Patent Application
20080035914
a and 1b show current-voltage characteristics of TFTs with N,N′-Bis(2-phenylethyl)perylene-3,4:9,10-bis(dicarboximide) (BPE-PTCDI).
The term “C1-C4-alkyl” embraces straight-chain and branched alkyl groups. These groups are in particular, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl. This applies also to all alkyl moieties in alkoxy, alkylamino, dialkylamino, alkylthio, etc.
C1-C4-alkylene embraces straight-chain and branched hydrocarbon chains with 1 to 4 carbon atoms, in particular CH2, CH2CH2, CH(CH3), CH2CH2CH2, CH(CH3)CH2, CH2CH(CH3), CH2CH2CH2CH2, CH(CH3)CH2CH2, CH2CH(CH3)CH2, CH2CH2CH(CH3), CH(C2H5)CH2, CH2CH(C2H5).
For the purposes of the present invention, the term “cycloalkyl” embraces both substituted and unsubstituted cycloalkyl groups, preferably C3-C8-cycloalkyl groups like cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl or cyclooctyl, in particular C5-C8-cycloalkyl. Substituted cycloalkyl groups can carry, for example, 1, 2, 3, 4, 5 or more than 5 substituents which are preferably selected independently of alkyl and substituents as defined above for “alkyl”. Substituted cycloalkyl groups carry preferably one or more, e.g. 1, 2, 3, 4 or 5, C1-C6-alkyl groups.
Examples of preferred cycloalkyl groups are cyclopropyl, cyclobutyl, cyclopentyl, 2- and 3-methylcyclopentyl, 2- and 3-ethylcyclopentyl, cyclohexyl, 2-, 3- and 4-methylcyclohexyl, 2-, 3- and 4-ethylcyclohexyl, 3- and 4-propylcyclohexyl, 3- and 4-isopropylcyclohexyl, 3- and 4-butylcyclohexyl, 3- and 4-sec.-butylcyclohexyl, 3- and 4-tert.-butylcyclohexyl, cycloheptyl, 2-, 3- and 4-methylcycloheptyl, 2-, 3- and 4-ethylcycloheptyl, 3- and 4-propylcycloheptyl, 3- and 4-isopropylcycloheptyl, 3- and 4-butylcycloheptyl, 3- and 4-sec.-butylcycloheptyl, 3- and 4-tert.-butylcycloheptyl, cyclooctyl, 2-, 3-, 4- and 5-methylcyclooctyl, 2-, 3-, 4- and 5-ethylcyclooctyl, 3-, 4- and 5-propylcyclooctyl.
For the purposes of the present invention, the term “cycloalkenyl” embraces unsubstituted and substituted monounsaturated hydrocarbon groups having 3 to 8, preferably 5 to 6, carbon ring members, such as cyclopenten-1-yl, cyclopenten-3-yl, cyclohexen-1-yl, cyclohexen-3-yl, cyclohexen-4-yl and the like. Suitable substituents for cycloalkenyl are the same as those mentioned above for cycloalkyl.
The term “bicycloalkyl” preferably embraces bicyclic hydrocarbon groups having 5 to 10 carbon atoms such as bicyclo[2.2.1]hept-1-yl, bicyclo[2.2.1]hept-2-yl, bicyclo[2.2.1]hept-7-yl, bicyclo[2.2.2]oct-1-yl, bicyclo[2.2.2]oct-2-yl, bicyclo[3.3.0]octyl, bicyclo[4.4.0]decyl and the like.
For the purposes of the present invention, the term “aryl” embraces monocyclic or polycyclic aromatic hydrocarbon radicals which may be unsubstituted or unsubstituted. Aryl is preferably unsubstituted or substituted phenyl, naphthyl, indenyl, fluorenyl, anthracenyl, phenanthrenyl, naphthacenyl, chrysenyl, pyrenyl, etc., and in particular phenyl or naphthyl. Aryl, when substituted, may carry—depending on the number and size of the ring systems—one or more (e.g. 1, 2, 3, 4, 5 or more than 5) substituents which are preferably selected independently of one another from among alkyl, alkoxy, thioalkyl, cycloalkyl, heterocycloalkyl, aryl, hetaryl, halogen, hydroxy, mercapto, COOH, carboxylate, SO3H, sulfonate, NE1E2, nitro and cyano, where E1 und E2, independently of one another, are hydrogen, alkyl, cycloalkyl, heterocycloalkyl, aryl or hetaryl. Aryl is in particular phenyl which, when substituted, generally may carry 1, 2, 3, 4 or 5, preferably 1, 2 or 3, substituents.
