JP2008140883A - Organic thin film transistor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an organic thin film transistor that can reduce a source/drain electrode and a charge injection barrier of an organic semiconductor layer and uniformise the state of boundary surface. <P>SOLUTION: The organic thin film transistor is provided with a metal constituting a source/drain electrode on a boundary surface between the source/drain electrode and an organic semiconductor layer; and a fused multi-ring aromatic group thiol represented by a sulfide joint formula (1). In the formula (1), R<SB>1</SB>, R<SB>2</SB>, R<SB>3</SB>, R<SB>4</SB>, R<SB>5</SB>, R<SB>6</SB>, R<SB>7</SB>, and R<SB>8</SB>are respectively independent hydrogen, alkyl group, alkenyl group, alkoxyl group, ether group, halogen group, ketone group, ester group, amino group, amide group, cyano group, cyril group, and a substituent selected from inductive groups comprised of these groups. n is an integer of 0 to 5, and m is an integer of 0 to 12. When n and m are two or more, R<SB>1</SB>, R<SB>2</SB>, R<SB>7</SB>, and R<SB>8</SB>are independently selected from the substituents. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an organic thin film transistor and an electrode surface treatment material used therefor.

In recent years, active matrix liquid crystal display devices using active elements such as thin film transistors have been used as display screens for flat-screen televisions, personal computer monitors, and mobile phones because of their advantages of high image quality, low power consumption, and space saving.
However, the manufacturing process of an amorphous silicon thin film conventionally used as a semiconductor layer requires an expensive vacuum apparatus and requires several steps because of using photolithography, which increases the manufacturing cost. . In addition, when an inorganic material as described above is used as a semiconductor, the manufacturing (film formation) temperature of a thin film transistor (TFT) requires a high temperature of 300 to 400 ° C., so that flexibility such as plastic and impact resistance are improved. Fabrication on a rich substrate is extremely difficult.

On the other hand, if an organic semiconductor material is used, it becomes possible to form a film at a temperature lower than the heat resistant temperature of the plastic. Further, if a semiconductor film can be formed by a solution process, cost reduction, large area, and flexibility can be achieved. Be expected.
Here, in the conventional organic thin film transistor, a noble metal having a large work function, such as gold (Au), platinum (Pt), palladium (Pd), or the like is used for the electrode in order to ensure good ohmic contact with the organic semiconductor material. . However, the electrical or physical contact between the metal constituting the electrode and the organic semiconductor layer is poor, and may have a specific resistance value comparable to or higher than the specific resistance of the semiconductor layer. Resistors make it difficult to make transistors finer and higher performance.

According to Non-Patent Document 1, the transistor structure disclosed in Non-Patent Document 1 cannot obtain sufficient contact resistance because the organic semiconductor layer is directly stacked on the source / drain electrodes. Note that Non-Patent Document 1 mainly measures the mobility evaluation of an organic transistor in an academic field under a high voltage of several tens to 100 V of the gate voltage and the drain voltage. It is assumed that the charge injection barrier is not a problem.
On the other hand, Patent Document 1 or Patent Document 2 discloses a transistor including a conductive organic material between a source / drain electrode and an organic semiconductor layer, but it cannot be said that contact resistance is sufficient. In addition, after forming the metal source / drain electrodes, the conductive organic layer needs to be subjected to patterning or impurity doping, which is not easy in terms of process.

Therefore, in order to drive a transistor within a practical driving voltage range, which is adaptable to a solution process that is simple in terms of cost, a source / drain electrode and an organic semiconductor are formed by a method as simple as possible. There is a need to improve the contact (contact resistance) between the layers.
CD Dimitrakopoulos, PRL Malenfant, ADVANCED MATERIALS, 14, 99 (2002) JP 2006-49577 A JP 2006-148131 A

  The present invention employs an interfacial contact layer that improves electrical and physical contact between the source / drain electrode and the organic semiconductor layer as described above, thereby making transistor operation and device performance in a more practical low-voltage region. An organic thin film transistor that can be made uniform is provided.

