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|Workshop in Honour of Carlo Taliani|
Bologna May 22,23,24 2011
Abstracts of Talks
HIGH TEMPERATURE SUPERCONDUCTIVITY: THE EXPLANATION
Loughborough University, United Kingdom
Soon after the discovery of the first high temperature superconductor 25 years ago by Georg Bednorz and Alex Mueller the late Sir Nevill Mott answering his own question “Is there an explanation?” [Nature 327 (1987) 185] expressed a view that the Bose-Einstein condensation of small bipolarons, predicted by us in 1981, could be the one. Since then the controversial issue of weather the electron-phonon interaction is crucial for high-temperature superconductivity or weak and inessential has been one of the most challenging problems of contemporary condensed matter physics. Here I outline the pioneering contribution of Carlo Taliani and his colleagues and several other groups in solving the problem. I also outline some recent developments in the bipolaron theory suggesting that the true origin of high-temperature superconductivity is found in a proper combination of the strong Coulomb repulsion with the significant Froehlich electron-phonon interaction. Acting together these interactions form a (bi)polaronic liquid in high-temperature superconductors protected from clustering.
SPECTRAL DIFFUSION IN SINGLET AND TRIPLET STATES OF
H. Bässler 1*, S. T. Hoffmannn1, S. Athanasopoulos2, D. Beljonne2, A. Köhler1,
1 Department of Physics, University of Bayreuth, Germany
2 Laboratory for Chemistry of Novel Materials, Center for Research in Molecular Electronics and Photonics University of Mons, Belgium
* Corresponding author: Baessler@staff.uni-marburg.de
We have employed quasi continuous fluorescence and phosphorescence spectroscopy within a temperature range between 10K and 500K to monitor the spectral diffusion of singlet and triplet excitons in a series of -conjugated polymers.  We investigated (i) how spectral diffusion is controlled by the degree of energetic disorder present in the amorphous film (that is reflected by the inhomogeneous broadening of the photoluminescence spectra), and (ii) how this process depends on the range of the electronic coupling (by comparing singlet exciton diffusion via long-range Förster transfer against triplet exciton diffusion by short-range Dexter transfer). The fluorescence spectra bear out a bathochromic shift upon cooling the sample down to a critical temperature below which the shift saturates. This is a signature of spectral relaxation. Random walk theory applied to excitation transport within a Gaussian density of states distribution is both necessary and sufficient to rationalize the experimental results in a quantitative fashion. The same behaviour is observed for triplets in weakly disordered systems, such as in a polymer containing platinum in the main chain and a ladder-type poly-phenylene. In contrast we observe a hypsochromic shift of the phosphorescence spectra below a characteristic temperature for triplets in systems with at least moderate energetic disorder. Monte-Carlo simulations prove that the hypsochromic shift is caused by frustration of triplet exciton relaxation because thermally activated exciton jumps that otherwise promote spectral diffusion become progressively frozen-out. The frustration effect is controlled by the jump distance, and thus it is specific for triplet excitations that migrate via short range coupling among strongly localized states as compared to singlet excitons.
 S. T. Hoffmann, H. Bässler et al., Phys. Rev. B, 81, 115103 (2010);  A. Köhler, H. Bässler, Mater. Sci. Eng. R, 66, 71 (2009).
POLARONS SELF ORGANIZATION IN COMPLEX FRACTAL MATERIALS
A. Bianconi1, N. Poccia1, and A. Ricci1*
1Department of Physics, Sapienza University of Rome, P.le Aldo Moro 2, Italy.
The high Tc superconductivity in the two component scenario is discussed. We show evidence for coexistence of polarons and free particles segregated in two different spatial domains of distorted and undistorted lattice. The recent work is focusing on mapping and control of the microstructure of high temperature copper oxide superconductors when there are oxygen interstitials or vacancies that strongly influences their bulk properties inducing a complex multiscale texture of two components. For example, the oxygen interstitials in the spacer layers separating the superconducting CuO2 planes undergo ordering phenomena in Sr2O1+yCuO2, YBa2Cu3O6+y and La2CuO4+y that induce remarkable transition temperature (Tc) enhancement with no hole concentration change. It is also known that complex systems often have a scale-invariant structural organization, but hitherto none had been found in high-Tc materials. Here we report that the ordering of oxygen interstitials in the La2O2+y spacer layers of La2CuO4+y high-Tc superconductors is characterized by a fractal distribution up to a maximum limiting size of 0.5 mm. Intriguingly, these fractal distributions of dopants appear to promote superconductivity at high temperature.
