CdO/Cu2O solar cells by chemical deposition

CdO and Cu₂O thin films have been grown on glass substrates by chemical deposition method. Optical transmittances of the CdO and Cu₂O thin films have been measured as 60–70% and 3–8%, respectively in 400–900 nm range at room temperature. Bandgaps of the CdO and Cu₂O thin films were calculated as 2.3 and 2.1 eV respectively from the optical transmission curves. The X-ray diffraction spectra showed that films are polycrystalline. Their resistivity, as measured by Van der Pauw method yielded 10⁻²–10⁻³ Ω cm for CdO and approximately 10³ Ω cm for Cu₂O. CdO/Cu₂O solar cells were made by using CdO and Cu₂O thin films. Open circuit voltages and short circuit currents of these solar cells were measured by silver paste contacts and were found to be between 1–8 mV and 1–4 μA.     

Temperature dependence of absorption coefficient spectra for μc-Si films by resonant photothermal bending spectroscopy

Resonant photothermal bending spectroscopy (R-PBS) has been developed for estimating absorption coefficient spectra of thin film semiconductors. This technique has been applied to hydrogenated microcrystalline silicon (μc-Si:H) films at different measurement temperatures. It is found that absorption coefficient of μc-Si:H films at 0.7–1.1 eV is relevant for the localized states and decreases with increasing measurement temperature. The localized state exists at 0.7 eV in the band gap from the band edge. The origin of the absorption is also discussed.     

Correlated transport and optical phenomena in Ga-doped CdO films

Series of samples of lightly Ga-doped CdO thin films (3%, 6%, and 9%) have been prepared by evaporation method on glass substrate. The prepared films were characterised by X-ray diffraction (XRD), UV–VIS–NIR absorption spectroscopy, and dc-electrical measurements. The investigation shows that Ga doping widens the energygap of CdO. The optical properties were easily explained by using Tauc et al. band-to-band transitions and classical Drude theory. The electrical behaviour of the samples shows that they are degenerate semiconductors. The 6% Ga-doped CdO sample shows increase its mobility by 3.2 times, increase its conductivity by 1.5 times, increase its intrinsic bandgap, and a slight increase its transmittance relative to undoped CdO film. Explanation was given concerning these variations. From transparent conducting oxide (TCO) point of view, Ga is not sufficiently effective for CdO doping comparing to other dopants like In, Sn, Sc, and Y.     

CdO–CdS nanocomposites with enhanced photocatalytic activity for hydrogen generation from water

Nanocomposites of CdO–CdS have been prepared in ethylene glycol water mixture followed by heating at 300 °C. TEM and XRD studies confirmed the atomic scale mixing of CdO and CdS nanoparticles, leading to the formation of CdSO₃ phase at the interfacial region between CdO and CdS. Photocatalytic studies for hydrogen generation from water show an enhanced activity for CdO–CdS composites compared to individual components namely CdO or CdS nanoparticles. Based on optical absorption, surface area measurements, steady state and time resolved fluorescence studies, it is established that, enhanced absorption in the visible region, higher surface area and increase in lifetime of the charge carriers are responsible for the observed increase in hydrogen yield from water when composite sample was used as the photocatalyst compared to individual components. The composite sample when combined with Pt as co-catalyst exhibit a large increase in the photocatalytic activity.     

Effect of tellurium doping on the structural, optical, and electrical properties of CdO

Te-doped CdO thin-films (1%, 3%, and 5%) have been prepared by a vacuum evaporation method on glass and silicon-wafer substrates. The prepared films were characterised by X-ray fluorescence, X-ray diffraction, UV-VIS-NIR absorption spectroscopy, and dc-electrical measurements. Experimental data indicate that Te ions doping slightly stresses the host CdO crystalline structure and changes the optical and electrical properties. The bandgap of the host CdO was suddenly narrowed by about 23% due to a little (1%) doping with Te ions. This bandgap shrinkage was explained by effects of trap levels overlapping with conduction band. The electrical behaviours of the Te-doped CdO films show that they are degenerate semiconductors with a bandgap of 1.7-2.2 eV. The 1% Te-doped CdO film shows increase its mobility by about 5 times, conductivity by ∼140 times, and carrier concentration by ∼27 times, relative to undoped CdO film. From transparent-conducting-oxide point of view, Te is sufficiently effective for CdO doping. Finally, the absorption in the NIR spectral region was studied in the framework of the classical Drude theory.     

