Lifetime and Efficiency of Blue Phosphorescent Organic-Light Emitting Diodes

Abstract

Organic light-emitting diodes (OLEDs) are poised to realize high performance for innovative display and 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 thesis aims at understanding the physical background of these issues and providing potential solutions. OLEDs produce photons via radiative recombination of electron-hole bound pairs, called excitons. Fluorescent OLEDs depend on emission from the singlet excitons achieving an electron-to-light conversion, or internal quantum efficiency (IQE), from 25% up to 62.5%. On the other hand, phosphorescent OLEDs (PHOLEDs) exploit the emission from triplet excitons, attaining nearly 100% IQE. In OLED-based products, red and green PHOLEDs are universally used due to their high efficiency and long operational lifetime, while fluorescent OLEDs are still used for the blue emitting component despite their low performance. Thus, the development of long-lived and high efficiency blue PHOLEDs is a key to the success of the technology. In the first part of this thesis, we investigate the nonradiative loss mechanism dominant in deep blue emitting phosphorescent materials. We identify the metal-centered ligand-field states (3MC states) as a major source of efficiency loss and a probability of thermal population to these states increases with the emission energy of the emitter. Thus, we develop tris-cyclometalated Iridium (III) complexes using N-heterocyclic carbene (NHC) ligands that render the energy of the 3MC states inaccessibly high while keeping a wide energy gap for deep blue emission. The NHC-ligand based Ir(III) complex can thereby minimize the nonradiative loss and achieve high IQE in deep blue. In PHOLEDs, the NHC-Ir(III) complexes are used as the emitters, as well as hole transporting and electron blocking components. This multiple use enables a very high brightness operation of deep blue PHOLEDs, potentially suitable for demanding display applications. In the second part of this thesis, we focus on understanding and solving the short lifetime of blue PHOLEDs. We identify the intrinsic mechanism of the device degradation is the bimolecular annihilation between the excited states in the emission layer (EML) that generates the energetically “hot” excited state. If such a hot excited state dissipates its energy on the EML molecule, the resulting chemical bond dissociation and its products permanently deteriorate device performance. The frequency of this failure process increases with the energy of the excited state, particularly severe in blue PHOLEDs compared to red and green emitting devices. Thus, we propose two solutions to this problem: (i) reducing the probability of the bimolecular annihilation via distributing the excited state density and (ii) bypassing the dissociative reaction via thermalizing the hot excited states on the ancillary 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.