For the purposes of the present invention heterocycloalkyl embraces nonaromatic, unsaturated or fully saturated, cycloaliphatic groups having generally 5 to 8 ring atoms, preferably 5 or 6 ring atoms, in which 1, 2 or 3 of the ring carbon atoms are replaced by heteroatoms selected from oxygen, nitrogen, sulfur, and a group —NR3—, said cycloaliphatic groups further being unsubstituted or substituted by one or more—for example, 1, 2, 3, 4, 5 or 6-C1-C6 alkyl groups. Examples that may be given of such heterocycloaliphatic groups include pyrrolidinyl, piperidinyl, 2,2,6,6-tetramethylpiperidinyl, imidazolidinyl, pyrazolidinyl, oxazolidinyl, morpholidinyl, thiazolidinyl, isothiazolidinyl, isoxazolidinyl, piperazinyl, tetrahydrothiophenyl, dihydrothien-2-yl, tetrahydrofuranyl, dihydrofuran-2-yl, tetrahydropyranyl, 1,2-oxazolin-5-yl, 1,3-oxazolin-2-yl, and dioxanyl.
For the purposes of the present invention heteroaryl embraces substituted or unsubstituted, heteroaromatic, monocyclic or polycyclic groups, preferably the groups pyridyl, quinolinyl, acridinyl, pyridazinyl, pyrimidinyl, pyrazinyl, pyrrolyl, imidazolyl, pyrazolyl, indolyl, purinyl, indazolyl, benzotriazolyl, 1,2,3-triazolyl, 1,3,4-triazolyl, and carbazolyl, which, when substituted, can carry generally 1, 2 or 3 substituents. The substituents are selected from C1-C6 alkyl, C1-C6 alkoxy, hydroxyl, carboxyl, halogen and cyano.
5- to 7-membered heterocycloalkyl or heteroaryl radicals bonded by a nitrogen atom and optionally containing further heteroatoms are, for example, pyrrolyl, pyrazolyl, imidazolyl, triazolyl, pyrrolidinyl, pyrazolinyl, pyrazolidinyl, imidazolinyl, imidazolidinyl, pyridinyl, pyridazinyl, pyrimidinyl, pyrazinyl, triazinyl, piperidinyl, piperazinyl, oxazolyl, isooxazolyl, thiazolyl, isothiazolyl, indolyl, quinolinyl, isoquinolinyl or quinaldinyl.
Halogene is fluorine, chlorine, bromine or iodine.
In a preferred embodiment R1 and R2 are selected from cyclopropane, cyclobutane and cyclopentane.
In a preferred embodiment R1 is selected from CH2—Ra, CH2CH2—Ra, CH2CH2CH2—Ra and CH2CH2CH2CH2—Ra. In a preferred embodiment R2 is selected from CH2—Rb, CH2CH2—Rb, CH2CH2CH2—Rb and CH2CH2CH2CH2—Rb.
Preferably Ra and Rb are selected from
wherein
Preferably, n is 1 or 2.
In a preferred embodiment, R1 and R2 have the same meaning.
Especially preferred are compounds of the formulae:
Step a) of the method for producing an OFET comprises providing a substrate with at least one preformed transistor site located on the substrate. It will be understood that when an element such as a layer, region or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. So e.g. a typical organic thin film transistor comprises a gate electrode on the substrate and a gate insulating layer on the surface of the substrate embedding the gate electrode.
In a special embodiment the substrate comprises a pattern of organic field-effect transistors, each transistor comprising:
In a further special embodiment a substrate comprises a pattern of organic field-effect transistors, each transistor comprising at least one organic semiconducting compound located on the substrate forms an or is part of an integrated circuit, wherein at least part of the transistors comprise at least one compound of the formula (I) as semiconducting compound. Preferably, all of the transistors comprise at least one compound of the formula (I) as semiconducting compound.
