JP5920806B2 - Organic semiconductor thin film formation method - Google Patents

Description

The present invention relates to a method for forming an organic semiconductor thin film that can be used in an organic semiconductor device, and more particularly to a method for forming an organic semiconductor thin film having high crystallinity .

  Organic semiconductors can be formed by a solution process that takes advantage of their solubility, and realization of a low-cost, low-environmental load process is expected. On the other hand, organic semiconductors have low mobility compared to inorganic semiconductors. This is because organic molecules are gathered by van der Waals forces, so the interaction between molecules is small. Organic thin films are generally polycrystalline, and there are many disordered molecules inside the grain boundaries. This is due to the point to do. Therefore, in order to improve the mobility of the organic semiconductor, it is necessary to form a thin film with high crystallinity and less disorder.

  A general organic semiconductor thin film has a problem that many trap levels due to molecular disorder are present in the grain boundary, which impedes charge transport. An object of the present invention is to reduce grain boundaries and molecular disorder in an organic semiconductor thin film, to form an organic semiconductor thin film with higher crystallinity, or to form an organic semiconductor single crystal itself directly on a substrate. . In the present application, an organic semiconductor single crystal formed in the thin film is also handled as a part of the organic semiconductor thin film.

According to one aspect of the present invention, an organic semiconductor thin film forming method including the following steps is provided.
(a) A two-layer structure comprising a polymer layer and an organic semiconductor layer is formed on a substrate.
(b) Solvent vapor annealing treatment is performed on the two-layer structure using a solvent in which the polymer and the organic semiconductor are soluble.
Here, the solubility of the polymer in the solvent may be 1 mg / mL or more.
The polymer layer may be formed on the substrate, and the organic semiconductor layer may be formed on the polymer layer.
The two-layer structure may be formed by a layer separation method.
The organic semiconductor layer may be formed on the substrate, and the polymer layer may be formed on the organic semiconductor layer.
The two-layer structure may be formed by separately forming the polymer layer and the organic semiconductor layer.
The molecular weight of the polymer may be 1000 or more.
The polymer may be selected from the group consisting of PMMA, PVP, PαMS, polystyrene, polyvinyl alcohol, and polyethylene glycol.
The solvent used for the solvent vapor annealing may be selected from the group consisting of toluene, xylene, tetralin, decalin, chlorobenzene, dichlorobenzene, trichlorobenzene, chloroform, THF, and cyclohexane.
The organic semiconductor may be selected from the group consisting of π-electron conjugated aromatic compounds, chain compounds, organic pigments, and organosilicon compounds.
The organic semiconductor may be selected from the group consisting of C8-BTBT, C10-BTBT, C12-BTBT, and TIPS-pentacene.
The thickness of the polymer layer may be in the range of 10 to 200 nm.
According to another aspect of the present invention, there is provided a semiconductor element that uses an organic semiconductor thin film formed by any of the methods described above as a semiconductor.
According to still another aspect of the present invention, there is provided an organic field effect transistor using an organic semiconductor thin film formed by any of the above-described methods as a channel material.
Here, a source and a drain may be disposed on a single single crystal of an organic semiconductor formed in the organic semiconductor thin film.

  According to the present invention, an organic semiconductor thin film having high crystallinity can be formed by using a simple process called solvent vapor annealing.

