Lower hole-injection barrier between pentacene and a 1-hexadecanethiol-modified gold substrate with a lowered work function

We used ultraviolet photoemission spectroscopy (UPS) to study the hole injection barrier at the interface between pentacene and a gold surface treated with 1-hexadecanethiol (HDT). Through these UPS in-situ experiments, we found that the energy barrier between HDT-modified gold and pentacene was 0.74 eV. This energy barrier was 0.11 eV smaller than that between bare gold and pentacene, despite the work function of HDT-modified gold being 1.08 eV lower than that of bare gold. This result does not follow the typical trend, whereby decreasing the work function of a metal increases the energy barrier. The observed behavior can be explained by two factors. First, the bare gold substrate exhibited a large interface dipole, whereas the HDT-modified gold did not. And second, pentacene on the HDT-modified gold substrate had a lower ionization energy than pentacene on bare gold. This finding can be explained in terms of the polarization energy related to the more crystalline structure of pentacene on the HDT-modified gold substrate, which was established by X-ray diffraction analysis. For comparison, we also measured the injection barrier between the amorphous organic semiconductor, N,N′-diphenyl-N,N′bis(1-naphthyl-1,1′-biphenyl-4,4′-diamine (α-NPD)), and HDT-modified gold.