Liquid crystals are a new type of organic semiconductors exhibiting molecular orientation in self-organizing manner, and have high potential for device applications. In fact, various device applications have been proposed so far, including photosensors, solar cells, light emitting diodes, field effect transistors, and so on.. However, device performance in those fabricated with liquid crystals is less than those of devices fabricated with conventional materials in spite of unique features of liquid crystals. Here we discuss how we can utilize the liquid crystallinity in organic transistors and how we can overcome conventional non-liquid crystalline organic transistor materials. Then, we demonstrate high performance organic transistors fabricated with a smectic E liquid crystal of Ph-BTBT-10, which show high mobility of over 10cm2/Vs and high thermal durability of over 200oC in OFETs fabricated with its spin-coated polycrystalline thin films.
We have fabricated polycrystalline thin films of liquid crystalline oligothiophene derivatives of ω,
ω'-dioctylterthiophene (8-TTP-8) and ω, ω'-dihexylquarterthiophene (6-QTP-6). In order to evaluate carrier transport properties of the polycrystalline films in lateral and vertical orientations, we measured them by time-of-flight (TOF)
experiments with sandwich type of liquid crystal cells and evaluating device performances of thin film transistors
(TFTs) fabricated on SiO<sub>2</sub>/Si, respectively. Because of the liquid crystallinity, we could observe non-dispersive transient
photocurrents in polycrystalline films of 8-TTP-8 in spite of thick sample of 16 μm, and determine hole mobility to be
0.3 cm<sup>2</sup>/Vs. Because of the same reason, in spin-coated thin film of 8-TTP-8, where 8-TTP-8 molecules sit
perpendicular to the substrate, the field effect transistor (FET) mobility was successfully determined to be 0.1 cm<sup>2</sup>/Vs.
In the same way, we have obtained the TOF and FET mobility for
6-QTP-6 to be 0.03 and 0.04 cm<sup>2</sup>/Vs, respectively.
On the basis of the present results, we discuss the benefits of the liquid crystallinity in fabricating polycrystalline films
as an organic semiconductor for device applications.
In these ten years after the discovery of electronic conduction in liquid crystals, a lot of effort has been made to characterize what the electronic conduction is in liquid crystals, understand the unique characteristics of charge carrier transport found, synthesize new materials having high mobility, and explore potential applications as a quality organic semiconductor. Judging from the accumulated data and understandings on properties in liquid crystals as a self-organizing molecular semiconductor, it is no doubt that the liquid crystal is very promising as a new type of quality organic semiconductor for opto-electronic devices, which enjoys both merits in molecularly disordered and ordered materials, i.e., large-area uniformity and high mobility that characterize each material, respectively. In this article, the new results found in this decade on charge carrier transport properties, new materials having a high mobility, theoretical modeling of charge carrier transport, which provides us with the physical basis to understand the unique features of charge carrier transport and with the scope and limitation of liquid crystals as an organic semiconductor, are reviewed briefly, and what the further study needs to be focused on for practical device applications are discussed.
The measurement and analysis of the current-voltage characteristics of a liquid-crystalline organic semiconductor 2-(4'-Octyphenyl)-6-dodecyloxynaphthalene (8-PNP-O12) in contact with electrodes of Pt, Au, ITO, Cr, and Al in the order of work function revealed that the injection of positive holes from the electrodes of Pt, Au, and ITO to 8-PNP-O12 took place according to the Schottky model, and that an electric double layers was formed at the interface between each of these electrodes and 8-PNP-O12, making it difficult to inject positive holes from the former to the latter.
We analyzed the experimental time-of-flight data for photoinjected holes in two smectic liquid crystals, the first consisting of a phenylnaphthalene derivative 8PNPO12, and the second consisting of a biphenyl derivative 6OBP6. We fit the time of flight transients for different electric field strengths to a multiple trapping model (MTM). From these fits we determined the distribution of trap depths, under the assumption that (i) linear response is valid, and (ii) the trap release rates are independent of field.
