Clark I. Bright, Bright Thin Film Solutions LLC, Tucson, AZ
Functional demands on transparent conductive materials (TCM) continues to increase because of the wide variety of applications. Transparent conductive oxides (TCO), such as indium tin oxide (ITO), are typically used as transparent electrodes in heating, EMI shielding, photovoltaic solar cells, displays, and many other applications. ITO deposited on glass with a high substrate temperature process, e.g., > 200 °C, has acceptable optical and electrical performance (85%-88%T/~10 Ohm/sq) for many applications. However, when ITO is vacuum deposited at low substrate temperature on plastic film, e.g., polyester, to permit flexible devices the electrical performance (85%T/~50 Ohms/sq) is much worse than on glass, limiting its applications. ITO deposited on plastic film with ~50 Ohms/sq is > 100 nm thick and easily cracked when strained, e.g., in bending, stretching, or subsequent processing. Therefore, ~10 Ohm/sq is not usually made on plastic film because of this concern. Clearly, a more conductive, highly transparent and more flexible TCM that is compatible with plastic substrates is needed to replace ITO. Very thin Ag and Ag/Au alloy metallic films, optically enhanced, using induced transmission Dielectric/Metal/Dielectric (DMD) designs were developed to address these performance limitations and to be compatible with roll-to-roll vacuum deposition on temperature-sensitive plastic film. Additionally, traditional inorganic metal oxides used as optical enhancement layers were replaced with vacuum evaporated organic "high index polymers" (in situ cured monomers), enabling their use in applications requiring bendable/flexible conductors and higher deposition speed. The fundamental transmission limitations of thin metallic films were absorption and inadequate optical enhancement. Very thin metal layers were used to mitigate absorption but if too thin, an island type structure occurred, increasing absorption and becoming discontinuous and nonconductive. To inhibit island formation various metal oxides were evaluated experimentally to nucleate Ag and Ag/Au thin films and enhance adhesion. Sputter deposited, discontinuous ZnO nanoclusters, (98%ZnO/2%Al2O3 target) enabled much thinner stable Ag-based layers with good conductive and low absorption. Using this method, DC magnetron sputter Polymer/Ag/Polymer TCM with ≥ 90%T/≤ 50 Ohms/sq and ≥ 87%T /~ 10 Ohms/sq, including substrate losses, were achieved on unheated polyester (PET) film (≤ 30°C), exceeding typical single layer ITO film performance. The PMP TCM corrosion resistance was enhanced by adding thin tin oxide "layers" and anticorrosion additives to the polymers and/or using a Ag/Au alloy layer. Typical PMP TCM tested by accelerated aging passed 120 hours @ 65°C/95% RH, and 72 hours @ 85°C/85% RH and demonstrated up to 50% extension without loss of conductivity and bending around a 3 mm radius of curvature without degradation.