Hybrid, organic and bio-materialsO
Computational modelling of organic semiconductors: from the quantum world to actual devices
Modelling organic semiconductors plays a crucial role for advancing organic electronics. By now the used methods have become exceedingly divers, which calls for a platform where scientists from the various involved areas can discuss their findings and link their expertise in hither to unexplored ways.
Using various types of advanced simulation approaches, tremendous insight into the properties of organic semiconductor materials and their application in devices has been generated. The great variety of the encountered problems requires the use of a very diverse pool of techniques ranging from highly-correlated quantum-chemistry approaches and band structure calculations (in many cases including many-body corrections) to more macroscopic techniques including molecular dynamics, coarse-grained, or micro-electrostatic modelling. For understanding the dynamics of charges and excitons or to analyze the formation of heterogeneous phases, statistical approaches including Monte-Carlo techniques are typically applied and, for understanding devices at their full level of complexity, drift-diffusion based calculations become necessary.
Recently, there has been a strong drive to combine these approaches in a hierarchical fashion, e.g., to use the results of molecular-dynamics modelling on complex structures as input for the quantum-mechanical simulation of electronic or optical properties. Beyond that, first attempts to establish tightly integrated multi-scale schemes are under way.
As a consequence, involved scientists typically have very diverse scientific backgrounds. This calls for the establishment of a platform for exchanging ideas, recent insights, and also the knowledge on specific computational tools. To aid in that, this symposium
(i) aims at providing an as comprehensive as possible overview of the strengths and limitations of modelling approaches used for describing organic semiconductors including the latest directly relevant methodological developments;
(ii) it will portray recent efforts in hierarchical and truly multi-scale modelling of these materials; and
(iii) above all, the symposium will provide a possibility for showcasing the insight into the intrinsic processes in organic semiconductors and devices that can be gained from computational studies.
Hot topics to be covered by the symposium:
spanning the length-scales:
- highly-correlated quantum-chemical and post density-functional theory methods
- quantum-mechanical techniques used for simulating complex structures
- molecular dynamics and coarse-graining applied to disordered and heterogeneous structures
- Monte Carlo techniques for simulating dynamics and growth
- simulation of device-structures at their full level of complexity by drift-diffusion based approaches
- hierarchical combination of simulations at multiple length-scales and true multi-scale modelling
phenomena in the focus of interest include (but are not limited to):
- understanding (charge-transfer) exciton formation/dissociation at hetero-interfaces
- modelling carrier injection/abstraction at electrodes
- quantifying the impact of disorder
- simulating dynamic processes from fs to minutes
- applying multi-scale approaches to charge-transport and exciton diffusion
- modelling film growth and phase dynamics
- understanding the impact of film heterogeneities on device performance
Invited speakers will be asked to submit feature articles as contributions to a special issue of Advanced Functional Materials. These will, naturally, be subject to strict peer reviewing.
Members of scientific committee (confirmed):
- Jean-Marie André, University of Namur, Belgium
- Jean-Luc Brédas, Georgia Institute of Technology, Atlanta, USA
- Gian Paolo Brivio, Universitá di Milano-Bicocca, Milano, Italy
- Paulette Clancy, Cornell University, Ithaca, USA
- Jerome Cornil, University of Mons, Belgium
- Claudia Draxl, Humboldt-Universität zu Berlin, Germany
- Dago de Leeuw, Max Planck Institute for Polymer Research, Mainz, Germany
- Elisa Molinari, CNRNano, Modena, Italy
- Mark Ratner, Northwestern University, Evanston, USA
- Zhigang Shuai, Tsinghua University, PR China
- Nobuo Ueno, Chiba University, Chiba, Japan
Invited speakers (confirmed):
- Denis Andrienko, Max Planck Institute for Polymer Research, Germany
- David Beljonne, Université de Mons, Belgium
- Clemence Corminboeuf, EPFL Lausanne, Switzerland
- Reinder Coehoorn, Philips Research Laboratories, The Netherlands
- Gianaurelio Cuniberti, TU Dresden, Germany
- Kristen A. Fichthorn, Penn State University, USA
- Geoffrey Hutchison, University of Pittsburgh, USA
- Hiroyuki Ishii, University of Tsukuba, Japan
- Jan Anton Koster, Rijksuniversiteit Groningen, The Netherlands
- Lingyi Meng, Xiamen University, PR China
- Jeff B. Neaton, Lawrence Berkeley National Lab, USA
- Chad Risko, Georgia Institute of Technology, USA
- Alice Ruini, Università degli Studi di Modena e Reggio Emilia, Italy
- Alexander L. Shluger, University College London, Great Britain
- Zoltán G. Soos, Princeton University, USA
- Alexandre Tkatchenko, Fritz Haber Institute of the Max Planck Society, Germany
- Claudio Zannoni, Universitá di Bologna, Italy
Institute of Solid State Physics
Graz University of Technology
Phone: +43 316 873 8475
Fax: +43 316 873 8466
Department of Materials and Interfaces
Weizmann Institute of Science
Phone: +972 8 9344993
Fax: +972 8 9344138
Institut für Physik, Humboldt-Universität zu Berlin
Phone: +49 30 2093 7537
Fax: +49 30 2093 7443
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Modelling non-covalent interactions : Egbert Zojer
Authors : Alexandre Tkatchenko
Affiliations : Fritz-Haber-Institut der MPG, Berlin, Germany
Resume : Van der Waals (vdW) interactions are ubiquitous in molecules and condensed matter, and play a crucial role in determining the structure, stability, and function for a wide variety of systems. The accurate prediction of these interactions from first principles is a substantial challenge because they are inherently quantum mechanical phenomena that arise from correlations between many electrons within a given system. We discuss the recently developed efficient method [1,2] that combines quantum and classical electrodynamics and accurately describes the nonadditive many-body vdW energy contributions arising from interactions that cannot be modeled by an effective pairwise approach. It is demonstrated that such contributions can significantly affect the behavior of biological (DNA), chemical (molecular crystals), and condensed (bulk, layered materials) systems. In most of these cases it is found that collective vdW interactions play a noticeable, if not crucial role, not just for quantitative values but also for the qualitative behavior [3,4].  A. Tkatchenko and M. Scheffler, Phys. Rev. Lett. 102, 073005 (2009).  A. Tkatchenko, R. A. DiStasio Jr., R. Car, and M. Scheffler, Phys. Rev. Lett. 108, 236402 (2012).  R. A. DiStasio Jr., O. A. von Lilienfeld, and A. Tkatchenko, Proc. Natl. Acad. Sci. USA 109, 14791 (2012).  V. V. Gobre and A. Tkatchenko, Nature Commun. 4, 2341 (2013).
Authors : J. X. Cantuaria, R. Lelis-Sousa
Affiliations : Departamento de Ciências Naturais, Licenciatura em Física, Universidade Federal do Tocantins, Campus de Araguaína, Rua Paraguai S/N, CEP 77824-838, Araguaína, Brazil
Resume : New devices based on organic materials have received a great deal of attention in the literature because of their potential advantages over inorganic semiconductors. Their interesting properties are largely determined by the active layer composed of organic semiconductor. Among the notable characteristics of organic devices, we can mention good flexibility combined with the possibility of covering large areas, low production cost, light weight and tuning of the electronic properties through gap engineering. Currently, pentacene and thiophene derivatives are of particular interest and have been used on fabrication of efficients devices . Although significant advances have been made in the understanding and control of fabrication of organic devices, many problems have not yet been resolved. One of the main problems which has puzzled many researchers is the efficiencies required for commercial applications. It was recently reported the synthesis of two new semiconductor  which are oxidation resistant and also exhibit high mobility: the DNTT and the DNSS. These crystals are formed by the junction of acene and thiophene rings and appears promising as good candidates for the active layer in electronic devices. Since these semiconductors was discovered recently, very little is known about their electronic properties. Here, we present a investigation of the structural and electronic properties of these new crystals, using first-principles method based on Density Functional Theory (DFT) as implemented in the Quantum-Espresso code . Theoretical modelling organic materials is a difficult task, since it requires a proper description of intermolecular long-range interactions and the correct accounting of such interactios it is important for a correct theoretical description of these systems. So that, we have use a standard DFT but with a semiempirical addition of dispersive forces ( or van der Waals forces (VDW)) to standard PBE functional. Here, we discuss about the importance of the inclusion of VDW correction to correct description of structural (atomic positons and crystal cell) and electronic properties of DNTT and DNSS.
Authors : Nicola Ferri (1), Robert A. DiStasio Jr. (2), Roberto Car (2), Alexandre Tkatchenko (1), and Matthias Scheffler (1)
Affiliations : (1) Theory Department, Fritz-Haber-institut der MPG, Faradayweg 4-6, 14195, Berlin, Germany; (2) Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA
Resume : The long-range van der Waals (vdW) energy is only a tiny part (0.001%) of the total energy, hence it is typically assumed to have a minor influence on the electronic properties. Although the vdW-DF functional includes the effects of non-local correlation energy on the electronic structure , a significant part of its energy arises from short distances. Here, we present a fully self-consistent (SC) implementation of the long-range Tkatchenko-Scheffler (TS) density functional , including its extensions to surfaces . The analysis of SC-TS effects on electron density differences for atomic and molecular dimers reveals quantitative agreement with correlated densities obtained from "gold standard" coupled-cluster quantum-chemical calculations. In agreement with previous work , we find a very small overall contribution from self-consistency in the structure and stability of vdW- bound molecular complexes. However, long-range vdW interactions turn out to significantly affect electronic properties of coinage metal (111) surfaces, leading to an increase of up to 0.3 eV in the workfunction in agreement with experiments. Furthermore, vdW interactions are also found to visibly influence workfunctions in hybrid organic/metal interfaces, changing Pauli push-back and charge transfer contributions.  T. Thonhauser et. al., Phys. Rev. B 76, 125112 (2007).  A. Tkatchenko and M. Scheffler, Phys. Rev. Lett. 102, 073005 (2009).  Ruiz et. al. Phys. Rev. Lett. 108, 146103 (2012).
Many-Body Perturbation-Theory : Leeor Kronik
Authors : Jeffrey B. Neaton
Affiliations : Department of Physics, University of California, Berkeley Molecular Foundry, Lawrence Berkeley National Laboratory Kavli Center for Energy Nanosciences at Berkeley
Resume : Organic semiconductors are a highly tunable, diverse class of cheap-to-process materials promising for next-generation optoelectronics, for example solar cells. Further development of new efficient and durable organic materials for such applications requires new intuition that links molecular-scale morphology to underlying excited-state properties and phenomena. Here, I will discuss the use of first-principles density functional theory and many-body perturbation theory within the GW approximation and the Bethe-Salpeter equation approach for computing and understanding spectroscopic properties of selected organic semiconductor crystals, including acenes from benzene to hexacene; PTCDA; perfloropentecene; and TIPS-pentacene. For both gas-phase and crystals, our quantitative calculations agree well with transport gaps extracted from photoemission and inverse photoemission data, and with measured polarization-dependent optical absorption spectra. Introducing a new analysis, we elucidate the nature of low-lying solid-state singlet and triplet excitons, which have significant binding energies and charge-transfer character in these systems, and assess the effects of dynamic disorder associated with room-temperature structural fluctuations. In addition, we rationalize trends in predicted singlet and triplet excitation energies in the context of recently proposed mechanisms for singlet fission. Insights into new materials, as well as implications for recent spectroscopic experiments, are discussed. This work is supported by the Department of Energy.
Authors : Ivan Duchemin (1), Carina Faber (2), Paul Boulanger (2), Xavier Blase (2,3)
Affiliations : (1) CEA-Grenoble/INAC/SP2M/L_Sim, UJF Grenoble; (2) Institut Neel CNRS Grenoble; (3) UJF Grenoble
Resume : Initially developed in the mid-eighties for inorganic semiconductors, the so-called GW and Bethe-Salpeter (BSE) formalisms, have been recently shown to yield electronic and optical (excitonic) properties of bulk and gas phase organic systems with remarkable accuracy. We will focus on optical excitations in organic systems, including the case of charge-transfer excitations in organic dyes and donor-acceptor complexes, with a discussion on the role of hot excitations. We will fiurther discuss the cyanines family case, for which TDDFT calculations systematically fail in providing accurate results, while BSE calculations yield an excellent agreement with high-level coupled-cluster calculations. Upcoming challenges, such as the development of specific embedding techniques, will conclude this presentation. The selected calculations have been performed with a recently developed Gaussian-basis GW and BSE package, the Fiesta code.  "Resonant hot charge-transfer excitations in fullerene-porphyrin complexes: a many-body Bethe-Salpeter study", I. Duchemin and X. Blase, Phys. Rev. B 87, 245412 (2013).  "Many-body Green's function study of coumarins for dye-sensitized solar cells", C. Faber, and al., Phys. Rev. B,86, 155315 (2012).  "Short-range to long-range charge-transfer excitations in the zincbacteriochlorin-bacteriochlorin complex: A Bethe-Salpeter study", I. Duchemin , T. Deutsch and X. Blase, Phys. Rev. Lett. 109, 167801 (2012).
