Detrimental Defects Research Articles

Effect of Self‐Seed Inducing on the Growth Mechanism and Photovoltaic Performance of Cu2ZnSnSe4 Thin Films

The self‐seed inducing grain growth mechanism for a Cu2ZnSnSe4 (CZTSe) thin film is explored via the existence of the fine‐grain CZTSe synthesized spontaneously at a relatively low soft‐selenization temperature of 300°C. Fine control over the elementary proportion and distribution of metal precursors is an important approach for obtaining the self‐seed inducing effect after soft selenization. Herein, a multiple‐cycle metal stack precursor, prepared by a weighting method, is developed to control the elementary proportion and distribution. By shortening the diffusion distance to produce a self‐seed inducing effect at soft selenization, the formation of detrimental intrinsic defects and secondary phases is effectively suppressed, contributing to the optimization of the CZTSe absorber back contact and surface. Consequently, the conductivity of the grain interiors and boundaries of the absorber layer is enhanced, the electric potential and roughness of the surface are reduced considerably, and the minority carrier lifetime of the layer is increased. When the number of stacked cycles increases from 1 to 7, the champion efficiency of CZTSe solar cell improves from 2.97% to 8.05% with a significant decrease in the open circuit voltage deficit from 0.831 to 0.617 V. These results may prove useful for developing a wider range of multinary compound semiconductor devices.

Competing Superior Electronic Structure and Complex Defect Chemistry in Quasi-One-Dimensional Antimony Chalcogenide Photovoltaic Absorbers

Antimony chalcogenides Sb2Se3 and Sb2S3 are an emerging class of quasi-one-dimensional van der Waals bonded materials with rapid progress and great promise for low-cost, high-efficiency, and scalable thin-film photovoltaics. Here using first-principles theoretical calculations, we show that antimony chalcogenides Sb2Se3 and Sb2S3 possess competing superior electronic structure and complex defect chemistry, limiting their photovoltaic performance. The high optical absorption, large Born effective charge, and high dielectric constant lead to low exciton binding energy and small hole effective mass, which ultimately benefit the photovoltaic performance. In contrast, quasi-1D antimony chalcogenides exhibit complex defect chemistry. The cation–anion antisites are the dominating native defects in the Sb rich condition, while in the Sb poor condition the anion–cation antisites are dominant. In both cases, the anion vacancies are also easy to form. These native defects introduce midgap transition levels which act as electron–hole recombination centers, limit the dopability, and lower the open circuit voltage. As a result, Sb2Se3 is a p-type semiconductor in Se rich condition, while Sb2S3 is an intrinsic semiconductor without significant free carrier density. Additionally, due to the unique quasi-1D structures and strong electron–lattice coupling, antimony chalcogenides are able to accommodate larger lattice relaxation than conventional 3D bulk crystals, resulting in negative-U behavior in their defect structures. Therefore, the competing superior electronic structure and complex defect chemistry suggest that, while antimony chalcogenides are excellent photoabsorbers, effort shall be made to suppress the formation of the detrimental native defects to further improve their open-circuit voltage, electronic transport, and, thus, power conversion efficiency.

Formation and Anisotropic Mechanical Behavior of Stacking Fault Tetrahedron in Ni and CoCrFeNiMn High-Entropy Alloy

Stacking fault tetrahedron (SFT) is a kind of detrimental three-dimensional defect in conventional face-centered cubic (FCC) structural metals; however, its formation and anisotropic mechanical behavior in a CoCrFeNiMn high-entropy alloy (HEA) remain unclear. In this work, we first performed molecular dynamics simulations to verify the applicability of the Silcox-Hirsch mechanism in the CoCrFeNiMn HEA. The mechanical responses of the SFT to shear stress in different directions and that of the pure Ni counterpart were simulated, and the evolutions of the atomic structures of the SFTs during shear were analyzed in detail. Our results revealed that the evolution of the SFT has different patterns, including the annihilation of stacking faults, the formation and expansion of new stacking faults, and insignificant changes in stacking faults. It was found that the effects of SFT on the elastic properties of Ni and HEA are negligible. However, the introduction of SFT would reduce the critical stress, while the critical stress of the CoCrFeNiMn HEA is much less sensitive to SFT than that of Ni.

