JP6934130B2 - Thienoacen single crystal organic semiconductor film - Google Patents

Description

The present invention relates to an organic semiconductor film and a method for producing the same. The present invention also relates to an organic semiconductor device provided with an organic semiconductor film.

Organic semiconductors have been inferior in electrical properties to inorganic semiconductors such as silicon, but recently, materials with excellent electrical properties are being developed, and research on crystal states and carrier mobility is progressing. I'm out. Organic semiconductors have the advantages of low cost, low environmental load, and large area as compared with the vacuum technology conventionally used for inorganic semiconductors. Further, since it can be manufactured near room temperature, it is possible to form a film by printing technology on a plastic substrate. Therefore, organic semiconductors are expected to be applied to next-generation electronic devices as post-silicon semiconductors.

As a method for producing an organic semiconductor film, various methods have been conventionally studied depending on the characteristics of the material, such as a vapor deposition method, a molecular beam epitaxial method, a solvent evaporation method, a melt method, and a Langmuir-Brojet method. Among these methods, the solvent evaporation method is a method of evaporating a solvent from an organic semiconductor solution to saturate the solution and precipitate crystals to form an organic semiconductor film, which is a simple method but has high performance. It is excellent in that an organic semiconductor thin film can be obtained.

Among the solvent evaporation methods, the solvent evaporation method based on a coating method using a solution such as an edge casting method, a drop casting method, a spin coating method, and a printing method (inkprint method or gravure printing method) is simple and inexpensive. This is a very promising method from the viewpoint that it can be manufactured near room temperature. In particular, the edge casting method is a promising technique in that a single crystal film can be produced as described in WO2011 / 0401155.

In the process of forming an organic semiconductor using the solvent evaporation method, it is necessary to dissolve the organic semiconductor in a solvent, but typical organic semiconductors such as acene derivatives and thiophene derivatives generally have poor solubility in a solvent. There is a problem that it easily gels. Therefore, many studies have been conducted to improve the solubility of organic semiconductor materials. In recent years, as described in Patent Document 1, it has also been proposed to solve the problem of solubility by combining a specific solvent and a specific organic semiconductor.

WO2011 / 0401155 Japanese Unexamined Patent Publication No. 2006-287224

It has been difficult to obtain a high-quality single crystal film by the conventional organic semiconductor film forming technology. The edge casting method described in Patent Document 1 is certainly promising as a technique capable of forming a single crystal film, but there is still room for improvement. It is considered that this is due to the fact that the material development and film formation technology of organic semiconductors are immature, and the problem of solubility of organic semiconductors in solvents has not been solved.

This problem has not been sufficiently solved by the technique described in Patent Document 2. According to the technique described in Patent Document 2, a stable solution of a polythiophene semiconductor can be prepared by using a halogen-containing aromatic compound as a solvent. However, although Patent Document 2 has specific data on the stability of the solution, it does not describe a specific example of actually forming a polythiophene semiconductor on a substrate, and there is no description about its crystallinity. According to the study results of the present inventor, even if a polythiophene semiconductor is formed on a substrate by the technique described in Patent Document 2, fine cracks are formed in the obtained thin film, and a high-quality single crystal thin film is obtained. It turned out that I couldn't get it. Since the presence of cracks adversely affects the carrier mobility, which is an important electrical property of the semiconductor, it is desirable to provide an organic semiconductor thin film in which cracks are suppressed. Further, the smaller the number of domains of the obtained organic semiconductor film, the higher the quality as a single crystal. However, in the prior art, only an organic semiconductor film having a large number of domains has been obtained.

The present invention has been created in view of the above circumstances, and one of the problems is to provide a single crystal organic semiconductor film in which cracks are suppressed and high homogeneity is provided. Another object of the present invention is to provide a method suitable for producing such an organic semiconductor film. The present invention also relates to an organic semiconductor device provided with such a single crystal film of an organic semiconductor.

As a result of diligent studies to solve the above problems, the present inventor prepares a solution of an organic semiconductor using 3-chlorothiophene as a solvent, and forms an organic semiconductor film by the solvent evaporation method to suppress crack generation. It was found that an organic semiconductor film with high homogeneity in which the number of domains is suppressed can be obtained. In particular, when a solution of an organic semiconductor is prepared using 3-chlorothiophene as a solvent and an organic semiconductor film is formed by the edge casting method, the occurrence of cracks is remarkably suppressed and a single crystal film having a high coverage can be obtained. I found that. Such an effect was not obtained by the similar compound 2-chlorothiophene, and further, it was not obtained by other halogen-based aromatic solvents or non-halogen-based aromatic solvents. It is considered to be a specific effect. The present invention has been completed based on the above findings, and is exemplified below.

[1]
A single crystal organic semiconductor film in which the number of domains existing in an area per 1 mm ^{2 is 10 or less.}

[2]
The single crystal organic semiconductor film according to [1], which has a coverage of 0.98 or more.

[3]
The single crystal organic semiconductor film according to [1] or [2], wherein the distribution range of the crystal axis is within 10 °.

[4]
The single crystal organic semiconductor film according to any one of [1] to [3], wherein the standard deviation of the crack angle is 0.5 rad or less.

[5]
The single crystal organic semiconductor film according to any one of [1] to [4], wherein the organic semiconductor is polythiophene or thianoacen.

[6]
The single crystal organic semiconductor film according to any one of [1] to [5], which has an average mobility of 5 cm ^{2 / Vs or more.}

[7]
The single crystal organic semiconductor film according to any one of [1] to [6], wherein the mobility coefficient of variation is 40 or less.

[8]
The single crystal organic semiconductor film according to any one of [1] to [7], which has an average film thickness of 1 μm or less.

