A George Washington University researcher helped design and construct a prototype for a new solar cell that integrates multiple cells stacked into a single device capable of capturing nearly all of the energy in the solar spectrum.

The new design, which converts direct sunlight to electricity with 44.5 percent efficiency, has the potential to become the most efficient solar cell in the world.

The approach is different from the solar panels commonly seen on rooftops or in fields. The new device uses concentrator photovoltaic (CPV) panels that use lenses to concentrate sunlight onto tiny, micro-scale solar cells. Because of their small size -- less than one millimeter square -- solar cells that utilize more sophisticated materials can be developed cost effectively.

The study, "GaSb-based Solar Cells for Full Solar Spectrum Energy Harvesting," was published in the journal Advanced Energy Materials.

The stacked cell acts almost like a sieve for sunlight, with the specialized materials in each layer absorbing the energy of a specific set of wavelengths, said Matthew Lumb, lead author of the study and a research scientist at the School of Engineering and Applied Science. By the time the light is funneled through the stack, just under half of the available energy has been converted into electricity. By comparison, the most common solar cell today converts only a quarter of the available energy into electricity.

"Around 99 percent of the power contained in direct sunlight reaching the surface of Earth falls between wavelengths of 250 nanometers and 2,500 nanometers, but conventional materials for high-efficiency multi-junction solar cells cannot capture this entire spectral range," Dr. Lumb said. "Our new device is able to unlock the energy stored in the long-wavelength photons, which are lost in conventional solar cells, and therefore provides a pathway to realizing the ultimate multi-junction solar cell."

Scientists have worked to develop more efficient solar cells for years, however this approach has two novel aspects. It uses a family of materials based on gallium antimonide (GaSb) substrates, which are usually found in applications for infrared lasers and photodetectors. These GaSb-based solar cells are assembled into a stacked structure along with high efficiency solar cells grown on conventional substrates that capture shorter wavelength solar photons. In addition, the stacking procedure uses a technique known as transfer-printing, which enables three dimensional assembly of these tiny devices with a high degree of precision.

This particular solar cell is very expensive, but researchers believe it was important to show the upper limit of what is possible in terms of efficiency. Despite the current costs of the materials involved, the technique used to create the cells shows promise, researchers say. Eventually a similar product enabled by cost reductions from very high solar concentration levels and technology to recycle the expensive growth substrates could be brought to market.

The research builds off of the advancements made by the MOSAIC Program, a $24 million research project funded by the Advanced Research Projects Agency-Energy that funds 11 separate teams across the United States seeking to develop technologies and concepts to revolutionize photovoltaic performance and reduce costs. Funding for this type of research is essential for developing viable commercial technology in the future, the researchers said.