Defect Levels Research Articles

Prolonging Charge Carrier lifetime via Intraband Defect Levels in S-scheme Heterojunctions for Artificial Photosynthesis.

S-scheme heterostructure photocatalysts, distinguished by unique charge-transfer pathways and exceptional catalytic redox capabilities, have found widespread applications in addressing challenging chemical processes, including the photocatalytic reduction of CO2 with a high reaction barrier. Nevertheless, the influence of intraband defect levels within S-scheme heterojunctions on charge separation, carrier lifetime, and surface catalytic reactions has, for the most part, been overlooked. Herein, we develop a tunable defect-level-assisted strategy to construct an electron reservoir, effectively prolonging the lifetime of charge carriers through the rapid capture and gradual release of photoelectrons within WO3-x/In2S3 S-scheme heterojunctions, as authenticated by femtosecond transient absorption spectroscopy and theoretical simulations. The surface photoredox mechanism, unraveled by Gibbs free energy calculations, demonstrates that oxygen-vacancy-induced defect states in WO3-x/In2S3 heterojunctions unlock the rate-determining H2O oxidation into free oxygen molecules by forming metastable oxygen intermediates, contributing to the facilitation of H2O photooxidation. This distinct role, combined with the extended carrier lifetime, results in boosted CO2 photoreduction with nearly 100% CO selectivity in the absence of any photosensitizer or scavenger. Our work sheds light on the role of controllable defect levels in governing charge transfer dynamics within S-scheme heterojunctions, thereby inspiring the development of more advanced photocatalysts for artificial photosynthesis.

Energy Band Alignment and Defect Synergistic Regulation Enable Air-Solution-Processed Kesterite Solar Cells With the Lowest VOC Deficit.

The major challenge in preparing high-performance Cu2ZnSn(S,Se)4 solar cells is the large open circuit voltage deficit (VOC-def). A new strategy utilizing the synergistic substitution of Ag and In dual cations has been proposed to simultaneously address the problems of undesirable interface band alignment and high-density detrimental bulk defects, obtaining decreased carrier recombination rate and increased minority carrier lifetime. The shorter In-S/Se bonds move the CBM higher by generating stronger repulsive force than the Sn-S/Se bonds, thus adjusting the interface band alignment. Ag substitution can effectively suppress Cu─Zn disorder, while Ag, In dual substitution can further passivate Sn-related defects and solve the issue of low carrier concentration in Ag single-substituted samples. Besides, the superior carrier property of In-Se materials significantly enhanced the device carrier lifetime and minority carrier diffusion length. The state-of-the-art air-solution-processed CZTSSe device without any addition treatment with 14.33% efficiency and 580mV VOC is obtained, possessing the lowest VOC-def value currently available in the CZTSSe field (VOC/VOC SQ = 64.7%). This work emphasizes the synergistic modulation of band alignment, defect level, grain growth, and carrier transportation by dual cation substitution, which paves a convenient and effective way to realize high-performance solar cells and photovoltaic devices.

Constructing surface oxygen vacancy in the [Bi2O2]2+ layer defects mediated Bi2MoO6 enhanced visible light responsive photocatalytic activity.

Bi2MoO6 nanospheres with surface oxygen vacancies (SOVs) controlled by the calcination process were prepared in this study. Performance testing revealed that the Bi2MoO6-4 sample (Bi2MoO6 calcined at 350 °C for 4h) with SOVs achieved a remarkable photocatalytic degradation efficiency up to 99.16% for Rhodamine B (RhB) within 50min, which is 2.19 times higher than that of pure Bi2MoO6. The higher photocatalytic performance of the Bi2MoO6-4 sample is attributed to the SOVs' defect level located at the Bi2MoO6 bandgap, narrowing the bandgap to effectively promote the photogenerated charge separation. The promotion of photocarrier separation and electron were transferred due to the Bi-O bond breakage in the Bi2MoO6-4 [Bi2O2]2+ layer, which mediates the defect level of SOVs in the band structure. The density functional theory calculation results reveal the possible formation site of the oxygen vacancy and the vacancy-induced defect states. This study provides a new approach for fabricating new photocatalysts with surface oxygen defects.

