A team of researchers from the University of Kansas have managed to extend the lifetime of excited electrons in graphene by several hundred times. This could have a significant meaning for the development of ultrathin and flexible solar cells with high efficiency, they said.
Professor Hui Zhao and graduate student Samuel Lane worked this out by connecting a graphene layer with two other atomic layers (molybdenum diselenide and tungsten disulfide). This extended the lifetime of excited electrons in graphene. Electrons in graphene move at a speed of 1/30 of the speed of light. This suggests that it can be used for solar cells to convert sunlight into electricity.
To be able to keep the electrons stay mobile for a longer time, beyond the usual time of about 1 picosecond, the researchers call for using van der Waals materials. "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," said Zhao. "We basically took 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."
To be able to do this, they designed a tri-layer material by putting single layers of MoSe2, WS2 and graphene on top of each other. The researchers used an ultrashort laser pulse (0.1 picosecond) to liberate some of the electrons in MoSe2. Using another ultrashort laser pulse, they were able to monitor these electrons as they move to graphene. They found that these electrons move through the "hallway" in about 0.5 picoseconds on average. They then stay mobile for about 400 picoseconds — a 400-fold improvement than a single layer of graphene, which they also measured in the same study.
According to the team, this process determined the lifetime of the mobile electrons. So, by choosing different "hallway" layers, this time can be controlled for various applications.
The research findings, explained in the paper, Unipolar optical doping and extended photocarrier lifetime in graphene by band alignment engineering, will be published in the September edition of Nano Futures journal.
Potential use of graphene in solar cell manufacturing is something manufacturers are slowly waking up to for various jobs. Chinese module maker ZNShine has set up a 500 MW production capacity for graphene coated solar modules. These panels, it said, have self-cleaning characteristics improving power output by 0.5% to 1% (see ZNShine To Supply Modules To BHEL).