Novel transport phenomena in organic electronic devices: heat, spin and thermoelectricity

Resume : A considerable effort is being devoted at exploring new spin functionalities through specifically designed molecules. The excited spin states of these molecules are moreover potentially appealing for quantum computing and data storage technology. Despite this exciting prospect, the design of hybrid metal-molecule devices is hampered by the interaction of the molecular spin with the electrons of the metal. Decoupling layers [1,2,3] as well as superconductors [4] have been successfully used to preserve the molecular spin, but introduce important constraints in the choice of the molecular environment. Inelastic electron tunneling spectroscopy within the junction of a scanning tunneling microscope (STM) uses current-driven spin-flip excitations for an all-electrical characterization of the spin state of a single object. Here we studied the exceptionally efficient spin-flip excitations of a double-decker nickelocene molecule (Nc) adsorbed directly on Cu(100) by means of inelastic tunneling spectroscopy (IETS), X-ray magnetic circular dichroism (XMCD) and density functional theory calculations (DFT). The results show that the molecule preserves its magnetic moment and magnetic anisotropy not only on Cu(100), but also in different metallic environments including the tip apex. Taking advantage of this robusteness we are able to functionalize the microscope tip with a Nc, which can be employed as a portable source of inelastic excitations as exemplified by a double spin-excitation process. [1] Hirjibehedin et al., Science 317, 1199 (2007) [2] Tsukuhara et al., Phys. Rev. Lett. 102, 167203 (2009) [3] Rau et al., Science 344, 988 (2014) [4] Heinrich et al., Nat. Phys. 9, 765 (2013)

Resume : Molecular spins have become key enablers for exploring magnetic interactions, quantum information processes and many-body effects in metals. Metal-organic molecules, in particular, let the spin state of the core metal ion to be modified according to its organic environment1 allowing localized magnetic moments to emerge as functional entities with radically different properties from its simple atomic counterparts. Here, using transition metal phthalocyanine high-spin complexes, we demonstrate the magnetic doping of a thin film of gold, effectively creating a new ground state2. We primarily demonstrate our concept using copper (II) phthalocyanine (CuPc), a paramagnetic metal-organic molecule with spin angular momentum S=1/2 arising from the oxidation state of the core metal ion, Cu2+ (ref. 1). We show the effects of the doping by electrical transport measurements that are sensitive to the scattering of itinerant electrons with magnetic impurities3,4, such as Kondo effect5 and weak anti-localization6. We complement this study using cobalt (II) phthalocyanine (CoPc), and perform control experiments with nickel (II) phthalocyanine (NiPc), phthalocyanine (Pc) and Cu. Our work expands in a simple and powerful way the classes of materials that can be used as magnetic dopants, and opens a new channel to couple the wide range of molecular properties with spin phenomena at a functional scale2. REFERENCES 1. Rawat, N. et al. Spin Exchange Interaction in Substituted Copper Phthalocyanine Crystalline Thin Films. Sci. Rep. 5, 16536–11 (2015). 2. Atxabal, A. et al. Spin doping using transition metal phthalocyanine molecules. Nat. Commun. 7, 13751 (2016). 3. Gang, T. et al. Tunable doping of a metal with molecular spins. Nat. Nanotechnol. 7, 232–236 (2012). 4. Ataç, D. et al. Tuning the Kondo effect in thin Au films by depositing a thin layer of Au on molecular spin-dopants. Nanotechnology 24, 375204–8 (2013). 5. Schrieffer, J. R. The Kondo Effect—The Link Between Magnetic and Nonmagnetic Impurities in Metals? J. Appl. Phys. 38, 1143–1150 (1967). 6. Bergmann, G. Measurement of the Magnetic Scattering Time by Weak Localization. Phys. Rev. Lett. 49, 162–164 (1982). 7. Hikami, S., Larkin, A. I. & Nagaoka, Y. Spin-Orbit Interaction and Magnetoresist ance in the Two Dimensional Random System. Prog. Theor. Phys. 63, 707–710 (1980).

Resume : Recent developments in molecular spintronics indicate that the deposition of aromatic organic molecules on the strongly reactive surfaces of ferromagnetic metals leads to a change in the local magnetic properties of the atoms hybridized with the molecule, such as exchange interaction, magnetic moments, and magneto-crystalline anisotropy [1]. In this talk I will show that the extreme multi-functionality of organic molecules can be used to functionalize the spin properties of the more general class of surfaces with a spin-texture induced by strong spin-orbit coupling. In particular, I will present our results on the following two-dimensional electronic systems: the surface states of the topological insulator (TI) Bi2Se3 [2], and the Rashba-split surface states of a Pb-Ag surface alloy [3]. In the case of the TI surface states, we have used theoretical calculations to guide the choice and chemical synthesis of appropriate molecules that customize the spin-texture of Bi2Se3. The theoretical predictions are then verified in angular-resolved photoemission (ARPES) experiments. We show that by tuning the strength of molecule-TI interaction, the surface of the TI can be passivated, the Dirac point can energetically be shifted at will, and Rashba-split quantum-well interface states can be created [2]. In the case of the Pb-Ag alloy, we have studied the influence of specific chemical bonds - as formed by the organic molecules CuPc and PTCDA- on the electronic structure of the surface alloy using momentum microscopy [4]. We have found that delocalized van der Waals or weak chemical pi-type bonds are not strong enough to alter the alloy. On the other hand, localized sigma-type bonds lead to a vertical displacement of the Pb surface atoms, which on turn leads to pronounced changes in the alloy’s surface band structure, including its spin texture [3]. Our results provide an exciting platform for tailoring spin-textured surface states using the controlled hybridization of organic molecules. [1] M. Cinchetti. Nature Nanotechnology 9, 965 (2014); [2] S. Jakobs, A. Narayan, B. Stadtmüller, A. Droghetti, I. Rungger, Y. S. Hor, S. Klyatskaya, D. Jungkenn, J. Stöckl, M. Laux, O. L. A. Monti, M. Aeschlimann, R. J. Cava, M. Ruben, S. Mathias, S. Sanvito and M. Cinchetti. Nano Letters 15, 6022 (2015); [3] B. Stadmüller, J. Seidel, N. Haag, L. Grad, C. Tusche, G. van Straaten, M. Franke, J. Kirschner, C. Kumpf, M. Cinchetti and M. Aeschlimann. Physical Review Letters 117, 096805 (2016); [4] B. Yan, B. Stadtmüller, N. Haag, S. Jakobs, J. Seidel, D. Jungkenn, S. Mathias, M. Cinchetti, M. Aeschlimann and C. Felser. Nature Communications 6, 10167 (2015).

Resume : Natural heat and waste heat are substantial, equivalent to more than half the solar/fossil/nuclear energy sources upon conversion into electricity. Thermoelectric generators could transform few percents of this heat loss into electricity if thermoelectric materials based on elements of high abundance could be designed. Conducting polymers based on carbon, oxygen and sulfur, such as poly(3,4-ethyelenedioxythiophene) PEDOT, display promising thermoelectric efficiency, although still far from the state of the art Bi2Te3 alloys. In the first part, we discuss the thermoelectric properties for the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT). In this conducting polymer, the charge carriers are electronic in nature. PEDOT displays a metallic conductivity (1500S/cm) and a large Seebeck coefficient (55μV/K) in its pristine form. The thermal conductivity is coupled to the electrical conductivity via Wiedemman-Franz law with a Lorentz factor equals to twice the value of an electron gas[1]. The power factor can be enhanced by controlling its state of oxidation[2]. This optimization together with the intrinsic low thermal conductivity of the polymer (0.37 Wm-1K-1) yields ZT=0.25 at room temperature[3]. We rationalize why PEDOT has good thermoelectric properties compared to polyaniline. We identify the role of the doping species and the link of the density of states for a (bi)polaron network with the Seebeck coefficient. Further, we show that enhancing the molecular order in PEDOT leads to an increase of both Seebeck coefficient and electrical conductivity. We believe that this observation reflects a transition from a Fermi glass to a semi-metal[4]. In the second part, we further study the thermoelectric properties for wet PEDOT films. Wet conducting polymers have been widely investigated in electrochemical devices, where their unique properties can be ascribed to both from electronic and ionic charge carriers. Here we found that both electrical conductivity and Seebeck coefficient for ionic conductive PEDOT films increase with humidity. A large thermo-induced voltage up to several hundreds of μV/K is observed and rationalized as the sum of an electronic thermopower and ionic thermopower[5]. We observed an extra thermogenerated current from this ionic seebeck effect, which vanishes with time. This is rationalized from an internal reorganization of electronic and ionic charge carriers, as well as the blocking of ions at the metal electrodes. Finally, we investigate the ionic thermoelectric effect in polymer electrolytes. We found out that the ionic Seebeck coefficient can reach 10 mV/K, i.e. close to 100 times larger than electronic Seebeck coefficient. In a first application, the ionic thermoelectric effect is used to charge a supercapacitor for energy harvesting of intermittent heat sources [6]. In a second application, we demonstrate the possibility to switch a transistor with a single ionic thermoelectric leg, thus creating a smart pixel that could in the future be integrated in temperature sensor arrays for e-skin and infra-red camera applications [7]. [1] Advanced Materials 2015, 27, 2101-6. [2] Journal of the American Chemical Society 2012, 134, 16456-16459. [3] Nature Materials 2011, 10, 429-433. [4] Nature Materials 2014, 13, 190–194. [5] Advanced Energy Materials, 2015, 5, 1500044. [6] Energy Environ. Sci., 2016,9, 1450-1457 [7] Nature Communications, in print

Resume : Physical vapour deposition is widely used to prepare organic amorphous films for many applications in electronics such as organic light emitting diodes, photovoltaics, transistors and other devices. In order to manage and understand the thermal transport in these devices it is crucial to know the thermal conductivity of the conforming active layers. For many organic glass-formers the deposition plays a crucial role in determining glass properties, such as thermal stability, density or molecular orientation among others. To our knowledge, thermal conductivity has not yet been investigated as a function of substrate temperature. Here, we use a silicon nitride membrane-based sensor operated with the 3?-Völklein method to measure in-situ, during deposition, the in-plane thermal conductivity of thin films of TPD, an organic semiconductor used in OLEDs, as a function of the deposition temperature. We observe how the thermal conductivity changes with substrate temperature achieving a decrease of ca. 15 % between the highest and the lowest values, obtained respectively at 95 % and 65 % of the glass transition temperature of the material. This behaviour is attributed to variations in the molecular alignment with respect to the substrate, a statement further supported by recent works. The possibility of tuning the properties of each active layer in an organic semiconductor device allows tailoring the thermal gradients without the need of new materials or complex architectures.

