A possible mechanism for enhanced electrofluorescence emission through triplet-triplet annihilation in organic electroluminescent devices

We demonstrate here that luminance increased more than linearly with an increase in current density of tris(8-hydroxyquinoline) aluminum (Alq(3))-based electroluminescent (EL) devices and the EL efficiency reached similar to5 cd A(-1) at 250 mA cm-2 when electron and hole injection was well balanced. The luminance-current curves were well fitted with a combination of a linear and a quadratic function of the current. The quadratic component can be attributed to additional singlet excited state ((1)Alq(3)(*)) formation through triplet-triplet (T-T) annihilation of triplet excited states ((3)Alq(3)(*)). The requirement of the well-balanced charge injection implies that the long-lived (3)Alq(3)(*) was quenched efficiently by energy transfer to excess and colored Alq(3)(-.) anion or Alq(3)(+.) cation radicals in the emission zone when the charge injection was unbalanced. The short-lived (1)Alq(3)(*) was not quenched appreciably. (C) 2002 American Institute of Physics.