a) is a polarization microscope image of a dioctylbenzothienobenzothiophene (C8-BTBT) thin film spin-coated on the SiO ₂ substrate of Comparative Example 1. Inset is the molecular structure of C8-BTBT. b) Polarized light microscope image of the result of solvent vapor annealing of the C8-BTBT thin film spin-coated on the SiO ₂ substrate shown in a). a) is a polarizing microscope image of a C8-BTBT thin film spin-coated on poly (methyl methacrylate) (PMMA) in Comparative Example 2. b) Polarization microscope image of the result of solvent vapor annealing of the C8-BTBT thin film spin-coated on PMMA shown in a). a) A polarization microscope image of a C8-BTBT / PMMA thin film having a layer separation structure formed by applying a mixed solution of C8-BTBT and PMMA by spin coating according to the present invention. b) Polarized microscope image of the layer-separated C8-BTBT / PMMA thin film shown in a) after solvent vapor annealing. a) Polarized light microscope image of a thin film having a two-layer separation structure formed by applying a mixed solution of C8-BTBT and PMMA. Only the upper C8-BTBT was removed by scratching the center. Inset is a conceptual diagram of a two-layer separation structure. b) A graph showing the measurement result of the surface shape indicating the film thickness height of each layer, wherein the surface height is measured in the direction of the arrow in a). a) to f) are polarization microscope images showing the results of solvent vapor annealing for 3 to 120 minutes, respectively, on the thin films having the two-layer separation structure of C8-BTBT and PMMA. g) AFM image of C8-BTBT single crystal formed by solvent vapor annealing. h) is an AFM image in which the tip of the needle crystal is enlarged, and the facet plane of the crystal can be observed. i) is a diagram showing a cross-sectional shape of the C8-BTBT single crystal in g) taken along the white line from the upper left to the lower right from the center slightly right. a) Schematic of the fabricated single crystal organic FET. b) Microscopic image of the channel region of the fabricated single crystal organic FET. In order to measure a single crystal, the remaining crystal is patterned (cut) by a laser (dotted line portion in the figure). c) is a graph showing drain current-gate voltage characteristics of a single crystal organic FET. a) is a histogram showing the mobility distribution of the fabricated single crystal organic FET. b) Histogram showing the threshold voltage distribution of the same FET. a) is a graph showing the temperature dependence of the drain current-gate voltage characteristics of the fabricated single crystal organic FET. b) A graph showing the temperature dependence of the mobility and threshold voltage of the same FET. A polarization micrograph of the result of subjecting a C8-BTBT film formed on three types of polymer substrates by vacuum sublimation to solvent vapor annealing using three types of solvents for about 15 hours. The polarizing microscope photograph of a PMMA film | membrane and the PMMA film | membrane surface profile figure after the formed single crystal removal. (a) is a polarized light micrograph immediately after spin coating. (b) is a polarization micrograph after solvent vapor annealing, which shows a swelling effect. (c) is a polarization micrograph of the sample after rinsing the formed single crystal with cyclohexane. The dark part (blue in the original polarization micrograph) of the edge of the elongated area (where the single crystal was formed) is the part that was not removed by rinsing, so this part is thick PMMA, not C8-BTBT I understand that. (d) and (e) are the surface profile scan results across the portion of the sample where the C8-BTBT single crystal has been removed. (d) shows a thin (33 nm) PMMA film and (e) shows a thick (155 nm) PMMA film. PMMA climbs the edge of the C8-BTBT crystal. It has been shown to sink. (a) to (f) are polarization micrographs after C8-BTBT on PMMA films of various thicknesses d were subjected to solvent vapor annealing for about 15 hours with chloroform under the same conditions. Single crystals were not seen when d was less than 10 nm. When d exceeded 200 nm, the crystal size did not become uniform, but as shown in (e) and (f), a single crystal exceeding 1 mm in length was produced. (g) is a graph showing the size and crystal depth of the generated single crystal as a function of the PMMA film thickness d, indicating the general tendency that the thicker the PMMA film, the longer the crystal and the deeper the crystal depth. . The broken lines in the graph are for making it easier to see the trend. It is a polarization micrograph showing the result of a solvent vapor annealing process performed using chloroform for about 15 hours. As a sample, (a) shows C10-BTBT on PMMA, (b) shows C12-BTBT on PMMA, (C) and (d) show the case where TIPS-pentacene on PαMS is used. The inset in (c) is an enlarged photograph of the TIPS-pentacene crystal. The large crystal of TIPS-pentacene shown in (d) is formed on a thick PαMS layer. The photograph in FIG. 12 uses an orthogonal polarizer to show local deformation of the polymer substrate around the single crystal (the part that appears dark in FIG. 12, blue in the original polarization micrograph). The process of darkening the background is not performed. A photograph of the results of solvent vapor annealing treatment of TIPS-pentacene spin-coated on a Si / SiO ₂ substrate for 15 hours. (a) is a photograph when a polarizer is not used, and (b) is a photograph when a polarizer is used. (a)-(d) are optical micrographs of C10-BTBT transistors. (e) is a graph showing the results of measuring the transfer characteristics of this transistor. The data plotted with a small circle (○) in the graph is a small square (□) on the left vertical axis (I _d / A). The data plotted in ( ¹ ) corresponds to the right vertical axis (sqrt (I _d ) / A ^1/2 ).

  In the formation of an organic semiconductor film, the present invention forms a two-layer structure of a polymer and an organic semiconductor, and a solvent vapor annealing method is applied thereto, thereby obtaining an organic semiconductor film having good crystallinity. . The thickness of the organic semiconductor layer is conveniently about several tens of nanometers, and if it is thicker than this, problems such as crystals becoming too thick arise. However, since the process itself is possible even at 100 nm or more, the upper limit of the thickness is not necessarily several tens of nm. If the polymer layer is about 10 to 200 nm thicker, the thicker the film, the higher the effect of solvent vapor annealing. However, if the polymer layer is too thick, the solvent will be taken in too much, causing problems such as crystal flow.

  As long as the solvent vapor annealing treatment is performed, the organic semiconductor must naturally be dissolved in the solvent used in the treatment, but the polymer must be soluble in the same solvent. Specifically, the solubility of the polymer in the solvent used for the solvent vapor annealing treatment in the present invention is preferably 1 mg / mL or more.

  In producing the two-layer structure of polymer layer-organic semiconductor, an appropriate method may be used as appropriate, and it is not necessary to limit to a specific method. As a non-limiting example, a two-layer structure may be realized by spin-coating a polymer film on a substrate and depositing an organic semiconductor film thereon by a vacuum sublimation method or the like.