[9]
Step 1 of preparing an organic semiconductor solution by dissolving an organic semiconductor in 3-chlorothiophene,
Step 2 of adhering the organic semiconductor liquid on the surface of the substrate,
Step 3 of evaporating 3-chlorothiophene from the organic semiconductor liquid on the surface of the substrate, and
A method for producing an organic semiconductor film including.

[10]
Step 1 of preparing an organic semiconductor solution by dissolving an organic semiconductor in 3-chlorothiophene,
Step 2 of preparing a substrate on which a contact member having an end face that stands up against the surface of the substrate is placed, and
Solution obtained in step 1 was supplied to the substrate, a step 3 of forming droplets that simultaneously contact the substrate surface and the end face,
Step 4 to evaporate 3-chlorothiophene from the droplets obtained in step 3 and
A method for producing an organic semiconductor film including.

[11]
The method for producing an organic semiconductor film according to [9] or [10], wherein the organic semiconductor is a thiophene derivative.

[12]
The method for producing an organic semiconductor film according to any one of [9] to [11], wherein the concentration of the organic semiconductor in the organic semiconductor solution is 0.2% by mass or less.

[13]
The method for producing an organic semiconductor film according to any one of [9] to [12], wherein the solution is supplied onto the substrate by a printing method.

[14]
The method for producing an organic semiconductor film according to any one of [9] to [13], wherein the organic semiconductor film is single crystal.

[15]
An organic semiconductor device comprising a substrate and a single crystal organic semiconductor film according to any one of [1] to [8] formed on the surface of the substrate.

[16]
A transistor provided with the single crystal organic semiconductor film according to any one of [1] to [8] as a semiconductor channel.

According to the present invention, an organic semiconductor film having a low number of domains without cracks and having a high coverage is provided. This makes it possible to manufacture an organic semiconductor device having excellent quality stability and high carrier mobility with a high yield.

It is a schematic perspective view for demonstrating the process of the edge casting method. It is a schematic perspective view for demonstrating the process following FIG. It is a schematic cross-sectional view for demonstrating the process of the edge casting method. It is a schematic cross-sectional view for demonstrating the modification of the edge cast method. It is a figure which shows typically the optical system for measuring a crystal axis. It is a schematic cross-sectional view for demonstrating the process of the continuous edge casting method. It is another schematic sectional view for demonstrating the process of the continuous edge casting method. This is an example of a photograph of an organic semiconductor film according to the present invention (Experiment No. 1). This is an example of a photograph of an organic semiconductor film according to a comparative example (Experiment No. 2).

<1. Coverage>
In one embodiment, the single crystal organic semiconductor film according to the present invention is characterized in that cracks in the film are suppressed and regions where the film is defective are also suppressed, so that the coverage of the film on the substrate is high. If cracks occur in the film or defect areas occur in the film-forming region, the coverage decreases. When the coverage is lowered, the yield is lowered and the quality stability of the organic semiconductor device is lowered. According to the present invention, the coverage of the organic semiconductor film can be 0.98 or more, preferably 0.99 or more. Here, the coverage is defined as the average value of the ratio occupied by the film when a range of ^{10 4} μm ² arbitrarily extracted from the region where the film is formed is observed at a plurality of places.

<2. Number of domains>
Further, since the single crystal organic semiconductor film according to the present invention has high single crystal property in one embodiment, the number of crystal domains can be extremely reduced. Specifically, the ^{number of domains per 1 mm 2} can be 10 or less, preferably 5 or less, and more preferably 1. When the number of domains is 1, it means that it is a so-called single domain single crystal, and it means that the crystal orientations are uniform.

<3. Crack angle>
In one embodiment of the single crystal organic semiconductor film according to the present invention, even if cracks occur, the cracking directions (angles) are aligned with the crystal axes having high mobility. Since the current usually flows along the direction in which the mobility is high, there is an advantage that the crack does not significantly affect the mobility or the transistor characteristics of the transistor. Specifically, the standard deviation of the crack angle can be 0.5 rad or less, preferably 0.3 rad or less, more preferably 0.2 rad or less, and even more preferably. Can be 0.15 rad or less, for example 0.1 to 0.5 rad.

<4. Crystal axis>
In one embodiment, the organic semiconductor film according to the present invention is a single crystal film having high uniformity of crystal axis. Specifically, the distribution range of the crystal axis can be within 10 °, preferably within 8 °, and even more preferably within 7 °. Such a single crystal film is advantageous for application to a large area electronic device.

In the present invention, the distribution range of the crystal axis is measured by the following procedure. The optical system shown in FIG. 5 is constructed, and low-angle incident X-ray diffraction (GIXD) measurement is performed. Specifically, while rotating 360 degrees around an axis φ perpendicular to the surface of the semiconductor film, X-rays (beam diameter is about 1.5 cm) from the light source L are diffracted by the organic semiconductor film to detect diffracted light. Detected by vessel D. Thereby, the distribution of the diffraction intensity (y-axis) when the thin film is rotated in the φ direction is measured. 2θ (x-axis) is set to 22.56 degrees. The standard deviation of the obtained diffraction intensity distribution is defined as the distribution range of the crystal axis.

<5. Carrier mobility (average value)>
Mobility is an important characteristic that affects the response speed of semiconductor devices. The low mobility of an organic semiconductor has been a problem to be solved for practical use, but the organic semiconductor film according to the present invention can have an excellent mobility higher than that of amorphous silicon in one embodiment, which is preferable. In the embodiment, the mobility can be comparable to that of polycrystalline silicon. Specifically, the organic semiconductor film according to the present invention ^{can have an average mobility of 5 cm 2} / Vs or more, preferably 7 cm ² / Vs or more in one embodiment. more preferably can have a 8 cm ² / Vs or more average mobility, even more preferably can have a 10 cm ² / Vs or more average mobility, for example, an average mobility of 8~12cm ² / Vs Can have.

