Even though plastic solar cells are cheaper to produce and they have some advantages like lightweight and flexibility, they still miss one important thing: the efficiency. Researchers are optimistic that in the years to come, the polymer solar cells will be able to compete from the efficiency point of view with the silicon solar cells, taking into account the fact that plastic solar cell have improved their efficiency each year.
Researchers led by a professor of physics at the University of California-Santa Barbara, Alan Heeger, have reported recently the development of a polymer solar cell that could convert about 6.1 % of the energy from the sun into electricity. This is a big step forward to the 10 % efficiency target which the plastic solar cell would need to reach in order to have an impact on the market. In comparison, conventional silicon solar cells have a 15 % efficiency.
Heeger is cofounder of Konarka, a plastic solar cell company, in which he is as well chief scientist. It seems that Konarka has made possible an increased efficiency of the polymer solar cell by using different materials than the cells made by Heeger in his lab. The increase was up to 6.4 %. A similar efficiency was reported by a Konarka competitor called Solarmer Energy.
Plastic solar cells have limited efficiency because of the polymer that can only absorb narrow bands of light. In order to increase the efficiency, films of polymer must be stacked and designed in such a way to catch different bands of light. But this would only make them expensive as the silicon cells but with lower efficiency.
To have a real improvement of polymer cells, each photon that gets absorbed by the polymer must be converted into an electron that can be collected. The electrons movement depends on the quality of the interface between the two components that make the film: California University cell used a conductive polymer and a version of a soccer-ball-shaped carbon compound called a fullerene while Heeger’s group used the same mixture but in different ratios with a different solvents for processing them. The result is a cell with nearly perfect internal efficiency.
But with these materials the 10 % efficiency will not be achieved. Heeger says “we can get there by synthesizing new materials that respond to more of the energy spectrum”.
Most of the energy coming from the sun is placed in the infrared spectrum, so scientists are now working to develop a material that is able to absorb this band. Luping Yu, a professor of organic chemistry at the University of Chicago who is collaborating with Solarmer Energy, said that he had developed a polymer which could absorb shorter-wavelength light, and which make the solar cells achieve more than 7 percent efficiency.