High Defect Density Research Articles

All‐Inorganic Perovskite Solar Cells: Modification Strategies and Challenges

Cesium‐based all‐inorganic wide‐bandgap perovskite solar cells (AIWPSCs) have been demonstrated with exceptional optoelectronic properties such as intrinsic optical wide‐bandgap and high thermal stability, which make them suitable candidates for the front sub‐cells of tandem solar cells (TSCs). Passivation of perovskite surface and interface is a matter of common interest in this community since all‐inorganic perovskites always suffer from non‐ideal crystallization such as phase impurity, high defect density, and non‐uniform morphology. Despite these shortcomings, numerous efforts have been devoted in recent years to pursuing high‐performance AIWPSCs, which exhibit an abruptly increased power conversion efficiency (PCE) from 2.9% to over 21.0%. In view of not having a thorough summary about the advancements on AIWPSCs, herein, a comprehensive review is given to highlight the recent device performance progress of AIWPSC, particularly focusing on the strategies to passivate the defects of all‐inorganic perovskite, namely, additive engineering, solvent engineering, interface modification, and the exploration of new charge transport materials (CTMs) for improving the phase stability and PCE of AIWPSCs. Finally, a conclusive outlook on AIWPSCs will be given to provide our perspectives aiming to inspire the further development of AIWPSCs.

Magnetocaloric effect in Gd-Sc solid solutions

Development of magnetic cooling technology needs effective and suitable solid state refrigerants whose properties meet rigorous functional and market requirements. One of the important issues hindering progress in this field is the lack of a clear understanding of the mechanisms underlying the giant magnetocaloric effect that some materials demonstrate. Considering systems based on one magnetic component seems to be a promising and effective way to unraveling this puzzle. This study is aimed at partially filling this gap by inspecting the magnetic and magnetocaloric properties of a series of GdSc alloys containing up to 25 at.% Sc. It has been established that all obtained alloys form single-phase disordered solid solutions with an hexagonal closed-packed crystal lattice. We found that doping with scandium induces strong distortion effects in the crystal structure due to huge atomic size mismatch. The studied GdSc alloys are ultra-soft ferromagnets with high Curie points. The magnetocaloric response in the alloys is not large in magnitude compared to Gd but the magnetocaloric effect (MCE) extends over a wide temperature interval exceeding 135 K, resulting in the relative cooling power of 930 – 986 J/kg in a magnetic field of 5 T. Such behavior in the MCE is associated with the strongly distorted crystalline structure which provokes complicated magnetic exchange correlations. High density of structural defects in GdSc alloys, induced by large atomic size mismatch, is probably the determining factor in improving the refrigerant capacity.

Electrical transport in epitaxially grown undoped and Si-doped degenerate GaN films

This study investigates the electrical transport properties of undoped and Si-doped, degenerate GaN heteroepitaxial films grown on sapphire by reactive rf sputtering of GaAs (and Si) in Ar-N2 mixture. The room temperature electrical measurements showed that the resistivity of undoped GaN film grown at 100% N2 was ∼2 × 105 Ω cm, which reduced to ∼1 Ω cm in Si-doped film, revealing the effect of Si doping. With decrease of N2 from 100% to 75%, the carrier concentration of Si-doped films increased from ∼7 × 1018 cm−3 to ∼2 × 1019 cm−3, but remained practically unchanged as N2 was decreased to 20%, which is explained by effects due to saturation of Si doping and increase of Ga interstitials as well as compensation by N interstitials and Ga vacancies. Undoped and Si-doped films grown below 20% N2 displayed similar carrier concentrations (∼1020 cm−3), due to dominance of Ga interstitials. Both undoped and Si-doped films were degenerate and displayed increase of mobility with carrier concentration and temperature, which was analyzed by the combined effect of ionized impurity and dislocation scattering, using compensation ratio as fitting parameter. At carrier concentrations ≲1019 cm−3, the mobility was governed by both ionized impurity and dislocation scattering, while at higher carrier concentrations, ionized impurity scattering was found to dominate, limited by compensation due to acceptors. In spite of the degenerate character, the films displayed a small decrease of carrier concentration with temperature, along with a nearly linear decrease of mobility, which are explained by a marginal increase of compensation ratio with decrease of temperature, taking into account the band edge fluctuation effects. These features of electrical transport have not been much explored for heteroepitaxial, n-type degenerate GaN films, possessing high density of dislocations and point defects.

