Electrical and optical properties of Al doped cadmium oxide thin films deposited by radio frequency magnetron sputtering

Cadmium oxide thin films with different percentages of aluminum doping have been synthesized via radio frequency magnetron sputtering technique. Thin films were deposited on glass and silicon substrates with different percentages of aluminum at a substrate temperature of 573 K and pressure of 0.1 mbar in Ar+O₂ atmosphere. The deposited films were characterized by studying their structural, electrical and optical properties. The X-ray diffraction pattern revealed good crystallinity with preferred (1 1 1) orientation in the films. Aluminum doping in CdO thin films were confirmed by X-ray photoelectron spectroscopic studies and actual doping percentages were also measured from it. The optical band gap was found to decrease first and then increase with increasing percentages of aluminum concentrations. The electrical conductivity was found to increase with increase of aluminum doping concentration up to 5% but for higher doping concentration (>5%) the conductivity was found to decrease.     

Highly efficient hematite films via mid-/ex-situ Sn-doping for photoelectrochemical water oxidation

In this study, we report the facile fabrication of Sn-doped hematite film via mid-situ and ex-situ doping methods for efficient photoelectrochemical (PEC) performances. The morphology of Sn-doped Fe₂O₃ films was varied with Sn precursor in the mid-situ doping process. The addition of SnCl₂ rendered a smooth-surfaced and well-distributed nanorod morphology, but SnCl₄ gave a deformed nanorod structure covered with layered coalescence of SnO₂ particles. The former demonstrated much higher photoelectrochemical performances as photoanodes than the latter. The photocurrent can be further improved by surface modification with SnCl₄ through spin-coating method. The effects of Sn precursors on the morphology, surface characteristics and the PEC properties of the photoanodes are investigated.     

Band gap optimization of tin tungstate thin films for solar water oxidation

Semiconducting ternary metal oxide thin films exhibit a promising application for solar energy conversion. However, the efficiency of the conversion is still limited by a band gap of a semiconductor, which determines an obtainable internal photovoltage for solar water splitting. In this report the tunability of the tin tungstate band gap by O₂ partial pressure control in the magnetron co-sputtering process is shown. A deficiency in the Sn concentration increases the optical band gap of tin tungstate thin films. The optimum band gap of 1.7 eV for tin tungstate films is achieved for a Sn to W ratio at unity, which establishes the highest photoelectrochemical activity. In particular, a maximum photocurrent density of 0.375 mA cm⁻² at 1.23 V_RHE and the lowest reported onset potential of −0.24 V_RHE for SnWO₄ thin films without any co-catalyst are achieved. Finally, we demonstrate that a Ni protection layer on the SnWO₄ thin film enhances the photoelectrochemical stability, which is of paramount importance for application.     

Transparent conductive Sn-doped indium oxide thin films deposited by spray pyrolysis technique

Films of indium oxide doped with tin (ITO) are prepared using the low cost spray pyrolysis technique. The effect of tin doping on the physical properties of In2O3 are studied. In this study the polycrystalline ITO films with the different Sn concentration of 1 to 100-wt% SnCl₂ were prepared on Corning 7059 glass substrate. These films were confirmed to show the high crystallinity by X-ray diffraction technique. Low sheet resistance (25Ω/▪) and high visible transmission (˜82%) were obtained when the films were deposited at the tin concentration of 2-wt%.     

Chemically grown Bi2S3 nanorod films for hydrogen evolution

Bi₂S₃ nanorod films were grown on ITO-coated glass substrates through chemical bath deposition (CBD) and annealing in a sulfur atmosphere. The as-deposited films were amorphous/nanocrystalline, with a particle size of 20 nm and a direct optical band gap of 1.87 eV. Upon annealing at 350 °C, the films exhibited a nanorod morphology with a length of 300 nm. Further increasing the temperature from 400 to 450 °C resulted in an increased diameter of nanorods. The direct optical band gap decreased from 1.68 to 1.47 eV upon increasing the annealing temperature from 350 to 400 °C. Photoelectrochemical (PEC) measurements showed that the nanorod films grown on ITO-coated glass substrates exhibited significantly increased PEC activity owing to their nanorod structures. The Bi₂S₃ nanorod films formed at 400 °C exhibited a maximum photocurrent density of 6.1 mA/cm² at 1 V, which was 2.5 times higher than that of the as-deposited films. The enhancement in the photocurrent density could be due to the effective visible-light absorption of Bi₂S₃ nanorods as a result of the increased crystallinity and decreased band gap. This study demonstrates the synthesis route involving a simple and inexpensive CBD method of Bi₂S₃ nanorod films for the optimized PEC water-splitting applications.     

