Understanding the influence of doping in efficient phosphorescent organic light-emitting diodes with an organic p-i-n homojunction

We report on the development and detailed investigation of highly efficient p-i-n phosphorescent organic light-emitting diodes (PhOLEDs) using 4,40-bis(carbazol-9-yl)-biphenyl (CBP) as a single organic semiconductor matrix. Following optimization of doping concentration of both the phosphorescent emitter molecule and of the p-and n-type dopants, an external quantum efficiency (EQE) of 15% and a power efficiency (PE) of 28 lm/W are realized at a luminance of 1000 cd/m(2). These values are comparable to the state-of-the-art for conventional complex multilayered PhOLEDs. By analyzing the device characteristics (i.e. electroluminescence spectra, the current density-voltage behavior of single carrier devices, the transient electroluminescent decay, and the impedance spectroscopy response), we find that the device performance is closely linked to the charge carrier balance in the device, which in turn is governed by the interplay of the conductivities of the doped layers and the transport of each charge carrier species within the emitting layer. (C) 2013 Elsevier B.V. All rights reserved.