Temperature dependence of optical properties of pentacene thin films probed by spectroscopic ellipsometry

The pentacene thin films with thickness of around 80 nm were grown on glass substrates by the thermal evaporation technique. Their optical properties were determined in the spectral region of 0.73-9.0 eV and at temperatures between 200 and 450 K by spectroscopic ellipsometry. The parameters of the dispersive structures were derived by numerical fitting of the experimental data to the stacked layer model. The room-temperature optical absorption spectrum shows a direct band gap of about 1.90 eV. With increasing temperature, the overall blueshift in the optical band gap reflects on the modification of the electron-phonon interactions. Moreover, the Davydov splitting of the pentacene thin films is decreasing from 0.13 eV at 200 K to 0.08 eV at 450 K. We interpret these results in terms of molecular reorientations that cause changes in mutual molecular overlap within the unit cell.