Any material suitable for the production of semiconductor devices can be used as the substrate. Suitable substrates include, for example, metals (preferably metals of groups 8, 9, 10 or 11 of the periodic table, e.g. Au, Ag, Cu), oxidic materials (like glass, quartz, ceramics, SiO2), semiconductors (e.g. doped Si, doped Ge), metal alloys (e.g. on the basis of Au, Ag, Cu, etc.), semiconductor alloys, polymers (e.g. polyvinylchloride, polyolefines, like polyethylene and polypropylene, polyesters, fluoropolymers, polyamides, polyurethanes, polyalkyl(meth)acrylates, polystyrene and mixtures and composites thereof), inorganic solids (e.g. ammonium chloride), and combinations thereof. The substrate can be a flexible or inflexible solid substrate with a curved or planar geometry, depending on the requirements of the desired application.
A typical substrate for semiconductor devices comprises a matrix (e.g. quartz or polymer matrix) and, optionally, a dielectric top layer (e.g. SiO2). The substrate also may include electrodes, such as the gate, drain and source electrodes of the OFETs which are usually located on the substrate (e.g. deposited on the nonconductive surface of the dielectric top layer). The substrate also includes conductive gate electrodes of the OFETs that are typically located below the dielectric top layer (i.e., the gate dielectric).
According to a special embodiment, a gate insulating layer is formed on a part of the surface of the substrate or on the entire surface of the substrate including the gate electrode(s). Typical gate insulating layers comprise an insulating substance, preferably selected from inorganic insulating substances such as SiO2, SiN, etc., ferroelectric insulating substances such as Al2O3, Ta2O5, La2O5, TiO2, Y2O3, etc., organic insulating substances such as polyimides, benzocyclobutene (BCB), polyvinyl alcohols, polyacrylates, etc. and combinations thereof.
Source and drain electrodes are located on the surface of the substrate at a suitable space from each other and the gate electrode with the copper semiconducting compound, at least one compound of the formula (I) being in contact with source and drain electrode, thus forming a channel.
Suitable materials for source and drain electrodes are in principal, any electrically conductive materials. Suitable materials include metals, preferably metals of groups 8, 9, 10 or 11 of the periodic table, e.g. Pd, Au, Ag, Cu, Al, Ni, Cr, etc. Preferred electrically conductive materials have a resistivity lower than about 10−3, more preferably lower than about 10−4, and most preferably lower than about 10−6 or 10−7 ohm metres.
According to a special embodiment, the drain and source electrodes are deposited partially on the organic semiconductor rather than only on the substrate. Of course, the substrate can contain further components that are usually employed in semiconductor devices or ICs, such as insulators, resistive structures, capacitive structures, metal tracks, etc.
The application of at least one compound of the formula (I) (and optionally further semiconducting compounds) can be carried out by known methods. Suitable are lithographic techniques, offset printing, flexo printing, etching, inkjet printing, electrophotography, physical vapor transport/deposition (PVT/PVD), chemical vapor deposition, laser transfer, dropcasting, etc.
In a preferred embodiment, the compound of the formula (I) (and optionally further semiconducting compounds) is applied to the substrate by physical vapor deposition (PVD). Physical vapor transport (PVT) and physical vapor deposition (PVD) are vaporisation/coating techniques involving transfer of material on an atomic level. PVD processes are carried out under vacuum conditions and involve the following steps:
The process is similar to chemical vapour deposition (CVD) except that CVD is a chemical process wherein the substrate is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. It was surprisingly found that compounds of the formula I can be subjected to a PVD essentially without decomposition and/or the formation of undesired by-products. The deposited material is obtained in high purity and in the form of crystals or contains a high crystalline amount. The deposited material is obtained in high homogeneity and a size suitable for use as n-type semiconductors. Generally, for physical vapor deposition, a solid source material of at least one compound of the formula (I) is heated above its vaporization temperature and the vapor allowed to deposit on the substrate by cooling below the crystallization temperature of the compound of the formula (I).
