Se Phase Research Articles

Ex-situ and in-situ analyses on reaction mechanism of CuInSe 2 (CIS) formed by selenization of sputter deposited CuIn precursor with Se vapor

Formation mechanism of CIS thin films by selenization of sputter deposited CuIn precursor with Se vapor was investigated by ex-situ and in-situ phase analysis tools. Precursor films were composed of In, CuIn and Cu 2In compounds, and their relative fractions were systematically changed with Cu/In ratios. Those films were found to have a double layered structure with nearly pure In particles (top layer) placed on the flat Cu-rich bottom layer, and the morphologies were also significantly affected by Cu/In ratio. At the initial stage of selenization, the outer In-rich layer reacted with Se vapor to form In–Se binary, which is the first selenide phase appeared, and inner Cu-rich phases acted as a Cu source to supply Cu to outer In–Se phase to form ordered vacancy compounds (OVC). As these reactions continues, in conjunction with Se incorporation into inner Cu-rich region, the films gradually changes from OVC to α-CIS, reflecting that the formation route of CIS is closely related to the elemental and phase distribution in precursor films. Selenized CIS films were further processed to fabricate CIS thin film solar cells, resulting in the best cell efficiency of 10.44%.

Facile, one-step controlled synthesis of Se nanocrystals in the presence of l-tyrosine

Abstract Se with different morphologies was synthesized using l -tyrosine as reducing agent and soft template by means of hydrothermal method. The method was simple and convenient to handle. The reaction process was monitored using ultraviolet–visible spectroscopy (UV–vis) and Fourier-transform infrared spectroscopy (FTIR). The morphology and crystalline phase were determined by transmission electron microscopy (TEM), scanning electronic microscopy (SEM) and X-ray diffraction (XRD) pattern. The results show that the morphologies and the crystalline phases of Se can be easily controlled. By varying the concentration ratio of l -tyrosine to selenious acid, the morphologies and crystalline phases of Se were not changed, but the diameter of Se was different. Se nano-rods are obtained in the lower temperature, and there is a transformation of microspheres into nanorods of Se with the increase of reaction time or reaction temperature. In addition, we discuss the possible mechanism of the reduction of SeO 3 2− ions by l -tyrosine. The eco-friendly, biogenic synthesis strategy could be widely used for preparing inorganic/organic biocomposites.

Effects of Cu/In ratio of electrodeposited precursor on post-sulfurization process in fabricating quaternary CuIn(S,Se) 2 thin films

This paper reports the analysis of S diffusion into electrodeposited CuInSe 2 (CISe) precursors during post-sulfurization treatment at 500 °C in an Ar/H 2S ambient. The characterizations of the sulfurized films by X-ray diffraction, grazing-incidence X-ray diffraction, Auger electron spectroscopy and micro-Raman spectroscopy allow the observation of the strong dependence of S incorporation into these films on the Cu/In ratio of the precursor. AES profiles reveal higher S content along the depth of Cu-rich film than Cu-poor film after sulfurization. Raman Scattering shows that copper sulfoselenides Cu–(Se,S) are only detected in Cu-rich samples. The re-crystallization of films during sulfurization was analyzed and it is presumed that quasi-liquid Cu–Se phases, which are related to Cu/In ratio of precursor, promote continuous incorporation of S into these films.

Synthesis and characterization of nickel selenide nanoparticles: size and shape determining parameters

Nickel selenide nanoparticles of different sizes and shapes were synthesized using a modified solvothermal method. The effects of the reaction time, pH and solvent on the properties of nickel selenide were also explored. The particles synthesized in 1 h yielded mono-dispersed spheres with a Ni 0.95Se phase, whilst the particles synthesized in 3 h resulted in a mixture of spherical and fused cylindrical nanoparticles with a single Ni 0.95Se phase. Further increments of time (6 and 24 h) resulted in mixtures of particle sizes and shapes and both the samples displayed a Ni 3Se 4 phase. The pH had effected very little changes on the properties of the nanoparticles though at highly acidic pH the particles evolved from spherical shaped Ni 0.95Se to cubic shaped Ni 3Se 4. The effect of the solvent was also investigated. The water synthesis resulted in mono-dispersed nanospheres with a Ni 0.95Se crystal phase whilst the ethylenediamine synthesized particles resulted in very small particles engulfed in an ethylenediamine layer with a Ni 6Se 5 crystal phase. The particles synthesized in pyridine on the other hand resulted in poly-dispersed Ni 5Se 5 nanoparticles with spherical and cubic morphologies.