An effective method for detecting a trace amount of chemical impurity, e.g., a few ppm or less, that degrades the charge carrier transport properties in smectic liquid crystalline (SmLC) semiconductors was investigated with a model system i.e., a 2-phenylnaphthalene derivative of 2-(dodecyloxy)-6-(4-octylphenyl)naphthalene (8-PNP-O12) and a terthiophene derivative of 2,5-bis(5-hexylthiophene-2-yl)thiophene (6-TTP-6). A transient photocurrent measurement could detect a chemical impurity of a few ppm or less that conventional analytical methods such as high-performance liquid chromatography (HPLC) and gas chromatography (GC) failed to detect: the slow transit induced by drift of ionized impurity molecules allowed us to detect it, which was clearly distinguished from the fast transit induced by the electronic conduction in the host SmLC semiconductor. This systematic study provided a semiquantitative basis for evaluating the contamination of chemical impurity.
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<sup>-2</sup> cm<sup>2</sup>V<sup>-1</sup>s<sup>-1</sup> 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<sup>-4</sup>, 10<sup>-3</sup> and 10<sup>-1</sup> cm<sup>2</sup>V<sup>-1</sup>s<sup>-1</sup>, respectively. Furthermore, even in a phthalocyanine derivative that is well known as a typical p-type organic semiconductor, i.e., octaoctylphthalocyanine (8H<sub>2</sub>Pc), a high electron mobility of 0.3 cm<sup>2</sup>V<sup>-1</sup>s<sup>-1</sup> was established, while the highest bulk hole mobility of 0.2 cm<sup>2</sup>V<sup>-1</sup>s<sup>-1</sup> 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.
Recently it has been discovered that some types of liquid crystals, which believed to be governed by ionic conduction, exhibit a very fast electronic conduction. Their charge carrier transport is characterized by high mobility over 10<sup>-2</sup> cm<sup>2</sup>/Vs independent of electric field and temperature. Now, the liquid crystals are being recognized as a new class of organic semiconductors. In this article, a new aspect of liquid crystals as a self-organizing molecular semiconductor are reviewed, focused on their basic charge carrier transport properties and discussed in comparison with those of molecular crystals and amorphous materials. And it is concluded that the liquid crystal is promising as a quality organic semiconductor for the devices that require a high mobility.
We have investigated the hole transport in smectic mesophases by Monte Carlo simulation based on a 2D hopping transport in Gaussian-distributed density of states and time-of-flight experiments. We found that their unique carrier transport properties such as non-Poole-Frenkel type of behavior i.e., field-and-temperature independent mobility, is well explained by the 2D disorder model with a small Gaussian width of 50-60 meV. Furthermore, we found the Pool-Frenkel type of behavior in a biphenyl derivatives and at a low temperaure range below ambient temperature in a therthiphene derviative and determined the Gaussian width to be 100-120meV and 50 meV, respectively. We came to a conclusion that the charge carrier transport in smectic mesophases can be explained by a 2D disorder model with a small Gaussian width of the density of states σ, where a value of σ/kT plays important role to determien its behavior at a given temperature.
Electro-optical characteristics were studied on the organic semiconductor materials exhibiting molecular alignment based on molecular self-organization in their liquid crystal phases. It was confirmed that these organic semiconductor materials exhibit very fast electron transport in the smectic mesophases as well as hole transport, both of which were independent on electric field and temperature. Above all, the ambipolar carrier transport in these materials make it possible to fabricate a simple electroluminescence cell without a layered structure required in conventional organic electroluminescence cells. In this paper, we fabricated electroluminescence cell with surface type of molybdenum or aluminum electrodes using hexyldodecylterthiophene (6-TTP-12) as a light emitting semiconducting material. The electroluminescence cell shows a green light emission when a dc bias is applied to the electrode. It was revealed that the light emission took place in the vicinity of the cathode interface, indicating that the hole is the majority carrier in the cell when the molybdenum or aluminum electrodes were applied. In addition, the light emission luminance of the cell is controllable in the external electrode which formed under insulating film as a gate electrode.
