Professor Hui Zhao and graduate student Samuel Lane, both of the Department of Physics & Astronomy, connected a graphene layer with two other atomic layers, molybdenum diselenide and tungsten disulfide, extending the lifetime of excited electrons in graphene by several hundred times. This will help speed the development of ultrathin and flexible solar cells with high efficiency.
Excited electrons move in graphene at a speed of 1/30 of the speed of light, much faster than other materials, but with an ultrashort lifetime of 1ps. One of the biggest challenges to achieving high efficiency in solar cells with graphene is that the liberated electrons lose energy quickly and become immobile.
“The number of electrons that can contribute to the current is determined by the average time they can stay mobile after they are liberated by light,” said Prof Zhao. “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. 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.”
The tri-layer material built with single layers of MoSe2, WS2 and graphene on top of each other tackles that challenge.
“When light strikes the sample, some of the electrons in MoSe2 are liberated. They are allowed to go across the WS2-layer to the graphene. Once in graphene, they have no choice but to stay mobile and hence contribute to electric currents,” he said.
To test out the material the 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. They found that these electrons move through the WS2 layer in about 0.5 picosecond on average and then stay mobile for about 400 ps in the graphene, a 400-fold improvement than a single layer of graphene, which they also measured in the same study.
The researchers also confirm the electrons tunnel back into the MoSe2 layer, and using different intermediate layers can control the tunnelling time for various applications.