Resume : The intrinsically low thermal conductivity of an organic semiconductor is one of its main advantages for thermoelectric applications, while it is a limiting factor for applications where high heat dissipation is demanded. Understanding, thus, the mechanisms that govern thermal transport in these kind of materials is very interesting from both an applied and a fundamental point of view. However, in spite of its importance, thermal transport in organic semiconductors has been poorly investigated, and has been focused in very specific systems. In this talk we will present a detailed study, performed by means of atomistic simulations, of the thermal transport properties of a set of structurally different organic semiconductors. The objective has been to establish a link between the structure of the considered organic materials and their thermal transport properties, by performing a detailed study of their vibrational properties. In practice three different effects on the thermal conductivity have been studied: (i) anisotropy (poly- para-phenylene), (ii) cut-chain torsions (biphenyl) and (iii) constrained chains (fluorene).

Resume : Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) has the highest reported thermoelectric figure of merit ZT=0.42 of all candidate organic thermoelectric materials but the relationship between the Seebeck coefficient and electrical conductivity does not follow the S∝-lnσ behaviour that follows from a homogeneous, single-band model with Boltzmann statistics nor the empirical S∝σ^(-1/4) observed in other conducting polymers. By measuring the temperature dependence of both quantities in the same direction through the same thin film samples we identify heterogeneous, fibrillar current pathways punctuated by Coulombic barriers and fluctuation assisted tunnelling through these barriers as the underpinning model. The number of fibrillar pathways and the detail of the Coulombic barriers are controlled by the self-organised microcrystalline-domain embedded in an amorphous matrix structure.

Resume : Thermoelectric studies can complement conventional charge transport methods as they give unique and direct insight into the charge carrier energetics. We conducted an experimental thermopower investigation of highly ordered N2200 monolayer OFETs. Surprisingly, we find that the Seebeck coefficient S decreases with increasing energetic disorder, contrary to what is expected from conventional theory where S is the quotient of the difference between transport and Fermi energy and lattice temperature. A subsequent kinetic Monte Carlo investigation revealed that this behavior is a direct consequence of the quasi-2D charge carrier confinement and inter-carrier Coulomb interaction, which forces carriers to occupy “unnaturally” high energies in the DOS, thus spreading the carrier distribution in energy. We show that this effect can be quantitatively captured by an effective temperature Teff of the carrier distribution, which is obtained from a fit of the Fermi-Dirac distribution to the occupation probability. Teff can be up to several times larger than the lattice temperature and replaces it as the denominator in the expression for S. Increasing the disorder of the DOS leads to an increased spreading of the carrier distribution, a larger Teff and thus to a smaller S, as observed experimentally.

Resume : Novel high-mobility materials based on organic molecules bring a host of advantages in spintronic applications. In organics, spin- and charge dynamics can be intimately linked. Until present simple stochastic models have been employed to model the spin dynamics in these complex systems, without linking them directly to the charge transport. Ideally, such materials should be theoretically modeled to understand simultaneously the charge and the spin transport simultaneously. This should be done using realistic structural models with atomic resolution, and field- and spin-orbit coupling (SOC) effects calculated from state-of-the-art first-principles theory. We present a multi-scale framework for modeling of spin-dynamics in organics, implemented on top of the electron dynamics given by the VOTCA-CTP package[1]. This development allows us to accurately treat the balance of SOC phenomena in realistic morphologies, while observing the link between charge- and spin-dynamics directly. Our results include a complete map of the spin-relaxation in Alq3 - the organic spintronics fruit-fly - as a function of charge concentration and temperature. Additionally, insights into the highly spin conducting polymer PBTTT will be presented, along with developments towards treating the spin Hall and Nernst effects. 1. V. Rühle et al., J Chem. Theory Comput. 7, 3335 (2011)

Resume : The small spin-orbit interaction of carbon atoms in graphene and single-walled carbon nanotubes (SWNTs) promises a long spin diffusion length and the potential to create a spin field-effect transistor. However, for this reason, they were largely overlooked as a possible spin-charge conversion material. In this presentation, an electric gate tuning of the spin-charge conversion voltage signal in single-layer graphene and SWNTs is reported [1,2]. Using spin pumping from an yttrium iron garnet ferrimagnetic insulator and ionic liquid top gate, we determined that the inverse spin Hall effect is the dominant spin-charge conversion mechanism in them. From the gate dependence of the electromotive force we showed the dominance of the intrinsic over Rashba spin-orbit interaction, a long- standing question in graphene research. Our study shows a simple spatial inversion symmetry breaking is not sufficient for generating the inverse Rashba-Edelstein effect, which is contrary to a conclusion in the other study [2]. The detail of the results will be introduced in the presentation. [1] S. Dushenko, M. Shiraishi et al., Phys. Rev. Lett. 116, 166102 (2016). [2] E. Shigematsu, M. Shiraishi et al., submitted. [3] J.B.S. Mendes et al., Phys. Rev. Lett. 115, 226601 (2015).

Resume : There is a growing interest in spintronics for technological applications. An indispensable building block for this are pure spin currents, a flow of electrons’ spin angular momentum without a net flow of charge. Organic semiconductors have recently been found to have a comparably large spin diffusion time and length [1], which makes them ideal candidates for spintronic devices. However, spin properties in organic semiconductors have yet to be fully understood. The efficiency of spin injection driven by ferromagnetic resonance (FMR) from a ferromagnetic material (FM) into an adjacent non-magnetic material (NM) is given by the spin mixing conductance g_eff. It can be quantified by measuring the linewidth broadening of the FMR absorption of the FM due to an increase in Gilbert damping caused by spin injection into the NM. Here, we use this technique to systematically study spin injection from a ferrimagnetic insulator, YIG, and a metallic ferromagnet, Ni_80Fe_20, into different conjugated polymers and small molecules. In particular, we investigate the influence of the orientation of the packing of the organic semiconductor with respect to the ferromagnetic substrate on the spin injection efficiency. [1] S. Watanabe*, K. Ando* et al., Nature Physics, 10, 308−313 (2014)

Resume : We report an electrode-dependent robust and electrically-controllable tunneling electroresistance (TER) of magnitude up to 10000% at room temperature in FTJs with 3 nanometer-thick, spin-coated films of ferroelectric co-polymer P(VDF-TrFE) (70:30) on conducting-oxide electrodes. A conductive-tip atomic force, piezo-force microscopy and transport measurements on P(VDF-TrFE) thin films covering Indium-Tin-Oxide (ITO), La0.67Sr0.33MnO3 (LSMO), Nb-doped SrTiO3 (Nb-STO) and Au electrodes clearly demonstrate that TER effect appears due to modulation of barrier height caused by electrically-induced polarization reversal in P(VDF-TrFE) molecules. Time-dependent measurements by Kelvin probe microscope reveal that surface potentials in P(VDF-TrFE) are most stable on Nb-doped STO, followed by LSMO, ITO and Au. Whether this relaxation of surface potential is associated with polarization relaxation of the ferroelectric layer, has been investigated using electric field induced photo-electron spectroscopy measurements at the oxide-ferroelectric interface. Modulation of electronic properties due to polarization switching of the P(VDF-TrFE) has been observed and was monitored over extended period of time. The results clearly indicate that long time data retention can be achieved in organic ferroelectric tunnel junctions without external power supply that makes them a potential candidate for future non-volatile, energy-efficient memory elements.

Resume : Single molecule magnets (SMMs) are molecules comprising only a handful of magnetic ions, which show some of the properties of bulk magnets and at the same time those of low-dimensional systems. These, for instance, include magnetic hysteresis together with quantum tunneling of the magnetization. A typical way to characterize the magnetism of SMMs is through relaxation experiments, where an ensemble of SMMs is polarized along the direction of an external field and then the relaxation of the magnetization is monitored in time. Surprisingly, many experiments measure a relaxation time considerably faster then what expected from the measured zero-field splitting, and such discrepancy usually increases with the strength of the anisotropy barrier. This surprising result collides with the most common SMMs design rule, which consists in engineering the ligand field of the magnetic ions so to increase the magnetic anisotropy. Here we will demonstrate that spin-phonon coupling can account for such discrepancy and in particular that phonon dissipation is key to explain the under-barrier relaxation. Our calculations, combining advanced post Hartree-Fock theory, a master equation approach to the spin-dynamics and a stochastic model for phonon dissipation, show that the relevant energy scale for the spin relaxation is given by the lower-lying phonon modes interacting with the spin system. These provide a channel for spin relaxation at energies lower than that needed to overcome the anisotropy barrier, hence producing a fast spin relaxation mechanism. Such finding introduces a second design criterion for highly stable SMMs, namely the accurate design of the vibrational spectrum of the molecules.

Resume : Poly(3,4-ethylenedioxythiophene) (PEDOT) is certainly the most known and most used conductive polymer since it is commercially available under its doped form and it shows great potential for organic electronic, photovoltaic and thermoelectric applications. Studies dedicated to PEDOT films have led to high conductivity enhancements.1 Recently, an exhaustive understanding of the mechanisms governing such enhancement has been attempted. We reported the development of highly conductive PEDOT films by controlling the crystallization of the PEDOT chains and by a subsequent dopant engineering approach using iron (III) trifluoromethanesulfonate as oxidant, N-methyl pyrrolidone as polymerization rate controller and sulfuric acid as dopant.2 XRD, HRTEM, Synchrotron GIWAXS analyses and conductivity measurements down to 3 K allowed us to unravel the organization, doping and transport mechanism of these highly conductive PEDOT materials. We found that N-methyl pyrrolidone promotes bigger crystallites and structure enhancement during polymerization while sulfuric acid treatment allows the replacement of triflate anions by hydrogenosulfate leading to the increase of the charge carrier concentration. We proposed a charge transport model that fully corroborated our experimental observations. These polymers exhibited conductivities up to 5400 S cm-1 and transmittance at 550 nm between 80 and 96 %. With their promising electric, thermoelectric, optoelectronic and thermal behavior which will be discussed briefly, they open routes to industrial applications and hence long term stability is highly expected. A comprehensive study of their ageing and stability when subjected to thermal stress and various environmental stresses such as humidity, sun illumination, dark and inert atmospheres will be presented. The results regarding the transport mechanisms and the changes in the films’ structure will also be discussed.

Resume : A numerical study is performed to investigate heat transfer and pressure drop in a rectangular channel with alternating porous baffles mounted on both upper and lower walls. Time-dependent three-dimensional turbulent flow with constant thermo physical properties is assumed for air and Reynolds number ranging from 10000 to 50,000. A detailed analysis is carried out to investigate flow pattern, Nusselt number, and friction factor for various obstacle height, width, and spacing. The results show a transition to unsteady flow at a moderate value of the Reynolds number. For the unsteady case, the flow becomes periodic in time with vortex-shedding oscillations.

