Bulk Defect Concentration Research Articles

A numerical approach to optimize the efficiency of a novel HTL-free Sr3Ti2S7 Ruddlesden-Popper perovskite solar cell

The exceptional environmental and structural stability of two-dimensional Ruddlesden-Popper perovskites has made them appealing candidates for high-performance perovskite solar cells. Besides, Ruddlesden-Popper perovskites can be synthesized using low-cost solution processing and have tunable bandgaps as compared to their conventional three-dimensional counterparts. Therefore, a Ruddlesden–Popper perovskite solar cell has been reported in this investigation and the SCAPS program has been used to theoretically study the device performance of the proposed lead-free, environmentally friendly perovskite solar cell. A novel Sr3Ti2S7 material has been utilized as the light absorber for the first time in such a theoretical study. Two different configurations, employing WS2 and C60 electron transport layers have been presented and device simulations have been performed to optimize the solar cell. The utilization of C60 as an electron transport material did not prove to be beneficial for the solar cell as it suffered from major recombination losses. It was also found that higher doping concentration of the electron transport materials could help these cells achieve improved fill factor. The diffusion length of the electrons and holes with respect to absorber defect density was also determined. Mott-Schottky analysis proved that the ITO/C60/Sr3Ti2S7/Au structure had a lower flatband potential compared to ITO/WS2/Sr3Ti2S7/Au. Additionally, from the impedance spectroscopy results, the recombination lifetime of the ITO/WS2/Sr3Ti2S7/Au configuration was found to be 46.7 μs compared to only 12.9 μs for the ITO/C60/Sr3Ti2S7/Au architecture. It was also established that if the thickness of the absorber and the bulk defect concentration were optimized the ITO/WS2/Sr3Ti2S7/Au and ITO/C60/Sr3Ti2S7/Au solar cells could achieve conversion efficiencies close to 11.0 % and 10.0 % respectively.

Multiscale Simulations Facilitate the Design of High‐Performance Perovskite Solar Cells

Organic‐inorganic hybrid perovskite solar cells (PSCs) have swiftly emerged as a prominent contender in the photovoltaic industry, owing to their unparalleled optoelectronic capabilities. Nevertheless, the commercial viability of organic‐inorganic hybrid PSCs is significantly hindered by their limited hygrothermal stability. Therefore, based on multiscale simulation technology, we systematically investigated the microscopic properties of CsPbBrxI3−x (0 ≤ x ≤ 3) perovskite materials and the corresponding photovoltaic device performance. Multiscale simulation technology is numerical simulation method that combines the density functional theory (DFT) with finite element method (FEM). DFT is used first to study the energy band structure, density of states, and optoelectronic parameters of CsPbBrxI3−x (0 ≤ x ≤ 3). Then, FEM is used to study the optical properties of all‐inorganic PSCs based on the results of DFT simulation. Finally, the bulk defect concentration (Nt) of the perovskite material, and the defect concentration between the perovskite and the charge transport layer (CTL) on the photovoltaic performance of the device are calculated and analyzed using CsPbI3 PSCs as an example. We finally designed the CsPbI3 all‐inorganic PSCs with a theoretical efficiency of 22.65%. Our theoretical simulation methods and corresponding results present a groundbreaking approach to crafting highly efficient and exceptionally stable all‐inorganic PSCs.

Surface Defect Suppression for High Color Purity Light‐Emitting Diode of Free‐Standing Single‐Crystal Perovskite Film

AbstractHigh color purity is one of the important features for single‐crystal metal halide perovskite light‐emitting diode (LED). Despite single‐crystal perovskite showing low bulk defect concentration, single‐crystal perovskite LEDs do not exhibit high color purity advantage due to the absence of effective surface defect passivation. Herein, one fully wrapped structure is proposed to passivate the surface of the free‐standing CsPbBr3 single‐crystal films. The surface of CsPbBr3 single‐crystal films is wrapped by ultra‐thin polymethyl methacrylate, precisely controlling the thickness. The single‐crystal perovskite film device can achieve high color purity with a full width at half maximum (FWHM) of 15.8 nm) and a large luminescent area of 2.25 mm2. It is observed that surface passivation is due to interaction of CO bond in polymer chains with the Lewis acid PbBr2. The passivated perovskite single‐crystal films significantly improve carrier lifetime and suppress surface defects. It is noteworthy that the passivated free‐standing single‐crystal perovskite films are feasible to build up a vertical LED device structure, avoiding the edge glowing and short‐circuiting of the LED device. This study demonstrates the large luminescent area of the high‐quality millimetre‐scale free‐standing single‐crystal films for wide color gamut display and vertical optoelectronic devices.

