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Author: Fuhrmann, Jürgen × End date: 2024 × Start date: 2020 × Type: article × WGL Institute: WIAS ×

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Now showing 1 - 5 of 5
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    Experimental proof of Joule heating-induced switched-back regions in OLEDs
    (London : Nature Publishing Group, 2020) Kirch, Anton; Fische, Axel; Liero, Matthias; Fuhrmann, Jürgen; Glitzky, Annegret; Reineke, Sebastian
    Organic light-emitting diodes (OLEDs) have become a major pixel technology in the display sector, with products spanning the entire range of current panel sizes. The ability to freely scale the active area to large and random surfaces paired with flexible substrates provides additional application scenarios for OLEDs in the general lighting, automotive, and signage sectors. These applications require higher brightness and, thus, current density operation compared to the specifications needed for general displays. As extended transparent electrodes pose a significant ohmic resistance, OLEDs suffering from Joule self-heating exhibit spatial inhomogeneities in electrical potential, current density, and hence luminance. In this article, we provide experimental proof of the theoretical prediction that OLEDs will display regions of decreasing luminance with increasing driving current. With a two-dimensional OLED model, we can conclude that these regions are switched back locally in voltage as well as current due to insufficient lateral thermal coupling. Experimentally, we demonstrate this effect in lab-scale devices and derive that it becomes more severe with increasing pixel size, which implies its significance for large-area, high-brightness use cases of OLEDs. Equally, these non-linear switching effects cannot be ignored with respect to the long-term operation and stability of OLEDs; in particular, they might be important for the understanding of sudden-death scenarios. © 2020, The Author(s).
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    Electrothermal Tristability Causes Sudden Burn-In Phenomena in Organic LEDs
    (Weinheim : Wiley-VCH, 2021) Kirch, Anton; Fischer, Axel; Liero, Matthias; Fuhrmann, Jürgen; Glitzky, Annegret; Reineke, Sebastian
    Organic light-emitting diodes (OLEDs) have been established as a mature display pixel technology. While introducing the same technology in a large-area form factor to general lighting and signage applications, some key questions remain unanswered. Under high-brightness conditions, OLED panels were reported to exhibit nonlinear electrothermal behavior causing lateral brightness inhomogeneities and even regions of switched-back luminance. Also, the physical understanding of sudden device failure and burn-ins is still rudimentary. A safe and stable operation of lighting tiles, therefore, requires an in-depth understanding of these physical phenomena. Here, it is shown that the electrothermal treatment of thin-film devices allows grasping the underlying physics. Configurations of OLEDs with different lateral dimensions are studied as a role model and it is reported that devices exceeding a certain panel size develop three stable, self heating-induced operating branches. Switching between them causes the sudden formation of dark spots in devices without any preexisting inhomogeneities. A current-stabilized operation mode is commonly used in the lighting industry, as it ensures degradation-induced voltage adjustments. Here, it is demonstrated that a tristable operation always leads to destructive switching, independent of applying constant currents or voltages. With this new understanding of the effects at high operation brightness, it will be possible to adjust driving schemes accordingly, design more resilient system integrations, and develop additional failure mitigation strategies. © 2021 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH
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    Modeling Polycrystalline Electrode-electrolyte Interfaces: The Differential Capacitance
    (Bristol : IOP Publishing, 2020) Müller, Rüdiger; Fuhrmann, Jürgen; Landstorfer, Manuel
    We present and analyze a model for polycrystalline electrode surfaces based on an improved continuum model that takes finite ion size and solvation into account. The numerical simulation of finite size facet patterns allows to study two limiting cases: While for facet size diameter dfacet →0 we get the typical capacitance of a spatially homogeneous but possible amorphous or liquid surface, in the limit 1[nm] < dfacet, an ensemble of non-interacting single crystal surfaces is approached. Already for moderate size of the facet diameters, the capacitance is remarkably well approximated by the classical approach of adding the single crystal capacities of the contributing facets weighted by their respective surface fraction. As a consequence, the potential of zero charge is not necessarily attained at a local minimum of capacitance, but might be located at a local capacitance maximum instead. Moreover, the results show that surface roughness can be accurately taken into account by multiplication of the ideally flat polycrystalline surface capacitance with a single factor. In particular, we find that the influence of the actual geometry of the facet pattern in negligible and our theory opens the way to a stochastic description of complex real polycrystal surfaces. © 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited.
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    Drift–diffusion simulation of S-shaped current–voltage relations for organic semiconductor devices
    (Dordrecht : Springer Science + Business Media B.V., 2020) Doan, Duy Hai; Fischer, Axel; Fuhrmann, Jürgen; Glitzky, Annegret; Liero, Matthias
    We present an electrothermal drift–diffusion model for organic semiconductor devices with Gauss–Fermi statistics and positive temperature feedback for the charge carrier mobilities. We apply temperature-dependent Ohmic contact boundary conditions for the electrostatic potential and discretize the system by a finite volume based generalized Scharfetter–Gummel scheme. Using path-following techniques, we demonstrate that the model exhibits S-shaped current–voltage curves with regions of negative differential resistance, which were only recently observed experimentally. © 2020, The Author(s).
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    Entropy and convergence analysis for two finite volume schemes for a Nernst–Planck–Poisson system with ion volume constraints
    (Berlin ; Heidelberg : Springer, 2022) Gaudeul, Benoît; Fuhrmann, Jürgen
    In this paper, we consider a drift-diffusion system with cross-coupling through the chemical potentials comprising a model for the motion of finite size ions in liquid electrolytes. The drift term is due to the self-consistent electric field maintained by the ions and described by a Poisson equation. We design two finite volume schemes based on different formulations of the fluxes. We also provide a stability analysis of these schemes and an existence result for the corresponding discrete solutions. A convergence proof is proposed for non-degenerate solutions. Numerical experiments show the behavior of these schemes.