JP2012049291A - Method for manufacturing single-crystal organic semiconductor thin film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a single-crystal organic semiconductor thin film by a printing method, of which nearly entire region is composed of a single crystal.SOLUTION: A method for manufacturing a single-crystal organic semiconductor thin film comprises: preparing a first ink obtained by dissolving an organic semiconductor in an organic solvent having high affinity to the organic semiconductor, and a second ink composed of an organic solvent having low affinity to the organic semiconductor; and forming a region for reserving an ink obtained by mixing the first ink and the second ink on a substrate. A part of the region for reserving the ink is shaped to generate a seed crystal with a high degree of efficiency, and from a starting point of the part, a single crystal is formed over a nearly entire region of the region for reserving the ink.

Description

  The present invention relates to a method for producing a monocrystalline organic semiconductor thin film.

  Organic electronics using high molecular and low molecular organic semiconductors has attracted attention as a major next-generation technology for producing flat panel displays and electronic paper. In recent years, research and development of organic semiconductor thin film field effect transistors that use switching elements for active matrix in addition to organic electroluminescent diodes that have already been commercialized have made great progress.

  One of the major features of organic electronics is that electronic devices can be manufactured using solution processes that do not require vacuum, and advanced printing technology. For this reason, large-area flat panel displays and electronic paper can be manufactured at low cost and energy compared to conventional inorganic semiconductor devices that require high costs for large-scale vacuum equipment and high-temperature equipment as the product size increases. It is expected to be possible.

  In these organic semiconductor devices, the organic semiconductor layer that is the core of device operation is an amorphous thin film with no regular arrangement of molecules, or a polycrystalline thin film consisting of microcrystals with a size of several micrometers or less in which molecules are regularly arranged. Is being used. On the other hand, as a prototype device for investigating the state of carrier transport in an organic semiconductor layer, many studies of single crystal devices using single crystals having a size of several hundred micrometers or more have been conducted. Single crystal devices have been reported to show excellent carrier transport properties because there are no grain boundaries that can be a cause of hindering carrier transport, and the regularity of the molecular arrangement extends to the entire region of the semiconductor. Patent Document 1).

Furthermore, in recent years, research has been conducted to construct a single crystal device exhibiting excellent characteristics on a substrate using a process technology and to utilize it as a practical device.
For example, in a large number of source / drain electrode arrays fabricated on a substrate, surface treatment is performed selectively with a silane coupling agent with octadecyltriethoxysilane (OTS) only between the electrodes, and an organic semiconductor single crystal is formed only on the treated region. Non-Patent Document 2 proposes a method of growing by a vapor phase method.

For the construction of a single crystal organic semiconductor device using a solution method, 2,7-dioctyl [1] benzothieno [3,2-b] [1] benzothiophene (hereinafter referred to as “C8-BTBT”) exhibiting high crystallinity. Non-Patent Document 3 proposes a method of gradually growing a C8-BTBT single crystal by bringing the end of a silicon wafer or the like for supporting a droplet onto the substrate into contact with the substrate and tilting the entire substrate. Has been.
However, the method described above is insufficient from the viewpoint of producing a single crystal organic semiconductor thin film with a controllability at a level capable of industrial mass production. In particular, regarding the application of printing technology for utilizing the features of organic electronics, no effective method is known for single crystal organic semiconductor devices.

On the other hand, one of the methods for constructing a low-molecular organic semiconductor thin film by a printing technique is an ink-jet printing method that uses ink having a relatively low viscosity. In the ink jet printing method, a solution (ink) in which an organic semiconductor is dissolved at a high concentration in an organic solvent or the like is dropped from an ink head onto a substrate, and an organic semiconductor layer is deposited by subsequent evaporation of the organic solvent.
In Patent Document 1 related to the previous patent application, as a method of forming an organic semiconductor thin film having a high film thickness uniformity, a double-shot inkjet printing method using an organic semiconductor whose solubility varies greatly depending on the type of organic solvent is proposed. Has been. In this method, an ink obtained by dissolving an organic semiconductor in a high concentration in an organic solvent and an ink composed of an organic solvent that hardly dissolves the organic semiconductor are ejected simultaneously or alternately from each ink head and mixed on the substrate. Thus, it is shown that the precipitation of the organic semiconductor proceeds before the evaporation of the organic solvent, and an organic semiconductor thin film with high homogeneity can be obtained.

  However, when an organic semiconductor material with high crystallinity is thinned by the above method, after landing on the substrate, each part of the stored ink droplets, particularly at each part of the outer edge of the droplets where the evaporation rate of the solvent is high Since seed crystals are randomly generated, the obtained organic semiconductor thin film is a polycrystalline organic semiconductor thin film composed of several to several hundred microcrystals. For this reason, it is difficult to obtain a monocrystalline organic semiconductor thin film in which the regularity of the molecular arrangement covers almost the entire region of the thin film by the printing method.

