Thermally Activated Delayed Fluorescence

TADF materials can replace phosphor materials at a lower cost, but they can't do it alone.

How do OLEDs produce light?

All OLEDs work by forcing high-energy electrons into one side of the device and removing low-energy electrons from the other side. The device is engineered so that the electrons can only relax and lose their energy by using the molecules in the emissive layer, producing light. In order for that to happen, though, the electron needs to have the same properties ("spin") as the electron that's missing from the lower energy level. When an excited electron is on the same molecule as a missing electron (or "hole") with the same spin, that is called a "singlet exciton," which can easily relax to emit light. When the spins are different and the electron can't relax to emit light normally, that is called a "triplet exciton." Triplets are produced over singlets at a 3:1 ratio, so this has a massive impact on device efficiency.

How was the efficiency problem first addressed?

Phosphors were unique in that they could use triplet excitons to produce light via phosphorescence, which made OLEDs viable for high-value applications like AMOLED displays (left). Phosphor molecules are built around a very large metal atom, which allows triplets to relax by flipping their spins in a process called "spin-orbit coupling". The downsides of phosphors are that the large metal atoms tend to make the materials very expensive and hard to produce, and that the light emitted in this process is not pure in colour. With phosphors as the best available emitter, OLEDs were limited to expensive products like smartphones, and the OLEDs themselves needed built-in colour filters to achieve decent colour contrast, which harms their efficiency - and your phone's battery life.

What does TADF do instead?

Thermally Assisted Delayed Fluorescence-capable molecules have another way of using triplet excitons that doesn't depend on a large metal atom. They are very carefully and intelligently designed to ensure that their triplet and singlet energy levels are so close together, ambient thermal energy can nudge the electron from a state triplet to a singlet state; hence, "thermally assisted." This process takes some time, but results in a singlet that can relax normally and produce light: "delayed fluorescence."

However, some of the same factors that make a TADF molecule good at converting triplets to singlets can also make them poorer at actually letting the electron relax and emit light. Additionally, TADF molecules tend to emit light with the same sort of broad, impure colour spectrum that a phosphor produces. Since their properties need to be so carefully balanced, tuning a TADF molecule to do both jobs in an OLED device, work with all the other materials, and emit the right colour of light, is very difficult. They cost much less to produce than phosphors, but that's only half the battle. To make a real next generation OLED with better performance at a lower cost, the job of actually producing light must be given to a partner material.

What does the partner material need to do?

Pairing a fluorescent molecule, which can use singlets to make light, with a TADF molecule, which turns triplets into singlets, leads to a "hyperfluorescence" or "co-emissive" light emission system. In this process, pioneered by Prof. Chihaya Adachi of Kyushu University, the TADF molecule converts triplets into usable singlets and passes their energy to the fluorescent molecule, which then produces light normally. Separating these processes to two molecules lets both be optimized for their specific roles, and takes some of the burden of design off the TADF molecules. The fluorescent emitter can then be optimized to produce very pure light with a high efficiency, while overlapping its energy levels with those of the TADF material to ensure it can recive the delivered energy effectively. Amber Molecular specializes in producing these finely tuned emitter materials.