Hole Block Layer
This layer confines holes to the emitting layer.
Hole Injection Layer
An efficient hole injection layer facilitates electron injection from the anode to the organic layer.
Highest Occupied Molecular Orbital
An Orbital is the wave function of an electron that describes the place in which an electron with a certain probability is located. In a molecule there are different molecular orbitals which can be occupied with electrons. The HOMO is the occupied molecular orbital with the highest energy.
Hole Transport Layer
This layer transports the holes from the anode to the emitting layer.