GaAs solar cell to challenge efficiency records

Earlier this year, Magnolia Optical Technologies revealed it is working with DARPA to develop high efficiency GaAs-based solar cells for military portable power as well as potential drone and space applications.

Following many years of research on thin film structures, Ashok Sood, President of Magnolia, is confident the $1.5 million project, part the of DARPA Phase II Small Business Innovation Research Program, will deliver the cost-effective, highly efficient solar cell that industry craves.

"The goal of the program is to develop high-efficiency, thin GaAs-based solar cells that are cost-effective to produce for soldier's portable power applications and maintain performance over changing environmental conditions," he says. "We are in the process of demonstrating this capability with DARPA... we know it can be done."

Today's technology-of-choice for photovoltaics in space, the III-V multi-junction solar cell, packs a powerful punch for its weight, with decades of research having raised efficiencies to more than 30%.

Multi-junction devices use multiple bandgaps, or junctions, that are tuned to absorb a specific region of the solar spectrum. A high-bandgap top cell absorbs high energy photons while a slightly lower bandgap material, placed beneath, absorbs lower-energy, longer-wavelength photons. Typical multi-junction cells use two or three absorbing junctions, with the maximum efficiency increasing with the number of junctions.

But while multi-junction solar cell efficiencies have raised many an eyebrow, so have the costs. The devices' many thick, complex epitaxial layers are expensive to fabricate, leaving this high-cost solar cell largely up in Space.

What's more, multi-junction solar cells are highly sensitive to changes in the solar spectrum; not so helpful back on Earth, where the spectral distribution of incident light varies wildly throughout the day, and year.

Efforts to tackle these issues are gathering momentum, with, for example, NASA having funded research to increase photon collection at the top cell as well as integrate III-V cells onto silicon. However, Sood and Magnolia chief technology officer, Roger Welser, are taking a different approach.

Focusing on single-junction solar cells, rather than multi-junction devices, the researchers are exploiting nanostructures to deliver cost-effective, efficient III-V devices with a favourable power-to-weight ratio. To this end, they are embedding novel designs of quantum dots or wells into III-V absorber layers, with these nanostructured absorber layers then being incorporated to the single-junction cells.

Efficiencies of today's GaAs-based single-junction devices come in at more than 26%, and using absorber layers can extend the infrared absorption of such devices, boosting conversion efficiencies to a theoretical high of 40%. The figure would be fantastic but as Sood puts is: "If we can get anywhere around 30% on a single junction with these quantum structures, then that will be a very big event."

The problem with III-V nanostructures

Efficient photovoltaic energy generation demands device structures that can absorb a wide spectrum of incident radiation - including infra-red wavelengths - while extracting the photogenerated carriers at high voltages. Nanostructured quantum well and quantum dot solar cells hold vast promise here, because, as Sood and Welser emphasise: "The nano-enhanced III-V absorber layers provide a pathway to extend infrared absorption."

Given this, Sood and colleagues have been designing solar cells with novel quantum well profiles that they reckon will raise efficiencies towards 30% and higher.

Tailoring the compositional profile of InGaAs wells is known to suppress the recombination losses that deplete efficiency. So with this in mind, the researchers have been growing various types of step-graded quantum wells and inserting these into absorbing layers to inhibit this phenomenon further and extend infrared absorption.

According to Sood, their experiments on solar cells with novel step-graded well structures show promise, although he won't be drawn on performance figures yet.

"We are working with DARPA and demonstrating a prototype device to show that we can really do this," he says. "If we can use our quantum structures and quantum dots in single-junction solar cells to get the same kind of performance as triple-junction devices, that is going to be a big thing."

Following the DARPA Phase II SBIR Program, Sood and colleagues intend to work with industry players to develop their technology further for portable power and Space applications for the military. "The photovoltaic market is growing rapidly with a wide range of defense and potential commercial applications," says Sood. "Commercialisation has to be our next step."

Image credit: David Kamm, NSRDEC