Capture Coefficient Research Articles

Design and Simulation for Minimizing Non-Radiative Recombination Losses in CsGeI2Br Perovskite Solar Cells.

CsGeI2Br-based perovskites, with their favorable band gap and high absorption coefficient, are promising candidates for the development of efficient lead-free perovskite solar cells (PSCs). However, bulk and interfacial carrier non-radiative recombination losses hinder the further improvement of power conversion efficiency and stability in PSCs. To overcome this challenge, the photovoltaic potential of the device is unlocked by optimizing the optical and electronic parameters through rigorous numerical simulation, which include tuning perovskite thickness, bulk defect density, and series and shunt resistance. Additionally, to make the simulation data as realistic as possible, recombination processes, such as Auger recombination, must be considered. In this simulation, when the Auger capture coefficient is increased to 10-29 cm6 s-1, the efficiency drops from 31.62% (without taking Auger recombination into account) to 29.10%. Since Auger recombination is unavoidable in experiments, carrier losses due to Auger recombination should be included in the analysis of the efficiency limit to avoid significantly overestimating the simulated device performance. Therefore, this paper provides valuable insights for designing realistic and efficient lead-free PSCs.

Characteristics of hollow micron zero valent iron and its transport properties in groundwater: Effect of key engineering parameters and retention mechanism

In this study, a hollow micron zero-valent iron (H-mZVI) was synthesized, and its transport and retention property in saturated porous media was determined via a series of column experiments. Furthermore, the maximum migration distance (Lmax) and sedimentation rate coefficient (Kdep) models of H-mZVI in saturated porous media were established using statistical methods. The results revealed a distinct hollow structure in H-mZVI, with a density of 1.03±0.03 g/cm3, significantly lower than solid micron zero-valent iron (4.57±0.15 g/cm3). FTIR and XRD analyses indicated no formation of new functional groups on H-mZVI's surface, with iron being the main component. The column experiment demonstrated that the Lmax of H-mZVI in saturated porous media was 4.15 times that of solid micron zero-valent iron (mZVI) under the same conditions. The prediction model of Lmax aligned with the linear model, where Lmax correlated positively with particle size, injection velocity, and H-mZVI concentration, but inversely with ionic strength. Medium particle size and injection velocity were the main engineering parameters to control H-mZVI. The prediction model of Kdep accorded with the quadratic model, and an interaction was observed between medium particle size and injection velocity, which jointly affected the deposition rate of H-mZVI. Moreover, the single particle capture coefficient (η0) was hereby calculated and analyzed using the T-E theory. Interception primarily governed the precipitation of H-mZVI in saturated porous media, with gravity sedimentation contributing minimally to η0.

C60/CZTS Junction Combination to Improve the Efficiency of CZTS-Based Heterostructure Solar Cells: A Numerical Approach

This work presents a copper zinc tin sulfide (CZTS)-based solar cell structure (AI/ITO/C60/CZTS/SnS/Pt) with C60 as a buffer layer, developed using the SCAPS-1D simulator by optimizing each parameter to calculate the output. Optimizing the parameters, the acceptor concentration and thickness were altered from 6.0 × 1015 cm−3 to 6.0 × 1018 cm−3 and 1500 nm to 3000 nm, respectively. Although, in this simulator, we can tune the value for the acceptor concentration to 6.0 × 1022, higher doping might present an issue regarding adjustment in the physical experiment. Thus, tunable parameters need to be chosen according to the reliability of the experimental work. The defect density varied from 1.0 × 1014 cm−3 to 1.0 × 1017 cm−3 and the auger hole/electron capture coefficient was determined to be 1.0 × 10−26 cm6 s−1 for the maintenance of the minorities in theoretical to quasi-proper experimental measurements. Although the temperature was intended to be kept near room temperature, this parameter was varied from 290 K to 475 K to investigate the effects of the temperature on this cell. The optimization of the proposed structure resulted in a final acceptor concentration of 6.0 × 1018 cm−3 and a thickness of 3000 nm at a defect density of 1.0 × 1015 cm−3, which will help to satisfy the desired experimental performance. Satisfactory outcomes (VOC = 1.24 V, JSC = 27.03 mA/cm2, FF = 89.96%, η = 30.18%) were found compared to the previous analysis.

