[CO.sub.2]-laser treatment of indium tin oxide nanoparticle coatings on flexible polyethyleneterephthalate substrates.
Keywords Indium tin oxide (ITO), Nanoparticles, Laser treatment, Oscillatory bending tests
Transparent electrodes are essential for display applications such as touch screens, electroluminescence lamps, and organic light-emitting diodes (OLED). As coating, indium tin oxide (ITO) is almost exclusively used because of its superior combination of low electrical resistance (1-3 x [10.sup.-4] [OMEGA] cm) and high transparency (> 90% at a film thickness of 100 nm) in the visible range. (1)
For flexible display applications and reel-to-reel processing which are more and more in the focus, polymer films have to be used as substrate material. States of the art for the deposition of ITO on polymer substrates are PVD techniques, such as sputtering (2) or vapor deposition. (3) However, these techniques are very costly due to the vacuum process, the additional structuring, and the related wastage of the expensive ITO. Wet deposition techniques, such as the sol-gel method (4-6) or the coating with nanoparticle dispersions, (6-8) offer a simpler process and, therefore, a cost-reduction potential by avoiding vacuum conditions and making the direct patterning by printing is possible.
However, ITO nanoparticle coatings have to be annealed at high temperatures to enhance their conductivity. For polymer substrates, the annealing temperatures are limited by their low thermal stability. New treatments have to be applied to improve the electrical conductivity of ITO nanoparticle coatings without thermally damaging the substrate.
Investigations of coatings of ITO nanoparticles on flexible polymer substrates are rarely described in the literature and focus predominantly on the UV irradiation of ITO nanoparticle sols modified with UV-curing binders.
Al-Dahoudi et al. (9) investigated coatings of ITO nanoparticle sols. 3-Glycidoxypropyltrimethoxysilane (GPTS) and 3-methacryloxypropyltrimethoxysilane (MPTS) were added as UV-curing binders. Polymethylmethacrylate (PMMA) and polycarbonate (PC) substrates were coated by spin, dip, and spray coating processes. A Specific resistance of 0.25 [OMEGA] cm corresponding to a stable sheet resistance of 5000 [[OMEGA]/[??]] for a layer thickness of 570 nm was achieved using UV irradiation followed by a low-temperature heat treatment under air and in reducing atmosphere. Puetz et al. (10) described a low-temperature curing technique for coatings of ITO nanoparticles on PMMA, PC, and PET substrates using UV irradiation as well. MPTS together with a UV photostarter was added to the dispersion. A specific resistance of 0.1 [OMEGA] cm was achieved after heat treatment in a reducing atmosphere. In a further study, Puetz et al. (11) investigated direct gravure printing for the fabrication of transparent-conductive ITO nanoparticle coatings based on the modified ITO nanoparticle dispersion described above.
In this study, [CO.sub.2]-laser treatment was used to improve the conductivity of ITO nanoparticle coatings on PET substrates. The aim was to generate ITO nanoparticle coatings on flexible PET substrates which fulfill the properties required for transparent flexible electrodes in optoelectronic applications. A sheet resistance below 1000 [[OMEGA]/[??]] and a transmission in the visible range of at least 80% are the aims. In addition, with regard to the application as flexible electrodes, the stability of the electrical conductivity of [CO.sub.2]-laser-treated ITO nanoparticle coatings under oscillatory bending was investigated and compared to commercial sputtered ITO coatings using a special device described by Koniger et al. (12)
As a substrate material, the polyester film Hostaphan GN 4600 from Mitsubishi Polyester Films with a thickness of 96 [micro]m was chosen. The PET film offers a high dimensional stability which is required for printable electronic applications. Furthermore, the surface is very smooth supporting the generation of homogeneous coatings. (13)
As a coating solution, an ethanolic dispersion of ITO nanoparticles with a solid content of 35.5 wt% from Evonik Degussa GmbH was used. The arithmetic mean value of the volumetric grain-size distribution is about 100 nm measured by dynamic light scattering.
At first, a plasma treatment of the PET film was conducted to improve the wettability. The plasma chamber plasmaclean 4 from Ilmvac GmbH was used. The plasma is generated by an ac voltage of 50 kHz at a pressure of 1 mbar. The plasma treatment was carried out for 30 s under air conditions.
