Classification of Lattice Defects in the Kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 Earth‐Abundant Solar Cell Absorbers

The kesterite‐structured semiconductors Cu₂ZnSnS₄ and Cu₂ZnSnSe₄ are drawing considerable attention recently as the active layers in earth‐abundant low‐cost thin‐film solar cells. The additional number of elements in these quaternary compounds, relative to binary and ternary semiconductors, results in increased flexibility in the material properties. Conversely, a large variety of intrinsic lattice defects can also be formed, which have important influence on their optical and electrical properties, and hence their photovoltaic performance. Experimental identification of these defects is currently limited due to poor sample quality. Here recent theoretical research on defect formation and ionization in kesterite materials is reviewed based on new systematic calculations, and compared with the better studied chalcopyrite materials CuGaSe₂ and CuInSe₂. Four features are revealed and highlighted: (i) the strong phase‐competition between the kesterites and the coexisting secondary compounds; (ii) the intrinsic p‐type conductivity determined by the high population of acceptor Cu_Zn antisites and Cu vacancies, and their dependence on the Cu/(Zn+Sn) and Zn/Sn ratio; (iii) the role of charge‐compensated defect clusters such as [2Cu_Zn+Sn_Zn], [V_Cu+Zn_Cu] and [Zn_Sn+2Zn_Cu] and their contribution to non‐stoichiometry; (iv) the electron‐trapping effect of the abundant [2Cu_Zn+Sn_Zn] clusters, especially in Cu₂ZnSnS₄. The calculated properties explain the experimental observation that Cu poor and Zn rich conditions (Cu/(Zn+Sn) ≈ 0.8 and Zn/Sn ≈ 1.2) result in the highest solar cell efficiency, as well as suggesting an efficiency limitation in Cu₂ZnSn(S,Se)₄ cells when the S composition is high.     

Oxidation state and site selection of gold in (YBa2Cu3O7 −δ)(1 − x)Au x

Recent experiments on the effect of Au substitution on the superconductivity of YBa₂Cu₃O_{7 –} (YBCO) have revealed that Au, unlike most other dopants, enhances the superconducting transition temperature, T _c, by 2 K at 10 mole % Au. In the present study. X-ray photoelectron spectra, nominally of 4, 6, 8 and 10 mole % Au-doped YBCO, have been obtained. The Au spectra display a relatively intense peak which is the oxidized component associated with the Au 4f_7/2 core level, and an additional component that is most probably due to unreacted gold. The binding energy of the oxidized component is consistent with Au⁺. This observation, together with previous studies, leads us to postulate a structural model in which Au⁺ substitutes into a two-ligand c-axis co-ordinated Cu(1) site at a Ba-O (4) surface layer.     

Microstructure and interface evolution of Sn-2.5Bi-1.4In-1Zn-0.3Ag/Cu joint during isothermal aging

In this work, isothermal aging of Sn-2.5Bi-1.4In-1Zn-0.3Ag (in wt% hereafter) solder with Cu substrate for 0, 0.5, 2, 10, 100 and 200 h were conducted, and the variation of intermetallic compounds (IMCs) and microhardness were investigated. For the as-soldered sample, the Cu₆Sn₅ and Cu₅Zn₈ layers formed at the interface and Cu₆Sn₅, Cu₅Zn₈, Ag–Sn–In, Ag–Zn–In IMCs and Bi particles were observed in the solder matrix. During aging treatment, the Cu₅Zn₈ layer decomposed and finally disappeared at the interface, and a new Cu₃Sn layer appeared just below the Cu₆Sn₅ layer. Besides, in the solder matrix phase transformation from Cu₅Zn₈ into CuZn proceeded and the content of In in the Ag–Sn–In and Ag–Zn–In IMCs increased and Bi particles dispersed uniformly with the increasing aging time. Furthermore, the microhardness of the solder changed during aging owing to the observed phase evolution. These evolutions are always considered harmful to the reliability of the solder joints, especially the appearance and the growth of the Cu₃Sn layer.     

Structural, mechanical, hardness indentation measurements of Sn65−x Ag25Sb10Cu x solder alloys rapidly solidified from melt at cooling rate 1.1×105 K s−1

Structural, mechanical properties, and hardness indentation measurements of Sn_65–x Ag₂₅Sb₁₀Cu _x (x=0, 0.5, 1.0, 1.5, 2.0, and 2.5 wt %) solder alloys have been studied and analyzed. The alloy exhibits mechanical properties superior to those in both the Sn–Ag₂₅ binary and Sn–Ag₂₅Sb₁₀ ternary solder alloys. The addition of small amounts of Cu is found to refine the effective grain size, while retaining the uniform distribution of Ag₃Sn, SnSb, and Cu₁₀Sn₃ precipitates in the solidification microstructure, thus significantly improving the ductility and strength.     

