How a photon becomes four charge carriers

Illustration of exciton cleavage in the organic semiconductor pentacene consisting of five benzene rings. Instead of the usually two free charge carriers, four free charge carriers, represented by orange orbits, are generated by absorbing a photon in pentacene.
Photo Credit: Technical University of Berlin

Some materials convert photons into more charge carriers than would be expected. With an ultra-fast film, researchers have now been able to get an idea of this process. Physicists from the University of Würzburg were there.

Photovoltaics, i.e. The conversion of light into electricity is a key technology in the sustainable generation of energy. Since Max Planck and Albert Einstein, it has been known that both light and electricity occur in tiny, quantized packages: on the one hand in the form of photons and on the other hand as elementary charges in the form of electrons and holes.

Better solar cells thanks to exciton splitting

In the material of a conventional solar cell, the energy of a single photon is transferred to two free charges, nothing more. However, some molecular materials such as pentacene show an exception to this rule and instead convert a photon into four charges. This excitation doubling, which is referred to as exciton fission, is of great benefit for the highly efficient photovoltaics, in particular to improve the prevailing technologies based on silicon.

A team of researchers from the Fritz Haber Institute of the Max Planck Society, the Technical University of Berlin and the Julius Maximilians University in Würzburg has now deciphered the first step in this process by making an ultra-fast film about the conversion of photons into free ones Cargo and ended a decades-old debate about the mechanism of this process.

"When pentacene is stimulated by light, the charges in the material react quickly," explains Professor Ralph Ernstorfer, one of the main authors of the study. “It was an open and highly controversial question whether an absorbed photon directly excites two electrons and holes or initially only an electron-hole pair that then shares its energy with another charge pair.“ Ernstorfer is head of a Max Planck research group at the Fritz Haber Institute and professor for experimental physics at the TU Berlin.

Snapshots of one billionth of a millionth of a second

To decipher this riddle, the researchers used time- and angle-resolved photoemission spectroscopy, a state-of-the-art technique for observing the dynamics of electrons on the time scale of femtoseconds, that is: one billionth of a millionth of a second. With this ultra-fast electron film camera, they were able to take pictures of the volatile excited electrons for the first time.

"Seeing these pairs of load carriers was crucial to decipher the process," says Alexander Neef from the Fritz Haber Institute and first author of the study. “An excited electron-hole pair not only has a certain energy, but also assumes certain spatial distribution patterns called orbitals. In order to understand the process of splitting the singular, it is therefore important to identify the orbitals of the charge carriers and to see how their occupation changes over time."

Using the images of the ultra-fast electron film, the researchers deciphered the dynamics of the excited charge carriers for the first time based on these orbital properties. "We can now say with certainty that only one pair of electron-holes is excited immediately after the photon excitation and have identified the mechanism that doubles the generated charge carriers," added Alexander Neef.

Crucial for the use of organic semiconductor

"The clarification of this first step in the splitting of excitons is crucial for the successful use of organic semiconductor in innovative photovoltaic applications and thus for the further increase in the conversion efficiency of today's solar cells", explains Professor Jens Pflaum, whose working group at the University of Würzburg provided the high-quality molecular crystals for this study.

The scientists are convinced that such progress will have an enormous impact, since solar energy and its generation by these third-generation cells will be a dominant source of energy in the future.

Published in journal: Nature

Research Material: Nature has a commentary article.

Source/Credit: University of Würzburg

Reference Number: phy041423_01

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