Optically detected magnetic resonance studies of luminescence-quenching processes in π-conjugated materials and organic light-emitting devices

It is widely recognized that nonradiative quenching of excitons by other excitons and polarons become the dominant decay mechanism of these excitons at high excitation densities. These quenching processes cause the roll-off in the efficiency of organic light-emitting devices (OLEDs) and prevent lasing at high injection current densities. This review presents the optically-detected magnetic resonance (ODMR) evidence for these photoluminescence- and electroluminescence-quenching processes. And while it provides such evidence for quenching of singlet excitons by polarons and triplet excitons, it reveals the central role of the strongly spin-dependent annihilation of triplet excitons by polarons, since under normal excitation conditions the steady-state polaron and triplet exciton populations are 100104 times the singlet exciton population. In addition, it also suggests that quenching of singlet excitons by bipolarons, likely stabilized by a counterpolaron or countercharge at specific sites, may also be a significant quenching mechanism that also affects the charge transport properties.