Observation of temperature-dependent capture cross section for main deep-levels in β-Ga2O3

Direct observation of the capture cross section is challenging due to the need for extremely short filling pulses in the two-gate Deep-Level Transient Spectroscopy (DLTS). Simple estimation of the cross section can be done from DLTS and admittance spectroscopy data but it is not feasible to distinguish temperature dependence of pre-exponential and exponential parts of the emission rate equation with sufficient precision conducting a single experiment. This paper presents experimental data of deep levels in β-Ga2O3 that has been gathered by our group since 2017. Based on the gathered data, we propose a derivation of apparent activation energy (Eam) and capture cross section (σnm) assuming the temperature dependent capture via the multiphonon emission model, which resulted in a strong correlation between Eam and σnm according to the Meyer–Neldel rule, which allowed us to estimate low- and high-temperature capture coefficients C0 and C1 as well as capture barrier Eb. It also has been shown that without considering the temperature dependence of capture cross section, the experimental values of σn are overestimated by 1–3 orders of magnitude. A careful consideration of the data also allows to be more certain identifying deep levels by their “fingerprints” (Ea and σn) considering two additional parameters (EMN and σ00) and to verify the density functional theory computation of deep-level recombination properties.

Impacts of vacancy complexes on the room-temperature photoluminescence lifetimes of state-of-the-art GaN substrates, epitaxial layers, and Mg-implanted layers

For rooting the development of GaN-based optoelectronic devices, understanding the roles of midgap recombination centers (MGRCs), namely, nonradiative recombination centers and deep-state radiative recombination centers, on the carrier recombination dynamics is an essential task. By using the combination of time-resolved photoluminescence and positron annihilation spectroscopy (PAS) measurements, the origins of major MGRCs in the state-of-the-art GaN epilayers, bulk crystals, and Mg-implanted layers were identified, and their concentrations were quantified for deriving the capture coefficients of minority carriers. In this article, potential standardization of the room-temperature photoluminescence lifetime for the near-band-edge emission (τPLRT) as the concentration of major MGRCs well below the detection limit of PAS is proposed. For n-GaN substrates and epilayers grown from the vapor phase, τPLRT was limited by the concentration of carbon on N sites or divacancies comprising a Ga vacancy (VGa) and a N vacancy (VN), [VGaVN], when carbon concentration was higher or lower, respectively, than approximately 1016 cm−3. Here, carbon and VGaVN act as major deep-state radiative and nonradiative recombination centers, respectively, while major MGRCs in bulk GaN crystals were identified as VGa(VN)3 vacancy clusters in Na-flux GaN and VGa or VGaVN buried by a hydrogen and/or VGa decorated with oxygen on N sites, VGa(ON)3–4, in ammonothermal GaN. The values of τPLRT in n-GaN samples are compared with those of p-GaN, in which τPLRT was limited by the concentration of VGa(VN)2 in Mg-doped epilayers and by the concentrations of VGaVN and (VGaVN)3 in Mg-implanted GaN right after the implantation and after appropriate activation annealing, respectively.

The role of hydrogen in nonradiative recombination in CsPbI3: a first-principles investigation

Defect-induced nonradiative recombination is the main factor hindering efficiency improvement in CsPbI3 perovskite solar cells. It has been recently claimed that the unintentionally incorporated H impurity can potentially cause nonradiative losses due to the deep levels. Using first-principles approaches, we show that, as a matter of fact, the H impurity has a negligible effect on carrier recombination in CsPbI3 due to its small nonradiative capture coefficient and low density. This insight rationalizes why the addition of hydriodic acid does not reduce carrier lifetime and could prevent acidic additives being discarded as potential candidates for assisting film growth.

