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- W2022650549 abstract "The electroluminescence (EL)-, electrical current density $(J)ensuremath{-},$ and photoluminescence (PL)- detected magnetic resonance (ELDMR, EDMR, and PLDMR, respectively) of tris-(8-hydroxyquinoline) aluminum $({mathrm{Alq}}_{3})$-based organic light-emitting devices (OLEDs) and ${mathrm{Alq}}_{3}$ films is described. At low temperatures, a positive spin-1/2 resonance is observed, i.e., the changes in $J,$ the EL intensity ${I}_{mathrm{EL}},$ and the PL intensity ${I}_{mathrm{PL}}$ are positive $(ensuremath{Delta}J/J,$ $ensuremath{Delta}{I}_{mathrm{EL}}{/I}_{mathrm{EL}},$ and $ensuremath{Delta}{I}_{mathrm{PL}}{/I}_{mathrm{PL}}>0).$ $ensuremath{Delta}J/J$ and $ensuremath{Delta}{I}_{mathrm{EL}}{/I}_{mathrm{EL}}$ are insensitive to the nature of the ${mathrm{Alq}}_{3}/mathrm{cathode}$ interface. They weaken with increasing $T$ and become unobservable above 60 K. $ensuremath{Delta}{I}_{mathrm{PL}}{/I}_{mathrm{PL}}$ 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 ${mathrm{Alq}}_{3}$ 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 ${mathrm{Alq}}_{3}$-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 $Tensuremath{approx}60mathrm{K},$ another spin-1/2 resonance, which reduces both $J$ and ${I}_{mathrm{EL}}$ (but is unobservable in the PL), emerges and grows with increasing $T.$ This negative EDMR and ELDMR is sensitive to the buffer layer between ${mathrm{Alq}}_{3}$ 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 ${I}_{mathrm{EL}}.$ However, at 295 K, the ratio $|ensuremath{Delta}{I}_{mathrm{EL}}{/I}_{mathrm{EL}}|$ in ${mathrm{Alq}}_{3}{/mathrm{A}mathrm{l}mathrm{O}}_{x}/mathrm{Al}$ devices to that in ${mathrm{Alq}}_{3}/mathrm{C}mathrm{s}mathrm{F}/mathrm{A}mathrm{l}$ devices is significantly lower than the ratio of $|ensuremath{Delta}J/J|$ in these devices. Hence we suspect that other mechanisms, unidentified at this point, are also contributing to the negative ELDMR." @default.
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- W2022650549 date "2004-04-15" @default.
- W2022650549 modified "2023-10-10" @default.
- W2022650549 title "Magnetic resonance studies of tris-(8-hydroxyquinoline) aluminum-based organic light-emitting devices" @default.
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- W2022650549 doi "https://doi.org/10.1103/physrevb.69.165311" @default.
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