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.     

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.     

Optical properties of sol–gel dip-coated Ta2O5 films for electrochromic applications

Amorphous Ta₂O₅ films were prepared by sol–gel dip process on different substrates. The dip-coating technique was used to prepare amorphous Ta₂O₅ films by hydrolysis and condensation of tantalum ethoxide, Ta(OC₂H₅)₅, precursor. Stable coating solutions were prepared using acetic acid as a chelating ligand and catalyzer. Single layer and multi-layered Ta₂O₅ films were fabricated at a dipping rate of 107 mm/min. The microstructure, stoichiometry and optical properties of these films were investigated as a function of the film thickness. Room temperature CV measurements clearly revealed a protonic conductor behavior for Ta₂O₅ films. Optical properties such as refractive index, extinction coefficient and optical band gap value of the Ta₂O₅ films were calculated from optical transmittance measurements. It was found that the refractive index and extinction coefficient values were affected by the thickness of the coatings. The refractive index at a wavelength of 550 nm increased from 1.70 to 1.72 with increasing film thickness. The optical band gap value (3.75±0.12 eV) of the coating was unaffected by the film thickness. These results indicate that sol–gel-deposited Ta₂O₅ films have a promising application as proton conductors in electrochromic devices.     

Optical, structural and photoelectron spectroscopic studies on amorphous and crystalline molybdenum oxide thin films

Optical, structural and electronic properties of amorphous and crystalline molybdenum oxide thin films have been investigated. As-deposited amorphous films got crystallized into a layered orthorhombic phase on annealing at 350°C. Refractive index (n) and extinction coefficient (k) of as-deposited films and films annealed at 150°C, 240°C and 350°C have been calculated using reflectance and transmittance data. Spectral dependence of absorption coefficient has been explained on the basis of charge transfer transition mechanism. Optical band gap of amorphous MoO_3−x is 3.16 eV and it has increased by 0.11 eV on crystallization. XPS core level analysis reveals the presence of Mo⁺⁴, Mo⁺⁵ and Mo⁺⁵ oxidation states in amorphous films, proving oxygen deficiency in as-deposited films. Same studies on crystalline films show the presence of only Mo⁺⁶ states. Valence band spectrum of amorphous films reveal emission from Mo4d levels, which is absent in crystalline films. Complete correlation is seen between the optical properties and XPS data.     

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.     

Deposition of hydrogenated amorphous silicon (a-Si:H) films by hot-wire chemical vapor deposition (HW-CVD) method: Role of substrate temperature

Hydrogenated amorphous silicon (a-Si:H) thin films were deposited from pure silane (SiH₄) using hot-wire chemical vapor deposition (HW-CVD) method. We have investigated the effect of substrate temperature on the structural, optical and electrical properties of these films. Deposition rates up to 15 Å s⁻¹ and photosensitivity 10⁶ were achieved for device quality material. Raman spectroscopic analysis showed the increase of Rayleigh scattering in the films with increase in substrate temperature. The full width at half maximum of TO peak (Γ_TO) and deviation in bond angle (Δθ) are found smaller than those obtained for P-CVD deposited a-Si:H films. The hydrogen content in the films was found <1 at% over the range of substrate temperature studied. However, the Tauc's optical band gap remains as high as 1.70 eV or much higher. The presence of microvoids in the films may be responsible for high value of band gap at low hydrogen content. A correlation between electrical and structural properties has been found. Finally, the photoconductivity degradation of optimized a-Si:H film under intense sunlight was also studied.     

Structural and optical properties of hot wall deposited CdSe thin films

Cadmium selenide (CdSe) films were prepared by hot wall deposition technique using optimized tube length under a vacuum of 6 mPa on to well-cleaned glass and ITO substrates. The X-ray diffraction analysis revealed that the films are polycrystalline in nature for lower thickness and at lower substrate temperatures, but with increasing thickness and increasing substrate temperature a more preferred orientation along (0 0 2) direction was observed. The crystallite size (D), dislocation density (δ) and strain () were calculated. An analysis of optical measurements revealed a sharp absorption around 700 nm and a direct allowed transition. The band gap was found to be around 1.7 eV. The effect of thickness and substrate temperature on the fundamental optical parameters like band gap, refractive index and extinction coefficient are studied.     

