The researchers from the Faculty of Physics of the Moscow State University have grown organic crystals that allow creating flexible electronic devices. Credit: Dmitry Yu. Paraschuk et al.
Scientists from Faculty of Physics Moscow State University have grown organic semiconductor crystals which can reduce the cost of the process of creating light, flexible and transparent light-emitting electronic devices of the new generation. It has high light-emitting efficiency that promise a bright future for wet-processed organic optoelectronics. They also made a double breakthrough using much simpler and cheaper technologies that previously were considered impractical.
The organic optoelectronics is a rapidly growing area including organic light-emitting transistors (OLET) and organic lasers pumped by electric current. This is, indeed, a very promising area: the organic semiconductors due to their availability can even replace silicon in some electronic applications.
It was believed organic semiconductor crystals grown from the vapor phase are much preferred than the ones grown from solution, since those obtained from vapor are purer and free of structure impurities. Prof. Dmitry Paraschuk did not share this opinion, and preferred the solution growing for several reasons, particularly because of much simpler and cheaper technologies stemming from this method.
METHOD: Thiophene-phenylene oligomers were chosen as work-horse molecules. Crystals were grown from solution of these molecules. Their luminescent and electrical properties were measured there as well.
RESULTS: The solution-grown crystals are more luminescent than their analogues obtained from vapor. Their quantum yield (i.e. the ratio of the number of photons emitted to the number of absorbed ones) reached 60%, whereas vapor-grown crystals presented no more than 38%. Such a striking difference in the luminescence efficiency is explained, particularly, by the fact that, perhaps, during the solution-growing some internal radiationless channels that take some part of the excitation energy are suppressed in crystals. But, apparently, this is not the only explanation. “We have found reasons for such a high quantum yield, but we are not ready to publish these yet. This is a matter of our future studies, “- told Professor Paraschuk.
“We have shown that it is possible to grow crystals on the surface of the liquid in different ways. Roughly speaking, after we place a solution with molecules in a vessel and then begin to cool it, under some certain conditions we allow molecules to be deposited on the surface at the “air-liquid” interface. Because the liquid surface is almost perfect, the crystals grown on it are of good quality, and owing to their high electronic performance they are much more preferable to the vapor-grown ones. Moreover, the surface of the solution-grown crystals is molecularly smooth with angstrom-scale roughness, which allows us to create field-effect transistors on their basis, and its quality is irreplaceable in this case” — says the professor.
Dmitry Paraschuk emphasizes that the applicability of the crystals in light-emitted transistors, and therefore in organic optoelectronics is just a suggestion, the validity of which is still to be proved. The same situation is with the possibility of creating lasers controlled by electric current obtained on the same basis.
“People dream to get lasers that can ignite by simple connection of the film to the power source, but they haven’t achieved success yet,”- told Paraschuk. — “We hope that with the help of organic crystals we can reach this goal. The combination of good conductivity and high efficiency allows us to hope that the first electrically pumped laser will be created on the basis of such crystals.” http://www.eurekalert.org/pub_releases/2016-02/lmsu-oca020316.php
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