Structural and electrical properties of LiCoO2 thin-film cathodes deposited on planar and trench structures by liquid-delivery metalorganic chemical vapour deposition

The electrochemical properties of annealed-LiCoO₂ cathodes deposited on planar and trench structures by liquid-delivery metalorganic chemical vapor deposition are investigated for various deposition temperatures and input Li:Co mole ratios. With the planar structure, the best crystallinity of the films is obtained at a deposition temperature of 450 °C and an input Li:Co mole ratio of 1.0. The deposition window for optimum initial discharge capacity and capacity retention is a deposition temperature of 450–500 °C and an input Li:Co mole ratio of 1.0, and an input Li:Co mole ratio of 1.0–1.2 at a deposition temperature of 450 °C. The initial discharge capacity and capacity retention of LiCoO₂ thin films deposited with an input Li:Co mole ratio of 1.2 at 450 °C are approximately 25 μAh/cm² μm and 77%, respectively. The initial discharge capacity of films deposited on a trench structure shows an increase of approximately 130% compared with that of films deposited on a planar structure with an input Li:Co mole ratio of 1.2. The rechargeabilities of films deposited in a trench structure are inferior to those in a planar structure because conformal growth in the trench structure is poor. Thus, a trench structure can improve the initial discharge capacity and capacity retention of lithium microbatteries.     

Deposition of ZrO2 film by liquid phase deposition

In this study, undoped ZrO₂ thin films were deposited on single-crystal silicon substrates using liquid phase deposition. The undoped films were formed by hydrolysis of zirconium sulfate (Zr(SO₄)₂·4H₂O) in the presence of H₂O. A continuous oxide film was obtained by controlling adequate (NH₄)₂S₂O₈ concentration. The deposited films were characterized by SEM, FT-IR, XRD and DTA. Typically, the films showed excellent adhesion to the substrate with uniform particle diameter about 150 nm. The thicknesses of ZrO₂ film were about 200 nm after 10 h deposition at 30 °C. These films shows single tetragonal phase after heat treated at 600 °C. High annealing temperature (e.g. 750 °C) may result in the phase transformation of (t)-ZrO₂ into (m)-ZrO₂.     

A thin film silicon anode for Li-ion batteries having a very large specific capacity and long cycle life

A thin film of Si was vacuum-deposited onto a 30 μm thick Ni foil from a source of n-type of Si, the film thickness examined being 200–1500 Å. Li insertion/extraction evaluation was performed mainly with cyclic voltammetry (CV) and constant current charge/discharge cycling in propylene carbonate (PC) containing 1 M LiClO₄ at ambient temperature. The cycleability and the Li accommodation capacity were found to depend on the film thickness. Thinner films gave larger accommodation capacity. A 500 Å thick Si film gave a charge capacity over 3500 mAh g⁻¹ being maintained during 200 cycles under 2 C charge/discharge rate, while a 1500 Å film revealed around 2200 mAh g⁻¹ during 200 cycles under 1 C rate. The initial charge loss could not be ignored but it could be reduced by controlling the deposition conditions.     

A vacuum deposited Si film having a Li extraction capacity over 2000 mAh/g with a long cycle life

We have found that a Si film vacuum deposited on a Ni foil has a Li insertion capacity over 2000 mAh/g with cycleability over 1000 cycles, but a great issue was its difficulty to obtain a sufficiently thicker film capable of high current charge/discharge. In the present paper the examination of the high current charge/discharge performance of thicker Si film in relation to the film formation condition. The electrochemical evaluation was performed with cyclic voltammetry (CV) and constant current charge/discharge test with various loading currents in PC containing 1 M LiClO₄.A Si film prepared with a rapid deposition rate gave a discharge capacity over 2000 mAh/g even with a very high charge/discharge rate over 10 C. In addition, the surface roughening of the substrate foil was found to play an important role to provide a thick film capable of high current performance. The constant discharge curve gave a wide plateau in the potential range between 200 and 500 mV versus Li/Li⁺. The XRD pattern of the deposited film gave no peaks due to Si, indicating the film to be amorphous. The SEM image of the deposited film was rather homogeneous, and after 500 cycles it still covered the entire surface of the Ni substrate though the surface became inhomogeneous.     

