Degradation of blue-phosphorescent organic light-emitting devices involves exciton-induced generation of polaron pair within emitting layers
Journal
Nature Communications
Date
2018.03.23
Abstract
Despite enormous successes in green and red electroluminescence, applications of cyclometalated Ir(III) complexes into blue-emitting devices have been retarded by short operation lifetimes. Previous studies indicated that intrinsic degradation of emitting layers might be responsible for the instability, but chemical mechanisms underlying the degradation processes have yet to be fully understood to date. We proposed that irreversible chemical degradation would be evoked by generation of radial ion pairs (RIPs) of Ir(III) complex dopants and organic host molecules upon device operation. To examine this hypothesis, we employed a new series of triscyclometalated Ir(III) complexes having N-heterocyclic carbene (NHC) ligands based on benzoimidazole and imidazo[4,5-b]pyrazine scaffolds. Ground-state and excited-state electrochemical potentials of the complexes were determined by steady-state photoluminescence spectroscopy and cyclic and differential pulse voltammetry. Comparison of the values with the potentials of 3,3？-biscarbazolyl-5-cyanobiphenyl, a host molecule, revealed a positive driving force for generation of RIP through electron transfer from Ir(III) complexes to the excited-state host. We directly monitored the formation and decay of RIP, employing laser flash photolysis. Spectral assignments of the radical species were supported by spectroelectrochemical measurements and quantum chemical calculations based on time-dependent density functional theory. In particular, rate constants for the electron-transfer processes were determined and analyzed with corresponding driving forces based on the classical Marcus theory of electron transfer. Furthermore, blue electrophosphorescence devices comprised on the Ir(III) complexes were fabricated, and their operation lifetimes were correlated with the electro-transfer behaviors. This analysis revealed importance of suppression of RIP and acceleration of radical recombination. We believe our research will enable novel strategies for creating blue-phosphorescent electroluminescence devices with long lifetimes and high efficiencies.