Tunable Polymer Could Make Truly White OLED

Doping an organic polymer with platinum atoms makes the light it emits tunable, which could lead to the realization of cheaper, more efficient and truly white OLEDs.

Existing white OLED displays use different organic polymers that emit different colors, arranged in red, green and blue pixels that must be combined or converted to make white light. “This new polymer has all those colors simultaneously, so no need for small pixels and complicated engineering to create them,” said Z. Valy Vardeny, a physicist at the University of Utah.

Vardeny and a team of international colleagues inserted platinum metal atoms at different intervals along a chainlike organic polymer and found that they could tune the wavelengths of light it emitted.

University of Utah physicist Z. Valy Vardeny works in a glove box where light-emitting polymers are studied under clean conditions. The polymers hold promise for use in a new generation of OLEDs. Courtesy of Lee J. Siegel, University of Utah.

These polymers aren’t quite OLEDs, because they only emit light when stimulated by other light; a true OLED is a polymer that emits light when stimulated by electrical current.

Polymers have two kinds of electronic states: The first is a “singlet” state that can be stimulated by light or electricity to emit higher-energy, fluorescent blue light; until now, OLEDs derived their light only from this state, allowing them to convert only 25 percent of energy into light — better than incandescent bulbs, but far from perfect. The second state is a usually inaccessible “triplet” state that theoretically could emit lower-energy, phosphorescent red light, but normally does not, leaving 75 percent of electrical energy that goes into the polymer inaccessible for conversion to light.

A sample of a platinum-rich polymer emits light as a laser beam hits it. The light appears white because the polymer emits a combination of broad-spectrum violet and yellow, which combine to appear white. Courtesy of Tek Basel, University of Utah.

White OLEDs made using the enhanced polymers would not only produce true white light but also be much more energy efficient because they would take advantage of both fluorescence and phosphorescence, Vardeny said.

For the study, the researchers used two versions of the same semiconducting polymer. One version had a platinum atom in every unit or link in the chainlike polymer; it emitted violet and yellow light. The other version had a platinum atom every third unit and emitted blue and orange light.

“What is new here is that we can tune the colors the polymer emits and the relative intensities of those colors by changing the abundance of this heavy atom in the polymer,” Vardeny said. “The idea, ultimately, is to mix this polymer with different platinum units so we can cover the whole spectrum easily and produce white light.”

A “platinum-rich pi-conjugated polymer” tuned to emit white light when stimulated by light should come in about a year, he added, and it should be about two years until development of true white OLEDs. The new polymer also has potential applications in solar cells or even in computer memory systems.

The study was published online in Scientific Reports.

For more information, visit: www.unews.utah.edu