2019.08.07

TDK Front Line

Vol.8 Various Applications with Ag-Stacked Film

Transparent conductive films, which are optically transparent and electrically conductive, make an essential component as transparent electrodes such as touch sensors. Indium tin oxide (ITO) is the most commonly used material for transparent electrodes. Recently, however, with the increase in size for use in digital signage displays and the increase in use cases for applications other than touch sensors, there has been an increasing demand for transparent conductive films with further lower resistance which ITO cannot fulfill the required characteristics. To meet such demand, TDK has newly developed Ag-stacked film, or Ag-stacked transparent conductive film, in which Ag-alloy thin-film is uniformly layered over plastic film. It has a wide variety of possible applications, including electrodes for light control windows (smart windows), which can control the amount of light passing through a window, transparent film antennas, electrodes for organic EL lighting, and anti-condensation heaters for automobile windows.

Types of transparent conductive films

ITO films are characterized by high transparency within the visible light region and a relatively high level of conductivity. However, there is an incompatible trade-off between optical transmittance (i.e. transparency) and conductivity. Making a film thinner to increase optical transmittance increases the surface resistance, resulting in poorer conductivity, while making films thicker ensures high conductivity at the expense of visibility. For instance, while optical transmittance of approximately 90% is required for touch panels (touch sensors) used in smartphone applications, it is difficult to reduce the surface resistance of the ITO films to 100 Ω/sq. or less. Also, ITO films cannot be used for large-screen displays where low resistance is required.
To break through this technological limitation, TDK has explored new solutions using Ag (silver), which features the highest conductivity among metals, instead of using rare metal indium.

Ag-based transparent conductive films are produced by different methods: a wire ink method in which needle-shaped Ag wires are mixed in ink, and then applied and printed on a substrate, and a metal mesh method in which thin metal (such as Ag and Cu) wires are arranged in a matrix and formed on a substrate. These methods, however, do not provide high surface smoothness and good visibility because the fine unevenness of the wire is formed on the film surface.
In contrast, TDK has achieved a transparent conductive film with excellent balance of characteristics and usability by forming a thin Ag alloy film uniformly on the surface of a plastic film using a sputtering method. This is called Ag-stacked film.

Features of TDK Ag-Stacked Film

Metals are highly conductive thanks to free electrons. Free electrons reflect light on surface with little penetration inside, creating a unique metallic shimmer. Ag films that have a thickness of about 100 nm or greater have high light reflectivity and therefore used in luxury mirrors and other products; however, when the film thickness is reduced to 20 to 30 nm or smaller, transmission of visible light increases, enabling them to be used as transparent conductive films that simultaneously realize high conductivity and high transmittance. However, when the thickness of Ag film is reduced to 20 nm or less, it becomes unstable due to the effects of oxygen, water vapor, or heat if no measures are taken.

To solve this issue, adopting a highly conductive Ag-alloy material and a highly transparent proprietary oxide film (protective film), TDK has developed Ag-stacked film forming a transparent conductive film that has a multilayer structure of "oxide layer/Ag alloy layer/oxide layer" on a film substrate, using an advanced sputtering method. The Ag-stacked film has also realized a reduction in resistance down to 1/10 or less than that of ITO films used for touch panels while maintaining a high transmittance equivalent to that of ITO films, through nano-order thickness control of the layer.

Continuous layer consisting of protective-layer/Ag alloy film/protective-layer is deposited on the film substrate using an advanced sputtering method.

In addition to its high optical transmittance in combination with a low resistance far superior to that of ITO films, TDK Ag-stacked film presents a good balance of characteristics with no weakness such as high storage reliability and heat stability through the use of an original oxide film materials, high durability of repeated bending, and high surface smoothness that cannot be achieved through the Ag wire ink method or the metal mesh method.
Another feature of Ag-stacked film is excellent thermal insulation since it has high light reflectivity in the infrared region. These features enable the use in a wide array of applications that could not have been achieved by conventional transparent conductive films. The following are prominent case examples for using Ag-stacked film.

Transparent electrodes for light control films such as smart windows

TDK Ag-stacked film can be used as transparent electrodes for a light control film, which can switch between transparency and opacity instantly, or change the level of transparency successively.
As a light control film, a device having a structure in which a light control layer with liquid crystal molecules dispersed is sandwiched between transparent conductive layers has been put to practical use. Incident light is scattered and the device becomes opaque since its liquid crystal molecules are usually dispersed. When a voltage is applied from the transparent electrodes, these molecules are aligned and light can be transmitted through the film. The film may be used as a light control partition and a substitute for projection screens in office floors, and is also possible to use the show window as a digital signage. Meanwhile, when used as a light control sunroof of automobiles, the film can control the amount of sunlight, reducing the load on the air conditioning unit and improving fuel efficiency and power consumption costs. It may also be used as light shielding windows in railroad vehicles, airplanes, and marine vessels.

Some type of Ag-stacked film can change the level of transparency successively, enabling light control windows (smart windows) to control the amount of light collection depending on the season, time, or weather. In addition, it is also suitable for transparent electrodes for organic EL lighting and flexible displays, and it can be expected to be used in smart houses and smart buildings.

Applicable as transparent film antennas

In addition to uses as transparent electrodes, TDK Ag-stacked film may also be used as transparent functional films. One of the applications is transparent film antennas.
In an office building where it can be harder to receive communication signals, indoor antennas connected with the base station may be installed on ceilings to ensure a better communication environment. Conventional types of antenna units, in which an antenna is placed inside a plastic container, has been said to spoil the appearance of the interior space. Transparent film antennas, on the contrary, are less conspicuous, lightweight, and crack-resistant. They are the best solution to improve signal reception in office buildings, hotels, event halls, public facilities, and healthcare facilities.

Meanwhile, for 5G (fifth-generation) communication with ultra-high speed and large capacity, a number of small-sized antennas are installed within the area covered by a base station to eliminate “blind zones” of wireless communication. Transparent film antennas, which do not spoil the appearance of their installation sites, are also suitable for smart antennas in the 5G era.

Transparent film heater suitable for a defogger for resin windows

In an effort to make automobiles even more lightweight, the world is seeing increasingly more automobile windows made from resins such as polycarbonate and acrylic. However, thermal conductivity of these transparent resins is only about 20% of that of glass, and electrically-heated wires used as a defogger do not serve their purposes sufficiently.
When the resistance of its conductive film is lowered, TDK Ag-stacked film acts as a heating element with current applied to it, which allows it to work as a transparent film heater for the defogging system, instead of electrically-heated wires.
Transparent film heaters generate heat quite evenly, in contrast to electrically-heated wire heaters with uneven heat distribution. They can be used for front or rear windshield, as well as plastic lenses of security cameras. They can also be combined with other Ag-stacked film-based products such as light control windows and transparent film antennas.

Combining high transmittance and low resistance and showing a good balance of characteristics, TDK Ag-stacked film provides outstanding performance as a next-generation transparent conductive film and is expected to be used for a wide variety of applications in various fields.

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