Researchers at Kyushu University have - with just a tiny tweak - greatly increased the device lifetime of OLEDs that use a recently developed class of molecules to convert electricity into light with the potential for increased efficiency at a lower cost in future displays and lighting. The easily implemented modifications can also potentially increase the lifetime of OLEDs currently used in smartphone displays and large-screen televisions.
According to the Researchers typical OLEDs consist of multiple layers of organic films with various functions. Until recently, the light-emitting molecules were either fluorescent materials, which can be low cost but can only use about 25% of electrical charges, or phosphorescent materials, which can harvest 100% of charges but include an expensive metal such as platinum or iridium.
The Researchers at Kyushu University's Center for Organic Photonic and Electronics Research (OPERA) changed this in 2012 with the demonstration of efficient emitters based on a process called thermally activated delayed fluorescence (TADF).
Through molecular design, these TADF materials can convert nearly all of the electrical charges to light without the expensive metal used in phosphorescent materials, making both 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 concerns remained that the lifetime of early TADF devices was still on the short side.
The newly developed modification was to put two extremely thin (1-3 nm) layers of the lithium-containing molecule Liq on each side of the hole blocking layer, which brings electrons to the TADF material, the green emitter 4CzIPN in this case, while preventing holes from exiting the device before contributing to emission.
Figure: Kyushu University - OLED with the TADF emitter 4CzIPN
Applying additional optimisations that have been previously reported, the 5% drop was further delayed to more than 1,300 hours, over 16 times that of the initial devices.
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.
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. Though they are still trying to completely unravel the degradation mechanism.
These defects were observed 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, a process 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 and are also still difficult using phosphorescence.
Daniel Tsang, lead author on the study, said, "While our initial TADF devices lost 5% of their brightness after only 85 hours." Daniel added, "we have now extended that more than eight times just by making a simple modification to the device structure."
Professor Chihaya Adachi, director of OPERA, said, "What we are finding is that the TADF materials themselves can be very stable, making them really promising for future displays and lighting." Chihaya Adachi added, "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."
This work was supported by the Kumamoto Collaborations on Organic Electronics under the Regional Innovation Strategy Support Program sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT).
Daniel Ping-Kuen Tsang | Chihaya Adachi
Scientific Reports 6 | Article number: 22463 (2016) | doi:10.1038/srep22463
Received: 22 October 2015 | Accepted: 11 February 2016 | Published online: 01 March 2016
Organic light-emitting diodes (OLEDs) under constant current operation suffer from a decrease of luminance accompanied by an increase of driving voltage. We report a way to greatly improve the stability of OLEDs having a green emitter exhibiting thermally activated delayed fluorescence (TADF), (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN), by introducing ultrathin (1 to 3 nm) interlayers of 8-hydroxyquinolinato lithium (Liq) between hole-blocking layer and its surrounding emissive and electron-transport layers. Under constant current operation starting at a luminescence of 1,000 cd/m2, the time to reach 90% of initial luminance (LT90) increased eight times, resulting in LT90 = 1,380 hours after insertion of the interlayers. Combining this new concept and mixed host system, LT95 was further extended to 1315 hours that is 16 times of reference device. This is the best value reported for TADF-based OLEDs and is comparable to the operational lifetimes of well-established phosphorescence-based OLEDs. Thermally stimulated current measurements showed that the number of deep charge traps was reduced with the insertion of the ultrathin Liq interlayer, indicating that reducing the number of deep traps is important for improving the operational lifetime and that exciton-polaron annihilation may be a source of the device degradation.