Chemical Aging, Charge Transport, and Electroluminescence in Alq_3-based Organic Light-Emitting Diodes

摘要

Conduction in aluminum (III) 8-hydroxyquinoline (Alq_3) was modeled based on trap-charge limited conduction of electrons in the bulk. The evolution of a narrow Gaussian distribution of localized trap states below the lowest unoccupied molecular orbital (LUMO) of Alq_3, lying against a natural exponential background, was used to explain changes in the current-voltage characteristic and external quantum efficiency with time observed by many researchers for organic light-emitting diodes. Based on the change of the shape of the J-V curve, the depth of the electron trap states that were formed during aging was about 0.25 eV below the LUMO of Alq_3. An increase in drive voltage and decrease in efficiency is predicted with aging by this model for current densities in a reasonable range, assuming that the evolved trap states are non-emissive and also non-quenching. The products of chemical aging can account for the generation of traps at the observed depth.