The temperature of the substrate material during the deposition should be less than the temperature corresponding to the vapor pressure. The deposition temperature is preferably from 20 to 250° C., more preferably from 50 to 200° C. It was surprisingly found, that it is advantageous to increase the temperature of the substrate during deposition, (e.g. for formation of a film). In general, the higher the temperature during deposition, the higher the intensity of the diffraction peaks obtained by X-ray diffraction (XRD) of the obtained semiconducting material, the larger the grain sizes, and as a result the higher the charge carrier mobility.
The obtained semiconducting layer in general should have a thickness sufficient for ohmic contact between source and drain electrode.
The deposition can be carried out under inert atmosphere, e.g. under nitrogen, argon or helium atmosphere.
The deposition can be carried out under ambient pressure or reduced pressure. A suitable pressure range is from about 0.0001 to 1.5 bar.
Preferably, the compound of the formula (I) is applied to the substrate in a layer, having an average thickness of from 10 to 1000 nm, preferably of from 15 to 250 nm.
Preferably, the compound of the formula (I) is applied in at least partly crystalline form. In a first embodiment, the compound of the formula (I) can be employed in form of preformed crystals or a semiconductor composition comprising crystals. In a second embodiment, the compound of the formula (I) is applied by a method that allows the formation of an at least partly crystallographically ordered layer on the substrate. Suitable application techniques that allow the formation of an at least partly crystalline semiconductor layer on the substrate are sublimation techniques, e.g. the aforementioned physical vapor deposition.
According to a preferred embodiment, the applied compound of the formula (I) comprises crystallites or consists of crystallites. For the purpose of the invention, the term “crystallite” refers to small single crystals with maximum dimensions of 5 millimeters. Exemplary crystallites have maximum dimensions of 1 mm or less and preferably have smaller dimensions (frequently less than 500 μm, in particular less than 200 μm, for example in the range of 0.01 to 150 μm, preferably in the range of 0.05 to 100 μm), so that such crystallites can form fine patterns on the substrate. Here, an individual crystallite has a single crystalline domain, but the domains may include one or more cracks, provided that the cracks do not separate the crystallite into more than one crystalline domain.
The stated particle sizes of the crystals of the compounds of the formula (I), the crystallographic properties and the crystalline amount of the applied compounds can be determined by direct X-ray analysis. During the pretreatment and/or the application of the compound of the formula (I), preferably appropriate conditions e.g. pretreatment of the substrate, temperature, evaporation rate etc. are employed to obtain films having high crystallinity and large grains.
The crystalline particles of the compounds of the formula (I) may be of regular or irregular shape. For example, the particles can be present in spherical or virtually spherical form or in the form of needles. Preferably the applied compound of the formula (I) comprises crystalline particles with a length/width ratio (L/W) of at least 1.05, more preferably of at least 1.5, especially of at least 3.
Organic field-effect transistors (OFETs), wherein the channel is made of an at least partly crystallographically ordered compound of the formula (I) as organic semiconductor material will typically have greater mobility than a channel made of non-crystalline semiconductor. Larger grains and correspondingly less grain boundaries result in a higher charge carrier mobility.
Preformed organic semiconductor crystals in general and especially crystallites can also be obtained by sublimation of the compound of the formula (I) prior to application. A preferred method makes use of physical vapor transport/deposition (PVT/PVD) as defined in more detail in the following. Suitable methods are described by R. A. Laudise et al in “Physical vapor growth of organic semiconductors” Journal of Crystal Growth 187 (1998) pages 449-454 and in “Physical vapor growth of centimeter-sized crystals of α-hexathiophene” Journal of Crystal Growth 182 (1997) pages 416-427. Both of these articles by Laudise et al are incorporated herein in their entirety by reference. The methods described by Laudise et al include passing an inert gas over an organic semiconductor substrate that is maintained at a temperature high enough that the organic semiconductor evaporates. The methods described by Laudise et al also include cooling down the gas saturated with organic semiconductor to cause an organic semiconductor crystallite to condense spontaneously.
According to a preferred embodiment, the organic field-effect transistor according to the invention is a thin film transistor. As mentioned before, a TFT has a thin film structure in which a source electrode and a drain electrode are formed on a semiconductor film layer, and an insulating film is formed if necessary. The source and drain electrode materials generally should be in ohmic contact with the semiconductor film.
