H. Stubb

Molecular field‐effect transistors using conducting polymer Langmuir–Blodgett films

Thin‐film field‐effect transistors (FETs) have been prepared using poly(3‐hexylthiophene)/ arachidic acid and quinquethiophene/arachidic acid Langmuir–Blodgett (LB) films with thicknesses ranging from a monolayer to some ten monolayers. The effect of the number of layers on the mobility and conductivity has been studied. This is to our knowledge the first demonstration of a LB FET utilizing organic semiconductors as the active material.

Langmuir—Blodgett light-emitting diodes of poly(3-hexylthiophene) : electro-optical characteristics related to structure

Poly(3-hexylthiophene) Langmuir-Blodgett (LB) films have been used as the emitting layer in light-emitting diodes (LEDs). In order to develop further the device and increase the quantum efficiency by balancing the number of injected holes and electrons, two different approaches have been taken. Following the more conventional way, LB films of hole- and electron-transporting materials have been used in a separate layer and/or in a blend with the emitting material. Insulating polyaniline LB films at the electrode interfaces have also been used in order to control the operation. A simple model relating the electro-optical characteristics of the LED to its structure has been proposed.

Melt and solution processable poly(3-alkylthiophenes) and their blends

Abstract We report on the properties of melt and solution processable poly(3-alkylthiophenes), P3AT, and on the fabrication of electrically conducting polymer blends consisting of P3AT, specifically poly(3-octylthiophene), POT, and a thermoplastic matrix polymer made using ordinary melt processing techniques. The POT used in this work was synthesized using two different chemical polymerization techniques. Films made from as-synthesized POT powder, either by solution casting or melt processing techniques were found to be easily doped by electron acceptors to conductivities of 20–30 S/cm. Blending of POT with thermoplastics such as ethylenevinylacetate copolymer, EVA, in the molten state yields polymer blends with excellent mechanical properties that might be doped to conductivities exceeding 1 S/cm.

Photoluminescence quenching in a polymer thin‐film field‐effect luministor

We report the observation of photoluminescence quenching in thin films of poly(3‐hexylthiophene) and Langmuir–Blodgett films of poly(3‐hexylthiophene)/arachidic acid by the injection of positive charges in the polymer. Charge injection was made in a polymer field effect transistor, used in what we name the luministor mode. The quenching is discussed in terms of polarons/bipolarons acting as recombination centra for the excitons, suppressing the photoluminescence. The inverse phenomenon, luminescence enhancement by depletion of charges, has also been achieved.

Synthesis and properties of FeCl4-doped polythiophene

Abstract Electrochemically synthesized polythiophene films containing BF 4 − -anions, PT-BF 4 , have been carefully reduced and re-oxidized with FeCl 4 -anions to obtain highly conducting PT-FeCl 4 -films. The maximum conductivity we have observed for this complex is 50 Scm −1 which corresponds to a maximum dopant concentration of approximately 26 mol-%. Thermoelectric power (TEP) measurements of lightly and heavily doped PT-FeCl 4 show that the positive TEP decreases drastically from 614 μV/K to 10.5 μV/K when the conductivity increases from 1.1 10 −5 to 10.1 Scm −1 . In heavily doped samples TEP is slightly overlinear, a phenomenon which is explained by a model based on conducting strands separated by thin potential barriers. PT-FeCl 4 -films exhibit promising stability properties, the conductivity remaining in the metallic regime for at least 16 months under ambient conditions.

Electrical conductivity of synthetic DOPA-melanin polymer for different hydration states and temperatures

The dependence of the d.c. conductivity on the hydration and temperature (293-343 K) for synthetic DOPA-melanin polymer is presented. The hydration state of the melanin has been changed by varying the humidity conditions around the sample. It has been shown, that in the range of relative humidity values (0-100%), changes in the hydration state of melanin have predominant influence on electrical conductivity (10(-13)-10(-5)S cm-1) in comparison to temperature. The influence of the two forms of water on the conductivity--the first form adsorbed mainly on the melanin surface and easily removed by drying, and the second one incorporated into the inner structure of the polymer, has been investigated. The temperature dependence of the conductivity in vacuum (0.8 and 0.04 mb) and thermal activation energy values (0.49-0.76 eV) for cooling and heating curves have been determined. The relationship between thermal activation energy and preexponential factor sigma 0 (compensation effect) and possible charge transport mechanisms are discussed.

THERMOCHROMISM AND OPTICAL ABSORPTION IN LANGMUIR-BLODGETT FILMS OF ALKYL-SUBSTITUTED POLYTHIOPHENES

Thermochromism and optical absorption in mono‐ and multilayers of Langmuir–Blodgett films of poly(3‐alkylthiophenes), poly(3‐octyl‐2,2’‐bithiophene), and poly(3’‐octyl‐2,2’;5’,2‘‐terthiophene) were studied. In sparsely alkylated polythiophenes the magnitude of the thermochromic shift was smaller than in poly(3‐alkylthiophenes) and roughly proportional to the sidechain concentration. Results of Valence Effective Hamiltonian calculations were compared with the experimental results of thermochromism. A vibronic structure was found in the absorption spectra of Langmuir–Blodgett films at room temperature. The vibronic splitting in poly(3‐hexylthiophene) was approximately 0.18 eV as previously has been observed in poly(3‐alkylthiophenes) but in poly(3’‐octyl‐2,2’;5’,2‘‐terthiophene) it was 0.20–0.25 eV. The vibronic peaks stay approximately at constant energies and vanish at elevated temperatures.

Aligned polymer chain field effect transistors

Abstract The anisotropy of the charge carrier mobility in stretch oriented thin films of undoped poly(3-octylthiophene) has been investigated. The mobility has been measured using a field effect transistor onto which the polymer film is coated. The anisotropy increases with increasing orientation, reaching values of 4. Causes of this anisotropy, including residual dopant distributions, conjugation length anisotropy, intrinsic anisotropy of transport on and between chains, as well as crystallinity and monodisperse conjugation lengths are discussed.

Operating principle of polymer insulator organic thin-film transistors exposed to moisture

The operating principle of a hygroscopic insulator field-effect transistor has been investigated. The semiconductor poly(3-hexylthiophene) shows an increased conductivity after applying the poly(vinylphenol) (PVP) insulator, which is attributed to an interfacial dipole layer caused by the permanent dipole moment in PVP. The effects of solvents on the transistor device indicate that low molecular weight protic solvents such as water, methanol, and ethanol affect the I-V characteristics significantly, in contrast to 1-propanol and aprotic solvents such as acetonitrile. The presence of a protic solvent inside the device insulator gives rise to an ion-assisted modulation of the transistor source-drain current.

Field-effect conduction in polyalkylthiophenes

Abstract Semiconducting poly(3-alkylthiophenes) with butyl, hexyl, octyl and decyl side chains have been electrically characterized using a thin film field-effect transistor structure. The mobility of the p-type charge carriers depends on the side chain length. The mobility is also affected by additional impurities and defects.