Authors : Noa Marom
Affiliations : Department of Physics and Engineering Physics, Tulane University, New Orleans, LA 70118, USA
Resume : Organic photovoltaics are attractive for large area, low cost applications and for flexible modules. However, their relatively low efficiency leaves much to be desired. Insight from computation may help improve device performance by designing new organic semiconductors and ordered nanostructured interfaces. It is important to obtain an accurate description of the electronic structure of organic semiconductors, including the fundamental gaps and absolute positions of the donor highest occupied molecular orbital (HOMO) and the acceptor lowest unoccupied molecular orbital (LUMO) at the interface. This requires going beyond ground state density functional theory (DFT). Many-body perturbation theory is often used for this purpose within the GW approximation, where G is the one particle Green function and W is the dynamically screened Coulomb interaction. Typically, GW calculations are performed as a non-self-consistent perturbative correction to DFT eigenvalues, known as G0W0. The predictive power of G0W0 is limited by a strong dependence on the DFT starting point. Self-interaction errors (SIE), spurious charge transfer, and incorrect ordering and hybridization of molecular orbitals may propagate from the DFT level to G0W0. These issues may be addressed by judiciously choosing a hybrid DFT starting point or by going beyond G0W0 to a higher level of self-consistency. Here, this is demonstrated for prototypical organic semiconductors, such as phthalocyanines and anhydrides.
Authors : Alice Ruini
Affiliations : Dipartimento di Scienze Fisiche, Informatiche e Matematiche, Universita' di Modena e Reggio Emilia and Istituto CNR-NANO-S3, Modena, Italy
Resume : We here present results on C-based systems, namely organic conjugated polymers and graphene-based nanostructures. Both systems offer an ideal scenario for the study of electronic and excitonic confinement, being composed of quasi-one-dimensional structures. In the case of organic polymers, we mainly address the electronic, transport and optical properties of PPV-based systems: the effect of solid-state chain packing, of fluorine-functionalization and the presence of keto-related defects are investigated, mainly by means of ab initio calculations based on the many-body perturbation theory. Very recent results on graphene-based polymers will be also presented also in combination to cutting-edge experiments, with special emphasis on the optical properties of nanoribbons with selected edge shapes on a metal substrate. For such structures, we also investigated, by means of an hybrid (ab-initio semiempirical) approach, the role of edge covalent functionalization as well as the effect of pi-pi coupling. Our whole study suggests that (and how) controlling aggregation, functionalization and defects can provide tunable parameters in view of the design of efficient electronic and optoelectronic devices.
DFT applications and challenges : Jeffrey Neaton
Authors : Clemence Corminboeuf, Riccardo Petraglia, Eric Bremond, Peter Tentscher, Stephan Steinmann
Affiliations : Laboratory for Computational Molecular Design, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
Resume : Our research focuses on the realization of efficient and accurate computational methods capable of simulating π-conjugated systems relevant to the field of organic electronics.[1-4] The apparent simplicity of the chemical composition of π-conjugated systems contrasts the complexity of their electronic structure, where the most commonly used framework suffer from obvious failures. The talk provides an overview of our recent activity, which includes addressing lingering difficulties in describing energies and geometries of systems relevant to the field of organic electronics.[1-4] Emphasis will be placed on improved DFT and SCCDFTB-based treatments of charged radical oligomers (i.e., charge carrier precursor), organic charge-transfer complexes, large-scale stacks of oligomers,[3,5,6] as well as exciton coupling between oligomeric chains. We will also describe strategies to fine-tune the specific intermolecular interactions and orientations between π-conjugated moiety to achieve better charge transport.[5,6]  Steinmann, S. N.; and Corminboeuf, C. J. Chem. Theory Comput. 2011, 7, 3567.  Petraglia, R.; Corminboeuf, C. J. Chem. Theory Comput. 2013, 34, 2242.  Steinmann, S. N.; Corminboeuf, C. J. Chem. Theory Comput. 2012, 8 4305.  Steinmann, S. N.; Piemontesi, C.; Delachat, A.; Corminboeuf, C. J. Chem. Theory Comput. 2012, 8 1629.  Frauenrath, H. et al. ACS Nano, 2013, 7, 8498.  Frauenrath, H. et al. submitted to Nature Materials.
Authors : Kristian Baruël Ørnsø, Juan Maria Garcia-Lastra and Kristian Sommer Thygesen
Affiliations : Center for Atomic-scale Materials Design, Department of Physics, Technical University of Denmark
Resume : The search for sustainable energy sources has been intensified during the past years and Dye Sensitized Solar Cells (DSSC) have since the emergence of the first efficient system in 1991 received extensive attention due to the promising nature of these in terms of cost efficiency and flexibility. In DSSCs the ability of dye molecules to strongly absorb visible light is exploited and commonly, either porphyrins or ruthenium based dyes are used combined with the I-/I_3- redox pair as electrolyte in an acetonitrile solvent. The advantage of the porphyrin based dyes are the large absorption of visible light and the straight-forward customizability by introducing side groups. We exploit this feature by presenting the calculated frontier energy levels, orbitals, and optical gaps for more than 4000 systematically functionalized porphyrin dye candidates. Based on this we investigate trends in the selective tuning of energy levels and orbital shapes and estimate a (loss-less) DSSC level alignment quality of the candidate molecules. Furthermore, we use the constructed database to propose new DSSC setups and use Molecular Dynamics to investigate the regeneration of the porphyrin dyes.
Authors : G. Volonakis (1), L. Tsetseris (2), S. Logothetidis (1)
Affiliations : (1) Lab for Thin Films, Nanosystems and Nanometrology, Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece; (2) Department of Physics, National Technical University of Athens, Athens, Greece
Resume : Over the last years, the power conversion efficiency of organic photovoltaics (OPVs) is continuously rising and has recently reached values larger than 11%. However, exposure of OPVs to ambient conditions is known to significantly deteriorate the performance of the devices. Yet the atomic-scale details and mechanisms that govern this degradation remain unclear. Here we employ first-principles calculations based on the density-functional theory to (a) reveal the favorable defective configurations formed when O2 and H2O molecules intrude the molecular crystals of prototype OPV semiconductors P3HT and PCBM and (b) to find the effects of such impurities on the electronic properties of the materials. Specifically, molecular H2O is found to physisorb intact with minimal effects on the electronic properties. On the other hand, our results show that O2 can either physisorb intact or may chemisorb and dissociate on PCBM and P3HT. We thus identify the preferable configurations, in agreement with pertinent experimental data. These conformations are found to create states inside the band gaps, which can act as charge carrier traps (deep and shallow) or as charge recombination centers and consequently are expected to have detrimental effects on the performance of the materials. The reported defective configurations are consistent with available reports on oxidation of both materials. These results help clarify the role of O2 and H2O impurities on the degradation mechanisms in OPVs.
Authors : David A. Egger (1,2), Shira Weissman (1), Sivan Refaely-Abramson (1), Sahar Sharifzadeh (3), Matthias Dauth (4), Roi Baer (5), Stephan Kümmel (4), Jeffrey B. Neaton (3), Egbert Zojer (2), Leeor Kronik (1)
Affiliations : (1) Weizmann Institute of Science, Israel; (2) Graz University of Technology, Austria; (3) Lawrence Berkeley National Laboratory, USA; (4) University of Bayreuth, Germany; (5) Hebrew University of Jerusalem, Israel
Resume : Density functional theory with optimally-tuned range-separated hybrid (OT-RSH) functionals has been recently suggested [Phys. Rev. Lett. 109, 226405 (2012)] as a non-empirical approach to accurately predict the outer-valence electronic structure of organic molecules. The OT-RSH scheme relies on separating the inter-electron interaction into short- and long-range, and tuning the remaining parameters in the density functional from first-principles conditions, without recourse to fitting to experimental data. Here, we provide a quantitative evaluation of the OT-RSH approach and examine its performance in predicting the outer-valence electron spectra of the prototypical gas-phase aromatic rings benzene, pyridine and pyrimidine. For a range of up to several eV away from the frontier orbital energies, we find that the outer-valence electronic structure obtained from the OT-RSH method agrees very well (typically within 0.1-0.2 eV) with both experimental photoemission data from the literature and our many-body perturbation theory data in the GW approximation. The sole exception found is a high-symmetry orbital that is particular to small aromatic rings and occurs relatively deep inside the valence state manifold. We conclude that the OT-RSH approach is an efficient device for reliable electronic-structure calculations of finite systems, a feature we find to prevail also for more complex and practically relevant molecules such as terpyrimidinethiol and copper-phthalocyanine.
Authors : C.A. Rozzi (1), M. Amato, A. Rubio (2), and E. Molinari (1,3)
Affiliations : (1) Istituto Nanoscienze - CNR, Centro S3, 41125 Modena, Italy; (2) Nano-Bio Spectroscopy Group and ETSF Scientific Development Centre, Universidad del País Vasco, Centro de Física de Materiales CSIC-UPV/EHU-MPC and DIPC, 20018 San Sebastián, Spain; (3) Dipartimento di Scienze Fisiche, Matematiche e Informatiche, Università di Modena e Reggio Emilia, via Campi 213a, 41125 Modena, Italy.
Resume : The initial quantum dynamics leading to electron transfer from the photoexcited polymer to the fullerene in P3HT-PCBM is investigated ab-initio by time-dependent density functional theory simulations. We find coherent electron transfer between donor and acceptor and oscillations of the transferred charge with a period matching that of observed vibrational modes in ultrafast optical spectroscopy . Our results show that the coherent coupling between electronic and nuclear degrees of freedom is of key importance in triggering charge delocalization and transfer not only in covalently bonded molecules  but also in this prototype non-covalently bonded system of relevance for photovoltaics.  SM Falke, CA Rozzi, D Brida, M Amato, A De Sio, A Rubio, G Cerullo, E Molinari, and C Lienau, submitted.  CA Rozzi, SM Falke, N Spallanzani, A Rubio, E Molinari, D Brida, M Maiuri, G Cerullo, H Schramm, J Christoffers, C Lienau, Quantum coherence controls the charge separation in a prototypical artificial light-harvesting system, Nat Commun 4, 1602 (2013).
Authors : Daniele Fazzi, Mario Barbatti, Walter Thiel
Affiliations : Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr.
Resume : One of the major debate in organic photovoltaics concerns the role played by charge transfer states at the donor/acceptor (D/A) interface, in particular the driving force and the time scales bringing to exciton dissociation and charge generation . Several pictures emerged involving hot exciton dissociation  and electrostatic effects at the D/A interface . Despite huge efforts spent to understand the photophysics in OPV, a deep understanding is still missing. We present a photophysical investigation for thiophene based systems (P3HT and PCPDTBT [2b]) to understand which are the prevalent deactivation mechanisms after photoexcitation and their relaxation time scales. TD-DFT nonadiabatic excited states dynamics (different XC functionals) have been performed on single chains and molecular cluster. The role played by electron delocalization, energy and charge transfer processes have been explored. For pristine materials, ultrafast deactivation and exciton localization has been calculated in the range of 100 fs, reproducing the experimental data . On these grounds, exciton dissociation should occur before the relaxation process in the D phase, as recently reported [2b]. General structure-property relationships are drawn to suggest materials with improved efficiency. 1 Brédas JL, et al., Acc Chem Res, 2009, 42, 1691. 2 Grancini G, et al., Nat Mater, 2013, 12, 29. 3 Shane RY, et al., J Phys Chem C, 2013, 117, 12981. 4 Banerji NJ, et al., J Mater Chem, 2013, 1, 30.