In a preferred embodiment, the method according to the invention comprises the step of depositing on the surface of the substrate at least one compound (C1) capable of binding to the surface of the substrate and of binding at least one compound of the formula (I). A first aspect is a method, wherein a part or the complete surface of the substrate is treated with at least one compound (C1) to obtain a modification of the surface and allow for an improved application of the compounds of the formula (I) (and optionally further semiconducting compounds). A further aspect is a method for patterning the surface of a substrate with at least one compound of the formula (I) (and optionally further semiconducting compounds). According to this aspect, a substrate with a surface has a preselected pattern of deposition sites or nonbinding sites located thereupon is preferably used. The deposition sites can be formed from any material that allows selective deposition on the surface of the substrate. Suitable compounds are the compounds C1 mentioned below. Again, PVD can be used for the application of the compounds of the formula (I) to the substrate.
A special embodiment of step b) of the method according to the invention comprises:
The free surface areas of the substrate obtained after deposition of (C1) can be left unmodified or be coated, e.g. with at least one compound (C2) capable of binding to the surface of the substrate and to prevent the binding of at least one compound of the formula (I).
A further special embodiment of step b) of the method according to the invention comprises:
The free surface areas of the substrate obtained after deposition of (C2) can be left unmodified or be coated, e.g. with at least one compound (C1) capable of binding to the surface of the substrate and of binding at least one compound of the formula (I).
For the purpose of the present application, the term “binding” is understood in a broad sense. This covers every kind of binding interaction between a compound (C1) and/or a compound (C2) and the surface of the substrate and every kind of binding interaction between a compound (C1) and at least one compound of the formula (I), respectively. The types of binding interaction include the formation of chemical bonds (covalent bonds), ionic bonds, coordinative interactions, solvatophobic interaction, Van der Waals interactions (e.g. dipole dipole interactions), etc. and combinations thereof. In one preferred embodiment, the binding interactions between the compound (C1) and the compound of the formula (I) is a non-covalent interaction.
Suitable compounds (C2) are compounds with a lower affinity to the compounds of the formula (I) than the untreated substrate or, if present, (C1). If a substrate is only coated with at least one compound (C2), it is critical that the strength of the binding interaction of (C2) and the substrate with the compound of the formula (I) differs to a sufficient degree so that the compound of the formula (I) is essentially deposited on substrate areas not patterned with (C2). If a substrate is coated with at least one compound (C1) and at least one compound (C2), it is critical that the strength of the binding interaction of (C1) and (C2) with the compound of the formula (I) differs to a sufficient degree so that the compound of the formula (I) is essentially deposited on substrate areas patterned with (C1). In a preferred embodiment the interaction between (C2) and the compound of the formula (I) is a repulsive interaction. For the purpose of the present application, the term “repulsive interaction” is understood in a broad sense and covers every kind of interaction that prevents deposition of the crystalline compound on areas of the substrate patterned with compound (C2).
In a first preferred embodiment, the compound (C1) is bound to the surface of the substrate and/or to the compound of the formula I via covalent interactions. According to this embodiment, the compound (C1) comprises at least one functional group, capable of reaction with a complementary functional group of the substrate and/or the compound of the formula (I).
In a second preferred embodiment the compound (C1) is bound to the surface of the substrate and/or to the compound of the formula (I) via ionic interactions. According to this embodiment, the compound (C1) comprises at least one functional group capable of ionic interaction with the surface of the substrate and/or a compound of the formula (I).
In a third preferred embodiment the compound (C1) is bound to the surface of the substrate and/or to the at least one compound of the formula (I) via dipole interactions, e.g. Van der Waals forces.
