In two just-released studies, scientists have announced new ways of making solar cells less expensive and more efficient. In one of the projects, researchers from the University of Toronto demonstrated that nickel can work just as well as gold for electrical contacts in colloidal quantum dot solar cells. In the other, a team from California’s Lawrence Berkeley National Laboratory added selenium to zinc oxide, dramatically increasing the oxide’s efficiency in absorbing solar light. Both developments could result in more practical, affordable solar technology.

Quantum dot solar cells are already on the less-expensive side, as the dots themselves (also known as nanocrystals) are nanoscale bits of a semiconductor material, created using low-cost chemical reactions. The current produced by those dots has traditionally been collected via gold electrical contacts, but researchers from the U of T’s Photovoltaics Research Program have now successfully used nickel contacts to get the job done just as well.

When the team first tried nickel, it intermixed with the quantum dots, forming a compound that blocked the flow of the electrical current. By adding just one nanometer of lithium fluoride between the nickel and the dots, however, a barrier was created that kept the two from mixing, while still allowing the current to flow from the dots to the nickel.

The scientists claim that by using nickel, the material costs of quantum dot solar cells will be reduced by 40 to 80 percent. They plan to commercialize the technology once they can boost the cells’ overall efficiency.

Like nanocrystals, zinc oxide is also a relatively inexpensive material. While it has shown promise as a solar power conversion medium, the challenge has been to boost its efficiency at collecting solar energy. The team from Berkeley Lab has succeeded in doing that, by embedding selenium in it. Even just a nine percent concentration of selenium in a mostly zinc oxide base was found to have a pronounced effect on its ability to absorb light.

"Researchers are exploring ways to make solar cells both less expensive and more efficient; this result potentially addresses both of those needs," said Marie Mayer, of Berkeley Lab’s Solar Materials Energy Research Group.

The research from both studies will appear in the journal Applied Physics Letters.