How can we get ever-brighter digital displays?
Your smartphones and televisions are getting smaller, thinner and more energy-efficient — and you can thank OLEDs (organic light-emitting diode) for that.
Like a constellation of tiny flashlights, OLED displays emit millions of points of light to produce the sharp and constantly changing images demanded by high-definition video.
As impressive as they are, however, current OLEDs lose more than 40 percent of their potential brightness. But Stanford researchers may have found a fix. “If your smartphone or VR headset display is bright and beautiful today, just think what better OLED displays could mean,” says Majid Esfandyarpour, first author of a paper in the journal Nature Communications and a postdoctoral scholar in the lab of Mark Brongersma, professor of materials science and engineering and the paper’s senior author.
Like most light-emitting devices, going all the way back to Edison’s incandescent bulb, OLEDs have two electrodes that carry electricity to and away from the light-emitting material. In OLEDs, these electrodes are two of the layers in a multi-layer stack. At the top: a transparent electrode, which allows the light to exit the diode. (In a TV display this would be the outward facing side of the screen.) At the bottom, a reflective electrode, typically made of a metal that is both highly conductive and reflective so it can pass electricity and act as a mirror.
The problem is that the bottom electrode behaves differently when the light-emitting diode is placed close to a smooth, reflective surface. A considerable fraction of potential light gets trapped and absorbed instead of reflected. That trapped light also creates unwanted heat, which limits the OLED’s lifespan.
Brongersma and Esfandyarpour devised a novel way around these problems. Instead of using a smooth metallic surface on the bottom of their OLEDs, they corrugated the reflective layer with nanoscale ripples like a crinkle-cut potato chip. Although these ripples are almost unimaginably tiny, they cause more light to be reflected outward. This potentially doubles the visible brightness of OLEDs while also increasing their life span by reducing heat-induced degradation.
Because these OLEDs are light, bright, fast and thin, they’re especially promising for application in virtual/augmented reality headsets and smartphones. Brongersma says they’ve already had contacts with outside parties that may try to implement these new electrodes in future displays. “They can also be made on flexible materials,” adds Esfandyarpour, “leading to some cool possibilities in wearable or roll-up displays, too.”
Additional authors include visiting professors Alberto G. Curto and Pieter G. Kik, and Professor Nader Engheta of the University of Pennsylvania.
Financial support for this research was provided by a Multi University Research Initiative from the Air Force Office of Scientific Research, a gift from Konica Minolta Laboratory and a Marie Curie International Outgoing Fellowship.