Improving the Classification of Defect Levels in Main Electrical Equipment Through Advanced Prompting on a Large Language Model

The stable operation of the power system is closely related to the national economy and people’s livelihoods. Therefore, the timely detection, qualitative assessment, and handling of major equipment defects are crucial. The classification of defect levels in main electrical equipment is a fundamental task in this process, which is often manually completed, supplemented by knowledge bases or expert systems. However, this approach is time-consuming, labor-intensive, involves challenging human–machine interaction, and relies on expert experience. Conversational large language models, such as ChatGPT, ERNIE Bot, ChatGLM, have garnered widespread recognition in various domains. However, these models may have errors in the reasoning process, resulting in biased or even erroneous outputs, which is referred to as the “hallucinations”. The hallucinations’ problem of large language models poses challenges in specific fields. To mitigate the hallucinations in large language models, researchers often seek to incorporate domain-specific knowledge into these models through methods like fine-tuning or prompt learning. In order to enhance model performance while minimizing computational costs, this study adopts the prompt learning approach. Specifically, we propose a large language model prompt learning framework based on knowledge graphs, aiming to provide the large language model with reasoning support by leveraging specific information stored within the knowledge graph and receive explainable reasoning result. Experimental results demonstrate that our module achieved superior results on the power defect dataset compared to the non-prompt method.

Spraying-Deposited Transparent p-Type Sn-Doped CuI Film and Its Ultrahigh-Speed Self-Powered Photodetector.

The exploitation of simply processed p-type semiconductors and photodetectors with promising optoelectrical properties remains challenging yet essential for current and future advanced optoelectronic applications. Transparent p-type CuI and Sn-doped CuI (Cu-Sn-I) films and their self-powered photodetectors have been successfully fabricated by the spraying method. It is found that the incorporation of Sn dopants enhances the optical, electrical, and photoelectric properties of CuI thin films as well as their corresponding self-powered heterojunction photodetectors. This improvement of the optoelectrical properties of the Cu-Sn-I film and its photodetector can be attributed to the adjustment of the acceptor defect level and increased hole concentration resulting from Sn doping. The Cu-Sn-I/n-Si photodetector exhibits a responsivity of 10.7 mA/W, a detectivity of 6.79 × 1011 Jones, and a response time of 77 μs/30 μs (0 V bias). The response time exhibits the fastest rise and decay times compared with the other CuI-based self-powered UV photodetectors in recent years, showcasing promising applications in the realm of transparent electronics moving forward. This study also presents an effective strategy for enhancing the electrical properties of p-type semiconductors and devices through effective doping.

Enhanced photocatalytic performance of TiO2 with tunable oxygen vacancies induced by H2/N2 mixture treatment

Surface treatment can effectively modulate the surface properties of a photocatalyst to hasten the separation and transfer of photoinduced charges, ultimately achieving high photocatalytic performance. Herein, surface treatment of anatase-phase TiO2 prepared by a hydrothermal method was performed under H2/N2 mixed atmosphere. The experimental results demonstrate that roasting TiO2 in a H2/N2 atmosphere can effectively reduce partial Ti4+ to Ti3+, thereby facilitating the production of tunable oxygen vacancies (OVs) by disrupting Ti-O-Ti bonds. OVs can construct defective energy levels to bring about a decrease in the bandgap of TiO2 and an extension of light absorption range. Moreover, OVs remarkedly improve the separation of photoinduced charges of TiO2 and accelerate the photocatalytic reaction as active sites. The photocatalytic experimental results demonstrate that TiO2 exhibits the highest performance when roasting TiO2 in a H2/N2 atmosphere for 2 h, and the photocatalytic degradation rate constant of rhodamine B (RhB) and tetracycline (TC) on this sample under simulated solar light irradiation is 2.24 and 1.11 times higher than that on the reference TiO2, respectively. These findings provide valuable insights for designing highly efficient photocatalysts for effective water pollution treatment.

