Structure and electronics of donor-acceptor blends

[ES]: Las mezclas moleculares donor-aceptor se utilizan en dispositivos electrónicos de inyección, como las células solares orgánicas. Las interfaces metal/molécla en estos dispositivos son clave para su funcionamiento, ya que el alineamiento de los niveles moleculares con respecto al nivel de Fermi del metal determina las barreras de inyección/extracción de electrones y huecos. En este trabajo se estudian la estructura y el alineamiento energético de monocapas moleculares que son mezclas de dos moléculas conjugadas de pequeño tamaño, una dadora y otra aceptora. Las moléculas utilizadas son la ftalocianina de cobre (CuPc) y el pentaceno (PEN) -moléculas dadoras bien conocidas por su uso en dispositivos- y sus homólogas fluoradas, FCuPc y PFP, que son moléculas aceptoras. Las combinaciones dador-aceptor estudiadas son CuPc+PFP y PEN+FCuPc, y los substratos utilizados, las superficies (111) de los metales Ag y Cu. Del trabajo anterior, se sabe que para las monocapas mixtas sobre Au(111), los niveles moleculares están anclados al nivel de vacío, lo cual produce un desplazamiento característicos en energía de ligadura de los niveles de las moléculas (dador a menor energía de ligadura, aceptor a mayor energía de ligadura) en función de la concentración relativa de moléculas dadoras y aceptoras en cotacto con el metal. El anclaje al nivel de vacío es de esperar en sistemas poco interactivos, en los que o existe transferncia de carga. Por eso las mezclas sobre Au(111) siguen de cerca los desplazamientos arriba mencionados. Sin embargo, cuando utilizamos substratos más reactivos, como Ag(111) y Cu(111), existirán efectos adicionales que debemos tener en cuenta y observaremos que los desplazamientos de los niveles moleculares divergen de aquéllos predichos por el modelo de anclaje al nivel de vacío. Este trabajo busca entender estas ciscrepancias y explicarlas con efectos como la transferencia de carga y los cambios en la distancia de absorción.[EN]: The growing potential of organic layers as the main component in electronic devices has led to a boom in the study of organic metal interfaces. Both the structure of organic overlayer and its electronic energy level alignment with the metal are defining characteristics of the system. In this thesis we explore both structural and electronic properties of organic monolayers on single crystal surfaces. Copper phthalocyanine (CuPc) and pentacene (PEN) are organic semiconductors well known for their successful integration into optoelectronic devices. These, along with their uorinated counterparts fluorinated copper phthalocyanine (FCuPc) and perfluoropentacene (PFP) are molecules employed in this work. In Chapter 3 a purely structural study of the dislocation network formed at low temperature in the PFP/Ag(111) system is made. Theoretical calculations based on the experimental structural parameters obtained by scanning tunneling microscopy (STM) rationalize the generation of dislocations by taking into account the effects of intermolecular, molecule-substrate and substrate mediated interactions. The remaining two chapters deal with the main focus of this dissertation, this being the structure (Chapter 4) and electronics (Chapter 5) of monolayer donor-acceptor blends made up of CuPc and PEN (donors) and their fluorinated counterparts FCuPc and PFP (acceptors) on noble metal (111) substrates. STM measurements show that, regardless of the substrate, the molecular blends|CuPc+PFP and FCuPc+PEN|share a common structure that tends to maximize donor-acceptor contact, due to the leading role of hydrogen bonding in the self-assembly process. The new environment of the molecules is found to produce changes in the electronic levels of the molecules, thereby a ecting the energy level alignment between the organic layer and the metal. Photoelectron spectroscopy using X-ray and UV radiation allows probing the molecules' occupied levels, whereas X-ray absorption allows probing the unoccupied levels. For weakly interacting systems a simple vacuum level pinning scenario surfaces to explain the changes in the molecular levels. For more strongly interacting systems the effects of charge transfer and of conformational changes (assessed by X-ray standing wave measurements) must be taken into account in order to understand the changes taking place in these mixed layers and provide a complete picture of the energy level alignment in such systems.Peer Reviewe