  Alternatively, this two-layer structure can be formed in one step by using a layer separation method. Here, a method for creating a two-layer structure by a layer separation method will be described. First, a layer separation structure of a polymer and a semiconductor is formed. Specifically, when a polymer having a molecular weight of about 1000 or more and a low molecular organic semiconductor are simultaneously dissolved in a solvent and applied onto a substrate by a method such as spin coating, a polymer-rich layer is formed due to the difference in molecular weight between the two. And separate into a low molecular semiconductor rich layer. The interface between the two layers formed by such a layer separation process becomes very smooth.

  As for the formation of the above two-layer structure by the layer separation method, for example, in Non-Patent Document 3, an organic layer is utilized by utilizing a layer separation phenomenon that occurs when TIPS-Pentacene and poly (a-methylstylene) are mixed and applied. It has been reported that FET characteristics and uniformity have been improved.

  When solvent vapor annealing is performed on a thin film formed in a two-layer structure, organic semiconductor molecules move on the polymer layer to reconfigure the thin film. As a result, a thin film with few grain boundaries or an organic semiconductor single crystal can be formed on the substrate.

  Here, the solvent vapor annealing will be described. Solvent vapor annealing is performed by exposing the sample to an environment saturated with solvent vapor. The resulting functional material reorganization is due to partial remelting of the deposited layer, which triggers molecular rearrangement and improves the material morphology.

  For example, Non-Patent Document 1 reports that by performing solvent vapor annealing on a triethylsilylethynyl anthradithiophene (TES ADT) thin film, the thin film crystallizes and the characteristics of the organic FET are improved. Non-Patent Document 2 also describes crystallization of an organic thin film by solvent vapor annealing and application to an organic FET.

  In general, a higher solvent concentration in the vapor is preferred. In order to achieve this, it can be achieved by using a solvent having high solubility and high vapor pressure, or by adjusting the temperature (Non-Patent Documents 11, 15, and 16). However, the solvent vapor annealing is a complicated interaction between the solvent and the substrate, between the molecule and the solvent, and between the molecule and the substrate (Non-patent Document 16), and its analysis involves many difficulties. So far, significant research has been conducted on vapor annealing on inorganic substrates, but such research on organic substrates has been very delayed.

  The inventor of the present application uses molecular mobility when a layer (specifically, a polymer layer, hereinafter also referred to as a polymer substrate) acting as an organic substrate has a good solubility in the solvent in the solvent vapor annealing process. It has been found that this increases the formation of single crystals of organic semiconductors such as C8-BTBT. That is, as a result of the study by the present inventors, a correlation between the crystallinity of the organic semiconductor film and the affinity between the solvent and the polymer substrate was found. In the crystallization process of the present invention, the solubility of the polymer substrate in the solvent subjected to the solvent vapor annealing treatment is determined by the polarity and boiling point (non-patent document 14), which are decisive factors in the case of an inorganic insoluble substrate. , 15). This general method can be used to make organic single crystals directly on a substrate using any miscible polymeric substrate and solvent vapor annealing solvent.

  In the examples described below, C8-BTBT is mainly described as the organic semiconductor, but the effect of the present invention is not limited to C8-BTBT, and can be understood from the results of Experiment 5 and Experiment 6. As shown, it is effective for all low molecular organic semiconductors. For example, π-electron conjugated aromatic compounds, chain compounds, organic pigments, organosilicon compounds, and the like can be given. More specifically, examples include acene (TIPS-pentacene, etc.), thiophene, perylene, fullerene, porphyrin, and the like. The material of the polymer layer formed so as to overlap with the organic semiconductor layer is not limited as long as it is soluble in the solvent used for solvent vapor annealing and has a molecular weight of about 1000 or more. Of course, there is no intention to limit this to individual substances. For example, PMMA, polystyrene, polyvinyl alcohol, polyethylene glycol, poly (4-vinylphenol) (PVP), α-methyl polystyrene (PαMS), etc. can be used. . The solvent used for solvent vapor annealing also has a large or small effect depending on the type, but it does not matter. As a solvent used for solvent vapor annealing, since both organic semiconductor and polymer material molecules are soluble in the solvent, specifically, toluene, xylene, tetralin, decalin, chlorobenzene, dichlorobenzene, trichlorobenzene, Examples include chloroform, tetrahydrofuran (THF), cyclohexane and the like. Further, in the following examples, an organic FET having a top contact type structure was taken as an example. However, the effect of the present invention is to improve the crystallinity of the channel region. Therefore, the type and structure of the organic device essentially has no effect. It doesn't matter.

  It should be noted that the direction of the two-layer structure to be processed does not affect the solvent vapor annealing process in the present invention. As can be seen from the examples described below, to the cover of the Petri dish in “○ Crystal growth and measurement” in an experiment (for example, [Experimental conditions etc.]) in which solvent vapor is applied with the surface of the two-layer structure facing downward. The sample was attached to the sample), and the experiment was carried out by applying the solvent vapor with the surface facing upward (for example, see the sample installation for video recording in the same location). It was the same. Although not specifically explained, it has been confirmed that there is no influence on the result of the solvent vapor annealing even when the sample is placed vertically (that is, the surface exposed to the solvent vapor is placed vertically). It was.