Resume : Since the conventional complementary metal-oxide-semiconductor (CMOS) technology is approaching to the fundamental limits of scaling down, alternative architectures or materials have been extensively explored to give the solution beyond CMOS scaling. In terms of new materials, the low dimensional semiconducting materials are aggressively researched such as nanotubes, nanowires and 2-dimensional (2-D) sheets. At the early stage of 2-D materials studies, graphene was largely investigated with its extremely high carrier mobility and the ambipolarity. However, graphene is very limited for a various application in electronics owing to zero energy band gap. Rather, 2-D transition metal dichacogenides (TMDs) materials are emerging not only with a finite energy band gap and its engineering in respect of a thickness, but also with the layered structure which can materialize the various device architectures by stacking the layers in atomic scale. Weak van der Waals interaction between interlayers of TMDs makes vertical integration with different materials of TMDs as modifying the energy structure, but at the same time, the weak bonding of TMD channel to gate dielectric layer causes the severe carrier fluctuation at interfaces in the device such as a configuration of a transistor, which degrades the performance largely. For TMDs transistors, the large amount of interfacial impurities perturbs the channel carriers owing to the poor interface and a large surface area for a thickness of a few nanometers. On considering the established theory for investigation of carrier fluctuation in conventional MOSFETs, through the study of low-frequency (LF) noise of 1/f features, we can understand the origin of current variance in TMDs transistors. Different to the LF noise analysis in MOSFETs using the basic models of carrier number fluctuation (CNF) or Hooge mobility fluctuation (HMF), neither can explain LF noise characteristics of the TMD transistor, in our study for MoTe₂ field effect transistors (FETs). Based on the previous studies of carrier transport in TMDs FETs and practical fitting of LF noise models to our data, we found out the interfacial impurities attribute the current fluctuation predominantly in MoTe₂ FETs, which is described by CNF model. To account for the large discrepancy in a low current regime, which cannot be explained even with CNF model, the effect of carrier refraction by interfacial charged traps was regarded. The model of CNF CMF (correlated mobility fluctuation) described the whole current regime very well for MoTe₂ FETs, which was introduced to MOSFETs for explaining the magnified interfacial effect on ultra-thin channels of devices. Interfacial Coulomb scattering parameter (α), which is one of LF noise parameters, was highly dependent on the accumulated carrier density with following α∝(N_acc)^-γ in ranged between 1.18~1.64 of γ. Compared with γ = 0.5 for MOSFETs, we can figure out the interfacial Coulomb scattering by charged traps are intensified largely, in particular for a low current regime for MoTe₂ FETs. The interface trap density of N_ST from LF noise parameters also exhibited around 10¹²~10¹³ eV^-1cm^-2, which signifies quite poor interface compared to MOSFETs. Thus, top passivating Al₂O₃ layer on MoTe₂ channel effectively reduced N_ST. As the alternative strategies to decline the interfacial carrier fluctuations in TMDs FETs, we tried to modify the channel thickness and gate structures in order to place the most of carriers within the area uninfluential to traps. We studied the correlation between the carrier mobility and the channel thickness in multilayer ambipolar MoTe₂ FETs by synthetic analysis of the static and the LF noise. The interplay of N_ST and α exhibited the maximum carrier mobility at a thickness of ~10nm. At ~10nm, N_ST showed the minimum value, and α decreased up to 10nm and saturated above 10nm in MoTe₂ FETs. The Coulomb-scattering-limited carrier mobility μ_C peaked near 10nm according to μ_C ∝ N_ST^-1 x α^-1. This is explained by the charge distribution modified by the channel thickness, which is further verified in a configuration of double gate (DG) FET. As changing both gate fields from top and bottom systematically in a multilayer MoTe₂ DG FET, it was rationalized that the modification of charge distribution by gate fields reduces the interfacial carrier fluctuation in addition to the enhanced controllability of channel charges. The improvement of device performance was examined by the betterment of the carrier mobility, contact resistance, flat band voltage, and N_ST. To sum up, LF noise study in MoTe₂ FETs revealed the interfacial carrier fluctuations of trapping/detrapping or scattering at the interface trap sites are predominant to modify the carrier transport inside the MoTe₂ channel. The experimental results advocated that the moderate channel thickness and the balanced bilateral gate fields weaken the carrier fluctuation and enhance the device performance largely in TMDs materials, as suggesting the pictures for future electronics using TMDs materials.

Resume : For the prediction of the trend of the catalytic behaviour in transition and noble metals, the most widely used model known to be the so called d-band center model, which correlates the adsorption energy to the average energy of the d electrons (d band center) of the metal surface, and has been proved successful in understanding the chemical bonding in heterogeneous interfaces various solid-gas and solid-liquid phases. Here, we demonstrate that this model, which was initially constructed for studying the adsorption of small molecules on the so-called platinum group of metals, is inadequate in capturing the complete catalytic activity of the magnetically polarized transition metal (TM) surfaces and propose its generalization by invoking two distinct band centers for spin majority and spin minority d-electrons. We demonstrate the potency of this extended d band model by comparing the adsorption energies of NH3 molecule on 3d-TMs obtained through spin-polarized density functional theory with the ones that are predicted by the model.

Resume : With tunable electrical conductivity and an intrinsically low thermal conductivity, organic materials present promising properties for thermoelectric applications. Among them, polythiophenes such as PEDOT:PSS and P3HT are p-type materials with reasonable ZT. However, due to anisotropic conductivities in thin films (electrical and thermal), the measurements of these parameters are challenging. In our study, we use a method that measures the in-plane thermoelectric properties to observe the influence of doping on polythiophenes. Dopants increase the charge carrier concentration, but can also modify the morphology of the thin films, and this work probes this effect. We additionally study morphologies of the n-type organic thermoelectric material TTF:TCNQ. Temperature-dependence of the thermoelectric properties is used to probe charge transport mechanisms in the materials studied as well as the Wiedemann-Franz law.

Resume : The N atom encapsulated fullerene (N@C60) has attracted great experimental and theoretical research attention due to its unique electronic and magnetic properties. The encapsulated N atom at the center of C60 cage has S = 3/2 spin quartet quasi-atomic states. The C60 cage shielding the N atom from the environment results in some excellent magnetic resonance properties, which makes N@C60 molecule a potential candidate for applications in biomedical sciences, quantum information processing, and spin labeling. Different from the metallic endohedral fullerene, N@C60 shows semiconductor properties with band gap 1.63 eV. It is meaningful to reveal the electron transport mechanism under electric field besides magnetic field. Due to hard purification of N@C60, little work has ben done for the preparation of the N@C60 films and crystals and their application in electrical devices. Here, the field-effect study and photodective applications of non-metallic endohedral fullerene are reported in the first time. The solution-grown needle-like single crystals of N@C60 are self-aligned in an orderly fashion. OFETs based on N@C60 single crystals exhibit electron mobilities as high as 2.23 cm2 V-1 s-1 which is about three times higher than the devices based on C60 under the same experimental condition. Compared to C60 single crystals, N@C60 crystals present a denser intermolecular packing, which was certificated by the lattice constant extracted from selected area diffraction patterns. Furthermore, the electrical properties showa band-like charge transport from 180K to 300K. Also, photodectors based on N@C60 single crystals exhibit highly sensitive photo-conductive properties with responsitivity of 177.3 A W-1 in the near-infrared (NIR) region, which is a more satisfactory value compared with those of other reported photodectors working under the similar conditions. The excellent electron mobility and photodetection of the N@C60 single crystals will enable significant advancements for the high-mobility n-type OFETs and highly sensitivite photosensing applications.

Resume : Molecular-scale electronic and optical devices are attracting numerous attention in the rapidly evolving information technologies. In particular, molecules that can be reversibly switched between two or more stable states show great potential for data storage and logical components. Recently, a novel molecular switch has been observed in experiment, in which the anthradithiophene (ADT) molecule can be reversibly switched by simply applying opposite charge carriers. Here, by using dispersion-inclusive density-functional theory computations we find that such a binary switch is based on the so-called extrinsic precursor state, where the adsorption sites at the physisorbed state and chemisorbed state are different. Our computations also reveal that the switching behavior of ADT originates from anthracene moieties. We can tune the stability and transition barrier by substituting the functionalized parts. Finally, we predicted a similar switching behavior in trans-ADS on the Cu(111) surface.

Resume : Utilizing organic light emitting diodes (OLEDs) for lighting requires a homogeneous operation across large areas. The ultimate suppression of profound local variations in temperature and current density across such devices requires an in-depth understanding of the interplay between the peculiar thermal and charge transport properties of organic semiconductors. Here, we identify and discuss major challenges for theoretical simulations geared towards revealing the impact of such inhomogenieties on the performance of entire devices Utilizing a drift-diffusion model including heat transport, we illustrate using selected examples crucial requirements that self-consistent simulations of coupled thermal and electric transport have to cope with in order to give valid results for all relevant operating conditions. Amongst these challenges are (i) the choice of relevant thermoelectric effects and (ii) the treatment of self-heating due to the strong temperature dependence of thermal and charge transport, causing instabilities not only in real devices, but also in simulations. Moreover, quantitatively correct results demand (iii) full, complex 3D simulations and, finally, the correct consideration of the dependence of the charge transport on inhomogeneous fields.

Resume : P3HT doped by F4TCNQ is a promising material in organic thermoelectrics. One big problem to achieve higher conductivity is that it requires a large amount of F4TCNQ which heavily destroys the crystal morphology of P3HT film, and impedes the charge transport. This makes it also almost impossible to make devices by solution-processing. To overcome this drawback, we indicated a simple way, that surface doping can achieve higher conductivity and do not destroy the crystal morphology of P3HT. Based on this method, we can get a conductivity of around 287 S/m and power factor ≈ 6 µW/m·K-2, a new record for P3HT doped by F4TCNQ. Furthermore, we can achieve higher power factor with double layers by surface doping. It is also an effective way to tune Seebeck coefficient by surface doping. We blended P3HT with PTB7 and by using surface doping we found that with increasing fraction of PTB7 the Seebeck coefficient increases from 141 µV/K (pure P3HT) to 1138 µV/K at 90 wt% of PTB7 and then dropped with even higher PTB7 fraction. This reason is that the fermi energy and transport energy shift uncoordinatedly with increasing PTB7 fraction. We conclude that surface doping is promising way to achieve high power factor and high Seebeck coefficient.

