work ahead to overcome technical challenges and make these inventions ready for prime time. For example, more research is needed on the environmental, health, and safety aspects of nano-materials, said the National Academy of Sciences in a 2012 report that looked broadly at nanotechnology, not at solar applications in particular. (See related pictures: "Seven Ingredients for Better Car Batteries.")

But Luke Henley, a University of Illinois at Chicago chemistry professor who received a 2012 National Science Foundation grant to develop a solar-related nanotechnology project, predicts there will be major advances over the next five to ten years. "It’s potentially a game changer," he says. Here are five intriguing recent nanotechnology innovations that could help to boost solar power.

Billions of Tiny Holes

To reduce the amount of sunlight that is reflected away from silicon solar cells and wasted, manufacturers usually add one or more layers of antireflective material, which significantly boosts the cost. But late last year, NREL scientists announced a breakthrough in the use of nanotechnology to reduce the amount of light that silicon cells reflect. It involves using a liquid process to put billions of nano-sized holes in each square inch of a solar cell’s surface. Since the holes are smaller than the light wavelengths hitting them, the light is absorbed rather than reflected. The new material, which is called "black silicon," is nearly 20 percent more efficient than existing silicon cell designs. (See related photos: "Spanish Solar Energy.")

The "Nano Sandwich"

Organic solar cells, made from elements such as carbon, nitrogen, and oxygen that are found in living things, would be cheaper and easier to make than current silicon-based solar cells. The tradeoff, until now, is that they haven’t been as efficient. But a team of Princeton University researchers, led by electrical engineer Stephen Chou, has been able to nearly triple the efficiency of solar cells by devising a nanostructured "sandwich" of metal and plastic. In technical lingo, their invention is called a plasmodic cavity with subwavelength hole array, or PlaSCH. It consists of a thin strip of plastic sandwiched between a top layer made from an incredibly fine metal mesh and a bottom layer of the metal film used in conventional solar cells.

All aspects of the solar cell’s structure—from its thickness to the spacing of the mesh and diameter of the holes—are smaller than the wavelength of the light that it collects. As a result, the device absorbs most of the light in that frequency rather than reflecting it. "It’s like a black hole for light," Chou explained in a Princeton press release in December. "It traps it." Another plus: researchers say the PlaSCH cells can be manufactured cost-effectively in sheets, using a process developed by Chou years ago that embosses the nanostructures over a large area, similar to the way newspapers are printed.

Mimicking Evolution

One of the big difficulties in coming up with more energy-efficient solar cells is the limitations of the researchers’ own imaginations. But in a January 2013 article published in Scientific Reports, Northwestern University mechanical engineering professors Wei Chen and Cheng Sun and their graduate students Shuangcheng Yu and Chen Wang introduced a method that might be superior to human brainstorming. Using a mathematical search algorithm based on natural biological evolution, they took dozens of design elements and then "mated" them over a series of 20 generations, in a process that mimicked the evolutionary principles of crossover and genetic mutation.

"Our approach is based upon the biologically evolutionary process of survival of the fittest," Chen explained in an article on Northwestern University’s website.

The result: An evolution-inspired organic solar cell—that is, one that uses carbon-based materials rather than silicon crystals—in which light first enters a 100-nanometer-thick scattering layer with an unorthodox geometric pattern. The researchers say this should enable it to absorb light more efficiently. The U.S. Department of Energy's Argonne National Laboratory will fabricate an actual working version of the new cell for testing.

Tiny Antennae

We’re used to thinking of solar energy as something that we collect with panels. But even the latest-generation silicon panels can take in light from only a relatively narrow range of frequencies, amounting to about 20 percent of the available energy in the sun’s rays. The panels then require separate equipment to convert the stored energy to useable electricity. But researchers at the University of Connecticut and Penn State  are working on an entirely new approach, using tiny, nanoscale antenna arrays, which would take in a wider range of frequencies and collect about 70 percent of the available energy in sunlight. Additionally, the antenna arrays themselves could convert that energy to direct current, without need for additional gear.

Scientists have been thinking about using tiny antennae for a while, but until recently, they lacked the technology to make them work, since such a setup would require electrodes that were just one or two nanometers apart—about 1/30,000 the width of a human hair. Fortunately, University of Connecticut engineering professor Brian Willis has developed a fabrication technique called selective area atomic-layer deposition, which makes it possible to coat the electrodes with layers of individual copper atoms, until they are separated by just 1.5 nanometers. "This new technology could get us over the hump and make solar energy cost-competitive with fossil fuels," Willis explained in February. "This is brand new technology, a whole new train of thought."

Solar-Collecting Paint

No matter what sort of solar energy-collecting technology you employ, there’s still the problem of building a bunch of the devices and hooking them up in places with sun exposure. But University of Southern California chemistry professor Richard L. Brutchey and postdoctoral researcher David H. Webber have devised a technology that could turn a building into a solar collector.

