Synthetic Metals (v.161, #23-24)
Imaging nanostructures in organic semiconductor films with scanning transmission X-ray spectro-microscopy
by Benjamin Watts; Christopher R. McNeill; Jörg Raabe (pp. 2516-2520).
► We image organic semiconductor films with scanning transmission X-ray microscopy. ► NEXAFS spectroscopy provides a powerful contrast mechanism for organic materials. ► STXM generates quantitative composition maps. ► Novel detectors provide simultaneous surface and bulk information. ► We image molecular conformation via linear dichroism.Thin films of organic semiconducting materials are of increasing technological importance in optoelectronic devices such as light emitting diodes (LEDs), lasers, field effect transistors (FETs) and solar cells. However, the morphology of such films is complex, often displaying three dimensional composition structure or molecular alignment effects. The structure of the polymer film incorporated into a device can strongly affect its performance characteristics, e.g. via the connectedness of polymer domains and to the device electrodes, or due to anisotropic material properties due to molecular alignment.Scanning transmission X-ray spectro-microscopy (STXM) has been demonstrated to be an excellent tool for the study of organic materials due to its high spatial resolution (down to about 20nm) and strong contrast based on a variety of spectroscopic mechanisms. In particular, tuning the probing X-ray beam to resonances in the near edge X-ray absorption fine structure (NEXAFS) spectra provides a mechanism for molecular-structure based contrast which is a very powerful tool for studying blends of organic components. A further advantage of STXM is that strategic use of spectroscopic information allows quantitative compositional analysis of imaged areas. Recent work at the PolLux STXM has demonstrated two new developments in the imaging of thin organic films: simultaneous surface and bulk imaging via an additional channeltron electron detector, and molecular orientation mapping via anisotropic near edge resonances.
Keywords: X-ray; Microscopy; NEXAFS; STXM; Polymer; Molecular orientation; Thin film; Surface
Energy dispersive X-ray reflectivity applied to the study of thermal stability of self-assembled organic multilayers: Results on phosphonic acids
by Muriel de Pauli; Carlos A. Pérez; Mariana C. Prado; Douglas H.C. Araújo; Bernardo R.A. Neves; Angelo Malachias (pp. 2521-2525).
► The temperature evolution of self-assembled organic multilayers is studied by energy dispersive X-ray reflectivity (EDR). ► Octadecylphosphonic acid multilayers present bilayer periodicities of 50Å (straight bilayer) and 34Å (tilted bilayer). ► Thermal stability studies show that the tilted bilayer configuration is more stable than the straight arrangement. ► A gradual switching from 50Å to 34Å configuration is observed for temperatures above 45°C. ► The energy difference between both configurations, still not quantified for this system, is evaluated as 0.99meV/molecule.The temperature evolution of self-assembled phosphonic acid multilayers was investigated by energy dispersive X-ray reflectivity and angular-resolved reflectivity. Energy dispersive measurements were performed in an experimental setup specially designed for the X-ray fluorescence beamline of the Brazilian Synchrotron Light Laboratory. It allows the precise monitoring of phase transitions observed in organic thin film and multilayer systems. The studied multilayers – obtained from dip coating of a solution of octadecylphosphonic acid – present different bilayer periodicities of 50Å (straight bilayer) and 34Å (tilted bilayer). Energy dispersive and angular-resolved data evidence re-organization of the lamellar ordering of octadecylphosphonic acid multilayers as a function of temperature. The energy dispersive technique presents many advantages over conventional methods such as short acquisition time, possibility to vary external parameters and high flux, making it suitable for light scatterers as polymers and other organic molecules.
Keywords: Energy dispersive reflectivity; Synchrotron radiation; Self-assembled organic molecules
Visualizing photovoltaic nanostructures with high-resolution analytical electron microscopy reveals material phases in bulk heterojunctions
by Martin Pfannmöller; Harald Flügge; Gerd Benner; Irene Wacker; Wolfgang Kowalsky; Rasmus R. Schröder (pp. 2526-2533).
► We investigate the morphology of standard bulk heterojunctions by TEM. ► We combine low energy-loss spectroscopic imaging with spectral pattern recognition. ► The analysis permits the creation of morphological maps with reliable domain sizes. ► A third composite phase of P3HT and PCBM is discernable by spectral classification. ► The technique is superior to conventional TEM producing imaging artifacts.The efficiency of bulk heterojunctions (BHJs) depends largely on their 3D morphology at the nanoscale. So far bright-field transmission electron microscopy imaging was used to elucidate their structure, applying imaging conditions, which may compromise reliability of image interpretation. We provide evidence that conventional electron micrographs cannot be used for assessing domain sizes when visualizing BHJs. Instead, we describe an analytical method to distinguish materials by their spectral characteristics, i.e. to visualize a material by its specific electronic excitations. Using analytical microscopy of ultra-thin films combined with optimized data analysis, the electronic excitations can directly be linked to materials and their spatial distribution. This is illustrated for an annealed film of [6,6]-phenyl C61 butyric acid methyl ester and poly(3-hexyl-thiophene), where we create a morphological map of the film based on functional contrast.
Keywords: Abbreviations; PCBM; [6,6]-phenyl C61 butyric acid methyl ester; P3HT; poly(3-hexyl-thiophene); OPV; organic photovoltaic; BHJ; bulk heterojunction; GIXS; grazing incidence X-ray scattering; TEM; transmission electron microscopy; ESI; electron spectroscopic imaging; LLE; local linear embedding; EELS; electron energy-loss spectroscopy; CTF; contrast transfer functionOrganic photovoltaics; Bulk heterojunction; Analytical electron microscopy; Low energy-loss electron spectroscopic imaging; Multivariate statistical analysis
Methods in determination of morphological degradation of polymer:fullerene solar cells
by Vida Turkovic; Sebastian Engmann; Gerhard Gobsch; Harald Hoppe (pp. 2534-2539).
The changes in the blend morphology of the thin films accelerate when thermally annealed at increased temperatures. Tapping-mode atomic force microscopy (AFM) measurements provide deeper insight into the nano- and micro-meter scale of the phase separation observable on the surface of the film. Furthermore, to prove the coarsening of phase separation on various length scales, optical microscopy, UV–Vis, photoluminescence and measurements were conducted. Of special interest is tracking down the formation of fullerene aggregates and correlation of their growth in time. We demonstrate ways to detect characteristic stages of phase separation directly using simple optical measurements.
Keywords: Morphology; Degradation; UV–Vis spectroscopy; Atomic force microscopy; Optical microscopy; Photoluminescence; P3HT; PCBM; Bulk heterojunction; Thermal annealing
Aging of polymer/fullerene films: Temporal development of composition profiles
by Sebastian Engmann; Vida Turkovic; Harald Hoppe; Gerhard Gobsch (pp. 2540-2543).
► Shape of fullerene domains by ellipsometric means. ► Phase segregation near the substrate interface. ► Fullerene cluster formation near the film surface.We apply spectral ellipsometry (SE) to analyze the in-depth morphology of a polymer/fullerene film. Observed fullerene concentration gradients within the film are investigated with respect to temporal changes. The changes within the fractional composition of the polymer/fullerene film are resolved and can be correlated to the formation of fullerene segregations near the surface.
Keywords: Ellipsometry; Effective medium approximation; Morphology; Degradation; Polymer; Fullerene
Co-solvent effects on the morphology of P3HT:PCBM thin films
by F. Reisdorffer; O. Haas; P. Le Rendu; T.P. Nguyen (pp. 2544-2548).
► We investigate P3HT:PCBM films using co-solvents of chlorobenzene and toluene. ► The choice of solvents is based on their close boiling points and on different polarities. ► The result points out the key role of the PCBM solubility on the film organization. ► P3HT regioregularity has no influence on morphology with dominant chlorobenzene solvents.Morphology control of the active layer in bulk hetero junction devices is essential for obtaining high-energy conversion efficiency. For poly(3-hexylthiophene) (P3HT)–[6-6]-phenyl-C61-butyric acid methyl ester (PCBM) based organic photovoltaic cells, the low performance is assigned to the formation of PCBM aggregates, whose process has not been clearly established. In this work, we have investigated P3HT:PCBM composite films, which were prepared by dissolving the mixture in co-solvents of chlorobenzene and toluene of different proportions. The solvents have been chosen for their close boiling points and for their different polarities, which impact on the solubility of organic components. Optical (absorption, photoluminescence), surface (Atomic Force Microscopy, X-ray photoelectron spectroscopy) and structural (X-ray diffraction) characterization techniques have been performed on blend films prepared by co-solvents. The result analysis has clearly pointed out the key role of the PCBM solubility on the film organization. Conversely, the regioregularity of P3HT would not influence the film morphology when using dominant chlorobenzene solvents but strongly affected it when using dominant toluene solvents.
Keywords: Organic solar cells; P3HT:PCBM; Co-solvent; Morphology
Core level energy differences between the surface and bulk of organic semiconductor films: The effect of electrostatic polarization energy
by Hiroyuki Yoshida; Eisuke Ito; Masahiko Hara; Naoki Sato (pp. 2549-2553).