Increasing surface band gap of Cu(In,Ga)Se_2 thin films by post depositing an In-Ga-Se thin layer

We have developed a simple approach to fabricate wide band gap surface layer for Cu(In,Ga)Se2 (CIGS) thin film. The Cu depleted surface layer was reconstructed by an In-Ga-Se post deposition treatment at different temperatures, which was monitored by a light controlling method. A desirable Cu concentration in surface layer has been achieved after depositing a 80 nm thick In-Ga-Se layer at 400°C and the corresponding device performance is remarkably improved compared with device without surface modification. Additionally, the excess Cu(2-x)Se phase on the surface could also be eliminated by this method in case of high Cu/(In+Ga).

Real-Time Raman Scattering Analysis of the Electrochemical Growth of CuInSe2 Precursors for CuIn(S,Se)2 Solar Cells

This work reports the in-situ analysis of the electrochemical growth of CuInSe2 precursors for the production of low cost CuIn(S,Se)2 based solar cells by Raman scattering measurements performed at real time conditions. The measured data point out the existence of three stages in the growth of the layers: (a) initial stage characterised by the preferential growth of elemental Se and Cu rich phases, (b) intermediate stage where the preferential growth of the main CuInSe2 phase takes place, and (c) final stage with a gradual increase in the spectral contributions from Se and CuSe secondary phases. This correlates with the spectral evolution of Cu poor ordered vacancy compound phases present in the layers, in spite of the overall Cu excess conditions typically used in these processes. These in-situ measurements allow elucidating the main growth mechanisms involved in the electrochemical synthesis of CuInSe2. Existence of different growth stages during the process determines a non-homogeneous in-depth distribution of the secondary phases in the deposited layers that are determining for the final efficiency of the devices.

Thermally induced effects on structural and electrical properties of selenium-rich Cd-Se thin films

The effect of annealing in nitrogen atmosphere on structural and electrical properties of selenium rich CdSe (SR-CdSe) thin films deposited by thermal evaporation onto glass substrates were studied. X-ray diffraction (XRD) patterns showed that the as-prepared films were amorphous, whereas the annealed films were polycrystalline. Analyzing XRD patterns reveals the coexistence of both Se and CdSe crystalline phases which exhibits a hexagonal structure. The microstructure parameters (crystallite size, microstrain and dislocation density) were calculated for annealed films. Temperature dependence (300–500 K) of d.c. conductivity was studied for as-prepared and annealed thin films. The experimental results indicate that the electrical conduction taking place through thermally activated process. At higher temperatures, electrical conduction for as-prepared film is taking place in the extended states while localized states conduction in the band tails is most likely to take place for annealed films. Regarding the lower temperature range, conduction by hopping in the localized states near the Fermi level is found to be dominant. Thus, conductivity data in this range was analyzed using Mott's variable range hopping conduction, where Mott's parameters were calculated for SR-CdSe thin films.

Enhanced electrical properties of pulsed laser-deposited CuIn 0.7Ga 0.3Se 2 thin films via processing control

Polycrystalline CuIn 0.7Ga 0.3Se 2 thin films were prepared on soda-lime glass substrates using pulsed laser deposition (PLD) with various process parameters such as laser energy, repetition rate and substrate temperature. It was confirmed that there existed a limited laser energy, i.e. less than 300 mJ, to get phase pure CIGS thin films at room temperature. Particularly, even at room temperature, distinct crystalline CIGS phase was observed in the films. Crystallinity of the films improved with increasing substrate temperature as evidenced by the decrease of FWHM from 0.65° to 0.54°. Slightly Cu-rich surface with Cu 2− x Se phase was confirmed to exist by Raman spectra, depending on substrate temperature. Improved electrical properties, i.e., carrier concentration of ∼10 18 cm −3 and resistivity of 10 −1 Ω cm at higher substrate temperature for the optimal CIGS films are assumed to be induced by the potential contributions from highly crystallized thin films, existence of Cu 2− x Se phase and diffusion of Na from substrates to films.

