KR20080075648A - Organic thin film transistor with a controlled threshold voltage - Google Patents

Abstract

An organic thin film transistor is provided to control a threshold voltage of the TFT(Thin Film Transistor) by changing a work function of a gate electrode by forming an ion film on a surface of the gate electrode. An organic thin film transistor includes a substrate, a gate electrode, an insulation layer, source and drain electrodes, and an organic active layer. An ion film is formed on a surface of the gate electrode, such that a work function of the gate electrode is changed. A cation film is formed by treating an acid liquid on the surface of the gate electrode. The acid liquid is selected from the group consisting of phosphoric acid, sulfuric acid, and acetone. An anion film is formed by treating a base liquid on the surface of the gate electrode.

Description

Organic Thin Film Transistor with a Controlled Threshold Voltage

1A to 1C are cross-sectional views of an organic thin film transistor, and FIG. 1A is a top-contact of a source-drain electrode formed on an organic active layer among lower gates, and FIG. 1B illustrates a source-drain of an organic active layer among lower gates. A bottom-contact deposited on the electrode and the insulating layer, and FIG. 1C is a cross-sectional view of the upper gate organic thin film transistor;

2 is a view showing the surface energy change of the gate electrode formed with a cationic film by treatment with an acidic aqueous solution according to one embodiment of the present invention;

3 is a view showing the surface energy change of the gate electrode formed with an anion film by treating with a basic aqueous solution according to one embodiment of the present invention;

4 is a graph illustrating electrical characteristics of a gate electrode on which an ion film is formed according to an embodiment of the present invention.

The present invention relates to an organic thin film transistor (OTFT) capable of adjusting a threshold voltage, and more particularly, to an organic thin film transistor including a substrate, a gate electrode, an insulating layer, a source and a drain electrode, and an organic active layer. An ion film is formed on the surface of an organic thin film transistor, and the work function of the gate electrode is changed.

Recently, there is an increasing need for materials capable of excellent image quality and a large screen, low power consumption, light weight, and miniaturization as information display materials in various fields such as TVs, monitors, mobile phones, and video mobile communications. However, conventional inorganic thin film transistors such as TFTs (thin-film transistors) mostly use silicon (silicon) as an inorganic layer as an active layer, and silicon is not bendable due to its characteristics, and it is fragile and expensive to manufacture, so there is a limitation in application.

As one of the next generation materials to solve this problem, an organic thin film transistor (OTFT) is emerging. OTFT uses an organic film instead of a silicon film as an active layer, and according to the material of the organic film, a low molecular weight organic thin film transistor such as oligothiophene, pentacene, etc., and a polymer organic thin film such as polythiophene series It is divided into transistors.

OTFT has 10 to ³ times lower charge mobility than conventional single crystal silicon semiconductors, which limits the application of high-speed switching devices, but it is easy to large area, transparent and flexible, and relatively low temperature and low cost. It has many advantages, including the possibility of manufacturing.

Therefore, research for using OTFT as a drive circuit of a liquid crystal display device, an organic electroluminescent display using a plastic substrate, etc. is actively progressing. In addition, the application range of organic thin film transistors to an integrated circuit has been extended to display devices such as electronic price tags, stamps, radio frequency identification tags, smart cards, and electronic paper. .

However, the OTFT has a high threshold voltage due to defects in the organic active layer thin film, and thus, the low mobility and the high threshold voltage have limitations in practical use of electronic circuits despite many advantages of the OTFT.

Accordingly, attempts have been made to reduce the threshold voltage of the OTFT by applying an insulator having a high dielectric constant to the device. However, since the conductive channel for operating the OTFT is formed at the interface between the insulator and the organic semiconductor, when the device is continuously driven, residual charge is present at the interface due to the high dielectric properties of the insulator, which causes the threshold voltage to be severely shifted, thereby causing electrical There is a problem of instability.

Therefore, there is a high need for a technology that can electrically control the threshold voltage of the organic thin film transistor while being stable. Accordingly, the present invention provides a technique of controlling the threshold voltage by forming an ion film on the surface of the gate electrode to change the work function of the gate electrode.

