Dr. Lumb Leads Successful Eﬀort to Create Highly Eﬃcient Solar Cell

Dr. Matthew Lumb and a team of researchers are working to develop an innovative and highly efﬁcient solar cell that is capable of capturing nearly all of the wavelengths in the solar spectrum.

In a study published last July in the journal Advanced Energy Materials, the team announced that it had designed and constructed a prototype for a new solar cell that integrates multiple cells stacked into a single device. The new design is capable of capturing long wavelength light in the solar spectrum typically wasted in conventional solar cells. The prototype converts direct sunlight to electricity with 44.5 percent efﬁciency. The world record for efﬁciency currently stands at 46 percent, but with further development, the new approach holds the promise of signiﬁcantly higher efﬁciencies in the future due to the greater light capture.

The new device uses concentrator photovoltaic (CPV) technology that employs lenses to concentrate sunlight onto tiny, micro-scale solar cells. Because of their small size—less than one-millimeter square—solar cells utilizing more sophisticated materials can be developed cost-effectively. The stacking procedure employs micro-transfer printing to precisely assemble conventional, high efﬁciency solar cell materials with compounds based on the GaSb (gallium antimonide) platform, more commonly found in infra-red (IR) applications such as IR lasers and photodetectors.

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. By the time the light is funneled through the stack, just under half of the available energy has been converted into electricity.

“Around 99 percent of the power contained in direct sunlight reaching the surface of Earth falls between wavelengths of 250 and 2500 nanometers, but conventional materials for high-efﬁciency multi-junction solar cells cannot capture this entire spectral range,” explained Dr. Lumb, the lead author of the study and a research scientist in the Department of Electrical and Computer Engineering. “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.”

This particular solar cell is very expensive. However, Dr. Lumb and his team 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 much promise. Eventually, a similar product may be brought to market, enabled by cost reductions from very high solar concentration levels and technology to recycle the expensive growth substrates.

In December of last year, Dr. Lumb was awarded a $1.4 million Advanced Research Projects Agency-Energy (ARPA-E) grant to lead a consortium of researchers in a new research effort to develop a high-performance CPV module. This module will contain mechanically stacked solar cells assembled using the transfer printing approach, but unlike current commercial CPV modules, it will be fully integrated with conventional ﬂat plate photovoltaic technology. The goal of this marriage of concentrator and ﬂat plate components is to produce an extremely high efﬁciency solar module capable of capturing both direct and diffuse light.

“These new hybrid photovoltaic modules have the potential to harvest more energy from the sun than ever demonstrated before and increase the commercial competitiveness of concentrator photovoltaic technologies in a broad range of applications,” said Dr. Lumb.

Working alongside him under the new grant are his collaborators, the Naval Research Laboratory, Northwestern University, MIT, and the companies Veeco and X-Celeprint.