Authors : Caterina Cocchi (1,2), Deborah Prezzi (1), Alice Ruini (1,3), Elisa Molinari (1,3), Carlo A. Rozzi (1)
Affiliations : (1) Centro S3, CNR-Istituto Nanoscienze, Via Campi 213A, I-41125 Modena, Italy; (2) Humboldt-Universität zu Berlin, Institut für Physik und IRIS Adlershof, Zum Grossen Windkanal 6, 12489 Berlin, Germany; (3) Dipartimento di Scienze Fisiche, Informatiche e Matematiche, Università di Modena e Reggio Emilia, I-41125 Modena, Italy
Resume : Optical limiting (OL) is a prototypical nonlinear process which is relevant for an entire class of devices, related to the protection of light-sensitive elements including the human eye from intense light sources. While extensively studied experimentally, an accurate theoretical investigation of this phenomenon is still missing. In the framework of time-dependent density functional theory, taking advantage of the octopus code , we present a fully ab initio, non-perturbative description of the OL properties of a metal-free phthalocyanine, a prototype of macrocyclic organic compounds, by simulating the effects of a broadband electric field of increasing intensity. Our results confirm reverse saturable absorption as the leading mechanism for OL phenomena in this class of systems and reveal that a number of dipole-forbidden excitations are populated by excited-state absorption at more intense external fields. The excellent agreement with the available experimental data supports our approach as an effective and powerful tool to describe and predict optical limiting , in view of applications.  A. Castro et al., Phys. Status Solidi B 243, 2465 (2006).  C. Cocchi et al., submitted (2013)
Electronic Structure of Molecular Solids : Elisa Molinari
Authors : Chad Risko
Affiliations : Georgia Institute of Technology
Resume : The field of plastic electronics, where devices have active semiconductor layers composed of π-conjugated organic molecules or polymers, offers the potential to revolutionize the broader electronics industry as materials with specific function, form, and processability can be conceived through chemistry. There remains a great challenge, however, to understand how molecular assembly in the bulk and at interfaces with other materials impact the pre-designed molecular characteristics e.g., redox potentials, absorption and emission features that evolve in the solid state to provide the final materials properties that ultimately define device performance. Here we will discuss the application of primarily quantum-chemical approaches to address the relationships between molecular structure, solid-state packing, and materials-scale properties. In particular, we will examine how the nature of the substituents in rubrene affects the planarity of the molecules in the bulk vs. the gas phase, and how the triisopropylsilylethynyl substituents on TIPS-pentacene influence the bulk electronic polarization vs. pentacene.
Authors : Sivan Refaely-Abramson (1), Sahar Sharifzadeh (2), Manish Jain (3), Roi Baer (4), Jeffrey B. Neaton (2), Leeor Kronik (1)
Affiliations : (1) Department of Materials and Interfaces, Weizmann Institute of Science, Rehovoth, Israel; (2) Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, USA; (3) Department of Physics, Indian Institute of Science, Bangalore, India; (4) Fritz Haber Center for Molecular Dynamics, Institute of Chemistry, Hebrew University, Jerusalem, Israel
Resume : Fundamental gap renormalization due to electronic polarization is a basic phenomenon in molecular crystals. Despite its ubiquity and importance, all conventional approaches within density-functional theory completely fail to capture it, even qualitatively. Here, we present a new screened range-separated hybrid functional, which, through judicious introduction of the scalar dielectric constant, quantitatively captures polarization-induced gap renormalization, as demonstrated on the prototypical organic molecular crystals of benzene, pentacene, and C60. This functional is predictive, as it contains system-specific adjustable parameters that are determined from first principles, rather than from empirical considerations [Phys. Rev. B 88, 081204(R) (2013)].
Authors : Daniel Lueftner (1), Sivan Refaely-Abramson (2), Michael Pachler (1), Michael G. Ramsey (1), Leeor Kronik (2), Peter Puschnig (1)
Affiliations : (1) Institute of physics, University of Graz, Austria; (2) Department of Materials and Interfaces, Weizmann Institute of Science, Israel
Resume : Quinacridone is an organic molecule (C20H12N2O2) utilized in the formation of organic pigments. It has also been discussed for usage in organic electronics particularly due to its stability under ambient conditions and its tendency to form self-assembled supramolecular networks. Here, we report on its electronic structure, both, for the isolated molecule as well as for the alpha- and beta- bulk molecular crystal polymorphs. We employ an optimally tuned range-separated hybrid functional (OT-RSH) within density functional theory as well as GW corrections within a many-body perturbation theory framework. A comparison of the theoretical results obtained with the dierent levels of theory and a subsequent comparison with experimental data from angle-resolved photoemission spectroscopy emphasize the need for going beyond simple semi-local DFT-functionals in order to obtain the correct orbital ordering. Furthermore the comparison indicates that the results obtained with OT-RSH greatly improve those of standard DFT functionals and achieve an agreement with experiment at the level of GW calculations, thus making the OT-RSH an alternative to the computationally more expensive GW approach.
Modelling Charge Transfer and Charge-Transfer Excitations : Clemence Corminboeuf
Authors : David Beljonne
Affiliations : Chemistry of Novel Materials University of Mons Place du Parc, 20 B-7000 Mons, Belgium
Resume : Primary photoexcitations in organic materials are strongly bound singlet Frenkel-like excitons coupled to a local reorganization of the lattice (polaronic excitons). In some molecular crystals such as oligoacenes, singlet excitons undergo an efficient spin-conserving fission process yielding two triplet excitons. Besides being of interest from a fundamental point of view, this process opens new opportunities to break the Schockley-Queisser limit in single junction organic solar cells. Singlet fission has thus retained lots of attention recently. We will review some recent works done in our lab and elsewhere on the fundamentals of singlet fission and point in particular to the importance of charge-transfer excitations in mediating the process, through either admixture of charge-transfer character into the lowest exciton states or via a super-exchange mechanism. The same effects also affect the energy transfer rates and diffusion coefficients of the resulting triplets. We will present a first-principle study of triplet transport in an organic crystal taking into account such effects in addition to thermally activated crystal phonons and molecular vibrations. Finally, if time permits, a brief discussion of the mechanisms by which bound polaron pairs can escape from their Coulomb attractive well will be presented and analyzed in the light of recent experimental results.
Authors : Reinhard Scholz, Till Jägeler-Hoheisel, Johannes Widmer, Christian Koerner, Karl Leo
Affiliations : Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Str. 1, D-01069 Dresden, Germany
Resume : Charge transfer (CT) states around the donor-acceptor interface in an organic solar cell determine the device performance in terms of the open circuit voltage. Photovoltaic cells containing different donor-acceptor combinations are characterized electrically, revealing a dependence of the open circuit voltage on temperature and illumination intensity. The low-temperature limit of the open circuit voltage compares favourably with absorption and PL bands assigned to CT states. The results are analysed with a constrained density functional tight-binding scheme allowing to localize opposite elementary charges on the ionized donor and acceptor molecules. As an alternative, with any DFT functional and basis set, the CT energy can be derived from the ionization potential of the donor, the electron affinity of the acceptor, and the Coulomb interaction between the Mulliken charges of the pair of ionized molecules. The embedding polarizable medium reduces the CT gap by substantial polarization shifts for the cationic donor and the anionic acceptor, whereas the screening by the dielectric constant of the medium reduces the Coulomb interaction significantly. Hence, for CT states at typical donor-acceptor interfaces, the barrier towards charge separation shrinks to about half an eV. The proposed computational scheme reproduces the observed trends of the observed open circuit voltages in photovoltaic devices relying on several donor-acceptor blends.
Authors : Gabriele D'Avino and Matthieu J. Verstraete
Affiliations : Université de Liège, Institut de Physique, Allée du 6 Août, 17 Sart-Tilman, B-4000 Liège, Belgium
Resume : Mixed-stack charge transfer (CT) crystals (e.g. TTF-CA) are one of the few examples of ferroelectrics among organics materials . In these quasi-1D systems, electron donor (D) and acceptor (A) molecules pack in an alternating pattern, DADA, and a fractional charge, q, is transferred from D to A. In some systems, a lattice instability is triggered by the increase of CT above the neutral-ionic boundary (q~0.5), leading to a low-temperature polar phase (Tc=81 K in TTF-CA). We present a detailed theoretical investigation on a novel series of hydrogen-bonded CT crystals, for which room temperature ferroelectricity has been claimed . Our analysis is based on the Peierls-Hubbard model, which proved successful in describing the transition in different CT crystals . A novel approach to the model parameterization, based on first-principle calculations, is here proposed, and its accuracy validated by reproducing the transition of TTF-CA. Our calculations show that, contrary to what was originally reported, the CT crystals in  are all largely neutral (q<0.1) and hence very unlikely to present a polar lattice. Experimental data reported in  confirm our estimate for q and do not allow one to draw a definitive conclusion about the ferroelectricity of these materials.  S. Horiuchi, Y. Tokura, Nature Materials 7, 357 (2008);  A. Tayi et al., Nature 488, 485 (2012);  G. DAvino et al., Phys. Rev. Lett. 99, 156407 (2007); G. DAvino et al., Phys. Rev. B 83, 161105R (2011);
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Modelling Interfaces I : Alice Ruini
Authors : David Gao, Matthew Watkins, Alexander Shluger
Affiliations : Department of Physics and Astronomy, University College London, UK
Resume : We will discuss the results of theoretical modelling of adsorption of individual organic molecules and molecular aggregates at insulating surfaces motivated by high resolution non-contact AFM imaging of surface structures. We aim at understanding how the molecular structure, the polar end-groups, as well as the substrate lattice constant and ionic species influence the formation of supra-molecular networks on insulating surfaces. As one such example, we discuss differences in the adsorption, binding, and mobility of Co-Salen molecules at NaCl and NiO (001) surfaces and the theoretical methods used in these studies. As the second example, we present the results of modelling the assembly of CDB molecules at several alkali halide surfaces. These, much larger molecules are composed of a central aromatic part surrounded by two lateral decyloxy chains (O-(CH2)9-CH3). The central part consists of three phenyl rings ended by cyano (CN) groups. We are developing multi-scale methodology for modelling the structure and growth of such networks. This includes embedded cluster QM molecular dynamics of molecules at surfaces, which is used for Genetic Algorithm powered development of force-fields for molecule-surface interactions. These force-fields are then used for modelling the atomistic structure and dynamics of larger aggregates of molecules at surfaces. This information is used for modelling AFM images of surface structures and the mechanisms of their assembly, including the effect of surface defects.
Authors : Oliver T. Hofmann (1), Viktor Atalla (1), Georg Heimel (2), Patrick Rinke (1), and Matthias Scheffler (1)
Affiliations : (1) Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany; (2) Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D-12489 Berlin, Germany
Resume : To describe charge transfer at inorganic/organic interfaces two different scenarios are usually invoked. Either, all molecules in the first organic monolayer become fractionally charged, or the charge localizes on only a fraction of the molecules. Which scenario dominates depends on the strength of the interaction between substrate and adsorbate. Inserting an insulating layer between a metal and the organic therefore provides a way to tune the interaction from the weakly to the strongly interacting (covalently bonded) regime. Experimentally, charge-transfer can be investigated by means of photoelectron spectroscopy, but its resolution might not suffice if the films are disordered or the charge transfer is small. Here, we study these systems using density functional theory (DFT). We first ascertain the suitability of DFT to describe both charge transfer regimes by carefully examining the parameter space of hybrid functionals. We then find that the two charge-transfer scenarios exhibit a qualitatively different evolution of the interface dipole with respect to the coverage of the organic molecules. The difference becomes more pronounced for smaller charge transfer. Our electronic structure calculations are therefore an ideal complement to photoelectron spectroscopy. Furthermore, we investigate how the two mechanisms affect the electrostatic potential above the surface, and discuss the potential implications for organic electronic devices.
Authors : Elisabeth Wruss (1), David A. Egger (1), Yuli Huang (2,3), Tomas Bučko (4,5,6), Wissam A. Saidi (7), Satoshi Kera (3), Egbert Zojer (1)
Affiliations : (1) Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria; (2) Department of Physics, National University of Singapore, 2 Science Drive 3, 117542, Singapore; (3) Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan; (4) Department of Physical and Theoretical Chemistry, Faculty of Natural Sciences, Comenius University, Mlynska Dolina, SK-84215 Bratislava, Slovakia; (5) Slovak Academy of Sciences, Institute of Inorganic Chemistry, Dubravska cesta 9, SK-84236 Bratislava, Slovakia; (6) Faculty of Physics, University of Vienna, Sensengasse 8/12, 1090 Vienna, Austria; (7) Department of Chemical and Petroleum Engineering, University of Pittsburgh, 1249 Benedum Hall, Pittsburgh, PA 15261, U.S.A.
