Space Charge Limited Current Research Articles

Electrical characterization of silicon nitride interlayer-based MIS diode

In this study, silicon nitride (Si3N4) thin film on p-type GaAs wafer was deposited by RF magnetron sputtering. The surface morphology of Si3N4/GaAs structure was analyzed by atomic force microscopy (AFM). The electrical characteristics of the fabricated Au/Si3N4/p-GaAs metal–insulator-semiconductor (MIS) diode were investigated by using current–voltage (I–V) measurements at room temperature. The electronic parameters such as ideality factor (n) and barrier height (Φbo) of the MIS diode were derived using thermionic emission (TE). The Φbo was also extracted from Norde method. The barrier height values obtained from both TE and Norde were found to be in harmony with each other. The interface state density (Nss) and series resistance (Rs) parameters of the MIS diode were determined from the measured I–V data. In addition, the dominant current conduction mechanisms of the MIS diode were also investigated by forward bias ln(IF) − ln(VF) and reverse bias ln(IR) − VR0.5 plot. At high forward bias, the current conduction was associated with the space charge limited current (SCLC). At reverse bias region, the current conduction was associated with the Schottky emission (SE).

(Invited) Electrical Properties of Vapor-Deposited Organic Semiconductor Nanowires By Conductive Atomic Force Microscopy

Understanding how organic semiconductors self-assemble and create nanoscale morphologies is integral for the development of organic electronics. In particular, one-dimensional (1D) molecular nanostructures are of interest because of their unique optoelectronic properties as well as their potential role in fundamental charge transport research. Here, we investigate four small molecules (TpCPD, TpDCF, AcCPD, and AcDCF) possessing anisotropic, non-planar structures and large dipole moments, and establish robust algorithms to control their molecular self-assembly via physical vapor deposition. We find that only molecules containing the fused ring system form 1D nanowires due to the stronger van der Waals associations of the long, planar acenaphthene moieties. We examine the kinetics of self-assembly of AcDCF and create diverse 1D morphologies. Finally, using conductive AFM (c-AFM), we show that 1D wires support higher current densities relative to randomly-oriented clusters lacking long-range order through space-charge limited current (SCLC) measurements. Figure 1

A reaction-and-assembly approach using monoamine zinc porphyrin for highly stable large-area perovskite solar cells

Inhibiting the irreversible escape of organic cations and iodide species in perovskite films is crucial for the fabrication of efficient and stable perovskite solar cells (PSCs). Here, we develop a reaction-and-assembly approach using monoamine zinc porphyrin (ZnP) to modify methylammonium (MA+) lead iodide perovskite film. The amine group in ZnP reacts with MA+ and I− ions to yield monoammonium zinc porphyrin (ZnP-H+I−). The resultant films show no escape of iodide when immersed in ether solutions. Measurements from space-charge limited currents and transient photoluminescence indicate the modified films have reduced density of defects. These results suggest the formed ZnP-H+I− is bound on the surface and grain boundary of perovskite film to retard migrations of ions. DFT calculations also show that the energy alignment between ZnP-H+ and perovskite facilitates the electron transfer and reduces charge recombination at the perovskite grains. Furthermore, post-treating the ZnP-doped film with ZnP again results in the formation of a one dimension zig-zag coordination polymer on the surface of the perovskite film. The single crystal structure of ZnP shows the polymer layer is formed through the coordination interaction between the Zn(II) metal center and a neighboring monoamine. The polymer facilitates the interfacial charge transfer, and reduces the escape of organic cations and iodide species in perovskite films, thereby keeping the excellent cell performance (20.0%) and further realizing the ion encapsulation. Finally, the modified PSCs retain over 90% of its original efficiency over 2,000 h at 85 °C or AM 1.5 G continuous illumination, or over 6,000 h in 45% humidity without encapsulation. This work affords a new strategy to achieve the efficient ions immobilization and encapsulation by in situ reaction and coordination assembly of mono-amine zinc porphyrin.

