Low Frequency Noise Correlation between Electrical and Optical Signals for Predicting Degradation in Organic Light Emitting Diodes

Abstract

The Low Frequency Noise (LFN) characteristics of Organic Light Emitting Diodes (OLEDs) were investigated in relation to device degradation. The standard layer structure of indium tin oxide (anode), hole transport layer, Alq3, Lithium Fluoride and Aluminium (cathode) was used. With duration of operation, the device degradation is characterized by an eventual drop in the luminance level and the electroluminescence efficiency. Additionally, the driving voltage at a fixed current and the LFN increase gradually during device degradation, accompanied with the formation of non-emissive areas (dark spots). It is found that the coefficient of correlation between voltage fluctuations across the device and low frequency fluctuations of the optical signals remains constant for an initial period and then decreases exponentially with duration of operation and is a sensitive parameter to predict OLED device lifetime. For a number of OLEDs driven at constant current, the device with higher initial correlation coefficient possesses a longer lifetime. The direct relation between LFN correlation and device lifetime can be explained by carrier recombination mechanisms at the microscopic level. An increase in trap density can reduce the internal radiative recombination rate which at the macroscopic level is reflected by a decrease in the correlation coefficient.