As-electrodeposited Films Research Articles

Electroreduction of Si(NCO)4 for Electrodeposition of Si

The electrochemical reduction and concomitant suitability of tetraisocyanatosilane, Si(NCO)4, as a precursor for Si electrodeposition has been examined. At T = 25 °C, voltammetric data indicated that the electroreductive pathway of Si(NCO)4 paralleled the electroreduction of SiCl4, albeit with attenuated current densities. At T = 150 °C, the measured current density increased significantly due to coupled reaction likely involving an electropolymerization of the tetraisocyanato ligand. At either temperature, the electrochemical response of Si(NCO)4 did not strongly depend upon the identity of the anion in the supporting electrolyte. A combination of chronoamperometric, electron microscopy, and X-ray photoelectron spectroscopy data were used to study the morphology and composition of the electrodeposited Si films at room temperature. The composition and morphology of the as-electrodeposited films were consistent with the formation of amorphous Si when using solid n +-Si electrodes, similar to what is observed during the electroreduction of SiCl4. Attempts to use Si(NCO)4 for Si crystal growth by the electrochemical liquid-liquid-solid (ec-LLS) process with a liquid e-GaIn electrode resulted in no appreciable crystalline Si product. Although Si(NCO)4 can be used for electrodeposition of amorphous Si onto solid electrodes at room temperature, more work is needed to identify conditions that would facilitate its use for ec-LLS.

Electrodeposition in Ionic Liquids for Controlled Synthesis of New Class of Ionic Semiconductors Based on Binary and Ternary Chalcogenide Perovskites for Thin Film Photovoltaic Solar Cells

Thin film photovoltaic (PV) solar cells based on Cu(In,Ga)(S,Se)2 (CIGS), CdTe and Cu2ZnSnS4 (CZTS) have been in the forefront of the pursuit for sustainable and pervasive solar electricity utilization. Recently, researchers have focused on alternative semiconductor photo-absorbers that can be synthesized as thin films by simple low cost solution based methods. Efforts in this direction led to solar cells based on the hybrid organic-inorganic perovskite photo-absorbers having the general formula RNH3MX3 where R= CnH2n+1, M= Pb, Sn and X= Cl, I or Br showing significantly high (24.2%) conversion efficiency. However, the light induced instability, mechanical durability and thermal degradation problems though largely elucidated, still require intensive efforts towards alleviation. The key factors for high efficiency of hybrid perovskite cell are high absorption coefficient, defect-tolerance and large carrier diffusion lengths. This is attributed to their ionic character derived from higher cation-anion coordination resulting in enhanced Coulomb interaction. The entirely inorganic chalcogenide perovskites are the ionic semiconductor counterparts which are purported to have similar optoelectronic properties sans the stability issues but have not been investigated in detail. The perovskite ionic semiconductors with formula unit ABX3 (A=Ca, Ba or Sr ; B=Ti or Zr; X= S, Se) have direct 0.8-2.4 eV bandgap, high photo-absorption and large carrier mobility. These appear attractive for photovoltaic solar cell application but are needed to be investigated as thin films by emphasizing the synthesis-property correlations. In this work we have investigated electrodeposition synthesis in ionic liquid medium of the transition metal layered chalcogenide TiS2 and p-type CaTiS3 perovskite chalcogenide semiconductor thin films which have application in solar cells, the former as a hole conductor and the later as an efficient photo-absorber.In this work we describe an ab initio study of scalable electrodeposition of TiS2 and CaTiS3 polycrystalline semiconductor thin films from choline chloride–urea eutectic based ionic liquid electrolytes at 80°C and establish their structure and optoelectronic properties dependent upon the electrodeposition variables. The ionic liquid electrodeposition, not studied in detailed for semiconductor layers, is unique due to large stable potential window, high solubility for metal salts and devoid of hydrogen interference usual in aqueous medium. Further, it enables electrodeposition of semiconductor with elements having high reduction potential like Ca and Ti. We synthesized these films by electrodeposition in a 3-cell assembly with Pt reference and counter electrode and transparent conducting glass (FTO) as substrate. Precursors TiCl4, CaCl2 and Na2S2O3 served as source of Ti, Ca and S and initial cyclic voltammetry (CV) studies with variable precursor composition have established deposition potentials of Ca and Ti as -0.68V and -0.79V vs Pt, respectively.Detailed CV study with variable Ca:Ti precursor ratio coupled with film composition analysis establishes deposition potential and precursor composition for stoichiometric CaTiS3 film synthesis. X-ray diffraction showed as-electrodeposited films are polycrystalline. The Raman spectra includes numerous peaks in the 50-1200 cm-1 range which are interpreted as characteristic vibration modes of CaS at 170 and 220 cm-1. The variation in the intensity of the A1g mode at 359 cm-1 originating from TiS2 was used to calibrate the deposition potential relative to film composition. Tauc analysis of the uv-vis transmission spectra showed direct bandgap of 1.5 eV which is ideal for terrestrial solar radiation absorption. A correlation of the electrochemical deposition potential with the inclusion of secondary phases was noticed. This also resulted in the increase in the bandgap energy indicating possibility for adjusting bandgap for specific solar cell structures. The TiS2 layered chalcogenide with potential use as hole transport layer in chalcogenide perovskites was likewise electrodeposited. Detailed analysis of the reduction peaks in CV plots shows peak at -0.76V vs Pt corresponding to TiS2 formation and a rapidly rising current wave corresponds to under potential deposition. The direct electrodeposition of TiS2 is by reduction of Ti at substrate and instant sulfurization in presence of S2- ions. The granular growth is directed by FTO which provides electrochemical reduction and nucleation sites for TiS2 polycrystalline film growth as established by X-ray diffraction. All Raman peaks are interpreted belonging to TiS2 and partial phase mixing with the TiS3 depending on the electrodeposition potential was seen. Thus, the observed decrease in the bandgap with increase in potential can be used for integrating TiS2 in solar cells as hole conductor as well as an absorber depending on the configuration, This paper will report on the detailed effect of the variables used in ionic liquid electrodeposition and consequent structural and optoelectronic properties of the two important ionic semiconductors for assessment of their use in solar cells.

