Thermal annealing induced changes in optical spectroscopic properties of tetrapyridyl-porphyrin thin films for energy applications

Optical properties of uniform and homogeneous thermally evaporated thin films of 5,10,15,20-tetra(4-pyridyl)21H, 23H-porphyrin (TPyP) have been demonstrated in this paper. X-ray diffraction analysis indicated the presence of a nanocrystalline structure of TPyP films which enhanced by increasing the annealing temperature up to 473K. Optical studies were investigated using spectrophotometric measurements of the transmittance and the reflectance of pristine and annealed TPyP films and from which, optical constants and related parameters were calculated. The absorption spectra of TPyP films characterize different electronic transitions bands locate in the ultra-violet and visible spectral regions which are high intense Soret (B) band, four quasi-electronic states (Q bands) and three weak bands called N, L and C bands. The edges of absorption bands were analyzed to estimate the type of transition bands, the optical energy gap and the Urbach energy tail of TPyP thin films. A single oscillator model was applied to a normal dispersion district of refractive index spectrum of TPyP films to determine many spectral dispersion parameters. Moreover, the surface and volume energy loss functions of TPyP films were determined. The annealing temperature-induced significant changes in the structural formation and optical spectroscopic features of TPyP thin films. The results of this study show that TPyP thin films could be used in energy conversion devices as a promising absorbing medium.