Meanwhile, as a method for adjusting the work function of the gate electrode, a method of using the material of the gate electrode as a material having various work functions may be considered, but a material which can be used as the gate electrode of the OTFT is Au, Al, Cr, etc. Limited to metals, this approach is practically limited.

Accordingly, an object of the present invention is to solve the problems of the prior art as described above and the technical problems that have been requested from the past.

The inventors of the present application, after in-depth study and various experiments, when forming an ion film on the surface of the gate electrode, it is possible to control the threshold voltage by changing the work function of the gate electrode and to maintain an electrically stable characteristic It was confirmed that the applicability as a next-generation electronic device can be maximized, and the present invention has been completed.

Therefore, the organic thin film transistor (OTFT) according to the present invention is an organic thin film transistor (OTFT) composed of a substrate, a gate electrode, a source and drain electrode, an insulating layer, and an organic active layer, and an ion film is formed on the surface of the gate electrode. The work function of the gate electrode is changed.

In the present specification, 'the work function of the gate electrode is changed' means that the work function of the material itself used as the gate electrode is reduced or increased by the formation of the ion film.

The change of the work function of the gate electrode enables the adjustment of the work function difference between the gate electrode and the organic semiconductor, thereby controlling the flat band voltage of the OTFT and thereby controlling the threshold voltage of the device. Therefore, by effectively reducing the threshold voltage, it is possible to lower the power consumption of the display, the sensor, and the electronic element to which the OTFT is applied. In addition, since the increase in the threshold voltage can be easily realized, it is possible to selectively increase the threshold voltage of the element element as necessary in implementing various integrated circuits.

Specifically, the shift of the threshold voltage of the OTFT is influenced by the flat band voltage, as shown in the following equation (1), and the flat band voltage is one between the gate electrode and the organic semiconductor, as shown in the following equation (2). Affected by functional differences or interfacial charges.

(1)

(One)

(2)

(2)

: p-type 반도체의 벌크 포텐셜(bulk potential for p-type material); l _dep : 공핍층 길이(depletion length); ΔΦ _MS : 게이트 전극과 유기 반도체의 일함수 차이(gate-semiconductor work function difference); 및 Q _if : 계면 전하(interface charge)를 나타낸다. _Wherein : V _FB : flat band voltage; N _A : density of ionized acceptors; d : thickness of the organic semiconductor film; C _ins : gate insulator capacitance per unit area of the gate insulator;

: The bulk potential of the p-type semiconductor (bulk potential for p -type material) ; l _dep: Length depletion (depletion length); ΔΦ _MS : gate-semiconductor work function difference between the gate electrode and the organic semiconductor; And Q _if : Represents an interface charge.

Therefore, in order to increase the threshold voltage, a cation film can be formed in the device using the p-type organic semiconductor so as to increase the work function of the gate electrode, whereas in the case of using the n-type organic semiconductor, the work function of the electrode can be reduced. To form an anion membrane.

That is, even when a high dielectric constant gate insulator is not used, the threshold voltage can be ultimately controlled by adjusting the work function difference between the gate electrode and the organic semiconductor by adjusting the work function of the gate electrode and thereby adjusting the flat band voltage. In addition, it is possible to fundamentally prevent the problem of electrical instability that may occur when using a high dielectric constant gate insulator.

The method for forming the ion film according to the present invention is not particularly limited, and in one preferred example, the surface of the gate electrode may be formed so as to form a uniform ion film that is easy to process a large area gate electrode. It can be formed by treating an acidic or basic solution. That is, the cationic film may be formed by treating an acidic solution on the surface of the gate electrode, or the anionic film may be formed by treating a basic solution.

When the gate electrode is treated with an acidic solution, the threshold voltage can be increased because the work function of the gate electrode is increased, and when the gate electrode is treated with the basic solution, the threshold voltage is reduced because the work function of the gate electrode is decreased. can do.

The method of treating the acidic or basic aqueous solution on the surface of the gate electrode is not particularly limited, and for example, spray coating, spinning, dip coating, gravure coating, microcontact printing, The ion layer can be formed by treating the gate electrode surface using various known methods such as ink jet printing, stamping, transfer printing, and deposition.

