New bandgap boundaries could boost electronics

Ashley N. Paddock, ashley.paddock@photonics.com

A new layer-by-layer growth technique that reduces the bandgap of complex metal oxides by 30 percent could improve the performance of solar cells, LEDs, displays and other electronic devices.

For years, complex transition metal oxides have held promise for a variety of information and energy applications, but the challenge has been to devise a method to reduce bandgaps of those insulators without compromising the material’s useful physical properties.

The bandgap, a major factor in determining electrical conductivity in a material, directly determines the upper wavelength limit of light absorption. Wide bandgap tunability is highly desirable for optoelectronic devices and energy materials.

Ho Nyung Lee of Oak Ridge National Laboratory developed the layer-by-layer growth technique. For his achievement, he received the Presidential Early Career Award for Scientists and Engineers. He and his colleagues then used the method to achieve a 30 percent reduction in the bandgap of complex metal oxides, surpassing previous accomplishments of 6 percent – or 0.2 eV – and opening the door to new approaches to controlling bandgap in complex oxide materials.

“Our approach to tuning bandgaps is based on atomic-scale growth control of complex oxide materials, yielding novel artificial materials that do not exist in nature,” Lee said. “This ‘epitaxy’ technique can be used to design entirely new materials or to specifically modify the composition of thin-film crystals with subnanometer accuracy.”

ORNL’s materials scientists developed a synthesis strategy for discovering novel complex oxide thin films for stronger solar light absorption. Courtesy of ORNL.

With this discovery, the potential exists for oxides with band-gaps to be continuously controlled over 1 eV by site-specific alloying developed by the ORNL team.

“This work exemplifies how basic research can provide technical breakthroughs that will result in vastly improved energy technologies,” said Michelle Buchanan, associate lab director for ORNL’s Physical Sciences Directorate.

A patent is pending for the technology. The research was funded initially by the Laboratory Directed Research and Development program and later by the US Department of Energy’s Office of Science. Optical measurements were performed in part at the Center for Nanophase Materials Sciences, a DOE-BES user facility at ORNL.

The findings were outlined in Nature Communications (doi: 10.1038/ncomms1690).