The search for lead‐free alternatives to lead‐halide perovskite photovoltaic materials resulted in the discovery of copper(I)‐silver(I)‐bismuth(III) halides exhibiting promising properties for optoelectronic applications. The present work demonstrates a solution‐based synthesis of uniform CuxAgBiI4+x thin films and scrutinizes the effects of x on the phase composition, dimensionality, optoelectronic properties, and photovoltaic performance. Formation of pure 3D CuAgBiI5 at x = 1, 2D Cu2AgBiI6 at x = 2, and a mix of the two at 1 < x < 2 is demonstrated. Despite lower structural dimensionality, Cu2AgBiI6 has broader optical absorption with a direct bandgap of 1.89 ± 0.05 eV, a valence band level at ‐5.25 eV, improved carrier lifetime, and higher recombination resistance as compared to CuAgBiI5. These differences are mirrored in the power conversion efficiencies of the CuAgBiI5 and Cu2AgBiI6 solar cells under 1 sun of 1.01 ± 0.06% and 2.39 ± 0.05%, respectively. The latter value is the highest reported for this class of materials owing to the favorable film morphology provided by the hot‐casting method. Future performance improvements might emerge from the optimization of the Cu2AgBiI6 layer thickness to match the carrier diffusion length of ≈40–50 nm. Nonencapsulated Cu2AgBiI6 solar cells display storage stability over 240 days.

Solution Processable Direct Bandgap Copper‐Silver‐Bismuth Iodide Photovoltaics: Compositional Control of Dimensionality and Optoelectronic Properties

Lead-free perovskite-inspired materials (PIMs) are gaining attention in optoelectronics due to their low toxicity and inherent air stability. Their wide bandgaps (≈2 eV) make them ideal for indoor light harvesting. However, the investigation of PIMs for indoor photovoltaics (IPVs) is still in its infancy. Herein, the IPV potential of a quaternary PIM, Cu2 AgBiI6 (CABI), is demonstrated upon controlling the film crystallization dynamics via additive engineering. The addition of 1.5 vol% hydroiodic acid (HI) leads to films with improved surface coverage and large crystalline domains. The morphologically-enhanced CABI+HI absorber leads to photovoltaic cells with a power conversion efficiency of 1.3% under 1 sun illumination-the highest efficiency ever reported for CABI cells and of 4.7% under indoor white light-emitting diode lighting-that is, within the same range of commercial IPVs. This work highlights the great potential of CABI for IPVs and paves the way for future performance improvements through effective passivation strategies.

Enhancing the Microstructure of Perovskite-Inspired Cu-Ag-Bi-I Absorber for Efficient Indoor Photovoltaics.

Robust bonding of Cu quasi-nanoparticles sintering for Ag coated chip and bare copper substrate was achieved. The effect of temperature, pressure and time on the sintering bonding strength and microstructural evolution was deeply studied. 36.5 MPa shear strength was achieved when applied 5 MPa pressure. By increasing to 30 MPa, it shows the best die shear strength of 116 MPa, accomplished with a sintering temperature of 250 °C for 3 min. Temperature also influenced the shear strength a lot. Between 210 °C and 230 °C can already provide strength over 30 MPa. When increased to 270 °C, the strength was extremely enhanced to over 120 MPa. Inspection on the fracture behaviors and cross-section of sheared off samples was conducted by SEM, EDS, and XRD. It is found that low bonding performance is due to both of the incomplete burnt out of organics and incomplete Cu QNPs sintering. In addition, high bonding is account for the positive effect of pressure and temperature on promoting the necking growth, sintering networking formation, pores isolation and brittle-ductile fracture transformation. The recommended sintering profile is 250 °C, 3 min, 20 MPa. Finally, the feasibility for SiC MOSFET power electronics DA was verified by testing its static characteristics at both room temperature and 150 °C. 

Microstructural evolution, fracture behavior and bonding mechanisms study of copper sintering on bare DBC substrate for SiC power electronics packaging

Paste based on Cu nanoparticles (NPs) is a promising die-attachment material for high-power electronic devices due to their high conductivity and inexpensive-cost. However, the rapid oxidation of Cu NPs during sintering in air hinders their application. In this work, we synthesized stable Cu NPs with a diameter of 50~110 nm for die-attachment in air. Unlike most of the previous reports about copper pastes, nano-Cu paste prepared from the synthesized Cu NPs can be sintered at low temperatures without a protective atmosphere. By investigating the effect of different solvents on the conductivity of the sintered copper film, a mixture of glycerin and ɑ terpineol was selected as a carrier solvent to prepare nano-Cu paste. After sintering, the formed Cu-Cu joints exhibited the high shear strength, which reaches a maximum of 44.5 MPa at 275°C with a pressure of 10 MPa in air. Thus, this work shows a feasibility of the sintering of nano-Cu paste in air and the great potential of nano-Cu paste in the packing of the next-generation power devices.

Synthesis of Air-Sinterable Copper Nanoparticles for Die-Attachment

Low-toxicity perovskite-inspired Cu 2 AgBiI 6 is a potential candidate for indoor photovoltaics. Cu 2 AgBiI 6 -based photovoltaics with an optimized mesoporous TiO 2 thickness ensure high fill factor and a power conversion efficiency of 4.64% at 200 lux. 

