A study on the effect of the macrocycle ring size and R-group chain length on the optical properties of the CuPc thin films

Optical properties of spin-coated films of copper phthalocyanine (CuPc) are investigated by using UV-visible spectroscopy. The absorption spectra were recorded in the UV-visible region and showed two absorption bands of the phthalocyanine molecules, namely, the Soret (B) and the Q-band. The UV-visible spectrum is used to calculate the physical parameters of the thin film under investigation such as absorption A, reflectance R, refractive index n, extinction coefficient k, optical conductivity sigma(opt) and optical band gap E-g values. The obtained refractive index and the extinction coefficient values vary from 1.5 to 2.5 and from 0.1 to 4.5, respectively. We have observed that the direct allowed gaps have been found to be 1.5 eV and 3 eV for CuPc I; 2.4 eV, 3.2 eV and 3.65 eV for CuPc II. On the other hand, for CuPc III thin films, they have been observed as 1.55 eV, 2.55 eV and 2.9 eV. Differences in optical properties are attributed to different chemical structures of the CuPc molecules in terms of their macrocycle ring size and R-group chain length.