Extrinsic limiting factors of carrier transport in organic field-effect transistors

Extrinsic factors to disturb the carrier transport in pentacene field-effect transistors (FETs), as a representative of the high-mobility organic FETs (OFETs), have been comprehensively analyzed by using atomic-force-microscope potentiometry (AFMP), microscopic four-point-probe field-effect transistor (MFPP-FET) measurement, and other techniques. In the first part, by mainly using AFMP as a powerful tool to reveal the potential distribution in working OFETs, we show how and how much the formation of source/drain electrodes influences the apparent field-effect mobility both for top- and bottom-contact configurations. In the second part, we show the influence of irregular grain structures and regular grain boundaries. The films grown both at very low and high temperature ranges contain distinctive insulating parts, which make the apparent mobility very low. Within the moderate growth temperature range, the intrinsic field-effect mobility obtained by MFPP-FET measurement is proportional to the average grain size. This behavior is well explained by the polycrystalline model with the diffusion theory. According to the observations in this work, it is obvious that these extrinsic limiting factors must be carefully excluded to discuss the intrinsic mechanism of the carrier transport in OFETs.     

Ambipolar organic field-effect transistors on unconventional substrates

In this paper we report on the realization of flexible all-organic ambipolar field-effect transistors (FETs) realized on unconventional substrates, such as plastic films and textile yarns. A double layer pentacene-C₆₀ heterojunction was used as the semiconductor layer. The contacts were made with poly(ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) and patterned by means of soft lithography microcontact printing (μCP). Very interestingly growing C₆₀ on a predeposited pentacene buffer layer leads to a clear improvement in the morphology and crystallinity of the film so it obtains n-type conduction despite the very high electron injection barrier at the interface between PEDOT:PSS and C₆₀. As a result, it was possible to obtain all-organic ambipolar FETs and to optimize their electrical properties by tuning the thicknesses of the two employed active layers. Moreover, it will be shown that modifying the triple interface between dielectric/semiconductor/electrodes is a crucial point for optimizing and balancing injection and transport of both kinds of charge carriers. In particular, we demonstrate that using a middle contact configuration in which source and drain electrodes are sandwiched between pentacene and C₆₀ layers allows significantly improving the electrical performance in planar ambipolar devices. These findings are very important because they pave the way for the realization of low-cost, fully flexible and stretchable organic complementary circuits for smart wearable and textile electronics applications.     

Hyperspherical approach to description of electron correlation in negative ion of exciton

The binding energy of excitonium negative ion for the ground^1,3S-states in bulk semiconductors GaAs and ZnO in the hyperspherical coordinate method was found. Angular and radial correlations between electrons in gerade and ungerade states were taken into account by channel functions, that are the eigenfunctions of Hamiltonian on the surface of the sphere in the three-dimensional configuration space. Energy values were calculated using the adiabatic and Born-Oppenheimer approximations. The obtained energy values are in agreement with those obtained using variational method.     

Highly-efficient solution-processed phosphorescent multi-layer organic light-emitting diodes investigated by electromodulation spectroscopy

We report on electromodulation (EM) spectroscopy studies of phosphorescent multi-layer organic light-emitting diodes (OLEDs) that are processed from solution. Compared to conventional single-layer OLEDs, they comprise an additional layer of a crosslinkable, oxetane-functionalized triphenylamine-dimer (XTPD) that is inserted between the PEDOT:PSS anode and the emissive layer. Devices with optimized stack architecture feature reduced operating voltages and reach a current efficiency approaching 40 cd/A—twice as much as the corresponding single-layer device. Using EM measurements, we quantify the electric field in the XTPD layer and the emissive layer of such a multi-layer OLED and also measure the average electric field in a single-layer reference device. By comparing the dependence of the internal field on the applied voltage for devices with and without the XTPD layer, we find that in the device containing the XTPD layer there is an increased accumulation of electrons at the anode side of the emissive layer. This accumulation enhances the recombination probability and supports the injection of holes into the emissive layer which explains the observed efficiency improvement and reduction in operating voltage compared to conventional single-layer OLEDs.     

