Energetic disorder impacts energy-level alignment of alpha-sexithiophene on hydrogen-terminated silicon and silicon oxide

Abstract

The energy-level alignment at hybrid organic-inorganic interfaces is decisive for the performance of (opto-)electronic devices. We use ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS) to measure the energy-level alignment of vacuum-sublimed α-sexithiophene (6 T) thin films with HF-etched n-type Si(100) and with Si with a native oxide layer (SiOx). The 6 T thin films induce a small (<0.1 eV) downwards band bending into both substrates as shown by XPS. The well-ordered growth of 6 T on Si leads to a relatively narrow density of states (DOS) distribution of the highest occupied molecular orbital (HOMO) as shown by UPS. Furthermore, the Fermi-level comes to lie at rather mid-gap position and, consequently, no energy-level bending occurs in the 6 T layer. Structural disorder in the 6 T thin film on SiOx leads to a broad HOMO DOS distribution and to tailing states into the energy gap. Consequently, downwards energy-level bending (by around 0.20 eV) takes place in the 6 T layer.