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- ItemFeel the heat: Nonlinear electrothermal feedback in organic LEDs(Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2013) Fischer, Axel; Koprucki, Thomas; Gärtner, Klaus; Tietze, Max L.; Brückner, Jacqueline; Lüssem, Björn; Leo, Karl; Glitzky, Annegret; Scholz, ReinhardFor lighting applications, Organic light-emitting diodes (OLED) need much higher brightness than for displays, leading to self-heating. Due to the temperature-activated transport in organic semiconductors, this can result in brightness inhomogeneities and catastrophic failure. Here, we show that due to the strong electrothermal feedback of OLEDs, the common spatial current and voltage distribution is completely changed, requiring advanced device modeling and operation concepts. Our study clearly demonstrates the effect of negative differential resistance (NDR) in OLEDs induced by self-heating. As a consequence, for increasing voltage, regions with declining voltages are propagating through the device, and even more interestingly, a part of these regions show even decreasing currents, leading to strong local variation in luminance. The expected breakthrough of OLED lighting technology will require an improved price performance ratio, and the realization of modules with very high brightness but untainted appearance is considered to be an essential step into this direction. Thus, a deeper understanding of the control of electrothermal feedback will help to make OLEDs in lighting more competitive.
- Item3D electrothermal simulations of organic LEDs showing negative differential resistance(Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2017) Liero, Matthias; Fuhrmann, Jürgen; Glitzky, Annegret; Koprucki, Thomas; Fischer, Axel; Reineke, SebastianOrganic semiconductor devices show a pronounced interplay between temperature-activated conductivity and self-heating which in particular causes inhomogeneities in the brightness of large-area OLEDs at high power. We consider a 3D thermistor model based on partial differential equations for the electrothermal behavior of organic devices and introduce an extension to multiple layers with nonlinear conductivity laws, which also take the diode-like behavior in recombination zones into account. We present a numerical simulation study for a red OLED using a finite-volume approximation of this model. The appearance of S-shaped current-voltage characteristics with regions of negative differential resistance in a measured device can be quantitatively reproduced. Furthermore, this simulation study reveals a propagation of spatial zones of negative differential resistance in the electron and hole transport layers toward the contact.
- Itemp-Laplace thermistor modeling of electrothermal feedback in organic semiconductors(Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2015) Liero, Matthias; Koprucki, Thomas; Fischer, Axel; Scholz, Reinhard; Glitzky, AnnegretIn large-area Organic Light-Emitting Diodes (OLEDs) spatially inhomogeneous luminance at high power due to inhomogeneous current flow and electrothermal feedback can be observed. To describe these self-heating effects in organic semiconductors we present a stationary thermistor model based on the heat equation for the temperature coupled to a p-Laplace-type equation for the electrostatic potential with mixed boundary conditions. The p-Laplacian describes the non-Ohmic electrical behavior of the organic material. Moreover, an Arrhenius-like temperature dependency of the electrical conductivity is considered. We introduce a finite-volume scheme for the system and discuss its relation to recent network models for OLEDs. In two spatial dimensions we derive a priori estimates for the temperature and the electrostatic potential and prove the existence of a weak solution by Schauder's fixed point theorem.
- ItemSelf-heating, bistability, and thermal switching in organic semiconductors(Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2012) Fischer, Axel; Pahner, Paul; Lüssem, Björn; Scholz, Reinhard; Koprucki, Thomas; Gärtner, Klaus; Glitzky, AnnegretWe demonstrate electric bistability induced by the positive feedback of self-heating onto the thermally activated conductivity in a two-terminal device based on the organic semiconductor C60. The central undoped layer with a thickness of 200 nm is embedded between thinner n-doped layers adjacent to the contacts minimizing injection barriers. The observed current-voltage characteristics follow the general theory for thermistors described by an Arrhenius-like conductivity law. Our findings including hysteresis phenomena are of general relevance for the entire material class since most organic semiconductors can be described by a thermally activated conductivity.
- ItemSelf-heating effects in organic semiconductor devices enhanced by positive temperature feedback(Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2012) Fischer, Axel; Pahner, Paul; Lüssem, Björn; Leo, Karl; Scholz, Reinhard; Koprucki, Thomas; Fuhrmann, Jürgen; Gärtner, Klaus; Glitzky, AnnegretWe studied the influence of heating effects in an organic device containing a layer sequence of n-doped / intrinsic / n-doped C60 between crossbar metal electrodes. A strong positive feedback between current and temperature occurs at high current densities beyond 100 A/cm2, as predicted by the extended Gaussian disorder model (EGDM) applicable to organic semiconductors. These devices give a perfect setting for studying the heat transport at high power densities because C60 can withstand temperatures above 200ʿ C. Infrared images of the device and detailed numerical simulations of the heat transport demonstrate that the electrical circuit produces a superposition of a homogeneous power dissipation in the active volume and strong heat sources localized at the contact edges ...