Charge transfer effects in organic field-effect transistors containing a donor/acceptor heterojunction

Heterojunction organic field-effect transistors (OTFTs) based on copper phthalocyanine (CuPc) as p-type and on copper hexadecafluorophthalocyanine (F₁₆CuPc) or 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) as n-types were fabricated and studied. They were shown to operate in unipolar mode where hole current was recorded with values approaching those of CuPc-based single-layer device. However, due to a charge transfer phenomenon between CuPc and F₁₆CuPc, the occurrence of a build-in electrical field at the CuPc/F₁₆CuPc interface is shown to influence the transport properties at low drain-source voltages and to induce a non-ideal ohmic behavior in OTFT characteristics. This effect is not observed in the case of CuPc/TCNQ because of a larger energy difference between HOMO and LUMO energy levels of CuPc and TCNQ, respectively.     

Depletion- and ambipolar-mode field-effect transistors based on the organic heterojunction composed of pentacene and hexadecafluorophtholocyaninatocopper

We investigated heterojunction organic field-effect transistors (OFETs) using pentacene and hexadecafluorophtholocyaninatocopper (F₁₆CuPc) as double active layers. Two operation modes including depletion- and ambipolar-type were observed depending on the deposition order of two organic films. Depletion-mode was firstly observed from that devices with pentacene as the bottom layer and F₁₆CuPc as the top layer, which was attributed to dipole effects originated from the pentacene/F₁₆CuPc interface. Then improved device performances were obtained with mobility from 0.87 to 1.06 cm²/V s, and threshold voltage shifted from ?20 to +25 V as compared with conventional pentacene-based devices. Furthermore, the heterojunction OFETs exhibited typical ambipolar transport when alternating the deposition order of two films, which exhibited ambipolar mobilities with 0.06 cm²/V s for electron and 0.0025 cm²/V s for hole, respectively. All results implied the utilization of heterojunction can effectively improve the device performances of OFET, and operation mode strongly on the deposition order of two films.     

The application of SnS nanoparticles to bulk heterojunction solar cells

Tin mono-sulphide (SnS) nanoparticles were synthesized by a facile method. Reactions producing narrow size distribution SnS nanoparticles with the diameter of 5.0–10 nm were carried out in an ethylene glycol solution at 150 °C for 24 h. Bulk heterojunction solar cells with the structure of indium tin oxide (ITO)/polyethylenedioxythiophene:polystyrenesulphonate (PEDOT:PSS)/SnS:polymer/Al were fabricated by blending the nanoparticles with a conjugated polymer to form the active layer for the first time. Current density–voltage characterization of the devices showed that due to the addition of SnS nanoparticles to the polymer film, the device performance can be dramatically improved, compared with that of the pristine polymer solar cells.     

Electrochemically prepared heterojunction of polyaniline on n-SrTiO3

A p-type polyaniline (PAn) film was electrochemically polymerized directly on an n-type strontium titanate (SrTiO₃) ceramic and thus an n-SrTiO₃/p-PAn heterojunction was fabricated. It is demonstrated experimentally that the heterojunction shows rectifying characteristics. The voltage-sensitive varistor coefficient and the ideality factor of the diode were calculated to be 3.76 and 3.68, respectively. Surface treatment of SrTiO₃ ceramics, interfacial contact of the composite and some other factors affecting the current-voltage (I-V) characteristic of the n-SrTiO₃/p-PAn heterojunction were studied.     

Characterization and performance of graphene-PbSe thin film heterojunction

An infrared detector with high responsivity based on graphene-PbSe thin film heterojunction was reported.High-quality PbSe thin film and graphene were prepared ...     

The effect of a dilution agent on the dipping exothermic reaction process for fabricating a high-volume TiC-reinforced aluminum composite

A synthesis of in situ formed TiC/Al composites utilizing the reaction between molten aluminum and preforms consisting of Ti, C, Al and TiC powder was studied. The addition of TiC powder as a dilution agent controlled the explosive exothermic reaction and played a key role in the homogeneous distribution of reinforcement.     