Resume : Van der Waals (vdW) interactions crucially impact the adsorption of organic molecules on metallic surfaces. Their contribution to the overall binding mechanism becomes especially significant for flat-lying organic molecules, which are often referred to as being only weakly bound to metallic substrates. When modelling such systems using (semi-)local density-functional-theory, vdW interactions are neglected due to their inherent nonlocal character and, therefore, adsorption geometry and electronic properties are often not described accurately. To circumvent this shortcoming, we apply the PBE vdWsurf  approach, which has been shown to strongly improve the modelling of metal-organic interfaces. We investigate phthalocyanine molecules and pentacene derivatives on coinage metal surfaces at an atomistic level. To correctly describe the electronic structure of the interfaces especially with respect to the ordering of the orbitals, we apply hybrid-functionals that help mitigating the orbital self-interaction error. Our results demonstrate the multitude of interactions responsible for the formation of hybrid metal-organic interfaces, highlighting, in particular, the important contribution of vdW interactions. Finally we analyse whether there is a gradual or abrupt transition between ?strong? (chemisorptive) or ?weak? (physisorptive) bonding upon changing the substrate work-function.  V. G. Ruiz et al, Phys. Rev. Lett. 108, 146103 (2012).
Modelling Charge Transport I : Hiroyuki Ishii
Authors : Gianaurelio Cuniberti Frank Ortmann Sebastian Radke
Affiliations : Institute for Materials Science, Max Bergmann Center of Biomaterials, and Center for Advancing Electronics Dresden, TU Dresden, 01062 Dresden, Germany
Resume : Novel organic materials are at the focus of current experimental and theoretical activities in order to improve the efficiency of their charge transport characteristics. Theory can provide guidelines to realize high mobility materials either through a molecular picture or through simulations of condensed phases. While the molecular point of view can provide some qualitative tendencies, significant improvements however have still to be accomplished to achieve predictive power of transport simulations in real devices. We present here recent work on transport simulations in organic materials by combining high level electronic structure calculations with molecular dynamics simulations to parametrize effective models for charge transport. Especially the influence of static and dynamical disorder in the electronic structure of the organic systems on the charge mobilities will be addressed.
Authors : Jacob Gavartin, Matthew Halls
Affiliations : Schrodinger Inc.
Resume : It is widely accepted that static and dynamic disorder significantly degrades (or moderates) carrier mobility in organic semiconductors. Yet the quantification of such degradation and ultimately its control is proved to be difficult . Experimentally, no perfectly ordered reference system can be synthesized and thus no systematic separation of various aspects of disorder is possible. Theoretically, different aspects of disorder are often considered within different level of approximations making a comparative analysis of the effects of on-site and off-site, static and dynamic disorder difficult. Towards the quantification of effects of disorder we present the Born-Oppenheimer molecular dynamics analysis considering structural disorder as well as electron-phonon coupling in molecular semiconductor materials relevant for optoelectronic applications. The presented approach helps to understand merits and limitations of popular phenomenological models, such as uncorrelated Gaussian disorder, as well as to separate controllable and intrinsic parameters of carrier mobility. 1. V. Coropceanu, J. Cornil, D. A. da Silva Filho, Y. Olivier, R. Silbey, and J.-L. Brèdas. Chem. Rev. 2007, 107, 926-952.
Authors : S. Ciuchi (1), S. Fratini (2)
Affiliations : (1) Dipartimento di Scienze Fisiche e Chimiche Universita` dellAquila, L'Aquila, Italy; (2) Inst. NEEL, F-38042 Grenoble, France
Resume : We explore the charge transport mechanism in organic semiconductors based on a model that accounts for the thermal intermolecular disorder at work in pure crystalline compounds, as well as extrinsic sources of disorder that are present in current experimental devices. Starting from the Kubo formula , we describe a theoretical framework that relates the time-dependent quantum dynamics of electrons to the frequency-dependent conductivity [2,3]. The emergence of a ``transient localization'' phenomenon is shown to be a general feature of organic semiconductors, that is compatible with the bandlike temperature dependence of the mobility observed in pure compounds. This regime imply a breakdown of semiclassical transport without polaron formation [2,3]. Carrier trapping by extrinsic disorder causes a crossover to a thermally activated behavior at low temperature, which can be tuned by tuning disorder parameters trough molecular tailoring .  S. Fratini and S. Ciuchi, Phys. Rev. Lett. 103, 266601 (2009)  S. Ciuchi S. Fratini and D. Mayou, Phys. Rev. B 83, 081202 (2011)  S. Ciuchi, S. Fratini, Phys. Rev. B 86, 245201 (2012)  Nikolas A. Minder, Shaofeng Lu, Simone Fratini, Sergio Ciuchi, Antonio Facchetti, Alberto F. Morpurgo, Adv. Mat. (2013)
Authors : Denis Andrienko
Affiliations : Max Planck Institute for Polymer Research
Resume : Interest in the field of organic electronics is largely provoked by the possibility to fine-tune properties of organic semiconductors by varying their chemical structure. Often, compound design is solely guided by chemical intuition, even though material development would benefit from more rigorous structure-property relationships, which link the chemical structure, material morphology and macroscopic properties. To formulate such relationships, an understanding of physical processes occurring on a microscopic level as well as the development of methods capable of scaling these up to macroscopic dimensions are required . The aim of computer simulations is to facilitate this by zooming in on the behavior of electrons and molecules and by bridging micro- and macroscopic worlds. Here, we describe the current status of methods which allow the linking of molecular electronic structure and material morphology to the mesoscopic/microscopic dynamics of charge carriers and excitons. Special attention is paid to the challenges these methods face when aiming at quantitative predictions [2,3].  V. Ruehle, A. Lukyanov, F. May, M. Schrader, T. Vehoff, J. Kirkpatrick, B. Baumeier, D. Andrienko, J. Chem. Theory Comput., 3335-3345, 2011  F. May, B. Baumeier, C. Lennartz, D. Andrienko, Phys. Rev. Lett., 109, 136401, 2012  M. Schrader, R. Fitzner, M. Hein, C. Elschner, B. Baumeier, M. Riede, K. Leo, P. Baeuerle, D. Andrienko, J. Am. Chem. Soc., 134, 6052-6056, 2012
Modelling Charge Transport II : Gianaurelio Cuniberti
Authors : Hiroyuki Ishii (1), Nobuhiko Kobayashi (2), Kenji Hirose (3)
Affiliations : (1) University of Tsukuba and JST-PRESTO; (2) University of Tsukuba; (3) NEC
Resume : Organic semiconductors are expected to be one of the candidates for future device applications. In disordered organic semiconductors, the charge carrier is localized in a molecule. The hopping of carrier has been observed in experiments as thermally activated behavior of low mobility. The transport property has been understood using the Marcus theory. On the other hand, it has been shown by recent experiments that carrier extends spatially over ten molecules in single-crystal organic semiconductors, supporting the band-transport theory. The unified theoretical description from hopping transport behavior to band transport behavior represents a very challenging problem. We have developed the time-dependent wave-packet diffusion method , where we evaluate the mobility using the Kubo formula based on the quantum wave-packet dynamics combined with the molecular dynamics simulation. We applied the method to the pentacene and rubrene crystals and clarified the transition from the hopping transport behavior to the band-like behavior in our unified way. These calculated results indicate that competition among the thermal fluctuation induced by the molecular vibrations, polaron formation and static disorder provides important clues to understanding the transport mechanism of realistic organic materials. In my presentation, I will talk about these calculated results in detail. Financial support was provided by JST-PRESTO.  H. Ishii, et al., Phys. Rev. B 85, 245206 (2012)
Authors : P. Vancsó (1,4), G. I. Márk (1,4), D. Kvashnin (2), V. A. Demin (2), L. Chernozatonskii (2), Ph. Lambin (3), A. Mayer (3) and L. P. Biró (1,4)
Affiliations : (1) Institute of Technical Physics and Materials Science, Research Centre for Natural Sciences, PO Box 49, H-1525 Budapest, Hungary, www.nanotechnology.hu; (2) N.M.Emanuel Institute of Biochemical Physics, Russian Academy of Sciences (IBCP), 4 Kosygin St., Moscow, 119991, Russian Ferderation; (3) Department of Physics of Matter and Radiation, University of Namur (FUNDP) 61, Rue de Bruxelles, B-5000 Namur, Belgium; (4) Korean-Hungarian Joint Laboratory for Nanosciences, PO Box 49, H-1525 Budapest, Hungary
Resume : According to our recent massive DFT simulations, when creating holes in a bilayer graphene, the two layers tend to zip together along the edges. This phenomenon has also been recently observed by transmission electron microscopy (TEM) . If the holes are arranged periodically, the optimized structure turns to be a carbon nanotube superlattice like closed sp2 carbon network. By varying the size and distance of the holes, a very rich configuration space of systems with diverse electronic structures can be created, which includes both conducting and localized states. Utilizing our wave packet dynamical simulation code  we studied the local time- and energy dependent dynamics of the system. By comparing the results using a jellium potential and a local pi-band pseudopotential  we were able to separate the geometrical and electronic structure effects, which are intermixed in the band structure.  G. Algara-Siller, et al., Carbon 65 (2013) 80-86  G. I. Mark, et al., Phys. Rev. B 85(2012) 125443  A. Mayer, Carbon 42, (2004) 2057
Authors : Kurt Stokbro, Anders Blom, and Soren Smidstrup
Affiliations : QuantumWise A/S, Lerso Parkalle 107, 2100 Copenhagen, Denmark
Resume : The topic of this paper is the simulation of the electronic properties of interfaces from first principles, on the atomic scale. For the calculation we use a Non-Equilibrium Greens Function approach as implemented in the ATK package, which allows us to study an isolated interface, opposed to supercell approaches which simulate an array of interfaces. We present results for the I-V characteristics of the gold-pentacene interface. The calculation shows a strong rectifying for transport across the interface.To explain the simulations we extend the conventional Schottky barrier theory to include the effect of narrow bands in the organic crystal, and show how the calculations can rationalize recent experimental data References: 1. A. Blom and K. Stokbro, Atomistic modeling of semiconductor interface, Journal of Computational Electronics 12(4), 623 (2013). 2. Atomistic ToolKit, www.quantumwise.com 3. K. Stokbro and S. Smidstrup, Electron transport across a metal-organic interface: Simulations using nonequilibrium Green's function and density functional theory, Phys. Rev. B 88, 075317 (2013). 4. Z. Liu, M.Kobayashi, B. C. Paul, Z. Bao, and Y. Nishi, Contact enginnering for organic semiconductor devices via Fermi level depinning at the metal-organic interface Phys. Rev. B 82, 035311 (2010)
Modelling Interfaces II : Alexandre Tkatchenko
Authors : Kristen A. Fichthorn, Shafat Mubin
Affiliations : Department of Chemical Engineering; Department of Physics
Resume : Organic thin films have been the subject of intense research because of their suitability for applications in molecular electronics. The beneficial properties of these films, such as the charge mobility, exciton diffusion length, energy-level alignment at the organic-substrate interface, and optical properties are all sensitive to the structure of the film. While it is generally agreed that highly ordered films have the most favorable properties, predicting and controlling organic thin-film structures is still a significant challenge. From a fundamental perspective, first-principles calculations cannot probe large enough lengths or long enough times to describe the link between thin-film deposition and processing conditions and film structure. In this talk, I will discuss a multi-scale approach aimed at quantifying the structures and dynamics of thin films of benzene, PTCDA, and NTCDA on noble-metal surfaces. In this approach, we developed force fields to describe the interaction of these molecules with the metal substrates and we applied these to describe several aspects of these systems, including their order-disorder phase transitions and their desorption and surface diffusion kinetics. An interesting interplay between the delocalized electrons in these pi-conjugated molecules, van der Waals interactions, and substrate-mediated interactions by both bulk and surface-state electrons of Ag produces several interesting physical phenomena that have been (or should be) experimentally observed in these systems, including the inverse melting transition and significantly higher desorption preexponential factors than are conventionally assumed.
Authors : Jérôme Cornil, Victor Geskin, and Colin Van Dyck
Affiliations : University of Mons Laboratory for Chemistry of Novel Materials
Resume : Understanding the alignment of molecular orbitals and corresponding transmission peaks with respect to the Fermi level of the electrodes is a major challenge in the field of molecular electronics. In order to design functional devices, it is of utmost importance to assess whether controlled changes in the electronic structure of isolated compounds are preserved once they are inserted in the molecular junctions. We shed light here on this central issue by performing Density Functional Theory calculations on junctions including diarylethene-based molecules. We demonstrate that the chemical potential equalization principle allows us to rationalize the existence or not of a Fermi level pinning (i.e., same alignment in spite of varying ionization potentials of the isolated compounds) and points to the essential role played by Metal Induced Gap States (MIGS). We also evidence that the degree of level pinning is intimately linked to the degree of orbital polarization when a bias is applied between the two electrodes. C. Van Dyck et al., PhysChemChemPhys
Authors : Veronika Obersteiner (1) , David A. Egger (1) , Georg Heimel (2), Egbert Zojer (1)
Affiliations : (1) Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria; (2) Institut für Physik, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 6, 12489 Berlin, Germany;
Resume : In the area of molecular electronics it is increasingly understood that there are fundamental differences between the transport through individual molecules and through continuous, extended, two-dimensional monolayers [1-3]. Here, we aim at relating those differences to effects that originate from the collective electrostatic interactions between the molecules in an extended system . This is theoretically demonstrated by varying the coverage in self-assembled monolayers (SAMs) consisting of organic molecules with dipoles distributed along their backbones. These coverage dependent calculations are done using density functional theory (DFT). To determine the transport properties, the transmission coefficients are calculated applying a Greens function approach. Besides varying the coverage, collective effects are additionally studied for molecular clusters of different size. To study the impact of the docking chemistry (that is known to also strongly affect electrical transport ), we moreover study collective effects in conjunction with different anchoring groups forming the contact between the SAMs and the gold electrodes.  Y. Selzer et al, Nano Lett. 2005, 5, 61.  M. G. Reuter, et al, Nano Lett. 2011, 11, 4693.  A. Landau et al, J.Comput.Theor.Nanosci. 5.4 2008, 535-544.  D. A. Egger et al, Adv. Matter. 2012, 24, 4430.  L. A. Zotti et al, Small 2010, 6, 1529 .
