September 20, 2003
Princeton Research Group Makes Organic Photovoltaic Breakthrough

Princeton University researchers have developed techniques that may finally make organic photovoltaics cheaper than existing silicon-based photovoltaic solar cells.

PRINCETON, N.J. -- Princeton electrical engineers have invented a technique for making solar cells that, when combined with other recent advances, could yield a highly economical source of energy.

The results, reported in the Sept. 11 issue of Nature, move scientists closer to making a new class of solar cells that are not as efficient as conventional ones, but could be vastly less expensive and more versatile. Solar cells, or photovoltaics, convert light to electricity and are used to power many devices, from calculators to satellites.

The new photovoltaics are made from "organic" materials, which consist of small carbon-containing molecules, as opposed to the conventional inorganic, silicon-based materials. The materials are ultra-thin and flexible and could be applied to large surfaces.

Organic solar cells could be manufactured in a process something like printing or spraying the materials onto a roll of plastic, said Peter Peumans, a graduate student in the lab of electrical engineering professor Stephen Forrest. "In the end, you would have a sheet of solar cells that you just unroll and put on a roof," he said.

Peumans and Forrest cowrote the paper in collaboration with Soichi Uchida, a researcher visiting Princeton from Nippon Oil Co.

The cells also could be made in different colors, making them attractive architectural elements, Peumans said. Or they could be transparent so they could be applied to windows. The cells would serve as tinting, letting half the light through and using the other half to generate power, he said.

Because of these qualities, researchers have pursued organic photovoltaic films for many years, but have been plagued with problems of efficiency, said Forrest. The first organic solar cell, developed in 1986, was 1 percent efficient -- that is, it converted only 1 percent of the available light energy into electrical energy. "And that number stood for about 15 years," said Forrest.

Forrest and colleagues recently broke that barrier by changing the organic compounds used to make their solar cells, yielding devices with efficiencies of more than 3 percent. The most recent advance reported in Nature involves a new method for forming the organic film, which increased the efficiency by 50 percent.

Researchers in Forrest's lab are now planning to combine the new materials and techniques. Doing so could yield at least 5 percent efficiency, which would make the technology attractive to commercial manufacturers. With further commercial development, organic solar devices would be viable in the marketplace with 5 to 10 percent efficiency, the researchers estimated. "We think we have pathway for using this and other tricks to get to 10 percent reasonably quickly," Forrest said.

By comparison, conventional silicon chip-based solar cells are about 24 percent efficient. "Organic solar cells will be cheaper to make, so in the end the cost of a watt of electricity will be lower than that of conventional materials," said Peumans.

The technique the researchers discovered also opens new areas of materials science that could be applied to other types of technology, the researchers said. Solar cells are made of two types of materials sandwiched together, one that gives up electrons and another that attracts them, allowing a flow of electricity. The Princeton researchers figured out how to make those two materials mesh together like interlocking fingers so there is more opportunity for the electrons to transfer.

The key to this advance was to apply a metal cap to the film of material as it is being made. The cap allowed the surface of the material to stay smooth and uniform while the internal microstructure changed and meshed together, which was an unexpected result, said Forrest. The researchers then developed a mathematical model to explain the behavior, which will likely prove useful in creating other micromaterials, Forrest said.

"We've shown a very new and general process for reorganizing the morphology of materials and that was really unanticipated," Forrest said.

The research was supported by grants from the Air Force Office of Scientific Research, the National Renewable Energy Laboratory and the Global Photonic Energy Corp.

Some day advances in fabrication techniques will so lower the cost of making photovoltaic solar cells that they will become cost-effective to generate a substantial portion of our electric power. The big question is when will this happen?

Share |      Randall Parker, 2003 September 20 03:37 PM  Energy Solar

Patrick said at September 23, 2003 12:02 AM:

The other point is that a solar cell is a light emitting diode in reverse. Well not exactly but if you can make one then you should be able to make the other. Ultra cheap sheet LEDs have a lot of uses. How about your entire ceiling being a light? The white lines on the road being illuminated? Glowing billboards? (OK, forget the last idea.)

Randall Parker said at September 23, 2003 12:50 AM:

I've made previous posts on the progress to make solid state replacements for conventional light bulbs. My guess is that we are going to witness the beginning of the phase-out of incandescent and flourescent bulbs starting within a decade.

Ken Novak said at September 25, 2003 11:02 AM:

About PV costs: My bottom line estimate is for grid-competitive PV products selling into some markets in 2007, and being mass marketable in 2011. I've just posted my rationale on this, at

Projections on solar phototaic cell prices

Ken Novak

ps. glad to see you list me in your blog roll. i'll have to start checking yours out...

Randall Parker said at September 25, 2003 12:21 PM:


Went and read your post. My comments:

As for the US government speeding market adoption: I just so totally disagree I don't know where to begin.

In a nutshell: I think the US government should fund the basic research at a much higher rate. University research labs should be getting orders of magnitude more funds to look at all manner of photovoltaic materials.

But as for production credits: Since the types of materials that are likely to lead to the greatest cost reductions are not even in production yet why subsidize current refinement of current processes? It provides revenue to current manufacturers. Most of that revenue goes toward using current processes, not toward making new processes, Plus, those manufacturers may not be the ones that come up with the cost breakthrus. How many of the companies you listed in your own post are not even making photovoltaics for sale as of yet?

Also, the really cool advances are going to allow the creation of materials that are not separate from roofs or sidings. There is no point in refining ways to make solar panels that go on roofs if future roof tiles will themselves be the photovoltaic cells.

Government can usefully speed up basic research and applied research toward understanding and discovering new materials. But anything that moves into the realm of technological development will happen quite quickly with private money once the basic research produces breakthrus that venture capitalists see will make possible a viable technology.

