Formation Of Space Charge Region Research Articles

Performance improvement mechanisms of perovskite solar cells by modification of NiO hole-selective contacts with self-assembled-monolayers

This study investigates effects of the modification of NiOx hole-selective contacts (HSCs) with self-assembled monolayers (SAMs), [2-(9H-carbazol-9-yl) ethyl]phosphonic acid (2PACz), on the characteristics of perovskite solar cells, used for perovskite–silicon tandem cells. After the NiOx layers were fabricated by RF sputtering, the 2PACz modification was performed by the spin-coating of a 2PACz solution and subsequent annealing at 100 °C for 10 min. The 2PACz-modified NiOx layer improved the solar cells' open-circuit voltage and short-circuit current density. Photoelectron yield spectroscopy measurements imply that the 2PACz monolayers increase the work function of the NiOx HSCs, thereby enhancing field-effect passivation at the perovskite–HSC interface. Density functional theory calculations and electron spin resonance measurements indicate that an increase in the work function results from a vacuum level shift due to the electric dipole moment of 2PACz molecules and formation of space-charge region at the NiOx–2PACz interface. The performance improvement is explained as a result of enhanced field-effect passivation caused by the vacuum level shift and thus the increase in the work function. These findings, which clarify SAM modification effects of HSCs on solar-cells’ performance, can contribute to the development of highly efficient and reliable perovskite–silicon tandem solar cells.

New stepwise potentiostatic charge protocol for preventing dendrite formations in Li metal batteries

Here, we report a new charge protocol for dendrite-free Li-metal battery based on the understanding the effects of over-potential on Li dendritic growth. We demonstrate that when a large over-potential is applied, the deposited Li metal shows a needle-like morphology even though a current density of a cell does not reach the limiting current density. This clearly indicates over-potential leads to the early formation of space charge region near a negative electrode, and thereby the Li dendritic growth can be easily triggered. As a result, control of the over-potential is very important for dendrite-free Li metal batteries. Through this understanding, we propose a new stepwise potentiostatic charge protocol that can keep the over-potential below a certain critical value. By applying new charge protocol, we clearly demonstrate that Ni-rich layered materials can be fully utilized without triggering the Li dendritic growth. The understanding and finding provide a novel guideline and stimulate further research to develop ways to safely operate high energy density Li metal battery.

Influence of Inorganic Glass Ceramic Particles on Ion States and Ion Transport in Composite Single-Ion Conducting Gel Polymer Electrolytes with Varying Chain Chemistry

A major goal of next-generation battery development is the engineering of nonflammable solid-state electrolytes with high enough ionic conductivity to compete with traditional liquid electrolytes. Composite polymer electrolytes (CPEs), which combine inorganic fillers or electrolytes with a polymer matrix, are seen as a strategy to boost the ionic conductivity of flexible polymer electrolytes while overcoming the brittle aspect of inorganic electrolytes. In this work, we examine the impact of polymer backbone chemistry on Li+ ion conduction within crosslinked single-ion conducting gel polymer electrolytes (SIPEs) that contain a lithium ion-conducting glass ceramic electrolyte (LICGC). Certain SIPE compositions based on poly(tetrahydrofuran) diacrylate (PTHFDA) crosslinking macromonomers exhibit a significant increase in conductivity with the inclusion of LICGC, a result unexpected from prior literature. With the use of Raman spectroscopy, small angle X-ray scattering, and particle-induced gamma-emission spectroscopy (PIGE), it is proposed that the enhanced conductivity comes from the formation of percolated LICGC particles sheathed in an ion-rich domain. This region develops in the pre-polymer solution due to interactions between the LICGC particle surface and the ionic comonomer, much like the formation of space-charge regions in soggy-sand liquid electrolytes, and persists post-polymerization to yield a CPE of enhanced conductivity. The particle–ionic monomer interactions are modulated by the crosslinking macromonomer polarity, polymer casting solvent, and particle surface area. While there is ample room for continued optimization, the best SIPEs in this study are capable of Li metal dissolution/deposition, and they reach Li+ conductivities greater than 2 × 10–4 S/cm at 25 °C, surpassing the practical use threshold.

