Optical Properties of Pentacene and Perfluoropentacene Thin Films

TL;DR

This study investigates the optical anisotropy and electronic transitions of pentacene and perfluoropentacene thin films, revealing significant shifts in absorption spectra due to crystalline environment effects.

Contribution

It provides detailed optical characterization and analysis of the anisotropic properties of PEN and PFP thin films using spectroscopic ellipsometry and structural methods.

Findings

Strong optical anisotropy observed in both films

Significant HOMO-LUMO transition shifts due to crystalline environment

Identification of a second absorption band in PFP with a blueshift

Abstract

The optical properties of pentacene (PEN) and perfluoropentacene(PFP) thin films on various SiO2 substrates were studied using variable angle spectroscopic ellipsometry. Structural characterization was performed using X-ray reflectivity and atomic force microscopy. A uniaxial model with the optic axis normal to the sample surface was used to analyze the ellipsometry data. A Strong optical anisotropy was observed and enabled the direction of the transition dipole of the absorption bands to be determined. Furthermore, comparison of the optical constants of PEN and PFP thin films with the absorption spectra of the monomers in solution shows significant changes due to the crystalline environment. Relative to the monomer spectrum the HOMO-LUMO transition observed in PEN (PFP) thin film is reduced by 210 meV (280 meV). Surprisingly, a second absorption band in the PFP thin film shows a slight blueshift (40 meV) compared to the spectrum of the monomer with its transition dipole perpendicular to that of the first absorption band.