WO2003023506A2

WO2003023506A2 - Aligned discotic liquid crystals and their applications

Info

Publication number: WO2003023506A2
Application number: PCT/GB2002/004180
Authority: WO
Inventors: Anick Van De Craats; Natalie Stutzmann; Richard Henry Friend; Henning Sirringhaus
Original assignee: Cambridge University Technical Services Limited
Priority date: 2001-09-12
Filing date: 2002-09-12
Publication date: 2003-03-20
Also published as: WO2003023506A3; AU2002324190A1; GB0122022D0

Abstract

A method for forming an electronic device having a semiconductor active layer comprising a discotic conjugated molecule, the method comprising aligning the columnar stacks of the discotic molecules parallel to the direction of current flow in the electronic device.

Description

ALIGNED DISCOTIC LIQUID CRYSTALS FOR DEVICES

This invention relates to discotic liquid crystals, especially discotic liquid crystals suitable for use in devices such as organic thin film transistors, and methods of aligning discotic liquid crystal molecules.

Semiconducting organic field-effect transistors (FETs) have potential applications as key elements of integrated logic circuits (C. Drury, et al., APL 73, 108 (1998)) and optoelectronic devices (H. Sirringhaus, et al., Science 280, 1741 (1998)) based on solution processing on flexible plastic substrates. One main criterion to obtain high charge carrier mobilities has been found to be a high degree of structural order in the active semiconducting polymer.

For some semiconducting conjugated polymers it is known to be possible to induce uniaxial alignment of the polymer chains in thin films by using processing techniques such as Langmuir-Blodgett (LB) deposition (R. Silerova, Chem. Mater. 10, 2284 (1998)), stretch alignment (D. Bradley, J. Phys. D 20, 1389 (1987)), or rubbing of the conjugated polymer film (M. Hamaguchi, et al., Appl. Phys. Lett. 67, 3381 (1995)). Polymer FET devices have been fabricated with uniaxially aligned polymer films fabricated by stretch alignment ((P. Dyreklev, et al., Solid State Communications 82, 317 (1992)) and LB deposition (J. Paloheimo, et al., Thin Solid Films 210/21 1 , 283 (1992)). However, the field-effect mobilities in these studies have been low (< 10^"5 cm²/Vs). Some conjugated polymers and small molecules exhibit liquid-crystalline (LC) phases. By definition, a liquid-crystalline phase is a state of matter, in which the molecules have a preferential orientation in space. This alignment is conventionally regarded as being alignment with respect to a vector called the director. Unlike in the solid, crystalline state the positions of the molecules in the LC phase are randomly distributed in at least one direction. Depending on the type of orientational and residual positional order one distinguishes between nematic, cholesteric and smectic LC phases. The nematic phase possesses long- range orientational order but no positional order. Smectic phases are characterized by a two-dimensional (2D) layered structure, in which the molecules self-assemble into a stack of layers each with a uniform orientation of the molecules with respect to the layer normal, but either no positional order or a reduced degree of positional order in the 2D layers. LC phases occur mainly in polymers / molecules with a significant shape anisotropy. Examples of conjugated LC polymers are main-chain polymers with a rigid-rod conjugated backbone and short flexible side chains, so-called hairy-rod or rigid-rod polymers. Examples are poly-alkyl-fluorenes (M. Grell, et al., Adv. Mat. 9, 798 (1998)) or ladder-type poly- paraphenylenes (U. Scherf, et al., Makromol. Chem., Rapid. Commun. 12, 489 (1991 )). Another type of LC polymers are side-chain polymers with a flexible non- conjugated backbone and rigid conjugated units in the side chains.

A special class of liquid-crystalline organic molecules are disc-shaped molecules with a rigid 2D conjugated core and flexible side chains such as hexabenzocoronenes (HBC) (P. Herwig, et al., Adv. Mater. 8, 510 (1996)) or triphenylenes (D. Adam, et al. Nature 371 , 141 (1994)) (Fig. 1 ). Discotic liquid

crystals are known to self-assemble in columnar aggregates with strong π-π

intermolecular interactions, which leads to the formation of an efficient one- dimensional pathway for electronic charge. High charge carrier mobilities on the order of 0.2 - 1 cm²V^"1s^"1 along the columns have been inferred previously from pulse radiolysis experiments on polycrystalline solids of substituted hexa-per/^'- hexabenzocoronenes (HBCs) which form discotic liquid crystalline phases that are thermodynamically stable over large temperature regions [Van de Craats, et. al. Adv. Mater. 11 , 1999, 1469].

Alignment of LC polymers and nematic small molecules as used in liquid crystal display devices has been investigated intensively. These molecules can be uniaxially aligned on top of suitable alignment layers. In an aligned sample the orientation of the director, that is, for example, the preferential orientation of the polymer chains in a main-chain LC polymer, is uniform over a macroscopic

