Hiroaki Iino

Liquid crystals for organic thin-film transistors

Crystalline thin films of organic semiconductors are a good candidate for field effect transistor (FET) materials in printed electronics. However, there are currently two main problems, which are associated with inhomogeneity and poor thermal durability of these films. Here we report that liquid crystalline materials exhibiting a highly ordered liquid crystal phase of smectic E (SmE) can solve both these problems. We design a SmE liquid crystalline material, 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10), for FETs and synthesize it. This material provides uniform and molecularly flat polycrystalline thin films reproducibly when SmE precursor thin films are crystallized, and also exhibits high durability of films up to 200u2009°C. In addition, the mobility of FETs is dramatically enhanced by about one order of magnitude (over 10u2009cm2u2009V−1u2009s−1) after thermal annealing at 120u2009°C in bottom-gate-bottom-contact FETs. We anticipate the use of SmE liquid crystals in solution-processed FETs may help overcome upcoming difficulties with novel technologies for printed electronics.

Availability of Liquid Crystallinity in Solution Processing for Polycrystalline Thin Films

Figure 1 . Chemical structures of the liquid crystalline materials used in the present experiments, i.e., ω , ω ’-dioctylterthiophene (8-TTP-8) and 2,7-didecylbenzothienobenzothiophene (C 10 -BTBT). The research of materials for organic fi eldeffect transistors (OFETs) has been extended to the exploration for new materials suitable for the fabrication of polycrystalline thin fi lms by solution processes such as spincoating and ink-jet techniques instead of costly vacuum evaporation techniques. In fact, solution-processed polycrystalline thin fi lms from precursor materials, [ 1 , 2 ] such as 6,13-bis(triisopropylsilylethynyl)-pentacene, [ 3 ] and alkylated oligothiophene, [ 4–6 ] hexabenzocoronene, [ 7 ] and benzothienobenzothiophene (BTBT) [ 8 ] derivatives, attained high fi eld-effect transistor (FET) mobility over 1 cm 2 V − 1 s − 1 comparable to that of vacuum evaporated fi lms of pentacene. [ 9 ]

Liquid crystalline thin films as a precursor for polycrystalline thin films aimed at field effect transistors

We have investigated the availability of liquid crystalline thin films as a precursor of quality polycrystalline thin films for organic field effect transistors (OFETs) by solution process. The experiments of spin-coating at various temperatures proved that the morphology of resulting polycrystalline thin films at room temperature strongly depend on what the films immediately after spin-coating and solvent evaporation at those temperatures were, i.e., crystal, liquid crystal, or isotropic liquid. The polycrystalline films spin-coated at the temperatures for crystal and isotropic liquid phases were much roughened owing to many small crystal flakes and droplets. On the other hand, the thin films spin-coated at liquid crystal temperatures were very uniform in a whole area and showed high FET mobility having a small variation from device to device.

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.

Bulk mobility of polycrystalline thin films of quaterthiophene derivatives

Although polycrystalline thin films of organic materials are applied for organic field effect transistors, their bulk electrical properties have not been characterized. In this study, we investigated hole transport properties in the polycrystalline thin films of a series of quaterthiophene derivatives by both time-of-flight experiments and field effect measurements. The time-of-flight bulk mobility was almost same or smaller than field effect mobility. Both mobilities showed a similar trend as a function of side chain length. We discussed these results from the view point of the intrinsic nature of quaterthiophenes, which gave a fundamental knowledge of charge transport properties in the organic polycrystalline thin films.

Charge Carrier Transport Properties in Liquid Crystalline 2-Phenylbenzothiazole Derivatives

We synthesized liquid crystalline 2-phenylbenzothiazole derivatives and investigated their charge carrier transport properties by time-of-flight experiments. These materials show less ordered phases such as smectic A, smectic C, and nematic phases at temperature range lower than 100°C, and their mobility was relatively small, from 10−5 cm2/Vs to 10−4 cm2/Vs. In addition, the mobility depends on temperature, while it did not depend on electric fields. According to these results, we estimated the eneregy distribution of density of states responsible for condition, σ of Gaussian width, to be 78 ∼ 118 meV.

