Formation of CuInSe2 thin films on flexible substrates by electrodeposition (ED) technique

This work reveals the formation of electrodeposition (ED) of CuInSe₂ (CIS) film on flexible substrate. Ternary compounds were co-deposited on Au coated plastic substrate from an aqueous acidic solution containing 1 mM CuCl₂, 5 mM InCl₃ and 1 mM SeO₂ adjusted to pH=1.65. It was found that the film stoichiometry improves when the growth solution consisted of 1 M triethanolamine (TEA) and 0.1 M Na-citrate. The optimal ED-CIS film was obtained after annealing at 150°C for 1 h in a nitrogen (N₂) atmosphere. Optical absorption study showed that the energy gap of the annealed material is 1.18 eV. Good and reliable quality ED-CIS film was grown in this research with the potential use in fabricating flexible solar cells. This was supported by various analytical techniques, including energy dispersive spectroscopy (EDS), X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), ultraviolet/visible spectroscopy, Fourier transform infrared spectrometer (FTIR) and photoelectrochemical (PEC) characterization.     

Deposition of CuInS2 films by electrostatic field assisted ultrasonic spray pyrolysis

We studied CuInS₂ (CIS) film growth using high electrostatic field assisted ultrasonic spray (HEFAUS) deposition. CIS films were fabricated with various precursors and substrate temperatures. All the as-sprayed CIS films were observed to be grown with mixed ordering mode, where chalcopyrite (CH) and CuAu (CA) orderings coexisted. It was found that application of additional sulfurization to sprayed CIS films induced re-crystallization of films accompanied by enhancement of CH ordering. After the post-sulfurization, the most improved film showed nearly the same CH-fraction as that for a CIS film which was made by sulfurization of sputtered Cu-In alloy film. These results indicate that our modified spray deposition could be used for fabrication of CIS photoabsorbing layer instead of high-cost vacuum-based process. All fabricated films were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscope and energy dispersive X-ray analysis measurements.     

Chemical-bath ZnO buffer layer for CuInS2 thin-film solar cells

ZnO buffer layers were grown by a chemical-bath deposition (CBD) in order to improve the interface quality in p-CuInS₂ based solar cells, to improve the light transmission in the blue wavelength region, but also as an alternative to eliminate the toxic cadmium. The process consists of immersion of different substrates (glass, CIS) in a dilute solution of tetraamminezinc II, [Zn(NH₂)₄]²⁺, complex at 60–95°C. During the growth process, a homogeneous growth mechanism which proceeds by the sedimentation of a mixture of ZnO and Zn(OH)₂ clusters formed in solution, competes with the heterogeneous growth mechanism. The mechanism consists of specific adsorption of a complex Zn(II) followed by a chemical reaction. The last process of growth results in thin, hard, adherent and specularly reflecting films. The characterization of the deposited CBD-ZnO layers was performed by X-ray diffraction (XRD), optical transmittance, scanning electron microscopy, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The as-deposited films on glass show hexagonal zincite structure with two preferred orientations (1 0 0) and (1 0 1). High optical transmittance up to 80% in the near-infrared and part of the visible region was observed. The low growth rate of the films on CIS suggests an atomic layer-by-layer growth process.The device parameters and performance are compared to heterojunction with a standard CdS buffer layer.     

Effects of annealing on CuInSe2 films grown by molecular beam epitaxy

CuInSe₂ (CIS) thin films with a range of Cu/In ratios were grown by molecular beam epitaxy on GaAs (0 0 1) at substrate temperatures of T_s = 450–500°C and the effects of annealing under various atmospheres have been investigated. Photoluminescence spectra obtained from an ex-situ vacuum annealed CIS film at a temperature of T_A = 350°C showed a red-shift and a broadening of an emission peak (peak c) which originally appeared at 0.970 eV before annealing and the red-shifted peak c was found to consist of two overlapping peaks. The excitation power dependence of these overlapping peaks indicated the radiative recombination processes associated with the emissions to be a conduction band to acceptor transition (peak at 0.970 eV) and a transition due to donor-acceptor pairs (peak at 0.959 eV), indicating the formation of a shallow donor-type defect during the vacuum annealing process. The origin of this defect has tentatively been attributed to Se vacancies. On the other hand, the molar fraction of oxygen increased with increasing annealing temperature in dry-air. An epitaxially grown In₂O₃ phase was found both in Cu-rich and In-rich films annealed at T_A 350°C, which was not observed in the films annealed in Ar atmosphere. Thermodynamic calculations based on the Cu---In---Se---O---N system showed In₂O₃ to be the most stable phase in good agreement with the experimental results.     

