Lipase immobilized on nanostructured cerium oxide thin film coated on transparent conducting oxide electrode for butyrin sensing

Nanostructured cerium oxide (CeO₂) thin films were deposited on transparent conducting oxide (TCO) substrate using spray pyrolysis technique with cerium nitrate salt, Ce(NO₃)₃·6H₂O as precursor. Fluorine doped cadmium oxide (CdO:F) thin film prepared using spray pyrolysis technique acts as the TCO film and hence the bare electrode. The structural, morphological and elemental characterizations of the films were carried out using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray analysis (EDX) respectively. The diffraction peak positions in XRD confirmed the formation of highly crystalline ceria with cubic structure and FE-SEM images showed uniform adherent films with granular morphology. The band gaps of CeO₂ and TCO were found to be 3.2 eV and 2.6 eV respectively. Lipase enzyme was physisorbed on the surface of CeO₂/TCO film to form the lipase/nano-CeO₂/TCO bioelectrode. Sensing studies were carried out using cyclic voltammetry and amperometry, with lipase/nano-CeO₂/TCO as working electrode and tributyrin as substrate. The mediator-free biosensor with nanointerface exhibited excellent linearity (0.33–1.98 mM) with a lowest detection limit of 2 μM with sharp response time of 5 s and a shelf life of about 6 weeks.     

A study of crystallographic phases in non-stoichiometric (oxygen deficiency) indium oxide thin films

Indium oxide is a well-known transparent conductive oxide (TCO) in its stoichiometric composition (In₂O₃). Its electrical and optical properties are strongly influenced by the chemical composition. This work focuses on an experimental investigation of the crystallographic phases in non-stoichiometric (oxygen deficiency) compositions of indium oxide thin films. The thin films were deposited at 300 °C by reactive sputtering of pure indium target at different oxygen gas flow rates on Si substrates. Two different phases are identified only in the non-stoichiometric compositions: metallic indium- and crystalline indium-rich oxide. The metallic indium phase appears as nano-crystals, a few nano-meters in diameter, evenly dispersed and occupies only 1 vol. % of the film. These metallic nano-particles have a negligible effect on the optical transparency and electrical conductivity of the films. The indium-rich oxide (In_xO_y) phase which occupies about 99 vol. % of the film has the bixbyite crystallographic structure and average grain size of about 50 nm. This phase has a pronounced effect on improving the TCO figure-of-merit (FM) relative to stoichiometric crystalline In₂O₃ films due to a higher increase of the electrical conductivity than the decrease of the optical transparency.     

Cubic PbS thin films on TCO glass substrate by galvanic technique

We report an electrochemical deposition of PbS thin films on SnO₂:F coated Transparent Conducting Oxide (TCO) glass substrates from a solution of EDTA, Pb(OAc)₂ and Na₂S₂O₃. A Pb strip acted as a sacrificial anode, while the TCO glass was the cathode. No external bias was applied. The deposition of PbS thin films was pH sensitive and a pH around 3 was found to be optimum for film deposition. The deposition was carried out at 80 °C, with stirring of the solution. X-ray diffraction studies revealed that the PbS films were of cubic phase. Scanning electron microscope image analysis showed a highly compact surface morphology. IR spectrum yielded an energy band gap around 0.4 eV. A.C. current–voltage (I–V) measurements were carried out using gold as one of the contacts. It was found that the PbS film formed an ohmic contact when the other electrode was also gold, however, a Schottky junction resulted, when the second contact was TCO.     

TiO2 thin films using organic liquid materials prepared by Hot-Wire CVD method

TiO₂ thin films prepared by Hot-Wire CVD method have been studied as a protecting material of transparent conducting oxide (TCO) against atomic hydrogen exposures for the fabrications of Si thin film solar cells. It was found that electrical conductivity of the films at room temperature reached a value of 0.4 S/cm. This value is 2-3 orders of magnitude higher than that of TiO₂ films prepared by RF magnetron sputtering and electron-beam evaporation methods in our previous works. The conductivity improvement seems to be partly due to the enlargement of TiO₂ crystallites.     

