Rice University researchers are describing a new way to harvest solar energy with a new paper this week in the journal Science.

Naomi Halas, Rice’s Stanley C. Moore Professor in Electrical and Computer Engineering, the paper’s lead researcher explains,  “We’re merging the optics of nanoscale antennas with the electronics of semiconductors. There’s no practical way to directly detect infrared light with silicon, but we’ve shown that it is possible if you marry the semiconductor to a nanoantenna. We expect this technique will be used in new scientific instruments for infrared-light detection and for higher-efficiency solar cells.”

Infrared Absorbing Antennas Embedded in Silicon.

Keep in mind that more than a third of the solar energy arriving on Earth comes in the spectrum of infrared light.  This energy is what warms the earth, feels good when basking in the sun and is in part re radiated back into space each night.  Is a new take on thermal energy gathering.

Silicon based solar cells convert mostly the visible spectrum of sunlight into electricity in the vast majority of today’s solar panels, but the technology doesn’t capture infrared light energy.

The semiconductor class of materials has spectrum gaps where light below a certain frequency passes directly through the material and is unable to generate an electrical current.  This is from the spectrum at the infrared being longer or simply bigger than semiconductors can react to.  This is where the antennae idea comes in.

The Rice team is showing they can extend the frequency range for electricity generation into the infrared by attaching a metal nanoantenna specially tuned to interact with infrared light to the silicon.

When infrared light passes into the antenna, it creates a “plasmon,” a wave of energy that sloshes through the antenna’s ocean of free electrons where the wave can create a current flow.

The study of plasmons is one of Professor Halas’ specialties, and the new paper resulted from basic research into the physics of plasmons that began in her lab years ago.

It’s been known that plasmons decay and give up their energy in two ways; they either emit a photon of light or they convert the light energy into heat. The heating process begins when the plasmon transfers its energy to a single electron, also known as a ‘hot’ electron.

Rice graduate student Mark Knight who is lead author on the paper, together with Rice theoretical physicist Peter Nordlander, his graduate student Heidar Sobhani, and Halas set out to design an experiment to directly detect the hot electrons resulting from plasmon decay.

By patterning a metallic nanoantenna directly onto a semiconductor to create a “Schottky barrier,” Knight showed that the infrared light striking the antenna would result in a hot electron that could jump the barrier, which creates an electrical current. This works for infrared light at frequencies that would otherwise pass directly through the device.

Here’s the payoff –according to Knight, “The nanoantenna-diodes we created to detect plasmon-generated hot electrons are already pretty good at harvesting infrared light and turning it directly into electricity. We are eager to see whether this expansion of light-harvesting to infrared frequencies will directly result in higher-efficiency solar cells.”

This opens up some big questions.  Foremost is how good is “pretty good”?  An estimated percentage or proportion would be worth knowing.  Next up would be the costs to add the antennas to a silicon photovoltaic cell.  That in turn asks if one even needs sunlight or would ant hot radiating source do?  Would it be efficient to just forego the photovoltaic costs all together?

It’s easy to see this could a lot of different directions.  When more is known, such as the characteristics of the current generated and the efficiency another look will be worthwhile.

One does wonder if there is a chart about the energy density of the points along the spectrum arriving on Earth.  (If you know leave a comment with a a link, please.)  At high efficiency and high density this technology could be very important.


2 Comments so far

  1. Alex. H on August 4, 2011 6:00 AM

    Hey super Infos zum Thema Energie.
    Wo hast Du all die Informationen herbekommen?

  2. Srijita B on June 13, 2012 5:57 PM

    It of-course have lots of commercial impacts as in middle-east and India and lot more other countries in that region are suffering from handling not so handy and ineffective solar-cell battery. I guess if transferred to efficient machine will gain popularity in those areas and will be great help to them.

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