Emission zone in organic light-emitting diodes under AC operation

Organic light-emitting diodes consisting of a diamine/Al-quinolinol junction, a LiF/Al cathode, and an ITO anode were operated under application of rectangular voltage pulses alternated between 0 V (off-period) and 8 V (on-period) with frequencies in a range from 1 to 100 kHz. The emission intensity in the on-period was found to be lowered, if the off-period is longer than or equal to 100 μs. From the analyses of the response of emission and the light-emitting zones in the devices under different operating conditions, it was concluded that, just after the turn-on, the light-emitting zone distributes in the Al-quinolinol layer deep from the diamine/Al-quinolinol interface. The region expands deeper as the off-period becomes longer. The expanded zone approaches the interface in the on-period of about 10 μs. Such a change in the light-emitting zone with the operational conditions was attributed to the change in the energy barrier for the injection of electrons from the cathode into the Al-quinolinol layer, which is caused by the electric field applied to the organic layers.