Determination of crystal orientation in organic thin films using optical microscopy

Fesenko, Pavlo; Rolin, Cédric; Janneck, Robby; Bommanaboyena, S. P.; Gaethje, Heiko; Heremans, Paul; Genoe, Jan

doi:10.1016/j.orgel.2016.06.011

Cited by 19 publications

(27 citation statements)

References 33 publications

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“…For an isolated para-phenylene molecule, its polarization is parallel to the long molecule axis. 43,44 Since the molecule orientation also determines the optical indicatrix and polarization properties of fibers and wings, 33,52 a spatially resolved polarization analysis reveals the projection of the long molecule axis onto the substrate surface. From the orientation of the long fiber or wing axis, y orient , the angle of the long molecule axis with respect to the long aggregate axis is calculated, b mol = f pol À y orient .…”

Section: Resultsmentioning

confidence: 99%

Assembly of diverse molecular aggregates with a single, substrate-directed molecule orientation

et al. 2016

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Oriented, fluorescing organic nanoaggregates derived from 1,4'''-dicyano-functionalized para-quaterphenylene (CNP4) are obtained upon vacuum deposition on muscovite mica. Two types of aggregates are observed with fiber- and wing-like shape, respectively, both growing along distinct substrate directions. The polarization of the emitted fluorescence, their morphology, and their electric surface potential differ, reflecting different polymorphs. The wings are chiral twins. The molecules orient within ±5° along the same direction, templated by the substrate.

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Section: Resultsmentioning

confidence: 99%

Assembly of diverse molecular aggregates with a single, substrate-directed molecule orientation

et al. 2016

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“…DIC is an optical technique that can probe optical anisotropy of thin films. 17 As shown in Fig. 2c, fast and slow axes n 1 , n 2 obtained from the DIC signal correspond to the crystallographic axes b and a, respectively.…”

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confidence: 88%

Arrays of Pentacene Single Crystals by Stencil Evaporation

Fesenko

Flauraud

Xie

et al. 2016

Crystal Growth & Design

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In order to realize high-performance organic thin-film transistors (TFT), two parameters of the organic semiconducting layer are desired: single crystallinity for high mobility, and patterning for low off currents. High-quality single crystals can be fabricated using vapor techniques such as physical vapor transport (PVT) but they require high temperatures close to thermodynamic 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 equilibrium, for example, 240°C for pentacene. Such high temperatures are not ideal for TFT fabrication on plastic substrates and limit the use of PVT in flexible electronics applications.In this work arrays of pentacene single crystals were directly deposited at low temperature of 40°C by vacuum thermal evaporation through micro-fabricated stencil masks (stencil lithography). By decreasing the stencil aperture size down to 1 µm x 1 µm, we were able to limit the nucleation area until only one grain per aperture is nucleated and grown. We studied systematically scaling effects for large singe crystal growth and discuss details of the growth morphology. We found for instance that the formed pentacene crystals are one monolayer thick and the crystal area is much larger than the aperture size. This can be explained by the diffusion of adsorbed molecules on the surface laterally under the shadow mask, where they are protected from other impinging molecules. The diffusion away from the impinging area under the aperture affects the nucleation density inside this area and was used to calculate the diffusion length to nucleation λ N = 0.66 ± 0.11 µm of pentacene on SiO 2 at 40°C. Our diffusion-driven growth of organic single crystals by stencil lithography is a direct method to grow patterned arrays of single crystalline organic thin-film semiconductor layers.

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“…The most promising methods such as meniscus‐guided coating techniques or thermal gradient annealing, all employ a unidirectional solidification approach that separates nucleation and subsequent growth. Even though these techniques yield thin films with large single crystalline regions, these layers exhibit many morphological defects such as cracks, voids, or parallel ribbon growth that affect the film over multiple length scales. Such defects lower the electrical performance and are a major cause for spread in transport characteristics.…”

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confidence: 99%

“…We distinguish three main types of defects that can be formed during unidirectional crystallization. First, cracks in the semiconductor film are often caused by thermal contraction or a phase change when the film cools down after processing above room temperature. Second, in meniscus‐guided coating techniques, a stick–slip motion of the meniscus gives rise to defect patterns parallel to the receding contact line .…”

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confidence: 99%

“…Parallel ribbons have the same crystal orientation when they stem from the same nucleation event. In C 8 ‐BTBT, ribbons tend to grow either along the [100] or the [110] direction that is set by the propagation of fast‐growing facets in a mass‐supply limited system . Depending on the orientation of the initial nucleus, the direction best aligned with the coating direction is favored.…”

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Highly Crystalline C8‐BTBT Thin‐Film Transistors by Lateral Homo‐Epitaxial Growth on Printed Templates

et al. 2017

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Highly crystalline thin films of organic semiconductors offer great potential for fundamental material studies as well as for realizing high-performance, low-cost flexible electronics. The fabrication of these films directly on inert substrates is typically done by meniscus-guided coating techniques. The resulting layers show morphological defects that hinder charge transport and induce large device-to-device variability. Here, a double-step method for organic semiconductor layers combining a solution-processed templating layer and a lateral homo-epitaxial growth by a thermal evaporation step is reported. The epitaxial regrowth repairs most of the morphological defects inherent to meniscus-guided coatings. The resulting film is highly crystalline and features a mobility increased by a factor of three and a relative spread in device characteristics improved by almost half an order of magnitude. This method is easily adaptable to other coating techniques and offers a route toward the fabrication of high-performance, large-area electronics based on highly crystalline thin films of organic semiconductors.

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Determination of crystal orientation in organic thin films using optical microscopy

Cited by 19 publications

References 33 publications

Assembly of diverse molecular aggregates with a single, substrate-directed molecule orientation

Assembly of diverse molecular aggregates with a single, substrate-directed molecule orientation

Arrays of Pentacene Single Crystals by Stencil Evaporation

Highly Crystalline C8‐BTBT Thin‐Film Transistors by Lateral Homo‐Epitaxial Growth on Printed Templates

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