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 contribution of the redshifted emission spectrum component of a dopant or a separate film in multilayer structure can be changed by electric-field-induced quenching of its excited states and/or voltage evolution of the recombination zone. The effect is demonstrated on N,N'-diamine (TPD) single-layer LEDs nonuniformly doped with a perylene bisimide pigment (PBP) and double heterostructure TPD/Alq(3)/PBP with 8-hydroxyquinoline aluminum, Alq(3). While in the first class of LEDs the color changes from red to blue dependent on the applied voltage and average concentration 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 different color emission bands to the driving current and applied electric field as well as to the composition and structure parameters of LEDs such as dopant concentration or thickness of the layers. (C) 1998 American Institute of Physics.