Dopant gives graphene solar cells highest efficiency yet

May 21, 2012 by Lisa Zyga feature
Graphene-based Schottky junction solar cells: (a) undoped, (b) doped, and (c) an image of a doped solar cell showing contacts and contact leads. Image credit: Miao, et al. ©2012 American Chemical Society

( -- By taking advantage of graphene’s favorable electrical and optical properties, and then adding an organic dopant, researchers have achieved the highest power conversion efficiency yet for a graphene-based solar cell. The 1.9% power conversion efficiency of the undoped devices increases by more than four times to 8.6% after doping.

The researchers, led by Sefaattin Tongay and Arthur F. Hebard at the University of Florida in Gainesville, have published their study on the high-efficiency solar cells in a recent issue of Nano Letters.

“Here, not only we have taken advantage of graphene's beautiful optical transparency, but also we have reduced graphene's electrical resistance by adjusting the Fermi level of graphene using a cheap and environmentally stable organic coating layer,” Tongay told “During this step, Nature favored us by yielding a higher rectification and electric field at the interface, further improving the solar cell’s .”

In the new solar cells, a single layer of graphene placed on top of a silicon wafer serves as a Schottky junction, the main component of simple photovoltaic devices called Schottky junction solar cells.

Under illumination, electron-hole pairs are photogenerated in the silicon. The photogenerated electrons and holes are separated by the Schottky junction’s built-in electric potential and collected by the oppositely charged graphene and semiconductor contacts. This one-way flow of current (electrons flowing in one direction and holes in the other) is a defining property of the Schottky junction and enables the generation of power from the device.

While graphene-based Schottky junction solar cells have been demonstrated in the past, here the researchers took an extra step and doped the graphene with the organic chemical TFSA using a simple spin-casting method.

Doping allowed the researchers to adjust graphene’s Fermi level (a measure of electron potential energy), which resulted in two changes that improved the solar cells’ overall efficiency: a reduction in the graphene’s resistance and an increase in the solar cell’s built-in potential, which leads to a more efficient separation of the electron-hole pairs generated by the absorbed photons.

With their 8.6% efficiency, the doped devices provide a significant efficiency improvement over other graphene-based Schottky junction solar cells, which have so far demonstrated efficiencies ranging from 0.1% to 2.86%.

Compared with Schottky junction solar cells that use indium tin oxide, those that use graphene have several advantages. For instance, the ability to tune graphene’s properties enables researchers to optimize solar cell efficiency and use the graphene layer on other semiconductors besides silicon.

The researchers hope that the methods used here, which are simple and scalable, can lead to further device improvements and practical applications in the future.

“We expect that the efficiency can be further improved by engineering the interface, using different organic coating layers yielding higher doping effects, improving the graphene quality and graphene transfer procedure, using anti-reflection layers, and numerous other methods known by the solar cell community,” Tongay said. “This is just a beginning.”

Hebard added that further discoveries of graphene physics should lead to more efficient and inexpensive solar cells.

“Our described power conversion efficiency increase with the simple application of a stable organic overlayer is just a beginning,” he said. “Graphene and its derivatives continue to surprise us with unusual properties (strength, flexibility, diffusion barrier, tunable Fermi energy, linear electronic spectrum, etc). Further advances will come with a deeper understanding of the physics of how incoming photons efficiently create electrons and holes, which are then separated and collected in our described configuration. This knowledge should be applicable to finding alternative substrates to silicon (organics and polymers come to mind), which are less expensive and can be applied to large areas.

“It's clear that research on graphene and its derivatives is already in the sunlight; we expect that our work on will keep it there.”

Explore further: Study reveals new characteristics of complex oxide surfaces

More information: Xiaochang Miao, et al. “High Efficiency Graphene Solar Cells by Chemical Doping.” Nano Letters. DOI: 10.1021/nl204414u

Journal reference: Nano Letters search and more info website

4.5 /5 (12 votes)

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1.2 / 5 (12) May 21, 2012
why do we care about a solar cell that is only 10% effiecient at best??
5 / 5 (17) May 21, 2012
Because it may be dirt cheap? And because this is a very crude way of doping (spin cast). If history is anything to go by then more exact doping methods would get an even better efficiency.