In the present invention, the mobility is calculated based on a commonly used FET mobility evaluation method (FET method). That is, the FET current type
Id = (W / 2L) ・ μ ・ Cox (Vg-Vt) ^ 2 ・ ・ ・ In the case of saturation region (in the formula, Id: drain current, Vg: gate voltage, Vt: threshold voltage, μ: mobility, Cox : Oxide film capacity, W: Channel width, L: Channel length.)
The mobility is determined from the measured value of IdVg of the FET by transforming from the equation relating to the mobility μ.

<6. Carrier mobility (coefficient of variation)>
High homogeneity of the organic semiconductor film is an important property for achieving a high yield, but a small coefficient of variation of mobility is advantageous from the viewpoint of homogeneity. In this respect, the organic semiconductor film according to the present invention has a controlled mobility coefficient of variation in one embodiment. Specifically, in one embodiment, the organic semiconductor film according to the present invention has a mobility fluctuation coefficient of 40 or less, preferably a mobility fluctuation coefficient of 35 or less, and more preferably a mobility fluctuation coefficient. Is 30 or less, and even more preferably, the mobility variation coefficient is 30 or less, for example, the mobility variation coefficient is 20 to 30.

In the present invention, the coefficient of variation of mobility is calculated by the following equation from the mobility data obtained when the average value of mobility is calculated.
(Floor coefficient of mobility) = (standard deviation of mobility) / (average value of mobility) x 100

<7. Film thickness>
The film thickness required for the organic semiconductor film varies depending on the application and is not particularly limited, but in general, it is desirable that the film thickness is thin in order to reduce resistance outside the channel region. The organic semiconductor film according to the present invention can have an average film thickness of 1 μm or less in one embodiment, 100 nm or less in another embodiment, and 50 nm or less in yet another embodiment. Can be. The organic semiconductor film according to the present invention can have an average film thickness of 20 nm or less in one preferred embodiment, 15 nm or less in another embodiment, and 10 nm or less in yet another embodiment. Can be.

<8. Organic semiconductors>
The type of organic semiconductor constituting the organic semiconductor film according to the present invention is not particularly limited, but a material having a high self-condensing function is desirable. The self-condensing function means that when a molecule precipitates from a solvent, it spontaneously aggregates and tends to crystallize easily. Further, a thiophene derivative is preferable from the viewpoint of compatibility with 3-chlorothiophene as a solvent. Examples of the thiophene derivative include polythiophene and thienoacene (thiophene-containing polycyclic aromatic compound), and examples thereof include compounds having the following chemical structure. From the viewpoint of obtaining a single crystal, it is necessary to use the organic semiconductor alone, and it is not desirable to use a mixture of a plurality of types of organic semiconductors.

式（ｉ）中、Ｒ¹及びＲ²はそれぞれ独立に水素原子又は炭素数が４〜１０のアルキル基である。アルキル基はヘテロ原子（典型的には酸素原子及び硫黄原子から選択される。）を含んでもよい。また、Ｒ¹及びＲ²は一緒になって環を形成することもできる。自己凝集能の理由により、好ましくは、Ｒ¹及びＲ²はそれぞれ独立に水素原子又は炭素数が５〜８のアルキル基である。より好ましくはＲ¹及びＲ²はそれぞれ独立に水素原子又はヘキシル基である。 (A. Polythiophene semiconductor)
Since the polythiophene semiconductor has a high on / off ratio (conductivity in the on state with respect to the conductivity in the off state) and high mobility, it can be suitably applied to semiconductor devices such as transistors.

In formula (i), R ¹ and R ² are independently hydrogen atoms or alkyl groups having 4 to 10 carbon atoms. Alkyl groups may contain heteroatoms (typically selected from oxygen and sulfur atoms). R ¹ and R ² can also be combined to form a ring. For reasons of self-aggregating ability, R ¹ and R ² are preferably hydrogen atoms or alkyl groups having 5 to 8 carbon atoms, respectively. More preferably, R ¹ and R ² are independently hydrogen atoms or hexyl groups, respectively.

n represents an integer of 5 to 100. n indicates the average number of thiophene monomer units in the polythiophene semiconductor, that is, the length of the polythiophene chain. From the viewpoint of forming a single crystal film, n is preferably 50 or less.

In one embodiment of the polythiophene semiconductor, in the above formula (i), R ¹ is a hydrogen atom and R ² is an alkyl group having 4 to 10 carbon atoms. For the reason of self-aggregation ability, R ¹ is preferably a hydrogen atom and R ² is an alkyl group having 5 to 8 carbon atoms in the above formula (i). More preferably, in the above formula (i), R ¹ is a hydrogen atom and R ² is a hexyl group.

Specific examples of preferable polythiophenes include P3HT (poly (3-hexylthiophene)), P3OT (poly (3-octylthiophene)), and P3DT (poly (3-decylthiophene)).

式（ｉｉ）中、Ｒ³、Ｒ⁴、Ｒ⁵及びＲ⁶はそれぞれ独立に、水素原子又は炭素数が１〜１４のアルキル基である。アルキル基はヘテロ原子（典型的には酸素原子及び硫黄原子から選択される。）を含んでもよく、アルキル基中の水素原子はハロゲン原子等の置換基で置換されていてもよい。自己凝集能の理由により、Ｒ⁴＝Ｒ⁵であることが好ましく、Ｒ³＝Ｒ⁶であることが好ましい。溶解性の観点から、好ましくは、Ｒ⁴及びＲ⁵が水素原子であり、Ｒ³及びＲ⁶がそれぞれ独立に炭素数が１〜１４のアルキル基であるか、又は、Ｒ³及びＲ⁶が水素原子であり、Ｒ⁴及びＲ⁵がそれぞれ独立に炭素数が１〜１４のアルキル基である。より好ましくは、Ｒ³及びＲ⁶が水素原子であり、Ｒ⁴及びＲ⁵がそれぞれ独立に炭素数が１〜１４のアルキル基である。自己凝集能の理由により、アルキル基の好ましい炭素数は４〜１２であり、より好ましくは６〜１０である。 (B. Thieno Asen)
Thienoacen represented by the following formulas (ii) to (ix) is preferable because it has high atmospheric stability and high mobility.

