New trends to grow the n-CdZn (S1−xSex)2/p-CuIn(S1−xSex)2 heterojunction thin films for solar cell applications

The n-CdZn(S_1−xSe_x) and p-CuIn(S_1−xSe_x)₂ thin films have been grown by the solution growth technique (SGT) on glass substrates. Also the heterojunction (p–n) based on n-CdZn (S_1−xSe_x)₂ and p-CuIn (S_1−xSe_x)₂ thin films fabricated by same technique. The n-CdZn(S_1−xSe_x)₂ thin film has been used as a window material which reduced the lattice mismatch problem at the junction with CuIn (S_1−xSe_x)₂ thin film as an absorber layer for stable solar cell preparation. Elemental analysis of the n-CdZn (S_1−xSe_x)₂ and p-CuIn(S_1−xSe_x)₂ thin films was confirmed by energy-dispersive analysis of X-ray (EDAX). The structural and optical properties were changed with respect to composition ‘x’ values. The best results of these parameters were obtained at x=0.5 composition. The uniform morphology of each film as well as the continuous smooth thickness deposition onto the glass substrates was confirmed by SEM study. The optical band gaps were determined from transmittance spectra in the range of 350–1000 nm. These values are 1.22 and 2.39 eV for CuIn(S_0.5Se_0.5)₂ and CdZn(S_0.5Se_0.5)₂ thin films, respectively. J–V characteristic was measured for the n-CdZn(S_1−xSe_x)₂/p-CuIn(S_1−xSe_x)₂ heterojunction thin films under light illumination. The device parameters V_oc=474.4 mV, J_sc=13.21 mA/cm², FF=47.8% and η=3.5% under an illumination of 85 mW/cm² on a cell active area of 1 cm² have been calculated for solar cell fabrication. The J–V characteristic of the device under dark condition was also studied and the ideality factor was calculated which is equal to 1.9 for n-CdZn(S_0.5Se_0.5)₂/p-CuIn(S_0.5Se_0.5)₂ heterojunction thin films.     

Optical and electrical studies on molybdenum sulphoselenide [Mo(S1−xSex)2] thin films prepared by arrested precipitation technique (APT)

Nanocrystalline stoichiometric [Mo(S_1−xSe_x)₂] thin films were deposited by using arrested precipitation technique (APT) developed in our laboratory. The precursors used for this are namely, molybdenum triethanolamine complex, thioacetamide and sodium selenosulphite; and various preparative conditions are finalised at the initial stages of deposition. Formation of [Mo(S_1−xSe_x)₂] semiconducting thin films are confirmed by studying growth mechanism, optical and electrical properties. X-ray diffraction analysis showed that the composites are nanocrystalline being mixed ternary chalcogenides of the general formula [Mo(S_1−xSe_x)₂]. The optical studies revealed that the films are highly absorptive (α×10⁴ cm⁻¹) with a band-to-band direct type of transitions and the energy gap decreased typically from 1.86 eV for pure MoS₂ down to 1.42 eV for MoSe₂. The thermoelectrical power measurement shows negative polarity for the generated voltage across the two ends of semiconductor thin films. This indicate that the [Mo(S_1−xSe_x)₂] thin film samples show n-type conduction.     

The influence of hydrogen in the incorporation of Zn during the growth of Cu2ZnSnS4 thin films

Cu₂ZnSnS₄ (CZTS) is a p-type semiconductor, candidate to replace Cu(In,Ga)Se₂ as absorber layer in thin film solar cells. The best solar cells based on CZTS present efficiencies up to 6.8%. These results were improved when metallic Zn was replaced by ZnS, which may imply a different chemical path for the formation of CZTS. In this study it is compared with the diffusion of Zn on Cu₂SnS₃ by introducing metallic Zn or ZnS. For this CZTS films were grown by sulphurization of Cu₂SnS₃, some with a Zn layer and others with a ZnS layer. The influence of H₂ during the annealing process is also studied and for this some sulphurizations were done in the presence of a partial atmosphere of H₂.The SEM micrographs of the samples show a columnar growth structure of the films with different degrees of compactness. The compactness is improved in the samples where a ZnS layer was present in the precursor and the sulphurization was done in the presence of H₂. EDS chemical profiling revealed regular zinc distribution for the samples with metallic Zn whilst the ones with ZnS exhibited a Zn-rich surface. X-ray diffraction (XRD) indicated the presence of CZTS and Cu_2−xS phases in all samples. These results were confirmed by Raman scattering.It was concluded that the sulphurization of Cu₂SnS₃ films with the use of ZnS layers under H₂ atmosphere produces better quality CZTS thin films, since it promotes Zn diffusion and avoids Zn losses by evaporation.     