Preparation and Surface Characterization of Nanostructured MoO3/CoxOy and V2O5/CoxOy Interfacial Layers as Transparent Oxide Structures for Photoabsorption

A simple low-cost electrochemical deposition technique has been used to grow molybdenum oxide (MoO3) and vanadium oxide (V2O5) nanostructures separately on cobalt oxide (CoxOy) thin film to form two different bilayer films. The samples were characterized by some surface probing techniques. Morphological and crystal structural studies revealed that the deposited α-MoO3 and V2O5 layers completely laminated the underlying cubic CoxOy film to form compact structures without any significant lattice distortion. The average particle sizes were estimated to be between 34 nm and 41 nm. Optical studies revealed the presence of two energy band transitions in the spinel CoxOy structure which can be attributed to the occurrence of ligand-to-metal charge transfer (LMCT). The electrical studies showed a trap-filled limit voltage (VTFL) in the CoxOy layer which was explained on the basis of space charge limited current (SCLC) behavior of porous film, and it was corrected with lamination by the overlayer. This study also reaffirmed the role of transition metal oxides as a recipe for window and buffer layers capable of minimizing photon absorption loss and recombination of charge carriers between absorber and the contact electrodes in optoelectronic devices.

Memristor switching and integration in ensembles of silicon nanocrystallites

We suggest a memristor device that is based on ensembles of Si nanocrystallites that are embedded in an SO2 matrix, for which we show that its operation is well accounted for by the theory of space charge limited currents. This operation consists of a memristive film that exhibits electronic charge integration properties as well as a potential to provide a quantum confinement controlled spiking behavior. As such, the present system is probably the closest available two-terminal electronic film analog that may imitate the neuron’s function. This system can be considered then, not only as “purely electronic” and “bio-realistic” but also as having the great advantage of being compatible with the silicon microelectronic technology. Corresponding devices have the potential to become practical by their downscaling, on the one hand, and by providing a controllable spiking mechanism on the same device, on the other hand.

Greatly enhanced breakdown strength of Pt/LNO/BST/Au thin films by regulating the space charge limited current though the dielectrics/electrode interface modification

Abstract Carrier injection is the most common and critical issue which significantly affects the electric breakdown strength (Eb) of the dielectric films since the dielectric thin films usually work under a rather higher applied electric field compared with its bulk forms. As so far, many efforts have been devoted to gain a more insightful understanding of the conduction mechanism behind the carrier injection. Recently growing interest is focused on using proper method of regulating these mechanisms to restrain the carrier injection for an ultrahigh Eb in the dielectric films. Here, a simple but effective approach is introduced that aims at regulating the space charge limited current (SCLC) for a restrained electrons injection by the dielectrics/electrode interface modification. An ultrathin SrTiO3 (STO) layer was inserted between the Pt/LaNiO3/Ba0.67Sr0.33Ti0.989Mn0.003Y0.008O3 (Mn/Y doped BST) dielectric thin films and the Au electrode. With the interface modification, the potential barrier at the dielectrics/electrode interface for electrons injection was significantly increased, and the SCLC was retrained. It was also found that the STO layer can further reduce the dielectric loss of the films. Consequently, the leakage current of the thin film was decreased by at least two order magnitude and the breakdown strength of the thin film was at least twice as high as that of untreated films with little effect on the dielectric constant. It should be helpful as a potential strategy to improve the breakdown strength and dielectric performance of dielectric thin films simultaneously.

Oxide-based selector with trap-filling-controlled threshold switching

A cross-point memory architecture will be adopted to make the most of the suitability of resistance change memories for high integration. To achieve this, the development of a selector device that is connected in series to the memory and helps to remove the sneak path current flowing through unselected and half-selected cells is an urgent requirement. In this Letter, we developed a selector based on a simple sandwiched structure of Pt/CoO/ITO and confirmed the nonpolar threshold resistance switching that does not require the first fire process. The selector showed an excellent performance of a very small dispersion of the threshold voltages and resistance in the OFF state with the relative standard deviations of less than 2% each, as well as the quick transition between the OFF and ON states within 50 ns. The scaling law was confirmed in the current, both in the ON and OFF states, ION and Ith. It allows us to analyze ION and Ith, based on which we can reduce the area of the device. The current conduction of the Pt/CoO/ITO selector is ruled by the trap-controlled space charge limited current (SCLC), and the threshold switching from ON to OFF states and vice versa is caused by the transition from the trap-unfilled to the trap-filled SCLC and vice versa, respectively.

Characteristics of analog memristor on thin-film Pt/Co0.2TiO3.2/ITO

In this work, thin films of Pt/Co0.2TiO3.2 (23 nm)/ITO were investigated showing their memristive capacity related to analog switching properties. A temporal analysis of current and voltage in nonlinear I–V graphs shows pinched hysteresis loop with gradual distribution in its electrical conductivity, typical of analog systems. The presence of trap centers such as oxygen vacancies suggests that the specific conduction mechanism is space-charge-limited current (SCLC). In addition, we study its performance under repetitive voltage pulses as well as at different durations. We further conducted data retention tests in the memristor high resistance state (HRS) and low resistance state (LRS).