Study of optical properties of some sol–gel derived films of ZnO

The optical transmission and reflection data for aluminum doped zinc oxide films have been analyzed. The optical absorption coefficient (α) and hence the refractive index (n), extinction coefficient (k) and the band gap (E_g) have been determined for these films using different methods. The films are prepared by sol–gel technique and are optically transparent. It is observed that the band gap increases with aluminum doping from 3.19 to 3.24 eV, but is less than the bulk ZnO crystal. A qualitative explanation has been put forward for the band gap widening with doping. The width of the band tail states which is connected to localized states in the band gap is least for 1 at% doped aluminum film. The porosity of the films as deduced from the refractive index seems to be of the same order in all the cases, but relatively higher for 1 at% aluminum doped films.     

Optical properties of TiO2 thin films deposited by DC sputtering and their photocatalytic performance in photoinduced process

TiO₂ thin films were deposited by DC Sputtering varying the deposit time. These films were characterized by XRD, AFM, photoluminescence, UV–Vis, ellipsometry and XPS. The optical properties of TiO₂ thin films with different thickness, influenced their photocatalytic behavior in two photoinduced process. When TiO₂ thin films were irradiated with a UV light, midgap states were generated and the electrons were placed in lower energies than its band gap, favoring the photocatalytic hydrogen production and CO₂ photoreduction. From PL technique analyses it was observed that electrons occupied midgap states between the bands, with lower energies than the band gap. With these results it was possible to propose an energy diagram in order to correlate with photoinduced processes results. The presence of Ti³⁺ species was reconfirmed by means of XPS analyses. These species could be found in the midgap states, generated by the interaction between the UV irradiation and the film surface, which contributed to the photocatalytic activity of the films. The hydrogen production was similar for all the thin films studied (33–35 μmol) associated to the presence of similar energy midgap states. In the case of CO₂ photoreduction, all films produced CH₂O (8951 and 6252 μmol/g) and the films with a thickness of 330 and 420 nm generated CH₃OH (970 and 292 μmol/g). The extinction coefficient confirmed the XRD results for the film with greatest deposited time, which exhibited the highest crystallinity. All photocatalytic results did not show any dependence with the thin film thickness.     

Some physical properties of Cd1−xSnxS films used as window layer in heterojunction solar cells

CdS has been proved to be an ideal material for use as the window layer for heterojunction solar cells especially with n-CdS/p-CdTe. CdS, Cd_0.9Sn_0.1S and Cd_0.8Sn_0.2S films were deposited onto glass substrates at 300 °C substrate temperature by using ultrasonic spray pyrolysis technique (USP). The effect of Sn concentration on some structural, optical and electrical properties of the films was presented. The crystal structure and orientation of the films were investigated by X-ray diffraction (XRD) patterns. XRD patterns showed that films have polycrystalline nature with a hexagonal structure. The grain size of the films decreased with increasing x values. The optical band gap values were obtained from optical absorption spectra of the films. The optical band gap values of the films were found to be between 2.44 and 2.45 eV. The variations of conductivity of Cd_1−xSn_xS (0 ≤ x ≤ 0.2) films have been investigated depending on applied voltage in dark and under illumination. The resistivity significantly decreased with increasing tin concentration and under illumination.     

Comparative study of Cu2ZnSnS4 film growth

We fabricated Cu₂ZnSnS₄ (CZTS) thin films using two different methods, spray pyrolysis and sulfurization of Cu-Zn-Sn metallic films. Spray pyrolysis was carried out under air ambient with modified ultrasonic spray system. Sulfurizations of metallic Cu-Zn-Sn films were done for stacked metallic films, Cu/Sn/Zn/glass, Cu/Sn/Cu/Zn/glass and Sn/Cu/Zn/glass, which were prepared by sputtering method in high vacuum chamber. The sprayed films were not observed to be grown well with good crystallinity, compared with CZTS films made by sulfurization of stacked metallic films. However, it was found that application of additional sulfurization to sprayed CZTS films induced great improvement of crystallinity to the level of the sulfurized metallic films. This implicates that spray pyrolysis with additional sulfurization is a good method for fabrication of CZTS films, especially as a low-cost fabrication technique. All CZTS films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and Raman spectroscopy measurements.     

