J. Paloheimo

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.

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.

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.

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.

Polymer Field-Effect Transistors for Transport Property Studies

Thin film field-effect transistors have been prepared of poly(3-alkyl-thiophenes) by using spin-coating techniques. The devices are used in the determination of the charge carrier mobility μ, dc conductivity σ, and the carrier concentration p0. Poly(3-hexylthiophene) is characterized in a wide temperature range T = 130–430 k, and possible transport mechanisms are discussed.

Polymeric light-emitting diodes from molecularly thin poly(3-hexylthiophene) Langmuir-Blodgett films

Light‐emitting diodes based on molecularly thin films of a conjugated polymer have been fabricated. A few Langmuir–Blodgett layers of poly(3‐hexylthiophene) have been used as the active emitting material of the devices. Even the thinnest devices with only 3 monolayers, each having a thickness of ≊3 nm, yield the same luminance as thicker ones. The emission results from excitons in the polymer chains, and has a spectrum similar to that of photoluminescence, with a maximum at about 2.0 eV.

Conductivity, thermoelectric power and field-effect mobility in self-assembled films of polyanilines and oligoanilines

Abstract Thin conducting films of polyanilines and oligoanilines were deposited by using a layer-by-layer self-assembly process. Conductivity, thermoelectric power and field-effect measurements were performed on the films. The highest conductivities were around 1 S cm −1 at room temperature. The conductivity was studied as a function of temperature and was found to obey the T − 1 2 law of variable-range hopping. The thermoelectric power of the films was initially n-type in air at room temperature, but changed its sign to p-type under vacuum or with time. A linear temperature dependence was observed and related to the contribution from metallic polymer chain segments. The field effect, measured using thin film transistor structures, was always n-type and the field-effect mobilities about 10 −6 -10 −3 cm 2 V −1 S −1 . The Coulomb gap model gives a good description of the system still lacking an expression for the field effect.

Molecular light‐emitting diodes using quinquethiophene Langmuir–Blodgett films

We report extended studies on electroluminescence from molecularly thin semiconducting Langmuir–Blodgett films of quinquethiophene. The effect of the thickness of active layer and cathode electrode material has been investigated. From all these devices, greenish electroluminescence (visible in a dark room) has been observed with quantum efficiencies of the order of 10−3%. Even five or nine layers of Langmuir–Blodgett films yield the same luminance as thicker films. The electroluminescence spectrum showed a profile identical to the photo‐ luminescence spectrum.