Display Omitted► C1s energy differences between the surface and bulk of organic films are examined. ► We employed a new experimental method we have recently developed. ► Pentacene and F-pentacene are compared to discuss the origin of the C1s energy differences. ► The electrostatic polarization energy predominantly affects the energy level difference.The difference in the C1s core level energies between the surface and bulk regions of an organic semiconductor thin film is examined for pentacene and perfluoropentacene (F-pentacene). The experimental method we have recently developed allows us to determine precisely the differences as well as the thickness of the surface region. The C1s binding energies in the outermost layer turn out to be larger than those in the bulk by 0.32eV (pentacene) and 0.25eV (F-pentacene). The results are successfully explained by the interaction energy between the point charge and induced dipole moments suggesting the electrostatic polarization energy plays a main role in the difference of the core level energy.
Keywords: Organic semiconductor film; Ionization energy; Photoemission spectroscopy; Pentacene; Perfluoropentacene; Target factor analysis
Comparison between the density-of-states of picene transistors measured in air and under vacuum
by C. Voz; A. Marsal; C. Moreno; J. Puigdollers; R. Alcubilla (pp. 2554-2557).
► Picene OTFTs on polystyrene dielectric present high field-effect mobilities in air (1.3cm2V−1s−1). ► Density-of-states in the gap of picene was calculated by analysing the electrical characteristics. ► Narrow distributions of hole traps located at 0.28 and 0.42eV from the HOMO level. ► Ionized oxygen molecules partly neutralize hole traps in devices exposed to air.Picene has recently attracted much attention as the active layer in organic thin-film transistors because of its good performance in air. In this work, we have fabricated picene thin-film transistors that exhibit field-effect mobilities up to 1.3cm2V−1s−1 and on/off ratios above 105 in ambient conditions. These devices have been electrically characterized over the temperature range 300–360K in air and also under vacuum conditions. In particular, the thermal activation energy of the channel conductance as a function of the gate bias has been measured. The dependence of the activation energy on the gate bias corresponds to a gradual shift of the Fermi level towards the HOMO level as more gap states are filled by trapped holes. The density-of-states can be estimated from the derivative of the activation energy with respect to gate bias. The calculated density-of-states is compared for devices measured in air and under vacuum conditions. These results can help to understand the gas sensing capability of picene, together with its enhanced electrical performance after air exposure.
Keywords: Picene; Organic semiconductors; Thin-film transistors; Density-of-states
Temporal and thermal properties of optically induced instabilities in P3HT field-effect transistors
by Lorenz A. Kehrer; Stefan Winter; Rene Fischer; Christian Melzer; Heinz von Seggern (pp. 2558-2561).
► Optically induced changes in P3HT based organic field-effect transistors. ► Oxygen induces charge transfer states inside the P3HT layer. ► Threshold voltage shifts and p-doping of the P3HT layer. ► Investigation of the temporal relaxation as a function of the temperature. ► Electronic discharge of trap states and oxygen diffusion.In this contribution we report on temporal and thermal properties of an optically induced instability in poly(3-hexylthiophene) (P3HT) based organic field-effect transistors (OFET) fabricated on a flexible polyethylene terephthalate (PET) substrate. By illuminating depleted p-type top-gate P3HT field-effect transistors with visible light a substantial shift of the threshold-voltage of up to +20V and an increase in the off-current by three orders of magnitude has been observed. Both phenomena, the threshold-voltage shift and the increase of the off-current, require the presence of oxygen and are persistent for days at room temperature. The effect is explained by the formation of a charge-transfer-complex (CTC) of P3HT and oxygen known from literature to act as an electron trap. Here, we discuss the temporal and thermal stability of the detrapping of photo-generated charge carriers from such traps and the loss of trapping sites by diffusion of oxygen out of the P3HT layer. It is demonstrated that the thermally activated detrapping of electrons from the oxygen induced CTC states is much faster than the removal of the oxygen induced traps.
Keywords: P3HT; Organic field-effect transistor; Oxygen; Charge transfer complex; Diffusion; Stability
Reproducibility and stability of C60 based organic field effect transistor
by Rizwan Ahmed; Michael Sams; Clemens Simbrunner; Mujeeb Ullah; Kamila Rehman; Günther Schwabegger; H. Sitter; Timm Ostermann (pp. 2562-2565).
► Reproducibility and stability measurements of organic field effect transistors (OFET). ► Biased stress stability of n-type OFET. ► The role of reproducibility and stability in organic electronic circuit. ► Providing well defined limits for the modelling of circuit design.A comprehensive study concerning the reproducibility and stability of organic n-type field effect transistors is presented. C60 based OFETs were chosen to investigate the fabrication reproducibility and the long term stability because C60 is a high mobility n-type material. We fabricated 48 transistors and each transistor was measured for 24h inside the glove box. To test for life time stability – long term measurements up to three months have been undertaken. We report about the fluctuations in the device parameters of all investigated transistors by comparing the transfer characteristics, and on/off ratio for short time and long time measurements. C60 based OFETs showed good reproducibility and stability for short time measurements and a decay for long time measurements.
Keywords: Reproducibility and stability of OFET; C; 60; n-Type OFET; Long time measurement
The effect of the active layer thickness on the performance of pentacene-based phototransistors
by A. El Amrani; B. Lucas; B. Ratier (pp. 2566-2569).
► We discussed the fabrication and characterization of pentacene-based phototransistors. ► We characterized the pentacene photoelectric properties under UV. ► We observed the higher shift in the threshold voltage for thinner pentacene films. ► A high photosensitivity of 6.5×104 was obtained.In this paper, we discuss the fabrication and characterization of pentacene-based phototransistors using indium tin oxide as a transparent electrical gate and PMMA as a transparent dielectric gate. The photoelectric properties with different pentacene film thicknesses were characterized under ultraviolet (365nm) illumination. We observed that for the thinner pentacene films, the threshold voltage upon UV illumination was shifted from its initial value in the dark to a positive voltage of more than 16V, whereas the shift was only of 3V for thicker films. Thus, we obtained a higher photosensitivity of 6.5×104 for thinner pentacene films, which indicates that the organic thin film transistors could find use in photodetector applications.
Keywords: Pentacene; UV illumination; Phototransistor; Responsivity; Photosensitivity
Surface vs bulk phase transitions in semiconducting polymer films for OPV and OLED applications
by A. Roigé; M. Campoy-Quiles; J.O. Ossó; M.I. Alonso; L.F. Vega; M. Garriga (pp. 2570-2574).
► Monitoring of crystallization transition of polymer films. ► Differences between bulk and surface transition temperatures are evidenced. ► In-situ AFM evidences transition as a roughness increase. ► Raman peaks suffer a blue-shift and intensity increase upon crystallization.Post deposition annealing treatments are considered one of the most important and effective ways to increase the performance of polymer-based thin films for organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs).Hence, thermally induced morphological changes such as phase transitions are key phenomena which can have a determinant influence on the final properties and stability of the materials and devices based upon them. In this work, we have successfully proven that in-situ atomic force microscopy (AFM) and Raman spectroscopy can be used to measure the cold crystallization transition temperature of the widely studied blue-emitting polymer poly(9,9-dioctylfluorene) (PFO) and the workhorse system for photovoltaics based on mixtures of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Raman, as a bulk probe, evidences lower crystallization temperatures for PFO and P3HT films compared to those obtained at the surface with AFM which suggest the existence of morphological and/or molecular mobility differences between the bulk and the surface.
Keywords: Organic thin films; Raman spectroscopy; Phase transitions; Polythiophene; Polyfluorene; In-situ AFM
The Li3PO4/Al electrode: An alternative, efficient cathode for organic light-emitting diodes
by Andrea Gassmann; Christian Melzer; Heinz von Seggern (pp. 2575-2579).
► Li3PO4/Al introduced as efficient electron injecting contact for OLEDs and PLEDs. ► Li3PO4/Al is competitive to the LiF/Al benchmark and yields long-lived devices. ► Already 5–7Å thin Li3PO4 layers capped with Al are optimal for electron injection. ► No charge-transfer doping between the organic semiconductor Alq3 and Li3PO4. ► Interface reaction: formation of dipolar layer and distinct work function lowering.In this contribution an alternative, efficient cathode for organic light-emitting diodes (OLEDs) is introduced consisting of a sub-nanometer thin lithium phosphate (Li3PO4) layer capped with aluminium. This cathode is competitive to the LiF/Al state-of-the-art cathode and performs equally well in small molecule and polymer OLEDs but yields a higher device lifetime. The functional principle of the Li3PO4/Al cathode is based on the in situ formation of a dipolar interface. Partly oriented AlPO4 dipoles are the reason for a potential drop at the interface and a work function lowering of about 500meV. The work function of the Li3PO4/Al cathode is found to be comparable to the work function of calcium resulting in a promotion of electron injection. As the reaction products of the interface reaction are environmentally stable and chemically attached to the interface diffusion of these species into the organic layer is unlikely. This might be the reason for the observed enhanced device lifetime for devices with a Li3PO4/Al cathode.
