Bandgap engineering of Cu2InxZn1−xSn(S,Se)4 alloy films for photovoltaic applications

Bandgap engineering of Cu₂In_xZn_1?xSn(S,Se)₄ alloy films for photovoltaic application has been investigated. Cu₂In_xZn_1?xSn(S,Se)₄ (0 ≤ x ≤ 0.6) alloy films with different In contents and a single kieserite phase were fabricated by using a simple low-cost sol-gel method. The influence of In content on the structure, morphology, and optical and electrical properties of Cu₂In_xZn_1?xSn(S,Se)₄ films was investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscope (TEM), X-Ray photoelectron spectroscopy (XPS), optical absorbance, and room-temperature Hall measurements. The results of XRD, TEM, and XPS demonstrated the substitution of some Zn atoms by In in Cu₂In_xZn_1?xSn(S,Se)₄ films. The Hall measurements show that the carrier concentration of the Cu₂In_xZn_1?xSn(S,Se)₄ (0 ≤ x ≤ 0.6) decreases with increasing In content and that the p-type Cu₂In_xZn_1?xSn(S,Se)₄ films with preferable electrical properties can be obtained by adjusting the In content during film deposition. The optical measurements indicate that the bandgap of Cu₂In_xZn_1?xSn(S,Se)₄ films with kesterite structure can be continuously tuned in the range of 1.13–1.01 eV as x is increased from 0 to 0.6. Our results show that the Cu₂In_xZn_1?xSn(S,Se)₄ alloy is a potentially applicable material for bandgap grading absorption layers in high-power-conversion-efficiency solar cells.     

Nanoscale observation of surface potential and carrier transport in Cu2ZnSn(S,Se)4 thin films grown by sputtering-based two-step process

Stacked precursors of Cu-Zn-Sn-S were grown by radio frequency sputtering and annealed in a furnace with Se metals to form thin-film solar cell materials of Cu₂ZnSn(S,Se)₄ (CZTSSe). The samples have different absorber layer thickness of 1 to 2 μm and show conversion efficiencies up to 8.06%. Conductive atomic force microscopy and Kelvin probe force microscopy were used to explore the local electrical properties of the surface of CZTSSe thin films. The high-efficiency CZTSSe thin film exhibits significantly positive bending of surface potential around the grain boundaries. Dominant current paths along the grain boundaries are also observed. The surface electrical parameters of potential and current lead to potential solar cell applications using CZTSSe thin films, which may be an alternative choice of Cu(In,Ga)Se₂.PACS number: 08.37.-d; 61.72.Mm; 71.35.-y     

Synthesis and investigation of environmental protection and earth-abundant kesterite Cu2MgxZn1-xSn(S,Se)4 thin films for solar cells

We have synthesized Cu₂Mg_xZn_1–xSn(S,Se)₄ (0?≤?x?≤?0.6) thin films by a facile sol-gel method, and studied the influence of Mg concentration on the crystal structure, surface morphology and photoelectric performance of Cu₂Mg_xZn_1–xSn(S,Se)₄ thin films systematically. It was shown that the smaller Zn²⁺ in Kesterite phase Cu₂ZnSn(S,Se)₄ will be replaced by larger Mg²⁺, forming uniform pure phase Cu₂Mg_xZn_1–xSn(S,Se)₄. The band gap of Cu₂Mg_xZn_1–xSn(S,Se)₄ films can be adjusted from 1.12 to 0.88?eV as the x value changes from 0 to 0.6. Furthermore, the Cu₂Mg_xZn_1–xSn(S,Se)₄ thin films with large grain size, smooth surface and less grain boundaries was obtained at an optimized condition of x?=?0.2. The carrier concentration of Cu₂Mg_xZn_1–xSn(S,Se)₄ thin film reaches the maximum 6.47?×?10¹⁸ cm^?3 at x?=?0.2, which is a potential material to be the absorption layer of high efficiency solar cells.     

