Trap concentration dependence of thermally stimulated currents in small molecule organic materials

Abstract

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.

Hole transport in the organic small molecule material α-NPD: evidence for the presence of correlated disorder

“…31,[38][39][40] The relatively high glass transition temperature, 95°C, is viewed as beneficial to the OLED stability. In none of the earlier studies of the hole-mobility of ␣-NPD, based on the steady-state current density versus voltage ͓J͑V͔͒ curves, 16,24,41,42 time-of-flight measurements [43][44][45][46][47][48][49] or impedance measurements, 50 the carrier-density dependence of the mobility was taken into account.…”

Low temperature thermally stimulated current characterization of nanoporous TiO2 films

“…On the contrary, the TSC signal measured with the nc‐TiO 2 (for technical reasons is obtained as the sum of two sequential scans) is too broad to be discussed in terms of a standard TSC theory, by means of a few discrete intra‐gap energy levels. In the case of a continuous distribution of levels we can resort to a fractional TSC where, after priming, a set of contiguous TSC scans is used to separate emissions among different temperature/energy ranges . As in a standard TSC, the sample is first primed at a low temperature T 0, by exposing it to a photon source during a filling time, and then a decay time is waited to achieve a quasi‐equilibrium regime of conduction.…”

Low Temperature Thermally Stimulated Currents in nanostructured TiO2

“…With this method, by a progressive depletion of the emitting levels with the contiguous TSC scans it is possible to relate the number of states , proportional to Q, to the activation energy, and then to reconstruct the DOS [7]. A variant of the method was performed by Weise et al [5] through a temperature scan followed by a heating up to the maximum temperature (about RT) at each step, the priming being performed before each step. In this manner he left out the uncertainty due to defects emitting at higher temperatures.…”