Monitoring Positive Charges in Solar Materials

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Transition metal oxides such as Zinc Oxide (ZnO) are at the center of the recent surge in research and development on solar energy conversion into electrical (photovoltaics) or chemical (photocatalytic) forms, but also of applications such as detectors of high-energy radiation. All of these applications rely on the generation of negative (electrons) and positive (holes) charges, and the understanding of their evolution as a function of time is crucial for these applications.

While electrons have been detected by various techniques, holes have so far escaped observation. Various reasons are behind this: the signal of holes is obscured by that of the electrons and/or element-selective strategies cannot be implemented because they require working under vacuum, i.e. conditions which remote from the practical ones, e.g. the solution phase.

The lab of Majed Chergui at EPFL, within the Lausanne Centre for Ultrafast Science, along with scientists from the Paul-Scherrer-Institut and the Argonne National Laboratory (Chicago) have now successfully detected holes and identified their trapping sites after above band-gap photoexcitation using time-resolved element-selective techniques. The researchers used a novel dispersive X-ray emission spectrometer, combined with X-ray absorption spectroscopy. The technique allowed them to directly detect the trapping of holes with a resolution of 80 picoseconds (1 picosecond is a millionth of a millionth of a second).

The data, supported by computer simulations, revealed that photo-excited holes become trapped in the substrate at singly charged oxygen vacancies. The hole trapping turns the latter into doubly charged vacancies, which causes four zinc atoms around them to move outwards by approximately 15%. The hole traps then recombine radiatively with the delocalized electrons of the conduction band, which generates the green luminescence that is commonly detected when ZnO is used as a detector of high-energy radiation. Identifying the hole traps and their evolution opens up new insights for the future development of devices and nanodevices based on transition metal oxides.

"This is only the beginning," says Majed Chergui. "With the launch of the new Swiss X-ray free electron laser, SwissFEL at the Paul-Scherrer-Institut, a new era is opening before us."