Steady-state Current-voltage Characteristics Research Articles

Effect of Single Walled Carbon Nanotubes on the Series Resistance and Trap Energy of Malachite Green Dye Based Organic Device

In this paper, we have estimated the series resistance (Rs) and the trap energy (Ec) of the sandwiched type Malachite Green (MG) dye-based organic device and have also observed the influence of single-walled carbon nanotubes (SWCNT) on both of these parameters. To form the organic device, we have used Indium Tin Oxide (ITO) coated glass as the front electrode and Aluminium (Al) as a back electrode by using the spin coating technique. The values of series resistance are measured from both I-V characteristics and by utilizing Cheung Function due to the non ideal behavior of organic devices. We have also extracted the values of Rsby using H (I) versus I plot and verified the values with the measured values of Rsfrom the Cheung function. The extracted values of series resistance using these three processes remain consistent with each other in showing that the values of series resistance have been reduced considerably in the presence of SWCNT. The trap energy has been estimated from the steady-state current-voltage characteristics. There is a significant correlation in between series resistance and the trap energy of the organic device. The presence of Single-Walled Carbon Nanotubes reduces the trap energy from 0.086 eV to 0.057 eV. Lowering of the trap energy of the metal-organic layer interface in presence of Single Walled Carbon Nanotubes attributes to the reduction of the value of the series resistance. The extracted value of Rsdecreases from 0.154 MΩ to 0.0389 MΩ in presence of SWCNT. Decrease in the value of both of these parameters in the presence of SWCNT will definitely improve the charge transport mechanism of the organic device and thereby the conductivity.

Study on the effect of singlewalled carbon nanotubes on junction properties of Safranin –T dye-based organic device

In this work, junction properties of metal-organic dye contact have been studied and alteration of these properties in presence of single walled carbon nanotubes has been observed. Junction properties of metal-organic interface significantly influence the device performance. Present work studies barrier potential and trap energy under the influence of single walled carbon nanotubes. Barrier inhomogeneity and the effect of image charge on lowering of barrier potential have also been studied. Formation of the organic device has been done by sandwiching Safranin - T dye in between two electrodes which are Indium Tin Oxide coated glass substrate and Aluminium respectively. Charge flow at the metal–organic layer interface has been analyzed by Richardson –Schottky thermionic emission theory. Both barrier potential and trap energy have been estimated from the steady-state current-voltage characteristics of the device. The incorporation of single walled carbon nanotubes lowers both of these parameters results in improvement of current flow at the metal-organic contact.

Study on the Effect of Zinc Oxide Nanoparticles on Injection Barrier Height of Crystal Violet Dye Based Organic Device

Organic devices have achieved substantial progress due to its flexibility, cost effectiveness, simple processing and wide customizability. But there are certain limitations of these devices. One of the main limitations is the lower charge injection from metal electrodes to the organic layer which affects the performance of these devices. The charge injection process is strongly dependent on the injection barrier height at metal- organic layer interface. Due to high barrier height at metal – organic layer interface, the charge injection is reduced at the interface. Attempts need to be made to reduce the injection barrier height at metal – organic layer interface to improve the charge injection process. In the present work, we have studied the injection barrier height of Indium tin oxide coated glass/Crystal violet dye/Aluminium based organic device and subsequently we have also observed the effect of zinc oxide nanoparticles on injection barrier height of this device. Indium tin oxide coated glass and aluminium are used as front electrode and back electrode respectively to form this device. The device has been prepared with and without zinc oxide nanoparticles by using spin coating technique. We have analyzed the steady state current-voltage characteristics of the device to determine the barrier height of the device. Barrier height is reduced from 0.87 eV to 0.78 eV in the presence of zinc oxide nanoparticles. The barrier height is also estimated by using another alternative method which is known as Norde method. By using Norde method, barrier height is measured which reduces from 0.83 eV to 0.77 eV in presence of zinc oxide nanoparticles. Both the methods show good consistency with each other. Reduction of the injection barrier height in presence of zinc oxide nanoparticles indicates better charge injection through the metal - organic dye interface. The higher electron mobility of zinc oxide nanoparticles facilitates efficient charge injection at the interface. Thus, this work will be informative to study the effect of zinc oxide nanoparticles on injection barrier height as it reduces the barrier height to improve the charge flow at metal – organic dye interface.

