Influence of phosphorescent dopants in organic light-emitting diodes with an organic homojunction

We report on phosphorescent homojunction organic light-emitting diodes (HJOLEDs) using p-i-n structures based on a single ambipolar organic semiconductor, 4,4'-Bis(carbazol-9-yl)-biphenyl, as matrix organic materials. In HJOLEDs, the phosphorescent dopant molecules play an important role in controlling the charge balance inside the emissive layer. We observe a four-fold enhancement in the luminous efficacy at 1000 cd/m(2) from 3.7% to 12.9% by varying the emitter molecule. The influence of the energy level of the emitter molecule on charge balance is investigated by analyzing current density vs. voltage curves with the trap-limited current theory and by analyzing the electroluminescence spectra. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772014]