2nd tyc energy materials workshop – 7 - 8th june 2012 king ... · 2nd tyc energy materials...
TRANSCRIPT
2nd
TYC Energy Materials Workshop – 7 - 8th
June 2012
King’s College London
Abstracts
Photogeneration and mobility of charges in photovoltaic materials
based on conjugated polymers and covalent organic frameworks Laurens D.A. Siebbeles Optoelectronic Materials Section, Department of Chemical Engineering, Delft University of
Technology, The Netherlands
[email protected] We studied the mechanism of charge carrier photogeneration in blend films of the polymer poly(3-hexyl-thiophene) (P3HT) and the electron accepting fullerene PCBM by ultrafast optical and terahertz spectroscopy. Photoexcitation leads to direct formation of free mobile electrons and holes with a yield that is independent on temperature.1,2 This implies that the Onsager-Braun model with an initial electron-hole distance of the order of nanometers is inadequate. This is atributed to charge delocalization causing the electron-hole Coulomb attraction to be negligible. The near IR part of the solar spectrum can be harvested by adding PbS quantum dots to yield ternary P3HT:PCBM:PbS blends. Using ultrafast optical specroscopy, we found that photoexcitation of PbS quantum dots leads to hole transfer to P3HT and electron transfer to PCBM. However, we found from terahertz photoconductivity measuerements that the excess charges in the organic components are immobile. This is explained by Coulomb interaction with charge-induced dipoles in the highly polarizable quantum dots. A new class of materials with great promise for application in organic opto-electronics consists of Covalent Organic Frameworks (COFs). We studied the dynamics of excitons and charges within COFs consisting of phthalocyanine units that are strongly coupled by pi-pi stacking in an eclipsed configuration. The charge mobility is virtually independent on temperature, which is typical for a band-like mechanism of charge motion, in contrast to hopping via localized states. According to quantum mechanical simulations, eclipsed stacking of the phthalocyanine units can lead to a high charge mobility of ~100 cm2/Vs, which largely exceeds that for conventional organic semiconductors. References
1 J. Piris, T.E. Dykstra, A.A. Bakulin, P.H.M. van Loosdrecht, W. Knulst, M.T. Trinh, J.M. Schins and
L.D.A. Siebbeles, J. Phys. Chem. C, 2009, 113, 14500
2 W.J. Grzegorczyk, T.J. Savenije, T.E. Dykstra, J. Piris, J.M. Schins and L.D.A. Siebbeles, J. Phys. Chem. C, 2010, 114, 5182
3 Delocalization and Mobility of Charge Carriers in Covalent Organic Frameworks, S. Patwardhan, A.A. Kocherzhenko, F.C. Grozema and L.D.A. Siebbeles, J. Phys. Chem. C, 2011, 115, 11768.
Charge separation and recombination in organic photovoltaics
interfaces Alessandro Troisi Department of Chemistry, University of Warwick, U.K.
[email protected] The key process in organic photovoltaics cells is the separation of an exciton, close to the donor/acceptor interface into a free hole (in the donor) and a free electron (in the acceptor). In an efficient solar cell, the majority of absorbed photons generate such hole-electron pairs but it is not clear why such a charge separation process is so efficient in some blends (for example in the blend formed by poly(3-hexylthiophene) (P3HT) and a C60 derivative (PCBM)) and how can one design better OPV materials. The electronic and geometric structure of the prototypical polymer:fullerene interface (P3HT:PCBM) is investigated theoretically using a combination of classical and quantum simulation methods. It is shown that the electronic structure of P3HT in contact with PCBM is significantly altered compared to bulk P3HT. Due to the additional free volume of the interface, P3HT chains close to PCBM are more disordered and, consequently, they are characterized by an increased band gap. Excitons and holes are therefore repelled by the interface. This provides a possible explanation of the low recombination efficiency and supports the direct formation of “quasi-free” charge separated species at the interface. This idea is further explored by using a more general system-independent model Hamiltonian. This talk will discuss how and when a combination of computational and theoretical models can truly contribute to organic electronics and will provide few examples of genuine material properties predictions based on computational chemistry methods. References
1 T. Liu, D.L. Cheung and A. Troisi A, Phys. Chem. Chem. Phys. 2011, 13, 21461.
2 D.P. McMahon, D.L. Cheung, Troisi A, J. Phys. Chem. Lett. 2011, 2, 2737. 3 A. Troisi, Chem. Soc. Rev. 2011, 40, 2347. 4 A. Troisi, Organic Electronics, 2011, 12, 1988.
Electronic structure of organic photovoltaic compounds from first principles Ismaila Dabo
Université Paris-Est, CERMICS, Projet Micmac ENPC-INRIA, 6-8 avenue Blaise Pascal, Marne-la-
Vallée, France
The fundamental study of photoconversion mechanisms in organic photovoltaic devices from first principles raises important conceptual and computational challenges. Before addressing the dynamical properties of strongly bonded electron-hole pairs in organic semiconductors of low dielectric constant, the first difficulty is to overcome the predictive deficiency and computational cost of conventional first-principles calculations in describing the electronic and dielectric properties of donor-acceptor complexes. To address current limitations, orbital-dependent density-functional theory (OD-DFT) methods represent promising alternatives. In this presentation, we demonstrate that OD-DFT based upon Koopmans’ condition1 is apt at describing donor and acceptor levels within 0.1-0.4 eV and 0.2-0.6 eV relative to experiment, which is comparable to the predictive performance of many-body perturbation theory methods. Furthermore, Koopmans-compliant dielectric responses for semiconducting polymers are predicted in close agreement with more expensive wave-function methods. This level of predictive performance allows the physically accurate and computationally tractable electronic-structure description of photoactive donor-acceptor materials from first principles.