Molecular spintronics using single-molecule magnets under irradiation

We theoretically investigate a single-molecule magnet (SMM) grafted to a quantum dot in contact with metallic leads and interacting with a resonant electromagnetic radiation. We explore both the explicit time-dependent behavior and the steady-state current-voltage characteristics of the device when the source lead is ferromagnetic. At zero-bias voltage, a net current is pumped through the device with the source spin current being reversed and amplified in the drain lead; this effect also persists for nonzero bias. We explain this effect in terms of spin transitions in the nanomagnet induced by the resonant radiation followed by their subsequent relaxation via spin-asymmetric charge-transfer processes. We demonstrate that the same effects are recovered in the time-averaged current when the device interacts with pulsed resonant radiation. Moreover, within the pulsed irradiation regime, appropriate choices of pulse length and wait times are shown here to allow the detection of coherent Rabi oscillations of the SMM spin states, via time-averaged spin current measurements.

Copper oxide nanoparticle doped bulk-heterojunction photovoltaic devices

Organic and metal oxide-based hybrid solar cells have drawn attention for their improved device performance along with mechanical stability and cost-effectiveness. We investigated the incorporation of various concentrations of copper oxide (CuO) nanoparticles (NPs) blended with poly 3-hexylthiophene: [6,6]-phenyl-C61-butyric methyl ester (P3HT:PCBM) bulk-heterojunction devices. With an optimum concentration of CuO NPs in P3HT: PCBM the devices power conversion efficiency is about 35% larger than without CuO NPs. The overall improvement in the photovoltaic properties of the device is attributed to the improved optical absorption, crystallinity and roughness of the photoactive layer confirmed by the AFM and XRD characteristics of the blended films. Beyond optimum nanoparticle concentration deteriorates the device performance from high trapped charge density as evidenced by steady-state current-voltage characteristics and charge-extraction-by-linearly-increasing-voltage (CELIV).

Investigation of Traps in Thin-Film Organic Semiconductors Using Differential Analysis of Steady-State Current–Voltage Characteristics

A study of traps in thin organic semiconductor films is described through the analysis of their impact on current voltage ( ${J}-{V}$ ) characteristics of two terminal diode-like structures. The ${J}-{V}$ characteristics follow a power law whose exponent exhibits voltage dependence as a result of change in trap occupancy with applied voltage. A method of analysis of ${J}-{V}$ characteristics using the product of differential resistance and current is described which enables different unique regions of device characteristics to be visually delineated and facilitates extraction of threshold voltage, marking onset of trap-filled regime. Simulation results are presented to validate and highlight the usefulness of the proposed concepts and techniques. The experimental results obtained with polymers P3HT, blend of P3HT with phenyl C61 butyric acid methyl ester, and small molecule pentacene organic semiconductor are used to illustrate the insight offered by the proposed method. Both simulation and experimental results indicate that analysis based on double-log plots may lead to large overestimation of voltage marking onset of trap-filled regime.

Equivalent circuit model of Ge/Si separate absorption charge multiplication avalanche photodiode

The equivalent circuit model of Ge/Si Separate Absorption Charge Multiplication Avalanche Photodiode (SACM-APD) is proposed. Starting from the carrier rate equations in different regions of device and considering the influences of non-uniform electric field, noise, parasitic effect and some other factors, the equivalent circuit model of SACM-APD device is established, in which the steady-state and transient current voltage characteristics can be described exactly. In addition, the proposed Ge/Si SACM APD equivalent circuit model is embedded in PSpice simulator. The important characteristics of Ge/Si SACM APD such as dark current, frequency response, shot noise are simulated, the simulation results show that the simulation with the proposed model are in good agreement with the experimental results.