Wide range tuning of electrical conductivity of RF sputtered CdO thin films through oxygen partial pressure variation

The effect of oxygen partial pressure variation on the electrical conductivity and the optical transparency of CdO thin films, deposited through RF magnetron sputtering were studied in detail. Thin films of CdO have been deposited through radio frequency magnetron sputtering of a prefabricated CdO target at a fixed pressure 0.1 mbar and at a substrate temperature 523 K. It was found that the electrical conductivity of the CdO films could be varied over three decades for a variation of oxygen partial pressure of 0–100%, without introducing any extrinsic dopants. X-ray diffraction (XRD) studies showed that the films were polycrystalline in nature with a preferential orientation along (1 1 1) plane. Compositional information was obtained by X-ray photoelectron spectroscopic studies. This wide range of variation of electrical properties was explained through the oxygen vacancies formation.     

Synthesis and characterization of aluminum-doped CdO thin films by sol–gel process

Aluminum-doped cadmium oxide (CdO:Al) thin films are deposited on glass substrates by the sol–gel dip-coating method, taking cadmium acetate dihydrate as the precursor material. Aluminum nitrate has been taken as a source of Al-dopant. XRD pattern reveals the good crystallinity of CdO thin films. SEM micrograph showed the presence of faceted crystallites. Optical study shows 40–85% transparency with a bandgap value lying in the range 2.76–2.52 eV, depending upon the Al content in the films. Optimum percentage of Al was 5.22 for a maximum room temperature conductivity of 2.81×10³ (Ω cm)⁻¹. Hall measurement confirmed that the material is of n-type, with mobility and carrier concentrations lying in the range 413–14.7 cm²/V s, and 3.4×10¹⁹–8.11×10²⁰ cm⁻³, when percentage of Al varies in the range 1.32–7.24.     

Hydrogen storage and PL properties of novel Cd/CdO shelled hollow microspheres prepared under NH3 gas environment

Here we report the first ever fabrication of metal/semiconductor Cd/CdO shelled hollow microspheres with average diameter of 20–30 μm via heating cadmium metal powder inside horizontal tube furnace at 500 °C for 40 min under ammonia gas flow of 150 sccm. Vapor-solid (VS) based growth mechanism was proposed for the formation of Cd/CdO shelled hollow microspheres (CCOSHMs). The as-prepared product was characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Room temperature photoluminescence (PL) studies exhibited a UV emission band at 395 nm (E = 3.13 eV) which may be ascribed to combined effect of near band edge emission of CdO and Cd related radiative recombination of electrons in s, p conduction band near Fermi surface and the holes in the d bands generated by Xe light excitation. The hydrogen absorption properties of CCOSHMs were investigated at three different temperatures 373, 473, and 573 K. The maximum hydrogen absorption of 1.30 wt% was observed at 573 K which is better than many other materials. This indicates the potential of Cd/CdO shelled hollow microspheres for applications in light emitting devices as well as an interesting material for hydrogen storage research.     

Temperature dependence of the optical band gap and refractive index of poly(ethylene terepthalate) oligomer–DDQ complex thin film

The temperature dependence of the optical band gap and refractive index dispersion of thin film of poly(ethylene terepthalate) oligomer–DDQ charge transfer complex has been investigated. The absorption edge shifts to the lower energy as consequence of the thermal annealing on film and the fundamental absorption edge corresponds to a direct energy gap. The temperature coefficient of the optical band gap for the film was found as dE_g/dT = − 3.15 × 10⁻³ eV/K. The temperature dependence of the refractive index has also been investigated and it is observed that the refractive index changes by annealing temperatures.     