Effect of FePO4 coating on electrochemical and safety performance of LiCoO2 as cathode material for Li-ion batteries

LiCoO₂ was surface modified by a coprecipitation method followed by a high-temperature treatment in air. FePO₄-coated LiCoO₂ was characterized with various techniques such as X-ray diffraction (XRD), auger electron spectroscopy (AES), field emission scanning electron microscope (FE-SEM), energy dispersive spectroscopy (EDS), transmission electron microscope (TEM), electrochemical impedance spectroscopy (EIS), 3 C overcharge and hot-box safety experiments. For the 14500R-type cell, under a high charge cutoff voltage of 4.3 and 4.4 V, 3 wt.% FePO₄-coated LiCoO₂ exhibits good electrochemical properties with initial discharge specific capacities of 146 and 155 mAh g⁻¹ and capacity retention ratios of 88.7 and 82.5% after 400 cycles, respectively. Moreover, the anti-overcharge and thermal safety performance of LiCoO₂ is greatly enhanced. These improvements are attributed to the FePO₄ coating layer that hinders interaction between LiCoO₂ and electrolyte and stabilizes the structure of LiCoO₂. The FePO₄-coated LiCoO₂ could be a high performance cathode material for lithium-ion battery.     

Photoluminescence properties of stoichiometric CuInSe2 crystals

We studied photoluminescence (PL) properties of stoichiometric CuInSe₂ (CIS) single microcrystals. Temperature and laser power dependencies of the PL spectra were measured. Two bands at 0.973 (A-band) and 0.991 eV (B-band) governed the obtained PL spectra. Measured dependencies showed very similar properties for both bands: the j-shift that was generated by altering the laser power was 2 meV per decade for both bands and thermal activation energies were 46 and 32 meV for A- and B-band, respectively. Based on these results, we assume that both bands come from the same shallow donor–shallow acceptor recombination process with different bandgap energies for CIS. The solid solution phase of CIS with K or Na is proposed as the source of distinctive bandgap.     

Investigating the solid electrolyte interphase using binder-free graphite electrodes

Binder-free (BF) electrodes simplify interpretation of solid electrolyte interphase (SEI) data obtained from studies of graphite surfaces. In this work, we prepared BF-graphite electrodes by electrophoretic deposition (EPD), and the SEI layers formed on the electrode in lithium cells containing LiPF₆- and LiF₂BC₂O₄-bearing electrolytes were examined by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The results showed that the dominant SEI species were lithium alkyl carbonates (ROCO₂Li) and lithium alkoxides (ROLi); Li₂CO₃ was conspicuously absent. Trigonal borate oligomers are most likely present in the SEI of graphite samples cycled in LiF₂BC₂O₄ electrolyte, while lithium fluorophosphates are present on graphite samples cycled in LiPF₆ electrolyte. The SEI layer coverage was greater on graphite samples cycled in LiF₂BC₂O₄ electrolyte than in the LiPF₆ electrolyte. Our results demonstrate that BF-graphite electrodes prepared by EPD are suitable for the study of SEI layer formed in various electrolyte systems.     

Cu(In,Ga)Se2 based photovoltaic structure by electrodeposition and processing

CuIn_1−xGa_xSe₂ (CIGS) thin films were formed from an electrodeposited CuInSe₂ (CIS) precursor by thermal processing in vacuum in which the film stoichiometry was adjusted by adding In, Ga and Se. The structure, composition, morphology and opto-electronic properties of the as-deposited and selenized CIS precursors were characterized by various techniques. A 9.8% CIGS based thin film solar cell was developed using the electrodeposited and processed film. The cell structure consisted of Mo/CIGS/CdS/ZnO/MgF₂. The cell parameters such as J_sc, V_oc, FF and η were determined from I–V characterization of the cell.     

