Preparation, Optical Spectroscopy, and Fluorescence of Molecular Organic Composites for Light-Emitting Diodes

Abstract

Electroluminescence from small organic molecules has been known for some time. Thirty years ago, Helfrich and Schneider reported blue-violet electroluminescence in anthracene with an external quantum efficiency as high as 8%.1 This quantum efficiency is much better than that for the best polymer-based light-emitting diode (LED) reported to date.2 In spite of the superior quantum efficiency of molecular-based electroluminescent devices, no major progress was achieved until fairly recently when Tang and VanSlyke reported the first low-voltage organic LED with an external quantum efficiency of 1% (number of photons per electron).3 The emitting layer in this device consists of a thin layer of the metal complex, tris (8-hydroxyquinolinato) aluminum (AlQ3). Later, Littman and Martie showed an enhancement in the electroluminescence quantum efficiency of AlQ3 by doping it with the highly fluorescent laser dyes, Coumarin 540, Coumarin 343, and DCM.4 The paper focuses on a new class of organic composites consisting of highly fluorescent guest molecules dispersed in AlQ3. The electronic and optical properties of organic nanostructures based on these materials are studied, as functions of the luminescent center concentration, via optical and fluorescence spectroscopies. Photoluminescence quantum yields are measured and used to probe the efficiency of energy transfer between host and guest molecules.