Electrochemical Display Switches Between Emissive, Reflective Modes

Daniel S. Burgess

A display technology demonstrated at the Chinese University of Hong Kong promises to combine the benefits of organic LED and liquid crystal devices by operating in both emissive and reflective modes.

The technique suggests applications in portable consumer electronics such as MP3 players, for which the ability to switch between modes as required by the ambient light conditions could extend battery life.

The dual-mode display features two polymer thin films that are selectively addressed by the polarity of the driving voltage. Under a positive bias, the electroluminescent layer generates light (top). Under a negative bias, the electrochromic layer turns a dark color (bottom). Courtesy of King Y. Wong, Chinese University of Hong Kong.

The display is an electrochemical cell featuring two polymer thin films that are blended with a solid electrolyte and sandwiched between electrodes. One film exhibits electroluminescence under a positively biased voltage. The other demonstrates electrochromism under a negatively biased voltage. Therefore, altering the polarity of the driving voltage causes the device to switch between modes of operation.

In their proof-of-principle experiments, the scientists employed materials that produced orange light in the emissive mode and that turned dark blue in the reflective mode. They propose that the proper choice of materials, deposited in an active or passive matrix, would enable the fabrication of full-color displays. The current at the operating voltage for the reflective mode was two orders of magnitude lower than that for the emissive mode, confirming that the dual-mode display could conserve power for portable applications.

King Y. Wong, associate professor in the physics department at the university, said that the work has significance beyond its potential for display applications. Light-emitting electrochemical cells, which operate based on ion migration, receive less attention than purely electronic organic LEDs because the latter have longer operating lifetimes and shorter switching times. The additional functionality of the reflective mode, however, may encourage research into electrochemical displays, Wong said.

The demonstration device, which was not optimized for performance, also exhibited these drawbacks of light-emitting electrochemical cells, with an operating lifetime of only a few days and a switching time on the order of a second. Wong nevertheless expressed optimism, noting that the first polymeric organic LEDs were short-lived and inefficient. The researchers are investigating the mechanisms behind these limitations.

Applied Physics Letters, Sept. 12, 2005, 113502.