High Efficiency Red Phosphorescent Organic Light-Emitting Diodes with Simple Structure

After the first report of electroluminescence in anthracene organic materials in monolayer devices in 1963 by Pope et al. (Pope et al., 1963) and by Helfrich and Schneider in 1965 (Helfrich & Schneider, 1965), this phenomenon remained of pure academic interest for the next two decades owing to the difficulty of growing large-size single crystals and the requirement of a very high voltage ( 1000 V) to produce the luminance. The evolution of OLED devices is summarized in Fig. 1. Tang and his group demonstrated that the poor performance of the monolayer early device was dramatically improved in two layers device by the addition of a hole transport layer (HTL) with the thin amorphous film stacking in the device structure (VanSlyke & Tang, 1985; Tang et al., 1988). Organic electroluminescent devices having improved power conversion efficiencies by doping the emitting layer were also realized around the same time by the Kodak group. Subsequently, heterostructure configurations to improve the device performance were implemented by inserting several layers like buffer layer between anode and hole transport layer (HTL) (VanSlyke et al., 1996; Shirota et al., 1994; Deng et al., 1999) electron transport layer (ETL), hole blocking layer (HBL) (Adamovich et al., 2003) or interlayer between cathode and ETL (Hung et al., 1997; Kido and Lizumi, 1998) in the device structure. Such multilayer device structure often enhances the drive voltages of OLEDs. Usually, the operating voltage for higher brightness was much higher than the thermodynamic limit which is 2.4 eV for a green device. Chemical doping with either electron donors (for electron transport materials) or electron acceptors (for hole transport materials) can significantly reduce the voltage drop across these films. These devices with either HTL or ETL doped layer show improved performance; but the operating voltages were still rather higher than the thermodynamic limit. Subsequently, Leo and his group proposed the concept of p-type doped HTL and ntype doped ETL (J. Huang et al., 2002). These p-i-n structure devices show high luminance and efficiency at extremely low operating voltages. Indeed all these devices have multilayer structure with high currentand power-efficiencies, but thin emitting layer. Nevertheless, narrow thickness of emitting layer in p-i-n OLEDs and complex design architecture of phosphorescent OLEDs are not desirable from the manufacturing perspective.