Qdot Breakthrough Could Mean Cheaper Solar Panels

A new method for producing quantum dots -- molecular specks of semiconductors -- could clear the way for the material to create better, cheaper solar panels than conventional ones made with silicon.

The research, by scientists at Rice University's Center for Biological and Environmental Nanotechnology (CBEN), describes a breakthrough chemical method for making four-legged cadmium selenide quantum dots, which previous studies have shown to be particularly effective at converting sunlight into electrical energy.

Research by Michael Wong and scientists at Rice University has revealed a breakthrough method for producing quantum dots -- molecular specks of semiconductors -- that could clear the way for producing better, cheaper solar energy panels. (Photo: Rice University)

"Our work knocks down a big barrier in developing quantum-dot-based photovoltaics as an alternative to the conventional, more expensive silicon-based solar cells," said principal investigator Michael Wong, assistant professor of chemical and biomolecular engineering.

Quantum dots are "megamolecules" of semiconducting materials that are smaller than living cells. They interact with light in unique ways to give off different-colored light or to create electrons and holes, due partly to their tiny size, partly to their shape and partly to the material they're made of. Scientists have studied quantum dots for more than a decade, with an eye toward using them in medical tests, chemical sensors and other devices.

One way to produce cheaper solar cells is to make them out of quantum dots. Other scientists' research has shown that four-legged quantum dots, which are called tetrapods, are many times more efficient at converting sunlight into electricity than regular quantum dots. But, Wong said, the problem is that there is still no good way to produce tetrapods. Current methods lead to a lot of particles with uneven-length arms, crooked arms and even missing arms. Even in the best recipe, 30 percent of the prepared particles are not tetrapods, he said.

CBEN's formula, which was developed by Wong and his graduate student Subashini Asokan, with CBEN Director Vicki Colvin and graduate student Karl Krueger, produces same-sized particles in which more than 90 percent are tetrapods. Significantly, these tetrapods are made of cadmium selenide, which until now have been very difficult to make.

The essence of the new recipe is to use cetyltrimethylammonium bromide instead of the standard alkylphosphonic acid compounds. Cetyltrimethylammonium bromide happens to be safer -- it's used in some shampoos, for example -- and it's much cheaper than alkylphosphonic acids. For producers looking to eventually ramp up tetrapod production, this means cheaper raw materials and less purification steps, Wong said.

"One of the major bottlenecks in developing tetrapod-based solar cell devices has been removed, namely the unavailability of high-quality tetrapods of the cadmium selenide kind," Wong said. "We might be able to make high-quality nanoshapes of other compositions also, using this new synthesis chemistry."

The research appears in the journal Small; Wong was co-author of the paper. The work was funded by the National Science Foundation, 3M Corp., Advanced Aromatics LP, the Air Force Office of Scientific Research and Rice.

For more information, visit: www.rice.edu