In formula (ii), R ³ , R ⁴ , R ⁵ and R ⁶ are independently hydrogen atoms or alkyl groups having 1 to 14 carbon atoms. The alkyl group may contain a hetero atom (typically selected from an oxygen atom and a sulfur atom), and the hydrogen atom in the alkyl group may be substituted with a substituent such as a halogen atom. For reasons of self-aggregation ability, R ⁴ = R ⁵ is preferable, and R ³ = R ⁶ is preferable. From the viewpoint of solubility, preferably, R ⁴ and R ⁵ are hydrogen atoms, R ³ and R ⁶ are independently alkyl groups having 1 to 14 carbon atoms, or R ³ and R ⁶ are independent alkyl groups. It is a hydrogen atom, and R ⁴ and R ⁵ are independently alkyl groups having 1 to 14 carbon atoms. More preferably, R ³ and R ⁶ are hydrogen atoms, and R ⁴ and R ⁵ are independently alkyl groups having 1 to 14 carbon atoms. For reasons of self-aggregating ability, the alkyl group preferably has 4 to 12 carbon atoms, more preferably 6 to 10 carbon atoms.

Specific examples of the suitable compound represented by the formula (ii) include 3,9-didecyldinafto [2,3-b: 2', 3'-d] thiophene represented by the following formula (iii). ..

式（ｉｖ）中、Ｒ⁷、Ｒ⁸、Ｒ⁹及びＲ¹⁰はそれぞれ独立に、水素原子又は炭素数が１〜１４のアルキル基である。アルキル基はヘテロ原子（典型的には酸素原子及び硫黄原子から選択される。）を含んでもよく、アルキル基中の水素原子はハロゲン原子等の置換基で置換されていてもよい。自己凝集能の理由により、Ｒ⁷＝Ｒ⁹であることが好ましく、Ｒ⁸＝Ｒ¹⁰であることが好ましい。溶解性の観点から、好ましくは、Ｒ⁷及びＲ⁹が水素原子であり、Ｒ⁸及びＲ¹⁰がそれぞれ独立に炭素数が１〜１４のアルキル基であるか、又は、Ｒ⁸及びＲ¹⁰が水素原子であり、Ｒ⁷及びＲ⁹がそれぞれ独立に炭素数が１〜１４のアルキル基である。より好ましくは、Ｒ⁸及びＲ¹⁰が水素原子であり、Ｒ⁷及びＲ⁹がそれぞれ独立に炭素数が１〜１４のアルキル基である。自己凝集能の理由により、アルキル基の好ましい炭素数は６〜１３であり、より好ましくは８〜１０である。

In formula (iv), R ⁷ , R ⁸ , R ⁹ and R ¹⁰ are independently hydrogen atoms or alkyl groups having 1 to 14 carbon atoms. The alkyl group may contain a hetero atom (typically selected from an oxygen atom and a sulfur atom), and the hydrogen atom in the alkyl group may be substituted with a substituent such as a halogen atom. For reasons of self-aggregation ability, R ⁷ = R ⁹ is preferable, and R ⁸ = R ¹⁰ is preferable. From the viewpoint of solubility, preferably, R ⁷ and R ⁹ are hydrogen atoms, or R ⁸ and the carbon number R ¹⁰ are each independently an alkyl group having from 1 to 14, or, R ⁸ and R ¹⁰ is It is a hydrogen atom, and R ⁷ and R ⁹ are independently alkyl groups having 1 to 14 carbon atoms. More preferably, R ⁸ and R ¹⁰ are hydrogen atoms, and R ⁷ and R ⁹ are independently alkyl groups having 1 to 14 carbon atoms. For reasons of self-aggregating ability, the alkyl group preferably has 6 to 13 carbon atoms, more preferably 8 to 10 carbon atoms.

Specific examples of the suitable compound represented by the formula (iv) include 2,9-didecyldinafto [2,3-b: 2', 3'-f] thieno [3, represented by the following formula (v). 2-b] Thiophene (C _10- DNTT) can be mentioned.

式（ｖｉ）中、Ｒ¹¹、Ｒ¹²、Ｒ¹³及びＲ¹⁴はそれぞれ独立に、水素原子又は炭素数が１〜１４のアルキル基である。アルキル基はヘテロ原子（典型的には酸素原子及び硫黄原子から選択される。）を含んでもよく、アルキル基中の水素原子はハロゲン原子等の置換基で置換されていてもよい。自己凝集能の理由により、Ｒ¹¹＝Ｒ¹³であることが好ましく、Ｒ¹²＝Ｒ¹⁴であることが好ましい。溶解性の観点から、好ましくは、Ｒ¹¹及びＲ¹³が水素原子であり、Ｒ¹²及びＲ¹⁴がそれぞれ独立に炭素数が１〜１４のアルキル基であるか、又は、Ｒ¹²及びＲ¹⁴が水素原子であり、Ｒ¹¹及びＲ¹³がそれぞれ独立に炭素数が１〜１４のアルキル基である。より好ましくは、Ｒ¹²及びＲ¹⁴が水素原子であり、Ｒ¹¹及びＲ¹³がそれぞれ独立に炭素数が１〜１４のアルキル基である。自己凝集能の理由により、アルキル基の好ましい炭素数は５〜１２であり、より好ましくは８〜１０である。