Recently it has been discovered that some types of liquid crystals exhibit very fast electronic conduction characterized by high mobility over 10<sup>-2 </sup>cm<sup>2</sup>/Vs, with is 1000 to 10000 times higher than that of the amorphous organic semiconductors practically used. Now, the liquid crystals are being recognized as a new class of organic semiconductors, that is, <i>Self-organizing molecular semiconductors</i>. The liquid crystalline materials enjoy crystal-like self-organizing molecular alignment and liquid-like fluidity. These unique nature provide us with a good basis for their application to the photo-voltaics in terms of quality photo-electrical properties and feasibility of large-area application. The general aspects related to photoconductivity in liquid crystalline materials, especially charge carrier generation and carrier transport properties are reviewed on the basis of our experimental results on the smectic liquid crystals and discuss their high potential as a new type of photo-voltaic materials.
Recently is has been discovered that some types of liquid crystals exhibit very fast electronic conduction whose charge carrier transport are characterized by high mobility over 10<sup>-2</sup> cm<sup>2</sup>/Vs independent of electric field and temperature. Now, the liquid crystals are being recognized as a new class of organic semiconductors, in addition to the recognition as the display material. The liquid crystal enjoys superior properties to conventional amorphous materials in terms of the organic semiconductors for large-area devices, thanks to its liquid-like fluidity and crystal-like molecular alignment. In this articles, a new aspect of liquid crystals as a Self-organizing molecular semiconductors are reviewed on the basis of our experimental results on smectic liquid crystals and discuss their high potential for organic electronic devices.
Recently it has been discovered that some types of liquid crystals exhibit fast electronic conduction, whose carrier transport properties are characterized by high mobility over 10<SUP>-2</SUP>cm<SUP>2</SUP>Vs independent of electric field and temperature. Theses materials feature crystal-like self- organizing molecular alignment and liquid-like fluidity, providing us with a good basis of the excellent photo- electrical properties and large-are uniformity for practical application. The general aspects of carrier transport properties in the present materials, which we call Self- organizing molecular semiconductors, are reviewed on the basis of our experimental results on the smectic liquid crystals and discus their high potential as a new type of organic semiconductors.
The spectral sensitization and photosensitizer efficiency of a liquid crystalline photoconductor, 2-(4'-octylphenyl)-6- dodecyloxy-naphthalene (8-PNP-O12) with C<SUB>70</SUB> were investigated by steady-state and transient photocurrent measurements in terms of temperature, electric field, and doping concentration of C<SUB>70</SUB>. The C<SUB>70</SUB>-doped liquid crystal cell exhibited a photoresponse in visible region of 400-700nm corresponding to the optical absorption of C<SUB>70</SUB>. In the time-of-flight measurement, the fast transient photosignals with fast rise and decay on the order of microseconds were obtained even in a bulk excitation condition, which is governed by the ambipolar carrier transit. In visible region, the same photogeneration efficiency for hole and electron indicated that C70 can inject both electron and hole into 8-PNP-O12 when photoexcited. The phase transition temperature did not change by doping C<SUB>70</SUB> but the phase transition was found to have a great influence to the photogeneration yield. In the high ordering SmB phase, the photosensitization yield was found to be about two orders larger than that in the low ordering SmA phase and isotropic phase, where different interaction of C<SUB>70</SUB> was obvious in optical absorption and texture is under polarized microscope.
The photo-hole generation process in the smectic phases of a liquid crystalline photoconductor, 2-(4'-octylpheny)-6- dodecyoxylnaphthalene (8-PNP-012) was investigated by using steady-state and transient photocurrent measurements. It was revealed that the photo-generation of holes was governed by two different processes: Onsager type of photo-carrier generation in the bulk and electrode-enhanced hole injection induced by the charge transfer from photo-excited 8-PNP-012 molecules to the electrode interface. In the former process, for fairly high light intensity, the collected photo- generation charge was found to be approximately proportional to the square of the light intensity irrespective of the phase, suggesting the contribution of exciton-exciton interactions. And in the latter process, the photogeneration quantum yield can be one order of magnitude larger than that in the bulk generation process. We will discuss totally the photo-carrier generation process in mesophases of 8-PNP-012, in terms of exciton-exciton and exciton-electrode interactions including the effect of electrode materials and the disorder of molecular arrangement.