Resume : Spin-polarized charge transfer between a ferromagnetic metal (FM) and a molecule can promote molecular ferromagnetism [1,2] and interface states [3,4]. Observations of high spin-polarization of Fermi level states at room temperature [5] designate such interfaces as a very promising candidate toward achieving a highly spin-polarized, nanoscale current source at room temperature, when compared to other solutions such as half-metallic systems and solid-state tunnelling over the past 25+ years. Such interfaces are also called organic spinterfaces. The aforementioned molecular ferromagnetism describes a magnetic coupling at these interfaces imposed by the ferromagnetic metal’s magnetic order, such that the molecule’s non-magnetic C and N atoms acquire a magnetic moment. Furthermore, in the case of metal phthalocyanines, the central atom, e.g. paramagnetic Mn, is also coupled to the underlying FM surface[6]. Separately, in bulk form, metal phthalocyanines are known to exhibit 90º superexchange between the metal sites, leading to correlations in magnetic fluctuations along molecular chains whose sign and strength depend on the molecular stacking geometry [7]. To explore the magnetic interplay between an organic spinterface and nominally paramagnetic spin chains within ultrathin MPc films, we have deployed x-ray absorption spectroscopy + x-ray magnetic circular dichroism (XAS/XMCD) techniques, ab-initio density functional theory, magnetooptic kerr effect as well as magnetotransport experiments on Co/MPc/Co nanojunctions. We find that the Co/MnPc organic spinterface can stabilize an antiferromagnetic spin chain of at least 3 unit lengths away from the interface at room temperature and low applied magnetic field[8]. This hybrid magnetic bilayer exhibits a low-temperature response akin to either exchange bias or so-called ‘magnetic hardening’[9]. Finally, these spin chains considerably enrich the magnetic field dependence of magnetotransport across nanojunctions[10,11]. Indeed, we observe (a) effective high coercive fields that can be electrically tuned; (b) concurrent tunnel magnetoresistance and tunneling anisotropic magnetoresistance; and (c) bias voltage dependencies of spin-polarized transport in the nanojunctions several spintronic states that indicate an interaction between spin-polarized transport and spin flip excitations within a MPc chain[12]. These observations of magnetic ordering within the organic tunnel barrier imply that it is structurally ordered. Thus, taken together, these results affirm the growing maturity of the organic spintronics research field. Indeed, they mark the significant milestone in 2015 of transitioning from amorphous to ordered organic tunnel barriers, as was achieved in 2001 for inorganic MTJs upon transitioning from amorphous Al2O3 to MgO tunnel barriers[13]. That transition ushered in the concept of electronic symmetry channels of solid-state tunnelling alongside their spin channel counterparts. Based on our results, the rich palette of physical effects within solid-state tunnelling that results from spin chains within a structurally ordered organic tunnel barrier, and the ability to tune a spin chain’s exchange interactions[7], augur important research opportunities for the field of organic spintronics at the interface with quantum physics. [1] A. Scheybal et al., Chemical Physics Letters 411, 214 (2005) [2] H. Wende et al., Nat. Mater. 6, 516 (2007) [3] C. Barraud et al., Nature Physics 6, 615 (2010) [4] S. Sanvito, Molecular spintronics, Chem. Soc. Rev. 40, 3336 (2011). [5] F. Djeghloul et al., Sci. Rep. 3, 1272 (2013); F. Djeghloul et al, Carbon 87, 269 (2015); F. Djedhloul et al, J. Phys. Chem. Lett. 7 2310 (2016). [6] M. Rastei et al, Phys. Rev. Lett. 101 116602 (2008); S. Javaid et al, Phys. Rev. Lett. 105 077201 (2010); J. Brede et al, Phys. Rev. Lett. 105 047204 (2010) [7] M. Serri et al., Nature Communications 5, 3079 (2014) [8] M. Gruber et al, Nature Materials 14, 981, (2015). [9] K.V Raman et al, Nature 493 7433 (2013). [10] C. Barraud et al., Phys. Rev. Lett. 114 206603 (2015). [11] C. Barraud et al, Dalton Trans. 45 16694 (2016). [12] X. Chen et al., Phys. Rev. Lett. 101 197208 (2008). [13] M. Bowen et al, Appl. Phys. Lett. 79 1655 (2001).

Resume : The stretchable electronics (i.e. foldable smartphones), and thin film sensors markets have huge growth potential in the nearby future. For such stretchable electronics we need suitable flexible conductors and a perfect candidate for this is the Poly(3,4-ethylenedioxythiophene) poly(4-styrenesulfonate) (PEDOT-PSS) polymer. Most recently it was found that post-treatment with sulfuric acid of PEDOT-PSS thin films result in electrical conductivity increase and a UV absorption decrease due to the replacement of majority of PSS with sulfate ions. However, the correlation between thermal, electrical, and optical properties of the acid treated PEDOT-PSS thin films are not very well understood. In this project, PEDOT-PSS thin films were deposited by either drop-casting or spin-coating onto microscopic slides and then submerged into sulfuric acid for 10-60 minutes. Preliminary thermal measurements show that the acid treatment up to 60 minutes increases the thermal conductivity by a factor of 3 compared to the non-treated films. These trends are consistent with the electrical conductivity measurements, which show an increase in the electric carrier density. This carried density increase is also proven by an observed shift in plasmon resonance frequency. Correlation between thermal and optical properties will be presented.

Resume : The study of the spin properties of ferromagnetic metal-organic interfaces has recently received considerable attention because of the prospect of developing new hybrid and multi-functional molecular devices and to the very specific fundamental phenomena which occur at the ferromagnet/molecule interface. In particular, highly spin-polarized interface states at the Fermi energy could be observed in very recent spin-resolved photoemission studies of phthalocyanine films and amorphous carbon films on ferromagnetic Co(001) [1,2]. The question is now raised whether this property is confined to these two systems, or whether these results are indicative of a broader pattern. The goal of the present study is to show how general the phenomenon of highly spin-polarized interface states in ferromagnetic metal-organic interfaces is. We show that different ferromagnetic metal-organic interfaces behave quite similar with respect to their spin-polarized interfacial electronic structure, regardless of the choice of ferromagnet; of whether the molecule is conjugated or not; and of whether it is planar or not. This high spin polarization at room temperature of states near the Fermi level thus appears to be a generic feature of spin-polarized charge transfer at the interface between a ferromagnetic metal (Co, Fe) and a molecule[3]. [1] F. Djeghloul et al., Sci. Rep. 3, 1272 (2013) [2] F. Djeghloul et al., Carbon 87, 269 (2015) [3] F. Djeghloul et al., J. Phys. Chem. Lett 7, 2310 (2016)

Resume : DFT calculations, carried out with different basis sets, for the static longitudinal linear polarizability αL, and second order hyperpolarizability γL, of small doubly charged tetradecahepta-ene chains, are presented. To demonstrate the stability of several conformations studies of different reaction profiles, determination of rate constants of isomerization reactions and activation energies were made. A theoretical study of a new range based molecules substituted hexadecaocta-ene having the structure D-Π-A by the methods DFT, AM1, PM3 and PM6 finally we can highlight: • The effectiveness of the methods used to calculate the polarizabilities αL, hyperpolarizabilities γL and their components. • The correlations found between the various physical parameters and electronic substituted hexadecaocta-ene and its value exceeds 0.88 average polarizability allowing us and precise quantitative determination of the polarizability for systems with more than 80 atoms by simple extrapolation. • The PM6 method is an effective alternative methods CPHF / TDHF, DFT for the calculation of polarizabilities. The results obtained for positive and negative chains show that the ionization state effect decreases more rapidly, as the chain length is increased, for αL than for γL. Or both types of charged chains, the incorporation of the electron correlation increase the αL, and γL values, as compared to the DFT values. A comparison between the results obtained using the standard 6-31G basis set and augmented versions of this set, obtained by the addition of diffuse and polarization functions, shows that 6-31G basis set does not provide a good description of the negative chains studied here and that the addition of extra diffuse functions on the basis set is needed in order to obtain reliable estimates for polarizabilities, specially for γL. Key words : polarizabilities; hyperpolarisability; tetradecahepta-ene; DFT, AM1, PM3 , PM6

Resume : In this study, we used quantum chemistry calculations in order to determine some kinetic parameters of the isomerization reaction of the substituted tetradecahepta-ene. The studied molecules are: (C14H16, C14H8F8, C14H8Cl8, C14H8Br8 et C14H8I8) Cis and Trans. One of the adopted ways to access these parameters (activation energy, rate constant, etc ...) is looking for the transition state that is based on the exploration of intermediaries during the passage of Cis-Trans isomerization process. The study of a ten molecules series gives the following results:  The Trans conformer is more stable than the Cis. The activation energy changes very greatly depending on the size and nature of the substituent according to the reaction profile.  The constants of the isomerization reaction rates are in the following order: kC14H16 >> k C14H8F8 >> k C14H8Cl8 >>k C14H8Br8 >> k C14H8I8. The geometrical parameters vary considerably according to intermediate products The calculation methods are DFT and Ab-initio methods at STO-3G (d,p) levels Keywords: substituted tetradecahepta-ene, kinetics; isomerisation, Ab-initio, DFT

Resume : In organic semiconductors, band-like transport is peculiar to charge carriers that are delocalized over more than a single molecular unit and it is associated with an increase in carrier mobility upon decreasing of temperature. Hitherto, how such type of transport is related to molecular design is not fully understood and, generally, a comprehensive picture of its origin is lacking[1]. In this work, we used two dicyanoperylenecarboxydiimide derivatives (PDI) molecules with different lateral chains that were either fluorinated, PDIF-CN2 or alkylated, PDI8-CN2. The former derivative would exhibit band-like transport whilst hopping transport was identified as the main conduction mechanism for the latter. Both molecules were exposed to comparable extrinsic disorder that is considered to stem from the substrate1. A comparative study that includes multiple characterization techniques such as inelastic neutron scattering, THz and PE spectroscopy, GIWAXS, electrical characterization but also modeling and simulations allows us to unravel the origin of band-like transport mechanism by looking at the molecular fluctuations and their effect on the phonon modes of both compounds. [1] N. A. Minder, S. Ono, Z. Chen, A. Facchetti, A. F. Morpurgo, Adv. Mater. 2012, 24, 503.

