Article pubs.acs.org/crystal
Crystal Morphology and Growth in Annealed Rubrene Thin Films Thomas R. Fielitz and Russell J. Holmes* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *
ABSTRACT: While controlled crystallization of organic thin films holds great potential for enhancing the performance of electronic devices, quantitative understanding of the processes involved is limited. Here, we characterize the thin film crystal growth of the organic semiconductor rubrene during annealing using polarized optical microscopy with a heated stage for in situ measurements, followed by atomic force microscopy and X-ray diffraction. During annealing, the film undergoes transitions from predominant growth of a polycrystalline triclinic crystal structure to single crystal orthorhombic, followed by polycrystalline growth of the orthorhombic polymorph. Observation of crystal morphology with time allows determination of the crystal orientation, which is used in conjunction with crystal size measurements to determine the crystallization activation energies for the observed growth phases and crystal planes.
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small molecule organic semiconductor thin films allows facile in situ observation of quantitative and qualitative effects of temperature, time, and other film perturbations. In this work, the roles of these factors are elucidated for the organic semiconductor rubrene. Annealing rubrene thin films presents an intriguing case study since these initially amorphous films are known to crystallize upon annealing, yet little is documented about what happens as crystals grow in terms of nucleation, growth mode, and morphology evolution, making it an ideal candidate for in situ exploration. Because of its polymorphism and relatively simple molecular structure, information gained from studying this archetypical organic semiconductor may lend insight into the wider class of small organic molecules, both semiconducting and otherwise. Furthermore, because of its anisotropic fieldeffect mobility,27 simple determination of grain orientation is key to more accurate quantification of thin film transistor performance limits in crystalline rubrene and other molecular thin films, providing impetus for development of more facile crystal orientation determination techniques.
INTRODUCTION The ability to precisely order and crystallize organic semiconductor thin films has been widely exploited to realize effective charge and exciton transport, as well as enhanced optical absorption.1−5 Improvements due to molecular orientation may be manifested in greater orbital overlap to aid charge transfer,6 reduced film disorder to improve energy transfer,7 and alignment of more strongly absorbing molecular axes with optical stimuli,8 each of which has practical implications for electronic devices. In addition to the literature showing the efficacy of crystalline films as transistors,9−11 recent work has also demonstrated the ability to grow highly crystalline thin films using a template layer for use in organic photovoltaic cells.12−16 Since vapor-deposited films relevant to organic electronics can be glassy or amorphous in nature,17−19 it is essential to devise processing approaches that induce crystallinity in a controlled and predictable manner. One route to the realization of crystalline thin films is thermal annealing, wherein the film is heated to allow molecules sufficient time and energy to reorder into an energetically favored structure. Previous work in the solid phase crystallization of small molecular weight organic semiconductors has characterized in situ film and powder annealing behavior using X-ray diffraction (XRD) and differential scanning calorimetry (DSC), respectively. These techniques elucidate crystallization progress and thermodynamic information regarding the phase transition, but yield limited insight into the spatial evolution of thin-film crystal growth because of the relatively large sample volumes of these techniques.20−22 Polarized optical microscopy (POM) has been demonstrated to be a valuable technique for quantifying micron-scale crystal growth;23−25 however POM studies have been largely confined to melts of insulating polymers or pharmaceuticals with a few exceptions.26 Applying this technique to study crystal growth in © 2016 American Chemical Society
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EXPERIMENTAL METHODS
Sample Preparation. All films were grown on glass slides coated with indium−tin-oxide (ITO) having a sheet resistance of 15 Ω/□ or on single-polished (110) silicon wafers with nominal native oxide. All substrates were degreased with detergent and solvents. The ITO-glass substrates were exposed to a UV-ozone ambient for 10 min prior to deposition of the organic layers and held at 295 K during deposition. All layers were deposited by high vacuum thermal evaporation (
