Electrochromism and Electrophoresis – What’s the Difference?
While electrophoretic displays and electrochromic displays are both examples of reflective displays, their underlying technologies are very different. Electrochromic displays use ultra-thin polymers that change color in response to an applied electric field. Electric fields cause chemical oxidation and reduction of electrochromic materials. This change requires very little energy and is relatively stable, so refresh requirements are low.
These displays are made by printing a material stack of electrodes, polymers and electrolytes in thin layers (Figure 3). These displays are only a few hundred microns thick, making them highly flexible. Electrochromic displays can switch at voltages as low as 3 V, which means they can often be driven directly by standard CMOS products without the need for specialized display drivers that generate higher switching voltages.
Figure 3. Electrochromic display material stack.Image courtesy of Ynvisible
In electrophoretic displays, applying an electric field causes tiny microcapsules filled with colored pigments to move. Negatively charged white pigment ink particles are attracted to the positively charged top electrode (Figure 4). Positively charged black pigment ink particles are attracted to the negatively charged bottom electrode. Viewed from the top, the pixel will appear white.
Eink electrophoretic display pixel technology
Figure 4. Electrophoretic display pixel operation.Image courtesy of Eink
Applying an electric field of opposite polarity switches the states and the pixel appears black. These are bistable. Therefore, not only is the transition energy low, but no refresh is required. Pixels will maintain their condition almost indefinitely, but may experience some degradation over time. Electrophoretic displays require 15 V for switching, so a dedicated display driver can step up the standard CMOS system voltage of 3 or 5 V to the 15 V required for switching.
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