The ultraviolet-ozone effects on organic thin-film transistors with double polymeric dielectric layers

Organic thin-film transistors (OTFTs) were fabricated based on the double polymeric insulators consisting of polyvinyl alcohol (PVA) and polystyrene (PS). The using of double dielectric layers exhibited an improved device performance comparing with the conventional devices with single dielectric layer. By exposing the polymer insulating layer (PS) under ultraviolet (UV) light for different times (0, 5, 10 and 15 min), the surface properties of the PS film has been further optimized for the growth of pentacene. The grain size and crystallization of pentacene film grown on PS are apparently improved as the increase of surface energy of PS. As a result, a field-effect mobility of 1.38 cm² V^?1 S^?1 and an on/off current ratio of 10⁴ had been obtained by UV treatment for 10 min. All results indicate the surface energy of dielectric layer is a crucial factor to influence the devices performance that can be well tuned by UV treatment. The polymeric materials pair consisting of PS and PVA is a promised candidate as the dielectric layers for high-performance OTFTs.     

Fabrication and characterization of polyaniline/porous silicon heterojunction

Heterojunction between polyaniline (PANI) and porous silicon (PS) was fabricated by making a layer of PANI on PS, using spin coating method. PS was fabricated by electrochemical etching process. PS was characterized by photoluminescence (PL), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) while the PANI was characterized by FTIR and absorption (UV–VIS) spectroscopy. Current–voltage and capacitance–voltage measurements were done to determine the electrical properties of the heterojunction structure. The ideality factor of the heterojunction was found to be 4.2, which was considered high due to large defect density at the interface. Built-in potential was measured by both I–V and C–V and was found to be Φ_b(I–V) = 0.41 V and Φ_b(C–V) = 0.28 V respectively. The discrepancy in the values of the built-in potential was discussed. Band discontinuity in conduction band and valence band were found to be 0.65 and 1.27 eV respectively. Solar response of the heterojunction was also observed at AM (air mass) 1.0 and it showed a promising behavior as a photovoltaic device.     

Electrical and photovoltaic properties of FeTPPCl/p-Si heterojunction

Hybrid heterojunction cell based on thermally evaporated 5,10,15,20-tetraphenyl-21H, 23H-porphine iron (III) chloride (FeTPPCl) as the organic semiconductor and p-Si wafer as the inorganic semiconductor have been investigated. This device showed rectification behaviour like diode. The conduction mechanisms and the diode parameters have been studied using current–voltage (I–V) characteristics in the temperature range (298–373 K) and capacitance–voltage (C–V) characteristics at room temperature. This cell exhibited photovoltaic characteristics with a short-circuit current (I_sc) of 2.8 mA, an open-circuit voltage (V_oc) of 0.475 V, a fill factor FF = 32%.     

N-doped C-coated MoO2/ZnIn2S4 heterojunction for efficient photocatalytic hydrogen production

Designing a heterojunction photocatalyst to improve the separation efficiency of photogenerated electrons and holes is of great significance to improve the hydrogen production efficiency.In this work,we report a rational design to grow ZnIn₂S₄ on Mo-MOF-derived N-doped C-coated MoO₂(MOZIS),and it has excellent photocatalytic hydrogen production with triethanolamine(TEOA) as sacrificial agent.N-doped C improves the electron transport efficiency between MoO₂     

The application of soluble and regioregular poly(3-hexylthiophene) for organic thin-film transistors

We have investigated the effect of solvent medium, thermal annealing and dedoping reducing agent such as hydrazine on the performance of poly(3-hexylthiophene) (P3HT) device. The performance of P3HT device varied significantly from different solvents, among them, p-xylene proved to be the best solvent medium, and also hydrazine as dedoping agent made progress in the mobilities by one or two orders of magnitude. Especially, in our experiments, solution preparing, deposition and device measurements were all performed in the air for the application of P3HT without further purification to the ink-jet printing, spin coating or solution casting process for large area and flexible organic thin-film transistors (OTFTs).     

The effect of copper hexadecafluorophthalocyanine (F16CuPc) inter-layer on pentacene thin-film transistors

We report the impact of a copper hexadecafluorophthalocyanine (F₁₆CuPc) layer inserted between the electrode (Ag and Au) and the pentacene active layer in organic thin-film transistors (OTFTs). The electrical characteristics of the transistor are highly improved with the insertion of F₁₆CuPc in devices having either an Ag or an Au electrode. The threshold voltage was reduced remarkably from ?26.20 V to ?3.72 V after inserting the F₁₆CuPc layer in the device with an Ag electrode. In order to examine the effect of the F₁₆CuPc layer, we evaluated the energy level alignments at the interfaces of pentacene/Ag and pentacene/F₁₆CuPc/Ag by in situ ultraviolet photoelectron spectroscopy. The hole-injection barrier from the Ag electrode to the pentacene layer is 0.55 eV with F₁₆CuPc, while it is 0.80 eV without F₁₆CuPc. The enhancements in the threshold and turn-on voltages are understood with the energy level alignments at the electrode–pentacene interface.     