A short review on heterojunction photocatalysts: Carrier transfer behavior and photocatalytic mechanisms

Construction of heterojunctions by coupling two semiconductors together is one of the most efficient ways to realize the electron-hole pair separation. In this short paper, I systematically discuss the properties of heterojunction photocatalysts in all cases. The formation of space charge regions, built-in electric field and potential barriers at the interface regions of thermally equilibrated heterojunctions are analyzed in details. When the heterojunctions are used for photocatalysis, the transfer behavior and mechanism of photo-excited non-equilibrium carriers between the constituent semiconductors are discussed. It is demonstrated that the heterojunction properties, carrier transfer behavior and photocatalytic mechanism depend highly on the semiconductivity (N-type or P-type), work function (or Fermi level), and CB/VB potentials of the constituent semiconductors. All types of the heterojunction photocatalysts are expected to exhibit enhanced photocatalytic performances. This paper provides an important insight into the understanding of heterojunction photocatalytic mechanism and design of excellent heterojunction composite photocatalysts.

Modeling how defects affect conductivity of capacitor materials

A computational framework reveals how the formation of space charge regions controls the conductivity of polycrystalline materials and resultant properties in electronic devices.

Thermal Poling of New Double-Hole Optical Fibers

Fused silica are common fiber materials which have macroscopic central symmetry without second-order nonlinearity. Studies have shown that thermal poling of fused silica fibers can destroy this macroscopic central symmetry, resulting in second-order nonlinearity or linear electro-optical effects. In this paper, a new type of double-hole optical fiber is designed. A two-dimensional (2D) numerical model is used to simulate the movement of ions and the formation of space charge region by finite element analysis. It is found that the single round square hole structure of the new double-hole fiber promotes the thermal poling process. The effective second-order nonlinear coefficient χ eff ( 2 ) of the new double-hole poled fiber is 0.28 pm/V at the core center, which is 0.05 pm/V higher than that of the circular double-hole poled fiber. In the fiber core, the radial distribution of the internal electric field and of χ eff ( 2 ) is calculated and analyzed. The results of this paper are of great significance for the application of thermally poled fibers on nonlinear all-fiber devices.

Phase-pure VO2 nanoporous structure for binder-free supercapacitor performances

Vanadium oxides are anticipated as a high-performance energy storage electrode due to their coupled double layer and pseudo-capacitative charge storage mechanism. In the present work, we investigated the influence of different structural phases of as-grown VO2 nanoporous structure and corresponding oxidation states on the supercapacitor performance. This nanoporous structure facilitates fast ion diffusion and transport. It is shown that stoichiometric monoclinic VO2, with V oxidation state of +4, provides superior charge storage capacity with a capacitance value of 33 mF/cm2, capacitance retention of 93.7% and Coulombic efficiency of 98.2%, to those for VO2 structures with mixed oxidation states of V5+ and V4+. A comparable high energy density is also recorded for the sample with all V4+. Scanning Kelvin probe microscopy results clarify further the formation of space charge region between VO2 and carbon paper. These key findings indicate the potentiality of binder-free single phase monoclinic VO2 porous structure towards the next-generation micro-supercapacitor application.

Ionic conduction in sodium azide under high pressure: Experimental and theoretical approaches

Alkali metal azides can be used as starting materials for the synthesis of polymeric nitrogen, a potential material of high energy density. In this letter, we report the ionic transport behavior in sodium azide under high pressure by in situ impedance spectroscopy and density functional theory calculations. The ionic transportation consists of ion transfer and Warburg diffusion processes. The ionic migration channels and barrier energy were given for the high-pressure phases. The enhanced ionic conductivity of the γ phase with pressure is because of the formation of space charge regions in the grain boundaries. This ionic conduction and grain boundary effect in NaN3 under pressures could shed light on the better understanding of the conduction mechanism of alkali azides and open up an area of research for polymeric nitrogen in these compounds and other high-energy-density polynitrides.

