Concentration quenching of electroluminescence in neat Ir(ppy)3 organic light-emitting diodes

We studied concentration quenching of electroluminescence (EL) in organic light-emitting diodes with a neat fac-tris (2-phenylpyridinato-N, C2') iridium (III) [Ir(ppy)(3)] emitting layer of different thicknesses sandwiched between electron and hole blocking layers. The intensity of the green emission decreased rapidly with increasing Ir(ppy)(3) thickness and was reversely correlated with the tail band emission. The overall light output power reached the minimum at 4 nm, and attained a saturated value for Ir(ppy)(3) thicker than 6 nm. These results are interpreted as evidence that concentration quenching in Ir(ppy)(3) originates from both short and long-range energy transfer between excited and ground states of molecules. The EL quenching magnitude was found to be independent of the injection current, indicating that biexcitonic annihilation plays a minor role. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3504599]