Solar cells have been combined in a way that doesn't just break, but shatters the record for converting unconcentrated sunlight into electricity. The method is unlikely to be suitable for rooftop installation, but could prove useful for other solar applications.
Sunlight contains a spectrum of wavelengths, and solar engineers face a never-ending challenge: They can collect most wavelengths, but waste a lot of energy from the shortest, or only harvest the higher energy wavelengths. Either way, efficiency is low.
Multi-junction solar cells represent one possible solution. These stack layers are designed to collect different wavelengths. The top layer collects the highest-energy wavelengths, allowing lower energy light to be collected by those below. Some low-energy light always gets blocked, but this method has made possible the collection of 40 percent of the energy in light under test conditions.
Now, the team that set that record has added a new one, this time of 34.5 percent efficiency, but doing it without concentrating the sunlight first. Previous efforts used large banks of mirrors to focus light onto a small cell – impractical in many environments. Dr. Mark Keevers of the University of New South Wales (UNSW) took a different approach, using a prism to direct low-energy wavelengths at a traditional silicon cell, while higher energy wavelengths fell on a triple-layered multi-junction cell.
A schematic of the way infrared light (confusingly shown in yellow) is directed to the silicon cell, while visible colors fall on the multi-junction cell. UNSW
Dr. Keevers told IFLScience: “Triple junction cells are fabricated in one block of material, but they can't incorporate silicon cells.” Consequently, it is better to have the two cell types side-by-side, with the prism directing light between, rather than stacked.
Solar power records are often set using tiny cells covering less than 1 square centimeter (0.16 inches2), with makers struggling to replicate their success at larger scales. Keevers told IFLScience that the 34.5 percent record, confirmed by the U.S. National Renewable Energy Laboratory, was achieved over 28 cm2 (4.3 inches2), “comparable in size to a single solar cell.”
The current record for efficiency with unconcentrated light on a large scale is just 24 percent. To break this record, Keevers will need to link enough cells together to cover 800 cm2 (124 inches2). “There are always losses,” he told IFLScience. “But we think we can get over 30 percent easily.”
Keevers said cells designed with this sort of light-splitting are unlikely to be appropriate in the short term for rooftop use, but could be suitable for projects such as the one being developed by UNSW's commercial partner RayGen to focus light from fields of mirrors onto multi-junction cells atop a tower.
Sadly, the project's future is in doubt. The research was made possible by a grant from the Australian Renewable Energy Agency (ARENA). However, Keevers told IFLScience that both the Australian government and opposition have adopted policies of shifting ARENA to a model where it would take equity in solar projects rather than making grants. This move would prevent ARENA from supporting early-stage research such as this, an outcome Keevers described as “devastating.”