References
1 I. Dabo, A. Ferretti, N. Poilvert, Y. L. Li, and N. Marzari, Phys. Rev. B, 2010, 82, 115121. (selected as an Editors' suggestion)
Exciton and charge transfer dynamics at organic material interfaces Peter J. Rossky Institute for Computational Engineering and Sciences and Department of Chemistry &
Biochemistry, 1 University Sta. A5300, University of Texas at Austin, Austin, Texas 78712 USA
[email protected] In order to develop a working chemical intuition about organic electronic materials, and especially to develop design principles for organic photovoltaic materials, it appears to be imperative to understand the relationship between molecular-level structure and the electronic excited state phenomena of exciton migration and charge separation dynamics both within conjugated polymers and at organic donor/acceptor interfaces. In this talk, I will describe recent progress in using a mixed quantum/classical non-adiabatic molecular dynamics simulation approach that employs an all-atom description of the intermolecular interactions coupled with a semi-empirical (PPP) electronic Hamiltonian. Results exploring several systems at ambient temperature will be discussed. These systems will include phenylene-vinylene and thiophene oligomers, as well as cyanine and fullerene components. The roles of molecular structural fluctuations and intermolecular electronic couplings, as well as the roles of donor and acceptor excited state alignments and of intrinisic interfacial fields, will be discussed.
Simulating charge carrier mobilities in n-type semiconducting organic
solar cell components Harald Oberhofer Theoretical Chemistry, TU Munich, Lichtenbergstr. 4, D-85747 Garching, Germany
Organic solar cells are envisaged as a promising alternative to silicon based solar cells. They are cheap and easy to produce, light and flexible, and easily deployed on walls or roofs. Unfortunately, these advantages currently come at the price of small photo-electric conversion efficiencies. One of the loss mechanisms is inefficient charge transfer in the semiconducting layers. To gain atomistic insight into this process, we used advanced density functional theory (DFT) based methods to investigate the electron-conducting properties of modified fullerene crystals, which form in the nano-crystalline domains of the n-type semiconducting layer in organic solar cells.
Based on the DFT results we employed an analytically solvable kinetic hopping model which allowed us to predict macroscopic electron mobilities, also accessible to experiments.
In our contribution we will first briefly discuss the techniques used to estimate electron mobilities from computer simulations. Then we present our calculations on modified fullerene crystals. We studied C60 and [6,6]-phenyl-C61-butyricacid-methyl-ester (PCBM) for a number of different crystal lattices which are commonly found in experiments. These results can represent a starting point for a microscopic understanding of structure-mobility relationships in these important materials.
Theoretical Modelling of the Key Electronic Processes in Organic Solar
Cells Jérôme Cornil Laboratory for Chemistry of Novel Materials, University of Mons, Belgium.
[email protected] In this presentation, I will illustrate that theoretical modeling at the atomistic scale can prove very useful to understand and optimize all key electronic processes governing the operation of organic solar cells [1]. In such devices, light is converted into charges following the succession of many different events: (i) absorption of the incident light; (ii) migration of the excitations to interfaces between organic donor and acceptor units, in analogy with the p-n junctions in inorganic solar cells; (iii) dissociation of the excitations into charge carriers; (iv) escape of the charges from their mutual Coulomb attraction and charge transport towards the electrodes; (v) collection of the charges at the electrodes. Quantum-chemical calculations are exploited to tailor the optical properties of the materials in order to optimize the absorption of the solar light and to describe the parameters controlling energy transfer processes in supramolecular architectures, excitation dissociation into charge carriers [2], and charge transport properties. Monte-Carlo simulations are then used to propagate excitations and charges in supramolecular architectures on the basis of the parameters obtained at the quantum-chemical level and estimate exciton diffusion range [3] and charge mobility [4], respectively. We will give also a special emphasis to the nature of the electronic interactions occurring between the donor and acceptor moieties in the solar cells and the resulting implications for device performance [5]. Finally, we will rely on quantum-chemical calculations and micro-electrostatic models to shed light into the driving force responsible for the generation of free carriers in the cells [6]. References
1 J.L. Brédas, J.S. Norton, J. Cornil, and V. Coropceanu, Acc. Chem. Res. 2009, 42, 1691.
2 V. Lemaur, M.C. Steel, D. Beljonne, J.L. Brédas, and J. Cornil, J. Am. Chem. Soc. 2005, 127, 6077. 3 L. Viani, L. Poulsen Tolbod, M. Jazdzyk, G. Patrinoiu, F. Cordella, A. Mura, G. Bongiovanni, C. Botta,
D. Beljonne, J. Cornil, M. Hanack, H.J. Egelhaaf, and J. Gierschner, J. Phys. Chem. B, 2009, 113, 10566.
4 S. Coropceanu, J. Cornil, D.A. da Silva Filho, Y. Olivier, R. Silbey, and J.L. Brédas, Chem. Rev. 2007, 107, 926.
5 D. Beljonne, J. Cornil, L. Muccioli, C. Zannoni, J.L. Brédas, and F. Castet, Chem. Mater. 2011, 23, 591. 6 S. Verlaak, D. Beljonne, D. Cheyns, C. Rolin, M. Linares, F. Castet, J. Cornil, and P. Heremans, Adv.
