Graphene device makes ultrafast light to energy conversion possible

By Colin Jeffrey

Original link:  http://www.gizmag.com/graphene-ultrafast-light-energy-conversion-photodetector-semiconductor/37005/

Using layers of graphene, scientists claim to have created a
photodetector that converts light to energy in less than 50 quadrillionths
of a second (Image: ICFO/Achim Woessner)

Converting light to electricity is one of the pillars of modern electronics, with the process essential for the operation of everything from solar cells and TV remote control receivers through to laser communications and astronomical telescopes. These devices rely on the swift and effective operation of this technology, especially in scientific equipment, to ensure the most efficient conversion rates possible. In this vein, researchers from the Institute of Photonic Sciences (Institut de Ciències Fotòniques/ICFO) in Barcelona have demonstrated a graphene-based photodetector they claim converts light into electricity in less than 50 quadrillionths of a second.

Graphene has already been identified as a superior substance for the transformation of photons to electrical current, even in the infrared part of the spectrum. However, prior to the ICFO research, it was unclear exactly how fast graphene would react when subjected to ultra-rapid bursts of light energy.

To test the speed of conversion, the ICFO team – in collaboration with scientists from MIT and the University of California, Riverside – utilized an arrangement consisting of graphene film layers set up as a p-n (positive-negative) junction semiconductor, a sub-50 femtosecond, titanium-sapphire, pulse-shaped laser to provide the ultrafast flashes of light, along with an ultra-sensitive pulse detector to capture the speed of conversion to electrical energy.

When this arrangement was fired up and tested, the scientists realized that the photovoltage generation time was occurring at a rate of better than 50 femtoseconds (or 50 quadrillionths of a second).

According to the researchers, this blistering speed of conversion is due to the structure of graphene which allows the exceptionally rapid and effective interaction between all of the conduction band carriers contained within it. In other words, the excitation of the molecules of graphene by the laser pulses causes the electrons in the material to heat up, and stay hot, while the carbon lattice underlying the structure remains cool. And, as the electrons in the laser-excited graphene do not cool down rapidly because they do not easily recouple with the graphene lattice, they remain in that state and transfer their energy much more rapidly.

As such, constant laser-pulse excitation of an area of graphene quickly results in superfast electron distribution within the material at constantly elevated electron temperatures. This rapid conversion to electron heat is then converted into a voltage at the p-n junction of two graphene regions.

Significantly, this "hot-carrier" generation is quite different from the operation of standard semiconductor devices. This is because their operation is dependent upon overcoming of the binding electron energy inherent in the material for an incoming photon to dislodge an electron and create an electrical current. In the ICFO device, the continued excitation of electrons above this band-gap level results in the much faster and easier movement of them when subjected to incoming photons to create an electric current.

Though it is early days in the study of such devices, the practical upshot of this research may be in the eventual production of novel types of ultrafast and extremely effective photodetectors and energy-harvesting devices. And, given that the basic operating principles of hot-carrier graphene devices are substantially different from traditional silicon or germanium semiconductors, an entirely new stream of electronic components that take advantage of this phenomenon may evolve.

The findings of this work have recently been published in the journal Nature Nanotechnology.

Source: ICFO