Ag, Ti dual-cation substitution in Cu2ZnSn(S,Se)4 induced growth promotion and defect suppression for high-efficiency solar cells

A dual-cation-substituted Cu2ZnSn(S,Se)4 involving Ag and Ti to separately occupy partial Cu and Sn is developed. Such a new strategy can simultaneously promote crystal growth and suppress detrimental defects, leading to a high efficiency of 12.73%.

Quinary Nanocrystal‐Based Passivation Strategy for High Efficiency and Stable Perovskite Photovoltaics

Despite the rapid progress of perovskite materials in emerging perovskite photovoltaic devices, they still suffer from the polycrystalline nature associated with grain boundaries (GBs) which are vulnerable to moisture permeation and/or ion migration. Besides, charge carrier recombination of GBs through defect states plays a crucial role in restricting the performance and stability of perovskite photovoltaics. To address such detrimental issues, quinary kesterite nanocrystals, namely Cu2NiSn(S,Se)4 (CNTSSe), having narrow size distribution below 10 nm by a facile hot‐casting method are rationally designed and employed as a passivation agent for the GBs/surface of perovskite films. This passivation strategy greatly reduces defect states at perovskite GBs and promotes continuity between adjacent grains, resulting in accelerated hole transport ability and suppressed interfacial recombination. Thereupon, champion power conversion efficiency of 20.8% (20.5 ± 0.3% in average) (Cs0.05(FA0.90MA0.10)0.95Pb(I0.90Br0.10)3), 18.9% (MAPbI3), and 18.7% (FAPbI3) is achieved with a negligible hysteresis and outstanding stability by retaining over 85% of initial performance under ambient conditions with continuous illumination over 900 h. Herein, not only a universal approach to effectively passivate the GBs of the perovskite films by inorganic nanocrystals is presented, but also a deep understanding of detrimental defects on the photovoltaic performance and stability of perovskite solar cells is ensured.

Hydrogen-Anion-Induced Carrier Recombination in MAPbI3 Perovskite Solar Cells.

Identification and passivation of defect-induced electron-hole recombination centers are currently crucial for improving the efficiency of hybrid perovskite solar cells. Besides general intrinsic defects, experimental reports have indicated that hydrogen interstitials are also abundant in hybrid perovskite layers; however, few reports have evaluated the effect of such defects on the charge carrier recombination and device efficiencies. Here, we reveal that under I-poor synthesis conditions, the negatively charged monatomic hydrogen interstitial, Hi-, will form in the prototypical CH3NH3PbI3 perovskite layer, acting as a detrimental deep-level defect, which leads to efficient electron-hole recombination and lowers the cell performance. We further rationalize that Br doping can mitigate the large atomic displacement caused by the presence of Hi- and hence suppress the formation of the deep localized state. The results advance the knowledge of the deep-level defects in hybrid perovskites and provide useful information for enhancing solar cell performance by defect engineering.

In-Situ polymerization of PEDOT in perovskite Thin films for efficient and stable photovoltaics

Solution-processed polycrystalline perovskite films have detrimental defects in the bulk and surfaces/grain boundaries, especially the uncoordinated lead ions (Pb2+) with low formation energy, limiting both the performance and stability of the photovoltaic devices. In this work, a conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) is incorporated into a bulk perovskite film by a facile in-situ polymerization method, where 5-bromo-2,3-dihydro-thieno[3,4-b][1,4]dioxine (BEDOT) as monomer is directly added to a perovskite precursor solution and then in-situ polymerized within the perovskite film during the film formation. The PEDOT is chemically anchored at the surface and grain boundaries of the perovskite film via interaction of sulfur and oxygen atoms in PEDOT with uncoordinated lead ions in the perovskite, passivating the surface and bulk defects and also improving the charge extraction efficiency in the solar cell. Moreover, robust and hydrophobic properties of PEDOT can enhance the thermal and moisture resistance of the perovskite film. As a result, a champion power conversion efficiency (PCE) of 22.58% is achieved, along with significantly enhanced environmental, thermal, and light-soaking stability.