The interaction between (C1) and the substrate and/or between (C1) and the compounds of the formula (I) is preferably an attractive hydrophilic-hydrophilic interaction or attractive hydrophobic-hydrophobic interaction. Hydrophilic-hydrophilic interaction and hydrophobic-hydrophobic interaction can comprise, among other things, the formation of ion pairs or hydrogen bonds and may involve further van der Waals forces. Hydrophilicity or hydrophobicity is determined by affinity to water. Predominantly hydrophilic compounds or material surfaces have a high level of interaction with water and generally with other hydrophilic compounds or material surfaces, whereas predominantly hydrophobic compounds or materials are not wetted or only slightly wetted by water and aqueous liquids. A suitable measure for assessing the hydrophilic/hydrophobic properties of the surface of a substrate is the measurement of the contact angle of water on the respective surface. According to the general definition, a “hydrophobic surface” is a surface on which the contact angle of water is >90°. A “hydrophilic surface” is a surface on which the contact angle with water is <90°. Compounds or material surfaces modified with hydrophilic groups have a smaller contact angle than the unmodified compound or materials. Compounds or material surfaces modified with hydrophobic groups have a larger contact angle than the unmodified compounds or materials.
Suitable hydrophilic groups for the compounds (C1) (as well as (C2)) are those selected from ionogenic, ionic, and non-ionic hydrophilic groups. Ionogenic or ionic groups are preferably carboxylic acid groups, sulfonic acid groups, nitrogen-containing groups (amines), carboxylate groups, sulfonate groups, and/or quaternized or protonated nitrogen-containing groups. Suitable non-ionic hydrophilic groups are e.g. polyalkylene oxide groups. Suitable hydrophobic groups for the compounds (C1) (as well as (C2)) are those selected from the aforementioned hydrocarbon groups. These are preferably alkyl, alkenyl, cycloalkyl, or aryl radicals, which can be optionally substituted, e.g. by 1, 2, 3, 4, 5 or more than 5 fluorine atoms.
In order to modify the surface of the substrate with a plethora of functional groups it can be activated with acids or bases. Further, the surface of the substrate can be activated by oxidation, irradiation with electron beams or by plasma treatment. Further, substances comprising functional groups can be applied to the surface of the substrate via chemical vapor deposition (CVD).
Suitable functional groups for interaction with the substrate include:
In a preferred embodiment, the compound (C1) is selected from alkyltrialkoxysilanes and is in particular n-octadecyl triethoxysilane. In a further preferred embodiment, the compound (C1) is selected from hexaalkyldisilazanes and is in particular hexamethyldisilazane (HMDS). In a further preferred embodiment, the compound (C1) is selected from C8-C30-alkylthiols and is in particular hexadecane thiol. In a further preferred embodiment the compound (C1) is selected from mercaptocarboxylic acids, mercaptosulfonic acids and the alkali metal or ammonium salts thereof. Examples of these compounds are mercaptoacetic acid, 3-mercaptopropionic acid, mercaptosuccinic acid, 3-mercapto-1-propanesulfonic acid and the alkali metal or ammonium salts thereof, e.g. the sodium or potassium salts. In a further preferred embodiment the compound (C1) is selected from alkyltrichlorosilanes, and is in particular n-(octadecyl)trichlorosilane.
Additionally to or as an alternative to deposition of said compound (C1) on the substrate, the substrate can be contacted with at least one compound (C2) capable of binding to the surface of the substrate as well as of interaction with the compound of the formula (I) to prevent deposition of (S) on areas of the substrate not patterned with compound (C1). According to a suitable embodiment, the compounds (C2) are selected from compounds with a repulsive hydrophilic-hydrophobic interaction with (S).
The compounds of the formula (I) can be purified by recrystallization or by column chromatography. Suitable solvents for column chromatography are e.g. halogenated hydrocarbons, like methylene chloride. In an alternative embodiment, the compounds of the formula (I) can be recrystallized from sulfuric acid.
In a preferred embodiment, purification of the compound of the formula (I) can be carried out by sublimation. Preferred is a fractionated sublimation. For fractionated sublimation, the sublimation and/or the deposition of the compound is effected by using a temperature gradient. Preferably the compound of the formula (I) sublimes upon heating in flowing carrier gas. The carrier gas flows into a separation chamber. A suitable separation chamber comprises different separation zones operated at different temperatures. Preferably a so-called three-zone furnace is employed. A further suitable method and apparatus for fractionated sublimation is described in U.S. Pat. No. 4,036,594.
