Why does the cathode in an OLED require a low work function and the anode require a high work function? 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.
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 &quot;unnatural&quot; configuration is inevitably higher.