JP2007234974A - Organic thin film transistor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an organic thin film transistor that obtains excellent transistor characteristics by controlling an OFF current small and reducing a contact resistance between source/drain electrodes and an organic semiconductor in order to solve a problem that the source/drain electrodes of the organic thin film transistor formed by the print method that has a greater contact resistance with the organic semiconductor than that of source/drain electrodes made of gold formed by the photolithography and the ON/OFF ratio of the transistor is decreased because a sufficient ON current cannot be obtained. <P>SOLUTION: The organic thin film transistor disclosed herein has the source/drain electrodes 14 formed particularly by the print method, the contact area between the organic semiconductor layer 15 and the source/drain electrodes 14 is increased by making the film thickness of the layer 15 thick from 50 nm or over and 500 nm or below so as to decrease the contact resistance thereby obtaining a sufficient ON current and a high ON/OFF ratio. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an organic thin film transistor.

  In recent years, electronic paper, RFID (Radio Frequency Identification) tags, and the like have attracted attention, and cost reduction, flexibility, weight reduction, and the like are required. From these viewpoints, research on organic semiconductors that replace inorganic semiconductors such as silicon as a semiconductor has been actively conducted. In general, when an organic semiconductor is used, a liquid process is possible, and thus there are advantages such as an increase in area, application of a printing method, use of a plastic substrate, and the like (see Non-Patent Document 1).

  For this reason, a transistor using a printing method, particularly an organic transistor, has attracted attention. This method is attracting attention for the following reasons.

  Since processing at a low temperature is possible, a resin film can be used for the substrate. In addition, since a semiconductor is an organic substance, a semiconductor layer pattern can be formed using a solution in which the semiconductor is dissolved in a solvent, so that a high-temperature and high-vacuum process is unnecessary and a transistor can be manufactured at low cost. Is possible.

  In order to form a semiconductor layer from a solution, a spin coating method, a dip method, an ink jet method, or the like can be used. Among them, in the spin coat method or the dip method, in a transistor array in which a plurality of transistors are arranged, current flows in the semiconductor layer between the elements, so that the current value in the off state increases and the on / off ratio decreases. There's a problem.

  In order to keep the off-state current small, it can be overcome by reducing the thickness of the semiconductor layer in addition to element isolation. It has been estimated that the effective thickness of the organic semiconductor layer is about 3 nm (Non-Patent Document 2), and a film thickness larger than that is a factor that increases the off-current.

  On the other hand, in order to realize cost reduction and flexibility, it is desirable that not only a semiconductor but also an electrode and an insulating film are formed by a printing method or the like. In the formation of an electrode using a printing method, research on the formation of an electrode pattern using a conductive polymer, a metal colloid solution, or the like has been conducted (Non-patent Document 3).

However, a source / drain electrode generally formed by a printing method has a larger contact resistance with an organic semiconductor than a source / drain electrode such as gold formed by photolithography, and sufficient on-current cannot be obtained. Therefore, the on / off ratio becomes small.
Science Vol. 265, 1684 (1994) Digest of Tech. Papers of AM-LCD04, 31 (2004) Thin Solid Films Vol. 279, 438 (2003)

  In order to obtain excellent transistor characteristics by using source / drain electrodes formed by a printing method, it is necessary to suppress off-state current and to reduce contact resistance with a semiconductor.

  The present invention has been made in view of such circumstances, and particularly in an organic thin film transistor having a source / drain electrode formed by a printing method, an organic thin film transistor capable of obtaining a sufficient on-current and a high on / off ratio. The purpose is to provide.

  As a result of the study, the present inventors have found that the above object can be achieved by increasing the contact area with the source / drain electrodes by increasing the thickness of the organic semiconductor layer and decreasing the contact resistance, thereby completing the present invention. did.

  The invention according to claim 1 is an organic thin film transistor in which at least a gate electrode, a gate insulating film, a source / drain electrode, and an organic semiconductor layer are formed on an insulating substrate, and the thickness of the organic semiconductor layer is 50 nm or more and 500 nm An organic thin film transistor characterized by the following.

  The invention according to claim 2 is the organic thin film transistor according to claim 1, wherein the thickness of the organic semiconductor layer is not less than 100 nm and not more than 500 nm.

