  The nucleation mechanisms during solution deposition of organic semiconductor
thin films determine the grain morphology and may influence the crystalline
packing in some cases. Here, in-situ optical spectromicroscopy in reflection
mode is used to study the growth mechanisms and thermal stability of
6,13-bis(trisopropylsilylethynyl)-pentacene thin films. The results show that
the films form in a supersaturated state before transforming to a solid film.
Molecular aggregates corresponding to subcritical nuclei in the crystallization
process are inferred from optical spectroscopy measurements of the
supersaturated region. Strain-free solid films exhibit a temperature-dependent
blue shift of optical absorption peaks due to a continuous thermally driven
change of the crystalline packing. As crystalline films are cooled to ambient
temperature they become strained although cracking of thicker films is
observed, which allows the strain to partially relax. Below a critical
thickness, cracking is not observed and grazing incidence X-ray diffraction
measurements confirm that the thinnest films are constrained to the lattice
constants corresponding to the temperature at which they were deposited.
Optical spectroscopy results show that the transition temperature between Form
I (room temperature phase) and Form II (high temperature phase) depends on the
film thickness, and that Form I can also be strain-stabilized up to
135$^\circ$C.
