Trap concentration dependence of percolation in doped small molecule organic materials

The thermally stimulated current (TSC) technique is used to investigate the effect of doping of organic glassy thin films of the hole transport material N,N′-di(1-naphthyl)N,N′-diphenylbenzidine (α-NPD) with various concentrations of 4,4′,4″-tris(N-(1-naphthyl)-N-phenylamino)triphenylamine (1-NaphDATA). The mobility is estimated from current-voltage characteristics. At small dopant concentrations a TSC peak appears at about 200K. Increasing the dopant concentration to about 4vol% leads to a peak shift towards higher temperatures, related to decreasing mobility. When increasing the dopant concentration further, the peak shifts again to lower temperatures towards the peak position for pure 1-NaphDATA. The energy distribution of the trap structure is obtained utilizing the fractional TSC technique. In accordance to a higher-lying highest occupied molecular-orbital level of 1-NaphDATA as compared to the α-NPD matrix, the activation energy of a deep trap level of about 0.5eV was reported previously for low dop...

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