Schottky barriers to semiconducting polymers

Schottky barrier diodes are made from virtually all semiconducting polymers. Application of Schottky barriers on the development of electronic devices built from semiconducting polymers prompted this research. The article investigated the dc and ac admittance of Schottky barrier which occur at the interface between aluminum and poly(3-methyl thiophene) made ready by electropolymerisation. The experiment revealed that the interfacial layers occurring in polymer Schottky barriers is significant in the response of the controlling device.

Electrical characterization of semiconducting polymers

Schottky diodes resulting from an intimate contact of aluminum on electrodeposited poly(3-methylthiopene) were studied by admittance spectroscopy, capacitance-voltage measurements and voltaic and optically-induced current and capacitance transients. The loss tangents show the existence of interface states that can be removed by vacuum annealing. Furthermore, the C-V curves contradict the idea of movement of the dopant ions.

Schottky barrier diodes from semiconducting polymers

Schottky barrier diodes based on Al/poly(3-methylthiophene)/Au have been fabricated and their electrical behaviour investigated. I-V characteristics revealed a dependence on the fabrication conditions, specifically on the time under vacuum prior to evaporation of the rectifying contact and post-metal annealing at elevated temperature. The available evidence is consistent with the formation of a thin insulating layer between the metal and the polymer following these procedures. Long periods under vacuum prior to deposition of the aluminium electrode reduced the likelihood of such a layer forming. Capacitance-voltage plots of the devices were stable to voltage cycling, so long as the forward voltage did not exceed similar to 1 V. Above this a small degree of hysteresis was observed, which is attributed to the filling/emptying of interface states or traps in the polymer.

Schottky barriers to semiconducting polymers

Schottky barrier diodes are made from virtually all semiconducting polymers. Application of Schottky barriers on the development of electronic devices built from semiconducting polymers prompted this research. The article investigated the dc and ac admittance of Schottky barrier which occur at the interface between aluminum and poly(3-methyl thiophene) made ready by electropolymerisation. The experiment revealed that the interfacial layers occurring in polymer Schottky barriers is significant in the response of the controlling device.

Electrical characterization of semiconducting polymers

Schottky diodes resulting from an intimate contact of aluminum on electrodeposited poly(3-methylthiopene) were studied by admittance spectroscopy, capacitance-voltage measurements and voltaic and optically-induced current and capacitance transients. The loss tangents show the existence of interface states that can be removed by vacuum annealing. Furthermore, the C-V curves contradict the idea of movement of the dopant ions.

Schottky barrier diodes from semiconducting polymers

Schottky barrier diodes based on Al/poly(3-methylthiophene)/Au have been fabricated and their electrical behaviour investigated. I-V characteristics revealed a dependence on the fabrication conditions, specifically on the time under vacuum prior to evaporation of the rectifying contact and post-metal annealing at elevated temperature. The available evidence is consistent with the formation of a thin insulating layer between the metal and the polymer following these procedures. Long periods under vacuum prior to deposition of the aluminium electrode reduced the likelihood of such a layer forming. Capacitance-voltage plots of the devices were stable to voltage cycling, so long as the forward voltage did not exceed similar to 1 V. Above this a small degree of hysteresis was observed, which is attributed to the filling/emptying of interface states or traps in the polymer.

Charge accumulation and transport in degenerately doped semiconducting polymers with mixed ionic and electronic conductivity