Effect of TiN diffusion barrier layer on residual stress and carrier transport in flexible CZTSSe solar cells

The major drawback of flexible Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells is the inevitable residual stress in CZTSSe that considerably limits the efficiency and flexibility of these cells. Hence, in this work, TiN layers with varying thicknesses were sputtered between flexible Ti substrates and back contact Mo layers as diffusion barriers. The TiN barrier layer relieved residual stress, facilitated grain growth, and decreased the porosity of CZTSSe, thereby effectively suppressing the formation of carrier recombination paths and improving the mechanical strength of CZTSSe. Meanwhile, the band alignment of the CZTSSe/CdS heterojunction could be significantly tailored, leading to an improved ‘‘cliff-like’’ conduction band offset from ?0.49 eV to ?0.33 eV. Under the optimized TiN layer thickness of 50 nm, the power conversion efficiency of the fabricated flexible CZTSSe solar cell increased considerably from 3.43% to 4.85% along with high bending stability. Therefore, introducing the TiN diffusion barrier into traditional flexible CZTSSe solar cells improves the efficiency and flexibility of these devices. Moreover, this method could be a promising pathway for the large-scale production of smart, flexible, and portable electronic devices.     

High-performance copper selenide nanocomposites for power generation

Cu_1.97Se-x wt.% In₂O₃ (x = 0–1) bulk composites are fabricated by combining high-temperature melting and spark plasma sintering technology. The doping of In³⁺in Cu vacancies tunes the carrier concentration and modifies the band structure of Cu_1.97Se. Additionally, excessive In₂O₃ particles benefit the construction of multiple lattice defects to strengthen phonon scattering and suppress the long-range migration of Cu ions, resulting in obviously reduced thermal conductivity and improved service stability. Ultimately, a peak ZT value of 1.43 is obtained at 873 K for the Cu_1.97Se-0.5 wt.% In₂O₃ bulk specimen, which is almost 150% higher than that of pristine sample. The thermoelectric conversion efficiency and mechanical properties of the composites sample are also measured. The results suggest that Cu_1.97Se-based nanocomposites offer potential for use in power generation device assembly.     

A new method for measuring wetness of flowing steam based on surface plasmon resonance

Stacked precursors of Cu-Zn-Sn-S were grown by radio frequency sputtering and annealed in a furnace with Se metals to form thin-film solar cell materials of Cu₂ZnSn(S,Se)₄ (CZTSSe). The samples have different absorber layer thickness of 1 to 2 μm and show conversion efficiencies up to 8.06%. Conductive atomic force microscopy and Kelvin probe force microscopy were used to explore the local electrical properties of the surface of CZTSSe thin films. The high-efficiency CZTSSe thin film exhibits significantly positive bending of surface potential around the grain boundaries. Dominant current paths along the grain boundaries are also observed. The surface electrical parameters of potential and current lead to potential solar cell applications using CZTSSe thin films, which may be an alternative choice of Cu(In,Ga)Se₂. PACS number: 08.37.-d; 61.72.Mm; 71.35.-y     

Influence of the selenization time on the properties of (Na0.1Cu0.9)2ZnSn(S,Se)4 thin films and their photovoltaic solar cells

(Na_0.1Cu_0.9)₂ZnSn(S,Se)₄ thin films with a single kesterite phase were synthesized using a sol-gel spin-coating method accompanied by rapid post-annealing. In this study, we investigated the effect of selenization time on the crystal quality and photoelectric performance of the (Na_0.1Cu_0.9)₂ZnSn(S,Se)₄ films. It was found that the crystallinity and morphology of the films was enhanced, and some of bigger Se substituted for the S site in (Na_0.1Cu_0.9)₂ZnSn(S,Se)₄ with increasing the selenization time. The bandgap of the film can be regulated from 1.04 eV to 0.99 eV by varying the selenization time. In addition, all films showed p-type conductive characteristics, and films with optimal electrical performance could be obtained by optimizing the selenization time. Finally, the (Na_0.1Cu_0.9)₂ZnSn(S,Se)₄ thin film with the best crystal quality and optical-electrical characteristics was obtained at an optimized selenization time of 15 min. A high power conversion efficiency (PCE) of 3.92% was obtained for the (Na_0.1Cu_0.9)₂ZnSn(S,Se)₄ device, which is 42% higher compared to that of the undoped Cu₂ZnSn(S,Se)₄ (CZTSSe) device.     

