Quantum Dot Solar Panels

By Jade Boyd

Better, cheaper solar energy panels may soon be possible thanks to arrays of molecular specks of semiconductors called quantum dots. The idea of quantum-dot-based voltaics is not new. It’s long been known that four-legged cadmium selenide quantum dots, in particular, are effective at converting sunlight into electrical energy. The problem has been in finding a manufacturing method that makes high-quality dots in sufficient quantities.

That problem may have been solved by research conducted at Rice’s Center for Biological and Environmental Nanotechnology. “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 — giving off different-colored light or creating 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 achieve cheaper solar cells is to make them out of quantum dots. Prior research has shown that four-legged quantum dots — called tetrapods — are many times more efficient at converting sunlight into electricity than are regular quantum dots. But Wong said that current methods of producing tetrapods lead to a lot of particles with missing arms or arms that are of uneven length or crooked. Even in the best recipe, 30 percent of the prepared particles are not tetrapods.

"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,”

-Michael Wong

CBEN’s formula, developed by Wong and graduate student Subashini Asokan with CBEN director Vicki Colvin and graduate student Karl Krueger, produces same-sized particles, more than 90 percent of which are tetrapods. Significantly, these tetrapods are made of cadmium selenide, a compound that has been very difficult to produce in this configuration. The method is not only cheaper but safer than conventional methods because it uses cetyltrimethylammonium bromide instead of the normally used alkylphosphonic acid compounds. Cetyltrimethylammonium bromide is used in some shampoos, and for producers looking to ramp up tetrapod production, this means cheaper raw materials and fewer purification steps.

The research was funded by the National Science Foundation, 3M Corp., Advanced Aromatics LP, the Air Force Office of Scientific Research and Rice University. It appeared online May 1 in the journal Small.