  Furthermore, it was found that the order of the polymer layer and the organic semiconductor layer in the two-layer structure subject to the solvent vapor annealing treatment does not affect the treatment result. That is, in the following experiment, the polymer vapor film was formed on the substrate side, and the organic semiconductor film was formed thereon, so that the solvent vapor annealing treatment was performed in a state where the organic semiconductor appeared on the sample surface. However, by reversing the stacking order, first, an organic semiconductor film is first formed on a substrate, and a solvent vapor-soluble polymer film formed thereon is formed with a solvent vapor. Surprisingly, when the annealing process is performed, the stacking order is reversed during the process, and a polymer film is formed on the substrate side, and an organic semiconductor film having a good crystallinity (a large single crystal is grown) is formed on the surface side. A formed two-layer structure was obtained. This phenomenon is considered to be the result of the layer separation caused by the solvent given by the treatment in parallel with the solvent vapor annealing.

[Experiment 1]
As shown below, organic semiconductor thin films were formed according to the conventional method (Comparative Examples 1 and 2) and the method of the present invention, and these films were compared.

  An organic semiconductor thin film was formed on a highly doped silicon wafer having a silicon oxide film of 50 nm formed thereon as a substrate, according to an embodiment of the present invention. Here, in Experiment 2, the highly doped silicon functions as a gate electrode, and the silicon oxide film functions as a gate insulating layer. C8-BTBT (Nippon Kayaku Co., Ltd.) and PMMA (Sigma-Aldrich Corporation, USA) were dissolved in chlorobenzene at a ratio of 1: 1 to obtain a 2 wt% solution. By spin coating this solution on a substrate, C8-BTBT and PMMA were separated into layers by the difference in molecular weight, and a two-layer structure of PMMA in the lower layer and C8-BTBT in the upper layer was formed. The polarization micrograph is shown in FIG. Further, when the substrate was subjected to solvent vapor annealing for 120 minutes with chloroform vapor at room temperature, C8-BTBT gathered in a self-organized manner on the substrate, and a needle-like single crystal was formed as shown in FIG.

  On the other hand, as Comparative Example 2, a polarizing micrograph of a C8-BTBT monolayer film formed by spin-coating a chlorobenzene solution in which the same amount of C8-BTBT is dissolved on the same substrate under the same conditions as above. Is shown in FIG. Further, FIG. 1 b) shows a polarization micrograph of the result of solvent vapor annealing for 120 minutes with chloroform vapor under the same conditions as above.

  Furthermore, as Comparative Example 2, a PMMA thin film was first formed on the same substrate by spin coating, and a C8-BTBT thin film was formed thereon under the same conditions as in Comparative Example 1. This polarized light micrograph is shown in FIG. Thereafter, a polarizing micrograph of the result of solvent vapor annealing with chloroform vapor for 120 minutes under the same conditions as above is shown in FIG. 2 b).

  Comparing the experimental example according to the present invention with comparative examples 1 and 2, in particular the polarization microscope after each 120 minute solvent vapor anneal, the organic semiconductor crystals in comparative examples 1 and 2 are much smaller than those according to the present invention. It can be seen (note that the magnification of the photograph in FIG. 3b) is approximately half that of b) in FIG. 1 and b) in FIG. Not only is the organic semiconductor crystal according to the method of the present invention much larger, but as can be readily seen by comparing these photographs, the crystal shape is much better according to the present invention, and its internal disc It shows that there are few orders.

  The polarization micrograph shown in a) of FIG. 4 shows a state in which the lower layer PMMA appears by scraping the central portion of the surface of the organic semiconductor film formed by the same method as in b) of FIG. In addition, since no thin film is formed on the left portion of this photograph, SiO2 on the substrate surface is exposed. The result of measuring the surface height of this surface along the line indicated by the white dashed arrow in the figure is shown in FIG. In particular, the area where C8-BTBT is scraped (ie, the area where PMMA is exposed) is rough due to the nature of this treatment, but the thickness of the C8-BTBT layer and PMMA layer is still about 50 nm. , About 50-60 nm.

  Furthermore, it was investigated how the organic semiconductor thin film formed changes when the solvent vapor annealing time is changed in forming the organic semiconductor thin film according to the present invention. Polarized micrographs of the organic semiconductor thin films obtained under the same conditions as in FIG. 3 and with the solvent vapor annealing time of 3 minutes, 5 minutes, 15 minutes, 25 minutes, 40 minutes, and 120 minutes are shown in FIGS. ) Respectively. As can be seen from these photographs, the organic semiconductor thin film that has been subjected to solvent vapor annealing for 120 minutes is the best in this range because a larger organic semiconductor single crystal is completed and the shape of the single crystal is in order. I understood it.

