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We present here a method for the quantitative prediction of the spectroscopic specular reflectivity line-shape in anisotropic layered media. The method is based on a 4 &times, 4 matrix formalism and on the simulation from the first principles (through density functional theory&mdash, DFT) of the anisotropic absorption cross-section. The approach was used to simulate the reflectivity at the oxygen K-edge of a 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) thin film on Au(111). The effect of film thickness, orientation of the molecules, and grazing incidence angle were considered. The simulation results were compared to the experiment, permitting us to derive information on the film geometry, thickness, and morphology, as well as the electronic structure.

In this review, the technique of resonant soft X-ray reflectivity in the study of magnetic low-dimensional systems is discussed. This technique is particularly appealing in the study of magnetization at buried interfaces and to discriminate single elemental contributions to magnetism, even when this is ascribed to few atoms. The major fields of application are described, including magnetic proximity effects, thin films of transition metals and related oxides, and exchange-bias systems. The fundamental theoretical background leading to dichroism effects in reflectivity is also briefly outlined.

Resonant soft x-ray absorption was applied at the S L2,3 (2p) edges of thin films of 1,4-benzenedimethanethiol (BDMT) on gold and of sexithiophene (6 T) on flat and patterned CaF2 surfaces. Linear dichroism effects were clearly observed depending on the orientation of the electric field vector of the incoming radiation. They were related to the organization of the molecules. For the interpretation of the angular dependencies of the spectral features, density functional theory (DFT) simulations of the absorption cross sections were calculated. It was possible to confirm the generally accepted chemisorption geometry of BDMT on Au(111), composed of molecules nearly vertical at the surface with one of the S atoms participating in the bonding with Au and the other left pending at the outermost surface and contributing mostly to the measured x-ray absorption. For 6 T on CaF2 a new chemisorption phase was identified on ridge-patterned CaF2(110), coexisting with a previously known phase given by 6 T crystallites highly oriented along the ridges. The new phase being composed of molecules oriented perpendicular to the ridges and lying on their edges.

In this paper a novel approach is presented to prepare flexible and transparent conducting films, whose components can be separated and recovered via a recycling process. The fabrication method is based on low power sputtering of ultrathin gold layers on sodium alginate free-standing films. The resulting foils are thin, easy to handle, and shape, while showing good conductive properties. In particular, they show excellent resistance to mechanical stress, like bending or rubbing, and are highly stable in ambient atmosphere over several months. Therefore they may represent a very promising candidate to be employed in green electronics, thanks to the reduced energy consumption required for their fabrication, the absence of toxic components or chemicals that are derived from oil, and the possibility to disassemble the devices at the end of their life in environmentally friendly conditions.

Resonant soft X-ray reflectivity at the carbon K-edge was applied to a trigonal tetracene single crystal. The angular resolved reflectivity was quantitatively simulated describing the tetracene crystal in terms of its dielectric tensor, which was derived from the anisotropic absorption cross section of the single molecule, as calculated by density functional theory. A good agreement was found between the experimental and theoretically predicted reflectivity. This allows us to assess the anisotropic optical constants of the organic material, probed at the carbon K-edge, in relation to the bulk/surface structural and electronic properties of the crystal, through empty energy levels.

Interference of light and material particles is described with a unified model which does not need to assume the wave-particle duality. A moving particle is associated with a region of spatial correlated points named coherence cone. Its geometry depends on photon or particle momentum and on the parameters of the experimental setup. The final interference pattern is explained as a spatial distribution of particles caused by the coherence cone geometry. In the present context, the wave front superposition principle, wave-particle duality and wave-collapse lose their meaning. Fits of observed single electron and single molecule interference patterns together with the simulation of expected near-field molecule interference (Talbot carpet) demonstrate the model validity.

UV-visible spectrophotometric properties of gold nanoparticles fabricated by magnetron sputtering on free-standing sodium alginate (SA) membranes have been recorded and simulated. The simulation implied the tuning of the collision frequency as well as the introduction of the Local Surface Plasmon (LSP) resonance in optical constants of the nanostructured material, which are based on a Drude-Lorentz model. The nanostructured film is treated by means of the Effective Medium Approximation. The film stack is simulated using the Generalized Transfer Matrix Method. The results show that the actual material can properly be treated, and quantitative results are obtained. A red shift in the LSP spectral position is observed for more interconnected material.

