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We evaluated emission behaviors of crystallized films of 1,4-bis(5-phenylthiophene-2-yl)benzene (AC5) in detail which was a representative thiophene/phenylene co-oligomer. The crystallized AC5 films were prepared by vapor deposition onto a substrate and thermal treatment. The AC5 films consisted of a crystalline domain with the size of several tens of micrometers. We used femtosecond laser pulses for the excitation of the AC5 films. As a result, the femtosecond laser pulses did not induce re-absorption above excitation energy densities of their laser threshold. The obtained gain value for AC5 crystallized film was large, over 150 cm-1. Furthermore, the emission cross section of the crystallized AC5 film was nearly 10-16 cm2.

Organic light-emitting diodes are fabricated by heterojunction of thiophene/phenylene co-oligomer films using biphenyl-capped bithiophene (BP2T) and its cyano-substituted derivative (BP2T-CN). Strong electron-withdrawing cyano-groups in BP2T-CN transform the p-type BP2T into n-type. Photoluminescence and electroluminescence from their bilayered films dominantly result from the BP2T-CN layer since the lying molecular orientation of BP2T-CN facilitates surface emission while the standing orientation of BP2T is not suitable for the device configuration. The current density and electroluminescence intensity are considerably increased by carrier doping with MoO3 and Cs2CO3 into the BP2T and BP2T-CN films, respectively.

One-dimensional (1D) structures of 2,5-bis(4-biphenylyl)thiophene (BP1T) crystals are fabricated for light amplification and field-effect transistor (FET) measurements. A strip-shaped 1D structure (10 m width) made by photolitography of a vapor-deposited polycrystalline film shows amplified spontaneous emission and lasing oscillations under optical pumping. An FET fabricated with this 1D structure exhibits hole-conduction with a mobility of h = 8.0×10−3 cm2/Vs. Another 1D-structured FET is fabricated with epitaxially grown needle-like crystals of BP1T. This needle-crystal FET exhibits higher mobility of h 034 cm2/Vs. This improved hole mobility is attributed to the single-crystal channel of epitaxial needles while the grain boudaries in the polycrystalline 1D-structure decrease the carrier transport.

We have improved the operation method of organic light-emitting field-effect transistors by applying a square wave to the gate electrode. A thiophene/phenylene co-oligomer crystal was used as the organic layer. Compared with the sinusoidal wave gate bias application, the square-wave bias produces the emission intensity ten times as large as that of the former. The effective emissions take place through electrons injection from the source contact when the gate bias traverses 0 V so as to be positive. When asymmetric electrodes were used for the source and drain contacts, the resulting emission exhibited the narrowed spectral line at 491.5 nm with its FWHM approximately 1.1 nm. The line narrowing is expected to be a consequence of the emission intensity increment caused by the enhanced electrons injection from the Ag source contact. The location of the emission line is closely related to those of the multimodes due to the laser oscillation by cavity resonance.

We have estimated the optical gain for a crystal of a thiophene/phenylene co-oligomer, 1,4-bis(5-phenylthiophen-2-yl)benzene. We prepared the crystal by a vapor phase growth method and measured emission spectra by exciting it with a pulse laser and changing the pumped stripe lengths. With increasing the stripe lengths, the emission intensity was increased and the emission spectra were gain narrowed around 516 nm with its full width at half maximum down to approximately 6 nm. The optical gain spectrum was determined from the emission spectra that varied as a function of the pumped stripe lengths. The gain spectrum was peaked at 516 nm with the maximum net optical gain coefficient being 75 cm(-1). This peak location is in excellent agreement with the peak position of the gain narrowed emission spectra. The results indicate that the determination of the gain spectrum is important for laser applications of the material. The oligomer crystal in the present studies is a good candidate for them.

We present several aspects of thiophene/phenylene co-oligomers (TPCOs). TPCOs are regarded as a newly occurring class of organic semiconductors. These materials are synthesized by hybridizing thiophene and phenylene rings at the molecular level with their various mutual arrangements. These materials are characterized by the super-controlled nanoscale sizes and molecular shapes. These produce peculiar crystallographic structures and high-performance optical and electronic properties. The crystals of TPCOs were obtained through both vapor phase and liquid phase. In the TPCO crystals, the molecules take upright configuration. These cause large carrier mobilities of field-effect transistors and laser oscillations under optical excitations. Spectrally-narrowed emissions (SNEs) were also achieved under weak optical excitation using a mercury lamp. The light-emitting field-effect transistors using these crystals for an active layer have shown the current-injected SNEs when the device was combined with an optical cavity and operated by an alternating-current gate-voltage method. Thus the TPCO materials will play an important role in the future in the fields of nanoscale technology and organic semiconductor materials as well as their optoelectronic device applications.

We report spectrally-narrowed emissions that take place from an organic semiconductor slab crystal of 2,5-bis(4-biphenylyl)thiophene (BP1T) under a low excitation-intensity regime. These emissions are caused with a mercury lamp that operates on a household power supply with an electric current approximately 1 A. The BP1T slab crystal is equipped with a distributed Bragg reflector. To complete this structure the slab crystal is attached to a diffraction grating that is engraved on a surface of a quartz glass substrate. The diffraction gratings have precisely been formed using a focused ion beam with a nanometer-defined precision. The spectral narrowing accompanied by the emission intensity increment is related to the strong mode-coupling between the forward electromagnetic wave and the backward (i.e., reflected) wave within the grating zone. Using a laser we also carried out the emission measurements on the BP1T crystals under a high excitation-intensity regime. The emissions are characterized as the longitudinal multimode laser oscillation, enabling us to determine the group refractive index of 4.56 for the BP1T slab crystal. Under both the low and high excitation-intensity regimes excitons are dominant species of the emission. Their participation in the spectrally-narrowed emissions is briefly discussed.