Japanese Patent Application No. 2010-182945

Science and Technology of Advanced Materials, Vol.10, 024314, 2009 Nature, Vol. 444, 913, 2006 Applied Physics Express, Vol.2, 111501 pages, 2009

  In view of the situation as described above, an object of the present invention is to produce a single crystalline organic semiconductor thin film in which almost the entire region of the thin film is composed of a single single crystal by a printing method.

The above problem is solved by the following means.
(1) Prepare a first ink obtained by dissolving the organic semiconductor at a high concentration in an organic solvent having a high affinity for the organic semiconductor, and a second ink comprising an organic solvent having a low affinity for the organic semiconductor. A method for producing a single crystalline organic semiconductor thin film comprising: a step; and a step of mixing the first and second inks on a substrate and forming a region for storing the ink,
A single crystal property characterized in that a part of a region where the ink is stored has a shape in which a seed crystal is generated with high efficiency, and the single crystal is grown over almost the entire region where the ink is stored starting from the shape. Manufacturing method of organic semiconductor thin film.
(2) Prepare a first ink obtained by dissolving the organic semiconductor in a high concentration in an organic solvent having a high affinity for the organic semiconductor, and a second ink comprising an organic solvent having a low affinity for the organic semiconductor. A method for producing a single crystalline organic semiconductor thin film comprising: a step; and a step of mixing the first and second inks on a substrate and forming a region for storing the ink,
The area for storing the ink includes a small reservoir for generating seed crystals, a large reservoir for growing a single crystal from the seed crystals, and a constriction for suppressing convection between the reservoirs and selecting the crystal growth direction. A method for producing a monocrystalline organic semiconductor thin film comprising a region.
(3) The method for producing a monocrystalline organic semiconductor thin film according to (1) or (2), wherein the first and second inks are ejected simultaneously or alternately from the respective ink heads onto the substrate. .
(4) The region in which the ink is stored is defined by hydrophilizing a part of the substrate surface that has been hydrophobized and then further hydrophobizing the part of the substrate surface that has been hydrophilized. (1) The manufacturing method of the single-crystal organic-semiconductor thin film in any one of (3).

  According to the present invention, a seed crystal is first generated in one part of the ink droplet stored on the substrate, and this can serve as a nucleus to grow a single crystal thin film over the entire region of the droplet. A single crystalline organic semiconductor layer made of a single single crystal over a wide area of × 100 μm or more can be produced. This makes it possible to build an organic semiconductor device that exhibits excellent device characteristics without crystal grain boundaries that cause device characteristic deterioration by a printing method. In addition, it can be used in a manufacturing process that is suitable for mass production at low cost with rolls, tows and rolls.

(A) Example of hydrophilic (lipophilic) / hydrophobic patterning for forming a single crystalline organic semiconductor thin film, (b) generation position of seed crystal (indicated by ● mark) and growth direction of single crystal (indicated by arrow) (C, d) Polarized micrographs of single crystal thin films obtained in C8-BTBT. (A) Photomicrograph of C8-BTBT single crystal thin film, (b) Atomic force microscope image and (c) height profile of thin film surface, and (d) Crystal structure of C8-BTBT single crystal.

Embodiments of the present invention will be described below in detail with reference to the drawings.
C8-BTBT having high crystallinity was used as the organic material constituting the organic semiconductor layer. An organic semiconductor layer was formed by the double shot ink jet printing method described in Patent Document 1 using an ink formed by dissolving the above organic semiconductor at a high concentration and an ink using an organic solvent that hardly dissolves the organic semiconductor.

  The area where ink deposited on the substrate by inkjet printing is stored is defined by patterning by hydrophilic (lipophilic) and hydrophobic treatments on the surface of the substrate. A surface-treated product having a shape designed in advance so that the evaporation of the solvent proceeds relatively quickly and a seed crystal is generated is used.

Hereinafter, the present invention will be described in more detail based on examples.
As inks used in double shot ink jet printing, C8-BTBT 10 mg (21.5 μmol: molecular weight 464.77) dissolved in 0.8 ml of orthodichlorobenzene (DCB) at a concentration of 26.9 mmol / l (A ink), and C8-BTBT Ink (B ink) made of dimethylformamide (DMF), which hardly dissolves water was used. Both DCB and DMF used as raw materials are available at low cost. In addition, the viscosity of each of the obtained inks was 2 to 3 mPa · s, and showed suitable properties as an ink used for inkjet printing.

  The ink jet printing apparatus used was capable of arbitrarily ejecting fine droplets of about 100 picoliters for each of the two types of inks in an accurate amount and in an accurate position without contact by digital control. A ink containing C8-BTBT and B ink consisting only of a low affinity solvent were respectively applied to the same positions on the substrate and mixed in a solution state.

  In this example, a p-type silicon wafer coated with a silicon oxide film having a thickness of 100 nm was used as a substrate, and a source / drain gold electrode formed thereon was used. As treatment methods for the substrate surface, the hydrophobic treatment on the insulating film surface was performed with hexamethyldisilazane (HMDS), and the hydrophobic treatment on the gold electrode surface was performed with a thiol treatment. In order to form a hydrophilic (lipophilic) region after the hydrophobization treatment, only a partial surface of the substrate was exposed by a mask pattern having the shape shown in FIG. 1A and selectively hydrophilized by UV / ozone treatment. Thereafter, the hydrophilic surface was surface-treated with lipophilic phenethyltrichlorosilane (PTS). As a result, a surface treatment pattern was obtained in which ink droplets deposited on the substrate could be confined in the lipophilic region having the shape shown in FIG.