Posters : Heimel, Zojer, Kronik
Authors : Xing Gao, Hua Geng, Qian Peng, Yuanping Yi, Dong Wang, Zhigang Shuai
Affiliations : MOE Key Laboratory of Organic OptoElectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China; Key Laboratory of Organic Solids, Beijing National Laboratory for Molecular Science (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China
Resume : Understanding the carrier transport processes and predicting the carrier mobility from first principle in organic electronic materials have been a longstanding challenge. We have applied the nonadiabatic Ehrenfest dynamics coupled with density functional tight binding (DFTB) to investigate the carrier motion in the donor-acceptor type polymer for photovoltaics. The equations of motion for the electrons are evolved under the fixed subspace spanned by the active molecular orbitals during each nuclear time step and the feedback from charge to the nuclei motions, namely, the polaronic effect is considered. Then, we use this methodology to investigate the charge transport dynamics for the ladder-type poly(p-phenylenes) (LPPP) and poly(diketopyrrolo-pyrrole (DPP)) series with ~2×10^3 atoms. The carrier mobilities are evaluated as diffusion process. It was found that the diffusion abilities are determined by the magnitude of transfer integrals and localization length for frontier orbital which caused by the self-trapping effects (polaron) arising from the double bond stretching and twisting motions. This method can be useful in exploring the underlying charge transport behavior and improving the structure design of materials in organic electronics.
Authors : Xiaoyan Zheng, Dong Wang*, Zhigang Shuai*
Affiliations : Department of Chemistry, Tsinghua University
Resume : The softness of van der Waals bonded organic semiconductors makes controlling their electronic performance by external pressure possible. Recently, solution-sheared thin-films with nonequilibrium single-crystal domains presenting much higher charge mobilities than the unstrained ones were discovered (Nature 2011, 480, 504-508; Nat. Mater. 2013, 12, 665-671). However, efficient and targeted modulation of charge transport in organic semiconductors still poses huge challenges, and requires detailed microscopic understanding of the structure-property relationship. Herein, we are motivated to elucidate the lattice strain-molecular packing-charge carrier mobility relationship in TIPS-P crystals, for a general understanding of the underlying mechanism of lattice strain controlled charge transport in organic semiconductors. By developing a multiscale approach combining quantum chemistry, quantum model of charge transfer and nonequilibrium molecular dynamics, we investigated lattice strain controlled charge transport in 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-P). We show that shear-strained TIPS-P indeed exhibits one-dimensional charge transport, and the hole mobility is shown to increase with increasing shear strain, which is in qualitative agreement with the experimental observations (Nature 2011, 480, 504-508). And we predict that either shear or tensile strain leads to mobility enhancement, but with strong anisotropy. By combining shear with normal strains, we realize almost isotropic charge transport in TIPS-P crystal with the hole mobility improved by at least one order of magnitude. Our approach, which bridges the gap between the molecular packing at the atomistic level and the charge transport at the mesoscopic scale, enables a deep understanding of strain-mobility relationship over a wide range of organic semiconductors. This structure-property relationship is urgently needed for the realization of soft electronics or pressure-sensitive integrated circuits such as electronic-skin.
Authors : Jinyang Xi, Dong Wang, Zhigang Shuai
Affiliations : Department of Chemistry, Tsinghua University
Resume : The electron-phonon couplings and charge transport properties of α-and γ-graphynes were investigated from first-principles by using the density-functional perturbation theory and Boltzmann transport equation. The effective deformation potential constants were extracted for the corresponding electron-phonon couplings in both carbon allotropes. Corresponding results of graphene were also examined for comparison. In order to obtain the ultra-dense sampling of electron-phonon coupling matrix elements throughout the Brillouin zone, the Wannier-interpolation-based techniques were applied. We demonstrated that the intrinsic electron-phonon scatterings in these two-dimensional carbon materials are dominated by low-energy longitudinal-acoustic phonon scatterings over a wide range of temperatures. By contrast, the high-energy optical phonon scatterings play a more and more important role at high temperatures, due to the significant coupling strength and increasing excitations of optical phonons. With larger longitudinal-acoustic phonon scatterings, the electron mobilities of α-and γ-graphynes are predicted to be ~ 10^4 cm^2V^-1s^-1 at room temperature, which is one order of magnitude lower than graphene.
Authors : Navid Abedi , Georg Heimel
Affiliations : Institut für Physik, Humboldt-Universität zu Berlin
Resume : Its wide band-gap, high transparency and tunable morphology introduced ZnO as a useful material for optoelectronic devices but also as a challenging material to control and understand. Especially its surfaces which, in turn, define the properties of interfaces between ZnO and other materials, are often ill-defined. Identifying the surface structure and stoichiometry of ZnO is, therefore, an important prerequisite for knowledge-guided device optimization. While X-ray photo-emission spectroscopy (XPS) is, in principle, an extremely useful experimental technique for achieving just that, the interpretation of experimental results is not always straightforward in practice. To establish a clear-cut correlation between surface composition and expected XPS signal, we performed density functional theory (DFT) based calculations of the most probable structures of Zinc oxide's complex (0001), (000-1) and (10-10) surfaces. Chemical core-level shifts are then obtained from total-energy differences and Slater's transition-state theorem. Applying the experimental approach to estimate surface stoichiometry in reverse, we also provide XPS intensities based on tabulated values for electron mean free paths. In the end, we produce peak shapes with a combination of Lorentzian and Gaussian functions, allowing direct comparison with experimental spectra.
Authors : Jedrzej Szmytkowski
Affiliations : Faculty of Applied Physics and Mathematics, Gdansk University of Technology, Gdansk, Poland
Resume : Nowadays, organic solar cells attract extensive attention due to their potential attractive properties. In order to achieve higher efficiencies of such devices, it is necessary to recognize all phenomena which cause the reduction of photocurrent. Nongeminate recombination seems to be a dominant process leading to the loss of photocurrent in organic donor-acceptor bulk heterojunction solar cells. Previously, it has been observed that an order of recombination is greater than two what value is typical for the Langevin bimolecular recombination. In this work, we demonstrate a description of this effect and we discuss the origin of higher recombination orders reported in experiments. The influence of charge carriers concentration on the mobility is analyzed.
Authors : Elisabeth Verwüster, David A. Egger and Egbert Zojer
Affiliations : Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria
Resume : The deposition of self-assembled monolayers (SAMs) is an efficient way of tuning the properties of interfaces. In spite of such systems being stabilized by covalent links to the (in our case metallic) substrate, their atomistic and, consequently, electronic structures are impacted by several effects. These involve reconstructions of the substrate surfaces, the packing density of the adsorbate layer, electrostatic interactions between polar elements of neighboring chains and the possibility to form inter-molecular bonds. Moreover, the structures of the SAMs are also strongly impacted by van der Waals interactions, which are difficult to describe by semi-local density functional theory. To investigate such effects in detail, we performed slab-type band-structure calculations employing van der Waals corrections on aromatic SAMs. We systematically varied the coverage, considered surface vacancies and various types of adatoms and bridging atoms, and compared tail-group substituents of electron-donating (-CH3) and -withdrawing (-CN, -CF3) character. We find a significant impact of coverage and tail-group substitution on the investigated properties, like molecular tilt-angle, work-function modification, and electronic level alignment with the observed variations typically increasing when including vdW interactions.
Authors : Gernot J. Kraberger, David A. Egger, Egbert Zojer
Affiliations : Institute of Solid State Physics, Petersgasse 16, A-8010 Graz, Austria
Resume : Finding novel electronic applications based on the seminal material graphene is a relatively new, but quickly growing field. In this context, controlling the electronic properties of graphene through modifying its structure and chemical composition is an appealing strategy. Here, we explore the possibility to control the electronic structure of graphene through chemically induced collective electrostatic effects. As a first attempt to show the potential of this concept, we substitute specific carbon atoms by boron and nitrogen, thereby creating in-plane lines of dipoles. Using standard density functional theory calculations, the resulting dipolar field is shown to cause a shift in the electrostatic potential, thus changing the energy of the graphene electronic states with respect to the Fermi level in the vicinity of the BN lines. When installing two oppositely oriented close-lying dipole chains, a localization of the occupied or the unoccupied states either between or outside the two chains can be realized. Furthermore, the role of different geometries for incorporating the B and N atoms (e.g., in a zigzag- or armchair-like fashion) and the impact of different distances between the two dipole lines are investigated. Finally, we examine whether similar effects can be realized by self-assembling organic molecules containing dipolar units onto the graphene layer without chemically modifying the graphene sheet itself.
Authors : O. G. Ziogos, D. N. Theodorou
Affiliations : Department of Chemical Engineering, National Technical University of Athens, 15780 Athens, Greece
Resume : Organic disk-shaped molecules that have the ability to self-organize into columnar phases are promising as building blocks for one dimensional molecular wires. Amongst relatively small organic mesogens, HBC-C12 molecules exhibit the highest intra-columnar charge mobility in the crystalline phase, ranging up to one third of graphite's mobility in the direction perpendicular to the graphitic sheets. Recent experimental studies revealed that the functionalization of alkyl-substituted HBC mesogens with polar groups affects the structural and dynamical properties of the molecular crystals. Charge mobility in these systems is sensitive to structural defects and to the thermal motion of their constituents. Handling and final packaging into an electronic device most probably brings them to a strained state. A theoretical study of the impact of external stress on the local structure and ordering of the molecules could be beneficial for the selection of suitable mesogens that can form crystals robust to external mechanical perturbations. To this end, we calculate the elastic response to normal and shear stress of HBC-C12 and HBC molecular crystals and examine how peripheral substitution affects their mechanical properties. The Molecular Dynamics method is employed with empirical force fields in order to examine structural properties and study the response to finite normal and shear deformations within the elastic regime.
Authors : Thiago B. de Queiroz, Stephan Kümmel
Affiliations : Theoretical Physics IV, University of Bayreuth
Resume : Charge transfer excitations play a prominent role in the field of molecular electronics. At the same time they have developed a reputation for being hard to predict with the otherwise predominant method for calculating molecular excitations, (time-dependent) density functional theory. Recently, it has been demonstrated that range-separated hybrid functionals with an "optimally tuned" range separation parameter describe charge-transfer excitations reliably. Many studies of this kind focused on molecules in vacuum. Here, we investigate the influence of solvation on the electronic excitations of thiophene oligomers, i.e. paradigm low gap systems, and investigate how the tuned range-separated hybrid concept can (not) be combined with solvation models. We take into account molecular geometries obtained from molecular dynamics (MD) in which dioxane solvent molecules have been explicitly included. The geometrical distortions that are present in solvated thiophenes at room temperature increase absorption energies up to 0.5 eV. We further investigate the concept of optimally tuning the range separation parameter in the presence of a solvent as modeled with a continuum solvation method. Possible limitations with respect to system size, treating solvation consistently, and the influence of short-range exchange are discussed. This study is thus a step on the way towards a more consistent comparison between experimental data and first-principles theory.
