• Inhalt ausklappen Inhalt einklappen Nanoscale Adv.: "Pentacene/perfluoropentacene bilayers on Au(111) and Cu(111): impact of organic–metal coupling strength on molecular structure formation†"Nanoscale Adv.: "Pentacene/perfluoropentacene bilayers on Au(111) and Cu(111): impact of organic–metal coupling strength on molecular structure formation†"

    Qi Wang, Jiacheng Yang, Antoni Franco-Cañellas, Christoph Bürker, Jens Niederhausen, Pierre Dombrowski, Felix Widdascheck,
    Tobias Breuer, Gregor Witte, Alexander Gerlach, Steffen Duhm and Frank Schreiber
    Nanoscale Adv., 3, 2598 - 2606 (2021), •Doi: 10.1039/d1na00040c

    As crucial element in organic opto-electronic devices, heterostructures are of pivotal importance. In this context, a comprehensive study of the properties on a simplified model system of a donor-acceptor (D-A) bilayer structure is presented, using ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and normal-incidence X-ray standing wave (NIXSW) measurements. Pentacene (PEN) as donor and perfluoropentacene (PFP) as acceptor material are chosen to produce bilayer structures on Au(111) and Cu(111) by sequential monolayer deposition of the two materials. By comparing the adsorption behavior of PEN/PFP bilayers on such weakly and strongly interacting substrates, it is found that: i) the adsorption distance of the first layer (PEN or PFP) indicates physisorption on Au(111), ii) the characteristics of the bilayer structure on Au(111) are (almost) independent of the deposition sequence, and hence, iii) in both cases a mixed bilayer is formed on the Au substrate. This is in striking contrast to PFP/PEN bilayers on Cu(111), where strong chemisorption pins PEN molecules to the metal surface and no intermixing is induced by subsequent PFP deposition. The results illustrate the strong tendency of PEN and PFP molecules to mix, which has important implications for the fabrication of PEN/PFP heterojunctions.

  • Inhalt ausklappen Inhalt einklappen ACS APPL. MATER. INTERFACES: "Charge Transfer Excitation and Asymmetric Energy Transfer at the Interface of Pentacene−Perfluoropentacene Heterostacks"ACS APPL. MATER. INTERFACES: "Charge Transfer Excitation and Asymmetric Energy Transfer at the Interface of Pentacene−Perfluoropentacene Heterostacks"

    Anna-Katharina Hansmann, Robin C. Döring, Andre Rinn, Steffen M. Giesen, Melanie Fey, Tobias Breuer, Robert Berger, Gregor Witte, and Sangam Chatterjee
    Appl. Mater Interfaces,13, 5284 - 5292 (2021) •Doi: 10.1021/acsami.0c16172

    High-performance solar cells demand efficient charge-carrier excitation, separation, and extraction. These requirements hold particularly true for molecular photovoltaics, where large exciton binding energies render charge separation challenging at their commonly complex donor–acceptor interface structure. Among others, charge-transfer (CT) states are considered to be important precursors for exciton dissociation and charge separation. However, the general nature of CT excitons and their formation pathways remain unclear. Layered quasiplanar crystalline molecular heterostructures of the prototypical donor–acceptor system pentacene–perfluoropentacene studied at cryogenic temperatures are a paramount model system to gain insights into the underlying physical mechanism. In particular, a detailed experiment–theory analysis on a layered heterojunction featuring perfluoropentacene in its π-stacked polymorph and pentacene in the Siegrist phase indicates that exciton diffusion in unitary films can influence the formation efficiency of CT excitons localized at internal interfaces for these conditions. The correlation of the structural characteristics, that is, the molecular arrangement at the interfaces, with their absorption and photoluminescence excitation spectra is consistent with exciton transfer from pentacene to the CT exciton state only, whereas no transfer of excitons from the perfluoropentacene is detected. Electronic structure calculations of the model systems and investigation of coupling matrix elements between the various electronic states involved suggest hampered exciton diffusion toward the internal interface in the perfluoropentacene films. The asymmetric energy landscape around an idealized internal donor–acceptor interface thus is identified as a reason for asymmetric energy transfer. Thus, long-range effects apparently can influence charge separation in crystalline molecular heterostructures, similar to band gap bowing, which is well established for inorganic pn-junctions.

  • Inhalt ausklappen Inhalt einklappen APPL. ELECTRON. MATER.: "B3N3‑Substituted Nanographene Molecules: Influence of Planarityon the Electronic Structure and Molecular Orientation in Thin Films"APPL. ELECTRON. MATER.: "B3N3‑Substituted Nanographene Molecules: Influence of Planarityon the Electronic Structure and Molecular Orientation in Thin Films"

    Katharina Greulich, Axel Belser, Daniel Bischof, Felix Widdascheck, Marie S. Sättele, Peter Grüninger, Holger F. Bettinger, Gregor Witte, Thomas Chassé, and Heiko Peisert
    APPL. ELECTRON. MATER, 3, 825-837 (2021), •Doi: 10.1021/acsaelm.0c00967

