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Illumination & Displays

Making thin films more transparent, more conductive

A coating that offers 85% transmittance and excellent durability can be used as a transparent electrode in flexible displays or as a thermal barrier in energy-saving windows.
12 September 2006, SPIE Newsroom. DOI: 10.1117/2.1200608.0340

Transparent electrodes are conductive thin films deposited on transparent substrates. These thin-film systems are used for flat-panel displays, photovoltaic cells, electrochromic devices, electroluminescent lamps, as well as electromagnetic-interference and radio-frequency shielding. For these applications, transparent electrodes exhibit three important qualities: high optical transparency, electrical conductivity, and mechanical durability, including flexibility. The films can also be used in energy-saving windows because they transmit visible light while reflecting infrared energy and can electrically defrost the panes.1 Material properties of earlier transparent electrodes significantly limited functionality in each of the three quality areas. We created a new thin-film system that displays excellent transparency, conductivity, and durability, which makes our technology, the EclipseVISTEC, a superior transparent electrode.

Other transparent electrodes use materials characterized by high optical transparency and high electrical conductance, such as transparent conductive oxides (TCOs), including indium tin oxide (ITO), zinc oxide doped with aluminum, and tin oxide doped with fluorine. But TCOs are limited to about 60% transparency, and increasing the conductance tends to reduce the film's durability.

TCOs also trade off conductance and durability. To maximize the conductance of ITO and achieve the best quality film, film thickness must either be increased or the substrate must be heated during deposition. Both activities reduce the durability of transparent electrodes. Increasing the thickness is not ideal because ITO has low tensile strength and displays micro-cracks when flexed, and the problem is worse with thicker films. But heating is also problematic because the most popular substrates—including glass, ceramics, polyethylene terephthalate (PET), and polypropylene—are sensitive to temperature and display thermal shrinkage.1

We designed a coating that displays significantly higher transparency and conductivity than TCOs, without requiring heated deposition or displaying microcracks upon application of tensile force. Our EclipseVISTEC multilayered thin-film system has a conductive layer that works like a transparent metal layer, plus an overcoat that protects the conductive layer from environmental degradation, blocks UV light, and minimizes reflections (see Figure 1). We can also optimize layer thicknesses to provide specific electrical and reflective characteristics: thicker coatings provide higher conductance but slightly lower transparency, while thinner systems provide increased transparency but slightly lower conductance.

Figure 1. High-resolution micrographs of indium tin oxide (ITO) and EclipseVISTEC films at the same resolution show that the surface topography of the latter coating is much smoother.

The conductive layer acts much like a polymer and demonstrates high optical transparency and mechanical flexibility. The system can be applied easily to rigid or flexible substrates—such as glass or PET—at room temperature in vacuum via direct thermal evaporation or sputtering techniques.1

We performed optical and mechanical tests on the EclipseVISTEC and comparable TCO thin-film systems that are currently used in industry as transparent electrical conductors (see Figure 2). We performed optical tests of the films' transmittance, film-side reflectance, and substrate-side reflectance using a PerkinElmer UV/Vis/Near IR Lambda 9 spectrophotometer. A Tencor Alpha 500 profilometer measured film thicknesses. We also performed conductance, thermal stability, environmental, flexibility, and vibration testing.

Figure 2. Comparative plots show transmittance (T%) spectra of ITO and EclipseVISTEC coatings, as well as the spectra reflected from the film side (Rf) and the substrate side (Rs) of both coatings.

Optical transmittance results for EclipseVISTEC demonstrate greater than 85% photopic transparency. We measured sheet resistance values less then 2.6Ω/square, which is much lower than ITO's sheet resistance of 10Ω/square. When we optimized the coatings for transparent electrical conductors, they still exhibited 3–5 Ω/square sheet resistance, which is equivalent to 2 × 105S/cm. When deposited on PET, EclipseVISTEC is rollable around a 0.5cm diameter without losing its high conductance and transparency. When we subjected the coating to environmental tests that included UV testing and 95% humidity and 55°C for a week, it survived without degrading. Optical and electrical performances stay the same even when heated to 300°C for 30min. We also confirmed that one can perform photolithographic etching and laser patterning on EclipseVISTEC.

Our coating compares favorably with ITO and other commercially employed transparent electrodes. EclipseVISTEC demonstrates 25% more heat-reflection capability and about 50% less electrical resistance than ITO. It also offers significantly more thermal and environmental durability and can survive on flexible substrates.

The demonstrated spectral performance also makes our thin-film system an excellent coating for energy-saving windows. These windows need to transmit visible light while reflecting in the thermal IR. The coating can be applied directly on the inside of double-pane glass windows or coated on PET hanging inside of a double-pane low-emissivity window to form a thermally insulated triple-pane window.

Hulya Demiryont
Eclipse Energy Systems, Inc.
St. Petersburg, Florida
Hulya Demiryont is currently chief scientist at Eclipse Energy Systems. She has over 30 patents, 100 publications, 18 years of industrial experience, and 23 years in academia. She received her BS, MS, PhD, and full professorship in physics from Istanbul University, where she also headed the Physics Department. In addition, she has written several invited papers for SPIE conferences.