Organic Electronics 2004 Volume 5 №01-03

Elsevier. — 156 p. — ISSN: 1566-1199.«Organic Electronics» is a journal whose primary interdisciplinary focus is on materials and phenomena related to organic devices such as light emitting diodes, thin film transistors, photovoltaic cells, sensors, memories, etc.Papers suitable for publication in this journal cover such topics as photoconductive and electronic properties of organic materials, thin film structures and characterization in the context of organic devices, charge and exciton transport, organic electronic and optoelectronic devices.«Organic Electronics» provides the forum for applied, fundamental and interdisciplinary contributions spanning the wide range of electronic properties and applications of organic materials. A Letters section is included for rapid publication of short articles announcing significant and highly original results.1-6
Raffaele Guido Della Valle, Aldo Brillante, Elisabetta Venuti, Luca Farina, Alberto Girlando, Matteo Masino. Exploring the polymorphism of crystalline pentacene
Abstract:
Starting from each of the five complete X-ray structures published so far for crystalline pentacene, we have computed the structures of minimum potential energy, and obtained two local minima of the potential energy, i.e., two different ‘‘inherent structures’’ of mechanical equilibrium. This behavior, which has been found to be independent of the details of the potential model, indicates that there are at least two different single crystal polymorphs of pentacene. One of the two polymorphs corresponds to the structure originally determined by Campbell et al. [Acta Cryst. 14 (1961) 705]. The other polymorph corresponds to the structure found in all more recent measurements. The calculations predict significant differences between the corresponding Raman spectra of the lattice phonons, which we have checked experimentally, confirming the existence of two polymorphs. The correct identity of the samples, initially assigned only by matching experimental and calculated spectra, has been verified directly with X-ray diffraction measurements. Finally, we have obtained theoretical information on the global stability of the minima by systematically sampling the potential surface of crystalline pentacene, and found that the two polymorphs correspond to the two deepest minima. Further deep minima with layered structures, which might correspond to the thin film polymorphs found to grow on substrates, are also predicted.7-22
A. Thierry, J.C. Wittmann, B. Lotz, V. da Costa, J. Le Moigne, M. Campione, A. Borghesi, A. Sassella, H. Plank, R. Resel. Organisation, structure and morphology of organic thin films via electron microscopy
Abstract:
Recently, molecular materials have emerged as challenger to inorganic materials for the active layers in opto-electronic devices. However, if they require comparatively low operating voltages, it is still difficult and a major challenge to grow organic macroscopic single crystals. Therefore, these molecular materials are incorporated in the devices as ordered thin films rather than as single crystals. Different procedures, among which epitaxial growth, are used to elaborate ‘‘single-crystal like’’ thin films as perfect as possible. The macroscopic performances of these films rely strongly on their quality. Therefore, both their morphology and their structure have to be carefully characterised and understood. Among the various ex-situ techniques commonly available to control thin films, electron microscopy (EM) that can examine both the real and the reciprocal space is most appropriate. Combining the various EM modes one can visualise the crystal habits and morphology down to a few nanometres, the sizes, number and shapes of micro-defects or grain boundaries at the heart of the crystallites. The potentiality of EM as a versatile tool to characterise the fine structure of thin films is illustrated via the description of some distinctive cases of epitaxially grown thin films. We first recall the geometrical matching rules for usual epitaxy and then describe two cases of specific selection rules: a topographic selection for some polyolefins, a molecular selection for some diacetylenes. Finally, the morphology, structure and quality of ordered thin films for opto-electronic will be reported. Though very diverse at first sight, the four chosen examples illustrate the process of understanding step by step the epitaxial growth of organic thin films on orienting substrates. These examples demonstrate that the origin of the orientation of a crystal onto another crystal can be very diverse. The major and ultimate goal is to yield large well-oriented thin films based on polymer or molecular crystals. This can only be achieved by a careful selection of the depositsubstrate pair. Such films will optimise the opto-electronic response of organic materials at the macroscopic scale, a prerequisite for future development.23-27