    BN-substituted nanographene molecules are currently the focus of interest because the substitution of C–C units by isoelectronic and isosteric BN units is a straightforward way of changing the electronic properties of nanographenes. Another parameter influencing the electronic structure, orientation, and growth mode of nanographene molecules is the planarity of the molecules. The electronic structure, orientation, and film growth of the related molecules B3N3-hexa-peri-hexabenzocoronene (BN-HBC), B3N3-hexabenzotriphenylen (BN-HBP), and B3N3-hexabenzotriphenylen-2H (BN-HBP-2H) on Au(111) have been studied by photoelectron spectroscopy (PES), X-ray absorption spectroscopy (XAS), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). XA spectra were simulated using time-dependent density functional theory (TDDFT). The calculation of C 1s excitation spectra allows the assignment of individual transitions and the examination of the degree of cross-linking between biphenyl units. It is shown that the planarity of the molecules distinctly affects the electronic structure, interface properties a well as growth in thin films.

  • Inhalt ausklappen Inhalt einklappen PCCP: "Selective Saturation of Step-Edges as a Tool to Control the Growth of Molecular Fibres"PCCP: "Selective Saturation of Step-Edges as a Tool to Control the Growth of Molecular Fibres"

    Maximilian Dreher and Gregor Witte
    PCCP, 23, 8023-8029 (2021) •Doi: 10.1039/D0CP06725C

    The concept of bottom-up self-organisation has become a promising alternative for structuring molecular materials, which is hardly accessible by conventional top-down approaches such as lithography due to their limited chemical robustness. While these materials often tend to form three-dimensional, crystalline islands or fibres upon film growth, the size and orientation of such fibres are mainly governed by appropriate preparation conditions as well as microscopic interactions at the interface with the supporting surface. Substrate surface defects such as vacancies or step-edges, which cannot be completely ruled out on real surfaces on the mesoscopic scale, can act as prefered nucleation sites for molecules that leads to parasitic film growth competing with their intrinsic alignment prevailing on an ideal surface. In the present study we demonstrate for the case of para-quaterphenyl (p-4P), that the presence of azimuthally disordered, fibres on Ag(111) surfaces can be understood as a superposition of step-mediated nucleation and the intrinsic epitaxial fibre growth on ideal surfaces. We validate the concept by purposely exposing the silver substrates briefly to oxygen or even ambient air to passivate the more reactive step-sites, which hampers subsequenty grown molecular films to nucleate at these step-edges. This yields a truly epitaxial alignment as well as an enlargement of the fibres present on the whole sample.

  • Inhalt ausklappen Inhalt einklappen EURJOC: "Synthesis and Molecular Properties of Partially Fluorinated DNTTs"EURJOC: "Synthesis and Molecular Properties of Partially Fluorinated DNTTs"

    Matthias W. Tripp, Daniel Bischof, Maximilian Dreher, Gregor Witte, and Ulrich Koert
    EurJOC, 1295-1304 (2021) •Doi: 10.1002/ejoc.202001635

    1,2,3,4-Tetrafluoro-dinaphtothienothiophene (F4DNTT)
    and 1,2,3,4,8,9,10,11-octafluoro-dinaphtothienothiophene (F8DNTT)
    were synthesized via bisthiomethyl alkene intermediates which were
    accessible by McMurry coupling or Wittig olefination of partially
    fluorinated naphthalene precursors. DFT-based electronic structure
    calculations, near edge X-ray absorption fine structure (NEXAFS)
    spectroscopy and UV/Vis measurements were used for
    HOMO/LUMO gap determination and to analyze the electronic
    structures of the partially fluorinated DNTTs. Reduced exciton binding was observed in thin films.

  • Inhalt ausklappen Inhalt einklappen CHEM SCI: "Engineering of TMDC–OSC hybrid interfaces: the thermodynamics of unitary and mixed acene monolayers on MoS2"CHEM SCI: "Engineering of TMDC–OSC hybrid interfaces: the thermodynamics of unitary and mixed acene monolayers on MoS2"

    Stefan R. Kachel, Pierre-Martin Dombrowski, Tobias Breuer, J. Michael Gottfried and Gregor Witte
    CHEM SCI, 12, 2575–2585 (2021) •Doi: 10.1039/d0sc05633b

    Hybrid systems of two-dimensional (2D) materials such as transition metal dichalcogenides (TMDCs) and organic semiconductors (OSCs) have become subject of great interest for future device architectures. Although OSC–TMDC hybrid systems have been used in first device demonstrations, the precise preparation of ultra-thin OSC films on TMDCs has not been addressed. Due to the weak van der Waals interaction between TMDCs and OSCs, this requires precise knowledge of the thermodynamics at hand. Here, we use temperature-programmed desorption (TPD) and Monte Carlo (MC) simulations of TPD traces to characterize the desorption kinetics of pentacene (PEN) and perfluoropentacene (PFP) on MoS2 as a model system for OSCs on TMDCs. We show that the monolayers of PEN and PFP are thermally stabilized compared to their multilayers, which allows preparation of nominal monolayers by selective desorption of multilayers. This stabilization is, however, caused by entropy due to a high molecular mobility rather than an enhanced molecule–substrate bond. Consequently, the nominal monolayers are not densely packed films. Molecular mobility can be suppressed in mixed monolayers of PEN and PFP that, due to intermolecular attraction, form highly ordered films as shown by scanning tunneling microscopy. Although this reduces the entropic stabilization, the intermolecular attraction further stabilizes mixed films.

