Space charge and carrier trapping effects on the transient photocurrents of organic materials using the time-of-flight technique

We apply the time-of-flight (TOF) technique to study space charge and carrier trapping effects in the organic materials N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1-biphenyl-4,4'-diamine (TPD) and tris(8-hydroxyquinolato) aluminum (Alq(3)). This is carried out by changing the applied electric field, the laser pulse intensity, and the repetition rate together with environmental conditions in air or in vacuum. We report for the first time, to the best of our knowledge, a clearly nondispersive electron transport in TPD due to the absence of deep traps. Conversely, Alq(3) exhibits a dispersive electron transport. However, this can be partially recovered by leaving the sample in high vacuum for several hours. This behavior is ascribed to a reduced concentration of O-2-related deep traps.