Power Efficiency Improvement of White Phosphorescent Organic Light-Emitting Diode with Thin Double-Emitting Layers and Hole-Trapping Mechanism

Abstract

This study is carried out to discuss how to reduce the driving voltage of blue phosphorescent organic light-emitting diodes (PHOLEDs) by using a thin double-emission layer. A hole transport-type host (TCTA) is inserted between the hole transport layer (TAPC) and the emitting layer (EML), constituting a buffer layer between them with the aim of improving charge carrier balance. Furthermore, in this study, we also utilize the interface between double light-emitting layers of devices by codoping them with a red phosphorescent dopant [Os(bpftz)(2)(PPh(2)Me)(2)]. An Os complex with a high-lying highest occupied molecular orbital (HOMO) energy level (trapping holes) is codoped at the interface between emitting layers and an exciton-formation zone is expanded to obtain a white PHOLED with high efficiency. From the results, the optimal structure of the white device exhibits a yield of 35 cd A(-1), a power efficiency of 22 lm W(-1), and CIE coordinates of (0.33; 0.38) at a luminance of 1000 cdm(-2). Furthermore, the power efficiency can be improved to 30 lm W(-1) by attaching the outcoupling enhancement film. (C) 2011 The Japan Society of Applied Physics