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Spectroscopic insight with nanoscale resolution on model photovoltaic systems

Final Report Summary - NANOPV (Spectroscopic insight with nanoscale resolution on model photovoltaic systems)

Organic p-n semiconducting interfaces are envisaged as a promising alternative to the more expensive inorganic photovoltaic cells, and this project aimed at the synthesis and characterization of several of those model systems. Among the targeted systems we find covalently linked organic heterostructures with well separated moieties with differentiated electronic properties, as well as bicomponent molecular blends comprising both donor and acceptor semiconducting molecules held together in ordered structures by non-covalent interactions.
For the former, adequate routes for the covalent coupling of appropriate precursor molecules have been first studied, with particular emphasis on precursors that could potentially render graphene nanoribbons. In particular, two chemical approaches have been followed.
On the one hand, molecular precursors that are linked on the surface by Ullman coupling are being studied. These subsequently lead to the formation of graphene nanoribbons by dehydrogenation. Previously published results from Fasel, Muellen et al. with the precursor DBBA have been reproduced, and by using different precursors we have also been able to synthesize wider nanoribbons with a consequently narrower bandgap (ACS Nano 2013, 7, 6123). In addition to bringing the bandgap into technologically more useful magnitudes, the new precursor is designed so as to allow easy further functionalization (e.g. doping by electron accepting substituents) and also to be chemically compatible with DBBA, thereby setting the base for a variety of GNR heterostructures that we have subsequently characterized in great detail (manuscript is currently under review in Nature Nanotechnology).
On the other hand we have studied molecular precursors that are to form radicals and allow polymerization by isomerization reactions (Bergman cyclization) and that could later afford graphene nanoribbons by dehydrogenation. While the chemical changes observed on the surface do not correspond to those anticipated, the complexity of these unexpected chemical reactions, together with the detailed visualization of the covalent bond rearrangements, have provided unprecedented insight into surface-supported chemical reactions and the associated manuscript has been accepted for publication in Science (Science 2013, 340, 1434). Furthermore, the gained understanding of the underlying chemistry has been used for an optimized design of molecular precursors allowing the synthesis of polymeric acelylene-derivatives. While this was demonstrated on Au(111) substrates (Nano Letters 2014, 14, 2251), much more complex chemical scenarios occur on Ag(100), including alkyne homo-coupling reactions and dissipation-related stabilization of intermediate species (manuscript in preparation).
For both approaches, various different substrates have been studied, including different facets of metal single crystals and also insulating thin films such as NaCl films (although the latter with little success).
Regarding the synthesis and characterization of molecular p-n heterojunctions in mixtures of separate donor and acceptor molecules, the knowledge gained from studies prior to the beginning of this fellowship have been complemented with a long list of systematically varied systems. These include combinations of fluorinated (acceptor molecules) and non-fluorinated (donor molecules) pentacene and copper-phthalocyanine on Au, Ag and Cu (111) surfaces. Apart from specific deviations in certain systems that have been relatively well understood, an important trend showing donor-acceptor blend stoichiometry-dependent charge injection barriers has been found that does not only provide a better understanding of molecular mixtures, but also a handle on crucial parameters for their functionality, such as the interfacial energy level alignment. This work has been published in ACS Nano 2013, 7, 6914.
Detailed crystalline-electronic structure correlations have additionally been studied in more detail combining synchrotron based X-ray Standing Waves with STM, as is the dependence of the molecule adsorbate induced interfacial dipole with the molecule-substrate distance. These results have been published in Phys. Rev. Lett. 2014, 112, 117602.
Summarizing, the most important finalized results so far include the synthesis of atomically precise graphene nanoribbons wider than those previously reported and with an associated narrower bandgap, the growth of nanoribbon heterostructures, together with their detailed spectroscopic characterization, the unprecedented direct visualization of chemical bond structure modification in surface-supported chemical reactions, its use for precursor optimization and the creation of conjugated organic polymers on surfaces, the understanding of stoichiometry-dependent interfacial energy alignments in donor-acceptor blends on metal surfaces, as well as important structure-property relations between adsorption height and interface dipoles, consequently with a direct impact on interfacial energy level alignment.
Forthcoming publications in relatively advanced stages include the detailed study of spectroscopic fingerprints of phthalocyanine charging through supramolecular environment controlled work function variations, or the analysis of new enediyne precursor cyclization reactions to optimize our understanding and control of that new reaction pathway for the synthesis of functional materials directly on surfaces.
The finalized and forthcoming results all have important implications for the development and understanding of organic-based electronic components, thus pointing to a significant potential impact of this project for the improvement of these novel technologies.
Besides, from a completely different perspective, this project has served to train Dimas G. de Oteyza in several interesting scientific aspects, as is the use of state-of-the-art, low-temperature scanning probe microscopies and spectroscopies, or in the preparation of insulating buffer layers on metallic crystals and their subsequent use as substrates for other materials atop. This knowledge, gained within a leading group in the field as is that of Mike Crommie in the University of California at Berkeley, will definitely mark Dimas’s future scientific activities and therefore help in the development of European scientific output.