Visible light-induced degradation of blue textile azo dye on TiO2/CdO–ZnO coupled nanoporous films

The photodegradation of a typical textile blue azo dye, followed by UV–VIS spectra analysis, has been carried out successfully under white light illumination on TiO₂/CdO–ZnO nanoporous coupled thin films. A relatively fast degradation occurs in dye solutions with concentrations of 100 mg/l (pH=3), at temperatures of 85°C, and with the aid of 400 mg/l hydrogen peroxide. Photodegradation also occurs on nanoporous TiO₂ films but with significant lower efficiency than on TiO₂/CdO–ZnO coupled nanoporous films. Dye photodegradation does not occur on TiO₂/CdO or TiO₂/ZnO nanoporous films, suggesting that both CdO and ZnO components are required on the sensitization of TiO₂ nanoporous films. A combined effect of new sensitizing interband states (response to white illumination) and/or rectification phenomena (improved charge separation) may be responsible of the higher photocatalytic activity of the TiO₂/CdO–ZnO nanoporous films. Similarly, the alternative route for visible degradation, the photosensitized degradation mechanism, could also benefit from the coupled nanoporous films due to a higher driving force for electron injection (dye oxidation).     

In doped CdO films: Electrical,optical, structural and surface properties

Recently, there has been a lot of work on the production and investigation of the physical properties of Transparent Conducting Oxide (TCO) materials which have common application area in photovoltaic solar cells and some optoelectronic devices. In this work, CdO film which is a material belongs to TCO family has been produced by Ultrasonic Spray Pyrolysis technique on microscope glass substrates at the substrate temperature of 250 ± 5 °C. Electrical, optical, structural and surface properties of undoped and In doped (at 1.3 and 5%) CdO films and the effect of In doping percentage on the physical properties of CdO films have been investigated. It has been determined that electrical conductivity of CdO film is high and this value has been decreased by In doping. After the optical investigations, it has been observed that the transmittances of the films are about 30% and decreased dramatically by In doping. XRD investigations showed that, films have polycrystalline structure and good crystallinity levels. It has been found that In element hasn't got an important effect on the morphology of the films after the examination of surface micrographs. It has been determined that Cd and O elements are present in the solid film by using EDS. After all investigations, it has been concluded that In doping has an important effect on the electrical, optical, structural and surface properties of CdO films.     

An effective use of nanocrystalline CdO thin films in dye-sensitized solar cells

Thin films of cadmium oxide (CdO) were synthesized by layer-by-layer deposition method on indium doped tin oxide (ITO) substrates. Post-deposition annealing at 250 °C for 24 h produced pure phase CdO films by removal of trace amount of cadmium hydroxide, as confirmed from X-ray diffractogram. First time employment of CdO in place of TiO₂ in dye-sensitized solar cells is reported to check feasibility and cell performance. A dye-sensitized nanocrystalline CdO photo-electrode was obtained by adsorbing cis-dithiocyanato (4,4′-dicarboxylic acid-2,2′-bipyridide) ruthenium (II) (N3) dye by keeping at 45 °C for 20 h. The efficiency of dye-sensitized nanocrystalline CdO thin film solar cell was increased from 0.24% to 2.95% due to dye adsorption. This must be highest reported conversion efficiency for other metal oxides than TiO₂based dye-sensitized solar cells.     

