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 6·10−4cm2/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.