Like a hall of mirrors, nanostructures trap photons inside ultrathin solar cells (w/ Video)

Apr 22, 2014

In the quest to make sun power more competitive, researchers are designing ultrathin solar cells that cut material costs. At the same time they're keeping these thin cells efficient by sculpting their surfaces with photovoltaic nanostructures that behave like a molecular hall of mirrors.

"We want to make sure light spends more quality time inside a solar cell," said Mark Brongersma, a professor of materials science and engineering at Stanford and co-author of a review article in Nature Materials.

Brongersma and two Stanford colleagues—associate professor of and engineering Yi Cui and professor of electrical engineering Shanhui Fan—surveyed 109 recent scientific papers from teams around the world.

Their overview revolves around a basic theme: looking at the many different ways that researchers are trying to maximize the collisions between and electrons in the thinnest possible layers of . The goal is to reveal trends and best practices that will help drive developments in the field.

Solar energy is produced when photons of light collide with the electrons in a photovoltaic crystal. As loose electrons move through the crystal, they generate an electrical current.

Today's are already thin. They are made up of layers of photovoltaic materials, generally silicon, that average 150 to 300 micrometers, which is roughly the diameter of two to three human hairs.

As engineers continue to shave down those dimensions they have to develop new molecular traps and snares to ensure that photons don't simply whiz through their ultrathin solar cells before the electrical sparks can fly.

"A lot of the excitement now is about using the principles of photonics to manage light waves in the most efficient way," Fan said. "There are perhaps hundreds of groups in the world working on this."

The review article provides a high level view of how scientists are trying to design structures to facilitate interactions between the infinitesimal instigators of solar current, the photons and the electrons.

Research face enormous challenges in trying to architect nanostructures attuned to catch light. Sunlight consists of many colors. When we see rainbow, what we see is result of atmospheric moisture acting as a prism to bend light into its constituent colors. Creating different nanostructures to catch the pot of photons at the end of each color of the rainbow is part of what this research is about.

Nevertheless, scientists are already reporting some success

"We are seeing systems that use one one-hundredth as much photovoltaic material as today's solar cells while getting 60 percent to 70 percent of the electrical output," Brongersma said.

The most common photovoltaic material is a refined form of silicon similar to that found in computer chips. This material accounts for 10 percent to 20 percent of a solar cell's cost. Lowering those expenses 100-fold would therefore have a considerable effect on the overall cost-efficiency of production.

But Cui says lowering material costs is only part of the push behind ultrathin solar. Another benefit is flexibility. Because of the thickness of the light-catching silicon layer, today's solar cells must be kept rigid lest their crystal lattice be damaged and the flow of electrons disrupted.

"But at 10 micrometers of thickness silicon has a high degree of mechanical flexibility," said Cui, citing a dimension less than one-tenth the thickness of the photovoltaic layer inside today's solar cells.

Cui, who has made just such an experimental material, shows a movie of flapping this thin silicon like a piece of paper and cutting it with a scissors (see separate videos; flapping /above/ and cutting /below/). Those thin silicon strips incorporate some of the photon-trapping nanostructures described in the Nature Materials article. Cui says the light-to-energy conversion efficiency of thin silicon is approaching that of the rigid silicon in today's solar cells.

This video is not supported by your browser at this time.

This video is not supported by your browser at this time.

Flapping silicon isn't just a science project. Such flexibility would pay a dividend when it comes to installation, which accounts for roughly one-third of the total cost of a rooftop solar array. "These thin silicon cells can be embedded into flexible plastic, making installation like rolling out a carpet," Cui said.

Yet even as researchers succeed in getting more from less, many hurdles remain according to Fan, who develops computer models to study how different nanostructures and materials will affect photon-electron interactions.

"There are an infinite number of structures, so it isn't possible to model them all," he said, alluding to what he called the "theoretical bottlenecks" that impede scientific understanding of this ethereal realm where light and matter intersect.

