Chemical potential shifts at organic device electrodes induced by grafted monolayers

Charge injection from an electrode into an organic semiconductor is strongly dependent on the height of the energy barrier between the electrode workfunction and the energy level of the HOMO or LUMO molecular orbitals. In the case of the well-known emitter tris-(8-hydroxyquinoline) aluminium (Alq) on clean indium tin oxide (ITO), an energy barrier of about 1 eV hinders both negative and positive charge carriers to be injected from the oxide electrode. By grafting molecules with appropriate dipole moments on ITO we have constructed light-emitting diodes based on Alq to demonstrate that the ITO workfunction can be modified in such a way that holes and even electrons can be efficiently injected.