Inverted Bottom-Emitting White Organic Light-Emitting Diodes with Power Efficiency over 220 lm W-1

Abstract The white organic light-emitting diodes (WOLEDs) have great potential in the fields of solid-state lighting. Continued efforts have been made to catch up with the power efficiency (PE) of inorganic light-emitting diodes. Apart from low operating voltage and near unity internal quantum efficiency, improvement the efficiency of light extraction, especially suppression of surface plasmon (SP) modes is essential for realizing ultimate PE. In contrast to corrugated structure which could suffer from spectral selectivity, higher leakage current and potential degradation issue stemmed from the rough structure, increasing the distance between the emitter and the metallic electrode becomes an effective avenue towards suppressing SP modes while maintaining the planar structure. In this work, an inverted bottom-emitting structure with thick hole transport layer (HTL) composed of multi-period organic heterojunctions is demonstrated to effectively extract the SP modes without any electrical property degradation in the current density range for lighting. Investigation on electrical properties of thickened HTL with increased count of organic heterojunctions indicates no increase of driving voltage at same current density in lighting application condition. By attaching an matched high refractive index lens, a record maximum forward-viewing PE of 228.4 lm W-1 is obtained for the thick WOLED fabricated on high refractive index glass substrate, exhibiting significant enhancements of 57% compared to the counterpart of the conventional thin WOLEDs. An unambiguous picture is given to depict the regularity derived from increase of emitter-to-metal distance by combination of a full optical simulation and experimental measurements excluded the influence of electrical property degradation.