Sulphurization of single-phase Cu11In9 precursors for CuInS2 solar cells

Using Rutherford backscattering (RBS), X-ray diffraction (XRD), and scanning electron microscopy (SEM), the sulphurization of single-phase Cu₁₁In₉ precursors to be employed as light absorbing CuInS₂ (CIS) layers in CIS-CdS heterojunction thin-film solar cells has been investigated. The Cu₁₁In₉ precursor films were produced by DC-sputtering from a single-phase Cu₁₁In₉ target. The sulphurization at 500 or 300 °C was performed by adding different amounts of elemental sulphur with heating rate and sulphurization time as additional parameters. During sulphurization at 500 °C, up to 50% of the indium initially present in the precursor is lost. We relate the In-loss to the volatile In₂S compound, the formation of which is favoured by the phase transition of Cu₁₁In₉ to Cu₁₆In₉ at 307 °C. Consequently, the In-loss can be suppressed by employing a sulphurization temperature of 300 °C. At this temperature, a prolonged sulphurization time and a large sulphur excess are necessary in order to obtain stoichiometric CIS beneath a CuS_x surface phase.     

The formation of the thin-film solar cell absorber CuInS2 by annealing of Cu-In-S stacked elemental layer precursors — A comparison of selenisation and sulfurisation

We present results of in-situ X-ray diffraction experiments on the formation of CuInS₂ thin film solar cell absorbers. The experiments have been performed while annealing Cu-In-S stacked elemental layer precursors produced by sputtering and thermal evaporation to investigate the crystallisation process of the chalcopyrite CuInS₂. Rietveld refinement has been performed to obtain the quantitative phase evolution of crystalline phases while annealing. The annealing process is characterised by a rapid sulfurisation of the initially present intermetallic alloy Cu₁₁In₉ forming the sulfide phases CuS, Dg-Cu_2 − xS, InS and/or CuIn₅S₈. The chalcopyrite CuInS₂ crystallises at elevated sample temperatures by the consumption of these sulfide phases as educts. Three different chalcopyrite formation reactions have been identified by an analysis of the quantitative phase evolution. A comparison to earlier investigations on the formation of CuInSe₂ from Cu-In-Se precursors is presented to show similarities and differences of sulfurisation and selenisation processes. The chalcopyrite forms from chalcogenide educts in both cases. However, distinct differences concerning the chalcogenisation kinetics of sulfur and selenium containing Cu-In precursors have been revealed. The chalcogenisation of the intermetallic alloy phase Cu₁₁In₉ proceeds extremely fast for Cu-In-S precursors as compared to Cu-In-Se samples.     

Role of Na in reaction pathways and kinetics of CuInSe2 formation from stacked binary precursors

Binary stacked In₂Se₃/CuSe precursors were prepared onto substrates with different types of sodium (Na) sources, i.e., soda-lime glass (SLG), sodium-free glass (SFG), SFG with Na-doped Mo layers (Mo:Na), in a co-evaporation system. SIMS depth profiles for In₂Se₃ precursors deposited at 400 °C demonstrated that the different amounts of Na diffused out of each Na source. High-temperature XRD experiments revealed that there was no significant effect of Na on the reaction path of CuInSe₂ formation from In₂Se₃/CuSe stacked precursor. The reaction rate of the precursor without Na (i.e., SFG/Mo/In₂Se₃/CuSe) was found to be higher than that of those with Na (i.e., SLG/Mo/In₂Se₃/CuSe and SFG/Mo:Na(750 nm)/Mo/In₂Se₃/CuSe).     

Structural and chemical analyses of sputtered InxSy buffer layers in Cu(In,Ga)Se2 thin-film solar cells

Sputtered In_xS_y layers deposited on borosilicate glass and Si at substrate temperatures ranging from about 60 °C to 340 °C were analyzed by means of X-ray diffraction, energy-dispersive X-ray spectrometry, and optical transmission and reflection measurements. With increasing substrate temperature, the In_xS_y layers exhibit increasing sulfur concentration and also increasing absorption-edge energies. In_xS_y layers on Cu(In,Ga)Se₂(CIGS)/Mo/glass stacks were additionally studied by scanning and transmission electron microscopy. With increasing substrate temperature, Cu, Ga, and In interdiffusion between CIGS and In_xS_y becomes more enhanced. At 340 °C, CuIn₅S₈ forms instead of In_xS_y. The CuIn₅S₈ formation at elevated temperatures may be the reason for the very low efficiency of solar cells with indium sulfide buffers deposited at temperatures above about 250 °C by various techniques.     

