Photovoltaics from any semiconductor

Jul 26, 2012
The SFPV technology was tested for two top electrode architectures: (A) the top electrode is shaped into narrow fingers; (B) top electrode is uniformly ultrathin. Image courtesy of Berkeley Lab

A technology that would enable low-cost, high efficiency solar cells to be made from virtually any semiconductor material has been developed by researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley. This technology opens the door to the use of plentiful, relatively inexpensive semiconductors, such as the promising metal oxides, sulfides and phosphides, that have been considered unsuitable for solar cells because it is so difficult to taylor their properties by chemical means.

"It's time we put bad materials to good use," says physicist Alex Zettl, who led this research along with colleague Feng Wang. "Our technology allows us to sidestep the difficulty in chemically tailoring many earth abundant, non-toxic semiconductors and instead tailor these materials simply by applying an electric field."

Zettl, who holds joint appointments with Berkeley Lab's Division and UC Berkeley's Physics Department where he directs the Center of Integrated Nanomechanical Systems (COINS), is the corresponding author of a paper describing this work in the journal . The paper is titled "Screening-

Engineered Field-Effect ." Co-authoring it were William Regan, Steven Byrnes, Will Gannett, Onur Ergen, Oscar Vazquez-Mena and Feng Wang.

Solar cells convert sunlight into electricity using semiconductor materials that exhibit the photovoltaic effect – meaning they absorb photons and release electrons that can be channeled into an electrical current. Photovoltaics are the ultimate source of clean, green and renewable energy but today's technologies utilize relatively scarce and expensive semiconductors, such as large crystals of silicon, or thin films of cadmium telluride or copper indium gallium selenide, that are tricky or expensive to fabricate into devices.

"Solar technologies today face a cost-to-efficiency trade-off that has slowed widespread implementation," Zettl says. "Our technology reduces the cost and complexity of fabricating solar cells and thereby provides what could be an important cost-effective and environmentally friendly alternative that would accelerate the usage of solar energy."

Alex Zettl (left) and Will Regan can make low-cost, high efficiency solar cells from virtually any semiconductor material. Credit: (Photo by Roy Kaltschmidt)

This new technology is called "screening-engineered field-effect photovoltaics," or SFPV, because it utilizes the electric field effect, a well understood phenomenon by which the concentration of charge-carriers in a semiconductor is altered by the application of an electric field. With the SFPV technology, a carefully designed partially screening top electrode lets the gate electric field sufficiently penetrate the electrode and more uniformly modulate the semiconductor carrier concentration and type to induce a p-n junction. This enables the creation of high quality p-n junctions in semiconductors that are difficult if not impossible to dope by conventional chemical methods.

"Our technology requires only electrode and gate deposition, without the need for high-temperature chemical doping, ion implantation, or other expensive or damaging processes," says lead author William Regan. "The key to our success is the minimal screening of the gate field which is achieved through geometric structuring of the top electrode. This makes it possible for electrical contact to and carrier modulation of the semiconductor to be performed simultaneously."

Under the SFPV system, the architecture of the top electrode is structured so that at least one of the electrode's dimensions is confined. In one configuration, working with copper oxide, the Berkeley researchers shaped the electrode contact into narrow fingers; in another configuration, working with silicon, they made the top contact ultra-thin (single layer graphene) across the surface. With sufficiently narrow fingers, the gate field creates a low electrical resistance inversion layer between the fingers and a potential barrier beneath them. A uniformly thin top contact allows gate fields to penetrate and deplete/invert the underlying semiconductor. The results in both configurations are high quality p-n junctions.

Says co-author Feng Wang, "Our demonstrations show that a stable, electrically contacted p-n junction can be achieved with nearly any semiconductor and any electrode material through the application of a gate field provided that the electrode is appropriately geometrically structured."

The researchers also demonstrated the SFPV effect in a self-gating configuration, in which the gate was powered internally by the electrical activity of the cell itself.

"The self-gating configuration eliminates the need for an external gate power source, which will simplify the practical implementation of SFPV devices," Regan says. "Additionally, the gate can serve a dual role as an antireflection coating, a feature already common and necessary for photovoltaics."

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Shakescene21
4.7 / 5 (3) Jul 26, 2012
So photovoltaics could be made from semi-conductor materials that are cheaper, more plentiful, less toxic, or easier to work with...
Every few weeks photovoltaics take another step forward.
Bewia
1.8 / 5 (5) Jul 26, 2012
Every few weeks photovoltaics take another step forward.
But these steps are getting gradually smaller and smaller, so we can ask, when we should finish with sponsoring of photovoltaic research from taxes and mandatory fees. It's like the progress in Windows or MS Office development - you can get new version with revamped user interface each year, but its utility value raises way slower.
PPihkala
5 / 5 (2) Jul 26, 2012
It would be interesting to get numbers. What is efficiency and what will be the cost?
Grallen
4 / 5 (5) Jul 26, 2012
Every few weeks photovoltaics take another step forward.
But these steps are getting gradually smaller and smaller, ...

WHAT? This is a huge leap forward. Cost has what has been holding back photovoltaics for ages! The ability to use abundant materials instead of rare ones is amazing.
CapitalismPrevails
2.3 / 5 (3) Jul 26, 2012

WHAT? This is a huge leap forward. Cost has what has been holding back photovoltaics for ages! The ability to use abundant materials instead of rare ones is amazing.