Wide-optical bandgap with improved conductivity p-μc-Si:Ox:H films prepared by Cat-CVD

Boron-doped hydrogenated microcrystalline silicon oxide (p-μc-Si:O_x:H) films have been deposited using catalytic chemical vapor deposition (Cat-CVD). The single-coiled tungsten catalyst temperature (T_fil) was varied from 1850 to 2100 °C and films were deposited on glass substrates at the temperatures (T_sub) of 100–300 °C. Different catalyst-to-substrate distances of 3–5 cm and deposition pressures from 0.1 to 0.6 Torr were considered.Optical and electrical characterizations have been made for the deposited samples. The sample transmittance measurement shows an optical-bandgap (Eg_opt) variation from 1.74 to 2.10 eV as a function of the catalyst and substrate temperatures. One of the best window materials was obtained at T_sub=100 °C and T_fil=2050 °C, with Eg_opt=2.10 eV, dark conductivity of 3.0×10^?3 S cm^?1 and 0.3 nm s^?1 deposition rate.     

Effect of chitosan on stabilization of acetates-containing solution: A novel precursor for LiMn2O4 film deposition

Due to the adequate viscosity of the chitosan-added precursor solutions, the films deposited from the chitosan-added precursor solution showed a higher deposition rate than the ones from the PVP-added solution under the same coating parameters. Furthermore, the chitosan-added precursor solution remained stable without any precipitation for at least 10 months. On the other hand, without the addition of chitosan, the precursor solution showed apparent precipitation after being stirred for 12 h. The enhanced stability of the precursor solution by the addition of chitosan is attributed to the complexation between metal ions and the –NH₂ groups of chitosan. And the electrochemical behavior for the deposited films calcined at 700 °C for 1 h was also characterized by charge–discharge test. The result revealed that the film deposited from chitosan-containing precursor solution possesses an initial discharge capacity of 134 mAh g⁻¹ and about 9% capacity loss after 50 charge/discharge cycles, which is better than the one deposited from chitosan-free precursor solution with an initial discharge capacity of 108 mAh g⁻¹ and 24% capacity loss after 50 cycles.     

Solid-phase crystallization of amorphous silicon on ZnO:Al for thin-film solar cells

The suitability of ZnO:Al thin films for polycrystalline silicon (poly-Si) thin-film solar cell fabrication was investigated. The electrical and optical properties of 700 -nm-thick ZnO:Al films on glass were analyzed after typical annealing steps occurring during poly-Si film preparation. If the ZnO:Al layer is covered by a 30 nm thin silicon film, the initial sheet resistance of ZnO:Al drops from 4.2 to 2.2 Ω after 22 h annealing at 600 °C and only slightly increases for a 200 s heat treatment at 900 °C. A thin-film solar cell concept consisting of poly-Si films on ZnO:Al coated glass is introduced. First solar cell results will be presented using absorber layers either prepared by solid-phase crystallization (SPC) or by direct deposition at 600 °C.     

Nanocrystalline tin oxides and nickel oxide film anodes for Li-ion batteries

Tin oxides and nickel oxide thin film anodes have been fabricated for the first time by vacuum thermal evaporation of metallic tin or nickel, and subsequent thermal oxidation in air or oxygen ambient. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements showed that the prepared films are of nanocrystalline structure with the average particle size <100 nm. The electrochemical properties of these film electrodes were examined by galvanostatic cycling measurements and cyclic voltammetry. The composition and electrochemical properties of SnO_x (1<x<2) films strongly depend on the oxidation temperature. The reversible capacities of SnO and SnO₂ films electrodes reached 825 and 760 mAh g⁻¹, respectively, at the current density of 10 μA cm⁻² between 0.10 and 1.30 V. The SnO_x film fabricated at an oxidation temperature of 600 °C exhibited better electrochemical performance than SnO or SnO₂ film electrode. Nanocrystalline NiO thin film prepared at a temperature of 600 °C can deliver a reversible capacity of 680 mAh g⁻¹ at 10 μA cm⁻² in the voltage range 0.01–3.0 V and good cyclability up to 100 cycles.     

