P. Kuivalainen

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.

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.

Conductive Langmuir-Blodgett multilayer films of poly(3-octylthiophene)

Abstract Multilayer films of poly(3-octylthiophene)-arachidic acid were prepared and characterized using ellipsometry, microscopy, UV-visible absorption and capacitance measurements. When doped with iodine or SbCl 5 gas the conductivity of the films increased by a factor of 10 5 up to 2 × 10 −4 S cm −1 . Removal of the dopant from the films was quite rapid in dynamic vacuum and in air.

Spin-Dependent Transport in Heavily Mn-Doped GaAs

The theory of magnetotransport in heavily Mn-doped GaAs, a p-type ferromagnetic semiconductor, is developed in the weak coupling limit by calculating the temperature and magnetic field dependences of the spin disorder scattering-limited hole mobility, The theory is based on the exchange interaction between the itinerant holes and the localized 3d spins of the Mn ions, and it is developed by using Zubarevs double-time Greens functions. The relaxation time of the charge carriers is calculated from the imaginary part of the self-energy appearing in the derived hole propagator. Adding a contribution from impurity scattering the total hole mobility is estimated. Comparison to the experimental resistivity data shows that a good agreement with the measured results is obtained only if the short-range order and the consequent T dependence of the critical scattering are taken into account in the paramagnetic region. The measured resistivity peak near Curie temperature is broader than the calculated one, which together with the estimated large exchange parameter J pd = 2.3 eV calls for an extension of the present theory to the intermediate or strong coupling cases.

Thermal stability of conductivity in electorn irradiated blends of FeCl3-doped poly(3-octylthiophene) and ethylenevinylacetate

Abstract The stability of conductivity in FeCl 3 -doped polymer blends of poly(3-octylthiophene) and ethylenevinylacetate has been studied over a wide temperature range (10–410 K). In contrast to the large conformational changes in undoped materials the dc conductivity behaviour was found to be practically reversible and stable up to the melting point of ethylenevinylacetate (350 K). A spontaneous conductivity increase above 350 K was interpreted to be related to additional doping by dopant molecules trapped inside a network of partially crosslinked poly(3-octylthiophene). Crosslinking of matrix polymer by electron irradiation did not improve the stability of conductivity. The electrical properties of heavily irradiated samples indicated an increase in the average size of conducting islands which was related to an irradiation-induced crosslinking of poly(3-octylthiophene) aggregates.

Electrical properties of FeCl3-doped polyethylenevinylacetate-polyoctylthiophene blends

Abstract We have studied the electrical transport properties of a new polymer blend consisting of polyoctylthiophene (POT) melt processed into a polyethylenevinyl-acetate (EVA) matrix. The EVAPOT blend doped with FeCl 3 has a room temperature dc conductivity σ dc (300K) varying from 10 −8 Ω −1 cm −1 to 1Ω −1 cm −1 with increasing doping. The temperature dependence of σ dc fits σ dc (T) = σ 0 exp [-( T 0 / T ) 1 2 ] , especially in highly conducting samples where the conduction can be described by the granular metal model. This conclusion is supported by the small metallic thermoelectric power which varies linearly with temperature.

Electrical properties of ion implanted polythiophene

Abstract Polythiophene as a free standing film has been ion implanted with 25 keV F + ions. Rutherford backscattering measurements showed that the surface of the film was modified to a depth of 0.4 μm. The dc-conductivity increase was more than 7 orders of magnitude, the highest value being 1.3·10 −2 Ω −1 cm −1 for a dose of 2·10 17 F + /cm 2 . The increased conductivity, which was stable with time was accompanied by a drastic decrease in thermoelectric power which showed metallic-like behaviour. The temperature dependence of conductivity and thermoelectric power points to variable range hopping between localized states caused by the implantation. Electron spin resonance data support the concept of localized charge carriers.

Lcao-Gáspár-Kohn-Sham energy bands of trans-polyacetylene-chain

Abstract The band structure of the dimerised trans-polyacetylene-chain ( d CC = 1.35 A , d CC = 1.46 A , ∡ CCC = 120° , d CH = 1.09 A ) is calculated with the (parameter-free, non-self-consistent) linear-combination-of-atomic-orbitals method using the Gaspar-Kohn-Sham (GKS) potential for the exchange and correlation. The calculated dimerisation gap of 1.6 eV agrees closely with experiment and also with other GKS calculations when the degree of dimerisation is taken into account. The calculated density of states agrees closely with the X-ray photoemission spectrum of Brundle.

Free-carrier absorption of radiation in magnetic semiconductors due to spin-disorder scattering

The free-carrier absorption in the case of spin-disorder scattering is calculated using the density matrix method developed by Kohn and Luttinger (1957). A quantum mechanical equation is derived and then solved by iterations to lowest order. The absorption coefficient alpha at various frequencies is calculated for non-degenerate and degenerate ferromagnetic semiconductors. alpha shows anomalous behaviour at the frequency corresponding to the splitting of the conduction band into spin-up and spin-down sub-bands.