Keywords: OLED; Electron injection; Li; 3; PO; 4; /Al; Interface reaction; Dipolar layer
Highly efficient solution-processed white organic light-emitting diodes based on novel copolymer single layer
by Carmen Coya; Angel Luis Álvarez; Mar Ramos; Rafael Gómez; Carlos Seoane; José Luis Segura (pp. 2580-2584).
► White-emitting polymer for solid state lighting. ► Single component, single-layer, solution processed devices. ► Blue-emitting poly(fluorine-alt-phenylene) containing co-monomers bearing red-emitting 1,8-naphtalimide pendant groups. ► Efficiency of 9.42Cd/A for the device with a fraction x=0.0005 of monomers with chromophore, at low driving current, 47μA.We fabricate by solution processed methods organic light emitting diodes with single-layer structure (ITO/(PEDOT:PSS)/co-polymer/Ba/Al) and study the transport properties of the final devices. The co-polymer is novel poly(fluorene-alt-phenylene) (PFP) derivatives containing co-monomers, involving red-emitting 1,8-naphthalimide units as pendant groups (0.0005, 0.005, 0.02 and 0.08wt%) covalently attached. All the devices exhibited emission at very low driving currents in the μA range (47–73μA). White emission with luminous efficiency of 9.42Cd/A at 50μA is obtained for the co-polymer with the smallest amount of chromophore. Commission Internationale de L’Eclairage (CIE) coordinates evolve from almost pure white color (0.26, 0.30) for low currents to stable cool white (0.21, 0.23). Increasing naphthalimide contents leads to stable green and orange emission with 3.07, 19.5 and 6.7Cd/A efficiencies. The current–voltage response of the devices is analyzed by means of a numerical model that includes an injection mechanism based in the microscopic hopping theory and a field-dependent carrier mobility for the bulk transport regime. The fitting results allow to estimate the dependence of carrier mobility on polymer composition in the diodes.
Keywords: Solid state lighting; WOLEDs; Solution processed; Charge transport
Photocurrent spectroscopy of ion-implanted organic thin film transistors
by B. Fraboni; A. Scidà; A. Cavallini; S. Milita; P. Cosseddu; A. Bonfiglio; Y. Wang; M. Nastasi (pp. 2585-2588).
► Photocurrent (PC) spectroscopy of pentacene thin films and thin film transistors (OTFT) of different thicknesses. ► PC spectroscopy identifies the film active thickness range through the distribution density of electrically active states. ► Effects of ion implantation on organic thin film transistors. ► Non-invasive, direct study of the active semiconductor film in fully operational OTFTs.In this paper we investigate the distribution of the electrically available states near the band-edge in pentacene thin films of different thicknesses, aiming to the identification of the active thickness of pentacene layers in fully operational devices such as organic thin film transistors (OTFTs). The film structure has been studied by X-ray diffraction technique, while their relative electronic density of states distribution (DOS) around the band-edge has been investigated by photocurrent (PC) spectroscopy analyses. The effects of ion implantation on OTFTs have been investigated by PC analyses of OTFTs implanted with N+ ions of different energy and doses. We show how PC spectroscopy has the remarkable ability to detect modifications of the DOS distribution in a non invasive way, thus allowing the direct study of the active semiconductor film in fully operational OTFTs.
Keywords: Organic thin film transistor; Density of electronic states distribution; Ion implantation
Effect of the morphology on the optical and electrical properties of polycarbonate film doped with aniline derivatives monomers
by Florin Stanculescu; Anca Stanculescu; Mihaela Girtan; Marcela Socol; Oana Rasoga (pp. 2589-2597).
► Polycarbonate of bisphenol A matrix embedding amidic monomers, as composite material. ► The polycarbonate films shown good photoluminescence in the green region. ► Matrix doped with –CN substituent monomer shown non-linear optical phenomena.In this paper we present some comparative studies between the optical and electrical properties of the films prepared by vacuum evaporation from amidic monomeric compounds obtained from maleic anhydride and aniline derivative with different functional groups [CN; NO2], and the composite films prepared by spin-coating from a polycarbonate of bisphenol A matrix embedding clusters of these amidic monomers. We have emphasised the effect of the polymeric matrix on the properties of the synthesised monomers with the purpose to change the physical properties of the films (optical transmission, luminescence, electrical conduction, optical non-linear phenomena). The correlation between the morphology of the films and the optical and electrical properties has also been investigated.
Keywords: Nanocomposite films; Optical properties; Electrical properties
Crystallization of pentacene thin films on polymeric dielectrics
by Armin Moser; Heinz-Georg Flesch; Alfred Neuhold; Marco Marchl; Simon J. Ausserlechner; Matthias Edler; Thomas Griesser; Anja Haase; Detlef-Matthias Smilgies; Ján Jakabovič; Roland Resel (pp. 2598-2602).
► Crystallography and morphology of pentacene thin films on organic dielectrics. ► Morphology and crystallography are found to be linked with each other. ► The influence of the substrate surface parameters is studied. ► The nanoscopic surface morphology is important for the growth.Thin films of pentacene have been deposited on five different organic dielectric layers of three different classes of organic materials: a conventional polymer, a photoreactive polymer and a self assembled monolayer. The morphology and crystallographic properties of the pentacene films were investigated with atomic force microscopy and grazing incidence X-ray diffraction. It is found that the films form well known crystallographic structures but show unusual crystal orientations with respect to the surface. The (001) lattice plane of the Campbell phase is tilted approximately 13° from the substrate surface normal while tilts between 4° and 7° are observed for the thin film phase. The island shape and density are found to be inhomogeneous on all substrates. In films where mainly large dendritic grains are observed the thin-film phase is the prevalent crystallographic phase while films with a high island density and small irregular grains are dominated by the Campbell phase. It is observed that the mean surface roughnesses and mean surface energies of the substrate play a minor role for the crystallization while the substrate's morphology on nanoscopic scale is crucial for the thin film growth.
Keywords: Pentacene thin film; Organic dielectric; Crystal structure; Preferred orientation; Morphology
Stability to photo-oxidation of rubrene and fluorine-substituted rubrene
by Sureeporn Uttiya; Luisa Raimondo; Marcello Campione; Luciano Miozzo; Abderrahim Yassar; Massimo Moret; Enrico Fumagalli; Alessandro Borghesi; Adele Sassella (pp. 2603-2606).
► Rubrene and substituted-rubrene molecules degradation by reaction with oxygen under light makes the solutions and thin films become colorless. ► The decrease of absorption intensity in the visible range and the rise of a band at the higher energy identify photo-oxidation. ► Fluorine atoms as substituted of hydrogen atoms in rubrene enhance the stability of the molecule to photo-oxidation.Rubrene, among organic semiconductors, displays very high charge carrier mobility in the solid state and this is why it has become a very promising material for various device applications. Unfortunately, rubrene also exhibits a rather low stability, affecting device lifetime, because it is very prone to photo-oxidation. In order to limit this problem, a new substituted-rubrene was synthesized, which should be more stable, still having a good transport behaviour in the solid state.In this paper, we show how molecule degradation by reaction with oxygen under light makes the solutions and thin films become colorless, as observed by monitoring in time the evolution of absorbance. We found that the degradation of rubrene in both solutions and thin films is faster than that of the fluorine-substituted molecule. These results demonstrate that the introduction of fluorine can be a way to enhance the stability of rubrene to photo-oxidation.
Keywords: Rubrene; Oxidized rubrene; Photo-oxidation; Optical properties; Organic molecular beam epitaxy
Interplay between solid-state organization and optical properties of thin films of poly-arylene-vinylene and -difluorinated vinylene: Fullerene blends
by Elena Dilonardo; Antonio Cardone; Carmela Martinelli; Maria M. Giangregorio; Maria Losurdo; Pio Capezzuto; Gianluca M. Farinola; Francesco Babudri; Francesco Naso; Giovanni Bruno (pp. 2607-2611).
► Analysis of a novel acceptor–donor copolymer. ► Optical characterization of ultra-thin films. ► Interplay between morphology/aggregation in the solid state and optical properties.Nowadays considerable efforts are devoted to the synthesis of low bandgap conjugated polymers for application in organic polymer solar cells. A large variety of low bandgap polymers are prepared by alternating copolymerization of electron-donating donor and electron-withdrawing acceptor units. The interaction between these two units can reduce the polymer bandgap, increasing the sunlight absorption. Benzothiadiazole is commonly used as acceptor block unit in low bandgap polymers. In this contribution we investigate the supramolecular organization and optical properties of thin films of conjugated polymers consisting of benzothiadiazole and thiophene with electron-withdrawing difluorovinylene, and electron-donating vinylene substituents. Atomic force microscopy and spectroscopic ellipsometry are exploited for the analysis of the morphology and optical transitions, respectively. It is found that F-atoms in the vinylene unit yield a blue-shift of the absorption peaks of 0.2eV respect to the hydrogenated polymer and an increase in the absorption coefficient of fluorinated polymers, which indicates their potential application as photovoltaic material. The morphology evolution of the conjugated polymers blended with a fullerene derivate ([6,6]-phenyl C61-butyric acid methyl ester, PCBM) is also investigated by atomic force microscopy.
