How is thermally activated delayed fluorescence different from traditional fluorescence? Are there any real practical applications of TADF materials or is it just a scientific curiosity?

The basic concepts are still the same. TADF is still fluorescence. Energy absorption,charge separation recombination and emission are occuring just like they do normally. But TADF materials allow emissive entities to absorb thermal energy from surroundings and change state before emission. Why is this relevant?

Fluorescent emission happens when the excited state electron has the opposite spin quantum number as the ground state electron.The system of electrons is paired having net spin 0. In this case a smooth transition occurs between excited and ground state leading to an emission. But in practice things rarely go smooth.

As many as 75% of electrons can change their spin state when electrically excited and become what are known as triplets (system of electrons with same spin). These triplets are forbidden to return to ground and loose energy as heat (phosphorescent glow might also occur but its too slow and at too low an intensity to be useful in lighting applications). This is where TADF comes in.

TADF allows triplet electrons to absorb thermal energy and undergo a change of state to singlet which allows them to relax to ground state and emit light. Theoretically 100% efficiency is possible because now triplet electrons are not lost to heat. And this is why there is a flurry of research going on to understand and engineer TADF devices.

TADF has practical applications as emitters in OLED. Industry experts consider third generation of OLEDs to be based on TADF. Personally I believe that TADF like entities would be better suited in traditional solid state LED as opposed to OLEDS which are usually solution based. Because TADF molecules have to absorb heat and also emit in visible range the chances of degradation remain high. Solid state materials would help in handling excess heat increasing lifetime of the device. But either way TADF emitters promise new and exciting possibilities for lighting applications.

#engineering #light

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Notes by Akshat Jiwan Sharma

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