An improved method for extraction of mobility from space charge limited current in organic semiconductor films

Estimation of mobility in thin film organic semiconductors from steady state current-voltage characteristics using Murgatroyd expression is attractive because of its simplicity but is accompanied by significant errors. These errors, resulting from neglecting diffusion current, are especially large at lower applied voltages, being more than 50% for voltages under 5 V. Using higher voltages to reduce errors is impractical in organic thin films not only due to possibility of degradation of the device but also due to distortion arising from parasitic series resistance caused by higher device currents. The present work describes an improved expression of space charge limited current for field dependent mobility that incorporates the effect of diffusion by modifying the quadratic dependence of J on voltage to J∝V(V+Vβ), where inclusion of voltage term Vβ compensates for neglecting diffusion current. Since the diffusion related voltage Vβ depends on small built-in voltage that may be present under experimental conditions and, thus, is not known a priori, a self consistent method of extraction is described that allows extraction of Vβ and mobility parameters with reduced errors up to less than 10% even for voltages under 2 V. Simulation and experimental results obtained with small-molecule Pentacene based devices are presented that illustrate the basic concept and usefulness of the proposed approach.

(Invited) Simulation of the Effects of As-Grown Defects on GaN-Based Power HEMTs

With the wide band-gap of Gallium Nitride (GaN) and high mobility of 2 dimensional electron gas(2DEG), AlGaN/GaN high electron mobility transistors (HEMTs) are considered ideal candidates for high speed and power applications ranging from hybrid vehicles to space communication. Developments in the fabrication of GaN-based blue LEDs have been incorporated into the processing of AlGaN/GaN HEMT structures leading to improvements in amount of defects and dislocations resulting from heteroepitaxial growth. But evident degradation of dynamic performance of such devices due to the presence of as-grown defects still remains a subject of major concern. Using FLorida Object Oriented Device Simulator (FLOODS) TCAD, this work looks to analyze the impact of variation of defect properties as well as their spatial distribution and concentration on the steady-state and dynamic behavior of AlGaN/GaN HEMTs. FLOODS TCAD’s defect modeling capability will be applied to examine how multiple defect levels, of both donor and acceptor-like nature, can influence device performance. Particular emphasis will be on transient switching, such as gate-lag and drain-lag experiments, in consideration of the slow nature observed frequently in literature. While comprehensive investigation of the role of charged defects at the AlGaN/passivation layer interface on performance degradation has already been performed, contribution by as-grown defects, expected to be in significant quantity in the GaN buffer layer, is yet to be investigated. Building on experimental results, which suggest the presence of multiple acceptor-like defects, impact on steady-state current-voltage characteristics will be observed followed by an examination of how such defects will affect dynamic performance by isolating their contribution to transient and switching behavior. Figure: (a) Drain current against gate voltage at VDS=0.5V for HEMT A with 1 acceptor-like level (nT=1014 cm-3, Et=Ec-0.7eV) and HEMT B with 2 acceptor levels(Level 1: nT=1014 cm-3, Et=Ec-0.7eV; Level 2: nT=1015 cm-3, Et=Ec-0.5eV) (b) Optical pumping like detrapping of electrons, biasing at VDS=1V and VGS=0V. HEMT A(Level 1: nT=1013 cm-3, Ec-0.2eV, τ=1s; Level 2: nT=4x1013 cm-3, Ec-0.5eV, τ=1ms), HEMT B(Level 1: nT=1013 cm-3, Ec-0.2eV, τ=1s; Level 2: nT=1013 cm-3, Ec-0.5eV, τ=1ms) Figure 1

Transport through a correlated interface: Auxiliary master equation approach

We present improvements of a recently introduced numerical method [Arrigoni etal, Phys. Rev. Lett. 110, 086403 (2013)] to compute steady state properties of strongly correlated electronic systems out of equilibrium. The method can be considered as a non-equilibrium generalization of exact diagonalization based dynamical mean-field theory (DMFT). The key modification for the non-equilibrium situation consists in addressing the DMFT impurity problem within an auxiliary system consisting of the correlated impurity, $N_b$ uncorrelated bath sites and two Markovian environments (sink and reservoir). Algorithmic improvements in the impurity solver allow to treat efficiently larger values of $N_b$ than previously in DMFT. This increases the accuracy of the results and is crucial for a correct description of the physical behavior of the system in the relevant parameter range including a semi-quantitative description of the Kondo regime. To illustrate the approach we consider a monoatomic layer of correlated orbitals, described by the single-band Hubbard model, attached to two metallic leads. The non-equilibrium situation is driven by a bias-voltage applied to the leads. For this system, we investigate the spectral function and the steady state current-voltage characteristics in the weakly as well as in the strongly interacting limit. In particular we investigate the non-equilibrium behavior of quasi-particle excitations within the Mott gap of the correlated layer. We find for low bias voltage Kondo like behavior in the vicinity of the insulating phase. In particular we observe a splitting of the Kondo resonance as a function of the bias voltage.