New natively textured surface Al-doped ZnO-TCOs for thin film solar cells via magnetron sputtering

Natively textured surface aluminum doped zinc oxide (ZnO:Al) thin films were directly deposited via pulsed direct current (DC) reactive magnetron sputtering on glass substrates. During the reactive sputtering process, the oxygen gas flow rate was varied from 8.5 sccm to 11.0 sccm. The influences of oxygen flow rate on the structural, electrical and optical properties of naturally textured ZnO:Al TCO thin films with milky surface were investigated in detail. Gradual oxygen growth (GOG) technique was developed in the reactive sputtering process for textured ZnO:Al thin films. The light-scattering ability and optical transmittance of the natively textured ZnO:Al TCO thin films can be improved through gradual oxygen growth method while maintaining a low sheet resistance. Typical natively textured ZnO:Al TCO thin film with crater-like surface exhibits low sheet resistance (R_s ～ 4 Ω), high transmittance (T_a > 85%) in visible optical region and high haze value (12.1%).     

Galvanic deposition of hexagonal ZnO thin films on TCO glass substrate

We report an electrochemical deposition of ZnO thin films on SnO₂:F coated Transparent Conducting Oxide (TCO) glass substrates from a solution of Zn(NO₃)₂. A Zn rod acted as a sacrificial anode, while the TCO glass was the cathode. No external bias was applied. The deposition of ZnO thin films was pH sensitive and a pH between 5 and 6 was found to be optimum for film deposition. The deposition was carried out at 60 and 80 °C, with stirring of the solution. X-ray diffraction studies revealed that the ZnO films were of hexagonal phase. Scanning electron microscope (SEM) images showed prominent hexagonal micro-grains with vertical columnar growth. Optical absorption spectra yielded an energy band gap around 3.3 to 3.4 eV. A.C. current–voltage (I–V) and capacitance–voltage (C–V) measurements were carried out using colloidal silver paste as one of the contact. It was found that the ZnO film formed a Schottky junction and the TCO/ZnO/Ag system could be used as an electrical switching device.     

Applications of microcrystalline hydrogenated cubic silicon carbide for amorphous silicon thin film solar cells

We demonstrated the fabrication of n-i-p type amorphous silicon (a-Si:H) thin film solar cells using phosphorus doped microcrystalline cubic silicon carbide (μc-3C-SiC:H) films as a window layer. The Hot-wire CVD method and a covering technique of titanium dioxide TiO₂ on TCO was utilized for the cell fabrication. The cell configuration is TCO/TiO₂/n-type μc-3C-SiC:H/intrinsic a-Si:H/p-type μc- SiC_x (a-SiC_x:H including μc-Si:H phase)/Al. Approximately 4.5% efficiency with a V_oc of 0.953 V was obtained for AM-1.5 light irradiation. We also prepared a cell with the undoped a-Si_1−xC_x:H film as a buffer layer to improve the n/i interface. A maximum V_oc of 0.966 V was obtained.     

Growth and characterization of transparent conducting nanostructured zinc indium oxide thin films

Transparent conductive oxide (TCO) films have been widely used in various applications, such as for transparent electrodes in flat-panel displays, and in solar cells, optoelectronic devices, touch panels and IR reflectors. Among these, tin doped zinc oxide (ZTO) and indium doped zinc oxide (ZIO) have attracted considerable attention. Particularly, IZO thin film is the best candidate for high-quality transparent conducting electrodes in OLEDs and flexible displays. In this work zinc indium oxide (ZIO) thin films were deposited on glass substrate with varying concentration (ZnO:In₂O₃ — 100:0, 90:10, 70:30 and 50:50 wt.%) at room temperature by flash evaporation technique. These deposited ZIO films were annealed in vacuum to study the thermal stability and to see the effects on the physical properties. The XRF spectra revealed the presence of zinc and indium with varying concentration in ZIO thin films, while the surface composition and oxidation state were analyzed by X-ray photoelectron spectroscopy. The core level spectra were deconvoluted to see the effect of chemical changes, while the valance band spectra manifest the electronic transitions. The surface morphology studies of the films using atomic force microscopy (AFM) revealed the formation of nanostructured ZIO thin films. The optical band gap was also found to be decreased for both types of films with increasing concentration of In₂O₃.     