Keywords: Donor–acceptor polymer; Conjugated polymer thin films; Optical properties; Morphology
Optical and electrical properties of arylenevinylene compounds thin films prepared by vacuum evaporation
by O. Rasoga; L. Vacareanu; M. Grigoras; M. Enculescu; M. Socol; F. Stanculescu; I. Ionita; A. Stanculescu (pp. 2612-2617).
► Two arylenevinylene oligomers thin films deposited by vacuum evaporation. ► The best injector contact: 1,4-bis [4-(N,N’-diphenylamino)phenylvinyl]benzene/p-type silicon. ► Higher roughness for 3,3′-bis(N-hexylcarbazole)vinylbenzene film.This paper discusses the properties of two arylenevinylene oligomers thin films deposited by vacuum evaporation on different substrates. The morphology of the thin films was investigated by scanning electron microscopy and atomic force microscopy, and the optical properties by UV–Vis and fluorescence spectroscopy. The optical nonlinear phenomenon of two-photon absorption induced fluorescence has been evidenced in the thin films of these compounds. We have also investigated the electrical properties of semiconductor/insulator/semiconductor (ITO/oligomer/Si) heterostructures in correlation with the contact energetic barriers, morphological and structural properties of the layers.
Keywords: Organic thin films; Conjugated oligomers; Optical properties; Electrical properties
Inkjet printed perylene diimide based OTFTs: Effect of the solvent mixture and the printing parameters on film morphology
by Immacolata Angelica Grimaldi; Mario Barra; Anna De Girolamo Del Mauro; Fausta Loffredo; Antonio Cassinese; Fulvia Villani; Carla Minarini (pp. 2618-2622).
► Inkjet printed film uniformity is controlled by solvent mixtures. ► The crystalline microstructure influences the performance of OTFT. ► PDI-8CN2 semiconductor is inkjet printed in a bottom-gate bottom-contact OTFT. ► The optimized solvent mixture improves the OTFT performances.In the present work, we report the influence of the solvents on the morphology and the uniformity of inkjet printed n-type (electron-transporting) perylene diimide (PDI-8CN2) semiconductor films on SiO2 substrates. In particular, a solvent mixture composed by o-dichlorobenzene and chloroform was employed and the semiconductor crystalline structure was investigated as a function of the mixing ratio of the component solvents. For each mixture composition, the printing parameters such as substrate temperature and drop overlapping degree, were optimized to improve the reproducibility of the deposition process and the structural quality of the final films. Organic thin film transistors were fabricated and electrically characterized. The electrical measurements suggest that for the devices with larger active areas, the solvent mixture approach improves the performances of OTFTs in comparison with the use of pure o-dichlorobenzene solution.
Keywords: Inkjet printing; Organic thin film transistor; n-Type organic semiconductor
Hybrid films based on silicon nanowires dispersed in a semiconducting polymer for thin film solar cells: Opportunities and new challenges
by Joël Davenas; Gisèle Boiteux; David Cornu; Andrzej Rybak (pp. 2623-2627).
► Fabrication of hybrid solar cells from blends of silicon nanowires and P3HT in solutions. ► Evaluation of the charge pair dissociation efficiency and photovoltaic characterization. ► Identification of the limiting processes arising from electron confinement in silicon nanostructures. ► Main role of the polymer/silicon electronic processes demonstrated.Hybrid devices based on silicon nanowires dispersed in poly(hexylthiophene: P3HT) thin films have been realized. The different steps, starting from the production of n type silicon nanowire, dispersion of the organic–inorganic components in solution to produce hybrid thin films, which have been integrated in simple photovoltaic diode structures, have been developed. The n type doping of the nanowires is indicated by Raman spectroscopy whereas their crystalline structure is shown by HRTEM. A broadening of the silicon absorption spectrum towards the near infrared is induced by an electronic confinement effect for nanowire diameters of the order of 10nm. Effective dissociation of the photogenerated charge pairs is shown by a partial quenching of P3HT photoluminescence with increasing SiNWs content, whereas an open circuit voltage of 0.68V is deduced from the electrical characteristics under visible light illumination. However photocurrents remain low for the investigated material combinations. The high series resistance appears the most critical point limiting the device performances. The high densities of SiNWs electronic surface states acting as trapping centers can account for the large reverse current. To confirm the high potential of this new type of hybrid films for energy conversion, improvements are needed for a better control of the silicon nanowire/polymer interface, which appears the critical point.
Keywords: Silicon nanowire; P3HT; Hybrid films; Solar cells; Silicon/polymer interface
Enhanced performance in organic light-emitting diode by utilizing MoO3-doped C60 as effective hole injection layer
by Ye Zou; Zhenbo Deng; Denghui Xu; Jing Xiao; Maoyang Zhou; Hailiang Du; Yongsheng Wang (pp. 2628-2631).
► MoO3-doped C60 is used as an efficient hole injection layer for OLED. ► Driving voltage is significantly reduced and luminance is greatly enhanced. ► Power efficiency is increased by more than 40% at 10 and 100mA/cm2, respectively. ► The enhancement is ascribed to charge transfer complex formed between MoO3 and C60.We report an efficient hole injection layer (HIL) composed of MoO3-doped C60 for organic light-emitting diodes (OLED). The structure of the OLED device is ITO/MoO3:C60 (5nm:5nm)/NPB (45nm)/Alq3 (55nm)/LiF (0.5nm)/Al. Compared with normal device without a HIL, the device using MoO3-doped C60 as HIL can significantly enhance both hole injection efficiency and electroluminescence. The power efficiency has been increased by approximately 40.7% and 41.7% at the current density of 10mA/cm2 and 100mA/cm2, respectively, for the device using MoO3-doped C60 as HIL than the control device. The cause for the enhancement was ascribed to the charge transfer complex formed by co-evaporation of MoO3 and C60. Hole-only devices were fabricated to confirm the hole injection enhancement. Ultraviolet/visible/near-infrared absorption spectra were measured to confirm the formation of the charge transfer complex.
Keywords: Hole injection; Molybdenum oxide; p-Doping; Charge transfer complex
Surface electrical conductivity of poly(methyl methacrylate) thin films: Observation of conductivity switching
by Bipul Biswas; Aloke Kumar Chakraborty; Manisree Majumder; Avijit Chowdhury; Manik Kr. Sanyal; Biswanath Mallik (pp. 2632-2637).
► Surface electrical conductivity in poly(methyl methacrylate) (PMMA) thin films. ► Conductivity switching at a threshold voltage ( Vth) in PMMA thin films. ► Conduction mechanisms in the voltage regions lower and higher than Vth are different. ► Conductivity switching arises possibly due to structural reorientation in PMMA. ► Reversibility of the system demonstrates its suitability for electronic devices.We present here the experimental results of surface electrical conductivity/current of poly (methyl methacrylate) [PMMA] thin films prepared from chloroform solvent. The switching in current/conductivity in PMMA thin films at a certain bias voltage (threshold voltage) has been observed from the current–voltage characteristics under certain experimental conditions. Such switching in current has been observed to depend on the sample cell temperature and illumination of light. Interestingly the conductivity switching was not noticed at room temperature for the measurements under dark conditions. The mechanisms for the electrical conduction in the two voltage regions, i.e. the voltage regions lower and higher than the threshold voltage are different. The results have been interpreted on the basis of creation of conduction paths at a certain bias voltage due to the structural reorganization/reorientation in PMMA matrix.
Keywords: Polymer; Thin film; Conductivity switching; Light illumination; Hoping model
Synthesis of novel white flake polyaniline
by Hamidreza Ghafouri Taleghani; Mahdi Aleahmad; Hossein Eisazadeh (pp. 2638-2640).
► Synthesis of novel polyaniline with white color. ► Change in polyaniline color has led to an interest in its use for new applications. ► Adding acetic acid in the reaction solution was proposed to prepare white polyaniline.A novel polyaniline have been successfully prepared by in situ emulsion polymerization using KIO3 as an oxidant. White polyaniline was observed after 3h the color of the polyaniline is usually black or green. This change in polyaniline color has led to an interest in its use for display devices, packaging and other applications. Scanning electron microscopy was used to characterize the morphology of polyaniline. The chemical structure of products was determined by Fourier Transform Infrared Spectroscopy (FTIR) and UV–vis spectroscopy. The results show that the type of surface active agent affects the intensive of peaks. This process leading to white flake polyaniline plate with 0.015–140μm particle size and approximate yield of 40%.
Keywords: White polyaniline; Morphology; Chemical structure
Investigations into the electrochemical characteristics of nickel oxide hydroxide/multi-walled carbon nanotube nanocomposites for use as supercapacitor electrodes
by S.-J. Kim; G.-J. Park; B.C. Kim; J.-K. Chung; G.G. Wallace; S.-Y. Park (pp. 2641-2646).
► Supercapacitor electrodes of NiOOH/MWNT nanocomposite were chemically precipitated. ► Specific capacitance (2436Fg−1) and surface area (274.32m2g−1) were found for 50wt.% MWNTs at 10mVs−1. ► Energy density and power density were 82.8Whkg−1 and 8.5kWkg−1, respectively.We have fabricated supercapacitor electrodes consisting of nickel oxide hydroxide (NiOOH) and multi-walled carbon nanotubes (MWNTs). The presence of the carbon nanotubes provides a conducting network and also increases the specific surface area. A specific capacitance of 2436Fg−1 and a specific surface area of 274.32m2g−1 were obtained at a scan rate of 10mVs−1 on the 50wt.% of MWNTs. Energy density and power density were 82.8Whkg−1 and 8.5kWkg−1, respectively.