This thesis is part of the fields of Material Physics and Organic Electronics and aims to determine the charge carrier density and mobility in the hydrated conducting polymer–polyelectrolyte blend PEDOT:PSS. This kind of material combines electronic semiconductor functionality with selective ionic transport, biocompatibility and electrochemical stability in water. This advantageous material properties combination makes PEDOT:PSS a unique material to build organic electrochemical transistors (OECTs), which have relevant application as amplifying transducers for bioelectronic signals. In order to measure charge carrier density and mobility, an innovative 4-wire, contact independent characterization technique was introduced, the electrolyte-gated van der Pauw (EgVDP) method, which was combined with electrochemical impedance spectroscopy. The technique was applied to macroscopic thin film samples and micro-structured PEDOT:PSS thin film devices fabricated using photolithography. The EgVDP method revealed to be effective for the measurements of holes’ mobility in hydrated PEDOT:PSS thin films, which resulted to be <μ>=(0.67±0.02) cm^2/(V*s). By comparing this result with 2-point-probe measurements, we found that contact resistance effects led to a mobility overestimation in the latter. Ion accumulation at the drain contact creates a gate-dependent potential barrier and is discussed as a probable reason for the overestimation in 2-point-probe measurements. The measured charge transport properties of PEDOT:PSS were analyzed in the framework of an extended drift-diffusion model. The extended model fits well also to the non-linear response in the transport characterization and results suggest a Gaussian DOS for PEDOT:PSS. The PEDOT:PSS-electrolyte interface capacitance resulted to be voltage-independent, confirming the hypothesis of its morphological origin, related to the separation between the electronic (PEDOT) and ionic (PSS) phases in the blend.

Interface morphology snapshots of vertically segregated thin films of semiconducting polymer/polystyrene blends

We present atomic force microscopic images of the interphase morphology of vertically segregated thin films spin coated from two-component mixtures of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) and polystyrene (PS). We investigate the mechanism leading to the formation of wetting layers and lateral structures during spin coating using different PS molecular weights, solvents and blend compositions. Spinodal decomposition competes with the formation of surface enrichment layers. The spinodal wavelength as a function of PS molecular weight follows a power-law similar to bulk-like spinodal decomposition. Our experimental results indicate that length scales of interface topographical features can be adjusted from the nanometer to micrometer range. The importance of controlled arrangement of semiconducting polymers in thin film geometries for organic optoelectronic device applications is discussed. (c) 2007 Elsevier Ltd. All rights reserved.

Effet de la microstructure sur les propriétés excitoniques des polymères semi-conducteurs semi-cristallins

Les polymères semi-conducteurs semicristallins sont utilisés au sein de diodes électroluminescentes, transistors ou dispositifs photovoltaïques organiques. Ces matériaux peuvent être traités à partir de solutions ou directement à partir de leur état solide et forment des agrégats moléculaires dont la morphologie dicte en grande partie leurs propriétés optoélectroniques. Le poly(3-hexylthiophène) est un des polymères semi-conducteurs les plus étudiés. Lorsque le poids moléculaire (Mw) des chaînes est inférieur à 50 kg/mol, la microstructure est polycristalline et composée de chaînes formant des empilements-π. Lorsque Mw>50 kg/mol, la morphologie est semicristalline et composée de domaines cristallins imbriquées dans une matrice de chaînes amorphes. À partir de techniques de spectroscopie en continu et ultrarapide et appuyé de modèles théoriques, nous démontrons que la cohérence spatiale des excitons dans ce matériau est légèrement anisotrope et dépend de Mw. Ceci nous permet d’approfondir la compréhension de la relation intime entre le couplage inter et intramoléculaire sur la forme spectrale en absorption et photoluminescence. De plus, nous démontrons que les excitations photogénérées directement aux interfaces entre les domaines cristallins et les régions amorphes génèrent des paires de polarons liés qui se recombinent par effet tunnel sur des échelles de temps supérieures à 10ns. Le taux de photoluminescence à long temps de vie provenant de ces paires de charges dépend aussi de Mw et varie entre ∼10% et ∼40% pour les faibles et hauts poids moléculaires respectivement. Nous fournissons un modèle permettant d’expliquer le processus de photogénération des paires de polarons et nous élucidons le rôle de la microstructure sur la dynamique de séparation et recombinaison de ces espèces.