A.Yu. Andreev, C. Teichert, G. Hlawacek, H. Hoppe, R. Resel, D.-M. Smilgies, H. Sitter, N.S. Sariciftci. Morphology and growth kinetics of organic thin films deposited by hot wall epitaxy
Abstract:
In this work we have used atomic force microscopy and X-ray diffraction by synchrotron radiation to investigate the growth kinetics and morphology of para-sexiphenyl layers. The results of our investigations can be summarized as follows: (a) para-sexiphenyl grows on mica epitaxialy; (b) a rearrangement from randomly distributed small parasexiphenyl islands with compact shape to elongated islands occurs during the growth if the critical island density is reached; (c) with further increase of the growth time the islands become elongated, quickly reaching a fixed asymptotic width while their height remains much smaller than their length and width.29-34
A. Guti errez-Llorente, G. Horowitz, R. P erez-Casero, J. Perri ere, J.L. Fave, A. Yassar, C. Sant. Growth of polyalkylthiophene films by matrix assisted pulsed laser evaporation
Abstract:
Films of regio-random and regio-regular poly(3-hexylthiophene) were grown by matrix assisted pulsed laser evaporation (MAPLE), where the target is a frozen solution of the polymer in orthoxylene. Analysis by MALDI mass spectroscopy confirms that the deposition does not result in any degradation of the polymer. UV–visible absorption spectroscopy and AFM images show that the structure of the films strongly depends on the temperature of the substrate during deposition. Best results are obtained when the substrate is held at low temperature.35-43
Th. Wagner, A. Bannani, C. Bobisch, H. Karacuban, M. Stohr, M. Gabriel, R. Moller. Growth of 3,4,9,10-perylenetetracarboxylic-dianhydride crystallites on noble metal surfaces
Abstract:
The growth of organic crystallites formed by 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) has been investigated by scanning tunneling microscopy on Cu(1 1 0), Cu(1 1 1) and Au(1 1 1). For the chosen parameters PTCDA exhibits Stranski–Krastanov growth. If the average coverage exceeds about 3–5 monolayers the formation of crystalline islands begins. Working at very low tunneling currents it has been possible to obtain molecular resolved images of crystallites formed by up to 100 molecular layers, while the overall coverage has a value of about 10–20 monolayers. Since the side walls typically exhibit a moderate slope in the range of 14–20 their structure may be as well analyzed as the flat top layer. As will be presented it is not only possible to determine the unit cell within the molecular layer but also their relative displacement between subsequent layers. A comparison between the 3D structures of several crystals will be given.45-51
R. Resel, I. Salzmann, G. Hlawacek, C. Teichert, B. Koppelhuber, B. Winter, J.K. Krenn, J. Ivanco, M.G. Ramsey. Structure and morphology of sexiphenyl thin films grown on aluminium (1 1 1)
Abstract:
The epitaxial growth of sexiphenyl (C₃₆H₂₆) on an atomically clean aluminium (1 1 1) surface is studied on the basis of thin films grown at different substrate temperatures. Thin films with an average thickness of 350 A are investigated by X-ray diffraction techniques and atomic force microscopy. Grown at room temperature the films show epitaxial order. It is found that the aromatic planes of sexiphenyl are oriented parallel to Al(1 1 1) and the long axes of the sexiphenyl molecules are aligned along a 1-10 azimuthal direction at the Al(1 1 1) surface. The films grow in islands with average lateral dimensions up to of 200 nm and to 50 nm in height, the areas between the islands are observed as rather flat. The films grown at 150 ° C show a completely different morphology: occasional large islands and irregularly distributed segmented needles with a relatively constant width of about 0.5 μm, lengths up to 100 μm and heights of about 600 nm. Although the film appears as crystalline, no epitaxy could be verified.53-58
