JP2010056476A - n-TYPE AND HETERO-JUNCTION ORGANIC THIN-FILM TRANSISTOR - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an organic thin-film transistor (TFT) capable of being actuated at a low voltage using a highly-performing n-type organic semiconductor material in a semiconductor layer on a substrate with an electron-absorbing group induced therein at both ends of an organic compound with an advanced high-planarity molecular frame of a π-electron system. <P>SOLUTION: A compound represented in a formula is used in the semiconductor layer on the substrate. Here, the character X denotes hydrogen, halogen, a cyano group, a nitro group, an alkyl group, an alcoxy group, or an alkylsilylethynyl group. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an n-type organic thin film transistor using an organic compound having an anthradithiophene skeleton useful as an n-type semiconductor material operating at a low voltage, and a heterojunction organic thin film transistor.

  Thin film transistors (TFTs) are widely used as pixel drive elements for active matrix liquid crystal displays, as represented by polycrystalline silicon, amorphous silicon, and the like. This active matrix drive using a silicon TFT is also applied to an organic EL display which has recently been activated, and an 11-inch class organic EL television has been commercialized. However, the manufacture of these silicon-based TFTs requires complicated and high-temperature processes, and does not meet the demands of the era of cost reduction and energy saving. In contrast, research on TFTs using organic compounds in semiconductor layers has been continued with the aim of simplifying the manufacturing process and lowering the temperature. The low temperature process enables not only cost but also TFT construction on plastic film. The plastic substrate can realize a flexible and lightweight display compared to a glass substrate.

Early research on organic TFTs uses polymers such as polyacetylene and polypyrrole, and the obtained carrier mobility is small, which is very practical in the range of 10 ^-5 -10 ^-6 cm / Vs. It was not a thing. However, 10 ⁻² −10 ⁻¹ cm ² / Vs has been reported for organic TFTs using low molecular weight α-sexiphenyl in the active layer (see Non-Patent Document 1). Furthermore, a field effect mobility that exceeds 1 cm ² / Vs of amorphous silicon in a pentacene vapor-deposited film that is a polycyclic aromatic has been reported (for example, see Non-Patent Document 2). The on / off ratio is 10 ⁶ . The majority of organic TFTs reported in the world are p-types in which charge carriers flowing in organic semiconductors are holes, and there are very few examples of n-type organic TFTs in which electrons flow. Examples reported so far include fluorinated phthalocyanines, fluorinated pentacenes, perylene derivatives, and recently, organic semiconductors in which electron withdrawing groups are introduced on both sides of a thiazolothiazole skeleton or a bithiophene skeleton.

Here, the n-type organic TFT will be described. The basic structure of the organic TFT includes a top contact type and a bottom contact type shown in FIG. The organic semiconductor is formed with a thickness of 30 nm to 100 nm on the gate insulating film. By applying a voltage to the gate electrode, electrons are injected from the source electrode into the organic semiconductor layer and accumulated at the interface of the gate insulating layer. In this state, by applying a positive voltage to the drain electrode, electrons corresponding to the gate voltage flow from the source electrode side to the drain electrode side. In addition, the operating voltage of n-type organic TFTs reported to date is quite high, and the threshold voltage is 50-70 V (see Non-Patent Documents 3 to 5).
G. Horowitz, D. Fichou, XZ Peng, ZG Xu, F. Garnier, Solid State Commun. 72, 381 (1989) Y.-Y.Lin, DJ Gundlach, SF Nelson, and TN Jackson, IEEE Electron Device Letters, 18, 606 (1997) S. Ando, R. Murakami, J. Nishida, H. Tada, Y. Inoue, S. Tokito, and Y. Yamashita, J. Am. Chem. Soc. 127, 14996 (2005) S. Tatemichi, M. Ichikawa, T. Koyama, and Y. Taniguchi, Appl. Phys. Lett. 89, 112108 (2006) D. Kumaki, S. Ando, S. Shimono, Y. Yamashita, T. Umeda and S. Tokito, Appl. Phys. Lett. 90, 053506 (2007)

  Many of the organic semiconductors reported so far are p-type, and there are very few examples of n-type. In the future, development of high-performance n-type organic semiconductors will be indispensable when considering application to CMOS or the like combining p-type and n-type, or integrated circuits. Therefore, there is a demand for how to develop an organic TFT that has a high electron mobility and operates at a low voltage, which is a problem.