Effect of Etching Duration on the Morphological and Opto-Electrical Properties of Silicon Nanowires Obtained by Ag-Assisted Chemical Etching

Silicon nanowires (SiNWs) were obtained on p-Si (100) substrate by Ag-assisted chemical etching method in two-step process. The influence of the etching duration on the morphological, optical and electrical properties of SiNWs samples was investigated. SEM images show clearly the presence of nanowires and the existence of porous silicon structure especially for low etching duration. Fourier Transformed Infrared spectroscopy (FTIR) measurements allow identifying several particles on the SiNWs surfaces such as oxygen and hydrogen elements. The optical properties of the SiNWs layer were investigated by Photoluminescence spectroscopy (PL). A strong broad PL band was observed at room temperature for an etching time of 30 s. The PL spectra exhibit multi-band profile in the red-green region. The luminescence of SiNWs is mainly attributed to Si-oxide interface states and oxygen deficient centers. Ag/SiNWs/Si Schottky diodes were analyzed by X-ray diffraction (XRD) and studied by current - voltage (I-V) measurements. Diode parameters such as ideality factor (n), series resistance (Rs), saturation current (Is) and energy barrier (φb) were determined by analyzing the I-V characteristics. Ag/SiNWs/Si diode for an etching duration of 30 s exhibits a rectifying behavior with a factor ideality of 2.8 and a low threshold voltage of about 0.44 V in forward bias. Space charge limited current (SCLC) model is the dominant transport mechanism through Ag/SiNWs/Si diode. The trap states, on the Ag/SiNWs interface, play in important role in the conduction phenomenon through these structures.

Efficient inverted perovskite solar cells with preferential orientation and suppressed defects of methylammonium lead iodide by introduction of phenothiazine as additive

High crystalline quality of perovskite thin film is essential for efficient metal halide perovskite solar cells (PSCs). In this investigation, we report a simple anti-solvent washing treatment using a versatile electron-rich heterocycle (phenothiazine, PTZ) as additive in sec-butyl alcohol to improve methylammonium lead iodide (MAPbI3) perovskite crystal growth. As a result, the PTZ-modified PSCs exhibit average power conversion efficiency (PCE) of 15.50%, obviously surpassing 14.38% of the reference device, which is due to the synergistic effects of the PTZ on the crystallization orientation and defects passivation of the perovskite films. The strong interaction between PTZ and Pb2+ ions not only promotes the size of the MAPbI3 crystals to increase from 0.58 μm to 0.73 μm and thus reduces the grain boundaries, is but also beneficial for the formation of a preferential orientation perovskite film, thereby delivering improved electronic properties. Moreover, as Lewis base, PTZ passivates the defects among perovskite crystals consistent with the decreased defect density calculated using the space charge limited current (SCLC) method. The result indicates using PTZ as additive into the perovskite precursor solution a highly effective and facile method to obtain high performance PSCs.

Effect of the Interface Improved by Self-Assembled Aromatic Organic Semiconductor Molecules on Performance of OLED

This work focuses on characterization the performance of enhanced interface of organic light emitting diode (OLED) device by Self-assembled Monolayer (SAM) technique. SAM technique is popular in order to overcome the weak bonding at the organic/inorganic interface in OLED. New generation of SAM molecules, phenyl-benzoic-acid (PBA, 4-(9H-carbazol-9-il) benzoic acid (MZ39), 4-(2,5-di-2thienyl-1H-pyrrol-1-il) benzoic acid (MZ25) were coated on between Indium Tin Oxide (ITO). The two configuration of ITO/SAM/TPD/Al and ITO/TPD/Al diode were fabricated as hole-only device to show the contribution of SAM layer on the hole mobility calculated by Space Charge Limited Current (SCLC) technique. The optical characterization of OLED devices with configuration ITO/TPD/Alq3/Al and ITO/SAM/TPD/Alq3/Al was performed to see the effect of aromatic SAM molecules on the luminance and quantum efficiency. Especially, the SAM modified OLED has a maximum luminance of 397 cd m−2. All devices containing SAM layer showed better performance than reference one.

