Optimization of white OLEDs based on charge carrier conduction properties of phosphorescent emitting layers

Abstract

The charge conduction properties of the organic phosphorescent emission layer doped with iridium-based green and red phosphorescent emitters, fac-tris(2-phenylpyridine) iridium(III) (Ir(ppy)₃) and bis(2-(2'-benzo [4,5-a]thienyl)pyridinato-N,C3')iridium(acetyl-acetonate) (btp₂Ir(acac)), were studied and compared to those of the reference host of 4,'-N,N'-dicarbazole-biphenyl (CBP). In the CBP host layer, both dopants act as hole traps but they affect the electron transport differently. Compared with the pristine CBP film, the electron mobility is similar for Ir(ppy)₃-doped CBP but it is more than two orders of magnitude lower for btp2Ir(acac)-doped CBP. Because of such difference in the electrical conduction properties between the Ir(ppy)₃- and btp2Ir(acac)-doped CBP, the main recombination zone position and the electron-hole balance changes. Based on these findings, we optimized white organic light emitting diodes (OLEDs) with multi-emitting layer (EML) structures in which CBP layers doped with Ir(ppy)₃ and btp2Ir(acac) and fluorescent dopant of 4,4'-bis[2-{4-(N,N-diphenylamino)phenyl}vinyl]biphenyl (DPAVBi) were used as green (G), red (R), and blue (B) EMLs, respectively. The white OLEDs with the R/G/B EML sequence show improved electron and hole balance, resulting higher efficiency, better color stability and longer lifetime compared to the G/R/B EML sequence. A high luminous current efficiency of 13.5 cd/A at 100 cd/m² was achieved with the R/G/B EML sequence.