In order to solve the above-mentioned problems, the present inventor has intensively studied. As a result, the source / drain electrode of the organic thin film transistor including the gate electrode and the gate insulating layer, the source / drain electrode, and the organic semiconductor layer, the organic semiconductor layer, A structure in which the condensed polycyclic aromatic thiol represented by the formula (1) is sulfide-bonded at the interface between the metal constituting the source / drain electrode and sulfur of the thiol is useful in at least a part of the region between The present invention has been completed.
That is, the present invention
1. A condensed polycyclic aromatic thiol represented by the formula (1) which is sulfide-bonded to a metal constituting the electrode is present at the interface between the source / drain electrode and the organic semiconductor layer. Organic thin film transistor.

式（１）中、Ｒ_１、Ｒ_２、Ｒ_３、Ｒ_４、Ｒ_５、Ｒ_６、Ｒ_７、Ｒ_８は、各々独立に水素、アルキル基、アルケニル基、アルコキシル基、エーテル基、ハロゲン基、ケトン基、エステル基、アミノ基、アミド基、シアノ基、シリル基、およびこれらの基から構成される誘導基から選ばれる置換基であり、ｎは０〜５の整数、ｍは０〜１２の整数である。尚、ｎ、ｍが２以上の場合のＲ_１、Ｒ_２、Ｒ_７、Ｒ_８は各々が独立に上記置換基から選ばれる。
２、基板上にゲート電極、ゲート絶縁層、ソース／ドレイン電極を記載の順に配置してなるトランジスタ用基板の表面を、
（１）１に記載の縮合多環芳香族チオールの溶液で表面処理して製造する事を特徴とする界面コンタクト層の製造工程と、
（２）引き続き、有機半導体の溶液で表面処理して製造する事を特徴とする有機半導体層の製造工程と、
を経て製造されることを特徴とする１に記載の有機薄膜トランジスタの製造方法である。

In formula (1), R ₁ , R ₂ , R ₃ , R ₄ , R ₅ , R ₆ , R ₇ , R ₈ are each independently hydrogen, alkyl group, alkenyl group, alkoxyl group, ether group, halogen group , A ketone group, an ester group, an amino group, an amide group, a cyano group, a silyl group, and a substituent selected from these groups, n is an integer of 0 to 5, and m is 0 to 12 Is an integer. In the case where n and m are 2 or more, R ₁ , R ₂ , R ₇ and R ₈ are each independently selected from the above substituents.
2. A surface of a transistor substrate in which a gate electrode, a gate insulating layer, and a source / drain electrode are arranged on the substrate in the order described,
(1) A process for producing an interface contact layer, characterized by being produced by surface treatment with a solution of the condensed polycyclic aromatic thiol according to 1;
(2) Subsequently, a manufacturing process of an organic semiconductor layer characterized in that it is manufactured by surface treatment with an organic semiconductor solution;
2. The method for producing an organic thin film transistor according to 1, wherein the organic thin film transistor is produced through a process.

In the present invention, a condensed polycyclic aromatic thiol is disposed between a source / drain electrode and an organic semiconductor layer, and the sulfur of the thiol is sulfide-bonded to a metal constituting the source / drain electrode (hereinafter referred to as a condensed poly thiol). By forming the interface contact layer (also referred to as a derivative of a ring aromatic thiol), it was possible to reduce the charge injection barrier from the source / drain electrode to the organic semiconductor layer and to reduce the contact resistance between them. .
In addition, the interface contact layer made of a derivative of a condensed polycyclic aromatic thiol improves the wettability when an organic semiconductor layer is formed using an organic semiconductor solution. As a result, the physical contact between the organic semiconductor layer and the interface contact layer is improved. It is possible to improve the general adhesion, to achieve uniform and stable transistor performance, and to obtain a good organic thin film transistor.

In this embodiment, the condensed polycyclic aromatic thiol represented by the following general formula (1) is formed on the surface of the source / drain electrode of the transistor substrate in which the gate electrode, the gate insulating layer, and the source / drain electrode are arranged in the order described. An organic thin film transistor having a structure in which an organic semiconductor layer is formed through an interface contact layer made of a derivative.
The condensed polycyclic aromatic thiol derivative forming the interface contact layer of the present invention is composed of a thiol having the structure of the following general formula (1).

