Measurements Of Current-voltage Characteristics Research Articles

Scanning probe microscopy of laser-graphitized diamond-like carbon films

A complex technique of scanning probe microscopy/spectroscopy (SPM/SPS) based on the microscopy of lateral forces and registration of a local electrical conductivity in combination with measurements of the microrelief has been developed for studies of laser-graphitized carbon microstructures. The method includes multiple direct and reverse probe scanning with the subsequent correction of the map position, their pixel-by-pixel subtraction and averaging, and a statistical processing of resulting data arrays concerning the distribution of lateral forces (friction forces). In addition, based on the measurements of currentvoltage (I–V) characteristics, a distribution of the electrical conductivity is built in the probe-sample circuit. Carbon structures based on hydrogenated diamond-like films of a-C:H type, which were deposited onto Si substrates, are used as the objects for studies. A local graphitization of the surface has been carried out by the irradiation of the films with an excimer KrF laser according to a preset microscopic pattern. Based on the resulting data, it is found that the reaction of the lateral forces (friction forces) in the laser-graphitized region is reversed to the temperature variations: when the temperature increases (from room to ∼120°C), the distribution of the friction forces shifts towards higher values and returns practically to the initial values when the temperature decreases to the initial level, which proves the influence of a water adsorbate on the friction properties of laser-graphitized regions on the film surface. It is also found that the laser-graphitized region is structurally inhomogeneous, which is proven by a decrease in the electrical conductivity from the center to the periphery.

A novel study for determining early life degradation of multi-crystalline-silicon photovoltaic modules observed in western Himalayan Indian climatic conditions

The results of a first time study are reported on the degradation of a roof mounted multi-crystalline-silicon photovoltaic system after 2.5years of outdoor exposure in a western Himalayan location in India. The main objective of the study is to identify frequently occurring defects in modules to study the change in electrical performance parameters and correlate these with identified defects under Indian climatic conditions. A comprehensive test campaign is conducted by visual inspection, thermal imaging and current–voltage characteristic measurements. Early degradation and recently reported defect: snail trails are observed in all modules along with non-uniform single cell browning and junction box failure. Thermal imaging analysis is carried out which shows that three modules developed serious defects like hotspots, disconnected cells and string interconnect ribbons and a significant result that all solar cells located directly on top of junction boxes were 3–8°C hotter than other cells indicating higher thermal degradation rate of these cells than others. Peak power degradation of modules is quantified by measuring performance parameters under standard test conditions as a function of field exposure time, using class-A sun-simulator. Three modules showed 50% degradation while remaining seven modules showed 0.6–2.5% degradation in peak power after 2.5years. PV modules certified as per International Electro-technical Commission (IEC) standards have shown considerable degradation indicating the need to enforce quality control and review qualification standards for Indian climatic conditions if modules have to perform reliably for more than 20years under field conditions. The outcome of the study will be of importance to enhance the knowledge of climate specific field PV degradation mechanism and to provide inputs to Indian/IEC standards, the improvements of which are being considered currently.

The equipment for controlling the structure and functional properties of nanostructured composite films

The article is devoted to the creation of an instrumental system allowing evaluating the functional properties and current-voltage characteristics of nanostructured composite films at different temperatures and other environmental parameters. The system is based on an assessment of current-voltage characteristics of a nanostructured film material. The main components of the system are a chamber and a unit for current-voltage characteristics measuring. The stage with the test material and the contact system are provided with a heating element and a cooling system thus allowing warming to 150 °C and fast cooling to negative temperatures by liquid nitrogen circulating. The chamber body leak proofness against the external environment allows forming a composition of the atmosphere at a predetermined humidity level, which is essential for the measurement of current-voltage characteristics of polymer materials. The article describes the design features of the instrumental system and results of its application used for determining the properties of polymer nanostructured composite films.

Electrochemically assisted mechanically controllable break junction studies on the stacking configurations of oligo(phenylene ethynylene)s molecular junctions

We demonstrate an electrochemically assisted mechanically controllable break junction (EC-MCBJ) approach for current-voltage characteristic (I-V curve) measurements of metal/molecule/metal junctions. A series of oligo(phenylene ethynylene)s compounds (OPEs), including those involving electron withdrawing substituent group and different backbone lengths, had been successfully designed, synthesized, and placed onto the fabricated nanogap to form molecular junctions. The observed evolution in the measured conductances of OPEs indicates that there is a dependence of conductance on molecular length and substituent group. Compared with those extracted from conductance histogram construction, the conductances of OPEs measured from I-V curves are considerably lower. Based on the transmission spectra of OPEs that calculated by density functional theory (DFT) combined with non-equilibrium Green’s function (NEGF) method, this difference was attributed to our distinct experimental operation, which may give rise to a stacking configuration of two OPE molecules.

