Electrochemical models for the radical annihilation reactions in organic light-emitting diodes

Bilayer organic light emitting diodes (OLEDs), based upon vacuum deposited molecules, or single layer OLEDs, based upon spin-cast polymeric materials, doped with these same molecules, produce light from emissive states of the lumophores which are created through annihilation reactions of radical species, which can be modeled through solution electrochemistry. Differences seen in solution reduction and oxidation potentials of molecular components of OLEDs are a lower limit estimate to the differences in energy of these same radical species in the condensed phase environment. The light emitted from an aluminum quinolate (Alq(3))/triarylamine (TPD)-based OLED, or an Alq(3)/PVK single layer OLED, can be reproduced from solution cross reactions of Alq(3)(-)./TPD+. (or other comparable cation radical species). The efficiency of this process increases as the oxidation potential of the TPD increases, due to added substituents. Radical cations and anions of solublized version of quinacridone dopants (DIQA) which have been used to enhance efficiencies in these OLEDs, are shown to be electrochemically more stable than Alq(3)(-.) and Alq(3)(+.), and DIQA radical annihilation reactions produce the same emissive state as in the quinacridone-doped OLEDs. Electrochemical studies demonstrate the ways in which other dopants might enhance the efficiency and shift the color output of OLEDs, across the entire visible and near-IR spectrum. Chemical degradation pathways of these same molecular components, which they may undergo during OLED operation, are also revealed by these electrochemical studies.