Thiophene/phenylene Co-oligomer Research Articles

Organic light-emitting diodes based on layered films of thiophene/phenylene co-oligomers

We have fabricated p–n junctions using a combination of p- and n-type thiophene/phenylene co-oligomer (TPCO) thin films and applied them to organic light-emitting diodes (OLEDs). We made two types of devices using the different p-type TPCOs. Both the devices showed current-injected light emissions and high rectification ratios (~102–104). The one type of the devices produced brighter surface emissions and higher external quantum efficiencies (0.17%) than the other. The results indicated that the p–n junction comprising layered TPCOs showed good functionality.

Energy Dispersions of Anisotropic Refractive Indices of Thiophene/Phenylene Co-Oligomer Crystals

We have determined the energy dispersions of the anisotropic refractive indices along the three a-, b-, and c-crystal axes of the thiophene/phenylene co-oligomer (TPCO) crystals with higher precision. The refractive indices were calculated from interference fringes observed in the vertical- and oblique-incidence transmission spectra of single crystal thin films. The optical constants of the TPCO crystals have large optical anisotropy and strong energy dispersion because of the molecular transition dipole moments oriented as H-aggregates. In particular, the refractive index of the c-axis and its energy dispersion are much larger than those of the other axes. The strong energy dispersion of the refractive indices results in the large effective refractive indices of the TPCO crystals. The determined refractive indices along the three crystal axes of BP1T crystals are very useful for optical device applications.

Giant organic single-crystals of a thiophene/phenylene co-oligomer toward device applications

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 7.6×4.4mm. 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.

Light-emitting field-effect transistors made of single crystals of an ambipolar thiophene/phenylene co-oligomer

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.

Amplified Pulse Emissions with Variable Delay Times in Vibronic Transition Bands of Thiophene/Phenylene Co-Oligomer Single Crystals

Time-resolved photoluminescence of thiophene/phenylene co-oligomer (TPCO) single crystals has been measured at 7 K. The amplified pulse emissions with time delay are demonstrated in the 0–2 vibronic transition band of the TPCO single crystal under high-intensity excitation. We identify the delayed pulse emission to the 2A1 vibration line. The pulse peak of the 2A1 line exhibits a maximum delay time of 70 ps at 7 K. We consider that the amplified pulse emissions with time delay in single crystals occur at a vibronic transition band with the largest spontaneous emission intensity in the PL spectrum under weak excitation.

Microdisk lasers and field effect transistors of thiophene/phenylene co-oligomers by using high temperature deposition method

Microdisk lasers of the organic semiconductor material thiophene/phenylene co-oligomer (TPCO) have been fabricated on Si/SiO 2 substrates. Quality of the organic films is improved by using the high temperature deposition method, and microcavities without grain boundaries and cracks are obtained. As a result, microdisks with 1 μm diameter are fabricated successfully. Comparing with the bulk film part, the lasing threshold is reduced to 2% of the annealed bulk film parts. A part of the reduction comes from the improved quality of crystal films and another part of the reduction from the small disk size. To our knowledge, this is the smallest microdisk cavity made from organic crystals ever reported. The TPCO crystal films fabricated by using the high temperature deposition method also show the good performance of p-channel field effect transistor operation.

Single-Crystal Growth and Charge Transport Properties of an Alternating Co-Oligomer Composed of Thiophene and Phenylene Rings

We grow single crystals of a thiophene/phenylene co-oligomer (TPCO) composed of seven alternating thiophene and phenylene rings (AC'7). These crystals are grown in a liquid phase directly onto silicon substrates covered with a silicon dioxide layer. The grown crystals are well-defined hexagons and are composed of a molecular layered structure along the molecular length. Top-contact field-effect transistors are fabricated by depositing metal electrodes on the AC'7 crystals grown on the substrates. We estimate the field effect mobilities in the transistor device configurations. The mobility data are associated with the crystallographic results of the material. The devices produce a maximum mobility of ∼0.1 cm2·V-1·s-1. This value is higher than that obtained for another TPCO composed of five alternating rings (AC5). The present results indicate that AC'7 crystals are potentially useful for electrical device applications.

Refractive Index along the Molecular Long Axis of an Orthorhombic Thiophene/Phenylene Co-oligomer Crystal

We have determined a refractive index along the crystal long axis of an orthorhombic thiophene/phenylene co-oligomer (TPCO) crystal. From the space group symmetry of the crystal, the three crystallographic axes are identical to the principal axes of the refractive indices. We estimated the index to be 3.76 on the basis of the mode intervals of the equally-spaced longitudinal multimode laser oscillation which is highly polarized along the crystal long axis (i.e., the a-axis). This index is much larger than those associated with the other axes. These experimental indices were compared to the semiempirically calculated ones, in qualitatively good agreement with them.

Optical selection rule for the lower Davydov excitons in co-oligomer single crystals

Intrinsic optical transitions of lower Davydov excitons in thiophene/phenylene co-oligomer (TPCO) crystals were investigated. Lower Davydov excitons are either optically allowed or forbidden depending on the number of thiophene rings constituting the TPCOs. The TPCO molecules are orientated nearly parallel to each other like $H$ aggregates and are aligned in herringbone fashion within a crystalline layer. The transition selection rules are dominated by the components of the molecular transition dipole moments lying in the herringbone planes of the respective crystals. The selection rules at the exciton band bottoms of the TPCOs differ from those of oligothiophenes in spite of these two compounds having similar molecular structures. The selection rules vary drastically with the molecular alignment because of the $H$ aggregation. TPCO and oligothiophene crystals are treated as $H$ aggregates as a first approximation. This treatment is very useful for understanding the optical transitions of conjugated materials.

Anisotropic Refractive Indices of Organic Crystals of Thiophene/Phenylene Co-Oligomers Determined by Microspectroscopic Measurements

We have measured and determined the anisotropic refractive indices of thiophene/phenylene co-oligomer (TPCO) crystals. The refractive indices were calculated from the crystal thicknesses and optical path lengths on the basis of the interference of the reflectance spectra measured along the crystal axes by a polarizing microscope with a white light source. The higher refractive indices were observed along the crystal short axis and this was attributed to a denser molecular packing along that axis.

Microdisk and Microring Lasers of Thiophene–Phenylene Co‐Oligomers Embedded in Si/SiO2 Substrates

Microdisk and microring lasers of the organic semiconductor material thiophene/phenylene co-oligomer (TPCO) have been fabricated on Si/SiO2 substrates. Laser emission with the whispering gallery modes (WGM) is observed in 0-1 vibration transitions. Comparing with the bulk film parts, a few times reduction of the lasing threshold has been observed in microdisks. Microring lasing threshold is reduced further when the width of the waveguide is less than 1 μm. WGM with waveguide confinement is supposed to contribute to the decrease of the threshold.

Laser oscillations of whispering gallery modes in thiophene/phenylene co-oligomer microrings

Laser oscillation of whispering gallery modes was observed in microring structures of semiconducting thiophene/phenylene co-oligomer (TPCO) crystals at room temperature. Microring structures were formed by dry etching from thin film crystals of TPCO. The thresholds for the laser oscillation of a microring and a thin film crystal are 200 and 1400μJ∕cm2 for picosecond excitation, respectively. Therefore, the threshold for the microring was reduced to 1∕7 of that for the thin film crystal. The dramatic reduction of threshold clearly demonstrates the importance of microcavity in making efficient organic semiconductor lasers.

Improved sublimation growth of single crystals of thiophene/phenylene co-oligomers

We have improved the sublimation recrystallization method using a handmade sublimation apparatus and optimizing various physical conditions. The improved apparatus comprises two closely aligned heaters and enables us to grow the large thiophene/phenylene co-oligomer (TPCO) crystals of high quality. We found it essential and effective for the crystal growth to adjust temperatures of these heaters and their temperature difference of ca. 40–50 °C. We fabricated organic field effect transistor (OFET) devices with the resulting crystals of a TPCO. Those devices show the p-type FET characteristics. The field effect mobilities increased with smaller thicknesses of the crystals; the maximum mobility recorded was 0.021 cm 2/Vs.