Study of the MDMO-PPV/metal interface and PCBM/metal interface by electroabsorption spectroscopy

Abstract

AbstractIn this work, we study the workfunction alignment of organic compounds and electrodes evaporated on top of these organic materials by means of electroabsorption (EA). The organic materials of this study are poly[2-methoxy-5-(3',7'-dimethyloctyloxyl)]-1,4-phenylene vinylene (MDMO-PPV) and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM). The electrodes are Al, Au, LiF/Au and LiF/Au, the latter two with a 1 nm thick layer of LiF, as well as with co-evaporation of LiF and the metal. In case of the MDMO-PPV, LiF enhances the built-in potential and thus reduces the electron injection barrier. Equal built-in potentials for a co-evaporated LiF:metal contact suggest doping is happening at the MDMO-PPV/metal interface. Unlike predicted by the Au and Al workfunction value, the built-in potentials for the PCBM/Al and PCBM/Au interface are equal. This suggests that Fermi level pinning occurs at these interfaces. The presence of a thin LiF layer screens the charge transfer from the metal to the PCBM.