Perovskite Sr(Fe1-xCux)O3-δ materials for chemical looping combustion applications

This paper presents the results of chemical looping combustion (CLC) research. Nowadays, CLC is one of the best prospective combustion technologies, because it enables the production of a concentrated carbon dioxide (CO₂) stream, following water condensation, without any energy penalty for its separation.The objective of this work was to study chemical looping reaction performance with the application of novel perovskite-type oxygen carriers (OCs). Sr(Fe_1-xCu_x)O_3-δ family members were tested for hydrogen combustion for power generation purposes. Sr(Fe_1-xCu_x)O_3-δ, which is a perovskite-type oxide, was prepared using the calcination method, where x = 0, 0.1 and 0.33. Reactivity tests were performed using a thermogravimetric analyser (TGA, Netzsch STA 409 PG Luxx) under isothermal conditions in multiple reduction–oxidation cycles. Both the temperature (600–800 °C) and number of redox cycles (five cycles) effects on the reaction performance of recently developed OC samples were evaluated in the study. TGA data were used for the assessment of the oxygen transport capacity value, redox reaction rates and stability. Sr(Fe_1-xCu_x)O_3-δ showed an excellent stable chemical looping performance. The changing of oxygen content (3.8–4.86 wt%) occurred within approximately 2 min, with the chemical properties of the material maintained during the cycling combustion tests.In addition, new SrFeO_3-δ OCs, doped with copper (Cu) perovskite-type materials, were analysed using multiple methods: X-ray powder diffraction (XRD); scanning electron microscopy (SEM); surface area by Brunauer–Emmett–Teller (BET) method; and melting behaviour study. In terms of physical properties, the new OCs can resist both high CLC process temperatures and mechanical forces, which are essentially useful. The analysis showed that Sr(Fe_1-xCu_x)O_3-δ carriers performed at extremely high melting temperatures (>1280 °C). Results of crushing strength testing showed that developed materials had a pronounced mechanical resistivity with a crushing strength higher than 1 N and will perform well in fluidized beds (4.31–6.23 N).In this paper, it was demonstrated that known mixed oxygen-ionic and electronic conducting membrane materials such as Sr(Fe_1-xCu_x)O_3-δ might also be applied as oxygen carriers. Overall results demonstrated Fe-Cu-based perovskites might be successfully used as OCs in the chemical looping combustion process.     

Structure, composition and optical properties of Cu2ZnSnS4 thin films deposited by Pulsed Laser Deposition method

Polycrystalline Cu₂ZnSnS₄ (CZTS) thin films have been directly deposited on heating Mo-coated glass substrates by Pulsed Laser Deposition (PLD) method. The results of energy dispersive X-ray spectroscopy (EDX) indicate that these CZTS thin films are Cu-rich and S-poor. The combination of X-ray diffraction (XRD) results and Raman spectroscopy reveals that these thin films exhibit strong preferential orientation of grains along [1 1 2] direction and small Cu_2−xS phase easily exists in CZTS thin films. The lattice parameters and grain sizes have been examined based on XRD patterns and Atom Force Microscopy (AFM). The band gap (E_g) of CZTS thin films, which are determined by reflection spectroscopy varies from 1.53 to 1.98 eV, depending on substrate temperature (T_sub). The optical absorption coefficient of CZTS thin film (T_sub=450 °C) measured by spectroscopic ellipsometry (SE) is above 10⁴ cm⁻¹.     