Charge transfer and recombination kinetics in dye-sensitized solar cell using static and dynamic electrical characterization techniques

This study investigates the steady-state current–voltage characteristics, electronic and ionic processes and dynamic response of a TiO2 nanoparticle based dye-sensitized solar cell (DSSC) using impedance spectroscopy and current–voltage measurements. A dye-sensitized solar cell is fabricated with 12.26μm thick TiO2 layer, having power conversion efficiency of 2.9% under AM1.5 spectrum with photo-generated current density (JSC) of 7.2mAcm−2 and open-circuit voltage (VOC) of 672mV. The observed electrochemical phenomenon at the TiO2–electrolyte interface, in diffusion of electrolyte and in charge transfer at the counter electrode is mathematically modeled using alternating current (AC) electrical equivalent circuit. The effect of temperature and illumination on the steady state and dynamic parameters of dye-sensitized solar cells is also studied. The dynamic resistance of DSSC decreases from 440.7Ω to 78.6Ω with increase in illumination level from 20mWcm−2 to 100mWcm−2.

Traps signature in steady state current-voltage characteristics of organic diode

Traps cause distortion in current-voltage (I−V) characteristics of an organic diode due to their voltage dependent occupancy. The present work describes a technique that uses the change in slope of I−V characteristics due to increased filling of traps to create a sharply defined peak that allows ready visualization of presence of traps even when their direct impact on I−V is not clearly evident. It is also shown that the signatures of exponential and single/Gaussian traps are significantly different thereby allowing them to be easily differentiated. Simulation and experimental results with P3HT and P3HT:PCBM diodes are presented that highlight the usefulness of the proposed technique.

Auxiliary master equation approach to nonequilibrium correlated impurities

We present a numerical method for the study of correlated quantum impurity problems out of equilibrium, which is particularly suited to address steady state properties within Dynamical Mean Field Theory. The approach, recently introduced in [Arrigoni et al., Phys. Rev. Lett. 110, 086403 (2013)], is based upon a mapping of the original impurity problem onto an auxiliary open quantum system, consisting of the interacting impurity coupled to bath sites as well as to a Markovian environment. The dynamics of the auxiliary system is governed by a Lindblad master equation whose parameters are used to optimize the mapping. The accuracy of the results can be readily estimated and systematically improved by increasing the number of auxiliary bath sites, or by introducing a linear correction. Here, we focus on a detailed discussion of the proposed approach including technical remarks. To solve for the Green's functions of the auxiliary impurity problem, a non-hermitian Lanczos diagonalization is applied. As a benchmark, results for the steady state current-voltage characteristics of the single impurity Anderson model are presented. Furthermore, the bias dependence of the single particle spectral function and the splitting of the Kondo resonance are discussed. In its present form the method is fast, efficient and features a controlled accuracy.

Analytical model of the electrical instability mechanism in multibarrier heterostructures with tunnel-opaque barriers

Through mathematical modeling of the conductivity of multibarrier heterostructures, the steady-state current-voltage characteristics, the S-shaped behavior of which is indicative of electrical instability, are obtained. To study its dynamic parameters, an analytical model of instability is developed using known approximations of the physics of semiconductor devices. In the steady-state case, the model yields an S-shaped current-voltage characteristic close to numerical simulation results. This fact is considered as confirmation of the adequacy of the developed analytical model. In the small-signal case, the latter is generalized to the situation with harmonic electrical perturbation. The resulting formula for the frequency dependence of the small-signal impedance indicates the possibility of dynamic-resistance negativity up to terahertz frequencies. A clear physical interpretation of the instability in terms of positive feedback in the unit cell of the multibarrier heterostructures under study is proposed. The results of measurements of the quasi-stead-state current-voltage characteristics of fabricated test multibarrier GaAs/AlGaAs structures with a pronounced portion of negative differential resistance are also presented.