CuInSe2 Formation from CuSe/In2Se3 and Cu2Se/In2Se3 Powders: Reaction Kinetics and Mechanisms

CuInSe₂ (CIS) powders were synthesized using CuSe, Cu₂Se, and In₂Se₃ as the raw materials. The formation mechanisms and reaction kinetics from CuSe/In₂Se₃ and Cu₂Se/In₂Se₃ powders in a selenium atmosphere were investigated. It was observed that the formation temperature of α‐CIS powders synthesized using Cu₂Se/In₂Se₃ as the raw materials was higher than that using CuSe/In₂Se₃. Both reactions for Cu₂Se/In₂Se₃ and CuSe/In₂Se₃ mixtures follow one‐dimensional diffusion‐controlled reactions with apparent activation energies of 124.3 and 73.2 kJ/mol, respectively. For both mixtures the indium‐rich β‐CIS phase resulting from Cu⁺ ion diffusion toward the In₂Se₃ phase was observed. The particle size and morphology of the newly formed CIS was similar to In₂Se₃, which indicated that the CIS formation kinetics may be dominated by the diffusion of Cu⁺ ions. The Cu–Se liquid phase resulting from the peritectic decomposition of CuSe₂ and CuSe at a relatively low temperature may promote Cu⁺ diffusion into the In₂Se₃ surface, assisting CIS formation.     

Structural and dielectric properties,and nonlinear electrical response of the CaCu3-xZnxTi4O12 ceramics: Experimental and computational studies

CaCu_3-xZn_xTi₄O₁₂ ceramics (x = 0, 0.05, 0.10) were successfully prepared by a conventional solid-state reaction method. Their structural and dielectric properties, and nonlinear electrical response were systematically inspected. The X-ray diffraction results indicated that single-phase CaCu₃Ti₄O₁₂ (JCPDS no. 75–2188) was obtained in all sintered ceramics. Changes in the lattice parameter are well-matched with the computational result, indicating an occupation of Zn²⁺ doping ions at Cu²⁺ sites. The overall tendency shows that the average grain size decreases when x increases. Due to a decrease in overall grain size, the dielectric permittivity of CaCu_3-xZn_xTi₄O₁₂ decreases expressively. Despite a decrease in the dielectric permittivity, it remains at a high level in the doped ceramics (~3,406–11,441). Besides retention in high dielectric permittivity, the dielectric loss tangent of x = 0.05 and 0.10 (~0.074–0.076) is lower than that of x = 0 (~0.227). A reduction in the dielectric loss tangent in the CaCu_3-xZn_xTi₄O₁₂ ceramics is closely associated with the enhanced grain boundary response. Increases in grain boundary resistance, breakdown electric field, and conduction activation energy of grain boundary as a result of Zn²⁺ substitution are shown to play a crucial role in improved grain boundary response. Furthermore, the XPS analysis shows the existence of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺, indicating charge compensation due to the loss of oxygen lattice. Based on all results of this work, enhanced dielectric properties of the Zn-doped CCTO can be explained using the internal barrier layer capacitor model.     

Influence of H2Se concentration on Se-rich CZTSSe absorbers sputtered with a ceramic quaternary target

Se-rich CZTSSe absorbers with large grains are important contributors to the performances of CZTSSe solar cells and can be fabricated by the selenization of as-sputtered CZTS precursors. To explore the effects of the H₂Se concentration in the annealing atmosphere on the growth of CZTSSe phases, sputtered CZTS precursors were subjected to annealing with a mixed gas of Ar and H₂Se at different concentrations. A series of characterization techniques were employed to investigate the morphologies, phase structures, surface states, and elemental compositions of the annealed samples. The results demonstrate that the H₂Se concentration in the annealing atmosphere can significantly affect the grain size, suppressing the decomposition of CZTSSe absorbers. When the H₂Se concentration in the annealing atmosphere reaches 4.5 vol%, a selenium-enriched CZTSSe absorber that is composed of the pure kesterite structure and that has densely packed large grains and a high concentration of selenium was obtained. The highest efficiency of 10.12% of CZTSSe solar cells was achieved herein.     

