xSe Thin Films Research Articles

Synthesis and characterization of temperature-dependent nanospherical Cu2−xSe thin films for photoelectrochemical cell application

Synthesis and characterization of temperature-dependent nanospherical Cu2−xSe thin films for photoelectrochemical cell application

The growth of self-intercalated Nb1+xSe2 by molecular beam epitaxy: The effect of processing conditions on the structure and electrical resistivity

We report on the synthesis of self-intercalated Nb1+xSe2 thin films by molecular beam epitaxy. Nb1+xSe2 is a metal-rich phase of NbSe2 where additional Nb atoms populate the van der Waals gap. The grown thin films are studied as a function of the Se to Nb beam equivalence pressure ratio (BEPR). X-ray photoelectron spectroscopy and x-ray diffraction indicate that BEPRs of 5:1 and greater result in the growth of the Nb1+xSe2 phase and that the amount of intercalation is inversely proportional to the Se to Nb BEPR. Electrical resistivity measurements also show an inverse relationship between BEPR and resistivity in the grown Nb1+xSe2 thin films. A second Nb-Se compound with a stoichiometry of ∼1:1 was synthesized using a Se to Nb BEPR of 2:1; in contrast to the Nb1+xSe2 thin films, this compound did not show evidence of a layered structure.

Rapid growth of Cu2−xSe thin films on brass with mixed acid solutions stabilized by using sodium silicate as a sealant

Composite Cu2−xSe thin films can be rapidly prepared on brass by liquid chemical deposition in 1 min. With the addition of acetic acid in the oxidizing solution containing SeO2/H2SO4/ H2O2, the sizes of the selenide particles become smaller and thus much smoother films are obtained, while the films become thinner and denser as seen from their scanning electron microscopy and EDS results. X-ray diffraction and XPS results reveal that copper acetate is spontaneously formed with the acetate acid addition, and the existence of copper acetate hinders the growth of copper selenides, leading to the decrease in film thickness; during their mutual deposition, copper acetate probably fills the spaces among selenide particles, and thus improves the compactness and decrease the roughness of the composite film. The miniaturization of the selenide particles with the acetate acid addition results in a great improvement in both the hardness and wear resistance of the composite films. When the composite films are sealed by sodium silicate, a successive interlayer is formed on their outer surfaces that result in the further improvement in their corrosive resistance as reflected by the electrochemical test and thermal humidity test.

High-performance thermoelectric silver selenide thin films cation exchanged from a copper selenide template.

Over the past decade, Ag2Se has attracted increasing attention due to its potentially excellent thermoelectric (TE) performance as an n-type semiconductor. It has been considered a promising alternative to Bi–Te alloys and other commonly used yet toxic and/or expensive TE materials. To optimize the TE performance of Ag2Se, recent research has focused on fabricating nanosized Ag2Se. However, synthesizing Ag2Se nanoparticles involves energy-intensive and time-consuming techniques with poor yield of final product. In this work, we report a low-cost, solution-processed approach that enables the formation of Ag2Se thin films from Cu2−xSe template films via cation exchange at room temperature. Our simple two-step method involves fabricating Cu2−xSe thin films by the thiol-amine dissolution of bulk Cu2Se, followed by soaking Cu2−xSe films in AgNO3 solution and annealing to form Ag2Se. We report an average power factor (PF) of 617 ± 82 μW m−1 K−2 and a corresponding ZT value of 0.35 at room temperature. We obtained a maximum PF of 825 μW m−1 K−2 and a ZT value of 0.46 at room temperature for our best-performing Ag2Se thin-film after soaking for 5 minutes. These high PFs have been achieved via full solution processing without hot-pressing.