distance of > μm-mm. This is the scale of practical channel lengths in FET

devices. Alignment can be induced by shear forces or flow or by depositing the LC polymer onto a substrate with an alignment layer exhibiting a uniaxial anisotropy in the plane of the substrate. The alignment layer may be a mechanically rubbed organic layer such as polyimide (M. Grell, et al., Adv. Mat. 9, 798 (1998)), a layer evaporated at an oblique angle onto the substrate, or a layer with a grooved surface. For a review of the various techniques which can be used to align LC molecules see for example, J. Cognard, J. Molec. Cryst. Liq. Cryst. Suppl. Ser. 1 , 1 (1982). Much less is known about the alignment of discotic liquid crystals (Mori, Hiroyuki, European patent application 94116645.6; Kamada, et al., European patent application 94114956.9.). The alignment mechanisms for a disc-shaped discotic molecule is considered to be very different from that of an elongated rigid-rod small nematic molecule. For a planar TFT device it is desirable that the orientation of the discotic LC is such that the columnar stacks are lying in the plane of the substrate, in which the current is transported. Ideally the stacks should be aligned uniaxially along the direction of current transport in order to make optimum use of

the fast charge transport along the π-stacked molecular columns. The more

frequently carriers have to hop from one column to a neighbouring column across the potential barrier of the insulating alkyl chains of the discotic molecule on their way between source and drain electrodes of the TFT the lower the charge carrier mobility will be.

PCT/GB00/02404 discloses an electronic device that comprises a semiconducting layer of an aligned liquid crystalline conjugated polymer or an aligned discotic conjugated molecule. The methods disclosed in PCT/GB00/02404 in order to produce such aligned layers of discotic conjugated molecules are based on conventional techniques to align liquid crystals such as depositing a layer of the liquid crystal onto a rubbed polymer alignment substrate such as polyimide or polyvinylalcohol (see for example, European patent application 94114956.9), and bringing such films into their thermotropic liquid crystalline phase by annealing at temperatures of typically 100-200 °C. Here we disclose a novel alternative deposition method that allows the fabrication of highly oriented films of discotic conjugated molecules for use in such devices. In contrast to conventional liquid crystal alignment techniques the method is based on the templated growth of discotic conjugated molecules from solution. The key feature of the technique is the adsorption of molecules from a solution onto a substrate capable of inducing alignment of the discotic molecules. A well- defined orientation of the discotic molecules on the substrate is induced by the atomic or molecular structure of the surface of the substrate. The discotic molecules may also be in lattice-matched, epitaxial relationship with respect to the atomic or molecular structure of the substrate surface. Alternatively, a well-defined orientation of the discotic molecules on the substrate can be induced by their interaction with a regular array of topographical features on the substrate (graphoepitaxy). A solid, highly oriented film grows at the interface between the templating substrate and the solution. By formulating the solution in a solvent in which the discotic conjugated molecule has a low solubility, and by controlling the temperature of the solution and the substrate homogeneous nucleation of the discotic molecules in the bulk of the solution can be prevented in favour of the templated adsorption of molecules from solution onto the substrate. Also, use of high-boiling temperature solvents are favourable to obtain better control of the precipitation of the discotic materials due to slow evaporation of such solvents. In this way we have achieved highly oriented growth of discotic liquid crystals films. After the solution growth the order and uniaxial alignment of the film may be improved further by an additional step of bringing the discotic film into the meso- phase, or even isotropic melt. Under suitable conditions our alignment technique results in films in which the discotic columns are aligned in the plane of the substrate. This is the ideal orientation for incorporation into field-effect transistor devices, in which the current transport is in the plane of the substrate, i.e. preferentially along the direction of the discotic columns. The molecular architecture within these films is ideally suited to achieve high charge carrier mobilities.

However, depending on the surface structure of the substrate and the deposition conditions other orientations, such as a vertical alignment or an oblique alignment of the columns can be induced, which is useful for other electronic device configurations in which current transport is normal to the plane of the substrate, such as vertical channel transistors or light-emitting or photovoltaic diode devices.

The electronic device is suitably a switching device. The electronic device is preferably a transistor, most preferably a thin-film transistor.

The said method may comprise forming source and drain electrodes of the transistor in locations relative to the active layer such that the channel of the transistor is oriented parallel to the alignment direction of the discotic columns.

The method preferably comprises depositing the discotic LC on top of an alignment layer capable of inducing the said alignment of the discotic LC. The method preferably comprises the step of forming the alignment layer. Preferably the columnar alignment of the discotic molecules extends over a

distance / domain size of at least 100 nm, more preferably at least 1 μm, most

preferably at least 10 μm.

It is preferred that the discotic molecules have monodomain, uniaxial alignment over the area of the electronic device. However, performance improvements may already be obtained if the alignment occurs only locally, that is, if the discotic is in a multidomain configuration with several domains with randomly oriented directors located within the active area of the device. In each domain the discotic columns would be aligned uniaxially parallel to the director, when brought into the LC phase. To produce films in a multilayer configuration no alignment layer is needed.

An aspect of the present invention also provides a logic circuit comprising a transistor as set out above. Such a logic circuit may also include at least one optical device. An aspect of the present invention also provides an active matrix display comprising a transistor as set out above, for example as part of voltage hold circuitry of a pixel of the display.

The present invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 shows molecular structures of the hexa-per/^'-hexabenzocoronene derivatives studied and a schematic representation of the columnar discotic liquid crystalline phase, Dh. The arrow along the columnar axis indicates the coaxially- insulated, conductive pathway provided by the aromatic cores of neighbouring discotic molecules within a columnar aggregate;

Figure 2 shows optical micrographs taken under crossed polarizers of HBC-PhC12 and HBC-C8.2 on PTFE-coated glass substrates. Highest transmitted light intensities were detected when the HBC films were positioned such that the PTFE alignment direction was at +/- 45° to the polarizers (left panels), and total extinction was found parallel and perpendicular to them (right panels);

Figure 3: shows electron diffraction patterns of the HBC films. The arrows indicate the alignment direction of the oriented PTFE chains and the columnar stacks.