Mobility overestimation due to gated contacts in organic field-effect transistors

Parameters used to describe the electrical properties of organic field-effect transistors, such as mobility and threshold voltage, are commonly extracted from measured current–voltage characteristics and interpreted by using the classical metal oxide–semiconductor field-effect transistor model. However, in recent reports of devices with ultra-high mobility (>40 cm2 V−1 s−1), the device characteristics deviate from this idealized model and show an abrupt turn-on in the drain current when measured as a function of gate voltage. In order to investigate this phenomenon, here we report on single crystal rubrene transistors intentionally fabricated to exhibit an abrupt turn-on. We disentangle the channel properties from the contact resistance by using impedance spectroscopy and show that the current in such devices is governed by a gate bias dependence of the contact resistance. As a result, extracted mobility values from d.c. current–voltage characterization are overestimated by one order of magnitude or more.

Current-dependent formation of Bechgaard salt nanocrystals using nanoscale electrocrystallization for the fabrication of organic nanodevices

Herein, the nanocrystal formation of tetramethyltetraselenafulvalene (TMTSF)-based nanocrystals based on nanoscale electrocrystallization is reported. Whereas direct current electrolysis produced nanocrystals over the entire anode surface, nanocrystals were only formed in the gap between the two electrodes using alternating current (AC) electrolysis. Consequently, a two-terminal device where nanocrystals bridge the source and drain electrodes was simply prepared via this one-step electrolysis process. The prepared nanocrystal had the same structure as the bulk crystal of (TMTSF)2·ClO4, which is known as a superconducting Bechgaard salt. Although the nanocrystal exhibited a metallic nature, weak gate-voltage-dependent characteristics were observed in the current-voltage characteristics measurements. Facile fabrication of organic nanodevices was thus enabled using this AC electrolysis method.

Localized Plasmon-Stimulated Nanochemistry of Graphene Oxide on a SERS Substrate.

In recent years, there has been remarkable progress in the reduction and functionalization of graphene oxide (GO) using nanoparticles and high-energy optical photons. Most of these reactions are carried out in solutions, whereas the local modification of GO on solid substrates still remains a challenge. In this work, we demonstrate the local reduction of GO and its further destruction, leading to the synthesis of polyaromatic hydrocarbons (PAHs) stimulated by localized surface plasmons (LSPs). The reduction of GO and the synthesis of PAHs have been carried out on a substrate designed for surface-enhanced Raman spectroscopy (SERS). We found that LSPs initiate the destruction of water molecules entrapped in the nanogaps between silver nanoparticles after the deposition of GO from the aqueous suspension. It was demonstrated that OH radicals, as a result of water decomposition, initiate the reduction of GO, leading to the synthesis of PAHs. The reactions have been observed in real time by using SERS. The measurement of current-voltage (I-V) characteristics through conductive atomic force microscopy (AFM), recorded in an LSP-stimulated area, have shown the increased electrical conductivity (more than ten times) compared with the conductivity of GO. The synthesis of new compounds in the LSP-stimulated area has been confirmed by the appearance of new peaks in the Raman spectra and nonlinear I-V characteristics typical for PAHs. We show that the used method allows the local modification of electrical properties of GO and controlled nanopattering of organic compounds on the surface.

Temperature dependent electrical characterization of thin film Cu2ZnSnSe4 solar cells

Impedance spectroscopy (IS) and current–voltage characteristics measurements were applied to study properties of a Cu2ZnSnSe4 (CZTSe) thin film solar cell. IS measurements were done in the frequency range 20 Hz to 10 MHz. The measurement temperature was varied from 10 K to 325 K with a step ▵T = 5 K. Temperature dependence of Voc revealed an activation energy of 962 meV, which is in the vicinity of the band gap energy of CZTSe and hence the dominating recombination mechanism in this solar cell is bulk recombination. Different temperature ranges, where electrical properties change, were found. Interface states at grain boundaries with different properties were revealed to play an important role in impedance measurements. These states can be described by introducing a constant phase element in the equivalent circuit.