  Fig. 5g) is an AFM image of a C8-BTBT single crystal formed by solvent vapor annealing, and Fig. 5h) is an AFM image obtained by enlarging the tip of the needle-like crystal. Can be observed. FIG. 5 i) shows a cross-sectional shape of the C8-BTBT single crystal in g) of FIG. 5 cut along the white line from the upper left to the lower right from the center slightly right.

[Experiment 2]
Using the organic semiconductor thin film formed according to the method of the present invention in Experiment 1, an organic field effect transistor (FET) was produced as an example of the organic semiconductor element, and its characteristics were evaluated.

  That is, 15 nm of molybdenum oxide and 50 nm of gold are further vacuum-deposited as source / drain electrodes on the organic semiconductor thin film formed under the same conditions as in b) of FIG. 3, and two or more crystals are formed between the electrodes. If present, the organic FET having a single crystal channel was formed by patterning (cutting) with a laser. In Experiment 1, each single crystal is very large and has good crystallinity. Therefore, a conventional organic semiconductor film is used even in a channel in which a plurality of crystals are connected in series. It is expected to show better characteristics than the fabricated organic FET. However, a channel consisting of a single single crystal should show better characteristics because there is no grain boundary, so measurement was performed with an organic FET having such a configuration. The reason why the plurality of single crystals are not arranged in parallel by cutting with a laser is that the characteristics of a single organic semiconductor single crystal channel are to be measured.

  A schematic diagram of the organic FET fabricated in this way is shown in FIG. Also, a micrograph of the channel region region between the source electrode and the drain electrode of the organic FET actually formed is shown in FIG. In this micrograph, the portion surrounded by the white broken line on the left side shows a crystal cut by a laser. The strip running diagonally to the right is a C8-BTBT single crystal. FIG. 6 c) is a graph showing the drain current-gate voltage characteristics of the single crystal organic FET. Here, the left and right arrows on the graph indicate on the left or right side of the graph the scale of the vertical axis of the base curve.

As a result of electrical measurement of the fabricated organic FET in a vacuum, a mobility of 3.0 cm ² / V s on average and a maximum of 9.1 cm ² / V s were obtained. As a result of evaluating the temperature dependence of the mobility of several devices, it was about 3 cm ² / V s at 300 K, but it increased to about 9 cm ² / V s at 80 K. The temperature characteristics were opposite to those of semiconductors, suggesting carrier transport by band conduction. FIG. 7 a) is a histogram showing the mobility distribution of the fabricated single crystal organic FET at 300 K, and FIG. 7 b) is a histogram showing the threshold voltage distribution of the same FET. 8a) is a graph showing the temperature dependence of the drain current-gate voltage characteristics of the fabricated single crystal organic FET, and FIG. 8b) is a graph showing the temperature dependence of the mobility and threshold voltage of the same FET. is there. In FIG. 8 b), white circles and black circles represent the mobility measured in the temperature lowering process and the temperature rising process, respectively, and white squares and black squares represent the threshold voltages measured in the temperature lowering process and the temperature rising process, respectively. In FIG. 8 b), the threshold voltage is constant with respect to the temperature, so it can be seen that there are very few trap levels that hinder charge transport in the crystal.

[Experiment 3]
Hereinafter, the results of experiments conducted to further examine the conditions for forming an organic semiconductor film having good crystallinity, particularly the conditions for the solvent vapor annealing treatment will be described. Detailed conditions after Experiment 3 will be described separately near the end of this specification.

  When C8-BTBT layer is provided on three kinds of polymer layers that serve as substrates and solvent vapor annealing treatment with three kinds of solvents is applied, single combinations of nine combinations of C8-BTBT are obtained. The degree is shown in FIG.

  In the experiment shown in FIG. 9, three types of polymers, PVP, PMMA, and PαMS, were used, and a C8-BTBT film was formed on each polymer layer by vacuum sublimation. The polymer-C8-BTBT bilayer sample thus formed was subjected to a solvent vapor annealing treatment with three kinds of organic solvents such as THF, chloroform and cyclohexane. The photograph shown in FIG. 9 shows that the sample of the two-layer structure using the polymer positioned in the column direction (that is, the upper side) is subjected to the solvent vapor annealing treatment with the solvent positioned in the row direction (that is, the left side) for 15 hours. It is a polarizing microscope photograph which shows the result. For example, (d) in the figure shows the result of solvent vapor annealing treatment of a sample in which a C8-BTBT film is formed on a PVP substrate with chloroform vapor for 15 hours.

  The three polymer substrates used here are all hydrophobic on the surface, and all three solvents dissolve C8-BTBT well when heated to 40 ° C or higher (approximately 1.5 wt%). . Therefore, the vapor pressure and relative polarity of the solvent (corresponding to the polarity index in Table 1) and the solubility of the polymer in the solvent should be considered as factors that determine the presence or absence of single crystallization by solvent vapor annealing. It was left as a kimono. As is clear from FIG. 9, in the combinations (a), (b), (c), (e), (f) and (i), the growth of C8-BTBT single crystals was observed by performing solvent vapor annealing. However, no single crystal growth was observed in other combinations. Table 1 shows a summary of these results together with candidate factors that influence whether or not single crystallization occurs.