Structural and photoluminescent properties of CuGaxIn1−xSe2 thin films prepared by close-spaced vapor transport technique

CuGaxIn1−xSe2 (CIGS) (x = 0, x = 0.3 and x = 1) absorber layers were deposited onto transparent conducting oxide (SnO2) substrates using close-spaced vapor transport technique (CSVT). The aim of this work was to assess the quality of CIGS absorber films with various gallium contents. Composition found by Energy Dispersive Spectroscopy (EDS) showed that the investigated samples were Cu-poor and CuGa0.3In0.7Se2 thin film had a composition close to 0.3 as planned. The structural properties were investigated by X-ray diffraction analysis (XRD) and Raman spectroscopy, while optical properties were investigated by measuring transmittance and photoluminescence spectroscopy. It was found that all investigated samples crystallize with (112) as a preferential crystallographic direction. The shift of the XRD peaks towards a higher value of 2θ with the increase of Ga composition was observed. The grain size determined from the width of 112 XRD peak was within 40–75 nm range with no clear correlation with Ga content. Raman spectra of samples with x = 0 and 0.3 featured mainly a peak assigned to the A1 mode at about 175 cm−1. In CuGaSe2 the secondary Cu2Se phase was detected both by XRD and Raman. The band gap as evaluated by optical transmission measurement is shifted towards lower than expected values due to substantial defect-related bands broadening. A large concentration of defects in the samples was also confirmed by low or absent PL signal in the vicinity of the bandgap, while the emission due to deep defects was prevailing in the spectrum. The optoelectronic properties as indicated by photoluminescence have to be improved if CSVT method of absorber preparation is to be used more widely in photovoltaics.

Sn1−xSe thin films with low thermal conductivity: role of stoichiometric deviation in thermal transport

Tin selenide (Sn1−xSe) polycrystalline thin films were prepared by thermal co-evaporation, and the thermoelectric properties of the Sn1−xSe thin films were investigated.

Synthesizing three-dimensional ordered macroporous CuInxGa1−xSe2 thin films by template-assisted electrodeposition from modified ionic liquid

Three-dimensional ordered macroporous (3DOM) CuInxGa1−xSe2 (CIGS) film is synthesized by one-step galvanostatic electrodeposition based on polystyrene templates from a special mixture of ionic liquid 1-butyl-3-methylimidazolium tetra-fluoroborate and ethanol. X-ray diffraction pattern, Raman spectroscopy and transmission electron microscopy present the CIGS film has ideal chalcopyrite crystallization. 3DOM CIGS film with high Ga content has photochemical activity and good photo-stability, its band gap is 1.34eV. This is a simple, feasible and additive free method without selenization process to synthesize 3DOM CIGS film, the film is qualified and available for absorber-layer fabrication or other fields.

Development, characterization, and study of new (Cd,Zn)Se-based hybrid semiconductive systems

This work describes the synthesis and study of two new hybrid (organic–inorganic) three-layered systems with improved semiconductive behavior. As far as the first system is concerned, CdSe-Fc-Zn x Cd1−x Se, the inner layer is CdSe, while the outer one is Zn x Cd1−x Se, both prepared by electrodeposition. The partial substitution of Cd by Zn in the outer layer leads to an environmentally friendlier product compared with those containing exclusively Cd. The organic layer of the hybrid system is ferrocene (Fc), which is enveloped between the two inorganic layers in a sandwich-like structure. Ferrocene is deposited on CdSe layer using the spin-coating technique. It was found that the as-prepared CdSe-Fc-Zn x Cd1−x Se system has improved properties and photoconductivity compared with the Zn x Cd1−x Se monolayers. In order to further enhance the photoresponse of the system described above, a second three-layer hybrid was developed, the CdSe-Fc-Zn x Cd1−x Se/oxalate. This system was produced with the electrodeposition of the outer layer Zn x Cd1−x Se in the presence of disodium oxalate, a selected organic additive in the bath. All the products were fully characterized via XRD, SEM–EDAX, band gap, and photoelectrochemical cell (PEC) measurements, and the results confirmed the development of the new hybrid semiconductive systems. The as-described CdSe-Fc-Zn x Cd1−x Se and CdSe-Fc-Zn x Cd1−x Se/oxalate systems, compared to the pure inorganic CdSe and Zn x Cd1−x Se thin films, present remarkably improved photoresponse due to the synergetic action of their components.

