The research breakthrough of ultra-thin solar cells, nano-tungsten and Molybdenum can improve the electrical conductivity of graphene
The graphene is a single-layer carbon atom, and has an ideal characteristic suitable for various applications. In the last ten years, graphene is the most popular material. Only in 2017, more than 30,000 research papers on graphene have been published worldwide. Such as the most desirable graphene solar cell. Unfortunately, the small part of the "graphene battery" news in the country, not seeing real-volume products. In fact, the only advantage of the graphene as the material of the battery is that the conduction is very fast, in addition to this bright spot. But it can be determined that scientists around the world have been trying to break through the structural improvement of the graphene material. Recently, two researchers from the University of Kansas in the United States joined the graphene layer and the two other transition metal single-layer two-dimensional material _ 2-selenide and tungsten disulfide, thereby prolonging the life cycle of the electron, and exciting the electronic performance of the graphene hundreds of times, The significance of this work is to promote the development of high-performance ultra-thin and flexible solar cells.
For electronic and optoelectronic applications, graphene has excellent charge transfer performance by converting energy from sunlight to electric energy. Electrons move in graphene at a speed of 1: 30 at graphite speed-much faster than other materials. This suggests that graphene can be used in solar cells, but graphene also has a major drawback, hindering this application-its ultra-short lifetime excited electrons are only about one picosecond (1/1000000 seconds). Professor Zhao, the lead director of the research team, and his student Ryan said: "when the sun activates electrons, electrons can move freely and steadily in the material, just like a group of students standing up from their seats. they are no longer confined to a fixed position, and all energetic students can move freely in the classroom-just like electricity." But at the same time, one of the biggest challenges to the high efficiency of solar cells using graphene as working material is the release of electrons, because graphene is a typical "zero band gap material", which can not stop and has a strong tendency to lose energy and become immobile. Professor Zhao said: "in graphene, electrons remain free for only one picosecond. This is too short for the accumulation of a large number of moving electrons. In other words, although electrons in graphene can be moved and can move quickly through light excitation, they can only keep moving for too short a time to contribute to electricity, and it is difficult to maintain heat for one second, which is an inherent property of graphene and has become a major constraint on the application of this material to photovoltaic or photosensitive.
In order to achieve high efficiency of graphene solar cells, the researchers designed a three-layer material, which overlapped monolayer molybdate selenide (MoSe2), tungsten disulfide (WS2) and graphene. We can think of the Molybdenum disulfide layer and the graphene layer as two rooms full of electrons, while the middle WS2 floor is a corridor separating the two rooms. When the light shines, electrons in the MoSE 2 layer are released. They are allowed to enter the graphene layer through the WS 2 corridor. Corridors act as buffer belts so that electrons leave their seats in MoSe2, to help convert light energy into current. "to prove that this idea is effective, the researchers used ultra-short laser pulse (0.1 picosecond) to release some electrons from MoSe2, and by using another ultra-short laser pulse, they were able to monitor these electrons as they moved to graphene. They found that the electrons moved on average in corridors of about 0.5 picoseconds. Then they kept moving about 400 picoseconds-400 times more than monolayer graphene. This contributes to the efficient storage and discharge of solar cells. Graphene, tungsten disulfide and molybdate selenide are all monolayer two-dimensional nanomaterials as thin as cicadas, so the sandwich structure can be used to fabricate very thin, very small and transparent solar cell panels, which are both efficient and stable to convert light into electronic. the results show that graphene, tungsten disulfide and molybdate selenide are all monolayer two-dimensional nanomaterials as thin as cicadas wings. At the same time, scientists believe that the adjustable band gap of the three materials can be used to construct multi-junction batteries, which can be used in electronic devices such as collapsible mobile phones.