Electronic Characterization of Au/DNA/ITO Metal-Semiconductor-Metal Diode and Its Application as a Radiation Sensor

Deoxyribonucleic acid or DNA molecules expressed as double-stranded (DSS) negatively charged polymer plays a significant role in electronic states of metal/silicon semiconductor structures. Electrical parameters of an Au/DNA/ITO device prepared using self-assembly method was studied by using current–voltage (I-V) characteristic measurements under alpha bombardment at room temperature. The results were analyzed using conventional thermionic emission model, Cheung and Cheung’s method and Norde’s technique to estimate the barrier height, ideality factor, series resistance and Richardson constant of the Au/DNA/ITO structure. Besides demonstrating a strongly rectifying (diode) characteristic, it was also observed that orderly fluctuations occur in various electrical parameters of the Schottky structure. Increasing alpha radiation effectively influences the series resistance, while the barrier height, ideality factor and interface state density parameters respond linearly. Barrier height determined from I–V measurements were calculated at 0.7284 eV for non-radiated, increasing to about 0.7883 eV in 0.036 Gy showing an increase for all doses. We also demonstrate the hypersensitivity phenomena effect by studying the relationship between the series resistance for the three methods, the ideality factor and low-dose radiation. Based on the results, sensitive alpha particle detectors can be realized using Au/DNA/ITO Schottky junction sensor.

Experimental determination of tunneling characteristics and dwell times from temperature dependence of Al/Al2O3/Al junctions

Measurements of current-voltage (I-V) characteristics of a high quality Al/Al2O3/Al junction at temperatures ranging from 3.5 K to 300 K have been used to extract the barrier properties. Fitting results using Simmons's model led to a constant value of barrier width s ∼ 20.8 Å and a continuous increase in the barrier height with decreasing temperature. The latter is used to determine the energy band gap temperature dependence and average phonon frequency ω = 2.05 × 1013 s−1 in Al2O3, which adds confidence to the precision of our measurements. The barrier parameters are used to extract the temperature dependent dwell times in tunneling (τD = 3.6 × 10−16 s at mid-barrier energies) and locate resonances above the barrier.

Synthesis of hybrid graphene–porphyrin micro/nanofiber structures by ionic self-assembly

Cationic mesa-tetra(4-pyridyl) porphine (TPyP)-graphene oxide (GO) hybrid structures have been synthesized by ionic self-assembly. The different aggregated structures shape were obtained through electrostatic and π–π stacking interactions, as indicated by redshift of porphyrin Soret band throughout the pH range of 1.5–9.1, fluorescence quenching, Raman spectra and Energy dispersive X-ray spectroscopy. A strong fluorescence quenching of the cationic porphyrin in the presence of graphene indicated that efficient electron or energy transfer occurred from the excited state of the cationic porphyrin to graphene. The investigation revealed a three-dimensional fiber structures formation under the acid conditions and leaf-like structures of the TPyP/GO assemblies under the alkaline conditions of the solution. The fibers morphology and cross-section was determined quantitatively by Scanning force microscopy; this revealed a diameter of the fibers up to hundreds of nanometers and the lengh up to hundreds of micrometers. The electrical properties of the fibers were investigated by spreading resistance and current–voltage characteristics measurements using Atomic Force Microscopy (AFM) with conductive cantilevers. The current–voltage characteristics fit to the Poole–Frenkel emission mechanism in the case of TPyP/GO sheets and Schottky emission dominate in the nanofibers case. This new graphene material shows a potential for energy, sensing and photonic applications.

Effect of through-plane polytetrafluoroethylene distribution in a gas diffusion layer on a polymer electrolyte unitized reversible fuel cell

The relationship between through-plan polytetrafluoroethylene (PTFE) distribution on a gas diffusion layer (GDL) and unitized reversible fuel cell (URFC) performance was investigated. Titanium (Ti) - felt was used for the oxygen-side GDL and treated with 10wt.% PTFE dispersion to enhance hydrophobicity. The dependence of PTFE distribution on the PTFE drying conditions was examined using scanning electron microscopy (SEM)-based energy dispersive X-ray spectroscopy (EDS) imaging. The EDS image maps revealed that the PTFE distribution strongly depended on the drying condition of the PTFE; drying under atmospheric pressure yielded a highly non-uniform PTFE distribution in the through-plane direction, whereas drying under vacuum pressure yielded a relatively uniform PTFE distribution. The cell performance of URFCs was then evaluated based on measured current–voltage characteristics during both electrolysis and fuel cell operation modes. Results verified that compared with non-uniform PTFE distribution, a uniform distribution in the Ti-felt GDL improved the fuel cell performance under the fully wet condition (relative humidity = 100%). By applying the Ti-felt GDL with uniform PTFE distribution in the through-plane direction, the current density at the cell voltage of 0.6 V was increased by a factor of about 1.9 compared with the Ti-felt with non-uniform PTFE distribution.

