JP5429784B2 - Organic thin film transistor and manufacturing method thereof - Google Patents

Description

  The present invention relates to an organic thin film transistor constituting an organic semiconductor device and a manufacturing method thereof.

Electronic devices made of organic semiconductors are attracting attention as inexpensive alternatives to silicon semiconductor devices. In particular, an organic semiconductor device can be manufactured at a lower cost than a silicon semiconductor device that requires a process that requires a significant manufacturing cost, and is useful when economy is a priority.
As other advantages of organic semiconductors, it is easy to make a large-area electronic device using a thin film, and it can be formed on a plastic substrate because a high-temperature process is not required in the manufacturing process. Since it has characteristics such as not deteriorating element characteristics against mechanical bending, it is possible to manufacture a large-area mechanically flexible electronic device that is impossible with a silicon semiconductor device.

  In particular, in order to realize a flexible large-area display and electronic paper, research and development of an organic semiconductor thin film field effect transistor shown in FIG. In FIG. 4, 1 is an electrode, 2 is an organic semiconductor, 4 is an insulating layer, and 5 is a gate electrode.

Currently, low molecular weight materials such as pentacene are the main research targets for organic semiconductor materials constituting organic thin film transistors. In many cases, the low molecular weight semiconductor thin film layer used in the organic thin film transistor is a polycrystalline thin film made of microcrystals having a size of several nanometers to several tens of micrometers. These thin films are generally manufactured on a gate insulating film such as a silicon oxide film by using a vacuum deposition method or a coating method.
Even if the same organic semiconductor material is used, the film quality such as the size of the microcrystals in the polycrystalline thin film, the filling rate, the crystal structure, the connectivity between the microcrystals varies greatly depending on the thin film manufacturing method and the surface state of the substrate. As a result, the performance of the organic semiconductor device depends critically. In order to obtain a high-performance organic thin film transistor, it is indispensable not only that the material itself has high performance, but also finding a thin film manufacturing method suitable for each material.

For example, in a pentacene thin film transistor, it is known that device characteristics are greatly improved by modifying the surface of a silicon oxide film used as a gate insulating film with a silane coupling agent. As the silane coupling agent, HMDS (Hexamethyldisilazane), OTS (Octadecyltrichlorosilane) or the like is used. These react with the SiOH group on the surface of the silicon oxide film to greatly change the substrate surface from hydrophilic to water-repellent.
In a pentacene thin film transistor, the mobility of an unprocessed substrate remains at 0.1 cm ² / Vs, but the mobility can be reduced to about 1 to ² cm ² / Vs by improving the water repellency of the substrate on which the thin film is grown. And greatly improved.

However, the above method is not effective for all low molecular weight materials. For example, HMTTF (Hexamethyletrahiafulvalene) having the chemical structural formula shown in FIG. 5 is known. (See Patent Document 1)
In the HMTTF single crystal transistor, mobility exceeding 10 cm ² / Vs has been reported (see Non-Patent Document 1), which has a high potential, but a thin film is grown on the surface of a silicon oxide film subjected to HMDS treatment or OTS treatment. In the method, it is stopped at about 0.08 cm ² / Vs. In this case, since the coverage (filling rate) of the thin film on the substrate is also low, it is difficult to obtain a good reproducibility result. In order to utilize the high potential of HMTTF, it is indispensable to construct a high-quality thin film with a large crystallite size and a high filling rate.

  On the other hand, as one method for controlling the organic semiconductor thin film layer, a method of producing an ultrathin insulating polymer layer having a thickness of several nanometers by a spin coating method has been attempted (see Patent Document 2 and Non-Patent Document 2). . This method is the same as the silane coupling agent in that it increases the water repellency of the substrate. However, the silane coupling agent is limited to the oxide film surface, but it can be applied to plastic substrates. Have. However, in a pentacene thin film transistor manufactured on a substrate processed by this method, the mobility is the same as or lower than that of the silane coupling agent.