Healing soft interface for stable and high-efficiency all-inorganic CsPbIBr2 perovskite solar cells enabled by S-benzylisothiourea hydrochloride

Interfacial regulation to heal the soft lattice induced defective nanostructured surfaces of perovskite films plays a crucial role in determining the stability and efficiency of perovskite solar cells (PSCs). Herein, we report a facile method to strengthen and passivate the inorganic PbI2-excess CsPbIBr2 surface with a multifunctional S-benzylisothiourea hydrochloride (SBTCl), which not only significantly reduces the detrimental defects through interaction between electron-donating groups and defects (as well as Cl doping as lattice “glue”) but also enables straightway unobstructed hole extraction and transfer arising from the reduced work function. Consequently, a higher efficiency of 10.56% with an open-circuit voltage of 1.327 V than control device with 7.72%-efficiency is obtained for inorganic CsPbIBr2 PSC free of hole transporting material, which is one cutting-edge value in this kind of PSC. In addition, the high hydrophobicity of SBTCl passivator hinders the permeation of moisture into perovskite film, resulting in improved long-term air-stability after storage in 5% relative humidity (RH) without encapsulation over 110 days and persistent light irradiation over 16 h in 50% RH condition.

Enhancing the Photovoltaic Performance of Cu2ZnSn(S,Se)4 Solar Cells with Ba Trace Doping: Large Chemical Mismatch Cation Incorporation

Being absent from larger mismatch of ionic radius and chemical coordination between the cations in Cu2ZnSn(S,Se)4 (CZTSSe) can aggravate the formation of detrimental cation antisite defects and clusters with low formation energy, which results in a serious open‐circuit voltage deficit and a challenge for higher power conversion efficiency of CZTSSe solar cells. External cation doping or alloying is a more effective strategy to solve this issue. Herein, samples of trace Ba doping into CZTSSe are fabricated by the sol–gel method and the mechanism of such trace Ba doping are studied combined by X‐ray diffraction (XRD), Raman, and photoluminescence (PL) measurements. The performance of CZTSSe solar cells is increased by 25.7% as 1% Ba is doped into CZTSSe to substitute Zn. The results highlight the roles of the trace Ba doping on the performance of CZTSSe solar cells, which is beneficial to the reduced formation of the detrimental defects and associated clusters. As a consequence, the bandgap fluctuations are reduced, which results in the improvement of V OC and thus the performance of the solar cell. The cation substitution engineering with larger chemical mismatched alkaline‐earth metal cations provides insights for further improvement of CZTSSe solar cells based on earth abundant elements.

Photoinduced Dynamic Defects Responsible for the Giant, Reversible, and Bidirectional Light-Soaking Effect in Perovskite Solar Cells.

Perovskite solar cells (PSCs) exhibit large, reversible, and bidirectional light-soaking effects (LSEs); however, these anomalous LSEs are poorly understood, limiting the stability engineering and commercialization. We present a unified defect theory for the LSEs in lead halide perovskites by reconciling their defect photochemistry, ionic migration, and carrier dynamics. We considered typical detrimental defects (IPb, Ii, VI) and observed that two atomic configurations were favored, where the carrier lifetime of one configuration was nearly 1 order of magnitude longer than that in the other. First-principles calculations showed that light illumination promotes ion-diffusion-assisted transitions from energetically stable configurations to metastable configurations, which are converted back to stable configurations in the dark. Fermi-level-dependent formation energies of stable/metastable configurations were used to rationalize contradictory experimental results of anomalous LSEs in PSCs observed in various studies, thus providing insights for minimizing the LSE to achieve high-performance stable PSCs.