Funct. Mat. 2009, 19, 3809.
Threaded molecular wires as model conjugated polymers with
controlled interstrand interactions Franco Cacialli Department of Physics and Astronomy, and London Centre for Nanotechnology, University College
London, Gower Street, WC1E 6BT, London, UK
[email protected] Threaded molecular wires made with conjugated-polymers-based polyrotaxanes offer an example of a “bottom-up” approach to electroluminescent nanostructures.1 This class of materials is engineered at a supramolecular level by threading a conjugated macromolecule, such as poly(para-phenylene), poly(4,4’-diphenylene vinylene) or poly(9,9’-fluorene) through α- or β-cyclodextrin rings, so as to reduce intermolecular interactions and solid-state packing effects, that red-shift and partially quench the luminescence. Such a supramolecular approach preserves the fundamental semiconducting properties of the conjugated wires, and is effective at both increasing the photoluminescence efficiency and blue-shifting the emission of the conjugated cores, in the solid state, while still allowing charge-transport and thus electroluminescence (EL). The reduced tendency for polymer chains to aggregate shows in both solid-state films and in solution (as probed by fluorescence decay dynamics) and allows solution-processing of individual polyrotaxane wires onto substrates, as revealed by scanning-force microscopy.2 Control of the threading ratio is possible, thereby resulting in fine tuning of the excitonic vs. aggregate contribution to the luminescence, as well as of the electro- and photo-luminescence efficiency.3 A particularly intriguing possibility afforded by supramolecular and nanoscale encapsulation of these soluble semiconductors is the suppression of energy transfer which enables both fabrication of white-emitting LEDs,4 and achievement of unprecedentedly broad gain bands, in “conjugated” blends of different semiconductors, with potential application to broad-band amplifiers and multi-colour lasers.5 Water solubility of rotaxanes carrying un-susbtituted cyclodextrins also enables their incorporation into stretchable matrices and thus strong polarisation of absorption and emission from such films.6 References
1 F. Cacialli, J.S. Wilson, J. J. Michels, C. Daniel, C. Silva, R. H. Friend, N. Severin, P. Samorì, J. P. Rabe, M. J. O'Connell, P. N. Taylor, H. L. Anderson. "Cyclodextrin-threaded conjugated polyrotaxanes as electroluminescent insulated molecular wires with reduced interstrand interactions". Nature Materials. 1, 160-164 (2002).
2 J.S. Wilson, M.J. Frampton, J. J. Michels, L. Sardone, G. Marletta, R. H. Friend, P. Samorì, H. L. Anderson, F. Cacialli. "Supramolecular complexes of conjugated polyelectrolytes with poly(ethylene oxide): multifunctional luminescent semiconductors exhibiting electronic and ionic transport ". Adv. Mat. 17, 2659–2663 (2005).
3 S Brovelli, G. Latini, M. J. Frampton, S. O. McDonnel, F. Oddy, O. Fenwick, H. L. Anderson and F. Cacialli. “Enhanced electroluminescence of threaded molecular wires via fine tuning of their threading ratio". Nano Letters 8, 4546-4551 (2008).
4 S. Brovelli, F. Meinardi, G. Winroth, O. Fenwick, G. Sforazzini, M. J. Frampton, L. Zalewski, J. A. Levitt, F. Marinello, P. Schiavuta, K. Suhling, H. L. Anderson, and F. Cacialli. “White electroluminescence by control of resonant energy-transfer in organic-soluble
polyrotaxane:polyfluorene blends”. Adv. Funct. Mat. 20, 272-280 (2010).
5 S. Brovelli, T. Virgili, M.M. Mroz, G. Sforazzini, A. Paleari, H.L. Anderson, G. Lanzani, and F. Cacialli. “Ultra-broad optical gain and two-colour amplified spontaneous emission in binary blends of insulated molecular wires”. Adv. Mat. 22, 3690-3694 (2010).
6 F. Di Stasio, P. Korniychuk, S. Brovelli, P. Uznanski, S. O. McDonnel, G. Winroth, H. L. Anderson, A. Tracz, F. Cacialli. "Highly-polarized emission from oriented films incorporating water-soluble conjugated polymers in a polyvinyl alcohol matrix”. Adv. Mat. 23, 1855-1859 (2011).
Atomistic simulations of thermal transport in nanostructured
semiconductors Giulia Galli Department of Chemistry and Department of Physics, University of California, Davis
We present the results of atomistic simulations of heat transport in realistic models of ordered and disordered semiconductors. In particular we discuss the thermal properties of Si and SiGe at the nanoscale (nano-wires [1] and nanoporous films [2]), as obtained from molecular dynamics simulations and Botlzman transport equation calculations. We also discuss recent results on disordered systems, including amorphous Si [3] and SiO2. References
1 M. K. Y. Chan, J. Reed, D. Donadio, T. K. Mueller, Y. S. Meng, G. Galli and G. Ceder, Phys. Rev. B 81, 174303 (2010); D. Donadio and G. Galli, Nano Lett. 10, 847 (2010); D.Donadio and G. Galli, Phys. Rev. Lett. 102, 195901 (2009).