Effects of step-deposition on structures and properties of transparent conducting aluminum-doped zinc oxide films prepared by DC magnetron sputtering

The 2 wt% aluminum-doped zinc oxide films (AZO) was sputtered on corning glass plate at temperatures of 30–200 °C by DC magnetron sputtering using ceramic target. The microstructures and electrical resistivity of thin films were investigated by scanning electron microscope (SEM) and the van der Pauw method. The optical transmittances of films were measured by UV visible spectrophotometer in the wavelength of 300–900 nm. It was found that the average optical transmittances of specimens were 88%. Highly oriented AZO films in the (0 0 2) direction was observed in specimens as increasing of the substrate temperature. The dense film increased as the temperature increases. In addition, craters of greater depth with more compactness were obtained by step-deposition. The lowest resistivity of 9×10⁻⁴ Ω cm with film thickness of 700 nm was found in specimen grown by step-deposition at 200 °C.     

Structural and electrochemical properties of nanocrystalline LixMn2O4 thin film cathodes (x = 1.0–1.4)

The process optimization of nanocrystalline lithium manganate thin films (Li_xMn₂O₄; x = 1.0–1.4) has been demonstrated by using a cost-effective solution growth technique. Films were first attempted with Pt–Si (Si/SiO₂/TiO₂/Pt) substrates but because of inter-diffusion of TiO₂ buffer layer with Pt at higher annealing temperature, phase impure LiMn₂O₄ films were obtained. Phase pure films on the basis of XRD analysis were found on Pt substrate at specified growth parameters. The annealing temperature and annealing time were varied, the films annealed at 700 °C for 2 h were found to be the best films. The nanocrystalline nature of the films was revealed by the SEM micrographs and the surface morphology studied using AFM. Finally, the electrochemical properties (cyclic voltammetry and constant current measurements) of these films were analyzed using a home made three-electrode cell and Gamry Battery tester instrumentation. The formation of a prominent layer of fluoride species deposited over the cathode surface during the repeated cycling was revealed by XPS measurements. Further experiments are in progress on identifying the exact composition of these unwanted species. The formation of the Jahn-Teller active Mn³⁺ during electrochemical cycling was completely ruled out from the XPS analysis. Also the very consistent value of [Mn³⁺/Mn⁴⁺] ratio before and after electrochemical cycling on the surface of the film revealed good quality of the films. Finally, the formation of the fluoride layer was concluded as a passive layer that causes the initial capacity drop during first few cycles of the cell performance.     

Cyclic voltammetric studies of the effects of time and temperature on the capacitance of electrochemically deposited nickel hydroxide

A cyclic voltammetric (CV) technique was used to study the combined effects of annealing temperature and time on the pseudocapacitance of thermally treated electroprecipitated nickel hydroxide thin films. Through the analysis of the areas of the CVs cycled between 0 and 0.35 V (versus Ag/AgCl) it is shown that the optimal treatment condition for maximum film capacitance occurs at 300°C for 3 h. On the other hand, using the anodic and cathodic peak currents of the CVs cycled between 0 and 0.5 V (versus Ag/AgCl), the maximum film capacitance is also shown to occur at a thermal treatment condition of 300°C and 3.5 h (or 320°C and 3.2 h for linear approximations). The two methods demonstrate simple ways of extracting useful information on the electrochemical performance properties of thin films.     

Comparative studies of nickel oxide films on different substrates for electrochemical supercapacitors

Thin nickel oxide (NiO) films were obtained by post-heating of the corresponding precursor films of nickel hydroxide (Ni(OH)₂) cathodically deposited onto different substrates, i.e., nickel foils, and graphite at 25 °C from a bath containing 1.5 mol L⁻¹ Ni(NO₃)₂ and 0.1 mol L⁻¹ NaNO₃ in a solvent of 50% (v/v) ethanol. The surface morphology of the obtained films was observed by scanning electron microscope (SEM). Electrochemical characterization was performed using cyclic voltammetrty (CV), chronopotentiometry (CP) and electrochemical impedance analysis (EIS). When heated at 300 °C for 2 h in air, the specific capacitance of the prepared NiO films on nickel foils and graphite, with a deposition charge of 250 mC cm⁻², were 135, 195 F g⁻¹, respectively. When the deposition charge is less than 280 mC cm⁻², the capacitance of both appears to keep the linear relationship with the deposition charge. The specific capacitance, cyclic stability of the NiO/graphite hybrid electrodes in 1 mol L⁻¹ KOH solution were superior to those on nickel foils mainly due to the favorable adhesion, the good interface behavior between graphite and the NiO films, and the extra pseudo-capacitance of the heated graphite substrates.     

