Morphological, electrical and optical properties of organic light-emitting diodes with a LiF/Al cathode and an Al-hydroxyquinoline/diamine junction

We report the results of the morphological, electrical and optical characterisation of double-layer Alq3-based organic emitting diodes with a lithium fluoride (LiF)/Al cathode. A detailed electron microscopy investigation of their cross-section shows the presence of LiF isolated grains underneath the Al film. Due to the introduction of the LiF layer, luminance was larger than 30,000 cd/m2 at a bias voltage VB = 25V with a maximum external luminous efficiency as large as 46lm/W at VB = 20V. Performing on/off VB cycles at a very low frequency, each time a recovery of the initial electric and luminous performance was observed. The non-exponential decay of both electric current and luminance during each on-cycle has been interpreted as the signature of dispersive transport controlled by multiple trapping on localised states in an amorphous system.