Polycrystalline Organic TFT Fabricated by Solution Process Using Liquid Crystalline Material

We have fabricated polycrystalline thin film transistors (TFTs) with a liquid crystalline material i.e., ω,ω′-dihexylquaterthipohene by solution process. The TFTs exhibited p-channel performance and its mobility was determined to be 0.04 cm2/Vs, which was comparable to that determined by time-of-flight experiments. We discuss that how “liquid crystallinity” helps fabricating uniform thin films on substrates by spin-coating and controlling grain boundaries not to across the conduction channels formed by self-aligned π-conjugated aromatic cores in liquid crystalline molecules, and conclude that liquid crystalline material is a good candidate for quality polycrystalline thin films for organic TFTs.

Liquid Crystalline Materials for Organic Polycrystalline Field Effect Transistors

We have investigated features of liquid crystalline materials for organic polycrystalline field effect transistors (OFETs) with a model liquid crystalline material of 5,5″-dioctylterthiophene (8-TTP-8). Bulk mobility in polycrystalline thin films of 8-TTP-8 via its liquid crystalline phase was easily determined by time-of-flight experiments. Furthermore, we could evaluate impurity contamination that caused electrically active trap states in the films by analysis of transient photocurrents in liquid crystal phase. We could fabricate uniform and less defective polycrystalline thin films for OFETs by spin-coating its solution at liquid crystal temperatures, with which we fabricated OFET exhibited high mobility of 0.18 cm2/Vs.

High Uniformity and High Thermal Stability of Solution-Processed Polycrystalline Thin Films by Utilizing Highly Ordered Smectic Liquid Crystals

We have investigated the fabrication of polycrystalline thin films of a liquid crystalline benzothienobenzothiophene derivative, i.e., 2-octylthienyl-benzothienobenzothiophene having a highly ordered smectic liquid crystal phase (SmE), the thermal stability of the films, and the performance of their field effect transistors (FETs). The polycrystalline thin films spin-coated at a temperature of SmE phase exhibited uniform textures and were molecularly flat. The FETs after solvent vapor anneal showed the high FET mobility of 1.1 cm2 V-1 s-1, while as-fabricated FETs did the low FET mobility of ca. 10-3 cm2 V-1 s-1. In addition, the thermal stability of the films improved up to 180 °C without melting. After thermal stress at 150 °C for 5 min, the FETs exhibited low mobility as in the as-fabricated FETs but they recovered to the high FET mobility of 1.0 cm2 V-1 s-1 after additional anneals. We compared these results with those of dialkyl benzothienobenzothiophene derivatives and discussed the role of highly ordered liquid crystal phase.

Charge Injection Enhanced by Guest Material in Molecularly Doped Liquid Crystalline Thin Films

We have investigated the charge injection into a host–guest type of molecularly doped liquid crystalline thin film in liquid crystal cells. A 2-phenylnaphthalene smectic liquid crystal of 2-(4-octylphenyl)-6-dodecyloxynaphthalene was used as a host material and a diketopyrrolopyrrole derivative of 1,4-diketo-N,N -dimethyl-3,6-bis(4-dodecyloxyphenyl) pyrrolo[3,4-c]pyrrole was used as a dopant material: the current–voltage characteristics of the thin films with Pt, indium–tin oxide (ITO), MgAg, and Al electrodes were measured and analyzed on the basis of the Schottky mechanism. By comparing the current–voltage characteristics, we determined the majority carriers dominating the current and estimated the barrier height for electrons and holes. We found that the barrier height was smaller than the energy difference between energy levels of the host liquid crystalline material and the work functions of electrode materials. We concluded that the current in the host–guest thin film was dominated by the guest material that has a narrow energy gap and is responsible for the charge injection into the host material.