Characterization of non-vacuum CuInxGa1 − xSe2 thin films prepared by low-cost sequential electrodeposition technique

Copper indium gallium diselenide (CuInxGa1−xSe2) thin films were grown on indium tin oxide coated glass substrate by a sequential electrodeposition technique. The deposition bath consisted of an aqueous solution of 10mM CuCl2, 10mM InCl3, 20mM GaCl3, 20mM H2SeO3 as precursors, and 200mM LiCl. The pH of the solution was adjusted to 1.7 by adding HCl. Cu, In, Cu, Ga, Cu, Se components were deposited sequentially. Cu/In/Cu/Ga/Cu/Se stacked layers were annealed at 250, 350, 450 and 550°C for 30 min. X-ray diffraction (XRD) studies showed the produced CIGS thin films had polycrystalline structure. From scanning electron microscopy studies, it was found that CIGS thin films exhibited different surface appearances depending on annealing temperature. Energy dispersive x-rays analysis showed that elemental ratio of Se decreased from 46.99 to 14.84 (%) with increasing of annealing temperature. Optical band gap of the CIGS films varied between 1.41 and 2.19 eV. Thicknesses and refractive indices of the produced CIGS thin films were calculated by fitting spectroscopic ellipsometric data (ψ and ∆) by using Cauchy model. Deposited CIGS thin films were p-type semiconductor with carrier concentration of ~1016 cm−3.

Toward low-cost large-area CIGS thin film III: Effect of Se concentration on crystal growth and defect formation of sequentially electrodeposited CIGS thin films

Nowadays, large area CuInxGa1−xSe2 (CIGS) thin film solar cells still face difficulties and challenges of the uniformity of composition and structure. This study illustrates how Se concentration affects the crystal growth and the defect formation of CIGS from low cost electrodeposited stacked Cu/In/Cu/Ga/Cu layers selenized in the rapid thermal process. Various crystal growth modes were observed in the film selenized with different Se concentrations. It was found that the crystal growth was strongly dependent on the Se concentration in the early stage of selenization between 250 and 350°C, which can lead to variations of film composition and structure including a phase separation of CuGaSe2 from CuInSe2 and formation of defect holes in a CIGS film at a higher selenization temperature.

Toward low-cost large-area CIGS thin film II: Out-of-plane compositional variations of sequentially electrodeposited Cu/In/Cu/Ga/Cu stacked layers selenized in rapid thermal process

Electrodeposition is a promising method to realize large-scale and low-cost CuInxGa1−xSe2 thin film solar cells. In this study, we investigate the out-of-plane compositional and structural variations of electrodeposited Cu/In/Cu/Ga/Cu stacked layers selenized at different temperatures in the rapid thermal process by employing cross-sectional scanning electron microscopy, grazing incidence X-ray diffraction, inductively coupled plasma atomic emission spectroscopy, confocal Raman spectroscopy, and Auger spectroscopy. A growth model has been suggested to describe the material structure and the chemical composition of the electrodeposited CIGS thin film during the selenization process on the basis of the experimental results.

Galvanic synthesis of Cu2−XSe thin films and their photocatalytic and thermoelectric properties

Cu2−XSe thin film with cubic berzelianite phase was prepared by a simple, low-cost two electrode electrochemical technique and the photocatalytic and thermoelectric properties of the thin films were investigated. The results showed that Cu2−XSe crystallized in the cubic berzelianite phase and found to possess both direct and indirect band gaps of 2.9 and 1.05eV respectively, covering almost the entire range of solar-spectrum. The photocatalytic discoloration of aqueous methylene blue (MB) and rose-bengal (RB) dyes over Cu2−XSe thin films were investigated under visible light irradiation. Cu2−XSe thin films showed higher catalytic activity for MB compared to RB in presence of H2O2. The photocatalytic discoloration followed first-order reaction kinetics. Complete removal of aqueous MB was realized after visible light irradiation for 150min with Cu2−XSe thin film catalyst in presence of H2O2. Thermoelectric performances through power factor and figure of merit have been evaluated. Carrier concentration obtained from thermoelectric power was used to evaluate the mobility of carriers from electrical conductivity measurement.

