The interfaces of poly(p-phenylene vinylene) and fullerene derivatives with Al, LiF, and Al/LiF studied by secondary ion mass spectroscopy and x-ray photoelectron spectroscopy: Formation of AlF3 disproved

Two mutually exclusive mechanisms have been proposed to explain the improved electron injection by the insertion of a LiF layer between the metal cathode and the active organic layer of organic photoelectronic devices: the dipole and the doping mechanism. The possibility of the doping mechanism was studied by investigating the interface of $\mathrm{poly}\text{[2-}\mathrm{methoxy}\text{-5-(3′,7′}\mathrm{dimethyl}-\mathrm{octyloxyl}\text{)-1,4-}\mathrm{phenylenevinylene}]$ (MDMO-PPV) or ${\mathrm{1-(3-(methoxycarbonyl)propyl)-1-phenyl[6,6]C}}_{\mathrm{61}}$ (PCBM) with Al, LiF, or Al/LiF. In this mechanism, Li dopes the organic layer, after liberation via the reaction ${\mathrm{Al+3LiF\to AlF}}_{\mathrm{3}}\mathrm{+3Li}.$ If this reaction takes place, ${\mathrm{AlF}}_{\mathrm{3}}$ should be detectable at the surface. However, SIMS measurements showed that ${\mathrm{AlF}}_{\mathrm{3}}$ is not present at the Al/LiF/MDMO-PPV and Al/LiF/PCBM interfaces. This is evidence that the proposed reaction does not occur. Other evidence that the doping mechanism cannot be the general mechanism to explain the enhanced electron injection comes from the presence of LiF on both organic surfaces. XPS measurements indicate that there is a reaction of Al with the carboxylic oxygen of PCBM, and that a LiF layer between PCBM and Al prevents this reaction.