  In order to increase the mobility in the organic semiconductor thin film, it is necessary to increase the overlap of electron orbits between molecules. For this purpose, a highly planar molecular skeleton that can easily form a stack structure is effective. A deeper minimum unoccupied molecular orbital (LUMO) is desirable to reduce the barrier for electron injection from the electrode. In general, the LUMO level becomes deeper as the energy gap of HOMO-LUMO is smaller. Therefore, it is necessary to take a molecular structure in which a π-electron conjugated system is developed. Therefore, an organic compound having an anthradithiophene skeleton in which a thiophene ring is condensed around anthracene which is a typical π electron system (for example, JG Laquindanum, HE Katz, and AJ Lovinger, J. Am. Chem. Soc. 1998, 120, 664). Substitution of electron-withdrawing trifluoromethylbenzene at both ends of this structure can be expected to achieve n-type properties (T. Yasuda, M. Saito, H. Nakamura, and T. Tsutsui, Appl. Phys. Lett. 89, 182108 (2006)). Addition of a thiophene ring on both sides of anthracene can avoid twisting with a benzene ring and can maintain a planar structure.

  In addition, by introducing bulky substituents at positions 5 and 11 of the anthradithiophene skeleton, the solubility in organic solvents is improved and application from a solution becomes possible. It was found that a thin film can be formed. This makes it possible to produce an organic TFT by an ink jet method or a flexographic printing method.

  The present invention is a low-voltage drive using a high-performance n-type organic semiconductor material in which electron-withdrawing groups are introduced at both ends of an organic compound having a highly planar molecular skeleton with a developed π-electron system for a semiconductor layer on a substrate. An organic thin film transistor (TFT) is provided.

That is, according to the present invention, the following is provided.
[1] An n-type organic thin film transistor, wherein a syn and anti isomer mixture of a compound represented by the following formula [Chemical Formula 1] is used for a semiconductor layer on a substrate.

(However, X represents hydrogen, halogen, cyano group, nitro group, alkyl group, alkoxy group, or alkylsilylethynyl group.)
[2] A heterojunction organic thin film transistor comprising a mixture of syn and anti isomers of the compound according to [1] as an n-type organic semiconductor layer, which is combined with a p-type organic semiconductor layer.

BEST MODE FOR CARRYING OUT THE INVENTION

  The present invention is an organic compound having an anthradithiophene skeleton, which is a compound represented by the above formula [Chemical Formula 1] in which electron-withdrawing fluoromethylbenzene is substituted at both ends in the long axis direction. Mainly shows electronic properties excellent in electron transport properties. Here, the substituent X is hydrogen, halogen (fluorine among fluorine, chlorine, bromine and iodine), cyano group, alkyl group (especially alkyl group having 1 to 20 carbon atoms, such as methyl group, hexyl group, etc.) ), An alkoxy group (particularly, an alkoxy group having 1 to 20 carbon atoms, such as a methoxy group), or an alkylsilylethynyl group (particularly one having an alkyl group having 1 to 20 carbon atoms, such as a methyl group, ethyl Group having isopropyl group, etc.).

  More specifically, examples of preferred molecular structures are shown below.

  As shown in FIG. 1, an organic TFT using the n-type anthradithiophene derivative of the present invention for a semiconductor layer is formed by laminating a gate electrode, a drain electrode, a source electrode, a gate insulating film, and a semiconductor layer thereof. The TFT which can be illustrated can be illustrated. In terms of a fine organic TFT, a bottom contact type is advantageous, and after forming a source electrode and a drain electrode by a photolithography process, an organic semiconductor layer is formed.