Numerical analysis of the thin film solar cell modelled based on In doped CdS semiconductor

In this study, pure and 1%, 2% and %3 In-doped CdS thin films were produced by spray pyrolysis method. CdS is an n-type (II-VI group) semiconductor material and used as a buffer layer in solar cells. By doping In into CdS thin film, it was investigated how optical and crystalline behavior of thin film are changed. Using Moss and Herve&Vandamme and Ravindra relations, refractive indices and dielectric coefficients were investigated depending on the band gap of the obtained CdS sample. It has been observed that In element decreases the band gap of CdS thin film, improved its crystal structure and reduced its roughness. Therefore, 3% In doped CdS has gained a more ideal feature for use as an n-type semiconductor in solar cells. CIGS/In doped CdS solar cell was modelled and analysed by SCAPS-1D simulation program by using the physical parameters of the semiconductor layers that make up solar cells as imputs of program. Photovoltaic parameters of solar cell based on donor defect density, the neutral interface defect density and Auger electron/hole capture coefficient which were calculated by using In %3 doped CdS thin film, which has the most ideal n-type semiconductor properties.

In‐gap States and Carrier Recombination in Quasi‐2D Perovskite Films

In‐gap states and their effect on recombination rates in quasi‐2D lead–iodide‐based perovskites, intercalated with various spacer molecules, are studied using a combination of scanning tunneling spectroscopy and temperature‐dependent photoconductivity measurements. The results are further analyzed by a Shockley–Read–Hall model. Indications for shallow in‐gap states, positioned at about 0.15–0.2 eV below the bottom of the conduction band, are found. These states are identified as dominating the recombination route of photogenerated carriers in these systems, with a relatively large capture coefficient of about 10−5–10−6 cm3 s−1 at room temperature. First‐principles calculations based on density functional theory imply that these states are not an intrinsic effect of the inclusion of the spacer molecules, but rather one that arises from chemical defect formation or structural deformation of the perovskite layers. The results suggest that further improvement of the performance of solar cells that are based on quasi‐2D perovskites requires, along with enhancing carrier mobility, efforts to suppress the concentration of these detrimental defect states.

Modeling and formation of a single-walled carbon nanotube (SWCNT) based heterostructure for efficient solar energy: Performance and defect analysis by numerical simulation

This work introduces a new highly efficient heterostructure solar cell that shows the supremacy of the single-walled carbon nanotube as an absorber layer and platinum (Pt) as a back contact. This article focuses on the most important process: optimizing the thickness and acceptor concentration of the absorber layer. Another novel fact in this work is that minorities have been included as a replica of defects and the Auger hole/electron capture coefficient, and the variations in defects have been shown with some flawless contour plots. As the whole study has been carried out using a simulator, it might not be completely realistic, but it shows outcomes close to reality. In addition, the use of minorities takes this simulation work closer to the physical one. A contemporary model—Al/ZnO/TiO2/SWCNT/SnS/Pt—has been investigated in this work for efficient performance. At the end of tuning, the input parameters are set at thickness (W) = 1.5 µm, acceptor concentration (NA) = 1 × 1020 cm−3, and defects = 1 × 1015 cm−3. Under these optimum conditions, this model has shown outstanding outcomes: VOC = 1.04 V, JSC = 41.91 mA cm−2, FF = 72.12%, and η = 31.57%. Although an efficiency of 32.86% was achieved at NA = 1 × 1021 cm−3, it is difficult to keep the acceptor concentration high in reality. Hence, the optimum value of the acceptor concentration is considered at 1 × 1020 cm−3.