Resume : The development of solution-processable components for organic field-effect transistors (OFETs) is in high demand for technological applications such as the backbone of flat panel displays, electronic paper (e-paper), radio-frequency identification (RFID) tag, etc. Considerable effort over the past decades has been focused on improving the carrier mobility in organic thin-film transistors by optimising the organic materials. Together with the integration of high performance organic materials, another enduring challenge in OFET development is the improvement and control of the injection of the charge carriers into the organic channel. Graphene, a two-dimensional layer of covalently bonded carbon atoms, is steadily making progress into applications relying on heterointerfaces with organic semiconductors. Here, we demonstrate the versatile operation of solution-processed organic transistors both in lateral and vertical geometry by exploiting the weak-screening effect and work function modulation properties of the graphene electrodes. The lateral field-effect transistor that we have studied is based on graphene as source and drain contacts and Polyera ActiveInkTM N2200 polymer as a solution-processed polymeric semiconductor. In this proof-of-concept device we have found a much superior performance as compared to devices with gold contacts in similar device configurations. Moreover, in our prototype vertical field-effect transistor with the N2200 polymer channel and graphene source-electrode, the experimental results show an effective energy-barrier modulation of up to ~500 meV and a transconductance of up to three orders of magnitude, applicable for low power electronics. Our results demonstrate a general strategy for overcoming traditional noble metal electrodes, and to integrate graphene with solution-processed N2200 polymer transistors for high-performance devices suitable for future plastic electronics.

Resume : Thin hole transport layers (HTL) are crucial elements in organic semiconductor based devices. Metal oxides are an encouraging material class for this purpose. Metal oxides can be used to modify either the two contacts in a device for improved wettability as well as chemical and electronic compatibility of the contacts with the organic layer. Several materials like NiO, V2O5, WO3 and MoO3 have demonstrated encouraging prospectives for performing as efficient charge transport layers. Especilly MoO3 attracted extensive interest due to its superior performance. In order to evaluate charge transport properties of annealed semiconductor films, devices are required to be stable at high annealing temperature. Whereas PEDOT:PSS has generally proper charge injection and extraction properties, these may significantly change upon heating above certain temperature. In this contribution, we demonstrate that a MoO3 interlayer can efficiently substitute PEDOT:PSS as hole transport layer within single carrier hole only devices, due to its increased stability at elavated annealing temperature.

Resume : The efforts to constantly improve data storage technology and the prospect of quantum computing have lead to the study of spin-related effects in molecules. Among available molecules, the metallocenes (MCp2, where M = Fe, Co, Ni etc. and Cp is a cyclopentadienyl ring) represent an ideal playground to experimentally validate new concepts in this domain. Theoretical studies [1-3] showed in fact that metallocenes could be used as spin filters and that the filtering efficiency could be improved by spin doping the molecules using transition metal atoms. We present here the first experimental evidence of such a spin doping by focusing on common metallocenes like ferrocene (FeCp2) and nickelocene (NiCp2) adsorbed on a Cu(100) surface. By combining scanning tunnelling microscopy (STM) and density functional theory (DFT) calculations we show that the on-surface reaction of these metallocenes with single transition atoms produces new magnetic molecules. In particular, the combination of a Co atom with ferrocene leads to a molecule exhibiting a spin 1/2 Kondo effect [4], while the combination of a Ni atom with nickelocene leads to a molecule exhibiting a high magnetic anisotropy [5]. References [1] L. Zhou et al., J. Am. Chem. Soc., 130, 4023–4027 (2008) [2] L. Wang et al., Nano Lett. 8, 3640 (2008) [3] Z. Yi et al., ACS Nano 4, 2274 (2010) [4] Ormaza et al., Nano Lett. 16, 588 (2016) [5] Bachellier et al., in preparation

Resume : Molecular junctions are a versatile test bed for investigating nanoscale thermoelectricity and contribute to the design of new cost-effective environmentally friendly organic thermoelectric materials. It was suggested that transport resonances associated with discrete molecular levels could play a key role in thermoelectric performance, but no direct experimental evidence has been reported. Here we study single-molecule junctions of the endohedral fullerene Sc3N@C80 connected to gold electrodes using a scanning tunnelling microscope. We find that the magnitude and sign of the thermopower depend strongly on the orientation of the molecule and on applied pressure. Our calculations show that Sc3N inside the fullerene cage creates a sharp resonance near the Fermi level, whose energetic location, and hence the thermopower, can be tuned by applying pressure. These results reveal that Sc3N@C80 is a bi-thermoelectric material, exhibiting both positive and negative thermopower, and provide an unambiguous demonstration of the importance of transport resonances in molecular junctions.

Resume : Thermoelectric (TE) materials transform heat flow into electricity. The process efficiency is expressed by the figure of merit (FoM), which is proportional to the electrical conductivity σ, the Seebeck coefficient S, and to the inverse of the thermal conductivity κ. Having an intrinsic low κ, organic semiconductors (OSC) have great potential for flexible, low-cost and large-area TE applications. The optimisation of FoM of OSC is usually obtained by doping the material to increase the charge carrier density, which is not really efficient because it increases σ but reduces S. On the other hand, the nano-confinement of OSC has to be considered an appealing alternative to increase the FoM because it has shown to improve the charge mobility, then σ, without affecting S and it is expected to reduce κ. Here, we compare TE properties of polymeric continuous films, obtained by spin coating, with those of nano-stripes which are obtained by using the Lithographically Control Wetting (LCW) technique [1]. The studies of two commercial n-type P(NDI2ODT2) and p-type IIDDT-C3 OSCs are reported. TE properties are studied by using the field-effect transistors (OFETs) geometry, which allows the investigation of the trade-off relationship among σ, S and carrier concentration without doping the materials [2]. [1] D. Gentili, F. Valle, C. Albonetti, F. Liscio, M. Cavallini, Acc. Chem Res 47, 2692 (2014) [2] F. Zhang, Y. Zang, D. Huang, C. Di, X. Gao, H. Sirringhaus, D. Zhu, Adv Funct Mat 25, 3004 (2015)

Resume : Organic thin film transistors offer great potential for use in flexible electronics. Much of this potential lies in the solution processability of the organic polymers enabling both roll coating and printing on flexible substrates and thus greatly reducing the material and fabrication costs. We present flexible organic power transistors prepared by fast (20 m min−1) roll-to-roll flexographic printing of the drain and source electrode structures, with an interspace below 50 um, directly on polyester foil[1]. The devices have top gate architecture and were completed by slotdie coating of the organic semiconductor poly3hexylthiophene and the dielectric material polyvinylphenol before the gate was applied by screen printing. All the processing was realized in ambient air on a PET flexible substrate. We explore the footprint and the practically accessible geometry of such devices with a special view toward being able to drive large currents while handling the thermal aspects in operation together with other organic printed electronics technologies such as large area organic photovoltaics (OPV) and large area electrochromic displays (EC). We find especially that an elevated operational temperature is beneficial with respect to both transconductance and on/off ratio. We achieve high currents of up to 45 mA at a temperature of 80 °C. Finally, we observe a significant temperature dependence of the performance, which can be explored further in sensing applications. [1] Francesco Pastorelli, Thomas M. Schmidt, Markus Hösel, Roar R. Søndergaard, Mikkel Jørgensen and Frederik C. Krebs, " The Organic Power Transistor: RolltoRoll Manufacture, Thermal Behavior, and Power Handling When Driving Printed Electronics", Volume 18, Issue 1, pages 51–55, January 2016, doi: 10.1002/adem.201500348

Resume : Graphene, a single sheet of carbon atoms forming a honeycomb lattice (HCL), is a gapless semiconductor with low-energy excitations that are described by a Dirac-Weyl Hamiltonian with pseudospin S = 1/2. The T3 or dice lattice, is described by the Dirac-Weyl Hamiltonian resembling the one describing the HCL, but with pseudospin S = 1. The band structure of the T3 lattice is that of graphene with an additional flat band. The significance of the α-T3 model lies in the fact that it interpoles, via a parameter α, between HCL (α=0) and dice lattice (α=1) [1]. Recently, a paper [2] showed that Hg1-xCdxTe has an α-T3 structure for a parameter α=1/√3. The energy band spectrum of the α-T3 model is independent of α. However, the parameter α has an effect on the band coupling and the Berry phase. Here we theoretically investigate how the parameter α could affect the electron transmission through a potential barrier in the α-T3 model. We found that the potential barrier is more transparent on the values of α that tend towards 1 (dice lattice) than on those that tend towards 0 (HCL). Furthermore, the results obtained highlight three important phenomena: Fabry-Perot resonance, Klein tunneling and super Klein tunneling. References: [1] A. Raoux, M. Morigi, J.-N. Fuchs, F. Piéchon, and G. Montambaux, Phys. Rev. Lett. 112, 026402 (2014). [2] J.D. Malcolm and E. J. Nicol, Phys. Rev. B 92, 035118 (2015).

Resume : Thermoelectric (TE) materials can convert heat into electricity when a temperature gradient is present. The investigation of conductive polymers such as polyaniline (PANI) and poly(3,4-ethylenedioxythiophene) (PEDOT) as active materials for thermoelectric generators in the room temperature range is gaining interest because of several key advantages offered by these materials. The relative easiness of solution processing, their mechanical stability and flexibility together with low density and low thermal conductivity make conductive polymers suitable for integration in a thermoelectric generator. Polymers offer remarkably low thermal conductivity values but modest Seebeck coefficient and electrical conductivity. Polymer/inorganic composites of PANI were produced by adding high-conductive single wall carbon nanohorns (SWCNHs) or thermoelectric metal chalcogenides nanoparticles belonging to the tetrahedrite family Cu12Sb4S13 as fillers to improve the thermoelectric performance by means of polymer matrix/inorganic particle electronic junction. Electrical conductivity, Seebeck coefficient and thermal conductivity were estimated on PANI-based composite films and pressed pellets. The thermal stability was evaluated by means of thermogravimetric analysis/differential scanning calorimetry coupled with residual gas analysis.

Resume : Over recent years several new classes of conjugated polymer and molecular semiconductors have shown relatively high carrier mobilities. We have been studying the Seebeck coefficient of a range of these materials using gated field-effect transistor configurations, both to better understand the transport physics of these materials but also to understand better their suitability for thermoelectric applications as well as relevant the structure-property relationships. We have developed novel strategies for bulk doping of self-organised conjugated polymers, for which the incorporation of dopant molecules does not degrade the structural order of the polymer. Polymers doped in this way exhibit excellent charge transport and thermoelectric properties. We are also investigating the transmission of pure spin currents through these materials.

