Electroluminescence-detected magnetic resonance studies of Pt octaethyl porphyrin-based phosphorescent organic light-emitting devices

Abstract

The electroluminescence (EL)-detected magnetic resonance (ELDMR) of 0, 1, 2.5, 6, and $20\mathrm{wt.}\%$ Pt octaethyl porphyrin (PtOEP)-doped tris(8-hydroxyquinolinate) Al $\left(\mathrm{Al}{\mathrm{q}}_3\right)$-based phosphorescent multilayer organic light-emitting devices (OLEDs) is described. In $1\mathrm{wt.}\%$-doped devices, the ELDMR from the PtOEP and $\mathrm{Al}{\mathrm{q}}_3$ emission are both very similar to that of undoped devices. They exhibit a positive (EL-enhancing) spin-$\frac{1}{2}$ polaron resonance at $10⩽T⩽50\mathrm{K}$, whose magnitude $\Delta I_{\mathrm{EL}}∕I_{\mathrm{EL}}$ increases with current and weakens with increasing $T$, and a negative (EL-quenching) resonance at $50\mathrm{K}⩽T$, which grows with $T$. At $295\mathrm{K}$, $\mid \Delta I_{\mathrm{EL}}∕I_{\mathrm{EL}}\mid$ decreases with current. The enhancing resonance is attributed to the magnetic-resonance reduction of singlet exciton (SE) quenching by a reduced population of polarons and host triplet excitons (TEs). The reduction in the TE and polaron populations is, in turn, due to the spin-dependent annihilation of host TEs by polarons, which is enhanced under magnetic resonance conditions. Since the polaron and host TE populations are much greater than the SE population, the polaron-host TE interaction is identified as one of the major interactions which govern the dynamics of the excited states in OLEDs. The quenching resonance is attributed to magnetic resonance enhancement of formation of dianions at the organic/cathode interface, which increases the charge density at that interface, and consequently the rate of field-induced host SE dissociation. Both the enhancing and quenching resonances weaken as the PtOEP concentration increases; at $6\mathrm{wt.}\%$, the enhancing resonance is undetectable and the quenching resonance is very weak $(\mid \Delta I_{\mathrm{EL}}∕I_{\mathrm{EL}}\mid \sim 2\times {10}^{-5})$. The results can be explained by assuming that the ELDMR of the guest emission is due to the effect of magnetic resonance conditions on the host SEs. A rate equation model is established to explain the evolution of the ELDMR with dye concentration. Since the foregoing quenching mechanisms are believed to be responsible for the drop in the efficiency ${\eta }_{\mathrm{ext}}$ of fluorescent OLEDs at high current, the present results indicate that they are also responsible for the drop in ${\eta }_{\mathrm{ext}}$ of phosphorescent OLEDs at high current. In the $20\mathrm{wt.}\%$-doped devices, the spin-$\frac{1}{2}$ polaron resonance is negative at all $T$, and $\mid \Delta I_{\mathrm{EL}}∕I_{\mathrm{EL}}\mid$ and the resonance linewidth decrease with increasing $T$; $\mid \Delta I_{\mathrm{EL}}∕I_{\mathrm{EL}}\mid$ is weakly current dependent at both $20\mathrm{K}$ and $295\mathrm{K}$. This behavior is consistent with the dianion model, if the dianion density decreases with increasing $T$. This is probably due to a low barrier for thermal dissociation of the dianions.