  Also, as shown in FIG. 2, a heterojunction organic transistor exhibiting bipolar characteristics, that is, CMOS characteristics can be constructed by stacking two layers of an n-type organic semiconductor and a p-type organic semiconductor of the present invention, for example, pentacene.

  The gate electrode according to the present invention includes metals such as gold, platinum, chromium, palladium, aluminum, indium, molybdenum, low-resistance polysilicon, low-resistance amorphous silicon, tin oxide, indium oxide, and indium / tin oxide (ITO ) And the like are generally used, but of course, it is not limited to these materials, and two or more of these materials may be used. Here, as a method of forming an electrode, there are a method of vapor deposition, sputtering, plating, and various CVD growth methods. Further, depending on the purpose of use, it is also possible to use a conductive plate such as a silicon wafer, a stainless steel plate, or a copper plate, serving both as a gate electrode and a substrate.

  Examples of the source electrode and drain electrode according to the present invention include metals such as gold, platinum, palladium, aluminum, chromium, titanium, molybdenum, indium, low resistance polysilicon, and low resistance amorphous silicon, tin oxide, indium oxide, and indium. Although tin oxide (ITO) or the like is generally used, it is of course not limited to these materials, and two or more kinds of these materials may be used. Here, as a method of forming an electrode, there are a method of vapor deposition, sputtering, plating, and various CVD growth methods.

  As the gate insulating film according to the present invention, any inorganic or organic material can be used as long as it is insulative. Generally, silicon oxide, silicon nitride, aluminum oxide, aluminum nitride, titanium oxide, polyethylene are used. Polyester, polyimide, polyphenylene sulfide, polyparaxylene, polyacrylonitrile, various insulating resins and the like may be used, and two or more of these materials may be used in combination. There are no particular limitations on the method for producing these insulating films, and examples include CVD, plasma CVD, plasma polymerization, vapor deposition, spin coating, dipping, cluster ion beam vapor deposition, and LB method. Can also be used. When a silicon wafer is used as both a gate electrode and a substrate, a silicon oxide film obtained by a silicon thermal oxidation method or the like is preferable as the gate insulating film.

  As a method for producing an organic semiconductor thin film to be a semiconductor layer, a dry process such as a vacuum deposition method, a molecular beam epitaxial growth method, or an ion cluster beam method can be used. In the case of an organic semiconductor that is soluble in an organic solvent and can be formed into a thin film, a coating method such as a spin coating method, an ink jet method, a screen printing method, an imprinting method, or the like can be used. Although there is no restriction | limiting in particular as the film thickness of an organic-semiconductor layer, Generally about 50 nm is preferable.

  As the p-type organic semiconductor material according to the present invention, pentacene, anthradithiophene, sexithiophene, polythiophene, or the like is generally used, but it is not limited to these materials.

  Any insulating material can be used for the substrate according to the present invention. Specifically, glass, alumina sintered body, polyimide film, polyester film, polyethylene film, polyphenylene sulfide film, polyparaxylene film. Various insulating plastics such as can be used. Specific examples will be described below, but the present invention is of course not limited thereto.

Next, a method for synthesizing a compound having an anthradithiophene skeleton used in the organic thin film transistor of the present invention will be described as follows.
First, as Scheme 1,

As shown in the above, compound (1) is reacted with compound (2) in the presence of a palladium catalyst to give compound (3).
Pd (PPh ₃ ) ₄ , PdCl ₂ (PPh ₃ ) ₂ , PdCl ₂ (dppf) and the like can be used as the palladium catalyst used in Scheme 1 (where PPh ₃ is triphenylphosphine and dppf is diphenylphosphinoferrocene). Pd (PPh ₃ ) ₄ is particularly preferable from the viewpoint of synthesis time and yield. In the compound (2), M represents an organometallic substituent such as alkyl tin or dihydroxy boron. In particular, dihydroxyboron is preferable. The amount of compound (2) to be used is 1.0 equivalent to 5.0 equivalents, preferably 1.1 equivalents to 1.5 equivalents, relative to compound (1). The solvent used is tetrahydrofuran, dioxane, diethyl ether, toluene, N, N-dimethylformamide, etc., preferably tetrahydrofuran. The reaction temperature is 0 ° C. or higher and 100 ° C. or lower, preferably 50 ° C. or higher and 70 ° C. or lower. The reaction time is 1 hour or more and 20 hours or less, preferably 10 hours or more and 15 hours or less. After completion of the reaction, the target compound (3) can be obtained by performing extraction with an organic solvent and purification. Purification methods include recrystallization and silica gel column chromatography.