Resume : The effect of ferromagnetic scanning tunneling microscope (STM) cobalt tip on the electronic, magnetic and electronic transport properties of a Co-phthalocyanine(CoPc)/Co(111) junction has been investigated in the framework of density functional theory in conjunction with the Landauer-Bütteker formula and the nonequilibrium Green function formalism. The calculations show that by reducing the distance between the STM tip and the CoPc molecule, i.e., when passing from the tunneling regime to the contact regime the spin magnetic moment of CoPc molecule is flipped. This spin flip of the CoPc molecule led to a change of the sign of the tunneling magneto-resistance ratio (TMR). The change from the tunneling regime to the contact regime also let to high modification of the total and spin-polarized I-V characteristics.

Resume : Organic electronics devices have demonstrated their large potential for many fields of applications (OLEDs, OPVs...) but their fabrication requires special care due to the high sensitivity of organic semiconductors to contaminants or damage. X-ray exposure, for instance during electron gun operation in a deposition chamber, may modify the optoelectronic properties of organic semiconductor (OSC) layers, as evidenced by several authors. We present a systematic study of the influence of electron-gun exposure of ITO/PEDOT:PSS/NPB/MADN/Al OLED structures as a function of electron-gun acceleration voltage, exposure time and nature of the e-beam target (Al or Co). By comparison with pristine samples, a dramatic change of the I,L(V) characteristics of OLEDs is reported with a drift of the onset electroluminescence to higher voltage (typically from 5 V to 18 V) and an important drop of current and emission. Photoluminescence (PL) spectra also reveal significant changes of peak emission in terms of energy, intensity and width. All these effects are indicative of the creation of localized new energy levels related to the presence of traps in the OSC gap. Identification and assignment of the PL peaks is performed through temperature and excitation-dependent characterizations. Magnetoresistance measurements are in progress in order to investigate the role of such trap states on organic magnetoresistance (OMAR).

Resume : Magnetic field effects observed in various organic devices, abbreviated as OME, are of great scientific interest because of their large magnetoresistance (MR) up to 10% at room temperature and under small magnetic fields of approximately 10 mT without ferromagnetic contacts [1]. In this work, we studied the effect of traps on OME of sexithiophene devices by impedance spectroscopy. Two types of trap were introduced by electrical stressing and by doping with pentacene molecule. The results show that these traps caused different effects on OME of sexithiophene devices, which is further discussed based on the impedance spectroscopy data.

Resume : We investigated the feasibility of using an ultrathin MgO(001) interlayer for developing a long spin transport channel in organic spintronic devices of Si(001) \ 5 nm MgO(001) \ 15 nm Fe(001) \ 1.6 nm MgO(001) \ t nm Cu-phthalocyanine (CuPc) (t = 1.2~ 5.0) \ 25 nm Co magnetic tunnel junctions (MTJs). Correlation between experimentally observed structural properties of the MgO\CuPc and CuPc\Co interfaces and spin transport properties was explored by direct nanoscale visualization and magnetoresistive measurement. An asymmetric dependence of magnetoresistance on the bias polarity was clearly observed. Because the interfaces are crucial for the tunneling magnetoresistance amplitude and voltage dependence, the MTJ structures with and without an MgO interlayer were intensively examined by cross-sectional TEM. The TEM observations indicate that the CuPc molecules were deposited on the MgO film in a planar configuration in a rather ordered manner. In addition, the reaction layer at the CuPc\Co interface appears to consist of nano-sized Co grains on top of which large columnar Co grains grew. The interface properties were also clarified by the surface sensitive technique, metastable helium de-excitation spectroscopy (MDS). It is shown that the controlled growth of CuPc films on Fe(001) layers by insertion of an ultrathin MgO(001) layer is the key factor influencing both spin injection and transport processes.

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Resume : The use of organic spin valves in planar lateral geometry makes it possible to avoid undesired shunt resistances and to achieve gate control over spin currents. However, results always remain controversial owing to the short spin diffusion length and large metal-organic interface resistance values that make spin valve magnetoresistance signals possibly elusive. Here, we investigate electrolyte-gated organic field-effect transistors with channel length down to sub-100nm scale, thus matching well the window allowed for spin injection and detection. Limited short-channel effects have been observed, with good saturation of the output curve and ON/OFF ratio from 10^6 up to 10^10. We show how the remarkably small contact resistance values result from current crowding, as charges are also injected and collected away from the edges of electrodes, up to a crowding length scale of several hundreds of nm. This uneven spatial current distribution therefore extends over distances that exceed the spin flip length in organics. By performing calculations based on Valet-Fert model, we show that current crowding increases the effective channel length and decreases the magnetoresistance values by typically one order of magnitude, even for the longest spin flip length and shortest crowding lengths values reported in the literature. Our findings indicate that current crowding effects have a clear detrimental impact on lateral organic spin valves performance and therefore cannot be ignored.

Resume : Theoretical modeling is an indispensable complement to experimental work in the field of molecular spintronics. Spin dynamics in particular are often difficult to directly access experimentally. Ideally, theoretical models of spin dynamics combine atomistic resolution with material-relevant ensemble effects, and the interplay of heat- and charge-dynamics with magnetic field-, morphological- and spin-orbit coupling effects. We pursue this ideal by implementing spin-dynamics on top of charge-dynamics obtained using the VOTCA-CTP [1] software. VOTCA-CTP is a multi-scale modeling framework for soft matter. Our implementation uses a semi-classical kinetic Monte Carlo model with spin relaxation mechanisms modeled and calculated from first-principles electronic structure theory. As a first application, we calculated the spin relaxation time in bulk Alq3 - the fruit-fly molecule of molecular spintronics - as a function of temperature and charge concentration. The result is an unprecedented view of the balance of all spin relaxation mechanisms in this system. Ongoing work includes applications to the PBTTT polymer, and spin-off simulations of spin-relaxation in thiophene-based high-mobility organic semi-conductors. In addition to these results, we present an outlook on future extensions to so-called spinterfaces, and complex spin phenomena such as the Spin Hall- and Nernst effects. 1. V. Rühle et al., J Chem. Theory Comput. 7, 3335 (2011)

Resume : The field-induced time-resolved microwave conductivity (FI-TRMC) technique has recently been introduced as a promising approach to probe the intrinsic charge carrier mobility over short length and time-scales at the semiconductor/dielectric interface by microwave-based dielectric loss measurements. We will report on the structural and electronic properties of four isomers of didodecyl[1]benzothieno[3,2-b][1]benzothiophene (C12-BTBT) varying by the isomerism of the alkyl side-chains.[1] This work highlights that the molecular packing, driven by the molecular structure of the isomers, has not only a strong impact on the charge carrier mobility but also on the ionization potential due to changes in the magnitude of electronic delocalization and electronic polarization effects. The FI-TRMC measurements yield a strikingly high average interfacial mobility of 170 cm2/Vs at room temperature for 2,7-didodecyl[1]benzothieno[3,2-b][1]benzothiophene approaching for the first time the hole mobility of inorganic semiconductors. Quantum-chemical calculations demonstrate that the transport operates within the band regime in this isomer, which we associate with the 2D character of the crystal and the limited thermal fluctuations in electronic transfer integrals. Such a record mobility holds great promise for the field of organic electronics and efforts shall now be invested to further understand how to achieve these performances in contact-based technologies. [1] Adv. Mater. 2016, 28, 7106

Resume : Metal-organic interfaces play a crucial role in organic electronic devices. Recent experiments show that the energy barriers at such interfaces can be determined precisely with hot electron transistors. This establishes a novel spectroscopy tool, so-called hot-electron in-device spectroscopy. In addition, hot-electron transistors exhibit unconventional conductance behavior. We have developed theoretical models to describe transport features of such novel devices, which include the tunneling and scattering distributions of the hot electrons in the base. This paper presents an approach, which is able to describe all essential transport phenomena observed in current multi-level hot-electron devices. Thereby, the device parameters are clearly related to molecular parameters – a great asset for such spectroscopy. We further study the influence of disorder and other characteristics, which may influence the collector current as a function of the hot-electron energy.

Resume : We present an analytical model to calculate the conductivity and Seebeck coefficient, and concomitantly the power factor, of doped organic semiconductors. The model describes, with a single set of parameters, experiments covering a wide range of doping concentrations and temperatures for various polymers in which doping gives rise to integer charge transfer. The key ingredient of our model is the fact that the electrostatic potential of ionized dopants broadens the original density of states (DOS) of the organic semiconductor, giving rise to a new, deep tail of trap sites. As a consequence of the competition between DOS broadening and state filling, the effective mobility becomes a non-monotonous function of dopant concentration. For increasing doping concentration, the conductivity and power factor increase while the Seebeck coefficient decreases. Interestingly, for initially low-disorder materials, the continuous broadening of the DOS with increasing doping concentration gives rise to an apparent power law relation between conductivity and Seebeck coefficient with slope around -1/4, in good agreement with many recent experiments, including our own. We used our model to assess attainable performances for doped organic semiconductors. Using realistic-optimistic parameters we find that, within the model framework, the thermoelectric figure of merit ZT will not easily become 0.1 or more.

Resume : Polymer nanostructuration can modify the material properties due to changes produced by size reduction, crystallization, orientation, etc. The confinement of polymers inside the nanocavities of anodic porous alumina has shown to tailor the dimension of many polymers and their composites[1,2]. All these properties changes at the nanoscale need to be studied and understood, which is very challenging. In this work, we present the results on the synthesis and thermal conductivity properties of ordered Polycarbonate nanostructures 1D (nanowires), 3D (nanowires networks) compared to bulk reference samples. The synthesis of the samples consisted in the thermal infiltration of Polycarbonate inside 2D- and 3D-porous alumina membranes acting as templates[3]. The structural and morphological characterization was performed by SAXS and SEM techniques respectively. And we will show how the thermal conductivity of this polymer is affected by means of Scanning Thermal Microscopy (SThM) technique[4]. The results on the alteration of thermal properties of the synthesized nanostructures will be discussed and correlated with the detected modifications on the structural and morphologically nanostructured samples compared to bulk references. [1] Mijangos, C., et al. Progress in Polymer Science, 2016, 54-55, 148. [2] M. M. Rojo, et al. Nanoscale, 2014, 6, 7858. [3] J. Martín, et al. Nature Communications 2014, 5, 5130. [4] A. A Wilson et al. Nanoscale, 2015, 7, 15404.