CHARGE GENERATION FROM EXCITONS IN MOLECULAR SEMICONDUCTORS
Cavendish Laboratory; JJ Thomson Avenue, Cambridge CB3 0HE, UK
Excitons in molecular semiconductors generally show high Coulomb binding energies, of order 0.5 eV, because dielectric screening is low. They also show comparable values for the exchange energy between spin-singlet and spin-triplet configurations. These both present challenges and opportunities for the use of such materials in solar cells. I will illustrate this with examples from the Cambridge group:
Current designs for organic photovoltaic diodes depend on the ionisation of photogenerated excitons at the heterointerface between electron-accepting and hole-accepting semiconductors. It is very well established that photoinduced electron transfer across the heterointerface can be very rapid and very efficient. However, these excitations can stabilise as coulombically-bound charge-transfer excitons that are not easily separated to fully-separated charge carriers, and can intersystem cross to lower energy spin triplet excitons. I will show for some polymer-polymer heterojunctions (formed between F8BT and PFB) that the Coulomb binding between electron and hole in the charge transfer state is of order 0.2 eV. This can be determined from both the magnetic field dependence of photocurrent. For solar cells made using fullerenes as electron acceptors, charge separation is easier, though transient optical absorption measurements provide clear evidence for electron-hole trapping that is reduced when an internal electric field is present.
Large exchange energies allow scope for multiple exciton generation for materials for which the triplet exciton energy is less than one half of the singlet exciton energy, since this favours energetically the fission of a photogenerated singlet to a pair of triplet excitons. We have shown that this process is effective in pentacene and that the resultant triplets can be ionised against a heterojunction formed with fullerene. This offers scope for enhanced solar cell efficiencies.
JOURNEY FROM THE DISCOVERY OF PROPERTIES OF ORGANIC SEMICONDUCTORS TO STATE-OF-THE-ART DEVICES AND CIRCUITS
Since the beginning of the nineties, organic semiconductors have been integrated into devices with ever growing economic and scientific impact: OLEDs, organic photovoltaic cells and organic transistors. We will go into the state of the art of some of these devices, that build upon the pioneering work of Carlo Taliani.
Pentacene is mostly used as exemplary transistor material. It was not only used for building good understanding of charge transport and its relation to morphology, but also to develop a thin-film semiconductor technology intended primarily for backplanes of flexible displays. The level of control in this technology is today sufficient for integrating thousands of transistors into circuits, as an example of which we will show a functional organic microprocessor on plastic foil.
In the field of photovoltaics, the construction of heterojunctions between organic semiconductors was recognized early on as a key to exciton splitting, in particular using C60 as acceptor. The electronic structure of organic heterojunctions is, however, still not fully revealed. We will share some new insights on the problem, which have been obtained by microelectrostatic calculations, and which shed new light on the splitting of geminate pairs at the interface between donors and C60 acceptors.
DISPERSIVE RAMAN LINES IN CARBON NANOPHASES
University of Vienna, Faculty of Physics; 1090 Wien, Strudlhofgasse 4, A
In this report a review will be given on the application of the dispersion effect of Raman lines in carbon nanophases and in conducting polymers. Starting from the classical examples of polyacetylene the phenomenon will be discussed for nanodiamond, graphite, single-walled carbon nanotubes, and graphene. It will be demonstrated in what sense the dispersion originates from photo-selective resonance scattering or from a double resonance Raman process. The dispersion can be linear or oscillatory and allows obtaining information on the structure of the material under investigation. The importance of isotope effects will be elucidated.
4D ULTRAFAST OPTICAL MICROSCOPY IN SEMICONDUCTING POLYMERS
Center for Nano Science and Technology (CNST) of IIT@polimi Milano
The talk will start with a warm hearth memory of the happy time spent in Bologna, with a brief mentioning to the scientific problems hot at that time (1992-94). Solitons, polymer vs oligomers, correlation vs electron-phonon coupling. Then I will go ahead along a path leading to very recent developments in ultrafast spectroscopy introducing the 4D ultrafast optical microscopy. We study a test-blend of PFO in PMMA, for photonic applications, and the prototypical photovoltaic blend P3HT-PCBM. We obtain space and time resolution, suggesting the existence of a long lived CT state at the interfaced between crystalline domain with peculiar polarization properties, well reproduced by theoretical calculations.
CHARGE TRANSFER EXCITONS IN ORGANIC BULK HETEROJUNCTIONS FOR PHOTOVOLTAIC APPLICATIONS
M. A. Loi
University of Groningen, Zernike institute for Advanced Materials,
Nijenborgh 4, NL-9747 AG Groningen, The Netherlands
Photovoltaic devices based on organic bulk heterojunctions have recently gained renewed interest thanks to the achievement of power conversion efficiencies up to 8%. Such achievement showed that this class of devices can have a future outside the research laboratories. Nevertheless, it is also clear that not all the physical questions on their working mechanism are answered and that the use of new narrow band-gap polymers pose new interesting questions.