Effect of bath temperature on the properties of CuInxGa1−x Se2 thin films grown by the electrodeposition technique

Electrodeposition is a promising and low cost method to synthesize CuInx Ga1−x Se2 (CIGS)thin films as an absorber layer for solar cells. The effect of bath temperature on the properties of CIGS thin films was investigated in this paper. CIGS films of 1 μm thickness were electrodeposited potentiostatically from aqueous solution, containing trisodium citrate as a complexing agent, on Mo/glass substrate under a voltage of −0.75 V, and bath temperatures were varied from 20 to 60°C. The effects of bath temperature on the properties of CIGS thin films were characterized by X-ray diffraction (XRD) and scanning electron microscopy. XRD revealed the presence of the CuIni0.7Ga0.3 Se2 phase, the optimal phase for application in solar cells. The grain dimensions and crystallizability increase along with the increase of the bath temperature, and the films become stacked and homogeneous. There were few changes in surface morphology and the composition of the films.

Growth of CuInSe<SUB>2</SUB> Thin Film Using Non-Vacuum Solution Deposition Technique

Polycrystalline thin films of CuInSe 2 were prepared successfully using the chemical spray pyrolysis technique as a function of Cu/In atomic ratios and variation of selenium ratios. For the compositional and structural analysis, X-ray diffraction, UV-visible spectroscopy, the Hall effect measurement and scanning electron microscopy were carried out. The effects of the Cu/In ratio and selenium concentration were observed in the XRD, SEM, UV-visible spectrum and the Hall effect measurement analysis for fabricated thin films. The result of XRD and UV-visible analysis showed an increase or decrease of crystallinity and shift of transmittance tendency depending on the copper and selenium content. The grain size of thin films was increased with the increase of copper content. In the thin films, the variation of Cu/In ratios between 0.8 to 1 showed an increase on hole density, but a decrease of Hall mobility and resistivity. We suggest the formation of Cu x Se phase particles or layers depends on the spray solution. Based on these results, we will fabricate and characterize CISe/CIGSe thin film solar cells in further work.

Enhancement in as-grown CuInSe 2 film microstructure by a three potential pulsed electrodeposition method

P-type copper indium diselenide (CuInSe 2) films have been prepared onto ITO substrates by an electrodeposition method, that sequentially applies potential pulses at the deposition potential of each element Cu, Se and In, and then step it back in cyclically to induce the solid state reaction between the elements. Two electrolyte concentrations as well as three different pulse durations were assessed. The resulting films were compared with those deposited at fixed electrode potentials. As-grown films are nanocrystalline and have an E g ∼ 0.95 eV. Raman spectroscopy shows that Se and Cu–Se contents decrease while pulse duration increases and electrolyte concentration decreases. Cu–Se phases are even absent for films grown at the low electrolyte concentration. These results represent a great improvement in the film phase purity reducing the need of post-deposition treatments.

Electrocrystallization Mechanism of Cu–In–Se Compounds for Solar Cell Applications

Chalcogenide semiconductors, such as , are promising materials for optoelectronic applications such as solar cells. Conversion efficiencies up to 14% based on Cu–In–Se layers electrodeposited have been achieved. However, the electrocrystallization of this multinary system, whose components exhibit different electrochemical and chemical properties, is a complex process. In the present work, the electrocrystallization mechanism of is investigated in dilute acidic solutions by in situ electrochemical investigation (polarization, electrochemical quartz crystal microbalance, and impedance spectroscopy) and ex situ chemical and structural analyses. A detailed X-ray photoelectron spectroscopy analysis is carried out to determine the surface composition and the chemical environment of the elements. Marked differences between surface and bulk concentrations are evidenced. At low polarizations, binary Cu–Se phases are obtained, which facilitate the reduction of Se(IV) species into elemental selenium. An intermediate phase is formed which, in turn, enables the incorporation of indium beyond a potential threshold close to −0.6 V/mercurous sulfate electrode. In this potential zone, major transitions are observed in composition, morphology, structure, and electrochemical behavior. A reaction path is proposed, which accounts for the main experimental observations. It emphasizes the role of surface reactions, occurring via passivating and nucleation sites, in the incorporation of indium.

Selenium Biogeochemical Cycling and Fluxes in the Hyporheic Zone of a Mining-Impacted Stream

The influence of hyporheic exchange on selenium (Se) biogeochemistry and mobility in sediments is unknown. A multiscale investigation of Se biogeochemistry in the hyporheic zone of East Mill Creek (EMC), southeastern Idaho, USA, was performed using in situ surface water and pore water geochemical measurements, a field-based stream tracer test, and energy-dependent micro synchrotron X-ray fluorescence (mu-SXRF) measurements of Se speciation in sediments. The active hyporheic zone was determined to be 12 +/- 3 cm. Pore water redox profiles indicated that a transition to suboxic conditions begins at approximately 6 cm. Modeling pore water Se and solid phase analysis suggested Se uptake is occurring. Micro-SXRF analysis of sediments showed reduced elemental Se or selenides throughout the profile and selenite in surface sediments. Field geochemical measurements and microscale analysis both support the hypothesis that reduction in the hyporheic zone promotes sequestration of surface water Se.

