The organic light-emitting diode (OLED) technology has become the state-of-the-art for mobile displays, is penetrating the TV display sector, and is a light-source with further application potential in the area of automotive lighting, indoor lighting, and wearable and health-monitoring electronics. Recently, J. Rafols-Ribe et al. showed that the efficiency and lifetime of OLEDs can be increased by the layer-by-layer optimization of its fabrication [J. Rafols-Ribe et al., Sci. Adv., 4 (2018)]. This was possible by heating the substrate during deposition. In this work, we investigate how each heated OLED-layer influences the overall device-performance change and compare it to post-annealing treatment by studying the optoelectrical and photoluminescent characteristics of the OLEDs. Additionally, we employ impedance spectroscopy to calculate the giant surface potential of the thin films in an attempt to learn more about the nanoscopic changes of the layer morphologies. Here, different emitter systems, i.e., phosphorescence- or thermally activated delayed fluorescence-based ones, are tested. We find a clear correlation between the surface potential of a transport layer and efficiency of the OLED. Furthermore, we discuss that the efficiency is improved because of a reduced non-radiative decay rate. We show that the in-situ heating increases the efficiency only for OLEDs containing polar layers. These findings contribute to the understanding of the in-situ heating of OLED layers and provide further levers for OLED improvement.
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