Structural and optical properties of Tris(8-hydroxyquinoline) aluminum (III) (Alq 3) thermal evaporated thin films

Abstract

X-ray diffraction (XRD), transmission electron microscope (TEM) micrographs and optical properties of Tris(8-hydroxyquinoline) aluminum (III) (Alq 3) have been studied. XRD of powder Alq 3 showed that the material has a polycrystalline nature with triclinic structure. The crystal structure and morphology of the as-deposited and annealed (at 473 K for 2 h.) Alq 3 thin films indicated that the as-deposited film is amorphous in nature, while the annealed film has a polycrystalline nature with amorphous background. The molecular structure of the Alq 3 was confirmed by the analysis of (FTIR) spectra. The optical constants such as the refractive index, n, the absorption index, k and the absorption coefficient, α, of both the amorphous and polycrystalline Alq 3 films were determined using spectrophotometric measurements of transmittance ( T) and reflectance ( R) in the wavelength range (200–2500 nm). The analysis of the data showed an indirect allowed transition energy gaps E g i n d of 2.66 eV and 2.28 eV for the as-deposited and the annealed Alq 3 thin films, respectively. As well as another probability of direct allowed transition was carried out with energy gaps E g 1 d of 2.82 eV, and E g 2 d of 4.14 eV for the as-deposited film and E g 1 d of 2.59 eV, E g 2 d of 3.88 eV for the annealed films, respectively. Some optical parameters namely molar extinction coefficient ( ɛ molar ), oscillator strength ( f) and electric dipole strength ( q 2) have been evaluated. According to the single oscillator model (SOM), some related parameters such as oscillation energy ( E 0), the dispersion energy ( E d ), the optical dielectric constant ( ɛ ∞ ), the lattice dielectric constant ( ɛ L ) and the ratio of free carrier concentration to its effective mass ( N/ m*) were estimated. Graphical representation of both the surface and volume energy loss functions and the real optical conductivity as a function of photon energy supports the existence of the mentioned optical transitions.