Efficient InGaN‐Based Red Light‐Emitting Diodes by Modulating Trench Defects

AbstractTrench defects in multi‐quantum wells (MQWs) have been considered as flawed structures that severely degraded the internal quantum efficiency (IQE) of light‐emitting diodes (LEDs) in the past. In this research, trench defects are innovatively utilized to enhance the efficiency of red InGaN LEDs. Specifically, dual‐color MQWs structures are applied to modulate trench defects. The upper red MQWs, grown on top of green MQWs with a high density of trench defects, exhibit a significant wavelength redshift of 68 nm and ≈ 6‐fold luminescence enhancement compared to those without intentionally introduced trench defects. Red InGaN LEDs with an IQE of 16.4% are achieved with this epitaxy growth strategy. Such wavelength redshift is attributed to the more indium incorporation due to the strain relaxation effect of trench defects. Moreover, the luminescence enhancement originates from the strong emission of the red MQWs inside trench defects, mainly attributed to strain relaxation and defect shielding by the wide and deep trenches. Achieving red emission by modulating trench defects is simple and reproducible without requiring additional substrate designs, which provides a novel way toward high‐efficiency red InGaN LEDs.

Crystallization-induced formation of two-dimensional carbon nanosheets derived from sodium lignosulfonate for fast lithium storage

Sodium lignosulfonate, an abundant natural resource, is regarded as an ideal precursor for the synthesis of hard carbon. The development of high-performance, low-cost and sustainable anode materials is a significant challenge facing lithium-ion batteries (LIBs). The modulation of morphology and defect structure during thermal transformation is crucial to improve Li+ storage behavior. Synthesized using sodium lignosulfonate as a precursor, two-dimensional carbon nanosheets with a high density of defects were produced. The synergistic influence of ice templates and KCl was leveraged, where the ice prevented clumping of potassium chloride during drying, and the latter served as a skeletal support during pyrolysis. This resulted in the formation of an interconnected two-dimensional nanosheet structure through the combined action of both templates. The optimized sample has a charging capacity of 712.4 mA h g−1 at 0.1 A g−1, which is contributed by the slope region. After 200 cycles at 0.2 A g−1, the specific charge capacity remains 514.4 mA h g−1, and a high specific charge capacity of 333.8 mA h g−1 after 800 cycles at 2 A g−1. The proposed investigation offers a promising approach for developing high-performance, low-cost carbon-based anode materials that could be used in advanced lithium-ion batteries.

Quantifying defects in graphene oxide structures

Oxidation of graphite and subsequent exfoliation leads to single layer graphene oxide (GO). One of the key structural features of GO is the presence of different kinds of defects that dictates their various physical and chemical properties. Molecular dynamics simulations with ReaxFF force fields have been widely used to generate realistic models of GO. In these simulations, the extent and distribution of the defects are varied by changing the initial oxygen (O)/carbon (C) ratio while the defect density is often measured by the total number of non-graphitic carbon (non-gC) atoms. Our calculations suggest that this parameter overestimates the defect densities at low O/C ratio. Herein, we employ the relative area of the defects as an alternative metric to gauge the defect density. Being exclusive to the defects and sensitive to the structural irregularities, this metric works well at both low and high defect densities. In another example, we consider the reduction of GO to reduced graphene oxide (rGO) for different O/C ratios, where the decrease in the number of non-gC atoms is associated with the formation of comparatively larger defects. Hence, it is unclear whether the extent of reduction in the defect density (if at all) should vary monotonically with the O/C ratio. The defect area, unlike the count of the non-gC atoms, mirrors this ambiguity and the change with respect to O/C ratio is not strictly monotonic. Additionally, we also investigate the dependence of the defect distribution and defect area on the size of the simulation cell.

Hematite (α-Fe2O3) with Oxygen Defects: The Effect of Heating Rate for Photocatalytic Performance.

