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.
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Source: Phys.org