An electrochemical technique to deposit thin films of PbTe

PbTe thin films were deposited electrochemically on transparent conducting oxide coated (TCO) glass substrates from a solution of lead acetate and TeO₂ at low pH. A lead (Pb) strip was used as a sacrificing anode and the TCO glass acted as the cathode, which were short-circuited externally. Depositions were carried out at different temperatures of the bath to study the growth kinetics and grain growth. X-ray diffraction technique, scanning electron microscopy, infrared spectroscopy and resistivity measurements were carried out to characterize the deposited films. The films were polycrystalline in nature with a cubic phase.     

Optical and electrical properties of Sn-doped CdO thin films obtained by pulse laser deposition

Transparent tin-doped cadmium oxide (Sn-CdO) thin films with different Sn concentration were deposited on quartz glass substrates by pulse laser deposition (PLD) at 400 °C. The film’s crystallographic structure, optical and electrical properties were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), UV-VIS spectrophotometer and Hall system. Results show that doping of Sn enhances the film’s [111] preferred orientation and causes slight shift in the (200) Bragg angle towards higher value. The optical band gaps (E_g) of the Sn-doped films were found to increase with the increase of Sn doping concentration. In addition, proper doping of Sn evidently improves the electrical properties of CdO, such as the resistivity of the CdO film with 2.9 at% Sn doping is about one-twelfth of that of the CdO film, while the carrier concentration is about 13 times of that of the undoped. The improvements both in optical and electrical properties endow that the Sn-CdO thin films have potential application as TCO material for different optoelectronic device applications.     

Negative ions in reactive magnetron sputtering

Negative ions are present in magnetron sputtering if electronegative elements are involved. The majority of the negative ions impinging on transparent conductive oxide (TCO) films during growth is O^? produced at the oxidised target surface while it is rather unimportant whether it is bulk oxide or a surface oxide formed in reactive sputtering. O^? bombards the film with energies equivalent to the target voltage and by far exceed 100 eV. It is hence apt to cause radiation damage in sensitive TCO films. This is shown by the lateral distribution of the highenergy O^? flux in planar magnetron sputtering that exhibits the same pattern imaging the erosion groove as the resistivity of TCO films. This lateral inhomogeneity strongly depends on the erosion groove depth. The emission of O^? further depends on the sputtered material, for TiO₂ deposition it is much less than for other TCO materials. The emission probability correlates to the secondary electron emission coefficient of the oxide.     

Application of hydrogen-doped In2O3 transparent conductive oxide to thin-film microcrystalline Si solar cells

Hydrogen-doped In₂O₃ (IO:H) films with high electron mobility and improved near-infrared (NIR) transparency have been applied as a transparent conducting oxide (TCO) electrode in substrate-type hydrogenated microcrystalline silicon (μc-Si:H) solar cells. The incorporation of IO:H, instead of conventional Sn-doped In₂O₃, improved the short-circuit current density (J_sc) and the resulting conversion efficiency. Optical analysis of the solar cells and TCO films revealed that the improvement in J_sc is due to the improved spectral sensitivity in the visible and NIR wavelengths by reduction of absorption loss caused by free carriers in the TCO films.     

Substitution of transparent conducting oxide thin films for indium tin oxide transparent electrode applications

The present status and prospects for further development of reduced or indium-free transparent conducting oxide (TCO) materials for use in practical thin-film transparent electrode applications such as liquid crystal displays are presented in this paper: reduced-indium TCO materials such as ZnO-In₂O₃, In₂O₃-SnO₂ and Zn-In-Sn-O multicomponent oxides and indium-free materials such as Al- and Ga-doped ZnO (AZO and GZO). In particular, AZO thin films, with source materials that are inexpensive and non-toxic, are the best candidates. The current problems associated with substituting AZO or GZO for ITO, besides their stability in oxidizing environments as well as the non-uniform distribution of resistivity resulting from dc magnetron sputtering deposition, can be resolved. Current developments associated with overcoming the remaining problems are also presented: newly developed AZO thin-film deposition techniques that reduce resistivity as well as improve the resistivity distribution uniformity using high-rate dc magnetron sputtering depositions incorporating radio frequency power. In addition, stability tests of resistivity in TCO thin films evaluated in air at 90% relative humidity and 60 °C have demonstrated that sufficiently moisture-resistant AZO thin films can be produced at a substrate temperature below 200 °C when the film thickness was approximately 200 nm. However, improving the stability of AZO and GZO films with a thickness below 100 nm remains a problem.     