JP 2005-223175 A JP 2005-228968 A

Yukihiro Takahashi et al. “High mobility Organic Field-Effect Transistor Based on Hexamethylenetetrathiafulvalene with Organic Metal Electrodes”, Chemical Materials 19, 6382 (2007). Choongik Kim et al. “Gate Dielectric Microstructural Control of Pentacene Film Growth Mode and Field-Effect Transistor Performance”, Advanced Materials, Vol.19, pp. 2561-2566 (2007)

  In view of the above situation, the present invention has an object to realize an organic thin film transistor comprising a high-quality thin film using a low molecular organic semiconductor having a potentially high mobility and having a cyclic alkyl group site. To do.

In order to solve the above problems, the present invention provides the following organic thin film transistor and a method for manufacturing the same.
(1) An organic thin film transistor having a channel region made of a low molecular weight organic semiconductor having a cyclic alkyl group portion on a gate insulating film through a polymer thin film having high affinity with an alkyl chain.
(2) The organic thin film transistor according to (1), wherein the low molecular organic semiconductor is HMTTF.
(3) The organic thin film transistor according to (1) or (2), wherein the polymer thin film is any one of polystyrene, polycarbonate, PMMA, PVA, polyethylene, and polypropylene.
(4) A step of forming a polymer thin film by applying a polymer solution having a high affinity with an alkyl chain dissolved in toluene on a silicon oxide film constituting the gate insulating film, and having a cyclic alkyl group site on the polymer thin film. And a method of manufacturing an organic thin film transistor, including a step of forming a low molecular weight organic semiconductor film constituting the channel region.
(5) Manufacture of an organic thin film transistor including a step of forming a polymer thin film by applying a polystyrene solution dissolved in toluene on a silicon oxide film constituting a gate insulating film and a step of forming an HMTTF organic semiconductor film on the polymer thin film Method.

In the present invention, a low molecular weight organic semiconductor having a cyclic alkyl group site in the molecule like HMTTF is formed on a polymer thin film having a high affinity with an alkyl chain. Exercise is more efficient. Thereby, film quality such as microcrystal size, filling rate, crystal structure, and bonding between microcrystals is improved, and a high-performance organic thin film transistor having a mobility exceeding 6 cm ² / Vs can be obtained.

It is a cross-sectional structure schematic diagram of the organic thin-film transistor which concerns on this invention. It is a characteristic view of the organic thin-film transistor which concerns on this invention. It is an AFM image of the obtained HMTTF thin film. It is a cross-sectional structure schematic diagram of the conventional organic thin-film transistor. It is a chemical structural formula of HMTTF.

1 Electrode 2 Organic Semiconductor 3 Polymer Thin Film 4 Silicon Oxide Film 5 Gate Electrode

Embodiments of the present invention will be described below in detail with reference to the drawings.
FIG. 1 is a schematic sectional view of an organic thin film transistor according to the present invention. An HMTTF film, which is an organic semiconductor, is laminated on a substrate constituting the gate electrode 5 via a silicon oxide film 4 having a thickness of 300 to 500 nm and a polymer thin film 3 having a thickness of 4 nm constituting a gate insulating film. An electrode 1 constituting a drain electrode is formed to constitute an organic thin film transistor.
The polymer thin film 3 in FIG. 1 is a polystyrene thin film.

Gate voltage dependence of the source-drain current I _DS when the drain-source voltage of the organic thin film transistor according to the present invention (V _D) was -1V Figure 2 shows a graph of (transfer characteristics). On / off ratio of the organic thin film transistor from Fig., It is estimated to be of the order of 10 ^6.
The mobility μ is calculated from the following equation.
I _DS = (WC _i / 2L) μ (V _G −V ₀ ) ²
However, _{C i} is the insulating capacity, 10nF ^/ cm 2, _{V G} is the gate _{voltage, I DS} is the source-drain current, _{V 0} is the extrapolated threshold voltage.
From the above formula, the mobility of the HMTTF thin film transistor by the polystyrene thin film coated substrate was calculated to be 6.4 cm ² / Vs.

