Dye-sensitized solar cell based on optically transparent TiO2 nanocrystalline electrode prepared by atomized spray pyrolysis technique

Preparation of crack-free thin films of interconnected and non-agglomerated TiO₂ nanoparticles on electronically conducting fluorine doped tin oxide surfaces is instrumental in designing and developing transparent dye-sensitized solar cells (DSCs). A novel technique called “Atomized Spray Pyrolysis” (ASP) has been designed and developed to achieve such perfectly transparent thin films. Optical transmittance of TiO₂ films produced on FTO surface by this ASP method has been compared with those obtained by doctor-blading and by hand spray methods and found that the atomized spray pyrolysis technique give films with high transparency. Dye adsorption per gram of TiO₂ is 2.16 times higher in the sample produced by the ASP method when compared to the film produced by the hand spray method and is 1.60 times higher than that produced by the doctor-blading method using a commercially available TiO₂ nanocrystalline paste. SEM studies show the presence of interconnected discrete particles in the film produced by the ASP method. The fill factor (ff) remains almost constant for the cells with thickness from 6 μm to 13 μm but the highest photovoltage and photocurrent were found in ∼10 μm film based DSC which gave 8.2% conversion efficiency at AM 1.5 irradiation for cells of 0.25 cm² active area.     

Optimization of the dye-sensitized solar cell performance by mechanical compression

In this study, the P25 titanium dioxide (TiO₂) nanoparticle (NP) thin film was coated on the fluorine-doped tin oxide (FTO) glass substrate by a doctor blade method. The film then compressed mechanically to be the photoanode of dye-sensitized solar cells (DSSCs). Various compression pressures on TiO₂ NP film were tested to optimize the performance of DSSCs. The mechanical compression reduces TiO₂ inter-particle distance improving the electron transport efficiency. The UV–vis spectrophotometer and electrochemical impedance spectroscopy (EIS) were employed to quantify the light-harvesting efficiency and the charge transport impedance at various interfaces in DSSC, respectively. The incident photon-to-current conversion efficiency was also monitored. The results show that when the DSSC fabricated by the TiO₂ NP thin film compressed at pressure of 279 kg/cm², the minimum resistance of 9.38 Ω at dye/TiO₂ NP/electrolyte interfaces, the maximum short-circuit photocurrent density of 15.11 mA/cm², and the photoelectric conversion efficiency of 5.94% were observed. Compared to the DSSC fabricated by the non-compression of TiO₂ NP thin film, the overall conversion efficiency is improved over 19.5%. The study proves that under suitable compression pressure the performance of DSSC can be optimized.     

TiO2 micro-flowers composed of nanotubes and their application to dye-sensitized solar cells

TiO₂ micro-flowers were made to bloom on Ti foil by the anodic oxidation of Ti-protruding dots with a cylindrical shape. Arrays of the Ti-protruding dots were prepared by photolithography, which consisted of coating the photoresists, attaching a patterned mask, illuminating with UV light, etching the Ti surface by reactive ion etching (RIE), and stripping the photoresist on the Ti foil. The procedure for the blooming of the TiO₂ micro-flowers was analyzed by field emission scanning electron microscopy (FESEM) as the anodizing time was increased. Photoelectrodes of dye-sensitized solar cells (DSCs) were fabricated using TiO₂ micro-flowers. Bare TiO₂ nanotube arrays were used for reference samples. The short-circuit current (J_sc) and the power conversion efficiency of the DSCs based on the TiO₂ micro-flowers were 4.340 mA/cm² and 1.517%, respectively. These values of DSCs based on TiO₂ micro-flowers were higher than those of bare samples. The TiO₂ micro-flowers had a larger surface area for dye adsorption compared to bare TiO₂ nanotube arrays, resulting in improved J_sc characteristics. The structure of the TiO₂ micro-flowers allowed it to adsorb dyes very effectively, also demonstrating the potential to achieve higher power conversion efficiency levels for DSCs compared to a bare TiO₂ nanotube array structure and the conventional TiO₂ nanoparticle structure.     

