Power efficiency improvement of white phosphorescent organic light-emitting diode with thin double-emitting layers and hole-trapping mechanism

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(bpft_z)₂(PPh₂Me)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 lmW^-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 lmW^-1 by attaching the outcoupling enhancement film.