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(α-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.

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The undoped host material α-NPD also contains traps (see Ref. 13), which are intrinsic to the material. Therefore, not all injected charge carriers are free charge carriers. However, this situation is already reflected by the mobility μ of the host material and thus in the current–voltage dependence of the undoped device, to which the doped devices are compared to. The considered trapping parameter θ reflects only the additional traps introduced by doping.
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