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Long Live OLEDs
June 25, 2010 20:14


Russian physicists developed a technique for making organic light-emitting diodes live longer. For this purpose they added to OLEDs long-living nano-crystals of cadmium chalcogenides, inorganic luminescent sites, which help organic chromophores and simplify production technology of OLEDs with required emission spectrum.

Organic light-emitting diodes (OLEDs) have many advantages over other light-emitting structures. First, organic chemistry offers a wide variety of light-emitting substances; second, synthesis of new organic substances doesn’t require complex growth setup, like beam epitaxy or precision precipitation does. However, OLEDs have heir own peculiarities. OLEDs cannot live long, because lifetime of their emitting sites – chromophores – is too short. Russian scientists found a solution to this problem.

Physicists suggest replacing organic chromophores with inorganic luminescent centres, based upon semiconductor nano-crystals. Nano-crystals of cadmium chalcogenides (chalcogenides are chemical combinations of elements of 6th group of the periodic table with metals – with cadmium here) have strong bonds, thus do not collapse during exploitation. Moreover, these nano-crystals have a very interesting property – when their size change, luminescence wavelength also changes, so it is easy to get required emission wavelength.

Production technology of OLEDs with embedded inorganic nano-crystals has only one difference from normal OLED-production technology – a step is added, when colloid solution of semiconductor nano-crystals is mixed with organic semiconductor. In that case, organic part works as a semiconductor, and long-living inorganic component is responsible for luminescence. However, in order for nano-crystals to emit light, they should attract electrons and “holes” for recombination and emission of a light photon.



Scientists discovered an interesting effect – even if they put a potential barrier around a quantum dot, which provides perfect conditions for recombination of an electron and a “hole” in a system’s centre, surface states still have to be taken into consideration, research supervisor says. Effect of surface can be seen, when a system is permanently irradiated with a laser – scientists detected so-called “blinking” fluorescence instead of normal one. “Blinking” fluorescence is characterized by a rapid state “on/off” transition – “on” means emitting light, “off” is lack of fluorescence. Physicists are currently studying electron processes in quantum dots by means of recently bought unique research equipment – a scanning confocal microscope.

One of the main application areas for organic light-emitting diodes is “organic” display creation technology. “Organic” displays have better characteristics than plasma and liquid-crystal monitors: they consume less energy, they are smaller and can be “soft” and etc, that is why making OLEDs live longer is useful in many aspects. However, Russian physicists, being fans of fundamental science, believe that cadmium quantum dots are interesting objects for studying not only as parts of OLEDs, but as themselves.

Sources: Institute of Physics

Kizilova Anna

Tags: Russian technologies     

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