Herein, we describe the use of soft X-ray reflectivity at the carbon K-edge to study the molecular organization (orientation, structure, and morphology) of pentacene active films in a top-contact transistor geometry. This technique is not affected by sample charging, and it can be applied in the case of insulating substrates. In addition, the sampling depth is not limited to the near-surface region, giving access to buried device interfaces (metal/organic and dielectric/organic). Spectral lineshape simulations, based on ab-initio calculations using a realistic 3D layer-by-layer model, allow us to unravel the details of the molecular organization in all the specific and crucial areas of the active film, overcoming the limitations of conventional approaches. The tilt angle of the long molecular axis in the whole film is found to progressively decrease with respect to the substrate normal from 25° to 0° with the increasing film thickness. A full vertical alignment, optimal for in-plane charge hopping, is reached only after the complete formation of the first five monolayers. Remarkably, starting from the first one in contact with the dielectric substrate, all the monolayers in the stack show a change in orientation with the increasing thickness. On the other hand, at the buried interface with a gold top-contact, the molecules assume a flat orientation that only propagates for two or three monolayers into the organic film. Top-contact devices with the highest performances can thus be obtained using films of at least ten monolayers. This explains the observed thickness dependence of charge mobility in pentacene transistors.

The synthesis of two new thieno(bis)imide (TBI, N) end functionalized oligothiophene semiconductors is reported. In particular, trimer (NT3N) and pentamer (NT5N) have been synthesized and characterized. Two different synthetic approaches for their preparation were tested and compared namely conventional Stille cross coupling and direct arylation reaction via C-H activation. Theoretical calculations, optical and electrochemical characterization allowed us to assess the role of the ?-conjugation extent, i.e., of the oligomer size on the optoelectronic properties of these materials. In both {TBI} ended compounds, due to the strong localization of the {LUMO} orbital on the {TBI} unit, the {LUMO} energy is almost insensitive to the oligomer size, this being crucial for the fine-tailoring of the energy and the distribution of the frontier orbitals. Surprisingly, despite its short size and contrarily to comparable TBI-free analogues, {NT3N} shows electron charge transport with mobility up to ?N = 10-4 cm2 V-1 s-1, while increasing the oligomer size to {NT5N} promotes ambipolar behavior and electroluminescence properties with mobility up to ?N = 0.14 cm2 V-1 s-1 and to ?P = 10-5 cm2 V-1 s-1.

Organic light-emitting transistors (OLETs) are multifunctional optoelectronic devices that hold great promise for a variety of applications, including flat panel displays, integrated light sources for sensing and optical communication systems. The narrow illumination area within the device channel is considered intrinsic to the device architecture and is a severe technological drawback for all those applications where a controlled, wide and homogeneous emission area is required. Here it is shown that not only the position but also the extension of the emission area is voltage-tunable, and the entire channel of the transistor can be homogeneously illuminated. The modeling of the exciton distribution within the channel at the different bias conditions coupled to the modeling of the device emission profile highlights that excitons are spread through the entire channel width and across the bulk of the central emission layer of the p-channel/emitter/n-channel trilayer active heterostructure.

α-Sexithiophene (6T) ultrathin films have been grown on CaF2(111)/Si(111) planar surfaces and on CaF2(110)/Si(001) ridged surfaces by molecular beam epitaxy. The growth mode has been studied by means of atomic force microscopy (AFM), photoemission, and near edge X-ray absorption fine structure (NEXAFS). AFM reveals a substantial difference in the film morphology on the two substrates: on CaF2(111) large islands with flat terraces form with no in-plane preferential growth direction; on CaF2(110) narrow and elongated islands develop following the substrate corrugation. Photoemission and X-ray absorption at Ca L2,3 and F K edges indicate that the interaction with the substrate is negligible. Near-edge X-ray absorption (NEXAFS) flanked by DFT calculations of the angular-resolved absorption cross section of 6T at the carbon K-edge reveal that the molecules on both substrates have their long axis vertically oriented with respect to the substrate plane. In addition, in-plane anisotropy of the molecular orientati...

