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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