May 10, 2013
TMDC "wallpaper" produces electricity
Researchers have taken another important step towards making flexible photovoltaics and solar cells thanks to a new class of two-dimensional crystals called semiconducting transition-metal dichalcogenides (TMDCs). These ultra-thin heterostructures might be used as "wallpaper" on any surface exposed to sunlight to produce electricity – in contrast to current technologies that are thick, heavy and brittle.
"If this technology is developed further, it has a chance of becoming a game changer in the area of solar energy," says Antonio Helio Castro Neto of the National University of Singapore.
The electrons in TMDCs interact exceptionally strongly with light. This means that even though these materials are just a few atoms thick, a large portion of absorbed photons can be used to produce electric current.
Castro Neto and colleagues made a heterostructure, or "high-tech sandwich", as they call it, containing atomically thin materials, each with a well-defined role. The first component, boron nitride, which is a one atom-thick transparent insulator, encapsulates the entire ensemble and can be thought of as the "bread" in the sandwich. Next up is graphene, a 2D sheet of conducting carbon that plays the role of the "lettuce". It is used to collect the electrons produced by the TMDCs (which are the "meat" in the sandwich).
"We also employed gold nanoparticles – that you can think of as the 'pepper'," explained Castro Neto. "Although not strictly needed, these particles do 'spice' things up by increasing the amount of light absorbed by the structure thanks to a phenomenon called plasmonics.
In this high-tech sandwich, the materials are all different and have different electronic properties. Separately, they are not particularly good for photovoltaics applications but put them all together in a certain combination and you get a very 'juicy' photovoltaic device."
As a case in point, the researchers succeeded in fabricating extremely efficient flexible devices with a photoresponsivity above 0.1 A/W, which is equivalent to an external quantum efficiency of above 30%.
According to the team, which includes researchers Andre Geim and Kostya Novoselov from the University of Manchester in the UK and colleagues in Portugal, Korea and Germany, it is so-called van Hove singularities in the TMDCs that allow for enhanced light-matter interactions, leading to improved photon absorption and more electron-holes pairs (or excitons) being created in the device. These excitons are responsible for producing electricity – when the electrons and holes then separate.
Van Hove singularities are named after Belgian physicist Léon van Hove who discovered, in 1953, that electrons travelling freely through certain crystals could come to a standstill at specific wave frequencies and wavelengths. The electrons essentially "freeze" and their speed reduces to zero. "In this standstill state, the electrons become extremely sensitive to any kind of external stimulus," says Castro Neto.
"When light with the right frequency then hits electrons in this unique van Hove state, they respond massively. This is what we observed in our experiments, and we exploited the singularity to boost light absorption and create electric current," he told nanotechweb.org.
The researchers say that they are now busy looking for materials with stronger van Hove singularities so that they can increase light absorption even further and improve overall quantum efficiencies. "We are also keen to produce such materials artificially," revealed Castro Neto. "At the moment, we extract the atomically thin layers from 3D crystals but we know that there are ways to grow 2D layers artificially, and this will be fundamental for technological applications.
This is a field that it is very much in its infancy," he added. "If the graphene field is young – less than 10 years – then this new one is even younger. There is much to do and explore."
The present work is detailed in Science.
About the author
Belle Dumé is contributing editor at nanotechweb.org.