Cu(In,Ga)Se2 thin films prepared from stacked precursors by post-selenization process

The copper–indium–gallium metallic precursors were fabricated corresponding to the sequence CuGa/CuIn/CuGa/soda-lime-glass by sequential direct current magnetron sputtering. The as-sputtered precursors were comprised of Cu₁₁In₉, Cu₁₆In₉, In, Cu₉Ga₄, and CuGa₂ phases, which may be closely correlated to the deposition sequences of multi-layered metallic precursors. Cu(In, Ga)Se₂ (CIGS) absorber films were prepared from the stacked precursors by post-selenization process with solid Se powder, and the morphological, structural, and compositional properties were investigated. The as-selenized CIGS film exhibited a smooth, compact, and densely packed morphology with well-defined and faceted crystal grains. The film was Cu-deficient and had a low Ga content. X-ray diffractometer results indicated the formation of single-chalcopyrite structure CIGS absorber layers. Depth-resolved Raman patterns showed the formation of a dominant CIGS phase in the as-selenized layer, and an ordered vacancy compound phase like Cu(In, Ga)₃Se₅ at the surface and inner region. With increasing the sputtering time, the full width at half maximum of the chalcopyrite A₁ Raman peak increased. Band broadening can be interpreted as a result of a higher density of defects within the chalcopyrite CIGS phase. The A₁ peak shifts with increasing sputtering depths were not apparent, which was related to a uniform distribution of Ga in the CIGS thin film.     

The influence of gallium on phase transitions during the crystallisation of thin film absorber materials Cu(In,Ga)(S,Se)2 investigated by in-situ X-ray diffraction

Chalcopyrite based photovoltaic materials Cu(In_xGa_1 − x)(S_ySe_1 − y)₂ (CIGSSe) are substituted in the cation and anion lattice to adopt the semiconductor bandgap to the terrestrial solar spectrum. In-situ X-ray diffraction (XRD) investigations on the crystallisation of thin film absorber materials Cu(In,Ga)(S,Se)₂ while annealing stacked elemental layers (SEL) show phase transitions proceeding during the chalcopyrite synthesis.Thin layers of metals with elemental ratio Cu:In:Ga = 3:2:1 are deposited onto Mo-coated polyimide foil by DC-magnetron sputtering. The metal precursor is covered with S and subsequently Se by thermal evaporation of the elements in chalcogen excess (S + Se) / (Cu + In + Ga) = 2.3. Investigated chalcogen ratios reach from pure Se to pure S. Crystalline phases formed during the annealing of SEL are qualitatively determined. The results are compared to conclusions drawn from previous experiments on Ga-free CuIn(S,Se)₂ absorbers. The presence of Ga and S influences significantly the time-scale and the temperatures of phase transitions, i.e. the sulfoselenisation of precursor phases Cu₁₆(In,Ga)₉ and Cu₉(Ga,In)₄ proceeds faster with increasing S and is shifted to higher temperatures as compared to Ga-free Cu₁₁In₉/Cu₁₆In₉.     

Structure and phase relations in the 2(CuInS2)-Cu2ZnSnS4 solid solution system

The intermixing of roquesite (CuInS₂) and kesterite (Cu₂ZnSnS₄), i. e. Cu(In_x(ZnSn)_1−xS₂ was investigated by a combination of neutron and X-ray powder diffraction. Samples with 0 ≤ × ≤ 1 were synthesized by a solid state reaction of the pure elements in evacuated silica tubes at 800 °C and cooled with a 10 K/h rate after the final annealing. The structural parameters of CuIn_x(ZnSn)_1−xS₂ were determined by simultaneous Rietveld refinement of neutron and X-ray diffraction data. The microstructure and chemical composition of the samples were investigated by electron microprobe analysis. A broad miscibility gap exists in the region 0.4 ≤ × < 0.8 indicated by the coexistence of two phases, an In-rich (x ~ 0.77) and a Zn-Sn-rich (x ~ 0.33) phase. Cu(In_x(ZnSn)_1−xS₂ mixed crystals with 0 ≤ x < 0.4 crystallize in the kesterite type structure, and with 0.8 ≤ × ≤ 1.0 in the chalcopyrite type structure. In the latter In, Zn and Sn are disordered on the In site. In the mixed crystals the lattice constant a and c show a linear dependence on chemical composition, whereas the tetragonal deformation Δ = 1−c/2a varies nonlinearly. Moreover in the mixed crystal with x ~ 0.15 the tetragonal deformation is equal zero and thus its structure is characterized by a pseudo-cubic unit cell.     