So where are the numbers? If it were a big deal they would be confident enough to publicize the efficiency % and guesstimated $/kWh.
antialias_physorg
4 / 5 (2) Jul 27, 2012
This is pretty cool. I hope they figure out a way to not only use cheap materials but also have a cheap production process that scales well (I'm not sure this type of structure is suitable for a roll-on process - which is what factories are always looking for)
But that is a problem for another day. I'm sure once they crunch the numbers of how much they can save on materials - and become independent of world market price fluctuations by using ubiquitous ones - research into scaleable construction will be tackled with force.
mrCalvin
5 / 5 (1) Jul 27, 2012
The opening sentence of the summary is ". . . .enable low-cost, high efficiency solar cells to be made from virtually any . . .. " Yet the aricle pays scant attention to cost (Other than cheap material / process) and skips over efficiency entirely.
Satene
3 / 5 (1) Jul 27, 2012
This is a huge leap forward. Cost has what has been holding back photovoltaics for ages! The ability to use abundant materials instead of rare ones is amazing.
The cost of material is marginal at the case of silicon solar cells (the primary source of silicon is just sand melted). What is expensive is another transparent layer (indium?!) and metallic electrode (gold, silver?), increased complexity and production cost and decreased stability and lifetime. The TCO will be increased with more complex construction and production of cells and their stability will be limited as the thin surface layers will be prone to contamination/damage. Without thorough calculation it's not possible to decide, whether the SFPV technology has some economical meaning or not.

Your schematic opinion clearly follows the PR presented in the article, but there are many other technical aspects in the game.
Satene
not rated yet Jul 27, 2012
If we take a look at the development of silicon solar cells during last twenty years, we may recognize easily, that the research of last twenty years influenced their construction in subtle way only. Why? Because most of clever improvements (advanced light trapping nanosurfaces and antireflective coatings, various nanorods, nanopillars and quantum dots, hot electron trapping layers etc) are decreasing the stability and lifetime of solar cells significantly. They're serving well like the salary generators for researchers involved, but they never leaved their labs from good reasons. http://energy.aol...ne-knows

BTW The same problem exist with stability of lithium batteries, which work well at the paper, but in reality they degrade way faster, than their manufacturers are promising. The customers can be fooled easily and their investments are wasted in this way.
daveib6
not rated yet Jul 27, 2012
But these steps are getting gradually smaller and smaller

The truth is that photovoltaics, as with all other semiconductor technologies are improving on an exponential curve, precisely the opposite of what Bewia claims. It really doesn't matter which aspect of solar technology you measure, over the last half century, if you plot that measure on a logarithmic scale, the graph is surprisingly smooth. This will continue into the foreseeable future until solar is supplying the majority of grid energy. The only point I would make in defense of Bewia's argument is that since the cost of solar has already crossed the line at which the energy produced is competitive with the average utility energy costs without adding in the soft costs of regulatory BS, the only place for the Federal government left is to reduce those soft costs so that industry can utilize the technology to its' fullest potential without all the regulatory BS as much as doubling the cost of the final installation.
daveib6
not rated yet Jul 27, 2012
The good news is that this is exactly what the Obama administration is attempting to do. http://www1.eere....id=18516
Shakescene21
not rated yet Jul 27, 2012
"Every few weeks photovoltaics take another step forward."

" But these steps are getting gradually smaller and smaller, ..."

@Bewia-- What evidence do you have that these steps are getting smaller? There is no reliable data series that I know of, but all indications are that the cost per KWH is going down and percentage conversion efficiency is going up. The flood of technological advancements will sustain those trends and also enable the use of materials that are more common, less toxic, longer-lasting, etc.

Eikka
not rated yet Jul 27, 2012
WHAT? This is a huge leap forward. Cost has what has been holding back photovoltaics for ages! The ability to use abundant materials instead of rare ones is amazing.


Yes. Infrastructure and installation cost. The cost of the photovoltaics themselves are very close to being neglible.
wwqq
not rated yet Jul 30, 2012
@Bewia-- What evidence do you have that these steps are getting smaller?


Going from 0.1% at the beginning of last century to 6% in 1954 is a factor 60 improvement, and this occured with almost no investment. Despite 50 years of fawning adulation the improvement since then has been a factor of a few, because the low hanging fruit has been picked and we're approaching physical limits of single- and multi-junction efficiency.

Cost of panels has been going down much faster, but that game is now over. O&M and balance-of-system costs aren't getting noticably cheaper and they already dominate overall costs.

That's ignoring the fact that the electrical grid is not a giant battery and that we can't keep burning natural gas forever. Actual storage would be very expensive and would not make much sense even if the power was gratis.
antialias_physorg
not rated yet Jul 30, 2012
Going from 0.1% at the beginning of last century to 6% in 1954 is a factor 60 improvement, and this occured with almost no investment.

However relative percentage increases are totally irrelevant. What's relevant is how much energy you get out (i.e. the absolute precentage of sunlight converted to electricity) vs cost.
Because when all is said and done: it's the energy you get out which you use.

New technologies can also resphape what is economical to add to existing materials. Right now installation adds a lot to the cost of PV.
But first companies are already adding photovoltaics to shingles. With transparent photovoltaics we can add this stuff to windows. It's just a matter of time until the adding of this capability becomes so cheap in the manufacturing process that the end product will be only marginally more expensive than the product without it.

And at that point people will go "why not have it in there?"
Shakescene21
not rated yet Aug 02, 2012

Going from 0.1% at the beginning of last century to 6% in 1954 is a factor 60 improvement, and this occured with almost no investment. Despite 50 years of fawning adulation the improvement since then has been a factor of a few...


In break-even analysis, going from 0.1% to 6% efficiency is usually less significant than going from 6% to 18%. This is especially true when the cost-per-square-foot is going down at the same time as efficiency is going up. Cost per kwh and percent efficiency are critical, as well as the price of coal-generated electricity.

As far as the costs of energy storage and system infrastructure, there are parallel improvements in these areas too, and the outlook for photovoltaics looks better every year.

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