  In the ink application by the ink jet method, first, the head filled with B ink is moved onto the above-mentioned lipophilic region, 40 drops of B ink are ejected and landed on the substrate, and then the head filled with A ink is quickly moved. First, the ink moved to the position where the B ink landed, and 12 drops of A ink were ejected to obtain droplets in which A ink and B ink were mixed on the substrate.

  The shape of the lipophilic region in FIG. 1A is composed of a small liquid reservoir part 1, a large liquid reservoir part 3, and a part 2 that becomes a constriction for suppressing convection between 1 and 3. Due to the limitation due to the contact angle, the volume of the droplets stored per unit surface area is smaller in the small liquid reservoir part 1 than in the large liquid reservoir part 3. Since the convection between 1 and 3 is limited to some extent, in the small liquid reservoir portion 1 having a large surface area / volume, the solvent is evaporated more efficiently per unit volume and a seed crystal is generated. The seed crystal generated in the liquid reservoir part 1 is selected from the polycrystals having the same growth direction through the constriction part 2 and becomes the nucleus of crystal growth in the liquid reservoir part 3, as shown in FIG. Single crystals grow from this point.

  Actually, when the state of crystal growth was observed with a microscope, the seed crystal generated at the small liquid reservoir site became a nucleus and the single crystal gradually grew from 1 to 3 at the large liquid reservoir site. Observed. FIG. 1 (c) is a polarizing micrograph of the obtained C8-BTBT.

  FIG. 2 shows experimental results showing that the obtained thin film is a single crystal thin film. FIG. 2A is an enlarged view of the micrograph, and it can be seen that a striped pattern is observed. FIG. 2B shows the result of observing the striped pattern with an atomic force microscope image, and FIG. 2C shows the result of the height profile. From this, it can be seen that the striped pattern seen in the micrograph captures a thickness change of 3 nm. This change in thickness is consistent with the thickness of the molecular layer by C8-BTBT shown in FIG. 2 (d), and it is concluded that a step-and-terrace structure in a single crystal was observed.

In addition, said Example is for making an understanding of this invention easy to the last, and is not limited to this Example. That is, modifications and other aspects based on the technical idea of the present invention are naturally included in the present invention.
For example, as an organic semiconductor, although the result regarding C8-BTBT was illustrated, what has a property which shows high crystallinity with another organic semiconductor may be sufficient. Moreover, it is good also as an organic molecule which improved the crystallinity in the layer form by substituting with an alkyl group or a substituent equivalent to it.

In addition, as a method for forming a highly homogenous organic semiconductor layer, a method using a double shot ink jet method in which two kinds of different droplets are mixed is exemplified. However, if a method for forming a highly homogeneous organic semiconductor layer is used, this method is used. Other methods may be used. The same effect can be obtained as long as seed crystals are efficiently generated from some of the droplets deposited on the substrate.
For example, in the case of growing a single crystal organic semiconductor by gradually evaporating an organic solvent by a substrate tilt in a solution method, the same effect can be obtained.

1 Small reservoir 2 Constriction site for convection suppression and crystal growth direction selection 3 Large reservoir site

Claims (4)

Preparing a first ink obtained by dissolving the organic semiconductor in a high concentration in an organic solvent having a high affinity for the organic semiconductor, and a second ink comprising an organic solvent having a low affinity for the organic semiconductor; A step of mixing the first and second inks on a substrate and forming a region for storing the inks,
A single crystal property characterized in that a shape of a seed crystal is generated at a part of a region where the ink is stored with high efficiency, and the single crystal is grown over almost the entire region where the ink is stored starting from the shape. Manufacturing method of organic semiconductor thin film. Preparing a first ink obtained by dissolving the organic semiconductor in a high concentration in an organic solvent having a high affinity for the organic semiconductor, and a second ink comprising an organic solvent having a low affinity for the organic semiconductor; A step of mixing the first and second inks on a substrate and forming a region for storing the inks,
The area for storing the ink includes a small reservoir for generating seed crystals, a large reservoir for growing a single crystal from the seed crystals, and a constriction for suppressing convection between the reservoirs and selecting the crystal growth direction. A method for producing a monocrystalline organic semiconductor thin film comprising a region.   3. The method for producing a single crystalline organic semiconductor thin film according to claim 1, wherein the first and second inks are ejected simultaneously or alternately from each ink head onto the substrate. The region in which the ink is stored is defined by hydrophilizing a part of the surface of the substrate that has been hydrophobized and then further lipophilically treating the part of the surface of the substrate that has been hydrophilized. Item 4. The method for producing a single crystalline organic semiconductor thin film according to any one of Items 1 to 3.