Improvement Performance of Planar Perovskite Solar Cells by Bulk and Surface Defect Passivation

The photovoltaic property of perovskite solar cells (PSCs) is affected by detrimental defects located at the bulk and surface of perovskite films. Furthermore, defect passivation of the perovskite ...

Impact of Cation Substitution in (AgxCu1−x)2ZnSnSe4 Absorber‐Based Solar Cells toward 10% Efficiency: Experimental and Theoretical Analyses

Solar cells based on kesterite Cu2ZnSnSe4 (CZTSe) compounds with earth‐abundant elements are highly desirable for the low‐cost and high‐efficiency production of renewable energy. However, the occurrence of intrinsic defects substantially impairs the photovoltaic properties of CZTSe. Herein, a cation substitution method to control and passivate the defect states in bandgap of kesterite CZTSe by incorporating Ag ions is introduced. Intensity‐dependent low‐temperature photoluminescence measurements show that Ag incorporation can reduce the density and depth of intrinsic defects in CZTSe. The results reveal that 10% Ag‐alloyed CZTSe provides the shallowest defect states and less nonradiative recombination. It is also confirmed by first‐principles calculations that Ag incorporation enables the formation and suppresses the beneficial and detrimental defects, respectively. Based on the theoretical results, the observed subband photoluminescence peaks can be assigned to the intrinsic point and cluster defects. The best power conversion efficiency of 10.2% is achieved for the 10% Ag‐alloyed CZTSe cell, along with an enhanced open‐circuit voltage. These results open up a new avenue for further improving the performances of CZTSe‐based device via defect engineering.

Regional temperature control in ceramic injection moulding: An approach based on cooling rate optimisation

The injection moulding of ceramic components with uneven wall thickness presents challenges due to differential cooling rates developing in the injected parts, which cause premature solidification of the feedstock at thin features and lead to detrimental defects, worsening in components from green to sintered states. To cope with this, suitable mould thermal control approaches have to be selected and validated, as current control methods are based on the achievement of a uniform cavity surface temperature, which is not tailored to such complex geometries. In this work, a novel thermal control system is proposed, based on regional mould temperatures, implemented with the use of Peltier modules, which locally and independently heat and cool different cavity features according to their thickness. The regional temperature profiles are optimised over time with the use of a coupled Finite Element - Particle Swarm Optimisation (FE-PSO), to achieve uniform cooling rates throughout the moulded components. The performance of this approach is compared to both constant ambient mould temperature and Rapid Heat Cycle Moulding (RHCM) techniques, which instead aim at achieving uniform temperatures throughout the mould cavity surface. Results show that the novel proposed method, based on regional temperature control and uniform cooling rates, promotes the simultaneous solidification of features with a 10-times difference in surface-to-volume ratio. Due to this, in terms of components quality, the novel method brings the advantages of higher dimensional control and reduction of differential shrinkage compared to the other analysed approaches, thus increasing the capability to use injection moulding to manufacture ceramic components characterised by non-uniform wall thickness.

Crystalline defect analysis in epitaxial Si0.7Ge0.3 layer using site-specific ECCI-STEM

Electron channeling contrast imaging (ECCI) is a powerful technique to characterize the structural defects present in a sample and to obtain relevant statistics about their density. Using ECCI, such defects can only be properly visualized, if the information depth is larger than the depth at which defects reside. Furthermore, a systematic correlation of the features observed by ECCI with the defect nature, confirmed by a complementary technique, is required for defect analysis. Therefore, we present in this paper a site-specific ECCI-scanning transmission electron microscopy (STEM) inspection. Its value is illustrated by the application to a partially relaxed epitaxial Si0.7Ge0.3 on a Si substrate. All experiments including the acquisition of ECCI micrographs, the carbon marking and STEM specimen preparation by focused ion beam, and the in-situ-subsequent-STEM-in-scanning electron microscopy (SEM) characterization were executed in one SEM/FIB-based system, thus significantly improving the analysis efficiency. The ECCI information depth in Si0.7Ge0.3 has been determined through measuring stacking fault widths using different beam energies. ECCI is further utilized to localize the defects for STEM sample preparation and in-situ-subsequent-STEM-in-SEM investigation. This method provides a correlative 2.5D defect analysis from both the surface and cross-section. Using these techniques, the nature of different line-featured defects in epilayers can be classified, as illustrated by our study on Si0.7Ge0.3, which helps to better understand the formation of those detrimental defects.

