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 -sexithiophene (6T) thin films with HF-etched n-type Si(100) and with Si with a native oxide layer (SiOx). The 6T thin films induce a small (<0.1 eV) downwards band bending into both substrates as shown by XPS. The well-ordered growth of 6T 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 6T layer. Structural disorder in the 6T 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 6T layer.

Advanced characterization of organic–metal and organic–organic interfaces: from photoelectron spectroscopy data to energy-level diagrams

“…For the heterostructure, the energy levels of the annealed TiOPc bilayer and these of subsequently deposited F 16 CuPc layers with QCM thicknesses of 4 Å and 48 Å are shown. In our ELDs we use the peak maxima for valence and core levels, although in literature ELDs often the onset of the HOMO-level is used, as it is the relevant parameter for charge transport and injection [101][102][103]. However, the determination of the onset can be ambiguous, in particular if spectral signatures from different layers overlap, e.g.…”

Chemical Defects and Energetic Disorder Impact the Energy‐Level Alignment of Functionalized Hexaazatriphenylene Thin Films

“…[ 21 ] In the HATNA spectrum of the thin film deposited with a low rate, the onset of the HOMO peak is at ≈2 eV BE and the peak is significantly broader ( σ of 870 meV) than the HOMO peak of HATCN and such a broad HOMO DOS points to a highly disordered thin film. [ 39–41 ] In contrast to the HATCN spectrum, no gap states are visible in the HATNA spectrum, which is a first hint that the additional phenyl rings are actually stabilizing the molecular structure. Moreover, gap states can be induced into the HATNA thin film by increasing the thermal energy during vacuum‐sublimation (high rate spectrum in Figure 2a) and this spectrum and the impact of the increased evaporation speed on the HATCN spectrum will be discussed further later.…”

The ionization energy of α-sexithiophene and p-sexiphenyl in 2D and 3D thin films grown on silicon oxide surfaces

“…The estimated PE values were 2.03 eV and 2.12 eV for the a-6T and p-6P thin films, respectively, which were clearly larger than that calculated for the pentacene thin films. The influence of the PYS measurements under air conditions on the IE and PE of the samples is probably insignificant because the IE values of the a-6T and p-6P thin films obtained via PYS under ambient air conditions were comparable to those obtained for the a-6T (IE = 4.8-5.0 eV 6,[70][71][72] ) and p-6P (IE = 5.6-5.7 eV 59,73,74 ) thin films with standing molecular arrangements measured via surface-sensitive UPS under ultrahigh vacuum conditions. Therefore, the differences in the PE between the pentacene thin films and the a-6T thin films and between pentacene thin films and the p-6P thin films can evidently exist.…”