Cómo un fotón se convierte en cuatro portadores de carga

Ilustración de la división de excitones en el semiconductor orgánico pentaceno, que consta de cinco anillos de benceno cada uno. En lugar de los dos portadores de carga gratuitos habituales, la absorción de un fotón en el pentaceno genera cuatro portadores de carga gratuitos, representados por pistas naranjas. Crédito: Universidad Técnica de Berlín

Algunos materiales convierten los fotones en más cargas gratuitas de lo esperado. Usando película de alta velocidad, los investigadores ahora han podido obtener imágenes de este proceso.

Investigadores del Instituto Fritz Haber de la Sociedad Max Planck, la Universidad Técnica de Berlín y la Julius-Maximilians-Universität Würzburg utilizaron película de alta velocidad para capturar el proceso de fisión de los excitones, lo que podría aumentar considerablemente la eficiencia de las células solares. Usando espectroscopía de fotoemisión resuelta en tiempo y ángulo, observaron la dinámica de electrones en una escala de tiempo de femtosegundos y resolvieron un debate de larga data sobre el mecanismo detrás de la conversión de un[{» attribute=»»>photon into four charges in materials like pentacene. The discovery is crucial for the use of organic semiconductors in photovoltaics and could lead to significant advancements in solar energy generation.

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 come in tiny, quantized packets called photons and elementary charges, the latter represented by electrons and holes.

Better solar cells thanks to exciton splitting

In a usual solar cell, the energy of a single photon is transferred to two free charges in the material, but no more than that. However, a few molecular materials like pentacene are an exception and show conversion of one photon into four charges, instead. This excitation doubling, which is called exciton fission, could be extremely useful for high-efficiency photovoltaics, specifically to upgrade the dominant silicon-based technologies.

A team of researchers at the Fritz Haber Institute of the Max Planck Society, the Technical University of Berlin, and the Julius-Maximilians-Universität Würzburg 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.

”When pentacene is excited by light, the charges in the material rapidly react,” explains Prof. Ralph Ernstorfer, a senior author of the study. “It was an open and highly disputed question whether an absorbed photon excites two electrons and holes directly or initially just one electron-hole pair, which subsequently shares its energy with another charge pair.” Ernstorfer is head of a Max Planck research group at the Fritz Haber Institute and Professor of Experimental Physics at the Technical University of Berlin.

Snapshots of one billionth of a millionth of a second

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 charge carrier pairs was crucial to decipher the process,” says Alexander Neef, from the Fritz Haber Institute and the first author of the study. “An excited electron-hole pair not only has a specific energy but also adapts distinct patterns, which are called orbitals. To understand the process of singlet fission it is such essential to identify the orbital shapes of the charge carriers and how these change over time.”

Crucial for the use of organic semiconductors

With the images from the ultrafast electron movie at hand, the researchers decomposed the dynamics of the excited charge carriers for the first time based on their orbital characteristics. “We can now say with certainty that only one electron-hole pair is excited immediately after photon excitation and identified the mechanism of the free charge carrier-doubling process,” adds Alexander Neef.

“Resolving this initial step in exciton fission is essential to successfully implement this class of organic semiconductors in innovative photovoltaic applications and, thus, to further boost the conversion efficiency of today’s solar cells,” states Prof. Jens Pflaum, whose group at the University of Würzburg has provided the high-quality molecular crystals for this study. Such an advance will have enormous impacts as solar energy and its generation by these third-generation cells will be a dominant energy source of the future.

Reference: “Orbital-resolved observation of singlet fission” by Alexander Neef, Samuel Beaulieu, Sebastian Hammer, Shuo Dong, Julian Maklar, Tommaso Pincelli, R. Patrick Xian, Martin Wolf, Laurenz Rettig, Jens Pflaum and Ralph Ernstorfer, 12 April 2023, Nature.
DOI: 10.1038/s41586-023-05814-1

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