Single molecule spectroscopy of oligofluorenes: how molecular length influences polymorphism

Polyfluorene represents a unique model to study the influence of intramolecular conformation on the electronic properties of chromophores with an extended π-conjugation. According to the degree of planarity between the adjacent repeat units the electronic and optical properties can change substantially. This peculiar spectroscopic behavior has been described by identifying different phases, namely the glassy, the γ- and the β-phase. Here, we present low-temperature single-molecule spectroscopy of a series of oligofluorenes differing in the number of monomeric units, in order to gain information on the influence of chain length on the polymorphism. By monitoring the energy of the 0-0 transition we have classified single molecules belonging to the different phases. We demonstrate that a large number of molecules start to form the β-phase only when more than 9 repeat units constitute the molecular chain. The implications for the control of morphology in polyfluorene thin films are discussed.     

Improvement of electrical properties of silicon-based thin-film transistors by modifying technological fabrication processes

This paper is a review of technological process evolution associated to electrical performance improvement of silicon-based thin-film transistors (TFTs) that were performed mainly in the GM/IETR laboratory. The main objective in agreement with the fields of applications is to fabricate TFTs at a temperature low enough to be compatible with the substrates, glass substrates in a first place and flexible substrates in a second one, which implies several approaches. In fact, the electrical properties of the TFTs, mainly field-effect mobility of carriers in the channel, I _on/I _off drain current ratio, and subthreshold slope, are strongly dependent on the quality and the nature of the channel material, on the material quality and thus on the density of states at the interface with the gate insulator, and on the quality of the gate insulator itself. All the improvements are directly linked to all these aspects, which means an actual combination of the efforts. For the glass substrate, compatible technology processes such as deposition techniques, or solid phase, or laser crystallizations of active layers were studied and compared. The paper details all these approaches and electrical performances. In addition, some results about the use of a silicon–germanium compound as channel active layer and airgap transistors for which the insulator is released, complete the presentation of the evolution of the silicon-based TFTs during the last twenty years.     

Volume structuring of high power LED encapsulates by femtosecond laser direct writing

We report on the micro-fabrication of diffractive optical elements (DOEs) such as 1D, 2D and concentric grating structures inside the volume of thin silicone films by femtosecond laser direct writing. In addition, we show that such structures can also be integrated into silicone films that act as encapsulation layers of high power light-emitting diodes. The latter strategy opens new possibilities to homogenize and to control the light emitted from such devices.     

Third order response to a multifrequency photon field in semiconductors

This paper contains a detailed calculation of the photoinduced current density at third order in the coupling between a semiconductor and a multifrequency photon field, starting from its standard textbook expression which reads in terms of a triple commutator. Due to a major intrinsic problem linked to this triple commutator, such a derivation has been made possible quite recently only, thanks to the tools developed in the composite-boson many-body theory we have recently constructed. The photoinduced current density is shown to ultimately read in a compact form, in terms of the “Pauli scatterings” and “Coulomb scatterings” for exciton-exciton interactions introduced in this theory. Representation of this third order response in Shiva diagrams, which visualize interactions between excitons, is also given to better grasp the physics of the various contributions.     

Absorption measurements in InSe:Ho single crystal under an electric field

InSe:Ho single crystal was grown by Bridgman-Stockberger method. Electric field effects on the absorption measurements have been investigated as a function of temperature in InSe:Ho single crystal. The absorption edge shifted towards longer wavelengths and a decrease of intensity in absorption spectra occurred under an electric field of 7.5 kV/cm. Using absorption measurements, steepness parameter and Urbach energy were calculated under electric field. Applied electric field caused an increase in the Urbach energy. At 10 K and 320 K, the first exciton energies were calculated as 1.322 and 1.301 eV for zero voltage and 1.245 and 1.232 eV for applied electric field, respectively.     

Enhanced luminance of organic light-emitting diodes with metal nanoparticle electron injection layer

Improvement of the performance of organic light-emitting diodes (OLEDs) was achieved by implementing Magnesium-Nickel nanoparticles at the cathode–organic interface using pulsed laser deposition technique. The small geometry of Mg-Ni nanoparticles acts to enhance the localized electric field around them, thus increasing electron injection through tunneling, from the cathode to the organic layer. Improved current and luminance characteristics were demonstrated for both small molecule and polymer-based OLEDs when the nanoparticle layer was incorporated.     