First-principles study on electronic structure, phase stability, and optical properties of In2X2O7 (X═C, Si, Ge or Sn)

In₂Ge₂O₇ and In₂Si₂O₇ are commonly used as scintillation materials. More studies on In₂X₂O₇ (X═C, Si, Ge, or Sn) are important to explore the possibility of using these materials for optoelectronic devices. This work presents results dealing with structural properties, electronic structure, chemical bonding, carrier effective masses, and optical spectra of polymorphs of In₂X₂O₇ obtained from first-principles calculations. The monoclinic phase of In₂Ge₂O₇, cubic and monoclinic phases of In₂Si₂O₇, as well as cubic phase of In₂Sn₂O₇ are known in scientific literature. From the total energy calculations at high pressure/strain we have found that the monoclinic phase of In₂Si₂O₇, In₂Ge₂O₇, and In₂Sn₂O₇ can be transformed into the cubic phase. The cubic phase of In₂Ge₂O₇ and In₂Sn₂O₇ is found to be more stable than the monoclinic phase. However, the monoclinic phase of In₂C₂O₇ and In₂Si₂O₇ is more stable than the cubic phase. The phase stability study suggests that In₂C₂O₇ is not stable, and that it might dissociate into corresponding binary oxides. Effective masses of electrons and holes have been estimated. Analysis of optical properties shows that in Si solar cells In₂Si₂O₇ and In₂Sn₂O₇ can be used as antireflection coating layer.     

Ultrasonically sprayed indium sulfide buffer layers for Cu(In,Ga)(S,Se)2 thin-film solar cells

Indium sulfide layers were grown by an ultrasonic spray pyrolysis method for application in Cu(In,Ga)(S,Se)₂ solar cells. X-ray diffraction measurements of layers on soda lime glass showed polycrystalline In₂S₃ with preferential orientation along the [103] direction and X-ray photoelectron spectroscopy revealed presence or absence of oxygen and chlorine impurities depending on the composition of the spray solution. For more quantitative chemical composition measurements In₂S₃ layers were sprayed on silicon substrates and analyzed with Rutherford backscattering spectrometry. The structural and chemical information on the In₂S₃ layer sprayed with different sulfur concentrations in the chemical precursor solution are correlated to the photovoltaic performance of solar cells. Best cell efficiency of 12.4% was achieved with an ultrasonically sprayed In₂S₃ buffer layer on a Cu(In,Ga)(S,Se)₂ absorber.     

Phase evolution during CuInSe2 electrodeposition on polycrystalline Mo

Using structural analyses means of ex-situ Raman spectroscopy and X-ray diffraction combined with electrical measurements, we study the phase evolution in the growth by electrodeposition technique of CuInSe₂ on polycrystalline Mo. For this purpose the growth was stopped at different stages, and then the different layers were analysed. First growth steps seem to be controlled by the deposition of secondary phases, like elemental Se and Cu₂Se binary. After the deposition of approximately 300 nm of material, CuInSe₂ ternary and ordered vacancy compounds start to adequately form. At a thickness close to 2000 nm, the formation of binary Cu_xSe is observed, remaining up to the final growth process (4350 nm). All these results are compared with the kinetic model of the system under the consideration of the experimental composition evolution.     

Cu2ZnSnS4 solar cells prepared with sulphurized dc-sputtered stacked metallic precursors

In the present work we report the details of the preparation and characterization results of Cu₂ZnSnS₄ (CZTS) based solar cells. The CZTS absorber was obtained by sulphurization of dc magnetron sputtered Zn/Sn/Cu precursor layers. The morphology, composition and structure of the absorber layer were studied by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and Raman scattering. The majority carrier type was identified via a hot point probe analysis. The hole density, space charge region width and band gap energy were estimated from the external quantum efficiency measurements. A MoS₂ layer that formed during the sulphurization process was also identified and analyzed in this work. The solar cells had the following structure: soda lime glass/Mo/CZTS/CdS/i-ZnO/ZnO:Al/Al grid. The best solar cell showed an open-circuit voltage of 345 mV, a short-circuit current density of 4.42 mA/cm², a fill factor of 44.29% and an efficiency of 0.68% under illumination in simulated standard test conditions: AM 1.5 and 100 mW/cm².     

