Graphene Converts Single Particles of Light Into Many Electrons

It sounds like free energy: A single incoming photon meets a sheet of graphene and, upon collision, produces multiple electrons. From this single particle of light, one of the infinitesimal points of energy behind the electromagnetic force, comes electric current.

This feat was ​recently demonstrated by researchers at the Swiss École Polytechnique Fédérale de Lausanne (EPFL) and, while not really offering free energy, it demonstrates the potential of graphene as a high-efficiency photovoltaic (solar energy-producing) material. The phenomenon, known properly as carrier multiplication, has the potential to nearly double the theoretical upper limit of solar energy conversion from 32 to 60 percent, ​reports IEEE Spectrum.

The efficiency boost comes from a simple enough idea. Typically, when a photon smashes into an electron in a photovoltaic material the result is an excited electron or an electron at a higher energy state than before. This electron might just pop loose from its atomic home and become freely moving electric current, which is then conducted away and harvested for power. The amount of energy needed to pop an electron loose varies by material, and is known as the band gap.

Where the inefficiency comes in is if a photon arrives with too much energy (above the gap), leaving some leftover to be dissipated as heat. In a carrier multiplication scheme, that excess goes instead toward popping out another electron. So: less waste.

Graphene, single atom-thick two-dimensional sheets of graphite, has all kinds of neat and highly desirable properties for the technology near-future, including very high thermal and electrical conductivity, incredible strength, and, indeed, an extremely strong interaction with light. Potential graphene applications include but are hardly limited to fuel cells, industrial lubricants, novel drug delivery devices, integrated circuits, desalinization filters, environmental sensing, and synthetic bone tissue.

The multiplicative properties of graphene were demonstrated first indirectly by researchers at the Barcelona-based Institute of Photonic Science last year. Here, the EPFL researchers go a step further, using a new technique called ultrafast time- and angle-resolved photoemission spectroscopy to observe photoconversion in action, a feat that until recently was impossible just given that photoconversion occurs on timescales in the femto-neighborhood of 10^-15 seconds.

The general idea behind the technique is that the energized material to be observed is hit with successive "probe" pulses of light, each one illuminating the evolving energy states of the electrons within the material. It's sort of like using a flash to photograph a dark room, only this particular room isn't dark so much as fast.

Graphene still isn't quite ready for photovoltaic prime-time, however. It ​turns out that the stuff doesn't absorb light very well in the first place. Being able to convert sunlight to electricity at very high efficiencies doesn't much matter if the material wants to ignore the light in the first place. And there is also the question of how exactly to harvest current from graphene, the ​edges of which tend to have different conductivity properties than the central zones and are also quite reactive when exposed.