Preparation of Cu2ZnSnS4 thin films by sulfurizing electroplated precursors

Cu₂ZnSnS₄ (CZTS) thin films were prepared by sulfurizing precursors deposited by electroplating. The precursors (Cu/Sn/Zn stacked layers) were deposited by electroplating sequentially onto Mo-coated glass substrates. Aqueous solutions containing copper sulfate for Cu plating, tin sulfate for Sn plating and zinc sulfate for Zn plating were used as the electrolytes. The precursors were sulfurized by annealing with sulfur at temperatures of 300, 400, 500 and 600 °C in an N₂ gas atmosphere. The X-ray diffraction peaks attributable to CZTS were detected in thin films sulfurized at temperatures above 400 °C. A photovoltaic cell using a CZTS thin film produced by sulfurizing an electroplated Sn-rich precursor at 600 °C exhibited an open-circuit voltage of 262 mV, a short-circuit current of 9.85 mA/cm² and an efficiency of 0.98%.     

Manganese oxide film electrodes prepared by electrostatic spray deposition for electrochemical capacitors from the KMnO4 solution

Manganese oxide film electrodes for electrochemical capacitors were deposited on the polished Pt foils by electrostatic spray deposition (ESD) from KMnO₄ precursor solution. The electrochemical properties of electrodes were systematically studied using cyclic voltammetry (CV), constant current charge–discharge tests, and electrochemical impedance spectroscopy (EIS). The specific capacitance (SC) of thick deposited film was 149 F g⁻¹ at the very high scan rate of 500 mV s⁻¹, in comparison with 209 F g⁻¹ at the low scan rate of 5 mV s⁻¹. The electrode shows good cyclic performance. The initial SC value was 163 F g⁻¹ and 103% of the initial SC can be retained after 10,000 cycles at the scan rate of 50 mV s⁻¹.     

Lithium cobalt oxide cathode film prepared by rf sputtering

Thin films of LiCoO₂ prepared by radio frequency magnetron sputtering on Pt-coated silicon are investigated under various deposited parameters such as working pressure, gas flow rate of Ar to O₂, and heat-treatment temperature. The as-deposited film was a nanocrystalline structure with (1 0 4) preferred orientation. After annealing at 500–700 °C, single-phase LiCoO₂ is obtained when the film is originally deposited under an oxygen partial pressure (P_O2) from 5 to 10 mTorr. When the sputtering process is performed outside these P_O2 values, a second phase of Co₃O₄ is formed in addition to the HT-LiCoO₂ phase. The degree of crystallization of the LiCoO₂ films is strongly affected by the annealing temperature; a higher temperature enhances the crystallization of the deposited LiCoO₂ film. The grain sizes of LiCoO₂ films annealed at 500, 600 and 700 °C are about 60, 95, and 125 nm, respectively. Cyclic voltammograms display well-defined redox peaks. LiCoO₂ films deposited by rf sputtering are electrochemically active. The first discharge capacity of thin LiCoO₂ films annealed at 500, 600 and 700 °C is about 41.77, 50.62 and 61.16 μAh/(cm² μm), respectively. The corresponding 50th discharge capacities are 58.1, 72.2 and 74.9% of the first discharge capacity.     

Effect of annealing temperature on electrochemical performance of thin-film LiMn2O4 cathode

Spinel LiMn₂O₄ thin-film cathodes were obtained by spin-coating the chitosan-containing precursor solution on a Pt-coated silicon substrate followed by a two-stage heat-treatment procedure. The LiMn₂O₄ film calcined at 700 °C for 1 h showed the highest Li-ion diffusion coefficient, 1.55 × 10⁻¹² cm² s⁻¹ (PSCA measurement) among all calcined films. It is attributed to the larger interstitial space and better crystal perfection of LiMn₂O₄ film calcined at 700 °C for 1 h. Consequently, the 700 °C-calcined LiMn₂O₄ film exhibited the best rate performance in comparison with the ones calcined at other temperatures.     

