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New Diode Materials
August 29, 2014   
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Researchers worldwide are searching for better luminescent materials for the production of organic light emitting diodes (OLED). Two new materials—compounds containing what are known as europium complexes—developed by Polish researchers from the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw display record red-light luminescence efficiency in their class, and their properties enable faster, low-cost manufacturing of thin OLED films, according to the researchers.

Europium is a chemical element with the symbol Eu and named after the continent of Europe. It is a ductile metal with a hardness similar to that of lead. Most applications of europium exploit the phosphorescence of europium compounds.

The researchers from the Institute of Physical Chemistry developed the compounds using phosphine oxides (oxidized organic compounds containing phosphorus-carbon bonds) as co-ligands in europium ion-based complexes. A research group from Scotland’s University of St. Andrews collaborating with the Institute of Physical Chemistry used the developed compounds to build prototype OLEDs generating nearly monochromatic red light.

“Both compounds, carefully designed by us, display record luminescence efficiency in their class,” said Prof. Marek Pietraszkiewicz from the Institute of Physical Chemistry. “As a matter of fact, we know red emitters with somewhat higher efficiency, containing iridium, but that’s a completely different type of materials.”

Red light emitted by europium complexes with phosphine oxides is of well-defined wavelength, about 612 nanometers (a nanometer is one billionth of a meter). The luminescence quantum yields of these compounds reach 90 percent.

Michał Maciejczyk, a doctoral student from an international doctoral studies program run by the Institute of Physical Chemistry, said, “The narrow emission wavelength range and the record efficiency result from our approach to molecular design. We attach extended, highly rigid phosphine oxides to europium complexes. As a result, the energy delivered to the molecule is not dissipated in unnecessary vibrations or rotations. Instead of delivering heat to the surroundings, we have higher efficiency and virtually monochromatic light.”

An important advantage of the luminescent materials developed and produced at the Institute of Physical Chemistry is their stability—they do not degrade when exposed to oxygen or light. Equally important is the possibility to produce films of these materials from solutions. Existing manufacturing technologies for the production of OLED films usually require the use of high vacuum evaporation and deposition methods. The vacuum deposition technique is very expensive, troublesome, and not always available. It also requires the material to be heated to 200-300 degrees Celsius, a temperature not well tolerated by all compounds. The problems disappear when the films can be deposited directly from the solution—and this is possible for phosphine oxides with europium complexes.

Potential applications of the new materials include not only OLED displays or lighting components, such as rear lights of mechanical vehicles, but also flexible dermal patches for use in treatment for skin cancer. The europium complex-based compounds incorporated in such patches would generate light of an exactly known wavelength that could locally activate appropriately selected active ingredients, delivered earlier with other methods to the patient’s damaged skin cells. During treatment, the dermal patch would require only a small power supply from a battery. The patient’s mobility would be affected only minimally, and hospitalization would no longer be needed.
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