New blue phosphorescent iridium complexes for OLEDs

Abstract

A new class of ligands for complexation with Ir(III) has been developed. Tris-homoleptic complexes derived from these ligands have been found to exhibit highly efficient blue phosphorescence with photoluminescent quantum yields in solution at room temperature of >0.9. These complexes have been applied as the emissive materials in OLEDs to give devices with efficiencies of up to 26 cd/A and an E.Q.E. of 17.4% at 1 mA/cm 2 . IntroductionThe development of organic light-emitting diodes (OLEDs) has revolutionized display and lighting technologies over the past 20 years. 1 OLED displays exhibit desirable properties including thinner structures, excellent contrast ratios, fast response times, and much wider viewing angles. 2 OLED lights have the potential for large-area energy-efficient illumination sources. 3 An essential feature for these applications is low power consumption and high efficiency. Fluorescent materials were initially investigated as emitting species for OLED applications. However, as fluorescence occurs from a singlet excited state and the statistical probability of a singlet exciton being formed is 25%, there is a significant waste of energy. In the late 1990s, Thompson and Forrest introduced heavy-metal complexes as triplet harvesters which was arguably the most significant breakthrough for emissive materials. 4 By exploiting the spin orbit coupling of a phosphorescent heavy-metal complex, it is possible to harvest the triplet excitons previously inaccessible to fluorescent emitters. Ir(III) complexes are considered to be the most promising triplet harvester. 5 They exhibit good photo and thermal stabilities, high phosphorescence quantum efficiencies, short phosphorescent lifetimes, and facile color tuning through ligand structure control. While red and green Ir(III) phosphors have seen success, the development of blue Ir(III) phosphors remains a significant challenge.

Blue–Green Electroluminescent Carbon Dots Derived from Fenugreek Seeds for Display and Lighting Applications

“…The threshold voltage ( V th ) for the current density was approximately 2.5 V, and the difference between V on and V th was relatively large (3.3 V) compared with that of high-performance blue OLEDs . We believe that holes begin to be injected into the CD emission layer at 2.5 V ( V th ), and electrons are injected into the emission layer at 5.8 V ( V on ) through carrier recombination and emission.…”