Award-Winning PV Cell Could Stride Toward 50% Efficiency

Award-Winning PV Cell Could Stride Toward 50% Efficiency

In their quest for progressively efficient photovoltaic devices, scientists in the III-V Multijunction Photovoltaics Group at the U.S. Department of Energy's (DOE) National Renewable Energy Laboratory (NREL) have been trying to prevail over the solar spectrums boundaries and unchanging rules since the 1990s, when they began their search for easy-to-grow materials with ideal band gaps.

In 2012, NREL and its industry partner, Solar Junction, defeated those solar rules so well they won an R&D 100 award from R&D Magazine for a world-record three-layered multijunction solar cell — the SJ3 — with 43.5% efficiency at 415 suns, as Solar Novus Today reported. Now, NREL has verified that record was outshone when a SJ3 cell reached 44% efficiency at 947 suns.

In concentrated photovoltaics (CPV) in particlular, cell efficiency has proven to be a powerful lever to drive down the cost of solar electricity and, thus, propelling the technology forward. “We have seen in the past that improvements in multijunction cell efficiency have catalyzed the introduction of new CPV systems companies and innovative approaches to lowering CPV costs,” says Daniel Friedman, manager of the NREL III-V Multijunction Photovoltaics Group. “I would anticipate the present  improvements in cell efficiency, as well as future improvements, to have a comparable catalyzing effect on the industry.”

According to NREL, with inventions such as the ultra-high-efficiency SJ3 cell, CPV technology could provide enough electrical energy to supply the entire US many times over. But what would have to happen for such a cogent clean energy solution to become reality? “Such an ambitious goal is very complex and involves more than just photovoltaics,” Friedman points out, referencing a recent NREL study that tackles big-picture questions on what large-scale renewables adoption could look like. “One component of this is to lower the cost of photovoltaics, and that is what the improved CPV cell efficiencies drive towards.”

Cost and long-term reliability remain major challenges on the path to more widespread implementation of CPV power systems. Friedman affirms this current research successfully addresses both. “This achievement [of the world-record 44%-efficient SJ3 cell] stands on the shoulders of several decades of R&D in multijunction photovoltaics,” the NREL researcher says. It was NREL's pioneering multijunction work that ultimately led to the Solar Junction SJ3 solar cell with tunable bandgaps, lattice-matched architecture and ultra-concentrated tunnel junctions.The Lab predicts there could be a 50%-efficient cell in the near future. “There are several multijunction approaches that have pathways to near-50% efficiencies. The SJ3 is one such, and the Inverted Metamorphic Multijunction approach is another,” Friedman reports. “In both cases, a key step towards near-50% efficiencies is the addition of a fourth junction with approximately 0.7 eV bandgap under the existing three-junction structures.” Furthermore, he believes it possible to enhance the performance of the existing junctions by careful analysis and improvement of their optoelectronic properties.

As solar researcher, Friedman’s outlook for solar energy is positive. “The multijunction solar cell field is in the midst of an exciting renaissance, where great strides are being made in the fundamental understanding of the subtleties of the device operation and in the application of this understanding towards obtaining higher-performance devices. I'm looking forward to continuing to work in the multijunctions field during these times,” the scientist concludes.