A model for redistribution of oppositely charged point defects under the stress field of dislocations in nonstoichiometric ionic solids: Implications in doped ceria

Point defect distribution in the vicinity of discontinuities plays important role in the transport properties of nonstoichiometric ionic solids. Here, considering dopants and oxygen vacancies as the major point defects in doped ceria, we develop a Monte Carlo model to examine how the stress field of edge dislocations affect point defect distribution in their surroundings. Point defects are considered to interact with the elastic stress field of dislocations due to their misfit volume, and the electrostatic interaction between the point defects is also taken into account. In contrast with a prevalent theory of chemo-mechanical equilibrium in solid solutions, the model developed here is consistent with classical elasticity in that the point defects do not interact through their self-stress fields. Stress effects both on the defect distribution, and on the electric potential, are examined for a single dislocation as well as a periodic array of like dislocations. In agreement with previous atomistic simulations, the model predicts that electrostatic interactions drive enrichment or depletion of defects of both types on either the compressive or tensile side of edge dislocations depending on the ionic radius of the dopant. The stress field of an array of like dislocations periodic in the direction of the Burgers vector is shown to result in different bulk defect concentrations and bulk electric potentials on the opposite sides of the array, whereas for an array with repeat direction normal to the Burgers vector, defect enrichment and depletion emerge in alternate regions limited to the vicinity of the dislocations.

Performance Enhancement of an MoS2-Based Heterojunction Solar Cell with an In2Te3 Back Surface Field: A Numerical Simulation Approach.

Researchers are currently showing interest in molybdenum disulfide (MoS2)-based solar cells due to their remarkable semiconducting characteristics. The incompatibility of the band structures at the BSF/absorber and absorber/buffer interfaces, as well as carrier recombination at the rear and front metal contacts, prevents the expected result from being achieved. The main purpose of this work is to enhance the performance of the newly proposed Al/ITO/TiO2/MoS2/In2Te3/Ni solar cell and investigate the impacts of the In2Te3 BSF and TiO2 buffer layer on the performance parameters of open-circuit voltage (V OC), short-circuit current density (J SC), fill factor (FF), and power conversion efficiency (PCE). This research has been performed by utilizing SCAPS simulation software. The performance parameters such as variation of thickness, carrier concentration, the bulk defect concentration of each layer, interface defect, operating temperature, capacitance-voltage (C-V), surface recombination velocity, and front as well as rear electrodes have been analyzed to achieve a better performance. This device performs exceptionally well at lower carrier concentrations (1 × 1016 cm-3) in a thin (800 nm) MoS2 absorber layer. The PCE, V OC, J SC, and FF values of the Al/ITO/TiO2/MoS2/Ni reference cell have been estimated to be 22.30%, 0.793 V, 30.89 mA/cm2, and 80.62% respectively, while the PCE, V OC, J SC, and FF values have been determined to be 33.32%, 1.084 V, 37.22 mA/cm2, and 82.58% for the Al/ITO/TiO2/MoS2/In2Te3/Ni proposed solar cell by introducing In2Te3 between the absorber (MoS2) and the rear electrode (Ni). The proposed research may give an insight and a feasible way to realize a cost-effective MoS2-based thin-film solar cell.

Machine Learning Approach to Delineate the Impact of Material Properties on Solar Cell Device Physics.

