Solar cells go organic

Although they are not particularly efficient, plastic solar cells that are flexible enough to be
sprayed on roofs or printed on clothes look like being remarkably cheap

From The Economist print edition

EVERY minute, the sun showers the earth with more energy than the world’s entire population
consumes in a year. Unfortunately, it is expensive to convert all that sunshine into electricity.
Most solar cells are made of inorganic silicon and, like computer chips, require laborious
manufacturing processes that involve costly clean rooms and vacuum chambers. As a result,
solar energy costs roughly three to four times as much as electricity from conventional sources.
The good news is that recent advances in plastics and nanotechnology are speeding up the
development of cheap, flexible cells that can be sprayed on walls or even printed on paper and
fabrics.

A solar cell is made of semiconductive film sandwiched between two electrodes. When light
strikes the semiconductor, electrons in the vicinity become excited enough to dislodge
themselves from their surroundings and leave “holes” behind. A built-in electric field between
the two electrodes coaxes the electrons (which carry a negative charge) and the holes (with a
positive charge) to move to opposite ends of the cell—a relatively easy process in inorganic
semiconductors such as silicon.

In plastics, however, the force that binds an electron to a hole is nearly 100 times stronger.
Moreover, organic materials often have structural “traps”, like roadblocks on a highway, that
impair the electron’s mobility. Another drawback is that organic materials such as plastic absorb
less light from the solar spectrum. So far, plastic cells have achieved efficiencies of only 2.5%,
against 10% for standard inorganic solar panels and 30% in the most expensive modules. But
because polymers can be manufactured cheaply in a solution at room temperature, researchers
at the University of Arizona in Tucson, who have been experimenting with printing cells on
paper or plastic via inkjets, think all the fuss is worth it.

Instead of going wholly organic, another team at the University of California at Berkeley has
recently created solar cells made of inorganic nanorods, seven nanometres (billionths of a
metre) in diameter and 60 nanometres long, embedded in a film of polymers. The idea is to try
to combine the flexibility of plastics with the proven efficiencies of inorganic semiconductors.

Because the rods are so tiny (ie, 10,000 times thinner than hair), they take advantage of
phenomena that work at the quantum level where the absorption of certain colours of light
increases by a factor of two. There are other benefits of scaling down—the amount of costly
inorganic semiconductive material is reduced. As a result, there is no need to use clean rooms.
Similar to pure organic cells, these hybrids are easily manufactured cheaply in a beaker.

These new solar cells are not without their problems. To be commercially viable, their
efficiency still needs to be doubled or tripled from present levels. Nevertheless, both research
teams are convinced they can make the leap within the next few years. Alan Heeger, who
shared a Nobel prize for the discovery of electrically conductive plastics, agrees. “Ten per cent
[efficiency] is possible,” he says.

Several companies have shown interest in commercialising the prototypes. If plastic solar cells
take off, as Berkeley’s Janke Dittmer envisages, they could even end up being used in “smart
clothes” that recharge your MP3 player, palm device or mobile phone as you walk around. In a
few years’ time, sunbathing might then take on a whole new meaning.

http://www.economist.com/science/tq/displayStory.cfm?story_id=1176099