Facile electrochemical preparation of three-dimensional porous Cu films by potential perturbation

A 3-dimentional (3D) micro-nano hierarchical porous Cu film was fabricated by surface rebuilding of smooth Cu substrates in a blank solution of 1 M NaOH with square wave potential perturbation. The potential step from 0.4 to −2.5 V (vs. SCE) and a frequency of 50 Hz were chosen for the fabrication. The pore formation and Cu nanostructure evolution were characterized by scanning electron microscopy. The fabrication process involved fast Cu electrochemical oxidation-reduction and suitable rate of H₂ releasing. During the repeated Cu oxidation-reduction, the Cu atoms were removable, forming dendrite-like structures. At the same time, the formed H₂ bubbles acted as a dynamic template to shape the formation of micropores. The effect of H₂ bubble as a template on the size of the formed micropores was demonstrated by adding a small amount of surfactant (cetyltrimethylammonium bromide, CTAB) into the basic solution to adjust the size of the bubbles. The as-prepared 3D porous Cu showed high electrocatalytic activity toward the reduction of NO₃⁻ and H₂O₂. The present in situ preparation method was green, convenient and required neither Cu(II) species and additives in solution nor post-treatment for template removal.     

CuInS2 quantum dot-sensitized TiO2 nanorod array photoelectrodes: synthesis and performance optimization

CuInS₂ quantum dots (QDs) were deposited onto TiO₂ nanorod arrays for different cycles by using successive ionic layer adsorption and reaction (SILAR) method. The effect of SILAR cycles on the light absorption and photoelectrochemical properties of the sensitized photoelectrodes was studied. With optimization of CuInS₂ SILAR cycles and introduction of In₂S₃ buffer layer, quantum dot-sensitized solar cells assembled with 3-μm thick TiO₂ nanorod film exhibited a short-circuit current density (I_sc) of 4.51 mA cm⁻², an open-circuit voltage (V_oc) of 0.56 V, a fill factor (FF) of 0.41, and a power conversion efficiency (η) of 1.06%, respectively. This study indicates that SILAR process is a very promising strategy for preparing directly anchored semiconductor QDs on TiO₂ nanorod surface in a straightforward but controllable way without any complicated fabrication procedures and introduction of a linker molecule.     

Electrochemical characterization of amorphous LiFe(WO4)2 thin films as positive electrodes for rechargeable lithium batteries

Amorphous LiFe(WO₄)₂ thin films have been fabricated by radio-frequency (R.F.) sputtering deposition at room temperature. The as-deposited and electrochemically cycled thin films are, respectively, characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and X-ray photoelectron spectra techniques. An initial discharge capacity of 198 mAh/g in Li/LiFe(WO₄)₂ cells is obtained, and the electrochemical behavior is mostly preserved in the following cycling. These results identified the electrochemical reactivity of two redox couples, Fe³⁺/Fe²⁺ and W⁶⁺/W^x+ (x = 4 or 5). The kinetic parameters and chemical diffusion coefficients of Li intercalation/deintercalation are estimated by cyclic voltammetry and alternate-current (AC) impedance measurements. All-solid-state thin film lithium batteries with Li/LiPON/LiFe(WO₄)₂ layers are fabricated and show high capacity of 104 μAh/cm² μm in the first discharge. As-deposited LiFe(WO₄)₂ thin film is expected to be a promising positive electrode material for future rechargeable thin film batteries due to its large volumetric rate capacity, low-temperature fabrication and good electrode/electrolyte interface.     

A diffusion kinetics study of Li-ion in LiV3O8 thin film electrode

The kinetic behaviors of Li-ion insertion/extraction in LiV₃O₈ thin film have been investigated using cyclic voltammetry (CV), potentiostatic intermittent titration (PITT) and electrochemical impedance spectroscopy (EIS) method. This LiV₃O₈ thin film with a mixed amorphous-nanocrystalline microstructure was fabricated by RF sputtering. For the first time, the intrinsic kinetics of LiV₃O₈ thin film electrode is obtained. The DLi+ value is about 10⁻¹³ cm²/s in mixed amorphous-nanocrystalline microstructure LiV₃O₈ thin film. Different to crystalline LiV₃O₈ thin film, the DLi+ values do not change a lot with the increase of cell potential which is due to the absence of structural phase transition behavior in mixed microstructure LiV₃O₈ thin film during Li⁺ insertion/extraction process. This is also the reason for excellent capacity retention performance of LiV₃O₈ film with a mixed microstructure.     