Role of metallic-like conductivity in unusual temperature-dependent transport in n-ZnO : Al/p-Si heterojunction diode

The current–voltage characteristics of an n-ZnO : Al(AZO)/p-Si heterojunction diode is investigated over a temperature range of 293 and 423 K. The measured current–voltage characteristics show good rectification behaviour at all temperatures. It is observed that the AZO/Si heterojunction exhibits different (unusual) types of charge conduction processes in the temperature range under consideration. In addition, temperature-dependent resistivity measurements performed on a AZO thin film grown on a glass substrate show metallic-like conductivity, which is explained on the basis of local annealing of defects, mainly vacancies, in the AZO layer. Finally, based on our experimental findings, we construct a parametric phase diagram to elucidate the transition from one to the other conduction mechanism. The present study will be useful to understand the effect of self-heating for AZO-based devices.

Localized Tail States and Electron Mobility in Amorphous ZnON Thin Film Transistors.

The density of localized tail states in amorphous ZnON (a-ZnON) thin film transistors (TFTs) is deduced from the measured current-voltage characteristics. The extracted values of tail state density at the conduction band minima (Ntc) and its characteristic energy (kTt) are about 2 × 1020 cm−3eV−1 and 29 meV, respectively, suggesting trap-limited conduction prevails at room temperature. Based on trap-limited conduction theory where these tail state parameters are considered, electron mobility is accurately retrieved using a self-consistent extraction method along with the scaling factor ‘1/(α + 1)’ associated with trapping events at the localized tail states. Additionally, it is found that defects, e.g. oxygen and/or nitrogen vacancies, can be ionized under illumination with hv ≫ Eg, leading to very mild persistent photoconductivity (PPC) in a-ZnON TFTs.

Experimental and Theoretical Analysis of Nanotransport in Oligophenylene Dithiol Junctions as a Function of Molecular Length and Contact Work Function.

We report the results of an extensive investigation of metal-molecule-metal tunnel junctions based on oligophenylene dithiols (OPDs) bound to several types of electrodes (M1-S-(C6H4)n-S-M2, with 1 ≤ n ≤ 4 and M1,2 = Ag, Au, Pt) to examine the impact of molecular length (n) and metal work function (Φ) on junction properties. Our investigation includes (1) measurements by scanning Kelvin probe microscopy of electrode work function changes (ΔΦ = ΦSAM - Φ) caused by chemisorption of OPD self-assembled monolayers (SAMs), (2) measurements of junction current-voltage (I-V) characteristics by conducting probe atomic force microscopy in the linear and nonlinear bias ranges, and (3) direct quantitative analysis of the full I-V curves. Further, we employ transition voltage spectroscopy (TVS) to estimate the energetic alignment εh = EF - EHOMO of the dominant molecular orbital (HOMO) relative to the Fermi energy EF of the junction. Where photoelectron spectroscopy data are available, the εh values agree very well with those determined by TVS. Using a single-level model, which we justify via ab initio quantum chemical calculations at post-density functional theory level and additional UV-visible absorption measurements, we are able to quantitatively reproduce the I-V measurements in the whole bias range investigated (∼1.0-1.5 V) and to understand the behavior of εh and Γ (contact coupling strength) extracted from experiment. We find that Fermi level pinning induced by the strong dipole of the metal-S bond causes a significant shift of the HOMO energy of an adsorbed molecule, resulting in εh exhibiting a weak dependence with the work function Φ. Both of these parameters play a key role in determining the tunneling attenuation factor (β) and junction resistance (R). Correlation among Φ, ΔΦ, R, transition voltage (Vt), and εh and accurate simulation provide a remarkably complete picture of tunneling transport in these prototypical molecular junctions.

Optimal Fabrication of P3HT:PCBM Based Organic Solar Cells with N-Type DMDCNQI Dopant.