Electrical and optical properties of Cu2ZnSnS4 thin films prepared by rf magnetron sputtering process

Cu₂ZnSnS₄ thin films were deposited on corning 7059 glass substrates without substrates heating by rf magnetron sputtering. The Cu/(Zn+Sn) ratio of the thin film sputtered at 75 W was close to the stoichiometry of Cu₂ZnSnS₄. However, the S/(Cu+Zn+Sn) ratio was less than the stoichiometry. The as-deposited films were amorphous and annealed in the atmosphere of Ar+S₂ (g). The annealed (1 1 2), (2 0 0), (2 2 0), (3 1 2) planes were conformed to all the reflection of a kesterite structure. A preferred (1 1 2) orientation was observed with the increase of the annealing temperature. The optical absorption coefficient of the thin film was about 1.0×10⁴ cm⁻¹. The optical band energy was derived to be 1.51 eV. The optical absorption coefficient of the sputtered Cu₂ZnSnS₄ thin films was less than that of CuInS₂ thin film, however, the band gap energy was more appropriate for photovoltaic materials.     

Synthesis and electro-optic properties of nanosized-boron doped cadmium oxide thin films for solar cell applications

Boron doped CdO thin films were prepared by sol–gel dip coating technique. Atomic force microscopy results indicate that the boron doped CdO films have the nanostructure. The influence of the boron doping on the film growth is resulted in a change of grain size. The optical band gap of the CdO films was significantly changed by boron dopant. The refractive index dispersion of the films obeys the single oscillator model. The dispersion parameters, oscillator and dispersion energy were changed by boron dopant. The optical absorption results show that the optical band gap of the CdO film can be engineered over a wide range of 2.27–2.45 eV by introducing B dopant. For solar cell applications of the CdO film, a p-Si/1% B doped n-CdO heterojunction solar cell was fabricated and the solar cell shows the best values of open circuit voltage, V_oc = 0.37 and short circuit current density, J_sc = 0.81 mA/cm² under AM1.5 illumination, despite the fact that V_oc and J_sc are lower than those reported in the literature without using frontal grid contacts and or post-deposition annealing. It is evaluated that this work is useful as a basis search for synthesis of the nanosized-boron doped cadmium oxide thin films for solar cell applications and more competitive p-Si/n-CdO based solar cells.     

Effect of compact structure on the phase transition in the oxides derived Cu2ZnSnSe4 thin films

Cu₂ZnSnSe₄ (CZTSe) films were prepared by selenization of oxides nanoparticles. A novel densification method was performed to improve the grain size and morphology of the CZTSe films. From absorption spectroscopy measurement, it was also found that the compressed CZTSe films showed Kesterite structure with a band gap of 0.92 eV, while the untreated CZTSe films showed partially disordered Kesterite structure with a band gap of 0.86 eV. The phase transition during the selenization of oxides nanoparticles is affected significantly by the compact density. The nucleation and growth of Kesterite phase is considered to be facilitated by the mass transfer around the particle contacts. The different characterization techniques show that the dense CZTSe layer with very large grain size can be achieved by using compression method.     

The structure and photocatalytic studies of N-doped TiO2 films prepared by radio frequency reactive magnetron sputtering

N-doped TiO₂ films were prepared by a radio frequency reactive magnetron sputtering (RF-MS) deposition method from an undoped TiO₂ target in a mixture of Ar/N₂ atmosphere on heated quartz glass substrates. The structures and properties of the N-doped were studied by XRD, Raman, XPS, TEM, ultraviolet (UV)-vis and PL spectroscopy. By analyzing the structures and photocatalytic activities of undoped and N-doped TiO₂ films under ultraviolet and visible light irradiation, the probable photocatalytic mechanism of N-doped TiO₂ films was investigated. Because many oxygen defects are caused in films by nitrogen doping, it is presumed that nitrogen doping and oxygen defect induced the formation of new states closed to the valence band and conduction band, respectively. The cooperation of nitrogen and oxygen defects leads to a significant narrowing of the band gap and greatly improves the absorption in the visible light region. It is found that the degradation efficiencies of N-doped TiO₂ films greatly decreased under ultraviolet irradiation, but slowly improved under visible light irradiation, compared with the undoped TiO₂ film. It is suggested that the N-doped TiO₂ films are formed for the nitrogen to occupy oxygen defect sites directly. The doped nitrogen ions and oxygen defects act as recombination centers that reduce the lifetime of photo-induced electrons and holes, thereby resulting in the decrease of photocatalytic activity under ultraviolet light illumination.     

