The effect of doping of thin organic glassy films of the hole transport material N,NЈ-di͑1-naphthyl͒N, NЈ-diphenylbenzidine ͑␣-NPD͒ with various concentrations of 4,4Ј, 4Љ-tris͑N-͑1-naphthyl͒-N-phenylamino͒-triphenylamine ͑1-NaphDATA͒ is investigated with respect to the resulting charge carrier mobility and the effect on thermally stimulated currents ͑TSC͒. The energetic trap structure is obtained utilizing the fractional TSC technique. At low doping concentrations of a few percent the activation energy of an about 0.5 eV deep trap level is measured, which is related to the higher lying highest occupied molecular orbital level of 1-NaphDATA compared to the ␣-NPD matrix. A strong shift of the TSC peak towards higher temperatures with increasing dopant concentration is observed. It can be attributed to re-trapping despite the low film thickness of 200 nm. Thereby the effect of re-trapping is theoretically investigated and found to be composed of a mobility change and a change of the trap depth, as it is measured in fractional TSC measurements. The latter is related to enhanced re-trapping into deeper traps. The maximal peak shift occurs at about 4 vol % of dopant concentration, corresponding to about 2.7% molecular concentration. For higher concentrations of 1-NaphDATA the mobility increases again, and the TSC peak gradually changes to that of pure 1-NaphDATA, explainable by a progressive change from a deep-trap-controlled transport to a percolating transport on the 1-NaphDATA molecules.

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 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.

KurzfassungDieser Beitrag zeigt auf der Basis bestehender Ansätze Möglichkeiten neuer Duktilitätsdefinitionen auf. Die dabei neu erstellten Ansätze bieten im besonderen Maße für umformkritische Leichtbaublechwerkstoffe die Möglichkeit zur differenzierten Werkstoffcharakterisierung. Die Beschreibung der Duktilität eines Werkstoffes dient nicht nur der Versagensvorhersage in der Bauteilsimulation, sondern zeigt sowohl dem Karosserieentwickler weitere Optimierungspotenziale als objektives Bewertungskriterium, als auch der Qualitätssicherung zusätzliche Prüfkriterien auf.