We have fabricated P3HT and PCBM-based organic photovoltaic cells using DMDCNQI as a guest dopant material and an n-type buffer layer. We have investigated the physical effects of the DMD-CNQI molecule as a guest dopant and an n-type charge transfer buffer layer on the performance of the device by examining measurements of light current-voltage and photoluminescence characteristics. The device using DMDCNQI as a dopant and a buffer layer exhibited a remarkable increase in short-circuit current density (Jsc) due to the ability of an electron acceptor and to the formation of a highly conducting charge transfer complex. In particular, the device consisting of ITO/PEDOT:PSS/P3HT:PCBM:DMDCNQI(1 wt%)/DMDCNQ/Al showed a maximum power conversion efficiency of 3.50%.

Current-Voltage Characterization of Gallium Arsenide Nanowires Using a Conductive Atomic Force Microscopy

Utilizing semiconductor nanowires for optoelectronics device requires exact knowledge of their current-voltage properties. In this report, we examine accurate on-top imaging and I-V characterization of individual vertical Gallium Arsenide Nanowires (GaAs NWs) using conductive atomic force microscopy without additional microscopy tools, thus allowing versatile application. The measured current-voltage characteristic of a single NW shows the typical performance of a Schottky contact, which caused by the contact between the metallic AFM tip and the top of NWs. The height of the Schottky barrier is dependent on the diameter of the nanowires. The linear part of the curve was used to calculate the differential resistance, which was found to be about 25 to 100 MΩ. Energy band gap for GaAs NW was found to be 1.5 eV by differential conductivity measurement.

Extraction of Propagation Delay-Correlated Mobility and Its Verification for Amorphous InGaZnO Thin-Film Transistor-Based Inverters

We found that field-effect mobility, which had been widely used in the evaluation of the mobility of an amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistor (TFT) failed to describe the effect of mobility on propagation delay (tPD) in an a-IGZO TFT-based circuit, and also proposed an extraction technique for the tPD-correlated mobility (μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">tPD</sub> ) considering both the subgap density-of-states and the voltage-dependent charge density. It is verified that the proposed μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">tPD</sub> is the best correlated with the measured tPD in IGZO TFT-based inverters other than various mobilities in the literature. Our results have revealed that it is possible to predict tPD only with the measured current-voltage characteristic of the a-IGZO TFT without measuring tPD in IGZO-based circuits.

Difference in charge transport properties of Ni-Nb thin films with native and artificial oxide

Here, we report on the properties of native and artificial oxide amorphous thin film on a surface of an amorphous Ni-Nb sample. Careful measurements of local current-voltage characteristics of the system Ni-Nb / NiNb oxide/Pt, were carried out in contact mode of an atomic force microscope. Native oxide showed n-type conductivity, while in the artificial one exhibited p-type one. The shape of current-voltage characteristic curves is unique in both cases and no analogical behavior is found in the literature. X-ray photoelectron spectroscopy (XPS) measurements were used to detect chemical composition of the oxide films and the oxidation state of the alloy components. Detailed analysis of the XPS data revealed that the structure of natural Ni-Nb oxide film consists of Ni-NbOx top layer and nickel enriched bottom layer which provides n-type conductivity. In contrast, in the artificial oxide film Nb is oxidized completely to Nb2O5, Ni atoms migrate into bulk Ni-Nb matrix. Electron depletion layer is formed at the Ni-Nb/Nb2O5 interface providing p-type conductivity.

Evaluation of pulsed laser annealing for flexible multilayer MoS2 transistors

To realize the proper electrical characteristics of field-effect transistors, the quality of the contact and interface must be improved because they can substantially distort the extracted mobility, especially for materials with low densities of states like molybdenum disulfide (MoS2). We show that mechanically flexible MoS2 thin-film transistors (TFTs) with selectively laser annealed source/drain electrodes achieve enhanced device performance without plastic deformation including higher field-effect mobility (from 19.59 to 45.91 cm2 V−1 s−1) in the linear regime, decreased subthreshold swing, and enhanced current saturation. Furthermore, numerical thermal simulations, measured current-voltage characteristics, and contact-free mobility extracted from the Y-function method suggest that the enhanced performance originated from a decrease in the Schottky barrier effect at the contact and an improvement of the channel interface. These results demonstrate that picosecond laser annealing can be a promising technology for building high performance flexible MoS2 TFTs in flexible/stretchable circuitry, which should be processed at low temperatures.