Electrodeposition of CuInxGa1−xSe2 thin films via pulse technique from Ionic liquid containing n-propyl alcohol

Stoichiometric Cu1.00In0.51Ga0.1Se2.04 (CIGS) thin films containing Cu(II), In(III), Ga(III) and Se(IV) are obtained for the first time in ionic liquid/n-propyl alcohol mixtures with controlled pulse electrodeposition parameters at 323K. The effects of deposition current density, deposition time, frequency and duty cycle are studied in detail. The band gap of obtained CuInxGa1−xSe2 (CIGS) thin film are approximately 1.35eV, as measured using UV–visible absorption spectra. The interplanar spacing and average Hall coefficient of the CIGS thin films are approximately 3.315Å and 1.011×10−1cm3/C, respectively.

Comparison studies on electrodeposited CdSe, SnSe and Cd x Sn1−x Se thin films

Electroplated cadmium stannous selenide (Cd x Sn1−x Se) thin films were coated on indium-doped tin oxide (ITO) conducting glass substrates from aqueous bath solutions containing CdCl2, SnCl2, and Na2SeO3. The x value was tuned by change in metal ion source concentration of the electrolytic bath solution. The various x values such as 1, 0.7, 0.45, and 0, of Cd x Sn1−x Se thin films were characterized by structural, morphological, compositional, and optical properties using X-ray diffraction, scanning electron microscopy, energy dispersive analysis by X-rays, and UV-vis-NIR spectrophotometer, respectively. X-ray diffraction (XRD) patterns revealed that the deposited films exhibit polycrystalline nature Cd x Sn1−x Se thin films. The microstructural parameters such as crystallite size, dislocation density, microstrain, and number of crystallites per unit area were calculated and presented. Morphological studies revealed that spherical-shaped grains were observed in cadmium-dominated films and nano-rod-shaped grains were observed in tin-dominated films. Optical properties of CdSnSe films were determined from optical transmittance data in the spectral range 400 to 1,100 nm. The optical direct transition energy band gap was estimated using conventional method, and band gap energy was lying between 1.02 and 1.83 eV. The refractive index, extinction coefficient, and real and imaginary parts of dielectric constants were calculated using optical transmission spectra of Cd x Sn1−x Se thin films.

Characterization of Cu2−xSe thin films synthesized from electrochemical deposition

• Cu 2− x Se films were electrodeposited on Mo substrate with CuSO 4 and H 2 SeO 3 . • The deposited films were near-stoichiometric Cu 2− x Se with FCC structure. • The direct band gap for Cu 2− x Se thin films are in the range of 2.14–2.18 eV. Copper selenide (Cu 2− x Se) thin films were potentiostatically deposited on molybdenum substrate with an aqueous solution of CuSO 4 and H 2 SeO 3 . The effect of the deposition potential on the microstructure and electrical properties of the thin films were studied using FESEM, HRTEM, and two probe conducting measurements. The morphologies of the films changed from densely packed nanoparticles to the plate-like grains growing perpendicular to the substrate. In addition, these plates were composed of nanoparticles of about 100–200 nm in size. The nanoparticles were verified to be the face centered cubic (FCC) Cu 2− x Se from XPS and HRTEM analysis. The direct band gap for the Cu 2− x Se thin films was in the range of 2.14–2.18 eV measured from UV–vis absorption edge. The electrical conductivity of the thin films increased from 4.51 × 10 −3 to 7.08 × 10 −3 (Ω m) −1 as the Cu/Se ratio decreased from 1.92 to 1.89.

Electrodeposited CuIn1−xGaxSe2 thin films from non-aqueous medium for solar cell applications

Polycrystalline thin films of group I-III-VI, especially copper indium diselenide and its alloys with gallium (CuInxGa1−xSe2, CIGS) have proven to be a suitable absorbers layer to obtain high-efficiency solar cells. We have deposited CuInxGa1−xSe2 (CIGS) thin films by cost effective electrochemical technique from non-aqueous bath onto fluorine doped tin oxide (FTO) coated glass substrates. The electrochemical parameters have been investigated by cyclic voltammetry in a non-aqueous solution of ethylene glycol, consisting precursors of Cu, In, Ga, and Se. The structural, morphological, compositional, and optoelectronic properties of CIGS thin films were studied. The prominent X-ray diffraction peak (112) is systematically shifted from 2θ = 26.60 to 27.26°, with increase in the contents of Ga in CIGS precursor layer. The atomic percentage of Ga in CIGS thin film was further confirmed by energy dispersive x-ray analysis technique. Preliminary solar cell devices (Glass/FTO/CdS/CIGS/Au) showed photoactive behavior with good diode characteristics in dark and under illumination conditions.