Novel hydrogen-doped SrSnO3 perovskite with excellent optoelectronic properties as a potential photocatalyst for water splitting

Developing and designing novel electrodes for photocatalytic water splitting using computational analysis has become a crucial interest recently through bulk and surface calculations of the investigated materials. Doping wide band gap metal oxides has proven to be an efficient method for optical properties enhancement and band gap engineering. Herein, first-principles calculations were employed to investigate the possibility to engineer the optical and structural properties of SrSnO₃ perovskite as a potential catalyst for photo-driven hydrogen production. Specifically, the synergistic effect of hydrogen doping and oxygen vacancies (O_V) on the optoelectronic properties of SrSnO₃ was for the first time investigated and discussed in detail. The interstitial hydrogen defects (H_i) are energetically favorable compared with the substitutional hydrogen defects. Mono- and co-hydrogen occupied oxygen vacancies sites were further examined. Interstitial hydrogen doping was found to introduce a shallow defect state below the conduction band minimum (CBM) forming a band tailing and increasing the dielectric constant. Thus, it could be used in gate dielectric applications. The created defect states upon doping were found to depend directly on the defect site and the defect concentration. At high concentration of oxygen vacancies defect, the H_OV-O_V structural configuration showed localized and shallow defect states with a band gap of 1.3 eV below the CBM. It also considerably increased the dielectric constant with optical absorption enhancement, compared to the pristine SrSnO₃ counterpart. With optimum Gibbs free energy of hydrogen evolution reaction (HER) and theoretical band gap straddling of the oxygen and hydrogen evolution potentials, low exciton binding energy, and high permittivity, the H_OV-O_V structure is an ideal novel candidate catalyst for photocatalytic water splitting.     

Optical and structural properties of – thin films

In this study, the sol–gel spin-coating method has been used to make Ta₂O₅–CeO₂ thin films. These films have been prepared in various composition ratios to observe changes in their optical and structural properties. Reflectance and transmittance spectra were collected in the spectral range of 300–1000 nm and were accurately fit using the Tauc–Lorentz model. Film thicknesses, refractive indices, absorption coefficients, and optical band gaps were extracted from the theoretical fit. The highest refractive index value was found at 5% CeO₂ doping. The structure of the films was characterized by X-ray diffractometry and Fourier transform infrared spectrometry, while the surface morphology was examined through atomic force microscopy.     

Influence of silane flow on structural, optical and electrical properties of a-Si:H thin films deposited by hot wire chemical vapor deposition (HW-CVD) technique

The electrical, structural and optical properties of hydrogenated amorphous silicon (a-Si:H) films deposited from pure silane (SiH₄) using hot wire chemical vapor deposition (HW-CVD) technique are systematically studied as a function of silane flow rate between 5 and 30 sccm. We found that the properties are greatly affected by the silane flow rate over the range we studied. The device quality a-Si:H films with a photosensitivity >10⁵ were deposited by HW-CVD at a deposition rate >10 Å s⁻¹ using low silane flow rate. However, a-Si:H films deposited at higher silane flow rate and/or higher deposition rates show degradation in their structural and electrical properties. The FTIR studies indicate that the hydrogen bonding in a-Si:H films shifts from mono-hydrogen (Si–H) to di-hydrogen (Si–H₂) and (Si–H₂)_n complexes when films were deposited at higher silane flow rate. The hydrogen content in the a-Si:H films increases with increase in silane flow rate and was found to be less than 10 at.%. The Raman spectra show increase in disorder and the Rayleigh scattering with increase in silane flow rate. The optical band gap also shows an increasing trend with silane flow rate. Therefore, only the hydrogen content cannot be accounted for the increase in the optical band gap. We think that the increase in the optical band gap may be due to the increase in the voids. These voids reduce the effective density of material and increase the average Si–Si distance, which is responsible for the increase in the band gap. Silane flow rate of 5 sccm, appears to be an optimum flow rate for the growth of mono-hydrogen (Si–H) bonded species having low hydrogen content (4.25 at%) in a-Si:H films at high deposition rate (12.5 Å s⁻¹), high photosensitivity (10⁵) and small structural disorder.     

Synthesis of highly conductive boron-doped p-type hydrogenated microcrystalline silicon (μc-Si:H) by a hot-wire chemical vapor deposition (HWCVD) technique

Boron-doped hydrogenated microcrystalline silicon (μc-Si:H) films were prepared using hot-wire chemical vapor deposition (HWCVD) technique. Structural, electrical and optical properties of these thin films were systematically studied as a function of B₂H₆ gas (diborane) phase ratio (Variation in B₂H₆ gas phase ratio, dopant gas being diluted in hydrogen, affected the film properties through variation in doping level and hydrogen dilution). Characterization of these films from low angle X-ray diffraction and Raman spectroscopy revealed that the high conductive film consists of mixed phase of microcrystalline silicon embedded in an amorphous network. Even a small increase in hydrogen dilution showed marked effect on film microstructure. At the optimized deposition conditions, films with high dark conductivity (0.08 (Ω cm)⁻¹) with low charge carrier activation energy (0.025 eV) and low optical absorption coefficient with high optical band gap (2.0 eV) were obtained. At these deposition conditions, however, the growth rate was small (6 Å/s) and hydrogen content was large (9 at%).     