  A third aspect of the present invention is the organic thin film transistor according to the first or second aspect, wherein the source / drain electrodes are formed by a printing method.

  The invention according to claim 4 is the organic thin film transistor according to claim 3, wherein the printing method is screen printing.

  The invention according to claim 5 is the organic thin film transistor according to claim 1 or 2, wherein the organic semiconductor layer is formed by a printing method.

  The invention according to claim 6 is the organic thin film transistor according to claim 5, wherein the printing method is any one of an ink jet method, a dispenser, and a reverse offset printing.

  As described above, according to the present invention, the contact area with the source / drain electrodes can be increased by increasing the film thickness of the organic semiconductor layer, and a sufficient on-current can be obtained by decreasing the contact resistance. Since an on / off ratio can be obtained, ideal transistor characteristics can be obtained.

Hereinafter, the present invention will be described in more detail.
As the film thickness of the organic semiconductor layer in the embodiment of the present invention, the optimum film thickness varies depending on the organic semiconductor material, the electrode material, and the electrode forming method to be used, but is generally about 50 nm to 500 nm, preferably 100 nm to 500 nm. When the source / drain electrodes are formed by screen printing, there are variations depending on the paste and screen plate used, but the arithmetic average roughness Ra may be about 1 micron and the 10-point average may be about 10 microns. Therefore, in order to increase the contact area between the organic semiconductor and the source / drain electrodes and increase the on-current in the bottom contact type element, it is preferable that the organic semiconductor layer is thicker. However, as described above, from the viewpoint of off-state current, it is preferable that the thickness of the organic semiconductor is small, so that it is preferably 500 nm or less.

  The structure of the transistor in the embodiment of the present invention is not particularly limited as long as it is a bottom contact type, and is used for a basic structure of a bottom gate / bottom contact (planar) type and a top gate / bottom contact (stagger) type. I can do it.

  The material used for the substrate in the embodiment of the present invention is not particularly limited. Examples of commonly used materials include polyethylene terephthalate (PET), polyimide, polyethersulfone (PES), polyethylene naphthalate (PEN), and polycarbonate. There are flexible plastic materials such as quartz, glass substrates such as quartz, and silicon wafers. However, considering flexibility and each process temperature, it is desirable to use PEN, polyimide, or the like as the substrate. Moreover, a barrier layer etc. can also be provided as needed.

  In the embodiment of the present invention, the material used as the electrode material is not particularly limited, but generally used materials include metal or oxide thin films such as gold, platinum, nickel, and indium tin oxide; poly ( Conductive polymers such as ethylenedioxythiophene) / polystyrene sulfonate (PEDOT / PSS) and polyaniline; thickness in which metal colloidal particles such as gold, silver and nickel are dispersed or metal particles such as silver are used as the conductive material There is a film paste. The method for forming the electrode is not particularly limited, and a dry film forming method such as vapor deposition or sputtering is also conceivable. However, in consideration of flexibility and cost reduction, it is desirable that the film is formed by a wet film forming method such as screen printing, reverse offset printing, flexographic printing, or an ink jet method. In particular, the effect of the present invention is remarkable when the electrode is thick as in screen printing.

  In the embodiment of the present invention, the material used as the gate insulating film is not particularly limited, but generally used materials include polymer solutions such as polyvinylphenol, polymethyl methacrylate, polyimide, polyvinyl alcohol, alumina, There is a solution in which particles such as silica gel are dispersed. A thin film such as PET, PEN, or PES can also be used as the gate insulating film.

  In the embodiment of the present invention, the material used as the organic semiconductor is not particularly limited, but generally used materials include polymers such as polythiophene, polyallylamine, fluorenebithiophene copolymer, and derivatives thereof. Organic semiconductor materials and low molecular organic semiconductor materials such as pentacene, tetracene, and derivatives thereof can be used, but it is preferable to use a material that is soluble in a solvent. However, carbon compounds such as carbon nanotubes or fullerenes, semiconductor nanoparticle dispersions, and the like can also be used as semiconductor materials. As a method for printing an organic semiconductor, known methods such as gravure printing, offset printing, screen printing, flexographic printing, a dispenser, and an ink jet method can be used. In general, since the organic semiconductor has low solubility in a solvent, it is desirable to use reverse offset printing, flexographic printing, an inkjet method, and a dispenser suitable for printing a low viscosity solution.