Theoretical Analysis of Aggregation in Block-Copolymer Films: The Optical Signature

We focus this work on the theoretical investigation of the block-copolymer poly [oxyoctyleneoxy-(2,6-dimethoxy-1,4phenylene-1,2-ethinylene-phenanthrene-2,4diyl) named as LaPPS19, recently proposed for optoelectronic applications. We used for that a variety of methods, from molecular mechanics to quantum semiempirical techniques (AMI, ZINDO/S-CIS). Our results show that as expected isolated LaPPS19 chains present relevant electron localization over the phenanthrene group. We found, however, that LaPPS19 could assemble in a pi-stacked form, leading to impressive interchain interaction; the stacking induces electronic delocalization between neighbor chains and introduces new states below the phenanthrene-related absorption; these results allowed us to associate the red-shift of the absorption edge, seen in the experimental results, to spontaneous pi-stack aggregation of the chains. (C) 2009 Wiley Periodicals, Inc. Int J Quantum Chem 110: 885-892, 2010

Exploiting the versatility of taste sensors based on impedance spectroscopy

The versatility of sensor arrays made from nanostructured Langmuir-Blodgett (LB) and layer-by-layer (LBL) films is demonstrated in two ways. First, different combinations of sensing units are employed to distinguish the basic tastes, viz. sweet, sour, bitter, and salty tastes, produced, respectively, by small concentrations (down to 0.01 g/mol) of sucrose, HCl, quinine, and NaCl solutions. The sensing units are comprised of LB and/or LBL films from semiconducting polymers, a ruthenium complex, and sulfonated lignin. Then, sensor arrays were used to identify wines from different sources, with the high distinguishing ability being demonstrated in principal component analysis (PCA) plots. Particularly important was the fact that the sensing ability does not depend on specific interactions between analytes and the film materials, but a judicious choice of materials is, nevertheless, required for the materials to respond differently to a given sample. It is also shown that the interaction with the analyte may affect the morphology of the nanostructured films, as indicated with scanning electron microscopy. For instance, in wine analysis these changes are not irreversible and the original film morphology is retrieved if the sensing unit is washed with copious amounts of water, thus allowing the sensor unit to be reused.

Insulator-to-metal transition driven by clustering of solitons in polyacetylene

It's believed that the simple Su-Schrieffer-Heeger Hamiltonian can not predict the insulator to metal transition of transpolyacetylene (t-PA). The soliton lattice configuration at a doping level y=6% still has a semiconductor gap. Disordered distributions of solitons close the gap, but the electronic states around the Fermi energy are localized. However, within the same framework, it is possible to show that a cluster of solitons can produce dramatic changes in the electronic structure, allowing an insulator-to-metal transition.

Caracterização de diodos tipo Schottky fabricados com poli-o-metoxianilina em diversas temperaturas

Technological advances achieved during the twentieth century strongly boosted the scientific research in the area of condensed matter physics, especially in the study and development of new semiconductor materials. In the segment, the development of semiconducting polymers for application in electronic devices promotes the field of organic electronics...(Complete abstract click electronic access below)

Synergy between polyaniline and OMt clay mineral in Langmuir-Blodgett films for the simultaneous detection of traces of metal ions

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Molecular origin of charge traps in polyfluorene-based semiconductors

The comprehensive control of morphology and structure is of extreme importance in semiconducting polymers when used as active layers in optoelectronic devices. In the work reported here, a systematic investigation of the structural and dynamical properties of poly(9,9-di-n-octyl-fluorene-alt-benzothiadiazole), known as F8BT, and their correlation with electrical properties is presented when the material is used as an active layer in optoelectronic devices. By means of X-ray diffraction, one observes that in thick layer films (thickness of about 4 μm) grown by drop-cast deposition, a solvent induced crystalline phase exists which evolves to a stable phase as the temperature is raised. This was not observed in thin films (thickness of about 250 nm) prepared by spin-coating within the investigated temperature range. By modeling the current-voltages characteristics of both thick and thin film devices, important information on the influence of crystallization on the trapping states could be drawn. Furthermore, the temperature dependence of the charge carrier mobility was found to be closely related to that of the molecular relaxation processes. The understanding of the nature of such molecular relaxations, measured by solid-state nuclear magnetic resonance methods, allows one to understand the importance of molecular relaxations and microstructure changes on the trap states of the system.