Lyapunov based Control Strategy for DFIG based Wind Turbines to Enhance stability and Power

The Doubly Fed Induction Generators (DFIG) based wind turbine is fed with maximum power point tracking is presented in this paper in proposed technique the proportional coefficient tuned adaptively as per wind changes and compare with traditional approaches. This novel method uses three control laws to adjust the proportional gain adaptively to wind speed variations. The intended electrical torque is determined via feedback linearization in the first control law, which makes the assumption that the power capture coefficient and target rotor speed are instantly determined. The second control law uses a Lyapunov-based analysis to determine the power capture coefficient as per changes in wind speed, and the third control law establishes the required rotor speed. As a result of these control principles, the operating point of the turbine shifts in a direction that increases the power capture coefficient, leading the rotor speed to adaptively adjust in the direction of the desired speed. The proposed maximum power tracking method differentiates itself from the perturb-and-observe strategy by removing the need to include a dither or perturbation signal and reliably slipstram the trajectory of maximum power points even in the circumstance of a sudden change in wind velocity, which can cause the perturb-and-observe practice to fail.

Tautomeric mixture coordination enables efficient lead-free perovskite LEDs.

Lead halide perovskite light-emitting diodes (PeLEDs) have demonstrated remarkable optoelectronic performance1-3. However, there arepotential toxicity issues with lead4,5 and removing lead from the best-performing PeLEDs-without compromising their high external quantum efficiencies-remains a challenge. Here we report a tautomeric-mixture-coordination-induced electron localization strategy to stabilize thelead-free tinperovskiteTEA2SnI4 (TEAI is2-thiopheneethylammonium iodide) by incorporating cyanuric acid. We demonstrate that a crucial function of the coordination is to amplify theelectronic effects, even for those Sn atoms that aren't strongly bonded with cyanuric acidowing to the formation of hydrogen-bonded tautomeric dimer and trimer superstructures on the perovskite surface. This electron localization weakens adverse effects from Anderson localization and improves ordering in the crystal structure of TEA2SnI4. These factors result in a two-orders-of-magnitude reduction in thenon-radiative recombination capture coefficient and an approximately twofold enhancement inthe exciton binding energy. Our lead-free PeLED has an external quantum efficiency of up to 20.29%, representing a performance comparable to that of state-of-the-art lead-containing PeLEDs6-12. We anticipate that these findings will provide insights into the stabilization of Sn(II) perovskites and further the development of lead-free perovskite applications.

Finite-size corrections to defect energetics along one-dimensional configuration coordinate

Recently, effective one-dimensional configuration coordinate diagrams have been utilized to calculate the line shapes of luminescence spectra and nonradiative carrier capture coefficients via point defects. Their calculations necessitate accurate total energies as a function of configuration coordinates. Although supercells under periodic boundary conditions are commonly employed, the spurious cell size effects have not been previously considered. In this study, we have proposed a correction energy formalism and verified its effectiveness by applying it to a nitrogen vacancy in GaN.

Design of hypersonic wavecatcher intake at Mach 12 with rectangular-to-elliptical shape transition

Wavecatcher (inward-turning) intake flows, at design Mach 12, are investigated numerically, to display the effect of wall temperature on flow structures and intake performance. Hypersonic experiments on shock wave/boundary layer interaction are used to validate the Spalart-Allmaras turbulence model for reproducing the features of hypersonic flow. Simulations of hypersonic intake flow are performed at different wall temperatures, including isothermal Tw = 300 K, Tw = 1000 K, Tw = 2000 K, and the adiabatic case. The shock structures, impinging shock positions, surface streamlines, and the development of internal streamwise vortices are discussed. The mass-averaged performance of intake flow shows that, when the wall temperature changes from Tw = 300 K to adiabatic, the mass capture coefficient decreases from 0.991 to 0.933, the total pressure recovery decreases from 0.200 to 0.083, while exit section Mach number decreases from 4.478 to 3.514. The results suggest that the osculating design method of wavecatcher intake design can successfully be extended to Mach 12, while capturing all airflow at isothermal wall conditions.

