Influence of doping on spin-dependent exciton formation in Alq3 based OLEDs

We have used electrically detected magnetic resonance (EDMR) to investigate the effect of doping and trapped charges on spin-dependent exciton formation in aluminum(III) 8-hydroxyquinoline (Alq3) based OLEDs. As dopant we have used 4-(dicyanomethylene)-2-methyl-6-{2-[(4-diphenylamino)-phenyl]ethyl}-4H-pyran (DCM-TPA). We observed significant differences (in g-factor, ΔHpp and amplitude) between EDMR signals from DCM-TPA doped and undoped devices at room temperature. The signal from the DCM-TPA doped OLED was found to be strongly dependent on temperature, with signal intensity increasing by two orders of magnitude below 200 K. It was attributed to spin-dependent exciton formation, which our results indicate that takes place between a trapped electron in the dopant and a hole from Alq3, in the current/bias range investigated. The observed differences between doped and undoped devices are discussed in terms of a change in exciton precursor reaction rates and/or decrease in spin coherence time. Preliminary results from rubrene doped OLEDs indicate a similar process.