Analysis of electrical and photoelectrical properties of ZnO/p-InP heterojunction

A ZnO/p-InP heterojunction has been fabricated by dc sputtering of ZnO on p-InP. It has been observed that the device has a good rectification. The electrical properties of the device such as ideality factor, barrier height, series resistance have been calculated using its current-voltage (I-V) measurements between 300 and 380 K with 20 K intervals. The short current density (J_sc) and open circuit voltage (V_oc) parameters have been determined between 40 and 100 mW/cm². The photovoltaic parameters of the device have been also determined under 100 mW/cm² and AM1.5 illumination condition.     

A microscopic view of charge transport in polymer transistors

Charge transport in polymer transistors has been investigated experimentally on a sub-micrometer length-sale by means of scanning-probe techniques.First, we present measurements of the microscopic potential landscape inside operating polythiophene and polyfluorene transistors, which reveal details about the current flow. Transistors based on poly[(9,9-dioctylfluorene-2,7-diyl)-co-(bithiophene)] (F8T2) in a nematic glassy state show a distinct microstructure in the potential. This microstructure has been analyzed quantitatively, and can be explained by a mobility anisotropy of ≈10 for current flow parallel and perpendicular to the alignment direction of the F8T2 chains. Furthermore, we find that domain boundaries are not limiting the charge transport in F8T2 transistors.Secondly, the dynamics of the charge accumulation process is discussed on the basis of time-resolved measurements of the charge carrier density.Finally, we introduce light-assisted scanning potentiometry as a tool to investigate trap states in polymer transistors. The technique allows a direct mapping of the density of trapped charges. Photon-assisted trap-release in transistors based on poly[(9,9-dioctylfluorene-2,7-diyl)-alt-phenylene-(N-(p-2-butylphenyl)imino-phenylene)] (TFB) and a SiO₂ gate dielectric has been studied. We find that the efficiency of this release process is governed by the absorption properties of TFB. This indicates that trapping occurs in the polymer.     

Leakage conduction mechanism of top-contact organic thin film transistors

This paper presents an analysis of the leakage conduction mechanism of top-contact organic thin film transistors. According to the experimental result, the author found that the dominant leakage conduction mechanism of top-contact devices is the Schottky emission and the gate leakage current dramatically increases with an increase in applied voltage. It is important to identify the gate leakage effect for understanding the actual device operation mechanism and enhancing the device performance.     

The formation of protective nitride surfaces for PEM fuel cell metallic bipolar plates

The selective gas nitridation of model nickel-based alloys was used to form dense, electrically conductive and corrosion-resistant nitride surface layers, including TiN, VN, CrN, Cr₂N, as wellas a complex NiNbVN phase. Evaluation for use as a protective surface for metallic bipolar plates in proton exchange membrane fuel cells (PEMFC) indicated that CrN/Cr₂N based surfaces holdpromise to meet U.S. Department of Energy (DOE) performance goals for automative applications. The thermally grown CrN/Cr₂N surface formed on model Ni−Cr based alloys exhibited good stability and low electrical resistance in single-cell fuel cell testing under simulated drive-cycle conditions. Recent results indicate that similar protective chromium nitride surfaces can be formed on less expensive Fe−Cr based alloys potentially capable of meeting DOE cost goals.     

The origin of hole injection improvements with MoO3/Al bilayer electrodes in pentacene thin-film transistors

The electronic structures of pentacene/MoO₃/Al and pentacene/Al were studied using in situ photoemission spectroscopy. The secondary electron cutoffs, highest occupied molecular orbitals (HOMOs) and core level changes were measured upon deposition of the pentacene and MoO₃ to study the electronic structures at the interface. We obtained complete energy level diagrams for each interface, showing that the insertion of the MoO₃ layer between pentacene and Al induces the HOMO level shift of pentacene toward lower binding energies compared to that without MoO₃. This results in reduction of the hole injection barrier, which is responsible for the improvements in electronic device performance.