Scientists dealing with solar energy, although they are constantly improving photovoltaic cells, still have a lot of work to do. On the other hand, the problem is that the energy produced by solar cells is captured and stored so that it is available when the cells are unable to produce it (at night or when it is cloudy). On the other hand, the problem lies in the amount of energy that cells generate. The latter problem was recently taken up by scientists from the Fritz Haber Institute and the Technical University of Berlin.
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It has been known for more than a hundred years that light consists of particles called photons, and electric charge consists of electrons and electron holes. Photons incident on different materials are transformed into free charges. In a typical solar panel, the energy carried by a single photon is transferred to two free charges in the material. However, there are many materials in which the situation is much more interesting. Take pentacene, for example: a single photon incident on it transfers energy to four free charges. In this case, the so-called double excitation and exciton splitting. In physics, an exciton is a quasiparticle consisting of an electron-hole pair bound together by electrostatic forces.
How do you visualize the formation of electric charges?
As hard as it is to believe, scientists in Germany have managed to create an ultra-fast video that shows the conversion of a photon into an electric charge.
As a.d. Ralf Enrstorfer, lead author Article published in the journal NatureWhen a photon hits the surface of pentacene, the electric charges interact instantly. Until recently, scientists didn’t know what this process looked like: Did a photon first excited a pair of electron holes, and then shared its charge with the other, or did both pairs get excited simultaneously?
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Using the latest scientific advances, that is, photoemission spectroscopy methods recorded on a femtosecond time scale, physicists were able to capture the picture of the rushing electrons. Wait a minute, what is a femtosecond? Simply put, to record such a process, the researchers had to use a camera capable of recording frames every billionth of a millionth of a second.
Thanks to this amazing video, the scientists were able to analyze in detail the dynamics of excited charge carriers. It turns out that a photon incident on a material first excites a pair of electron holes. Moreover, the mechanism of subsequent doubling of free-charge carriers has been determined.
According to scientists, the knowledge obtained in this way may allow in the near future to use materials such as pentacene to produce highly efficient solar cells. Given the pace at which photovoltaics are already developing, but also the long-term plans for solar power plants in Earth orbit, understanding the processes that occur at the level of a single photon on a femtosecond scale could bring huge long-term benefits on a global scale.
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