Forward Current Conduction Mechanism of Mechanically Exfoliated β-Ga2O3/GaN pn Heterojunction Diode

In this letter, we fabricated the mechanically exfoliated β-Ga2O3/GaN pn heterojunction diode and carried out the electrical characteristics measurements. At room temperature, the diode shows a good rectifying property, with a turn-on voltage of 3.9 V–4.6 V and rectifying ratio greater than 106 @ ±20 V. From the 1/C2 vs V plot, the built-in voltage is determined to be 3.4 V and the energy band diagram of the heterojunction is also constructed. According to the temperature dependent I-V cures, three different forward current conduction mechanism can be identified, recombination-tunneling mechanism, trap charge limited space-charge-limited-current(SCLC) and SCLC mechanism for I < 10−7 A (region I), 10−7 < I < 10−4 A (region II) and I > 10−4 A (region III), respectively. While in region II, two different Et from exponential trap distribution model is determined to be 0.514 eV, 0.310 eV and the corresponding trap density is 1.63 × 1016 cm−3 and 1.71 × 1016 cm−3, respectively.

Synthesis, Doping and Characterization of new Molecular Semiconductors Containing (2E, 4Z)-5, 7-diphenylhepta-2, 4-dien-6-ynoic acids

This work refers to the synthesis and characterization of new (2E, 4Z)-5, 7-diphenylhepta-2, 4-dien-6-ynoic acids. We describe the nucleophilic addition of bis(trimethylsylil)ketene acetals (TMS) to aryl ynones substituted by halogen groups activated by boron trifluoride diethyl etherate (BF3·Et2O) for the stereoselective synthesis of dienynoic acid. The molecular materials were structurally characterized by IR spectroscopy, NMR spectroscopy and X-ray diffraction. After the characterization the synthesized acids were doped with indium(III) phthalocyanine chloride (In(III)PcCl) in order to generate a organic semiconductor that was characterized by UV–Vis spectroscopy to subsequently obtain their optical bandgap (Eg) values. The Eg value was compared to that obtained for the pure state dienynoic acids in order to evaluate the doping effect with the In(III)PcCl. The Eg diminished from values near 2.6 eV obtained for pure compounds to values around 1.4 eV for the same compounds, but now with doping. With the molecular semiconductors obtained were manufactured structures of disperse heterojunction which later were evaluated in their electric behavior. A behaviour ohmic at low voltages and Space Charge Limited Current (SCLC) at higher voltages was observe from the study J(V) carried out.

High hole mobility in room temperature discotic liquid crystalline tetrathienoanthracenes.

Tetrathienoanthracene (TTA), a new discotic core fragment, is explored that shows a remarkably high hole mobility (μh) of 4.22 cm2 V-1 s-1 at room temperature when used in space-charge limited current (SCLC) devices. Strong co-facial interactions (π-π, SS) among TTA cores along with a high tendency of the derivative to align homeotropically in the columnar mesophase over a large area in SCLC cells contributed to the top-class charge carrier mobility.

Electrical bistability in MoS2 nano-sheets doped polymeric nanocomposite films

Herein, we report the preparation and characterisation of nanocomposite films comprised of poly(ethylene oxide) (PEO) dispersed with exfoliated MoS2 nano-sheets. Few-layer nano-sheets of MoS2 with good stability and yield were prepared using liquid phase exfoliation method. Optical spectroscopic techniques and atomic force microscopy confirm exfoliation of MoS2 into few-layer nano-sheets. The current-voltage characteristics were investigated on two terminal devices by putting composite of PEO and MoS2 nano-sheets as the active material between ITO and aluminium electrodes. Electrical bistability observed with low resistance state (LRS) and high resistance states (HRS) were reversibly achieved by voltage sweeping. LRS (ON) and HRS (OFF) occurred at -3.5 V and 2 V respectively when the compliance was set at 50 µA. The resistance-ratio (Roff/Ron) ∼ 104 is achieved where the switching between resistance states is due to trap assisted space-charge limited current

Experimentally Validated Hopping-Transport Model for Energetically Disordered Organic Semiconductors