The manufacturing method of the organic thin-film transistor concerning this invention is demonstrated.
A silicon oxide thin film with a thickness of 300 nm formed on the surface of the N-type doped silicon substrate is used as the gate insulating film. Polystyrene, polycarbonate, PMMA, and PVA were used as polymers for modifying the insulating film surface. These polymers were coated by spin coating of a solution in which the polymer was dissolved.

  In polystyrene, toluene was used as a solvent and dissolved in a concentration of 0.11 wt% to obtain a solution. In polycarbonate, a solution in which monochlorobenzene was used as a solvent and dissolved to a concentration of 0.8 wt% was used. In PMMA, a solution in which toluene was used and dissolved in a concentration of 0.1 wt% was used. In PVA, a solution obtained by dissolving ultrapure water as a solvent to a concentration of 0.1 wt% was used.

Each solution is prepared by dispersing polymer powder in a solvent, using a rotator, and dissolving by heating to about 50 to 100 ° C. with a hot plate, and then performing filtration using a 0.5 micron membrane filter, Obtained by removing trash etc.
The polymer solution obtained by the above procedure was coated on the substrate by the following process.
(1) The organically cleaned substrate is irradiated with UV light for 30 minutes with a UV light irradiation device to decompose and clean the attached organic matter.
(2) A few drops of the polymer solution are dropped on the treated substrate and spin-coated for 1 minute at 8000 rpm.
(3) A polymer-coated substrate was obtained by drying in an electric oven at 100 ° C. for 1 hour and then gradually cooling.

An organic semiconductor thin film was formed on the substrate by a normal vacuum deposition method. The substrate is filled with about 10 to 20 mg of HMTTF material purified by a vacuum sublimation method in a heating vapor deposition source using a ceramic crucible, and evaporated by heating to about 200 ° C. under a vacuum of 2 × 10 ⁻⁵ Pa. About 50-100 nm was vapor-deposited on the top.

  FIG. 3A is an AFM image of the obtained HMTTF thin film surface, and FIG. 3B is a three-dimensional view thereof. FIG. 3 shows that a polycrystalline thin film having a high coverage is obtained on a polystyrene-coated substrate.

  This is because the surface diffusion motion of the HMTTF molecule having a cyclic alkyl group site in the molecule becomes efficient on the gate insulating film coated with a polymer thin film such as polystyrene having high affinity with the alkyl chain. . Compared with the thin film grown on HMDS, the thin film grown on the gate insulating film coated with a polymer thin film such as polystyrene, which has high affinity with the alkyl chain, has extremely high crystallite orientation. I understand.

As a material for the source and drain electrodes, TTF-TCNQ, which is a conductive organic material with high carrier injection efficiency into HMTTF, was used. Electrode patterning by TTF-TCNQ was produced by vacuum deposition using a shadow mask. The TTF-TCNQ material was heated to about 170 ° C. and evaporated under a vacuum of 10 ⁻⁴ Pa. The organic thin film transistor according to the present invention is obtained through the above steps. The device characteristics were measured using a prober in argon gas.

As described above, in the present invention, a low molecular organic semiconductor having a cyclic alkyl group site in the molecule like HMTTF is formed on a polymer thin film having a high affinity with an alkyl chain. Increases efficiency of molecular diffusion in As a result, film quality such as microcrystal size, filling rate, crystal structure, and connectivity between microcrystals was improved, and a high-performance organic thin film transistor having a mobility exceeding 6 cm ² / Vs could be fabricated. .

In the present specification, the present invention has been described by exemplifying a preferred embodiment. However, the above-described embodiment is only for facilitating understanding of the present invention, and the present invention is limited to this embodiment. is not.
That is, modifications and other aspects based on the technical idea of the present invention are naturally included in the present invention.

  For example, although a silicon oxide film is exemplified as the insulating film, a silicon nitride film or a polymer may be used as long as it has a good insulating property. Moreover, although polystyrene, polycarbonate, PMMA, and PVA were illustrated as a polymer, if it is a polymer with high affinity with alkyl chains, such as polyethylene and a polypropylene, it can substitute.

Claims (1)

The organic thin film transistor having a channel region formed of a organic semiconductor via a polymer film of polystyrene having a thickness of 4 nm HMTTF on the silicon oxide film.