In organic light-emitting diode displays, a new method promises to solve the blue emission bottleneck.
Japanese researchers have shown a promising method, using a new combination of emitter molecules, to finally solve a major obstacle for organic light-emitting diode displays: a blue light source that matches the excellent performance of red and green.
By separating the processes of energy conversion and emission between two molecules, researchers have achieved devices that produce high-efficiency pure blue emissions, retain brightness for a relatively long period of time, and contain no costly metal atoms – a collection of properties that have been difficult to achieve simultaneously.
Organic light-emitting diodes, or OLEDs, are known for their vivid colors and their ability to shape thin and even flexible devices and to transform electricity into light using carbon-containing molecules.
The different red, green and blue emitting pixels of an OLED display can be switched on and off completely individually, creating darker blacks and minimizing power consumption, unlike LCD technologies, which use liquid crystals to selectively block the emission of filtered backlighting covering several pixels.
In particular, blue OLEDs, however, have been a bottleneck in terms of efficacy and stability.
There are an increasing number of excellent performance options for red and green OLEDs, but devices that emit high-energy blue light are more challenging, with trade-offs almost always occurring between reliability, color purity, cost and lifetime,”There are a growing number of options for red and green OLEDs with excellent performance, but devices that emit high-energy blue light are more challenging, with trade-offs between efficiency, color purity, cost and lifetime almost always occurring,”
Although stable blue emitters are widely used in commercial displays based on a technique known as fluorescence, they suffer from low maximum efficiency. So-called phosphorescent emitters can achieve 100 percent ideal quantum efficiency, but usually have shorter lifetimes and require an expensive metal like iridium or platinum.
As an alternative, OPERA researchers have produced molecules that emit light based on the method of thermally activated delayed fluorescence (TADF), which without the metal atom may achieve excellent efficiency but also show broader color spectrum emissions.
Chihaya Adachi, director of OPERA, says, “The range of colors a display can produce is directly related to the purity of the red, green and blue pixels,” “If the blue emission is not pure and has a narrow spectrum, filters are needed to improve color purity, but that wastes the emitted energy.”
A promising way to overcome the purity problem based on a specific molecular design for a high-efficiency pure blue TADF emitter has recently been documented by Takuji Hatakeyama’s group at Kwansei Gakuin University, but the molecule, called v-DABNA, quickly degrades in action.
Working with Hatakeyama, OPERA researchers have now found that by combining v-DABNA with an additional TADF molecule developed at OPERA as an intermediate fast energy converter, the lifetime can be greatly improved while still achieving small emissions.
‘Three-quarters of the electric charges combine to form energy states called triplets in OLEDs, and these non-emitting triplets can be transformed into light-emitting singlets by TADF molecules,’ explains Masaki Tanaka, an OPERA researcher who worked on the study closely with Chan.
However, when converting high-energy triplets that often play a role in degradation, v-DABNA is somewhat sluggish.
We added an intermediate TADF molecule to get rid of the risky triplets quicker, which can convert triplets to singlets faster.
Although the intermediate molecule can transform triplets into singlets easily, it has a large range of emissions that generate sky-blue emissions. Nevertheless, in a high-energy state, the intermediate molecule can pass several of its singlets to v-DABNA, creating a quick and pure blue emission.
“The wavelengths that v-DABNA can absorb are very similar to the color it emits, relative to other emitters.
This unique property allows the broad-emitting intermediate to absorb most of the energy and yet emit pure blue, Chan says.
With this two-molecule form, referred to as hyperfluorescence, he