  In the embodiment of the present invention, a sealing layer or a light shielding layer may be used as necessary.

  FIG. 1 is a diagram for explaining an example of the manufacturing process of the organic thin film transistor of the present invention. In FIG. 1A, a gate electrode 12 is first provided on an insulating substrate 11, and a gate insulating film material 21 is placed in the direction of an arrow using a die coater 31 on the gate electrode 12 as shown in FIG. The gate insulating film 13 is provided by applying and drying. Next, as shown in FIG. 1C, the source / drain electrode material 22 is printed by screen printing using a screen printer 32 and dried to form the source / drain electrodes 14. Subsequently, as shown in FIG. 1D, the organic semiconductor material 23 is discharged using a dispenser 33 and dried to form the organic semiconductor layer 15. Finally, as shown in FIG. 1 (e), the sealing layer 16 is formed to obtain the organic thin film transistor of the present invention. The organic thin film transistor of the present invention is not limited to the embodiment shown in FIG. 1, and the gate insulating film 13 and the source / drain electrode 14 can be formed by methods other than those described above.

  Hereinafter, the present invention will be further described by examples.

Example 1
In this embodiment, a manufacturing method of a bottom gate / bottom contact type element will be described. Polyethylene naphthalate (PEN, manufactured by Teijin DuPont) was used as the substrate. A gate electrode was formed by depositing aluminum on the entire surface by vacuum deposition and forming a film with a thickness of 50 nm, photolithography, wet etching, and resist stripping. As the gate insulating film, polyimide (Neoprim manufactured by Mitsubishi Gas Chemical) was applied by a die coater and dried at 180 ° C. for 1 hour to form a gate insulating film. A silver paste (manufactured by Sumitomo Electric) was used as a source / drain electrode material, printed by screen printing, and dried at 180 ° C. for 1 hour to form source / drain electrodes. A fluorene bithiophene copolymer (F8T2) was used as an organic semiconductor material, printed with a dispenser, and vacuum dried at 90 ° C. for 90 minutes to form an organic semiconductor layer. At this time, the film thickness of the organic semiconductor layer was 58 nm. Thereafter, a fluororesin (Asahi Glass Cytop) was formed as a sealing layer by spin coating and dried at 90 ° C. for 3 hours. The transistor characteristics of this element are shown in Table 1. As a result, good transistor characteristics could be obtained.

(Example 2)
This example is the same as Example 1 except that the thickness of the organic semiconductor layer is 139 nm. The transistor characteristics of this element are shown in Table 1. As a result, good transistor characteristics could be obtained.

(Comparative Example 1)
This comparative example is the same as Example 1 except that the film thickness of the organic semiconductor layer is 37 nm. The transistor characteristics of this element are shown in Table 1. As a result, good transistor characteristics could not be obtained.

  Since the organic thin film transistor of the present invention can obtain a sufficient on-current, a high on-off ratio, and very good transistor characteristics, it is effective for various applications such as electronic paper and RFID tags.

(A)-(e) is a figure for demonstrating an example of the manufacturing process of the organic thin-film transistor of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Substrate, 12 ... Gate electrode, 13 ... Gate insulating film, 14 ... Source / drain electrode, 15 ... Organic-semiconductor layer, 16 ... Sealing layer, 22 ... Source / drain electrode material,
23 ... Organic semiconductor material, 31 ... Die coater, 32 ... Screen printer, 33 ... Dispenser.

Claims (6)

  An organic thin film transistor in which at least a gate electrode, a gate insulating film, a source / drain electrode, and an organic semiconductor layer are formed on an insulating substrate, wherein the organic semiconductor layer has a thickness of 50 nm to 500 nm. Thin film transistor.   The organic thin film transistor according to claim 1, wherein the organic semiconductor layer has a thickness of 100 nm to 500 nm.   The organic thin film transistor according to claim 1 or 2, wherein the source / drain electrodes are formed by a printing method.   4. The organic thin film transistor according to claim 3, wherein the printing method is screen printing.   The organic thin film transistor according to claim 1, wherein the organic semiconductor layer is formed by a printing method. 6. The organic thin film transistor according to claim 5, wherein the printing method is any one of an inkjet method, a dispenser, and reverse offset printing.

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