Optical Absorption in AlQ

In this paper, we report on optical absorption in thin films of tris(8-hydroxyquinoline)aluminium (AlQ) between 0.7 and 5 eV. We report on the first measurement of the sub-bandgap absorption in AlQ using Photothermal Deflection Spectroscopy (PDS) for different deposition conditions. The sub-bandgap absorption is not very sensitive to sample deposition temperature but increases on exposure to atmosphere. We present a simple model to explain the absorption data. We calculate the density of states in the gap to be about 6.1017/cm3 and present evidence that the fermi level lies in the upper half of the band gap close to the LUMO level in AlQ.