Realization of 11.5% Efficiency Cu2ZnSn(S,Se)4 Thin‐Film Solar Cells by Manipulating the Phase Structure of Precursor Films

Cu2ZnSn(S,Se)4 has been widely regarded as a promising thin‐film solar cell material. In recent years, the development of Cu2ZnSn(S,Se)4 solar cells has encountered a bottleneck, and the higher open‐circuit voltage deficit mainly caused by the secondary phase, CZTSSe/CdS interface recombination, deep‐level defects, and band‐tailing effects has been an outstanding issue. Herein, the influence of the composition and phase distribution of the precursor thin film on the defect and performance of Cu2ZnSn(S,Se)4 solar cells is studied. By modifying the distribution of composition and phase for precursor films, a Cu2ZnSn(S,Se)4 absorber layer without secondary phase and with fewer detrimental defects can be obtained from the pure‐phase precursor film. Thanks to the reduction of the band‐tailing effects, the increase in the depletion width for heterojunction, and the decrease in CZTSSe/CdS interface recombination, the photovoltaic performance of CZTSSe thin‐film solar cells is significantly improved. Finally, based on the excellent kesterite absorber layer, a Cu2ZnSn(S,Se)4 solar cell with 11.51% power conversion efficiency (the active area efficiency is 12.4%) is prepared.

Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

Origin of hydrogen passivation in 4H -SiC

Carbon vacancy ${\mathrm{V}}_{\mathrm{C}}$ is the dominant detrimental defect in SiC, and hydrogen passivation of ${\mathrm{V}}_{\mathrm{C}}$ is often used to facilitate its application in electronic devices. However, the exact nature of hydrogen passivation of ${\mathrm{V}}_{\mathrm{C}}$ in $4H$-SiC remains inconclusive in view of the available divergent experiment and theoretical findings. Here, using the Heyd-Scuseria-Ernzerhof screened hybrid density functional calculations, we demonstrate that the ${\mathrm{V}}_{\mathrm{C}}$ defect can capture up to four hydrogens, and the electrically active levels within the band gap can be entirely passivated, in line with recent reported experimental observations. This paper, thus, casts light on the role of hydrogen passivation in SiC.

Grain Boundary Perfection Enabled by Pyridinic Nitrogen Doped Graphdiyne in Hybrid Perovskite

AbstractThe solution processing in hybrid perovskite films inevitably results in the formation of detrimental defects at grain boundaries (GBs) that deteriorate the optoelectronic properties and bring about severe hysteresis as well as operational instability. Here, an effective scenario to alleviate the imperfection issue at perovskite GBs via incorporating pyridinic nitrogen‐doped graphdiyne (N‐GDY) is proposed. Taking full advantage of periodic acetylenic linkages and introduced pyridinic N atoms, the deep‐level trap states like Pb–I antisite defects and under‐coordinated Pb atoms are considerably passivated, thus diminishing the undesired non‐radiative recombination. Additionally, the spatial confinement coupling with electrostatic repulsion effect originated from the intrinsic 2D structure of N‐GDY, has been identified to deal with the halide ion migration behavior. Such contributions are further theoretically evidenced with the charge density delocalization as well as the ion migration energy barrier elevation. The authors unprecedentedly verified the superiorities based on the flexible chemical‐tailorability of atomic crystal GDY materials toward polycrystalline perovskite related energy conversion devices.