In this research, solar cell capacitance simulator-one-dimensional (SCAPS-1D) software was used to build and probe nontoxic Cs-based perovskite solar devices and investigate modulations of key material parameters on ultimate power conversion efficiency (PCE). The input material parameters of the absorber Cs-perovskite layer were incrementally changed, and with the various resulting combinations, 63,500 unique devices were formed and probed to produce device PCE. Versatile and well-established machine learning algorithms were thereafter utilized to train, test, and evaluate the output dataset with a focused goal to delineate and rank the input material parameters for their impact on ultimate device performance and PCE. The most impactful parameters were then tuned to showcase unique ranges that would ultimately lead to higher device PCE values. As a validation step, the predicted results were confirmed against SCAPS simulated results as well, highlighting high accuracy and low error metrics. Further optimization of intrinsic material parameters was conducted through modulation of absorber layer thickness, back contact metal, and bulk defect concentration, resulting in an improvement in the PCE of the device from 13.29 to 16.68%. Overall, the results from this investigation provide much-needed insight and guidance for researchers at large, and experimentalists in particular, toward fabricating commercially viable nontoxic inorganic perovskite alternatives for the burgeoning solar industry.

Facile preparation of anodized MoO3−x films and their boosted photocatalytic activity

Flexible MoO3−x films were prepared by anodic oxidation followed by annealing and liquid-phase reduction. The prepared samples were characterized by XRD, Raman spectra, SEM, UV–visible spectrophotometer, XPS, and EPR. The anodized amorphous MoO3 was transferred to the orthorhombic phase during the annealing. With the increase in annealing temperature, the MoO3 films’ photocatalytic activity increased. After liquid-phase reduction, defects, including oxygen vacancies, were introduced. The defect introduction enhanced the visible light absorption and increased the charge transfer efficiency to varying degrees. Compared with the as-annealed MoO3 films, the as-reduced MoO3 films’ photocatalytic activity rose by nearly four times at most. With the increase in reduction time, the bulk defect concentration increased. However, the surface defect concentration reached the maximum when the reduction time came to 2 h. The most excellent photocatalytic activity corresponded to the largest surface defect concentration, suggesting surface defects improved the photocatalytic activity. On the other hand, the bulk defects had the opposite effect. The photogenerated holes played a crucial role in the photocatalytic degradation of MB. Anodic oxidation followed by annealing and liquid-phase reduction is a facile and effective way to prepare MoO3 films with high photocatalytic activity.

High‐Temperature Photochromism of Fe‐Doped SrTiO3 Caused by UV‐Induced Bulk Stoichiometry Changes

AbstractThe impact of UV irradiation on Fe‐doped SrTiO3 (Fe:STO) single crystals is investigated at elevated temperatures. Illumination leads to incorporation of oxygen into the single crystals and thus to a decreasing oxygen vacancy concentration and oxidation of Fe3+ to Fe4+. The Fe4+ ions cause a color change from transparent/brownish to black. This photochromic blackening due to stoichiometry changes at elevated temperatures is irreversible at room temperature, but annealing at high temperatures, for example at 700 °C, can restore the original stoichiometry and color. Absorbance changes due to UV irradiation are monitored by ex situ and in situ UV–vis spectroscopy experiments and changes in electrical properties are measured by van der Pauw measurements and in‐plane electrochemical impedance spectroscopy. After 1140 min of illumination at 440 °C, for example, electrical measurements reveal a conductivity increase by more than a factor of 5 due to the enhanced hole concentration in blackened Fe:STO. In addition, UV illumination increases the oxygen chemical potential up to a calculated p(O2) of more than 109 Pa in Fe:STO. Hence, UV light can be used to tune the color, but also electrical properties of Fe:STO by directly impacting the bulk defect concentrations.

Oxygen vacancies in TiO2/SnOx coatings prepared by ball milling followed by calcination and their influence on the photocatalytic activity

We prepared TiO2/SnOx (x = 1, 2) composite coatings through ball milling followed by calcination. The composite coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), spherical aberration corrected scanning transmission microscopy (Cs-corrected STEM), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) and other techniques. Bulk oxygen vacancies in the form of vacancy clusters rather than individual sites were directly observed by Cs-corrected STEM. With an increase in the calcination temperature from 873 K to 1073 K or holding time from five hours to ten hours, the concentration of bulk defects also increased. On the other hand, the photocatalytic activity of the as-prepared samples for degrading MB dye increased with the concentration increase of bulk defects. Sample 973 K-10 h showed the best photocatalytic activity whether under the irradiation of UV light or visible light. With a further increase in the concentration of bulk oxygen vacancies, the photocatalytic activity decreased, although surface Ti3+ defects appeared. In other words, a moderate concentration of bulk defects could benefit the enhancement of the photocatalytic activity of the as-prepared samples. Ball milling followed by calcination is an effective method to introduce defects such as oxygen vacancies.

