The search for low-cost, large-area, flexible devices has led to a remarkable increase in the research and development of organic semiconductors, which serve as one of the most important components for organic field-effect transistors (OFETs). In the current review, we highlight deposition techniques that offer precise control over the location or in-plane orientation of organic semiconductors. We focus on various vapor- and solution-processing techniques for patterning organic single crystals in desired locations. Furthermore, the alignment of organic semiconductors via different methods relying on mechanical forces, alignment layers, epitaxial growth, and external magnetic and electric fields are surveyed. The advantages, limitations, and applications of these techniques in OFETs are also discussed.

Controlled Deposition of Crystalline Organic Semiconductors for Field-Effect-Transistor Applications

Experimental techniques for the fabrication and characterization of organic thin films for field-effect transistors.

We investigate the growth and field-effect transistor performance of aligned pentacene thin films deposited by zone-casting from a solution of unsubstituted pentacene molecules in a chlorinated solvent. Polarized optical microscopy shows that solution processed pentacene films grow as large crystalline domains with pronounced anisotropy in the substrate plane, in contrast to vacuum sublimed pentacene films, which consist of small crystalline grains with random in-plane orientation. The high structural alignment is confirmed by in-plane and out-of-plane X-ray diffraction analysis, with out-of-plane 00n reflections up to at least the seventh order, and a pronounced in-plane anisotropy with the a-axis of the triclinic unit cell predominantly aligned parallel to the zone-casting direction and the ab-plane parallel to the substrate. The average charge carrier mobility of the zone-cast pentacene devices depends strongly on the underlying dielectric. Divinylsiloxane-bis-benzocyclobutene (BCB) resin is found to b...

High-Mobility Aligned Pentacene Films Grown by Zone-Casting

The authors report preferential in-plane molecular orientation and charge transport anisotropy in pentacene thin film transistors achieved by using a photoaligned polyimide film with large in-plane anisotropy. Polarized infrared absorption spectra indicated that the molecular plane normal of the pentacene preferentially aligned along the average orientation direction of the underlying polyimide backbone structure. Atomic force microscope images showed that the alignment of the polyimide backbone structure significantly modified the pentacene growth process and remarkably increased the grain size. The charge carrier mobility along the polyimide alignment direction was about twice of that perpendicular to it.

Orientation control of pentacene molecules and transport anisotropy of the thin film transistors by photoaligned polyimide film

Inducement of Azimuthal Molecular Orientation of Pentacene by Imprinted Periodic Groove Patterns for Organic Thin‐Film Transistors

Pentacene was vapor-deposited on the NaCl substrate and friction-transferred PTFE layer. Pentacene molecules crystallized in bulk phase and/or thin film phase depending on the deposition condition. The polarized absorption spectra of pentacene/PTFE double layer showed an anisotropic feature. Pentacene oriented uniaxially taking edge-on state on a PTFE layer. The electrical property of pentacene/PTFE double layer also indicated anisotropic feature. The field effect mobility along to the traverse of the friction direction was 4.2 × 10−1 cm2/Vs.

Fabrication and Field Effect Transistor Characteristics of Well-Ordered Pentacene Film

The absorption spectrum of merocyanine (BTET) film deposited on a NaCl substrate showed a broad absorption band in the wavelength region between 450 nm and 550 nm and a narrow peak at 640 nm. The former bands were assigned to the absorption bands of α- and β-form crystal, the latter peak was assigned to J-band. BTET film was composed of square-shaped and rectangular crystals grown epitaxially. The square-shaped crystal was composed of α-form and the rectangular crystal was composed of α-form and J-aggregate. The formation mechanism of J-aggregate in film was discussed.

Hiroki Kihara

Papers

Fabrication and Field Effect Transistor Characteristics of Well-Ordered Pentacene Film

Fabrication of orientation-controlled merocyanine j-aggregates from vapor phase