Substrate-Independent Energy-Level Pinning of an Organic Semiconductor Providing Versatile Hole-Injection Electrodes

Abstract

Tailor-made electrode work functions are indispensable to control energy-level offsets at the interfaces of (opto-)electronic devices. We show by means of photoelectron spectroscopy that several nanometer thick layers of the organic semiconductor 1,4,5,8,9,3,6,7,10, on virtually all substrates provide holeinjecting electrodes with work functions of around 5.60 eV. This substrate-independent energy-level alignment is due to a relatively large density of gap states in HAT-CN thin films, which is clearly visible in the photoemission data. Furthermore, this additional density of occupied states makes the wide-gap semiconductor thin films sufficiently conductive for electrode applications. Moreover, our study highlights a quite intriguing energy-level alignment scenario as the Fermi-level in HAT-CN thin films is located far from the midgap position, this is rather uncommon for undoped organic semiconductor thin films.

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

“…The HOMO-level of F 16 CuPc is centered at 2.04 eV BE and does not show a thickness-dependent shift. The VL shifts upwards with increasing F 16 CuPc coverage, which is, most likely, due to pinning of the F 16 CuPc LUMO-level at E F [51][52][53][54]. Interestingly, despite the absence of charge carriers, Fermi-level pinning can take place at organic-organic interfaces and leads to a potential drop over the contact layer and direct charge transfer from the second layer to the (metal) substrate [32].…”

“…to the comprehensive review article 'The Impact of Dipolar Layers on the Electronic Properties of Organic/Inorganic Hybrid Interfaces' by Zojer et al [40]. Furthermore, for all layers in figure 1 (with the exception of the chemisorbed monolayer), E F is rather at mid-gap position and energy-level and VL-shifts due to Fermi-level pinning at the frontier molecular orbitals and/or gap states [ [51][52][53][54] are not considered. Noteworthy, the density of gap or tailing states is usually too low to be detected with conventional UPS-setups [55,56].…”

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

“…The almost rigid shift of VL, valence and core levels is a tell-tale sign for energy-level bending, which in organic thin films is often due to tailing states of the frontier molecular orbital DOS [9,10]. Indeed, for thin 6 T coverages the HOMOonset almost reaches E F (figure 1(c)) and Fermi-level pinning can be expected [48]. As mentioned above, the general broadening of the DOS in the 6 T thin film on SiO x is, most likely, due to disorder, which can be due to the coexistence of domains with different polymorphs on the surface or due to local defects, e.g., molecules with a different inclination angle with respect to surrounding molecules.…”

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

“…[60][61][62] The downward energy-level bending is somewhat unexpected for an electron acceptor and due to a reversed charge transfer from the gap states to the substrate, that is, HATCN can be regarded as a Janus-faced acceptor-donor. [21] Figure 4. Energy-level diagrams of a) HATNA and b) HATCN thin films on ITO, Au(111) and MoO x , respectively.…”

“…Thin films of 1,4,5,8,9,12-hexaazatriphenylene-2,3,6,7,10,11-hexacarbonitrile (HATCN) are textbook examples of substrate-independent ELA, as the effective WF of HATCN thin films on virtually all substrates is around 5.60 eV. [21][22][23][24][25] For low and moderate substrate WFs, this has been attributed to the high EA of HATCN thin films (5.41 eV as measured by inverse photoemission, IPES). [26] However, on high WF substrates like MoO x (WF around 6.70 eV), [27] deposition of HATCN leads to a downward energy-level shift due to interfacial charge transfer from a large density of occupied gap states just below the LUMO level, which could be evidenced by ultraviolet photoelectron spectroscopy (UPS) and was attributed to the presence of chemical defects.…”