Characterization of transparent conducting oxide thin films deposited on ceramic substrates

In this work, we investigate the optical and electrical properties of various transparent conductive oxide (TCO) thin films deposited on insulating ceramics for emerging optoelectronic applications. Thin films investigated include indium tin oxide (ITO), ruthenium oxide (RuO₂), and iridium oxide (IrO₂) on Al₂O₃ ceramic substrates. The conducting films have been deposited by various techniques including RF magnetron sputtering and low-cost spray pyrolysis. The morphological characteristics of the films were carried out using high magnification optical microscopy and atomic force microscopy (AFM). Optical and electrical characterization was carried out by optical absorbance/transmittance, van der Pauw, current-voltage (I-V), and Hall effect measurements. The results are presented in this paper.     

Formation of semitransparent CuAlSe2 thin films grown on transparent conducting oxide substrates by selenization

Wide band-gap semiconductors have been studied for applications as buffer layers in thin film solar cells and as top cell in tandem devices. CuAlSe₂ (CAS) thin films were deposited onto bare and two different transparent conducting oxide (TCO)-coated glass substrates, In₂O₃:Sn (ITO) and ZnO:Al (AZO), by a two stage process consisting on the selenization of metallic precursor layers. Homogeneous and crystalline formation of CAS thin films is not trivial and it is strongly influenced by selenization conditions, type of substrate and the film thicknesses. Under certain conditions, polycrystalline CuAlSe₂ thin films with chalcopyrite structure and preferential orientation along the (112) plane were obtained onto bare glass susbtrates. However, formation and crystallization of homogeneous CAS thin films was promoted by transparent conducting oxides (ITO and AZO)-coated glass substrates and take place in a wide range of thicknesses and Se amounts with high degree of reproducibility. TCO-coated substrates promoted larger grains when the CAS compound was formed. The band-gap energy, preferential orientation, crystallite size and the average surface roughness varied depending on the film thickness and type of substrate.     

Structural and optoelectronic properties of indium doped SnO2 thin films deposited by sol gel technique

Indium doped tin oxide (SnO₂:In) thin films were deposited on glass substrates by sol–gel dip coating technique. X-ray diffraction pattern of SnO₂:In thin films annealed at 500 °C showed tetragonal phase with preferred orientation in T (110) plane. The grain size of tin oxide (SnO₂) in SnO₂:In thin films are found to be 6 nm which makes them suitable for gas sensing applications. AFM studies showed an inhibition of grain growth with increase in indium concentration. The rms roughness value of SnO₂:In thin films are found to 1 % of film thickness which makes them suitable for optoelectronic applications. The film surface revealed a kurtosis values below 3 indicating relatively flat surface which make them favorable for the production of high-quality transparent conducting electrodes for organic light-emitting diodes and flexible displays. X-ray photoelectron spectroscopy gives Sn 3d, In 3d and O 1s spectra on SnO₂:In thin film which revealed the presence of oxygen vacancies in the SnO₂:In thin film. These SnO₂:In films acquire n-type conductivity for 0–3 mol% indium doping concentration and p type for 5 and 7 mol% indium doping concentration in SnO₂ films. An average transmittance of >80 % (in ultra-violet–Vis region) was observed for all the SnO₂:In films he In doped SnO₂ thin films demonstrated the tailoring of band gap values. Photoluminescence spectra of the films exhibited an increase in the emission intensity with increase in indium doping concentration which may be due structural defects or luminescent centers, such as nanocrystals and defects in the SnO₂.     

