Magnetic resonance studies of tris-(8-hydroxyquinoline) aluminum-based organic light-emitting devices

The electroluminescence (EL)-, electrical current density (J)-, and photoluminescence (PL)- detected magnetic resonance (ELDMR, EDMR, and PLDMR, respectively) of tris-(8-hydroxyquinoline) aluminum (Alq₃)-based organic light-emitting devices (OLEDs) and Alq₃ films is described. At low temperatures, a positive spin-1/2 resonance is observed, i.e., the changes in $J,$ the EL intensity IEL, and the PL intensity IPL are positive (ΔJ/J, ΔIEL/IEL, and ΔIPL/IPL>0). ΔJ/J and ΔIEL/IEL are insensitive to the nature of the Alq₃/cathode interface. They weaken with increasing T and become unobservable above 60 K. ΔIPL/IPL also decreases with $T,$ but is still observable at 250 K. Since the resonances all have the same g value, similar linewidths, and a similar dependence on T and the excitation level $(J$ or the laser power), they are all attributed to the same mechanism. That mechanism is either the reduction of singlet exciton (SE) quenching by a reduced population of polarons in the bulk of the Alq₃ layer (``the quenching mechanism''), or the enhanced formation of SEs from singlet polaron pairs at the expense of triplet excitons (TEs) (``the delayed PL mechanism''). However, the latter mechanism implies that the yield of SEs in Alq₃-based OLEDs is greater than 25%. Due to evidence to the contrary, and other evidence which is inconsistent with the delayed PL mechanism, we conclude that the positive spin-1/2 resonance is due to the quenching mechanism. At T≈60K, another spin-1/2 resonance, which reduces both J and IEL (but is unobservable in the PL), emerges and grows with increasing $T.$ This negative EDMR and ELDMR is sensitive to the buffer layer between Alq₃ and the cathode, and is attributed to the magnetic resonance enhancement of the spin-dependent formation of negative spinless bipolarons from spin-1/2 negative polarons at the organic/cathode interface. The increased trapping of injected electrons at the interface reduces J and consequently IEL. However, at 295 K, the ratio |ΔIEL/IEL| in Alq₃/AlOₓ/Al devices to that in Alq₃/CsF/Al devices is significantly lower than the ratio of |ΔJ/J| in these devices. Hence we suspect that other mechanisms, unidentified at this point, are also contributing to the negative ELDMR.