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

Abstract

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.

Optimizing the Charge Balance of Fluorescent Organic Light‐Emitting Devices to Achieve High External Quantum Efficiency Beyond the Conventional Upper Limit

“…One possible mechanism causing the high EQE is triplet-triplet annihilation (TTA). [15][16][17][18][19] Transient electroluminescence responses of the green and blue emitting devices are shown in Figure 5 . In both the green and blue emitting devices, the …”

Magnetic‐Field Effects in Organic Semiconducting Materials and Devices

“…It is noted that unbalanced bipolar charge injection reduces the e-h pairing for the formation of polaron pairs and excitons, and therefore increases the triplet exciton-charge reaction in organic light-emitting diodes. [114] The relevant outcome of triplet exciton-charge reactions is the generation of secondary electrons and holes by destroying the bound e-h pairs in excitonic states. The secondary electrons and holes can be recaptured to form polaron pairs with singlet and triplet configurations due to Coulombic attraction through germinate and nongerminate processes, generating secondary polaron pairs.…”

Correlation Between Triplet–Triplet Annihilation and Electroluminescence Efficiency in Doped Fluorescent Organic Light‐Emitting Devices

“…This leads to the maximum theoretical limit for the device quantum efficiency of 62.5% for OLEDs based on fluorescent emitters. [6] Such derivation for the maximum quantum efficiency is based on an assumption that the TTA process always produces singlet excitons. As, in reality, there can be additional TTA pathways that do not produce singlet excitons, such as T þ T !…”

“…Such effect was the focus of a number of investigations. [5][6][7][8] These studies, however, were primarily limited to OLEDs utilizing undoped (neat) emitter systems. A systematic study of TTA processes in OLEDs based on the technologically more important host:guest systems with fluorescent dopants is still lacking.…”