  In Table 1, in order to facilitate the correspondence with FIG. 9, the arrangement order of the solvent and the polymer is the same as FIG. In the table, section A shows the vapor pressure and polarity index of the solvent. Section B indicates whether or not C8-BTBT single crystals grew after solvent vapor annealing for each of the nine polymer-solvent combinations, as indicated by ○ and × (regardless of the size of the single crystal, the single crystal ) If observed. Section C also indicates whether the polymer is soluble in the solvent for each of the nine combinations by Y and n, respectively. For example, in section B of Table 1, when PVP was used as a substrate, single crystals were observed when THF was used as a solvent, but no single crystals were observed when chloroform or cyclohexane was used. It is shown.

  In Table 1, when section A and section B were compared, no correlation was observed between the presence or absence of single crystal formation and the vapor pressure or polarity index of the solvent. Note that in Section B of Table 1, it may appear that the PVP column is related to the order of the vapor pressure of the solvent. That is, a single crystal can be seen only in the solvent THF showing the maximum vapor pressure at the top of the PVP row. However, this regularity does not hold for the chloroform and cyclohexane rows in Table 1. For example, chloroform gives the same result that single crystals can be formed for PMMA and PαMS, but not the same result for PVP. Even when the polar factor is examined, a discrepancy similar to the vapor pressure of the solvent appears. For example, a PαMS substrate is subjected to a solvent vapor annealing treatment with cyclohexane, which is a nonpolar solvent, to produce a single crystal of C8-BTBT, similar to THF and chloroform, which are polar solvents. Therefore, neither the vapor pressure nor the polarity of the solvent is recognized as a dominant factor in the single crystal growth by the solvent vapor annealing treatment.

On the other hand, by comparing section B and section C in Table 1, the formation of single crystals in the solvent vapor annealing treatment is strongly related to the solubility of the polymer constituting the substrate in the solvent. I understand. That is, single crystal growth is observed in all cases where the polymer constituting the substrate is soluble in the solvent given as vapor, whereas no single crystal is observed when it is insoluble. This correlation suggests that a substrate that is soluble in the solvent used in the solvent vapor annealing process assists in recrystallization, possibly due to its increased fluidity. A study of solvent vapor annealing revealed that a thin layer of solvent (thickness of 10 nm) condensed on the substrate, which became a medium for the movement of molecules on the surface (see Non-Patent Document 16). The inventor of the present application examined a real-time video image of the sample surface during the solvent vapor annealing process, and the sample using PMMA as the substrate clearly observed the dissolution and reconstitution of the C8-BTBT film within 2 minutes at the earliest. However, in the sample in which C8-BTBT was provided on a bare (that is, not coated with a polymer) Si / SiO ₂ substrate, no reconstruction was observed even after 1 hour. From these observations, the presence of soluble PMMA has a dramatic effect on the amount of chloroform taken up (similar to many other solvents, but here we will use chloroform as an example) compared to the case without PMMA. Clearly increased. Chloroform incorporated into the polymer layer significantly increased the mobility of C8-BTBT molecules on the surface, which promoted recrystallization of C8-BTBT in a self-organized manner. By performing the solvent vapor annealing process for a long time, the average size of C8-BTBT crystals continues to increase due to Ostwald ripening (see Non-Patent Document 20). This process may continue for several days until the system reaches thermal equilibrium, with larger crystal sizes depending on the time of solvent vapor annealing.

  Further experiments were conducted to confirm the lower limit of the solubility of the polymer in the solvent used for solvent vapor annealing, which is required for organic semiconductor crystal growth. Specifically, a solvent was dropped into the polymer using a micropipette, and the amount of the solvent required to completely dissolve the polymer was estimated. For example, in the case of the combination of PVP and THF, it was found that THF was added dropwise to 53.4 mg of PVP at a time by 50 uL and dissolved in the ninth time, so that 400 to 450 μL of solvent was required. Since the solubility is the polymer weight / solvent amount, it was between 53.4 × 1000/450 and 53.4 × 1000/400, and in this case the solubility was calculated to be 118.7-133.5 mg / mL. This experiment was performed for nine combinations of three types of polymers (PVP, PMMA, PαMS) and three types of solvents (THF, chloroform, cyclohexane). The measurement results are shown in Table 2. In addition, in order to compare and contrast the solubility measured in this way with the experimental results on whether C8-BTBT single crystal growth was observed by solvent vapor annealing in the case of the polymer-solvent combination, The presence / absence of crystal growth is indicated in the rightmost column of Table 2 by ○ / ×.

