Study on hybrid blue organic light emitting diodes with step controlled doping profiles in phosphorescent emitting layer

Abstract

We have designed blue organic light emitting diodes (OLEDs) consisting of fluorescent and phosphorescent emitters whose phosphorescent emitting layers have three step-controlled doping profiles of 0, 4 and 8%. The different doping profiles determined the characteristics of electroluminescent and fluorescent intensity variation under various voltages. Also, it was found that emission channels separated by the non-doped region enhanced fluorescent emission. The phosphorescent doping concentration adjacent to the fluorescent emitting layer is strongly related to device efficiency and the location of the non-doped region, particularly, affected the electrical and optical characteristics of the blue OLEDs due to the hole transport type of the host material. Placing the region on the anode side had the effect of widening the hole transport region, leading to higher current density. Based on this effect, the device with the regularly stepwise doping concentration (0-4-8%) showed high current density and luminance but the phosphorescent emission was dominant. The lowest efficiency roll-off was observed from the device containing the separated emission channels for fluorescence and phosphorescence by the non-doped region, which gave rise to relatively stronger fluorescent emission.