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Making Solar Cells Cheaper — It Could Be Plastics

Like the advice given to Dustin Hoffman in The Graduate, some people think the future of solar cells lies in plastics. Here's why.
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Enough solar energy hits our Earth each day to satisfy all of the world’s energy demands throughout the year. A technology exists — photovoltaics — that is capable of transforming sunlight directly into electricity without any moving parts. The technology has been around for more than 50 years, yet it contributes only 0.1 percent of the all of the energy produced worldwide.

High cost has impeded greater market penetration.

Silicon, the first material discovered that is capable of converting enough sunlight directly into electricity for practical purposes, has dominated the photovoltaic market to this day. Ever since its discovery, people have been looking for a cheaper material or a less-expensive process for making silicon solar cells. From start to finish, changing the raw silicon to a solar cell has been very expensive.

In the 1970s, technologists found they could save money making silicon solar cells by bypassing costly “crystal growing” and casting silicon into ingots. But the bulkiness of working with large blocks of silicon that had to be cut into extremely thin wafers remained. To eliminate the time-consuming sawing and the waste it created, others chose to pull out — from a vat of molten silicon — a thin ribbon just thick enough for solar modules.

Others searched for alternative photovoltaic materials that could be deposited over a large area on a rigid surface in a continuous process. The so-called active layer — the part that generated a stream of electrons from sunlight — would be only several microns thick. These are called thin films.

Plastics have also entered in the race for cheaper solar cells. Their entrance had its beginnings in the late 1970s with the discovery of a new class that behaved more like metals or semiconductors than like traditional plastics, which could not conduct electricity at all.

Fifteen years later, it was also discovered that when one of these conductive plastics was joined with a carbon-60 molecule (a soccer-ball-shaped entity known as Buckminsterfullerene) and light hit the plastic, an electron transfer to the carbon molecule occurred, producing measurable amounts of electricity — hence, the transformation of conductive plastics to a plastic solar cell usually referred to as an organic solar cell or organic photovoltaics (“organic” in this sense referring to chemistry involving carbon compounds).

Organic photovoltaics can be mixed in solution to become ink- or paintlike. They lend themselves to printing on fabric or film, as in a newspaper-printing plant, or brushed or sprayed onto a surface like paint. Either way, production becomes ultra-fast, and they can conform to the shape of the structure they are stuck to.

Think of the applications: painting photovoltaic material on poles holding up and now powering street lamps and traffic lights; in similar fashion, the back of a cell phone could be covered with organic solar cell material for its source of power, as could building facades, windows and rooftops for generating electricity indoors. Choosing different molecules for richer color schemes could provide aesthetically pleasing clothing, awnings, drapery and handbags that would also act as electrical generators for portable electronics.

So the interest in organic photovoltaics has gained momentum in the last decade, spurred also by their extremely low cost. Beyond just higher costs, silicon and its inorganic thin-film relatives are relatively rigid in comparison, requiring structures to support them.

But will organic solar cells live up to their hype? Composed of molecules rather than atoms (as opposed to pure silicon), there’s more disorder. Extremely fast processing of organics, in one sense an advantage, adds to the problem, as there’s very little time to structurally arrange the device properly, and polymer chains can fold and tangle.

At that point, the charges produced can get caught up in this molecular clutter and not make it to collection points as harvestable electricity. Absorbing the right part of the spectrum for optimum power production also poses problems for organics. Designers of organic solar cells have yet to come up with cells that can put to use, as silicon does, the packets of energy coming from the sun that pack more power.

For all of these reasons, the amount of sunlight hitting an organic cell that becomes useful electricity is dwarfed by its silicon counterpart, which have an efficiency — the amount of solar energy that the solar cell converts into useful energy -- of 14 to 20 percent for commercial cells and up to 24.7 percent in the lab. In contrast, the first organic cells had an efficiency of 1 percent. Now, the best laboratory organic solar cells now can convert more than 6 percent of the incoming sunlight into electricity. The new record was accomplished by stacking two different organic materials in layers so that they absorb a wider range of the solar spectrum; please remember that all efficiencies reported for organic solar cells come from laboratory products.

“This is the highest level achieved for solar cells made from organic materials,” said Alan Heeger, who shared the Nobel Prize for co-discovering conducting plastics described earlier. The new record gives Heeger and others working with plastic solar cells the confidence that  “additional improvements” can be made that “will yield efficiencies sufficiently high” for their commercialization.

With such a dramatic rise in efficiency, there is no doubt that organic solar cells will one day provide significant amounts of clean, renewable power throughout the world.

By the same token, silicon’s continued room for improvement should not be shrugged off. Efficiency is rising by novel designs, such as placing all the electrical contacts on the back so the portion exposed to the sun has an unobstructed exposure to sunlight.

Others have combined thin-film amorphous silicon with more ordered crystalline to boost electrical output. Costs have also been brought down by making cells thinner, using less of the costly silicon to produce more electricity per cell.

Record growth rate of product around 40 percent a year over the last several years has also lowered production price — each doubling of output reduces costs by 20 percent. Even those converting silicon compounds into silicon pure enough for solar cells are now finding cheaper ways of making the feedstock.

Then there are the thin films like cadmium telluride and copper-indium-galllium- diselenide just coming onto the market; their manufacturers promise these products will significantly lower the price of photovoltaics.

With fossil fuels at record prices and the compelling need to produce electricity without causing greenhouse gas emissions, it seems there’s room for everything under the sun that can generate electricity without burning fossil fuels.

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