The realization of organic light-emitting transistors (OLETs) based on α,ω-dihexylcarbonylquaterthiophene (DHCO4T), an intrinsically ambipolar and luminescent semiconductor, is reported. In this de...

The relationships between the molecular structure, packing modalities, charge mobility and light emission in organic thin films is a highly debated and controversial issue, with both fundamental and technological implications in the field of organic optoelectronics. Thieno(bis)imide (TBI) based molecular semiconductors provide an interesting combination of good processability, tunable self-assembly, ambipolar charge transport and electroluminescence, and are therefore an ideal test base for fundamental studies on the structure-property correlation in multifunctional molecular systems. Herein, we introduce a new class of thieno(bis)imide quaterthiophenes having alkyl side chains of different shapes (linear, cyclic, branched) and lengths (C1-C8). We found that contrarily to what is generally observed in most molecular semiconductors, the length of the alkyl substituent does not affect the optical, self-assembly and charge transport properties of TBI materials. However, different electroluminescence powers are observed by increasing the alkyl side, this suggesting a potential tool for the selective modulation of TBI functionalities. A deep experimental and theoretical investigation on this new family of TBI materials is provided. This journal is © the Partner Organisations 2014.

Interference of light and material particles is described in this chapter with a unified model, which does not need to assume the superposition principle. A moving particle is associated with a region of spatial correlated points called a coherence cone . Its geometry depends on photon or material particle momentum and on the parameters of the experimental setup. The coherence cone geometry causes the spatial distribution of particles in preferential directions. After propagation, particles accumulate on the final observation plane to form interference peaks. In the context given here, the wave front superposition principle, wave–particle duality, and wave collapse are no longer significant. In addition, the present model describes light and material particle distributions in near- and far-field regions and in paraxial and nonparaxial approximations so that the paraxial Fourier and Wigner optics are included, as particular cases, in our more general model. Fits of observed single-electron and single-molecule interference patterns, together with the simulations of expected near-field molecule interferences, demonstrate the model validity. An interference experiment is suggested to realize molecular nanostructures with a vacuum sublimation process controlled by a shadow mask. A new scenario is envisaged to miniaturize electronic devices and to realize individual noninteracting nanodomains of well-chosen thickness.

Organic molecular semiconductors are key components for a new generation of low cost, flexible, and large area electronic devices. In particular, ambipolar semiconductors endowed with electroluminescent properties have the potential to enable a photonic field-effect technology platform, whose key building blocks are the emerging organic light-emitting transistor (OLET) devices. To this aim, the design of innovative molecular configurations combining effective electrical and optical properties in the solid state is highly desirable. Here, we investigate the effect of the insertion of a thieno(bis)imide (TBI) moiety as end group in highly performing unipolar oligothiophene semiconductors on the packing, electrical, and optoelectronic properties of the resulting materials. We show that, regardless of the HOMO-LUMO energy, orbital distribution, and molecular packing pattern, a TBI end moiety switches unipolar and nonemissive oligothiophene semiconductors to ambipolar and electroluminescent materials. Remarkably, the newly developed materials enabled the fabrication of single layer molecular ambipolar OLETs with optical power comparable to that of the equivalent polymeric single layer devices.

Optoelectronic properties of light-emitting field-effect transistors (LETs) fabricated on bottom-contact transistor structures using a tetracene film as charge-transport and light-emitting material are investigated. Electroluminescence generation and transistor current are correlated, and the bias dependence of the LETs external quantum efficiency is determined. The device's performance degrade rapidly upon gate biasing. The effect is attributed to charge trapping, which can be prevented by operating the devices in pulsed mode. A model for the electron injection mechanism in a p-type organic transistor is proposed. On the basis of this model, electrical and optical characteristics, as well as the dependence of the external quantum efficiency on drain- and gate-bias, are well reproduced.

The interplay between atomic geometry and magnetic properties of Fe atoms at the Fe–Ni(1 1 1) interface was studied. To this end dichroic optical absorption and reflectivity (both in energy and θ–2θ angular scan mode) were carried out in the L 2 , 3 edge region of Fe in (Fe 6 A /Ni 24 A (1 1 1)) 10 multilayer while the atomic geometry of the Fe site at the interface was studied for 1 atomic layer of Fe on Ni(1 1 1) by photo-electron diffraction (in angular scan mode and back-scattering regime). The Fe atom was found to be chemisorbed in an fcc hollow site and to stack pseudomorphically to Ni(1 1 1). Its magnetic moment was found to be in the range 2.9±0.9–1.93±0.6 μ B . Possibility of surface anisotropy is discussed.