In a specific example, the substrate on which the gate electrode is formed may be immersed in an acidic aqueous solution or a basic aqueous solution using a immersion method and treated for a predetermined time. Preferably, the acidic or basic aqueous solution treatment may be performed in a bath at a predetermined temperature. In addition, after the aqueous solution treatment, it may be subjected to an ultrasonic cleaning step and / or a drying step at a predetermined temperature in secondary distilled water of the same temperature.

The treatment temperature and treatment time of the acidic or basic aqueous solution may vary depending on the type and concentration of the aqueous solution and the like, and are not particularly limited.

The acidic aqueous solution and the basic aqueous solution for forming the ion membrane may be appropriately selected by varying the type and concentration according to the material of the gate electrode, and are not particularly limited. The acidic aqueous solution is preferably phosphoric acid or sulfuric acid. acid and acetone may be one or two or more selected from the group consisting of, and the basic aqueous solution is preferably tetrabutyl ammonium hydroxide, sodium hydroxide and potassium hydroxide. hydroxide) may be one or two or more selected from the group consisting of, but is not limited thereto.

In another preferred example for forming an ion film according to the present invention, the ion film may be formed by treating the surface of the gate electrode with a plasma generated by a reactive gas in a vacuum chamber. That is, the oxide on the surface of the gate electrode may react with the reactive plasma to form an ion layer.

The plasma treatment may be performed by placing a substrate on which a gate electrode is formed in the chamber using, for example, an apparatus including a chamber capable of forming a plasma. In this case, the power (watt) for forming the plasma can be changed for proper treatment, and the optimization effect can be exerted depending on the treatment time, so that the conditions can be secured.

The plasma treatment does not need to undergo a separate cleaning process or drying process, and in terms of maximizing the plasma treatment effect, it is preferable to minimize the time that the substrate is exposed to the atmosphere after the treatment to 30 minutes or less.

The reactive gas may be, for example, hydrogen, oxygen, fluorine-based gas, or inert gas oxygen, but is not limited thereto.

If the thickness of the ion film is too thin, it may not be possible to change the work function of the gate electrode to a desired level, on the other hand, if the thickness is too thick, there is a fear that the adhesion to the insulating layer is lowered, and the thickness of the device is unnecessary. Since there is a problem of increasing, the ion membrane may be preferably formed in a thickness of several ohms strong (nm) to several nanometers (nm), and the plasma surface-treated ion film is formed in a thickness of several ohms strong (Å) Can be. For example, the ion film may be formed to a thickness of 0.01 to 99 nm.

The density between the ions in the ion membrane can be variously adjusted by adjusting the concentration of the acidic or basic aqueous solution or by changing the reaction conditions such as the reaction time or the type of reactive gas during the plasma treatment, the work function of the desired gate electrode It can be set differently according to the degree of increase and decrease of.

The basic structure and driving principle of the OTFT according to the present invention is the same as that of a field-effect transistor (FET), and the basic component is an electrode (gate, source, drain), an organic active layer, and an insulating layer.

The electrode (gate, source, drain) is not particularly limited, a known electrode material can be used, for example, indium-tin oxide (ITO), chromium (Cr), copper ( Cu), molybdenum (Mo), aluminum (Al), aluminum alloy (Al alloy), tungsten (W), doped silicon, gold, silver, nickel, palladium, platinum, tantalum, barium, calcium and titanium It may be one or two or more selected, and may be used in various forms such as alloys, combinations, multilayers thereof, and the gate electrode may be preferably ITO. In addition, conductive polymers such as polyaniline, poly (3,4-ethylenedioxythiophene) / poly (styrene sulfonate) (PEDOT: PSS), and the like can also be used.

The substrate is not particularly limited, and a glass substrate, a plastic substrate, a metal substrate, and the like may all be used. The metal substrate may be, for example, steel use stainless steel (SUS), or the like, and the plastic substrate may be, for example, polyethersulphone (PES), polyacrylate, polyether imide (PEI), or the like. , polyetherimide, polyethylene naphthalate (PEN, polyethyelenennapthalate), polyethylene terephthalate (PET, polyethyeleneterepthalate), polyphenylene sulfide (PPS), polyimide, polycarbonate (PC), cellulose triacetate (TAC ), And cellulose acetate propionate (CAP), but may be one or two or more selected from the group consisting of, but are not limited thereto.

