Moritz Riede

Abstract:

Interfaces between organic electron-donating (D) and electron-accepting (A) materials have the ability to generate charge carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer:fullerene, small-molecule:C and polymer:polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT are excited. The best materials systems show an IQE higher than 90% without the need for excess electronic or vibrational energy.

Abstract:

For alternative energy technologies like photovoltaics (PV), the analysis of Research and Development (R&D) expenses is important to observe and understand market dynamics. This is, in turn, essential for policymakers. However, the quantitative evaluation of global corporate R&D investments is a challenging task due to unavailability or high scatter of precise data. Here we present a top-down approach to estimate the current and global historic cumulated PV R&D investments based on international PV patent applications. High growth rates of PV-related patents and R&D headcount accompany growth and development of the PV market and are an excellent indicator to analyze R&D investments. With this approach, current annual corporate PV R&D investments are found to be about 6000. m€ and show a rapidly increasing trend on a global scale. © 2013 Elsevier Ltd.

Abstract:

The performance of organic solar cells strongly depends on the nanoscale structure of the used mixed absorber layer. Utilizing photoconductive and conductive atomic force microscopy (pcAFM and cAFM), as well as transmission X-ray microscopy (TXM), we investigate the influence of different substrate temperatures T on the thin-film structure and local photocurrent in bulk-heterojunctions (BHJs) of vacuum deposited zinc phthalocyanine (ZnPc) and Buckminsterfullerene (C) mixed absorber layers. In this paper, we present topography maps, photocurrent maps under short-circuit current conditions, dark-current maps, and TXM images with high lateral resolution down to 25nm. We observe a strong influence of the substrate temperatures during deposition T on the nanoscopical segregation of the two components in the BHJ. This segregation leads to a spatial extension of the dark-current and a reduced short-circuit current at higher substrate deposition temperatures T. © 2013 Elsevier B.V.

Abstract:

In contrast to p-dopants, highly efficient molecular n-dopants are prone to degradation in air due to their low ionization potentials, limiting the processing conditions of doped functional organic devices. In this contribution, we investigate the air-stability of pure films of the n-dopant tetrakis(1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinato)ditungsten(II) (W2(hpp)4) and of C60 layers doped by W2(hpp)4. We find that 1/3 of the initial conductivity of the doped C60 thin films can be restored by thermal annealing in vacuum after a drop by 5 orders of magnitude upon air exposure. Furthermore, we show by ultraviolet photoelectron spectroscopy (UPS) and Seebeck measurements that the Fermi level shift toward the lowest unoccupied molecular orbital (LUMO) of C60 remains after air exposure, clearly indicating a conservation of n-doping. We explain these findings by a down-shift of the W2(hpp)4 energy levels upon charge-transfer to a host material with deeper lying energy-levels, facilitating a protection against oxidation in air. Consequently, the observed recovery of the conductivity can be understood in terms of a self-passivation of the molecular n-doping. Hence, an application of highly efficient n-doped thin films in functional organic devices handled even under ambient conditions during fabrication is feasible. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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.