Structural and photoluminescence study of the quaternary alloys system CuIn(SxSe1−x)2

The full composition range CuIn(S_xSe_1−x)₂ alloy system has been studied using 40 mm length crystal cuts from 10 mm diameter ingots grown by the classical Bridgman method. X-ray diffraction diffractographs show that the CuIn(S_xSe_1−x)₂ compounds have a chalcopyrite structure for each composition x, they exhibit an expansion on the unit cell characteristics by the tetragonal distortion which depends linearly on the electronegativity of the atoms. The photoluminescence spectra is investigated as a function of various compositions, temperature and excitation intensities. Photoluminescence spectra shows a wide variation in the dominant peak location and an overall blue shift with the increase of sulphur content. Photoluminescence CuInS₂ and CuIn(S_0.72Se_0.28)₂ have been studied in detail.     

Cu0.6Ni0.4Co2O4 nanowires,a novel noble-metal-free catalyst with ultrahigh catalytic activity towards the hydrolysis of ammonia borane for hydrogen production

In this work, we have prepared a series of Cu_xNi_1-xCo₂O₄ (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1) nanowires with a diameter of approximately 30 nm, which were characterized by X-ray powder diffractometry, scanning and transmission electron microscopy, and X-ray photoelectron spectrometry. For the first time, the catalytic activity of these Cu_xNi_1-xCo₂O₄ nanowires in ammonia borane (AB) hydrolysis was investigated, and it was found that a significant synergistic effect exists between NiCo₂O₄ and CuCo₂O₄ in the hydrolytic reaction. Among these Cu_xNi_1-xCo₂O₄ samples, the Cu_0.6Ni_0.4Co₂O₄ nanowires showed the highest turnover frequency (TOF) of 119.5 mol_hydrogen min^?1 mol_cat^?1. This value is 1.66 times as high as that for CoP nanoparticles, which is the most active noble-metal-free catalyst towards AB hydrolysis ever reported in the literature. Because of the low cost and high catalytic performance, the Cu_0.6Ni_0.4Co₂O₄ nanowires can be a robust catalyst towards AB hydrolysis in practical applications.     

Charge transfer behaviors over MOF-5@g-C3N4 with NixMo1−xS2 modification

The composite photocatalyst Ni_xMo_1?xS₂/MOF-5@g-C₃N₄ was successfully synthesized by means of hydrothermal with two step methods and the effective photocatalytic activity improvement was obtained. With the introduction of Ni_xMo_1?xS₂, the H₂ production reached the maximum about 319 μmol under continuous visible light irradiation for 5 h, which was 30 times higher than that of pure g-C₃N₄ photocatalyst. A series of characterization results shown that the MOF-5@g-C₃N₄ on the surface of Ni_xMo_1?xS₂ provided the more active sites and improved the efficiency of photo-generated charge separation with SEM, XRD, TEM, EDX, XPS, UV–vis DRS, BET, FTIR, transient fluorescence and electro-chemistry etc. and the results of which were in good mutual corresponding with each other. Furthermore, the reaction mechanism over the compound catalyst Ni_x-Mo_1?xS₂/MOF-5@g-C₃N₄ was proposed.     

Determination of Cu(In1−xGax)3Se5 defect phase in MBE grown Cu(In1−xGax)Se2 thin film by Rietveld analysis

Quantitative phase analysis of Cu(In_1−xGa_x)Se₂ (CIGS) thin film grown over Mo coated soda lime glass substrates was studied by Rietveld refinement process using room temperature X-ray data at θ-2θ mode. Films were found to contain both stoichiometric Cu(In_1−xGa_x)Se₂ and defect related Cu(In_1−xGa_x)₃Se₅ phases. Best fitting was obtained using crystal structure with space group I-42d for Cu(In_1−xGa_x)Se₂ and I-42m for Cu(In_1−xGa_x)₃Se₅ phase. The effects of Ga/III (=Ga/In+Ga=x) ratio and Se flux during growth over the formation of Cu(In_1−xGa_x)₃Se₅ defect phase in CIGS was studied and the correlation between quantity of Cu(In_1−xGa_x)₃Se₅ phase and solar cell performance is discussed.     

On energy levels and heat generation of electron—hole pairs in Ge alloyed with CuInSe2 and Al2Te3

On the assumption of the possibility of obtaining continuous and concentrated solid substitutional solutions of multicomponent diamond-type compounds, orientation numbers and positions of energy levels of defects formed in Ge by molecules of diluted CuInSe₂ and Al₂Te₃ are offered. Heat generation of electron-hole pairs in solid solutions of substitution of (Ge₄)_1-x(CuInSe₂)_x and (Ge₆)_1-x(Al₂Te₃)_x with x < 0.005 is discussed.     

