Distribution of occupied states in doped organic hole transport materials

In the present work, the effect of chemical dopants on the energetic trap structure and the resulting transport properties of organic light emitting diodes (OLEDs) has been studied. For trap level detection, an energy resolved thermally stimulated current (TSC) technique known as fractional TSC was applied to derive the density of occupied states (DOOS). For the doping experiments, the widely used hole transport layer (HTL) materials 4,4,4"-tris-(N-(1-naphthyl)-N-phenylamino)-triphenylamine (1-NaphDATA), 4,4',4"-tris-(N-(3-methylphenyl)-N-phenylamino)-triphenylamine (MTDATA) and N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (alpha-NPD) were selected whereby the latter molecule possesses a larger absolute HOMO energy than the other materials. TSC spectra of alpha-NPD-doped 1-NaphDATA single layer devices show only slight differences in current magnitude and peak position compared to undoped reference samples. In contrast, TSC spectra of 1-NaphDATA-doped alpha-NPD devices differ considerably from undoped references: a new TSC peak at 225 K emerges indicating the introduction of a new trap level. Apparently, the higher lying HOMO level of the embedded 1-NaphDATA molecules generates deep traps in the alpha-NPD layer whereas alpha-NPD as dopant does not create any new traps. The activation energies calculated from the TSC spectra correspond to the reported HOMO energy difference between dopant and matrix molecules. Similar results are obtained for MTDATA-doped alpha-NTD devices. Further, it will be demonstrated that such purposely introduced traps have a strong influence on the charge transport properties in OLED devices even at room temperature. Therefore, conventional temperature dependent current-voltage (I-V) and luminance-voltage (L-V) characteristics have been investigated. A reduced I-V and L-V characteristics of the 1-NaphDATA- and MTDATA-doped alpha-NPD device compared to the respective undoped samples can be reported. (C) 2002 Elsevier Science B.V. All rights reserved.