[Show abstract][Hide abstract] ABSTRACT: This photoemission study shows that the work function (Phi) of indium-tin-oxide (ITO) can be increased from 4.2 up to 6.5 eV upon the deposition of the molecular electron acceptors tetrafluoro-tetracyanoquinodimethane (F4TCNQ) and hexaazatriphenylene-hexacarbonitrile (HATCN). The evolution of sample Phi and the hole injection barrier upon subsequent deposition of the hole transport material N,N'-bis(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (alpha-NPD) was studied for different acceptor precoverages of ITO, corresponding to different initial Phi values. When Phi of the acceptor covered substrate exceeds a critical value Phi(crit), the highest occupied molecular level of multilayer alpha-NPD is found to be pinned 0.5 eV below the Fermi level (E-F). Noteworthy, Phi(crit) is found at 5.2 eV, which is 0.4 eV higher than expected for alpha-NPD (4.8 eV), and vacuum level alignment does not apply even before E-F-pinning sets in. An electrostatic model that accounts for nonuniformity of the substrate at acceptor submonolayer coverages and the associated local work function changes explains the origin of "delayed" E-F-pinning.
The Journal of Physical Chemistry C 10/2013; 117:22285-22289. DOI:10.1021/jp401919z · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: We introduce the material hexaazatriphenylene hexacarbonitrile (HATCN) as electron conducting window layer for separating the photoactive region from the cathode in organic p-i-n type solar cells. HATCN has a wide band gap of 3.3 eV and is thus transparent in the visible range of the solar spectrum. Its electrical properties can be tuned by means of molecular n-doping which leads to an increase of electron conductivity by several orders of magnitude up to 2.2 x 10(-4) S/cm. However, an application in photovoltaic devices is restrained by its exceptionally high electron affinity, estimated 4.8 eV, which introduces an electron injection barrier to the photoactive acceptor material C-60. Here, we present a strategy to remove this barrier by means of introducing doped and undoped C-60 intermediate layers, thus demonstrating the importance of energy level matching in a multiple layer structure and the advantages of Fermi level control by doping. (C) 2010 Elsevier B.V. All rights reserved.
Solar Energy Materials and Solar Cells 12/2011; 95:927-932. DOI:10.1016/j.solmat.2010.11.024 · 5.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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 HIBs 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.
[Show abstract][Hide abstract] ABSTRACT: We investigate how the thermodynamic equilibrium is reached when a strong electron acceptor molecule (hexaaza-triphenylene-hexacarbonitrile, HATCN) is deposited on Ag(111) precovered with a physisorbed tris(8-hydroxyquinolinato)aluminum (Alq3) monolayer. Photoemission measurements reveal that equilibrium is achieved via a charge transfer from the metal to the HATCN layer through the Alq3 monolayer. The work function increase observed upon HATCN adsorption is explained by the formation of an interface dipole resulting from the metal-to-acceptor charge transfer and an electric field induced reorientation of the Alq3 molecule dipoles within the interlayer.
The Journal of Physical Chemistry C 09/2011; 115(35):17503-17507. DOI:10.1021/jp2053376 · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A new K-region functionalized pyrene is presented which was used as a building block for the straightforward synthesis of hexaaryl[a,c,fg,j,l,op]tetracene via fourfold Stille coupling and subsequent cyclodehydrogenation. Electronic properties and crystal structures are provided and reveal a saddle conformation for the curved hexaarylated tetracenes.
[Show abstract][Hide abstract] ABSTRACT: For the synthesis of an ortho-dithienylpyrene, a K-region bromination of pyrene was developed which enabled the first reported, non-statistical asymmetric functionalization of pyrene at the 4, 5, 9 and 10 positions. Crystal structures, optical and electronic properties and FET characteristics have been investigated.
