Spanish researchers design all-in-one down-conversion chemistry for white LEDs eeNews Europe

Using elaborate chemical synthesis techniques, a team of Spanish researchers led by Rubén Costa of IMDEA Materials (Madrid) was able to synthetize all three Red Green Blue (RGB) colour down-converters typically used for the manufacture of white LEDs into one down-converter chemistry covalently and stably bonded within a silica framework.All organic white LEDs typically require a precise layered approach with several organic semiconductors and injection, charge transport/blocking, and light emission layers, while inorganic solutions call for the use of colour down-converting coatings made of rare-earth materials on top of a UV or blue-emitting chip. Even when thoroughly mixed at the right ratio to emit a sun-like spectrum, the discrete chemistries used in organic white LEDs show discrepancies in stability, affecting a white LED's useful lifetime. The use of rare-earth materials for the inorganic alternative implies higher production costs and uncertain availability. Colour tuneability through chemistry also remains a challenge. In a paper titled "White-emitting organometallo-silica nanoparticles for sun-like light emitting diodes" published in Materials Horizons, the researchers took a more direct approach to down-conversion, by synthesizing luminescent organometallo-silica nanoparticles bearing a mix of blue-, green-, and red-emitting organometallic Ir(III) complexes simultaneously encapsulated via sol-gel coordination chemistry inside the silica nanoparticles. The sol-gel coordination process allowed the researchers to kinetically control the covalent bonding of the three organometallic chromophores at the molecular level, prior to the formation of the silica matrix around them. The stable silica nanoparticles can then be blended into polymers or packaging material to coat a UV- or blue-emitting chip, yielding a so-called hybrid inorganic/organic white LED (HWLED). The sol-gel coordination chemistry they describe is based on the co-condensation of emissive organometallic complex bearing alkoxysilane terminal groups with a silica source, forming hybrid materials with the chromophore homogeneously and stably incorporated into the silica framework. Unlike previous attempts which focused on creating down-converter coatings with mixtures of individually synthesized single organometallic complexes, the researchers were able to control the formation of the white emitting organometallic dots (OD) built through synthesis prior to the growth of the silica nanoparticle (NP). The single silica-encapsulated three-in-one complex yields a much higher stability over time even under harsh environments (in excess of several thousand hours). The white LEDs fabricated on that basis also exhibited a high quality white emission with sun-like colour rendering. The authors report single-component hybrid nanoparticles white LEDs (NPHWLED) having a broad emission spectrum with a maximum at 590nm close flanked by shoulders at circa 500 and 620nm, perfectly matching with that of the natural sunlight (measured in Madrid in December last year). What's more, the NP-HWLEDs showed minimal changes with respect to the emission spectrum at different measuring angles, suggesting a uniform spherical light distribution. The paper reports a photoluminescence quantum yield value of 20% for the white-emitting nanoparticles, which once blended into a rubber-like down-converting coating, enabled the most stable single-component HWLEDs designed up to date. IMDEA Materials - www.materiales.imdea.org   Related articles: Liquid QDs for high efficiency LEDs Researchers design fast white light from hybridized inorganic/organic LED