Mo-doped,Cr-Doped,and Mo–Cr codoped TiO2 thin-film photocatalysts by comparative sol-gel spin coating and ion implantation

Uniformly codoped anatase TiO₂ thin films of varying (equal) Mo and Cr concentrations (≤1.00 mol% for each dopant) were fabricated using sol-gel spin coating and deposited on fused silica substrates. All films were annealed at 450 °C for 2 h to recrystallise anatase. Undoped anatase films have been subjected to dual ion implantation for the first time, using Mo, Cr, and sequential Mo + Cr at 1 × 10¹⁴ atoms/cm². The films were characterised by GAXRD, AFM, SIMS, XPS, and UV–Vis and the performance was assessed by dye degradation. Despite the volumetric doping by sol-gel and the directional doping by ion implantation, neither method resulted in homogeneous dopant distributions. Both methods caused decreasing crystallinities and associated partial amorphisation. The XPS signal of the uniformly codoped films is dominated by undissolved dopant ions, which is not the case for the ion-implanted films. Increasing Ti valences are attributed to the fully oxidised condition of the Ti⁴⁺ ions that diffuse to the surface from Ti vacancy formation compared to the Ti valence of the bulk lattice, which contains Ti³⁺. Increasing O valence is attributed to the electronegativity of O^2?, which is higher than that of Ti⁴⁺. Detailed structural mechanisms for the solubility and energetics mechanisms involve the initial formation of Mo and Cr interstitials that fill the two voids adjacent to the central Ti ion in the TiO₆ octahedron, followed by integrated solid solubility (ISS) and intervalence/multivalence charge transfer (IVCT/MVCT). The sequential order of the last two is reversed for the two different doping methods. These two effects are likely to be the source of synergy, if any, between the two dopant ions. The photocatalytic performances of the uniformly codoped films are relatively poor and correlate well with the band gap (E_g). The performances of the ion-implanted films do not correlate with the E_g, where TiO₂–Mo performs poorly but TiO₂–Cr and TiO₂–Mo–Cr outperform the undoped film. These results are interpreted in terms of the competition between the effects of Mo doping, which causes partial amorphisation and/or blockage of active sites, and Cr doping, which may cause Mo–Cr synergism, Cr-based heterojunction formation, and/or improved charge-carrier separation owing to the surface-deposition nature of ion implantation.     

Effect of Mo-doping in SnO2 thin film photoanodes for water oxidation

New semiconducting metal oxides of various compositions are of great interest for efficient solar water oxidation. In this report, Mo-doped SnO₂ (Mo:SnO₂) thin films deposited by reactive magnetron co-sputtering in the Ar and O₂ gas environment are studied. The Sn to Mo ratio in the films can be controlled by changing the O₂ partial pressure and the deposition power of the Sn and Mo targets. Increasing the Mo concentration in the film leads to the increase in the oxygen vacancy density, which limits the maximum achievable photocurrent density. The thin films exhibit a direct band gap of 2.7 eV, the maximum achievable photocurrent density of 0.6 mA cm⁻² at 0 V_RHE and the onset potential of 0.14 V_RHE. The incident photon to current transfer (IPCE) efficiency of 22% is shown at a 450 nm wavelength. The initial performance of the Mo:SnO₂ thin films is evaluated for solar water oxidation.     

In situ tuning of band gap of Sn doped composite for sustained photocatalytic hydrogen generation under visible light irradiation

The significance of Sn dopant on the photocatalytic performance of Iron/Titanium nanocomposite towards photocatalytic hydrogen generation by water splitting reaction is investigated. Iron/Titanium nanocomposite modified by Sn⁴⁺ dopant acts as a suitable photocatalyst for induced visible light absorption facilitating pronounced charge separation efficiency. Various characterization techniques reveal the heterojunction formation of hematite Fe₂O₃ with anatase - rutile mixed phase of TiO₂ employing Sn doping, where Sn⁴⁺ dopant accomplishes the phase transformation of anatase to rutile, entering into the TiO₂ lattice. This extended the lifetime of photogenerated charge carriers and enhanced the quantum efficiency of the photocatalyst. The band gap of the nanocomposite is tuned to ~2.4 eV, favoring visible light absorption. A hydrogen generation activity of 1102.8 μmol, approximately five times higher than the lone system (216.5 μmol) is achieved for the 5% Sn doped system for an average of 5 h. Heterojunctions of hematite with anatase-rutile mixed phase, generated as a consequence of tin doping facilitated the enhanced hydrogen generation activity of photocatalyst.     