The atomic geometry and growth mechanism of Fe films (0.5-20 monolayers (ML)) epitaxially grown on Ni(001) have been investigated by a multitechnique approach with the twofold aim of assessing the reliability of quick structural techniques (primary-beam diffraction modulated electron emission (PDMEE), secondary electron imaging (SEI)), suitable for on-line monitoring of film growth, and studying the structural evolution of the Fe/Ni system. To this end, samples were analysed by PDMEE and SEI together with synchrotron radiation photoelectron diffraction (PD), and extended x-ray absorption fine structure (EXAFS). Results show the effectiveness of the approach by quick techniques. In the early stage of growth, Fe arranges in a strained fcc(001) structure. Transition to the bce phase occurs through nucleation of bcc(110) domains with the bce < 111 > //fcc < 110 > orientation. Quantitative analysis based on PD and EXAFS data is also presented.

We investigated the evolution during film growth of the electronic properties of Fe/Ni(111) ultrathin films with electron energy loss spectroscopy from the valence band and from the Fe L 2,3 ionization edges, relating them with structural modifications monitored with low energy electron diffraction. At ca. 4 monolayers of Fe a fcc to bcc phase transition was observed with the major changes in the electronic structure occurring in the 3d band region.

An analytical model for the electric field associated with a periodic array of alternating p- and n-doped stripes lying in a half-plane, tilted with respect to the specimen edges and thus better representing the actual experimental set-up is presented. With respect to a previous treatment, relative to the case of stripes perpendicular to the edge, a more physical derivation is outlined, and the calculated phase shift is used to interpret the main features of holography and Lorentz microscopy images, allowing a quantitative assessment of the influence of the specimen edge on them.

Despite their favorable electronic and structural properties, the synthetic development and incorporation of thiazole-based building blocks into n-type semiconductors has lagged behind that of other pi-deficient building blocks. Since thiazole insertion into pi-conjugated systems is synthetically more demanding, continuous research efforts are essential to underscore their properties in electron-transporting devices. Here, we report the design, synthesis, and characterization of a new series of thiazolethiophene tetra- (1 and 2) and hexa-heteroaryl (3 and 4) co-oligomers, varied by core extension and regiochemistry, which are end-functionalized with electron-withdrawing perfluorohexyl substituents. These new semiconductors are found to exhibit excellent n-channel OFET transport with electron mobilities (mu(e)) as high as 1.30 cm(2)/(V center dot s) (I-on/I-off > 10(6)) for films of 2 deposited at room temperature. In contrary to previous studies, we show here that 2,2'-bithiazole can be a very practical building block for high-performance n-channel semiconductors. Additionally, upon 2,2'- and 5,5'-bithiazole insertion into a sexithiophene backbone of well-known DFH-6T, significant charge transport improvements (from 0.0010.021 cm(2)/(V center dot s) to 0.200.70 cm(2)/(V center dot s)) were observed for 3 and 4. Analysis of the thin-film morphological and microstructural characteristics, in combination with the physicochemical properties, explains the observed high mobilities for the present semiconductors. Finally, we demonstrate for the first time implementation of a thiazole semiconductor (2) into a trilayer light-emitting transistor (OLET) enabling green light emission. Our results show that thiazole is a promising building block for efficient electron transport in ?-conjugated semiconductor thin-films, and it should be studied more in future optoelectronic applications.

Herein is described a multidisciplinary approach to understand the performance limitations of small molecule organic light emitting transistors (OLETs) based on a layered architecture, an innovative architecture potentially competitive with the state of the art and more flexible for spectral emission control. The processes of charge injection and field-effect transport at metal/organic and organic/organic interfaces are analysed using microscopic and spectroscopic techniques in coordination. Atomic force microscopy and ultrasonic force microscopy are employed to characterize the interface morphology and the initial growth stages of organic films where charge transport actually occurs. X-ray diffraction and near edge X-ray dichroic absorption with linearly polarised light allow to determine the unit cell packing and the molecular orientation at the active organic interfaces, as well as the amount of non-ordered domains. Moreover, chemical reactivity at the interfaces is measured by X-ray photoelectron spectroscopy. It is found that a strong reaction occurs at the metal-organic interfaces, with molecular fragmentation. Additionally, the transport properties strongly depend on the nature of the materials forming the organic stack. Specifically, amorphous conjugated films as bottom layers can promote an increased molecular disorder in the upper active layer, with a concomitant deterioration of the conduction properties.