Keywords: Supercapacitor; Nickel oxide hydroxide; Multi-walled carbon nanotubes; Nanocomposite
Universality of Zener tunneling in carbon/polymer composites
by Linxiang He; Sie-Chin Tjong (pp. 2647-2650).
► Current–voltage relationship of carbon nanofiber/high density polyethylene composites. ► Internal emission of electrons results from Zener tunneling. ► Zener tunneling may be universal as an internal electrical transport process for the carbon/polymer systems.Zener tunneling is identified in carbon nanofiber (CNF)/high density polyethylene (HDPE) composites exposed to external electric field. It is caused by the intense internal field strength due to the field enhancement effect. By combining Zener tunneling effect with percolation theory, a novel current density–electric field relationship is developed, which is applicable to carbon/polymer composites both below and above the percolation threshold. This Study shows that Zener tunneling may be universal as an internal electrical transport process for the carbon/polymer systems.
Keywords: Zener tunneling; Nolinear; Electrical conductivity; Composite
Preparation and characterization of magnetic multi-walled carbon nanotubes/ferrite nanocomposite and its application for the removal of aniline from aqueous solution
by Mohamed Abdel Salam; Mohamed A. Gabal; Abdullah Y. Obaid (pp. 2651-2658).
► Ferrite nanoparticles (NiFe2O4) were prepared using sol–gel auto-combustion method. ► Different characterization tools confirm the preparation of the ferrite nanoparticles. ► Magnetic multi-walled carbon nanotubes (MWCNTs)/NiFe2O4 nanocomposite was prepared by physical mixing. ► Characterization techniques verified that nanocrystallites of the ferrite aggregated around the MWCNTs homogenously. ► The prepared magnetic nanocomposite was then used for the removal of aniline from aqueous solution.Magnetic multi-walled carbon nanotubes/ferrite (NiFe2O4) nanocomposite was prepared by physical mixing. X-ray diffraction and Fourier transform infrared spectroscopy confirm the preparation of the ferrite nanoparticles. Different characterization techniques were used to study the morphological structure and surface analysis of the prepared magnetic multi-walled carbon nanotubes/ferrite nanocomposite, which verified that nanocrystallites of the ferrite aggregated around the multi-walled carbon nanotubes homogenously. The prepared magnetic multi-walled carbon nanotubes/ferrite nanocomposite was then used for the removal of aniline as an example of the organic pollutants from aqueous solution. The effect of nanocomposite mass, contact time, solution pH and solution temperature on the removal process was investigated. The magnetic nanocomposite showed a great efficiency for the removal of aniline with the ease of separation of the nanoparticles from the aqueous solution using an ordinary magnet.
Keywords: Carbon nanotubes; Ferrite; Nanocomposite; Aniline; Removal; Aqueous solution
Telechelic or side-functionalized diorganosiloxanes with ferrocenylimine groups
by Angelica Vlad; Maria Cazacu; Constantin Turta; Radu Ionut Tigoianu; Anton Airinei; Adina Arvinte (pp. 2659-2668).
Display Omitted► New telechelic or side-functionalized oligodiorganosiloxanes with aminopropyl groups were prepared. ► Aminopropyl groups attached to the siloxane were converted in ferrocenylimine ones. ► Iron spin state, electrochemical behavior and solvent effect were investigated by57Fe Mossbauer spectroscopy, cyclic voltammetry, and UV–Vis absorption spectroscopy, respectively.Formyl-ferrocene, FFc, was prepared and used as agent to attach ferrocenyl moiety to siloxanes through imine group. Thus, 1,3-bis(3-aminopropyl)tetramethyldisiloxane, L1, α,ω-bis(3-aminopropyldimethylsiloxy)oligodiphenylsiloxane, L2, and poly[dimethyl-methyl(3-aminopropyl)siloxane], L3 (the last two prepared by us) were converted in the corresponding azomethine derivatives: 1,3-bis(3-ferrocenyliminepropyl)tetramethyldisiloxane, L1Fc, α,ω-bis(ferrocenyliminepropyldimethylsiloxy)oligodiphenylsiloxane, L2Fc, and poly[dimethyl-methyl(3-ferrocenyliminopropyl)siloxane], L3Fc. The iron spin state was determined by Mossbauer spectroscopy, emphasizing the presence of mixed-valence iron of interest for molecular electronics. Cyclic voltammograms of the compounds, both in solution and as films deposited on glassy carbon electrodes, showed quasi-reversible oxidation/reduction waves making them suitable for sensing applications. The solvent effect was studied by UV–Vis spectra.
Keywords: Siloxane; Ferrocene; Imine; Mossbauer spectrum; Cyclic voltammetry; Mixed valence; Electronic absorption spectra
Detailed study of N,N′-(diisopropylphenyl)-terrylene-3,4:11,12-bis(dicarboximide) as electron acceptor for solar cells application
by J. Gorenflot; A. Sperlich; A. Baumann; D. Rauh; A. Vasilev; C. Li; M. Baumgarten; C. Deibel; V. Dyakonov (pp. 2669-2676).
► Properties of terrylene-3,4:11,12-bis(dicarboximide) as electron acceptor for OPV. ► Blended with P3HT efficient charge generation was observed. ► Time of flight electron mobility of 1e−3cm2/s/V comparable to PCBM. ► Some spectral regions with strong absorption do not participate solar cells’ EQE. ► Morphology studies: formation of a hole blocking layer limiting extraction.We report on terrylene-3,4:11,12-bis(dicarboximide) (TDI) as electron acceptor for bulk-heterojunction solar cells using poly(3-hexyl thiophene) (P3HT) as complementary donor component. Enhanced absorption was observed in the blend compared to pure P3HT. As shown by the very efficient photoluminescence (PL) quenching, the generated excitons are collected at the interface between the donor and acceptor, where they separate into charges which we detect by photoinduced absorption and electron-spin resonance (ESR). Time-of-flight (TOF) photoconductivity measurements reveal a good electron mobility of 10−3cm2V−1s−1 in the blend. Nevertheless, the photocurrent in solar cells was found to be surprisingly low. Supported by the external quantum efficiency (EQE) spectrum as well as morphological studies by way of X-ray diffraction and atomic force microscopy, we explain our observation by the formation of a TDI hole blocking layer at the anode interface, which prevents the efficiently generated charges to be extracted.
Keywords: Organic semiconductors; Polymers; Charge generation; Charge transport; Bulk heterojunction; Solar cell; Morphology
Lifetime study of single layer and stacked white organic light-emitting diodes
by Kyoung Soo Yook; Oh Young Kim; Jun Yeob Lee (pp. 2677-2681).
Display Omitted► Correlation of the lifetime with the device structure of white PHOLEDs. ► Long lifetime in orange/blue/orange stack structure. ► Little color change in the single layer white device.The lifetime of phosphorescent white organic light-emitting diodes (PHWOLEDs) was studied by changing the device structure of the PHWOLEDs. Four PHWOLEDs with different emitting layer structures were fabricated and the lifetime was investigated. Long lifetime was obtained in the device with orange/blue/orange stacked emitting structure, while the color change after lifetime test was minimized in the single emitting layer PHWOLED.
Keywords: Lifetime; White organic light-emitting diodes; Device structure; Color stability
Poly (ɛ-caprolactone)/siloxane biohybrids with application in “smart windows”
by L.C. Rodrigues; M.M. Silva; M.J. Smith; A. Gonçalves; E. Fortunato (pp. 2682-2687).
► The authors have prepared ionically-conducting membranes for the specific requirements of an electrochromic device. ► Organic/inorganic hybrid materials with potential application in practical devices have been characterized. ► The thermal and electrochemical stabilities of these hybrid materials were found to be appropriate for the prototype device developed.In view of the potential technological impact of solid polymer electrolytes (SPEs) in the domain of solid-state electrochemistry, particularly in the production of advanced batteries, sensors and electrochromic and photoelectrochemical devices, the sub-class of SPE materials has attracted considerable interest during the last two decades. Li+-doped ormolyte systems obtained using the sol–gel method have been investigated in the last few years. In this study we have used conductivity measurements, cyclic voltammetry at a gold microelectrode and thermal analysis to characterize an electrolyte system based on a lithium perchlorate (LiClO4)-doped di-urethane cross-linked poly(ɛ-caprolactone)(PCL)/siloxane hybrid matrix. The PCL/siloxane host hybrid matrix represents an attractive alternative, as it is expected to have lower environmental impact than electrolytes currently used in commercial devices. The biocompatible, biodegradable and air-permeable properties of the PCL matrix have led to extensive application of this material in the manufacture of suture cord, artificial skin, re-sorbable prosthetic devices and as a container for drug delivery. The amorphous hybrid materials characterized in this study combine acceptable levels of ionic conductivity (9.58×10−6 and 2.53×10−4Scm−1 at 25 and 90°C, respectively) with low environmental impact.The authors of a previous publication, have concluded that organic–inorganic systems based on poly(ɛ-caprolactone) may be used as dual-function electrolyte/adhesive component in prototype solid-state electrochromic devices. This component provides significant advantages in optical performance, cycle lifetime and durability of the electrochromic devices relative to conventional liquid electrolytes.