Phases in copper-gallium-metal-sulfide films (metal=titanium, iron, or tin)

The incorporation of metal impurities M (M = Ti, Fe, or Sn) into CuGaS₂ films is investigated experimentally as a function of impurity concentration. Films are synthesized by thermal co-evaporation of the elements onto glass/Mo substrates heated to 400 °C-570 °C. The compositions of the resulting films are measured by energy-dispersive X-ray spectroscopy and the structures of the present phases are studied by X-ray diffraction. The formation of Cu-M-S ternary phases is observed in a wide range of conditions. Films of Cu-Ga-Ti-S, synthesized at 500 °C, show the presence of a cubic modification of CuGaS₂ and Cu₄TiS₄. Alloying of CuGaS₂ and tetragonal Cu₂SnS₃ is observed for substrate temperatures of 450 °C. A miscibility gap opens at 500 °C and above with separate Sn-rich and Ga-rich phases. Similarly, alloys of CuFeS₂ and CuGaS₂ are only found in Cu-Ga-Fe-S films synthesized at lower substrate temperature (400 °C), whereas at 500 °C a miscibility gap opens leading to separate Fe-rich and Ga-rich phases.     

Characterisation of ultrasonically sprayed InxSy buffer layers for Cu(In,Ga)Se2 solar cells

In order to replace chemical bath deposited (CBD) CdS buffer layers in Cu(In,Ga)Se₂ (CIGS) solar cells by an alternative material, In_xS_y thin-film buffer layers were prepared by ultrasonic spray pyrolysis at various substrate temperatures. X-ray Diffraction measurements confirmed that the films contained primarily the tetragonal In₂S₃ phase. X-ray Photoelectron Spectroscopy measurements revealed a small concentration of chlorine impurity throughout the In_xS_y layer. By depositing the indium sulphide layer as buffer layer in the CIGS solar cell configuration, a maximum solar cell efficiency of 8.9% was achieved, whilst the reference cell with CdS/CIGS on a similar absorber exhibited 12.7% efficiency. Additionally, light soaking enhanced the efficiency of In_xS_y/CIGS cells primarily by improvements in fill factor and open circuit voltage.     

Synthesis of nanostructured M/Fe3O4 (M = Ag,Cu) composites using hexamethylentetramine and their electrocatalytic properties

Nanoscaled Ag/Fe₃O₄ hybrids with different Ag contents and Cu/Fe₃O₄ nanoshpere and microsphere were successfully synthesized with assistance of sodium citrate and (CH₂)₆N₄ via a hydrothermal process. The as-prepared samples were identified and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), and X-ray photoelectron spectroscopy (XPS), respectively. All samples were used as electrocatalysts modified on a glassy carbon electrode for p-nitrophenol reduction in a basic solution. The catalytic activity of Ag/Fe₃O₄ samples increased first and then decreased by increasing Ag content from 0% to 8%, and the one with 6% Ag displayed the highest catalytic activity. All the Cu/Fe₃O₄ samples exhibited enhanced catalytic activity by comparison with a glassy carbon electrode, and the one prepared with the molar ratio of Cu²⁺, Fe³⁺, citrate anion, and (CH₂)₆N₄ with 1:1:3:5 exhibited the highest catalytic activity.     

Synthesis of GaN nanorods by ammoniating Ga2O3 films on In2O3 layer deposited on Si (111) substrates

GaN nanorods were synthesized by ammoniating Ga₂O₃/In₂O₃ thin films deposited on Si (111) with magnetron sputtering. X-ray diffraction, Scanning electronic microscope and high-resolution TEM results show that they are GaN single crystals, the sizes of which vary from 2 to 7 μm in length and 200 to 300 nm in diameter. In₂O₃ middle layer plays an important role in the GaN nanorod growth.     

Synthesis and characterization of novel nanostructured fishbone-like Cu(OH)2 and CuO from Cu4SO4(OH)6

The Cu₄SO₄(OH)₆ was synthesized by a simple hydrothermal reaction with a yield of ~ 90%. Using Cu₄SO₄(OH)₆ as the starting material, novel fishbone-like Cu(OH)₂ was produced by a direct reaction of Cu₄SO₄(OH)₆ with NaOH solution. The Cu(OH)₂ consists of many needle-like nanorods parallel to each other and perpendicular to the direction of backbone, forming fishbone-like structure. Using the fishbone-like Cu(OH)₂ as the sacrificial precursor, CuO with similar size and morphology was obtained through a simple heat treatment. X-ray diffraction, scanning electron microscopy, energy dispersive X-ray, X-ray photoelectron spectroscopy, BET nitrogen adsorption, and UV-Vis absorption spectroscopy were employed to characterize the as-prepared samples. The conversion of the Cu₄SO₄(OH)₆ to the fishbone-like Cu(OH)₂ was visualized by time-dependent SEM images. A mechanism was also proposed based on the observed results.     