Light intensity dependent characteristics of micro-textured Si/PEDOT:PSS heterojunction solar cell

Heterojunction solar cells made of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and Si have attracted a lot of attention towards low-cost and efficient devices. Here, heterojunction solar cell using pristine PEDOT:PSS on micro-textured n-Si has been fabricated. Effect of light intensity on photovoltaic characteristics of PEDOT:PSS/Si cells has been investigated via current (J) versus voltage (V) characteristics and quantum efficiency (EQE) measurements. It is found that photocurrent (Jsc) of such cells deteriorates drastically at higher light intensities resulting in quite low Jsc (~ 15.0 mA/cm2) in J–V response under intense white light than that obtained from EQE measurements (> 30.0 mA/cm2) under low intensity monochromatic light. The observed effect of light intensity on the cell performances is reversible. Structural stability of PEDOT:PSS layer is also investigated by Raman spectroscopy. No post high intensity light exposure structural transformation in the PEDOT:PSS layer is observed. The observed light sensitive photoresponse of such cells has been attributed to possible light induced instantaneous structural changes in the PEDOT:PSS layer leading to reduced charge carrier dynamics and hence the formation of space charge region at PEDOT:PSS/Si interface at higher intensities. Further, the EQE with varying intensity is done which can help to optimize the illumination condition for such cell. The present study opens up new area of intensive research required in order to optimize polymer layer properties and improving the performance of PEDOT:PSS/Si solar cell leading to efficient, stable and low cost heterojunction solar cell technology.

The influence of Au nuclei layer on formation and photoelectrochemical properties of Cu2O thin films

The smooth and uniform Cu2O thin films were prepared by electrodeposition with the introduction of Au nuclei layer. The influence of Au nuclei layer on the formation of these thin films was investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and the optical absorption (UV–vis/DR) methods. Smooth and uniform Cu2O thin films without any islands and pinholes are formed when the electrodeposition is conducted after sputtering Au nuclei layer on ITO. Au nuclei layer provides much larger surface area, enhances the Cu2+ ion migration, and consequently prompts the deposition of Cu2O nuclei on Au nuclei layer to form smooth and uniform Cu2O thin films. Further, the Au nuclei layer plays an important role in improving photoelectrochemical performance of Cu2O thin films. The samples with Au nuclei layer exhibits increased absorption and red shift of absorption edge. The observed steady-state photocurrent of samples with Au nuclei layer is much higher than that of samples without Au nuclei layer as a result of the formation of space charge region in Cu2O thin films when the Au and Cu2O are in contact, which greatly improves the production and separation of photo-induced electrons and holes.

Buried Interfaces Effects in Ionic Conductive LaF3–SrF2 Multilayers

In multiphase/multilayer solid electrolytes, the composition, reactivity, and structure of interfaces between materials and phases play a fundamental role for fast ion‐conduction. Here, the properties of buried interfaces in prototypical fast ion‐conducting LaF3/SrF2 epitaxial multilayers are investigated. Photoelectron spectroscopy—both with soft‐X and high‐energy photons—is applied to separate composition and reactivity of buried interfaces with respect to the outermost surface. X‐ray reflectivity, high‐energy electron diffraction, X‐ray diffraction, atomic force and transmission electron microscopies are used to study morphology, layer crystallinity, epitaxy relations, and buried interface structure. It is found that while the alternated layers present good crystallinity and high lattice matching, with formation of almost ideal sharp interfaces, buried interfaces show a sizeable reduction of the energy barrier for F vacancy formation with respect to bare materials. A density higher by a factor of six of fluorine vacancies is observed at buried interfaces in multilayers with respect to the bare materials. This is correlated to the formation of space charge regions, favoring ion conduction. The formation of F depleted La fluoride regions at interfaces is also promoted by annealing. This is associated to the increase of ion conductivity in annealed heterostructures reported in literature.

Charge Transfer at Organic/Inorganic Interfaces and the Formation of Space Charge Regions Studied with Infrared Light

We present in situ infrared spectroscopy as a powerful tool for the qualitative and quantitative analysis of the charge transfer through the prototypical interface between the organic semiconductor 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) and MoO3 that in organic electronic devices is often used to improve their performance. Due to the different infrared vibrational spectra, charged and neutral species of CBP molecules can be well distinguished, which allows the measurement of the amount of charged species in the vicinity of the interface. The quantitative analysis of CBP thickness-dependent infrared transmission spectra delivered the extension of the space charge region from the interface into the CBP on a nanometer scale. The clear influence of the deposition sequence on these interface properties was clarified by further studies of the inverted layer structures.