Layer-by-layer assembly of functional silica and Au nanoparticles for fabricating electrogenerated chemiluminescence sensor

We described the use of silica nanoparticles as building blocks for the immobilization of electrogenerated chemiluminescence (ECL) reagent Ru(bpy)₃²⁺ and the fabrication of layer-by-layer assembly film by alternating the deposition of the Ru(bpy)₃²⁺-doped silica nanoparticles and Au nanoparticles. UV-vis absorption spectroscopy, scanning electron microscopy (SEM), cyclic voltammetry and ECL were used to characterize the uniform growth of the multilayer film. Since Ru(bpy)₃²⁺ could still maintain its ECL property when doped into the silica nanoparticles, the as-prepared multilayer film could be used as an effective ECL sensor, and the sensor showed high sensitivity and good stability.     

Electroless plated SnO2–Pd–Au composite thin film for room temperature H2 detection

Extremely thin SnO₂ nanosheets with high surface area were fabricated through a one-pot hydrothermal method. In this work, gas sensing property of the SnO₂ nanosheets was studied. SnO₂–Pd–Au mixed thin films were prepared by electroless deposition of Pd, Au, and nanostructured SnO₂ onto the surface of a high resistance alumina substrate. The whole fabrication process was carried out at room temperature without any thermal treatment required. The films deposited on the alumina substrate were characterized by SEM and EDS. The co-deposited Au improved the electric conductance of the sensing film. A relatively large amount of Pd (Pd/Sn ratio around 1:1) was obtained for the film instead of the usually low doping value of Pd (∼0.1% level) for SnO₂ hydrogen sensor. It has been found that the SnO₂–Pd–Au composite film sensor has fast response in the range of 134–1469 ppm toward hydrogen gas at room temperature. The sensor also shows good stability and repeatability. Effects of annealing condition of the sensing film on H₂ gas sensing performance was investigated as well. A possible machnism for SnO₂–Pd room temperature hydrogen sensing is proposed.     

Nanofabrication on monocrystalline silicon through friction-induced selective etching of Si3N4 mask

A new fabrication method is proposed to produce nanostructures on monocrystalline silicon based on the friction-induced selective etching of its Si₃N₄ mask. With low-pressure chemical vapor deposition (LPCVD) Si₃N₄ film as etching mask on Si(100) surface, the fabrication can be realized by nanoscratching on the Si₃N₄ mask and post-etching in hydrofluoric acid (HF) and potassium hydroxide (KOH) solution in sequence. Scanning Auger nanoprobe analysis indicated that the HF solution could selectively etch the scratched Si₃N₄ mask and then provide the gap for post-etching of silicon substrate in KOH solution. Experimental results suggested that the fabrication depth increased with the increase of the scratching load or KOH etching period. Because of the excellent masking ability of the Si₃N₄ film, the maximum fabrication depth of nanostructure on silicon can reach several microns. Compared to the traditional friction-induced selective etching technique, the present method can fabricate structures with lesser damage and deeper depths. Since the proposed method has been demonstrated to be a less destructive and flexible way to fabricate a large-area texture structure, it will provide new opportunities for Si-based nanofabrication.     

LiCoO2 thin film cathode fabrication by rapid thermal annealing for micro power sources

The rapid thermal annealing (RTA) process was employed to obtain crystalline LiCoO₂ thin films. XRD analyses of the LiCoO₂ thin film show increased crystallinity with an increase in the RTA time. The Auger electron spectroscopic analysis of the LiCoO₂ film strongly suggests that the RTA process is more advantageous to obtain a stable inter layer between the substrate and the deposited film and between each deposited layer than the conventional annealing process. All-solid-state thin film cells composed of Li/lithium phosphorous oxynitride (Lipon)/LiCoO₂ systems were fabricated using the LiCoO₂ cathode treated with RTA. The optimum condition of RTA would be 900 s at 650 °C, which exhibited a good rate capability for high power applications. Two cells were connected in parallel to obtain a higher discharge current, and they showed a specific capacity of 38.4 μAh cm⁻² μm⁻¹ even at a 25C rate (current density: 7.96 mA cm⁻²).     

The effect of compression on electron transport and recombination in plastic TiO2 photoanodes

The compression method was applied for the preparation of plastic TiO₂ porous films on a conductive indium–tin oxide (ITO)-coated polyethylene naphthalate (PEN) substrate at low temperature for the generation of high-efficiency plastic dye-sensitized solar cells (DSCs). The compression parameters, including pressure and time, were varied in order to determine their effect on the photovoltaic performance of the plastic DSCs. The results from electrochemical impedance spectroscopy (EIS) showed that charge transport resistance in the porous TiO₂ films (R_t) gradually decreased when the applied pressure was increased from 0 MPa to 150 MPa, which indicated a better connection between the TiO₂ nanoparticles and electron transport in the TiO₂ films. In addition, a longer press time led to an increased resistance of electron recombination (R_ct) and an increased charge-collection efficiency. After optimization of the compression parameters, the efficiency of energy conversion was increased by approximately 81.6%. In addition, the efficiency of energy conversion was increased by an additional 4.65% under AM1.5 illumination.     