In a further embodiment at least one compound of the formula (I) is subjected to purification and/or crystallization by physical vapor transport. Suitable PVD techniques are those mentioned before. In a physical vapor transport crystal growth, a solid source material is heated above its vaporization temperature and the vapor is allowed to crystallize by cooling below the crystallization temperature of the material. The obtained crystals can be collected and afterwards applied to specific areas of a substrate by known techniques, as mentioned above. A further aspect is a method for patterning the surface of a substrate with at least one compound of the formula (I) (and optionally further organic semiconducting compounds) by PVD. According to this aspect, a substrate with an unmodified surface, or a surface being at least partly covered with a substance that improves deposition of at least one compound of the formula (I) or a surface that has a preselected pattern of deposition sites located thereupon is preferably used. The deposition sites can be formed from any material that allows selective deposition on the surface of the substrate. Suitable compounds are the aforementioned compounds C1, which are capable of binding to the surface of the substrate and of binding at least one compound of the formula (I).
The invention will now be described in more detail on the basis of the accompanying figures and the following examples.
BPE-PTCDI was synthesized form perylene-3,4:9,10-tetracarboxylic acid bisanhydride and phenethylamine by known methods. The purification was carried out by three consecutive vacuum sublimations using a three-temperature-zone furnace (Lindberg/Blue Thermo Electron Corporation). The three temperature zones were set to be: 400° C., 350° C. and 300° C. and the vacuum level during sublimation was 10−6 Torr or less while the starting material was placed in the first temperature zone.
Highly doped n-type Si wafers (2.5×2.5 cm) with a thermally grown dry oxide layer (capacitance per unit area Ci=10 nF/cm2) as gate dielectric were used as substrates. The substrate surfaces were cleaned with acetone followed by isopropanol. Afterwards, the surface of the substrate was left unmodified (a) or was modified with n-octadecyl trimethoxysilane (b) or hexamethyldisilazane (c):
Top-contact devices were fabricated by depositing gold source and drain electrodes onto the organic semiconductor films through a shadow mask with channel length of 2000 μm and channel width of 200 μm. The electrical characteristics of the obtained organic thin film transistor devices were measured using a Keithley 4200-SCS semiconductor parameter analyzer. Key device parameters, such as charge carrier mobility (μ), on/off current ratio (Ion/Ioff), were extracted from the drain-source current (Id)-gate voltage (Vg) characteristics. The morphology of BPE-PTCDI thin films was determined using an atomic force microscope (AFM) (Multimode Nanoscope III, Digital Instrument Inc.) in tapping mode. Out-of-plane x-ray diffraction (XRD) measurement was carried out with a Philips X'Pert PRO system. The beam wavelength was 1.5406 Å operated at 45 KeV and 40 mA. Cyclic voltammetry data were obtained from a saturated solution in anhydrous methylene chloride under argon with 0.1 M tetrabutyl ammonium hexafluorophosphate as supporting electrolyte. The scan rate was 50 mVs−1. A silver wire was used as pseudoreference electrode. The ferrocene/ferrocenium redox couple was used as reference (Fc/Fc+E1/2=0.56 V in the used system).
Typical current-voltage characteristics (Ids−Vds for various Vg) of a BPE-PTCDI TFT are shown in
The following table 1 gives a summary of average field effect mobilities (cm2/Vs) over at least five devices, on/off ratio and treshhold voltage for BPE-PTCDI, deposited at various substrate temperatures.
The out-of-plane XRD patterns of 40 nm BPE-PTCDI thin film deposited at a temperature of 150° C. on a plain substrate and substrates where the surface was treated with n-(octadecyl)trimethoxysilane (OTS) and hexamethyldisilazane (HMDS) are shown in
Air-stability measurements of BPE-PTCDI TFTs are shown in
a), left axis: charge carrier mobility (dots: exposed to air only; squares: exposed to air and ambient light), right axis: relative humidity (curve)
b): on/off ratio
Air-stability measurements were carried out by monitoring the charge carrier mobility (
DME-PTCDI was purified by three consecutive vacuum sublimations using a three-temperature-zone furnace (Lindberg/Blue Thermo Electron Corporation). The material used was collected from the second temperature zone (T2) after the third purification.
