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Introduction Organic materials, primarily composed of carbon, hydrogen, and oxygen, are insulators and have very high resistivity at room temperature. Organic molecules with sp2 hybridization and delocalized π-electrons, such as conjugated hydrocarbons, metal phthalocyanines, and oligothiophenes, have semiconducting properties.1 Organic charge-transfer compounds, based on organic donors and acceptors,2 might become conducting or even superconducting. These extraordinary properties, which are unexpected in conventional non-conjugated organic materials, make organic semiconductors the subject of intense recent exploration. Organic semiconducting thin films are used in numerous applications such as organic field-effect transistors (OFETs),3 organic light-emitting diodes (OLEDs),4 and organic solar cells5 because of their light weight, flexibility, material solubility, low-temperature processability, large-scale yields, and low cost. Spin coating, drop casting, or printing techniques can be applied in the production of prototype electronic devices. However, grain boundaries,6 defects,7 dislocations,8 and impurities9 make polycrystalline organic films not suitable for the investigation of the intrinsic properties of organic semiconductors. Instead,

organic single crystals that can be prepared with high purity and low density of defects are ideal model compounds.10–13 They can be used to investigate structure-performance relationships,14 intrinsic15 and anisotropic16 properties, and photoconductivities,17,18 and can be used in computational simulations and calculations.19,20 Some potential applications for organic single crystals include inverters,21 logic circuits,22 and sensors.23 Van der Waals forces exist in organic semiconductors, and these weak intermolecular interactions give rise to different properties in organic semiconductors compared to inorganic semiconductors in which metallic or covalent bonds are predominant. The physical properties of organic semiconductors differ substantially from inorganic semiconductors. Melting points and sublimation temperatures are mostly much lower in organic semiconductors; therefore the methods of crystal growth of organic semiconductors are also substantially different from those used for inorganic materials. Many semiconducting organic molecules have been synthesized thus far, but only a few have been processed into solid samples for electrical charge transport measurements, and even fewer have been crystallized into freestanding single crystals large enough for electrical measurements. For electrical

Hui Jiang, School of Materials Science and Engineering, Nanyang Technological University, Singapore; [email protected] Christian Kloc, School of Materials Science and Engineering, Nanyang Technological University, Singapore; [email protected] DOI: 10.1557/mrs.2012.308

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MRS BULLETIN • VOLUME 38 • JANUARY 2013 • www.mrs.org/bulletin

© 2013 Materials Research Society

SINGLE-CRYSTAL GROWTH OF ORGANIC SEMICONDUCTORS

measurements, single crystals a few micrometers in size have