Electric potential mapping by thickness variation: A new method for model-free mobility determination in organic semiconductor thin films

Charge transport, with charge carrier mobility as main parameter, is one of the fundamental properties of semiconductors. In disordered systems like most organic semiconductors, the effective mobility is a function of the electric field, the charge carrier density, and temperature. Transport is often investigated in a space-charge limited current (SCLC) regime in thin film single carrier devices, where an electric current is driven in the direction perpendicular to the surface. Direct evaluation of the current-voltage characteristics, however, is problematic, because parasitic contributions from injection or extraction barriers can falsify results. Here, we present a novel measurement and evaluation technique for key transport parameters. First, it allows for the direct determination of the potential profile in single carrier devices. It is obtained from a series of steady-state current-voltage measurements from devices with varying thickness ("electric potential mapping by thickness variation", POEM). Second, the data can be evaluated to obtain the effective charge carrier mobility μ(F, n) as a function of the electric field F and the charge carrier density n. Single carrier transport is achieved by sandwiching the organic material under investigation between equally doped layers, i.e. p-i-p (resp. n-i-n) devices for hole (electron) transport investigations. The POEM concept is validated using drift-diffusion simulation data. It is furthermore experimentally applied to small molecular organic semiconductors, where the hole transport in a blend of zinc phthalocyanine (ZnPc) and C60 is characterized. In the measured range of F ≈ (1-5) × 105 V/cm and hole densities of approx. (1-5) × 1016 cm-3, the hole mobility is found to be in the range of (10-7-10-5) cm2/V s, comprising a pronounced field activation with an activation constant of 0.01cm/V. A dependence of the mobility on the charge carrier density in the given range is not observed. The POEM approach does not require a given mobility function as input, i.e. it constitutes a model-free determination of the effective mobility μ(F, n). It is especially suitable for semiconductors which require complex mobility models, like hopping or trap-dominated transport in disordered systems, and relatively low mobilities, like e.g. neat or mixed organic semiconductors. © 2013 The Authors. Published by Elsevier B.V. All rights reserved