Effect of orientational order of tris(8-hydroxyquinolinato)aluminum(III) on electroabsorption

The electroabsorption (EA) spectrum was analyzed in terms of the orientational order of tris(8-hydroxyquinolinato)aluminum(III) (Alq3) molecules, where the orientational order parameter 〈Pn(cosθ)〉 (θ: the tilt angle of molecule; Pn: nth Legendre polynomial; 〈〉: thermodynamic average) was used to quantify the anisotropic orientational order of Alq3. It was shown that the EA intensity is a function of S1 and S2 at the modulation frequency ω, whilst it is a function of S2 at the second-harmonic frequency 2ω. The EA spectrum for the vacuum deposited Alq3 was obtained from the optical transmittance measurement. The results suggested that the spontaneous polarization of Alq3 is generated in the evaporated films.