2 Y. He, D. Donadio and G. Galli, Nanolett. 2011 (accepted); Y. He, D. Donadio, Joo-H. Lee, J. C. Grossman, and G. Galli, ACS Nano 5, 1839 (2011).
3 Y. He, D. Donadio and G. Galli, Appl. Phys.Lett. 98, 144101 (2011).
Nanoscale thermoelectric metrology: model requirement for accurate measurements. Alexandre Cuenat
National Pysical Laboratory, Hampton Road, Teddington, UK
Significant improvements in the coefficient of performance of thermoelectric materials have been recently made using nanostructures. However, the performance of devices based on these emerging materials has proven to be particularly difficult to measure with precision. In typical operating conditions, parts of these materials are under strongly non-equilibrium conditions: - large temperature gradient and large current density. Furthermore, standard transport measurements assume homogeneous properties or distribution of carriers. In this contribution we present our latest effort to validate the metrology tools required to measure the distribution of local Seebeck coefficients accurately enough to enable the integration of these new materials into commercial devices. Most of the industry-standard softwares are based on drift-diffusion equation. They are important because they are used to optimize not only materials , but structures and processes as well. With this in mind, a generalized drift–diffusion equation in the relaxation-time approximation is presented and numerically solved. We argue that the model describe accurately the spatially localized the Seebeck coefficient. Numerical results for degenerate materials, where the thermoelectric power factor is maximized, are presented and contrasted with atomic force microscopy results. Based on this model, we discuss how local measurement can reduce uncertainty in bulk measurements. We conclude by discussing further modelling requirements to enable the faster development of new improved thermoelectric materials based on energy-filtering.
A dynamical approach to thermoelectric energy conversion Roberto D’Agosta, Nano-bio spectroscopy group and Universidad del Pais Vasco, San Sebastian, Spain.
The modeling of the thermoelectric energy conversion in nano-structured materials makes use of a few well-known static results for calculating the figure-of-merit. Recently, it has been shown that dynamical effects can improve the overall efficiency. In this talk, I present a novel way that allows for a direct evaluation of the Seebeck coefficient without reverting to any approximation. I will cast the theory in the framework of time-dependent dynamics, which in principle allows also for the calculation of the electrical and thermal conductance of the electrons. I will discuss a possible extension for the calculation of the vibrational thermal contribution to show how we can build a complete theoretical and numerical approach to the investigation of thermoelectric energy conversion.
Thermal conductivity in thin Silicon membranes: electrons and confined
phonons contributions Clivia M. Sotomayor Torresa Centre for Thermal Sciences (CETHIL), CNRS, National Institute of Applied Sciences (INSA) of Lyon,
France
[email protected] Charge transfer in thermoelectric nanostructures plays an essential role not only in mobility but also in thermal conductivity since thermal transport has contributions from electrons and phonons. But to what degree and under what circumstances? While much of the electronic transport in two- and one-dimensional systems has been actively investigated, phonon transport has been assumed to exhibit bulk behaviour. We have chosen to study experimentally thermal transport in Si nanowires (minimum width 100 nm) and light scattering in Silicon-on-Insulator membranes (minimum thickness 8 nm) in order to: (a) ascertain the validity limits of the Wiedemans-Franz law and (b) determine the contribution of confined acoustic phonons to the thermal conductivity. In the first instance, the theoretical work uses the elastic continuum and Green functions for the projected local density of phonon states. We have found that already at feature size of 500 nm there is a decrease in the effective thermal conductivity [1], which dramatically depends on shape, interfaces, frequency and temperature. Further experiments on confined phonon lifetimes [2] temperature are providing a picture to compare charge and lattice vibrations transfer in nanostructures materials as a step towards a better understanding of thermoelectricity in the nanoscale. References
1 P.-O. Chapuis et al, Proc. Intl. Workshop on Thermal Investigation of ICs and Systems THERMINICS15 (2010), EDA Publishing/THERMINICS 2010, ISBN: 978-2-35500-012-6
2 J A Johnson et al. Proc. MRS Spring meeting Symp. Nanoscale Heat Transport-from Fundamentals
to Devices, Vol 1347 DOI:10.1557/opl.2011.1333 (published online 23 August 2011)
Datamining High-Throughput DFT calculations in the search for new thermoelectric materials Georg K. H. Madsen ICAMS, Ruhr-Universität Bochum, Bochum, Germany
It has been shown that new thermoelectric materials can be discovered by screening known structures for favourable thermoelectric properties by ab initio methods.[1,2,3] However there remains a large challenge in discovering unknown phases computationally. As ternary and higher compounds are considered, a combinatorial explosion of potential structures and combinations must be considered.
Based on the data produced by the newly developed high throughput environment[4], and using the Ca-Zn-Mg-Si system as an example we show how data-mining techniques can be used to predict the stabilities of unknown compounds. Furthermore, it will be discussed how the correlations can be interpreted in terms of simplified models of the electronic structure, thereby leading to new insights into the chemistry of the system.
References
1 G. K. H. Madsen, J. Am. Chem. Soc. 2006, 128, 12140.
2 A. Bentien, S. Johnsen, G. K. H. Madsen, B. B. Iversen, and F. Steglich, Europhys. Lett. 2007, 80, 17008.
3 L. Bjerg, G. K. H. Madsen, and B. B. Iversen, Chemistry of Materials 2011, 23, 390,
4. I. Opahle, G. K. H. Madsen, R. Drautz, In preparation
Electrocaloric cooling Neil D Mathur Materials Science, University of Cambridge, Cambridge, CB2 3QZ, UK.