The organic active layer is, for example, pentacene (tetracene), tetracene (tetracene), anthracene (anthracene), naphthalene (naphthalene), alpha-6- thiophene, perylene (rubylene), rubrene, Coronene, perylene tetracarboxylic diimide, perylene tetracarboxylic dianhydride, polythiophene, polyparaperylenevinylene, polyfluorene, Polythiophenevinylene, polyparaphenylene, polythiophene-heterocyclic aromatic copolymer, oligoacene of naphthalene, oligothiophene of alpha-5-thiophene, phthalocyanine with or without metal, pyromellitic Dianhydrides, pyromellitic diimides, perylenetetracarboxylic acid dianhydrides, naphthalene tetracarboxylic acid diimides, naphthalene tetracarboxylic acid dianhydrides, and derivatives thereof It may be one or two or more selected from the group consisting of, but is not limited to these.

The insulating layer is not particularly limited, and may be an inorganic insulating layer, an organic insulating layer, or an organic-inorganic hybrid film. The inorganic insulating layer may include, for example, a nitride film such as silicon nitride (SiN _x ), an oxide film such as silicon oxide (SiO _x ), strontate, tantalate, titanate, zirconate, aluminum oxide, or tantalum oxide. , Titanium oxide, barium titanate, barium strontium titanate, barium zirconate titanate, zinc selenide, zinc sulfide and the like.

The organic insulating layer is selected from, for example, polyvinylidene fluoride (PVDF), cyanocellulose, epoxy, benzocyclobutene (BCB, benzocyclobutene), polyimide, parylene and polyvinylphenol (PVP, polyvinylphenol) It may be one or two or more. In addition, an organic layer having a high dielectric constant may also be used, for example, Ba _x Sr _1-x TiO ₃ (Barium Strontium Titanate, BST), Ta ₂ O ₅ , Y ₂ O ₃ , TiO ₂ , PbZr _x Ti _{1 -x} O ₃ (PZT), Bi ₄ Ti ₃ O ₁₂ , BaMgF ₄ , SrBi ₂ (Ta _1-x Nb _x ) ₂ O ₉ , Ba (Zr _1-x Ti _x ) O ₃ (BZT), BaTiO ₃ , SrTiO ₃ , Bi ₄ Ti ₃ O ₁₂ , and the like.

In one preferred embodiment, the insulating layer is an organic-inorganic mixed insulator of a polymer and inorganic particles, the organic-inorganic mixed insulator is a structure in which inorganic particles of high dielectric constant is evenly dispersed in the insulating polymer, or on the polymer layer It is composed of a structure in which a high dielectric constant inorganic film is formed, and may be electrically stable and compensate for low dielectric properties of the polymer. An insulating layer of such a structure is disclosed in Korean Patent Application No. 2006-0056692 to the applicant, which is incorporated by reference in the context of the present invention.

The structure of the OTFT is not particularly limited, and as shown in FIG. 1A, a source-drain electrode is formed on the organic active layer (top-contact), or as shown in FIG. 1B, an organic active layer is formed on the source-drain electrode and the insulating layer. A bottom-gate structure of a bottom-contact, or as shown in FIG. 1C, the gate may be a top-gate structure formed on the insulating layer and the organic active layer.

In some cases, the organic thin film transistor according to the present invention may further include a passivation layer, an encapsulation layer, a buffer film, and the like.

The present invention also provides a method of manufacturing an organic thin film transistor composed of a substrate, a gate electrode, an insulating layer, a source and drain electrode and an organic active layer, in order to produce a gate electrode having a changed work function, an acidic solution, a basic solution or a plasma It provides a manufacturing method including the step of forming an ion film by surface treating a gate electrode.

The ion film forming process may be preferably performed by a continuous process in a state where a gate electrode is formed on a substrate.

In a specific example, when manufacturing a top-contact OTFT in which a source-drain electrode is formed on an organic active layer,

i) providing a substrate;

ii) forming a gate electrode on the substrate;

iii) treating the gate electrode material with an acidic or basic solution or surface treatment with plasma to form an ion film;

iv) forming an insulating layer on the gate electrode on which the ion film is formed;

v) applying an organic active layer to the insulating layer; And

vi) forming a source electrode and a drain electrode to contact the organic semiconductor layer.

