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Published on August 9th, 2016 | by Tina Casey


Brace Yourself: “Hot Electron” Solar Cell Breaks Shockley-Queisser Limit Thanks To Inventor Of Photocopier

August 9th, 2016 by  

File this one under F for From Russia, with Solar. A team based at Drexel University has demonstrated how to break the Shockley-Queisser limit, which dictates the maximum amount of sunlight that a solar cell can convert to electricity. The Russia part comes in with team member Valdimir M. Fridkin, a Russian physicist who staked out winning turf in the race to invent the photocopier almost 50 years ago. Now he has a solar energy notch on his belt.

The new solar cell breakthrough is a significant step forward in solar cell R&D, as teams around the world vie to squeeze the maximum amount of energy from solar-friendly material whilst juggling conversion efficiency with size, weight, flexibility, durability and cost.

new solar cell shockley-queisser

New Solar Cell Breaks Through Shockley-Queisser

The new research deals with the field of “hot” electrons. In a solar cell material, hot electrons temporarily carry an extra load of energy.

Normally that extra energy is shed as heat, but if you could find a way to grab it off before it thermalizes, there’s your barrier-breaking conversion efficiency (the International Society for Optics and Photonics offers a plain language “hot carrier” primer in case you want more details).

Fridkin observed the hot electron phenomenon about 47 years ago, when he recorded an energy conversion pathway that differed from conventional solar cells. This “bulk photovoltaic effect” is weak, but it demonstrates that not all of the excess energy in a solar cell needs to be lost as heat. Here’s the explainer from Fridkin himself:

“The main result — exceeding [the energy gap-specific] Shockley-Queisser [power efficiency limit] using a small fraction of the solar spectrum — is caused by two mechanisms…The first is the bulk photovoltaic effect involving hot carriers and second is the strong screening field, which leads to impact ionization and multiplication of these carriers, increasing the quantum yield.”

By “screening field,” Fridkin is referring to the naturally occurring, reversible electric field that characterizes ferroelectric materials. The Drexel team found that their nanoscale electrode enhanced the screening field, enabling it to drive a cascading domino effect in which one excited electron accelerates and triggers the release of other nearby electrons.

If this is beginning to ring a bell, last year the National Renewable Energy Laboratory applied the principle to perovskite solar cells.

Of Course, There’s A Catch…

Of course, there’s a catch — the new research is based on barium titanate, which primarily works under ultraviolet light.

Also, the team recorded its observations on a rather small device…

We present data for devices that feature a single-tip electrode contact and an array with 24 tips (total planar area of 1 × 1 μm2) capable of generating a current density of 17 mA cm–2 under illumination of AM1.5 G. In summary, the BPVE at the nanoscale provides an exciting new route for obtaining high-efficiency photovoltaic solar energy conversion.

…so, a little scaling up is in order.

Still, the new study indicates yet another path forward to improving solar cell efficiency. Here’s Drexel professor Jonathan E. Spanier enthusing over the possibilities:

“Barium titanate absorbs less than a tenth of the spectrum of the sun. But our device converts incident power 50 percent more efficiently than the theoretical limit for a conventional solar cell constructed using this material or a material of the same energy gap.”

For (many) more details, check out the team’s paper in the journal Nature Photonics under the heading, “Power conversion efficiency exceeding the Shockley–Queisser limit in a ferroelectric insulator.”

Go, Navy!

In addition to Drexel and Fridkin’s home base, the Shubnikov Institute of Crystallography of the Russian Academy of Sciences, the research team also included the University of Pennsylvania and the U. S. Naval Research Laboratory.

If you’re surprised to see the Navy in the mix, you’re probably not alone. However, the Navy has been powering its way through the foundational clean tech research field. One notable example is a microbial fuel cell that gained a slot in Time Magazine’s Top 50 Inventions for 2009.

The Navy is also pumping funds into ready-for-market clean tech. It has played a key role in building the market for third-generation biofuel, and in 2013 it pumped $30 million in funding into Hawaii’s “Energy Excelerator.” Startups in the Excelerator’s portfolio are working on everything from ebikes to desalination and micro-concentrating solar cells.

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Image (screenshot): “…artist’s concept of optically-generated non-thermalized electrons and their collection in a ferroelectric crystal” by Ella Marushchenko. 
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About the Author

specializes in military and corporate sustainability, advanced technology, emerging materials, biofuels, and water and wastewater issues. Tina’s articles are reposted frequently on Reuters, Scientific American, and many other sites. Views expressed are her own. Follow her on Twitter @TinaMCasey and Google+.

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