1
$\begingroup$

I am currently doing some research on organic light emitting diodes and am just trying to get my head around some of the terminology and concepts because I am more from the organic chemistry side don't have a strong material sciences/physics background.

My reading indicates that the work function is the amount of energy required to extract an electron from a material. Electrons are injected via the cathode and electrons are extracted from the anode (i.e hole injection)

I keep reading that for OLEDs, the cathode should have a low work function to match the energy of the LUMO of the electroluminescent layer, and the anode should have a high work function to match the HOMO. I've read this in many journals and review papers but none of them explain why.

Any help would be appreciated.

$\endgroup$
2
  • $\begingroup$ Basically you want electrodes already near the levels you want to extract / inject electrons to. Therefore the electrode at which the material is reduced, the cathode, must have oxidation potential / work function near the LUMO energy level. Opposite, to work on the extraction of electrons from the HOMO, a metal lining down at higher ox potential is needed. Note that this can reduce the existence of energetic barriers at the interfaces, and in all cases reduces the potential needed for the functioning. If this does not answer your question I'll come back when I'll have time. $\endgroup$ – Alchimista Jan 10 at 10:50
  • $\begingroup$ Thank you for your response, that does point me in the right direction. I completely forgot about redox reactions (not a part of chemistry that I enjoy that much) but now that you mention it, it makes a lot more sense. $\endgroup$ – disafear Jan 16 at 2:36
2
$\begingroup$

The operation of light emitting diodes involves the electrochemical creation of radical - cations and -anions that upon recombination lead to molecules in their emitting excited states.

The formation of the above charged species happen when the applied forward potential is positive enough to inject (reduction) electrons from the cathode to the LUMO of the molecule and extract electrons (oxidation) from the HOMO to the anode.

Therefore, the electrodes must have work functions about the levels you want to operate onto.

This means that the cathode must have oxidation potential / work function near the LUMO energy level. Opposite, to work on the extraction of electrons from the HOMO, a metal lining down at higher oxidation potential is needed.

Note that this design not only reduces the potential needed for the functioning of the LED, but minimise energy barriers (if any) at the emitting material / electrode interfaces.

Also note that a reverse order can still lead to light emission, but the ON potential to be applied to this "unnatural" configuration is inevitably higher.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.