New class of materials for organic electronics:
Charge carriers in polymeric carbon nitrides always take paths perpendicular to the sheets, as Merschjann’s group has now shown. Light creates an electron-hole pair. The opposite happens when an electron and hole meet under certain conditions (forming a singlet exciton) and emit light (fluorescence). A graphene lattice is shown in the background. © C. Merschjann
Joint project details charge transport in polymeric carbon nitride for first time
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, 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 synthesised 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.
Light creates charge carriers
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.
New material for organic electronics
Polymeric carbon nitrides are not only non-toxic and cost-effective, they are also extremely durable because they are chemically very stable and can withstand temperatures of up to about 500 °C. Components made of these kinds of compounds might therefore be employed in environments that are unsuitable for today’s organic electronics. Merschjann finds the prospect of growing these compounds on ordered substrates, such as graphene for example, especially interesting though. This is because graphene possesses extremely high in-plane conductivity, while carbon nitrides primarily conduct perpendicular to the sheets. “Carbon nitrides need not fear the competition with conventional organic semiconductor materials. On the contrary, completely new kinds of all-organic optoelectronic components might be built using their property of being essentially one-dimensional semiconductors”, Merschjann hopes. He is currently working on making direct measurements of the charge carriers in a DFG-funded research project at Freie Universität Berlin.
The cooperation was initiated by the BMBF-Cluster-Projekct „Light2Hydrogen“.
The results have been published in the renowned periodical Advanced Materials: Complementing Graphenes: 1D Interplanar Charge Transport in Polymeric Graphitic Carbon Nitrides
DOI: 10.1002/adma.201503448
- Thin nickel films boost green hydrogen productionUltra-thin nickel oxide coatings can significantly improve the performance of anion exchange membrane (AEM) water electrolysers, a promising technology for producing green hydrogen. This was demonstrated by a team led by HZB scientist Dr. Michelle Browne. The results show that a nickel oxide film only 10 nanometres thick outperformed conventional nickel oxide powder electrodes while using less than 1% of the catalyst material. The study is published in the Journal of Materials Chemistry A,
- Green hydrogen with PEC electrolysers: New insights into transport processesOne method of storing solar energy is to use PEC electrolysers to produce hydrogen. However, scaling up this technology remains challenging. Now, a team at the HZB Institute for Solar Fuels has used 2D fluorescence imaging and particle velocimetry to observe the movement of ions and dissolved gases within the electrolyte during electrolysis. These new insights may prove useful in the development of larger PEC electrolysers.
- From Colombia to Berlin: Finding My Way in a New WorldIt was almost 11 p.m. when I arrived in Berlin. After a long journey from Colombia, all I wanted was to get to my accommodation, take a shower, and finally sleep.
Instead, I missed my train. Thinking it would follow the same route as the previous one as it would in my hometown of Medellín I confidently boarded the next train. About twenty minutes later, I realized something was wrong. I was heading in the wrong direction.
As if that was not enough, my phone battery was almost empty. Suddenly, I found myself alone in a city I had never visited before, late at night, speaking a language I did not understand, with no idea how to get back.
This was not how I had imagined the beginning of my first international trip....