本発明は、界面コンタクト層に上記の縮合多環芳香族チオールを用いることにより、有機薄膜トランジスタを構成する際に、界面コンタクト層を構成する当該チオールの硫黄原子が、ソース／ドレイン電極表面の金属原子とスルフィド結合を形成することで縮合多環芳香族チオール誘導体となり、界面に電極表面から界面コンタクト層への電荷授受が引き起こされ、結果として界面双極子を発生させることができると推察する。
これにより前記ソース／ドレイン電極表面金属と界面コンタクト層分子との電荷移動量は分子構造を変更することで調整可能と考えられるため、界面状態の制御も可能となる。

In the present invention, when the organic thin film transistor is formed by using the above condensed polycyclic aromatic thiol for the interface contact layer, the sulfur atom of the thiol constituting the interface contact layer is a metal atom on the surface of the source / drain electrode. It is inferred that a condensed polycyclic aromatic thiol derivative is formed by forming a sulfide bond with the compound, and charge transfer from the electrode surface to the interface contact layer is caused at the interface, resulting in generation of an interface dipole.
Thereby, it is considered that the amount of charge transfer between the metal on the surface of the source / drain electrode and the interface contact layer molecule can be adjusted by changing the molecular structure, so that the interface state can be controlled.

If electrons move from the source / drain electrode surface metal to the interface contact layer, an interface dipole from the electrode surface to the interface contact layer is generated as the Fermi energy of both layers reaches an equilibrium state. At this time, the vacuum level specific to the interface contact layer molecule is increased by the energy corresponding to the interface dipole compared to the vacuum level of the source / drain electrode, so that the work function of the source / drain electrode is caused by the interface dipole. It can be increased by the increased vacuum level difference.
For example, when a p-type organic semiconductor layer is formed on the source / drain electrode whose work function is increased by the effect of the interface contact layer, the highest occupation of the p-type organic semiconductor molecule from the Fermi level of the source / drain electrode. The energy barrier for charge injection into the orbit (HOMO) is reduced, and charge can be injected into the organic semiconductor layer with a relatively small drain voltage.

In addition, since the source / drain electrode surface and the interface contact layer are connected by a sulfide bond (chemical bond), the electronic state of the interface is stabilized, and the transistor performance varies and the contact between the organic semiconductor layer and the source / drain electrode. The defect is eliminated, and as a result, the reliability of the transistor performance is improved.
The interface contact layer itself has low electrical conductivity, and charge injection from the source / drain electrode to the organic semiconductor layer is caused by the tunnel effect, so it is considered that there is a more effective interface contact layer thickness. As a layer, in formula (1), it is calculated | required that n is 0 or more and 5 or less, More preferably, it is an integer of 1 or more and 4 or less. For the same reason, in formula (1), m is 0 or more and 12 or less, More preferably, it is an integer of 0 or more and 5 or less.

  In order to form such an interface contact layer, the thiol represented by the formula (1) is dissolved in an appropriate solvent such as toluene, chlorobenzene, alcohol, and the source / drain electrodes arranged in advance at a predetermined distance from each other. A transistor substrate comprising: a gate electrode disposed corresponding to a position of a channel region formed in the organic semiconductor layer; and a gate insulating layer disposed between the gate electrode and the source / drain electrode. The metal constituting the electrode and sulfur of the thiol are easily sulfided on the surface of the source / drain electrode by immersing in the thiol solution for about 0.5 hours to 2 hours or allowing the thiol solution to be applied to the transistor substrate and allowing it to stand. A bonded interface contact layer can be formed.

As a method for applying the thiol solution, a spin coat method, a dip method, a drop cast method, or the like is desirable. Note that the interface contact layer does not necessarily have a uniform continuous layered structure (a structure that completely covers the source / drain electrode), and at least one of the interfaces at the interface between the source / drain electrode and the organic semiconductor layer. An interface contact layer may be formed on the part.
The present invention further includes applying a solution of an organic semiconductor material with an aromatic solvent on the transistor substrate coated with the source / drain electrodes with the interface contact layer or immersing the substrate, and then adding the aromatic solvent. By removing the organic semiconductor layer, an organic semiconductor transistor is formed.