They’ve created a stable, electricity-conducting liquid filled with solar-collecting nanocrystals, which can be painted or printed like an ink onto surfaces such as window glass or plastic roof panels. The nanocrystals, made of cadmium selenide instead of silicon, are about four nanometers in size—about 250 billion of them could fit on the head of a pin—so they are capable of floating in a liquid solution.  (See related pictures: "A New Hub for Solar Tech Blooms in Japan.")

Brutchey’s and Webber’s secret to getting the technology to work? Finding an organic molecule that could attach to the nanocrystals and stabilize them and prevent them from sticking together, without hindering their ability to conduct electricity.

The researchers aim to work on nanocrystals built from materials other than cadmium, a toxic metal. "While the commercialization of this technology is still years away, we see a clear path forward toward integrating this into the next generation of solar cell technologies," Brutchey says. (See related video: "Toxic Land Generates Solar Power.")

This story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.

An earlier version of this story referred to Cheng Sun as a graduate student. He is assistant professor of mechanical engineering at Northwestern.

9 comments
Michael Remick
Michael Remick

i think they are going in the wrong direction to make solar cells usage more useful to society they need to develop a a method to use solar power with out using the sun that is where the future of solar cell creation should be going!!!

Alec Sevins
Alec Sevins

I sure hope this works, and I'd like to see eyesore wind turbines greatly scaled down and dismantled in many locations. It seems insane to not have solar apparatus on every structure that can possibly handle it, including the roofs of cars.

I think solar is by far the best renewable energy "savior" we have. People forget that wind is actually solar power as well. Fossil fuels are, too. We've been depleting that ancient solar capital for too long and it's time to get true energy income. Economists should understand that concept.

Boris De Denko
Boris De Denko

Mr. Wright:  I have a different viewpoint about efficiency and its place in the argument.  For the individual homeowner, space is not unlimited, so efficiency is an important concern.  One may not necessarily have 1,000 sq. feet of roof area to dedicate to solar power generation for a household.  This would be the situation where the homeowner would likely be willing to pay the "dollar for 10% efficiency", simply because he doesn't have enough space to buy the less expensive material to generate the same amount of energy. 

Paul Wright
Paul Wright

Can we please have a discussion about renewable energy that does not use the irrelevant term "efficiency"?

Efficiency, per se, is absolutely irrelevant to whether pv "works economically."  There is no fuel cost for the Sun's light, so it simply does not matter how efficiently that free fuel is converted into useful forms of energy.  If I could get 1% efficiency with a device that costs a penny, I would take that any day over a device that gives 10% efficiency but costs a dollar.

The most accurate comparative measure for various forms of energy, when converted to electricity, is cost per kwh over the entire life cycle of the equipment.  Utilities mislead the public by using other measures to hide the fact that renewable energy costs less than fossil energy over the life cycle.  Utilities do this for several reasons.

First, utilities are uncomfortable with renewable energy because the investment is more capital intensive than fossil plants.  A pv plant costs a lot up front but then there is very little cost afterward, whereas a fossil plant is relatively inexpensive up front but the major cost is paid over time for fuel.  Electric utilities have limited ability to amass capital, so they historically seek to maximize the wattage generating capacity, rather than minimize life-cycle cost.  The focus on capital cost rather than life-cycle kwh cost is exacerbated by the fact that cost is the consumers' problem, not the utilities', under our regulatory scheme in which cost is passed onto consumers.

Second, utilities do not like renewable energy because there is too much opportunity for consumers to generate electricity, and thus limit the profits utilities make.  Essentially, electric utilities make their money by buying huge amounts of fuel at lower prices than consumers are charged.  That tremendous revenue stream on fossil fuel is the life blood of the electric utility industry and they will not give it up without a fight.  Even after President Bush made net metering the law of the land in the Energy Policy Act of 2005, for example, the utilities are still able to prevent consumers from using renewable energy to generate electricity.

Simply stated, utilities do this by using their monopoly power in the market for distributing electricity in order to monopolize the market for generating electricity: "you cannot compete with my generating system by connecting your generating system to my grid, because I say so."  I have spoken with lawyers from the DoJ Antitrust Division who understand this, and are looking for a case to prosecute.  So, any renewable-energy entrepreneur who has experieinced efforts to shut down renewable projects (like the 80+ pv projects shut down in South Carolina last year) should contact the DoJ Antitrust Division.

Alec Sevins
Alec Sevins

@Paul Wright

I think you're missing the point. Efficiency matters a lot because it reduces the SIZE and installation cost of solar panels and boosts their practical usage on existing structures, which to me is far wiser than covering huge swaths of desert with solar farms. The idea should be to constrain Man's physical footprint as much as possible, not just the "carbon footprint."

