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

Vapor-deposited Alq(3) is used as the green emitting layer in a class of or ganic light-emitting diodes. In this paper, the time dependence of the fluo rescence 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 Alq(3) film is concluded. They are identified as sha llow and deep trapping sites. The average shallow trap excitation energy is located about 160 cm(-1) below the (quasi-) exciton band, while the deep t rap excitation energy is lower by approximately 2000 cm(-1). Above 75 K, th e shallow trap excitation is thermally depleted, via the (quasi-) exciton b and, to fast-trapping non-emissive sites. At liquid helium temperatures, th e depletion of the highest-lying shallow traps is frozen out. At room tempe rature, the lifetime of the shallow traps varies from about 1 to 20 ps; the lifetime of the deep traps is about 10 ns. (C) 2000 Elsevier Science B.V. All rights reserved.