The crystallisation of Cu2ZnSnS4 thin film solar cell absorbers from co-electroplated Cu-Zn-Sn precursors

The best CZTS solar cell so far was produced by co-sputtering continued with vapour phase sulfurization method. Efficiencies of up to 5.74% were reached by Katagiri et al. The one step electrochemical deposition of copper, zinc, tin and subsequent sulfurization is an alternative fabrication technique for the production of Cu₂ZnSnS₄ based thin film solar cells. A kesterite based solar cell (size 0.5 cm²) with a conversion efficiency of 3.4% (AM1.5) was produced by vapour phase sulfurization of co-electroplated Cu-Zn-Sn films. We report on results of in-situ X-ray diffraction (XRD) experiments during crystallisation of kesterite thin films from electrochemically co-deposited metal films. The kesterite crystallisation is completed by the solid state reaction of Cu₂SnS₃ and ZnS. The measurements show two different reaction paths depending on the metal ratios in the as deposited films. In copper-rich metal films Cu₃Sn and CuZn were found after electrodeposition. In copper-poor or near stoichiometric precursors additional Cu₆Sn₅ and Sn phases were detected. The formation mechanism of Cu₂SnS₃ involves the binary sulphides Cu_2 − xS and SnS₂ in the absence of the binary precursor phase Cu₆Sn₅. The presence of Cu₆Sn₅ leads to a preferred formation of Cu₂SnS₃ via the reaction educts Cu_2 − xS and SnS₂ in the presence of a SnS₂(Cu₄SnS₆) melt. The melt phase may be advantageous in crystallising the kesterite, leading to enhanced grain growth in the presence of a liquid phase.     

Synthesis, structure and exchange bias in Cr2O3/CrO2/Cr2O5 particles

We report on the synthesis, structure and magnetic properties of a novel exchange bias system with Cr₂O₃/CrO₂/Cr₂O₅ interfaces. Chromium oxide particles with mixed chromium valences were prepared by sintering CrO₃ in air. X-ray diffraction patterns show that CrO₃ lost its oxygen gradually with increasing temperature and time through Cr₃O₈, Cr₂O₅, CrO₂, and finally Cr₂O₃ at temperatures above 760 K. X-ray photoelectron spectra indicate a low CrO₂ content and a binding energy of 579.3 eV for Cr 2p_3/2 photoelectrons in Cr₂O₅. Chromium dioxide was found to stably coexist with Cr₂O₃ and Cr₂O₅ in the particles. Magnetic measurements show hysteresis loop shifts in the sample, indicating an exchange bias induced by antiferromagnetic Cr₂O₃/Cr₂O₅ in ferromagnetic CrO₂. An exchange bias of 9 mT at 5 K and a coercivity of 26.3 mT were observed in the chromium oxide particles containing CrO₂.     

Preparation of Cu2ZnSnS4 thin films by sulfurization of stacked metallic layers

Stacked precursors of Cu, Sn, and Zn were fabricated on glass/Mo substrates by electron beam evaporation. Six kinds of precursors with different stacking sequences were prepared by sequential evaporation of Cu, Sn, and Zn with substrate heating. The precursors were sulfurized at temperatures of 560 °C for 2 h in an atmosphere of N₂ + sulfur vapor to fabricate Cu₂ZnSnS₄ (CZTS) thin films for solar cells. The sulfurized films exhibited X-ray diffraction peaks attributable to CZTS. Solar cells using CZTS thin films prepared from six kinds of precursors were fabricated. As a result, the solar cell using a CZTS thin film produced by sulfurization of the Mo/Zn/Cu/Sn precursor exhibited an open-circuit voltage of 478 mV, a short-circuit current of 9.78 mA/cm², a fill factor of 0.38, and a conversion efficiency of 1.79%.     

Photoluminescence properties of the In2O3 octahedrons synthesized by carbothermal reduction method

In₂O₃ octahedrons were synthesized by carbothermal reduction method. The products were characterized by X-ray diffraction (XRD), energy dispersive X-ray (EDX), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction analysis (SAED) and room-temperature photoluminescence (PL) spectra. The results show that the products are single-crystalline In₂O₃ octahedrons with the arrises length in the range of 400-3000 nm. The PL spectra displays blue and green emission peaks which can be indexed to default and oxygen vacancies; blue-shift and intensity decrease was observed when excitation wavelength decreases from 380 nm to 325 nm. The growth mechanism of the In₂O₃ octahedrons is discussed.