Figure 4 shows columnar arrangement within the thin HBC films on PTFE- coated substrates.

Figure 5 shows optical micrographs of HBC-PhC12 films deposited onto rubbed polyimide (left) and PTFE alignment layers (right) from a xylene solution.

Figure 6 shows X-ray diffraction measurements on an aligned film of HBC- C8,2 on a PTFE alignment layer (a), and three different epitaxial (b,c) and graphoepitaxial (d) orientations of the HBC lattice with respect to the PTFE substrate.

Figure 7 shows device configurations for an organic field-effect transistor based on a uniaxially aligned discotic liquid crystal active semiconducting layer. Top gate configurations (a) and (b) as well as bottom gate configurations (c) and (d) are shown. In (c) and (d) the alignment layer is used as the dielectric layer as well. Figure 8: Transfer (left graph) and output characteristics (right graph) of a FETs of HBC-C8.2 on a PTFE-coated silicon/silicon dioxide transistor substrate.

Hexa-pe/7-hexabenzocoronenes (HBCs) belong to the family of discotic liquid crystals that are characterised by a flat aromatic core to which several alkyl chains are attached. The aromatic core of HBC consists of 13 fused six-membered carbon rings forming a flat, disk-like macrocycle with a diameter of ca. 11 A. When this aromatic macrocycle is peripherally substituted with long alkyl chains, the molecules self-assemble into columnar aggregates and form discotic liquid crystalline phases. In the liquid crystal phase, termed D_h, the side chains are effectively molten but the cores remain ordered in columnar stacks. The macrocycles are stacked orthogonally to the columnar axis and the columns are arranged in a two-dimensional hexagonal lattice as shown in Figure 1.

The hexabenzocoronene compounds that we studied in the present work are an alkyl-phenyl derivative, termed HBC-PhC12 (IUPAC name: hexa(3,7-dimethyl- octanyl)hexa-per/^'-hexabenzocoronene), and a molecule with branched alkyl side chains, HBC-C8.2 (IUPAC name: hexa(4-dodecyl-phenyl)hexa-pet7- hexabenzocoronene), see Figure 1. Despite the structural similarities of the two hexabenzocoronenes studied in the present work their crystallization/alignment characteristics seem likely to rest on principally different mechanisms since, at room temperature, the alkyl-phenyl derivative HBC-PhC12 forms a liquid- crystalline phase, denoted D_h, in which the side chains are effectively molten but the cores remain ordered in columnar stacks (Fig. 1 B), whereas the HBC-C8.2 with its branched alkyl chains is a polycrystalline solid (K-phase) and only enters

the D_h-mesophase at approximately 81 °C. For both compounds decomposition

occurs above ca. 300 °C without the formation of an isotropic liquid phase.

We show here uniaxial alignment of discotic liquid crystalline material that forms a polycrystalline solid phase at room temperature, which converts into a discotic liquid crystalline phase at ca. 80 °C. A similar alignment procedure is applicable for uniaxial alignment of a discotic liquid crystalline material that has a thermodynamically stable D_h phase at room temperature.

As an alignment layer for HBC we used highly oriented thin films of poly(tetrafluororethylene) (PTFE) on glass that were fabricated by the friction- transfer method described previously [Wittmann, Smith, Nature 352, 414 (1991 )]. PTFE-coated silicon or silicon/silicon oxide wavers are also applicable. Also, the PTFE layer may be coated with a thin addition layer, such as poly(p-xylylene) (PPX), or polyethylene (PE) for changing the surface properties (e.g. hydrophobicity) and/or interaction with the material to be adsorbed. Friction- transferred oriented PTFE layers have been used previously for the alignment of rigid rod polymers as well as nematic and smectic small molecule liquid crystals (US 5,180,470). However, alignment of discotic LCs has not been demonstrated previously using friction-transferred PTFE. PTFE alignment layers have also been used in TFT devices (US 5912473). However, alignment in this case was found to be limited to elongated conjugated rigid rod molecules such as thiophene and phenylene vinylene oligomers and polymers. Alignment of discotic LCs was not demonstrated. As discussed above the alignment mechanisms for a disc-shaped discotic molecule is very different from that of an elongated rigid-rod small nematic molecule.

The PTFE polymer layers consist of highly oriented polycrystalline films of a few tens of nanometers thickness. Detailed studies have shown that these substrates exhibit a (0 1 0) single-crystal orientation. The surface consists of a plane of parallel polymer chains with an interchain distance of ca. 5 A. These substrates were used as an alignment layer for the liquid crystalline HBCs.