When the ion membrane is formed by treating with acid or basic aqueous solution in step iii), it may be subjected to a bath treatment at a predetermined temperature. The bath temperature may be appropriately adjusted in consideration of the type of aqueous solution, the evaporation temperature of the aqueous solution, the amount of toxic substances generated during heating, and the like.

In addition, after the aqueous solution treatment, it may be subjected to an ultrasonic cleaning process and / or a drying process at a predetermined temperature for a few seconds (sec) with a cleaning liquid of the same temperature.

The cleaning process may be performed, for example, for about 20 seconds with secondary distilled water to minimize side reactions with the ion membrane. The drying process may be performed at, for example, about several seconds to several minutes in an electric oven at a temperature such that the cleaning solution may be evaporated without causing deformation of the substrate. For example, when the cleaning process is performed with distilled water, the drying temperature may be performed at approximately 100 to 150 ° C. in a range below the glass transition temperature of the substrate.

The method of depositing a gate electrode, an insulating layer, a source and a drain electrode, and an organic active layer on the substrate is not particularly limited, and may vary depending on whether it is a lower gate method or an upper gate method. The method for forming thin film electrodes, such as the gate electrode, the source electrode, and the drain electrode, is known, for example, by physical vapor deposition such as vacuum deposition, thermal evaporation, sputtering, or ink jet printing. The method can be used.

In addition, when using a polymer or oligomer dissolved in a solvent as the organic active layer, a solution process such as spin coating, solution casting, printing, Langmuir-Blodgett method may be used.

Patterning of the electrodes can be performed by known methods such as shadow masking, photolithography, printing, microcontact printing, and pattern coating.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the following Examples are provided to illustrate the present invention, and the scope of the present invention is not limited thereto.

Hereinafter, in order to evaluate the performance of the organic thin film transistor (OTFT) according to the work function change of the gate electrode, as shown in FIG. 1A, a source-drain electrode is formed on the organic active layer in the lower gate structure. contact) was manufactured.

 Example 1

An ITO gate electrode was formed on the glass substrate by the sputtering method. Then, 1 wt% Phosphoric acid aqueous solution of concentration was treated by immersion for 4 minutes to form a cationic membrane. At this time, the aqueous solution was soaked in 30 ~ 40 ℃, after treatment, ultrasonic cleaning was performed for 20 seconds in distilled water, and dried at approximately 150 ℃.

The surface energy change of the gate electrode on which the cation film was formed was measured and shown in FIG. 2. As shown in FIG. 2, it was found that a cationic film was formed through absorption of protons by treatment with an aqueous solution of phosphoric acid, thereby increasing the surface energy of the gate electrode.

An insulating film was formed on the gate electrode using a spin coating method of PVP on the gate electrode on which the cationic film was formed. Then, a p-type OTFT was prepared by thermally depositing pentacene as an organic semiconductor film, and finally forming a source / drain electrode by thermally depositing Au.

Example 2

Except that the gate electrode was treated for 3 minutes by immersion using tetrabutyl ammonium hydroxide at a concentration of 40 wt% instead of an aqueous phosphoric acid solution to form an anion film on the surface of the gate electrode. An organic thin film transistor (OTFT) was manufactured in the same manner as in Example 1.

The surface energy change of the gate electrode on which the anion film was formed was measured and shown in FIG. 3. As shown in FIG. 3, it can be seen that an anionic membrane was formed through absorption of hydroxide ions by treatment with an aqueous tetrabutyl ammonium hydroxide solution, thereby reducing the surface energy of the gate electrode.

Comparative Example 1

An organic thin film transistor (OTFT) was manufactured in the same manner as in Example 1, except that the aqueous phosphoric acid treatment was not performed on the gate electrode.

 Experimental Example 1

In general, the performance of OTFT is evaluated by threshold voltage, subthreshold slope, on / off current ratio, electric field mobility, etc. The overall performance of OTFTs according to Examples 1 and 2 and Comparative Example 1 is evaluated. The comparison is shown in Table 1 below.