Solvent Extraction of Indium,Gallium, and Zinc Ions with Acidic Organophosphates having Bulky Alkyl Groups

The extraction equilibria of In³⁺, Ga³, and Zn²⁺ with bis(4‐ethylcyclohexyl)phosphoric acid (D4ECHPA), bis(4‐cyclohexylcyclohexyl)phosphoric acid (D4DCHPA), and bis(2‐ethylhexyl)phosphoric acid (D2EHPA) were investigated in acidic aqueous sulfate media. The order of extractability of metal ions is D4DCHPA > D2EHPA > D4ECHPA, which corresponds to the lipophilicity (log P) of the extractants. The separation factors, β_(In/Ga) and β_(Ga/Zn), of D4ECHPA and D4DCHPA are greater or comparable than that of D2EHPA, because of the steric hindrance of the bulky cyclohexyl groups. In³⁺ can be therefore separated from simulated liquor containing a high concentration of Zn²⁺ by D4DCHPA.     

Enhanced thermoelectric properties of Cu2Se via Sb doping: An experimental and computational study

In this study, Cu₂Se_1?xSb_x (x = 0.000, 0.005, 0.010, and 0.015) thermoelectric materials were synthesised using a solid-state reaction technique. A first-principles calculation indicated that the formation energy of the substitution of antimony (Sb) on the Se site is negative and more stable than those of copper (Cu) sites. Sb doping enhanced the lamellar orientation, decreased the grain size, and created an acceptor impurity level. The electrical resistivity and Seebeck coefficient decreased with increasing Sb doping. A minimum reduction in the thermal conductivity by approximately three times that of the undoped sample was obtained at x = 0.005 with a value of 0.40 W/m K at 523 K. The maximum figure of merit (ZT) was obtained at x = 0.005 with a value of 0.47 at 523 K. These findings indicate that substituting Sb into Se sites is an efficient approach for improving copper selenide (Cu₂Se) thermoelectric materials.     

Novel synthesis method for quaternary Cd(Cu,Zn)Se thin films and its characterizations

Nowadays, the search for novel compounds by chemical synthesis is in trend. Herein, we report the deposition of Cd_1-x-yZn_xCu_ySe (0.025 ≤ x = y ≤ 0.15) films by facile, industry-oriented chemical synthesis. The Cd_1-x-yZn_xCu_ySe thin films were deposited at the optimized growth conditions (temperature = 70 ± 0.1 °C, pH = 10.3 ± 0.1, substrate rotation speed = 70 ± 2 rpm and time = 100 min). As-synthesized thin films were characterized for physical, chemical, topographical and electrical attributes. The study of vibrational modes in Cd_1-x-yZn_xCu_ySe thin films was done with the help of Raman spectroscopy. Improvement in surface topography with the integration of Cu²⁺ and Zn²⁺ into the CdSe lattice has been noticed by the atomic force microscopy (AFM). The electrochemical impedance spectroscopy revealed lower values of R_s and R_ct for x = y = 0.05 composition. Chemical deposition of Cd_1-x-yZn_xCu_ySe thin films may offer an excellent way to fabricate quaternary chalcogenide-based absorber materials for solar cells.     