Exact 3D simulation of Scanning Electron Microscopy images of semiconductor devices in the presence of electric and magnetic fields

Simulations of Scanning Electron Microscopy images of semiconductor devices in the presence of electric fields are usually too simplistic, since they just rely on approximated solutions of the Poisson equation. In this paper, the 3D Poisson equation is solved in a TCAD environment, which accounts for realistic boundary conditions, as well as for complex physical effects like the formation of space charge regions in semiconductors and the polarization of dielectrics. The calculated solution is then passed to a Monte Carlo code that implements a new electron tracking engine optimized for speed, stability, and accuracy. After introducing the new tracking engine, three simulation examples are presented dealing with the presence of an extraction field, self-charging of the irradiated sample, and potential contrast in a biased silicon junction.

Dielectric spectroscopic studies of boron subphthalocyanine chloride thin films

Boron subphthalocyanine chloride (Cl-BsubPc) thin film have been deposited by thermal evaporation technique and studied for structural and dielectric properties. AFM study indicates the formation of uniform and cracks free films. To study the dielectric properties as function of both frequency and voltage, Al/Cl-BsubPc/Al heterostructure has been fabricated. The impedance-frequency study indicates the formation of space charge region in lower frequency range. The conduction mechanism in Cl-BSubPc film, under applied ac field, found to be electronic hopping. The mobility and charge carrier concentration of Cl-BsubPc films have also been measured using capacitance techniques.

Dielectric spectroscopic evidence of space-charge dynamic effects in lead oxyfluoroborate glasses during evolution to β-PbF 2 electrocrystallization

Understanding of the mechanisms behind electrocrystallization in glassy electrolytes has been the subject of permanent research. For lead oxyfluoroborate glasses, evidence exists that this phenomenon involves redox-type electrochemical reactions that develop at the glass–electrode interfaces, promoting β-PbF 2 crystallites nucleation. Here, especially, the form by which these glasses dielectrically act in response to the electric field in the stage preceding crystallization incidence was investigated through applying electrochemical impedance spectroscopy and polarization/depolarization current techniques. This study shows that formation of space-charge regions, which reveal a peculiar dynamics and should incorporate the electroactive species susceptible to redox reactions, is the very first detectable reaction of these glasses during evolution to electrocrystallization.

Mechanism Of Trace Level H2S Gas Sensing Using Rf Sputtered SnO2 Thin Films With Cuo Catalytic Overlayer

Abstract H2S gas sensing response characteristics of bare SnO2 thin films and heterostuctures with nanolayer (10 nm) of Cu and CuO are studied. Changes in resistance values, occurring with integration of Cu and CuO nanolayers on SnO2 is acquired real-time, and compared. Rise in sensor resistance after introduction of Cu and CuO nanolayers on SnO2 sensing layer is understood to enhance the sensing response characteristics. Formation of space charge region between p-type CuO and n-type SnO2 and difference in work-function values between catalyst and sensing layer are shown to govern the increased value of starting resistance. Increase in starting resistance and lowering of resistance in presence of H2S due to spill-over of dissociated H2S gas molecule are playing crucial role in influencing the sensing response.