Organic–inorganic hybrid composites for photovoltaics: Organic guest molecules embedded in μc-Si and ZnSe host matrices

The concept of organic–inorganic hybrid composites for bulk sensitization of inorganic semiconductors by organic dye molecules is introduced. The idea is either to increase the absorptivity of e.g. indirect semiconductors as μc-Si or to expand in a two-step process the absorption spectrum of wide gap semiconductors to photons of energy smaller than the band gap. The composites are prepared by vacuum-based codeposition. Raman and optical spectroscopy, and photoemission have been used to prove the stability of the organic molecules ZnPc and F₁₆ZnPc for the applied growth conditions. Enhancement of photoconductivity has been shown for ZnPc–Si bilayer. As a crucial parameter for the transfer of excited charges, the alignment of dye HOMO–LUMO states versus semiconductor band edges has been determined using photoelectron spectroscopy.     

Au–Cu/p–CdTe/n–CdO/glass-type solar cells

The heterostructure design proposed by us for the photovoltaic (PV) solar cell is: Au–Cu/p–CdTe:Sb/n–CdO:F/glass. The CdO:F films were grown by the sol–gel method, in conditions in order to get low resistivity 4.5×10⁻⁴ Ω-cm and an optical transmission higher than 85%. The CdTe:Sb films were prepared by means of the RF sputtering technique, in conditions to get resistivity value around 10⁶ Ω-cm, high crystalline quality and higher grain size. The Au–Cu contacts were thermally evaporated. For the study of PV-heterostructure a systematic variation of the preparation parameters were carried out. The parameters involved in the manufacture of the cell, in order to look for the highest efficiency were: (A) For the deposit of the p-CdTe:Sb films, a low argon pressure of 2.5 m Torr and high substrate temperature of 450 °C. The CdTe:Sb film thickness was varied in the interval 4.5–11 μm. (B) For the activation of the heterostructure: (i) The treatment temperature in vacuum, after the CdTe is deposited, was varied in the 350–550 °C range and (ii) the treatment temperature in Ar atmosphere, after the heterostructure is dipped in CdCl₂ solution, was studied in the 400–510 °C range. (C) Optimization of the Cu–Au contact with the adequate Cu-film thickness. The highest energy conversion efficiency (η) value was 5.48%. This work reports a systematic study of the parameters involved in the solar cell manufacture, for the search of a better value of η.     

Effect of heat treatment on physical properties of CdO films deposited by sol–gel method

CdO film has been deposited by sol–gel spin coating method on the glass substrate and then the film has been annealed at 400, 500, 600 °C for 1 h. Effect of annealing temperature on the structural and optical properties of the film has been investigated. The crystal structure and orientation of the as-grown and annealed CdO films have been investigated by X-ray diffraction method. Annealed CdO films are polycrystalline with (111) preferential orientation. The information on strain and grain size is obtained from the full width-at-half-maximum (FWHM) of the diffraction peaks. Texture coefficient and lattice constant have been calculated. The surface morphology of the films has been analyzed. The optical band gap value decreased with increasing the annealing temperatures.     

A new route for fabricating CdO/c-Si heterojunction solar cells

CdO/c-Si solar cells have been made by depositing CdO thin films on p-type monocrystalline silicon substrate by means of the rapid thermal oxidation (RTO) technique using a halogen lamp at 350 °C/45 s in static air. Results on structural, optical, and electrical properties of grown CdO films are reported. The electrical and photovoltaic properties of CdO/Si solar cells are examined. Under AM1 illumination condition, the cell shows an open circuit voltage (V_OC) of 500 mV, a short circuit current density (J_SC) of 27.5 mA/cm², a fill factor (FF) of 60%, and a conversion efficiency (η) of 8.84% without using frontal grid contacts and/or post-deposition annealing. Furthermore, the stability of solar cells characteristics is tested.     

