Femtosecond fluorescence upconversion spectroscopy of vapor-deposited tris(8-hydroxyquinoline) aluminum films.

Abstract
Vapor-deposited Alq3 is used as the green emitting layer in a class of organic light-emitting diodes. In this paper, the time dependence of the fluorescence from thin Alq3 films has been studied by means of the femtosecond fluorescence upconversion technique. From the temporally resolved emission spectra, obtained after spectral reconstruction, the existence of different emissive sites in the Alq3 film is concluded. They are identified as shallow and deep trapping sites. The average shallow trap excitation energy is located about 160 cm−1 below the (quasi-) exciton band, while the deep trap excitation energy is lower by approximately 2000 cm−1. Above 75 K, the shallow trap excitation is thermally depleted, via the (quasi-) exciton band, to fast-trapping non-emissive sites. At liquid helium temperatures, the depletion of the highest-lying shallow traps is frozen out. At room temperature, the lifetime of the shallow traps varies from about 1 to 20 ps; the lifetime of the deep traps is about 10 ns.