Intermetallic compound layer formation between Sn–3.5 mass %Ag BGA solder ball and (Cu, immersion Au/electroless Ni–P/Cu) substrate

The growth kinetics of intermetallic compound layers formed between eutectic Sn–3.5Ag BGA (ball grid array) solder and (Cu, immersion Au/electroless Ni–P/Cu) substrate by solid-state isothermal aging were examined at temperatures between 343 and 443 K for 0–100 days. In the solder joints between the Sn–Ag eutectic solder ball and Cu pads, the intermetallic compound layer was composed of two phases: Cu₆Sn₅ (-phase) adjacent to the solder and Cu₃Sn (-phase) adjacent to the copper. The layer of intermetallic on the immersion Au/electroless Ni–P/Cu substrate was composed of Ni₃Sn₄. As a whole, because the values of the time exponent (n) are approximately 0.5, the layer growth of the intermetallic compound was mainly controlled by a diffusion-controlled mechanism over the temperature range studied. The growth rate of Ni₃Sn₄ intermetallic compound was slower than that of the total Cu–Sn(Cu₆Sn₅+Cn₃Sn). The apparent activation energy for growth of total Cu–Sn(Cu₆Sn₅+Cu₃Sn) and Ni₃Sn₄ intermetallic compound were 64.82 and 72.54 kJ mol^–1, respectively.     

Structural,optical and electrical characterization of undoped ZnMgO film grown by spray pyrolysis method

Undoped Zn_1−X Mg _X O poly crystalline films were successfully grown by a spray pyrolysis method at 500 °C. The samples indicated high quality because (0002) orientation was strongly observed in the X-ray diffraction (XRD) pattern below Mg content X = 0.3. A lattice constant of c axis decreased linearly with increasing Mg content, indicating that the lattice constant of c axis followed Vegard’s law. It was deduced that Mg atoms could be successfully substituted in Zn site from lattice constants and optical bandgap.     

Fabrication and microstructures of sequentially electroplated Au-rich, eutectic Au/Sn alloy solder

An Au-rich, eutectic Au/Sn alloy was fabricated by sequential electroplating of Au and Sn, and reflowing the as-deposited Au/Sn/Au triple-layer film at 320–350 °C. Microstructures and phase compositions for the as-deposited Au/Sn/Au triple-layer film and the reflowed Au-rich, eutectic Au/Sn alloys were studied. Two Si wafers, each with the Au-rich, eutectic Au/Sn alloy solder, were bonded together. For the deposited Au/Sn/Au triple-layer film, reaction between Au and Sn occurs at room temperature leading to the formation of AuSn and AuSn₄. After reflowing at 320 °C, two phases remain, AuSn and Au₅Sn, with the AuSn particles distributed randomly in the Au₅Sn matrix. There are also some micropores and microcracks in the reflowed alloy. If the annealing temperature is increased to 350 °C, the Au/Sn alloy is denser and contains fewer micropores. However, microcracks remain, forming preferentially along the Au₅Sn/AuSn interface. After reflowing at 320 °C under a pressure of 13 kPa, two Si wafers are joined using the Au-rich, eutectic Au/Sn alloy solder. The solder is in intimate contact with the Si wafers; however, there are some micropores within the solder. After reflowing at 350 °C, the bond is quite good, without microcracks or micropores at the Si wafer/solder interface or within the solder.     

Combinatorial synthesis and rapid characterization of Mo1−xSnx (0x1) thin films

Thin films of Mo_1−xSn_x, continuously and linearly mapped for 0<x<1, have been prepared by d.c. magnetron sputter deposition under various growth conditions. X-ray diffraction results indicate that as x in high-pressure deposited Mo_1−xSn_x increases from 0 to approximately 0.45, the bcc lattice expands and no new phases are formed. At low deposition pressures, Mo₃Sn, a β-tungsten structured phase, is formed along with the bcc Mo–Sn solid solution for 0.1<x<0.3. The variation of the lattice parameter for this intermetallic phase also indicates that solid solutions, possibly of the form Mo_3+ySn, are being formed. These materials are of special interest as anode candidates in lithium-ion batteries.     