Influence of dysprosium doping on the electrical and optical properties of CdO thin films

Lightly Dy-doped CdO thin films (molar 0.5%, 1%, 2%, and 2.5%) have been prepared by a vacuum evaporation method on glass and Si wafer substrates. The prepared films were characterised by X-ray fluorescence, X-ray diffraction, UV-vis-NIR absorption spectroscopy, and dc-electrical measurements. Experimental data indicate that Dy³⁺ doping slightly stretchy-stresses the CdO crystalline structure and changes the optical and electrical properties. The bandgap of CdO was suddenly narrowed by about 20% due to a little doping with Dy³⁺ ions. Then, as the Dy doping level was increased, the energygap was also increased. This variation was explained by the effect of Burstein-Moss energy shift (or bandgap widening effect) together with a bandgap shrinkage effect. The electrical behaviour of the samples shows that they are degenerate semiconductors. However, the 2% Dy-doped CdO sample shows an increase in its mobility by about 3.5 times, conductivity by 35 times, and carrier concentration by 10 times relative to undoped CdO film. From transparent conducting oxide point of view, Dy is sufficiently effective for CdO doping.     

Engineering the Mg–Mg2Ni eutectic transformation to produce improved hydrogen storage alloys

The technique of trace element doping to modify the solidification mechanism of faceted/non-faceted eutectics has been applied to the Mg–Mg₂Ni alloy system. It is demonstrated that the micro- and nano-structure of cast hypoeutectic Mg–Mg₂Ni alloys can be varied by trace additions of Na, Ca or Eu to the liquid prior to solidification. As a result, the reversible hydrogen absorption capability was in excess of 90% of the theoretical value of 6.8 wt.% under the absorption parameters of 350 °C and 1 MPa for 24 min and subsequent desorption at 0.2 MPa for 24 min after activation. The hydrogen absorption kinetics have been dramatically improved under realistic industrial conditions, and show no sign of reduced capacity over 200 cycles. This processing route results in a non-pyrophoric material that may be produced in large quantities at comparatively low cost.     

Thermodynamic analysis of monomethylamine–water solutions in a single-stage solar absorption refrigeration cycle at low generator temperatures

A theoretical analysis of the coefficient of performance COP was undertaken to examine the efficiency characteristics of the monomethylamine–water solutions for a single-stage absorption refrigeration machine, using low generator temperatures (60–80°C), which allows the use of flat plate solar collectors. The thermodynamic analysis considers both, basic and refined cycles. The refined absorption cycle included a sensible heat recover exchanger (that is a solution heat exchanger). The thermal coefficients of performance COP_h for the basis cycle and COP_SHE for the refined cycle were calculated using the enthalpies at various combinations, at the operating temperatures and concentrations. The flow ratio F_R has been calculated as additional optimization parameter. Due to the relative low pressure and the high coefficients of performance, the monomethylamine–water solutions present interesting properties for their application in solar absorption cycles at moderate condenser and absorber temperatures (25–35°C), with temperatures in the evaporator from −10°C to 10°C which are highly usable for food product preservation and for air conditioning in rural areas.     

Effect of substrate temperature on the structure and optical properties of CdTe thin film

The present work deals with the preparation, structure and optical characterization of cadmium telluride (CdTe) thin films. These films are formed by vacuum evaporation on the well-cleaned glass substrates. The compositional analyses are made by energy dispersive analysis by X-ray. The thicknesses of the samples are measured by multiple beam interferometry. The samples are prepared at different substrate temperatures. The X-ray diffraction has been employed to study the structure of the film. The structures of the samples are found to be crystalline and the crystallite size increases with the increase of substrate temperature. The d-spacing and lattice parameters of the samples are calculated and the results are also discussed. Optical characteristics of the CdTe samples have been analyzed using spectrophotometer in the wavelength range of 400–800 nm. The transmittance is found to decrease with the increase of film thickness. The transmittance falls steeply with decreasing wavelength. It reveals that CdTe films are having considerable absorption throughout the wavelength region (400–800 nm). The optical band gap energy has been evaluated from the plot of α² vs. hν. Two possible direct transitions have been observed for all the CdTe films in visible region. The observed allowed transition may be attributed due to the spin orbit splitting of the valence band.