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 SiO2/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 cm2/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 cm2/Vs.
In the same way, we have obtained the TOF and FET mobility for
6-QTP-6 to be 0.03 and 0.04 cm2/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.
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.
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.