Randall Parker said at September 25, 2003 12:35 PM:

Also, with regard to the period of cost-halving for solar cells: The rate of halving of solar cell cost has been running at a rate of longer than a decade per cost halving. The rate of price decline was much faster back in the 70s but has slowed.

To reiterate: the refinement of current approaches is not going to get us there. We need basic research into materials that have the potential to be inherently easier to fabricate.

Ken Novak said at September 25, 2003 1:06 PM:

O well, i guess we'll disagree. key points:

- no argument on basic science, of course that's worthwhile. it will have unexpected affects in many areas, including ones that are not environmentally benign, but on balance certainly worth more money. (The NNI is a good example)

- as i said in posting, the costs that will go down with government market promotion are the integration and "balance of system" costs. also, there will be more actors ready to adopt cheaper cells as they become available (dealers, installers, roofing material developers, etc). these matter regardless of the substrate of the cell.

- declarations of government policy are obviously helpful; just check how the wind production tax credit has performed (and the disruptions that on-again/off-again politics has had)

- roofs are key. all cells today can be integrated into roofing materials, all future cells will be too. the substrate is not the issue, the coating and other integration costs are much the same. it's the substrate choice that may change.

- many installations of PV have decent payback now (in addition to traffic lighting, marine applications, communications towers, etc). Production subsidies will promote those applications faster than suburban rooftop applications.

- as for the rate of doubling: your DOE report uses market sale averages, which do not directly track the technology, but rather the types of units that are produced in the US for their buyers. these are not the low cost producers now, which are mostly in asia. also, i was looking at retail prices for unmouted cells, not cost as they leave the factory, which is why i looked at $3 prices today rather than the lower prices in your sources.

anyway, as you mention, for global warming time scales, nano will probably eventually kick in (although policy measures might cut the window from say, 30 years to 20). and for today's geopolitical purposes (ie, replacing mideast oil), there's no possible change for 10 years minimum, so it doesn't change anything there either.


Randall Parker said at September 25, 2003 1:55 PM:

Ken, Substrate costs? I'm talking about materials advances where the roofing material would be the photovoltaic material. There wouldn't be a substrate. There'd be a single manufacturing process in a single factory that would produce the covering for a roof. Roofing material would be unrolled onto the top of the house and and at the end of the unrolled material there'd be wires coming out to carry off the juice. The roofing covering makers would be the photovoltaic materials makers. In an even more advanced version there wouldn't even be underlying wood panels. The roof would be a single material that was layed down on top of the wood beams (though the wood beams would themselves will eventually be replaced by materials made from nanotubes).

Same for sidings on houses and other buildings. You won't go and buy aluminum siding for your house. You'll buy photovoltaic siding that came from a factory that made each siding panel in its entirety. The obstacle for learning how to install it would be quickly overcome by market forces if the stuff was cheap enough to lower people's air conditioning bills. You'd just have an electrician working with the roofing and siding people to connect up the wires as the stuff was put up.

My view of market subsidies is that they return far less in terms of progress per dollar spent than basic research and applied research return. See the last comment I have at this post where I include an excerpt from a draft of the 2002 "House Energy and Water Appropriations Language". The total spent on university research (if we include the thin films money) is less than $30 mil per year. I'd rather take the whole renewable energy budget and take all that money and just give it to university researchers. Concentrators, technology demonstrators, Stirling engines, and other gimmicks do not hold out the prospect of orders of magnitude reductions in costs.

As for the possibility of change in 10 years: I agree with Nobel Laureate Richard Smalley that a Manhattan Project approach could lead to enormous breakthroughs in energy technology (see the bottom of that post for Smalley's comments at a Congressional hearing).

I'm advocating a far more radical approach to accelerating scientific and technological advance. Subsidizing markets is not the way to produce radical advances.

Randall Parker said at September 25, 2003 2:00 PM:

BTW, I do no see the point of having a wind tech subsidy. Wind power price drops are not going to be able to keep up with solar power price drops. Wind is going to lose out in solar since making structure surfaces be photovoltaics will eventually result in tiles and sidings that are no more expensive than current tiles and sidings. The incremental costs of photovoltaics will be incredibly low. Though we still need a way store photovoltaic energy the same is true for wind as well.

A. Fischer said at October 16, 2004 8:09 PM:

Are these materials available now? And, if so, how can I obtain them?

Stephen said at January 11, 2006 9:35 PM:

I'm doing a project on this, can anyone tell me the most efficient polymer thats being used right now, Im talking like in chemical formulas. Forgive me if what im saying is incorrect, I'm only a high school student.

Stephen said at January 11, 2006 9:35 PM:

I'm doing a project on this, can anyone tell me the most efficient polymer thats being used right now, Im talking like in chemical formulas. Forgive me if what im saying is incorrect, I'm only a high school student.

Stephen said at January 11, 2006 9:35 PM:

I'm doing a project on this, can anyone tell me the most efficient polymer thats being used right now, Im talking like in chemical formulas. Forgive me if what im saying is incorrect, I'm only a high school student.

Stephen Y said at January 11, 2006 9:35 PM:

I'm doing a project on this, can anyone tell me the most efficient polymer thats being used right now, Im talking like in chemical formulas. Forgive me if what im saying is incorrect, I'm only a high school student.

Stephen Y said at January 11, 2006 9:35 PM:

I'm doing a project on this, can anyone tell me the most efficient polymer thats being used right now, Im talking like in chemical formulas. Forgive me if what im saying is incorrect, I'm only a high school student.

ted lahti said at March 5, 2006 11:46 AM:

We have built a solar powered passenger rail vehicle.
Does anyone know the best manufactures of
organic solar cells? This would be perfect
for our Soltrain. Ted Lahti

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