Hematite (α-Fe2O3) emerges as an enticing material for visible-light-driven photocatalysis owing to its remarkable stability, low toxicity, and abundance. However, its inherent shortcomings, such as a short hole diffusion length and high recombination rate, hinder its practical application. Recently, oxygen vacancies (Vo) within hematite have been demonstrated to modulate its photocatalytic attributes. The effects of Vo can be broadly categorized into two opposing aspects: (1) acting as electron donors, enhancing carrier conductivity, and improving photocatalytic performance and (2) acting as surface carrier traps, accelerating excited carrier recombination, and deteriorating performance. Critically, the generation rate, distribution, role, and behavior of Vo significantly differ for synthesis methods due to differences in formation mechanisms and oxygen diffusion. This complexity hampers simplified discussions of Vo, necessitating careful investigation and nuanced discussion tailored to the specific method and conditions employed. Among various approaches, hydrothermal synthesis offers a simple and cost-effective route. Here, we demonstrate a hydrothermal synthesis method for Vo introduction to hematite using a carbon source, where variations in the heating rate have not been previously explored in terms of their influence on Vo generation. The analyses revealed that the concentration of Vo was maximized at a heating rate of 16 °C/min, indicative of a high density of surface defects. With regard to photocatalytic performance, elevated heating rates (16 °C/min) fostered the formation of Vo primarily on the hematite surface. The photocatalytic activity was 7.1 times greater than that of the sample prepared at a low heating rate (2 °C/min). These findings highlight the crucial role of surface defects, as opposed to bulk defects, in promoting hematite photocatalysis. Furthermore, the facile control over Vo concentration achievable via manipulating the heating rate underscores the promising potential of this approach for optimizing hematite photocatalysts.

Shallow defects and variable photoluminescence decay times up to 280 µs in triple-cation perovskites.

Quantifying recombination in halide perovskites is a crucial prerequisite to control and improve the performance of perovskite-based solar cells. While both steady-state and transient photoluminescence are frequently used to assess recombination in perovskite absorbers, quantitative analyses within a consistent model are seldom reported. We use transient photoluminescence measurements with a large dynamic range of more than ten orders of magnitude on triple-cation perovskite films showing long-lived photoluminescence transients featuring continuously changing decay times that range from tens of nanoseconds to hundreds of microseconds. We quantitatively explain both the transient and steady-state photoluminescence with the presence of a high density of shallow defects and consequent high rates of charge carrier trapping, thereby showing that deep defects do not affect the recombination dynamics. The complex carrier kinetics caused by emission and recombination processes via shallow defects imply that the reporting of only single lifetime values, as is routinely done in the literature, is meaningless for such materials. We show that the features indicative for shallow defects seen in the bare films remain dominant in finished devices and are therefore also crucial to understanding the performance of perovskite solar cells.

Deuterium retention and transport in ion-irradiated tungsten exposed to deuterium atoms: Role of grain boundaries

The influence of grain boundaries on deuterium (D) retention and transport was investigated in nanocrystalline tungsten (W) by exposing the samples to sub eV D atoms. Thin tungsten films with nanometer-sized grains were produced by pulsed laser deposition on tungsten substrates. Their grain size was increased up to one micrometer by thermal annealing in vacuum up to 1223 K. Irradiation damage was created by 20 MeV W ions at 290 K. The transmission electron microscopy analysis showed one order of magnitude larger dislocation density in nanometer-grained samples compared with the larger-grained samples. The samples were after W irradiation exposed to 0.3 eV D atoms at 600 K. D retention and D depth profiles were measured by nuclear reaction analysis. In the as-deposited nanometer-grained samples, D populated the damaged region more than three times faster than in the samples with larger grains, indicating that grain-boundaries increase D transport through the material. The concentration of defects was assessed by the final D concentration in the samples. The sample with the smallest grain size showed slightly larger D concentration in the irradiated area, but the difference in the D concentration was not substantial between different-grained samples. A large D concentration in the non-irradiated nanometer-grained sample was measured which is an indication for a high defect density in the initial material. From our observations, it can be postulated that the nanocrystalline microstructure did not substantially influence the generation of irradiation-induced defects by defect annihilation at grain boundaries.

