Electrochromic and Electrophoretic Technologies Shine in Low-Power Displays

Smart products are proliferating with the continuing advance of low-power electronics technology. This has included putting electronics into applications like wearables and labels that must be thin, lightweight, flexible, and robust. While liquid crystal display (LCD) products have been around for decades providing low power at low cost, they have a limited viewing angle and are too thick to bend in wearables or to be used in labels. Two display technologies (electrochromic and electrophoretic) are replacing LCDs in thse types of applications (Figure 1).

 

Figure 1. Low-power flexible 7-segment displays. Image courtesy of RISE

 

Display Types: Transmissive, Reflective, and Transflective

When we think of electronic displays, we naturally think of televisions, computer monitors, and smartphone screens. These are all examples of transmissive displays. They rely on an active backlight source to illuminate the display as shown at left in Figure 2. By providing their own source of light, they work great in low light but poorly in bright sunlight. But generating that light means power, typically lots of it.

 

Figure 2. Comparison of display methodologies. Image used courtesy of Samsung Display

 

As an alternative to transmissive displays, reflective displays rely on an external light source to illuminate the display (Figure 2, center). The external ambient light is reflected back to the viewer by a reflector at the back of the screen. Reflective displays require significantly less power than transmissive displays, reduce glare, and minimize eye strain due to the very high contrasts possible. However, they are not effective in low-light conditions. The most well-known type of reflective electronic display is the electrophoretic displays used in Amazon Kindle e-readers.

Transflective displays are a mix of transmissive and reflective technologies as illustrated at right in Figure 2. Providing a partially reflective backing and an active light source. Transflective displays can be employed in both low and bright light environments but do not provide the high contrast and very low power possible with purely reflective displays.

 

Electrochromic and Electrophoretic—What’s the Difference?

While both electrophoretic and electrochromic displays are examples of reflective displays, the underlying technologies are quite different. Electrochromic displays employ ultra-thin polymers that change color in response to an applied electric field. The electric field causes the electrochromic material to undergo chemical oxidation and reduction. This change requires very little energy and is relatively stable, so refresh requirements are low.

The displays are manufactured by printing the material stack of electrodes, polymers, and electrolytes in thin layers (Figure 3). These displays can be only a few hundred microns thick, which allows them to be highly flexible. Electrochromic displays can be switched at voltages as low as 3 V, which means they can often be driven directly by standard CMOS products without requiring specialized display drivers that generate higher switching voltages.

 

Figure 3. Electrochromic display materials stack. Image courtesy of Ynvisible

 

In electrophoretic displays, the application of an electric field causes tiny microcapsules filled with colored pigments to move. The negatively charged white pigment ink particles are drawn to the positively charged top electrode (Figure 4). The positively charged black pigment ink particles are drawn to the negatively charged bottom electrode. Viewed from the top, this pixel would appear white.

 

Figure 4. Electrophoretic display pixel operation. Image courtesy of Eink

 

Applying an electric field of the opposite polarity would switch the state, and the pixel would appear black. These are bistable states. So, not only is the transition energy low, but it will not require any refresh. The pixel will maintain its state almost indefinitely, but there can be some degradation over time. Electrophoretic displays require 15 V to switch, so specialized display drivers that step up the standard CMOS system voltages of 3 or 5 V to the 15 V needed for switching.

 

Applications for Flexible Displays

The unique properties (low power, thin, and flexible) of both electrochromic and electrophoretic displays will allow the introduction of dynamic displays into applications that have traditionally been limited to static, printed labels. Figure 5 provides examples of an retail display electronic shelf label (left) and a smart monitoring label (right).