Unipolar space-charge limited current through layers with a disparate concentration of shallow traps: Experiment and model

The influence of the spatial distribution of trap states on unipolar space-charge limited current (SCLC) is investigated experimentally and theoretically. Thin-layered films of the small molecule organic semiconductor N,N-'-di(1-naphtyl)-N,N-'-diphenylbenzidine (alpha-NPD) are vapor deposited on indium tin oxide, with aluminum as the counter electrode. The small molecule 4,4('),4(')-tris-[N-(1-naphtyl)-N-(phenylamino)]-triphenylamine (1-NaphDATA), which creates well-known shallow traps for holes, is used as dopant. The realized organic films consist of three layers, one of which is homogeneously doped. The influence of the spatial position of the doped layer on the current-voltage characteristics of the diodes is examined. Compared to an undoped device, the current density is strongly decreased and varies over orders of magnitude for the different spatial positions of the doped layer. It is shown that traps near the injecting electrode have the most pronounced effect on SCLC. A model for unipolar SCLC through a system of homogeneous layers with different trapping parameters for shallow traps is presented. The model quantitatively describes the experimental data and is used to calculate the spatial distributions of the charge-carrier density and the electric-field strength in the differently doped devices. (C) 2005 American Institute of Physics.