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 (Alq3)-based electroluminescent (EL) devices and the EL efficiency reached ∼5 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 (1Alq3*) formation through triplet–triplet (T–T) annihilation of triplet excited states (3Alq3*). The requirement of the well-balanced charge injection implies that the long-lived A4lq3* was quenched efficiently by energy transfer to excess and colored Alq3−⋅ anion or Alq3+⋅ cation radicals in the emission zone when the charge injection was unbalanced. The short-lived A3lq3* was not quenched appreciably.