In situ optical studies of thermal stability of iodine-doped polyazomethine thin films

Chemical vapour deposition (CVD) method, via polycondensation reaction, is used to obtain poly(1,4-phenylene-methylidynitrilo-1,4-phenylenenitrilomethylidyne) (PPI) thin films. The iodine (I-2) doping process of these films is observed in situ, by means of changes of the absorbance spectra. A distinct reduction of the energy gap, being a result of p-type doping, is connected with extracting electrons from pi-conjugated system and formation of the polaron states inside the gap. On the basis of UV-Vis-NIR(T) experiments, the heat treatment effect on the absorption coefficient spectra of iodine doped PPI thin films is reported, as a new method, to estimate thermal stability and to obtain the border temperature of doped polymer films. Moreover it is demonstrated that the analysis of absorption bands (shape, level and position) and absorption edge parameters, such as the Urbach energy (E-U) and the energy gap (E-G) can be used to evaluate the changes of polymer chain conformation and conjugation. (C) 2017 Elsevier Ltd. All rights reserved.