[email protected] A ferroelectric can be driven hot and cold by applying and removing a voltage near the Curie temperature. These electrocaloric effects are large in thin films1, and multilayer capacitor geometries have been proposed for cooling applications2. I will discuss progress. References
1 A. S. Mischenko, Q. Zhang, J. F. Scott, R. W. Whatmore and N. D. Mathur, Science, 2006, 311, 1270. 2 S. Kar-Narayan and N. D. Mathur, Appl. Phys. Lett., 2009, 95, 242903
Interfaces and Architectures for Efficient Light-to-Electrical Energy
Conversion with Dye-Sensitized Solar Cells Joseph T. Huppa,b a Dept. of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center,
Northwestern University, 2145 Sheridan Road, Northfield, IL 60208, U.S.A. b Argonne National Laboratory, 9800 South Cass Avenue, Argonne, IL 60439, U.S.A.
[email protected] The best existing dye-sensitized solar cells (DSCs) convert solar energy to electrical energy with about 12% efficiency – or about a third of what is theoretically achievable with cells of this kind (assuming rapid thermalization of injected electrons, and assuming that carrier multiplication behaviour is absent). This talk will focus on: a) understanding what limits the efficiency, and b) illustrating how nanostructured molecular-dye architectures and semiconductor-electrode architectures, together with interface tailoring, might be used to circumvent these limits. In particular, the presentation will draw upon recent studies (at Northwestern) that have utilized atomic-layer deposition (ALD) and related techniques to modify and define interfaces in highly spatially resolved fashion. From ALD-centered studies the competing roles of surface defect states, barrier-layer tunneling, barrier thermal population, and dye-facilitated super-exchange in determining electron interception dynamics and, therefore, photovoltages have been explored and, to a significant degree, explicated. Also described will be the results of studies that have focused on boosting light-harvesting and subsequent charge collection and photocurrent production, without sacrificing photovoltage.
Ab-initio methods in molecular design of dye-sensitized solar cells Ivano Tavernelli EPF Lausanne, Laboratory of Computational Chemistry and Biochemistry, Institute of Chemical
Sciences and Engineering, Switzerland.
The ab-initio design of new compounds and materials is a very promising area of research because it offers the possibility to guide, in a rational way, the search in the chemical space. In particular, ab-initio mixed quantum-classical molecular dynamics (MD) can provide the atomistic description of the structural changes necessary to target a desired chemico-physical property in materials, while specific ab-initio electronic properties can be used as guiding forces in the chemical space for the design of molecules with tailored characteristics. To this end, time-dependent density functional theory (TDDFT) offers a valuable framework for the calculation and the tuning of the spectroscopic properties of large molecular systems made of more than hundred atoms. Thanks to the recent advances in DFT/TDDFT functional design, a more accurate and reliable description of charge separated excited states (charge transfer excitations, and excitons) became recently available. This possibility opened new avenues in the field of ab-initio assisted molecular design, especially in the development of improved molecular dyes for solar cell devices. In this contribution, I will discuss some new theoretical developments for the design of novel organometallic dyes used in Graetzel-type solar cells. These are based on chemical modifications subjected to an appropriate selective pressure (in an evolutionary algorithm sense) or, to deterministic forces computed within TDDFT.
High electron lifetime in transparent TiO2 nanotubes-based photoanode for front-illuminated dye-sensitized solar cell A. Lambertia,b
a Istituto Italiano di Tecnologia IIT@POLITO - Center for Space Human Robotics, C.so trento 21,
10129, Torino, Italy. b
Politecnico di Torino - Applied Science and Technology Department, C.so Duca degli Abruzzi 24,
10129, Torino, Italy.
In the present work, the fabrication and characterization of non-curling, free-standing TiO2 nanotubes (nts) membranes and their integration in front-side illuminated dye-sensitized solar cells (DSCs) are reported. Vertically oriented TiO2 nt arrays were fabricated by a single step anodic oxidation of titanium foil. Free-standing nts membranes were easily separated by the metallic substrate without any crack, following a self-detaching procedure consisting in repeated rinsing in DI-water and ethanol. The membranes were then transferred and bonded on transparent FTO/glass substrates employing a layer of tape-casted commercial titania nanoparticles paste as well as a TiO2 sol as a binder before sintering at 500 ◦C for 1 hour. Stoichiometry, crystalline phase, quality and morphology of the film were investigated, evidencing the formation of a highly ordered 1D nts arrays, with a pure anatase crystalline structure. The as prepared photoanodes were TiCl4-treated in order to increase the surface area and to reduce recombination at the electrolyte/FTO interface. TiO2 nts-based DSCs were fabricated using both a reversible microfluidic architecture [1] and a standard thermoplastic irreversible sealing. Dye loading on the metal-oxide surface was analyzed with UV-Vis spectroscopy, and the dependence of the cell efficiency on nts thickness and dye incubation time was studied by I-V electrical characterization, incident-photon-to-electron conversion efficiency and impedance spectroscopy measurements under AM 1.5 illumination. Compared to the standard nanoparticle-based DSCs, the TiO2 nts-based devices show an marked increase in electrons lifetime, yielding an overall power conversion efficiency up to 8.2%.
