How high can they go?

The National Renewble Energy Laboratory is rightfully proud of the 44 percent solar efficiency record it helped set for a triple-junction solar cell. But that mark might not stand for long – and it’s another federally assisted effort that’s gunning for a bigger number.

The U.S. Naval Research Laboratory says that its scientists, working with academic and private-industry collaborators, have arrived at a scheme to shatter the 44 percent record and top the 50 percent conversion efficiency barrier.

solar cell efficiency naval research laboratory
Schematic of multijunction solar cell formed from materials lattice-matched to InP and achieving the bandgaps for maximum efficiency. (image via U.S. Naval Research Lab)

Robert Walters, an NRL research physicist, said in a statement that “it is generally accepted that a major technology breakthrough will be required” to go that high – but the he and his colleagues think they are on the right path.

“This research has produced a novel, realistically achievable, lattice-matched, multijunction solar cell design with the potential to break the 50 percent power conversion efficiency mark under concentrated illumination,” Walters said.

Because photons of sunlight arrive in a differentiated range, conventional solar cells with a single characteristic band gap energy are quite limited in their efficiency. High-energy photons lose their excess energy to the solar cell as waste heat; low-energy photons go uncollected by the solar cell, and their energy goes completely for naught.

The way around this problem is solar cells with multiple junctions, each of which has its own band gap energy.

From there, it’s a matter of pinpointing and structuring the correct materials.

The NRL research team said it has produced a design for a cell that can stretch the band gaps – from 0.7 to 1.8 electron volts (eV) – by using materials that are all lattice-matched to an indium phosphide (InP) substrate.

“Having all lattice-matched materials with this wide range of band gaps is the key to breaking the current world record” adds Walters. “It is well known that materials lattice-matched to InP can achieve band gaps of about 1.4 eV and below, but no ternary alloy semiconductors exist with a higher direct band-gap.”

In the announcement, the Naval Research Lab said its scientists worked with Imperial College London and the Illinois company MicroLink Devices on this proposed design. Work on turning it into a reality will proceed over the next three years under and ARPA-E grant with MicroLink and Rochester Institute of Technology of Rochester, N.Y., the NRL said.

This type of solar cell is aimed at utility-scale projects that use concentrating photovoltaics, in which lenses are used to ratchet up the intensity of the light hitting the solar cells.

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