Charge generation layer in stacked organic light-emitting devices

Three types of organic-based connection units were examined for use in stacked or tandem organic light-emitting devices, which include (i) Mg-doped tris(8-hydrooxyquinoline) aluminum(III) (Alq3)∕4,4′,4″-tris{N,-(3-methylphenyl)-N-phenylamino}-triphenylamine (m-MTDATA), (ii) Alq3/tetrafluorotetracyanoquinodimethane (F4-TCNQ)-doped m-MTDATA, and (iii) Mg-doped Alq3/F4-TCNQ-doped m-MTDATA. Device (iii) shows the highest current efficiency and the differences in device performance can be correlated with the electronic structure of the connection unit and its interface with the neighboring active layers. The working mechanisms of the connection-unit works are discussed in terms of band bending and charge carrier density. The electronic structures of the interface between layers in a connection unit are of particular importance to the device performance. Dopings of Mg in Alq3 and F4-TCNQ in m-MTDATA led to bipolar heterojunction. Removal of either the n-type or the p-type dopants suppresses the band bending and the formation of space charge regions. The charge density accumulated at this interface estimated from Poisson’s equation is 1018∕cm3, which is respectively 12 and 6 orders of magnitude higher than that in the Mg:Alq3/m-MTDATA and Alq3/F4-TCNQ:m-MTDATA connection units. Based on these results, the critical roles of dopants in an efficient connecting unit for stacked organic light-emitting diodes are elucidated.

Thickness-dependent dc conductivity of lithium borate glasses

Amorphous lithium borate films are prepared by ion-beam sputtering using a $0.2\phantom{\rule{0.3em}{0ex}}{\mathrm{Li}}_{2}\mathrm{O}∙0.8\phantom{\rule{0.3em}{0ex}}{\mathrm{B}}_{2}{\mathrm{O}}_{3}$ glass as a target material. The films of a thickness between 7 and $700\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ are deposited onto silicon substrates between two metallic AlLi electrodes serving as electrical contacts. dc conductivities of the glass layers of different thicknesses are determined by temperature-dependent impedance measurements. An increase of the specific dc conductivity of about 3 orders of magnitude is observed, when the thickness of the layers decreases from 120 down to $7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. This conductivity increase in the case of very thin films leads to an absolute conductance per unit area of $0.32\phantom{\rule{0.3em}{0ex}}{\ensuremath{\Omega}}^{\ensuremath{-}1}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ at $25\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, which is sufficient for technical application of the glass layers in solid-state ionic devices. Three alternative explanations are suggested to explain this nontrivial conductivity increase: structural modifications at the interfaces, the formation of space-charge regions, and the existence of randomly distributed ion-conducting channels.

Electrical barriers in the ZnO varistor grain boundaries

AbstractIn this paper a detailed nature of the electrical potential barriers in ZnO based varistors are presented. The excellent nonlinear current–voltage (I–V) characteristic of these varistors is attributed to the potential barriers formed between the successive ZnO grains during the processing cycle. These grain boundaries in the microstructure results in a few device‐related parameters such as built‐in‐potential, barrier height, barrier width, grain carrier density, Fermi level position in the grains, etc. Thermal activation energy is determined from the status of the leakage current of this device. Substantial studies conducted to comprehend possible conduction processes in the grain boundary regions under applied bias (electric field). It is understood that electron transport dominate the grain boundary regions based on electrical potential barrier consideration across the grain boundaries. The free electrons overcome potential barrier at elevated temperatures as well as under bias. These carriers fill trap sites (charged defect states) both in the bulk ZnO and in the grain surfaces (surface states). The formation of space charge region within the successive ZnO grains across the grain boundaries is a result of the equilibrium state of the device. Based on the findings an expression of the Schottky potential barrier width is proposed satisfying relevant expression of the current caused by the application of the electric field. It is observed that the potential barrier width is sharply reduced in the temperature range 320 K ≤ T ≤ 350 K. This reduction in the potential barrier width is likely to be associated with the relaxation process of the trap filled (trapped) carriers, usually monitored by the dielectric response (in the form of differential capacitance) of the device. Thus, the results provide an improved understanding of the nonlinear conduction behavior in ZnO varistors. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ionic conductivity of colloidal electrolytes

This paper reports an exploratory investigation on the effects of a colloidal phase on the conductivity of lithium-ion liquid electrolytes. The conductivities of liquid electrolytes comprising of organic solvents, lithium salts and a colloidal phase were characterized. The colloidal phase consisted of BaTiO 3 and Al 2O 3. The conductivity of a separator material impregnated with liquid and colloidal electrolytes was also characterized. The low temperature conductivity enhancement in colloidal electrolytes was attributed to a formation of space charge region between the colloidal phase and liquid electrolyte. The space charge region appears to be essential for the formation of a high conductivity phase.