Chemical Communications 06/2011; 47(24):6960-2. DOI:10.1039/c1cc11827g · 6.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In order to address the interplay of hydrogen bonding, dipolar interactions, and metal coordination, we have investigated the two-dimensional mono- and bicomponent self-assembly of three closely related diaminotriazine-based molecular building blocks and a complementary perylenetetracarboxylic diimide by means of scanning tunneling microscopy. The simplest molecular species, bis-diaminotriazine-benzene, only interacts via hydrogen bonds and forms a unique supramolecular pattern on the Au(111) surface. For the two related molecular species, which exhibit in addition to hydrogen bonding also dipolar interactions and metal coordination, the number of distinct supramolecular structures increases dramatically with the number of possible interaction channels. Deposition together with the complementary perylene species, however, always results in a single well-defined supramolecular arrangement of molecules. A detailed analysis of the observed mono- and bicomponent assemblies allows shedding light on the hierarchy of the competing interactions, with important implications for the fabrication of surface-supported supramolecular networks by design.
[Show abstract][Hide abstract] ABSTRACT: Atomically thin sheets of sp(2)-hybridized carbon--graphene--have enormous potential for applications in future electronic devices. Particularly promising are nanostructured (sub)units of graphene, the electronic properties of which can be tuned by changing the spatial extent or the specific edge termination of the carbon network. Processability and precise tailoring of graphene-derived structures are, however, still major obstacles in developing applications; both bottom-up and top-down routes are presently under investigation in attempts to overcome this limitation. Here, we propose a surface chemical route that allows for the atomically precise fabrication of tailored nanographenes from polyphenylene precursors. The cyclodehydrogenation of a prototypical polyphenylene on Cu(111) is studied using scanning tunnelling microscopy and density functional theory. We find that the thermally induced cyclodehydrogenation proceeds via several intermediate steps, two of which can be stabilized on the surface, yielding unprecedented insight into a dehydrogenative intramolecular aryl-aryl coupling reaction.
[Show abstract][Hide abstract] ABSTRACT: Functionalized polcyclic aromatic hydrocarbons (PAHs) are an interesting class of molecules in which the electronic state of the graphene-like hydrocarbon part is tuned by the functional group. Searching for new types of donor and acceptor molecules, a set of new PAHs has recently been investigated experimentally using ultraviolet photoelectron spectroscopy (UPS). In this work, the electronic structure of the PAHs is studied numerically with the help of B3LYP hybrid density functionals. Using the DELTA-SCF method, electron binding energies have been determined which affirm, specify and complement the UPS data. Symmetry properties of molecular orbitals are analyzed for a categorization and an estimate of the related signal strength. While SIGMA-like orbitals are difficult to detect in UPS spectra of condensed film, calculation provides a detailed insight into the hidden parts of the electronic structure of donor and acceptor molecules. In addition, a diffuse basis set (6-311++G**) was used to calculate electron affinity and LUMO eigenvalues. The calculated electron affinity (EA) provides a classification of the donor/acceptor properties of the studied molecules. Coronene-hexaone shows a high EA, comparable to TCNQ, which is a well-known classical acceptor. Calculated HOMO-LUMO gaps using the related eigenvalues have a good agreement with the experimental lowest excitation energies. TD-DFT also accurately predicts the measured optical gap. Comment: Journal of Molecular Spectroscopy, 20 pages, 2 figures, 3 tables, 48 references
[Show abstract][Hide abstract] ABSTRACT: We present a simple yet potentially universally applicable method for the solid–solid transfer of organic materials under ambient conditions for the fabrication of organic field-effect transistors. Thermal annealing of sprinkled powders of organic semiconductors on gold patterned SiOx surfaces yielded functional transistors with some of the characteristics comparable to those of solution-processed devices.
