Femtosecond ̄uorescence upconversion spectroscopy of vapor-deposited tris(8-hydroxyquinoline) aluminum ®lms

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 ̄uorescence from thin Alq3 ®lms has been studied by means of the femtosecond ̄uorescence upconversion technique. From the temporally resolved emission spectra, obtained after spectral reconstruction, the existence of di€erent emissive sites in the Alq3 ®lm is concluded. They are identi®ed as shallow and deep trapping sites. The average shallow trap excitation energy is located about 160 cmÿ1 below the (quasi-) exciton band, while the deep trap excitation energy is lower by approximately 2000 cmÿ1. 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. Ó 2000 Elsevier Science B.V. All rights reserved.