Lifetime and Efficiency of Blue Phosphorescent Organic-light Emitting Diodes
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Organic light emitting diodes (OLEDs) are poised to realize innovative display and high-performance lighting applications in the future. However, the development of suitable blue OLEDs remains a challenge which has impeded the progress of large-scale OLED commercialization for more than a decade. Blue devices are critical components for red-green-blue displays and white lighting but, to date, suffer from short operational lifetimes as well as a lack of efficient deep blue emitting materials. This presentation aims at understanding the physical background of these issues and providing potential solutions.
In the first part of this presentation, we investigate the nonradiative loss mechanism dominant in deep blue emitting phosphorescent materials. We identify that thermal population to metal-centered ligand-field states (3MC states) is a major source of efficiency loss. Thus, we develop tris-cyclometalated Iridium (III) complexes using N-heterocyclic carbene (NHC) ligands that render the energy of 3MC states inaccessibly high and achieve a high quantum efficiency in deep blue. In phosphorescent OLEDs (PHOLEDs), NHC-Ir complexes are used as emitters, as well as electron and exciton blocking components. This multiple use enables deep blue PHOLEDs with very high efficiency, potentially suited to demanding display applications.
In the second part of this presentation, we focus on understanding the short lifetime of blue PHOLEDs. We identify the cause of device degradation as the annihilation between excited states in the emission layer (EML), producing energetically "hot" excited states. If such a hot excited state dissipates its energy on the EML molecule, the bond dissociation ensues and resulting fragmented products permanently deteriorate the device performance. We propose two solutions to this problem: (i) reducing the probability of bimolecular annihilation via distributing the excited state density and (ii) thermalizing hot excited states on the ancillary, protective dopant in the PHOLED EML. The stability of the blue PHOLED employing both strategies is cumulatively improved and a theory is proposed to explain such lifetime enhancement.