Analysis of self-heating and trapping in organic semiconductor devices

Abstract

So far self-heating has only been of concern in large-area devices where the resistive transparent anode leads to a potential drop over the device resulting in inhomogeneous current, brightness and temperature distributions. In this work, we show that even small lab devices suffer from self-heating effects originating from the organic semiconductor layer. In admittance spectroscopy of organic semiconductor devices, negative capacitance values often arise at low frequency and high voltages. In this study we demonstrate the influence of self-heating on organic semiconductor devices with the aid of a numerical 1D drift-diffusion model that is extended by Joule heating and heat conduction. Furthermore the impact of trap states on the capacitance in combination with self-heating is demonstrated. The typical signature of self-heating might be overshadowed depending on the trapping dynamics. In a next step, we compare the negative capacitance vs. frequency for uni-and bipolar devices to quantify the different processes. We emphasize the impact of self-heating and trapping on OLEDs and organic solar cells. To ease the interpretation of the results we investigate simulations in the time domain as well as in the frequency domain. We have provided clear evidence of self-heating of organic semiconductor devices and conclude that a comprehensive model requires the inclusion of heat conduction and heat generation in the drift-diffusion model.