Optical and electrochromic properties of oxygen sputtered tungsten oxide (WO3) thin films

Thin films of tungsten oxide (WO₃) were deposited onto glass, ITO coated glass and silicon substrates by pulsed DC magnetron sputtering (in active arc suppression mode) of tungsten metal with pure oxygen as sputter gas. The films were deposited at various oxygen pressures in the range 1.5×10⁻²−5.2×10⁻² mbar. The influence of oxygen sputters gas pressure on the structural, optical and electrochromic properties of the WO₃ thin films has been investigated. All the films grown at various oxygen pressures were found to be amorphous and near stoichiometric. A high refractive index of 2.1 (at λ=550 nm) was obtained for the film deposited at a sputtering pressure of 5.2×10⁻² mbar and it decreases at lower oxygen sputter pressure. The maximum optical band gap of 3.14 eV was obtained for the film deposited at 3.1×10⁻² mbar, and it decreases with increasing sputter pressure. The decrease in band gap and increase in refractive index for the films deposited at 5.2×10⁻² mbar is attributed to the densification of films due to ‘negative ion effects’ in sputter deposition of highly oxygenated targets. The electrochromic studies were performed by protonic intercalation/de-intercalation in the films using 0.5 M HCl dissolved in distilled water as electrolyte. The films deposited at high oxygen pressure are found to exhibit better electrochromic properties with high optical modulation (75%), high coloration efficiency (CE) (141.0 cm²/C) and less switching time at λ=550 nm; the enhanced electrochromism in these films is attributed to their low film density, smaller particle size and larger thickness. However, the faster color/bleach dynamics is these films is ascribed to the large insertion/removal of protons, as evident from the contact potential measurements (CPD) using Kelvin probe. The work function of the films deposited at 1.5 and 5.2×10⁻² mbar are 4.41 and 4.30 eV, respectively.     

Optical and photoconductive properties of SnS thin films prepared by electron beam evaporation

Thin films of SnS have been prepared by electron beam evaporation. The films represent Herzbergite orthorhombic structure, established by their XRD patterns. The band gap energy and type of optical transitions were determined from transmission spectra and an optical band gap of E_g(tr)=1.23 eV for indirect transitions and E_g(tr)=1.38 eV for direct transitions were estimated. Using the dependence of photoconductivity from wavelength, a band gap of E_g(ph)=1.2 eV was determined as well. A thermal band gap of E_g(T)=1.29 eV was evaluated from the temperature dependence of the dark resistivity, and admixture level with activation energies (0.25 and 0.36 eV) were found. Roughness of the surface of SnS thin films was evaluated using atomic force microscopy.     

Heat treatment effects on the optical properties of Sol–gel Ta2O5 thin films

In this work optical properties of Ta₂O₅ thin films with respect to heat treatment temperature were investigated. Ta₂O₅ thin films were prepared by sol–gel process using dip-coated method with a constant speed of 107 mm/min. Optical properties have been calculated from optical transmission measurements as a function of heat treatment temperature. The refractive indices and absorption coefficients were affected by heat treatment. The refractive index at λ=550 nm increased from 1.84 to 2.04 and absorption coefficient increased from 241 to 5668 cm⁻¹ when heat treatment temperature increased from 100°C to 500°C. The thickness of the film decreased from 272 to 190 nm and their optical band gap decreased from 3.68±0.09 eV to 3.51±0.08 eV for the film heated from 100°C to 500°C.     

Effect of doping and heat treatment on the photoluminescence of CdS films deposited by spray pyrolysis

Large area thin films of n-type CdS were prepared by spray pyrolysis technique. The films were n-type doped by having [In/Cd] ion concentration ratio of 10⁻⁶, 10⁻⁵, 10⁻⁴, 10⁻³, and 10⁻² in the sprayed solution. These films were heat treated in N₂ atmosphere at 450°C for 1 h. The as-deposited undoped, doped, and heat treated were analyzed by photoluminescence (PL) at 5 K sample temperature. In general, the spectra displayed three main emission regions (green, yellow and red) with more than one band in each. The emission intensity is found to decrease with doping and the relative intensity of the bands is found to depend on the doping concentration level. The red band is only present in doped samples and its relative intensity is found to increase with doping. The effect of heat treatment in N₂ on the as-deposited undoped and the doped (10⁻⁴) samples on the relative intensity of the observed bands were compared and discussed. The results are compared with the electrical and morphological results and correlated with the probable changes in the concentration of shallow and deep radiative native defects and structural changes. These allow for better prediction of suitable doping and treatment conditions for good quality films.     

