Dynamics of triplet migration in films of N, N'-diphenyl-N, N'-bis(1-naphthyl)-1, 1'-biphenyl-4, 4''-diamine.

We study triplet migration properties in NPB (N, N'-diphenyl-N, N'-bis(1-naphthyl)-1, 1'-biphenyl-4, 4''-diamine) films using time resolved gated spectroscopy and dispersive migration theory as our main tools of analysis. We show that in NPB, a well-known hole transporter in organic light emitting diodes, at high excitation densities triplet migration follows two regimes--a dispersive non-equilibrium regime (distinguished by exciton energetical relaxation within the distribution of hopping sites and as a consequence the hopping frequency being time dependent) that evolves into a second, non-dispersive equilibrium regime. Further, we observe a third region, which we term acceleration. From the turning over time between dispersive and non-dispersive dynamics, we deduce the width of the triplet density of states (DOS). We observe how the DOS variance changes when one decreases the thickness of the NPB film and note how surface effects are becoming important. Furthermore, the DOS variance of NPB changes when another organic layer is evaporated on top, namely Ir(piq)3 (tris(1-phenylisoquinoline)iridium(III)). We believe that these changes are due to the different polarizable media in contact with the NPB film, either vacuum or Ir(piq)3. We also show in this paper that the triplet level when time approaches zero is much higher in energy than the relaxed triplet levels, as quoted in most published papers; these values are thus incorrect for NPB. Lastly, it is possible that even at room temperature, the dispersive regime might be important for triplet migration at high initial triplet concentrations and might affect the diffusion length of triplets to a certain extent. However, more experimentation needs to be performed in order to address this question. Overall, we have characterized the triplet migration dynamics of NPB fully and shown that it agrees with previously published observations for other organic semiconductors and theoretical considerations.