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In this paper, we investigate by photoemission the electronic properties of an organic/organic interface consisting in a strong electron acceptor hexaazatriphenylene-hexacarbonitrile (HATCN) deposited on a physisorbed N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (α-NPD) monolayer on Ag(111). At the first HATCN deposition steps (∼1 monolayer), the sample work function increases by 1.05 eV and the hole injection barrier (HIB) in the pre-adsorbed α-NPD monolayer is lowered by 0.65 eV. This results from HATCN diffusion to the silver surface through the α-NPD monolayer. Furthermore, this HATCN monolayer is proposed to form a compact chemisorbed monolayer, with a different structural arrangement than that observed on pristine Ag(111). In a second step, the additional deposited HATCN start growing on top of the α-NPD layer, and Fermi-level pinning, associated with the formation of HATCN negative polarons, is identified at the HATCN/α-NPD interface. Finally, HATCN is deposited on a α-NPD multilayer film. In this case, HATCN is also found to diffuse to the silver surface giving a similar HIB value in the α-NPD multilayer. This work demonstrates the importance of studying in detail the formation of organic heterostructures to understand the interplay between growth mode and electronic properties. [ABSTRACT FROM AUTHOR]

We demonstrate that graphene-based transparent and conductive thin films (GTCFs), fabricated by thermal reduction of graphite oxide, have very similar electronic and structural properties as highly oriented pyrolytic graphite (HOPG). Electron spectroscopy results suggest that the GTCFs are also semi-metallic and that the individual graphene sheets of the film are predominantly oriented parallel to the substrate plane. These films may therefore be considered as a technologically relevant analogue to HOPG electrodes, which cannot be easily processed into thin films. [ABSTRACT FROM AUTHOR]

We investigated the energy levels at organic heterojunctions comprising the donor diindenoperylene (DIP) on top of the acceptor C60 with photoelectron spectroscopy. The intermolecular interaction is weak as evidenced on a moderate work function electrode by a small interface dipole of 0.15 eV and flat energy levels on both sides of the junction. When a high work function electrode is used, the DIP levels become Fermi-level pinned and an electric field drops within the C60 layer underneath. The electric field distribution within an organic opto-electronic device may thus be adjusted by employing interfacial energy level pinning, even at physisorptive organic/organic interfaces. [ABSTRACT FROM AUTHOR]

Contacts between two molecular organic semiconductors [p-sexiphenyl (6P) and pentacene] and conducting polymers (CPs) were investigated with photoemission spectroscopy. The dependence of the hole injection barrier (HIB) at 6P/CP interfaces on substrate work function ([lowercase_phi_synonym]) exhibited a transition from almost Schottky-Mott limit-like behavior to Fermi-level pinning. For pentacene, no significant variation of the HIB as function of [lowercase_phi_synonym] was observed, despite the large range of [lowercase_phi_synonym] spanned by the CPs (4.4–5.9 eV). The results on contacts with CPs are compared to those with metals, where none of the two limiting cases for HIBs as a function of [lowercase_phi_synonym] was observed. [ABSTRACT FROM AUTHOR]

Ultraviolet photoemission spectroscopy is used to determine the energy level alignment at interfaces between three electroactive conjugated organic molecular materials, i.e., N,N'-bis-(1-naphthyl)-N,N'-diphenyll-1,1-biphenyll-4,4' -diamine; para-sexiphenyl; pentacene, and two high work function electrode materials, i.e., gold and poly(3,4-ethylenedioxythiophene)/ poly(styrenesulfonate). Although both electrode surfaces have a similar work function (∼ 5 eV), the hole injection barrier and the interfacial dipole barrier are found to be significantly smaller for all the interfaces formed on the polymer as compared to the metal. This important and very general result is linked to one of the basic mechanisms that control molecular level alignment at interfaces with metals, i.e., the reduction of the electronic surface dipole contribution to the metal work function by adsorbed molecules. [ABSTRACT FROM AUTHOR]

