Trap concentration dependence of percolation in doped small molecule organic materials

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 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.

Sol‐Gel‐Syntheses and Structural as well as Electrical Characterizations of Anatase‐ and Rutile‐Type Solid Solutions in the System IrO2−TiO2

“…Thus, for a nominal doping level of 5 mol % Ir (x = 0.05 or 5 mol %), 950 μmol titanium precursor and 50 μmol iridium precursor were used. The density of the liquid titanium precursor was considered to be ρ = 0.96 g/cm 3 . The size of the batch had an influence on the course of the syntheses.…”

“…In materials science, mathematical theories such as percolation theory [2] are used to explain the behavior, according to which only a certain amount of dopant is required to imprint the properties, such as metallic conductivity, of a dopant into the matrix component. [3] Thus, in some cases, intrinsically electrocatalytically inactive substances may be suitable for use in electrolyzers or fuel cells. [4] Besides this, other applications of doped titanium dioxides are often described as variants for pollutant degradation that show enhanced activity when irradiated with visible light.…”