Planar defects and strain distributions in polycrystalline BaFe2As2 superconductors synthesized by mechanochemical methods

In this study, the microstructure of polycrystalline Co-doped BaFe2As2 superconducting bulks was comprehensively analyzed. The synthesis process involved ball milling and subsequent spark plasma sintering (SPS). A strong correlation between the microstructure and synthesis conditions was observed. Specifically, when mechanochemical synthesis of raw metal powders was carried out at a ball milling energy (EBM) of 100 MJ/kg, the resulting Co-BaFe2As2 bulk contained nanograins with a high density of planar defects and exhibited a high critical current density (Jc) under both magnetic and self-fields. The findings suggest that planar defects are one of the influential factors on the superconducting properties. Atomic-resolution microstructural characterization revealed that the planar defects are parallel to (001) and exhibit distinctive atomic arrangements. Atomic-resolution elemental mappings displayed a remarkable barium concentration in planar defects and grain boundaries compared to the BaFe2As2 matrix. Local strain fields were also observed around planar defects, especially near their edges. These nanoscale strain fields could be considered as volumetric pinning centers, and effectively pin flux vortices in the random crystal orientation, thereby increasing Jc in the Fe-based superconducting bulk. Additionally, an extended investigation was performed to include a K-doped BaFe2As2 bulk that exhibits similar planar defects. Based on the results of the microstructural analysis, speculations were mode on the evolution process of the microstructure during the preparation of BaFe2As2 bulks.

Improved Air Stability for High-Performance FACsPbI3 Perovskite Solar Cells via Bonding Engineering.

Despite the fact that perovskite solar cells (PSCs) are widely popular due to their superb power conversion efficiency (PCE), their further applications are still restricted by low stability and high-density defects. Especially, the weak binding and ion-electron properties of perovskite crystals make them susceptible to moisture attack under environmental stress. Herein, we report an overall sulfidation strategy via introduction of 1-pentanethiol (PT) into the perovskite film to inhibit bulk defects and stabilize Pb ions. It has been confirmed that the thiol groups in PT can stabilize uncoordinated Pb ions and passivate iodine vacancy defects by forming strong Pb-S bonds, thus reducing nonradiative recombination. Moreover, the favorable passivation process also optimizes the energy-level arrangement, induces better perovskite crystallization, and enhances the charge extraction in the full solar cells. Consequently, the PT-modified inverted device delivers a champion PCE of 22.46%, which is superior to that of the control device (20.21%). More importantly, the PT-modified device retains 91.5% of its initial PCE after storage in air for 1600 h and over 85% of its initial PCE after heating at 85 °C for 800 h. This work provides a new perspective to simultaneously improve the performance and stability of PSCs to satisfy their commercial applications.

Stoichiometrically optimized eg orbital occupancy of Ni-Co oxide catalysts for Li-air batteries.

Li-air battery (LAB) technology is making continuous progress toward its theoretical capacity, which is comparable to gasoline. However, the sluggish reaction at the cathode is still a challenge. We propose a simple strategy to optimize the surface eg occupancy by adjusting the stoichiometric ratios of transition metal-based spinel structures through a controlled hydrothermal synthesis. Three distinct stoichiometries of Ni-Co oxides were used to demonstrate the direct correlation between stoichiometry and catalytic performance. The groundsel flower-like structure having a 1 : 1.4 Ni : Co atomic ratio with high surface area, high defect density, and an abundance of Ni3+ at the surface with semi-filled eg orbitals was found to benefit the structure promoting high catalytic activities in aqueous and aprotic media. The assembled LAB cells employing this cathode demonstrate an exceptional lifespan, operating for 3460 hours and completing 173 cycles while achieving the highest discharge capacity of 13 759 mA h g-1 and low charging overpotentials. The key to this prolonged performance lies in the full reversibility of the cell, attributed to its excellent OER performance. A well-surface adsorbed, amorphous LiO2/Li2O2 discharge product is found to possess high diffusivity and ease of decomposition, contributing significantly to the enhanced longevity of the cell.

Recent advances in artificial neuromorphic applications based on perovskite composites.