  In such a manufacturing process of an organic semiconductor layer by a so-called solution process, since the interface contact layer itself is a condensed polycyclic aromatic compound, the affinity with an aromatic solvent is improved, and the wettability of the solution is further increased. Is improved. In forming an organic semiconductor thin film by a solution process, wettability in a solution state is an extremely important factor, and a suitable organic semiconductor thin film layer is formed by providing the interface contact layer. Furthermore, since the organic semiconductor material used in the present invention also has a conjugated bond, the affinity of the interface is improved, and the adhesion between the source / drain electrode and the organic semiconductor layer can be improved.

By the way, the thiol used for forming the interface contact layer may be a condensed polycyclic aromatic thiol represented by the general formula (1), but R ₁ , R ₂ , R ₃ and R ₄ are Each independently selected from alkyl groups, alkenyl groups, alkoxyl groups, ether groups, ketone groups, ester groups, amino groups, amide groups, cyano groups, silyl groups, and derivative groups composed of these groups. (When m is 2 or more, R ₁ and R ₂ are also independently selected from the above groups), and R ₅ , R ₆ , R ₇ , and R ₈ are selected from any of hydrogen and halogen groups (In the case where n is 2 or more, R ₇ and R ₈ are each independently selected from hydrogen and halogen groups). Of these, benzenethiol, 2-naphthalenethiol, and 2-naphthalenemethanethiol are more preferable.

On the other hand, examples of the organic semiconductor material include both high polymers and low molecules. As the polymer, polyacetylene, polythiophene, polypyrrole, polythienylene vinylene, polyphenylene vinylene, polyfluorene, or derivatives thereof are desirable. For low molecules, thiophene oligomers, acene compounds typified by pentacene, phthalocyanines typified by copper phthalocyanine, anthradithiophene, bisdithienothiophene, or derivatives thereof are desirable.
As a method for forming the organic semiconductor layer, the organic semiconductor layer can be formed by a solution process as described above. For example, methods such as spin coating, dipping, screen printing, ink jet, blade coating, printing (lithographic printing, letterpress printing, intaglio printing, etc.) can be used. Examples of low molecular weight systems include vacuum vapor deposition, molecular beam epitaxy, sputtering, laser vapor deposition, vapor phase transport growth, and the like, but these production methods are difficult to apply to polymer systems. In view of this, and the apparatus to be used becomes complicated, it is preferable to form by a solution process.

Next, a transistor substrate as a base material on which the interface contact layer and the organic semiconductor layer of the present invention are arranged will be described.
The transistor substrate that can be used in the present invention is a substrate in which a gate electrode, a gate insulating layer, and a source / drain electrode are arranged in the order described. The details will be described below.
Materials constituting the gate electrode that can be used in the present invention include metals such as chromium, nickel, molybdenum, tungsten, gold, platinum, silver, copper, aluminum, and palladium, alloys using these metals, and polysilicon. Inorganic materials such as amorphous silicon, indium oxide, and indium tin oxide are preferable.

In the present invention, examples of the material constituting the gate insulating layer include SiO ₂ , SiNx, AlN, Ta ₂ O ₅ , Al ₂ O ₃ , and HfO ₂ . In addition to inorganic materials, polymer insulating film materials include polyvinyl chloride, polyvinyl alcohol, polyacrylonitrile, polyvinylidene fluoride, cyanoethyl pull column, polychloropyrene, polyethylene terephthalate, polymethyl methacrylate, polysulfone, polycarbonate, polyimide, etc. Organic materials are desirable.
And as a material which comprises a source / drain electrode, the metal in which ohmic contact with a semiconductor material is possible is preferable. For example, in the case of a p-type organic semiconductor in which the transport carrier is a hole, a metal having a large work function is required, and specific examples include gold, platinum, palladium, and the like. In the case of an n-type organic semiconductor whose transport carrier is an electron, a metal having a small work function is required, and examples thereof include aluminum, magnesium, and calcium.