Rib waveguides based on Zn-substituted LiNbO3 films grown by liquid phase epitaxy

The fabrication of epitaxially grown Zn-substituted LiNbO₃ (Zn:LiNbO₃) waveguide films and rib waveguides is reported and detailed investigations about microstructure, morphology and optical waveguide properties are provided. Zn:LiNbO₃ films were grown on congruent X-cut LiNbO₃ substrates by a modified liquid phase epitaxy in solid–liquid coexisting solutions. The homogeneously Zn-substituted films exhibit high crystalline perfection and extremely flat surfaces with averaged surface roughness of rms = 0.2–0.3 nm. At the film/substrate interface a Zn-containing transient layer has been observed, which allows the growth of elastically strained Zn:LiNbO₃ film lattices. X-ray diffraction reciprocal-space measurements prove the pseudomorphic film growth. The refractive index difference between substrate and film depends on the zinc substitution content, which increase with rising growth temperatures. For films with 5.3 mol% Zn (Δn_o ≈ +5 × 10⁻³) only ordinary ray propagation was observed, while for films with 7.5 mol% Zn (Δn_o ≈ +8 × 10⁻³, Δn_e ≈ +5 × 10⁻³) both modes, TM and TE propagate. Stress-induced refractive index changes are in the order of Δn ≈ 10⁻⁴. In rib waveguide microstructures singlemode propagation with nearly symmetrical field distribution has been observed. To demonstrate the potential of the proton exchange-assisted dry-etching technique interferometer microstructures were fabricated.     

Intermetallic compounds dynamic formation during annealing of stacked elemental layers and its influences on the crystallization of Cu2ZnSnSe4 films

A combined in-situ investigation using X-ray diffraction and differential scanning calorimetry during annealing was carried out to investigate the formation of intermetallic compounds in the stacked elemental layers and to reveal its influences on the crystallization of kesterite Cu₂ZnSnSe₄. The Mo/Cu/Zn, Mo/Cu/Sn/Zn, Mo/Cu/Zn/Se and Mo/Cu/Sn/Zn/Se stacked films were prepared with a composition resembling a typical kesterite Cu-poor and Zn-rich metallic composition. In-situ experiments during annealing of pure metallic stacked films reveal a dynamic intermetallic compounds formation of Cu₅Zn₈ → CuZn → Cu₂Zn → Cu₃Zn and Cu₆Sn₅ → Cu₄₁Sn₁₁. The CuZn and Cu₅Zn₈ layer formed at the interface of metals/Se may prevent the stacked metallic layers from selenization below 320 °C. On the other side, the dynamic formation of Cu–Zn phases in the stacked films is found to be an origin of a ZnSe gradual formation starting from 320 °C. Phase analysis suggests that the ternary Cu₂SnSe₃ phase forms almost immediately after the formation of Cu₂Se and SnSe. The formation of Cu₂SnSe₃ is indicated by the consumption of SnSe by the Cu₂Se which occurs at 530–540 °C. Crystallization of kesterite takes place above 540 °C. On a phenomenological basis of present results, consequences for the thin film kesterite fabrication for solar cell application are discussed.     

Microstructures formed from mixed solders on copper substrate

Abstract

With the development and use of a variety of Pb free solders, it is probable that some solder joints in electronic assemblies may be made with solders of two different compositions. To investigate possible microstructures resulting from such procedure, samples were prepared using small balls of four different Sn–Ag–Cu (SAC) Pb free solders, as well as Sn–Zn–Al solder, melted together with eutectic Pb–Sn solder paste and also various SAC solder pastes, on a copper substrate. It was observed that using eutectic Pb–Sn solder paste with an SAC solder ball introduced some Pb–Sn eutectic microstructure and changed the ternary eutectic present from Ag₃Sn–Cu₆Sn₅–Sn to Ag₃Sn–Pb–Sn. Use of an SAC solder paste with Sn–Zn–Al solder introduced an apparent Ag–Cu–Zn ternary compound, replacing Zn lamellae of the Sn–Zn eutectic. With eutectic Pb–Sn solder paste, the Pb–Sn–Zn ternary eutectic was formed. It was noted that use of a high Sn solder results in rapid dissolution of the copper substrate.     

Is there ferromagnetic to spin-glass transition in PdAuFe alloys?