Structural and electrochemical studies of undoped and In3+-doped co-binary Cu2-xTe and Bi2Te3 thin films for aqueous Na–S batteries

WO₃ electrodes coated with co-binary Cu_2-xTe and Bi₂Te₃ thin films were fabricated for sodium-sulfur (Na–S) batteries. Film fabrication was controlled by adjusting the pH of the solution and the indium doping concentration. The phases of orthorhombic CuTe and hexagonal Cu₂Te with rhombohedral Bi₂Te₃ were formed on the WO₃ electrode. After In³⁺ doping, In³⁺ ions act as Frenkel defects in the Cu_2-xTe structure. This indicated that In³⁺ ions are located at interstitial sites in the Cu_2-xTe structure with higher defect creation energy. Furthermore, more interconnected-like nanoparticles and reduced porosity were observed, thereby indicating that indium segregation with grain boundaries presented and contributed to an enhancement of the surface mobility, nucleation density, and a smoother surface. For electrochemical characteristics, a polysulfide solution was used as a redox electrolyte for ion transport. Optimization of the pH and indium concentration attributed to improve the exchange current density (J₀) and time responses for the colored and bleached states because of faster movement of Na⁺ and S²⁻ ions during inter/de-intercalation. Furthermore, optimization of the electrode by adjusting the pH and doping with indium is advantageous for both Na–S and rechargeable batteries because of long life cycle, reasonably high power and energy density of 306 W/kg and 9.35 Wh/kg, respectively. The highest specific capacity (C_s) values of the charge and discharge cycles for In³⁺-doped electrodes are ∼ 21 and 19 mAh/g, respectively with the coulombic efficiency approximates 100% (average value of ∼96%). This approach may provide a general path for the fabrication of undoped and In³⁺-doped co-binary Cu_2-xTe and Bi₂Te₃ films on WO₃ electrodes and may increase our knowledge regarding Na–S batteries for further performance improvement.     

Effects of annealing atmospheres on the structure and ferromagnetic properties of Fe0.12Cu0.02Zn0.86O thin films

Fe_0.12Cu_0.02Zn_0.86O thin film was deposited on a Si substrate using r. f. sputtering with no heating and an Ar/O₂ ratio of 10%. After deposition, the specimens were annealed at 400 °C for 1 h, in nitrogen and hydrogen atmospheres. The X-ray diffractometry (XRD) analysis results show that the crystallinity of Fe_0.12Cu_0.02Zn_0.86O thin film annealed in a nitrogen atmosphere is better than that of the film annealed in a hydrogen atmosphere. The X-ray photoelectron spectroscope (XPS) results show that there are more oxygen vacancies in the Fe_0.12Cu_0.02Zn_0.86O thin film annealed in a hydrogen atmosphere. The magnetic force microscope (MFM) analysis results also demonstrate that some magnetism particles are precipitated for the Fe_0.12Cu_0.02Zn_0.86O thin film annealed in a hydrogen atmosphere. This results in an improvement in the ferromagnetic properties, and the saturation magnetization is 70.2 emu/cm³, which is about 25% larger than that for the as-grown Fe_0.12Cu_0.02Zn_0.86O thin film.     

Cyclic voltammetry study of electrodeposition of Cu(In,Ga)Se2 thin films

The electrodeposition of Cu(In,Ga)Se₂ has been investigated by cyclic voltammetry (CV) in a DMF-aqueous solution that contained citrate as a complexing agent. The effects of the citrate ion on the reduction potentials of Cu²⁺, In³⁺, Ga³⁺ and H₂SeO₃ were examined for a unitary system. Furthermore, a cyclic voltammetry study was performed in a ternary Cu-In-Se system, a quaternary Cu-In-Ga-Se system, and binary Cu-Se, In-Se and Ga-Se systems. The insertion of In and Ga into the solid phase may proceed by an underpotential deposition mechanism, which involves two different routes: In³⁺ and Ga³⁺ reduction by a surface-induced effect from Cu₃Se₂ and/or reaction with H₂Se.     