Preparation and evaluation of Cu2ZnSnS4 thin films by sulfurization of E---B evaporated precursors

By sulfurization of E---B evaporated precursors, CZTS(Cu₂ZnSnS₄) films could be prepared successfully. This semiconductor does not consist of any rare-metal such as In. The X-ray diffraction pattern of CZTS thin films showed that these films had a stannite structure. This study estimated the optical band gap energy as 1.45 eV. The optical absorption coefficient was in the order of 10⁴cm⁻¹. The resistivity was in the the order of 10⁴ Ω cm and the conduction type was p-type. Fabricated solar cells, Al/ZnO/CdS/CZTS/Mo/Soda Lime Glass, showed an open-circuit voltage up to 400 mV.     

Electrochemical evaluation of double perovskite PrBaCo2-xMnxO5+δ (x = 0, 0.5, 1) as promising cathodes for IT-SOFCs

Mn-substituted double perovskites, PrBaCo_2-xMn_xO_5+δ (x = 0, 0.5, 1), are evaluated as cathode materials for intermediate-temperature solid oxide fuel cells. The effects of Mn substitution content on their structural and electrochemical properties including crystal structure, thermal expansion coefficient, and cathodic interfacial polarization resistance are investigated. The PrBaCo_2-xMn_xO_5+δ samples exhibit structural changes with increasing Mn contents from tetragonal (x = 0) to cubic (x = 0.5 and 1.0) symmetry. The thermal expansion coefficient decreases with the increasing Mn content while the cathodic performance increases with the increment of Mn content from x = 0 to x = 0.5 then decreases with the further increment of Mn content from x = 0.5 to x = 1.0. When using La_0.8Sr_0.2Ga_0.8Mg_0.15Co_0.05O₃ with 300 μm thickness as electrolyte and Sr₂Fe_1.4Ni_0.1Mo_0.5O_6-δ as anode, the maximum powder density of the x = 0.5 composite is 0.638 W cm^?2, which is higher than that of the other two samples with x = 0 (0.474 W cm^?2) and x = 1.0 (0.371 W cm^?2) at 800 °C.     

Preparation of Li2S-GeSe2-P2S5 electrolytes by a single step ball milling for all-solid-state lithium secondary batteries

Glass-ceramic and glass Li₂S-GeSe₂-P₂S₅ electrolytes were prepared by a single step ball milling (SSBM) process. Various compositions of Li_4−xGe_1−xP_xS_2(1+x)Se_2(1−x) with/without heat treatment (HT) from x = 0.55 to x = 1.00 were systematically investigated. Structural analysis by X-ray diffraction (XRD) showed gradual increase of the lattice constant followed by significant phase change with increasing GeSe₂. HT also affected the crystallinity. Incorporation of GeSe₂ in Li₂S-P₂S₅ kept high conductivity with a maximum value of 1.4 × 10⁻³ S cm⁻¹ at room temperature for x = 0.95 in Li_4−xGe_1−xP_xS_2(1+x)Se_2(1−x) without HT. All-solid-state LiCoO₂/Li cells using Li₂S-GeSe₂-P₂S₅ as solid-state electrolytes (SE) were tested by constant-current constant-voltage (CCCV) charge-discharge cycling at a current density of 50 μA cm⁻² between 2.5 and 4.3 V (vs. Li/Li⁺). In spite of the extremely high conductivity of the SE, LiCoO₂/Li cells showed a large irreversible reaction especially during the first charging cycle. LiCoO₂ with SEs heat-treated at elevated temperature exhibited a capacity over 100 mAh g⁻¹ at the second cycle and consistently improved cycle retention, which is believed to be due to the better interfacial stability.     

