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Science and Technology Highlights

Highlights in Research & Development

New III-V Device Concept Gets Two-for-the-Price-of-One Benefit

Combining and integrating two structures in semiconductor device leads to lower cost and more applications

NREL scientists, working with collaborators from the California Institute of Technology and University of New South Wales, have figured out how to integrate two design features of a solar cell to get better performance without increased deposition costs. The two features—a graded buffer and a Bragg reflector—have been merged into a graded buffer Bragg reflector (GBBR) that provides several key benefits to cell designers.

The image shows a three-junction solar cell stack. Each stack is divided into 4-5 rectangular sections (pink, white, green, and blue) showing how scientists are able to integrate two III-V device components into one in a multijunction III-V solar cell.

The graded buffer Bragg reflector (GBBR) combines two complex structures (GB and BR) into one thinner unit, enabling novel multijunction devices that may surpass current efficiency records with similar cost.

First, designers can select from a larger palette of III-V materials for the various layers in a multijunction solar cell. This is because the graded buffer can transition between two materials that are not lattice-matched—meaning that the crystal structures have different atomic spacings—without creating structural defects that can kill performance.

Second, the solar device can capture and use light in more than one pass through the cell stack. This is because the Bragg reflector can send light rays with specific wavelengths back into overlying layers so that they have a second chance to absorb the energy and convert it into electricity.

The innovative aspect of this work is that the scientists didn’t just stack the two features to get better performance. Instead, they integrated them so that the overall GBBR thickness is about the same as each typical individual feature, not the combined thicknesses. So, less material needs to be deposited, leading to a faster and less expensive fabrication process.

The initial three-junction solar device demonstrated an impressive 36.5% efficiency under terrestrial light conditions, and 32.4% under the AM0 space spectrum. This innovation can be further optimized for solar cells, but it can also benefit other opto-electronic applications such as lasers and light-emitting diodes.

Conceptually similar to the merger of phone and camera features into an integrated, well-functioning cell phone, the GBBR has brought two solar cell features together in a much-improved, new technology that will benefit applications within various markets.