High efficiency orange-red phosphorescent organic light emitting diodes based on a Pt(II)-pyridyltriazolate complex from a structure optimized for charge balance and reduced efficiency roll-off

We report a high power efficiency (PE) of 44.7 +/- 0.5 lm/W, high external quantum efficiency (EQE) of 19.7 +/- 0.1% at 500 Cd/m(2), and efficiency roll-off of only 4% of the peak value at a useful brightness of 1000 Cd/m(2) from orange-red emitting, organic light emitting diodes featuring 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as the hole transport layer/electron blocking layer, an emissive layer consisting of 65% bis[3,5-bis(2-pyridyl)-1,2,4-triazolato]platinum(II) (Pt(ptp)(2)) doped into 4,4'-bis(carbazol-9-yl)triphenylamine (CBP), and 1,3,5-tris(phenyl-2-benzimidazolyl)-benzene (TPBI) as the electron transport layer (ETL). Based on a comparison of these structures with devices that did not incorporate an electron/exciton blocker and devices featuring N,N'-dicarbazolyl-3,5-benzene (mCP) as the electron/exciton blocking layer, we ascribe the high efficiency and low efficiency roll-off to better charge balance, and enhanced confinement of excitons and the recombination zone to the emissive layer. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim