Yukiko Takayashiki

Fast Ambipolar Carrier Transport and Easy Homeotropic Alignment in a Metal-Free Phthalocyanine Derivative

We have investigated charge carrier transport in the various phases of the metal-free phthalocyanine derivative, 1,4,8,11,15,18,22,25-octaoctylphthalocyanine (8H2Pc), by the time-of-flight method. The discotic columnar phase of 8H2Pc easily self-organizes in columns perpendicular to the substrate when cooled from the isotropic phase. We found that well-defined transits were observed, not only for positive carriers, but also for negative carriers. For the columnar rectangular phase at 85°C the hole and electron mobilities were very high at 0.2 and 0.3 cm2/(Vs), respectively. These mobilities depend on neither electric field nor temperature. This ambipolar carrier transport was easily observed even in an ambient atmosphere. Interestingly, the mobility in the isotropic phase was of the order of 10-3 cm2/(Vs) and did not depend on temperature, which suggests that the electronic conduction is governed by charge carrier transport in that phase as well.

High electron mobility of 0.1cm2V−1s−1 in the highly ordered columnar phase of hexahexylthiotriphenylene

Negative charge carrier transport has been investigated in the columnar mesophases of 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT), (the best-known conducting discotic liquid crystal because of its high bulk hole mobility) using the time-of-flight method. It was shown that fast electron transport takes place in the highly ordered helical columnar phase. The electron mobility was 0.08cm2V−1s−1 at 45°C, which was the same as the hole mobility reported previously. Electron transport was also observed in the lower-order columnar hexagonal phase, but an additional slow transit appeared in the transient photocurrents, suggesting ionic transport originating from ionized impurities.

One-Dimensional to Three-Dimensional Electronic Conduction in Liquid Crystalline Mesophases

We have established the electronic conduction in the nematic phase of a small molecule of a 2-phenylbenzothiazole derivative, i.e., 2-(4′-octyloxyphenyl)-6-butoxybenzothiazole (8O-PBT-O4). This gives a new insight into the quest for the electronic conduction in liquid crystals, which was initiated by Kusabayashi and Labes in late 1960s and had succeeded over several decades, leading it to the end. In addition, it is clarified that the ionic conduction often observed in less ordered mesophases is induced with trace amounts of chemical impurities due to its low viscosity. The present result indicates that the charge carrier transport in the mesophase is electronic in its intrinsic nature irrespective of mesophases and molecular sizes, i.e., 1D-electronic conduction in columnar phase, 2D-electronic conduction in smectic mesophases, and 3D-electronic conduction in the nematic phase.

Self-Organization in Bulk Heterojunction Solar Cells with Small Molecules

We fabricated both conventional and inverted bulk heterojunction solar cells by spin-coating with a rod-like liquid crystal of 1,4-diketo-N,N-dimethyl-3,6-bis(4-dodecyloxyphenyl) pyrrolo[3.4-c]pyrrole as a donor and C61PCBM as an acceptor. In both cell structures, we found that molecular reorientation and self-organization of the pyrrolopyrrole derivative occurred so as to form conduction channels after the thermal treatment at 60 °C for 30 min and that the power conversion efficiency was improved by 30 times, up to 1%. These results prove that the bulk nature of rod-like liquid crystals is effective for ordered bulk heterojunction solar cells leading to the improvement of the cell performance.

Ambipolar Carrier Transport in Terphenyl Derivative

We have investigated the liquid crystalline behavior and charge carrier transport properties in the semectic phases of a terphenyl derivative, 4-4″-bis(dodecyloxy)-3-methyl-p-terphenyl (12O-TPMe-O12) having methyl side chain in the terphenyl core part. In its samples highly purified by recrystallization several times, we observed the transient photocurrents indicating electronic conduction not only for holes but also electrons and determined their mobilities to be 5 × 10−4 and 0.9 × 10−2 cm2/Vs in SmC and SmG phases, respectively. Thus, we concluded that the intrinsic nature of charge carrier transport in the terphenyl derivatives is electronic and ambipolar, where electrons and holes can be transported.

Synthesis and Characterization of Liquid Crystalline Semiconducting Materials, Dialkyl-2-phenylnaphthalene Derivatives—Mesomorphic behavior and Charge carrier transport

For establishment of high electronic charge carrier transport at ambient temperature, dialkyl derivatives of 2-phenylnaphthalene have been synthesized and characterized their liquid crystalline behaviors and photoelectrical properties. These materials exhibited smectic mesophases (Smectic A, B and E phases), at fairly low temperature range extended down below ambient temperature. Fast electronic transport of positive carriers was observed in SmE phase of 2-(4′-octylphenyl)-6-butylnaphthalene at 30°C, whose mobility was over 10−2 cm2/Vs.

Organic Light-Emitting Diode with Highly Ordered Polycrystalline Thin Film

We investigate organic light-emitting diodes with a polycrystalline thin film of a calamitic liquid-crystalline material. Compared to a non-liquid-crystalline material, the polycrystalline films fabricated via the liquid-crystalline phase showed high electrical current and light emission from an emissive dye dopant, in spite of a film thickness of over 1 µm. Judging from the X-ray diffraction patterns and polarized optical microscope images, well-controlled parallel molecular orientation and suppression of grain boundaries were achieved only in the liquid-crystalline material owing to the molecular orientation controllability of the liquid-crystalline materials. We conclude that the liquid-crystalline materials can be a good candidate for a polycrystalline organic semiconductor for not only thin-film transistors but also light-emitting diodes and solar cells.

Fast electron transport in discotic liquid crystalline semiconductors

We have re-investigated the negative charge carrier transport in discotic columnar phases of triphenylene derivatives and a phthalocyanine derivative by time-of-flight method in order to clarify the intrinsic nature of charge carrier transport in discotic liquid crystals. In a purified hexabutyloxytriphenylene (H4T), in which the fast hole transport was discovered previous reports, the transient photocurrents for negative carriers showed two transits in different time ranges, which were correspond to electron and ionic transports, respectively. The fast mobility corresponded to 10-2 cm2V-1s-1 comparable to the hole mobility reported previously. The fast electron transports were observed in purified hexahexyloxytriphenylene (H6T), hexapentyloxytriphenylene (H5T), and hexahexylthiotriphenylene (HHTT) as well, and the electron mobilities in these materials were 10-4, 10-3 and 10-1 cm2V-1s-1, respectively. Furthermore, even in a phthalocyanine derivative that is well known as a typical p-type organic semiconductor, i.e., octaoctylphthalocyanine (8H2Pc), a high electron mobility of 0.3 cm2V-1s-1 was established, while the highest bulk hole mobility of 0.2 cm2V-1s-1 was reported recently. Therefore, we conclude that the slow negative charge carrier transport reported in discotic liquid crystals previously originates from impurity-induced ionic transport, and that it is very likely for the intrinsic charge carrier transport in liquid crystalline semiconductors to be electronic and ambipolar, while it is very sensitive to the purity.