VOLTAGE-INDUCED EVOLUTION OF EMISSION-SPECTRA IN ORGANIC LIGHT-EMITTING-DIODES

A simple model for the voltage-induced alteration of emission spectra from single-as well as multilayer organic light-emitting diodes (LEDs) based on low-molecular-weight materials is reported. The relative con tribution of the redshifted emission spectrum component of a dopant or a separate film in multilayer structure can be changed by electric-fi eld-induced quenching of its excited states and/or voltage evolution o f the recombination zone. The effect is demonstrated on N,N'-diamine ( TPD) single-layer LEDs nonuniformly doped with a perylene bisimide pig ment (PBP) and double heterostructure TPD/Alq(3)/PBP with 8-hydroxyqui noline aluminum, Alq(3). While in the first class of LEDs the color ch anges from red to blue dependent on the applied voltage and average co ncentration of the PBP dye, the second-type structures operate in the red-green range as the blue emission from TPD is eliminated. Analytic considerations are presented relating the relative contribution of dif ferent color emission bands to the driving current and applied electri c field as well as to the composition and structure parameters of LEDs such as dopant concentration or thickness of the layers. (C) 1998 Ame rican Institute of Physics.