Reduced band gap & charge recombination rate in Se doped α-Bi2O3 leads to enhanced photoelectrochemical and photocatalytic performance: Theoretical & experimental insight

For better utilization of solar spectrum and complete redox of water for water splitting applications, it is required to have a semiconductor which is photoactive in visible region. In this study, we report theoretical and experimental investigations on morphological and opto-electronic modifications induced in α-Bi₂O₃ due to Selenium (Se) doping tested for photoelectrochemical (PEC) & photocatalytic properties. Density Functional Theory (DFT) calculations revealed band gap reduction and direct to indirect transitions in Se-doped α-Bi₂O_3. This reduction in band gap is attributed to hybridization of Se p & Bi s in valence band and Se d & Bi p orbital in conduction band. To support this finding experimentally, we synthesized Se-doped α-Bi₂O₃ using simple chemical precipitation method and measured its band gap using photoluminescence and UV–Vis spectroscopy. Experimental results also confirmed the reduction in band gap energy and recombination rate of charge carriers as compared to pristine α-Bi₂O₃ sample. PEC study of Se-doped α-Bi₂O₃ showed an increased photocurrent density, charge carrier density and lowered impedance, which indicates its efficient solar spectrum utilization and better hydrogen generation efficiency. Photocatalytic measurement also revealed higher rate of dye degradation with Se doped α-Bi₂O₃.     

Role of precursor solution in controlling the opto-electronic properties of spray pyrolysed Cu2ZnSnS4 thin films

Thin films of Cu₂ZnSnS₄, a potential candidate for application as absorber layer in thin film solar cells, were successfully deposited on soda lime glass substrates using spray pyrolysis and the effect of variation of precursor on the structural and opto-electronic properties was investigated. We used stannous as well as stannic chloride as precursors of tin in the spray solution. All the films exhibited kesterite structure with preferential orientation along the (1 1 2) direction. But crystallinity and grain size were better for stannic chloride based films. Also they possessed a direct band gap of 1.5 eV and the absorption coefficient was >10⁴ cm⁻¹. Carrier concentration and mobility could be enhanced and the resistivity reduced by two orders by using stannic chloride in spray solution. Junction trials were performed with CZTS films prepared using stannic chloride precursor as the absorber layer and indium sulfide as the buffer layer. XPS depth profiling of the junction was done. Formation of CZTS could be confirmed and also information about the junction interface could be obtained from the XPS results. We obtained an open-circuit voltage of 380 mV and short-circuit current density of 2.4 mA/cm².     

Photoelectrochemical studies on galvanostatically formed multiple band gap materials based on CdSe and ZnSe

The preparation of some multiple band gap semiconductor films based on CdSe and ZnSe has been carried out using galvanostatic electrochemical codeposition technique to investigate their photoelectrochemical characteristics on the basis of photoelectroconvertibility and photoaction spectral studies using I₂/I₃⁻ and [Fe (CN) ₆]³⁻/[Fe (CN) ₆]⁴⁻ redox couples. These composite systems show substantially improved photoelectrochemical properties compared to the constituent CdSe and ZnSe films prepared under comparable experimental conditions. These multiple band gap films were also found to exhibit enhanced resistance towards electrochemical corrosion.     

Influence of CdCl2 treatment on structural and optical properties of vacuum evaporated CdTe thin films

In this article we have discussed the structural, optical properties of vacuum evaporated CdTe thin films before and after CdCl₂ treatment. The CdTe thin films were prepared by vacuum evaporation. Films were prepared under the vacuum of 10⁻⁶ Torr. The structural studies have been performed by the X-ray diffraction (XRD) technique. The XRD analysis of vacuum evaporated CdTe films reveals that the structure of films is polycrystalline in nature. However, the crystallinity has been improved after the CdCl₂ treatment as shown by an increase of the diffraction peak intensities. This is due to the enhancement in the atomic mobility of CdTe. The optical properties of the CdTe thin films have been studied by the spectrophotometer in the 300–800 nm wavelength range. It is observed that the optical band gap energy is highly dependent on CdCl₂ treatments. The optical transitions in these films are found to be direct and allowed.