Commercial ZnO and its hybrid with Ag nanoparticles: Photocatalytic performance and relationship with structure

The excellent photocatalytic performance of a commercial ZnO (C-ZnO-1) was demonstrated. This ZnO showed approximately 3.8, 7.4 and 5.2 times greater activity than P25 TiO2 for the degradation of rhodamine B, methyl orange and methyl blue, respectively. After compounding with Ag, the catalyst showed record-high activities for the degradation of the three dyes (approximately 9.5, 13, and 50 times that of P25). The high activity of C-ZnO-1 is attributed to the low concentration of bulk defects (i.e., a high degree of crystallinity) and the relatively high number of surface defects (i.e., a higher ratio of surface hydroxyl oxygen).

(Invited) Towards an Improved Understanding of Electrochemical Oxygen Exchange Reactions on Mixed Conducting Oxides

It is generally agreed that the oxygen reduction reaction on oxide surfaces is strongly affected by defects, and the same is true for the counter reaction of oxygen evolution from an oxide. Relevant defects include the “standard” point defects also present in the bulk, i.e. oxygen vacancies and electrons or holes, but also defects within the ideal termination of an oxide, e.g. cation segregates, and higher dimensional defects such as dislocations or grain boundaries. However, it is far from trivial to clearly identify the role of specific defects on the oxygen reduction or release reaction. Often, one neither knows exact defect concentrations nor can one vary those in a defined manner without affecting the entity of all defects. This also hampers development of theoretical models describing the current voltage characteristics of oxygen reduction or evolution, i.e. oxygen exchange, at gas/solid interfaces. In this contribution, two types of experiments are employed to shed further light on the role of defects in oxygen exchange of mixed conducting oxides such as Sr doped LaCoO3 (LSC) and LaFeO3 (LSF). i) The first approach makes use of the fact that both oxygen partial pressure p(O2) and overpotential affect the oxygen chemical potential and thus defect concentrations in oxides. However, the oxygen reduction or evolution kinetics may still be affected very differently by overpotential and p(O2), since only the latter also changes the concentration of reacting molecules. By properly combining oxygen partial pressure changes and overpotential, we could tune the chemical potentials of LSF and gas phase independently and thus we extracted “true” reaction orders of O2 and defects in the oxygen reduction and evolution reaction. Additional bias dependent three point impedance measurements revealed the chemical capacitance of LSF and this was used for calculating the absolute bulk defect concentration in dependence of the oxygen chemical potential. This analysis also revealed that defect interaction strongly affects the thermodynamic data of LSF and a new defect chemical model including non-dilute defects is suggested. ii) The second type of experiments deals with the effect of certain cations in the topmost layer of LSC. It is already known that Sr segregation to the surface of mixed conducting oxides may reduce their activity towards oxygen reduction. A kind of SrO termination layer seems to be unavoidable on LSC at higher temperatures but still additional degradation may take place and reasons are not fully understood. A novel type of experiment was thus developed, with impedance spectroscopic analysis of samples inside a PLD (pulsed laser deposition) chamber (in-situ impedance PLD). In this manner, the surface of an LSC electrode in an YSZ based electrochemical cell could be decorated by very tiny amounts of other oxides (SrO, Co3O4, La2O3) and the effect on oxygen exchange kinetics could be detected immediately by impedance measurements. Already small fractions of monolayers of Sr strongly modified the polarization resistance, thus indicating that only a small number of very active sites was present on LSC and those became deactivated by Sr. Co, on the other hand, can significantly activate the LSC electrode surface.

Total Oxidation of Propane Using CeO2 and CuO-CeO2 Catalysts Prepared Using Templates of Different Nature

Several CeO2 and CuO-CeO2 catalysts were prepared using different methods, i.e., a homogeneous precipitation with urea, a nanocasting route using CMK-3 carbon as a hard template and a sol–gel process using Poly(methyl methacrylate) (PMMA) polymer as a soft template, and tested in the total oxidation of propane. The catalysts were characterized by a number of physicochemical techniques (XRD, N2 adsorption, TPR, XPS, Raman spectroscopy) showing distinct characteristics. For each series, Cu-Ce-O catalysts with low Cu-loadings (5 wt % CuO) showed the highest activity, higher than those samples either without copper or with high Cu-loading (13 wt % CuO). The incorporation of copper leads to an increase of the concentration of bulk defects but if the Cu-loading is too high the surface area drastically falls. The highest activity in the total oxidation of propane was achieved by Cu-containing ceria catalysts synthesized using a polymer as a template, as this method yields high surface area materials. The surface area and the number of bulk/sub-surface defects of the ceria seem to be the main properties determining the catalytic activity.

