Molecular developments to allow for more efficient electronic displays

Joseph Xu / Michigan Engineering
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By Ian Dillingham, Daily News Editor
Published September 24, 2014

Students often stare at their phones in class. They may not realize they’re actually looking at thousands or millions of tiny red, green and blue dots called pixels, but they most likely realize — as their batteries are slowly depleted — that these displays are major power drains for modern smartphones.

That is where researchers in the University’s Department of Electrical Engineering and Computer Science come in.

New findings published this week in Nature Communications may allow smartphones to run longer on less power thanks to advancements in light-emitting diodes — LEDs — the building blocks of color displays on smartphones and the source of countless other forms of artificial light.

The research was conducted in the lab of Stephen Forrest, the Paul G. Goebel Professor of Engineering and former vice president of research for the University.

Traditional LEDs are constructed with a mixture of indium, gallium, nitride and other inorganic compounds. In contrast, organic light-emitting diodes — OLEDs — are carbon-based.

In 1998, Forrest and his team demonstrated the first phosphorescent OLEDs, a subset of the OLED family that has been shown to be about four times more efficient. However, these PHOLEDs suffered from a crucial flaw: They degraded under blue light. While they performed well with red and green wavelengths, it turned out blue light was of too high an energy, breaking covalent bonds between molecules in the diode and rendering it useless.

Mobile phone manufacturers are currently able to incorporate the efficient PHOLEDs to emit red and green light on phone displays but have used the less-efficient fluorescent OLED for blue to extend the display life.

In the most recent study, however, Forrest’s lab demonstrated a longer-lasting version of the blue PHOLED, with a tenfold increase in lifespan compared to previous models. Yifan Zhang, a former student in the lab, and Jae Sang Lee, a current Ph.D. candidate, authored the recent publication.

Lee said he was inspired by his desire to create something very useful and efficient and to fill a gap in PHOLED production that the industry has been unable to address.

In standard OLEDs, electrons are essentially passed along the diode in a single line. The PHOLED takes advantage of what Forrest called a “quantum trick,” which allows four times more electrons to be transferred between molecules.

“What makes them so efficient is that what sits in the middle of each molecule is a heavy metal atom — iridium or platinum,” Forrest said. “That’s what made the OLED display industry possible in many respects, because they needed high efficiency.”

However, because these transfers excite the molecule into a state of high energy, they can result in collisions that can cause a single molecule to become twice as excited. This unstable state, which is determined by the energy of the electrons, has traditionally led to a breakdown of the diode in the case of blue light.

Forrest’s group overcame these constraints by increasing the area that the electrons had to spread out over the emissive layer of the diode, thereby lowering the probability of such collisions.

Implementation of the blue PHOLED into consumer electronics such as smartphones, tablets and televisions, could take three to five years, according to researchers. Once implemented, the new diodes could reduce energy consumption in these devices.

Forrest said the new PHOLEDs could easily be incorporated into current manufacturing techniques, as their structure is very similar to that of their predecessors.

“The equipment, the process, the time is basically identical in what is used today in mass-producing these appliances,” Forrest said.

Additionally, Forrest said OLEDs could eventually play a greater role in increasing energy efficiency, as they could be adopted in building and street lighting.

“I predict, that in the next five to 10 years, that OLEDs will be the dominant display technology used on the planet,” Forrest said.