  Furthermore, as Scheme 2,

As shown in the above, compound (3) is reacted with 1,4-cyclohexanedione in the presence of a base to give compound (4).
As the base used in Scheme 2, an aqueous potassium hydroxide solution, an aqueous sodium hydroxide solution, an aqueous lithium hydroxide solution, or the like can be used, but an aqueous potassium hydroxide solution is preferable from the viewpoint of increasing reactivity. The concentration of the aqueous solution is 1% or more and 20% or less, preferably 5% or more and 10% or less. Moreover, as a solvent to be used, there are water-soluble solvents such as tetrahydrofuran, methanol, and ethanol, but ethanol is preferred. The reaction temperature is 0 ° C. or higher and 50 ° C. or lower, preferably 10 ° C. or higher and 20 ° C. or lower. The reaction time is 1 hour or more and 20 hours or less, preferably 5 hours or more and 15 hours or less. After completion of the reaction, the objective compound (4) can be obtained by carrying out washing with an organic solvent and purification. The purification method includes washing with an organic solvent and sublimation.

  The resulting product is as described in the reported literature on the synthesis of similar anthradithiophenes (see JG Laquindanum, HE Katz and AJ Lovinger, J. Am. Chem. Soc., 120. 664 (1998)). In addition, although it is expected to be a mixture of syn and anti isomers, it is difficult to separate and analyze the mixture, so that it is used in the next reaction as it is as an isomer mixture.

  Next, as Scheme 3

As shown in (2), compound (4) is reduced in the presence of metallic aluminum to give compound (5).
The metal used in Scheme 3 is preferably aluminum or a metal containing aluminum, and the shape is preferably a wire having a diameter of 0.5 mm or more and 1.0 mm or less, rather than a lump or granule. The solvent used is an alcohol, preferably cyclohexanol. As the reaction initiator, CCl ₄ is preferable. The reaction temperature is 50 ° C. or higher and 200 ° C. or lower, preferably 100 ° C. or higher and 150 ° C. or lower. The reaction time is 10 hours to 50 hours, preferably 10 hours to 20 hours. It is. After completion of the reaction, the target compound (5) can be obtained by carrying out washing with an organic solvent and purification, as in Scheme 2. The purification method includes washing with an organic solvent and sublimation.

  Next, as Scheme 4

As shown in the above, the compound (4) is reacted with triisopropylsilylethynyllithium to obtain the compound (6).
Examples of the solvent used in Scheme 4 include ether solvents, and tetrahydrofuran is preferred. The reaction temperature during lithiation is −90 ° C. or higher and −50 ° C. or lower, preferably −80 ° C. or higher and −70 ° C. or lower. The reaction temperature in the reaction between the lithiated product and (4) is from −90 ° C. to −0 ° C., preferably from −50 ° C. to −10 ° C. The reaction time at the time of lithiation is 1 hour or more and 10 hours or less, preferably 1 hour or more and 2 hours or less. The reaction temperature in the reaction of the lithiated product with (4) is from 5 hours to 20 hours, preferably from 5 hours to 10 hours. After completion of the reaction, the target compound (6) can be obtained by performing extraction with an organic solvent and purification. Purification methods include recrystallization and silica gel column chromatography.

  The invention is illustrated in more detail by the following examples, but should not be limited thereto.

