Growth and characterisation of single crystals constitute a major field of materials science. In this feature article we overview the characteristics of organic single-crystal light-emitting field-effect transistors (OSCLEFETs). The contents include the single crystal growth of organic semiconductors and their application to transistor devices. First, we describe various single crystal growth methods that produce different morphologies and geometries of crystals. Using these single crystals we highlight construction and performance of the devices. The single crystal approach not only allows us to study the device performance that reflects the intrinsic nature of the organic semiconductors but also is advantageous to enhancement in the steady device operation. A current-injected laser oscillation in an electronic device configuration remains as a big challenge to be achieved. In this context we briefly mention the spectrally narrowed emissions as well as the possibility of light amplification in the OSCLEFETs.

•We synthesize a new series of alkyl-monosubstituted thiophene/phenylene co-oligomers.•The introduction of alkyl monosubstituents enhances the carrier mobility.•The highly-oriented thin films are obtained by the solution processing.•The bilayer structure is formed with thin films prepared by the solution processing.•The skim method produces large-sized free-standing thin films.A novel series of alkyl-monosubstituted thiophene/phenylene co-oligomers (TPCOs) has been synthesized and characterized. The introduction of alkyl chains to TPCOs improved the solubility in organic solvents. Thin films of these compounds were prepared by the vacuum-deposition, solution-cast, and skim methods. Of these, the solution-cast films and skim films consist of highly-ordered structures characterized by molecular bilayers. In particular, the skim method produced a large-sized free-standing thin film. We investigated the carrier transport of the thin films on the field-effect transistor (FET) configurations. These FET devices exhibited typical p-channel characteristics with clear saturation region. The introduction of alkyl monosubstituents is responsible for enhancement in the carrier mobility. The device characteristics are contrasted with those of alkyl-disubstituted compounds.

The thiophene/phenylene co-oligomers are ranked as a newly occurring class of organic semiconductors. The materials are characterised by that thiophenes and phenylenes are hybridised at the molecular level with their various mutual arrangements. These molecular arrangements produce peculiar morphological features in the solid state and excellent electronic and optical properties. In this review article we outline those characteristics in light of the structure/property relationship with central emphasis upon the crystal structure and its relevance to the leading-edge optoelectronic functionalities. These topics are most suitably approached by device studies including field-effect transistors and light-emitting devices. We describe the device characteristics in close connection with current-injected lasers.

We have developed an ambipolar thiophene/phenylene co-oligomer (TPCO) by introducing the trifluoromethyl group and the methoxy group to molecular terminals of the parent (unsubstituted) TPCO molecule. The crystals of the compound exhibit the ambipolar carrier transport on the field-effect transistors with homogeneous Au electrodes and heterogeneous MgAg and Au electrodes. The latter device definitively emits the light by applying sinusoidal-wave and square-wave alternating-current (AC) voltages to the gate electrode. Time-dependent emission measurements indicate that major emissions take place twice during each period of the AC gate bias.Graphical abstractResearch highlights► A light-emitting field-effect transistor comprises an organic semiconductor crystal. ► An ambipolar thiophene/phenylene co-oligomer is used for the LEFET device. ► The LEFET device is operated by applying AC voltages to the gate electrode. ► The LEFET device indicates external quantum efficiency up to 0.16 %.

We present the preparation of giant organic thin single-crystals of 2,5-bis(4-biphenylyl)thiophene (BP1T), a thiophene/phenylene co-oligomer (TPCO). Large-size well-defined hexagon crystals of BP1T were obtained by recrystallization from 1,2,4-trichlorobenzene solution using the constant-volume decreasing-temperature method under various conditions. The largest crystal had a size of 7.6 × 4.4 mm. We describe the detailed growth procedure and characterization of the resulting crystals. The polarizing microscope observation of the crystals confirms that they are present as single-crystals. In virtue of the large size of the crystal we were able to fabricate three field-effect transistor (FET) devices on one single crystal. The devices showed normal p-type FET characteristics. Furthermore, the emission features are examined. Major emission peaks were spectrally narrowed with increasing excitation energies. Several crystals exhibited the narrowed emission lines accompanied by many concentric fringes of various sizes. These observations strongly indicate that the emission is a coherent radiation.Graphical abstractHighlights• We prepare giant organic thin single-crystals of 2,5-bis(4-biphenylyl)thiophene (BP1T). The largest crystal is 7.6 × 4.4 mm in size. • We fabricate three field-effect transistor (FET) devices on a sole crystal at a time. These devices exhibit the normal device operation. • The crystals show spectrally narrowed emission lines at higher excitation energies. • The narrowed emission lines are accompanied by many fringes of concentric circles of various sizes. • These observations strongly indicate that the emission is a coherent radiation.

We propose a construction of an organic light-emitting field-effect transistor for producing spectrally-narrowed emissions (SNEs) under current injection. The device is characterized by the followings: a diffraction grating fabricated within the channel region, homogeneous source and drain electrodes made of a low work-function metal and a layered structure of organic semiconductor materials. The organic layered structure was composed of a p-type crystal and an n-type thin film. The latter film was deposited so as to cover the grating on the channel. We observed SNEs from the device when it was operated by applying square-wave alternating gate voltages. The observed spectra were peaking as a dominant line around 578 nm with a full width at half maximum less than 4 nm.

The thiophene/phenylene co-oligomers are ranked as a newly occurring class of organic semiconductors. The materials are characterised by that thiophenes and phenylenes are hybridised at the molecular level with their various mutual arrangements. These molecular arrangements produce peculiar morphological features in the solid state and excellent electronic and optical properties. In this review article we outline those characteristics in light of the structure/property relationship with central emphasis upon the crystal structure and its relevance to the leading-edge optoelectronic functionalities. These topics are most suitably approached by device studies including field-effect transistors and light-emitting devices. We describe the device characteristics in close connection with current-injected lasers.

Growth and characterisation of single crystals constitute a major field of materials science. In this feature article we overview the characteristics of organic single-crystal light-emitting field-effect transistors (OSCLEFETs). The contents include the single crystal growth of organic semiconductors and their application to transistor devices. First, we describe various single crystal growth methods that produce different morphologies and geometries of crystals. Using these single crystals we highlight construction and performance of the devices. The single crystal approach not only allows us to study the device performance that reflects the intrinsic nature of the organic semiconductors but also is advantageous to enhancement in the steady device operation. A current-injected laser oscillation in an electronic device configuration remains as a big challenge to be achieved. In this context we briefly mention the spectrally narrowed emissions as well as the possibility of light amplification in the OSCLEFETs.