Study of the effect of CdCl2 introduction on the high temperature activation treatment of CdTe:As polycrystalline thin films

V-doped CdTe polycrystalline films can achieve both doping activation and defect passivation by high-temperature CdCl2 heat treatment, but this requires simultaneous modulation of the amount of CdCl2 introduced to obtain high-quality films. It is found that increasing the CdCl2 introduction does not change the physical phase structure and lattice constant of CdTe:As thin films, but promotes grain recrystallisation, and can promote the formation of A-center, and inhibit the formation of Cd vacancy (VCd) defects, as well as the formation of deep energy level defects. The results provide guidance for the improvement of high-temperature CdCl2 heat treatment of V-doped CdTe polycrystalline thin films.

Deep level defect passivation for printable mesoporous CsSnI3 perovskite solar cells with efficiency above 10%

The lead-free inorganic perovskite CsSnI3 is considered as one of the best candidates for emerging photovoltaics. Nevertheless, CsSnI3-based perovskite solar cells experience a significant drop in performance due to the nonradiative recombination facilitated by trapping. Here, we show an electron donor passivation method to regulate deep-level defects for CsSnI3 perovskite with electron donor pyrrole. Experimental observations combined with theoretical simulations verify that the saturation of Tin dangling bonds with pyrrole on the CsSnI3 surface via a Lewis acid-base addition reaction can significantly reduce the density of deep-level defects. Consequently, the printable mesoporous perovskite solar cells with an FTO/compact-TiO2/mesoporous-TiO2/Al2O3/NiO/carbon framework device structure penetrated with CsSnI3 achieve a power conversion efficiency of up to 10.11%. To our knowledge, this represents the highest efficiency reported to date for lead-free perovskite-based printable mesoporous solar cells. Furthermore, the unencapsulated devices demonstrated remarkable long-term stability, retaining 92% of their initial efficiency even after 2400 h of aging in a nitrogen atmosphere.

High-Density Atomic Level Defect Engineering of 2D Fe-Based Metal-Organic Frameworks Boosts Oxygen and Hydrogen Evolution Reactions.

Electrocatalysts based on metal-organic frameworks (MOFs) attracted significant attention for water splitting, while the transition between MOFs and metal oxyhydroxide poses a great challenge in identifying authentic active sites and long-term stability. Herein, we employ on-purpose defect engineering to create high-density atomic level defects on two-dimensional Fe-MOFs. The coordination number of Fe changes from 6 to 4.46, and over 28% of unsaturated Fe sites are formed in the optimized Fe-MOF. In situ characterizations of the most optimized Fe-MOF0.3 electrocatalyst during the oxygen evolution reaction (OER) process using Fourier transform infrared and Raman spectroscopy have revealed that some Fe unsaturated sites become oxidized with a concomitant dissociation of water molecules, causing generation of the crucial *OH intermediates and Fe oxyhydroxide. Moreover, the presence of Fe oxyhydroxide is compatible with the Volmer and Heyrovsky steps during the hydrogen evolution reaction (HER) process, which lower its energy barrier and accelerate the kinetics. As a result, the optimized Fe-MOF electrodes delivered remarkable OER (259 mV at 10 mA cm-2) and HER (36 mV at 10 mA cm-2) performance. Our study offers comprehensive understanding of the effect of phase transformation on the electrocatalytic process of MOF-based materials.

Unraveling the Oxygen Vacancy-Performance Relationship in Perovskite Oxides at Atomic Precision via Precise Synthesis.