Chemical composition dependence of morphological and optical properties of Cu2ZnSnS4 thin films deposited by sol-gel sulfurization and Cu2ZnSnS4 thin film solar cell efficiency

The properties of Cu₂ZnSnS₄ (CZTS) thin films deposited by sol-gel sulfurization were investigated as a function of the chemical composition of the sol-gel solutions used. The chemical composition ratio Cu/(Zn+Sn) of the sol-gel solution was varied from 0.73 to 1.00, while the ratio Zn/Sn was kept constant at 1.15. CZTS films deposited using sol-gel solutions with Cu/(Zn+Sn)<0.80 exhibited large grains. In addition, the band gaps of these Cu-poor CZTS thin films were blue shifted. Solar cells with the structure Al/ZnO:Al/CdS/CZTS/Mo/soda lime glass were fabricated under non-vacuum conditions. The solar cell with the CZTS layer deposited using the sol-gel solution with Cu/(Zn+Sn)=0.80 exhibited the highest conversion efficiency of 2.03%.     

CuInxGa1−xSe2 thin films prepared by electron beam evaporation

CuIn_xGa_1−xSe₂ bulk compound of three different compositions x=0.75, 0.80 and 0.85 have been prepared using individual elements of copper, indium, gallium and selenium. Thin films of CuIn_xGa_1−xSe₂ have been deposited using the prepared bulk by electron beam evaporation method. The structural studies carried on the deposited films revealed that films annealed at 400 °C are crystalline in nature exhibiting chalcopyrite phase. The position of the (1 1 2) peak in the X-ray diffractogram corresponding to the chalcopyrite phase has been found to be dependent on the percentage of gallium in the films. The composition of the prepared bulk and thin films has been identified using energy dispersive X-ray analysis. The photoluminescence spectra of the CuIn_xGa_1−xSe₂ films exhibited sharp luminescence peaks corresponding to the band gap of the material.     

Hybrid functionals studies of structural and electronic properties of ZnxCd(1−x)S and (ZnxCd1−x)(SexS1−x) solid solution photocatalysts

The structural and electronic properties of the wurtzite Zn_xCd_1−xS and (Zn_xCd_1−x)(Se_xS_1−x) alloys are calculated using density functional theory calculations with HSE06 hybrid exchange-correlation functional. Special quasirandom structures are used to describe the disordered alloys, for x = 0.125, 0.25, 0.375, 0.5, 0.625, 0.75 and 0.875, respectively. Our calculations reveal that Zn_xCd_1−xS alloy with the appropriate Zn doping concentration not only causes the elevation of the conduction band minimum energy, but also increase the mobility of photogenerated holes and electrons, which well explains the high photocatalytic activity and stability of Zn_0.2Cd_0.8S alloy under a long-term light irradiation. Compared with Zn_xCd_1−xS alloy, (Zn_xCd_1−x)(Se_xS_1−x) alloy holds greater potential to simultaneously meet band gap, band edge, and mobility criteria for water splitting. Theoretical results predict that (Zn_xCd_1−x)(Se_xS_1−x) alloy with the ZnSe concentration in the range from 0.38 to 0.75 could be a more promising candidate than Zn_0.2Cd_0.8S alloy for photoelectrochemical hydrogen production through water splitting.     

Studies on Cu2ZnSnS4 (CZTS) absorber layer using different stacking orders in precursor thin films

Cu₂ZnSnS₄ (CZTS) thin films were deposited by sputtering on glass substrates using stacked precursors. The stacked precursor thin films were prepared from Cu, SnS₂ and ZnS targets at room temperature with different stacking orders of Cu/SnS₂/ZnS/glass (A), ZnS/Cu/SnS₂/glass (B) and SnS₂/ZnS/Cu/glass (C). The stacked precursor thin films were sulfurized using a tubular rapid thermal annealing system in a mixed N₂ (95%)+H₂S (5%) atmosphere at 550 °C for 10 min. The effects of the stacking order in the precursor thin films on the structural, morphological, chemical, electrical and optical properties of the CZTS thin films were investigated. X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy studies showed that the annealed CZTS thin film using a stacking order A had a single kesterite crystal structure without secondary phases, whereas stacking orders B and C have a kesterite phase with secondary phases, such as Cu_2−xS, SnS₂ and SnS. The annealed CZTS thin film using stacking order A showed a very dense morphology without voids. On the other hand, the annealed CZTS thin films using stacking orders B and C contained the volcano shape voids (B) and Sn-based secondary phases (C) on the surface of the annealed thin films. The direct band gap energies of the CZTS thin films were approximately 1.45 eV (A), 1.35 eV (B) and 1.1 eV (C).     

