Energy level alignment at the conjugated phenylenevinylene oligomer/metal interface

In this letter we report an investigation of the interfacial electronic str ucture formed by metals and conjugated oligomers using ultraviolet photoele ctron spectroscopy. Au and Ag were used as metal substrates for two five-ri ng phenylenevinylene oligomers: unsubstituted p-bis[(p-styryl)styryl]benzen e (P5V4) and the analogous oligomer with 2-methoxy-5-(2'-ethyl-hexyloxy) su bstitution on the central ring (MEH-P5V4). We found for all interfaces a lo wering of the energy levels of the organic overlayer by 0.4-1.2 eV. Remarka bly, this energy lowering, presumably due to interface dipole layers, was a lways such as to keep the hole injection barrier nearly constant and theref ore at most weakly sensitive to the work function of the metal or the ioniz ation potential of the oligomer. (C) 2000 American Institute of Physics. [S 0003-6951(00)01216-X].