I will show that the narrow band-gap polymer bulk heterojunctions offer a new opportunity to shed light on the charge separation phenomena and on the photo-excitation involved in the working mechanism of organic solar cells. I will report different examples were charge transfer excitons play a main role in the photoexcitation landscape and I will discuss their importance for the performance of bulk heterojunction solar cells.
Dorota Jarzab, Fabrizio Cordella, Jia Gao, Markus Scharber, Hans-Joachim Egelhaaf, Maria Antonietta Loi, Advanced Energy Materials, DOI: 10.1002/aenm.201100083 (2011); Maria Antonietta Loi, Stefano Toffanin, Michele Muccini, Michael Forster, Ulrich Scherf, Markus Scharber, Adv. Funct. Mat. 17, 2111 (2007);
ENERGY TRANSFER IN HYBRID ORGANIC/INORGANIC NANOCOMPOSITES
IBM Research GmbH, Zurich Research Laboratory, Säumerstrasse 4, 8803 Rüschlikon, Switzerland
The integration of optical components at the chip level demands for novel device concepts and material systems. Hybrid organic/inorganic materials are expected to exhibit large optical nonlinearities which could become essential in realizing fast all-optical switches. We present a study on the underlying electronic energy transfer processes in a model system constituting of an amorphous polyfluorene donor and an inorganic nanocrystal acceptor.
THEORY OF EXCIMERS AND EXCIPLEXES IN CONJUGATED POLYMERS
S. Mazumdar1, K. Aryanpour1, D. Psiachos1, C.-X. Sheng2, E. Olejnik2, B. Pandit2 , Z.V.Vardeny2
1Department of Physics, University of Arizona, Tucson, AZ 85721, USA;
2Department of Physics, University of Utah, Salt Lake City, UT 84112, USA.
We present a joint theory-experiment effort whose goal is to understand the difference between ordered films of $\pi$-conjugated polymers and disordered films and solutions. We report pressure effects on the transient picosecond and continuous wave photomodulation spectra of disordered and ordered MEH-PPV films up to 119 kbar. The ordered film exhibits two correlated photoinduced absorption (PA) bands absent in the disordered films, which dramatically blue-shift with pressure. We present correlated-electron calculations of excited state absorptions from interacting chains that establish that such blue-shift of PA bands is not expected from aggregates or polaron-pairs. Our calculations establish unambiguously that the primary photoexcitated species in ordered MEH-PPV films are excimers, whose PA bands are expected to show pressure-induced blueshift . We will revisit the longstanding problem the exciton binding energy of $\pi$-conjugated polymers. Time permitting, we will present a theory of exciplexes and photoinduced charge-transfer in donor-acceptor blends .
 K. Aryanpour et al., Phys. Rev. B, in press, 2011.
 K. Aryanpour, D. Psiachos and S. Mazumdar, Phys. Rev. B 81, 085407 (2010).
ABOUT SUPRAMOLECULAR ASSEMBLIES OF OLIGOMERS
Institute for Complex Molecular Systems,
Eindhoven University of Technology,
P.O. Box 513, 5600 MB Eindhoven, the Netherlands
Chemical self-assembly is an important tool in constructing functional nanoscopic materials. Knowledge about the presence and stability of multiple local supramolecular minima or substates in these synthetic self-assembled systems is crucial in order to drive the assembly towards the desired thermodynamic minimum. Both the existence and the factors that influence the formation of these substates have, however, barely been investigated. In the presentation, we show that the chemical self-assembly of chiral π-conjugated oligomers, sexithiophenes and oligo(phenylenevinylenes) operates via a nucleation – elongation pathway and hence is highly cooperative. As a result the solvent plays an essential role in the chemical self-assembly and strong evidence is found that the alkane solvents are co-organized with the oligomeric stack. These results are also of crucial importance for the discussion whether the chemical self-assembly creates the thermodynamically determined product or that is possible to form kinetically trapped structures as well. We will show that the self-assembly is strongly influenced by external stimuli. Impurities levels as low as 0.1% can direct the assembly, while only with cooling rates as low as 1 K/hr the assembly process occurs close to or under thermodynamic equilibrium. Multiple supramolecular substates have been probed, while stepwise cooling shows that annealing at different stages of the self-assembly influences the assembly process in different ways and indicates the changing energy landscape. Our findings reveal that chemical self-assembly has striking similarities with the crystallization of molecules and the conformational dynamics and aggregation of proteins. These insights will be used to study the amplification of chirality in self-assembling systems and how small enantiomeric excesses at the molecular level can lead to a high preference in the chirality at the supramolecular level.