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Polymeric carbon nitride is an organic material with interesting optoelectronic properties. As an inexpensive photocatalyst, it can be used to facilitate water splitting using sunlight. Joint research by Helmholtz-Zentrum Berlin, the University of Rostock, and Freie Universität Berlin as well as other partners has now investigated for the first time how light creates charge carriers in this class of materials and established details about charge mobility and lifetimes. They discovered surprising characteristics in their investigations that provide prospects for new applications, in conjunction with graphene for example.

Polymeric carbon nitrides are organic compounds synthesized to form a yellow powder of a myriad of nanocrystals. The crystalline structure resembles that of graphite because the carbon nitride groups are chemically bound only in layers, while just weak Van der Waals forces provide cohesion between these layers. It was already known that light is able to create an electron-hole pair in this class of materials. So there have already been numerous attempts to employ polymeric carbon nitrides as cost-effective photocatalysts for solar-powered water splitting. However, the efficiency levels so far have remained comparatively low.

Now a team headed by Dr. Christoph Merschjann (HZB and Freie Universität Berlin) and Prof. Stefan Lochbrunner (University of Rostock) have for the first time precisely probed the processes occurring during light-induced charge separation. "The most interesting result has been that charges are basically only transported along one dimension during this process, perpendicular to the graphite-like layers," explains Merschjann. The light creates an electron-hole pair that subsequently migrates in opposing directions. Using femtosecond spectroscopy as well as other spectroscopic time-domain methods, the researchers were able to make the first quantitative mobility and lifetime measurements on the charge carriers. This revealed that the charge mobility attains values similar to those in conventional organic semiconductor materials. Moreover, the charge carriers are long-lived before recombining again.

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