Nano Tungsten and Molybdenum Can Improve the Conductivity of Graphene

For electronic and photoelectric applications, solar cells convert energy from sunlight to electricity. Graphene has excellent charge transfer performance. Electrons move in graphene at 1 / 30 of the speed of graphite - much faster than other materials. This shows that graphene can be used in solar cells, but graphene also has a major disadvantage, which hinders this application - its ultra short life excited electrons are only about one picosecond (one millionth of a second).

Professor Zhao Zhao and his student Ryan, the main leader of the research team, think: "after the sun starts the electron, the electron can move freely in the material, just like a group of students standing up from their seats, they are no longer constrained in a fixed position, and all dynamic students can move freely in the classroom - just like the current. "

But at the same time, one of the biggest challenges to achieve high efficiency of solar cells with graphene as the working material is the electrons released, because the speed is extremely fast, and graphene is a typical "zero band gap material", which can not stop, has a strong loss of energy and become immobile. "In graphene, electrons remain free for only one picosecond," Zhao said. This is too short for accumulating a large number of mobile electrons. In other words, although the electrons in graphene can be moved by light excitation and can move rapidly, they can only keep moving for too short a time to contribute to the power, and it is difficult to keep the heat in one second, which is the inherent characteristic of graphene, and has become a great limiting factor for the application of this material in photovoltaic or photosensitive applications.

In order to achieve the high efficiency of graphene solar cells, researchers designed a three-layer material, which was composed of a single layer of molybdenum selenide (MoSe₂), tungsten disulfide (WS₂) and graphene.

We can think of the molybdenum disulfide layer and graphene layer as two rooms full of electrons, while the middle WS2 layer is the corridor separating the two rooms. When light strikes, electrons in the MoSe₂ layer are released. They are allowed to enter the graphene layer through the WS₂ layer corridor. Corridors act as buffers so that electrons leave their seats in MoSe₂, which helps convert light energy into electricity. "

To prove the idea, the researchers used ultrashort laser pulses (0.1 picosecond) to release some electrons in MoSe₂. By using another ultrashort laser pulse, they were able to monitor these electrons as they moved to graphene. They found that the electrons moved in an average corridor of about 0.5 picoseconds. Then they stay moving about 400 picoseconds - 400 times more than graphene alone. This is conducive to the efficient storage and discharge of solar cells.

Graphene, nano tungsten and molybdenum selenide are all single-layer and two-dimensional nano materials as thin as cicada wings, so the sandwich sandwich structure of the three can be used to make very thin, very small and transparent solar cell panels, which can transform light into electricity efficiently and stably. At the same time, the scientists believe that the adjustable band gap of three materials can be used to construct multi-layer batteries for electronic devices such as foldable mobile phones.

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