In formula (vi), R ¹¹ , R ¹² , R ¹³ and R ¹⁴ are independently hydrogen atoms or alkyl groups having 1 to 14 carbon atoms. The alkyl group may contain a hetero atom (typically selected from an oxygen atom and a sulfur atom), and the hydrogen atom in the alkyl group may be substituted with a substituent such as a halogen atom. For reasons of self-aggregation ability, R ¹¹ = R ¹³ is preferable, and R ¹² = R ¹⁴ is preferable. From the viewpoint of solubility, preferably, R ¹¹ and R ¹³ are hydrogen atoms, and R ¹² and R ¹⁴ are independently alkyl groups having 1 to 14 carbon atoms, or R ¹² and R ¹⁴ are respectively. It is a hydrogen atom, and R ¹¹ and R ¹³ are independently alkyl groups having 1 to 14 carbon atoms. More preferably, R ¹² and R ¹⁴ are hydrogen atoms, and R ¹¹ and R ¹³ are independently alkyl groups having 1 to 14 carbon atoms. For reasons of self-aggregating ability, the alkyl group preferably has 5 to 12 carbon atoms, more preferably 8 to 10 carbon atoms.

Specific examples of the suitable compound represented by the formula (vi) include 2,7-dioctyl [1] benzothiophene [3,2-b] [1] benzothiophene (C _{8) represented by the following formula (vi).} -BTBT).

式（ｖｉｉｉ）中、Ｒ¹⁵、Ｒ¹⁶、Ｒ¹⁷及びＲ¹⁸はそれぞれ独立に、水素原子又は炭素数が１〜１４のアルキル基である。アルキル基はヘテロ原子（典型的には酸素原子及び硫黄原子から選択される。）を含んでもよく、アルキル基中の水素原子はハロゲン原子等の置換基で置換されていてもよい。自己凝集能の理由により、Ｒ¹⁵＝Ｒ¹⁷であることが好ましく、Ｒ¹⁶＝Ｒ¹⁸であることが好ましい。溶解性の観点から、好ましくは、Ｒ¹⁶及びＲ¹⁸が水素原子であり、Ｒ¹⁵及びＲ¹⁷がそれぞれ独立に炭素数が１〜１４のアルキル基であるか、又は、Ｒ¹⁵及びＲ¹⁷が水素原子であり、Ｒ¹⁶及びＲ¹⁸がそれぞれ独立に炭素数が１〜１４のアルキル基である。より好ましくは、Ｒ¹⁶及びＲ¹⁸が水素原子であり、Ｒ¹⁵及びＲ¹⁷がそれぞれ独立に炭素数が１〜１４のアルキル基である。自己凝集能の理由により、アルキル基の好ましい炭素数は５〜１２であり、より好ましくは８〜１０である。

In the formula (viii), R ¹⁵ , R ¹⁶ , R ¹⁷ and R ¹⁸ are independently hydrogen atoms or alkyl groups having 1 to 14 carbon atoms. The alkyl group may contain a hetero atom (typically selected from an oxygen atom and a sulfur atom), and the hydrogen atom in the alkyl group may be substituted with a substituent such as a halogen atom. For reasons of self-aggregation ability, R ¹⁵ = R ¹⁷ is preferable, and R ¹⁶ = R ¹⁸ is preferable. From the viewpoint of solubility, preferably, R ¹⁶ and R ¹⁸ are hydrogen atoms, and R ¹⁵ and R ¹⁷ are independently alkyl groups having 1 to 14 carbon atoms, or R ¹⁵ and R ¹⁷ are respectively. It is a hydrogen atom, and R ¹⁶ and R ¹⁸ are independently alkyl groups having 1 to 14 carbon atoms. More preferably, R ¹⁶ and R ¹⁸ are hydrogen atoms, and R ¹⁵ and R ¹⁷ are independently alkyl groups having 1 to 14 carbon atoms. For reasons of self-aggregating ability, the alkyl group preferably has 5 to 12 carbon atoms, more preferably 8 to 10 carbon atoms.

Specific examples of a suitable compound represented by the formula (viii) include 3,11-didecyldinafto [2,3-d: 2', 3'-d'] benzo [1] represented by the following formula (ix). , 2-b: 4,5-b'] _{Dithiophene (C 10-} DNBDT).

The organic semiconductor film according to the present invention can be applied as a semiconductor layer of various organic semiconductor devices. In one embodiment of the organic semiconductor device, a substrate is provided, and a single crystal organic semiconductor film layer, various electrode layers, an insulating film layer, and the like are provided on the substrate. Here, the substrate refers to a member for forming a semiconductor layer on the surface thereof, and examples thereof include a glass substrate, a silicon substrate, a plastic substrate, a ceramic substrate, a metal substrate, and the like. Examples thereof include a substrate in which various device components such as an electrode layer and an insulating film layer are appropriately formed on the substrate. The substrate may be rigid or flexible. The types of semiconductor devices include, but are not limited to, semiconductor elements (diodes, rectifying elements, transistors, thermistas, varistor, thyristors, photoelectric conversion elements, etc.) and integrated circuits (linear ICs, digital ICs, etc.). For example, in one embodiment, the organic semiconductor device according to the present invention can be a transistor including the organic semiconductor film according to the present invention as a semiconductor channel.

Products using transistors include active matrix elements and drive circuits for driving pixels in flexible displays, logic circuits that can be manufactured on plastic, low-cost RF-ID tags, signal processing elements for flexible sensors, and high-speed response. Examples include pressure sensors.

<9. Film formation method>
A preferred method for producing an organic semiconductor film according to the present invention will be described. In order to produce a high-quality single crystal organic semiconductor film, the step of preparing an organic semiconductor solution is extremely important. As the solvent for the organic semiconductor, chloroform, which is a halogen-based solvent, chlorobenzene or o-dichlorobenzene, which is a halogen-based aromatic solvent, toluene, tetralin, which are non-halogen-based aromatic solvents, and the like have been used. Semiconductors have problems such as poor solubility in solvents and easy gelation. Even these solvents can be dissolved by heating the organic semiconductor, but it is difficult to produce a homogeneous single crystal film because crystallization is likely to occur rapidly due to the large difference in solubility depending on the temperature. Met.

