Space Charge Depletion Layers Research Articles

Mass transport and charge transfer through an electrified interface between metallic lithium and solid-state electrolytes

All-solid-state Li-ion batteries are one of the most promising energy storage devices for future automotive applications as high energy density metallic Li anodes can be safely used. However, introducing solid-state electrolytes needs a better understanding of the forming electrified electrode/electrolyte interface to facilitate the charge and mass transport through it and design ever-high-performance batteries. This study investigates the interface between metallic lithium and solid-state electrolytes. Using spectroscopic ellipsometry, we detected the formation of the space charge depletion layers even in the presence of metallic Li. That is counterintuitive and has been a subject of intense debate in recent years. Using impedance measurements, we obtain key parameters characterizing these layers and, with the help of kinetic Monte Carlo simulations, construct a comprehensive model of the systems to gain insights into the mass transport and the underlying mechanisms of charge accumulation, which is crucial for developing high-performance solid-state batteries.

Impact of positive space charge depletion layer on negatively charged and neutral centers in gold–diamond Schottky junctions

It has been widely accepted that conversion between negatively charged nitrogen-vacancy (NV−) center and neutral nitrogen-vacancy (NV0) center can be realized with the Fermi level crossing the transition level corresponding to the ground state of NV−. In this study, a suppressing effect was observed in the emission of NV− center; however, emission of NV0 center was not observed to be increasing as expected and was stable when lowering of the Fermi level was induced by a Schottky junction between gold and oxygen-terminated diamond containing NV centers. The relationship between the thickness of the depletion layer as well as that of the suppression layer, appearing when the Fermi level shifted below the ground state level of NV−, and the donor concentration was derived. Moreover, a model based on the assumption that an intermediate transition level (nonfluorescence state) existed between the ground state energy levels of NV− and NV0, was proposed to explain why suppressed NV− did not convert to NV0. The results of this study will aid in further understanding of the characteristics of NV centers. In addition, a Schottky junction can be developed to determine the charge state of unknown color centers and facilitate study of defects in diamonds.

Sintering and conductivity of BaCe 0.9Y 0.1O 2.95 synthesized by the sol–gel method

Ceramic powders of BaCe 0.9Y 0.1O 2.95 (BCY10) have been prepared by the sol–gel method. Barium and yttrium acetate and cerium nitrate were used as ceramic precursors in a water solution. The reaction process studied by DTA–TG and XRD showed that calcination of the precursor powder at T ≥ 1000 °C produces a single perovskite phase. The densification behaviour of green compacts studied by constant heating rate dilatometry revealed that the shrinkage rate was maximal at 1430 °C. Sintered densities higher than 95% of the theoretical one were thus obtained below 1500 °C. The bulk and additional blocking effects were characterized by impedance spectroscopy in wet atmosphere between 150 and 600 °C. A proton conduction behaviour was clearly identified. The blocking effect can be related to a space-charge depletion layer of protons in the vicinity of grain boundaries.

Effect of DC bias on electrical conductivity of nanocrystalline α-CuSCN

The grain boundary space charge depletion layer in nanocrystalline alpha phase CuSCN is investigated by studying electrical properties using impedance spectroscopic analysis in frequency domain. The measurements were performed at room temperature in wide frequency range 1 Hz to 1 MHz under various DC bias applied voltages ranges from 0 V to -2.1 V. The effect of bias on grain and grain boundary contribution electrical conductivity has been investigated by equivalent circuit model using non-linear least squares (NLLS) fitting of the impedance data. Three order of magnitude variation of grain boundary conductivity was observed for varying 0 V to -2.1 V. Variations in the σac clearly elucidate the DC bias is playing crucial role on grain boundary double Schottky barriers of nanocrystalline α-CuSCN.

