Investigation of the Effect of Various Dyes on OLEDs Electroluminescence

Abstract

In this work, five sets of samples have been prepared and studied. Every sample represents an Organic Light Emitting Diode OLED. We used Poly(9-vinylcarbazole) (PVK) doped with different dyes) R6G, Alq3, carbocyanine, oxadiazole and fluroscein (2% concentration. The measurements include I-V characteristics and variation of relative light intensity with the driving voltage. The variation of relative light intensity with the current and the conduction mechanism have been studied. The I-V characteristics are that of typical diode because the current increase exponentially with applied voltage. I-V characteristics and relative light intensity-voltage curves have approximately the same shape for the same sample. It is observed that, the lowest value of threshold voltage was for (ITO/PVK R6G (blend)/Al) sample and the highest was for ITO/PVK 1,2,4 oxadiazole(blend)/Al) sample (see table(.41)(. In general, the values obtained for are lower than those reported for conventional organic light emitting diode. Electroluminescence-voltage characteristics curve reveals that the sample (ITO/PVK 1,2,4 oxadiazole(blend)/Al)is bestsample exhibiting the maximum brightness which is achieved at an operating voltage of 10 V. By plotting the current-voltage curves in logarithmic scale, we ensured that the mechanism of conduction is ohmic at low voltage for all samples and at high voltage, space charge limiting conduction (SCLC) in most samples. Current-voltage in logarithmic scale curves demonstrate three regions for ITO/PVK R6G (blend)/Al, ITO/PVK carbocyanine(blend)/Al, ITO/PVK 1,2,4 oxadiazole(blend)/Al and four regions for ITO/PVK / Alq3 /Al (double layer), ITO/PVK fluroscein (blend)/Al. The thicknesses of thin films were measured using homemade variable angle spectroscopic ellipsometer (VASE). We also deposited thin film of PVK (48 nm) on Si substrate, and another thin film (18 nm) of Alq3 on Si substrate. We plot the variation of the refractive index of a Alq3 and PVK films with the variation of wavelength (visible spectrum). Finally, the experimental values of three Cauchy parameters (A, B, C) were calculated using a Matlab code.