The present inventor has found that 3-chlorothiophene not only exhibits higher solubility in organic semiconductors than other solvents, but also has a small difference in solubility depending on temperature. Therefore, it has been found that crystallization is unlikely to occur even at a relatively low temperature (60 to 70 ° C.), and coating at a low temperature is possible. In addition, coating at a low temperature makes it possible to improve the yield and increase the area of the device. Most surprisingly, when a solution of an organic semiconductor is prepared using 3-chlorothiophene as a solvent and the organic semiconductor film is formed by a solvent evaporation method, particularly an edge casting method, the occurrence of cracks is remarkably suppressed. The point is that a highly homogeneous high single crystal film with a small number of domains can be obtained.

The organic semiconductor solution can be prepared by putting the organic semiconductor in the heated solvent or by heating the organic semiconductor after putting it in the solvent. The form of the organic semiconductor to be put into the solvent is not particularly limited, but may be, for example, a powder form. From the viewpoint of the stability of the compound, the heating temperature of the solvent is preferably not more than the boiling point, more preferably 10 ° C. or less than the boiling point, and even more preferably 30 ° C. or less than the boiling point. Further, from the viewpoint of the stability of the compound and the solubility of the compound, the temperature is preferably room temperature to 135 ° C., more preferably 50 to 135 ° C., and even more preferably 70 to 90 ° C.

Further, the concentration of the organic semiconductor in the organic semiconductor solution is preferably 0.2% by mass or less, more preferably 0.1% by mass or less, and 0.025 from the viewpoint of obtaining a homogeneous thin film with good reproducibility. It is even more preferably ~ 0.05% by mass.

The edge casting method itself is a known method for forming an organic semiconductor, and is described in, for example, WO2011 / 040155 (the full text of the method is incorporated herein by reference). Hereinafter, preferred embodiments will be described with reference to FIGS. 1 to 4.

As shown in FIG. 1, electrodes and an insulating film are formed so that the organic semiconductor solution (hereinafter, also referred to as “raw material solution”) comes into contact with the end face of the contact member 2 and the surface of the substrate 1 at the same time. It is supplied onto the substrate 1 to form the droplet 3. By drying the droplet 3 in this state, the organic semiconductor film 4 is formed on the substrate 1.

The contact member 2 has an end surface 2a that stands up from the surface of the substrate 1 at a predetermined angle while being placed on the surface of the substrate 1. The end face 2a is typically planar. The droplet 3 is supplied so as to come into contact with the end surface 2a of the contact member 2. The contact member 2 can be formed of, for example, a resin, but any material other than the resin may be used as long as it appropriately fulfills the functions described below.

As a manufacturing process of the organic semiconductor film, first, as shown in FIG. 1, the contact member 2 is subjected to a substrate so that the end face 2a crosses a predetermined A direction of the substrate 1, preferably the end face 2a is orthogonal to the A direction. Place it on 1. The material of the substrate 1 is not limited, and examples thereof include glass, plastic, ceramic, and metal. Further, it is desirable to perform a treatment with a self-assembled monolayer containing a functional group exemplified by an alkyl group, an aryl group and an amino group so as to improve the wettability of the surface of the substrate 1. This is because a uniform single crystal film can be obtained when the contact angle between the far end portion of the raw material solution and the substrate 1 with respect to the contact member 2 is small during crystal growth. This contact angle is preferably 10 ° or less. The substrate 1 may be hard (rigid) or soft (flexible). When a soft substrate is used, it is possible to form a curved organic semiconductor film. In this state, the raw material solution is supplied onto the surface of the substrate 1 so as to come into contact with the end face 2a. The droplet 3 of the supplied raw material solution is held by the end face 2a and is in a state in which a constant force acts. The cross-sectional shape in this state is shown in FIG.

A drying process is performed with the droplet 3 held by the end face 2a to evaporate the solvent in the droplet 3. As a result, as shown in FIG. 3, in the droplet 3, the raw material solution becomes saturated due to the evaporation of the solvent at the far end edge portion from the end face 2a in the A direction, and the crystals of the organic semiconductor begin to precipitate. The movement of the far end edge of the droplet 3 with the evaporation of the solvent is shown by the broken lines e1 and e2. Crystallization of the organic semiconductor material progresses with the evaporation of the solvent, and as shown in FIG. 2, the organic semiconductor film 4 grows. That is, the crystal growth progresses toward the end face 2a along the A direction of the substrate 1, and the organic semiconductor film 4 is gradually formed.

In this drying process, the action of defining the crystal growth direction through contact with the end face 2a works depending on the state in which the droplet 3 of the raw material solution is attached to the end face 2a. As a result, it is considered that the effect of controlling the crystallinity is obtained, the regularity of the arrangement of the molecules of the organic semiconductor is improved, and it contributes to the improvement of the electron conductivity (mobility).

The drying process can be carried out in the air at a substrate temperature of 60 to 90 ° C, typically 70 to 80 ° C, depending on the type of organic semiconductor. The film forming speed can be about 15 to 25 μm / s, although it depends on the type of organic semiconductor. Further, the film thickness can be adjusted by changing the substrate temperature and the concentration of the organic semiconductor in the solution. Further, in the case of the continuous edge casting method described later, the film thickness also changes depending on the moving speed of the contact member.

As a modification of the above manufacturing method, as shown in FIG. 4, the substrate 1 is tilted and maintained at a predetermined angle, and the contact member 2 is preferably end face 2a so that the end face 2a crosses the tilt direction of the substrate 1. Is placed on the substrate 1 so as to be orthogonal to the inclination direction. In this state, the raw material solution is supplied onto the surface of the substrate 1 so as to come into contact with the end face 2a. The droplet 3 of the supplied raw material solution is held by the end face 2a and is suspended in the inclined direction of the substrate 1. By inclining the substrate 1, the size of the wet surface due to the droplets 3 can be controlled, and it becomes easy to obtain an organic semiconductor film having desired characteristics.