Resume : We present thermoelectric transport mechanism in polyaniline (PANI) filled with 1D (Single wall carbon nanotubes -SWCNT- and multi wall carbon nanotubes -MWCNT-) and 2D (Reduced graphene oxide -RGO-) carbonaceous nano-fillers. Chemically synthesized PANI/SWCNT, PANI/MWCNT, and PANI/RGO nanocomposites with various weight fractions of nano-fillers ranging from 0-21 wt-% have been characterized by using FE-SEM, and HR-TEM. XRD and Raman spectroscopy have been employed to investigate structural properties. The enhancement of the electrical conductivity is interpreted by using mixing rule and percolation model. Where the mixing rule indicates the modification of conducting state of PANI, the percolation study reveals that the formation of conducting networks is not a true statistical percolation process based on the random distribution of individual high aspect ratio fillers, but rather is attributed to the mutual attraction of the nano-fillers in the composites. It also explains that charge transfer between PANI and nano-fillers through π-π interaction is contributed to conducting network. The thermal conductivity of the composites does not show a percolation behavior. Also it can be observed that thermal conductivity value remains constant up to certain nano-fillers volume loading which fulfils the demand for excellent thermoelectric materials. Further, effective mass approximation (EMA) model is used in order to interpret the experimental data of thermal conductivity. The thermoelectric performance (ZT) is enhanced up to few orders of magnitude for the nanocomposites compared to that of pure PANI.

Resume : The introduction of p- and n-doped layers into the device architecture of organic opto-electronic devices can greatly improve their performance. Charge carrier extraction or injection from or into the organic photo-active layer is enhanced by doping, resulting in a reduction of ohmic losses. Transparent conductive layers can be realized by molecular doping of organic host materials moving the Fermi level to the appropriate position, enabling electron or hole selectivity. In this work, we use thermovoltage (Seebeck effect) and temperature-dependent conductivity measurements to determine the dominating type of charge carriers introduced by the dopant and to gain insight into the position of the transport level with respect to the Fermi level. The investigation of fullerene dopants with a high degree of fluorination in various amorphous host materials allows us to tune the energy level offset between host and dopant and to study their influence on Fermi level position and overall doping efficiency systematically. From the thermoelectric measurements it is possible to observe a clear influence of the energy level offset between matrix and dopant on doping efficiency. Combining thermoelectric measurements with Kinetic Monte Carlo simulations gives further insight into the influence of Coulomb interactions on the trapping of mobile charge carriers in doped organic semiconductors. We find that at low doping concentrations a high amount of charge carriers is immobilized in trap states, leading to a reduced doping efficiency. Upon increasing the doping concentration, the trap states are subsequently passivated and an increased doping efficiency can be observed. Furthermore the doping efficiency is increased upon fluorination of the dopant molecules and we find a correlation between the energy level offset and the doping efficiency, at the same molar concentration.

Resume : Due to new research efforts in the field of thermoelectrics with a focus on size effects, there is a growing need for measurement setups dedicated to samples with small geometrical dimensions like thin films and nanowires with considerably different physical properties than bulk material. The characterization of these samples is important to learn more about their structure and conduction mechanism but also important for technical applications e.g. in the semiconductor industry. We report on the development of a new system to simultaneous measure the electrical and thermal conductivity, the Seebeck Coefficient and the Hall Constant of a thin film sample in the temperature range from liquid nitrogen up to 300°C. Due to the nearly simultaneous measurement at only one sample, errors caused by different sample compositions, different sample geometries (thickness) and different heat profiles can be avoided. The system consists of two main parts, a structured Si-wafer and a suitable measurement setup. To measure the el. conductivity and the Hall constant, the wafer owns a structure with four electrodes to use the Van-der-Pauw method. For the Seebeck measurement an additional temperature gradient can be applied on a membrane Setup. The temperatures for the Seebeck calculation are measured with resistance thermometers on the chip. The thermal conductivity can be measured in plane using the Völklein Method, doing a steady state or transient measurement. Therefore a small heating/sensing stripe is deposited on a very thin nitride membrane. Depending on the material of investigation (organic or inorganic), the sample can be deposited on the front side or the backside of this membrane. To get a correct result, the measurement has to be done under vacuum (no heat transfer by conversion) in a thermal stabilized and controlled chamber (to avoid heat transfer by radiation, the ambient temperature has to be identical with the chip temperature). In order to meet these requirements a suitable vacuum chamber with sample holder and necessary ports has been designed. The sample holder can be cooled with liquid nitrogen and heated by joule heating on both ends to create either a constant temperature or a defined temperature gradient. To measure the Hall constant, the chamber is put between two spools of an electro magnet to apply a variable magnetic field with a maximum of +/-1 T. As proof of concept, a showcase study of Bi87Sb13 thin films in varying thickness has been performed and compared to previously published data [1]. References: [1] Linseis, V., Völklein, F., Reith, H., Woias, P. and Nielsch, K. (2016) ‘Platform for in-plane ZT measurement and Hall coefficient determination of thin films in a temperature range from 120 K up to 450 K’, Journal of Materials Research, 31(20), pp. 3196–3204. doi: 10.1557/jmr.2016.353.

Resume : Pi-conjugated organic molecules and polymers now provide a set of well-performing semiconductors that support devices, including light-emitting diodes (LEDs) as used in smart-phone displays and lighting, field-effect transistors (FETs) and photovoltaic diodes (PVs). These are attractive materials to manufacture, particularly for large-area applications where they can be processed by direct printing, so that the cost of materials and processing can be very low. This practical success is made possible by breakthroughs in the understanding and engineering of the underlying semiconductor science. The physics of organic semiconductors is often controlled by large electron-hole Coulomb interactions and by large spin exchange energies. Management of excited state spin is fundamental for efficient LED and solar cells operation. I will discuss in particular recent progress in the control of emissive spin singlet excited states and non-emissive spin triplet excited states.

Resume : Organic conductors are being evaluated for potential use in waste heat recovery through lightweight and flexible thermoelectric generators manufactured by means of cost effective printing processes. Assessment of the potentiality of organic materials in real devices still requires a deeper understanding of the physics behind their thermoelectric properties, which can pave the way for further development of the field. In this contribution we will report a detailed thermoelectric characterization of a set of inkjet printed thin films of commercially available poly(3,4-ethylenedioxythiophene) polystyrene sulfonate formulations, displaying different electrical conductivities in the range from 1 to 1000 S/cm. The dependence of thermopower with respect to temperature is analyzed and discussed for all samples, and its correlation with the density of states close to the Fermi level is investigated and discussed in the framework of Mott’s relation. Finally, with the aim of extracting a reliable thermoelectric figure of merit zT, thermal conductivity is extracted thanks to a simplified pump-probe spectroscopy based thermal conductivity measurement. A quantitative assessment of the potentialities of organic thermoelectrics for micro-thermoelectric generators is finally derived outlining minimum thermoelectric properties requirements to be achieved in order to pave the way for real applications.

Resume : Spin-dependent exciton recombination and charge transport play important roles in determining efficiencies of optoelectronic devices based on organic semiconductors. This talk deals with the fundamental question of organic magnetoconductance (MC): How spin mixing changes charge transport process in organic semiconductors? This is still the most exciting question and also the most debated controversy in the field of organic MC. It is of importance for improving the performance of organic MC devices, and for fundamental insights into the electronic/optoelectronic properties of organic semiconductors. We investigate the origin of small-field MC effects in poly(3-hexylthiophene):[6,6]-phenyl- C61-butyric acid methyl ester (P3HT:PCBM) photovoltaic (PV) devices. By using photoinduced absorption spectroscopy, we show that the MC is due to the charge recombination effect. We simulate the device operation using a drift-diffusion model incorporating spin-dependent rate equations, which clearly demonstrates that the MC is a result of competition between the charge dynamics and spin mixing, and convincingly explains the microscopic origin of the effects of voltage, temperature, and injection barriers on the MC. Our results provide mechanistic understanding that can help to improve the performance of organic magnetoconductance devices, and underline the importance of spin mixing and spin-dependent recombination in organic LEDs and PVs.

Resume : With the advantage of high-efficiency and ease in material processing, perovskites demonstrate the great promise to be the next-generation solar-cell absorbers and have triggered enormous scientific efforts in the field of photovoltaics. Exploiting the fundamental physics beneath the optical absorption of perovskite absorbers is of crucial importance to engineer it for better photovoltaic performance. In this work, we mainly investigate the influence of electron-phonon interaction on the electronic band structure and optical absorption of perovskite absorbers CsPbI3 and CH3NH3PbI3 from the atomic level using first-principles. This work can provide insight into the optical absorption of perovskite absorbers at varying temperatures. Based on the density functional perturbation theory, the effect of lattice vibration (including the zero-point vibration) on the electron’s energy state and lifetime is investigated. Then with the thermally perturbed electronic structure, the effect of electron-phonon interaction on optical absorption can be analyzed. Moreover, by comparing the inorganic and organic-inorganic hybrid perovskite, the role of organic cations in altering the electronic and optical properties is further explored. This work aims at unrevealing the electronic structure and optical absorption of perovskites at finite temperature and providing insight to guide the perovskite material design for better performance.

Resume : Spin-polarized charge transfer at the interface between a ferromagnetic metal and a molecule can lead to ferromagnetic coupling[1] and to a high spin polarization[2] at room temperature[3-4]. The magnetic properties of these interfaces can not only alter those of the ferromagnet[5], but can also stabilize molecular spin chains[6-7] with interesting opportunities toward quantum computing. With the aim to enhance an organic spintronic device’s functionality[8], an external control over this spin polarization may thus be achieved by altering the ferromagnet/molecule interface’s magnetic properties. To do so, we utilize the magnetoelectric properties[9] of an underlying ferroelectric/ferromagnetic interface. Switching the ferroelectric polarization state of a PbZr0.2Ti0.8O3 (PZT) bottom layer within a PZT/Co/FePc-based device alters the X-ray-magnetic circular dichroism of the Fe site within the phthalocyanine (Pc) molecular top-layer. We thus demonstrate how to alter the magnetic properties of an interface with high spin polarization at room temperature[4]. This expands electrical control over spin-polarized FM/molecule interfaces, which was first demonstrated using ferroelectric molecules[10], to all molecular classes. [1] A. Scheybal et al Chem. Phys. Lett. 2005, 411, 214. [2] C. Barraud et al, Nat. Phys. 2010, 6, 615. [3] S. Lach et al, Adv. Funct. Mater. 2012, 22, 989. [4] F. Djeghloul et al, Sci. Rep. 2013, 3, 1272. [5] K. V. Raman et al Nature 2013, 493, 509. [6] M. Gruber et al Nat. Mater. 2015, 14, 981. [7] C. Barraud et al Phys. Rev. Lett. 2015, 114, 206603. [8] S. Sanvito, V. A. Dediu, Nat. Nanotechnol. 2012, 7, 696. [9] O. Vlašín et al, ACS Appl. Mater. Interfaces 2016, 11, 7553. [10] S. Liang et al, Adv. Mater. 2016, 28 1521.