Understanding the fundamental effect of the oxygen vacancy atomic structure in perovskite oxides on catalytic properties remains challenging due to diverse facets, surface sites, defects, etc. in traditional powder catalysts and the inherent structural complexity. Through quantitative synthesis of tetrahedral (LaCoO2.5-T), pyramidal (LaCoO2.5-P), and octahedral (LaCoO3) epitaxial thin films as model catalysts, we demonstrate the reactivity orders of active-site geometrical configurations in oxygen-deficient perovskites during the CO oxidation model reaction: CoO4 tetrahedron > CoO6 octahedron > CoO5 pyramid. Ambient-pressure Co L-edge and O K-edge XAS spectra clarify the dynamic evolutions of active-site electronic structures during realistic catalytic processes and highlight the important roles of defect geometrical structures. In addition, in situ XAS and resonant inelastic X-ray scattering spectra and density functional theory calculations directly reveal the nature of high reactivity for CoO4 sites and that the derived shallow-acceptor defect levels in the band structure facilitate the adsorption and activation of reactive gases, resulting in more than 23-fold enhancement for catalytic reaction rates than CoO5 sites.

Anisotropy of radiation-induced defects in Yb-implanted β-Ga2O3

RE-doped β-Ga2O3 seems attractive for future high-power LEDs operating in high irradiation environments. In this work, we pay special attention to the issue of radiation-induced defect anisotropy in β-Ga2O3, which is crucial for device manufacturing. Using the RBS/c technique, we have carefully studied the structural changes caused by implantation and post-implantation annealing in two of the most commonly used crystallographic orientations of β-Ga2O3, namely the (-201) and (010). The analysis was supported by advanced computer simulations using the McChasy code. Our studies reveal a strong dependence of the structural damage induced by Yb-ion implantation on the crystal orientation, with a significantly higher level of extended defects observed in the (-201) direction than for the (010). In contrast, the concentration and behavior of simple defects seem similar for both oriented crystals, although their evolution suggests the co-existence of two different types of defects in the implanted zone with their different sensitivity to both, radiation and annealing. It has also been found that Yb ions mostly occupy the interstitial positions in β-Ga2O3 crystals that remain unchanged after annealing. The location is independent of the crystal orientations. We believe that these studies noticeably extend the knowledge of the radiation-induced defect structure, because they dispel doubts about the differences in the damage level depending on crystal orientation, and are important for further practical applications.

Oxidative Passivation of FAPbI3 via Femtosecond Laser for Enhanced Photoluminescence and Photoresponse Performance

AbstractThe presence of uncoordinated Pb atoms in perovskite materials, acting as non‐radiative recombination centers, significantly degrades the performance of perovskite‐based photoelectronic devices. Addressing this issue by passivating these deep energy level defects is essential for enhancing device performance. By now, the passivation methods are largely proposed by chemical additive engineering methods, which involve the oxidation of Pb through chemical additives. However, chemical passivation methods are often plagued by issues such as prolonged processing times, uncontrollable passivation regions uncontrollable, and environmental contamination. Here, a novel femtosecond laser oxidative passivation (FsLOP) technique for FAPbI3 is proposed, evidenced by X‐ray photoelectron spectroscopy (XPS) revealing the formation of Pb─O bonds. The role of peroxide anions in oxidation passivation is highlighted as pivotal. The oxidative passivation effect is further corroborated by a four‐fold increase in photoluminescence (PL) intensity and an exceptional improvement in photoresponse performance, achieving a detectivity of 6.77 × 1012 Jones. This work provides a deeper insight into the interaction between femtosecond lasers and perovskite materials, proposing an efficient, precisely controlled, and environmentally friendly way to improve the performance of perovskite photoelectronic devices.

Spectrum of Tunneling Transport through Phonon-Coupled Defect States in a Carbon-Doped Hexagonal Boron Nitride Barrier.

Defects in hexagonal boron nitride (h-BN) play important roles in tunneling transport through the h-BN barrier. Here, using carbon-doped h-BN (h-BN:C) as a tunnel barrier containing defects in a controlled manner, we investigated tunneling transport through defects in the h-BN:C/graphene heterostructures. Defect-assisted tunneling through a specific kind of carbon-related defect was observed in all measured devices, where the defect level was always located at ∼0.1 eV above the graphene's charge neutrality point. We revealed a phonon-assisted inelastic process in the defect-assisted tunneling, in which carriers tunnel through the defect with phonon emission. Furthermore, when the h-BN:C barrier was thick (12 layers, ∼4 nm), sequential tunneling through two defects became dominant, where the phonon-assisted inelastic process shows substantial effects between the two defects. This study reveals the contribution of phonons to defect-assisted tunneling transport, which is essential for the development of defect-related van der Waals (vdW) electronic techniques.