FexPb4−xSb4Se10: A New Class of Ferromagnetic Semiconductors with Quasi 1D {Fe2Se10} Ladders

A new family of quasi-one-dimensional ferromagnetic selenides with general formula Fe(x)Pb(4-x)Sb(4)Se(10) (0 < or = x < or = 2) was generated by isoelectronic substitution in octahedral positions of Pb atoms by Fe within the structure of Pb(4)Sb(4)Se(10). Two members of this family with x = 0.75 and x = 1 were synthesized as a single phase through direct combination of the elements at 823 K. Single crystal X-ray diffraction revealed that Fe(0.75)Pb(3.25)Sb(4)Se(10) crystallizes with the orthorhombic space group Pnma, whereas Fe(0.96)Pb(3.04)Sb(4)Se(10) adopts the lower symmetry monoclinic subgroup P2(1)/m (#11). Both compounds are isomorphous with Pb(4)Sb(4)Se(10), and their crystal structures consist of corrugated layers of edge-sharing bicapped trigonal prisms and octahedra around Pb atoms. Adjacent layers are interconnected by NaCl-type {SbSe} ribbons. The voids left by this arrangement are filled by the novel one-dimensional {Fe(2)Se(10)} double chains (ladder) of edge-sharing octahedra running along [010]. Temperature dependent magnetic susceptibility as well as field dependent magnetization isotherms showed that both Fe(0.75)Pb(3.25)Sb(4)Se(10) and FePb(3)Sb(4)Se(10) are ferromagnetic below 300 K and exhibit superparamagnetism at higher temperatures. A dramatic reduction in the magnetic moment per Fe(2+), approximately 0.40 micro(B), was observed in Fe(0.75)Pb(3.25)Sb(4)Se(10) and FePb(3)Sb(4)Se(10) suggesting that the Fe(x)Pb(4-x)Sb(4)Se(10) (0 < or = x < or = 2) phases are not ordinary ferromagnets where all the magnetic spins are parallel at low temperatures. Analysis of the magnetic coupling of spins located on adjacent Fe atoms (within a localized Fe(2+) moment picture) using Goodenough-Kanamori rules suggested that the magnetism within the ladder and ladder-single chain systems in Fe(x)Pb(4-x)Sb(4)Se(10) phases is controlled by competing interactions.

Non‐magnetic Kondo effect in M–As–Se (M = Zr, Hf, Th) phases

AbstractThermodynamic and transport properties of the tetragonal M–As–Se (M = Zr, Hf, Th) phases demonstrate that these materials are disordered metals with the electronic‐specific‐heat coefficient as small as, e.g., 0.3 mJ K−2 mol−1 for Th–As–Se. Remarkably, all the M–As–Se systems display a magnetic‐field‐independent AT1/2 term in the low‐temperature electrical resistivity. As observed for ZrAs1.595Se0.393, the magnitude of this extra term may vary by a factor of nearly 3 as well as the resistivity minimum is shifted from 10.8 to about 15.0 K, dispite virtually the same electron‐diffusion constant. On the other hand, a magnetic‐field (B ≤ 14 T) dependence of the electron transport in arsenide selenides is strongly material dependent: both the positive and negative sign of the magnetoresistivity being proportional to either B2 or B1/2 were found. These observations point to a non‐magnetic Kondo effect due to a random distribution of structural two‐level systems in the disordered As/Se sublattice.

Cathodic electrodeposition of mixed molybdenum–selenium oxides

Molybdenum–selenium oxides were electrochemically deposited onto indium–tin oxide (ITO) coated glass substrates from aqueous solutions containing molybdate ( Mo VI O 4 2 - ), selenate ( Se IV O 3 2 - ), and dimeric and tetrameric peroxo-polymolybdate (i.e., [Mo 2O 3(O 2) 4(H 2O) 2] 2−, [Mo 4O 9(O 2) 4] 2−) anions. Electrodeposition mechanisms were elucidated using chronocoulometry, cyclic voltammetry, spectroelectrochemistry, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and electrochemical quartz crystal nanogravimetry (EQCN). At relatively positive deposition potentials from −0.1 V to −0.4 V (versus Ag/AgCl) a substoichiometric molybdenum oxide phase, Mo 3O 8, co-deposits with an insulating phase of Se 0. At more negative potentials, mixed molybdenum–selenium oxides (Mo x Se 1− x O y , 0 < x < 0.4) are deposited. Competing side reactions involving hydrogen and hydrogen selenide influence the structure, composition, and morphology of the mixed molybdenum–selenium oxide deposits.