Highly oriented films of HBC-C8.2 and HBC-PhC12 were obtained by solution- casting onto poly(tetrafluororethylene)-coated substrates in ambient atmosphere. Polarized micrographs (see Figure 2) show uniform and highly birefringent films indicating a high degree of orientation of the HBC molecules. The homogeneous monodomains formed extend over more than hundreds of micrometers, and even centimeters in the case of HBC-C8,2 [Note that the PTFE film thickness is in the range of nanometers (ca. 40 nm) and therefore does not reveal any birefringence.] The strong influence of the PTFE template on the molecular alignment of HBC-PhC12 is clearly shown by the randomly oriented morphologies when the alignment layer is absent, as in the top left corner of the top left micrograph in Figure 2. For HBC-C8,2 thin films have been found that consist of uniformly aligned, oriented crystalites. Crystal growth of HBC-C8,2 is favoured on the PTFE-coated glass slide. Under the growth conditions used in this experiment, no deposition of the HBC molecules occurs on the bare glass substrate, as can be inferred from the lower right corner of the film depicted in the bottom left micrograph in Figure 2. The absence of any growth on the bare glass regions of the substrate is direct evidence that the growth of the film occurs by adsorption of HBC molecules from solution onto those regions of the substrate which allow a favourable surface interaction with the HBC molecules.

A uniaxially aligned thin film of HBC-PhC12 was produced by slowly evaporating a droplet of a 20 mg/ml p-xylene solution on a PTFE-coated substrate in a saturated atmosphere of xylene vapour. Using a 4 to 7 mg/ml HBC-Ph-C12 solution in

isodurene (tetramethylbenzene), a high-boiling temperature solvent (bp « 191 °C

compared to 135 °C for xylene), evaporation occurred slow enough rendering a drying in saturated atmosphere unnecessary. For HBC-C8,2 an aligned thin film was achieved by drying a droplet between two PTFE friction transfer coated glass slides. A ca. 4 to 9 mg/ml cyclohexanone solution was used for the latter.

The observed electron diffraction patterns of HBC-PhC12 and HBC-C8.2 are given in Figure 3. Well-defined, arc-shaped reflections have been found for HBC- PhC12, indicating a well-oriented film with an intracolumnar stacking distance of 3.5 A. For HBC-C8.2 a rich and sharp diffraction pattern is found with intra- and inter-columnar distances of 5.1 A and 19.5 A, respectively. For both HBC derivatives similar electron diffraction patterns were found throughout the entire film, which shows that the molecular ordering is not restricted to a sub-micrometer domain in a small region of the film. From the diffraction spots observed on the meridian, it appears likely that the orientation of the hexabenzocoronene is such that the columnar stacks lie parallel to the polymer chains, with the HBC macrocycles aligned orthogonally to the latter. From the half width at half maximum intensity of the 3.5 A -reflections, a maximum misalignment in the orientation of these supramolecular discotic columns was estimated to be around 11° to 13°. For the HBC-C8,2 films, a well-defined, rich diffraction pattern, typical of highly oriented, crystalline structures was obtained. We assign the present diffraction pattern to the [0 1 0] zone of the present hexabenzocoronene crystals; which would suggest that the columnar assemblies are viewed from the side with the discotic stacks being parallel to the substrate plane. Moreover, the HBC-C8.2 columns appear to be aligned in direction of the PTFE chains, as the 1.3 A - reflections of the oriented PTFE film are positioned perpendicular to the direction of the (h 0 1 ) reflections. The latter correspond to a 5.1 A - distance, which we assign to the stacking distance between neighboring hexabenzocronene cores within a columnar stack. Thus, the discs are probably tilted by 47° with respect to the column axis. Also, a hexagonal configuration of a unit cell distance of 39 A appears to be likely, which can be associated to a herring-bone structure with an intercolumnar distance between adjacent columns of opposite tilt of 19.5 A. From these structural investigations it can be inferred that in both cases the columnar stacks are oriented parallel to the underlying PTFE polymer chains as shows in Figure 4.

Highly oriented thin films of discotic liquid crystalline materials have been obtained by controlled molecular organisation from solution onto a crystalline polymer alignment layer. In the resulting, uniaxially aligned films, the columnar assemblies have been found to lie parallel to the substrate surface along the alignment direction of the polymer chains. This molecular orientation is established throughout elaborate monodomains which extend over hundreds of micrometers. The uniaxially aligned discotic liquid crystalline films have been used as the active semiconductive layer in organic field-effect transistors with the polymer alignment layer and an additional inorganic film as the insulating component of the device.

In order to elucidate the mechanism of directed crystal growth of the crystalline HBC compound, surface X-ray diffraction (SXRD) studies were undertaken (Fig. 6a). Evidence for meso-epitaxy, i.e. a combination of molecular epitaxy (commensurate growth) and surface-topology-induced epitaxy (graphoepitaxy) on the PTFE layer has been gathered. Various strong Bragg-diffraction peaks were recorded with half-width-at-half-maximum values of less than 1.5°; indicative of the high orientational order of this solution-processed, organic film. Three orientations of a quasi-hexagonal structure, denoted HBC-C8,2₀°, HBC-C8,2₁₉.r, and HBC-C8,2_{35 2}° (respectively in Fig. 6b, c, and d) have been found to coexist. The differences between these phases seem to lie in the arrangement with respect to the polymer alignment layer, which is probably related to dissimilar growth mechanisms. In all phases two different types of columnar stacks constitute a unit cell. This apparent inequivalence of the two columns in a unit cell is indicated in Figure 6b-d by different grey scales, and may possibly originate in opposite tilts of the hexabenzocoronene cores with respect to the columnar axes or the existence of glide-symmetry in the unit cell. The most abundant phases, HBC-C8,2₀» and HBC-C8,2ιg_.r, have only a small structural mismatch with the PTFE template of ca. 5 and 1 % respectively, and can therefore be assumed to have been formed by commensurate growth or molecular epitaxy. This film formation mechanism is generally assumed to be dominant for polymeric materials when lattice dimensions of the template and epilayer structure match within 15 %. For HBC-C8₀°, the intercolumnar distance, L, of about 19.8 A can adapt to a distance that corresponds to six PTFE chain-to-chain distances (Fig 6b). With a small tilt angle as in the HBC-C8,2ιg.r phase another favourable orientation can be achieved. Here, the (1 0 5)-plane is parallel to the surface with a repeat distance along the surface which matches nine PTFE chain-to-chain distances (Fig. 6c). In the third phase, HBC-C8,2_{35 2}«, which is much less abundant, the lattice makes a small angle of 5.2° with the substrate plane. We propose that this structure is formed via nucleation induced at macro-steps in the surface topology (graphoepitaxy) (Fig. 6(d)); a crystal growth mechanism previously found to occur on friction-transferred PTFE layers. Importantly, no reflections indicative of columnar stacks perpendicular to either the polymer molecules or to the substrate surface have been found in the HBC film analysed. This seems to imply that the HBC columns run parallel to the PTFE chains throughout the entire film.