Adsorption characteristics of metal ions by CO2-fixing Chlorella sp. HA-1

Single and binary metal systems were employed to investigate the removal characteristics of Pb²⁺, Cu²⁺, Cd²⁺, and Zn²⁺ by Chlorella sp. HA-1 that were isolated from a CO₂ fixation process. Adsorption test of single metal systems showed that the maximum metal uptakes were 0.767 mmol Pb²⁺, 0.450 mmol Cd²⁺, 0.334 mmol Cu²⁺ and 0.389 mmol Zn²⁺ per gram of dry cell. In the binary metal systems, the metal ions on Chlorella sp. HA-1 were adsorbed selectively according to their adsorption characteristics. Pb²⁺ ions significantly inhibited the adsorption of Cu²⁺, Zn²⁺, and Cd²⁺ ions, while Cu²⁺ ions decreased remarkably the metal uptake of Cd²⁺ and Zn²⁺ ions. The relative adsorption between Cd²⁺ and Zn²⁺ ions was reduced similarly by the presence of the other metal ions.     

Preparation and Characterization of CIGSe Solar Cells by Ink Printing on Alumina Substrates with Self‐Synthesized Selenide Powders via an Environment‐Friendly and Cost‐Effective Dry Synthesis Route

CIGSe solar cells with an ink‐printing absorber layer were prepared on Mo‐coated alumina substrates. The use of alumina substrates can extend the process window to higher temperatures. The inks contained single‐phase CIGSe powder, which was formed by firing different selenide powders of Cu₂Se, In₂Se₃, and Ga₂Se₃ at 800°C. All these powders were synthesized with an environment‐friendly and cost‐effective powder process. The printed inks were sintered at 600–800°C. The solar cells had power conversion efficiency of 0.50%, an open‐circuit voltage of 27 mV, a short‐circuit current density of 37 mA/cm², and a fill factor of 0.50.     

Uptake of Cu, Cd and Pb on Zn–Al layered double hydroxide intercalated with edta

Hydrotalcite-like compound [Zn₂Al(OH)₆]₂edta·nH₂O(ZnAl-edta) was obtained from the precursor [Zn₂Al(OH)₆]NO₃·nH₂O (ZnAl-NO₃), by the anion exchange method, with the aim of uptake Cu²⁺, Cd²⁺ and Pb²⁺ from the aqueous solutions by chelating process between edta and metal cations. The amount of Cu²⁺, Cd²⁺ and Pb²⁺ adsorbed was monitorized by atomic absorption technique at different contact time, pH and metal concentrations. The results indicate the very fast adsorption of the metal cations by ZnAl-edta reaching the equilibrium of the uptake reaction in two hours for Cu and Pb and 24 h for Cd. The shape of the adsorption isotherms suggests specific interaction and high host–guest affinity. At pH 5.5 and initial concentration C_i = 10 mM, the amount adsorbed was C_s = 1117, 375 and 871 μmol/g for Cu²⁺, Cd²⁺ and Pb²⁺, respectively.     

Preparation of thin film alloys with electrodeposition from heterotrinuclear M1–M2–M1 complexes (M1: NiII, CuII; M2: CuII, CoII, ZnII, CdII)

M₁ ^II–M₂ ^II–M₁ ^II type linear complexes (where M₁ = Ni²⁺, Cu²⁺ and M₂ = Cu²⁺, Zn²⁺, Co²⁺ and Cd²⁺ were prepared and dissolved in dimethyl sulfoxide. They were then electrodeposited on mild steel surfaces by the use of rotating disc electrodes. The deposition potentials were determined from cyclic voltammetric i-E scans. The metal films deposited on mild steel surfaces were characterized with atomic absorption spectrometer (AAS), X-ray fluorescence (XRF) and ion chromatography (IC) methods. Although the stoichiometric quantity of M₂ metal cation was half of the M₁ metal cation in the complex, the amount of M₂ metal deposited upon the surface was markedly greater. The amount of M₂ ion deposited on the surface was found to increase with the hardness of the ion. The X-ray diffraction (XRD) patterns showed that the excessively deposited metal on the surface was in metallic form as well as alloy. The size of the deposited film particles was investigated by the use of atomic force microscope (AFM) technique and the particles were observed to be bigger than nanoparticle size.