Tuning Band Gaps and Photoelectrochemical Properties of Electrodeposited CuO Films by Annealing in Different Atmospheres

The band gaps and photoelectrochemical properties of electrodeposited CuO films were tuned through annealing as-electrodeposited films in different atmospheres at various temperatures. The direct band gaps of the CuO film descended from 1.80 eV for as-electrodeposited film to 1.76 eV for the film annealed in O2 at 300 °C or 1.64 eV for the film annealed in N2 at 300 °C. The stable photocurrent density for CuO electrode annealed in O2 at 300 °C measured at −0.4 V vs Ag/AgCl under a simulated sun light reached to 0.44 mA cm−2 while the value was only 0.29 mA cm−2 for the electrode annealed in N2 in a solution with 10 mM potassium ferricyanide, 10 mM potassium ferrocyanide, and 0.5 M Na2SO4. The mechanism for photoresponse enhancement was studied and could be due to faster charge transfer and higher doping density for the CuO film annealed at 300 °C in O2.

Structural and optical properties of electrodeposited Cu2O thin films

Abstract In this work, we report on the synthesis of Cu2O thin films by electrodeposition technique. The as-electrodeposited films at −0.5 V potential for 15 min in basic solution (pH = 9) are well crystallized in cubic structure, with 〈1 1 1〉 as preferential orientation and with lattice parameter a = 4.26 A. The transmittance of Cu2O films reveals that they have high optical transmission (>70%) with direct band gap energy in order of 2.23 eV. The density functional theory using the plane-wave-self-consistent field (PWSCF) was carried out for Cu2O to determine some of their electronic properties. Theoretical analyses show that Cu2O has a direct band gap with value 2.01 eV, which is consistent with the experimental results.

Change of Crystallographic Orientation Characteristics of (001)-Oriented Electrodeposited Silver Films During Self-Annealing at Room Temperature

Self-annealing behaviors of the electrodeposited silver films which preferentially orient in the (001) direction were investigated by in situ EBSD analysis. Self-annealing started in storage for a few hours at room temperature (R. T.) and was almost complete after storage for 6h at R. T. In the initial stage of self-annealing, recrystallization of (001)-oriented grains progressed with the formation of twins with (212) orientation. Membrane stress in the as-electrodeposited film was compression stress of approximately 20MPa. The compression membrane stress seems to become driving force for self-annealing of the (001)-oriented electrodeposited silver film. Membrane stress in the film changed from compression stress to tensile stress by recrystallization in self-annealing.

Effect of Electrodeposition Potential on Composition of CuIn1−xGaxSe2Absorber Layer for Solar Cell by One-Step Electrodeposition

CIGS polycrystalline thin films were successfully fabricated by one-step cathodic electrodeposition on Mo-coated glass. In this study, we applied a galvanometry mode with three-electrode potentiostatic systems to produce a constant concentration electroplating solution, which were composed of CuCl2, InCl3, GaCl3, and SeO2. Then these as-electrodeposited films were annealed in argon atmosphere and characterized by X-ray diffraction. The results revealed that annealing treatment significantly improved the crystallinity of electrodeposited films and formed CIGS chalcopyrite structure, but at low applied deposition voltage (−950 mV versus SCE) there appeared second phase. The cross-section morphology revealed that applied voltage at −1350 mV versus SCE has uniform deposition, and higher applied voltage made grain more unobvious. The deposition rate and current density are proportional to deposition potential, and hydrogen was generated apparently when applying potential beyond −1750 mV versus SCE. It was found that the CIGS compound did not match exact stoichiometry ofCu : In : Ga : Se =1 : x : 1-x : 2. This result suggests the possibility of controlling the property of thin films by varying the applied potential during electrodeposition.