Authors : Caterina Cocchi, Claudia Draxl
Affiliations : Humboldt-Universität zu Berlin, Institut für Physik and IRIS Adlershof, Berlin, Germany
Resume : The spurious long-range behavior of time-dependent (TD) density functional theory (DFT) is a well known source of error in describing bound excitons in solids. Remarkably, TD-DFT is often able to capture the optical features of isolated systems, even with the most simple exchange-correlation kernels, like the TD local density approximation (LDA). With the example of molecular crystals, we aim at solving the puzzle when and why TD-DFT can be relied on. We answer this question by confronting TD-DFT with many-body perturbation theory (GW and Bethe-Salpeter equation), which is the most accurate methodology to describe optical excitations in solids. Our results are obtained with the all-electron code exciting (http://exciting-code.org), where all the quantities entering the two formalisms are treated on the same footing . In-depth analysis allows us to identify the shortcomings of TD-DFT in predicting the excitonic spectra of extended systems and to understand when this methodology is capable of providing correct results.  S. Sagmeister and C. Ambrosch-Draxl, Phys. Chem. Chem. Phys. 11, 4451 (2009)
Authors : Iris Hehn (1), Manuel Vieider (1), Otello Maria Roscioni (2), Luca Muccioli (2), David Egger (1), Swen Schuster (3), Michael Zharnikov (3), Claudio Zannoni (2), and Egbert Zojer (1)
Affiliations : (1) Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria; (2) Dipartimento di Chimica Industriale Toso Montanari, Università di Bologna; Viale del Risorgimento, 4, 40136 Bologna, Italy; (3) Applied Physical Chemistry, Heidelberg University, Im Neuenheimer Feld 253, 69120, Heidelberg, Germany
Resume : In this work we perform quantum mechanical simulations on mid-chain ester functionalized alkanethiols adsorbed on Au(111) substrates using density functional theory. These systems have already been studied extensively with a range of experimental methods . For simulations however, one has to bear in mind that with quantum-mechanically calculated forces and limited-size surface unit cells it is essentially impossible to sample the entire configuration space of the layers. The latter is particularly complex for alkanethiolates, as they are very flexible and relatively loosely packed on surfaces even at full coverage. We therefore perform molecular dynamics simulations on small unit cells containing only 4x4 molecules to generate reasonable starting geometries for the following density functional calculations, an approach we already successfully applied to model SAMs of different thiolates. In an alternative approach we perform molecular dynamics simulations on the same systems using far larger unit cells. In these calculations we account for the inevitable screening effects by a customized embedding procedure. The lengths of the alkyl segments above and below the carboxylate dipolar groups are systematically varied to study the impact of the molecular length on the order within the layers. The latter is quantified using the relevant structural parameters such as tilt, twist and azimuth angles.  Orlando M. Cabarcos et al., J. Phys. Chem. C 2008, 112, 10842
Authors : Philipp Herrmann, Georg Heimel
Affiliations : Institut für Physik, Humboldt Universität zu Berlin
Resume : Its wide bandgap and natural abundance make ZnO a promising material for optoelectronic devices. Particulary tempting is the use of ZnO in conjunction with organic semiconductors, but this requires detailed information on the interfaces in such hybrid systems. Given the complex phase diagram of ZnO surfaces, reliable means to control their structure and, thereby, the interfacial properties are needed for real applications. We therefore explore the functionalization and passiviation of ZnO surfaces with self-assembled monolayers (SAMs) of covalently attached organic molecules. It is important to investigate the stability of these SAMs under different environmental conditions. A typical procedure consists of the combination of quantum mechanical calculations to obtain binding energies and thermodynamical considerations to extrapolate to finite pressure and temperature. We highlight a problem regarding this approach when describing non-equilibrium conditions such as UHV and propose an extension to the thermodynamic part to deal with such situations. After periodic density functional calculations for simple SAM molecules on the non-polar ZnO 10-10 surface, this procedure is then applied to predict the structural phase diagram in a surrounding atmosphere consisting of hydrogen, oxygen and water.
Authors : Bitam Said, Abdelbaki Djebaili
Affiliations : Faculty of Sciences, University of Batna, -05000, Algeria
Resume : Investigation of the geometric and electronic structure of the polyacetylene with substituents of different strengths of donor and acceptor functional groups was carried out with 3-21G basis sets, by incorporating The rates of this geometrical isomerization and Arrhenius parameters. In the case of unsubstituted polyacetylene as the reference.. The values of the equilibrium constant for the reaction have also been determined at various temperatures between 300 and 500 K and the value of the energies change calculated. The results demonstrate that the isomerization energies are positive and the barrier heights ∆Ebarrier are expected to be sensitive for the magnitude of substituents effects. Keywords: polyacetylene, rate constant, equilibrium constant.
Authors : H.M.C. Barbosa, H.M.G. Correia, M.M.D. Ramos, L. Marques
Affiliations : Centre/Department of Physics, University of Minho Campus de Gualtar, 4710-057 Braga, Portugal
Resume : Recently, organic solar cells (OSC) have reached efficiencies above 10% at laboratory scale. However to become a reliable alternative to conventional inorganic solar cells, its necessary to understand the main mechanisms affecting device efficiency, degradation and failure. In this respect, computational simulations are an essential tool for further device optimization. In this work, we present an improved Monte Carlo model that takes into account a proper description of molecular organization at polymer active layer and considers the main physical processes mediating exciton and charge dynamics in OSC. This model was used to study the effects of polymer molecular properties, chain orientation , and interface nanostructuring on device efficiency. The model was also used to study OSC degradation mechanisms, leading to device failure and loss of efficiency, as a result of electrodes degradation. Our results show that changing electrodes work function affects simultaneously all optoelectronic mechanisms that rule exciton and charges dynamics, and thus the device performance.
Authors : Haoyuan Li，Lian Duan* and Yong Qiu
Affiliations : Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing, 100084, China
Resume : The time-of-flight (TOF) and dark-injection space-charge-limited current (DI-SCLC) measurements are both widely used to obtain the charge mobility in organic semiconductors. However, the charge transport in these cases is still not fully understood. In the TOF measurement, when the carrier density cannot be neglected, the current is said to be space-charge-perturbed (SCP). The effect of space-charge perturbation on the extracted charge mobility is unclear. The commonly assumed conclusion that the peak time in the DI-SCLC measurement equals 0.787 times the transit time in the TOF measurement also lacks validation for organic semiconductors. Using multi-particle Monte Carlo simulations, we have studied the transient currents in organic materials, under conditions varying from space-charge-free (SCF) to space-charge-limited (SCL). The results suggest that the SCP effect is shorter peak time and longer transit time. This conclusion has been confirmed by experiment. For transient SCL currents, the peak time is found to be much shorter than previously thought for systems with large energetic disorders. These results can be explained by the enhanced charge transport by higher carrier density and the non-uniform electric field, which is stronger at the carrier packet front and weaker at the injection part.
Authors : Chen Li, Lian Duan*, Haoyuan Li, Yongduo Sun, Yong Qiu
Affiliations : Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing, 100084, China
Resume : In order to unravel the effect of trap energy on carrier transport in organic disorder semiconductors, a comprehensive study was conducted on hole transport in a series of organic molecular hosts with explicit traps at different trap energies. The mobility measured by time-of-flight experiments was found to decrease significantly at shallow trapping, but hardly decreased at deep trapping, where a sharp decrease of carrier density was observed. By analyzing temperature dependence of the mobility, the decreased mobility at shallow trapping were found to originate from increased energetic disorder and activation energy, whereas both energetic disorder and activation energy are changeless at deep trapping. We find it reasonable to cover the different effects of deep trapping and shallow trapping in a universal mechanism based on the Miller-Abrahams hopping model, and carrier out multiple-carrier Monte Carlo simulations to elaborate how, within this universal mechanism, that deep and shallow traps affect energetic disorder, transporting trajectories and carrier density in different ways. The results suggest transporting at shallow trapping always involve in a multiple-trapping-release process owing to frequent thermal reactivation, thus lead to winding transporting trajectory, increased effective energetic disorder and increased activation energy; while deep traps tend to immobilize the carriers and act as ionized scattering centers, thus mainly decrease the carrier density and just elongate the trajectory slightly. Investigating the change of mobility with continuous trap energy suggests a transition region, rather than a strict borderline exists between deep traps and shallow traps, and in this region carrier transport is controlled by both deep traps and thermal reactivation from shallow traps. These results will be of great value for understanding of the mechanisms, as well as designing of the devices in organic electronics. References: The Journal of Physical Chemistry C, Under review. The Journal of Physical Chemistry C 2012, 116, 19748-19754.
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Multi-Scale Modelling : Zoltan Soos
Authors : Lingyi Meng
Affiliations : Collaborative Innovation Center of Chemistry for Energy Materials, Xiamen University, Xiamen 361005, P.R. China
Resume : We developed a novel multiscale computational method which combines quantum mechanics and classical electromagnetics to balance the accuracy and efficiency in theoretical and numerical simulations. In the quantum mechanics/electromagnetics (QM/EM) method, the region of the system where active electron scattering processes take place is treated quantum mechanically, while the surroundings and substrates are described by Maxwells equations and classical semiconductor models. QM and EM models are solved respectively for different regions of the system in a self-consistent manner. Potential distributions and current densities at the interface between QM and EM regions are employed as the boundary conditions for the quantum mechanical and electromagnetic simulations. Having been successfully implemented for static and time-dependent (time-domain and frequency-domain) fields, the QM/EM method could be readily extended to model the interaction between charge and light and thus be applied to study complicated systems and phenomena, like emerging materials, electronics, solar cells, plasmonics, biology, catalysis, and etc.
Authors : Ofer Sinai (1), Oliver T. Hofmann (2), Georg Heimel (3), Patrick Rinke (2), Matthias Scheffler (2), Leeor Kronik (1)
Affiliations : (1) Department of Materials and Interfaces, Weizmann Institute of Science, Herzl 234, Rehovoth 7610001, Israel; (2) Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin-Dahlem, Germany; (3) Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D-12489 Berlin, Germany
Resume : The electronic effects of semiconductor doping pose a unique challenge for first-principles simulations, owing to the intractably large system sizes needed for an explicit treatment of such effects. Passivated semiconducting surfaces may be simulated using "pseudo-atoms" with fractional nuclear charge. However, charged semiconductor surfaces further require a simulation scheme that describes both charge transfer between the surface and a bulk reservoir, and the effects of the associated space-charge region (SCR). Here, we present an efficient multi-scale technique that achieves both goals. It is based on the introduction of a fixed two-dimensional sheet of charge inside a slab-based surface calculation, which mimics the SCR-related field, along with free charge from a reservoir, such that the system is neutral overall. The amount of transferred charge is obtained from Fermi level equilibration between the bulk SCR, treated semi-classically, and the electronic states of the slab/surface, which receive a quantum-mechanical treatment. The method is implemented as a "wrapper program" around standard electronic structure codes, which we call CREST - the Charge-Reservoir Electrostatic Sheet Technique. By comparison to tight-binding calculations encompassing the full SCR, we show that the approach successfully predicts scenarios of no, partial, or full Fermi-level pinning. We further show that CREST can be employed fully self-consistently in the context of density functional theory.
Authors : Binit Lukose, Santoshkalyan Rayadhurgam and Paulette Clancy
Affiliations : School of Chemical & Biomolecular Engr., Cornell University Ithaca, NY 14853, U.S.A.
Resume : The self-assembly of PbSe nanocrystals into superlattices for solar PV applications typically involve coating the nanocrystals in long non-polar ligands. In this paper, we explore the viability of using organic semiconducting phthalocyanine molecules as "spacers" to prevent the nanocrystals from sintering, instead of ligands. The difficulty with this approach is the tendency of the phthalocyanine molecules to aggregate and potentially to separate the nanocrystals in a manner that deleteriously affects the charge transport characteristics of the system. In this presentation, we use a combination of ab initio calculations of the tendency of the phthalocyanine to bind to itself rather than to the nanocrystals and Molecular Dynamics simulations to uncover the tendency of phthalocyanine to aggregate in a PbSe superlattice. We explore the preference of the phthalocyanine to adhere to specific facets of the nanocrystals and hence affect the symmetry of the superlattice. The effect of the addition of a metal ion in the phthalocyanine to change the binding proclivities will offer guidance to experimentalists to optimize the charge transport characteristics of these novel systems.
Drift-Diffusion Simulations : Reinder Coehoorn
Authors : L.J.A. Koster
Affiliations : Zernike Institute for Advanced Materials University of Groningen Nijenborgh 4 9747 AG Groningen
Resume : The performance of organic bulk heterojunction solar cells is strongly dependent on the donor/acceptor morphology. The tremendous complexity of this morphology makes it difficult to include morphological effects in numerical device models. In this talk, I will show an extension of 1D approaches to include morphological effects and introduce a fully 3D drift-diffusion model. 1D drift-diffusion approaches treat the blend of acceptor and donor materials as one effective medium. This model can describe the current-voltage characteristics in terms of basic physics and material parameters but is silent on morphology. Inhomogeneities in the film can be included by sub-dividing the active layer into regions that can be treated in 1D separately. In this way, the effect of fullerene clusters in a polymer/fullerene blend can be described by treating the polymer-rich regions and the fullerene clusters separately. The 3D nanoscale morphology of polymer/zinc oxide solar cells was used as a direct input into a fully 3D optoelectronic model. This model includes the effects of exciton diffusion and quenching; space-charge; interfacial charge separation and recombination; and drift and diffusion of charge carriers. Given the experimental morphologies, the corresponding differences in performance could be reproduced with a single set of parameters. Several morphological aspects that determine the efficiency are discussed and compared to other organic solar cells.