The 500 mL 2-neck flask was purged with nitrogen, and 5-bromothiophene-2,3-dicarbaldehyde (1) (1.35 g, 6.16 mmol), 4- (trifluoromethyl) phenylboronic acid (2) (1. 29 g, 6.78 mmol), Pd (PPh ₃ ) ₄ (356 mg, 5 mol% based on (1)), tripotassium phosphate n-hydrate (7.0 g, special grade reagent manufactured by Kanto Chemical), and anhydrous tetrahydrofuran ( 130 mL) was added and reacted under reflux for 12 hours. After stopping the reaction by adding an aqueous ammonium chloride solution, the reaction solution was extracted with ethyl acetate, the organic phase was dried over anhydrous magnesium, filtered, and concentrated on a rotary evaporator. The residue was purified by silica gel column chromatography using a mixed solvent of hexane: ethyl acetate 4: 1 as a mobile phase, and 5- (4- (trifluoromethyl) phenyl) thiophene-2,3-di was obtained as a pale yellow solid. Carbaldehyde (3) (1.0 g, 57% yield) was obtained.
mp.> 300 ° C; ¹ H NMR (300MHz, CDCl ₃ ) δ7.74 (d, 2H, J = 8.40 Hz, ArH), 7.81 (d, 2H, J = 8.40 Hz, ArH), 7.89 (s, 1H , ThH), 10.42 (s, 1H, CHO), 10.51 (s, 1H, CHO); ¹³ C NMR (75 MHz, CDCl ₃ ) δ123.68 (q, J = 272.4 Hz), 126.18, 126.46 (q, J = 3.8 Hz), 126.72, 131.55, 131.99, 135.31, 144.40, 146.47, 150.69, 182.20 (CHO), 184.43 (CHO); ¹⁹ F NMR (282 MHz, CDCl ₃ ) δ-62.84; EI-MS method, m / z = 284 (M ⁺ )

In a 500 mL 2-neck flask, 5- (4- (trifluoromethyl) phenyl) thiophene-2,3-dicarbaldehyde (3) (800 mg, 2.81 mmol), 1,4-cyclohexanedione (157 mg, 1.41 mmol). ), Ethanol (120 mL) was added. 5% KOH aqueous solution (5.0 mL) was dripped there, and it was made to react at room temperature for 12 hours. Thereafter, ethanol was distilled off with a rotary evaporator, and water was added to the concentrate, followed by filtration. After washing with methanol and hexane, vacuum drying was performed to obtain 2,8-bis (4- (trifluoromethyl) phenyl) anthradithiophene-5,11-dione (4) (680 mg, 79%) as a brown solid. It was.
mp.> 300 ° C; EI-MS method, m / z = 608 (M ⁺ )

An aluminum wire (500 mg), mercury chloride (10 mg), carbon tetrachloride (0.5 mL), and anhydrous cyclohexanol (15 mL) were placed in a 100 mL three-necked flask and stirred at 100 ° C. for 2 hours. 2,8-bis (4- (trifluoromethyl) phenyl) anthradithiophene-5,11-dione (4) (500 mg, 0.821 mmol) was added thereto, and the mixture was reacted at 140 ° C. for 12 hours. 6N HCl (20 mL) was added, and the mixture was stirred for 1 hour and filtered. The product was washed with water, methanol, and tetrahydrofuran to obtain 180 mg of a crude product. The obtained crude product was purified by sublimation purification (argon flow, 30 Pa, 500 ° C.) to obtain the above 2,8-bis (4- (trifluoromethyl) phenyl) anthradithiophene (5) as a reddish purple solid. (80 mg, 17%) was obtained.
mp.> 300 ° C; MALDI-TOFMS method, m / z = 578 (M ⁺ )