In this work, we report on the fabrication and characterization of organic light-emitting transistors (OLETs) within an indium–tin-oxide (ITO)-free platform, using graphene-based transparent conductive electrodes in place of ITO as gate electrode. A direct comparison between twin bottom-gate/top-contacts OLETs, where a standard ITO layer is replaced with a film made of a few graphene layers, shows that comparable electrical characteristics can be obtained along with a clear improvement in the electroluminescence generation characteristics. Our experimental findings pave the way to the exploitation of graphene-based transparent conductive electrodes within this class of emerging devices on flexible substrates, further promoting the novel era of flexible organic electronics.

A novel method for mapping the charge density spatial distribution in organic field-effect transistors based on the electro-modulation of the photoluminescence is demonstrated. In field-effect transistors exciton quenching is dominated by exciton-charge carrier interaction so that it can be used to map the charge distribution in different operating conditions. From a quantitative analysis of the photoluminescence quenching, the thickness of the charge carrier accumulation layer is derived. The injection of minority charge carriers in unipolar conditions is unexpectedly evidenced, which is not displayed by the electrical characteristics.

Despite the variety of functional properties of molecular materials, which make them of interest for a number of technologies, their tendency to form inhomogeneous aggregates in thin films and to self-organize in polymorphs are considered drawbacks for practical applications. Here, we report on the use of polymorphic molecular fluorescent thin films as time temperature integrators, a class of devices that monitor the thermal history of a product. The device is fabricated by patterning the fluorescent model compound thieno(bis) imide-oligothiophene. The fluorescence colour of the pattern changes as a consequence of an irreversible phase variation driven by temperature, and reveals the temperature at which the pattern was exposed. The experimental results are quantitatively analysed in the range 20-200 degrees C and interpreted considering a polymorph recrystallization in the thin film. Noteworthy, the reported method is of general validity and can be extended to every compound featuring irreversible temperature-dependent change of fluorescence.

Target compound (V) is alternatively prepared via Stille coupling of bromo thiophene (I) with 5,5′-bis(tributylstannyl)-2,2′-bithiophene.

Interest in electronic properties of organic materials has been stimulated by emerging prospects of their application in electronics and photonics [1–4]. Among potential applications, one may envisage use of organic materials as active elements of molecular switches. According to a general definition, a switch is a mechanical, electrical, electronic, or optical device for opening or closing a circuit, or for diverting energy or charge from one part of a circuit to another. In this context, photochromic materials have been extensively studied.

1.1 OLED Vs. OLET Plastic electronics, i.e. electronics based on organic polymer and molecular semiconductors, is a key low-cost technology for new vast application areas. The scope of applications in plastic electronics is virtually endless. It ranges from opto-electronic devices and flexible organic light-emitting diodes (OLEDs) displays, to ultra-cheap, radio-frequency identification tags that can replace the now ubiquitous bar-codes, and to wearable computing and electronic bio-interfaces. The first demonstration of the OLED dates back to the 1960s when electrically driven light emission from non-crystalline organic materials was first observed. After that, several studies were carried out by academic groups and companies (Kodak, Pioneer, Motorola, NEC, etc...) both for fundamental physics comprehension and application purposes. Nowadays electronic products containing displays are becoming more and more portable. Therefore, they need some peculiarities like lightweightness, flexibility, brightness, etc... These, with many others, are the strong points of the OLEDs. In fact they are thinner, lighter and more flexible with respect to their inorganic counterpart. Moreover, OLEDs can be as bright as LEDs and they consume much less power. Due to the organic processability, they are easier to produce and can be made on larger area. Finally OLEDs have large fields of view, about 170 degrees, a significantly advantage over, for example, liquid crystal displays. Of course, these devices present also some disadvantages: they have typically shorter lifetime (in particular lifetime of the blue emitter is critical, about 1.000 hours), they are not very stable and can easily be contaminated by water or oxygen. In general, with respect to OLEDs, organic light-emitting transistors (OLETs) present some fascinating characteristics which overcome many physical and technical drawbacks in the realization of nano-scale integrated electroluminescent devices. The main difference between the vertical (OLED) and planar (OLET) device geometry is a direct consequence of the different device structures. In OLED, charge transport occurs perpendicular to the organic layers (bulk charge transport) while in OLET the transport occurs horizontally (field-effect charge transport). Under the typical biasing conditions, the electron and hole mobility can be about four orders of magnitude higher in OLETs than in OLEDs, thus affecting directly the material lifetime and exciton emission. In a typical OLED structure, the minority carriers travel only few tens

