Determination of thermal transition depth profiles in polymer semiconductor films with ellipsometry

Christian Müller, L. Mattias Andersson, Ovidio Peña-Rodríguez, Miquel Garriga, Olle Inganäs, Mariano Campoy-Quiles

    Research output: Contribution to journalArticleResearchpeer-review

    21 Citations (Scopus)

    Abstract

    Geometric confinement and interface effects can significantly alter the thermodynamic properties of thin polymer films. Phase transition temperatures have been shown to strongly depend on film thickness below a critical thickness threshold. It has been suggested that this behavior is due to an interface-induced continuous variation in phase transition temperatures throughout the depth of the films. Here we employ variable-temperature spectroscopic ellipsometry to demonstrate the existence of these depth profiles. We examine four different polymer semiconductors that are of interest for organic light-emitting diodes, solar cells, and field-effect transistors. In contrast to insulating polymers, these light-absorbing materials provide detailed information about structural changes as a function of depth due to wavelength-dependent attenuation. This concept enables us to investigate a broad range of thermodynamic processes including the glass transition, crystallization as well as crystalline and liquid-crystalline melting. In general, for the here investigated systems, higher transition temperatures are found at the free surface. Finally, the deduced profiles are used to predict the thickness dependence of the mean phase transition temperature. © 2013 American Chemical Society.
    Original languageEnglish
    Pages (from-to)7325-7331
    JournalMacromolecules
    Volume46
    Issue number18
    DOIs
    Publication statusPublished - 24 Sep 2013

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