The problem of efficiency definitely applies to wind turbines,which have become landscape-wrecking monsters with sporadic returns.

Paul Wright
Paul Wright

Can we please have a discussion about renewable energy that does not use the irrelevant term "efficiency"?

Efficiency, per se, is absolutely irrelevant to whether pv "works economically."  There is no fuel cost for the Sun's light, so it simply does not matter how efficiently that free fuel is converted into useful forms of energy.  If I could get 1% efficiency with a device that costs a penny, I would take that any day over a device that gives 10% efficiency but costs a dollar.

The most accurate comparative measure for various forms of energy, when converted to electricity, is cost per kwh over the entire life cycle of the equipment.  Utilities mislead the public by using other measures to hide the fact that renewable energy costs less than fossil energy over the life cycle.  Utilities do this for several reasons.

First, utilities are uncomfortable with renewable energy because the investment is more capital intensive than fossil plants.  A pv plant costs a lot up front but then there is very little cost afterward, whereas a fossil plant is relatively inexpensive up front but the major cost is paid over time for fuel.  Electric utilities have limited ability to amass capital, so they historically seek to maximize the wattage generating capacity, rather than minimize life-cycle cost.  The focus on capital cost rather than life-cycle kwh cost is exacerbated by the fact that cost is the consumers' problem, not the utilities', under our regulatory scheme in which cost is passed onto consumers.

Second, utilities do not like renewable energy because there is too much opportunity for consumers to generate electricity, and thus limit the profits utilities make.  Essentially, electric utilities make their money by buying huge amounts of fuel at lower prices than consumers are charged.  That tremendous revenue stream on fossil fuel is the life blood of the electric utility industry and they will not give it up without a fight.  Even after President Bush made net metering the law of the land in the Energy Policy Act of 2005, for example, the utilities are still able to prevent consumers from using renewable energy to generate electricity.

Simply stated, utilities do this by using their monopoly power in the market for distributing electricity in order to monopolize the market for generating electricity: "you cannot compete with my generating system by connecting your generating system to my grid, because I say so."  I have spoken with lawyers from the DoJ Antitrust Division who understand this, and are looking for a case to prosecute.  So, any renewable-energy entrepreneur who has experieinced efforts to shut down renewable projects (like the 80+ pv projects shut down in South Carolina last year) should contact the DoJ Antitrust Division.

Paul Wright
Paul Wright

Can we please have a discussion about renewable energy that does not use the irrelevant term "efficiency"?

Efficiency, per se, is absolutely irrelevant to whether pv "works economically."  There is no fuel cost for the Sun's light, so it simply does not matter how efficiently that free fuel is converted into useful forms of energy.  If I could get 1% efficiency with a device that costs a penny, I would take that any day over a device that gives 10% efficiency but costs a dollar.

The most accurate comparative measure for various forms of energy, when converted to electricity, is cost per kwh over the entire life cycle of the equipment.  Utilities mislead the public by using other measures to hide the fact that renewable energy costs less than fossil energy over the life cycle.  Utilities do this for several reasons.

First, utilities are uncomfortable with renewable energy because the investment is more capital intensive than fossil plants.  A pv plant costs a lot up front but then there is very little cost afterward, whereas a fossil plant is relatively inexpensive up front but the major cost is paid over time for fuel.  Electric utilities have limited ability to amass capital, so they historically seek to maximize the wattage generating capacity, rather than minimize life-cycle cost.  The focus on capital cost rather than life-cycle kwh cost is exacerbated by the fact that cost is the consumers' problem, not the utilities', under our regulatory scheme in which cost is passed onto consumers.

Second, utilities do not like renewable energy because there is too much opportunity for consumers to generate electricity, and thus limit the profits utilities make.  Essentially, electric utilities make their money by buying huge amounts of fuel at lower prices than consumers are charged.  That tremendous revenue stream on fossil fuel is the life blood of the electric utility industry and they will not give it up without a fight.  Even after President Bush made net metering the law of the land in the Energy Policy Act of 2005, for example, the utilities are still able to prevent consumers from using renewable energy to generate electricity.

Simply stated, utilities do this by using their monopoly power in the market for distributing electricity in order to monopolize the market for generating electricity: "you cannot compete with my generating system by connecting your generating system to my grid, because I say so."  I have spoken with lawyers from the DoJ Antitrust Division who understand this, and are looking for a case to prosecute.  So, any renewable-energy entrepreneur who has experieinced efforts to shut down renewable projects (like the 80+ pv projects shut down in South Carolina last year) should contact the DoJ Antitrust Division.

Barry Ramey
Barry Ramey

@Paul Wright Panel time is the only way to make all this work, exist the prolong cellular bounce and there is a glitch in which I have a system that will multiply the power. See you all soon! 

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