Solar panels are cheaper than ever these days, but installation costs can still be considerable for homeowners. More efficient solar panels can recapture the cost of their installation more quickly, so making panels that are better at converting sunlight into electricity is a key focus of solar research and development.

The silicon-based cells that make up a solar panel have a theoretical efficiency limit of 29 percent, but so far that number has proven elusive. Practical efficiency rates in the low-20-percent range have been considered very good for commercial solar panels. But researchers with Japanese chemical manufacturer Kaneka Corporation have built a solar cell with a photo conversion rate of 26.3 percent, breaking the previous record of 25.6 percent. Although it’s just a 2.7 percent increase in efficiency, improvements in commercially viable solar cell technology are increasingly hard-won.

Not only that, but the researchers noted in their paper that after they submitted their article to Nature Energy, they were able to further optimize their solar cell to achieve 26.6 percent efficiency. That result has been recognized by the National Renewable Energy Lab (NREL).

In the Nature Energy paper, the researchers described building a 180.4 cm2 cell using high-quality thin-film heterojunction (HJ)—that is, layering silicon within the cell to minimize band gaps where electron states can’t exist. Controlling heterojunctions is a known technique among solar cell builders—Panasonic uses it and will likely incorporate it into cells built for Tesla at the Solar City plant in Buffalo, and Kaneka has its own proprietary heterojunction techniques.

For this record-breaking solar cell, the Kaneka researchers also placed low-resistance electrodes toward the rear of the cell, which maximized the number of photons that collected inside the cell from the front. And, as is common on many solar cells, they coated the front of the cell with a layer of amorphous silicon and an anti-reflective layer to protect the cell’s components and collect photons more efficiently.

After describing the architecture of the solar cell, Kaneka researchers analyzed the energy losses that prevented the cell from reaching that 29-percent efficiency ideal, which could help future solar cell builders optimize their cells to get closer to the limit. Kaneka researchers estimated that overall efficiency was reduced by 0.5 percent due to resistive loss, 1 percent due to optical loss (the way the cell receives light), and 1.2 precent due to extrinsic recombination loss—where a free electron recombines with a positively charged hole rather than going on for current collection.

The paper noted that this solar cell was created using “industrial applicable” processes, like plasma-enhanced chemical vapor deposition (PECVD), which deposits thin films onto a solid wafer from a gas state. While the solar cell may be vapor-ware in the sense that chemical vapor helps create them, the industry-friendly process reduces the likelihood that the high-efficiency architecture will end up as something we’d call vaporware more colloquially. (Thanks folks, I'll be here all night.)

That said, the authors note that “further work is required before the individual cells can be assembled into a commercially available solar panel.” But further work seems likely. Kaneka’s research was funded by Japan’s New Energy and Industrial Technology Development Organization, abbreviated to NEDO, and according to IEEE Spectrum, the company will continue to work with NEDO to bring the levelized cost of solar cells down to $0.06 per kilowatt-hour by 2030.

Nature energy, 2017. DOI: 10.1038/nenergy.2017.32 (About DOIs).

Listing image by Photovoltaic & Thin Film Research Laboratories (Kaneka corporation)