Solar energy could provide a clean alternative to fossil fuels but solar cells aren’t cheap. The high cost of solar power has been one of the main factors in keeping solar energy adoption rates down. Researchers at Stanford could lead to less costly and more efficient green energy production. They have found that adding a single layer of organic molecules to a solar cell can increase its efficiency three-fold and could lead to cheaper, more efficient solar panels.

Professor of chemical engineering Stacey Bent first became interested in a new kind of solar technology two years ago. These solar cells used tiny particles of semiconductors called “quantum dots,” they have been a subject of research for several years due to their potential for providing much more efficiency than traditional cells using materials like silicon. Quantum dot solar cells are cheaper to produce than traditional ones, as they can be made using simple chemical reactions involved in their production are simpler than those used in building existing solar cells.

Unfortunately, these solar panels have yet to live up to their high-efficiency promise, and researchers are working on ways to change that. The very small dots on quantum dot cells can be adjusted in size to capture energy at different wavelengths. But solar cells made of a single material have a maximum efficiency of about 31 percent, a limitation of the fixed energy level they can absorb.

Stanford chemical engineering professor Stacey Bent, along with a team of researchers, found that, by adding a single layer of organic molecules less than a nanometer thick, the efficiency of these quantum dot cells can be tripled. The type of organic molecule used is not important, which surprised Bent.

“We thought it would be very sensitive to what we put down,” Bent said.

Bent and her students have been investigating ways to improve the efficiency of quantum dot solar cells, along with associate Professor Michael McGehee of the department of Materials Science and Engineering.

The researchers put a coating of titanium dioxide semiconductor in their quantum dot solar cell with a very thin single layer of organic molecules. The molecules  interactions caused them to pack together in an ordered way. The quantum dots were present at the interface of this organic layer and the semiconductor. Bent’s students tried several different organic molecules in an attempt to learn which ones would most increase the efficiency of the solar cells.

But she found that the exact molecule didn’t matter — just having a single organic layer less than a nanometer thick was enough to triple the efficiency of the solar cells. “We were surprised, we thought it would be very sensitive to what we put down,” said Bent.