Authors : Manfred Gruber (1), Egbert Zojer (2), Ferdinand Schuerrer (1), Karin Zojer (1)
Affiliations : (1) Graz University of Technology, Institute of Theoretical and Computational Physics, 8010 Graz, Austria; (2) Graz University of Technology, Institute of Solid State Physics, 8010 Graz, Austria
Resume : Despite the vast progress recently made regarding the performance and stability of organic thin-film transistors (OTFTs), they still await the realization of their full potential. Their optimization requires a thorough understanding of performance-limiting factors. A crucial quantity in this context is the contact resistance. Here, we present a method for the direct extraction of the contact resistance from drift-diffusion-based transport simulations. By considering all mechanisms relevant for the injection process, we show that, in contrast to common wisdom, the energetic offset between the electronic states in the electrodes and the semiconductor is only one of many factors determining the contact resistance. We show that it is, in fact, the combination of (i) the injection barrier height and (ii) internal fields, which set the rates of thermionic and tunneling injection. The fields, in turn are determined by the device layout (e.g., top- vs. bottom-contact), device dimensions, and the point of operation. Another crucial factor that can change the contact resistance by orders of magnitude is the charge-carrier mobility in the semiconductor, which governs interface recombination and back-drift at the contact. [1,2]  M. Gruber, E. Zojer, F. Schuerrer, K. Zojer, Adv. Funct. Mater. 23 (2013) 2941.  M. Gruber, F. Schuerrer, K. Zojer, Org. Electron. 13 (2012) 1887.
Authors : K. Gärtner (1), H. Doan (1), Th. Koprucki (1), A. Glitzky (1), A. Fischer (2), K. Leo (2), R. Scholz (2), B. Lüssem (3)
Affiliations : (1) Weierstrass Institute Berlin, Germany; (2) IAPP TU Dresden, Germany, (3) Kent State University, Ohio, USA
Resume : Vertical organic triodes (VOT) are vertical organic transistors realized by a central base electrode between an emitter and collector electrode separated by organic semiconducting layers. In analogy to vacuum triodes, the conducting base electrode should have insulated pin-holes filled with the organic semiconductor. This device concept has been experimentally studied by different groups. One realization of the base electrode is an oxidized granular aluminum layer. By drift-diffusion simulations, we study the switching behavior of such devices, assuming an ideal insulating base electrode. Here, the emitter is set to the ground voltage, and unipolar electron conduction is considered. The simulation reveals three operation modes: a) for low base voltages, complete depletion around the base and low off-current, b) increasing the voltage up to half of the collector-emitter voltage, the current increases exponentially, and c) dominance of the diffusion current in the base-collector region, leading to a saturation of the total current. Experimentally, the off-current is defined by the leakage current at the base which is a small fraction of the emitter-collector current in the on-state. A sensitivity study shows that all details of the pin-hole geometry can only weakly influence in operation mode b), and in the saturation regime c), their impact becomes very small.
Authors : Matteo Porro (1), Sebastiano Bellani (2), Nicola Martino (2), Maria Rosa Antognazza (3), Riccardo Sacco (1), Guglielmo Lanzani (2)
Affiliations : (1) Dipartimento di Matematica, Politecnico di Milano - Center for Nano Science and Technology @PoliMi, Istituto Italiano di Tecnologia; (2) Dipartimento di Fisica, Politecnico di Milano - Center for Nano Science and Technology @PoliMi, Istituto Italiano di Tecnologia; (3) Center for Nano Science and Technology @PoliMi, Istituto Italiano di Tecnologia
Resume : Recently, devices based on organic photovoltaic polymers have emerged as a promising new approach for the development of retinal prostheses. It has been demonstrated that a single-component film of poly(3-hexylthiophene) is able to trigger neuronal firing upon illumination and also restores light sensitivity to irradiance in the daylight range in explants of degenerated rat retinas. An established concept is that light absorption gives rise to generation of free charges and current flow in the device, and induces also heat production which might influence the electrical response of the neuronal cell. In order to properly understand working principles of the device and to better interpret experimental data, we devise a family of computational models of different degree of complexity. At first, the polymer/solution interface is described with a Drift-Diffusion model from which a simpler lumped parameter formulation is consistently derived. Then a thermal diffusion model is developed and coupled with the known dependency on temperature of ion channel conductivities and membrane capacitance. Simulations show good agreement with measurements in fast illumination regimes. Future work will consider in more detail phenomena occurring with prolonged illumination of the device.
Authors : D. Bartesaghi (1,2), M. Turbiez (3), L. J. A. K. Koster (1)
Affiliations : (1) Photophysics and Optoelectronics, Zernike Institute for Advanced materials, University of Groningen, Nijenborgh 4, NL-9747AG Groningen, The Netherlands; (2) Dutch Polymer Institute, P. O. Box 902, 5600AX Eindhoven, The Netherlands; (3) M. Turbiez: BASF Schweiz AG, Schwarzwaldallee 215, CH-4002 Basel, Switzerland
Resume : Quantitative modelling of the effect of active layer morphology on the performance of organic solar cells has proven difficult yet crucial for predictive modelling. Morphological parameters, such as the extent and the composition of donor- and acceptor-rich domains, influence both the charge generation and the charge transport throughout the active layer. In this work we analyse a polymer:fullerene system based on a small bandgap polymer mixed with a fullerene derivative that is capable of efficiencies higher than 5%. By changing the processing conditions, the morphology can be varied from a coarse separated morphology, with fullerene domains embedded in a polymer-rich matrix, to a completely mixed layer. We experimentally characterize the carrier transport and the strength of the bimolecular recombination and discuss their relation with morphological features. We then propose a model based on a drift-diffusion approach that combines electrical and morphological parameters; with this model, we quantify the contribution of each phase to the total current. Under operating conditions, we find that most of the current comes from the interfacial region between the phases, with holes travelling through the matrix and the electrons through the blobs. This device model consistently connects morphological features to overall device performance.
Kinetic Monte-Carlo Simulations : Jan-Anton Koster
Authors : R. Coehoorn (1,2), H. van Eersel (2,1), P.A. Bobbert (2,1) and R.A.J. Janssen (2,1)
Affiliations : (1) Philips Research Eindhoven, High Tech Campus 4, 5656 AE Eindhoven, The Netherlands; (2) Eindhoven University of Technology, P.O. 513, 5600 MB Eindhoven, The Netherlands
Resume : Organic Light Emitting Diode (OLED) technology for large area lighting is presently developing from design-focussed special-lighting applications towards general-lighting applications for homes and offices. Current developments are expected to enable the realization of added functionality such as flexibility, transparency in the off-state, and color-tunability. In this talk, an overview is given of the recent development in our laboratory of an advanced numerical method for simulating at a molecular scale the three-dimensional (3D) current density, the emission colour and the efficiency roll-off in state-of-the-art multilayer white and monochrome OLEDs. We use 3D Monte Carlo (MC) simulations, within which the energetic disorder and the spatially random positions of the host (matrix) and guest (emissive dye) molecules are taken into account. Also, all excitonic processes, such as radiative and non-radiative decay, exciton dissociation, exciton diffusion, exciton-polaron quenching, and exciton-exciton annihilation, are included. We show (i) how MC simulations can be used to analyze dedicated photoluminescence experiments, employed to obtain information about the excitonic interaction constants, (ii) how the efficiency roll-off depends on the charge transport and excitonic interaction parameters, and (iii) how the simulations can be used to design optimized OLED layer stacks.
Authors : Michael C. Heiber (1), Vladimir Dyakonov (1,2), Carsten Deibel (1)
Affiliations : (1) Experimental Physics VI, Julius-Maximilians-University of Würzburg, 97074 Würzburg, Germany; (2) Bavarian Center for Applied Energy Research (ZAE Bayern), 97074 Würzburg, Germany
Resume : The emergence of distinct electronic states called charge transfer (CT) states have been identified in organic donor-acceptor blends and have been shown to have a major impact on photogeneration, recombination, and the open-circuit voltage in organic photovoltaic devices. Such states are not found in the pure material components and arise in blends due to interactions between the donor and acceptor molecules. The weak optical absorption and emission of CT states has been measured to characterize their energy and their fundamental behavior. Here, we present recent updates on our efforts to simulate and model the absorption, electroluminescence, and photoluminescence spectra of the CT states measured in different donor-acceptor blends. By combining Marcus theory with energetic disorder and bulk heterojunction morphology models into a kinetic Monte Carlo simulation, we extract fundamental properties of the CT states from the experimental optical characterization techniques. This more detailed analysis of the CT state properties will allow a greater understanding of their importance to the operation of organic photovoltaic devices and allow scientists to target new material blends with optimum CT state properties.
Authors : Harm van Eersel (1,2), Peter Bobbert (1), René Janssen (1), Reinder Coehoorn (2,1)
Affiliations : (1) Eindhoven University of Technology; (2) Philips Research Eindhoven
Resume : It is well-known that the efficiency of organic light-emitting diodes (OLEDs) drops if operated at higher luminance levels by increasing the current-density: the so-called roll-off. Not well understood, however, is role of the excitonic loss mechanisms on the molecular scale: exciton-polaron quenching and exciton-exciton annihilation. There has been considerable debate in the literature on which mechanism is dominant [e.g. Adv. Mat. 25, 6801 (2013)]. Using a kinetic Monte Carlo (kMC) simulation of a 3D model of the complete device stack, we can now study these intrinsic processes on the molecular scale. By extending our electronic kMC model, as presented earlier [Nat. Mat. 12, 652 (2013)], with both radiative decay processes and non-radiative loss mechanisms, we can study the complex interplay of electrons and excitons in a complete multi-layer device. We have applied this methodology to two prototypical OLED stacks [Phys. Rev. B 77, 235215 (2008)] and show that we can describe both the experimental current-density versus voltage characteristics and the roll-off using only physical parameters as input for the simulation, i.e. without any fitting. Moreover, we can now study the role of the different molecular-scale mechanisms for different scenarios and determine which process is the dominant efficiency-limiting process.
Authors : Geoffrey Hutchison
Affiliations : Department of Chemistry University of Pittsburgh
Resume : A key challenge is to create computational methods that can properly handle charge transport in organic electronics and particularly solar cells, bridging across multiple length and time scales, in the presence of defects, traps, impurities, disorder, and particularly variations in nanomorphology. Our research has focused particularly on developing high-performance Monte Carlo charge transport simulations that explicitly treat disorder and defects of various kinds. Charge transport parameters are, where possible, derived from first-principals calculations, particularly the charge delocalization length. Our simulations are validated against in-house and external experimental measurements and have reproduced several non-obvious effects, including a negative differential resistance (NDR) effect in organic semiconductor mixtures. We have also developed a delocalized charge model capable of classically treating accurate electrostatic interactions in organic solar cells. Currently, a new database of idealized, realistic, and intermediate two-phase morphologies are being generated to correlate between materials architecture and charge transport properties in organic solar cells. We find that "obviously optimal" designs suggested by many in the community may perform worse than conventional bulk heterojunction morphologies. Progress towards identifying important structure-property relationships for optimal solar cell efficiencies will be discussed.