Structure and Electronic Properties of the A‐center in Si1−xGex Alloys: Insight from the Heyd–Scuseria–Ernzerhof Hybrid Functional Calculations

This study investigates the structure and electronic properties of the A‐center in Si1−xGex alloys (x = 0, 0.25, 0.50, 0.75, and 1), using the Heyd–Scuseria–Ernzerhof hybrid functional. It is found that the most stable structures form SiOSi bonds, with the other two atoms surrounding the A‐center being Ge atoms, which form a long bond between them. The behavior of the O atoms is significantly influenced by the Ge content. As it increases, the position of the O atom shifts relative to the surrounding atoms, thereby affecting the bond lengths and angles. The formation energy of the A‐center first decreases with increasing Ge content up to 0.75, which enhances the defect production; however, then it decreases from 0.75 to 1, thus suppressing defect production. The calculations are consistent with the experimental data, showing that the distance of the defect level from the conduction band minimum increases with Ge content, supporting the observed enthalpy changes in electron ionization.

Photoluminescence optical and thermal quenching in heavily doped Gd3(Ga,Al)5O12:Ce, Mg single crystals: Dependence on the Ga3+, Ce3+, and Mg2+ concentration

Heavily doped single crystals of Gd3GaxAl5-xO12:Ce, Mg (x = 2.46–2.95) with different concentrations of Ce (0.016–0.188 at.%) and Mg (0–0.083 at.%) are investigated by the X-ray diffraction, photoluminescence, and thermoluminescence methods. Dependences of the luminescence characteristics, as well as crystal lattice parameters and distances between the Ce3+ and Mg2+ ions, on the Ga, Ce, and Mg concentration are studied. Mechanisms of the processes, resulting in the photoluminescence optical and thermal quenching and acceleration of decay kinetics, and the influence of the crystal composition on these processes are discussed. The role of close {Ce3+ - Mg2+Ga} pairs in these processes is considered. At T > 400 K, the luminescence thermal quenching is caused by the crossover process, while in the 200–350 K temperature range, by the electron transfer from the 5d1 excited state of Ce3+ to nearby defect levels (electron traps) located between the 5d1 level and the conduction band. The latter process results also in the appearance of thermally stimulated luminescence, and its efficiency depends on the Ga3+ content and concentration of intrinsic defects. The optimum concentrations of Ga3+ and Mg2+ ions in the investigated crystals are determined.

Using ground state and excited state density functional theory to decipher 3d dopant defects in GaN

Using ground state density functional theory (DFT) and implementing an occupation-constrained DFT (occ-DFT) for self-consistent excited state calculations, we decipher the electronic structure of the Mn dopant and other 3ddefects in GaN across the band gap. Our analysis, validated with broad agreement with defect levels (ground-state calculations) and photoluminescence data (excited-state calculations), mandates reinterpretation and reassignment of 3ddefect data in GaN. The MnGadefect is determined to span stable charge states from (1-) inn-type GaN through (2+) inp-type GaN. The Mn(2+) is predicted to be ad2ground state spin triplet defect with a singlet excited state, isoelectronic with the defect associated with the 1.19 eV photoluminescence inn-type GaN. The combined analysis of defect levels and excited states invites reassessment of alld2-capable dopants in GaN. We demonstrate that the 1.19 eV defect, a candidate defect for optically controlled quantum applications, cannot be the Cr(1+) assumed in literature and instead must be the V(0). The combined ground-state/excited-state DFT analysis is shown to be able to chemically fingerprint defects.