Field-effect transistors (FETs) were assembled by depositing thin, aligned HBC films on PTFE-coated Si/Si0₂ wafers. The basic structure of a top-gate FET (Figure 7b) consists of a substrate 1 , comprising an alignment layer 2, and source-drain electrode 3. The conjugated discotic molecules 4 are deposited in contact with the source-drain electrodes and the alignment layer. An electrically insulating layer 5 acting as the gate dielectric and a gate electrode 6 are deposited on top of the discotic layer. Alternative top- and bottom gate configurations that can be used instead are depicted in Fig. 7.

In one embodiment of the invention a bottom gate FET was fabricated by shadow- mask evaporating gold source-drain electrodes on top of an HBC films deposited on top of a highly doped silicon wafer containing a 200 nm thick layer of Si0 , and a layer of friction transferred teflon. In Figure 7(c) the device configuration is depicted schematically. The Si0₂/PTFE layer structure acts as a double layer gate dielectric. The performance of a device based on a uniaxially aligned HBC-C8.2 film is shown in Figure 8. Devices prepared in ambient atmosphere show stable transistors with a clear field-effect for hole transport.

Approximately 40 devices with the channel parallel to the conductive, columnar pathway were tested and yielded ON/OFF ratios on the order of ca. 10⁴, FET mobilities of 0.5 - 1.0x10^~3 cm²/Vs (derived from the saturated regime), and a turn-on voltage of ca. -6 V. On comparing these devices to ones in which the channel is perpendicular to the alignment direction, an anisotropy in the source- drain current of more than two orders of magnitude has been observed (cf. curves I (channel along columns, source-drain voltage —40V), II (channel along columns, source-drain voltage -5V) and III (channel perpendicular to columns, source-drain voltage -40V) in Fig. 8a). However, this anisotropy does not necessarily reflect the unidirectional transport of charge within an aligned domain. Perpendicular to the alignment direction, ledges and voids are present in the PTFE film which are likely to retard charge transport significantly, because they will act as barriers or traps for charge carrier motion. Therefore, the observed anisotropy is thought to be mainly the result of structural irregularities in the film, rather than due to intrinsic, anisotropic charge transport properties within ordered domains.

For an isotropic, non-aligned HBC-C8,2 film generated by spin coating on oriented PTFE, significantly lower FET mobilities have been detected (curve IV (source- drain voltage -40V) in Fig. 8a). These results show that aligning the columnar aggregates is not simply invoked by the presence of an oriented PTFE layer. Additional processing conditions, such as slow evaporation of the solvent and a delicate control over heterogeneous nucleation at the interface as opposed to spontaneous aggregation in solution, are thought to be essential to allow an aligned HBC layer to develop.

We found that one of the important factors for the alignment of discotic LCs by solution deposition is the careful control over the adsorption and/or heterogeneous nucleation of the molecules at the interface with the alignment layer as opposed to spontaneous aggregation in solution. In general, it is considered advantageous if there is a strong energetically favourable interaction between the discotic molecules and the surface of the aligning substrate. The goal is to attract the discotic molecules towards the interface, and in this way induce an oriented, templated film growth. It is important to suppress as much as possible spontaneous nucleation/aggregation in solution, which would give rise to random orientation of crystallites on the substrate without uniaxial alignment.

In the films grown by the method disclosed here the discotic molecules have a defined orientation with respect to the surface structure of the alignment layer on the substrate. In most of the film the molecules arrange themselves in columnar stacks that are aligned uniaxially with respect to the surface structure of the alignment layer. In the case of HBC on PTFE there is a coexistence of several possible epitaxial or graphoepitaxial relationships of the HBC lattice with respect to the PTFE lattice, all of which have the columnar stacks aligned along the PTFE chains. In general, it is desirable to choose growth conditions such that one of these orientations dominates. The degree of orientation of the discotic film can be defined, for example, by considering the volume fraction of molecules, that are either part of a columnar stack that is misaligned by more than 40° with respect to the alignment direction imposed by substrate, or part of residual amorphous regions of the film. We consider the molecules of the film to have a defined orientation if this volume fraction is less than 50%. The volume fraction of misoriented molecules can be determined by analysis of X-ray or electron diffraction patterns of the film by comparing the intensity of diffraction peaks due to misaligned grains or amorphous regions with the intensity of diffraction peaks of aligned regions. The following methods can be used to achieve such favourable interface interaction. For each particular alignment layer/discotic LC combination the various factors may be acting in conjunction:

