Optical Properties of 3,4,9,10-Perylenetetracarboxylic Diimide (PTCDI) Organic Thin Films as a Function of Post-Annealing Temperatures

In this work, effect of post-annealing on the structural and optical properties of thermally evaporated PTCDI films deposited on quartz substrates is reported. The optical properties were investigated using spectrophotometric measurements in wavelength range 200-2500 nm for as-deposited and annealed films with the same thickness at different temperatures. The XRD studies confirm the films have orthorhombic structure (PNA21) space group. The optical constants were accurately determined using reflectance and transmittance spectra. The dispersion of the refractive index is discussed in terms of single oscillator model. In addition, the ratio of free carrier concentration to its effective mass was estimated. The absorption analysis has been also performed in order to determine the type of electronic inter-band transitions for the films. Both direct and indirect transitions are present. The direct and indirect bandgap energy decreases with increasing temperature. The decrease in the energy can be explained by increase of delocalized π electrons due to thermal annealing.