Advanced Functional Materials and Devices (AFMD) Group

Abstract:

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.

Abstract:

The maximum open-circuit voltage V of bulk-heterojunction solar cells is limited by the effective HOMO(donor)-LUMO(acceptor) gap of the photoactive absorber blend. We investigate blend layers comprising zinc-phthalocyanine (ZnPc) and the buckminster fullerene C with ultraviolet, x-ray, and inverse photoelectron spectroscopy. By varying the volume mixing ratio ZnPc:C from 6:1 to 1:6, we observe a linear increase of the HOMO(ZnPc)-LUMO(C) gap by 0.25 eV. The trend in this gap correlates with the change in the charge transfer energy measured by Fourier-transform photocurrent spectroscopy as well as with the observed open-circuit voltage of solar cells containing ZnPc:C as the photoactive absorber layer. Furthermore, the morphology of different ZnPc:C blend layers is investigated by grazing-incidence x-ray diffraction. As physical origins for the changed energy levels, a suppressed crystallization of the C phase in the presence of donor molecules as well as concentration-dependent growth modes of the ZnPc phase are suggested. © 2013 American Physical Society.

Abstract:

By combining computer simulations, grazing incidence, and powder X-ray-diffraction measurements we reconstruct the crystal structure of a thin film of terminally dicyanovinyl-substituted quaterthiophene (DCV4T). The crystal structure differs from the known single crystal structure of the same compound, but resembles the molecular packing of a methylated DCV4T. Charge transport simulations on the molecular level show that the 2 dimensional thin-film charge-transport network is well suited for hole transport in solar cells. © The Royal Society of Chemistry 2013.