A study of optical properties and annealing effect on the absorption edge of pristine- and iodine-doped polyazomethine thin films

Aromatic polyazomethine (PPI) thin film have been obtained by chemical vapor deposition (CVD) method, via polycondensation process and characterized by X-ray diffraction, AFM, FTIR and detailed UV–Vis–NIR studies. Optical transmission and fundamental reflectivity spectra of the film have been examined within the spectral range 200–2500 nm. The refractive index (n), film thickness (d) and parameters of the absorption edge i.e. the optical gap (EG) and the Urbach energy (EU) have been found for the PPI film before and after iodine (I2) doping. Then the annealing effect (from 25 °C every 25 °C up to 225 °C) on the absorption edge have been investigated during “in situ” spectral measurements. Amorphous character of the films allowed us to obtain the EG and EU values, in the way typical for amorphous semiconductors. As a result of iodine doping, the energy gap of the PPI film (2.19 eV) distinctly reduces to 1.73 eV, due to the polaron states and, simultaneously, the Urbach energy decreases. Thermal stability of the pure PPI film, being the typical feature of polyazomethines, was confirmed, while after iodine doping the film turned out to be thermo-stable only below 100 °C; then the distinct changes of the optical gap and the Urbach energy, connected with the iodine releasing process, have been presented and discussed.