Researchers from the University of Kansas have found yet another use for the versatile material graphene—this time, to help bolster the development of ultra-thin and flexible, highly efficient solar cells. A team from the university’s Department of Physics & Astronomy has paired graphene—a single layer of carbon atoms with high electrical conductivity—with two other atomic layers: molybdenum diselenide (MoSe2) and tungsten disulfide (WS2).
This marriage extends the lifetime of excited electrons in graphene by several hundred times, thus extending the energy possibilities for graphene as a material for electricity generation, Professor Hui Zhao, who led the research, said in a University of Kansas release. “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,” he explained. “The energized students move freely in the classroom—like human electric current,” he added.
Graphene already has an excellent charge-transport property, with electrons moving in graphene at a speed of 1/30 of the speed of light. This is much faster than other materials, researchers said, which opens the possibility for graphene to be used in solar cells.
|Researchers from the University of Kansas have connected a graphene layer with two other atomic layers (molybdenum diselenide and tungsten disulfide), thereby extending the lifetime of excited electrons in graphene by several hundred times. (Image source: Matthew Bellus)|
However, a hindrance to the application of graphene in photovoltaic or photo-sensing devices is the ultra-short lifetime of excited electrons in graphene (referring to the time the electrons stay mobile). In graphene, this time is only one picosecond or one-millionth of one-millionth of a second.
“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.”
In other words, he said, though light excitation can move electrons in graphene—and move them quickly—they stay mobile for too short a time to contribute to the generation of electricity. Zhao—working with graduate student Samuel Lane—has solved this problem, basically taking “the chairs away from the standing students so that they have nowhere to sit,” he explained. “This forces the electrons to stay mobile for a time that is several hundred times longer than before.”
Essentially, the researchers designed a tri-layer material by putting single layers of MoSe2, WS2, and graphene on top of each other. Keeping with the student analogy, the MoSe2 and graphene layers act as two classrooms full of students all sitting, while the middle WS2 layer acts as a hallway separating the two rooms, Zhao explained. When light strikes the sample, it liberates some of the electrons in MoSe2.
“They are allowed to go across the WS2-layer hallway to enter the other room, which is graphene,” he said. “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.”
Researchers demonstrated their idea by using an ultra-short laser pulse of 0.1 picoseconds to liberate some of the electrons in MoSe2. They then used another ultra-short laser pulse to monitor these electrons as they move to graphene. What they discovered is that the electrons move through the “hallway” in about 0.5 picoseconds (on average) and then stay mobile for about 400 picoseconds. This represents a 400-fold improvement over a single layer of graphene, which researchers also measured in their study.
Their experiments also confirmed that “seats” left in MoSe2 stay unoccupied for the same amount of time. This means that scientists applying their method can control this time, depending on their application, by choosing different “hallway” layers, Zhao said. The work—which researchers plan to continue—paves the way for the use of graphene in novel photovoltaics, potentially adding solar-energy generation to the growing list of applications for graphene, researchers said.
Elizabeth Montalbano is a freelance writer who has written about technology and culture for 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco, and New York City. In her free time, she enjoys surfing, traveling, music, yoga, and cooking. She currently resides in a village on the southwest coast of Portugal.
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