References
1 A. Lamberti, A. Sacco, S. Bianco, E. Giuri, M. Quaglio, A. Chiodoni, E. Tresso, Microelectron. Eng., 2011, 88, 2308.
TDDFT for light-matter interactions in strong coupling regimes Angel Rubio Nano-Bio Spectroscopy group and ETSF Scientific Development Centre, Universidad del Pais Vasco
UPV/EHU and Centro Mixto CSIC-UPV/EHU
[email protected] Despite the success of linear-response schemes to describe excitations of many electron systems, many physical processes stemming from the interaction of light with matter are nonlinear in nature. In this talk we will address the problems and open questions related to the description of this phenomena with the goal of providing a sound description of laser-induced-population processes within TDDFT. Through the exact solution of a few electron system interacting with a monochromatic laser we highlight some common deficiencies of all adiabatic density functionals within time-dependent density-functional theory to handle photoinduced processes leading to population changes of many-body states. One prototype case is Rabi oscillations between the ground and an excited state when a monochromatic laser with a frequency close to the resonance is applied. All adiabatic functionals are not able to discern between resonant and nonresonant (detuned) Rabi oscillations. Only the inclusion of an appropriate memory dependence can correct the fictitious time-dependence of the resonant frequency. We extend this description to dynamical induced charge transfer processes and many body tunneling. Adiabatic functionals will fail similarly in the description of all processes involving a change in the population of states. We will show our recent advances in deriving a new memory-dependent functional. The description of photo-induced processes in chemistry, physics, and biology and the new field of attosecond electron dynamics and high-intense lasers all demand fundamental functional developments going beyond the adiabatic approximation.
First-principles photoemission spectra of molecular photovoltaic interfaces Christopher E. Patrick a Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, UK
Nanostructured semiconductor films sensitized with molecular dyes are the fundamental building blocks of dye-sensitized solar cells (DSCs) [1]. In DSCs, photocurrent is generated via ultrafast electron injection from a photoexcited dye molecule to a semiconductor substrate [2]. Since this process occurs on a sub-nanometre length scale, the atomistic nature of the semiconductor/sensitizer interface plays a critical role in device performance [3]. A number of groups have attempted to characterise the interface with spectroscopic experiments, but often it is not obvious how the observed spectrum relates to the underlying atomistic structure. The aim of our research is to develop a first-principles understanding of semiconductor/sensitizer interfaces[4,5], with particular emphasis on using high-level computational methods to interpret published spectroscopic data [4]. In this talk I shall present calculations of core and valence photoemission spectra of semiconductor/sensitizer interfaces. Determining the quasiparticle properties of these large interfaces represents a considerable computational challenge. I shall describe the our methodology, based on density-functional theory and post-DFT techniques, and show that careful consideration of a wide range of physical effects is essential to understanding experimentally measured data.
References
1 B. O'Regan and M. Graetzel, Nature, 1991, 353, 737.
2 W. R. Duncan and O. V. Prezhdo, Annu. Rev. Phys. Chem., 2007, 58, 143.
3 F. de Angelis, S. Fantacci, A. Selloni, M. Graetzel and M. K. Nazeeruddin, Nano Lett., 2007, 7, 3189.
4 C. E. Patrick and F. Giustino, Phys. Rev. B, 2011, 84, 085330.
5 C. E. Patrick and F. Giustino, Adv. Funct. Mater., 2011, 21, 4663.
Electron and energy transfer dynamics at TiO2 interfaces: Time-domain
ab initio studies Oleg Prezhdo University of Rochester, RC Box 270216, Rochester NY 14627, USA
[email protected] Solar energy applications require understanding of dynamical response of novel materials on nanometer scale. Our state-of-the-art non-adiabatic molecular dynamics techniques, implemented within time-dependent density functional theory, allow us to model such response at the atomistic level and in real time. The talk will focus on photoinitiated charge transfer at the interfaces of bulk TiO2 with a variety of systems, including organic molecules, water, semiconductor quantum dots (QDs), and graphene. Photoinduced charge separation across molecular/bulk interfaces drives dye-sensitized semiconductor solar cells. It creates many challenges due to stark differences between molecular and periodic systems. QDs are quasi-zero dimensional structures with a unique combination of molecular and bulk properties. They exhibit new physical phenomena, including phonon bottleneck and multiple exciton generation, with the potential to increase solar cell efficiency. Graphene is an excellent conductor. At the same time, it has zero band-gap, and photoexcited electrons and holes can rapidly lose all energy by coupling to phonons, resulting in complete photovoltaic efficiency loss. We are able to characterize the rates and branching ratios of the competing processes. Our simulations provide a unifying description of quantum dynamics on nanoscale, resolve highly debated issues, and generate theoretical guidelines for development of novel systems for energy harvesting.