Roles of shallow energy level defect states on amplified spontaneous emission enhancement of CH3NH3PbBr3 perovskite crystals

Abstract In this work, we demonstrated the role of shallow energy level defect states on the emission of CH3NH3PbBr3 perovskite polycrystals under laser excitation. The perovskite polycrystals were synthesized by a simple, one-step, low-cost solution self-assembled method. By adjusting the sample preparation temperature from 303 to 373 K, we could manipulate the number of shallow energy level defect states, which were evaluated through low-temperature photoluminescence measurement. This led to an evolution of the CH3NH3PbBr3 perovskite polycrystals’ emission from amplified spontaneous emission to random lasing emission. As a result, the most efficient lasing threshold of 4 μJ mm−2 was achieved with the CH3NH3PbBr3 perovskite polycrystals synthesized at the optimum temperature of 333 K. Furthermore, the surface morphologies and the crystal structure of the CH3NH3PbBr3 perovskite polycrystals were also taken into consideration to unravel the role of defects in the CH3NH3PbBr3 perovskite polycrystals.

Thermoluminescence characteristics of UV-irradiated natural hackmanite

Hackmanite has received extensive attention from scholars in recent years. However, there is still no report on the thermoluminescence properties of natural hackmanites under UV irradiation, as well as its relative electron trap and luminescence center. In this paper, the structural defects and thermoluminescence spectroscopic properties of hackmanite are comprehensively characterized by XRD, TL spectroscopy, SEM, EPR, XPS and Raman spectroscopy. The traps depth of TL peaks are calculated by computerized glow curve deconvolution (CGCD) techniques. And the charge transition energy levels of intrinsic defects in Na8Al6Si6O24(Cl,S)2 are calculated by spin-polarized density-functional theory (DFT) in VASP. It shows that the low-temperature TL peaks of hackmanite are associated with Cl vacancies and photochromic properties. The high-temperature peak is caused by marginal oxygen vacancies. The results are conducive to deepening the understanding of structural defects of the hackmanites and linking the thermoluminescence with the phototropy.

The Influence of Etching Method on the Occurrence of Defect Levels in III-V and II-VI Materials.

The influence of the etching method on the occurrence of defect levels in InAs/InAsSb type-II superlattice (T2SLs) and MCT photodiode is presented. For both analyzed detectors, the etching process was performed by two methods: wet chemical etching and dry etching using an ion beam (RIE-reactive ion etching). The deep-level transient spectroscopy (DLTS) method was used to determine the defect levels occurring in the analyzed structures. The obtained results indicate that the choice of etching method affects the occurrence of additional defect levels in the MCT material, but it has no significance for InAs/InAsSb T2SLs.

Potentiostatic strengthening tactic for constructing defects rich battery-type nickel-cobalt selenate for enhancing energy storage

Reasonably designing transition metal composite electrode materials with high defect levels can overcome the disadvantage of relatively moderate energy density in supercapacitors. Herein, an appealing potentiostatic strengthening tactic for constructing defect rich electrode materials is proposed. Notably, the densely nanoneedle-like nickel–cobalt-based selenide (NCSe) undergoes in situ electrochemical reconstruction after potentiostatic modification, with significant changes in morphology, structure, and composition. The highly active materials NCSe-E, transforming from weakly active substances NCSe mentioned above, are rich in oxygen vacancies and a large number of exposed active sites. Subsequently, it is compared with typical cyclic voltammetry (CV) scanning techniques and the phase evolution process is explored using in situ and ex situ methods. The results show that the plentiful defects promote the adsorption/desorption of OH–, leading to superior rate performance of NCSe-E, with a specific capacity of 3830 mC cm−2 at 1 mA cm−2 and capacity retention of 72.0 % at 50 mA cm−2. Kinetic analysis and theoretical calculations further reveal that oxygen vacancies enhance ions adsorption energy in NCSe-E and promote charge transfer during charging/discharging processes. This work indicates that the novel defect construction strategy provides some new insights for achieving reasonable regulation of high-performance transition metal compound electrode materials.