Organometallic Materials for Electroluminescent and Photovoltaic Devices

Abstract

Electroluminescent devices, solar energy conversion technologies and light-emitting electrochemical cells represent a promising branch of modern optoelectronic industry based on organic dyes and polymers as the main working materials. Elementary processes like energy flow through an organic-inorganic interface and voltage control at a molecular level with peculiar electronic properties are now well understood and used in fabrication of new efficient and sophisticated optoelectronic devices. Today, organic light emitting diodes (OLEDs) are used commercially in displays and various lighting applications providing high external quantum efficiency (up to 19%) and low power consumption (Nazeeruddin et al. 2009). Electroluminescence of organic materials was observed for the first time by Martin Pope et al. (Pope et al. 1963) a half of a century ago. Twenty-four years later the pioneering work of Eastman Kodak Company (Tang & VanSylke 1987) provided the use of 8-hydroxyquinoline aluminum (Alq3) as electron-transporting and emissive material (Scheme 1) in an OLED device. Since then a growing progress has been witnessed in the field of organic optoelectronics through incorporation of various combinations of organic polymers, dyes and organometallic complexes (Yersin & Finkenzeller 2008, Kohler & Bassler 2009). Organic conjugated polymers, like poly(para-phenylene vinylene) (PPV) doped by various chromophores, are now used in OLEDs as they lend the possibility to create charge carrier recombination and formation of excitons with high efficiency of light emission. Typical OLEDs are fabricated by spin-coating, inkjet printing or by vacuum deposition of organic materials on an indium-tin-oxide (ITO)-coated glass and with a multilayer structure of the device including NPB (N,N’-Bis(naphthalene-1-yl)-N,N’-bis(phenyl)-benzidine) and Alq3 as the hole transport layer (HTL) and electron transport layer (ETL), respectively. These materials are presented here as typical examples. In between there is a doped emission layer (EML). Usually some additional layers which protect the ETL from reactions with the cathode material, or reduce the injection barrier and electron-hole quenching, are incorporated into the device architecture. These OLEDs are thin, flexible, stable, and energy