Afterward, the polymer substrates were coated by means of a doctor blade using an Erichsen Coatmaster 509 MC device. Coil blades with a wet thickness of 50 [micro]m were used to achieve a dry thickness of 3 [micro]m. The PET film was fixed to a heatable glass plate using an adhesive tape. The speed of the coil blade was set to 10 mm/s and the plate was heated to 60[degrees]C to get a quick evaporation of the ethanol. Under these conditions, a homogeneous coating was obtained.
The ITO nanoparticle coatings were annealed at 200[degrees]C for 20 min in air to enhance the adhesion to the PET film.
For the laser treatment, a Synrad Series 48 [CO.sub.2]-laser module with a wavelength of 10.6 [micro]m was used. The laser beam is extended by a 1:2 telescope, guided by a galvo scanner SK1020 of Scanlab and focused by a coated ZnSe F-theta lens. The F-theta lens corrects the spherical aberration by its special design.
The diameter of the laser spot is 0.65 mm. The laser spot can be moved with a scan velocity between 50 mm/s and 4.5 m/s. On the substrate, structures are processed by filling a selected part of the treatment plane with parallel laser lines. The distance of the lines, which is also called hatch distance, is minimal at 0.01 mm. The energy which is provided by the laser can be calculated as energy input per unit area by
[E.sub.a] = [[P.sub.L]/[[v.sub.S] * [h.sub.S]]]
The laser parameters are laser power, [P.sub.L], scan velocity, [v.sub.S] and hatch distance, [h.sub.S].
The morphology of the ITO nanoparticle films was characterized using the scanning electron microscope FE-SEM S4800 from Hitachi.
The layer thickness of the coatings was determined by means of white light confocal (WL-CF) microscopy. The WL-CF microscope "[mu]surf" from Nanofocus was used.
The sheet resistance of the coatings was measured using the four-point method setup of a Keithley SMU 236. The specific resistance was calculated by multiplying the sheet resistance with the layer thickness determined.
The infrared transmission was measured using the FTIR-Spectrometer Magna IR 750 from Nicolet.
For the characterization of the transmission in the visible range, the UV/VIS spectrometer Lambda 19 from Perkin Elmer was used.
Electrical conductivity under oscillatory bending
The stability of the electrical conductivity of the ITO nanoparticle coatings under bending was investigated using a device described in detail by Koniger et al. (12) The device enables measuring of the electrical resistance of conductive coatings under oscillatory bending. In this study, the application of tensile stresses within the coating was used for the investigation of the electrical conductivity of the ITO/PVP coatings under oscillatory bending (Fig. 1).
[FIGURE 1 OMITTED]
The polymer film is supported by four reels and flexibly fixed at its position by two springs on each side. The middle of the film sample is bent by a mandrel which can move up and down. The mandrel is mounted on a bar which is driven by an extender. The sample is bent around the mandrel so that the bending radius is defined by its geometry. The bending amplitude which relates to the strength of the bending can be set by the displacement of the extender. The number of revolutions of the extender controls the frequency of the oscillatory bending load. The electrical contact between the conductive coating and the electrical measuring device is achieved by a conductive adhesive copper tape to which the wires from the electrical measuring device are soldered.
For all the measurements in this study, a frequency of 0.1 Hz was chosen and the bending amplitude was set to 20 mm to ensure a complete bending around the mandrel.
Results and discussion
Transmission of the ITO nanoparticles and the PET film at 10.6 [micro]m
First of all, the transmission of the ITO nanoparticles and the PET film at the wave length of the laser (10.6 [micro]m) was investigated (Fig. 2).
[FIGURE 2 OMITTED]
The ITO nanoparticles (ITO nanoparticles in potassium bromide, "potassium bromide pellet") exhibit a transmission of 1% at the wave length of 10.6 [micro]m of the laser used (Fig. 2a). Much of the laser energy is absorbed by the ITO nanoparticles thereby reducing the risk of a thermal damage of the PET film. The PET film shows a transmission of 25% at 10.6 [micro]m (Fig. 2b) which is close to the maximum and contributes to a reduction of an energy input into the PET film. Especially, the high transmission of the PET film near the wave length of 10.6 [micro]n shows that the [CO.sub.2]-laser is an appropriate tool for the thermal treatment of ITO on PET films.
