Efficient green polymer light-emitting diodes with microcavity effect in electroluminescence spectrum but constant quantum efficiency

We report the efficient green polymer light-emitting diodes (LEDS) that exhibit a strong microcavity effect in the electroluminescence (EL) spectrum. The LEDs employ a double-layer structure, with poly-(3,4-ethylenedioxythiophene): polystyrene sulfonic acid as the hole-transporting layer and with a highly efficient polyfluorene-based green-emitting polymer as the electron-transporting and emitting layer. The EL spectra of the LEDs demonstrate a strong resonance effect with the thickness of the emitting layer varying from 30 to 280 nm. The turn-on voltage of the device increases with the increasing thickness of the emitting layer. However, the brightness and especially the external quantum efficiency of the devices are largely independent of the thickness from 100 to 280 nm, when the emitting layer is thick enough to avoid cathode quenching of the electroluminescence. (C) 2004 American Institute of Physics.