The growth of Cu2 − xSe thin films using nanoparticles

For low-cost fabrication applications, Cu2−xSe thin films were fabricated by a non-vacuum process and were then characterized in terms of quality. Cu–Se nanoparticles were synthesized by a low-temperature colloidal method and were confirmed to be Cu2−xSe nanoparticles by X-ray diffraction and Raman spectroscopy. The synthesized Cu2−xSe nanoparticles were deposited as films by a direct non-vacuum coating method, and the as-deposited Cu2−xSe thin films were selenized at different reaction temperatures, reaction times, and Se fluxes. The selenized films presented several distinct characteristics that depended on their fabrication conditions. Well-grown Cu2−xSe thin films were obtained at high reaction temperatures, long reaction times, and high Se fluxes. In addition, the selenized Cu2−xSe layer was observed to have a larger Cu content than the Cu2−xSe nanoparticles.

Smectic C liquid crystal growth through surface orientation by ZnxCd1–xSe thin films

A smectic C liquid crystal (LC) texture, consisting of distinct local single crystals (DLSCs) was grown using predefined orientation of ternary nanocrystalline thin films of ZnxCd1−xSe. The surface morphology and orientation features of the ZnxCd1−xSe films were investigated by AFM measurements and micro-texture polarization analysis. The ZnxCd1−xSe surface causes a substantial enlargement of the smectic C DLSCs and induction of a surface bistable state. The specific character of the morphology of this coating leads to the decrease of the corresponding anchoring energy. Two new chiral states, not typical for this LC were indicated. The physical mechanism providing these new effects is presented.

CuAlxGa1−xSe2 thin films for photovoltaic applications: Optical and compositional analysis

Wide-band gap chalcopyrite semiconductors have a great interest due to their potential application in multi-junction thin film solar cells or as window layers. Polycrystalline CuAlxGa1−xSe2 (CAGS) thin films have been prepared by selenization of evaporated metallic precursor layers on bare and Mo-coated soda lime glass substrates. The optical properties of CAGS films of 2 thicknesses have been analyzed by spectrophotometry in the visible-infrared (VIS-IR) and the compositional characteristics have been studied by energy dispersive analysis of X-rays (EDAX) and X-ray photoelectron spectroscopy (XPS). The optical transmission increases and the band gap energy shifts toward higher values as the Al content increases, which indicates the partial substitution of Ga by Al. The dependence of the band gap with the composition has resulted to be nonlinear and a bowing parameter of b=0.62 and b=0.54 for 0.6μm and 1.1μm-CAGS samples, respectively, has been obtained. XPS data have shown an Al, Ga and Se composition gradient in depth and a surface strongly oxidized. However, XPS reveals that the Cu composition remains constant in depth and the oxidation is relatively low in bulk increasing slightly in the interface with Mo/SLG. Moreover, samples with high Al content reveal a higher contribution of CuO in depth.

CuAlxGa1−xSe2 thin films for photovoltaic applications: Structural, electrical and morphological analysis

Wide-bandgap quaternary semiconductor chalcopyrites can be of interest for photovoltaic application. Novel CuAlxGa1−xSe2 (CAGS) (0≤x≤1) thin films have been achieved by selenization of evaporated metallic precursor layers in a wide range of Al content and for several film thicknesses. Influence of Al incorporation, x, and film thickness on the structural, electrical and morphological properties of the samples have been studied. Polycrystalline CAGS thin films have been obtained reproducibly with chalcopyrite structure. The lattice parameters, a and c, and the average crystallite size decreased with the increase of Al amount, x. All films showed random orientation with the degree of randomness reduced for increasing x values. All samples show photoconductivity and resistivities ranging from 101 to 104Ωcm depending on the film thickness and x value. Resistivity values increase and thermoelectric coefficient decreases with the increase of Al proportion, x. Surface morphology was studied by SEM images and roughness measurements. Grain size and the arithmetic average roughness decreases with the increase of x and with the decrease of film thickness.