[CO.sub.2]-laser treatment of ITO nanoparticles on PET films
First investigations showed a peeling of the ITO nanoparticle coating from the PET film after [CO.sub.2]-laser treatment for all the laser parameters applied. The reason for this effect is that because of the temporally short heating of the substrate by the laser irradiation, a softening of the PET film is hardly possible, which is a precondition for a good adhesion of the ITO nanoparticle coating to the PET substrate. Therefore, an annealing at 200[degrees]C for 20 min was conducted before the [CO.sub.2]-laser treatment to improve the adhesion of the ITO nanoparticle coating to the flexible PET film.
The annealed ITO nanoparticle coatings were laser treated using different energy inputs with the objective to achieve the lowest possible sheet resistance without thermally damaging the PET film. Investigations showed that the best results were achieved with the maximum scan velocity of 4.5 m/s and a hatch distance of 0.05 mm. A fast scan velocity avoids a thermal damage of the PET film resulting from a temporally long, focused energy input. A small hatch distance contributes to a homogeneous energy input.
In Fig. 3, the sheet resistance of an ITO nanoparticle coating on a PET film is plotted as a function of the energy input per unit area.
[FIGURE 3 OMITTED]
Energy inputs above 30 [kJ/[m.sup.2]] lead to sheet resistances below 1000 [[OMEGA]/[??]] without thermally damaging the PET film. With an energy input increasing up to 40 [kJ/[m.sup.2]], a decrease of the sheet resistance down to 400 [[OMEGA]/[??]] was observed which is about thirty times lower than the sheet resistance of the only thermally annealed ITO nanoparticle coatings. This corresponds to a specific resistance of 0.12 [OMEGA] cm for a film thickness of 3 [micro]m.
The reason for the improvement of the electrical conductivity of the ITO nanoparticle coating by [CO.sub.2]-laser treatment becomes obvious from the morphology of [CO.sub.2]-laser-treated ITO nanoparticle coatings. In Fig. 4, [CO.sub.2]-laser-treated ITO nanoparticle coatings are compared with ITO nanoparticle coatings annealed at 200[degrees]C.
[FIGURE 4 OMITTED]
In contrast to ITO nanoparticle coatings annealed only at 200[degrees]C, a slight sinter neck formation was observed for [CO.sub.2]-laser-treated (30-50 [kJ/[m.sup.2]]) ITO nanoparticle coatings. Moreover, a slight increase of particle size and a rounding of the particles were found. The sinter neck formation, which can be explained by a surface and grain boundary diffusion due to the energy input, lowers the grain boundary scattering of electrons, and thus improves the charge carrier mobility and the electrical conductivity, respectively.
For energy inputs around 45 [kJ/[m.sup.2]], a slight increase of the sheet resistance was measured, which can be explained by a crack formation. This becomes more pronounced for higher energy inputs (Fig. 5).
[FIGURE 5 OMITTED]
Inner tensions resulting from rising temperatures due to the high energy input and the mismatch of the coefficients of thermal expansion of the PET film and the ITO nanoparticle coating presumably cause the crack formation observed. For energies > 60 [kJ/[m.sup.2]], a thermal degradation of the PET film was observed (Fig. 6). The resulting surface damage explains the significant increase of the sheet resistance.
[FIGURE 6 OMITTED]
For layer thicknesses below 3 [micro]m, energy inputs of smaller than 40 [kJ/[m.sup.2]] cause a thermal damage of the PET film already, as less laser irradiation is absorbed by the ITO coating. Therefore, a layer thickness of at least 3 [micro]m has to be chosen to avoid a thermal damage of the PET film.
