AIMCAL Fall Technical Conference & 19th International Conference on Vacuum Web Coating

Transcription

1 AIMCAL Fall Technical Conference & 19th International Conference on Vacuum Web Coating A.G. Talma, R. Schlatmann, J.W.G. Mahy Akzo Nobel Chemicals bv, P.O. Box 9300, 6800 SB Arnhem, The Netherlands. Phone , fax , Novel roll-to-roll deposition of transparent electrode on plastic. Transparent electrodes are essential components in numerous consumer electronics products, such as displays, signage panels and lighting applications. Increasing demands on electrical and mechanical performance, cost, homogeneity and ruggedness have triggered the development and appearance on the market of new materials and production processes. This paper describes a novel roll-to-roll deposition process for a high performance transparent electrode material on plastic foil. Transparent conductive oxide (TCO) materials, such as ITO, SnOx:F, and Sb:SnO x (ATO), Cd:SnO x, Al:ZnO x and F:ZnO x are members of a class of doped semiconductors which exhibit interesting electrical and optical properties: they are optically transparent, electrically conductive and infrared reflecting. Their application areas are as thin vacuum sputtered layers on glass panels, as components in e.g. touch screens, LCD-based displays and smart low-e windows. Increasing demand for thinner, lighter and in particular more rugged and lower cost components has lead to TCO layers deposited on plastic substrates. These flexible substrates allow to produce TCO material in roll-to-roll processes, which in turn offers a low-cost perspective and increased homogeneity of the TCO over considerable web width and length. The key features of TCO films deposited on plastic are: Optical Properties: o Transmission (%T) in nm optical wavelength. o Absorption and Haze o Color. Electrical Properties: o Sheet resistivity (R s, expressed in units of Ohm/sq). o Linearity, usually express in %. Mechanical Properties: o Durability (scratch resistance and/or pen cycle lifetimes). o Flexibility (bending radius) o Thermal & Relative Humidity stability. o Substrate warping, usually expressed in mm. Cosmetic Defects, depending on the application. Direct and reactive sputtered tin-doped Indium oxide (ITO) on thermally stabilized and optically clear polyester (PET) foil between 75 and 175 µm thick is currently on the market, mainly for touch display and electroluminiscent lighting and signage applications in web widths ranging between 30 cm and roughly 150cm. The product faces a number of challenges however: as a thin, sputtered oxide layer it shows catastrophic failure of the electrical conductivity under tensile strain, e.g. by cracking and delaminating during bending [1,2]. Moreover, there is a need for lower sheet resistances (below 20 Ohm/square) required for current-driven and large surface area applications such as OLED-based displays [2]. The requirements of high conductivity (R s below 20 Ohm/sq) and high transparency (optical transmission of 85-90% on PET) are contradictory, unless the ITO is deposited at high temperature, which is incompatible with direct deposition on plastic substrates. In a novel approach, we developed a high temperature CVD method to grow TCO on a temporary (metal) substrate in a roll-to-roll process. For this purpose F-doped tinoxide (F:SnO x ) is used. The properties, such as layer thickness, morphology (crystallite size and shape) and electrical properties (e.g. dopant level) can be varied in a wide process window. The resulting layered structure is laminated roll-to-roll on a plastic foil at low (<150 C) temperature. During this lamination an adhesive (e.g. thermoset) can be applied between TCO and plastic, if required. This adhesive can be refractive index matched if so desired. Subsequently, the temporary substrate is removed in a third roll-to-roll

2 unit operation. Finally, the resulting flexible transparent layered electrode can be patterned and/or cut. Some typical key parameter values are shown in table 1. Table 1 Rs 5 25 Ohm/sq Absorption 12 - < 5% Thickness nm Thickness homogeneity +/- 2% As a result of the lamination process, excellent adherence of the TCO to the plastic foil can be obtained. This is shown in fig.1, in which a bending experiment similar to the ones reported in the literature [2] has been performed on TCO and compared with commercial ITO on PET. It can be seen that the TCO material shows rapidly increasing resistivity at bending diameters below 25 mm, which upon unbending decrease to their original value within 5%. On the other hand, the ITO material shows a substantial hysteresis which in the case of 100 Ohm/sq material remains above the onset value even after prolonged relaxation times TCO/PET ITO/PET (100 Ohm) ITO/PET (20 Ohm) 1000 R (ohm /square) Bending diameter (mm) Figure 1: bending test showing the logarithm of the sheet resistance as a function of bending diameter for our TCO material (labeled TCO/PET, 16 Ohm/sq), as compared to commercial ITO of nominal 100 and 20 Ohm/sq. The transmission of the TCO layer, compared with the as-obtained 175 µm PET foil is shown in fig. 2. From this transmission, it follows that a 9% lower transmission for this 20 Ohm/sq TCO can be obtained, relative to 90% transmission for the starting PET foil.

3 transmission TCO transmission % wavelength nm PET TCO Figure 2: optical transmission of TCO on PET versus as-obtained PET The microstructure of the TCO layer discussed in this paper consists of an isotropic array of columnar nanocrystals, grown on the temporary metal foil in a roll-to-roll CVD process. Due to the lamination process, the rough side of the TCO layer is mechanically anchored in the plastic foil (or in the applied adhesive), whereas the TCO-metal interface essentially determines the morphology of the external side of the TCO on plastic. As shown in fig. 3, the AFM scanning of the surface shows the milling texture of the standard grade (i.e. non-polished) metal closely, with a resulting RMS roughness of around 20 nm. RMS roughness: 20.9 nm Figure 3: AFM surface height plot of the TCO surface. From the performance characteristics described above, a number of conclusions can be drawn. A novel way of producing a high-performance TCO on plastic in a roll-to-roll platform has been demonstrated. This TCO derives its excellent conductivity at high optical transmission from the intrinsic characteristics of a high temperature crystal layer.

4 The compatibility of such a TCO with standard grade plastic foil deposition, is obtained by transferring the high-temperature grown TCO onto plastic foil by a low-temperature lamination process, using a temporary substrate approach. The wide process window and versatility of the roll-to-roll unit operations offer flexibility for various end-use applications, e.g. electrolumiscent lamps, signage and display. Its performance makes this TCO material excellently suited for applications in which high current/low resistivity is required. The high adherence of the TCO to the plastic foil and its microcrystalline nature makes the material well-suited for applications in which (repeated) bending, deformation and mechanical wear ( anti-scratch ) are relevant. [1] Cairns, D.R., Witte, R.P., Sparacin, D.K., Sachsman, S.M., Paine, D.C. and Crawford, G.P. (2000) Strain dependent electrical resistance of ITO on polymer substrates, Applied Physics Letters 76, :Gorkhali, S.; Cairns, D. R.; Crawford, G. P., Digest of Technical Papers - Society for Information Display International Symposium (2003), 34, ; Gorkhali, S. P.; Cairns, D. R.; Crawford, G. P., Journal of the Society for Information Display (2004), 12(1),45-49; Cairns, Darran R.; Paine, David C.; Crawford, Gregory P., Materials Research Society Symposium Proceedings 2001),666(Transport and Microstructural Phenomena in Oxide Electronics),F3.24/1-F3.24/12; Cairns, D. R.; Paine, D. C.; Crawford, G. P., Annual Technical Conference Proceedings - Society of Vacuum Coaters (2001), 44th, [2] Y. Leterrier, P. Bouten, X. Jiang, FLEXled-IST , deliverable D4PU FLEXled-epfl /Leterrier, September 2002.

5 BACK TO LIST