Perovskite-inspired Cu<sub>2</sub>AgBiI<sub>6</sub> for mesoscopic indoor photovoltaics under realistic low-light intensity conditions

There is a high demand for the implementation of metallic nanoparticle (NP) sintering technology for die attach in high-power electronics. The performance of this technology is superior to that of the technology involving the use of lead-free solders. Although Cu NP paste is potentially a low-cost material, it faces the challenge of oxidation during sintering. This may result in a significant deterioration of the mechanical, thermal, and electrical properties. Therefore, there are limited studies on the in-air sintering of Cu NP pastes. The present study demonstrated the in-air pressure-assisted low-temperature sintering of a commercial Cu NP paste. Furthermore, the sintering was performed without using a protective atmosphere, unlike that in most of the previously reported investigations. The sintering behavior was investigated at three levels of temperatures (200–240 °C) and five levels of pressures (5–25 MPa). The joints that were sintered at high temperatures and pressures exhibited condensed microstructures and high bonding strengths. High sintering temperatures accelerated the diffusion between Cu NPs, while high sintering pressure facilitated the removal of evaporated organic compounds and the air between NPs. This not only facilitated sintering but also prevented the oxidation of Cu. The optimal sintering conditions promoted the formation of 3D connections between the Cu NPs, thereby increasing the shear strength of the sample. The samples that were sintered at 240 °C and 10 MPa experiences the highest increase in the shear strength, furthermore, the microstructures were optimized under this condition. The shear strength of 28.1 ± 8.47 MPa was achieved under this condition, which satisfied the requirements for die attach in high power electronics applications, moreover, the sintering process was moderate and cost-effective. Therefore, the optimal sintering temperature and pressure for the in-air sintering of the Cu NP paste was concluded to be 240 °C and 10 MPa, respectively. The results indicated that in-air sintering with pressure assistance can be applied for die attach in the high-power electronics.

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In-air sintering of copper nanoparticle paste with pressure-assistance for die attachment in high power electronics

Recently, Cu2AgBiI6 semiconductor has been investigated due to the high absorption coefficient, direct bandgap, and low exciton binding energy, which are promising for eco-friendly photoelectric devices. Herein, pyridine is introduced as solvent additive to completely dissolve the solutes and form clear Cu2AgBiI6 precursor solution, which results in high-quality films and may provide a general approach for high-quality film growth of other bismuth-based metal halide semiconductors. In addition, the electronic structure of Cu2AgBiI6 has been demonstrated for the first time and shows an intrinsically weak n-type semiconductor. Furthermore, phenethylammonium iodide for surface passivation significantly improves the film quality, slightly n-dopes the material, and shifts up the band level. Finally, the photovoltaics and photodetector performance for n-i-p planar heterojunction devices have been investigated. The efficiency is up to 1%, highest for Cu2AgBiI6 solar cells and comparable with other lead-free bismuth based metal halide solar cells. Moreover, photodetectors with fast speed of rising and decaying time, especially the excellent specific photodetectivity of ∼1012 Jones within the wavelength of ∼350-600 nm, are achieved, which paves an alternative and promising strategy for the design of future commercial photodetectors that are self-powered, stable, nontoxic, etc.

Low-Temperature Solution-Processed Cu2AgBiI6 Films for High Performance Photovoltaics and Photodetectors.

Simulation of Cu pad expansion in wafer-to-wafer Cu/SiCN hybrid bonding

High-quality perovskite film is crucial to realizing high-efficiency stable perovskite solar cells. Doping is a direct and effective strategy to passivate defects and improve the crystal quality of perovskite films....

Multifunctional pseudohalide-based ionic liquid doping promotes efficient and stable perovskite solar cells

We introduce a new efficient spectral element approach to solve the two-dimensional magnetotelluric forward problem based on Gauss–Lobatto–Legendre polynomials. It combines the high accuracy of the spectral technique and the perfect flexibility of the finite element approach, which can significantly improve the calculation accuracy. This method mainly includes two steps: 1) transforming the boundary value problem in the partial differential form into the variational problem in the integral form and 2) solving large symmetric sparse systems based on the combination of incomplete LU factorization and the double conjugate gradient stability algorithm through the spectral element with quadrilateral meshes. We imply the spectral element method on a resistivity half-space model to obtain a simple analytical solution and find that the magnetic field solutions simulated by the spectral element approach matched closely to the exact solutions. The experiment result shows that the spectral element solution has high accuracy with coarse meshes. We further compare the numerical results of the spectral element, finite difference, and finite element approaches on the COMMEMI 2D-1 and smooth models, respectively. The numerical results of the spectral element procedure are highly consistent with the other two techniques. All these comparison results suggest that the spectral element technique can not only give high accuracy for modeling results but also provide more detailed information. In particular, a few nodes are required in this method relative to the finite difference and finite element methods, which can decrease the relative errors. We then deduce that the spectral element method might be an alternative approach to simulate the magnetotelluric responses in two- or three-dimensional structures.

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Boyao Zhang

Papers

In-air sintering of copper nanoparticle paste with pressure-assistance for die attachment in high power electronics

Low-Temperature Solution-Processed Cu2AgBiI6 Films for High Performance Photovoltaics and Photodetectors.

Simulation of Cu pad expansion in wafer-to-wafer Cu/SiCN hybrid bonding

Multifunctional pseudohalide-based ionic liquid doping promotes efficient and stable perovskite solar cells

An efficient spectral element method for two-dimensional magnetotelluric modeling