Subband gap optical absorption and defects in Tris(8 hydroxy quinolato) aluminium

In this paper, we report on measurements of the subband gap absorption of Tris(8 hydroxy quinolato) aluminium (Alq) in vapour, solution and in thin films. Using a combination of spectroscopies-transmission, photo-thermal deflection spectroscopy and luminescence excitation spectroscopy, we show that the bulk subband gap absorption in the solid state is very similar to that of an individual molecule. We also report on direct measurements of surface states in the thin film. The bulk absorption can be described by a Gaussian in the vicinity of the absorption peak (3.18 eV) and by an Urbach tail below 2.5 eV. We show that both these line shapes can be understood as a consequence of exciton–phonon interaction. These measurements help to clarify our understanding of defects in Alq.