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It is well-known that LEDs and transistors should not be connected in parallel as slight differences in resistance can lead to imbalanced current flow. This effect gets even stronger if the devices heat up as their resistance changes with temperature. For organic LEDs (OLEDs), this is a big issue: Every large-area OLED lighting panel can be understood as a parallel connection of numerous individual tiny OLEDs. As a consequence, these devices show inhomogeneous light emission if they heat up. A phenomenon that has been observed by researchers as well as industrial companies in the last couple of years, is a saturation of brightness that occurs even though the total applied current is continuously increased.
Now a team of researchers from TU Dresden and the Weierstrass Institute Berlin experimentally prove that OLEDs do not only saturate, they even show regions that are switched-back in brightness: Suddenly the OLED gets darker in a certain area although the total applied current is increased—clearly a counter-intuitive result. This so-called "switched-back" effect is directly related to the presence of a strong nonlinear electrothermal feedback in OLEDs that takes place upon heating up and which in turn induces negative differential resistance that makes the device prone to unstable operation.
Results - Impact - Stability - Applications - Brightness
The results have a strong impact on understanding long-term stability in applications with high brightness e.g. as they are found in the automotive sector. Here, OLEDs are now under consideration to replace LED technology for tail lights, signal lights, and brake lights due to their new design possibilities. One problem OLEDs are still facing is sudden-death phenomena. They are rarely described in literature due to their unpredictable and seemingly random occurrence. However, it is likely that the now proven switched-back regions are strongly related to such sudden-death phenomena. A better understanding of the OLED as a complex electrothermal system...
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