Electrical resistivity studies on several (Pd_1–x Au _x )_99.93Fe_0.07 alloys between 25 mK and 1 K are reported. The experiments reveal interesting features of the impurity spin polarization with the addition of Au atoms to pure PdFe_0.07 alloy. Forx<0.28 the ternary alloys exhibit a familiarT ² dependence for belowT _c , indicative of complex magnetic behavior. For still higher Au concentrations, resistivity maxima occur in these alloys, reflective of short-range magnetic ordering. Arguments are presented to show that this is strongly suggestive of spin-glass-type freezing at lower temperatures, as is observed in many canonical metallic spin-glasses.     

Effects of <Emphasis Type="Italic">β</Emphasis>-Sn grain <Emphasis Type="Italic">c</Emphasis>-axis on electromigration behavior in BGA Sn3.0Ag0.5Cu solder interconnects

The anisotropy of β-Sn grain can significantly affect the electromigration (EM) behavior in Sn3.0Ag0.5Cu (SAC305) solder interconnects. A real ball grid array (BGA) specimen with a cross sectioned edge row suffered electromigration for 600 h to investigate the effects of β-Sn c-axis on the behavior of electromigration in SAC305 solder interconnects. Scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) were used to obtain the microstructure and orientation of β-Sn grains in as-reflowed and low current density conditions. Besides, the orientation of c-axis had a great effect on the growth direction of IMCs in solder matrix. The solder interconnect with the Sn grain c-axis pointing the positive direction of ND would emerge serious electromigration phenomena. The density of Cu₆Sn₅ IMCs distributing at the surface of solder matrix increased obviously. However, when Sn grain c-axis was in the same direction with the opposite direction of ND, the original Cu₆Sn₅ IMCs in as-reflowed solder interconnect disappeared. Therefore, the results show that the solder interconnects will performance a different electromigration behavior due to the direction of c-axis in Sn grain: the growth direction of Cu₆Sn₅ IMCs in solder matrix will along the c-axis accompanied growing into solder matrix or gathering at the surface of the cross section.     

Phase Relations in the Zn3As2–ZnAs2–CdAs2–Cd3As2 System

Phase relations along the Cd₃As₂–ZnAs₂ and Zn₃As₂–CdAs₂ joins are studied by differential thermal analysis, x-ray diffraction, and microstructural analysis. The results, in conjunction with earlier data on the CdAs₂–ZnAs₂, Zn₃As₂–Cd₃As₂, Cd₃As₂–CdAs₂, and ZnAs₂–Zn₃As₂ binaries, are used to map out the phase diagram of the liquidus surface in the composition region Zn₃As₂–ZnAs₂–CdAs₂–Cd₃As₂ of the ternary system Cd–As–Zn. The ternary eutectic revealed in this region has an approximate composition of 26 at. % Cd + 65 at. % As + 9 at. % Zn and melts at 863 K.     

Characterization of wet processed (Ni, Zn)-ferrites for CO2 decomposition

Ultrafine (Ni, Zn)-ferrites were prepared by two different methods of coprecipitation and hydrothermal synthesis, and their oxygen-deficient ferrites (ODF) produced by hydrogen reduction were investigated on the efficiency of CO₂ decomposition. The crystalline sizes of (Ni, Zn)-ferrites were less than 30 nm with high Brunauer–Emmett–Teller (BET) surface areas, ranging from 77 to 172 m² g^–1. The (Ni, Zn)-ferrites by hydrothermal synthesis resulted in smaller crystalline sizes, higher BET surface areas and better efficiencies of CO₂ decomposition than by coprecipitation. Compared with the binary NiFe₂O_4– ferrite, the ternary (Ni _x , Zn_1–x ) Fe₂O_4– ferrites showed higher efficiency for CO₂ decomposition, indicating a potential catalyst for the reduction of CO₂ emission in the environmental atmosphere.     

Experimental study of the Ca–Mg–Zn system using diffusion couples and key alloys

Nine diffusion couples and 32 key samples were prepared to map the phase diagram of the Ca–Mg–Zn system. Phase relations and solubility limits were determined for binary and ternary compounds using scanning electron microscopy, electron probe microanalysis and x-ray diffraction (XRD). The crystal structure of the ternary compounds was studied by XRD and electron backscatter diffraction. Four ternary intermetallic (IM) compounds were identified in this system: Ca₃Mg_xZn_15−x (4.6 ⩽ x ⩽ 12 at 335 °C, IM1), Ca_14.5Mg_15.8Zn_69.7 (IM2), Ca₂Mg₅Zn₁₃ (IM3) and Ca_1.5Mg_55.3Zn_43.2 (IM4). Three binary compounds were found to have extended solid solubility into ternary systems: CaZn₁₁, CaZn₁₃ and Mg₂Ca form substitutional solid solutions where Mg substitutes for Zn atoms in the first two compounds, and Zn substitutes for both Ca and Mg atoms in Mg₂Ca. The isothermal section of the Ca–Mg–Zn phase diagram at 335 °C was constructed on the basis of the obtained experimental results. The morphologies of the diffusion couples in the Ca–Mg–Zn phase diagram at 335 °C were studied. Depending on the terminal compositions of the diffusion couples, the two-phase regions in the diffusion zone have either a tooth-like morphology or contain a matrix phase with isolated and/or dendritic precipitates.     