P-GaSb(Ox)/n-GaSb native-oxide heterojunctions: Non-vacuum process and photoelectric properties

A new non-vacuum technology is proposed and anisotype photosensitive heterojunctions (native oxide of a narrow-gap III–V semiconductor)-(binary compound) p-GaSb(Ox)/n-GaSb are fabricated for the first time. The developed technological process is based on the surface thermal interaction of a GaSb crystal with components of the normal Earth atmosphere. Based on original physical-technological studies of interaction in the GaSb/(air medium) system, it is found that p-GaSb(Ox) native-oxide films obtained in such a way exhibit high adhesion to the surface of the initial gallium antimonide n-GaSb. The steady-state current-voltage characteristics and spectral dependences of the relative photoconversion quantum efficiency of the obtained p-GaSb(Ox)/n-GaSb heterojunctions are first measured. On this basis, the systematic features of charge transport and photosensitivity are discussed. A new possible application of the non-vacuum thermal oxidation of GaSb films in the development of optical radiation photodetectors on substrates of homogeneous gallium antimonide n-GaSb crystals is first detected and implemented.

Effect of substitutional doping on temperature dependent electrical parameters of amorphous Se-Te semiconductors

Steady state current-voltage characteristics of the amorphous (Se80Te20)98Y2 (Y = Ag, Bi, Ge, Cd) semiconductors at different temperatures are reported. The measurements were performed using direct-current voltage bias to understand the basic conductivity mechanism and to evaluate the impact of each substituent on electrical response. The space charge limited conduction mechanism, and the density of states near Fermi level have been calculated. The difference in electrical response due to different substitutions in the glassy matrix is analyzed.

Estimation of built-in voltage from steady-state current–voltage characteristics of organic diodes

A technique for extraction of built-in voltage from the steady-state current–voltage characteristics of a two terminal organic diode is described which does not require assumption of quadratic dependence of current on voltage. The technique relies instead on current voltage characteristics being exponential below built-in voltage and power-law above it to generate a sharply defined peak at a voltage proportional to built-in voltage. Simulation results are used to validate the proposed method and experimental results obtained with P3HT and P3HT:PCBM blends are presented.

Anode dissolution of nickel in segregating oxide systems

The kinetics of anode dissolution of nickel in molten sodium borosilicate and calcium aluminosilicate containing 0.5–3 wt % NiO is studied. Data on the current efficiency of nickel during its anode dissolution under various experimental conditions are obtained. The steady-state current-voltage characteristics of a nickel electrode are analyzed.

Multiple steady state current–voltage characteristics in drift–diffusion modelisation of N type and semi-insulating GaAs Gunn structures

Theoretical and numerical investigations of carriers transport in N–Semi-Insulating (SI)–N and P–SI–P diodes is extended to the case of extrinsic (N type) or SI samples with Gunn like electric field dependent mobilities. The results obtained in a preceding publication [1] are valid as long as the bulk electric field does not increase above a threshold field E th associated with the beginning of negative electron differential mobility values: μ n,diff = ( dv n / dE) < 0, v n being the electron drift velocity. Convergence and stability problems occur only, for the steady state numerical simulation, in long N +–N–N + or N +–SI(N −)–N + diodes. SI(N −) characterizes a SI layer which keeps, under applied bias, a free electron concentration close to its thermal equilibrium value up to the beginning of electron space charge injection. A systematic study has been made by varying the contact boundary properties: flat band, metallic, N + or P +; the length of the sample and the electric parameters of the deep compensating trap of the SI layers. We show that these steady state numerical instabilities are related to the existence of multiple current–voltage solutions when numerical modelisation is made using the drift–diffusion model.

Charge-trapping effect of doped fluorescent dye on the electroluminescent processes and its performance in polymer light-emitting diodes

We have measured the temperature dependence of the steady-state current-voltage (I-V) characteristics and the transient electroluminescent (EL) characteristics in 4-(dicyanomethylene)-2-t-propyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) doped polyfluorene devices to study the charge-napping effect of DCJTB fluorescent dye on luminescence processes and on device performance Physical and chemical analyses prove that DCJTB molecules serve both as election and hole traps, and the charge-trapping effect is more sensitive against the electrons than the holes at the low dopant concentration This intrinsic characteristic causes the electron to be injected into the emitting layer first and then trapped in the bulk, producing a strong effect on device performance. (C) 2010 Elsevier B V All rights reserved