Measurement of the optical Stark effect in semiconducting carbon nanotubes

A strong optical Stark effect has been observed in (6,5) semiconducting single-walled carbon nanotubes by femtosecond pump-probe spectroscopy. The response is characterized by an instantaneous blueshift of the excitonic resonance upon application of pump radiation at photon energy well below the band gap. The large Stark effect is attributed to the enhanced Coulomb interactions present in these one-dimensional materials.     

Thermochromic effect in synthetic opal/polyaniline composite structures

The design and construction of a novel storage/indicator bilayer system is described where ammonia gas stored in a porous material can be used to dope a colour-changing polyaniline film. Both reversible and irreversible colour change effects are possible. A thin synthetic opal film is coupled to a polyaniline film in a parallel plate glass cell with ammonia gas adsorbed on the silica balls that form the opal structure. When heated and cooled, ammonia reversibly exchanges between the opal and polyaniline films causing a very distinct change in the colour of the polyaniline film. This thermochromic effect is also electrically detectable because of the large concomitant change in the resistivity of the polyaniline film that accompanies its colour change.     

Characterization of the electronic excitations in Alq3 using electron energy-loss spectroscopy

We have studied the electronic excitations of tris-(8-hydroxyquinoline)-aluminium(III) (Alq₃) thin films using electron energy-loss spectroscopy in transmission. This allowed us to derive an effective dielectric function of such films in a large energy range. Moreover, an analysis of the momentum dependence of the lowest lying excitation allowed us to gain information on its localization. We show that this excitation does not disperse, i.e., is localized in the condensed phase. In contrast to many other molecular organic semiconductors, its spectral weight does not follow the behavior of a simple dipole-allowed electronic transition.     

Unexpectedly high field-effect mobility of a soluble, low molecular weight oligoquaterthiophene fraction with low polydispersity

Layers made from soluble low molecular weight polythiophene PQT-12 with low polydispersity exhibit a highly ordered structure and charge-carrier mobilities of the order of 10⁻³ cm²/(V s), which we attribute to its proximity to monodispersity. We propose that polydispersity is a decisive factor with regard to structure formation and transport properties of soluble low molecular weight polythiophenes. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.     

MAPLE deposition of methoxy Ge triphenylcorrole thin films

Methoxy Ge Triphenylcorrole [Ge(TPC)OCH₃] has been recently synthesized and deposited as thin film by the Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique. In the last few years, corroles have been the object of an increasing number of studies and MAPLE technique seems to be a very promising deposition method for organic and polymeric films, producing good results for applications in chemical gas sensing layers production. In this work Ge(TPC)OCH₃ thin films were deposited by both spin coating and MAPLE techniques for comparison. The morphology of the films was investigated by Atomic Force Microscopy (AFM), while their optical properties were analyzed by photoluminescence (PL) and UV-vis absorption measurements and were compared with the ones of the starting solution. The film absorption spectrum presented the same peaks with the same relative intensities of that recorded in solution. The luminescence spectra were acquired periodically to evaluate the aging effects and no detectable variations were recorded over a period of 1 month.     

Flame front tracking in turbulent lean premixed flames using?stereo PIV and time-sequenced planar LIF of OH

This paper describes the simultaneous application of time-sequenced laser-induced fluorescence imaging of OH radicals and stereoscopic particle image velocimetry for measurements of the flame front dynamics in lean and premixed LP turbulent flames. The studied flames could be acoustically driven, to simulate phenomena important in LP combustion technologies. In combination with novel image post processing techniques we show how the data obtained can be used to track the flame front contour in a plane defined by the illuminating laser sheets. We consider effects of chemistry and convective fluid motion on the dynamics of the observed displacements and analyse the influence of turbulence and acoustic forcing on the observed contour velocity, a quantity we term as s _d ^2D. We show that this quantity is a valuable and sensitive indicator of flame turbulence interactions, as (a) it is measurable with existing experimental methodologies, and (b) because computational data, e.g. from large eddy simulations, can be post processed in an identical fashion. s _d ^2D is related (to a two-dimensional projection) of the three-dimensional flame displacement speed s _d , but artifacts due to out of plane convective motion of the flame surface and the uncertainty in the angle of the flame surface normal have to be carefully considered. Monte Carlo simulations were performed to estimate such effects for several distributions of flame front angle distributions, and it is shown conclusively that s _d ^2D is a sensitive indicator of a quantity related to s _d in the flames we study. s _d ^2D was shown to increase linearly both with turbulent intensity and with the amplitude of acousting forcing for the range of conditions studied.     

