How a single photon transforms into four charge carriers

A key technology for environmentally friendly energy is photovoltaics, which convert light into electricity. Since the times of Max Planck and Albert Einstein, we have realized that light and power come in small, quantized parcels called photons and rudimentary charges, the last option being addressed by electrons and openings.

Photons are transformed into more free charges than would be expected by some materials. Researchers have now been able to capture this process on ultrafast film. The results of their research have been published in Nature.

Better solar cells as a result of exciton splitting

Exciton splitting makes solar cells better. In a typical solar cell, only two free charges in the material receive the energy of a single photon. Pentacene, for example, is one such exception, demonstrating the conversion of one photon into four charges instead. Exciton fission, also known as excitation doubling, has the potential to be extremely beneficial for high-efficiency photovoltaics, particularly when it comes to upgrading the predominant silicon-based technologies.

“It is unclear if an absorbed photon directly excites two electrons and holes or first only one electron-hole pair, which then shares its energy with another charge pair. When pentacene is activated by light, the charges in the material react quickly.”

Prof. Ralph Ernstorfer, a senior author of the study. 

A group of specialists at the Fritz Haber Establishment of the Maximum Planck Society, the Specialized College of Berlin, and the Julius-Maximilians-Universität Würzburg have now unraveled the initial step of this interaction by recording an ultrafast film of the photon-to-power transformation process, settling a decades-old discussion about the component of the cycle.

According to senior study author Prof. Ralph Ernstorfer, “the charges in the material rapidly react when pentacene is excited by light.” The question of whether an absorbed photon directly excites two electrons and holes or only one electron-hole pair, which then shares its energy with another charge pair, was open and hotly debated. Ernstorfer is a professor of experimental physics at the Technical University of Berlin and the head of a Max Planck research group at the Fritz Haber Institute.

Time- and angle-resolved photoemission spectroscopy, a cutting-edge method for observing the dynamics of electrons on the femtosecond time scale, or one billionth of a millionth of a second, was used by the researchers to unravel this mystery. This ultrafast electron film camera empowered them to catch interesting pictures of the transiently invigorated electrons.

According to Alexander Neef, the study’s first author and a researcher at the Fritz Haber Institute, “Seeing these charge carrier pairs was crucial to decipher the process.” Not only does an excited electron-hole pair possess a particular energy, but it also adapts distinct patterns known as orbitals. To comprehend the course of singlet parting, recognizing the orbital states of the charge transporters and how these change after some time is so fundamental.”

Important for using organic semiconductors

Important for the use of organic semiconductors, “the researchers were able to decompose the dynamics of the excited charge carriers for the first time based on their orbital characteristics with the images from the ultrafast electron movie. Alexander Neef adds, “We have now identified the mechanism of the free charge carrier-doubling process and can now say with certainty that only one electron-hole pair is excited immediately after photon excitation.”

“Settling this underlying move toward exciton splitting is fundamental to effectively carrying out this class of natural semiconductors in creative photovoltaic applications and, subsequently, to additional lift the change productivity of the present sun-oriented cells,” states Prof. Jens Pflaum, whose gathering at the College of Würzburg has provided the excellent sub-atomic gems for this review. As solar energy and its generation by these third-generation cells will be the predominant energy source in the future, this advancement will have a significant impact.

More information: Alexander Neef et al, Orbital-resolved observation of singlet fission, Nature (2023). DOI: 10.1038/s41586-023-05814-1

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