HBL

Hole Block Layer

This layer confines holes to the emitting layer.

HIL

Hole Injection Layer

An efficient hole injection layer facilitates electron injection from the anode to the organic layer.

HOMO

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. 

HTL

Hole Transport Layer

This layer transports the holes from the anode to the emitting layer.