Microcrystalline silicon grown by VHF PECVD and the fabrication of solar cells

Intrinsic microcrystalline silicon has been deposited by very high frequency plasma enhanced chemical vapor deposition technique at frequency of 75 MHz. Different gas mixtures of silane and hydrogen were utilized, and the evolution of microstructure and phase in film were studied, while keeping the substrate temperature at 200 °C and the chamber pressure at 0.5 Torr. Optimised material was inserted in p–i–n solar cells: preliminary efficiency of 5.5% was reached for 1 μm-thick solar cells with the V_oc around 0.6 V.     

Anode-supported solid oxide fuel cell with yttria-stabilized zirconia/gadolinia-doped ceria bilalyer electrolyte prepared by wet ceramic co-sintering process

To protect the ceria electrolyte from reduction at the anode side, a thin film of yttria-stabilized zirconia (YSZ) is introduced as an electronic blocking layer to anode-supported gadolinia-doped ceria (GDC) electrolyte solid oxide fuel cells (SOFCs). Thin films of YSZ/GDC bilayer electrolyte are deposited onto anode substrates using a simple and cost-effective wet ceramic co-sintering process. A single cell, consisting of a YSZ (∼3 μm)/GDC (∼7 μm) bilayer electrolyte, a La_0.8Sr_0.2Co_0.2Fe_0.8O₃–GDC composite cathode and a Ni–YSZ cermet anode is tested in humidified hydrogen and air. The cell exhibited an open-circuit voltage (OCV) of 1.05 V at 800 °C, compared with 0.59 V for a single cell with a 10-μm GDC film but without a YSZ film. This indicates that the electronic conduction through the GDC electrolyte is successfully blocked by the deposited YSZ film. In spite of the desirable OCVs, the present YSZ/GDC bilayer electrolyte cell achieved a relatively low peak power density of 678 mW cm⁻² at 800 °C. This is attributed to severe mass transport limitations in the thick and low-porosity anode substrate at high current densities.     

A novel Al and Y codoped ZnO/n-Si heterojunction solar cells fabricated by pulsed laser deposition

Al and Y codoped ZnO (AZOY) transparent conducting oxide (TCO) thin films were first deposited on n-Si substrates by pulsed laser deposition (PLD) to form AZOY/n-Si heterojunction solar cells. However, the properties of the AZOY emitter layers are critical to the performance of AZOY/n-Si heterojunction solar cells. To estimate the properties of AZOY thin films, films deposited on glass substrates with various substrate temperatures (T_s) were analyzed. Based on the experimental results, optimal electrical properties (resistivity of 2.8 ± 0.14 × 10^?4 Ω cm, carrier mobility of 27.5 ± 0.55 cm²/Vs, and carrier concentration of 8.0 ± 0.24 × 10²⁰ cm^?3) of the AZOY thin films can be achieved at a T_s of 400 °C, and a high optical transmittance of AZOY is estimated to be >80% (with glass substrate) in the visible region under the same T_s. For the AZOY/n-Si heterojunction solar cells, the AZOY thin films acted not only as an emitter layer material, but also as an anti-reflected coating thin film. Thus, a notably high short-circuit current density (J_sc) of 31.51 ± 0.186 mA/cm² was achieved for the AZOY/n-Si heterojunction solar cells. Under an AM1.5 illumination condition, the conversion efficiency of the cells is estimated at only approximately 4% (a very low open-circuit voltage (V_oc) of 0.24 ± 0.001 V and a fill factor (FF) of 0.51 ± 0.011) without any optimization of the device structure.     

NiO–YSZ cermets supported low temperature solid oxide fuel cells

Solid oxide fuel cells with thin electrolyte of two types, Sm_0.2Ce_0.8O_1.9 (SDC) (15 μm) single-layer and 8 mol% Yttria stabilized zirconia (YSZ) (5 μm) + SDC (15 μm) bi-layer on NiO–YSZ cermet substrates were fabricated by screen printing and co-firing. A Sm_0.5Sr_0.5CoO₃ cathode was printed, and in situ sintered during a cell performance test. The SDC single-layer electrolyte cell showed high electrochemical performance at low temperature, with a 1180 mW cm⁻² peak power density at 650 °C. The YSZ + SDC bi-layer electrolyte cell generated 340 mW cm⁻² peak power density at 650 °C, and showed good performance at 700–800 °C, with an open circuit voltage close to theoretical value. Many high Zr-content micro-islands were found on the SDC electrolyte surface prior to the cathode preparation. The influence of co-firing temperature and thin film preparation methods on the Zr-islands’ appearance was investigated.