We report on an organic field effect transistor (OFET) with a photochromic dielectric layer, operating as an opto-electrical switch device. The structure contained a photochromic material dissolved in the polymer dielectric metastable forms; poly(methyl methacrylate) was a polymeric layer. The photochromic material was spiropyran exhibiting a large difference of the dipole moments of the stable and insulator; and an n-type perylene derivative was used as the organic semiconductor. Illumination of the structure with UV light resulted in a reversible increase of the source drain current, accompanied by a reversible decrease of the threshold voltage. The initial parameters were restored by a thermal relaxation in the dark or by illumination with visible light. The photoswitching ratio was found to be dependent on the gate voltage ranging between ca. 2 just above the threshold voltage and ca. 1.3 at the highest voltage employed (90 V). The switching has been attributed to reversible changes of dielectric properties of OFET's insulator (dielectric layer) due to a reversible light-triggered reaction of polar photochromic species, dissolved in the bulk of the dielectric layer. The contribution of dipoles aggregated on the semiconductor dielectric interface was estimated to be negligible at gate voltages exceeding ca. 10 V.

In this Article, we present a comprehensive study of organic field-effect transistors (OFETs) made of thin films of methyl, n-butyl, and n-hexyl end-substituted quaterthiophenes on a transparent substrate platform. This particular platform has been already used for organic light-emitting diodes (OLEDs) but rarely employed for OFETs. In perspective, this is a very promising route for the development of field-effect photonic applications such as organic light-emitting transistors (OLETs). A systematic characterization of the organic films has been made by means of atomic force microscopy (AFM) and X-ray diffraction (XRD) to correlate morphology, crystallinity and charge mobility to the alkyl chain length. In particular, a charge mobility value of 0.09 cm(2)/(V s) has been obtained in transparent OFETs with a large area channel for DH4T grown at room temperature. This mobility exceeds the one obtained on silicon-oxide substrates and is likely due to a more favorable interaction of the DH4T molecules with the PMMA layer employed as gate dielectric.

Astroglial cell survival and ion channel activity are relevant molecular targets for the mechanistic study of neural cell interactions with biomaterials and/or electronic interfaces. Astrogliosis is the most typical reaction to in vivo brain implants and needs to be avoided by developing biomaterials that preserve astroglial cell physiological function. This cellular phenomenon is characterized by a proliferative state and altered expression of astroglial potassium (K(+)) channels. Silk is a natural polymer with potential for new biomedical applications due to its ability to support in vitro growth and differentiation of many cell types. We report on silk interactions with cultured neocortical astroglial cells. Astrocytes survival is similar when plated on silk-coated glass and on poly-D-lysine (PDL), a well known polyionic substrate used to promote astroglial cell adhesion to glass surfaces. Comparative analyses of whole-cell patch-clamp experiments reveal that silk- and PDL-coated cells display depolarized resting membrane potentials (-40 mV), very high input resistance, and low specific conductance, with values similar to those of undifferentiated glial cells. Analysis of K(+) channel conductance reveals that silk-astrocytes express large outwardly delayed rectifying K(+) current (K(DR)). The magnitude of KDR in PDL- and silk-coated astrocytes is similar, indicating that silk does not alter the resting K(+) current. We also demonstrate that guanosine- (GUO) embedded silk enables the direct modulation of astroglial K(+) conductance in vitro. Astrocytes plated on GUO-embedded silk are more hyperpolarized and express inward rectifying K(+) conductance (K(ir)). The K(+) inward current increases and this is paralleled by upregulation and membrane polarization of K(ir)4.1 protein signal. Collectively these results indicate that silk is a suitable biomaterial platform for the in vitro studies of astroglial ion channel responses and related physiology. (c) 2010 Elsevier Ltd. Al! rights reserved.