TABLE 1

As shown in Table 1, the OTFT according to Example 1 in which the cationic film was formed on the gate electrode compared to the OTFT according to Comparative Example 1 increased the threshold voltage, whereas the anion film was formed on the gate electrode according to Example 2 It can be seen that the OTFT has significantly reduced the threshold voltage. Based on the above results, it was confirmed that the threshold voltage was easily controlled through the surface treatment of the gate electrode.

Experimental Example 2

In order to evaluate the electrical characteristics of the OTFT according to the formation of the ion layer on the gate electrode surface, changes in the drain-source current values of the OTFTs according to Examples 1 and 2 and Comparative Example 1 were measured and shown in FIG. 4. The channel length and width (L / W) were 90 μm / 200 μm, respectively, and the drain voltage (V _D ) was tested at −40 V.

Referring to FIG. 4, when the voltage of the gate electrode is -60 V, the drain-source current value of the OTFT according to Example 2 is approximately similar to that of Comparative Example 1, and the drain-source of the OTFT according to Example 1 The current value was 0.0025 A, which was greatly increased than that of Comparative Example 1 having a value of about 0.0020 A, indicating that the output characteristics were improved or at least the degradation of the output characteristics was prevented. Accordingly, it can be seen that the ion layer is formed on the gate electrode surface to be electrically stable even when the threshold voltage is increased or decreased.

The foregoing has been described above with reference to some embodiments according to the present invention, but those skilled in the art to which the present invention pertains to various applications and modifications within the scope of the present invention based on the above information. It will be possible.

As described above, the organic thin film transistor according to the present invention forms an ion film on the surface of the gate electrode to change the work function of the gate electrode, thereby controlling the threshold voltage and maintaining an electrically stable characteristic. The possibility of application as a large electronic device can be maximized.

Claims (14)

An organic thin film transistor (OTFT) composed of a substrate, a gate electrode, an insulating layer, a source and a drain electrode, and an organic active layer, wherein an ion film is formed on the surface of the gate electrode, so that the work function of the gate electrode is changed. Thin film transistor. The organic thin film transistor of claim 1, wherein the cationic layer is formed by treating an acidic solution on a surface of a gate electrode. The organic thin film transistor of claim 2, wherein the acidic aqueous solution is one or more selected from the group consisting of phosphoric acid, sulfuric acid, and acetone. The organic thin film transistor of claim 1, wherein the anion film is formed by treating a basic solution on a surface of a gate electrode. The organic thin film of claim 4, wherein the basic aqueous solution is at least one selected from the group consisting of tetrabutyl ammonium hydroxide, sodium hydroxide, and potassium hydroxide. transistor. The organic thin film transistor according to claim 2 or 4, wherein the ion film is formed to have a thickness of several ohms strong to several nanometers (nm). The organic thin film transistor according to claim 1, wherein the ion film is formed by treating a surface of a gate electrode with a plasma generated by a reactive gas in a vacuum. The organic thin film transistor of claim 7, wherein the reactive gas is hydrogen, oxygen, a fluorine-based gas, or an inert gas. 8. The organic thin film transistor according to claim 7, wherein the plasma treated ion film is formed to a thickness of several ohms strong. The method of claim 1, wherein the gate electrode is indium-tin oxide (ITO), chromium (Cr), copper (Cu), molybdenum (Mo), aluminum (Al), aluminum alloy (Al alloy), At least one selected from the group consisting of tungsten (W), doped silicon, gold, silver, nickel, palladium, platinum, tantalum and titanium. The organic thin film transistor of claim 10, wherein the gate electrode is ITO. In the method for manufacturing an organic thin film transistor composed of a substrate, a gate electrode, an insulating layer, a source and a drain electrode, and an organic active layer, the gate electrode may be formed of an acidic solution, a basic solution or a plasma to produce a gate electrode having a changed work function. Surface treatment to form an ion film. The method according to claim 12, wherein the gate electrode is surface treated by dipping to form an ion film by bathing the acidic solution or the basic solution. The method according to claim 12 or 13, further comprising a washing and / or drying step after treatment with the acidic or basic solution to form an ion membrane.

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