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End date: 2019 × DDC: 540 × Publisher: Weinheim : Wiley-VCH Verl. × WGL Institute: IPHT ×

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    Beer's Law-Why Integrated Absorbance Depends Linearly on Concentration
    (Weinheim : Wiley-VCH Verl., 2019) Mayerhöfer, Thomas G.; Pipa, Andrei V.; Popp, Jürgen
    As derived by Max Planck in 1903 from dispersion theory, Beer's law has a fundamental limitation. The concentration dependence of absorbance can deviate from linearity, even in the absence of any interactions or instrumental nonlinearities. Integrated absorbance, not peak absorbance, depends linearly on concentration. The numerical integration of the absorbance leads to maximum deviations from linearity of less than 0.1 %. This deviation is a consequence of a sum rule that was derived from the Kramers-Kronig relations at a time when the fundamental limitation of Beer's law was no longer mentioned in the literature. This sum rule also links concentration to (classical) oscillator strengths and thereby enables the use of dispersion analysis to determine the concentration directly from transmittance and reflectance measurements. Thus, concentration analysis of complex samples, such as layered and/or anisotropic materials, in which Beer's law cannot be applied, can be achieved using dispersion analysis. ©2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
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    Self-Assembled Graphene/MWCNT Bilayers as Platinum- Free Counter Electrode in Dye-Sensitized Solar Cells
    (Weinheim : Wiley-VCH Verl., 2019) Wahyuono, Ruri Agung; Jia, Guobin; Plentz, Jonathan; Dellith, Andrea; Dellith, Jan; Herrmann-Westendorf, Felix; Seyring, Martin; Presselt, Martin; Andrä, Gudrun; Rettenmayr, Markus; Dietzek, Benjamin
    We describe the preparation and properties of bilayers of graphene- and multi-walled carbon nanotubes (MWCNTs) as an alternative to conventionally used platinum-based counter electrode for dye-sensitized solar cells (DSSC). The counter electrodes were prepared by a simple and easy-to-implement double self-assembly process. The preparation allows for controlling the surface roughness of electrode in a layer-by-layer deposition. Annealing under N2 atmosphere improves the electrode's conductivity and the catalytic activity of graphene and MWCNTs to reduce the I3 − species within the electrolyte of the DSSC. The performance of different counter-electrodes is compared for ZnO photoanode-based DSSCs. Bilayer electrodes show higher power conversion efficiencies than monolayer graphene electrodes or monolayer MWCNTs electrodes. The bilayer graphene (bottom)/MWCNTs (top) counter electrode-based DSSC exhibits a maximum power conversion efficiency of 4.1 % exceeding the efficiency of a reference DSSC with a thin film platinum counter electrode (efficiency of 3.4 %). In addition, the double self-assembled counter electrodes are mechanically stable, which enables their recycling for DSSCs fabrication without significant loss of the solar cell performance. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.