Electrodeposition of Crystalline GaAs on Liquid Gallium Electrodes in Aqueous Electrolytes

Crystalline GaAs (c-GaAs) has been prepared directly through electroreduction of As(2)O(3) dissolved in an alkaline aqueous solution at a liquid gallium (Ga(l)) electrode at modest temperatures (T ≥ 80 °C). Ga(l) pool electrodes yielded consistent electrochemical behavior, affording repetitive measurements that illustrated the interdependences of applied potential, concentration of dissolved As(2)O(3), and electrodeposition temperature on the quality of the resultant c-GaAs(s). Raman spectra indicated the composition of the resultant film was strongly dependent on both the electrodeposition temperature and dissolved concentration of As(2)O(3) but not to the applied bias. For electrodepositions performed either at room temperature or with high (≥0.01 M) concentrations of dissolved As(2)O(3), Raman spectra of the electrodeposited films were consistent with amorphous As(s). X-ray diffractograms of As(s) films collected after thermal annealing indicated metallurgical alloying occurred only at temperatures in excess of 200 °C. Optical images and Raman spectra separately showed the composition of the as-electrodeposited film in dilute (≤0.001 M) solutions of dissolved As(2)O(3)(aq) was pure c-GaAs(s) at much lower temperatures than 200 °C. Diffractograms and transmission electron microscopy performed on as-prepared films confirmed the identity of c-GaAs(s). The collective results thus provide the first clear demonstration of an electrochemical liquid-liquid-solid (ec-LLS) process involving a liquid metal that serves simultaneously as an electrode, a solvent/medium for crystal growth, and a coreactant for the synthesis of a polycrystalline semiconductor. The presented data serve as impetus for the further development of the ec-LLS process as a controllable, simple, and direct route for technologically important optoelectronic materials such as c-GaAs(s).

Electrodeposition and Characterization of Eutectic Sn–Ag Alloy as Solder Bumps for Flip-Chip Interconnection

Eutectic Sn 3.5 wt % Ag alloy is one of the most promising lead-free solders in wafer bumping for flip-chip interconnection. Its electrodeposition in pyrophosphate-iodide-based baths was investigated. In particular, the effects of polyethylene glycol 600 and formaldehyde as surfactants were studied, and their addition proved to facilitate achieving a near-eutectic composition for the Sn–Ag alloy. Different deposited film morphologies were analyzed using scanning electron microscopy (SEM) and focused ion beam/SEM. X-ray diffraction investigations indicated that a biphasic structure of and was present in the as-electrodeposited film. To further confirm the efficacy of the electrodeposition process, fine pitch Sn–Ag solder bumps with smooth surface and homogeneous composition were achieved on glass test wafers.

XAS Study of Amorphous WO3Formation from a Peroxo-Tungstate Solution

Electrodeposition is a low cost method for WO3 thin film preparation. The starting dissolved tungsten precursor is a peroxo-compound which can be prepared by an equimolar mixing of sodium tungstate and hydrogen peroxide. The precursor preparation as well as the electrodeposition and precursor solution aging processes have been studied by X-ray absorption spectroscopy (XAS) at the W L3 edge. By extended X-ray absorption fine structure (EXAFS) analysis, the local structure of the first oxygen octahedral shell around the W absorber has been characterized both in the colloidal oxide formed after deposition solution aging and in electrodeposited films. The colloidal compound could be fitted with the same procedure as that successfully used with a monoclinic WO3 reference and involving two different W−O interatomic distances. The aging product is identified as amorphous WO3. If the spectra of as-electrodeposited films could not be fitted by the same procedure, the structural parameters of those cycled electroch...

Corrosion resistance in oxygen of electrolytic nickel and cobalt phosphorus coatings

Corrosion of NiP and CoP alloys in oxygen was investigated in the temperature range 350–800 °C on aselectrodeposited films and after annealing treatments. For as-electrodeposited films, both alloys oxidize by diffusion and the scales consist of an inner region rich in phosphorus and an outer region of oxides (NiO or Co 3O 4). Good corrosion resistance achieved after extended heating above 600 °C in vacuum, very well marked for NiP alloys, is explained in terms of crystallization and grain coarsening. This indicates that at these high annealing temperatures the crystals of phosphides formed a continuous layer with embedded nickel crystals rendering the alloy extremely resistant to oxygen penetration.