Highlights: [•] Electrical and morphological characterization of flexible OTFT devices under applied bending-stress is reported. [•] Bending-stress modifies the lateral spacing between individual pentacene crystallites. [•] The deterioration of the metal electrodes is found to be responsible for irreversibilities in the device characteristics. [ABSTRACT FROM AUTHOR]

Ultraviolet photoelectron spectroscopy was used to demonstrate organic/metal-contact charge injection barrier tuning by exploiting the orientation-dependent work function φ of a molecular acceptor [hexaazatriphenylene-hexanitrile (HATCN)] interlayer on Ag(111). The work function [φ of a flat-lying HATCN monolayer on Ag was 4.6 eV (similar to a pristine Ag electrode), whereas a layer of edge-on HATCN on Ag exhibited φ of 5.5 eV (comparable to a pristine Au electrode). The hole-injection barriers (HIBs) between HATCN-modified electrodes and the organic semiconductors tris(8-hydroxyquinoline)aluminum (Alq3) and N,N'-bis(1-naphtyhl)-N,N'-diphenyl-1,1'-biphenyl-4.4'-diamine (α-NPD) were reduced by more than 1 eV compared to pristine Ag and Au electrodes. Noteworthy, the HIBs determined with the flat-lying HATCN interlayer were lower than those obtained for pristine Ag substrates (φ of both electrodes is 4.6 eV), and the H1Bs with the edge-on HATCN on Ag were lower than those found for pristine Au (φ of both electrodes ca. 5.4 eV). This shows that acceptor interlayers are beneficial for charge injection in electronic devices even when the molecularly modified electrode φ is comparable to that of a pristine metal surface. It is argued that the molecularly modified electrodes are electronically more rigid than their pristine metal counterparts, i.e., the electron spill-out at the organic-terminated surface is less pronounced compared to Ag and Au surfaces. [ABSTRACT FROM AUTHOR]

Abstract: Ultraviolet photoelectron spectroscopy was used to investigate the energy level alignment at interfaces between the organic semiconductor poly(3-hexylthiophene) (P3HT) and poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDT:PSS) electrodes. The thickness of P3HT was varied between sub-monolayer and multilayer (20nm) coverage. The work function of PEDT:PSS decreased linearly from 4.90 to 4.35eV as function of P3HT coverage up to a full monolayer, and remained constant for larger thickness. In contrast, the low binding energy onset of the P3HT valence band shifted abruptly by 0.15eV towards higher binding energy between monolayer and multilayer. These results evidence the formation of an interface dipole confined to the intimate P3HT/PEDT:PSS contact, in full analogy to the already established model for small molecule/electrode interfaces. It is thus necessary to account for interface dipoles at polymer/electrode interfaces, because measurements on thick films may be compromised by additional band bending throughout the semiconductor polymer bulk. [Copyright &y& Elsevier]

Abstract: We report that solution-based treatment of Cu electrodes with strong electron acceptor molecules significantly decreases the contact resistance towards organic semiconductors, which is advantageous for applications such as organic field-effect transistors (OFETs). Spin-coating solutions of tetracyanoquinodimethane (TCNQ) or tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) onto Cu electrodes results in strongly chemisorbed acceptor (sub-)monolayers, which increase the electrode work function from 4.5eV (bare Cu) up to 5.2eV (F4-TCNQ-treated) even in air, as evidenced by X-ray photoelectron spectroscopy and photoelectron yield spectroscopy. The use of such modified electrodes in flexible OFETs with poly(3-hexylthiophene)-dithienyltetrafluorobenzene (P3HT-TFT) as semiconductor lead to a twofold increase of the on-current in the saturation regime and a decrease of the threshold voltage from −20V (bare Cu) to −7V (F4-TCNQ-treated). These results confirm that this simple solution-based process is viable for lowering organic/metal contact resistances in organic electronic devices. [Copyright &y& Elsevier]