Li diffusion in LiNi0.5Mn0.5O2 thin film electrodes prepared by pulsed laser deposition

Kinetic and transport parameters of Li ion during its extraction/insertion into thin film LiNi_0.5Mn_0.5O₂ free of binder and conductive additive were provided in this work. LiNi_0.5Mn_0.5O₂ thin film electrodes were grown on Au substrates by pulsed laser deposition (PLD) and post-annealed. The annealed films exhibit a pure layered phase with a high degree of crystallinity. Surface morphology and thin film thickness were investigated by field emission scanning electron microscopy (FESEM). The charge/discharge behavior and rate capability of the thin film electrodes were investigated on Li/LiNi_0.5Mn_0.5O₂ cells at different current densities. The kinetics of Li diffusion in these thin film electrodes were investigated by cyclic voltammetry (CV) and galvanostatic intermittent titration technique (GITT). CV was measured between 2.5 and 4.5 V at different scan rates from 0.1 to 2 mV/s. The apparent chemical diffusion coefficients of Li in the thin film electrode were calculated to be 3.13 × 10⁻¹³ cm²/s for Li intercalation and 7.44 × 10⁻¹⁴ cm²/s for Li deintercalation. The chemical diffusion coefficients of Li in the thin film electrode were determined to be in the range of 10⁻¹²-10⁻¹⁶ cm²/s at different cell potentials by GITT. It is found that the Li diffusivity is highly dependent on the cell potential.     

Composition-dependent characterization and optimal control of electrodeposited Bi2Te3 films for thermoelectric application

A method to control composition of Bi₂Te₃ films by mass transfer manipulation has been developed. The film composition can be varied by a diffusion-controlled method, which is related to the change of Bi³⁺/HTeO₂⁺ ratios in a controlled diffusion layer. A homogeneous and dense film with precise chemical composition could thus be obtained under constant electrode polarization. Meanwhile, the solo dependence of film properties on composition change of both Te-rich and Bi-rich films were investigated. Firstly, the studies of XRD and FE-SEM showed that different Te contents in deposit would lead to different dimensions of unit cell and grain sizes. The Seebeck coefficient increased apparently when the Te content was over 60 at.% Te. Te-rich films had higher carrier concentration but slower mobility than Bi-rich films. Inverse relations were observed between carrier concentration and carrier mobility and between Seebeck coefficient and conductivity. Therefore, an optimal power factor of 7 × 10⁻⁴ W/m K² was realized near the stoichiometric Bi₂Te₃.     

Electrochemistry of V2ON with lithium

V₂ON thin film has been successfully fabricated by reactive dc sputtering method and annealing process and was investigated for its electrochemistry with lithium. The reversible discharge capacities of V₂ON/Li cells cycled between 0.01 and 4.0 V were found in the range of 803–915 mAh g⁻¹ during the first 50 cycles. By using ex situ scanning electron microscopy, transmission electron microscopy, selected-area electron diffraction and X-ray photoelectron spectroscopy measurements, the reversible transformation between nanocrystalline V₂ON and well dispersed V, Li₂O, Li₃N nano-composites were revealed in the lithium electrochemical reaction. V₂ON thin film exhibits high reversible capacity and good cycle performance with remarkable lower polarization than VN thin film.     

Electrochemical reactivity of Co-Li2S nanocomposite for lithium-ion batteries

The fabrication of Co-Li₂S nanocomposite thin film is reported by pulsed laser deposition (PLD) for the first time. Li₂S-Co nanocomposite thin film is used as storing Li electrodes that have led to promising electrochemical activity and good electrochemical performance. The releasing Li process from the as-deposited Li₂S-Co nanocomposite thin films is confirmed by the ex situ high resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) measurements and may come from the decomposition of Li₂S with and without the interaction of metal Co into CoS₂ and S. The electrochemical reaction mechanism of Co-Li₂S nanocomposite film electrode involving both the formation and decomposition of Li₂S and the lithium extraction/insertion of CoS₂ after the initial charging process is proposed. Our results demonstrate the advantages of using Co-Li₂S nanocomposite in storage lithium materials.     

