Real-Time Measurement of Molecular Orientational Randomization Dynamics during Annealing Treatments by In-Situ Ellipsometry

The molecular orientational randomization dynamics of organic thin films during annealing were measured by real-time in situ ellipsometry. The spirofluorene derivatives used in this work formed amorphous thin films with the molecules oriented parallel to the substrates when the films were vacuum deposited at room temperature. However, the molecular orientations became random when the thin films were annealed at temperatures higher than the glass transition temperature because molecular migration occurred. Analysis of the ellipsometry results using a graded model showed that the randomization of the molecular orientations depended on the thickness of the thin film. This suggests that the surface glass transition temperatures are lower than the bulk glass transition temperatures in the thin films of these small molecules.