Electron spin resonance of thin films of organic light-emitting material tris(8-hydroxyquinoline) aluminum doped by magnesium

We have successfully observed electron spin resonance (ESR) signals of radical anions in thin films of tris(8-hydroxyquinoline) aluminum (Alq₃), a compound widely used as electron transporting and luminescent layers in organic light-emitting diodes. To obtain definitely defined radical-anion states in Alq₃, we doped Alq₃ with Mg by co-evaporating these materials. The obtained g value and peak-to-peak ESR linewidth ΔH_pp of Alq₃ radical anions are 2.0030 and 2.19 mT, respectively. Theoretical g value and hyperfine interactions were calculated by density functional theory method, which are in good agreement with the experimental results. A quantitative evaluation of doping concentration was performed. We confirmed that doped charges are localized at deep trapping sites by the lineshape analysis and temperature dependence of the ESR signals. Morphological investigation using transmission electron microscopy clarified that the co-evaporated Mg atoms form clusters.