Chemical and electronic structure of metal/organic interfaces

The chemistry, electronic structure and electron injection characteristics at interfaces formed between metals and organic semiconductors are crucial for the device performances of organic light emitting diodes (OLED) and other organic electronic devices. Metal and organic semiconductor interfaces are studied via X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IDES), and current-voltage (I-V) measurements.; We show that tris (8-hydroxyquinoline) aluminum (Alq ₃) and copper hexadecafluorophthalocyanine (F₁₆CuPc) undergo strong chemical interactions with Mg and Al, characterized by charge transfer and covalent bond formation. Au has been found to have no chemical reaction with the organic materials. In all the systems, diffusion of the metal atoms into the organic film broadens the metal-on-organic interface with respect to the organic-on-metal interface.; We study the electronic structures at metal/organic interfaces. We discussed the mechanisms that determine the energy level alignment at metal/organic interfaces. We study the origin and role of interfaces gap states at metal/organic interfaces. Chemical reaction and charge transfer between reactive metals and organic molecules generate new electronic states in the previous energy gaps of the organic materials. The gap states pin the Fermi level in the organic film close to the lowest unoccupied molecular orbital (LUMO) and therefore determine the energy level alignment at the metal/organic interface. For non-reactive metal atoms diffused in organic films, they dope the organic film by providing additional electronic states. This is convincingly demonstrated by p-type doping of F₁₆CuPC thin film by Au. Fermi level alignment is the main mechanism for energy level alignment in these cases.; We present a detailed study on the impact of deposition sequence on interface electronic properties, i.e. metal-on-organic vs. organic-on-metal. Four situations are discussed: in the presence of occupied gap states and Fermi level pinning; in the absence of occupied gap states; in the presence of an “insulating” layer created by metal diffusion into, and interaction with, the organic material; and, in the presence of non-ideal fabrication condition.; The phenomena and the underlying mechanisms that we discover in this work are general and should be applicable to most metal/organic interfaces.