Tailored Key Parameters of Perovskite for High-Performance Photovoltaics

ConspectusTo tackle the foreseen energy crisis and serious environmental problem caused by conventional energy resources, solar cells, or photovoltaics (PV), a technology that can directly convert sunlight to electricity has been considered to be one of the most important and cost-effective clean energy techniques that benefit the environment. So far, silicon-based PVs have dominated the market due to mature fabrication techniques and their relatively good power conversion efficiency (PCE) and excellent stability.In the past decades, metal halide perovskite PVs have been a major direction of current PV research owing to their intriguing optoelectronic performance. The record PCE of perovskite PV cells has unprecedently increased from an original 3.8 to 25.5%. The typical structure of a photovoltaic perovskite is ABX3, where A refers to a monovalent cation such as methylammonium (MA+), cesium (Cs+), or formamidinium (FA+); B refers to a divalent cation such as Pb2+ or Sn2+; and X refers to a halide anion. In general, a perovskite PV device consists of an active layer of the perovskite absorber, which is connected by a hole-transporting layer (HTL) and an electron-transporting layer (ETL). In the beginning, the strategies to improve the PCEs mainly focused on developing an effective method to grow a smooth and continuous polycrystalline film. After the improvement of the baseline PCE of the perovskite PVs, various techniques were employed to reduce the defects of perovskite formed at grain boundaries and surfaces, further pushing the PCEs of perovskite PV devices to over 20%. In the meantime, the discovery of suitable and low-cost interfacial materials such as electron-transporting materials and hole-transporting materials has appealed to the attention of the perovskite research community to accelerate the commercialization of the perovskite PVs.In this Account, we start from our efforts to develop facile and effective fabrication strategies to obtain smooth and continuous polycrystalline perovskite thin films, including vapor-assisted and moisture-assisted perovskite crystal growth. The strategies and the in-depth mechanisms of preventing the formation of specific detrimental defects accumulated at grain boundaries or the surface are also portrayed by designing various passivation molecules with certain functional groups, such as carbonyl groups, amine groups, or sulfonic groups. Some systematically designed electron- or hole-transporting materials to improve the charge extraction and collection efficiency in perovskite PV devices are discussed as well. Finally, future perspectives for further enhancing the long-term stability and PCE are proposed to accelerate the commercialization of perovskite PV devices in the next decade.

Influence of the base material on the mechanical behaviors of polycrystal-like meta-crystals

Architected lattice metamaterials offer extraordinary specific strength and stiffness that can be tailored through the architecture. Meta-crystals mimic crystalline strengthening features in crystalline alloys to obtain high strength and improved post-yield stability of lattice materials. This study investigates synergistic effects of the base material’s intrinsic crystalline microstructure and architected polycrystal-like architecture on the mechanical behavior of architected metamaterials. Four different polygrain-like meta-crystals were fabricated from 316L, Inconel 718 (IN718) and Ti6Al4V via laser powder bed fusion (L-PBF). While the elastic modulus of the meta-crystals did not vary significantly with the base material or the number of meta-grains, the strength of the meta-crystals showed strong increasing correlation with reducing the size of meta-grains. The differences between meta-crystals made by the three alloys were the most substantial in the post-yield behavior, where the 316L meta-crystals were the most stable while Ti6Al4V meta-crystals were the most erratic. The differences in the post-yield behavior were attributed to the base material’s ductility and intrinsic work-hardening. For all base materials, increasing the number of meta-grains improved the post-yield stability of meta-crystals. The tolerance to the processing defects also differed with the base material. Detrimental defects such as the high surface roughness on the downskin of the struts or the large, irregularly shaped pores near the surface of the struts led to early strut fracture in Ti6Al4V meta-crystals. In contrast, ductile IN718 was able to tolerate such defects, enabling the most significant synergistic strengthening across lengthscales to achieve architected materials of low relative density, but with a very high strength and an excellent energy absorption.