The substrate on which the gate electrode, the gate insulating layer, the source / drain electrode, the organic semiconductor layer, and the interface contact layer are formed may be an insulating material and can be suitably selected from a wide range. Examples thereof include inorganic materials such as glass and alumina sintered bodies, and insulating plastics such as polyimide films, polyester films, and polyethylene films. In particular, when a plastic substrate is used, a flexible organic thin film transistor that is lightweight and excellent in impact resistance can be produced.
The transistor substrate including the substrate, the gate electrode, the gate insulating layer, and the source / drain electrodes described above can be manufactured by a normal method.

Hereinafter, the structure of an organic thin film transistor according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a cross-sectional view of an organic thin film transistor according to an embodiment of the present invention.
In FIG. 1, a gate electrode 5 is formed on a substrate 6, and a gate insulating layer 4 is formed so as to cover the gate electrode 5. A source / drain electrode 2 is formed on the gate insulating layer 4 with the gate electrodes 5 being spaced apart from each other by a predetermined distance. An interface contact layer 3 disposed between the source / drain electrode 2 and the organic semiconductor layer 1 is formed.

FIG. 2 is a cross-sectional view showing an example of the structure of the organic thin film transistor of the present invention.
FIG. 3 is a cross-sectional view showing an example of the structure of the organic thin film transistor of the present invention.
FIG. 4 is a cross-sectional view showing an example of the structure of the organic thin film transistor of the present invention.
1 to 4 in the above drawings are cross-sectional views respectively showing examples of the structure of the organic thin film transistor of the present invention. Thus, the transistor substrate that can be used in the present invention basically has a gate insulating layer between the gate electrode and the source / drain electrode, and an organic semiconductor layer between the source / drain electrodes. In addition, as long as the interface contact layer exists between the source / drain electrode and the organic semiconductor layer, it is within the scope of the present invention.

As a means for specifying that the interface contact layer is formed, first, the metal atom of the electrode and sulfur form a sulfide chemical bond. The energy that the work function of the metal corresponds to the interface dipole Can be identified by confirming that only shift. Therefore, it can be confirmed that a chemical bond is formed on the electrode surface by analyzing the shift amount of the work function by an analysis method such as photoelectron spectroscopy.
Moreover, when specifying the molecular structure of the interface contact layer formed on the electrode surface, the element is specified by an X-ray photoelectron spectrometer or an Auger electron spectrometer, and surface mass spectrometry such as TOF-SIMS method is performed. By using it, the molecular weight of the interface contact layer molecule can be estimated. Furthermore, the vibration surface of the aromatic ring and the presence of other functional groups can be confirmed on the electrode surface by infrared spectroscopy, and the structure of the interface contact layer molecule is specified by combining the analysis results. It is possible.

  Examples of the present invention will be described below.

[Example 1]
First, a silicon substrate heavily doped with an n-type dopant (having a 200 nm thick thermal oxide film on the surface) was prepared, and a gold electrode pattern was formed on the surface as a source / drain electrode.
Next, naphthalene-2-methanethiol was used as a thiol for forming the interface contact layer, and dissolved in toluene at a concentration of about 3 mM as a solvent. The silicon substrate on which the electrode pattern is formed is immersed in the thiol derivative solution for about 1 hour, and then immersed in fresh toluene and ultrasonically cleaned to remove excess thiol, and an interface contact layer is formed on the surface of the source / drain electrode. Formed.

A mixture of 10 mg of pentacene powder and 15 g of 1,2,4-trichlorobenzene was heated to 180 to 200 ° C. under a nitrogen atmosphere to prepare a red-purple solution.
Under a nitrogen atmosphere, the pentacene solution was spread on a silicon substrate having an electrode pattern provided with the interface contact layer previously heated to 200 ° C., and the solvent was evaporated to form a pentacene thin film as an organic semiconductor layer.
For comparison, the pentacene solution was similarly developed on a substrate with an electrode pattern without an interfacial contact layer, and a pentacene solution was formed. Was not obtained. When the characteristics of the organic thin film transistor fabricated as described above were evaluated (channel width 500 mm, channel length 50 mm), the mobility of the pentacene thin film transistor having the interface contact layer was 0.10 cm ² / Vs on average and the threshold was −10 V. On the other hand, the mobility of the pentacene thin film transistor not provided with the interface contact layer was 0.03 cm ² / Vs on average and the threshold value was −19V.