Keywords: SPE; Poly(ɛ-caprolactone); LiClO; 4; Ionic conductivity; Thermal analysis; Electrochromic devices
Simultaneous increase in conductivity and Seebeck coefficient in a polyaniline/graphene nanosheets thermoelectric nanocomposite
by Yong Du; Shirley Z. Shen; Weidong Yang; Richard Donelson; Kefeng Cai; Philip S. Casey (pp. 2688-2692).
► Polyaniline/graphene thermoelectric composite pellets and films were prepared. ► The materials’ electrical conductivity and Seebeck coefficient were measured. ► Both the properties increase with the increasing of graphene content. ► The addition of GNs greatly increases the carrier mobility of the composites.Polyaniline/graphene nanosheets (PANI/GNs) thermoelectric (TE) bulk composite pellets and films have been prepared with PANI to GNs weight ratios ranging from 4:1 to 1:1. Their structure and morphology have been investigated by Raman spectroscopy, thermogravimetric analysis and field emission scanning electron microscopy. The TE properties of the pellets and films were measured at room temperature. As the weight ratio of PANI to GNs decreased from 4:1 to 1:1, both the electrical conductivity and the Seebeck coefficient of pellets and the films increased. This was attributed to a substantial increase in carrier mobility while carrier concentration was not significantly changed. As a result, the power factor of the pellets and films increased from 0.64 to 5.60 and 0.05 to 1.47μWm−1K−2, respectively. This is the first time the TE properties of PANI/GNs nanocomposites were reported.
Keywords: Polyaniline; Graphene; Thermoelectric properties; Composite
Predictive modeling of the vibrational quenching in emitting lanthanides complexes
by Angelo Monguzzi; Alberto Milani; Agniezka Mech; Luigi Brambilla; Riccardo Tubino; Carlo Castellano; Francesco Demartin; Francesco Meinardi; Chiara Castiglioni (pp. 2693-2699).
► A modeling of luminescence quenching in emitting lanthanides complexes is presented. ► Low NIR luminescence quantum yield is given to the presence of high-energy vibrational centers. ► NIR transitions are deactivated through electronic-vibrational energy transfer. ► The model has been tested on a purpose-made Er3+ complex with good results. ► It gives “a priori” information/predictions regarding the emission properties of new complexes.Organic complexes of lanthanides trivalent ions such as Nd3+ and Er3+ are receiving increasing attention in view of their use as dopants in polymeric telecommunication devices. Their employment is however currently limited by their small emission quantum yield (QY) due to non-radiative deactivation of near infrared (NIR) transitions through electronic-vibrational energy transfer (ET). A complete modeling of this quenching effect is presented here and successfully tested on a purpose-made Er3+ complex. This method provides “a priori” information/predictions regarding the emission properties of new complexes thus establishing useful guidelines for the design and synthesis of new molecules and for the engineering of materials suitable in technological applications.
Keywords: Near infrared emitters; Lanthanides complexes; Emission quenching; Vibrational intensities
Device characterization of organic nanostructure based on sodium copper chlorophyllin (SCC)
by M.E. Aydin; A.A.M. Farag; M. Abdel-Rafea; A.H. Ammar; F. Yakuphanoglu (pp. 2700-2707).
Molecular structure of SCC and I– V characteristics in dark and under illumination of Al/SCC/ITO device.Display Omitted► The energy band model was applied for the optical transitions of SCC. ► Al/SCC/ITO device exhibits good rectification behavior. ► The obtained ideality factor of the device shows non-ideal diode characteristics. ► Al/SCC/ITO device shows good performance and sensitivity under illumination.Thin films of nanocrystalline sodium copper chlorophyllin, SCC were prepared by spin coating technique at room temperature (300K). The films were identified by scanning electron microscopy (SEM), particle size analyzer (PSA) and X-ray diffraction (XRD). The energy band model was applied and the type of the optical transitions responsible for optical absorption was found to be direct allowed transition. Devices of Al/SCC/ITO were fabricated and the current density–voltage measurements showed diode characteristics. The photovoltaic parameters were calculated from the J– V characteristics under illumination through ITO and discussed in detail. Some junction parameters of the device such as ideality factor, barrier height, and series resistance were determined from J– V and C– V characteristics. The obtained ideality factor of the device shows non-ideal diode characteristics with the value of ideality factor greater than unity. This indicates the recombination process of electron-hole in the depletion region. The diffusion potential determined from the linear region of the J– V characteristics is found to be in close with that obtained from C– V measurements at 1MHz. A space charge limited conduction (SCLC) controlled by an exponential distribution of traps above the valence band edge was observed at higher voltage region. The interface state density ( Nss) was determined as a function of energy separation from the valence-band edge.
Keywords: Sodium copper chlorophyllin; Thin films; Nanocrystalline; Optical properties; Photovoltaic properties
The zwitterionic radical and its neutral radical derivative with interesting magnetic properties
by Yiman Zhang; Guoping Yong; Chongfu Li; Wenlong She; Yingzhou Li (pp. 2708-2713).
► A neutral radical was obtained via ring-opening reaction of zwitterionic radical. ► Two radicals exhibit conductivities and interesting magnetic behaviors. ► The zwitterionic radical displays a reversal of the net magnetization. ► The neutral radical exhibits a peculiar twice spin transition behavior. ► The neutral radical also possesses a high magnetic phase transition temperature.A new neutral radical was synthesized from a zwitterionic radical via radical oxidation ring-opening reaction under mild conditions. The zwitterionic radical displays interesting magnetic behavior, and it also possesses higher conductivity. The neutral radical exhibits unusual twice magnetic phase transition behaviors, including a high ferromagnetic to antiferromagnetic phase transition temperature ( ca. 382K), which can be attributed to the anisotropic contraction and non-uniform slippage of columnar stacking chain and the changes of close intermolecular contacts. These unique magnetic behaviors ascribe to their different stacking structures.
Keywords: Radical; 2-(Imidazo[1,2-; a; ]pyridin-2-yl)-2-oxo-; N; -(pyridin-2-yl)acetamide; Crystal structure; Magnetic property; Conductivity
The effect of the PEDOT:PSS surface energy on the interface potential barrier
by Mario Petrosino; Alfredo Rubino (pp. 2714-2717).
Display Omitted► Realization of 4 ITO/PEDOT:PSS/Al devices with 4 kinds of PEDOT:PSS. ► Measurements of the PEDOT:PSS surface energy. ► Device characterizations by the means of I− V and C− V measurements. ► Estimation of the carrier density and built-in potential. ► Discussion relating the built-in potential and the polar component of the PEDOT:PSS surface energy.Commercial PEDOT:PSS dispersions have been used to study the PEDOT:PSS interface properties; ITO/PEDOT:PSS/Al devices have been analyzed by the means of current–voltage and capacitance–voltage measurements where the effect of a barrier has been observed. Then by contact angle measurements employed on the PEDOT:PSS layer, the surface energy has been compared to the built-in potential obtaining an empirical law relating the polar component of the energy with the internal potential. The results have been interpreted in terms of interaction between the organic permanent dipoles and the metal that favours the tuning of the built-in potential.
Keywords: PEDOT:PSS; Surface energy; Interface dipoles
Development and characterization of poly(3,4-ethylenedioxythiophene)-coated poly(methylene blue)-modified carbon electrodes
by Somayeh Kakhki; Madalina M. Barsan; Esmaeil Shams; Christopher M.A. Brett (pp. 2718-2726).
► Electrosynthesis of PEDOT-coated poly(methylene blue) films on carbon electrodes. ► Electrosynthesis conditions by fixed potential or potential cycling and electrolyte optimized. ► Redox behaviour of poly(methylene blue) unaffected by PEDOT. ► Characterization by cyclic voltammetry and electrochemical impedance spectroscopy. ► PEDOT improves stability of poly(methylene blue) modified electrodes for sensors.Poly(3,4-ethylenedioxythiophene) (PEDOT) films have been formed by electropolymerization for the first time on top of poly(methylene blue) (PMB) modified GCE (PMB/GCE), in order to improve PMB modified electrode stability. Experimental conditions, such as electrolyte and fixed potential or potential cycling electrosynthesis have been optimized for PEDOT polymerization on bare GCE. Characterization of PMB modified together with PEDOT/PMB modified and bare GCE was done by cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The operational and storage stability of PEDOT/PMB/GCE was tested. Its good stability and the lack of influence of dissolved oxygen on its electrochemical properties, make it promising for sensor and biosensor applications.
Keywords: Poly(3,4-ethylenedioxythiophene); Poly(methylene blue); Conducting polymers; Electropolymerization
Exciton diffusion length in some thermocleavable polythiophenes by the surface photovoltage method
by J. Toušek; J. Toušková; Z. Remeš; J. Kousal; S.A. Gevorgyan; F.C. Krebs (pp. 2727-2731).