Na incorporation in Mo and CuInSe2 from production processes

Results of characterization of thin films of Mo deposited by DC magnetron sputtering on soda-lime glass (Mo/SLG) and CuInSe₂ (CIS) on Mo/SLG are presented. The primary objective of the work was to clarify the factors determining the concentration of Na in commercial-grade CIS. Mo films were deposited by three laboratories manufacturing CIS thin film solar cells. Analysis was by secondary ion mass spectrometry, scanning electron microscopy and X-ray diffraction. Changes in Mo deposition parameters in general affected the Na level but there was no obvious link to any single Mo deposition parameter. Oxygen content directly affected the Na level. The Na behavior was not obviously connected to film preferred orientation. Selenization of the Mo layers was also examined. Elemental Se vapor was found to produce significantly less selenization than H₂Se. The amount of selenization was also strongly dependent upon Mo deposition conditions, although a specific source of the change in reaction rate was not found. Na distributions in the CIS deposited on the Mo were not limited by the diffusivity of the Na. The Na concentration in the CIS was increased by annealing the Mo films both with and without intentionally added Na. The Na level in the CIS appears to be set more by the CIS deposition process than by the Na concentration in the Mo so long as the Mo contains sufficient Na to saturate the available sites in the CIS.     

CuInS2/SiNWs/Si composite material for application as potential photoelectrode for photoelectrochemical hydrogen generation

This work aims at developing a new composite material based on nanosized semiconducting CuInS₂ (CIS) particles combined with silicon nanowires grown on a silicon substrate (SiNWs/Si) for photoelectrochemical (PEC)-splitting of water. The CIS particles were prepared via a colloidal method using N-methylimidazole (NMI) as the solvent and an annealing treatment. The SiNWs were obtained by chemical etching of silicon (100) substrates assisted by a metal. The CIS/SiNWs/Si composite material was obtained by deposition of an aliquot of a suspension of CIS particles onto the SiNWs/Si substrate, using spin coating followed by a drying step. The XRD pattern demonstrated that CuInS₂ grows in the tetragonal/chalcopyrite phase, while SiNWs/Si presents a cubic structure. The SEM images show semi-spherical particles (～10 nm) distributed on the surface of silicon nanowires (～10 μm). The EIS measurements reveal n-type conductivity for CIS, SiNWs/Si and CIS/SiNWs/Si materials, which could favour the oxidation reaction of water molecules.     

Ex-situ and in-situ analyses on reaction mechanism of CuInSe2 (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 Cu₂In 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%.     

Mechanisms for adsorption,dissociation and diffusion of hydrogen in hydrogen permeation barrier of α-Al2O3: The role of crystal orientation

The mechanisms of adsorption of hydrogen on α-Al₂O₃(1-102) surface and of its diffusion in bulk are investigated, using first principles thermodynamics and kinetics, and compared with similar results obtained for the diffusion of hydrogen on α-Al₂O₃(0001) surface. Because of the different oxygen environments on both surfaces, the H binding energies on the (1-102) surface are 0.3–1.2 eV smaller than in the (0001) surface. The H₂ binding energies on (1-102) and (0001) surfaces are resembled. We have identified four main mechanisms, leading to dissociation of H₂, H migration on the surface, H diffusion into and inside the bulk. Equilibrium constant and activation barrier show that H₂ dissociation is the most favorable process and significant diffusion of H into the bulk can occur more readily from the (1-102) surface compared to the (0001) surface. Based on the hydrogen interaction with α-Al₂O₃(1-102) surface, a mechanism of α-Al₂O₃ suppressing H-permeation is identified.     

