Weise, Wieland ; Keith, Torsten ; Malm, Norwin von ; Seggern, Heinz von (2005)
Trap concentration dependence of percolation in doped small molecule organic materials.
In: Journal of Applied Physics, 98 (4)
doi: 10.1063/1.2005378
Artikel, Bibliographie

Kurzbeschreibung (Abstract)

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 200 K. Increasing the dopant concentration to about 4 vol % 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 0.5 eV was reported previously for low doping concentrations up to a few percent. At higher dopant concentrations the deep traps vanish from the trap structure. The behavior can be interpreted as a change from a trap-controlled transport for small doping concentrations to a percolating transport on the dopant molecules themselves. It is shown that the onset of percolation at rather low concentrations can be explained by hopping including not only nearest neighbors.

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