Patent Application
20100193351
This application claims the benefit of Korean Application No. 10-2009-0009167, filed Feb. 5, 2009, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.
1. Field
The present invention relates to a method for preparing a transparent conducting film coated with an AZO/Ag/AZO multilayer thin film with low resistivity and high light transmittance, and a transparent conducting film produced by the same method.
2. Description of the Related Art
Recently, along with the development of the optics and electronics fields, the industrial demand for a transparent conducting film with high light transmittance and electrical conductivity is increasing. Such a transparent conducting film is necessarily used for a flat panel display device, a solar cell, a transparent touch panel and the like.
The transparent conducting film should satisfy the following several conditions:
First, low resistivity (10−5 Ω-cm or less),
Second, high light transmittance (85% or more in a visible light wavelength of 550 nm),
Third, stable damp heat properties in the IEC 1646 standard (treatment for 10,000 hours under the condition of a temperature of 85° C. and a humidity of 85%: Wennerberg, et al. Solar Energy Materials and Solar Cells, 75, 47 (2003)), and
Fourth, stable flexibility in a bending test of the transparent conducting film.
As a transparent conducting film which satisfies the above conditions at present, SnO2:F, In2O3:Sn(ITO), Al-doped ZnO(AZO) thin films and the like are being spotlighted. Particularly, conventionally, ITO is widely used since it has a low resistivity (10−4 Ω-cm or less) and a high light transmittance of 85% in a visible light region. But, ITO has a limitation in its industrial use because of the price rise of ITO due to shortage of indium (In) as a raw material. Thus, the research is in progress on a new transparent conducting film which is low-priced and excellent in resistivity and light transmittance. In the meantime, since flexible organic light emitting diodes (OLEDs) attracting high interest currently should have a sheet resistance of 101 Ω/square or less, and a plasma display panel (PDP) optical filter should have a sheet resistance of 100 Ω/square or less, materials having properties suitable for the OLEDs and PDP optical filter are required.
According to this requirement, Liu et al. (Thin Solid Films, 441, 200 (2003)) published that a ZnS/Ag/ZnS multilayer thin film is formed on a quartz substrate using a deposition method by thermal evaporation. In the meantime, Sahu et al. (Solar Energy Materials and Solar Cells, 91, 851 (2007)) suggested that a AZO/Ag/AZO multilayer thin film is formed on a glass substrate using electron beam evaporation.
However, these multilayer thin films entail a problem in that that the light transmittance thereof is apt to decrease drastically as the visible light wavelength band increases, as well as Figure of Merit (Ω−1) indicating excellence of the transparent conducting film is less than 3.0×10−2 Ω−1, which is insufficient to be put into practical use of the transparent conducting film. In order to solve this problem, Sahu et al. suggested that these multiplayer thin films are thermally treated according to the temperature. However, such a thermal treatment method has a limitation that it can be applied to a glass or quartz substrate, but cannot be applied to a flexible substrate having no heat resistance like plastic. Furthermore, the thermal treatment method encounters a drawback in that it cannot also be applied to the flexible substrate having no heat resistance since a preparing process of such multilayer thin films is complicated and requires a high-temperature environment.
Additional aspects and/or advantages will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the invention.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a method for preparing a transparent conducting film coated with an AZO/Ag/AZO multilayer thin film which satisfies the conditions of the transparent conducting film including low resistivity, high light transmittance, etc., without any thermal treatment, and a transparent conducting film produced by the method.
Another object of the present invention is to provide a method for preparing a transparent conducting film which is made of a flexible polymer material and is coated with an AZO/Ag/AZO multilayer thin film, and a transparent conducting film of a flexible polymer material produced by the same method.
In order to accomplish the above objects, the present invention provides a method for preparing a transparent conducting film coated with an AZO/Ag/AZO multilayer thin film, and a transparent conducting film produced by the same method.
(1) Preparation Method of Transparent Conducting Film
The present invention is directed to a method for preparing a transparent conducting film coated with an AZO/Ag/AZO multilayer thin film, the method comprising the steps of: (a) forming a primary AZO thin film on a substrate using an AZO target doped with Al through a sputtering method; (b) depositing Ag on the primary AZO thin film using the sputtering method to form a deposited Ag layer; and (c) forming a secondary AZO thin film on the Ag thin film using the AZO target doped with Al through the sputtering method.
In this case, the thickness of the deposited Ag layer ranges from 5 to 15 nm, more preferably ranges from 7 to 11 nm. When the thickness of the Ag layer is less than 5 nm, the Ag layer is apt to be not evenly deposited on the substrate. On the contrary, when the thickness of the Ag layer is more than 15 nm, the Ag layer exhibits a drastic decrease in light transmittance.
In the present invention, the thicknesses of the primary AZO thin film and the secondary AZO thin film are sufficient as long as it is suited for a typical light transmittance, but preferably range from 10 to 100 nm, respectively. When the thickness of the AZO thin film is very thin, its electrical conductivity decreases whereas when the thickness of the AZO thin film is very thick, its light transmittance decreases, which causes a problem.
In the present invention, the substrate may be a non-flexible substrate such as a glass substrate, a quartz substrate and the like, may be a flexible polymer substrate made of polyethersulfone, polyethylene terephthalate, Polycarbonate, polyimide or polyethylene naphthalate.
(2) Transparent Conducting Film
The present invention is directed to a transparent conducting film coated with an AZO/Ag/AZO multilayer thin film prepared by the preparing method of the transparent conducting film.
The transparent conducting film according to the present invention exhibits a low resistivity of 10−5 Ω-cm or less and a high light transmittance of 85% or more at a wavelength band of a visible light region ranging from 300 to 800 nm. Particularly, in case of the figure of merit used as an index of indicting excellence of the performance, when the Ag layer has a deposition thickness of 9 nm, it exhibits the highest figure of merit of 4.0×10−2Ω−1. This figure of merit value is superior to the figure of merit values of 2.0×10−2Ω−1 and 2.87×10−2Ω−1 obtained by Lie et al. and Sahu et al. In addition, the sheet resistance and the light transmittance of the multilayer thin film are maintained stable without any change even after the damp heat treatment performed on the multilayer thin film for 1,000 hours under the condition where temperature is 85° C, and humidity is 85% as the IEC 1646 standard. Further, the result of the bending test of the transparent conducting film shows that there is no change in the adhesive force between the PES substrate and the multilayer thin film and the sheet resistances are the same within an error range.
These and/or other aspects and advantages will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:
Embodiments of the invention will be hereinafter described in detail with reference to the accompanying drawings. However, these embodiments of the present invention are merely illustrative of easy explanation on contents of the technical spirit and scope of the present invention, but the technical scope of the present invention is not limited or modified thereby. Also, it will be understood by those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the appended claims.
In the following embodiments, the thicknesses of primary and secondary AZO thin films were set to 45nm known as the condition most susceptible for the damp heat treatment so as to be suited for the worst condition. It will be of course understood by a person skilled in the art that since even an AZO/Ag/AZO multilayer thin film prepared under such a susceptible condition exhibits a good effect, the primary and secondary AZO thin film having a variety of different thicknesses prepared under a better condition than the susceptible condition also exhibits a better effect. Accordingly, in the present invention, the thicknesses of the primary and secondary AZO thin film are not limited to the thicknesses as described in the embodiment below.
(1) Formation of Primary AZO Thin Film
First, a Si substrate or a flexible polyethersulfone (PES) substrate (thickness: 200 μm) having excellent thermal properties was washed, and then foreign substances on the substrate surface were removed using N2 gas. Then, the substrate was deposited at room temperature using an RF sputtering method so that the thickness of the AZO thin film is about 45 nm. During the deposition, an AZO target (with a diameter of 2 inches) doped with 2 wt % Al was sintered at 1400° C. using a ceramic process. An RF power applied to the AZO target was 30 W, a working vacuum pressure was maintained at 1.5 mTorr, a distance between the target and the substrate was about 10 cm, and Ar flow rate of 40 sccm (standard cc/min) was used as a sputtering gas.
(2) Deposition of Ag
An Ag thin film was deposited to a thickness of 3 nm to 20 nm in-situ on the primary AZO thin film formed by the above method using the Ag target at different deposition times under the condition where DC power is 30 W, deposition pressure is 3 mTorr and Ar flow rate is 10 sccm
The thickness of the Ag thin film of the AZO/Ag thin film deposited on the Si substrate was identified using a transmission electron microscope (TEM). For the purpose of controlling of the thickness of the Ag thin film, the thin film was deposited thickly in a pre-test and its thickness was measured through a cross-section of the deposited film by the SEM. Thereafter, the deposition time was determined such that the relationship between time and film thickness are shown and the thickness of a thin film is formed on an extension line thereof. It is impossible to observe the AZO/Ag thin film deposited on the PES substrate using the SEM or TEM, but it can be presumed that since the Ag layer is deposited on the AZO thin film deposited on the substrate, it is not nearly influenced by a material of which the substrate is made and has the same thickness as that of the Ag layer deposited on the Si substrate.
In the embodiments below, an experiment in which a surface or cross-section was observed using the SEM or TEM was carried out using a multilayer thin film deposited on the Si substrate, and other experiments except this were conducted on a multilayer thin film deposited on the PES substrate unless specifically stated otherwise.
(3) Formation of Secondary AZO Thin Film
Subsequently, a secondary AZO thin film was deposited in-situ on the Ag thin film using the same method under the same condition as that of (1) formation of the primary AZO thin film to thereby form an AZO(45 nm)/Ag/AZO(45 nm)/PES multilayer thin film.
(1) Crystallinity
The crystal structure of the AZO and Ag layers was measured from the X-ray diffraction ((XRD, REGAKU D/MAX-RC) using a Cuka radiation and a nickel filer on the AZO/Ag/AZO multilayer thin film where the thickness of the Ag thin film prepared in the Embodiment 1 is 3, 5, 9, 15 and 20 nm, respectively, and the X-ray diffraction pattern was shown in
It can be seen from
(2) Structure
The microstructure of the Ag thin film which is deposited on the AZO thin film to a thickness of 5 nm, 9 nm and 20 nm, respectively, was observed by the SEM. Thereafter, photographs of the deposited Ag thin films were shown at the left side of
It can be found from the photographs of
(1) Light Transmittance
The light transmittance of the multilayer thin film prepared in Embodiment 1 was measured in the visible light region whose wavelength band ranges from 300 to 800 nm using a spectrophotometer (Shimadzu UV2450, Japan), and a spectrum of the light transmittance and a transmittance at a wavelength of 550 nm according to the thickness of Ag layers were shown in
As shown in
(2) Resistivity
Carrier concentration and mobility of the multilayer thin film prepared in Embodiment 1 was measured using the Van de Pauw method, and resistivity of the multilayer thin film was obtained by the following Equation. Thereafter, the measurement results thereof were shown in
Resistivity ρ=(neμ)−1
where n: carrier concentration, μ: carrier mobility, e: electron charge
It could be found from the graph of
(3) Figure of Merit
The transparent conducting film exhibits excellent properties as resistivity becomes lower and light transmittance becomes higher. However, since resistivity and light transmittance is not in a proportional relationship, figure of merit (Ω-1) is used as an index indicting excellence of the performance (Haacke, J. Appl. Phys. 47, 4086 (1976)). The figure of merit was calculated by the following Equation using the light transmittance measured in the above Embodiment 3 and sheet resistance, and the calculation result thereof was shown in
Figure of merit FTC=T10/Rs
where T is light transmittance measured at a wavelength band of 550 nm, and Rs is a sheet resistance of the multilayer thin film, which was measured within a precision of ±0.5 Ω/sq using the four-point probe method (model CMT-SR 1000).
It can be found from the graph of
In order to identify a damp heat resistance of the multilayer thin film of the present invention, damp heat treatment was performed on the multilayer thin film for 1,000 hours and then the property evaluation thereof was made under the condition where temperature is 85° C. and humidity is 85% as the IEC 1646 standard. The multilayer thin film was tested by using the AZO/Ag/AZO multilayer thin film containing a silver layer with a thickness of 9 nm which is excellent in figure of merit as a target. An AZO thin film with a thickness of 100 nm containing no silver layer was used as a control group.
A bending test was performed to identify the adhesive force between the substrate and the multilayer thin film. That is, as shown in a schematic view shown inside the graphs of
The measurement results of the sheet resistance according to the bending distance shown in
As described above, according to the present invention, it is possible to produce a transparent conducting film which exhibits a low resistivity of 10-5 Ω-cm or less and a high light transmittance of 85% or more without any thermal treatment, stability in damp heat treatment, and mechanical stability against bending stress unlike a conventional AZO/Ag/AZO multilayer thin film.
Furthermore, according to the present invention, it is possible to economically prepare a flexible transparent conducting film which exhibits a stable adhesive force on a flexible substrate while retaining the above mentioned characteristics, so that the flexible transparent conducting film can be utilized as a material of a variety of electronic devices such as a flat panel display device, a solar cell, a transparent touch panel and the like.
Although a few embodiments have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2009-0009167 | Feb 2009 | KR | national |