Poly-3-(2-methylhexyloxycarbonyl) dithiophene (P3MHOCT), poly-3-carboxydithiophene (P3CT) and polythiophene (PT) polymers were studied by optical and optoelectronic methods to find diffusion length as one of the important parameters characterizing them as candidates for solar cells. Their important property is that P3MHOCT can serve as a precursor which, after thermal annealing, converts into more rigid and insoluble P3CT and further thermal treatment produces native unsubstituted PT. Ellipsometric measurement yielded data on the thickness of the spin coated layers; absorption coefficients were obtained from transmittance and reflectance spectra. Our method of surface photovoltage extended to thin semiconducting layers was utilized for extraction of diffusion length of photogenerated excitons. Higher diffusion lengths in P3CT and PT materials as compared with that in P3MHOCT can probably be ascribed to the increase of the crystalline fraction. The highest diffusion length was found in P3CT polymer but its large resistivity represents a disadvantage in application in solar cells. Taking into account just these parameters, relatively low resistivity together with quite high diffusion length (13±2nm) predetermine the native polythiophene among the polymers studied in the present work as the best candidate for construction of solar cells.
Keywords: Thermocleavable polythiophenes; Photovoltage; Excitons; Diffusion length
Calculation of the activation volume in semi-conducting polypyrrole
by I. Sakellis; A.N. Papathanassiou; J. Grammatikakis (pp. 2732-2733).
► We investigate the negative sign of the activation volume υact. ► We investigate the fact that υact becomes more negative as the temperature is increased. ► The derived value of the activation volume is comparable to the value predicted from a thermodynamical model. ► Gibbs energy does not depend on the defect mechanism while it is only determined from the bulk material.We show that the recent experimentally derived value of the activation volume in semi-conducting polypyrrole is comparable to the value predicted from a thermodynamical model (Varotsos, 1976 ; Varotsos et al., 1978 ), which interconnects the activation Gibbs energy with the bulk elastic and expansivity data.
Keywords: PACS; 72.80.Ng; 77.22.−d; 66.30.hk; 63.50.LmAmorphous solids; Point defects; Activation volume; Thermodynamic properties
The performance of OLEDs based on sorbitol doped PEDOT:PSS
by Ali Kemal Havare; Mustafa Can; Serafettin Demic; Mahmut Kus; Siddik Icli (pp. 2734-2738).
Display Omitted► We report the performance of OLED consist of sorbitol-PEDOT:PSS. ► The OLED devices were tested in terms of electrical and optical characteristics. ► The annealed sorbitol-PEDOT:PSS anode device shows the best performance.We report the performance of OLED devices with a configuration of FTO/sorbitol-PEDOT:PSS (60nm)/TPD (50nm)/Alq3 (40nm)/Al (120nm) and FTO/plain-PEDOT:PSS (60nm)/TPD (50nm)/Alq3 (40nm)/Al (120nm) prepared from poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) undoped (plain-PEDOT:PSS) and doped by a constant concentration 5% of sorbitol, respectively. The OLED devices were tested in terms of electrical and optical characteristics in order to investigate the effect of sorbitol doping and thermal annealing on PEDOT:PSS thin film on FTO. The device fabricated from sorbitol-PEDOT:PSS with thermal annealing showed the maximum luminance of 130 (cdm−2). The devices with sorbitol doping showed a lowering of the operating voltage from 6.5 down to 3V. A constant luminous efficiency higher than 1.8cdA−1 was obtained in the range of current of 4–7 (mA/cm2) with doped sorbitol.
Keywords: Sorbitol; PEDOT:PSS; OLED; Luminance efficiency
Synchrotron X-ray scattering reveals early-stage crystallinity during the self-assembly of polyaniline nanotubes with rectangular cross-sections
by Cosmin Laslau; Bridget Ingham; Zoran D. Zujovic; Pavla Čapková; Jaroslav Stejskal; Miroslava Trchová; Jadranka Travas-Sejdic (pp. 2739-2742).
The use of synchrotron X-ray diffraction to study the crystallographic structure of nanostructure polyaniline is reported. It is shown to reveal unprecedented crystallographic information, particularly for early-stage self-assembled intermediate structures that are critical to the formation process. We discuss the new peaks, which are enabled here by specific advantages of synchrotron X-rays, including higher resolution diffraction patterns, and lower sample quantity requirements. The findings have application to the study of the structural evolution underpinning PANI nanotube formation.
Keywords: Polyaniline; Nanotube; Self-assembly; Synchrotron; Diffraction
Enzymatic synthesis of conductive polyaniline in the presence of ionic liquid
by Yesong Gu; Ju-Ya Tsai (pp. 2743-2747).
► Ionic liquid [bmim]OTF rescued the activity of HRP in the presence of SDBS. ► [bmim]OTF might reduce the CMC of SDBS. ► SDBS served as the template and doping agent for conductive polyaniline. ► PANI possessed excellent electrochemical properties by using both SDBS and [bmim]OTF.Enzyme catalyzed aniline polymerization has shown notable advantages, such as benign process and good solvent solubility for polyaniline. In this study, we further explored the possibility of using anionic surfactant sodium dodecylbenzenesulfonate (SDBS) as the template for the synthesis of conductive polyaniline by horseradish peroxidise (HRP), particularly in the presence of imidazolium-based ionic liquid [bmim]OTF. We found that the use of [bmim]OTF offered the protection for HRP against the inactivation by SDBS. Meanwhile, [bmim]OTF ensured the synthesis of conductive PANI in the presence of SDBS based on UV–vis spectrophotometer analysis. The conductive PANI was further characterized by FTIR, TGA, and cyclic voltammetry. Our results demonstrated that the enzymatic synthesized PANI in the presence of both SDBS and [bmim]OTF possessed excellent electrochemical properties.
Keywords: Enzymatic polymerization; Conductive polyaniline; Ionic liquid; Anionic surfactant
Effect of MoO3 as an interlayer on the performance of organic solar cells based on ZnPc and C60
by Wenjin Zeng; Kian Soon Yong; Zhi Ming Kam; Zhi-kuan Chen; Yuning Li (pp. 2748-2752).
Display Omitted► High efficiency of 3.43% was achieved from ZnPc:C60-based organic solar cells. ► Significant improvement in performance was observed after MoO3 inserted. ► Law of performance change was investigated by comparing device resistance.The effect of molybdenum trioxide (MoO3) as an interlayer is investigated on device performance of organic solar cells using zinc phthalocyanine (ZnPc):fullerene-C60 (C60) as the active layer. Our results indicate that the insertion of MoO3 has little effects on the surface topography or absorption properties, but noticeably increases the shunt resistance, which leads to the improvement of open-circuit voltage, short-circuit current and fill factor. As a result, the power conversion efficiency (PCE) of the devices can be improved by up to 25%. An optimum PCE of 3.43% was achieved with the insertion of 10nm MoO3.
Keywords: Organic solar cell; OPV; MoO; 3; Zinc phthalocyanine; Fullerene
Synthesis of polypyrrole/zinc oxide composites and study of their structural, thermal and electrical properties
by Aisha Batool; Farah Kanwal; Muhammad Imran; Tahir Jamil; Saadat Anwar Siddiqi (pp. 2753-2758).
► In this research work the synthesized composites have been characterized. ► There is a linear relation ship between DC conductivity and particle loading percentage. ► A good correlation among structural properties and conductivity was observed. ► Stability of polypyrrole increases with the increase in ZnO content in the matrix.Intrinsically conducting PPy/ZnO composites were synthesized with varying amounts of zinc oxide (0–20wt.%) via a facile chemical oxidation polymerization approach. These composites were characterized for their structure, thermal stability, surface morphology and DC electrical conductivity by XRD, FT-IR, TGA, DSC, SEM and four-probe method respectively. FT-IR results show broadening and shifts of peaks towards lower wave numbers in all composites suggesting better conjugation and some chemical interactions between PPy and ZnO particles. Powder X-ray diffraction analysis demonstrated the amorphous nature of PPy and its composites as well with no indication of crystalline peaks. The percolation threshold for conductivity of PPy/ZnO composites was 15wt.% of the volume fraction of PPy. SEM and TGA data suggest that PPy/ZnO composites have high thermal stability due to better conformation, compactness and reduction of grain boundary volume with zinc oxide particle loading that in parallel enhanced their DC conductivity.
Keywords: Polypyrrole; Composite; DC conductivity; FT-IR; XRD; SEM; TGA; DSC
Analysis of dark and photovoltaic characteristics of Au/Pyronine G(Y)/p-Si/Al heterojunction
by A.A.M. Farag; H.S. Soliman; A.A. Atta (pp. 2759-2764).