T.P. Nguyen, P. Le Rendu, P. L eveque, J. Ip, O. Gaudin, R.B. Jackman. Analysis of deep traps in 4,4'-bis(4-dimethylaminostyryl benzene) based light emitting diode devices
Abstract:
Charge based deep level transient spectroscopy (Q-DLTS), a modified version of the conventional DLTS technique, has been used to characterize light emitting diodes made with 4,40-bis(4-dimethylaminostryryl benzene) (DMASB), a poly(p-phenylene vinylene) (PPV) derivative containing amine groups. Analysis of the spectra obtained in ITODMASB-MgAg devices with different bias and temperature conditions has established that two types of traps exist within the band gap of the organic material. Traps of type I with a mean activation energy of 0.38 eV and a capture cross section of the order of 10 ^-17 cm² are identified as acceptor-like hole traps. These traps are believed to be distributed with a large deviation. Traps of type II have an activation energy of 0.50 eV and a capture cross section of the order of 10^-19 cm². They are assigned to donor-like electron traps. Comparing with the results obtained in PPV, we suggest that type I traps are related to the presence of the amine groups in the structure of the organic material.59-65
W. Porzio, U. Giovanella, C. Botta, M. Pasini, S. Destri, C. Provasi, V. Rossi. Efficient energy transfer in organic thin films by ultra-high vacuum evaporation
Abstract:
The energy transfer in organics, aiming to optimise the performance of many LED devices, has been exploited with two suitable oligomers deposited either in multi layered or in co-evaporated films by ultra high vacuum evaporation. We have chosen two components displaying the appropriate absorption/emission windows, in fact the emission maximum of the first, a thienyl-fluorene derivative, is centred in the region where the absorption of the second, tetrahexylsexithiophene, is intense, hence guaranteeing a complete Foerster transfer even at low content of the latter. Optical microscopy investigations together with XRD analysis allowed us to get insight onto both the structure orientation and the morphology of the film components. The optical aspects of this thin layers, 15–30 nm thick, are also discussed and compared with the mono-component films obtained in the same conditions. LED prototypes were prepared and compared with analogous devices obtained by spin-coating technique. The intimate structure/morphology can readily account for the better performance of LED devices obtained by high-vacuum evaporation.67-71
J. Puigdollers, C. Voz, A. Orpella, R. Quidant, I. Mart ın, M. Vetter, R. Alcubilla. Pentacene thin-film transistors with polymeric gate dielectric
Abstract:
Pentacene thin-film transistors have been obtained using polymethyl methacrylate as a gate dielectric. The maximum process temperature was 170 °C, which corresponds to the baking of the polymeric gate dielectric. These devices presented good electrical performances with field-effect mobilities of 0.01 cm² V^{- 1} s ^-1 and low threshold voltages (-15 V). Atomic force microscopy studies reveal that the microstructure of pentacene layers is strongly conditioned by the surface morphology of the dielectric.73-81
C. Goletti, G. Bussetti, P. Chiaradia, A. Sassella, A. Borghesi. The application of reflectance anisotropy spectroscopy to organics deposition
Abstract:
Reflectance anisotropy spectroscopy (RAS) has been recently used to investigate organic compounds, similarly to what has been already done in the case of inorganic semiconductors and metals. An important development is the application of RAS to monitor in real time the growth of thin organic layers in ultra-high vacuum (UHV). In this paper, we discuss the main characteristics of RAS spectroscopy, underlining its peculiar advantages with respect to other techniques. Then, we present recent results obtained by following in situ the deposition of an ordered organic film in UHV, namely α-sexithiophene films onto potassium acid phthalate.83-89
L. Luer, H.-J. Egelhaaf, D. Oelkrug, G. Cerullo, G. Lanzani, B.-H. Huisman, D. de Leeuw. Oxygen induced quenching of photoexcited states in polythiophene films
Abstract:
Oxygen causes reversible and irreversible detrimental effects to the performance of organic (opto-)electronic devices. In order to get some insight into the mechanisms of these effects, we investigated the kinetics of fluorescence quenching (FQ) in thin films (d ~ 100 nm–12 μm) of regioregular polyalkylthiophenes upon exposure to oxygen. The kinetics of FQ consists of a fast component, which is fully reversible, as well as a slow component, which is partially irreversible. The fast reversible component leads to a loss of fluorescence intensity of approximately 2% at an oxygen partial pressure of 1 bar within milliseconds. It is independent of the intensity of the exciting light and is ascribed to collisional quenching of excited singlet states after oxygen diffusion into the bulk of the film. The diffusion coefficients of oxygen and singlet excitons were determined as D(O₂) = 1:5(3) x 10 ^-7 cm² s ^-1 and D(S₁) = 5 x 10 ^-4 cm² s ^-1, respectively. The slow reversible component, whose amplitude depends on light intensity, occurs on a time-scale of minutes. It is assigned to the formation of charge-transfer complexes between excited singlet states of polythiophene and oxygen. Femtosecond pump-probe experiments show that effective quenching of excited singlet states by oxygen takes place on a picosecond time-scale, leading to the enhanced formation of charged states.91-97
Gerhard Weiser, Sven Moller. Influence of molecular anisotropy on the optical properties of nanocrystalline films of п-conjugated systems
Abstract
Absorption and electroabsorption spectra of nanocrystalline films of sexithiophene (T6), quaterthiophene (T4) and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) are presented and analysed by dielectric response theory and accounting for the coherent polarization induced by the incident light. Due to the anisotropic crystal structure absorption peaks do not occur at exciton transition energies but depend on the respective orientation of molecules and the wave vector of light. This directional dispersion causes the broad and rather featureless absorption spectra of nanocrystalline films. Electroabsorption spectra, however, resolve the fundamental exciton and its vibronic progression clearly. The analysis reveals that excited states interact via their dipole moments while exchange interaction and overlap of molecular states is negligible.99-105
R. Scholz, A.Yu. Kobitski, I. Vragovi c, H.P. Wagner, D.R.T. Zahn. Time-resolved photoluminescence in α-PTCDA single crystals: evidence for recombination via Frenkel excitons, charge transfer states, and excimers
Abstract:
The radiative recombination channels in α-PTCDA are analysed with time-resolved PL techniques in the 50 ns range between temperatures of T = 10 and T = 300 K. The resulting PL spectra are interpreted with calculations based on the transfer of Frenkel excitons and with time-dependent density functional theory applied to different deformed geometries of molecular dimers as microscopic models for self-trapped excitons. At low temperature, the lineshape and radiative lifetime of the most important PL channel can be assigned quantitatively to a vertical transition from the indirect minimum of the Frenkel exciton dispersion towards the electronic ground state. In an intermediate regime below about T = 100 K, charge transfer states involving an anionic and a cationic molecule dominate the PL spectra. Radiative recombination from an excimer state in a stack geometry has a rather weak temperature dependence, and due to nonradiative quenching of the other PL channels, this excimer PL band predominates above about T = 200 K.107-114
T. del Cano, V. Parra, M.L. Rodr ıguez-M endez, R. Aroca, J.A. de Saja. Molecular stacking and emission properties in Langmuir–Blodgett films of two alkyl substituted perylene tetracarboxylic diimides
Abstract
The structural and optical properties of Langmuir–Blodgett (LB) films of two perylene diimide derivates, N,N'-Bis(propyl)-3,4,9,10-perylenebis(dicarboximide) and N,N'-Bis(neopentyl)-3,4,9,10 perylenebis(dicarboximide), are reported. The surface pressure–area (п–A) isotherms have evidenced how the lateral chain substituents control the strength of the intermolecular interaction. Nevertheless, a head-on tilted molecular orientation on the water subphase was found for both compounds. This molecular orientation is preserved when the monolayers are transferred onto substrates. Molecular orientation in the films was extracted from comparative analysis of transmission and reflection–absorption infrared spectroscopy (RAIRS) data. Film structure has been studied by means of atomic force microscopy (AFM). The observed different emission profiles have been correlated with the ability of the derivatives to get coupled.115-127
