Interplay between molecular packing and charge transfer states in 6,13-bis(trisopropylsilylethynyl)-pentacene (TIPS-pentacene) organic semiconductor thin films

The optical properties of organic semiconductors are influenced by interference between neutral molecular excitations and charge transfer excitations. We present a comprehensive study of this effect for TIPS-pentacene in the temperature range from 25°C to 140°C via the influence of molecular packing on the optical and electronic properties, utilizing in-situ x-ray diffraction, polarized optical spectroscopy and density functional theory. In TIPS-pentacene, anisotropic thermal expansion causes neighboring molecules to “slide” relative to each other. Since charge transfer effects depend sensitively on the nodal structure of the highest occupied and lowest unoccupied molecular orbitals, optical excitations are modulated as the relative displacement is varied. We find that the transition energies of the lowest energy optical excitations are dramatically blue shifted as the temperature is increased, and that changes in the charge transfer integrals can be correlated with an enhancement of field-effect transistor mobility. These results suggest a new approach to improve carrier mobility in strained thin films by decreasing the sensitivity of the charge transfer integrals to dynamic disorder.