Ivo A. Hümmelgen

Electrodeposited p-type magnetic metal-base transistor

In this work the development of a magnetic metal-base transistor that operates by hole transport is reported. The transistor is constructed using p-type silicon as the collector, Co as the base, and Cu2O as the emitter. Both base and emitter are deposited using electrochemical procedures. The transistor shows a magnetic-field-dependent current gain and a magnetocurrent of ∼40% observed for a low emitter current value of 2 mA.

The use of tin oxide thin films as a transparent electrode in PPV based light-emitting diodes

Tin oxide (TO) thin films, nominally undoped, have been used as electrodes in poly(p-phenylene vinylene) (PPV) based organic electroluminescent devices. The evolution of the crystallinity and the electrical resistance of TO films submitted to the PPV thermal conversion conditions, have been investigated. It has been found that the electrical resistance is decreased whereas the crystallinity of the film is increased. It is shown in this work, that the photoluminescence of PPV converted on top of TO substrates is not as quenched as it is when converted on top of indium-tin oxide (ITO) substrates. The quantum efficiency of light-emitting diode is 0.07% at 17 V forward bias. It is also shown that the work function of TO films is very stable to different cleaning procedures, in contrast with previous results obtained for ITO films.

Charge transport in a blue-emitting alternating block copolymer with a small spacer to conjugated segment length ratio

We analyze current versus voltage data obtained using single carrier injection in several metal/polymer/metal sandwich structures. The polymer used in each case is a soluble blue-emitting alternating block copolymer. Our experimental results demonstrate that the electron transport is space-charge limited by the high density of traps having an exponential energy distribution (temperature dependent characteristic energy) in the copolymer. The electron mobility of 8×10−10 cm2/V s is directly determined using space-charge-limited current analytical expressions. Hole transport is also space-charge limited, with a mobility of 2×10−6 cm2/V s. A hole trap with energy 0.17 eV is observed. We compare these results with those obtained for related block copolymers with different spacer and conjugated segment lengths and discuss the influence of spacer length and conjugated segment length on the charge transport properties.

Temperature dependent tunnelling current at metal/polymer interfaces—potential barrier height determination

It has been demonstrated that the expression for the temperature dependent tunneling current through a triangular potential barrier at metal/polymer interfaces fits experimental data including the deviation from a straight line observed in Fowler–Nordheim plots. The fitting parameters can be used to calculate the potential barrier height at the metal/polymer interface, which is of crucial importance for the control of charge injection in organic light emitting diodes. The tunneling current density expression reproduces the observed dependence of the current with temperature.

Low-Voltage Poly(3-Hexylthiophene)/Poly(Vinyl Alcohol) Field-Effect Transistor and Inverter

We report on bottom-gate top-contact organic field-effect transistors that use regioregular poly(3-hexylthiophene) (rr-P3HT) as a channel semiconductor and poly(vinyl alcohol) as a gate insulator. We determine that the field-effect mobility of the rr-P3HT is 0.08 cm² ·V^-1·s^-1, the threshold voltage is -1.1 V, and the on/off current ratio is 10³, demonstrating the transistor operation in an inverter.

Preparation and characterization of transparent conducting tin oxide thin film electrodes by chemical vapour deposition from reactive thermal evaporation of SnCl2

A chemical vapour deposition technique has been developed for highly transparent conducting tin oxide thin films from the reactive thermal evaporation of SnCl2. We have demonstrated that good quality tin oxide thin films for electrode application in electronic devices can be produced by such a technologically simple and inexpensive way. Best results concerning optical transmittance, electrical conductivity and crystallinity (SnO2) were obtained in this way at substrate temperatures in the range of 500–550°C.

Influence of film thickness on charge transport of electrodeposited polypyrrole thin films

We report the investigation of charge transport properties of thin electrodeposited polypyrrole (PPy) films. The charge transport in these films is limited by thermionic injection and an expression for thermionic injection into low mobility materials is used to evaluate the energy barrier between electrodes (tin-oxide, Al and Ni) and PPy. Using the same expression, we estimate the positive charge carrier mobility, demonstrating that its value is higher for thinner films.

Charge injection and transport in electrochemical films of poly(3-hexylthiophene)

We have produced and characterized poly(3-hexylthiophene) (PHT) thin films by electrochemical deposition onto tin–oxide (TO) or gold covered glass substrates. PHT film thickness and roughness depend on deposition charge density and are also dependent on substrate material. From electrical measurements we determine an electrical conductivity of 9×10−5 S cm−1 when Ni and Au electrodes are used. An energy barrier of 0.8 eV for positive charge carrier injection into PHT at Al/PHT and TO/PHT interfaces was determined. We estimate the mobility of positive charge carriers in electrochemically deposited PHT as being 1×10−4 cm2 V−1 s−1. An energy diagram of PHT is proposed.We have produced and characterized poly(3-hexylthiophene) (PHT) thin films by electrochemical deposition onto tin–oxide (TO) or gold covered glass substrates. PHT film thickness and roughness depend on deposition charge density and are also dependent on substrate material. From electrical measurements we determine an electrical conductivity of 9×10−5 S cm−1 when Ni and Au electrodes are used. An energy barrier of 0.8 eV for positive charge carrier injection into PHT at Al/PHT and TO/PHT interfaces was determined. We estimate the mobility of positive charge carriers in electrochemically deposited PHT as being 1×10−4 cm2 V−1 s−1. An energy diagram of PHT is proposed.

Very high magnetocurrent in tris-(8-hydroxyquinoline) aluminum-based bipolar charge injection devices

Bipolar devices constructed using 60nm thick tris-(8-hydroxyquinoline) aluminum (Alq3) thin films sandwiched between a 200nm thick sulfonated polyaniline hole-injection electrode and Al∕Ca electron-injection electrode show very high (up to 103%) magnetocurrent values. True-hole-only and true-electron-only Alq3-based devices that make use of Si as charge carrier collecting electrode, and Al∕Ca as electron injecting electrode or Au as hole injecting electrode, are also proposed, prepared, and characterized. In these true-single-carrier devices magnetocurrent is not observed. This result provides strong evidence that bipolar injection is a necessary condition for very high magnetocurrent observation in Alq3.

A novel soluble poly(fluorenylenevinylene) conjugated polymer: synthesis, characterization and application to optoelectronic devices

We report the synthesis and characterization of poly(9,9-dioctyl-1,4-fluorenylenevinylene), PDO14FV. This polymer is a novel poly(arylenevinylene) with fluorene units and long chain substituents that ensure solubility in organic solvents. PDO14FV presents green photoluminescence and yellow–green electroluminescence. The estimated ionization potential of PDO14FV is 5.3 eV and its estimated electroaffinity 2.5 eV. Organic light-emitting diodes based on PDO14FV were constructed, showing electroluminescent intensity onset at ca. 6 V.