  In addition, in the part (100+, 50+) with a “+” symbol on the right side of the number of droplets dropped in Table 2, the polymer is completely removed even when the solvent is dropped the number of times indicated by the value. It was shown that it could not be dissolved. “ΜL” and “mL” represent “microliter” and “milliliter”, respectively.

  From the results shown in Table 2, the solubility of the polymer required to cause single crystal growth (solubility in the solvent used for solvent vapor annealing) is 1 mg / mL or more, more preferably 4 mg / mL or more. More preferably, it can be estimated to be 7 mg / mL or more.

[Experiment 4]
In addition to the increase in solvent absorption, C8-BTBT crystallization on polymer films such as PMMA depends on the swelling of PMMA in solvent vapor (see Non-Patent Document 19). FIG. 10 a) is a phase contrast photomicrograph showing the PMMA film as spin-coated and b) the PMMA film after solvent vapor annealing. From these photographs, it was found that pure PMMA films treated with solvent vapor annealing with chloroform caused significant swelling, especially around the edges and defects of the substrate. Thus, during the solvent vapor annealing process, the PMMA chains are greatly relaxed and redistributed locally with the C8-BTBT molecules above it, possibly due to molecular migration and recrystallization caused by π-π interactions. Support When these single crystals were removed by rinsing the sample on which the single crystals were grown by solvent vapor annealing with cyclohexane, the edges of the removed single crystals on the PMMA film were obtained as shown in the polarization micrograph of c) of FIG. There was a height deviation in the surrounding area. Furthermore, from the results of the surface profile scan shown in d) of FIG. 10, it was found that PMMA swelled during the solvent vapor annealing treatment, and the PMMA swollen by the capillary force raised the edge of the crystal. As a result, the dissolved C8-BTBT molecule also rises along the PMMA surface with the edge of the C8-BTBT single crystal and generally increases the thickness of this single crystal. In particular, it has been found that when a thicker PMMA film is used, the C8-BTBT single crystal is deeply embedded in the PMMA film, as shown in FIG.

  In general, under the same solvent vapor annealing treatment conditions, as shown in FIG. 11, the thicker the PMMA substrate, the longer the resulting single crystal and the greater the crystal depth. From this result, the following conclusion can be drawn. That is, the thick PMMA layer condenses and absorbs more chloroform in which C8-BTBT molecules are dissolved, and swells more. Therefore, by the time the thermal equilibrium is reached, the same amount of C8-BTBT molecules travels a longer distance and the recrystallization time becomes longer (see Non-Patent Document 16). Thus, the entire process of C8-BTBT single crystal formation is the result of solvent uptake and swelling of PMMA. When PMMA was made thicker, particularly long crystals (longer than 1 mm) were sometimes obtained as shown in e) and f) of FIG. However, on the other hand, by observing a moving image of the solvent annealing process, when the amount of condensed chloroform is increased, an effect of preventing stable crystallization due to excessive dissolution of molecules may be observed.

[Experiment 5]
In order to further verify whether the solubility of the polymer substrate always improves crystal formation, other three types of organic semiconductor molecules, specifically C10-BTBT, C12-BTBT (see Non-Patent Document 18) and 6 , 13-bis (triisopropyl-silylethynyl) pentacene (TIPS-pentacene) (see Non-Patent Document 21) was used to conduct additional experiments for crystal formation. C10-BTBT and C12-BTBT belong to Cn-BTBTs, but have a longer alkyl chain than C8-BTBT, and therefore have low solubility in chloroform. TIPS-pentacene has been extensively studied due to its high solubility and high FET mobility. As shown in FIG. 12, single crystal growth was observed by performing solvent vapor annealing on the PMMA or PαMS film. On the other hand, as shown in FIG. 13, even when solvent vapor annealing was performed on the organic semiconductor on the bare SiO ₂ substrate, such a single crystal was not obtained. When these organic semiconductor molecules were used, only a considerably small single crystal was obtained compared to C8-BTBT. This is probably because the solubility in chloroform is lower than that of C8-BTBT.

[Experiment 6]
A bottom gate / top contact transistor was fabricated using the C10-BTBT single crystal fabricated in Experiment 5. A transistor manufactured in this manner is shown in FIG. When the transfer characteristics of this transistor were measured, field effect mobility μ _FET = 6.0 cm ² V ⁻¹ s ⁻¹ was obtained.

  The results of Experiment 5 and Experiment 6 show a high possibility that the method of the present invention can be applied to crystallization of a general organic semiconductor. In principle, the method of the present invention can be applied to other organic semiconductors on a polymer substrate as long as both the substrate and the organic semiconductor are soluble in the solvent used for solvent vapor annealing.

[Experimental conditions]
Experiments after Experiment 3 were performed as follows.

