Improvement of the color-stability in top-emitting white organic light-emitting diodes by utilizing step-doping in emission layers

Abstract

Chromatic stability and purity cannot be acquired easily in the top-emitting white organic light-emitting diode (TWOLED) due to its special device structure with metal film as both anode and cathode. Blue/red/blue emission layers are utilized to optimize the chromaticity and to improve the stability. Then step-doping is introduced into the red emission layer to further improve the device stability. In order to explain the mechanism of the improvement in chromaticity stability, effects of step-doping on the device performances are analyzed in detail. Experimental results refleal that the built-in electric field induced by the step-doping changes the carrier drifting and exciton recombination in emission layers. When the doping concentration gradually increases from the anode side to the cathode side (increasing step-doping), the built-in electric field is facilitated to suppress the electron drafting and the exciton diffusing, which helps control the exciton recombination zone. Color stability can be improved in the TWOLED with increasing step-doping. Additionally, as an important factor influencing the emission spectra of TWOLEDs, microcavity resonance is calculated and used to explain the variation of spectra induced by device structural changes.