Authors : Marko Mladenovic, Nenad Vukmirovic
Affiliations : Scientific Computing Laboratory, Institute of Physics Belgrade, University of Belgrade
Resume : We have investigated the effects of dynamic disorder in side chains, main chains and both of them on the electronic structure of crystalline poly-3-hexylthiophene (P3HT). To obtain the atomic configurations of crystalline P3HT, we used Monte Carlo simulations. Electronic structure was found using the charge patching method , based on density functional theory, and overlapping fragments method . From 100 different realizations for 2 different stable configurations of crystalline P3HT we calculated: electronic density of states and hole localization in the backbone direction (one chain) and in the π-π stacking direction (different chains). We found that effect of side chains disorder is more pronounced in the more stable configuration of P3HT than in the other one due to wider distribution of the torsion angle between thiophene ring and side chain. Disorder of main chains affects the electronic structure more than disorder of side chains. Finally, in the case of disorder of both main and side chains, effects of disorder on the electronic structure are most pronounced and they are similar to the effects of complete disorder of P3HT in the amorphous phase.   N. Vukmirovic, L. Wang, J. Chem. Phys. 128, 121102 (2008)  N. Vukmirovic, L. Wang, J. Chem. Phys. 134, 094119 (2011)  N. Vukmirovic, L. Wang, J. Phys. Chem. B 115, 1792 (2011)
Authors : Niels J. van der Kaap, L. Jan Anton Koster
Affiliations : Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4 9747, AG, Groningen, The Netherlands
Resume : Kinetic Monte Carlo methods play an important role in organic semiconductor research. They allow the translation of nanoscale features into macroscopic properties like mobility and current density, by explicitly modeling phonon-activated hopping of particles through a discrete lattice. A proper description of the charge transport requires large simulation volumes, both for decreasing statistical errors and to be able to describe large morphological features. However, the serial nature of the algorithm and the linear scaling of runtime with volume impose a limit on the maximum volume that can still be solved in reasonable time. To overcome this linear scaling, we partition the volume into small cubes that are treated as separate simulations and can be executed in parallel. A checkerboard approach is used to manage carrier exchange between blocks in order to avoid boundary conflicts. To improve the performance of the Coulomb interaction calculations, the code is implemented to run massively parallel on a computer graphics board (GPU). This currently allows simulation volumes of up to 512x512x128 grid points, with full periodic boundary conditions. Without Coulomb interactions, the electric field, density and time dependence of the charge carrier mobility matches the results of a serial version of the algorithm. With Coulomb interactions enabled, about 10.000 charge hops can be evaluated per second at a density of 0.1, taking into account a cut-off radius of 31 lattice sites.
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Molecular-Dynamics Modelling : Paulette Clancy
Authors : Claudio Zannoni
Affiliations : Dipartimento di Chimica Industriale Toso Montanari, Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy.
Resume : Charge and energy transport in organic semiconductors, particularly in those that present liquid crystal (LC) phases, are strongly affected by molecular organizations and their orientational and positional order, particularly close to relevant interfaces [1,2]. With atomistic molecular dynamics we now start to predict realistic morphologies at various working conditions and here we present some applications to LC systems: e.g. cyanobiphenyls (5CB, 8CB) [3,4] and quinquephenyl  and to some more specific organic electronics problems [1,2], e.g. discotic LC solar cell design principles . We also tackle the problem of predicting alignment for LC close to a solid interface, e.g. for thin films of 5CB on H terminated silicon  and on crystalline (cristobalite) or amorphous silica surfaces of various roughness .  L. Muccioli et al., Topics Curr.Chem, (2013) DOI: 10.1007/128_2013_470  J. Cornil et al.,Accounts Chem. Res., 46,434 (2013)  G.Tiberio et al., ChemPhysChem, 10,125 (2009)  M. F. Palermo et al., J. Chem. Phys.,138, 204901 (2013)  Y. Olivier et al., ChemPhysChem, accepted (2014)  J.Idé et al., JACS, accepted (2014)  A. Pizzirusso et al., Chemical Science, 3, 573 (2012)  O. M. Roscioni et al., Langmuir, 29, 8950 (2013)
Authors : J. Spencer (1), F Gajdos (1), M Dupuis (2), J Blumberger (1)
Affiliations : (1) University College London, London, UK; (2) Pacific Northwest National Laboratory, Richland, USA
Resume : Historically, two descriptions for charge transport in organic semiconductors (OS) have been used, band-like and polaronic hopping. Neither of these limiting cases provide a fully satisfactory description. The band-like mechanism relies on Bloch-states and breaks down for ambient temperatures, while the hopping mechanism breaks down when reorganization energy becomes comparable to electronic coupling. The latter is e.g. the case for the important class of fullerene materials (C60,PCBM)^1. Clearly, in order to obtain a realistic picture of the conduction mechanism in OSs, the development of simulation techniques is required that does not make any assumptions about charge localization. In my presentation I will describe our recent efforts to develop such a methodology based on Tully fewest-switches surface hopping molecular dynamics. Here the wavefunction of the excess charge is propagated according to the t-dependent Schroedinger equation coupled to the classical equations of motions for the nuclei described by molecular force fields. The algorithm naturally takes into account diagonal and off-diagonal thermal disorder due to thermal fluctuations in full 3-dimensional space and allows charge to spontaneously spread or localize. First applications to fcc-C60 will be discussed and compared to the results of hopping models evaluated using kinetic Monte Carlo. References:  F Gajdos, H Oberhofer, M Dupuis, J Blumberger, J. Phys. Chem. Lett 4, 1012 (2013).
Authors : Claudia Caddeo (1,2), Alessandro Mattoni (2)
Affiliations : (1) Universita degli Studi di Cagliari, Dipartimento di Fisica, Cittadella Universitaria, 09042 Monserrato; (2) Istituto Officina dei Materiali del Consiglio Nazionale delle Ricerche (CNR-IOM), c/o Dipartimento di Fisica, Cittadella Universitaria, 09042 Monserrato
Resume : Since polymer-based devices are typically synthesized from solution, the polymer solubility plays a crucial role in determining their performances: for example, in solution processed poly(3-hexylthiophene) transistors the field effect mobility is markedly different depending on the polymer miscibility in solvents. The prediction of the solubility properties via atomistic simulations is an interesting challenge, but only few works exist on the topic. We apply the Flory-Huggins theory (that is typically used to study the miscibility of macromolecules) for the first time to study the solubility of conjugated conducting polymers in a typical organic solvent via atomistic simulations. The properties of the isolated solvent and polymer are correctly reproduced, and the calculated solubilities of the oligo(3-alkylthiophene)s in tetrahydrofuran as a function of their side chains lengths are in agreement with available experimental data. Present investigation shows that the atomistic approach based on molecular dynamics is a powerful tool to study the solubility of alkylthiophenes in molecular solvents.  J.-F. Chang et al., Chem. Mater. 2004, 16,4772-4776  P. J. J. Flory, J. Chem. Phys. 1942, 10, 51  C. Caddeo et al., Macromolecules 2013, 46 (19), 8003-8008
Macroscopic and Electrostatic Models : Georg Heimel
Authors : Davide Vanzo, Benjamin J. Topham and Zoltán G. Soos
Affiliations : Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA
Resume : The electronic polarizability of conjugated molecules is simply related to the dielectric properties of thin films used in organic electronic devices when intermolecular overlap is small. Films of nanoscale thickness combine discrete molecular layers with a continuous classical dielectric medium. Arrays of polarizable points with induced dipoles µ= a F describe molecular crystals or crystalline films, where a is given by quantum theory and the internal electric field F includes contributions from all dipoles. Dipole-field sums are evaluated directly for monolayer and multilayer films as well as for crystals with lower than cubic symmetry, taking into account the conditional convergence of 3D sums. Dipoles fields are related to depolarization and Lorentz factors L that govern the enhancement or reduction of F compared to the uniform applied electric field. Films based on cubic lattices achieve isotropic L = 1/3 at thickness d ~ 10nm, which is considerably greater than needed for uniform F. The dependence of L on d and lattice type is found at constant a per unit volume. Dipole field sums and Lorentz factors are extended to lattices whose unit cells have identical a tensors with the same or with different principal axes.
Authors : G. Ulisse, F. Brunetti
Affiliations : Department of Electronic Engineering, University of Rome "Tor Vergata"
Resume : Organic photovoltaic is one of the most important research topic and many scientific teams are continuously searching for new materials and new solar cell architectures. In particular the possibility of using a transparent conductive oxides (TCO) free substrate is one of the most challenging issue. In this context the use of graphene as semitransparent electrode has been proposed and photovoltaic devices have been already demonstrated  also for flexible devices . In this work we propose an approach in which the graphene electrode is 3D structured and acts not only as a contact, but also as photonic crystal. We simulated several solar cell architectures considering different materials, such as PEDOT or MoO3 as HTL and P3HT:PCBM or PTB7:PC70BM as active layer. The study has been performed using an electromagnetic simulator permitting the calculation of absorbed photons in the active layer for any geometrical configuration. For the optical modeling of the graphene, we considered measured values of refractive index shown in literature . We optimized the grating structure demonstrating an enhancement of the optical absorption up to 27% respect to a flat solar cell with same active layer thickness. The best performing gratings have a pitch with dimension in the order of hundreds of nanometer, compatible with available technologies such as nanoimprint lithography. 1 Angew. Chem. 120, 3032 3034, 2008 2 Adv. Mater. 25, 42964301, 2013 3 Appl. Phys. Lett. 97, 091904, 2010
Authors : Jasper J. Michels (1), Ellen Moons (2), Natalie Stingelin (3), Gerwin H. Gelinck (1), René A. J. Janssen (4), Dago de Leeuw (5), Paul W. M. Blom (5)
Affiliations : (1) Holst Centre/TNO, High Tech Campus 31, 5656 AE Eindhoven, The Netherlands; (2) Department of Engineering and Physics, Karlstad Univeristy, SE-65188 Karlstad, Sweden; (3) Department of Materials and Centre for Plastic Electronics, Imperial College, SW7 2AZ London, UK; (4) Molecular Materials and Nanosystems, Eindhoven University of Technology, 5612 AZ Eindhoven, The Netherlands; (5) Max Planck Institut für Polymerforschung, 55128 Mainz, Germany
Resume : This paper treats examples of active layers of various organic electronics devices, which morphology is controlled by solution-phase liquid-liquid de-mixing of components with different electronic properties. Specifically, focus will be on devices relevant to organic memory elements and photovoltaics. The interplay between bulk-interactions and substrate-induced effects will be treated, providing insight into why and under what conditions the initial stages of de-mixing, such as substrate-induced stratification, may still be visible in the morphology of the final dry layers. Time resolved two- and three-dimensional continuum simulations will be presented and benchmarked against experimental results with focus on gaining insight in the link between phase behavior and -purity and opto-electronic performance.
Authors : David Westerberg, Axel Lagerlöf, Peter Andersson Ersman, David Nilsson
Affiliations : Acreo Swesish ICT AB; EU FP7 project SmartEC
Resume : Accurate simulation models for active circuit elements in logic circuits facilitates circuit design and provides an insight in circuit functionality while reducing the need for prototyping. A PSPICE model for a standard silicon based transistor has been adapted to fit the behavior of organic electrochemical transistors (OECT, Acreo) and inorganic electrochemical thin film transistors (ECTFT, UNINOVA). OECTs based on PEDOT:PSS were screen printed and transfer, output and dynamic switch characteristics were measured and used to calibrate the PSPICE model, which was subsequently used to predict the behavior of logic circuits based on OECTs. The model was further modified to fit transfer, output and dynamic switch characteristics obtained from ECTFT measurements. Semi-printed prototypes of logic NOT- and NAND-gates comprising OECTs were fabricated and used in the construction of a decoder and a multiplexer circuit. A good agreement between simulations and measurements was observed for OECT characteristics and logic circuits based on OECTs regarding transient behavior, signal amplitudes and presence of logic hazards. A fairly good agreement was obtained between measured and simulated transfer, output and dynamic switch data of the ECTFT. The level 1PSPICE model is better suited for OECT simulation due to deviations between OECT and ECTFT behavior. Higher levels of the PSPICE model might improve simulation and measurement correlation for ECTFTs.
Authors : Damien Thompson
Affiliations : Dept of Physics & Energy and Materials & Surface Science Institute, University of Limerick
Resume : In this talk I will describe how a combination of electronic structure calculations and classical molecular dynamics computer simulations can aid experiments in the design of functional organic-inorganic interfaces. I will present recent results on experimental/simulation co-design of ferrocene-alkanethiolate self-assembled monolayer (SAM) films on silver; these films exhibit an atom-level sensitivity in their electrical properties.[1,2] I will also describe how simulations were used to inform the synthesis and interlinking of dendrimer-wrapped gold nanoparticles, potential interconnects in future molecular electronic devices.  References: 1. Li, Y.; Nerngchamnong, N.; Cao, L.; Hamoudi, H.; Roemer, M.; Sriramula, R.; Thompson, D.; Nijhuis, C.A. (2013) Fine-Tuning the Direction and Strength of Molecule-Electrode Coupling by van der Waals Interactions. submitted. 2. Nerngchamnong, N.; Li, Y.; Qi, D.; Jian, L.; Thompson, D.; Nijhuis, C.A. (2013) The Role of van der Waals Forces in the Performance of Molecular Diodes. Nature Nanotechnology, 8, 113-118. 3. Thompson, D.; Hermes, J.P.; Quinn, A.J.; Mayor, M. (2012) Scanning the Potential Energy Surface for Synthesis of Dendrimer-Wrapped Gold Clusters: Design Rules for True Single-Molecule Nanostructures. ACS Nano, 6, 30073017.
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