First-principles investigation of electronic and optical properties of Fe doped in CsBrO3 for enhanced photocatalytic hydrogen production

Self-consistent ab initio calculations carried out using full potential augmented plane wave (FP-LAPW) method were performed to study the electronic, optical and photocatalytic properties of CsBrO₃ and Fe doped CsBrO₃. Ground state and formation energy for CsBrO₃-perovskite are calculated and analyzed. The magnetic moment of Cs, Br, Fe and O are calculated in CsBrO3 and CsBr_0.34Fe_0.66O₃. The band structure, total and partial density of states (DOS) diagrams are discussed.The CsBrO₃ have semiconductor character with a wide direct gap. The value of gap energy is 4.24 eV of CsBrO₃. Fe doped in CsBrO₃ lead to band gap narrowing 1.02 eV for spin up and 1.434 eV for spin dn , which enhances the visible light catalytic activity. The conduction band minimum (CBM) and valence band maximum (VBM) potentials vs. normal hydrogen electrode (NHE) are calculated and analyzed. The general profiles of the optical spectra and the optical properties, including the real and imaginary part of dielectric function, reflectivity, absorption and optical conductivity are discussed. Our results predict that Fe doped CsBrO₃ is a promising visible light photo-catalyst for hydrogen production by water splitting.     

Bromine doping effect on some properties of CdS films

The suitability of bromine as an anionic dopant influencing the properties of CdS thin films is discussed in this paper. The as-deposited films were characterised respectively by XRD, SEM, UV-vis-NIR spectrophotometer and two-point probe setup. All the films appear to be polycrystalline in nature exhibiting hexagonal crystal structure with a (002) preferential growth texture. The 2θ value of the (002) plane shifts towards lower Bragg angle with doping inferring an expansion in their lattice volumes. Increased transparency and blue shift in optical band gap value are observed for the doped films. Electrical resistivity decreased with increase in Br-doping concentration. Increased transparency, widened band gap and decreased electrical resistivity values achieved confirm that Br is a suitable anionic dopant that can strongly influence the physical properties of pure CdS towards future optoelectronic devices.     

Optical characterization of a-C:N thin films deposited by RF sputtering

In this work we present the main results of the optical properties study of amorphous carbon nitride (a-C:N) thin films prepared by reactive radio frequency (RF) sputtering. The a-C:N films were deposited, at room temperature, onto glass substrates, from a graphite target, in a pure nitrogen plasma. During the deposition, the pressure of nitrogen and the power density were maintained at 10⁻² mbar and 0.79 W cm⁻², respectively. Optical properties of these films were deduced from optical transmission spectra in the ultraviolet–visible–near infrared (UV–Vis–NIR) range. The refractive index follows well the Cauchy law with an extrapolated value of 1.68 in the far IR region. The optical band gap of the a-C:N films is about 1.2 eV. This value is relatively high in comparison with that of amorphous carbon films (0.8 eV) obtained in similar conditions. The incorporation of nitrogen in the amorphous carbon network leads to an increase of the optical band gap.     

Structural phase change and optical band gap bowing in hot wall deposited CdSexTe1−x thin films

CdSe_xTe_1−x thin films of different compositions have been deposited on glass substrates by hot wall deposition method under conditions very close to thermodynamical equilibrium with minimum loss of material. The structural studies carried out on the deposited films revealed that they are crystalline in nature and exhibit either cubic zinc blende or hexagonal phase or both depending on the composition of the material. The lattice parameter values for both cubic and hexagonal phases have been determined and are observed to vary with composition according to Vegard’s law. The optical properties of the deposited CdSe_xTe_1−x thin films have been studied using transmittance spectra. The spectra shows a sharp fall in transmittance at wavelength corresponding to the band gap of the material. The optical band gap has been determined and found to be direct allowed. The band gap has been observed to strongly depend on film composition. The variation of band gap with composition has been observed to be quadratic in nature exhibiting a bowing behaviour.     

Preparation of highly conductive p-type μc-Si:H window layer using lower concentration of hydrogen in the rf glow discharge plasma

Boron doped p-type hydrogenated microcrystalline silicon (μc-Si:H) films have been prepared by radio-frequency glow discharge method. Highly conductive p-type μc-Si:H films can be obtained even with lower concentration of hydrogen in the rf glow discharge plasma if chamber pressure is low. Effects of increase in hydrogen (H₂) flow rate and chamber pressure have been studied. The structural properties of the films have been studied by X-ray diffractometry. The electrical and optical characterization have been done by dark conductivity, Hall measurements and optical absorption measurements respectively. Film with conductivity 0.1(Ω-cm)⁻¹ with band gap 2.1 eV has been obtained.