Structural Changes Underlying Field‐Cycling Phenomena in Ferroelectric HfO2 Thin Films

Since 2011, ferroelectric HfO2 has attracted growing interest in both fundamental and application oriented groups. In this material, noteworthy wake‐up and fatigue effects alter the shape of the polarization hysteresis loop during field cycling. Such changes are problematic for application of HfO2 to ferroelectric memories, which require stable polarization hystereses. Herein, electrical and structural techniques are implemented to unveil how cyclic switching changes nanoscale film structure, which modifies the polarization hysteresis. Impedance spectroscopy and scanning transmission electron microscopy identify regions with different dielectric and conductive properties in films at different cycling stages, enabling development of a structural model to explain the wake‐up and fatigue phenomena. The wake‐up regime arises due to changes in bulk and interfacial structuring: the bulk undergoes a phase transformation from monoclinic to orthorhombic grains, and the interfaces show changes in and diminishment of a nonuniform, defect rich, tetragonal HfO2 layer near the electrodes. The evolution of these aspects of structuring contributes to the increase in Pr and the opening of the constricted P–V hysteresis that are known to occur with wake‐up. The onset of the fatigue regime is correlated to an increasing concentration of bulk defects, which are proposed to pin domain walls.

Surface-assisted defect engineering of point defects in ZnO

Semiconductor surfaces facilitate the injection of highly mobile point defects into the underlying bulk, thereby offering a special means to manipulate bulk defect concentrations. The present work combines diffusion experiments and first-principles calculations for polar ZnO (0001) surface to demonstrate such manipulation. The rate behavior of oxygen interstitial injection varies dramatically between the Zn- and O-terminated ZnO surfaces. A specific injection pathway for the Zn-terminated surface is identified, and activation barrier determined from the first-principles calculations agrees closely with the experimental activation energy of 1.7 eV.

Evaluation of surface energy state distribution and bulk defect concentration in DSSC photoanodes based on Sn, Fe, and Cu doped TiO2

Electron transfer dynamics in the oxide layers of the working electrodes in both dye-sensitized solar cells and photocatalysts greatly influences their performance. A proper understanding of the distribution of surface and bulk energy states on/in these oxide layers can provide insights into the associated electron transfer processes. Metal ions like Iron (Fe), Copper (Cu) and Tin (Sn) doped onto TiO2 have shown enhanced photoactivity in these processes. In this work, the structural, optical and transient properties of Fe, Cu and Sn doped TiO2 nanocrystalline powders have been investigated and compared using EDX, Raman spectroscopy, X-ray Photoelectron spectroscopy (XPS), and Transient Absorption spectroscopy (TAS). Surface free energy states distributions were probed using Electrochemical Impedance spectroscopy (EIS) on Dye Sensitized Solar Cells (DSSC) based on the doped TiO2 photoanodes. Raman and XPS Ti2p3/2 peak shifts and broadening showed that the concentration of defects were in the order: Cu doped TiO2>Fe doped TiO2>Sn doped TiO2>pure TiO2. Nanosecond laser flash photolysis of Fe and Cu doped TiO2 indicated slower transient decay kinetics than that of Sn doped TiO2 or pure TiO2. A broad absorption peak and fast transient decay at 430nm for Fe doped TiO2 was ascribed to an increase in surface hole concentration resulting in poor current density in the Fe doped TiO2 photoanodes relative to pure TiO2, Sn or Cu doped anodes. The charge transfer capacitance and the calculated electron lifetimes correlated well with the trend in current density of the photoanodes (Sn>Cu>pure TiO2). The poor performance of Fe doped cells is due to faster recombination of injected electrons with surface holes while those of Sn and Cu were more influenced by the concentration of their bulk defects. These results demonstrate that the choice of selected metal ions doping onto TiO2 for a desired application should take into consideration the influence of bulk defect concentrations, the energy state distribution and the electron transfer properties in/on the oxide photoanodes.