Charge transfer to/from oxides: Role of interfaces Gennadi Bersuker SEMATECH, 257 Fuller Rd., Albany NY 12203
[email protected] Applications such as advanced microelectronics, energy storage and energy harvesting call for the introduction of new dielectric materials while maintaining nano-scale structure dimensions. The charge transfer to the dielectric structures comprised of nano-thick layers of different materials is controlled to a great degree by the interface regions; their structure and composition are determined by the inter-material interactions, which are strongly influenced by the fabrication process. These complex structures pose new challenges to interpreting electrical measurements to characterize charge transfer processes, which are sensitive to even extremely small concentrations of electrically active defects. The critical task is, thus, to link the structural and electrical characteristics of these multicomponent gate stacks to identify and eventually control defects affecting the charge transfer. In this presentation, we focus on analyzing oxide structural features responsible for the charge transfer by combining a variety of electrical measurement techniques with high time and spatial resolution that allows capturing fast transient charging processes and differentiating signals from different regions through the depth of the multi-layer stack. These data are used to fit the results of modeling of the physical processes underlying the electrical measurements to extract spatial and energy profiles of electrically active centers. The extracted characteristics are then compared to the atomic-level material modeling data to identify atomic and energy characteristics of the material responsible for the electrical properties.
First principles modelling of electron transfer between point defects in MgO Keith McKenna a Department of Physics, University of York, York, UK.
The quantum mechanical transfer of localised electrons between defects in metal-oxide materials plays an essential role in the fundamental processes of oxidation, photoluminescence and photochemical reaction [1-3], and underpins numerous technological applications, especially in relation to energy generation (batteries, photo-catalysts, solar cells) and electronics (transistors, spintronics) [4]. However, probing such electron transfer processes experimentally is extremely challenging owing to the very small length- and time-scales involved. In this talk, we will summarise some of our recent work on modelling electron transfer between oxygen vacancy defects in MgO using first principles techniques [5-7]. We show that the rate of electron transfer between defects depends on both their separation and crystallographic orientation. We also predict that as the separation between defects is increased, there is a crossover between delocalisation of electrons across defects, and activated transfer of localised electrons between defects, which occurs for defect separations in the range 7-10 Å [8]. These predictions have important consequences for modelling electron transport in wide-gap oxide materials, e.g. for simulating stress induced leakage current in metal oxide field effect transistors.
References
1 N. Cabrera and N.F. Mott, Rep. Prog. Phys., 1949, 12, 163.
2 G. H. Rosenblatt et al, Phys. Rev. B, 1989, 39, 10309.
3 E. Wahlstrom et al, Science, 2004, 303, 511.
4. M. Depas et al, IEEE Transactions on Electron Devices, 1996, 43, 1499.
5 H. Oberhofer, J. Blumberger, J. Chem. Phys., 2010, 133, 244105.
6 V. Tipmanee et al, J. Am. Chem. Soc., 2010, 132, 17032.
7 V. Tipmanee and J. Blumberger, Phys. Chem. B, 2012, 116, 1876.
8 K. McKenna and J. Blumberger, In preparation
Ultrafast Dynamics of Interfacial Electron Transfer Martin Wolf Fritz-Haber-Institut of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany
[email protected] Electron transfer across interfaces is of vital importance in various areas of physics, chemistry and biology. In these processes often an excess charge is transferred across an interface between different materials and is subsequently screened by the reorientation of a surrounding molecular environment. In polar molecular media, like water, this process is referred to as electron solvation, whereas in non-polar media, e.g. organic semiconductors, polaron formation may lead to the stabilization of injected excess charges. Here we use surface science techniques combined with time-resolved two-photon-photoemission (2PPE) spectroscopy to study the ultrafast dynamics of interfacial electron transfer at several hybrid interfaces: (1) Amorphous D2O or NH3 ice layers on a CU(11) substrate are used as a model system to study in detail the ultrafast dynamics of electron injection, localization and solvation in the polar adlayer as well as charge transfer back to the metal substrate.1,2 In further experiments (2) we employ 2PPE spectroscopy to investigate the occupied and unoccupied electronic structure and the electron dynamics at the pyridine/ZnO(10-10) interface as an example for a hybrid system of inorganic and organic semiconductors. Characterization of the pristine ZnO(10-10) surface yields the position of the valence and conduction band (CB) edge energies and significant surface band bending upon hydrogen termination leading to a metallic surface. Adsorption of pyridine leads to a substantial work function reduction up to ΔΦ = -2.7 eV which strongly affects the energy level alignment at the inorganic/organic interface. Furthermore we directly monitor the hot electron relaxation in the ZnO bulk CB on femtosecond timescales and electron capture at the semiconductor surface (>100 ps). Acknowledgement: This work was funded in part by the Deutsche Forschungs Gemeinschaft (DFG) through Sfb 951. References
1 J. Stähler et. al, Chemical Society Reviews 37, 2141 (2008)
2 J. Stähler, U. Bovensiepen and M.Wolf, in Dynamics at Solid State Surfaces and Interfaces,
Vol. 1, (Eds.) U. Bovensiepen, H. Petek, and M. Wolf. Wiley-VCH, Berlin (2010), p.359
Photocatalysis on metal oxide semiconductor electrodes Federico M. Pesci
Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, United
Kingdom
Concerns over CO2 atmospheric levels and the decrease in proven source of fossil fuels have led researchers to focus on the production of carbon free form of energy. Photocatalytic water splitting is a potentially inexpensive and environmentally friendly route to the storage of solar energy. Since the first report of photoelectrochemical water splitting using TiO2, a range of metal oxides photo-anodes and cathodes, including Fe2O3 and WO3 have been studied.1 In order to guide the design of the next generation of photocatalyst it is necessary to understand the mechanisms occurring on existing metal oxide electrodes. Transient absorption spectroscopy (TAS) is a powerful tool for the study of photogenerated charge carriers on semiconductors and we have demonstrated that processes such as electron/hole recombination, electron transport and water oxidation can be directly monitored in a complete photoelectrochemical cell.2 WO3 is considered to be a very promising photoanode for water splitting due to a reported high incident-photon-to-current efficiency.3,4 Despite several decades of research, significant questions regarding the mechanism of operation of WO3 electrodes in the most commonly studied electrolytes remain.5 Here we report TAS and electrochemical studies of WO3 which provide insights into the dynamics of the charge carriers on WO3.6 We are able to identify timescales of key processes and to provide important mechanistic insight into this widely studied material. Initial transient experiments that investigate the charge carriers dynamics on TiO2 and Cu2O are also reported.
