Transition from Non-Dispersive to Dispersive Hole Transport in a Small Molecule Organic Semiconductor Controlled by Mole
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0965-S14-03
Transition from Non-Dispersive to Dispersive Hole Transport in a Small Molecule Organic Semiconductor Controlled by Molecular Doping Arne Fleissner1, Hanna Schmid1, Christian Melzer1, Roland Schmechel1,2, and Heinz von Seggern1 1 Electronic Materials Department, Darmstadt University of Technology, Institute of Materials Science, Petersenstr. 23, Darmstadt, 64287, Germany 2 Institute for Nanotechnology, Forschungszentrum Karlsruhe (FZK), Karlsruhe, 76021, Germany
ABSTRACT The influence of charge carrier traps on charge carrier transport is studied in a small molecule organic semiconductor model system by means of an optical time-of-flight method. The model system consists of the hole transport material N,N’-di(1-naphtyl)-N,N’diphenylbenzidine (α-NPD, sometimes denoted as α-NPB) either undoped or doped with various concentrations of the small molecule 4,4’,4’’-tris-[N-(1-naphtyl)-N-(phenylamino)]triphenylamine (1-NaphDATA), which is known to create hole traps in α-NPD. In case of undoped α-NPD, non-dispersive hole transport is observed and the hole mobility is determined as 6⋅10-4 cm2/Vs in the examined electric field range, being in good agreement with published data. Depending on the intensity of the laser light employed for optical charge carrier generation, current transients both in the space-charge regime and in the small signal case are obtained. In the small signal case the current transients do not exhibit the expected flat current plateau before the characteristic kink that marks the transit time, but feature a cusp instead. A tentative mechanism for its formation is proposed. The influence of the trap concentration on charge carrier transport is studied by introducing 1-NaphDATA as a molecular dopant. It is demonstrated that the hole transport in α-NPD can be controlled by varying the doping concentration of 1-NaphDATA. Increasing the trap concentration, a transition from non-dispersive transport in undoped α-NPD to non-dispersive but trap-controlled transport with reduced mobility and further to dispersive transport is observed. INTRODUCTION Charge carrier transport through organic semiconductors is strongly affected by trap states in the energy gap. The fundamental understanding of their influence on charge transport is crucial for the design of high performance organic devices, such as organic light emitting diodes (OLEDs) [1] and organic field effect transistors (OFETs) [2]. In this work, the influence of the trap concentration on charge carrier transport mode and mobility in a small molecule organic semiconductor model system is studied by an optical time-of-flight method. The model system consists of the hole transport material N,N’-di(1-naphtyl)-N,N’diphenylbenzidine (α-NPD, sometimes denoted as α-NPB) molecularly doped with another small molecule 4,4’,4’’-tris-[N-(1-naphtyl)-N-(phenylamino)]-triphenylamine (1-NaphDATA). 1-NaphDATA is known to create about 0.4 eV-deep hole traps in α-NPD [3] due to the different energy of their HOMO (highest occupied molecular orbital) levels (5.4 eV an
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