Progress in electrodeposited absorber layer for CuIn(1−x)GaxSe2 (CIGS) solar cells

Thin film solar cells with chalcopyrite CuInSe₂/Cu(InGa)Se₂ (CIS/CIGS) absorber layers have attracted significant research interest as an important light-to-electricity converter with widespread commercialization prospects. When compared to the ternary CIS, the quaternary CIGS has more desirable optical band gap and has been found to be the most efficient among all the CIS-based derivatives. Amid various fabrication methods available for the absorber layer, electrodeposition may be the most effective alternative to the expensive vacuum based techniques. This paper reviewed the developments in the area of electrodeposition for the fabrication of the CIGS absorber layer. The difficulties in incorporating the optimum amount of Ga in the film and the likely mechanism behind the deposition were highlighted. The role of deposition parameters was discussed along with the phase and microstructure variation of an as-electrodeposited CIGS layer from a typical acid bath. Related novel strategies such as individual In, Ga and their binary alloy deposition for applications in CIGS solar cells were briefed.     

Improved CIGS thin-film solar cells by surface sulfurization using In2S3 and sulfur vapor

Surface sulfurization of Cu(In,Ga)Se₂ (CIGS) thin films was carried out using two alternative techniques that do not utilize toxic H₂S gas; a sequential evaporation of In₂S₃ after CIGS deposition and the annealing of CIGS thin films in sulfur vapor. A Cu(In,Ga) (S,Se)₂ thin layer was grown on the surface of the CIGS thin film after sulfurization using In₂S₃, whereas this layer was not observed for CIGS thin films after sulfurization using sulfur vapor, although a trace quantity of S was confirmed by AES analysis. In spite of the difference in the surface modification techniques, the cell performance and process yield of the ZnO:Al/CdS/CIGS/Mo/glass thin-film solar cells were remarkably improved by using both surface sulfurization techniques.     

Recent progress in scaling up highly efficient Zn(S,O)/Cu-chalcopyrite thin film solar cells and modules at HZB

In the present contribution we report on recent work covering Zn(S,O) buffer as heterojunction partner layer applied to pilot line low-gap Cu(In,Ga)(SSe)₂ (CIGSSe, E_g = 1.03 eV) and production scale wide-gap CuInS₂ (CIS, E_g = 1.54 eV). We highlight the crucial role that the processing control of the Zn(S,O) plays for the fabrication of Cu-chalcopyrite solar cells and modules. The analytical information obtained by the correlation with state-of-the art high resolution Transmission electron microscopy, X-ray photoemission and Auger spectroscopy (XPS and XAES) as well as L-edge XAS are discussed. A large number of efficient laboratory-scale solar cells and monolithically interconnected prototype CIGSSe and CIS modules are produced. The efficiencies are comparable to the CdS base line references or even higher. The electrical, electronic properties and the emerging phenomena in Cd-free devices such as light soaking are discussed.     

Bis (dibenzylidene acetone) palladium (0) catalyst for glycerol oxidation in half cell and in alkaline direct glycerol fuel cell

In this study, catalytic activity and performance of bis (dibenzylidene acetone) palladium (0) catalyst, Pd (DBA)₂, was evaluated toward glycerol oxidation reaction (GOR) in alkaline half cell and alkaline direct glycerol fuel cell (DGFC). The electrooxidation of glycerol on Pd (DBA)₂ was characterized in half cell by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA) techniques. Obtained results have highlighted the excellent electrocatalyst activity of Pd (DBA)₂ in terms of specific peak current density and onset potential compared to the results obtained by conventional Pd base catalysts. CVs results also demonstrate that Pd (DBA)₂ is still active even after 200 cycles.     

A nanocomposite photoelectrode made of 2.2 eV band gap copper tungstate (CuWO4) and multi-wall carbon nanotubes for solar-assisted water splitting

