High efficiency, low cost and long lifetime: pick two when it comes to making a blue OLED. Though not quite ready for commercial application, Japanese researchers may have found the solution.
OLED displays are commonplace these days, but that doesn’t mean the technology has reached a state of perfection. Manufacturers are more or less forced to pick blue pixels that, compared to their red and green counterparts, aren’t very good at producing light. More efficient blue OLEDs are available, but they’re either too expensive or their lifespan is not good enough for commercial application.
Researchers from Japan appear to have finally overcome this trade-off. They stacked two different OLED materials to create a stable and efficient organic emitter of deep blue light. The tandem structure effectively doubled both the emission at the same electrical current and the lifetime at high brightness.
Unruly teenagers
As the acronym implies, organic light-emitting diode displays utilize carbon-based materials that emit light in response to electrical current. As electrons move through the material from cathode to anode, they combine with holes to form excitons. The excess energy of these excited states can be released in different ways, and in OLEDs, it’s key to release as much as possible as visible light.
Zooming in a little more, excitons come in different flavors, with different light-emitting characteristics. Statistically, about 25 percent of the incoming electric charge produces so-called singlet state excitons, which readily return to the ground state by emitting radiation through a process called fluorescence. The other 75 percent of excitons, however, ends up in a triplet state, which is quantum mechanically forbidden to radiatively decay directly. It can still slowly emit light, which is called phosphorescence, but in general, triplets limit the efficiency of OLEDs.
Fortunately, it’s possible to convert triplet states into singlet states. By designing molecules in which the energy difference between the singlet and triplet state is relatively small, triplet excitons can be enticed by a little thermal energy to cross over to the singlet state and fluoresce after all.
This thermally activated delayed fluorescence (TADF) process is very effective, but in the specific case of blue OLEDs, there’s an additional problem. Blue light is in the most energy-rich part of the visible spectrum, meaning the associated excitons are also relatively high in energy. If left unchecked, like unruly teenagers, these energy-rich states can initiate chemical reactions that destroy delicate OLED materials.
This is what happened to a molecule designed by Kwansei Gakuin University. His TADF material, named ν-DABNA, produces pure blue light very efficiently. At the same time, it’s quite slow in converting triplet states into fluorescent singlet states. This gives the high-energy and long-lived triplet states plenty of time to start wreaking havoc: the material is electrochemically unstable.
Promising future
Working together with colleagues from Kyushu University, the Kwansei researchers then combined ν-DABNA with another TADF molecule which is very good at converting triplet states into singlets, but by itself emits a rather broad spectrum of blue light. The combination proved a match made in heaven. The added molecule acts as an intermediary, passing on many of its singlets to ν-DABNA, which dutifully emits them as a much narrower band of blue light. At the same time, relatively little ν-DABNA triplet states are present at any given time, making the combo a lot more stable chemically.
The researchers have named this tandem approach hyperfluoresence. “That this kind of approach can extend the lifetime of pure-blue emission from a molecule we previously developed is really exciting,” says Takuji Hatakeyama from Kwansei. His Kyushu collaborator Chihaya Adachi notes that although the characteristics of the dual system still fall short of practical applications, “stricter control of fabrication conditions often leads to even longer lifetimes, so these initial results point to a very promising future for this approach to finally obtain an efficient and stable pure-blue OLED.”