The method for forming the droplet 3 is not limited to the above method. For example, by taking out the substrate 1 together with the contact member 2 from the state of being immersed in the raw material solution, droplets adhering to the end face 2a can be formed.

As another modification, a method by a continuous edge casting method can be mentioned. The continuous edge casting method was introduced in (“Inch-Size Solution-Processed Single-Crystaline Films of High-Mobility Organic Semiconductors”, Junshi Soeda et al., Applied Physics Express 6, The Japan Society of Applied Physics, 2013, 076503). This is an advantageous method for increasing the area of the single crystal film. In the continuous edge casting method, as shown in FIGS. 6A and 6B, the direction parallel to the surface of the substrate 1 and the direction in which the contact member 2 is separated from the droplet 3 (directions of arrows X1 and X2 in the drawing). By continuously supplying the raw material solution 5 while moving the substrate 1 and the contact member 2 relative to each other, the solvent in the droplet 3 is evaporated and the contact member 2 is moved onto the substrate 1. Includes the continuous formation of the organic semiconductor film 4. The raw material solution 5 can be supplied, for example, by using the solution supply nozzle 6. As shown in FIGS. 6A and 6B, the contact member 2 is provided with a slight gap between the surface of the substrate 1 and the bottom surface of the contact member 2, and the tip of the nozzle 6 is arranged on the surface opposite to the end surface 2a of the contact member. , The raw material solution 5 can be supplied toward the gap.

According to this method, since the raw material solution 5 is continuously supplied, the crystal growth is not terminated by the evaporation of the solvent from the raw material solution 5. Therefore, the size of the organic semiconductor film 4 to be formed can be formed in a desired large area according to the width of the end face 2a of the contact member 2 and the moving distance.

It is desirable to continue supplying the raw material solution 5 so that the size of the droplets 3 is maintained within a predetermined range during the relative movement from the viewpoint of keeping the film thickness of the organic semiconductor film 4 constant. That is, the droplets 3 are maintained at the same size by supplying the raw material solution 5 at a rate equivalent to the evaporation rate of the solvent. Since the crystallization rate from the raw material solution 5 is about 1 mm / min to several cm / min according to the usual setting, the relative velocity between the substrate 1 and the contact member 2 may be adjusted to the same rate. By these operations, when the state shown in FIG. 6A progresses to the state shown in FIG. 6B, for example, the organic semiconductor film 4 is continuously formed on the surface of the substrate 1 after the contact member 2 has moved with a uniform film thickness. Will be done.

Examples of the present invention (invention examples) are shown below together with comparative examples, but these are provided for a better understanding of the present invention and its advantages, and are not intended to limit the invention. ..

<Organic semiconductor>
The following materials were prepared as organic semiconductors.
(1A) 3,11-didecyldinaft [2,3-d: 2', 3'-d'] benzo [1,2-b: 4,5-b'] dithiophene powder (TM026)
(1B) 3,9-dihexyldinafto [2,3-b: 2', 3'-d] thiophene powder (1C) 6,13-bis (triisopropylsilylethynyl) pentacene powder

<Solvent>
The following solvents were prepared.
(2A) 3-Chlorothiophene (2D) 2-Chlorothiophene (2E) 3-Phenoxytoluene (2F) 1,2-dimethoxybenzene (2G) 1,2-dimethoxybenzene (2H) Orthodichlorobenzene (ODCB)
(2I) Tetralin (2J) Butoxybenzene (2K) Thioanisole

<Formation of organic semiconductor film>
The organic semiconductor powder was put into a solvent in a beaker at a predetermined concentration and completely dissolved by heating to a predetermined temperature (melting temperature) on a hot plate. The obtained organic semiconductor solution was applied onto a glass substrate with a thin film electrode on a hot plate by an edge casting method, and heated to a predetermined temperature (deposition temperature) to evaporate the solvent to form an organic semiconductor thin film. The combination of the organic semiconductor and the solvent, the dissolution conditions of the organic semiconductor, and the temperature conditions of the hot plate when forming the organic semiconductor thin film are as shown in Table 1.