Resume : We present our recent work in theoretical understanding and modeling of thermoelectric properties for organic materials. The charge transport is calculated by considering both electron-phonon scattering and carrier-impurities scattering. The heat transport is modelled by first-principles molecular dynamics compared with phonon Boltzmann equation considering the anharmonic terms. We find that for polymeric materials, it is possible to realize phonon glass electron crystal by engineering the crystallinity at the sub-micron scale by virtue of difference in the mean-free path. We further investigate the electronic structure and thermoelectric poweer factors for the one-dimensional organo-metallic coordinated polymers.

Resume : Thermoelectric materials for energy generation have several advantages over conventional power cycles including lack of moving parts, silent operation, miniaturizability, and CO2 free conversion of heat to electricity. Excellent thermoelectric efficiency requires a combination of high thermopower (S, V/K), high electrical conductivity (σ, S/cm), and low thermal conductivity (κ, W/mK). To date the best materials available have been inorganic compounds with relatively low earth abundance and highly complex, vacuum processing routes (and hence greater expense), such as Bi2Te3. Molecular materials and hybrid organic-inorganics bring the promise of inexpensive, solution processible, mechanically durable devices. While highly conductive polymers are now commonplace, they generally demonstrate low thermopower. Ion conducting materials have previously been demonstrated to have very large Seebeck coefficients, and a major advantage of polymers over inorganics is the high room temperature ionic conductivity. Notably, PEDOT:PSS demonstrates a significant but short-term increase in Seebeck coefficient which is attributed to a large ionic Seebeck contribution. We have recently shown that doping with protic ionic liquids and other proton conductors can be used to control the thermoelectric power factor. I will discuss how electrochemistry can be utilized to stabilize the Seebeck enhancement leading to stable improvements to power factor in mixed conductor thermoelectrics.

Resume : Tetrathiafulvalene-extended dicarboxylate (TED) is a recently developed novel type of pure organic metal composed of a single molecule [1] utilizing protonic defects as a dopant [2], and it is not categorized in CT complexes and conducting polymers. It is prepared with solution process only and is chemically stable due to being a non-doped pure material, demonstrating high potential for printability in thin-film device applications. TED exhibits metallic conduction until low temperatures: the electrical conductivity of TED is 530 S/cm at 300 K and 1000 S/cm at 50 K [1]. The behavior of thermopower is also metallic exhibiting weak temperature dependence: the power factor is 14 μW/K2m at 300 K. The carrier concentration and mobility of TED were evaluated by the measurements of the Hall effect, which suggests that optimization of the two components is an effective way for developing good thermoelectrics. We have tuned the molecular structures of TED in order to optimize carrier concentration and mobility; partial substitution of hydrogen bonding proton by Li ion and addition of substituent on the extended TTF moiety in TED. The correlation between the molecular structures and thermoelectric properties will be presented. [1] Kobayashi, Y., Terauchi, T., Sumi, S. Matsushita, Y. Nature Mat., 2017, 16, 109. [2] Kobayashi, Y., Yoshioka, M., Saigo, K., Hashizume, D., Ogura, T. J. Am. Chem. Soc. 2009, 131, 9995.

Resume : In the last decade scientific interest for thermoelectric technology has extended to plastic electronics and the topic is now subject to exponential growth due to promising results published in the literature for conductive polymers and their composites. The concept may also extend to self-powered sensors that can be used for e-skin, soft robotics, wearable electronics, harvesting the voltage generated by a temperature gradient to sense a wide variety of stimuli including strain, pressure, humidity, light, pH and others. The scope of our study focus on demonstrating the self-powered strain and liquid sensing capability of commercial PEDOT:PSS, as well as making material improvements through the addition of different types and concentrations of streatchable elastomers to the casting solution, bringing significant enhancements in thermoelectric and mechanical properties such as electric conductivity, % strain at brake and elastic modulus, without significantly affecting the Seebeck coefficient, determinant for the self-powered sensing application.

Resume : Although the dream of manipulating quantum interference in single molecules has been discussed for many years, experimental evidence of the effect of quantum interference on the room-temperature electrical conductance of single-molecules was reported only recently[1]. In this talk, I will present a brief outline of current understanding of quantum interference in single-molecules[2] and then discuss recent results[3] demonstrating how room-temperature quantum interference and room-temperature phonon interference can be exploited to increase the thermoelectric performance of single molecules and assemblies of molecules connected to nano-gap electrodes. 1. J. Am. Chem. Soc., 2011, 133, 11426, Nature Nano. 2012, 7, 305; Nat. Nano., 2012, 7, 663, Phys. Rev. Lett. 2012, 109, 056801; Nano. Lett. 2012, 6, 1643-1647; JACS 2012, 134, 5262; Beilstein J. Nanotech. 2011, 2, 699 and refs. therein 2. Lambert, Chem. Soc. Rev. 44, 875-888 (2015); Geng, et al., J. Am. Chem. Soc. 137, 4469 (2015); Sangtarash et al., J. Am. Chem. Soc. 137 11425 (2015); D. Manrique, et al., Nature Comm. 6 6389 (2015); Berritta et al, Nanoscale 7 1096 (2015) 3. Evangeli et al., Nano Letters 13, 2141-2145 (2013); Garcia-Suarez et al, Nanotechnology 25, 205402 (2014); Sadeghi et al, Nano Lett. 15, 7467-7472 (2015); Manrique et al., Nano. Lett. 16, 1308−1316 (2016); Ismael et al, Nanoscale 7 17338 (2015); Rincón-García at al, Nature Materials, 15, 289–293 (2016); Han et al., Nature Comm. 7 11281 (2016)

Resume : Organic thermoelectrics offer the prospect of efficient waste heat recovery at around room temperature using non-toxic and Earth Crust abundant starting materials. Different alternative organic materials have been proposed, including single molecular crystals, charge transfer complexes, conducting polymers, carbon allotropes and mixtures thereof. Polymers offer low thermal conductivity and good processability. However, when doping polymers in order to enhance their electrical properties, the resulting solid often exhibit detrimental large phase separation of the molecular dopant and loss of mechanical properties (films become brittle). Carbon nanotubes (CNTs), on the other hand, can naturally provide high charge carrier mobilities and conductivities combined with a medium valued Seebeck coefficient, while processing and high thermal conductivities are a burden. Adding CNTs as fillers together with polymers taps in the advantages of both words: processing becomes easier due to the interaction between soluble polymers and CNTs; the polymer matrix acts as a binder for the CNTs making them less of a respiratory hazard; and thermoelectric properties can be synergistically optimized to obtain good performance retaining the mechanical benefits. In this contribution, I will describe the general behavior of polymer/CNT composites for thermoelectric applications, emphasizing our current understanding on why the composites follow a percolating behavior electrically but not thermally [1]. I will then discuss the electronic coupling between the two components (through charge transfer mechanisms). The strong electronic interaction opens the possibility to tune the performance of the composites depending on the stoichiometry. Moreover, since the degree of interaction can be modified depending on the processing schemes, I will show the switching of composites from p to n and vice versa by using UV irradiation during the drying [2, 3]. Finally, I will briefly summarize the effect of key parameters in the performance of the polymer/CNT composites for thermoelectricity, namely, type of CNT (chirality, multiwalled vs single walled, semiconducting vs metallic) and doping mechanism (during CNT synthesis, ammonolysis, or molecular doping through the polymer counterpart) [1-3]. [1] C. Bounioux, et al. Thermoelectric composites of poly(3-hexylthiophene) and carbon nanotubes with a large power factor. Energy Environ. Sci. 6, 918–925 (2013). [2] B. Dörling et al. Photoinduced p- to n-type Switching in Thermoelectric Polymer-Carbon Nanotube Composites. Advanced Materials, 28, 2782–2789 (2016). [3] B. Dörling et al. Exploring different doping mechanisms in thermoelectric polymer/carbon nanotube composites Synthetic Metals (2017) in press

Resume : A general method is proposed to produce oriented and highly crystalline conducting polymer layers by combining the controlled orientation/crystallization of polymer films by high-temperature rubbing with a soft-doping method. Doping highly aligned films of regioregular poly(3-alkylthiophene)s with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) yields highly oriented conducting polymer films. These doped layers display polarized UV-Visible-NIR absorption, anisotropy in charge transport and thermoelectric properties. Transmission Electron Microscopy and polarized UV-Vis-NIR spectroscopy help understand and clarify the structure of the films and the doping mechanism. The ordering of dopant molecules in the layers of alkyl side chains depends closely on the length and packing (interdigitated versus non-interdigitated) of the alkyl side chains. The high orientation results in anisotropic charge conductivity () and thermoelectric properties that are both enhanced in the direction of the polymer chains (Sigma=22±5 S/cm and S=-60±2 microV/K). The method of fabrication of such highly oriented conducting polymer films is versatile and is applicable to a large palette of semi-conducting polymers.

Resume : Organic-based devices are currently attracting great attention for applications where low-cost, large area coverage and flexibility are required. The best performing electronic devices to date are those comprising single crystals of organic semiconductors, although they are neither suitable for large-area applications nor compatible with fast high-throughput industrial-scale fabrication. Thus, engineering processing techniques that could give rise to highly crystalline and homogenous semiconducting films potentially resulting in reproducibly high mobility devices is a current challenge. We report here the bar-assisted solution shearing (BAMS) of organic semiconductor blends based on small semiconducting molecules and the insulating polymer polystyrene (PS).[1-3] This technique results in highly crystalline thin films that showed ideal OFET characteristics. Further, we investigated the influence of the deposition parameters and solution formulation on the thin film morphology and polymorphism, which in turn, has a crucial impact on the device performance. The best devices have also been successfully applied in water gated OFETs exhibiting a very high performance.[4] REFS: [1] F. G. del Pozo, S. Fabiano, R. Pfattner, S. Georgakopoulos, S. Galindo, X. Liu, S. Braun,M. Fahlman, J. Veciana, C. Rovira, X. Crispin, M. Berggren, M. Mas-Torrent, Adv. Funct. Mater. 2016, 26, 2379. [2] S. Georgakopoulos, F. G. del Pozo, M. Mas-Torrent, J. Mater. Chem. C 2015, 3, 12199. [3] I. Temiño, F. G. del Pozo, M. R. Ajayakumar, S. Galindo, J. Puigdollers, M. Mas-Torrent, Adv. Mater. Technol. 2016, 1,1600090. [4] F. Leonardi, S. Casalini, Q. Zhang, I. Temiño. D. Gutiérrez, S. Galindo, M. Mas-Torrent, Adv.Mater.(2016), DOI:10.1002/adma.201602479.