Charge transport in disordered organic semiconductors occurs by hopping of charge carriers between localized sites that are randomly distributed in a strongly energy dependent density of states. Extracting disorder and hopping parameters from experimental data like temperature dependent current-voltage characteristics typically relies on parametrized mobility functionals that are integrated in a drift-diffusion solver. Surprisingly, the functional based on the extended Gaussian disorder model (eGDM) has been extremely successful at this, despite it being based on the assumption of nearest neighbor hopping (nnH) on a regular lattice. We here propose a variable range hopping (VRH) model that has been integrated in a freeware drift-diffusion solver. The mobility model has been calibrated using kinetic Monte Carlo calculations and shows good agreement with the Monte Carlo calculations over the experimentally relevant part of the parameter space. The model is applied to temperature-dependent space charge limited current (SCLC) measurements of different systems. In contrast to the eGDM, the VRH model provides a consistent description of both p-type and n-type devices. We find a critical ratio of aNN/$\alpha$ (mean inter-site distance / localization radius) of ~3 below which hopping to non-nearest neighbors becomes important around room temperature and the eGDM cannot be used for parameter extraction. Typical (Gaussian) disorder values in the range 45-120 meV are found, without any clear correlation with photovoltaic performance when the same active layer is used in an organic solar cell.

Influence of top electrode on resistive switching effect of chitosan thin films

Abstract

Electrical Methods to Elucidate Charge Transport in Hybrid Perovskites Thin Films and Devices.

The panchromatic light absorption and excellent charge carrier transport properties in organo lead halide perovskites allowed to achieve an unprecedented power conversion efficiency in excess of 25 % for thin film photovoltaics fabrication. To understand the underlying phenomena, various comprehensive set of optical and electrical techniques have been employed to investigate the charge carrier dynamics in such devices. In this perspective, we aim to summarize the electrical transport properties of perovskite thin films by using (i) impedance spectroscopy (IS), (ii) space charge limited current (SCLC), (iii) field-effect transistors (FETs) and (iv) time-of-flight (TOF) methods. We have deliberated various equivalent circuit used to model the perovskite solar cells by means of IS. The SCLC technique provide vital electrical parameters such as mobility, activation energy, traps density and distribution, carrier concentration, density of states, etc. The TOF technique measures mobility as a primary parameter while the FETs configuration provide a valuable insight into the in-plane charge transport in perovskites thin films. We believe that these notable understanding will provide insights into charge carrier dynamics in perovskite materials and devices.

Impedance spectroscopy of Au/TiO2/n-Si metal-insulator-semiconductor (MIS) capacitor

In this study, the electrical properties of Au/TiO2/n-Si metal-insulator-semiconductor (MIS) capacitor were investigated by impedance spectroscopy (IS) technique. Impedance measurements were performed in the frequency range of 10 Hz-1 MHz for various bias voltages. The Cole-Cole plots show a single dielectric relaxation. The equivalent circuit was estimated from the shape of the Cole-Cole plots. The equivalent circuit of the MIS capacitor consists of a parallel resistor (Rp) and capacitor (Cp) in series with a resistor (Rs). It is observed that the Cole-Cole plots indicate a semicircle. The parameters of the equivalent circuit were determined by fitting the impedance measurement data. While the Rp value decreases with increasing the bias voltage, the Cp and Rs value are almost independent of the bias voltage. From the variation of log(Rp) with log(V), the dominant conduction mechanism of the MIS capacitor was determined as space-charge limited current (SCLC) mechanism.

Structure and electrical properties of Zn‐doped BiFeO3 films

AbstractBiFe1−xZnxO3 (x = 0, 0.5, 1, 1.5, 2 mol%) (BFZO) films were prepared on ITO/glass substrates by a sol‐gel method. The effects of different Zn contents on the structures and electrical properties of the BFZO films were investigated. From X‐ray diffraction (XRD), microstructure and X‐ray spectroscopy (XPS) results, the BFZO films with a Zn content of 1 mol% showed a better crystal structure and grain development, and the Fe2+ and oxygen vacancy concentrations in this sample were the lowest among all the evaluated BFZO films. The P‐E hysteresis loop indicated that the BFZO films with 1 mol% Zn had the highest remanent polarization (2Pr), which was 82.4 μC/cm2, along with a coercive field (2Ec) of 887 kV/cm at the tested electric field of 857 kV/cm. The BFZO film with 1 mol% Zn had the lowest leakage current density, which was 3.54 × 10−7 A/cm2 at the tested electric field of 200 kV/cm. Both at high and low electric fields, the space charge‐limited current (SCLC) conduction mechanism was the main leakage mechanism. When the test frequency was 105 Hz, the dielectric constant was 133, and the dissipation factor was 0.015.