  • Inhalt ausklappen Inhalt einklappen OrgEl: "Prepare with care: Low contact resistance of pentacene Field-Effect transistors with clean and oxidized gold electrodes"OrgEl: "Prepare with care: Low contact resistance of pentacene Field-Effect transistors with clean and oxidized gold electrodes"

    Yurii Radiev, Felix Widdascheck, Michael Göbel, Alrun Aline Hauke, Gregor Witte
    OrgEl, 89, 106030 (2021) •Doi: 10.1016/j.orgel.2020.106030

    In recent years, a number of researchers have voiced concerns regarding the increasing amount of publications which report record performance characteristics of organic field-effect transistors (OFETs). These devices often make use of elaborate architectures, ranging from surface treatment by self-assembled monolayers to contact-limited doping, injection layer introduction etc. While these techniques are claimed to improve the device performance, they often result in poorly defined device interfaces, such as that of the organic semiconductor with the dielectric and electrode surfaces, which render a rigorous theoretical description of the interface effects close to impossible. Furthermore, the effects of ambient exposure of prepared devices are often neglected during the analysis. In this work we introduce a completely high vacuum-based manufacturing and electrical characterization of OFETs using commercially available gold bottom-contact bottom-gate FET structures, which enables a rigorous exclusion of any air contact of the devices. Using the example of the prototypical and widely studied organic semiconductor pentacene, we compare various initial cleaning procedures of the FET-structures prior to the vacuum deposition of pentacene films on the device characteristics and complement the electrical in situ analysis by ex situ AFM measurements of the pentacene film morphology. We demonstrate that O2 plasma cleaning with subsequent thermal annealing under vacuum conditions results in well-defined interfaces of the pentacene film with clean surface of gold electrodes, which exhibit a remarkably low width-normalized contact resistance of 1.19 kΩcm. Additionally, we show that the formation of gold oxide on top of the electrodes as a result of O2 plasma cleaning without subsequent heating is beneficial for the charge carrier injection, resulting in a contact resistance of only 0.16 kΩcm – to the best of our knowledge, one of the lowest yet reported values for pentacene-gold systems. In addition, the comparative rigorous vacuum-based processing and analysis allows in particular to study also the effects of controlled exposure to ambient air – even for a short time – as well as aging under different storage conditions on the device performance.

  • Inhalt ausklappen Inhalt einklappen MATER. CHEM. C: "Stability of organic thin-film transistors based on ultrathin films of dinaphtho[2,3-b:2’,3’f]thieno[3,2-b]thiophene (DNTT)"MATER. CHEM. C: "Stability of organic thin-film transistors based on ultrathin films of dinaphtho[2,3-b:2’,3’f]thieno[3,2-b]thiophene (DNTT)"

    Rachana Acharya, Darius Günder, Tobias Breuer, Guido Schmitz, Hagen Klauk and Gregor Witte
    J. Mater. Chem. C, 9, 270-280 (2021), •Doi: 10.1039/D0TC04554C

    Organic thin-film transistors (TFTs) based on ultrathin semiconductor films are potentially useful as highly sensitive physical, chemical or biological sensors and may also help in the development of a better understanding of the relations between structural and charge-transport characteristics in thin films of organic semiconductors. A particularly promising small-molecule organic semiconductor is dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (DNTT). However, it was recently reported that ultrathin DNTT films spontaneously undergo dramatic morphological changes within minutes after deposition that lead to the disaggregation of the initially closed (or at least connected) single-monolayer films into disconnected multilayer islands. Here, we investigate how this spontaneous structural reconfiguration affects the characteristics of TFTs based on ultrathin DNTT films and explore the extent to which it can be prevented by cryogenic cooling or in-situ encapsulation. We fabricated inverted coplanar TFTs with a hybrid aluminum oxide / alkylphosphonic acid self-assembled monolayer gate dielectric and vacuum-deposited DNTT films with nominal thicknesses of 2.5 or 25 nm. Using atomic force microscopy (AFM) we monitored the spontaneous changes in the DNTT morphology in a quasi-continuous manner over a period of 12 hours after deposition. The charge-carrier mobility of the ultrathin DNTT TFTs was found to decrease rapidly, while the mobility of the TFTs with the thicker DNTT films is far more stable. We also found that the initial closed-monolayer morphology of the ultrathin DNTT films is preserved when the substrates are cooled to cryogenic temperatures immediately after the DNTT deposition, but that the morphological changes resume upon returning the substrates to room temperature. Furthermore, we fabricated TFTs in which the ultrathin DNTT films were encapsulated in-situ with a vacuum-deposited film of polytetrafluoroethylene, C60 or titanyl phthalocyanine immediately following the DNTT deposition and found that the encapsulation decelerates the structural reorganization of the ultrathin DNTT films and the concurrent degradation of the carrier mobility.