Very efficient high-luminance electrophosphorescent emission in polymer light-emitting diodes

The electrophosphorescent emission properties were investigated in polymer light-emitting diodes (PLEDs) based on a poly(9-vinylcarbazole) (PVK) doped with a green phosphorescent dye of fac-tris(2-phenylpyridine) iridium (III) [Ir(ppy)/sub 3/]. A green light peaked at 516 nm was emitted from devices with a configuration of ITO/PEDOT:PSS/PVK:Ir(ppy)/sub 3//BCP/Alq/sub 3//LiF/Al. The optimal doping concentration of Ir(ppy)/sub 3/ in PVK was found at 2% by weight, under which maximum current efficiency of 24.3 cd/A and peak external quantum efficiency of 6.8% were achieved at the high luminance of 4240 cd/m/sup 2/. The external quantum efficiency of 5% and current efficiency of 18 cd/A can be sustained even at the very high luminance of 35000 cd/m/sup 2/.

1. Introduction

Recently, effective external quantum efficiency of 19% photons/electron and luminance power efficiency of 70 L/W have been demonstrated in small molecule organic light-emitting diodes (OLEDs) [1], [2]. The reason is that nearly 100% internal emission quantum efficiency can be obtained by harvesting both singlet and triplet excitons in OLEDs doped with phosphorescent emitters containing heavy metals since the strong spin-orbit coupling of a heavy metal enhances inter-system crossing and mixes the single and triplet states [1]–[3].