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 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.

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.