Isabelle Robinson, AzO Cleantech
Recently, a research team at the University of Kansas developed another application for this versatile material in the form of solar cells.
Use of Graphene in Solar Cells
The researchers from the University’s Department of Physics & Astronomy have found that graphene is able to be used to create ultra-thin, flexible solar cells which are very efficient by forming them with a layer of molybdenum diselenide (MoSe2) and tungsten disulfide (WS2). Each layer is a single atom thick.
It is known that graphene has high charge-transport properties on its own, with electrons traveling at speeds of 1/30th the speed of light. This makes it an obvious choice for photovoltaic cells.
Professor Hui Zhao, the lead researcher stated in a University of Kansas release that “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. The energized students move freely in the classroom—like human electric current.”
Unfortunately, there is one large reason as to why graphene has not been applied to the photovoltaic market already -mainly due to the extremely short lifetime of the excited electrons in graphene - approximately one picosecond.
This is a very short time and means that although the light excitation can make electrons move very quickly through the graphene, they don’t stay excited for very long and this hinders the amount of electricity that can be generated.
“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 explains. “In graphene, an electron stays free for only one picosecond. This is too short for accumulating a large number of mobile electrons.”
By adding two the two atomic layers, the lifetime of the excited electrons within the graphene is extended by over 100 percent and therefore allows the material to be used for clean energy generation. Professor Zhao, with the aid of graduate student Samuel Lane, explained that they had solved this short lifetime problem by taking “the chairs away from the standing students so that they have nowhere to sit. This forces the electrons to stay mobile for a time that is several hundred times longer than before.”
In essence, the layer of molybdenum diselenide and graphene layer simulate two classrooms full of students all sitting. The tungsten disulfide is sandwiched in between and acts as a hallway separating the classrooms. This creates a tri-layer material, that when comes into contact with light, excites the electrons in the first layer of MoSe2.
“They are allowed to go across the WS2-layer hallway to enter the other room, which is graphene,” Zhao explains further. “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.”
A demonstration of this concept was performed by the researchers using an ultra-short laser pulse of 0.1 picoseconds. This excited the electron in the MoSe2 layer and caused them to move. Shortly after, they once again used the laser pulse, this time to monitor the displaced electrons moving through the graphene. The team discovered that the electrons moved through the WS2 layer in an average of 0.5 picoseconds. In addition to this, the electrons remained excited for approximately 400 picoseconds. This is a remarkable improvement on the lifetime of a single layer of graphene.
Conclusion
Zhoa believes that further study into the effects of different “hallway” layers must be undertaken, but the fact that these initial experiments show that scientists can control the time the “seats” lift in the MoSe2 layer stay unoccupied is positive. This research is hoped to inspire other research into the application of graphene in the photovoltaic market as well as other novel uses for the material.
Bibliography
Montalbano, E. (2018, August 17). Graphene's Conductivity Boosted for Better Solar-Energy Generating Technology. Retrieved from Design News: