CRT 패널상의 투명 전기전도성 ITO 코팅층 제조

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

조영상·홍정진↕·장성훈↕·양승만

한국과학기술원 생명화학공학과

화학공학의 이론과 응용 제8권 제1호 2002년

↕LG화학 기술연구원 신소재연구소

Preparation of Electrically Conductive and Transparent ITO Coating Layer on CRT Panel

Young-Sang Cho, Jeong-Jin Hong↕, Sung Hoon Jang↕, and Seung-Man Yang

Department of Chemical and Biomolecular Engineering, KAIST, Taejon 305-701, Korea

↕Advanced Materials Research Institute, LG Chem. Research Park, Taejon 305-380, Korea

Introduction

Transparent conductive oxides(TCOs) deposited on glass substrates are important materials in the field of optoelectric devices such as solar cells, liquid crystal displays, and electroluminescence. Among the various TCOs, tin-doped indium oxide(ITO) is widely used because it has the high figure of merit compared with the other n-type TCOs such as antimony-doped tin oxide(ATO) or fluorine doped tin oxide(FTO)[1].

One of the important application of ITO is CRT coating for shielding electromagnetic waves. Though 108□/□ of sheet resistance is satisfactory for preventing the accumulation of charge, it is necessary to form ITO film having a low resistivity not exceeding 106 □/□, and preferably in the order of 102 to 103 □/□ is desired[2].

There are several methods to deposit ITO on glass substrate such as CVD, sputtering, vacuum deposition, and sol-gel process. Among these various methods, the spin coating of redispersed ITO sol is now widely used for industrial purposes since this method is the most suitable for the mass production of coating layers as commercial process.

In this study, to deposit redispersed ITO sol on CRT panel, solutions containing ITO particles are prepared with the use of steric stabilizer. The electrical and optical properties were measured for  CRT coatings with ITO sol prepared by spin coater.

Experimental

The ITO powder used in this study was selected as commercialized one. The mixture of ethanol(Merck, 99.9%), DMF(N,N-dimethylformamide,  Duksan pharmaceutical Co. Ltd., 99.9%), and IPC(2-isopropoxyethanol, Aldrich, 99.9%) was used as a dispersion medium for ITO sol. The morphology of ITO particles was observed by Energy Filtering Transmission Electron Microscopy (EF-TEM, EM912 Omega). X-ray diffraction measurement was performed on a Rigaku D/MAX-RC for indium-tin oxide powders. The incident wavelength was Cu K□=0.154056 nm and the detector was moved between 2□=15∼75。. The scan speed was 3。/min. Colloidal suspension of ITO was prepared by dispersing ITO powders in the dispersion medium. Small amount of dispersant was used as steric stabilizer. To reduce the secondary particle size of ITO aggregates, solution containing ITO powder was vibratory-milled with zirconia bead. After vibratory milling processes, the green ITO sol was obtained and it was stable without any precipitation. The adsorption isotherm of dispersant was obtained by elemental analysis for separated ITO aggregates from ITO sol by centrifugation. Dynamic light scattering measurements were performed with a Brookhaven instrument at a wavelength of 674 nm at 90。 to measure the secondary particle size of ITO. To deposit coatings on 17-inch CRT panel, the ITO suspension was diluted and AS(antistatic) coating solutions were prepared. After the AS coatings were finished, AR(antireflective) coating solutions containing alkyl silicate were deposited as the secondary coating layer. Both AS and AR coatings were prepared by spin coater. This double-layered coating film was baked at 180 ℃ for 30 minutes. The sheet resistance and visible light reflectance of coating layer was measured by Loresta HP (MCP-T410, Mitsubishi Chemical) and Darsa Pro-5000 System (Professional Scientific Instrument), respectively.

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

Results and discussion

Fig. 1 and 2 shows the X-ray diffraction spectrum and TEM image of ITO powder, respectively. XRD spectrum of ITO was found to deviate slightly from that of pure indium oxide, and this clearly showed that the particles have the cubic bixbyite crystal structure without tetragonal tin oxide domain.

Fig. 1 XRD result of ITO powder     Fig. 2 TEM image of ITO powder

The primary particle size calculated from the Scherrer's equation using (222) peak was 22 nm[3], and this size is consistent with the particle size observed from TEM image.

