Electrode Materials for Touch Sensors: the Physical Properties to Consider
Electrodes are the backbone of capacitive touch sensors. So, the behavior of touch sensors heavily depends on the materials electrodes are made of.
Despite this well known fact, for a long time engineers overlooked experimenting with different transparent electrode materials. The default option of Indium Tin Oxide (ITO), which dominated the market for a long time seemed capable enough of providing adequate performance for all of their designs.
Newly developed trends in the touch sensor industry are changing this. With designers shifting their focus towards flexible, bent or large touch sensors, ITO’s inherent limitations force engineers to set their sights elsewhere for electrode materials.
There are a lot of emerging solutions out there that have shown promise. The most prominent ones include: metal mesh, silver nanowires, carbon nanotubes and PEDOT:PSS. However, many engineers are not sure if they check all the boxes of desired properties.
This blog post aims in providing a handy list of properties physical that a candidate material for transparent electrodes should have.
The properties of transparent electrode materials can be broken down to three main categories:
Electrical performance boils down to the resistivity and capacitance of the electrodes.
The capacitance is mainly affected by the geometrical properties of the electrodes. Specifically the shape and size of the electrodes play the most major role.
To compare resistivities, the industry uses the term sheet resistance. Sheet resistance needs to be as low as possible. This allows for faster response times, and use in large screens.
Another electrical property that should be taken under consideration is how much sheet resistance changes as the length of the electrode changes. This is crucial for stretchable devices, as the same behavior must be ensured in all lengths.
Optical performance is crucial when a screen is housed underneath the touch sensor. Great optical performance allows the user to have an unobstructed view of the screen. Focus lies on two properties here, transmittance and haze.
Optical transmittance is a measure of effectiveness of a material surface in transmitting light . In touch screens, visible light needs to be transmitted efficiently, so sometimes this measure is referred to as visible light transmittance, or VLT.
VLT’s values range from 0% to 100%. A value of 0% means that no light goes through (these materials are called opaque), while a value of 100% means that all light goes through (these materials are called transparent).
A good rule of thumb is that all VLT values above 85% are considered as acceptable by the touch screen industry.
It is important to mention that VLT here is measured on the whole stackup of the touchscreen, as optical losses happen on most of its components. That’s why, in electrodes VLT is usually required to be over 90%.
If anyone is not familiar with stackups, a good place to start is our StackUp guide.
Haze is caused by the reflection of light within the material. Its presence causes a “cloudy” look on the touchscreen, which decreases the viewing experience of the user. That’s why, haze must be limited typically to less than 2%.
The device’s design dictates the mechanical properties the electrodes should have. For example, if a device is bent, stretchable or flexible, then electrodes should also be able to bend, stretch or flex.
The main property engineers take into account when it comes to mechanical properties is that of brittleness. Brittleness expresses how much energy can a material absorb prior to fracture. Common measures of brittleness include the strain-stress graph and the material’s bending radius.
Trade-offs between properties
The main trade-off engineers have to optimize for is that between sheet resistance and transmittance. As we said, sheet resistance should be as low as possible, while transmittance as high as possible.
The easiest way to decrease sheet resistance is to increase the thickness of the electrodes. However, this lowers the optical performance of the electrodes.
That’s why, sheet resistance values for different electrode materials are referred to in correlation to the level of transmittance. An example would be that Silver Nanowires have a 10 Ohms/square sheet resistance at 94% transmittance. Thus, a very useful plot that engineers should be aware of and utilise is that of sheet resistance vs transmittance.
So, optimizing for this pair of properties requires knowledge of the properties of the materials, combined with design iterations and analysis, with the use of simulation software or prototyping.
ITO’s market share is expected to shrink in the next years, so ITO alternatives will become of greater importance.
Familiarizing yourself with ITO alternatives might look like a challenging task, but we hope that this blog post provides a solid start giving you pointers on what to look out for.
However, since information regarding electrodes is not readily available, we have also put together a handy electrode guide.
It includes a thorough comparison of ITO and 4 of its alternatives (metal mesh, silver nanowires, carbon nanotubes and PEDOT:PSS).
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