An image of a light-harvesting device produced with the superlattice. Image: Sanfeng Wu.
An image of a light-harvesting device produced with the superlattice. Image: Sanfeng Wu.

In the quest to harvest light for electronics, the focal point is the moment when photons encounter electrons. If conditions are right when this happens, an exchange of energy can occur; maximizing that transfer of energy is the key to efficient light harvesting.

"This is the ideal, but finding high efficiency is very difficult," said University of Washington (UW) physics doctoral student Sanfeng Wu. "Researchers have been looking for materials that will let them do this – one way is to make each absorbed photon transfer all of its energy to many electrons, instead of just one electron in traditional devices."

In traditional light-harvesting materials, energy from one photon only excites one electron at most, depending on the absorber's energy gap. This means that just a small portion of light energy is converted into electricity, with the remaining energy lost as heat. But in a recent paper in Science Advances, Wu, together with UW associate professor Xiaodong Xu and colleagues at four other institutions, reports one promising approach to coaxing photons into exciting multiple electrons.

Their approach exploits some surprising quantum-level interactions, and Wu and Xu, who has appointments in the UW's Department of Materials Science & Engineering and the Department of Physics, made this surprising discovery using graphene.

"Graphene is a substance with many exciting properties," said Wu, the paper's lead author. "For our purposes, it shows a very efficient interaction with light."

Graphene is a two-dimensional hexagonal lattice of carbon atoms bonded to one another, through which electrons are able to move easily. The researchers took a single, atom-thick layer of graphene and sandwiched it between two thin layers of a material called boron nitride.

"Boron nitride has a lattice structure that is very similar to graphene, but has very different chemical properties," said Wu. "Electrons do not flow easily within boron nitride; it essentially acts as an insulator."

Xu and Wu discovered that aligning the graphene layer's lattice with the layers of boron nitride produces a type of ‘superlattice’ with some intriguing new properties that rely on quantum mechanics. In particular, Wu and Xu detected unique quantum regions within the superlattice known as Van Hove singularities.

"These are regions of huge electron density of states, and they were not accessed in either the graphene or boron nitride alone," said Wu. "We only created these high electron density regions in an accessible way when both layers were aligned together."

When Xu and Wu directed energetic photons toward the superlattice, they discovered that those Van Hove singularities formed sites where one energized photon could transfer its energy to multiple electrons. By a conservative estimate, Xu and Wu report that within this superlattice one photon could excite as many as five electrons to flow as electric current.

With the discovery of a way to excite multiple electrons from the absorption of one photon, researchers may be able to create highly efficient devices for harvesting light and converting it into electricity. Future work will need to determine how to organize the excited electrons into electrical current and remove some of the more cumbersome properties of the superlattice, such as the need for a magnetic field. But they believe this efficient process between photons and electrons represents major progress.

"Graphene is a tiger with great potential for optoelectronics, but locked in a cage," said Wu. "The singularities in this superlattice are a key to unlocking that cage and releasing graphene's potential for light harvesting applications."

This story is adapted from material from the University of Washington, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.