Optimization of the Energy Capture Performance of the Lift-Drag Hybrid Vertical-Axis Wind Turbine Based on the Taguchi Experimental Method and CFD Simulation

In order to improve the energy capture performance of vertical axis lift wind turbines in a low wind speed environment, the drag wind turbine is employed to couple with the design of existing vertical axis lift wind turbines. In contrast to the existing literature, in this work, a computational model is proposed that can simulate the interaction between the turbine and the fluid. The effects of pitch angle (β), installation angle (θ), overlap ratio (ε) and diameter ratio (DL) on the energy capture performance of hybrid vertical axis wind turbines are systematically analyzed based on Taguchi and CFD methods. The results show that under the optimized parameter combination, the peak energy capture coefficient of the lift-drag hybrid wind turbine can be increased to 0.2328, compared with 0.0309 and 0.0287 of the pure lift and drag turbine, respectively. In addition, the result of the prototype test show that the optimized hybrid wind turbine not only has a better-starting performance but also has 2.0 times the output power of that of the lift wind turbine.

Metal Halide Perovskite Nanotubes for High-Performance Solar Cells with Ab Initio Analysis.

Compared to bulk metal halide perovskites, low-dimensional nanotubes can accommodate more intense atomic movement and octahedral distortion, leading to prompting the separation and localization of charge between the initial and final states and accelerating quantum coherence loss. Additionally, nonradiative carrier recombination is accompanied by weakened nonadiabatic coupling, which extends their lifetime by an order of magnitude. Common vacancy defects in perovskites act as nonradiative recombination centers, causing charge and energy loss. However, nanotubes and self-chlorinated systems can passivate and eliminate deep-level defects, resulting in a roughly two order of magnitude decrease in the nonradiative capture coefficient of lead vacancy defects. Simulation results demonstrate that the strategy of low-dimensional nanotubes and chlorine doping can provide helpful guidance and new insights for the design of high-performance solar cells.

Improved Carrier Lifetimes of CdSe Thin Film via Te Doping for Photovoltaic Application.

Cadmium selenide (CdSe) solar cells have proven to be a remarkable potential top cell for a silicon-based tandem application. However, the defects and short carrier lifetimes of CdSe thin films greatly limit the solar cell performance. In this work, a Te-doped strategy is proposed to passivate the Se vacancy defects and increase the carrier lifetime of the CdSe thin film. The theoretical calculation helps to reveal the mechanism of nonradiative recombination of the CdSe thin film in depth. After Te-doping, the calculated capture coefficient of CdSe can be reduced from 4.61 × 10-8 cm3 s-1 to 2.32 × 10-9 cm3 s-1. Meanwhile, the carrier lifetime of CdSe thin film is increased nearly 3-fold from 0.53 to 1.43 ns. Finally, the efficiency of the Cd(Se,Te) solar cell is improved to 4.11%, about a relative 36.5% improvement compared to the pure CdSe solar cell. Both theoretical calculations and experiments prove that Te can effectively passivate bulk defects and improve the carrier lifetime of CdSe thin films, deserving further exploration to improve solar cell performance.

Singular Time‐Dependent Photoconductivity Response of MAPbI3 Samples Deposited by Vacuum Processing on Different Substrates

Organo–inorganic halide perovskites continue to generate enormous interest for their potential use in various optoelectronic devices. Among the different fabrication methods, vacuum thermal deposition allows uniform films over large areas, providing good material quality without using organic solvents. However, the influence of the substrate on the properties of the material deposited by vacuum processing is a point that deserves further investigation. Herein, time‐dependent photoconductivity measurements are used to investigate the dynamics of charge carriers under illumination in planar films of CH3NH3PbI3. The samples are deposited by thermal sublimation on different substrates: PbI2, TaTm, C60, and glass. Experimentally, it is shown that the substrate has a profound effect on perovskite behavior and that photoconductivity is sensitive enough to probe these behaviors. Furthermore, a model based on Shockley–Read–Hall statistics is proposed, explaining the different photoconductivity responses by changing only the relative values of the different capture coefficients and the position of the Fermi level. It is argued that the substrate changes the stoichiometry of the evaporated material, affecting the transient photoconductivity response.