Effects of electrolyte in dye-sensitized solar cells and evaluation by impedance spectroscopy

Effects of the electrolyte of DSCs on impedance spectra were evaluated by changing concentration of redox couple, viscosity, and additives to electrolyte. The relation with current-voltage characteristics (I-V characteristics) was investigated. In many cases, the impedance component attributed to charge transfer at TiO₂|electrolyte interface demonstrated strong relation with the I-V characteristics. The recombination of electrons in TiO₂ with I₃⁻ in electrolyte was a key factor in determining performance of DSCs. To evaluate the effect of I₃⁻, diffusion-limiting current in the electrolyte for various viscosities was evaluated by cyclic voltammetry. When the short circuit current (SCC) was almost equal to the diffusion-limiting current, strong influence of the diffusion coefficient on the impedance spectra was observed: impedance arcs were enlarged as the diffusion coefficient was decreased. On the other hand, when the diffusion-limiting current was larger than the SCC, photo-excitation and electron injection processes became dominating factors in the DSCs performance. The SCC was regulated by the charge recombination process at TiO₂|electrolyte interface, and thus the impedance component ω₃ was related to the performance in such condition.     

A novel and facile route of ink-jet printing to thin film SnO2 anode for rechargeable lithium ion batteries

Thin film SnO₂ electrode has been prepared for the first time by using a novel facile and low-cost ink-jet printing technique. Wet ball-milling was employed to stabilize SnO₂ nano particles and conducting agent acetylene black (AB) using two kinds of polymeric hyperdispersants CH10B and CH12B, respectively, to prepare the stable colloid as “ink”. The morphology, structure, composition and electrochemical performance of SnO₂ thin film electrodes were investigated in detail by SEM, TEM, XRD, EDX, cyclic voltammograms (CV) and galvanostatic charge-discharge measurements. SEM images show uniform distribution of as-printed SnO₂ thin film electrodes. The thickness of monolayer thin film electrode was about 770-780 nm by TEM observation. The thickness of SnO₂ thin film could be increased by repeating the printing procedure on the Cu foil substrate. The average thickness of 10-layer SnO₂ thin-film electrode after compression for electrochemical measurement was about 2.3 μm. High initial discharge capacity about 812.7 mAh/g was observed at a constant discharge current density of 33 μA/cm² in a potential range of 0.05-1.2 V. It is expected that ink-jet printing is a very feasible, simple, convenient and inexpensive way to prepare thin film electrode for lithium ion batteries.     

Large-scale preparation of nanoporous TiO2 film on titanium substrate with improved photoelectrochemical performance

Fabrication of three-dimensional TiO₂ films on Ti substrates is one important strategy to obtain efficient electrodes for energy conversion and environmental applications. In this work, we found that hierarchical porous TiO₂ film can be prepared by treating H₂O₂ pre-oxidized Ti substrate in TiCl₃ solution followed by calcinations. The formation process is a combination of the corrosion of Ti substrate and the oxidation hydrolysis of TiCl₃. According to the characterizations by scanning electron microscopy (SEM), X-ray diffraction (XRD), and diffuse reflectance spectroscopy (DRS), the anatase phase TiO₂ films show porous morphology with the smallest diameter of 20 nm and possess enhanced optical absorption properties. Using the porous film as a working electrode, we found that it displays efficient activity for photoelectrocatalytic decolorization of rhodamine B (RhB) and photocurrent generation, with a photocurrent density as high as 1.2 mA/cm². It represents a potential method to fabricate large-area nanoporous TiO₂ film on Ti substrate due to the scalability of such chemical oxidation process.     

SnO2-coated multiwall carbon nanotube composite anode materials for rechargeable lithium-ion batteries

SnO₂-coated multiwall carbon nanotube (MWCNT) nanocomposites were synthesized by a facile hydrothermal method. The as-prepared nanocomposites were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The SnO₂/MWCNT composites, when combined with carboxymethyl cellulose (CMC) as a binder, show excellent cyclic retention, with the high specific capacity of 473 mAh g⁻¹ beyond 100 cycles, much greater than that of the bare SnO₂ which was also prepared by the hydrothermal method in the absence of MWCNTs. The enhanced capacity retention could be mainly attributed to good dispersion of the tin dioxide particles in the matrix of MWCNTs, which protected the particles from agglomeration during the cycling process. Furthermore, the usage of CMC as a binder is responsible for the low cost and environmental friendliness of the whole electrode fabrication process.