Furthermore, in order to estimate the contact resistance between the source / drain electrode and the organic semiconductor layer in order to investigate the effect of improving the contact between the source / drain electrode and the organic semiconductor layer, the channel length dependence of the channel resistance value was examined. As a result, at a gate voltage of −20 V, the resistance value of the transistor having the interface contact layer is 1.4 × 10 ⁵ Ωcm, whereas the transistor having no interface contact layer is 5.5 × 10 ⁵ Ωcm, which is about four times as large. A large value was shown.

[Example 2]
A mixture of 5 mg of 2-hexylpentacene powder and 7 mg of 1,2,4-trichlorobenzene was heated to 120 to 130 ° C. under a nitrogen atmosphere to prepare an organic semiconductor solution.
In a nitrogen atmosphere, the 2-hexylpentacene solution is preheated to 120 ° C., and then the solution is placed on a silicon substrate having a gold electrode pattern having an interface contact layer obtained by the same method as described in Example 1. By developing and removing the solvent by heating and evaporation, a 2-hexylpentacene thin film, which is an organic semiconductor layer, was formed to obtain a transistor structure.
The obtained transistor had good wettability of the organic semiconductor solution on the substrate having the interface contact layer as in Example 1. When the characteristics of the organic thin film transistor were evaluated, the mobility of the transistor having the interface contact layer was 0.08 cm ² / Vs on average and the threshold was −8.7 V on average. On the other hand, in the transistor having no interface contact layer, the average mobility was 0.005 cm ² / Vs, and the average threshold was 5.4 V. With respect to the 2-hexylpentacene thin film transistor, a reduction in contact resistance was confirmed by the interface contact layer, and no significant decrease in mobility was observed even in the short channel transistor.

  Although the present invention has been described with reference to the preferred embodiments, the present invention is not limited to the above embodiments.

  The present invention can be widely used in the field using transistors.

It is sectional drawing which shows the structure of the organic thin-film transistor which concerns on the Example of this invention. It is sectional drawing which shows the structure of the organic thin-film transistor of this invention. It is sectional drawing which shows the structure of the organic thin-film transistor of this invention. It is sectional drawing which shows the structure of the organic thin-film transistor of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor layer 2 Source / drain electrode 3 Interface contact layer 4 Gate insulating layer 5 Gate electrode 6 Substrate

Claims (2)

An organic thin film transistor, characterized in that a condensed polycyclic aromatic thiol represented by the following formula (1) which is sulfide-bonded to a metal constituting the electrode exists at an interface between the source / drain electrode and the organic semiconductor layer.

In formula (1), R ₁ , R ₂ , R ₃ , R ₄ , R ₅ , R ₆ , R ₇ , R ₈ are each independently hydrogen, alkyl group, alkenyl group, alkoxyl group, ether group, halogen group , A ketone group, an ester group, an amino group, an amide group, a cyano group, a silyl group, and a substituent selected from these groups, n is an integer of 0 to 5, and m is 0 to 12 Is an integer. In the case where n and m are 2 or more, R ₁ , R ₂ , R ₇ and R ₈ are each independently selected from the above substituents. A surface of a transistor substrate in which a gate electrode, a gate insulating layer, and a source / drain electrode are arranged on the substrate in the order described,
(1) A process for producing an interfacial contact layer, characterized by being produced by surface treatment with a solution of the condensed polycyclic aromatic thiol according to claim 1;
(2) Subsequently, a manufacturing process of an organic semiconductor layer characterized by being manufactured by surface treatment with an organic semiconductor solution;
The method for producing an organic thin film transistor according to claim 1, wherein the organic thin film transistor is produced through a process.

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