- Epitaxy: The alignment layer has preferably a highly crystalline aligned molecular structure. This allows the discotic molecule to deposit onto the alignment layer from solution with a coherent epitiaxial interface structure. Such form of molecular epitaxy is a crucial factor in achieving the alignment. Fig. 5 shows optical microscopy of HBC-PhC12 grown from p-xylene solution under identical conditions on top of a friction-transferred PTFE substrate and a rubbed polyimide substrate. The latter is a standard alignment substrate that imparts very efficient alignment onto most small molecule nematic LCs, but does not exhibit a highly crystalline surface structure. Under the conditions used in this experiment the HBC film on PTFE shows good uniaxial alignment, whereas on the polyimide substrate randomly oriented polycrystalline grains without preferential orientation are observed.

- Graphoepitaxy: The alignment layer may exhibit a regular topographic surface structure, such that interface energy can be minimized in a film growth mode in which the discotic columns adopt a well-defined orientation with respect to the topographic structure. A regular array of topographical features can be generated, for example, by a rubbing or friction-transfer process or by embossing a hard master containing an array of protruding features into the substrate. The master may contain an array of protruding parallel lines or wedges generating an array of parallel grooves on the substrate. The periodicity of the lines

is preferably less than 5μm, most preferably less than 1μrn. On such

a surface graphoepitaxial growth results in an aligned discotic film with orientation of the discotic columns along the grooves. Graphoepitaxial growth may also be initiated by sandwiching the solution of the discotic conjugated molecule in between the first substrate onto which the film is to be deposited, and a second substrate that contains a regular array of topographic features, such as an array of parallel wedges or grooves. The second substrate is preferably a substrate with a less favourable surface structure in respect of interaction with the discotic molecules, such that the deposition of the film occurs primarily onto the first substrate.

However, if the second substrate is brought in close proximity or contact with the first substrate during the growth of the film from solution, the discotic colums deposited on the first substrate align along the direction the topographic structures of the second substrate.

After the growth of the film the second substrate can be removed from the first substrate. The second substrate is made ideally from a flexible material that conforms well to the surface of the first substrate.

In order to accelerate the deposition process the second substrate may also have a finite permeability for the solvent of the solution to diffuse and evaporate through the second substrate. A possible material for the second substrate is polydimethylsiloxane (PDMS) with a surface relief fabricated by techniques analogous to techniques used for soft lithographic patterning (see for example, Y. Xia et al., Angewandte Chemie, Int. Ed. 37, 551 (1998)). This method can be used in conjunction with the deposition techniques described above.

- Density of nucelation sites: The alignment layer may contain a high density of nucleation sites that facilitate nucleation of the discotic molecules at the interface. These nucleation sites may be associated with a topographic structure, for example structural grooves, or sites associated with surface defects of a chemical, physical or structural nature.

- Surface energy: The alignment layer has preferably a low surface energy that allows formation of a low-energy interface structure with in-plane orientation of the discotic molecules, in which the flexible side chains attached to the discotic LC are in contact with the alignment layer. Alignment layers with a fluorinated or alkyl- terminated surface are particularly useful for a broad range of alkylated discotic LCs, such as HBCs or triphenylenes.

- Deposition from poor solvents: Deposition is preferably from a solvent in which the discotic LC is poorly or only moderately soluble. If a good solvent is used, the discotic molecules would favour intimate contact with the surrounding solvent molecules. In a moderate or poor solvent deposition at the interface is more favourable than interaction with surrounding solvent molecules. In the case of HBC-C8 cyclohexanone has been found to be a particularly suitable solvent. HBC-C8 is not soluble in cyclohexanone at room temperature. The solutions need to be heated to temperatures on the order of 50°C in order to dissolve the HBC molecules. The solubility requirement can be quantified using the Hildebrand solubility parameters (D.W. van Krevelen, Properties of polymers, Elsevier, Amsterdam (1990))

- A method that is particularly suitable in order to control the rate of heterogeneous nucleation at the interface with the aligning template is the deposition at elevated temperature. Organic solvents with a reasonably high boiling point higher than 130°C C, and preferably higher than 150°C are to be used, e.g. p-xylene and cyclohexanone. By carefully controlling the temperature of the substrate/solution adsorption/nucleation at the surface/solvent interface can be controlled and aggregation in solution can be suppressed. If the temperature of the substrate/solution is below a critical temperature Ti nucleation will occur in solution. If the temperature of the substrate/solution is higher than a critical temperature T₂ no heterogeneous nucleation at the interface will occur any more, and no film growth is observed. The aligning substrate and the solvent for deposition should be chosen such that T₂ is larger than T-i, and that the available growth window between temperatures Ti and T₂ is as wide as possible.