Any technology starts out small. If you always say "no good" when starting out and getting your first percent efficiency we'd NEVER get to anything worthwhile. Not that the solar cells which we use today and give us the highest efficiency also started out at 1-2%.

Research just doesn't produce 100% from a standing start.
2 / 5 (6) May 21, 2012
For all the news of breakthroughs and increases in efficiency when will it come to market??????
5 / 5 (11) May 21, 2012
For all the news of breakthroughs and increases in efficiency when will it come to market

This is Physorg. Not Endgadget.
5 / 5 (3) May 21, 2012
Right, people who do not understand the nature of research, probably shouldn't be reading this site.
5 / 5 (6) May 21, 2012
why do we care about a solar cell that is only 10% effiecient at best??

Because it's interesting research that might give us important insights in the future. Enjoy yourself a bit more. Science is supposed to be fun.
4.5 / 5 (2) May 21, 2012
I doubt it is 10% at best. I see that this work is coming from a research institute and unlike cooperate they got limited funding and resources. They have also pointed out other ways to further boost the efficiency. I wonder why not published in Nature?
4.8 / 5 (4) May 21, 2012
Another amazing Graphene story. Anyone who doesn't realize the significance of 8.6% efficiency this early in the research stage is ... The researchers have already identified numerous techniques that may could bump the efficiency quite a way. And it doesn't appear to use any rare metals.
5 / 5 (2) May 21, 2012
Why do you care about taking a dump when it is a 1E-34 th the mass of the universe?

"why do we care about a solar cell that is only 10% effiecient at best??" - ElNose

The organic cells used in most outdoor solar LED lighting is only 3% efficient or less.
3.7 / 5 (3) May 22, 2012
The Big Number that matters is $/Watt, not efficiency
1 / 5 (1) May 22, 2012
@ant -- dirt cheap but the cost is real estate -- how much space does this take up -- and 33% is the Shockley Queisser Limit for a single junction

@axe this isn't interesting -- its just a plug for graphene which turns out makes a terrible solar cell -- but is awesome at other things

@sciencewiz -- 10% was a high number -- the article states

The 1.9% power conversion efficiency of the undoped devices increases by more than four times to 8.6% after doping.

and you got voted up for doubting it was that low when its clearly stated in the article ???

@shakescene --
Anyone who doesn't realize the significance of 8.6% efficiency this early in the research stage is ...
is what ??? is knowledgable that in the past that signifies that they will probably max out around 15% ??? what did you wanna say?

@vendicar -- thank you for giving an answer to the question !!! up vote

@dirk -- if you like have you backyard full of solar panals that can only charge a flashlight

1 / 5 (1) May 22, 2012
I am really not trolling -- but everyone attacked me instead of answering the question with a knowledgeable and intelligent retort.

People implied I am stupid, that I didn't get science, that I am not supposed to pay attention to effeciency when talking about solar cells...

at least axemaster said it was interesting - I just happen to disagree, vendicar actually gave a reason that was non-obvious..

I remember being able to ask questions in this forum and get an answer, instead of an attack.
5 / 5 (1) May 22, 2012
dirt cheap but the cost is real estate

It really depends on the application.
If you have the real estate then great. Many people don't have enough money to plaster their entire roofs with the conventional kind of solar. A cheap alternative - while being more space intensive - may bring in similar net gains for less investment

Some applications (like solar panels for space probes) have very different specs (maximum watt per kg and resistance to harsh radiation environment).
Some areas are more prone to hurricanes/tornadoes/blizzards/hailstorm so a cheap/easily replaced solar panel may be more cost effective in the long run.

You never know what will fit your (economic and environment) situation best until you know all the options.
not rated yet May 22, 2012
The Big Number that matters is $/Watt, not efficiency

Obviously, $/Watt is critical, and this technology doesn't appear to need expensive materials -- but there are plenty of important numbers that matter: The amount of surface area available will often dictate that a panel needs to have 10% or more efficiency to produce enough electricity. Life expectancy of the panels, toxicity, ability to integrate with other technologies, maintenance requirements, and flexibility are all important characteristics.

One thing to remember about $/Watt is that tends to drop over time, unless the cells use rare and expensive materials.