Theory of the charge-transport properties of naphthyl diamine used in organic light-emitting devices

The electronic structures of a prototypical electroluminescent molecule, N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB), with various extra charges (+6 to −6 atomic units), have been theoretically studied by means of the PM3 and ab initio molecular orbital theories as well as density functional theory in combination with a decomposition of the density of states. It was found that, under positive charging, the essential distribution feature of the molecular orbitals at constituent atoms in the NPB molecule can still favor carrier transport, but cannot do so under negative charging. By explaining the efficient hole-transporting property of NPB, the present study elucidates the potential of the theoretical approach for the selection of optimum carrier-transporting organic materials.