References
1 Chen, X. B.; Shen, S. H.; Guo, L. J.; Mao, S. S. Chemical Reviews 2010, 110, 6503. 2 Cowan, A. J.; Barnett, C. J.; Pendlebury, S. R.; Barroso, M.; Sivula, K.; Grätzel, M.; Durrant, J. R.; Klug,
D. R. Journal of the American Chemical Society 2011, 133, 10134. 3 Alexander, B.; Kulesza, P.; Rutkowska, L.; Solarska, R.; Augustynski, J. Journal of Materials Chemistry
2008, 18, 2298. 4 Santato, C.; Odziemkowski, M.; Ulmann, M.; Augustynski, J. Journal of the American Chemical
Society 2001, 123, 10639. 5 Mi, Q.; Zhanaidarova, A.; Brunschwig, B. S.; Gray, H. B.; Lewis, N. S. Energy & Environmental Science
2012, 5, 5694. 6 Pesci, F. M.; Cowan, A. J.; Alexander, B. D.; Durrant, J. R.; Klug, D. R. Journal of Physical Chemistry
Letters 2011, 2, 1900.
First Principles Approach to Polaron Transport in the Solid State Michel Dupuis Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA
In this presentation we will review the computational characterization of polaron transport in the solid state using the Marcus/Hostein model and first-principles calculations. We will discuss ideas to extend the modeling to the mesoscale with a multi-state empirical polaron model.
Electronic excitations in Cu-based materials for thin-film solar cells Silvana Botti*a,b a Laboratoire des Solides Irradiés and ETSF, Ecole Polytechnique, CNRS, CEA-DSM, 91128 Palaiseau,
France. b
Université de Lyon, F-69000 Lyon, France and LPMCN, CNRS, UMR 5586, Universite´ Lyon 1, F-
69622 Villeurbanne, France.
During the past years, Cu(In,Ga)(Se,S)2 (CIGS) thin-film solar cells emerged as a technology that could challenge the current hegemony of silicon solar panels. CIGS compounds conserve to a very high degree their electronic properties in a large non-stoichiometric range and are remarkably insensitive to radiation damage or impurities. The family of kesterites Cu2ZnSe(S,Se)4 exhibits very similar electronic properties. Moreover, kesterites have the clear advantage of being composed of abundant, non-toxic, less expensive chemical elements. The origin of the exceptional electronic properties and the defect physics of these complex materials is still not completely understood, despite the large amount of experimental and theoretical work dedicated to that purpose. In particular, standard density functional theory yields often results in quantitative and qualitative disagreement with experiments. This is a serious problem when it comes to designing new materials for more efficient photovoltaic energy conversion. In this context, can ab-initio calculations of electronic excitations beyond ground-state density functional theory give a crucial contribution?
By presenting some examples of calculations, I will discuss which theoretical approaches are reliable, at a reasonable computational cost, and what is the physical insight that they allow to gain on electronic excitations in new materials for photovoltaics. Finally, I will present an example of application of material design, that represents at present the new frontier in the field of theoretical material science. Using the minima hopping method, we found and characterized new low-enthalpy phases of silicon with almost-direct band gaps and displaying strong absorption in the visible.
References
1 J. Vidal, S. Botti, P. Olsson, J-F. Guillemoles, and L. Reining, Phys. Rev. Lett., 2010, 104, 056401.
2 J. Vidal, F. Trani, F. Bruneval, M.A.L. Marques, and S. Botti, Phys. Rev. Lett., 2010, 104, 136401.
3 S. Botti, D. Kammerlander, and M.A.L. Marques, Appl. Phys. Lett., 2011, 98, 241915.
4 I. Aguilera, J. Vidal, P. Wahnón, L. Reining, and S. Botti Phys. Rev. B, 2011, 84, 085145.
Redox molecular junctions: Properties and functionalities Abraham Nitzan School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel
[email protected] Redox molecular junctions are molecular conduction junctions that involve more than one oxidation state of the molecular bridge. This property is derived from the ability of the molecule to transiently localize transmitting electrons. I will discuss the implications of this property in a system open to electron flux and their manifestations with regards to the nonlinear transport properties of such junctions. References
1 M. Galperin, M. A. Ratner and A. Nitzan, Hysteresis, switching, and negative differential resistance in molecular junctions: A polaron Model, Nano Letters, 2005, 5, 125-130
2 A. Migliore and A. Nitzan, Nonlinear charge transport in redox molecular junctions: a Marcus
perspective, ACS Nano, 2011, 5, 6669 3 M. Einax, M. Dierl and A. Nitzan, Heterojunction organic photovoltaic cells as molecular heat
engines: A simple model for the performance analysis, J. Phys. Chem. C, 2011, 115, 21396