We report on the characteristics of a host-guest lasing system obtained by coevaporation of an oligo(9,9-diarylfluorene) derivative named T3 with the red-emitter 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dye (DCM). We demonstrate that the ambipolar semiconductor T3 can be implemented as an active matrix in the realization of a host-guest system in which an efficient energy transfer takes place from the T3 matrix to the lasing DCM molecules. We performed a detailed spectroscopic study on the system by systematically varying the DCM concentration in the T3 matrix. Measurements of steady-state photoluminescence (PL), PL quantum yield (PLQY), time-resolved picosecond PL, and amplified spontaneous emission (ASE) threshold are used to optimize the acceptor concentration at which the ASE from DCM molecules takes place with the lowest threshold. The sample with a DCM relative deposition ratio of 2% shows an ASE threshold as low as 0.6 kW/cm(2) and a net optical gain measured by femtosecond time-resolved pump-and-probe spectroscopy as high as 77 cm(-1). The reference model system Alq(3):DCM sample measured in exactly the same experimental conditions presents an one-order-of-magnitude higher ASE threshold. The ASE threshold of T3:DCM is the lowest reported to date for a molecular host-guest energy-transfer system, which makes the investigated blend an appealing system for use as an active layer in lasing devices. In particular, the ambipolar charge transport properties of the T3 matrix and its field-effect characteristics make the host-guest system presented here an ideal candidate for the realization of electrically pumped organic lasers.

The potential of organic semiconductor-based devices for light generation is demonstrated by the commercialization of display technologies based on organic light-emitting diodes (OLEDs). Nonetheless, exciton quenching and photon loss processes still limit OLED efficiency and brightness. Organic light-emitting transistors (OLETs) are alternative light sources combining, in the same architecture, the switching mechanism of a thin-film transistor and an electroluminescent device. Thus, OLETs could open a new era in organic optoelectronics and serve as testbeds to address general fundamental optoelectronic and photonic issues. Here, we introduce the concept of using a p-channel/emitter/n-channel trilayer semiconducting heterostructure in OLETs, providing a new approach to markedly improve OLET performance and address these open questions. In this architecture, exciton-charge annihilation and electrode photon losses are prevented. Our devices are >100 times more efficient than the equivalent OLED, >2x more efficient than the optimized OLED with the same emitting layer and >10 times more efficient than any other reported OLETs.

We report on the characteristics of a host-guest lasing system obtained by co-evaporation of an oligo(9,9-diarylfluorene) derivative named T3 with the red-emitter 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dye (DCM). We demonstrate that the ambipolar semiconductor T3 can be implemented as active matrix in the realization of a host-guest system in which an efficient energy transfer takes place from T3 matrix to the lasing DCM molecules. We performed a spectroscopic study on the system by systematically varying the DCM concentration in the T3 matrix. Measurements of steady-state photoluminescence (PL), PL quantum yield (PLQY) and amplified spontaneous emission (ASE) threshold are used to optimize the acceptor concentration at which the ASE from DCM molecules takes place with the lowest threshold. Organic light-emitting diodes (OLEDs) implementing the DCM:T3 host-guest system as recombination layer are fabricated for verifying the optical properties of the optimised blend in real working devices. Indeed, the very low ASE threshold of T3:DCM makes the investigated blend an appealing system for use as active layer in lasing devices. In particular, the ambipolar charge transport properties of the T3 matrix and its field-effect characteristics make the host-guest system presented here an ideal candidate for the realization of electrically-pumped organic lasers.

The dynamics of triplet recombination in fluorene trimers have been studied using steady state photoinduced absorption (PA) spectroscopy. We investigated two type of oligomeric films, deposited by different techniques: thermal evaporation and spincoating. The different molecular arrangement in both films is manifested in a red-shift of the absorption, PL and T1-Tn triplet PA spectra of the sublimated film relative to the spincoated one. Triplet recombination dynamics follow a dispersive bimolecular recombination model away from the trap filling regime. Moreover we report on the characteristics of a host-guest lasing system obtained by co-evaporation of the most promising oligofluorene derivative (T3) with the red-emitter 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dye (DCM). The blend satisfies the necessary condition for an efficient Forster energy transfer to take place from T3 matrix to DCM molecules.