Effect of the soldering time on the formation of interfacial structure between Sn–Ag–Zn lead-free solder and Cu substrate

The evolution of interfacial structure between the Sn–3.7%Ag–0.9%Zn lead-free solder and Cu substrate were systematically explored for different soldering times (1, 5, and 10 min). According to microstructural observations, it is found that the longer the soldering time is, the thicker the soldered interface becomes. The interface soldered for 1 min is composed of the Cu₅Zn₈ intermetallic compounds (IMCs) layer locating above the Cu₆Sn₅ IMCs layer. The interfaces soldered for 5 and 10 min are mainly made up of the Cu₆Sn₅ IMCs with some bulk Ag₃Sn IMCs randomly distributing within it. The evolution of the IMCs layer in the soldered interface can be divided into three stages: the Cu₅Zn₈ IMCs firstly forms, then Cu₆Sn₅ IMCs separated out from the bottom (controlled by diffusion of Sn in the Cu₅Zn₈), finally, subsequent growth of the Cu₆Sn₅ IMCs layer is controlled by diffusion of Sn in Cu₆Sn₅ IMCs.     

The Effects of Substitution of Sn for Cu in Bi1.75Pb0.25Sr2CaCu2.3?xSnxSnxOy

The influence of Sn doping on superconductivity in the Bi-based 2212 phase is studied in this paper. For the samples R–T relations and magnetic hysteresis loops were measured. X-ray powder diffraction analysis was also performed. For Bi_1.75Pb_0.25Sr₂CaCu_2.3–x Sn _x O _y , the experimental results show that by adding the proper amount of Sn the superconductivity of the samples can be improved. As x = 0.15, the critical temperature T _c, the critical current density J _c, and the magnetic pinning force density F reach a maximum. At T = 11 K, the critical state parameters H _c1, H _c2, , , and are calculated and compared with the results reported by other researchers. The experimental results also show that the Sn doping is able to speed up the growth of the 2223 phase. In brief, Sn doping is an effective way of improving the superconductivity in Bi-based superconductors.     

A comparative study of different solvothermal methods for the synthesis of Sn<Superscript>2+</Superscript>-doped BaTiO<Subscript>3</Subscript> powders and their dielectric properties

Ternary oxides containing Sn²⁺ are rare and difficult to prepare by the conventional solid state reactions due to the disproportionation of Sn²⁺ to Sn⁴⁺ and Sn at high temperatures. In this article, Sn²⁺-doped barium titanate, Ba_1−x Sn _x TiO₃ (x = 0.00, 0.02, 0.05, and 0.10) nanopowders were successfully synthesized at a moderate temperature by a microwave-assisted solvothermal reaction (MSR) and a solvothermal reaction with rolling (SRR). The powders obtained using the MSR and SRR consisted of nanoparticles of 20–50 nm and 100–120 nm in diameter, respectively. The dielectric constant of the sample increased by doping with a small amount of Sn²⁺ (x ≤ 0.05), but decreased by doping in excess amounts of it.     

Phase Composition of Zn2TiO4–Zn2ZrO4Materials Prepared by Low-Temperature Plasma Synthesis and Ceramic Processing

Zn₂Ti_{1 – x} Zr _x O₄solid solutions were prepared by low-temperature plasma synthesis and solid-state reactions, and their properties were compared. The Zn₂TiO₄–Zn₂ZrO₄pseudobinary system was found to contain two broad solid-solution ranges with inverted spinel structures: phase at 0 < x< 0.3, witha= 8.474–8.555 Å, and phase at 0.5 < x< 1.0, with a= 8.615–8.740 Å and c= 8.733–9.120 Å. The conductivity of the solid solutions notably decreases upon substitution of Zr⁴⁺for Ti⁴⁺and varies exponentially with temperature.