AsianScientist (Mar. 15, 2016) – With a simple tweak, Japanese researchers have increased the device lifetime of organic light-emitting diodes (OLEDs) almost eightfold. These OLEDs use a recently-developed class of molecules to convert electricity into light, with the potential for increased efficiency at a lower cost. The research was published in Scientific Reports.
The easily implemented modifications can also potentially increase the lifetime of OLEDs currently used in smartphone displays and large-screen televisions.
Typical OLEDs consist of multiple layers of organic films with various functions. At the core of an OLED is an organic molecule that emits light when a negatively-charged electron and a positively-charged hole—which can be thought of as a missing electron—meet on the molecule.
Until recently, the light-emitting molecules were either fluorescent materials, which can be low cost but can only use about 25 percent of electrical charges, or phosphorescent materials, which can harvest 100 percent of charges but include an expensive metal such as platinum or iridium.
The researchers, from Kyushu University’s Center for Organic Photonic and Electronics Research (OPERA), have shown that thermally-activated delayed fluorescence (TADF) materials can convert nearly all of the electrical charges to light without the expensive metal used in phosphorescent materials, making high efficiency and low cost possible.
However, OLEDs under constant operation degrade and become dimmer over time, regardless of the emitting material. Devices that degrade slowly are key for practical applications, and a main concern was that the lifetime of early TADF devices was still on the short side. But with this leap in lifetime, many of those concerns can be addressed.
“While our initial TADF devices lost five percent of their brightness after only 85 hours, we have now extended that more than eight times just by making a simple modification to the device structure,” said lead author of the study, postdoctoral researcher Dr. Daniel Tsang.
The newly-developed modification was to put two extremely thin (1-3 nanometers) layers of the lithium-containing molecule Liq on each side of the hole-blocking layer. These molecule layers bring electrons to the TADF material—in this case, the green emitter 4CzIPN—while preventing holes from exiting the device before contributing to emission.
The devices will last even longer in practical applications, because the tests were performed at extreme brightness to accelerate the degradation.
Furthermore, after applying additional optimizations that have been previously reported, the five percent drop was delayed even more, to longer than 1,300 hours—over 16 times that of the initial devices.
“What we are finding is that the TADF materials themselves can be very stable, making them really promising for future displays and lighting,” said Professor Chihaya Adachi, corresponding author and director of OPERA.
The benefits of the Liq layers are not limited to TADF-based OLEDs as the researchers also found an improvement using a similar device structure with a phosphorescent emitter.
Though still trying to understand the degradation process, the researchers found that devices with the Liq layers contain a much lower number of traps—a type of defect that can capture and hold a charge, preventing it from moving freely in the device.
They observed these defects by measuring tiny electrical currents created when charges that were frozen in the traps at extremely cold temperatures escape by receiving a jolt of thermal energy as the device is heated. This process is called thermally stimulated current.
Having charges stuck in these traps may increase the chance for interactions with other charges and electrical excitations that can destroy the molecules and lead to degradation.
One of the next major challenges for TADF is stable and efficient blue emitting materials, which are necessary for full color displays.
“With the continued development of new materials and device structures, we think that TADF has the potential to solve the challenge of efficient and stable blue emission,” said Adachi.
The article can be found at: Tsang et al. (2016) Operational stability enhancement in organic light-emitting diodes with ultrathin Liq interlayers.
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Source: Kyushu University.
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