Display Omitted► The presented heterojunction device shows barrier inhomogenity. ► The conduction mechanism was explained on the basis of thermionic emission. ► The results support the device for photovoltaic sensor application.Thin films of Pyronine G(Y), PYR (G) were prepared on single crystalline p-Si by thermal evaporation technique. The temperature-dependence of the electrical characteristics of the Au/PYR (G)/p-Si/Al heterojunction diode in the temperature range of 300–400K was studied. The barrier height and ideality factor of the device were determined as a function of temperature. It is noticed that the bias barrier height Φ b increases and the ideality factor n decreases by increasing the temperature. Such behavior can be attributed to barrier inhomogeneities by assuming a high mismatch in PYR(G)/p-Si interface. The value of series resistance ( R s) was determined using Cheung's method and found to be a strongly temperature dependent. The variation of 1/ C2 with voltage shows a straight line at high frequency (1MHz) indicating the formation of barrier between PYR(G) and p-Si and the potential barrier height is about 0.92eV at 300K. The photovoltaic properties of Au/PYR(G)/p-Si/Al heterojunction were investigated under illumination with light intensity of 10mW/cm2. Discussion of the obtained results and their comparison with the previous published data were also given.
Keywords: Pyronine; Organic/inorganic hybrid structure; Photovoltaic
Synthesis, characterization, electrochemical behaviors and applications in the Suzuki–Miyaura cross-coupling reactions of N2S2O2 thio Schiff base ligand and its Cu(II), Co(III), Ni(II), Pd(II) complexes and their usage in the fabrication of organic–inorganic hybrid devices
by Hamdi Temel; Salih Pasa; Yusuf Selim Ocak; Ismail Yilmaz; Serpil Demir; Ismail Ozdemir (pp. 2765-2775).
Display Omitted► Thio Schiff base ligand and its transition metal complexes were synthesized. ► Catalytic property of the ligand was determined with Suzuki coupling reactions. ► Electrochemical properties of L, Cu(L) were investigated with cyclic voltammetry. ► The fabrication of organic–inorganic hybrid devices using L and its Co(III), Ni(II) and Pd(II) complexes and their electrical properties have been reported.The thio Schiff base ligand N, N′-bis( o-hydroxy-1-naphtaldehydene)-1,2-bis( o-amino phenilthio)ethane (L) was synthesized by the condensation of o-hydroxy-1-naphthaldehyde and 1,2-bis(2-amino thiophenol)ethane. The reactions of the ligand with different transition metal(II) salts under reflux system condition afforded a series of metal complexes such as Cu(II), Co(III), Ni(II) and Pd(II). Ligand and its complexes were characterized by a combination of elemental analysis, UV–vis spectroscopy, FT-IR,1H NMR, MS and TGA/DTA. The electrochemical properties of L and [Cu(L)] were investigated using cyclic voltammetry (CV) in the solutions of dimethyl sulfoxide (DMSO). The catalytic property of thio Schiff base ligand with Suzuki cross-coupling reactions were also determined with different substituted aryl halides in the presence of aryl boronic acid and Pd(AcO)2 as catalyst. In addition, the fabrication of organic–inorganic hybrid devices using L and its Co(III), Ni(II) and Pd(II) complexes and their electrical properties have been reported.
Keywords: 2-Amino Thiophenol; Suzuki coupling; Cyclic voltammetry; Organic–inorganic heterojunction
Two-dimensional extended π-conjugated anthracene-based molecules bearing 4-ethynyl-7-(5-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole at 2,6- and 9,10-substitution positions
by Jung A Hur; Sun Jae Kim; Kyung Hwan Kim; Tae Wan Lee; Kihyun Kim; Jicheol Shin; Kyung Seok Hwang; Byung Doo Chin; Dong Hoon Choi (pp. 2776-2784).
► Novel thiophenylbenzothiadiazole-containing conjugated molecules have been synthesized. ► The two π-conjugated molecules exhibited fairly high charge carrier mobilities. ► The photovoltaic cell with these molecules exhibited a power conversion efficiency of 0.55%.New 4-ethynyl-7-(5-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole-containing conjugated molecules were synthesized via the Sonogashira coupling reaction. The synthesized anthracene-based molecules showed good solubility in common organic solvents and had good self-film-forming properties. The semiconducting properties of the two synthesized π-conjugated molecules,4 and8, were evaluated by using the molecules in organic thin-film transistors (OTFTs). As-spun films of these molecules showed fairly high charge carrier mobilities, 0.010–0.10cm2V−1s−1 ( ION/ IOFF>103). The high charge carrier mobilities were attributed to the strong intermolecular interaction, which resulted in easy crystallization and dense coverage of the insulator surface. In addition, the two molecules were used to fabricate organic photovoltaic (OPV) cells with methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PC61BM), without thermal annealing. The best preliminary results were obtained for the photovoltaic cell containing8; the open-circuit voltage, short-circuit current density, and fill factor were 0.76V, 1.65mAcm−2, and 0.44, respectively, and the overall power conversion efficiency (PCE) was 0.55%.
Keywords: Conjugated molecules; Anthracene; Semiconductor; Mobility; Organic thin film transistor; Bulk heterojunction; Photovoltaic cell
Synthesis of new alternating conjugated copolymers consisting of tetrazine/carbazole or tetrazine/fluorene derivatives along with thiophene spacers for photovoltaic applications
by Myung-Jin Baek; Wooree Jang; Soo-Hyoung Lee; Youn-Sik Lee (pp. 2785-2791).
Display Omitted► Tetrazine-carbazole (PCTz) and tetrazine-fluorene polymers were synthesized. ► The bandgap and HOMO level of PCTz were 2.23 and −5.32eV, respectively. ► PCTz had a very high absorption coefficient. ► The open-circuit voltage of PCTz solar cells was very high. ► Under the same conditions, the power conversion efficiency was 2.13%.In order to investigate the relationships between chemical structures of conjugated polymers and their properties with respect to photovoltaic applications, two new copolymers consisting of an electron-deficient tetrazine unit and either an electron-rich carbazole (PCTz) or fluorene (PFTz) along with thiophene spacers were synthesized. The estimated electrochemical bandgap and HOMO level of PCTz and PFTz were 2.23/−5.32eV and 2.48/−5.60eV, respectively. PCTz had a very high absorption coefficient, which was even higher than poly(3-hexyltiophene) in chloroform solution. The bulk heterojunction photovoltaic devices fabricated using PCTz and [6,6]-phenyl-C71-butyric acid methyl ester had a high open-circuit voltage (1.0V) under AM 1.5G illumination at 100mW/cm2 with a power conversion efficiency of 2.13%.
Keywords: Solar cell; Conjugated polymer; Tetrazine; Carbazole; Fluorene; Deep HOMO
Alpha-sexthiophene/n− Si heterojunction diodes and solar cells investigated by I– V and C– V measurements
by Y. Takanashi; N. Oyama; K. Momiyama; Y. Kimura; M. Niwano; F. Hirose (pp. 2792-2797).
► The 6T/n−-Si junction has a current rectification characteristic explained by a Schottky heterojunction model. ► The Schottky barrier height and ideality factor were estimated to be 0.75–0.79eV and 2.5, respectively. ► The depletion layer is generated solely in the n−-Si layer on a sub-micron scale. ► This heterojunction allows for power generation with power conversion efficiencies up to 0.4% with a simulated solar light exposure of 50mW/cm2.The forward and reverse current density–voltage ( J– V) and capacitance–voltage ( C– V) characteristics of alpha-sexthiophene (6T)/n−-silicon heterojunction diodes were investigated to clarify the carrier conduction mechanism at the organic/inorganic heterojunction. These observation results indicate that the 6T/n−-Si junction has a current rectification characteristic explained by a Schottky heterojunction model. Diode parameters—the Schottky barrier height and ideality factor—were estimated to be 0.75–0.79eV and 2.5, respectively. A C– V analysis suggests that the depletion layer is generated solely in the n−-Si layer on a sub-micron scale from the junction at the zero bias and in the reverse bias condition and the diffusion potential was estimated to be 0.4eV. This heterojunction allows for power generation with power conversion efficiencies up to 0.4% with a simulated solar light exposure of 50mW/cm2.
Keywords: Organic; Inorganic; Heterojunction; Schottky; Solar cell
p–n junction organic photovoltaics fabricated by all solution processing
by Xin Zhang; Kang-Shyang Liao; Soniya D. Yambem; Nigel J. Alley; Seamus A. Curran (pp. 2798-2802).
► We prepared p–n junction organic photovoltaic cells by a solution processing. ► We compared the effects on p–n interface by different PCBM solvents. ► UV–Vis and AFM were conducted to study the morphology of active layer. ► We achieved the best device by processing with the lowest b.p. solvent for PCBM. ► The best device has a η 1.78%, Voc 0.44V, Jsc 9.4mA/cm2 and FF 0.43.We have prepared p–n junction organic photovoltaic cells using an all solution processing method with poly(3-hexylthiophene) (P3HT) as the donor and phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor. Interdigitated donor/acceptor interface morphology was observed in the device processed with the lowest boiling point solvent for PCBM used in this study. The influences of different solvents on donor/acceptor morphology and respective device performance were investigated simultaneously. The best device obtained had characteristically rough interface morphology with a peak to valley value ∼15nm. The device displayed a power conversion efficiency of 1.78%, an open circuit voltage ( Voc) 0.44V, a short circuit current density ( Jsc) 9.4mA/cm2 and a fill factor 43%.
Keywords: Organic photovoltaic; Solution processing; p–n junction solar cell