Excitonic diffusion in thin molecular films

The analyses of exciton diffusion in thin molecular films have shown that the diagonal elements of the diffusion tensor, usually called diffusion coefficients, depend on the layer index labeling layers in the direction of disturbed symmetry. The particular analysis was done for a thin film having four layers. In this structure only two layers are occupied by optical excitations. It means that in the four layer film two films occur in which optical excitations can travel. The subfilm contains a boundary layer that noticeably differs from the subfilm with internal layers. If the subfilm contains the boundary layer, the diffusion coefficient of this layer differs from the diffusion coefficient of any internal layer. If the subfilm contains two internal layers, the diffusion coefficient of these layers are equal, expectably from the viewpoint of physics. The exciton diffusion is very low due to the high exciton energies. This work was supported by the Serbian Ministry of Science and Technology: Grant No 1895.     

Quantitative analysis of charge-carrier trapping in organic thin-film transistors from transfer characteristics

A dynamic method for quantifying the amount and mechanism of trapping in organic field effect transistors (OFETs) is proposed. It exploits transfer characteristics acquired upon application of a triangular waveform gate sweep V _G. The analysis of the transfer characteristics at the turning point V _G=−V _max between forward and backward gate sweeps, viz. around the maximum gate voltage V _max applied, provides a differential slope Δm which depends exclusively on trapping. Upon a systematic change of V _max it is possible to extract the initial threshold voltage, equivalent to one of the observables of conventional stress measurements, and assess the mechanism of trapping via the functional dependence on the current. The analysis of the differential logarithmic derivative at the turning point yields the parameters of trapping, as the exponent β and the time scale of trapping τ. In the case of an ultra-thin pentacene OFET we extract β=1 and τ=10²–10³ s, in agreement with an exponential distribution of traps. The analysis of the hysteresis parameter Δm is completely general and explores time scales much shorter than those involved in bias stress measurements, thus avoiding irreversible damage to the device.     

Star-shaped oligofluorene nanostructured blend materials: controlled micro-patterning and physical characteristics

Star-shaped oligofluorene consists of highly-fluorescent macromolecules of considerable interest for organic electronics. Here, we demonstrate controlled micro-patterning of these organic nanostructured molecules by blending them with custom-synthesized photo-curable aliphatic polymer matrices to facilitate solventless inkjet printing. The printed microstructures are spherical with minimum dimensions of 12 μm diameter and 1 μm height when using a cartridge delivering ∼1 pL droplets. We evaluate the physical characteristics of the printed structures. Photoluminescence studies indicate that the blend materials possess similar fluorescence properties to neat materials in solid films or toluene solution. The fluorescence lifetime consists of two components, respectively 0.68±0.01 ns (τ ₁) and 1.23±0.12 ns (τ ₂). This work demonstrates that inkjet printing of such blends provides an attractive method of handling fluorescent nano-scaled molecules for photonic and optoelectronic applications.     

Subthreshold regime in rubrene single-crystal organic transistors

Organic field-effect transistors were fabricated with vapor-grown rubrene single crystals in a staggered top-contact configuration. The devices were electrically characterized by measuring the transfer curves at low drain voltage. In parallel to these measurements, a model is developed to account for the subthreshold regime of the transistors. The model is based on the multiple trapping and thermal release concept, which assumes that charge transport is limited by a single level of shallow traps located close to the transport band edge. It is shown that the threshold voltage no longer establishes at the transition between the depletion and accumulation regimes. Instead, the threshold corresponds to the point at which traps are filled. This results in a subthreshold current that varies linearly with gate voltage. Moreover, the subthreshold current at low drain voltages increases with drain voltage. These finding are in good agreement with the experimental data.