OLED ageing signature characterization under combined thermal and electrical stresses

OLED are appearing at the industrial scale for purposes such as decorative lighting as well as LCD screen back-lighting. One of their main advantages compared to LED is their reduced light intensity: the luminous flux is produced by a much greater surface. Nevertheless there are a number of strong constraints on their allowable operating conditions, which may otherwise dramatically reduce their lifetime. This aspect of OLEDs is up to now not well understood and generally requires the knowledge of the layers and the chemical compounds used to build the device. In this study we submitted commercial OLEDs to limit conditions with respect to their maximum current and temperature. Complete electrical, optical, photometric and thermic characterization was performed on new devices, and repeated at regular intervals during the ageing process. Electrical parameters were measured with a Solartron impedance analyzer and a Keithley Source meter applying current and voltage steps. By the analysis of the indicial response elements of the equivalent linear circuit were extracted. We will illustrate which parameters change the most with the ageing process. The purpose is to predict and quantify the useful lifetime of OLED under electrical and/or thermal stress. Another outcome of importance is related to OLED driver design, namely specifying the end-of-life electrical characteristics. The operating point's significantly drifted from their original values leading sometimes to a doubling of the operating voltage at the same current level.