Bulk and grain boundary resistivities of donor-doped barium titanate ceramics

The electrical properties of both the bulk and the grain boundaries of positive temperature coefficient (PTC) resistors have been determined as a function of temperature ranging from 25 to 400 °C by application of impedance spectroscopy. The resistances of the bulk and the grain boundaries as well as the capacitance of the grain boundary regions of n-conducting BaTiO 3 ceramics have been obtained by fitting an appropriate equivalent circuit to the impedance spectra. Samples of equal dimensions and composition but different grain sizes (2.8 and 6.6 μm) have been investigated. Whereas the bulk resistance remains almost constant throughout the whole temperature range, the grain boundary resistance shows a steep increase of several orders of magnitude (PTC effect) above the Curie temperature (around 90 °C). The grain boundary resistances of the disk-shaped samples (dimensions: 2.2 mm thickness, 8.0 mm diameter) vary typically between 35 Ω and 30 MΩ, and the bulk resistances are found to be approximately 4 Ω (bulk resistivity: 10 Ω cm). The variation of the grain boundary capacitance with temperature, depending remarkably on the grain size of the samples, follows the Curie–Weiss law. Both the steep increase of the grain boundary resistance and the Curie–Weiss behaviour of the grain boundary capacitance are described theoretically by application of an extended Heywang model. The defect chemistry in the grain boundary regions is modelled by assuming Schottky type space charge depletion layers. Optimized values for the temperature-dependent energy level and the density of the acceptor type grain boundary states are given.

The grain boundary impedance of random microstructures: numerical simulations and implications for the analysis of experimental data

The grain boundary impedance of polycrystalline materials is usually analyzed in terms of a simplified microstructure composed of cubic-shaped grains. In order to investigate the validity of such a “brick layer model”, impedance spectra of polycrystals with computer-generated microstructures (two-dimensional Voronoi diagrams) have been simulated. These calculations showed that the current density in polycrystals can be rather inhomogeneous and that the so-called grain boundary impedance depends on the bulk conductivity. Nevertheless, the corresponding grain boundary resistances, capacitances and relaxation frequencies are frequently close to the values expected from a brick layer model. The impact of these calculations on the analysis of grain boundary impedances caused by space charge depletion layers is discussed. It is shown in how far impedance parameters yield reliable space charge potentials. Measurements on polycrystalline SrTiO 3 illustrate the suggested approach. In the quantitative analysis of these experiments, capacitances have been calculated from constant phase elements.

Microcontact Impedance Spectroscopy at Single Grain Boundaries in Fe‐Doped SrTiO3 Polycrystals

Microcontacts on adjacent grains of polycrystalline Fe‐doped SrTiO3 samples have been used to locally investigate the properties of individual grain boundaries. Impedance spectroscopy was employed to separate bulk and grain boundary impedances. Experiments at about 30 different grain boundaries permit far‐reaching conclusions on the distribution of grain boundary resistances, capacitances, and peak frequencies measured between adjacent grains. The rather narrow distribution of the grain boundary peak frequencies indicates a narrow distribution of grain boundary resistivities. All features (e.g., nonlinear current–voltage characteristics, grain boundary thickness, temperature dependence) are in accordance with the assumption of space charge depletion layers (double Schottky barriers) as the origin of the enhanced grain boundary resistivity. The average barrier height measured was about 630 mV. For comparison conventional (macroscopic) impedance measurements on a polycrystal were also performed and a brick layer model was used to extract effective properties. The reasonable agreement between these effective parameters and the average of the locally obtained parameters demonstrates that, in this case, a brick layer analysis of conventional impedance experiments yields satisfying estimates of the grain boundary properties.

Influence of temperature and interface charge on the grain-boundary conductivity in acceptor-doped SrTiO3 ceramics

The temperature dependence of both the grain-boundary potential barrier height and the conductivity across the grain-boundary space-charge depletion layer in acceptor-doped SrTiO3 ceramics has been investigated by a numerical simulation technique. The underlying model is that of a back-to-back double Schottky barrier at the grain boundary. The influence of the amount of positively charged donorlike grain-boundary interface states on the charge transport behavior of the grain-boundary region is also discussed. An interpretation in terms of a defect chemistry model of the bulk and the space-charge depletion layer, which on both sides surrounds the grain-boundary core is presented. The temperature behavior of the potential barrier at the grain boundary can be divided into three different regimes: a linear regime, and subsequently, saturation and decreasing regimes. The theoretical explanation for this behavior is given. Two different spatial conductivity profiles at the grain boundaries have to be considered, which clearly can be identified by two different characteristic thermal activation energies of the effective grain-boundary conductivity. The barrier height itself is not equal to the the thermal activation energy of the effective grain-boundary conductivity. The electrical characteristics of the grain boundaries can be influenced deliberately by decorating the grain boundaries with suitable dopants.

Grain‐Boundary Defect Chemistry of Acceptor‐Doped Titanates: Inversion Layer and Low‐Field Conduction

The electronic and ionic conduction across grain‐boundary (GB) space‐charge depletion layers in acceptor‐doped SrTiO3 ceramics have been investigated by admittance analysis in the time domain. The dependence on the acceptor concentration, the oxygen partial pressure during equilibration, and the temperature dependence of the GB conductivity are discussed and interpreted in terms of a defect‐chemistry model of the GB region using a numerical simulation technique.