○ Preparation of substrate Unless otherwise specified, all experimental processes were performed in the atmosphere. The Si / SiO ₂ layer was cleaned by ultrasonic cleaning for 5 minutes each in deionized water, acetone and isopropanol, followed by treatment with UV ozone for 5 minutes. Polymer coatings on this substrate are available in various concentrations of PMMA (Fluka, Mw: 10,000) in chlorobenzene or anisole, PVP (Sigma-Aldrich Corporation, Mw: 25,000) in chloroform, or PαMS (Fluka, A Mw: 10,000) toluene solution was spin-coated on the Si / SiO ₂ layer to form a polymer film having a thickness of 40 nm to 210 nm.

-Crystal growth and measurement
C8-BTBT (manufactured by Nippon Kayaku Co., Ltd.) is vacuum-deposited (40 nm, 0.3 Å / s, <3 × 10 ^-4 Par) or spin coat (1-2 wt% chlorobenzene or anisole solution, 2000 rpm for 40 seconds). A TIPS-pentacene (Sigma-Aldrich Corporation) film was formed by spin coating of a toluene solution. For the solvent vapor annealing treatment, the sample was attached to a Petri dish cover covering a Petri dish containing half of chloroform with a tape with the upper surface of the sample facing down. For comparison between experiments using the same solvent, all the samples were accommodated in one Petri dish with the same size, so that the experiment was performed under the same vapor pressure (Non-patent Document 22). reference). Since the vapor pressure during the solvent vapor annealing process may be affected by the distance between the solvent surface and the sample, this distance was set to be approximately the same between the samples at the start of the experiment. In order to compare the growth of single crystals on various thicknesses of PMMA, PMMA solutions with various concentrations were spin-coated and annealed at 180 ° C for 2 minutes, and then the same C8-BTBT on these PMMA films. The solution (1 w% anisole solution) was spin coated. For real-time video recording, the sample was placed on a glass slide at the bottom of the Petri dish, covered with a piece of quartz (manufactured by Furuuchi Chemical Co., Ltd., 0.3 mm thickness), and then the entire configuration was placed under a microscope. Movie shooting and crystal length measurement were performed with a VHX-1000 digital microscope (manufactured by Keyence Corporation). The crystal depth measurement uses a P-16 + stylus profiler (KLA-Tencor), and each dot in the graph of FIG. 11 (g) is an average of the values of measurement results exceeding 10 times.

○ FET manufacturing and measurement
A highly doped n-type (100) silicon wafer having a 50 nm SiO ₂ layer was cleaned as described above, and after forming each layer of PMMA and C8-BTBT, a single crystal was obtained by solvent vapor annealing. The source electrode and the drain electrode were produced by vapor-depositing molybdenum oxide and gold (15/50 nm, 0.4 s / s) through a shadow mask in vacuum (<4 × 10 ⁻⁴ Par). The FET characteristics were measured in the atmosphere using an Agilent 4156C semiconductor parameter analyzer. The channel length (L) and width (W) were measured using a color three-dimensional laser scanning microscope (manufactured by Keyence Corporation, VK9510). μ _FET and V _th were obtained by I _d = (W / (2L)) C _i μ _FET (V _g −V _th ) ² from the transfer characteristics in the saturation region. The C _i (capacitance per unit area) of the insulating layer measured by capacitance-voltage measurement was 40 nFcm ⁻² at 50 Hz for the C10-BTBT sample.

  As described above, since an organic semiconductor film having higher carrier mobility than the conventional one and an organic semiconductor element using the same can be provided, it can be used in a wide range of applications including organic FETs, organic LEDs, and solar cells. The practical application of organic semiconductors can be promoted.

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Claims (7)

An organic semiconductor thin film forming method including the following steps.
(A) A polymer layer is formed on a substrate.
(B) After the polymer layer is formed, an organic semiconductor layer is formed thereon to form a two-layer structure on the substrate.
(C) by performing a solvent vapor annealing process using a solvent in which the polymer and the organic semiconductor are soluble in the two-layer structure and the solubility of the polymer is 1 mg / mL or more , A single crystal of the organic semiconductor is grown in the organic semiconductor layer . The method for forming an organic semiconductor thin film according to claim 1, wherein the polymer has a molecular weight of 1000 or more . The organic semiconductor thin film forming method according to claim 1 , wherein the polymer is selected from the group consisting of PMMA, PVP, PαMS, polystyrene, polyvinyl alcohol, and polyethylene glycol . The solvent used in the solvent vapor annealing is selected from the group consisting of toluene, xylene, tetralin, decalin, chlorobenzene, dichlorobenzene, trichlorobenzene, chloroform, THF, and cyclohexane . Organic semiconductor thin film forming method. The organic semiconductor thin film forming method according to any one of claims 1 to 4, wherein the organic semiconductor is an organic semiconductor selected from the group consisting of a π-electron conjugated aromatic compound, a chain compound, an organic pigment, and an organic silicon compound. . The organic semiconductor thin film forming method according to claim 5 , wherein the organic semiconductor is selected from the group consisting of C8-BTBT, C10-BTBT, C12-BTBT, and TIPS-pentacene . The method for forming an organic semiconductor thin film according to any one of claims 1 to 6, wherein the polymer layer has a thickness in a range of 10 to 200 nm .