Comparative study of Cu2ZnSnS4 film growth

We fabricated Cu₂ZnSnS₄ (CZTS) thin films using two different methods, spray pyrolysis and sulfurization of Cu-Zn-Sn metallic films. Spray pyrolysis was carried out under air ambient with modified ultrasonic spray system. Sulfurizations of metallic Cu-Zn-Sn films were done for stacked metallic films, Cu/Sn/Zn/glass, Cu/Sn/Cu/Zn/glass and Sn/Cu/Zn/glass, which were prepared by sputtering method in high vacuum chamber. The sprayed films were not observed to be grown well with good crystallinity, compared with CZTS films made by sulfurization of stacked metallic films. However, it was found that application of additional sulfurization to sprayed CZTS films induced great improvement of crystallinity to the level of the sulfurized metallic films. This implicates that spray pyrolysis with additional sulfurization is a good method for fabrication of CZTS films, especially as a low-cost fabrication technique. All CZTS films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and Raman spectroscopy measurements.     

Effect of Sr2+ doping on sintering behavior,microstructural development and electrical properties of LaPO4·nH2O nanorods prepared by dry mechanical milling

The effect of Sr²⁺ doping on the presence of second phases, sintering behavior, microstructural development, and electrical properties of LaPO₄·nH₂O nanorods (La_1?xSr_xPO_4?x/2 where x = 0.025 and 0.05) obtained by a dry mechanochemical milling process was investigated. When Sr²⁺ is present monazite-type La_1?xSr_xPO₄ nanopowders were obtained instead of rhabdophane-type LaPO₄·nH₂O. In addition, Sr²⁺ doping implies a larger P/La ratio and it enhances the formation of lanthanum tryoxophosphate (La(PO₃)₃), a thermodinamically stable phase, in doped samples. Dilatometric studies reveal a shift of the maximum shrinkage rate at lower temperatures for doped samples, with larger shifts with higher Sr²⁺ contents. This shift is related to the presence of oxygen vacancies but also to a higher content of La(PO₃)₃. Furthermore, the derivative of the linear shrinkage curves for all the samples showed peaks at temperatures higher than 1300 °C that are associated to the volatilization of P₄O₁₀ gas and to the recrystallization of monazite from the incongruent melting of La(PO₃)₃. After the dilatometric tests at 1500 °C the samples showed polygonal grains with a bimodal size distribution. For the doped samples the smaller grains do not present Sr²⁺ in their composition and it is related to those grains form from the recrystallization of monazite-LaPO₄ formed in turn from La(PO₃)₃. The total conductivity of the studied samples (x = 0.05) is higher for the samples sintered at 1000 °C for 1 h than for those sintered at 1500 °C without any dwell time. It can be due in part to the fact that the smaller grains of the samples sintered at 1500 °C do not contain Sr²⁺ and it can hinder the charge transport.     

Hydrogen storage in Zr0.9Ti0.1(Ni0.5Cr0.5-xVx)2 Laves phase,with x = 0, 0.125, 0.25, 0.375, 0.5. A theoretical approach

Density functional calculations were performed on Zr_0.9Ti_0.1(Ni_0.5Cr_0.5-xV_x)₂ Laves Phase, with x = 0, 0.125, 0.25, 0.375 and 0.5, in order to study its H absorption capacity. Binding energy, electronic structure and bonding were analyzed for the intermetallic compound with different V content and increasing amounts of hydrogen.The optimized geometry was found in good agreement with experimental data of the C14 Laves phase. Hydrogen locates preferentially in A₂B₂ tetrahedral sites in the AB₂ matrix (A = Zr, Ti; B = Ni, Cr, V) but AB₃ and B₄sites are also stable. The volume of the intermetallic and the H binding energy increases with vanadium content. Theoretically H absorption is possible up to 4.5 H/F.U. but the strongest binding energy is achieved with 3 H/F. U.The main contribution to density of states is due to d states of all components of the structure and an H-metal bonding is observed in the range ?10 to ?4 eV.