We report on the influence of the dielectric/organic interface properties on the electrical characteristics of field-effect transistors based on polyphenylenevinylene derivatives. Through a systematic investigation of the most common dielectric surface treatments, a direct correlation of their effect on the field-effect electrical parameters, such as charge carrier mobility, On/Off current ratio, threshold voltage, and current hysteresis, has been established. It is found that the presence of OH groups at the dielectric surface, already known to act as carrier traps for electrons, decreases the hole mobility whereas it does not substantially affect the other electrical characteristics. The treatment of silicon dioxide surfaces with gas phase molecules such as octadecyltrichlorosilane and hexamethyldisilazane leads to an improvement in hole mobility as well as to a decrease in current hysteresis. The effects of a dielectric polymer layer spin coated onto silicon dioxide substrates before deposition of the semiconductor polymer can be related not only to the OH groups density but also to the interaction between the dielectric and the semiconductor molecules. Specifically, the elimination of the OH groups produces the same effect observed with hexamethyldisilazane. The hole mobility values obtained with hexamethyldisilazane and polymer dielectrics are the highest reported to date for PPV-based field-effect transistors.

It is shown how the same physically appealing method can be applied to find analytic solutions for two difficult and apparently unrelated problems in optics and electrostatics. They are: (i) the diffraction of a plane wave at a perfectly conducting thin half-plane and (ii) the electrostatic field associated with a parallel array of stripes held at opposite potentials and lying in a half-plane. In the latter case, the solution of the problem is essential in order to calculate the electron optical phase shift experienced by the electron beam in electron microscopy experiments, where the model mimics an array of reverse biased p–n junctions. This paper is intended, in particular, for the undergraduate student who has completed his programme of calculus and electromagnetism, but it can also be of interest to the general physicist.

We report on the influence of the dielectric/organic interface properties on the electrical characteristics of field-effect transistors based on Poly-phenylenevinylene derivatives. We observe a direct influence of the dielectric surface on the field-effect mobility as well as on the charge injection at the source electrode, despite the fact that we used a top contact transistor structure. We find that the presence of traps at the dielectric surface, decreases the hole mobility and increases the threshold voltages. By treating the silicon dioxide dielectric surface with gas phase molecules such as octadecyltrichlorosilane (OTS) and hexamethyldisilazane (HMDS) the hole mobility improves and the threshold voltage slightly increases. The effects of a dielectric polymer layer spin coated onto silicon dioxide substrates before deposition of the semiconductor polymer can be related to the density of the oxydryl groups (-OH ), which are the most efficient traps for the charges flowing in the device. We use different polymer species such as polyvinylalchol (PVA), polymethylmetacrilate (PMMA) and a cyclotene derivative (B-staged bisbenzocyclobutene or BCB). The elimination of the -OH groups and of other traps, produces the same effect observed with HMDS coupled to a more pronounced enhancement of the threshold voltage, with the exception of PMMA. The electrical characteristics obtained with HMDS and PMMA polymer dielectrics are the highest reported to date for PPV-based field-effect transistors. We confirm that the purification of the active material is crucial to enhance the device performances and to achieve a better device to device reproducibility. We also investigated the effect of the dispersion of a phosphorescent dye into the active polymeric material. The electrical characteristics of OFETs with HMDS or PMMA dielectric with and without dye doping are compared.

We report on the triplet spectra and dynamics in two types of oligomeric films deposited by two different techniques: thermal evaporation and spin coating. The different molecular arrangement in both films is manifested in a red shift of the absorption, PL, and T-1-T-n absorption spectra of the sublimated film relative to the spin-coated one. Triplet recombination dynamics studied with steady-state photoinduced absorption (PA) spectroscopy follow a dispersive bimolecular recombination model away from the trap filling regime. We obtained values for the triplet bimolecular recombination ratio (beta) of 3.4 x 10(-14) and 1.1 x 10(-15) cm(3) s(-1) for evaporated and spin-coated film, respectively, the difference being attributed to diverse molecular arrangement in both films.