Online

Li, Yang

Graduate Program in Materials Science

2020

Ph. D.

Two of the most critical experimentally accessible properties of small-molecule organic semiconductor materials are the charge carrier mobility, which probes charge transport, and the optical absorption spectrum which probes the energy levels of excited states. The impact of molecular packing on the optical and charge transport properties are often treated separately. However, these effects are actually linked at a fundamental level, and it is of interest to understand the interrelationship between them, as well as how they respond to strain and thermal expansion. In this thesis, we highlight the fundamental relationship between these two phenomena in 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene). We present an extensive study of the optical and electronic properties of thin films utilizing in situ X-ray diffraction, optical spectroscopy and ab initio density functional theory. The influence of molecular packing on the properties are reported for thin films deposited in the temperature range from 25°C to 140°C, and for films that are strain-stabilized at their as-deposited lattice spacings after cooling to room temperature. Anisotropic thermal expansion causes relative displacement of neighboring molecules while maintaining a nearly constant stacking distance. This leads to a large blueshift in the absorption spectrum as the temperature increases. The blueshift largely reverses a redshift at room temperature compared to the solution absorption spectrum. A reduction in the ratio of the first two vibronic peaks relative to the solution spectrum is also observed. This combination of electronic and vibronic effects is a signature of charge transfer excitonic coupling with a positive coupling constant J_{CT} , which depend sensitively on the alignment of the nodes of the frontier molecular orbitals with those on neighboring molecules. These effects are also correlated with the sign and magnitude of electron and hole charge transfer integrals t_e and t_h calculated from density functional theory that provide additional evidence for charge transfer mediated coupling at room temperature, as well as insight into the origin of an experimentally observed enhancement of the field-effect transistor mobility in strain-stabilized thin films. The results suggest approaches to improve carrier mobility in strained thin films and for optical monitoring of electronic changes. The effect of average intermolecular distance on emission properties of TIPS-pentacene solutions and polystryene/TIPS-pentacene blend films were studied by varying the TIPS-pentacene concentration and the mass ratio of the host polymer to TIPS-pentacene. In the blend films, it is found that the excited TIPS-pentacene molecules give strong emission when the mean intermolecular spacing is larger than 6 nm but prefer to decay via a non-radiative way when mean intermolecular separation is reduced to 3 nm.