[Show abstract][Hide abstract] ABSTRACT: A series of five polymers containing different benzodithiophene isomers copolymerized with alkylated dithiophene have been synthesized and characterized in terms of their semiconducting properties. Because of the different bonding geometry of the benzodithiophene monomers, a varying degree of curvature is introduced into the polymer backbone chain. The influence of this curvature on the solubility, the electronic levels, the morphology in a film, and the charge carrier mobility in organic field-effect transistors has been investigated. It turns out that an increased degree of curvature improves the solubility, but decreases the order in the film. As a result, the polymer with an intermediate degree of curvature yields the highest charge-carrier mobilities. These findings shall serve as guideline for the rational design of other semiconducting polymers.
Chemistry of Materials 09/2010; 22(18). DOI:10.1021/cm101577j · 8.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A larger fused monomer in a polythiophene chain which can improve stability and device performance has been reported. The largest building block in a soluble polythiophene so far is tetrathienoacene (P3). This polymer is very stable toward oxidation in an organic field-effect transistor and also shows high charge-carrier mobilities. The polymerization using dibromodithiophene as comonomer was performed using Pd(PPh3)4 as catalyst in toluene. The reaction mixture was heated for 40 minutes in the microwave to 150 °C with a power density of 70 W/mL. In a field-effect transistor, a good device characteristic, which is low contact resistance and low hysteresis, have been measured. The field-effect mobility does not yet reach values needed for applications but is likely to be achieved after more processing optimizations.
[Show abstract][Hide abstract] ABSTRACT: Organic semiconductors are expected to play an important role in low-cost and energy-efficient electronic products, especially transistors. Our concept to improve the active materials is to introduce curvature into the polymer backbone. The beneficial contribution of the curvature is shown by a series of five isomeric polymers which demonstrate that an intermediate degree of curvature is the optimum compromise between good solubility and high order in the film. In this way, a polythiophene is developed that is easily synthesized, well soluble, exhibits high charge-carrier mobility using mild annealing conditions, and is stable under operation. When processed on a flexible substrate, it shows average mobilities as high as 0.5 cm2V-1s-1 .
Proceedings of SPIE - The International Society for Optical Engineering 08/2010; · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
[Show abstract][Hide abstract] ABSTRACT: Searching for new pi-conjugated charge-transfer systems, the electronic structure of a new acceptor-donor pair derived from coronene (C(24)H(12)) was investigated by ultraviolet photoelectron spectroscopy (UPS). The acceptor coronene-hexaone (C(24)H(6)O(6), in the following abbreviated as COHON) and the donor hexamethoxycoronene (C(30)H(24)O(6), abbreviated as HMC) were adsorbed as pure and mixed phases on gold substrates. At low coverage, COHON adsorption leads to the appearance of a charge-transfer induced interface state 1.75 eV below the Fermi energy. At multilayer coverage the photoemission intensity of the interface state drops and the valence spectrum of neutral COHON appears. The sample work function decreases from 5.3 eV (clean Au) to 4.8 eV (monolayer) followed by an increase to 5.6 eV (multilayer). The formation of a significant interface dipole due to charge-transfer at the metal-organic interface is possibly accompanied by a change in molecular orientation. HMC on Au exhibits no interface state and the sample work function decreases monotonically to ca. 4.8 eV (multilayer). The UPS spectra of individual donor and acceptor multilayers show good agreement with density functional theory modeling. In donor/acceptor mixed films the photoemission signal of the donor (acceptor) shifts to higher (lower) binding energy. This trend is predicted by the calculation and is anticipated when charge is transferred from donor to acceptor. We propose that mixed films of COHON and HMC constitute a weak charge-transfer system.
[Show abstract][Hide abstract] ABSTRACT: The adsorption of the molecular acceptor hexaazatriphenylene-hexacarbonitrile on Ag(111) was investigated as function of layer density. We find that the orientation of the first molecular layer changes from a face-on to an edge-on conformation depending on layer density, facilitated through specific interactions of the peripheral molecular cyano groups with the metal. This is accompanied by a rehybridization of molecular and metal electronic states, which significantly modifies the interface and surface electronic properties, as rationalized by theoretical modeling.