Transition Metal Oxides for Hole Injection in Organic Molecular Devices

摘要

Transition metal oxides (TMO) have recently generated considerable interest for their ability to enhance hole injection into typical hole transport materials. Applications in organic light emitting diodes or field effect transistors have been demonstrated. Hole injection enhancement has been linked to the high work function that is generally measured on TMO films. Concomitant to this property, TMO monomers (or trimers) have been found to have extremely high electron affinity (EA), and to behave as efficient acceptors (p-dopants) even in molecular materials with very deep valence states. As an example, molybdenum tri-oxide (MoO3) was found to p-dope N,N′- bis(1-naphtyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (-NPD), 4,4′- Bis(N-carbazolyl)-1,1′-biphenyl (CBP), which has an ionization energy (IE) of 6.2 eV. However, despite numerous reports related to the application of transition metal oxides in organic electronic devices, the underlying physical mechanisms pertaining to their electronic structure remain somewhat undefined. We report here recently published work that addresses this issue by carrying out ultraviolet and inverse photoemission spectroscopy (UPS and IPES) measurements of the electronic structure of MoO3 films formed under ultrahigh vacuum (UHV), and of their interfaces with a prototypical organic hole-transport material, i.e., N,N- di[(1-naphthyl)-N,N-diphenyl]-1,1-biphenyl-4 ,4- diamine (α-NPD).