Monitoring of the initial degradation of oxadiazole based blue OLED’s

A new type of blue emitting single active layer organic light emitting diode has been investigated, consisting of transparent PEDOT:PSS coated indium tin oxide back contact, a spin-coated small molecule active layer and an aluminium top contact. The active layer molecule contains an oxadiazole group as electron conductor and light emitter and a carbazole group as hole conductor. The initial degradation of this device under constant voltage conditions has been investigated by continuous monitoring of the diode current, the emitted light intensity and the optical emission spectrum. The monitoring results indicated no significant change in the active molecule layer emission properties, but a strong modification of the originally asymmetric charge carrier injection balance. A decrease of the threshold current for light emission and an increase of the differential efficiency has been observed after the stress test. Device heating effects could be clearly separated from real degradation of the light emitting diode