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Author: Leo, Karl × Publisher: Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik ×

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    Feel 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, Reinhard
    For 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.
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    Self-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, Annegret
    We 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 ...