Publications associated with Advanced Functional Materials and Devices Group

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

Organic Electronics: physics, materials, applications 14 (2013) 3460-3471

J Widmer, J Fischer, W Tress, K Leo, M Riede

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 C is characterized. In the measured range of F ≈ (1-5) × 10 V/cm and hole densities of approx. (1-5) × 10 cm, the hole mobility is found to be in the range of (10-10) cm/V s, comprising a pronounced field activation with an activation constant of 0.01 sqrt(cm / 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.

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