May 5th, 2013 by Tim Tyler
An atom-thin photovoltaic device with high quantum efficiency has been developed by an international team of researchers, researchers from the University of Boston, University of Oxford, and the National University of Singapore. The atom-thin photovoltaic device has a quantum efficiency of 30%, which is simply a measure of the number of photons converted into charge-carrying electrons.
To create the device, the researchers created a sandwich of a semiconducting transition metal dichalcogenide (TMDC) and sheets of graphene. “The work extends the range of new ‘heterostructures’ based on two-dimensional atomic crystals being developed by materials scientists and could lead to new types of flexible solar cells or phototransistors,” Simon Hadlington of Chemistry World notes.
Going on: “TMDCs are layered materials consisting of a triangular lattice of transition metal atoms sandwiched between two triangular lattices of chalcogen atoms – sulfur, selenium or tellurium. They are known to have potent photon–electron interactions because of the presence within the lattice of Van Hove singularities – a quantum mechanical feature which makes electrons peculiarly susceptible to external perturbations, such as stimulation by light.”
The researchers constructed the atom-thin photovoltaic device by using boron nitride as the outer, protective, insulating layer; then using graphene as a tough, transparent, and flexible conductor; then using TMDC as the “meat” (or tofu) of the sandwich.
“As light hits the TMDC it dislodges electrons, which are collected by the graphene electrodes, ready to be harnessed for carrying out work. The scientists also discovered that peppering the graphene surface with gold nanoparticles enhanced light absorption.”
According to team member Antonio Castro Neto of the University of Boston, who is working at the National University of Singapore at the moment:
Overall the device is no thicker than about 20Å. We wanted to see if something this thin could efficiently produce an electric current, and we showed that even something a few atoms thick can turn 30% of the energy of photons into electricity.
Another researcher, Henry Snaith, who is a photovoltaics expert at the University of Oxford, see’s a lot of potential with the new technology:
I think TMDC should make very good transistors, since the off current should be able to be very low – single atomic layer – and the on current should be very high due to high mobility. In addition, the possibility to interact with light makes phototransistors, and hence highly sensitive photodetectors, very possible.
Although, Snaith is less convinced that the system will result in new solar cells, as stacking the devices will cut its efficiency. Next on the agenda is to investigate other two-dimensional semiconductor crystals, to see if the system’s efficiency can be increased. The researchers plan to experiment with other combinations of materials in order to create novel heterostructures. They also plan to engineer the overall system to boost performance.
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