P. Petelenz. Theoretical models for electro-absorption spectroscopy
Abstract:
Interpretation of the physical mechanisms that are crucial for the understanding of theoretical approaches used to reproduce the electro-absorption (EA) spectra of organic molecular crystals is built step by step from the simplest case of an individual molecule, through a model dimer, then a crystal, and finally a film viewed as a disordered assembly of microscopically well ordered crystallites. The two-state model is used throughout to gain qualitative insight into the discussed effects. As an illustrative example, the calculations performed for sexithiophene where all the discussed effects are operative, are briefly discussed.129-134
F. Della Sala, G. Gigli, M.F. Raganato, M. Anni, D. Pisignano, R. Cingolani, L. Favaretto, G. Sotgiu, G. Barbarella, L. Antolini. Effects of intermolecular interactions on photoluminescence efficiency of crystalline thienylene-S,S-dioxide molecular semiconductors
Abstract:
The effects of intermolecular interactions on the photoluminescence (PL) quantum efficiency of differently substituted thienylene-S,S-dioxide derivatives in the crystalline phase are investigated. The exciton resonance interactions and the transfer integrals are evaluated using a quantum chemistry approach by considering the supramolecular structures obtained from X-ray data. Two classes of thienylene-S,S-dioxides are investigated: (1) rigid, fused ring compounds, for which the PL quantum efficiency can be well correlated with the rate of formation of non-radiatively decaying charge-transfer pairs upon photoexcitation; (2) flexible oligothiophenes for which the high PL quantum efficiency in the solid-state is assigned to a decrease of intramolecular internal conversion process, as confirmed by time-resolved PL measurements in solvents of different viscosity.135-140
J.O. Oss o, F. Schreiber, M.I. Alonso, M. Garriga, E. Barrena, H. Dosch. Structure, morphology, and optical properties of thin films of F₁₆CuPc grown on silicon dioxide
Abstract:
Thin films of hexadecafluorophthalocyanine (F16CuPc) were grown by organic molecular beam deposition (OMBD) under ultra high vacuum (UHV) conditions on oxidized Si(0 0 1) substrates. The film thicknesses ranged from 120 to 450 A and the substrate temperature was varied from )150 to 300 °C. X-ray diffraction and atomic force microscopy (AFM) were used to characterize the structure and morphology of the layers. Spectroscopic ellipsometry was employed to determine the dielectric properties and to locate characteristic transitions between 1.4 and 3.7 eV. The correlation of the electronic properties with the molecular packing in the films is discussed. The results are compared to the case of F₁₆CuPc films grown on stepped sapphire surfaces, which differ in their lateral ordering behavior.141-145
M. Campione, A. Borghesi, M. Moret, A. Sassella, B. Lotz, A. Thierry. Structural characterisation of polycrystalline α,ω-dihexyl quaterthiophene thin films by transmission electron microscopy
Abstract:
α,ω-dihexyl quaterthiophene thin films have been deposited by ultra-high vacuum sublimation on silica and potassium hydrogen phthalate substrates. The influence of the substrate on the structure of the films are investigated by transmission electron microscopy and the results are compared with the morphological and optical properties of the same samples.147-155
Ali N. Rashid, Peter Gunter. Self-assembled organic supramolecular thin films for nonlinear optics
Abstract:
We analyze the issues that must be considered on the road towards a practical fabrication method for organic nonlinear optical thin films. The advantages and disadvantages of the most useful techniques for producing nonlinear optical thin films are reviewed. We describe a promising molecular beam deposition techniques that has been developed in our laboratory for the growth of ordered supramolecular self-assembled thin films that are nonlinear optically active and maintain their polar order for large thicknesses.