Point defect determination by eliminating frequency dispersion in C– V measurement for AlGaN/GaN heterostructure

In this paper, an improved small-signal equivalent model is introduced to eliminate frequency dispersion phenomenon in capacitance–voltage ( C– V) measurement, and a new mathematic method is proposed to calculate the amount of bulk defect existing in the GaN buffer layer. Compared with photoluminescence (PL) and high resolution X-ray diffraction (HRXRD) data, it is identified that the main component of the bulk defect concentration is made up of the point defect concentration, rather than the edge dislocation concentration. All these results prove the accuracy of the improved C– V model and feasibility of the mathematic model.

ChemInform Abstract: Mechanistic Understanding and Electrochemical Modeling of Mixed Conducting (SOFC) Electrodes

AbstractReview: 66 refs.

Correlation between structural disorder and electric properties in YBa2Cu3O7−δ (YBCO)

The degradation of the surface resistance Rs of YBCO material by ion milling is usually supposed to be linked to oxygen loss and crystallographic defects. To study this effect, the structural and physical analysis of the milled YBCO thin film has been carried out using Ion Beam Analyses and SQUID measurements. The cationic composition and the oxygen quantity measured in the disordered layer on the top of the film are the same on milled and reference samples. An increase of the bulk defect concentration, especially in the oxygen sublattice, is observed in the layer underlying the disordered layer. From SQUID measurements, the milled sample shows a degradation of susceptibility vs temperature X(T) compared to the reference sample. This observation is in a good agreement with the structural analysis and the Rs measurements. After an oxygen annealing at 450°C on the milled samples, the cationic disorder decreased and the bulk defect concentration has not been affected. The X(T) characteristic has improved for the milled film showing its correlation with the evolution of the disordered layer. However the Rs value of the annealed material is not restored.

Photoresponse of gated p-silicon field emitter array and correlation with theoretical models

Improved fabrication and conditioning techniques for gated p-Si field emitter arrays (FEAs) (1×1mm2 in size with 50×50 emitters) greatly reduces the bulk damage and surface states. Consequently, only 0.9μA of the anode dark field-emission current was measured for 50V applied gate potential with a very low gate leakage current. At these low gate leakage currents, the transition between the tunneling-limited field-emission regime and the supply-limited saturation regime is clearly revealed in current-voltage plots. This transition is accompanied by a reduction in the anode current fluctuation. The standard deviation of the noise current drops from a few percent in the tunneling-limited regime to <1% in the supply limited regime at 50V. A photocurrent of 0.41mA was obtained for photoexcitation at 840nm wavelength. This corresponded to 40dB contrast ratio between the photocurrent and the dark current at room temperature. Additionally, our findings showed that surface states were fully depleted at low applied fields, the concentration of bulk defects was low and the area contributing to the dark current was confined to the emitter regions. In contrast, previously reported ungated Si field emitters had high concentrations of surface states and bulk defects. Furthermore, the contributing area to the dark current corresponded to the whole area of the device as opposed to only the emitter areas for a gated FEA. Photofield emission and dark current measurements from our gated p-Si FEA correlate well to theoretical models without requiring cooling to liquid-nitrogen temperatures.

Spatial uniformity of electron charge transport in high resistivity CdTe

Electron charge transport in high resistivity CdTe was investigated in terms of drift mobility, charge collection efficiency, and mobility-lifetime product. CdTe devices were produced from material grown by the Travelling Heater Method. Infrared microscopy was used to assess the quality of CdTe wafers, which showed a concentration of bulk defects and tellurium precipitates around the edges of the wafers. Laser-induced time of flight was used to measure the electron drift velocity, which was linear with respect to electric field at field strengths up to 200 V/cm. The measured electron drift mobility was 1040±20 cm 2/V s. Ion-beam induced charge (IBIC) imaging of the device cathode was carried out to produce high resolution maps of signal amplitude and electron drift time. Excellent spatial uniformity was observed in the sample, and a value of 6×10 −3 cm 2/V was measured for the electron mobility-lifetime product.