The 50 mL 3-neck flask was purged with nitrogen, triethylsilylacetylene (420 mg, 2.30 mmol) and anhydrous tetrahydrofuran (10 mL) were added, and the reaction solution was cooled to -78 ° C. A 1.54 M butyllithium hexane solution (1.34 mL, 2.07 mmol) was added dropwise and stirred for 1 hour. Then, a suspension of 2,8-bis (4- (trifluoromethyl) phenyl) anthradithiophene-5,11-dione (4) (350 mg, 0.575 mmol) in THF was added to the reaction solution at −20 ° C. 10 mL) was added dropwise and reacted at room temperature for 7 hours. Thereafter, water (1.5 mL) was added to stop the reaction, and SnCl ₂ .2H ₂ O (259 mg, 1.15 mmol) dissolved in 1N HCl (6 mL) was added dropwise, followed by stirring for 30 minutes. Thereafter, tetrahydrofuran was distilled off on a rotary evaporator, and then the concentrated solution was extracted with chloroform. The organic phase was dried over anhydrous magnesium, filtered, and concentrated on a rotary evaporator. The residue was purified by silica gel column chromatography using a mixed solvent of hexane: chloroform 3: 1 as a mobile phase to obtain [2,8-bis (4- (trifluoromethyl) phenyl) -5,11 as a black purple solid. -Di (triisopropylsilylethynyl)] anthradithiophene (6) (80 mg, 15% yield) was obtained.
mp.> 300 ° C; ¹ H NMR (300MHz, CDCl ₃ ) δ1.37 (m, 42H, i-Pr), 7.73 (d, 4H, J = 8.10 Hz, ArH), 7.74 (s, 2H, ThH) , 7.92 (d, 4H, J = 8.10 Hz, ArH), 9.11 (s, 2H, ArH), 9.14 (s, 2H, ArH); MALDI-TOFMS method, m / z = 938 (M ⁺ )
In Examples, for various measurements of the obtained compounds, 1H-NMR, 13C-NMR and 19F-NMR measurements were performed using BRUKER's Ultra Shield 300, and mass spectrometry was performed using Shimadzu Corporation's GCMS-QP2010, or MALDI. -Measured using TOFMS AXIMA-CFR plus.

As an example of the organic thin film transistor, a top contact type organic TFT using the organic semiconductor of the present invention was prototyped. A highly doped n-type silicon wafer was used as the substrate and gate electrode. As the gate insulating layer, a thermal oxide film (SiO ₂ ) having a thickness of 200 nm formed on a silicon wafer was used. The surface of the thermal oxide film was acid cleaned and then surface treated with hexamethyldisilazane. This substrate was set in a vacuum deposition apparatus and evacuated to a vacuum degree of 10 ⁻⁶ Torr, and then the organic semiconductor of the present invention (ie, 2,8-bis (4- (trifluoromethyl) represented by the above compound (5)). Phenyl) anthradithiophene)) was deposited to a thickness of 30 nm by vapor deposition. Finally, 50 nm thick gold source and drain electrodes were formed on the film by vacuum deposition using a shadow mask. In this case, the channel width was 1000 μm and the channel length was 50 μm.

FIG. 3 shows transistor characteristics when the organic semiconductor film is formed at room temperature and at 80 ° C. The drain current value was measured when the gate voltage was swept from -10V to 80V when the drain voltage was 80V. The threshold voltage was 33V when the substrate temperature was room temperature, but decreased to 18V at 80 ° C. It is considerably lower than the conventional threshold voltage. At the same time, an increase in drain current was observed, and a relatively high mobility of 0.1 cm ² / Vs was obtained. The on / off ratio of the current value was 5 × 10 ⁵ . Table 1 shows the performance values of each transistor.

It is a figure which shows the basic structure of organic TFT. It is a figure which shows the basic structure of heterojunction type organic TFT. FIG. 6 is a diagram showing transistor characteristics of TFTs produced in Example 2.

Claims (2)

An n-type organic thin film transistor, wherein a mixture of syn and anti isomers represented by the following formula is used for a semiconductor layer on a substrate.
(However, X represents hydrogen, halogen, cyano group, nitro group, alkyl group, alkoxy group, or alkylsilylethynyl group.)   A heterojunction organic thin film transistor comprising a mixture of the compound according to claim 1 and an anti isomer as an n-type organic semiconductor layer, which is combined with a p-type organic semiconductor layer.