Spectroscopic fingerprints of work-function-controlled phthalocyanine charging on metal surfaces

The electronic character of a π-conjugated molecular overlayer on a metal surface can change from semiconducting to metallic, depending on how molecular orbitals arrange with respect to the electrodes Fermi level. Molecular level alignment is thus a key property that strongly influences the performance of organic-based devices. In this work, we report how the electronic level alignment of copper phthalocyanines on metal surfaces can be tailored by controlling the substrate work function. We even show the way to finely tune it for one fixed phthalocyanine-metal combination without the need to intercalate substrate-functionalizing buffer layers. Instead, the work function is trimmed by appropriate design of the phthalocyanines supramolecular environment, such that charge transfer into empty molecular levels can be triggered across the metal-organic interface. These intriguing observations are the outcome of a powerful combination of surface-sensitive electron spectroscopies, which further reveal a number of characteristic spectroscopic fingerprints of a lifted LUMO degeneracy associated with the partial phthalocyanine charging.This work was supported by the Spanish Grant Nos. MAT2010-21156-C03-01, PIB2010US-00652, and the Basque Government Grant No. IT-621-13. We acknowledge funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant No. 226716.Peer Reviewe

Self-assembly of bicomponent molecular monolayers: Adsorption height changes and their consequences

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al.Codeposition of two molecular species [copper phtalocyanine (CuPc, donor) and perfluoropentacene (PFP, acceptor)] on noble metal (111) surfaces leads to the self-assembly of an ordered mixed layer with a maximized donor-acceptor contact area. The main driving force behind this arrangement is assumed to be the intermolecular C-Hâ̄F hydrogen-bond interactions. Such interactions would be maximized for a coplanar molecular arrangement. However, precise measurement of molecule-substrate distances in the molecular mixture reveals significantly larger adsorption heights for PFP than for CuPc. Most surprisingly, instead of leveling to increase hydrogen-bond interactions, the height difference is enhanced in the blends as compared to the heights found in single-component CuPc and PFP layers. The increased height of PFP in mixed layers points to an overall reduced interaction with the underlying substrate, and its influence on electronic properties like the interface dipole is investigated through work function measurements. © 2014 American Physical Society.This work was supported by the Spanish Grants No. MAT2010-21156-C03-01 and-C03-03, as well as No. PIB2010US-00652, and by the Basque Government (Grant No. IT-621-13). D. G. O. acknowledges support from the European Union under Grant No. FP7-PEOPLE-2010-IOF-271909. We acknowledge funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under Grant No. 226716.Peer Reviewe

Multi-component organic layers on metal substrates

Increasingly high hopes are being placed on organic semiconductors for a variety of applications. Progress along these lines, however, requires the design and growth of increasingly complex systems with well-defined structural and electronic properties. These issues have been studied and reviewed extensively in single-component layers, but the focus is gradually shifting towards more complex and functional multi-component assemblies such as donor–acceptor networks. These blends show different properties from those of the corresponding single-component layers, and the understanding on how these properties depend on the different supramolecular environment of multi-component assemblies is crucial for the advancement of organic devices. Here, our understanding of two-dimensional multi-component layers on solid substrates is reviewed. Regarding the structure, the driving forces behind the self-assembly of these systems are described. Regarding the electronic properties, recent insights into how these are affected as the molecule's supramolecular environment changes are explained. Key information for the design and controlled growth of complex, functional multicomponent structures by self-assembly is summarized.This work was supported by Spanish Grant No. MAT2013-46593-C6-4-P, and Basque Government Grant No. IT-621-13.Peer reviewe

Effect of surface reconstruction on the photoemission cross-section of the Au(111) surface state

