| STUDY |
A simple polymer-based electroluminescent device that can be switched between glowing red or green by reversing the direction of the current flow has been invented by scientists in the Netherlands. Typically, electroluminescent materials emit light of a fixed color, which is determined by the energy of the material's excited state (band gap). This means that a full-color display normally requires pixels made of three different materials that emit red, green, or blue light. Because the new device uses the same material to provide both red and green emission, it potentially could simplify the fabrication of luminescent displays, solid-state light sources, and color switches. It also could increase their brightness because for either red or green, a larger number of pixels would be illuminated. Key to the device is a semiconducting derivative of poly(phenylenevinylene) blended with a phosphorescent complex consisting of two ruthenium centers linked by a tetraphenylene bridge. The doped material is sandwiched between two electrodes, one of gold and one of indium tin oxide (ITO). When a positive voltage (4 V) is applied to the ITO electrode (forward bias), the light-emission process of the ruthenium complex is triggered, producing the complex's characteristic red glow, De Cola explains. When the bias voltage is reversed to -4 V, the direction of the current is reversed. Light emission from the metal complex ceases, and the polymer, in its excited state, now emits green light, which is determined by the polymer's band gap.
In their Nature paper, the researchers focus entirely on the fundamental aspects of the system. They propose an explanation for how the device works that "is based on a unique mechanism that has never before been reported for similar systems," according to De Cola, who is a chemistry professor
|