image: Fig.1 Molecular design of Th-DBN
Credit: ©Science China Press
Researchers have built a warm-white OLED from single-dopant light-emitting molecule. By linking two multi-resonance units with a thiophene bridge, the emitter glows blue-green on its own and orange-yellow of the excimer excited state. The two bands blend into warm white light, and the device reaches 27.1% external quantum efficiency, the best yet reported for a purely organic single-dopant white OLED.
White light, as we meet it every day, is almost always a mixture. The same is true of most white organic light-emitting diodes, or WOLEDs. To make a device glow white, engineers usually blend several emitters - red, green and blue, or sky blue and yellow - in one or more emissive layers, much as a painter mixes pigments.
That approach works, but it carries a cost. Every extra emitter adds steps to the vacuum deposition process and raises the price of the panel. Worse, different materials carry charge at different speeds, so as the driving current changes the region where excitons recombine shifts along with it. The result is white light that drifts in color as the screen dims or brightens. A cleaner solution would be to use a single light-emitting molecule that produces white light on its own. Researchers have tried this for years, using tricks such as excited-state proton transfer, room-temperature phosphorescence and switching molecular conformations. Yet every purely organic single-dopant WOLED reported so far has stalled below 15% external quantum efficiency, a ceiling set by how many excitons the molecule can actually harvest.
A team from Jilin University and Ji Hua Laboratory has now broken through that ceiling by exploiting a behavior that materials scientists usually try to suppress. Their starting material belongs to a family known as multi-resonance thermally activated delayed fluorescence emitters (MR-TADF). In these molecules, electron-donating nitrogen atoms and electron-withdrawing boron atoms sit inside a rigid fused ring framework. This arrangement separates the frontier molecular orbitals in space, which shrinks the gap between the singlet and triplet excited states. However, such flat, rigid molecules have a well-known side effect: at higher concentrations they tend to stack face to face and form an excimer, an excited pair of molecules that emits at a longer wavelength. Most researchers add bulky peripheral groups to prevent this. The team decided instead to encourage it.
The researchers linked two multi-resonance units with a thiophene bridge and extended the conjugated skeleton to create a molecule called Th-DBN. Quantum chemical calculations showed that a pair of Th-DBN molecules binds far more strongly than a pair of the smaller comparison molecule, so the stacked excimer forms readily and survives. The strategy delivered exactly what was hoped for. Isolated Th-DBN molecules emit blue-green light at 499 nm. Stacked pairs emit orange-yellow light at 588 nm. Together the two bands produce a warm, comfortable white with CIE coordinates of (0.35, 0.50) and a correlated color temperature near 4400 kelvin - close to the light of a domestic lamp.
Crucially, both bands behave as thermally activated delayed fluorescence. Transient measurements at 499 and 588 nanometers both show microsecond-scale delayed components, and the reverse intersystem crossing rates for the two states are comparable. In other words, the monomer and the stacked pair each run their own exciton-recycling channel, so triplets are harvested twice over. The doped film reached a photoluminescence quantum yield of 98.8%, meaning that forming the excimer did not open a new path for energy loss.
When Th-DBN was doped at 8% by weight into a conventional bipolar host, the device turned on at just 2.4 V and reached a maximum luminance above 17000 cd m-2. It achieved a maximum external quantum efficiency of 27.1%, a maximum current efficiency of 69.6 cd A-1 and a maximum power efficiency of 90.6 lm W-1. That efficiency is the highest yet reported for a purely organic single-dopant white OLED, nearly double the previous ceiling.
Durability matters as much as efficiency for lighting. Using a device architecture with an exciplex host that spreads the recombination zone, the team recorded an operating lifetime of 398 hours to 80 percent of the initial brightness at a starting luminance of 1000 candelas per square meter, along with a much reduced efficiency roll-off. Because the two emission bands leave gaps in the blue and green, the devices have a modest color rendering index; adding a second blue MR-TADF emitter filled the blue gap and produced standard white light with CIE coordinates of (0.33, 0.34) and a color rendering index of 63.
This is the first time an MR-TADF emitter has been used to generate white light from a single dopant. This work not only provides a highly efficient white OLED device with a simple structure, but also offers an approach to achieving efficient exciton utilization of OLED materials in the aggregated state by regulating the molecular transition behavior in the molecular aggregated state.
Journal
National Science Review