Understanding Device Performance of Blue Organic Light-Emitting Diodes with Fluorescent and Phosphorescent Emitters

Abstract

This thesis proposes blue organic light-emitting diodes (OLEDs) composed of fluorescent and phosphorescent emitting layers (EMLs) and the study was accomplished by engineering device architecture and dopant distribution in the EMLs. We investigated the electrical and optical properties when both types of emitters are applied as EMLs into a single device and determined the emission ratio and contribution between fluorescent and phosphorescent emitters by deconvoluting electroluminescent spectra with Gaussian fitting. The fluorescent EML is composed of 4,4 -bis(2,2 -diphenylvinyl)-1,1 –diphenyl (DPVBi) doped with 4,4'-bis(9-ethyl-3-carbazovinylene)-1,1'-biphenyl (BCzVBi) while Bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) is doped into 1,3-Bis(N-carbazolyl)benzene (mCP) for the phosphorescent EML. Firstly, dual-EML blue OLEDs are introduced and differentiated by doping styles, doping concentration and device structure to study the operating principles and changes in the efficiency and color characteristic in blue OLEDs. When both types of EMLs are used, the predominant emission is phosphorescence and the relative intensity of fluorescent and phosphorescent emission is controlled by the fluorescent doping concentration. With the incorporation of a step-graded doping profile in the EMLs, the trend of device performance is indicative of the effect of the location of the non-doped area. Blue OLEDs with the step-controlled doping concentration showed higher device efficiency and strengthened emission from fluorescence compared to the uniformly doped devices. Secondly, triple-EML blue OLEDs with the various combinations of EML host configurations were studied. The electroluminescent spectra of the devices were deconvoluted by Gaussian peak fitting to understand the origin of blue emission. The device performance is strongly influenced by the location of the n-type host material. Furthermore, effects of interlayer and EML thickness variation in devices were investigated to optimize device efficiency and color emission.