MSGID: 1:317/3 6455d7ec   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Exciton fission: One photon in, two electrons out    
      
     Date:   
         May 5, 2023   
     Source:   
         Fritz Haber Institute of the Max Planck Society   
     Summary:   
         Photovoltaics, the conversion of light to electricity, is a key   
         technology for sustainable energy. Since the days of Max Planck   
         and Albert Einstein, we know that light as well as electricity   
         are quantized, meaning they come in tiny packets called photons   
         and electrons. In a solar cell, the energy of a single photon is   
         transferred to a single electron of the material, but no more than   
         one. Only a few molecular materials like pentacene are an exception,   
         where one photon is converted to two electrons instead. This   
         excitation doubling, which is called exciton fission, could be   
         extremely useful for high-efficiency photovoltaics, specifically   
         to upgrade the dominant technology based on silicon. Researchers   
         have now deciphered the first step of this process by recording an   
         ultrafast movie of the photon-to-electricity conversion process,   
         resolving a decades-old debate about the mechanism of the process.   
      
      
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   FULL STORY   
   ==========================================================================   
   Photovoltaics, the conversion of light to electricity, is a key technology   
   for sustainable energy. Since the days of Max Planck and Albert Einstein,   
   we know that light as well as electricity are quantized, meaning they come   
   in tiny packets called photons and electrons. In a solar cell, the energy   
   of a single photon is transferred to a single electron of the material,   
   but no more than one. Only a few molecular materials like pentacene are   
   an exception, where one photon is converted to two electrons instead.   
      
   "When pentacene is excited by light, the electrons in the material   
   rapidly react," explains Prof. Ralph Ernstorfer, a senior author of the   
   study. "It was an open and very disputed question whether a photon excites   
   two electrons directly or initially one electron, which subsequently   
   shares its energy with another electron."  To unravel this mystery the   
   researchers used time- and angle-resolved photoemission spectroscopy,   
   a cutting-edge technique to observe the dynamics of electrons on   
   the femtosecond time scale, which is a billionth of a millionth of a   
   second. This ultrafast electron movie camera enabled them to capture   
   images of the fleeting excited electrons for the first time.   
      
   "Seeing these electrons was crucial to decipher the process," says   
   Alexander Neef, from the Fritz Haber Institute and the first author of the   
   study. "An excited electron not only has a specific energy but also moves   
   in distinct patterns, which are called orbitals. It is much easier to tell   
   the electron apart if we can see their orbital shapes and how these change   
   over time."  With the images from the ultrafast electron movie at hand,   
   the researchers decomposed the dynamics of the excited electrons for the   
   first time based on their orbital characteristics. "We can now say with   
   certainty that only one electron is excited directly and identified the   
   mechanism of the excitation- doubling process," adds Alexander Neef.   
      
   Knowing the mechanism of exciton fission is essential to using it   
   for photovoltaic applications. A silicon solar cell enhanced with an   
   excitation- doubling material could boost the solar-to-electricity   
   efficiency by one-third.   
      
   Such an advance could have enormous impacts since solar energy will be   
   the dominant power source of the future. Already today large investments   
   are flowing into the construction of these third-generation solar cells.   
      
       * RELATED_TOPICS   
             o Matter_&_Energy   
                   # Spintronics # Physics # Solar_Energy # Electricity   
                   # Materials_Science # Graphene # Energy_Technology #   
                   Energy_and_Resources   
       * RELATED_TERMS   
             o Solar_cell o Photoelectric_effect o Solar_power o   
             Electron_configuration o Electroluminescence o Atomic_orbital   
             o Electricity_generation o Electron_microscope   
      
   ==========================================================================   
   Story Source: Materials provided by   
   Fritz_Haber_Institute_of_the_Max_Planck_Society. Note: Content may be   
   edited for style and length.   
      
      
   ==========================================================================   
   Journal Reference:   
      1. Alexander Neef, Samuel Beaulieu, Sebastian Hammer, Shuo Dong, Julian   
         Maklar, Tommaso Pincelli, R. Patrick Xian, Martin Wolf,   
         Laurenz Rettig, Jens Pflaum, Ralph Ernstorfer. Orbital-resolved   
         observation of singlet fission. Nature, 2023; 616 (7956): 275 DOI:   
         10.1038/s41586-023-05814-1   
   ==========================================================================   
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2023/05/230505141618.htm   
      
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