ABSTRACT
The discovery of conjugated polymers has recently led to the development of high-efficiency organic light-emitting devices (OLEDs) [1,2], and many experimental and theoretical investigations have been undertaken to elucidate the underlying fundamental physical processes. Knowledge of the relative alignment of the energy levels at interfaces between organic materials and an injection electrode is crucial to an understanding of the device operation and, in particular, the physics of charge-carrier injection, transport, and radiative recombination, with the ultimate goal of improving electroluminescence (EL) efficiency. The alignment of the highest occupied molecular orbital (HOMO) of the organic materials of an OLED is usually estimated from the difference between the ionization potential (IP) of the polymer and the work function of the electrode, i.e., by using the SchottkyMott (SM) rule [3], which assumes that the energy levels correspond to a common vacuum level. This approximation thus neglects surface, interface, and charge transfer effects. The alignment of the lowest unoccupied molecular orbital (LUMO) is then estimated by adding the optical absorption edge energy to the HOMO energy, a procedure that neglects the exciton binding energy. This method for constructing OLED energy diagrams is a useful first approximation but can lead to significant errors. The applicability of the SM
rule is a topic of current debate. For instance, studies carried out on various conjugated polymers and small-molecule films using techniques such as ultraviolet photoemission spectroscopy (UPS) [4-9], internal photoemission [10,11], and scanning tunneling microscope (STM) spectroscopy [12-15] show clear evidence of significant deviations from the SM rule for organicmetal as well as for organic-organic interfaces. However, other reports of UPS studies [16,17] on thin films of precursor poly(p-phenylenevinylene) (PPV) or of poly(9,9-dioctylfluorene) (PFO) deposited on substrates with different work functions show that, at least in certain cases, the SM rule holds quite well and that the vacuum level misalignment at the polymerelectrode interface is only a few hundred meV.
