Abstract
Degradation of organic materials is responsible for the short operation lifetimes of organic light-emitting devices, but the mechanism by which such degradation is initiated has yet to be fully established. Here we report a new mechanism for degradation of emitting layers in blue-phosphorescent devices. We investigate binary mixtures of a wide bandgap host and a series of novel Ir(III) complex dopants having N-heterocyclocarbenic ligands. Our mechanistic study reveals the charge-neutral generation of polaron pairs (radical ion pairs) by electron transfer from the dopant to host excitons. Annihilation of the radical ion pair occurs by charge recombination, with such annihilation competing with bond scission. Device lifetime correlates linearly with the rate constant for the annihilation of the radical ion pair. Our findings demonstrate the importance of controlling exciton-induced electron transfer, and provide novel strategies to design materials for long-lifetime blue electrophosphorescence devices.
Highlights
Degradation of organic materials is responsible for the short operation lifetimes of organic light-emitting devices, but the mechanism by which such degradation is initiated has yet to be fully established
Previous studies established excitonlocalized homolysis and imbalance in charge carrier concentrations to be responsible for the generation of the radical species[16, 18, 22,23,24,25]
An optical bandgap energy (ΔEg) as large as 3.10 eV was determined, which enabled calculation of the excited-state oxidation (E*ox) and reduction (E*red) potentials of H according to the relationships E*ox = Eox − ΔEg and E*red = Ered + ΔEg
Summary
Degradation of organic materials is responsible for the short operation lifetimes of organic light-emitting devices, but the mechanism by which such degradation is initiated has yet to be fully established. It is possible that the exciton is reductively quenched by electron transfer from a nearby molecule with a shallow oxidation potential to form a radical ion pair (i.e., a polaron pair), due to the positive driving force of the electron transfer (Fig. 1) Such radical ion pair would rapidly undergo charge recombination to restore the original neutral states, but its labile nature can facilitate degradation of both the host and dopant materials. This mechanism may explain the greater instability of devices that emit blue light than those that emit green and red light, because it predicts faster formation and slower annihilation of the radical ion pair in blue emission layers (vide infra). The study reveals the importance of controlling the electrochemical potentials of a host exciton and its dopant for achieving long device lifetimes
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