Why does the cathode in an OLED require a low work function and the anode require a high work function?
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.