High-performance perovskite materials with excellent physical, electronic, and optical properties play a significant role in artificial neuromorphic devices. However, the development of perovskites in microelectronics is inevitably hindered by their intrinsic non-ideal properties, such as high defect density, environmental sensitivity, and toxicity. By leveraging materials engineering, integrating various materials with perovskites to leverage their mutual strengths presents great potential to enhance ion migration, energy level alignment, photoresponsivity, and surface passivation, thereby advancing optoelectronic and neuromorphic device development. This review initially provides an overview of perovskite materials across different dimensions, highlighting their physical properties and detailing their applications and metrics in two- and three-terminal devices. Subsequently, we comprehensively summarize the application of perovskites in combination with other materials, including organics, nanomaterials, oxides, ferroelectrics, and crystalline porous materials (CPMs), to develop advanced devices such as memristors, transistors, photodetectors, sensors, light-emitting diodes (LEDs), and artificial neuromorphic systems. Lastly, we outline the challenges and future research directions in synthesizing perovskite composites for neuromorphic devices. Through the review and analysis, we aim to broaden the utilization of perovskites and their composites in neuromorphic research, offering new insights and approaches for grasping the intricate physical working mechanisms and functionalities of perovskites.

A highly sensitive self-powered photodetector based on pinhole-free PEA0.2FA0.8SnI3 films with aminopyrimidine

The PEA0.2FA0.8SnI3-based photodetector with addition of 2-aminopyrimidine exhibited a high responsivity (0.4 A W−1) and detectivity (1.16 × 1012 Jones), while also ensuring excellent operation stability and repeatability after 104 on/off cycles.

Влияние примеси углерода на температуры плавления и кристаллизации наночастиц никеля: молекулярно-динамическое моделирование

The molecular dynamics method was used to study the processes of melting and crystallization of nickel nanoparticles and the influence of carbon impurities on these processes. The influence of the particle size and concentration of carbon atoms, which were initially randomly distributed in the volume of a nickel particle, was considered. The particle diameter varied from 1.5 to 12 nm, and the concentration of carbon atoms varied from 0 to 10 at.%. It has been shown that the melting and crystallization temperatures are inversely proportional to the particle diameter: as the particle size decreases and, accordingly, the free surface fraction increases, the temperatures of melting during heating and crystallization during cooling decrease. The presence of an impurity of carbon atoms reduces both the melting point and the crystallization temperature ‒ by approximately 150 K in the presence of 10 at.% carbon. Carbon atoms often formed clusters consisting of several carbon atoms in the metal. Such clusters distorted the nickel crystal lattice around them, which led to earlier melting of the metal near the carbon clusters. At cooling from the melt at a rate of 1012 K/s, the particle crystallized with the formation of a nanocrystalline structure containing a high density of grain boundaries and other defects. Impurity carbon atoms, especially carbon clusters, were fixed primarily at grain boundaries and triple junctions.

Glucose-based highly-porous activated carbon nanospheres (g-ACNSs) for high capacity hydrogen storage

The high density of sp2 defects and specific surface area result in an exceptional H2 storage capacity and stability of hollow carbon nanospheres.

Controllable construction of a 3D-honeycomb-like porous carbon network as a high-performance cathode for promoting Zn-ion storage capability.

Inheriting the energy storage mechanism of supercapacitors and rechargeable ion batteries, zinc ion capacitors (ZICs) greatly increase their energy density at high power without sacrificing their life span. However, sluggish kinetics and insufficient active sites for Zn2+ storage induced by the significant mismatch of charge carriers with limited pore size hinder the efficient Zn2+ storage and smooth application of carbonaceous cathode materials. Herein, a three-dimensional honeycomb-like porous carbon network (HPCN) was fabricated, which can reduce the diffusion barrier for fast kinetics, produce a high-density defect area, effectively increase active sites for charge storage, and generate a high nitrogen-doping content. Benefiting from these advantages, the optimized ZICs bring out a marvelous energy/power density (130 W h kg-1/11.7 kW kg-1) with an ultrahigh reliable cyclability of 97.8% after 50 000 cycles at a high current density of 5 A g-1. Importantly, systematic ex situ characterizations combined with theoretical calculations demonstrate that the outstanding Zn2+ storage capacity is attributed to the synergistic effect of physical co-adsorption of cations and reversible chemisorption. This work presents an attractive strategy for developing advanced carbon cathodes with suitable pores and accelerates the exploration of charge storage mechanisms, which may open a new avenue for practical supercapacitors.