Comparative study of physical properties of vapor chopped and nonchopped Al2O3 thin films

Aluminium oxide being environmentally stable and having high transmittance is an interesting material for optoelectronics devices. Aluminium oxide thin films have been successfully deposited by hot water oxidation of vacuum evaporated aluminium thin films. The surface morphology, surface roughness, optical transmission, band gap, refractive index and intrinsic stress of Al₂O₃ thin films were studied. The cost effective vapor chopping technique was used. It was observed that, optical transmittance of vapor chopped Al₂O₃ thin film showed higher transmittance than the nonchopped film. The optical band gap of vapor chopped thin film was higher than the nonchopped Al₂O₃, whereas surface roughness and refractive index were lower due to vapor chopping.     

Analysis of single junction a-Si:H solar cells grown on different TCO’s

In consequence of previous investigation of individual transparent conductive oxide (TCO) and absorber layers a study was carried out on hydrogenated amorphous silicon (a-Si:H) solar cells with diluted intrinsic a-Si:H absorber layers deposited on glass substrates covered with different TCO films. The TCO film forms the front contact of the super-strata solar cell and has to exhibit good electrical (high conductivity) and optical (high transmittance) properties. In this paper we focused our attention on the influence of using different TCO’s as a front contact in solar cells with structure as follows: Corning glass substrate/TCO (800, 950 nm)/p-type μc-Si:H (∼5 nm)/p-type a-Si:H (10 nm)/a-SiC:H buffer layer (∼5 nm)/intrinsic a-Si:H absorber layer with dilution R = [H₂]/[SiH₄] = 20 (300 nm)/n-type a-Si:H layer (20 nm)/Ag + Al back contact (100 + 200 nm). Diode sputtered ZnO:Ga, textured and non-textured ZnO:Al [3] and commercially fabricated ASAHI (SnO₂:F) U-type TCO’s have been used. The morphology and structure of ZnO films were altered by reactive ion etching (RIE) and post-deposition annealing.It can be concluded that the single junction a-Si:H solar cells with ZnO:Al films achieved comparable parameters as those prepared with commercially fabricated ASAHI U-type TCO’s.     

Optical properties of CeO<Subscript>2</Subscript> thin films

Cerium oxide (CeO₂) thin films have been prepared by electron beam evaporation technique onto glass substrate at a pressure of about 6 × 10⁻⁶ Torr. The thickness of CeO₂ films ranges from 140–180 nm. The optical properties of cerium oxide films are studied in the wavelength range of 200–850 nm. The film is highly transparent in the visible region. It is also observed that the film has low reflectance in the ultra-violet region. The optical band gap of the film is determined and is found to decrease with the increase of film thickness. The values of absorption coefficient, extinction coefficient, refractive index, dielectric constant, phase angle and loss angle have been calculated from the optical measurements. The X-ray diffraction of the film showed that the film is crystalline in nature. The crystallite size of CeO₂ films have been evaluated and found to be small. The experimental d-values of the film agreed closely with the standard values.     

Low-indium content bilayer transparent conducting oxide thin films as effective anodes in organic photovoltaic cells

Bilayer In-doped CdO/Sn-doped In₂O₃ (CIO/ITO) transparent conducting oxide (TCO) thin films were prepared by depositing thin ITO films by ion-assisted deposition on CIO films grown by metal-organic chemical vapor deposition. The optical, electrical, and microstructural properties of these bilayer TCO films were investigated in detail. A low sheet resistance of ~ 4.9 Ω/□ is achieved for the CIO/ITO (170/40 nm) bilayers without annealing. With a significantly lower In content (20 vs. ~ 93 at.%) and a much higher conductivity (> 12,000 vs. 3000-5000 S/cm) than commercial ITO, these bilayer films were investigated as anodes in bulk-heterojunction organic photovoltaic (OPV) devices having a poly(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene) + [6,6]-phenyl C₆₁ butyric acid methyl ester active layer. Device performance metrics in every way comparable to those of devices fabricated on commercial ITO are achieved, demonstrating that CIO/ITO bilayers are promising low-In content, highly conductive and transparent electrode candidates for OPV cells.