We report on the photoelectrochemical performances of a nanocomposite photoactive material made of copper tungstate (CuWO₄) and multi-wall carbon nanotubes (MWCNT). The purpose of this work was to create a light absorber/charge collector composite material with tunable electronic transport properties to minimize the bulk resistance of CuWO₄ material class. Nanocomposite thin films (typically 2.0 ± 0.1 μm) were fabricated by means of spray pyrolysis using solutions containing copper acetate, ammonium metatungstate and MWCNT. Spray-deposited polycrystalline CuWO₄ films were found to be porous, though crack-free, and made of CuWO₄ nanoparticles with dimensions in the 10–50 nm range. Tauc plots derived from UV–visible and photocurrent spectroscopy techniques led to a consistent band gap value of 2.20 (±0.05) eV. Electrochemical impedance spectroscopy performed in pH10 buffer solution under Air Mass 1.5 global (AM1.5_G) at 0.8 V vs. saturated calomel electrode (1.63V vs. reversible hydrogen electrode) pointed out a bulk resistance reduction by 30% on nanocomposites photoanodes when compared to un-modified CuWO₄ control samples. It is worth mentioning that the reduction in bulk resistance was achieved with an extremely low MWCNT:CuWO₄ weight ratio (1:10,000), in which MWCNT absorbed less than 2% of incoming light. Subsequent linear scan voltammetry (LSV) performed in the same conditions revealed a photocurrent density increase of 26% at 0.8 V_SCE (1.63 V_RHE) compared to control samples. Additional LSV and incident photon-to-current efficiency measurements demonstrated that MWCNT served as effective electron collectors distributed throughout the entire CuWO₄ bulk.     

Functionalization of TiO2 nanotubes with palladium nanoparticles for hydrogen sorption and storage

The use of hydrogen as an energy carrier is an attractive solution toward addressing global energy issues and reducing the effects of climate change. Design of new materials with high hydrogen sorption capacity and high stability is critical for hydrogen purification and storage. In this study, titanium dioxide nanotubes (TiO₂NTs) were modified with palladium nanoparticles (PdNPs) utilizing a facile photo-assisted chemical deposition approach. Electrochemical anodization was employed for the direct growth of TiO₂NTs. The PdNP functionalized TiO₂NTs (TiO₂NT/Pd) were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The hydrogen sorption behaviours and stability of the TiO₂NT/Pd nanocomposites were investigated and compared with nanoporous Pd networks that were deposited on a bulk titanium substrate (Ti/Pd) using cyclic voltammetry (CV) and chronoamperometry (CA). Our studies show that the TiO₂NT/Pd nanocomposites possess a much higher hydrogen storage capacity, faster kinetics for hydrogen sorption and desorption, and higher stability than the nanoporous Pd.     

CNT/PDMS composite membranes for H2 and CH4 gas separation

Polydimethylsiloxane (PDMS) composites with different weight amounts of multi-walled carbon nanotubes (MWCNT) were synthesised as membranes to evaluate their gas separation properties. The selectivity of the membranes was investigated for the separation of H₂ from CH₄ gas species. Membranes with MWCNT concentrations of 1% increased the selectivity to H₂ gas by 94.8%. Furthermore, CH₄ permeation was almost totally blocked through membranes with MWCNT concentrations greater than 5%. Vibrational spectroscopy and X-ray photoelectron spectroscopy techniques revealed that upon the incorporation of MWCNT a decrease in the number of available Si–CH₃ and Si–O bonds as well as an increase in the formation of Si–C bonds occurred that initiated the reduction in CH₄ permeation. As a result, the developed membranes can be an efficient and low cost solution for separating H₂ from larger gas molecules such as CH₄.     

Bacterial cellulose-assisted hydrothermal synthesis and catalytic performance of La2CuO4 nanofiber for methanol steam reforming

The perovskite-structured La₂CuO₄ nanocrystals with a fiber-like morphology were fabricated with bacterial cellulose nanofibers as the templates at a hydrothermal temperature of 120 °C. The La₂CuO₄ nanofibers were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), N₂ adsorption (BET). The catalytic performance of the La₂CuO₄ nanofiber was evaluated for steam reforming of methanol (SRM). At the low temperature of 200 °C methanol was completely converted into hydrogen and CO₂, without the generation of CO. Compared with La₂CuO₄ bulk powder, La₂CuO₄ nanofibers showed better catalytic activity for the SRM reaction. It is concluded that the special structure and unique morphology of the La₂CuO₄ nanofibers with larger specific area, are responsible for their excellent performance in catalyzing the SRM.