An artist’s illustration of the tri-layer made up of single layers of molybdenum diselenide (top), tungsten disulfide (middle) and graphene (bottom). Image: Matthew Bellus.
An artist’s illustration of the tri-layer made up of single layers of molybdenum diselenide (top), tungsten disulfide (middle) and graphene (bottom). Image: Matthew Bellus.

By connecting a graphene layer with two other atomic layers (molybdenum diselenide and tungsten disulfide), two researchers from the University of Kansas (KU) have extended the lifetime of excited electrons in graphene by several hundred times. Reported in a paper in Nano Futures, this work by Hui Zhao, a professor in the Department of Physics & Astronomy, and graduate student Samuel Laneis may speed the development of ultrathin and flexible solar cells with high efficiency.

For electronic and optoelectronic applications, graphene has excellent charge transport properties. According to the researchers, electrons move in graphene at 1/30th the speed of light – much faster than in other materials. This might suggest that graphene can be used in solar cells, which convert sunlight into electricity. But graphene has a major drawback that hinders such applications – the lifetime of its excited electrons (that is, the time an electron stays mobile) is very short, at about one picosecond (one-millionth of one-millionth of a second).

"These excited electrons are like students who stand up from their seats – after an energy drink, for example, which activates students like sunlight activates electrons," Zhao said. "The energized students move freely in the classroom – like human electric current."

According to Zhao, one of the biggest challenges to achieving high efficiency in solar cells with graphene as the working material is that liberated electrons have a strong tendency to lose their energy and become immobile, like students sitting back down.

"The number of electrons, or students from our example, who can contribute to the current is determined by the average time they can stay mobile after they are liberated by light," Zhao said. "In graphene, an electron stays free for only one picosecond. This is too short for accumulating a large number of mobile electrons. This is an intrinsic property of graphene and has been a big limiting factor for applying this material in photovoltaic or photo-sensing devices. In other words, although electrons in graphene can become mobile by light excitation and can move quickly, they only stay mobile too short a time to contribute to electricity."

In their new paper, Zhao and Lane report that this issue can be solved by using so-called van der Waals materials, like molybdenum diselenide (MoSe2) and tungsten disulfide (WS2). "We basically took the chairs away from the standing students so that they have nowhere to sit," Zhao said. "This forces the electrons to stay mobile for a time that is several hundred times longer than before."

To achieve this goal, working in KU's Ultrafast Laser Lab, they designed a tri-layer material by putting single layers of MoSe2, WS2 and graphene on top of each other.

"We can think of the MoSe2 and graphene layers as two classrooms full of students all sitting, while the middle WS2 layer acts as a hallway separating the two rooms," Zhao said. "When light strikes the sample, some of the electrons in MoSe2 are liberated. They are allowed to go across the WS2-layer hallway to enter the other room, which is graphene. However, the hallway is carefully designed so that the electrons have to leave their seats in MoSe2. Once in graphene, they have no choice but to stay mobile and hence contribute to electric currents, because their seats are no longer available to them."

To demonstrate that the idea works, the KU researchers used an ultrashort laser pulse (0.1 picosecond) to liberate some of the electrons in MoSe2. By using another ultrashort laser pulse, they were able to monitor these electrons as they move to graphene, finding that the electrons take an average of about 0.5 picoseconds to move through the ‘hallway’. The electrons then stay mobile for about 400 picoseconds – a 400-fold improvement over a single layer of graphene, which they also measured in the same study.

The researchers also confirmed that the ‘seats’ left in MoSe2 stay unoccupied for the same amount of time. In the classical world, these seats should stay empty forever. In quantum mechanics, however, the electrons ‘tunnel’ back to their seats. The researchers propose that this tunneling process determines the lifetime of the mobile electrons, which means that, by choosing different ‘hallway’ layers, this lifetime can be controlled for various applications.

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