"For instance, right now, we really don't have a way to know when we've gotten the most out of our photons," Fan said.

Explore further: Material scientist exploring ways to improve efficiency of solar cells

add to favorites email to friend print save as pdf

Related Stories

Recommended for you

Twisted graphene chills out

Sep 17, 2014

(Phys.org) —When two sheets of graphene are stacked in a special way, it is possible to cool down the graphene with a laser instead of heating it up, University of Manchester researchers have shown.

User comments : 9

Adjust slider to filter visible comments by rank

Display comments: newest first

Birger
not rated yet Apr 22, 2014
Ultrathin solar cells would also be useful for spacecraft, due to the low weight. However, they may need to be mounted on a metal sheet for structural strenght, being subjected to high g-forces during launch.
Returners
1 / 5 (1) Apr 22, 2014
"We are seeing systems that use one one-hundredth as much photovoltaic material as today's solar cells while getting 60 percent to 70 percent of the electrical output," Brongersma said.
The most common photovoltaic material is a refined form of silicon similar to that found in computer chips. This material accounts for 10 percent to 20 percent of a solar cell's cost. Lowering those expenses 100-fold would therefore have a considerable effect on the overall cost-efficiency of solar energy production.


Now:
Output/Input = 1

This:
0.7*Output/0.8Input = 0.875 = Less cost efficient.

Since land costs money, and you must buy more land to make up for the loss of the 30% output, then input costs go up by the additional land value costs, which are considerable, which is about 10 to 20% of total costs...

New Input = (1.43*Land costs + 0.8*Other Input)

0.7Output / 0.863 = 0.81 = less cost-efficient

0.7Output/0.926 = 0.76 = less cost-efficient
antialias_physorg
5 / 5 (2) Apr 22, 2014
However, they may need to be mounted on a metal sheet for structural strenght, being subjected to high g-forces during launch.

A flexible substrate could withstand g-forces better than a rigid one (which might bend or - in the case of rigid cells - shatter). In the end power output per kg is the figure of merit (with longevity under space conditions being another prime factor).
dvdrushton
not rated yet Apr 22, 2014
Since land costs money


Unless you are placing the panels on a roof - and only wanting to install enough power to offset your homes energy use. Even here in the U.S. where we use large amounts of a/c, and have pretty inneficient homes - we only need a portion of the roof covered.
Returners
1 / 5 (1) Apr 22, 2014
Unless you are placing the panels on a roof - and only wanting to install enough power to offset your homes energy use. Even here in the U.S. where we use large amounts of a/c, and have pretty inneficient homes - we only need a portion of the roof covered.


Industry requires massive amounts of land converted to solar farms, and in general the farms need to be relatively close to the location where they will be sending power.

Solar panels are much more cost-efficient than biofuels, but a 40% increase in land costs can only hurt their productivity for industry.
italba
not rated yet Apr 22, 2014
@Birger: If those cells are flexible enough can be folded or rolled into the satellite and unfolded once in orbit.
Lex Talonis
1 / 5 (1) Apr 22, 2014
Thinner cells, with better light trapping and transformation to energy ratios - are a good thing.

Especially if they can be made to work really well in a cost effective kind of a way.
antialias_physorg
5 / 5 (1) Apr 23, 2014
and in general the farms need to be relatively close to the location where they will be sending power.

Huh? That logic doesn't hold for nuclear or big water dams - which aren't sited close to consumers. Why should it hold for wind and solar? Please explain.

What's wrong with taking a few (hundred) square miles of desert and covering it in solar panels? Is aynone using that space now? Is the cost of desert land really that exorbitant?
PPihkala
not rated yet Apr 23, 2014
What's wrong with taking a few (hundred) square miles of desert and covering it in solar panels? Is aynone using that space now? Is the cost of desert land really that exorbitant?

Even deserts have plants and animals that are affected by any human activity. There is no energy generation with zero environmental effects. At least the raw material mining does damage.