The photoemission cross-section of the Shockley surface state of Au(111) is studied over a wide range of photon energies both experimentally and theoretically. The measurements are fully understood based on the theoretical analysis within a one-step abinitio theory of photoemission. The constant initial state spectrum is shown to be very sensitive to the structure of the topmost atomic layer. A maximum in the constant initial spectrum at 60eV is identified as a fingerprint of the Au(111) surface reconstruction. © 2012 IOP Publishing Ltd.This work was supported by the Spanish Ministerio de Ciencia e Innovación (Grants No. FIS2009-08355, FIS2010-19609-C02-02, MAT2010-21156-C03-01 and through the Research Program Ramón y Cajal) and the Basque Government (IT-257-07). The SRC is funded by the National Science Foundation (Award No. DMR-0084402).Peer Reviewe

Supramolecular environment-dependent electronic properties of metal-organic interfaces

Model donor-acceptor assemblies at metal-organic interfaces, namely, fluorinated copper-phthalocyanines (F 16CuPC) and pentacene (PEN) assemblies on the Au(111) surface, have been the focus of the present study. A full picture of the crystallographic and electronic structure of PEN and F 16CuPC monolayers as well as of their 1:1 binary mixture on the Au(111) surface has been explored by means of a variety of surface-sensitive techniques, providing important information on the intermolecular and molecule-substrate interactions governing the self-assembly process. A long-range ordered donor-acceptor network is observed for the mixture as a result of the greatly enhanced intermolecular interaction via C-F···H-C hydrogen bonds. Interestingly, the new supramolecular structure involves changes in the electronic structure of the molecular components. In particular, the strongest changes are observed at the C and F atoms of the F 16CuPc, as opposed to the F 16CuPc N, Cu, or PEN C atoms. With the aid of theoretical calculations, such effects are found to be at least partially related to an upward shift in energy of the F 16CuPc molecular orbitals, concomitant with a molecule-to-metal charge donation, not from the HOMO, but deeper lying orbitals. © 2012 American Chemical Society.This work was supported by the Spanish MICINN (MAT2010-21156-C03-01, C03-03, PIB2010US-00652) and the Basque Government (IT-257-07). The SRC is funded by the National Science Foundation (award no. DMR-0084402). B.P.D. thanks the ICTP (Trieste) for travel funding. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 226716. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007- 2013) through the Integrated Infrastructure Initiative “European Light Sources Activities − Synchrotrons and Free Electron Lasers” (grant agreement no. 226716).Peer Reviewe

Tailoring interactions in supramolecular networks by fluorination

Resumen del trabajo presentado al Symposium on Surface Science (3S), celebrado en Baqueira Beret, Lleida, Spain del 6 al 12 de marzo de 2011.Peer reviewe

Understanding energy-level alignment in donor-acceptor/metal interfaces from core-level shifts

The molecule/metal interface is the key element in charge injection devices. It can be generally defined by a monolayer-thick blend of donor and/or acceptor molecules in contact with a metal surface. Energy barriers for electron and hole injection are determined by the offset from HOMO (highest occupied) and LUMO (lowest unoccupied) molecular levels of this contact layer with respect to the Fermi level of the metal electrode. However, the HOMO and LUMO alignment is not easy to elucidate in complex multicomponent, molecule/metal systems. We demonstrate that core-level photoemission from donor-acceptor/metal interfaces can be used to straightforwardly and transparently assess molecular-level alignment. Systematic experiments in a variety of systems show characteristic binding energy shifts in core levels as a function of molecular donor/acceptor ratio, irrespective of the molecule or the metal. Such shifts reveal how the level alignment at the molecule/metal interface varies as a function of the donor-acceptor stoichiometry in the contact blend. © 2013 American Chemical Society.This work was supported by the Spanish MICINN (MAT2010-21156-C03-01 and -C03-03), the Basque Government (IT-621-13 and -578-13), European Research Council Advanced Grant DYNamo (ERC-2010-AdG Proposal No. 267374), and Spanish Grants FIS2010-21282-C02-01 and PIB2010US-00652. J.L.C. acknowledges financial support from the Mexican CONACyT program, D.J.M. from the Spanish “Juan de la Cierva” program (JCI-2010-08156), and D.G.O. from the European Union under FP7-PEOPLE-2010-IOF-271909.Peer Reviewe