However, the investigations show that for energy inputs in the range of 30-40 [kJ/[m.sup.2]] a [CO.sub.2]-laser treatment of ITO nanoparticle coatings on PET films leads to a significant improvement of the electrical conductivity without thermally damaging the PET film. For a layer thickness of 3 [micro]m, a sheet resistance of 400 [[OMEGA]/[??]] was achieved, which corresponds to a specific resistance of 0.12 [OMEGA] cm.
Application as a transparent electrode needs to be investigated, however, to determine how the [CO.sub.2]-laser treatment influences the transmission in the visible range.
The influence of a [CO.sub.2]-laser treatment on the transmission of annealed ITO nanoparticle coatings in the visible range is displayed in Fig. 7.
[FIGURE 7 OMITTED]
The uncoated PET film shows a transmission in the visible range of about 85%. An annealed 3-[micro]m thick ITO nanoparticle layer lowers the transmission to values between 80% and 83%. The [CO.sub.2]-laser treatment leads to a further decrease of the transmission which is the more pronounced at the higher energy input. For an energy input of 35 [kJ/[m.sup.2]], a maximum transmission of 81 %, and for an energy input of 46 [kJ/[m.sup.2]], a maximum transmission of 75% were observed.
The decrease of the transmission with increasing energy input can be explained by way of different effects. The crack formation described above for energy inputs of more than 40 [kJ/[m.sup.2]] leads to a scattering of incident light. In addition, a scattering caused by the slightly larger particle size and the densification of the film may also contribute to the small decrease. Furthermore, for energy inputs of more than 45 [kJ/[m.sup.2]], a slight thermal degradation of the PET film due to the laser irradiation causes a yellow tint.
However, for energy inputs in the range of 30-35 [kJ/[m.sup.2]], ITO nanoparticle coatings on PET films can be generated, which exhibit a sheet resistance below 1000 [[OMEGA]/[??]] and a transmission in the visible range of at least 80%, and thus fulfil the properties required for the sheet resistance and transparency if used as transparent electrodes.
Electrical conductivity under oscillatory bending
With respect to the application as flexible transparent electrodes, the stability of the electrical conductivity of [CO.sub.2]-laser-treated ITO nanoparticle coatings under oscillatory bending was investigated. The aim was to determine the bending radius for which the sheet resistance does not exceed the required value of 1000 [[OMEGA]/[??]]. In Fig. 8, the sheet resistance of a 3 [micro]m [CO.sub.2]-laser-treated ITO nanoparticle coating is plotted as a function of bending cycles for different bending radii. The tension mode described in "Electrical conductivity under oscillatory bending" under Experimental section was applied. The curves marked by symbols represent the sheet resistance under bending. The dotted lines give the initial sheet resistances.
[FIGURE 8 OMITTED]
A stronger increase of the sheet resistance under bending was observed with decreasing bending radius. For a bending radius of 3 mm. the sheet resistance increases from 700 to 4000 [[OMEGA]/[??]] after 300 bending cycles. For a bending radius of 10 mm, the sheet resistance is still below 1000 [[OMEGA]/[??]] after 300 bending cycles.
Smaller bending radii cause higher tensile stresses in the coating and therefore a more pronounced crack formation (12) (Figs. 9a and 9b), which leads to a greater increase of the resistance.
[FIGURE 9 OMITTED]
In addition, [CO.sub.2]-laser-treated ITO nanoparticle coatings were compared with commercial sputtered ITO coatings concerning the stability of the electrical conductivity under oscillatory bending. In Fig. 10, the sheet resistance is displayed as a function of bending cycles for a commercial sputtered ITO coating (125-[micro]m PET film; ~60 [[OMEGA]/[??]]; Cadillac Plastics) and the [CO.sub.2]-laser-treated ITO nanoparticle coating of 3-[micro]m thickness. For the investigation a bending radius of 10 mm was used.
[FIGURE 10 OMITTED]
Compared to the commercial sputtered ITO coatings, the [CO.sub.2]-laser-treated ITO nanoparticle coating shows a much higher stability of the electrical conductivity under oscillatory bending. For the sputtered ITO coating, a significant increase of the sheet resistance was observed, immediately after a few bending cycles. The sheet resistance under tension increases from 60 [[OMEGA]/[??]] to 15 [[OMEGA]/[??]] after 50 cycles. In contrast, the sheet resistance of the annealed laser-treated ITO nanoparticle coating just increases from 700 to 900 [[OMEGA]/[??]] after 50 cycles.