- Other factors that promote the growth of oriented films are the use of a saturated atmosphere of the solvent during drying of the solution- casted films and/or the use of very high boiling point solvents, such as isodurene, to allow for slow evaporation rates. This can be achieved for example by placing the solution in between the alignment substrate and a second substrate the main function of which is to reduce the evaporation rate. Preferably, the second substrate can be chosen such that no growth occurs on it, or that it adsorbs the material, too. In the case of HBC growth on a PTFE alignment layer, a bare glass substrate or silicon coated with a thin silicon oxide layer has been used successfully. In order to fabricate homogeneous films that are continuous both in the direction parallel and perpendicular to the aligning polymer chains, concentrations of at least 3 mg/ml are required; this could however vary with the compound used.

One combination of alignment layer and discotic LCs that exhibits a particularly low energy interface structure, and is therefore a preferred embodiment of the present invention, is a PTFE alignment layer and a discotic LC with fluoroalkyl side chains, such as a fluoroalkyl-substituted HBC.

The alignment process for discotic LC TFTs described in this invention is compatible with a broad range of TFT fabrication methods, such as the inkjet fabrication methods for all organic TFTs described in UK 0009915.0. In particular, deposition and patterning of the electrodes by direct ink-jet printing of a conducting polymer such as polyethylenedioxythiophene doped with polystyrene sulfonate (PEDOT/PSS) may be used. In order to allow for efficient hole injection from the source-drain electrodes the LC molecule should have a sufficiently low ionisation potential, preferably below 5.5 eV, that is well matched to the work function of common source-drain electrode materials such as inorganic metal electrodes (gold, platinum, aluminium, etc.) or conducting polymers such as PEDOT.

The alignment process described here can be used with a variety of different device configurations. Some of the possible device configuration are disclosed in Fig. 7.

According to another aspect of the present invention a method is disclosed in which the alignment layer that induces the molecular alignment of the discotic LC can also be used a dielectric layer of the TFT.

In the device shown in Fig. 8 the PTFE alignment layer forms the active interface between the discotic semiconducting layer and the dielectric. Since in the TFT current flow is confined to a few nanometers from the active interface, this structure makes optimum use of the alignment of the semiconducting layer which is expected to be highest at the interface with the alignment layer. However, this configuration requires a low density of interfacial defects that could otherwise act as electronic traps reducing the carrier mobility. Taking into account the crude mechanical method in which the PTFE is deposited, it is remarkable that the PTFE/discotic LC interface is of sufficiently high electronic quality as to sustain high mobility TFT operation. This is partly attributed to the highly crystalline surface structure of the PTFE, as well as to the self-healing capability of the discotic LC in its liquid crystalline phase.

In the present invention we have demonstrated alignment of discotic LCs with LC phases at room temperature, as well as discotic LCs exhibiting a crystalline phase at room temperature. We emphasize that both types of molecules are useful for device applications. In the case of molecules with an LC phase at room temperature care needs to be taken to avoid large electric fields under device operation in order to prevent reorientation of molecules and diffusion of ionic species under the action of an applied electric field. However, it has been shown that high charge carrier mobilities can be achieved in both a crystalline phase as well as a discotic LC phase [Van de Craats, et. al. Adv. Mater. 11 , 1999, 1469].

One of the characteristic features of the deposition method for the discotic LC as disclosed in the present invention is that if the solution of the discotic LC is removed from the substrate after a few minutes prior to evaporation of the solvent a continuous discotic LC film is present on the substrate. If film growth occurred by nucleation in the bulk of the solution this would not necessarily be the case.

Applications of discotic LC TFTs according to this invention are in organic TFT logic circuits (C. Drury, et al., APL 73, 108 (1998)) or as pixel drive transistors in high-resolution, active matrix displays (H. Sirringhaus, et al., Science 280, 1741 (1998)). Examples of such displays are active matrix polymer LED displays, liquid- crystal displays (LCD) or electrophoretic displays. The enhanced charge carrier mobility along the direction of preferential uniaxial alignment of discotic stacks compared to the mobility of the unaligned films can be used to increase the operation speed and the drive current capability of the TFTs.

The present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof, irrespective of whether it relates to the presently claimed invention. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1. A method for forming an oriented film of discotic conjugated molecules wherein the method comprises contacting a solution of said discotic conjugated molecules with a substrate having an oriented surface structure thereon, and growing the film of said discotic conjugated molecules by adsorbing at least some of said discotic conjugated molecules from the solution onto the surface of the substrate in a defined orientation with respect to the surface structure of the substrate.

2. A method for forming an oriented film of a discotic conjugated molecule as claimed in claim 1 wherein the surface structure is an atomic or molecular surface structure of said substrate and said discotic conjugated molecules are adsorbed in a defined orientation with respect to the atomic or molecular surface structure of said substrate.

3. A method for forming an oriented film of a discotic conjugated molecule as claimed in claim 2 in which the substrate has a crystalline or partially crystalline surface structure.

4. A method for forming an oriented film of a discotic conjugated molecule claimed in claim 2 or 3, in which the substrate comprises an alignment layer with an aligned atomic or molecular structure.

5. A method for forming an oriented film of a discotic conjugated molecule as claimed 4 wherein said alignment layer is made of a polymer with an aligned molecular structure.

6. A method for forming an oriented film of a discotic conjugated molecule as claimed in claim 5, in which the method comprises friction transferring said alignment layer to the substrate.

7. A method for forming an oriented film of a discotic conjugated molecule as claimed in claim 6, in which said alignment layer is made of friction transferred teflon.

