Structural and Electronic Properties of Interface between Metal and HAT-CN Molecule

Using {\em ab initio} density functional theory, we study the equilibrium structures and electronic properties of interface between metal surface and organic molecule, called 1,4,5,8,9,11- hexaazatriphenylene-hexacarbonitrile (HAT-CN), which is known for a strong electron acceptor and used as an efficient hole injection layer for organic light emitting diodes. We consider Ca (111) and Cu (111) surfaces, on which HAT-CN is placed. The geometrical and electronic properties of the interfaces are significantly different when HAT-CN is on one metal surface than when it is on the other. We find that the nitrogen atoms at the edge of HAT-CN have stronger interaction on metal surfaces than other atoms in the molecule. Due to the resultant charge transfer between the HAT-CN molecule and the metal surface, the surface dipole is formed at the interface. We discuss the dependence of the work function on the surface dipole formation and compare with the experimental results.