Low Frequency Noise Correlation between Electrical and Optical signals for predicting Degradation in Organic Light Emitt
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1195-B13-01
Low Frequency Noise Correlation between Electrical and Optical signals for predicting Degradation in Organic Light Emitting Diodes SJ Chua1,2, Huang Sha1 and KeLin2 1
Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602 Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576
2
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.
I. INTRODUCTION Organic Light Emitting Diodes (OLEDs) have many advantages over liquid crystal displays (LCDs) and are increasingly making inroads into display applications. Although the lifetime and reliability of OLEDs have improved greatly over the years, the pre-identification of device degradation mechanisms in any operating device remains difficult. At the present, a reduced luminance level and the formation of non-emissive areas (dark spots) are generally accepted as two observations for OLED degradation [1]. Overtime, as the defects in the device increase, the interaction between charge carriers and defects causes fluctuations in the device current and voltage contributing to extrinsic noise. Among the different types of extrinsic noise, 1/f noise, in particular, is closely related to the defects and irregularities at a microscopic level. These defects, acting as capture centers (traps), influence the physical parameters (the number of free carriers, the mobility, the minority carrier lifetime) and result in an overall electrical fluctuation at the macroscopic level. In these cases, 1/f noise measurement is a sensitive tool to examine the internal mechanism of device degradation [2]. Earlier studies suggest that low frequency noise is related to device lifetime and efficienc
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