Nonlinear Charge Transport in Acceptor‐Doped Titanates

AbstractThe field‐enhanced conductivity of grain boundary space charge depletion layers in acceptor‐doped SrTiO3 ceramics was investigated by impedance analysis in the time domain. The dependence of the GB conductivity on the external applied electrical field are discussed and interpreted in terms of a Schottky diffusion model.

Grain Boundary Defect Chemistry of Acceptor-Doped Titanates: High Field Effects

The field enhanced average conductivity in grain boundary space charge depletion layers in acceptor-doped SrTiO3 ceramics was investigated by impedance analysis in the time domain. The dependence of the grain boundary conductivity on the acceptor concentration, the temperature and the field dependence are discussed and interpreted in terms of a Schottky diffusion model combined with the influence of accumulated oxygen vacancies at the GB interface.

Electronic properties of grain boundaries in SrTiO3 and BaTiO3 ceramics

The grain boundary space charge depletion layer in acceptor-doped SrTiO3 and BaTiO3 ceramics is investigated by an impedance analysis in the time domain and is described in terms of a simplified back-to-back double Schottky barrier model. A numerical simulation is employed to analyze the defect chemistry in the depletion layer and to elaborate a refined Schottky model. The local distribution of the donor-type grain boundary states causing the depletion layer and the resulting band bending are discussed.

Grain Boundary Defect Chemistry of Acceptor‐Doped Titanates: Space Charge Layer Width

The grain boundary space charge depletion layers in acceptor‐doped SrTiO3 and BaTiO3 ceramics were investigated by impedance spectroscopy in the time and frequency domain. Based on the layer width and its dependence on the acceptor concentration, the temperature, and the oxygen partial pressure during annealing, a suggestion for a refined Schottky model is proposed. The local distribution of the donor‐type grain boundary states causing the depletion layer and the resulting band bending are discussed.

Theory of conduction and breakdown in perovskite thin films

The mechanism of the dc electrical conduction and breakdown of perovskite-type titanates was investigated by impedance analysis. Based on an acceptor doped SrTiO3 model material, samples of different microstructures-ceramics, single crystals, and thin films-were employed. This approach allows us to distinguish conduction contributions of the bulk lattice, grain boundaries, and electrode interfaces. Based on defect chemistry studies, a predominant ionic contribution due to mobile oxygen vacancies and an additional p-type conduction were revealed for the bulk. At interfaces, space charge depletion layers of 100-500 nm width are formed in which the local conductivity is reduced by approx. four orders of magnitude compared to the bulk. Thin films show a similar depression of the conductivity. The combination of these facts may be indicative for considering thin films as distributed Schottky barriers. The field enhancement of the conductivity of thin films and of the interface depletion layers is compared and ...

The surface photovoltage of polymethine semiconducting films

Recent solar cell work on Al/merocyanine/Ag sandwich devices has prompted the examination of the surface photovoltage of thin films (∼500 Å) of merocyanine and of four other closely related polymethine type dyes deposited on Ag rear contacts. All of these materials exhibited space charge depletion layers at the front surface with energy band bending changes resulting from a laser pulse ranging from 1 to 100 mV, and corresponding photovoltaic relaxation time constants on the order of 30 msec. The spectral dependence of the observed photovoltage was dominated by the nominally ’’forbidden’’ S0→T1 transition in the near IR. Much less photovoltage is observed at wavelengths corresponding to the allowed S0→S1 transition, even though light of this photon energy is strongly absorbed. This discrepancy between the relative intensity in the absorption and photovoltage spectra could be due to decreased dissociation of the bound excitons, and/or to increased recombination of the free carriers at the organic semiconductor free surface.

Electrochemical Behaviour of the Lithiated Nickel Oxide Crystal

Abstract The electrochemical properties of lithiated nickel oxide were studied using a single-crystal (100) face and a sintered body. The I–V curves of the Fe2+/Fe3+ system on the single-crystal electrode were almost the same as those on the sintered-body electrode when the electric resistivities of the crystals were equal. However, a high amount of charge was obtained at the sintered-body electrode compared with the single-crystal electrode when the crystals were oxidized anodically. The effect on these reaction rates of the space charge depletion layer formed on the electrode surface will be discussed. The dependence of the reaction rates on the electric resistivity of the crystal can be understood to a certain extent through this discussion.