Preparation and properties of CuInxGa1−xSe2 thin-film solar cell absorbers from selenization of Ga-rich electrodeposited precursors

This paper presents our studies on the preparation and properties of CuInxGa1−xSe2 (CIGS) thin-film solar cell absorbers grown from Ga-rich electrodeposited precursors followed by their selenization. X-ray fluorescence and scanning electron microscopy results show that the deposition potential of −0.7 V is suitable for obtaining a smooth CIGS precursor film with inclusion of more gallium (x = 0.4). X-ray diffraction studies reveal that a stoichiometric CuIn0.7Ga0.3Se2 phase that can be formed by selenization of the Ga-rich precursor is ascribed to a high Se vapor pressure. An optical study shows that the band-gap energy of the stoichiometric CIGS sample is 1.17 eV. The best photovoltaic devices showed a conversion efficiency of 2.0 ± 0.5%.

The influence of alloy phases in the precursors on the selenization reaction mechanisms

As precursors are deposited in different sequences, the alloy phases of Cu–In or Cu–Ga are observed. This leads to different selenized results, which consist of either the single quaternary CIGS or the phase-separated CIS and CIGS mixture. According to the analysis of experimental results, it can be concluded that the alloy phases in the precursors have a significant impact on the reaction mechanism and path in the selenization process. It is the Cu–Ga alloy phase in the precursors that suppresses the rapid transformation of Cu–In to CuInSe2. Hence at 450 °C the Cu–Ga and Cu–In can react together with Se to form CIGS. With higher temperature annealing at 560 °C, the single quaternary CuIn0.7Ga0.3Se2 phase can be obtained.

Structural characterization and phase transformation kinetics of Se58Ge42−xPbx (x=9, 12) chalcogenide glasses

The glass transition behavior and crystallization kinetics of Se58Ge42−xPbx (x = 9, 12) have been investigated using Differential Scanning Calorimetry (DSC) at five different heating rates under non-isothermal conditions. It has been observed that these glassy systems exhibit single glass transition and double crystallization on heating. The XRD pattern revealed that the considered glasses get crystallized into GeSe2 and PbSe/Se phases after annealing at 633–643 K for 2 h. The GeSe2 and Se phases were found to crystallize in monoclinic structure while, PbSe phase crystallizes in cubic structure. Besides this, a mixed phase was also observed in DSC thermograms after annealing. The kinetic studies include determination of various parameters such as Avrami exponent (n), frequency factor (Ko), dimensionality of growth (m), the activation energy for glass transition (Et) and for crystallization (Ec). The values of Et increases while that of Ec decreases after annealing. Also, dimensionality of growth decreases to one dimension from two and three dimensions after annealing.

Raman scattering and structural analysis of electrodeposited CuInSe2 and S‐rich quaternary CuIn(S,Se)2 semiconductors for solar cells

AbstractThis work reports the Raman scattering characterisation of CuInSe2 precursors grown by single step electrodeposition and the corresponding layers recrystallised under sulphurising conditions for solar cell devices. The analysis of the spectra measured on the as‐grown precursors has allowed identifying the main secondary phases in these layers with elemental Se, Cu–Se phases and chalcopyrite Cu‐poor ordered vacancy domains. To deepen in the identification of the Cu–Se phases, these measurements have been correlated with the analysis of binary Cu–Se layers. The experimental data indicate that formation of both Se and Cu–Se phases is likely controlled by the Se content in the layers. For values of stoichiometry below 1.15, excess Cu in the layers is accommodated in a phase with very low Raman efficiency (as Cu2Se). Increasing the content of Se leads to an increase in the spectral contribution from both Se and Cu2–x Se, being the formation of these phases likely favoured under high excess Se conditions. The characterisation of the corresponding recrystallised layers has allowed analysing the impact of the presence of the secondary phases in the as‐grown absorbers on the performance of the final solar cells. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)