B-Site Co-Doping Coupled with Additive Passivation Pushes the Efficiency of Pb-Sn Mixed Inorganic Perovskite Solar Cells to Over 17.

Pb-Sn mixed inorganic perovskite solar cells (PSCs) have garnered increasing interest as a viable solution to mitigate the thermal instability and lead toxicity of hybrid lead-based PSCs. However, the relatively poor structural stability and low device efficiency hinder its further development. Herein, high-performance manganese (Mn)-doped Pb-Sn-Mn-based inorganic perovskite solar cells (PSCs) are successfully developed by introducing Benzhydroxamic Acid (BHA) as multifunctional additive. The incorporation of smaller divalent Mn cations contributes to a contraction of the perovskite crystal, leading to an improvement in structural stability. The BHA additive containing a reductive hydroxamic acid group (O═C-NHOH) not only mitigates the notorious oxidation of Sn2+ but also interacts with metal ions at the B-site and passivates related defects. This results in films with high crystallinity and low defect density. Moreover, the BHA molecules tend to introduce a near-vertical dipole moment that parallels the built-in electric field, thus facilitating charge carrier extraction. Consequently, the resulting device delivers a champion PCE as high as 17.12%, which represents the highest reported efficiency for Pb-Sn-based inorganic PSCs thus far. Furthermore, the BHA molecule provides an in situ encapsulation of the perovskite grain boundary, resulting in significant enhancement of device air stability.

Effect of Eu2O3@C composite catalyst on hydrogen storage performance of Mg96La3Ni alloy

This study successfully synthesized Eu2O3@C composite catalyst by an improved chemical blowing method. The Eu2O3 nanoparticles in this catalyst are highly dispersed within a porous carbon matrix, exhibiting a well-developed mesoporous structure, a large specific surface area, and a high density of defects. Eu2O3@C was mixed with the Mg96La3Ni alloy through ball milling, and the impact of varying amounts of Eu2O3@C on the hydrogen storage performance of Mg96La3Ni was comprehensively investigated. The results showed that Eu2O3@C markedly enhances the de/hydrogenation kinetics of Mg96La3Ni, with the most substantial impact noted at a Eu2O3@C content of 2 wt %. At 360 °C, 77.3 % of the maximum hydrogen capacity could be absorbed within 2 min, while only 1.4 min was required to release 3 % of hydrogen. Additionally, Eu2O3@C markedly decreased the dehydrogenation activation energy of the alloys to 106.3 kJ/mol and reduced the endothermic peak temperature to 347 °C, while leaving thermodynamic properties unaffected. The Mg96La3Ni–Eu2O3@C composite exhibits remarkable cyclic stability, retaining its hydrogen storage capacity throughout the cycling process. This study pioneers a novel strategy to elevate the performance of magnesium-based hydrogen storage materials.

Vertical Graphene/Pentacene Single Crystal/Graphene Transistors for Self‐Powered Weak Light Detection and High‐Speed Imaging

AbstractOrganic semiconductors are promising candidates for next‐generation photodetectors owing to their unique properties including large absorption coefficient and intrinsic mechanical flexibility. However, limited by high defect density, large exciton binding energy, and short exciton diffusion length, many challenges remain for fabricating high‐performance organic photodetectors, especially for scenarios where weak light sensing and high‐speed imaging are required. Herein, by coupling single‐crystal pentacene with monolayer graphene, a vertical organic phototransistor with excellent zero‐bias photon detecting capacity is demonstrated. Under self‐powered weak light condition, the responsivity and specific detectivity are 2.86 AW−1 and 3.3 × 1011 Jones (based on experiment spectral noise density), respectively. It is demonstrated that weak light down to 25 nWcm−2 can be detected. When operating with an external bias, the responsivity of the device increases dramatically (up to 105 AW−1), and a better 3 dB bandwidth (≈74 kHz) is obtained. Finally, imaging functionality is demonstrated by employing this vertical transistor as a single‐sensing pixel. This work suggests organic single crystals exhibit enormous potential in advanced optoelectronic systems, and may provide a viable route for weak light detecting and high‐speed imaging.