We’ve been keeping tabs on the plasmonics scene for a while and we just got wind of an interesting new development from the Netherlands in partnership with Caltech. If the Netherlands connection doesn’t exactly ring your bells, recall that Caltech is the institutional home of NASA’s Jet Propulsion Laboratory, which is heavily invested in the related field of photonics.
As for the Netherlands angle, that would be FOM Institute AMOLF, which belongs to the Netherlands Organisation for Scientific Research. AMOLF has been partnering with Caltech on plasmonics for a number of years now, so let’s see what they’re up to in terms of influencing the next wave of low cost, high efficiency solar cells.
The Plasmonic Effect
For those of you new to the topic, plasmonics (aka the plasmo-electric effect) refers to the ability of certain metals to capture light and convert it to an electrical charge.
For more details you can check out an article on plasmonics at our sister site PlanetSave, but for now let’s just say that plasmonics refers to electrons that are in an excited state when exposed to light.
Today’s solar cell technology is based on silicon and other semiconductors, so introducing another class of materials into the mix could bust the solar field wide open in terms of cost, efficiency, and application.
Think of the new opportunities afforded by the emerging generation of flexible, lightweight thin film solar cells compared to conventional silicon solar cells and you can see what the future could bring.
The plasmonic effect has actually been around the block before, give or take a few centuries. As described in the AMOLF press materials, stained glass masters of early church architecture created vivid colors by embedding tiny particles of metal in the glass, to enhance light absorption and diffusion.
Some of the recent developments in the plasmonic field include a “black metal” solar cell from Lawrence Livermore National Laboratory, consisting of a surface etched with nanoscale pillars. A similar approach, described as a “forest of gold nanorods,” is under way at the University of California-Davis.
And yes, there is a plasmonic graphene angle but we’ll save that for another time.
The AMOLF-Caltech Plasmonic Collaboration
AMOLF and Caltech had previously examined the plasmonic effect in gold nanospheres exposed to a laser. That study found that the nanospheres exhibited the potential for a negative charge when exposed to blue light, and a positive potential under red light.
For the next step, which formed the basis of the new plasmonics study (just published in the journal Science, if you want to check it out), the team took a somewhat different path than that of Lawrence Livermore or UC-Davis.
Instead of creating rods or pillars, they engineered tiny circuits consisting of a film of gold only 20 nanometers thick, into which they “drilled” holes measuring only 100 nanometers in diameter (a nanometer is a billionth of a meter, for those of you keeping score at home).
The figure above, provided by AMOLF, shows (a) a rendering of a metal nanoparticle exposed to light, (b) an electron microscopy image of the nanoscale holes in gold film, (c) a chart of the measurements taken for different spacings between the holes, and (d) the same measurements interpreted for their electrical potential.
Here’s the happy recap of the study from AMOLF:
…these metal hole arrays possess bright plasmon resonances, of which the wavelength can be controlled by the spacing between the holes. When irradiating these circuits with a laser, and gradually tuning the color of the light from blue to red, a negative potential (-100 mV) was found for blue light and a positive potential (+100 mV) for red light.
Since When Is Gold Cheaper Than Coal?
When you see gold in connection with energy technology, you’ve got to be thinking that the cost factor is prohibitive. There’s certainly something to say for that line of thinking if you look at fuel cell technology, which has been bumping up against a price wall partly due to the use of expensive platinum catalysts.
So, don’t hold your breath for that shiny new golden solar cell to roll around.
On the other hand, foundational research at Caltech could lead to the discovery of other solar-friendly metals that could beat gold on cost-effectiveness, if not efficiency.
That brings us to the other angle to consider, which is cradle-to-cradle cost. Punching holes into a thin film, even at the nanoscale level, lends itself to the emerging crop of low cost, high throughput manufacturing technologies that are pushing the clean energy envelope. The use of metals that lend themselves to efficient, low-carbon recovery and recycling processes would round out the circle.
Now compare the cost of sunlight as an energy source to coal, or for that matter natural gas and oil. After a solar array is completed, the energy source comes beaming down from the heavens, free of charge. With fossil fuels, not so much. Aside from climate change issues, after you construct your fossil power plant you still have to account for a steady stream of expense to keep it running, including the local public health, economic and environmental risk factors related to fuel extraction, preparation, and transportation.