Impact of Ga and N Vacancies at the GaN m -Plane on the Carrier Dynamics of Micro-Light-Emitting Diodes

Micro-light-emitting diodes ($\text{\ensuremath{\mu}}\mathrm{LEDs}$) based on $\mathrm{Ga}\mathrm{N}$ are a key component for next-generation displays, but serious surface $e$-$h$ recombination deteriorates the device efficiency. To gain microscopic insights into the surface recombination, we use hybrid functional first-principles calculations to investigate $\mathrm{Ga}$ (${V}_{\mathrm{Ga}}$) and $\mathrm{N}$ (${V}_{\mathrm{N}}$) vacancies on the $\mathrm{Ga}\mathrm{N}$ $m$ plane that can be created considerably during the production of one-dimensional $\mathrm{Ga}\mathrm{N}$ structures. We find that the surface ${V}_{\mathrm{Ga}}$ is not critical for nonradiative Shockley-Read-Hall (SRH) recombination in light of the large formation energy (>2 eV) and its shallow levels. In contrast, the surface ${V}_{\mathrm{N}}$ exhibits a low formation energy (2 eV) and develops a deep defect state near the midgap, indicating the possibility of becoming useful SRH recombination centers. By constructing configuration-coordinate diagrams, we demonstrate that the energy barriers for electron and hole capture on the surface ${V}_{\mathrm{N}}$ are small enough to cause significant capture coefficients due to strong electron-phonon coupling.

A quantum model of charge capture and release onto/from deep traps.

The rapid development of optical technologies and applications revealed the critical role of point defects affecting device performance. One of the powerful tools to study the influence of defects on charge capture and recombination processes is thermoluminescence. The popular models behind thermoluminescence and carrier capture processes are semi-classic though. They offer a good qualitative description, but implicitly exclude the quantum nature of the accompanying parameters, such as frequency factors and capture cross sections. As a consequence, results obtained for a specific host material cannot be successfully extrapolated to other materials. Thus, the main purpose of our work is to introduce a reliable analytical model that describes non-radiative capture and release of electrons from/to the conduction band (CB). The proposed model is governed by Bose-Einstein statistics (for phonon occupation) and Fermi's golden rule (for resonant charge transfer between the trap and the CB). The constructed model offers a physical interpretation of the capture coefficients and frequency factors, and seamlessly includes the Coulomb neutral/attractive nature of traps. It connects the frequency factor to the overlap of wavefunctions of the delocalized CB and trap states, and suggests a strong dependence on the density of charge distribution, i.e. the ionicity/covalency of the chemical bonds within the host. Separation of the resonance conditions from the accumulation/dissipation of phonons on the site leads to the conclusion that the capture cross-section does not necessarily depend on the trap depth. The model is verified by comparison to the reported experimental data, showing good agreement. As such, the model generates reliable information about trap states whose exact nature is not completely understood and allows performing materials research in a more systematic way.

Understanding Auger recombination in perovskite solar cells.

Enhanced radiative efficiency, long carrier lifetimes, and high carrier mobilities are hallmarks of perovskite solar cells. Considering this, complete cells experience large nonradiative recombination losses that restrict their VOC considerably below the Shockley-Queisser limit. Auger recombination, which involves two free photo-induced carriers and a trapped charge carrier, is one potential mechanism. Herein, the effects of Auger capture coefficients in mixed-cation perovskites are analyzed employing SCAPS-1D computations. It is demonstrated that VOC and FF are severely decreased with an increase in the acceptor concentration and Auger capture coefficients of perovskites, thus reducing the device performance. When the Auger capture coefficient is increased to 10-20 cm6 s-1 under the acceptor concentration of 1016 cm-3, the performance is drastically lowered from 21.5% (without taking Auger recombination into account) to 9.9%. The findings suggest that in order to increase the efficiency of perovskite solar cells and prevent the effects of Auger recombination, the Auger recombination coefficients should be less than 10-24 cm6 s-1.