Fig. 3 shows the adsorption concentration of dispersant with equilibrium concentration in solvent phase. This isotherm is similar to Langmuir type implying the highly energetic adsorption at a low surface concentration[4]. According to Fig. 3, the adsorption concentration begins to saturate when the equilibrium concentration is 7.4 mol/m3. This condition corresponds to 2.5 wt. % of dispersant based on the weight of ITO powder.

Fig. 4 shows the secondary particle size of ITO sol with varying amount of dispersant and milling time. From this figure, we can find the clear trend that the secondary particle size decreases as the amount of dispersant increases. As the dispersant concentration decreases, the number of grafted steric stabilizer on the surface of ITO aggregate decreases. Thus the steric repulsion will also decrease, and the particles will aggregate due to van der Waals force.

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

Fig. 3 The adsorption isotherm of steric    Fig. 4 The secondary particle size stabilizer      on the surface ITO particle       with varying amount of dispersant	and milling time

To study the effect of secondary particle size on the physical properties of coating film, the electrical and optical properties of ITO coating layer were  measured. As the milling time increases, the contacting area between ITO aggregates increases since the secondary particle size decreases. Thus, the electrical conductivity of coating film decreases due to the increase of contact resistance. This behavior can be observed from Fig. 5. Fig. 6 shows the minimum reflectance of coating film deposited with ITO sol. As the milling time increases, the ITO aggregates in the film will be more closely packed. Thus, the visible light can scatter more easily through the interstitial void between ITO aggregates, and the minimum reflectance of film decreases with milling time[5]. The coating film showed excellent optical transparency since the minimum reflectance was less than 1 % in the whole range of milling time. The whitening of film was observed when the secondary particle size of ITO sol was larger than 130 nm due to coarse particles.

Fig. 5 The sheet resistance of coating film  Fig. 6 The minimum reflectance of coating film          with secondary particle size of ITO sol           with secondary particle size of ITO sol

Fig. 7 shows the sheet resistance of ITO coating film with varying amount of dispersant. The secondary particle size of ITO sol was 101±3.7 nm. Since most of dispersant can not be removed by thermal decomposition during the baking of coating panel and act as insulating layer between ITO aggregates.  Thus the additional electrical resistance due to the adsorbed surfactant layer can also be generated. This can be confirmed in Fig. 7, which shows the electrical resistance of film increases drastically with the increasing amount of dispersant.

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

To study the depletion flocculation behavior of ITO suspension, ITO sol was prepared by varying amount of dispersant and the secondary particle size was measured. Fig. 8 shows the increase of secondary particle size after several storage days. When the amount of dispersant was less than 4.2 wt.%, the increase of secondary particle size can be neglected. On the contrary, severe particle aggregation was observed when more than 4.2 wt.% of dispersant was used. This is because the unadsorbed dispersant dissolved in dispersion medium can cause the attractive osmotic pressure resulting in particle aggregation[6].

Fig. 7 The sheet resistance of coating film    Fig. 8 The change of ITO particle size with             with concentration of dispersant                  concentration of dispersant and storage day

Summary and conclusion

화학공학의 이론과 응용 제8권 제1호 2002년

The solutions containing ITO powder were prepared by using steric stabilizer and vibratory milling machine. The secondary particle size of ITO decreased with milling time and dispersant concentration.

ITO sol was spin-coated on the CRT panel. As the particle size of ITO dispersion becomes smaller, the sheet resistance increases and the reflectance decreases. When the particle size is larger than 130 nm, coating layer on CRT becomes opaque because of the light scattering by coarse particles.

As the concentration of dispersant increases, the sheet resistance increases since the adsorbed dispersant acts as insulating layer. The depletion flocculation behavior of ITO sol was observed when the amount of dispersant was larger than 4.2 wt. %.

References

1. R. G. Gordon, MRS bulletin, August 2000, 52-57

2. G. Frank, H. Kstlin, Appl. Phys. A, 1982, 27, 197

3. B. D. Cullity, Elements of X-ray Diffraction, 3rd ed., 2001, Prentice Hall

4. L. Wang et al., J Am. Chem. Soc., 2000, 83(4) 697-702

5. M. Kawata et al., US Patent 5,785,897

6. W. B. Russel et al., Colloidal Dispersion, 1999, Cambridge Univ. Press.