8. A method for forming an oriented film of a discotic conjugated molecule as claimed in claim 6 and 7, in which said alignment layer is made of a friction transferred polymer coated with at least one aligned molecular layer prior to the deposition of the solution of the discotic conjugated molecules.

9. A method for forming an oriented film of a discotic conjugated molecule as claimed in claim 1 wherein the surface structure comprises an array of topographic structures on the surface of the substrate and said discotic conjugated molecules are adsorbed in a defined orientation with respect to the array of topographic structures on the surface of the substrate.

10. A method as claimed in claim 9, wherein the substrate comprises an alignment layer containing an array of topographic structures.

11. A method as claimed in claim 9 or 10 wherein the method comprises the step of preparing said topographic structures by embossing a master containing a surface relief pattern into the substrate.

12. A method as claimed in claim 9 or 10 wherein the method comprises the step of preparing said topographic structures on the surface of the substrate by mechanical rubbing of the substrate.

13. A method as claimed in any preceding claim wherein the discotic conjugated molecules adsorbed onto the substrate are in the form of columnar stacks aligned along a uniaxial direction.

14. A method as claimed claim 13 wherein said uniaxial direction of alignment of the columnar stacks is essentially in the plane of the substrate surface.

15. A method as claimed in any of claims 1 to 14 wherein the discotic conjugated molecules are deposited from a solution in a solvent in which the solubility of the discotic conjugated molecules at room temperature is less than 10 grams per liter.

16. A method as claimed in any of claims 1 to 14 wherein the discotic conjugated molecules are deposited from a solution in a solvent in which the solubility of the discotic conjugated molecules at room temperature is less than 1 gram per liter.

17. A method as claimed in any of claims 1 to 16, wherein the discotic conjugated molecules are deposited from a solvent with a boiling point higher than 100°C.

18. A method as claimed in any of claims 1 to 16, wherein the discotic conjugated molecules are deposited from a solvent with a boiling point higher than 150°C.

19. A method as claimed in any preceding claim in which during the said adsorption step the substrate is kept at a temperature higher than the critical temperature for homogeneous nucleation of the discotic conjugated molecule in the bulk of said solution, but below the critical temperature for heterogeneous nucleation or adsorption from the solution onto the surface of said substrate.

20. A method as claimed in any preceding claim in which during the said adsorption step the substrate is kept at a temperature higher than 40°C, but below 130°C .

21. A method as claimed in any preceding claim in which during the said adsorption step the free surface of said solution deposited onto the substrate is kept at a temperature that is higher by more than 5°C than the temperature of the substrate.

22. A method as claimed in any preceding claim in which during the said adsorption step the free surface of said solution deposited onto the substrate is kept at a temperature that is higher by more than 10°C than the temperature of the substrate.

23. A method as claimed in any preceding claim in which during the said adsorption step the gas atmosphere above the free surface of the solution is saturated with the solvent from which the discotic conjugated molecules are deposited.

24. A method as claimed in any preceding claim comprising the step of sandwiching the solution of said discotic conjugated molecules between a first substrate onto which said discotic conjugated molecules are adsorbed and a second substrate that is not capable of adsorbing discotic conjugated molecules from solution onto its surface.

25. A method as claimed any of claims 1 to 23 comprising the step of sandwiching the solution of said discotic conjugated molecules between a first substrate onto which said discotic conjugated molecules are adsorbed and a second substrate, that contains a regular array of topographic structures.

26. A method as claimed in claim 24 or 25, wherein the distance between the first and second substrate is less than 10 microns.

27. A method as claimed in claim 24 or 25, wherein the distance between the first and second substrate is less than 1 micron.

28. An electronic device formed by a method claimed in any preceding claim.

29. An electronic device as claimed in claim 28, wherein the electronic device is an electronic switching device comprising a source electrode, a drain electrode, a semiconducting layer of a discotic conjugated molecule in contact with said source and drain electrodes, an electrically insulating layer in contact with said semiconducting layer, and a gate electrode on the opposite side of said electrically insulating layer from said semiconducting layer, wherein said semiconducting layer of discotic conjugated molecules is formed by any of the methods claimed in claims 1 to 27.

30. An electronic switching device as claimed in claim 29 wherein the semiconducting layer is in contact with an alignment layer on the opposite side of said semiconducting layer from said electrically insulating layer.

31. An electronic switching device as claimed in claim 29 wherein the semiconducting layer is in contact with an alignment layer that forms part of said electrically insulating layer.

32. An electronic switching device as claimed in any of claims 30 or 31 , in which said augment layer is made of friction transferred polymer.

33. An electronic switching device comprising a source electrode, a drain electrode, a semiconducting layer of a discotic conjugated molecule in contact with said source and drain electrodes, and a gate electrode forming an electrically rectifying contact with said semiconducting layer, wherein said semiconducting layer of discotic conjugated molecules is formed by any of the methods claimed in claims 1 to 27.

34. An electronic device as claimed in any of claims 28 to 33, wherein said molecules adsorbed onto the substrate are in the form of columnar stacks aligned along the direction of current flow in the device when in use.

35. An electronic device as claimed in any of claims 28 to 34 wherein said discotic conjugated molecules are in the solid state at the operating temperature of the device.

36. An electronic device as claimed in any of claims 28 to 34 wherein said discotic conjugated molecules are in the liquid crystalline state at the operating temperature of the device.

37. A logic circuit, display or memory device formed by any of the methods claimed in claim 1 to 27.