<Evaluation of organic semiconductor film>
The prepared thin film was evaluated according to the following method.
(1) Coverage The thin film was observed in a cross-nicoled state using an ECLIPSE LV100N POL type microscope manufactured by Nikon Corporation. Shooting was performed at an angle at which most of the thin film became brightest, using a 10x objective lens and a Sony NEX-5R camera under the conditions of ISO sensitivity of 200 and exposure time of 1.3 seconds. Based on the definition of the color information of the polarizing microscope photograph previously described coverage of crystals, and the average value was calculated by observing a plurality of locations the scope of any 10 ⁴ μm ^2.
Specifically, it was calculated based on the brightness information of green, which has the most remarkable relationship between the film thickness and the brightness in the above-mentioned polarized light microscope photograph. The decrease in coverage is mainly due to very fine cracks. In this case, the cracks existing in the thick film tend to appear brighter than the color of the thin film because the light of the film that appears bright spreads to the crack portion. Therefore, in order to obtain the coverage, it is necessary to compare the brightness of the pixels with the peripheral portion. First, the average crack interval is calculated by eye measurement from the photograph. For photographs with vertical cracks, calculate the average green brightness by half the crack spacing on each of the left and right sides of each pixel. If the average of both the left and right pixels is 3 or more darker than the target pixel, it is judged that the pixel is a crack.
In the case of Experiment No. 13, the coverage was clearly low visually, and the above judgment method was not applicable, so evaluation was performed by another method. In the obtained photograph, the brightness information of green is recorded in 256 steps from 0 to 255. Therefore, in the case of Experiment No. 13, the portion where the brightness of green was less than 30, which is the same as that of the substrate, was defined as the substrate portion, and the portion brighter than that was defined as the crystal.
(2) Average film thickness There is a high correlation between the pixel color information in the crystal taken at the brightest angle in the above-mentioned polarizing micrograph and the color information in the reflective micrograph and the thickness of the crystal measured by the atomic force microscope. I confirmed that. Using this relationship, the average film thickness was estimated by histogram analysis of polarizing micrographs for thin films with the same crystal direction. For thin films with irregular crystal directions, the average film thickness was estimated by histogram analysis of photographs taken with a reflective microscope.
(3) Carrier mobility A large number of field-effect transistors were manufactured using the formed crystal as an active layer, and the mobility was estimated from the transfer characteristics. First, the acceptor molecules 2,3,5,6-terafluoro-7,7,8,8-tetracyanoquinodimethane 2 nm and gold 30 nm were applied on the thin film using a shadow mask designed to have a channel length of 50 μm. Deposited as an electrode. In order to prevent current from flowing outside the channel, it was etched by a pulse laser having a wavelength of 355 nm so that the channel width was exactly 50 μm. As the insulating layer, silicon dioxide having a relative permittivity of 3.9 and a thickness of 200 nm was used. Using a Keithley 4200-SCS type semiconductor parameter analyzer, the gate voltage was swept in the range of + 10V to -50V under a drain voltage of -50V, and the transmission characteristics were measured. By fitting the gate voltage in the range of -25V to -50V in the obtained transfer characteristics, the carrier mobility in the saturation region was obtained from the measured value of IdVg of the FET described above. The measured mobility varies depending on the direction of the current flowing through the active layer, but here, the direction of the channel is set so that the current flows in the direction of the highest mobility.

The evaluation results are shown in Table 2. From Table 2, the organic semiconductor film according to the present invention is a homogeneous single crystal film having a high coverage. It can also be seen that the organic semiconductor film according to the present invention stably exhibits high mobility. That is, it will be understood that a high-performance organic semiconductor device can be manufactured with a high yield by using the organic semiconductor film according to the present invention.

(4) Number of domains For Experiment No. 1 (solvent: 3-chlorothiophene) and Experiment No. 2 (solvent: 2-chlorothiophene), the thin film was observed at multiple locations by the same method as the measurement of coverage (per field). 10 ⁴ range of [mu] m ^2), was measured number of domains crystals present per 1 mm ² by using a particle analysis capabilities WeveMetrics Co. Igor Pro based on the green color information. A region in which the brightness of green was continuously larger than 60 was defined as one domain. As a result, in Experiment No. 1, it was 1 domain / mm ² , whereas in Experiment No. 2, it was 5000 domains / mm ² .

(5) Crack angle For Experiment No. 1 (solvent: 3-chlorothiophene) and Experiment No. 2 (solvent: 2-chlorothiophene), the thin film was observed by the same method as the measurement of coverage (any 10 ^4). The angle of each crack was measured (in the range of μm ^2). FIG. 7 shows a polarizing micrograph obtained in Experiment No. 1, and FIG. 8 shows a polarizing microscope photograph obtained in Experiment No. 2. The streaks were cracks, and the length and angle of the cracks were measured by tracing over the cracks with a ruler tool from Adobe Photoshop. The standard deviation was calculated by weighting the crack angle with the crack length. As a result, in Experiment No. 1, it was 0.15 rad, whereas in Experiment No. 2, it was 0.83 rad. In the polarizing micrographs of FIGS. 7 and 8, it can be seen that the color of the thin film is changing, but this indicates the change in the thickness of the thin film, not a crack.

(6) Distribution of crystal axis For Experiment No. 1 (solvent: 3-chlorothiophene), the distribution range of the crystal axis was examined using a Rigaku model SaltLab GIXD device according to the method described above. The light source used was CuKα ray (1.5418 Å). By fitting a Gaussian curve to the measured intensity distribution, the standard deviation of the diffraction intensity distribution was obtained. As a result, the distribution range of the crystal axis was 7 °.

1 Substrate 2 Contact member 2a End face 3 Droplet 4 Organic semiconductor film 5 Raw material solution 6 Solution supply nozzle L Light source D Detector

Claims (9)

In a polarizing micrograph taken in a cross-nicol state, the area per 1 mm ² is defined as one domain in which the area where the green brightness is continuously larger than 60 out of the 256 levels of brightness from 0 to 255. A single crystal organic semiconductor film of thienoacen having 5 or less domains present therein, an average mobility of 5 cm ² / Vs or more, and a coefficient of variation of 30 or less. The single crystal organic semiconductor film of thienoacene according to claim 1, which has a coverage of 0.98 or more. The single crystal organic semiconductor film of thienoacene according to claim 1 or 2, wherein the distribution range of the crystal axis is within 10 °. The single crystal organic semiconductor film of thienoacene according to any one of claims 1 to 3, wherein the standard deviation of the crack angle is 0.5 rad or less. The single crystal organic semiconductor film of thienoacene according to any one of claims 1 to 4, wherein the average mobility is 7 cm ^{2 / Vs or more.} The single crystal organic semiconductor film of thienoacene according to any one of claims 1 to 5, wherein the average film thickness is 50 nm or less. The single crystal organic semiconductor film of thienoacen according to any one of claims 1 to 6, wherein thienoacen is represented by the following formula (viii).

In the formula (viii), R ¹⁵ , R ¹⁶ , R ¹⁷ and R ¹⁸ are independently hydrogen atoms or alkyl groups having 1 to 14 carbon atoms. An organic semiconductor device comprising a substrate and a thienoacen single crystal organic semiconductor film formed on the surface thereof according to any one of claims 1 to 7. A transistor comprising the thienoacen single crystal organic semiconductor film according to any one of claims 1 to 7 as a semiconductor channel.

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