Article pubs.acs.org/JPCC
Real-Time Measurement of Molecular Orientational Randomization Dynamics during Annealing Treatments by In-Situ Ellipsometry Takeshi Komino, Hiroko Nomura, Masayuki Yahiro, and Chihaya Adachi* Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka, 819-0395, Japan S Supporting Information *
ABSTRACT: 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.
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INTRODUCTION Organic amorphous films are frequently used in organic thin film optoelectronic devices because of their large area homogeneous photonic and electronic properties. However, their properties are inferior to those of crystalline films with well-ordered and orientated molecules. One promising strategy to improve the properties of organic amorphous films is control of their molecular orientation. For example, in comparison with randomly oriented films, our group previously demonstrated an improvement of nearly 1 order of magnitude in the carrier mobility perpendicular to the substrate in films with parallel molecular orientation.1 A pronounced decrease in the amplified spontaneous emission threshold of up to 50% was also observed.2 Temperature is the dominant factor in the control of molecular orientation.1−3 The glass transition temperature (Tg) is particularly important, because a glass transition in oriented molecular films leads to randomization by molecular migration and subsequent aggregation. It is therefore important to investigate Tg in amorphous films. With the discovery of a surface glass transition temperature [Tg(s)] in thin polymer films4 that is lower than the bulk glass transition temperature [Tg(b)], numerous studies of Tg in thin polymer films have been reported.5−20 These studies have revealed the crucial properties of the glass transition in polymer films, such as a depression of Tg in thin films, but most of these studies have been conducted by indirect methods. Despite many studies of polymer films, the distribution of Tg in neat films of small molecules has not yet been clarified. In our previous studies, using small rod-like molecules, we reported that the molecules were orientated parallel to the substrates in vacuum deposited films at room temperature, and randomization was observed even at substrate temperatures © 2012 American Chemical Society
that were much lower than the Tg(b) value estimated from differential scanning calorimetry (DSC) measurements.1,2 These results imply that Tg(s) is lower than Tg(b), even in the small molecule thin films. To investigate the actual Tg distributions, changes in the molecular orientations with temperature must be investigated. Spectroscopic ellipsometry is a useful method for this study, because the measurements can be conducted in real-time when heating the thin film. The ellipsometry measurement essentially provides the average Tg across the thickness of the film. Previous investigations of Tg using ellipsometry have only observed the changes in the ellipsometric angles (Δ and Ψ).5−13 However, if a variety of analytical procedures are used in spectroscopic ellipsometry, an appropriate optical model can be constructed and information about the molecular orientation at the surface can be extracted, even if inhomogeneous changes in the molecular orientation occur in the films. In this study, we directly investigate the Tg distributions in small molecule thin films by observation of their molecular orientation changes through spectroscopic ellipsometry.
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EXPERIMENTAL METHODS Spirofluorene derivatives of 2,7-bis(4′-hexyl-[1,1′biphenyl]-4yl)-9,9′-spirobi[fluorene] (BHBPSF) and 4,4′-(9,9′-spirobi[fluorene]-2,7-diyl)bis(N,N-diphenylaniline) (SFBDPA) were used in this study. Sublimed BHBPSF was purchased from NARD Institute (Osaka, Japan), while SFBDPA was synthesized as described elsewhere2 and then purified by sublimation before use. The vacuum deposited thin films fabricated from these compounds are amorphous because of the bulky and Received: March 5, 2012 Published: May 7, 2012 11584
dx.doi.org/10.1021/jp302158k | J. Phys. Chem. C 2012, 116, 11584−11588
The Journal of Physical Chemistry C
Article
Figure 1. Film thickness and transient characteristics of Δ during film deposition (region I) and annealing (region II) in (a) BHBPSF and (b) SFBDPA films. The film thicknesses were obtained from simulated ellipsometric angles. The black circles show the experimental Δ, while the red and blue lines show the simulated Δ with a simple uniaxial anisotropic model (anisotropic material/Si substrate) and a graded model of isotropic and anisotropic materials (graded material/Si substrate), respectively. The substrate temperature and fraction of the isotropic material relative to the total material at the surface, middle and film/substrate interface are also shown for (c) BHBPSF and (d) SFBDPA films. The specific temperatures at these positions with different isotropic fractions are summarized in Table 1.
Figure 2. Spectra of optical constants before and after [(a) BHBPSF at 25 min and (c) BHBPSF at 70 min, (b) SFBDPA at 25 min and (d) SFBDPA at 100 min] annealing of the films. Data from the spectra were used for the simulations of the ellipsometric angles (Figures 1, parts a and b). The subscript o represents “ordinary” (parallel to the substrate), while e represents “extraordinary” (perpendicular to the substrate).
BHBPSF and 433 K for SFBDPA with a heating rate of