The different mechanical stabilities of the electrical conductivity under oscillatory bending can be explained as due to the morphologies of the coatings. In contrast to the porous [CO.sub.2]-laser-treated ITO nanoparticle coating (Fig. 11a), the sputtered ITO coating is very brittle due to its dense structure (Fig. 11b) which favors a crack formation.
[FIGURE 11 OMITTED]
A detailed comparison of the crack formation in sputtered ITO coatings and ITO nanoparticle coatings is given by Koniger et al. (13)
The investigations concerning the stability of the electrical conductivity under bending show that the [CO.sub.2]-laser-treated ITO nanoparticle coatings exhibit a higher stability of the electrical conductivity under oscillatory bending compared to that of commercial sputtered ITO coatings, which is beneficial from the point of view of their use as flexible electrodes in optoelectronic applications.
In this study, the [CO.sub.2]-laser treatment is presented as a new method to improve the electrical conductivity of ITO nanoparticle coatings on flexible PET substrates.
A post-treatment of the previously thermally annealed ITO nanoparticle coatings on PET films using a [CO.sub.2]-laser leads to a significant improvement of the electrical conductivity without damaging the PET substrate. A sheet resistance of 400 [[OMEGA]/[??]] at a layer thickness of 3 [micro]m (specific resistance of 0.12 [OMEGA] cm) was achieved, which is smaller than a factor of 30 as compared with the only annealed ITO nanoparticle coatings. The improvement of the electrical conductivity can be explained by a slight sinter neck formation which lowers the grain boundary scattering and thus raises the electrical conductivity.
For a layer thickness of 3 [micro]m and energy inputs up to 35 [kJ/[m.sup.2]], the transparency of the [CO.sub.2]-laser-treated ITO nanoparticle coatings on PET films is about 80%. Thus, ITO-nanoparticle coatings on PET films can be generated with sheet resistances below 1000 [[OMEGA]/[??]] and a transmission in the visible range of ~80%.
In addition, the stability of the electrical conductivity of [CO.sub.2]-laser-treated ITO nanoparticle coatings under oscillatory bending was investigated and compared to that of commercial sputtered ITO coatings. The ITO nanoparticle coatings show an increase of the sheet resistance under bending with growing number of cycles due to a crack formation. A bending radius of 10 mm can be applied without exceeding the sheet resistance of 1000 [[OMEGA]/[??]] required after 300 bending cycles. Compared to commercial sputtered ITO coatings, [CO.sub.2]-laser-treated ITO nanoparticle coatings show a much higher stability of the electrical conductivity under oscillatory bending, which can be explained by the lower brittleness due to its more porous structure.
All in all, the [CO.sub.2]-laser treatment of ITO nanoparticles provides a great potential to improve the electrical conductivity of ITO nanoparticle coatings on flexible PET substrates. This method brings flexible transparent electrodes on the basis of printable ITO nanoparticle coatings close to a technical realization. It has been demonstrated that flexible electrically conducting films can be made, which fulfil the requirements with respect to the electrical conductivity and transparency for simple optoelectronic applications such as electroluminescence lamps and touch screens.
Acknowledgments The authors thank the German Research Foundation for their financial support, Evonik Degussa GmbH for providing the ITO nanoparticle dispersion, and Mitsubishi Polyester Films company for providing the PET films.
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[C] FSCT and OCCA 2009
T. Koniger ([??]), I. Al-Naimi, H. Munstedt
Institute of Polymer Materials, Martensstr. 7, D-91058 Erlangen, Germany e-mail: firstname.lastname@example.org
T. Rechtenwald, T. Frick, M. Schmidt
Bayerisches Laserzentrum GmbH, Konrad-Zuse-Str. 2-6, D-91052 Erlangen, Germany
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|Author:||Koniger, Tobias; Rechtenwald, Thomas; Al-Naimi, Ihab; Frick, Thomas; Schmidt, Michael; Munstedt, Hel|
|Date:||Mar 1, 2010|
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