Effect of thermal annealing on physical properties of vacuum evaporated In2S3 buffer layer for eco-friendly photovoltaic applications

The present communication reports the effect of thermal annealing on the physical properties of In₂S₃ thin films for eco-friendly buffer layer photovoltaic applications. The thin films of thickness 150 nm were deposited on glass and indium tin oxide (ITO) coated glass substrates employing thermal vacuum evaporation technique followed by post-deposition thermal annealing in air atmosphere within a low temperature range 150–450 °C. These as-deposited and annealed films were subjected to the X-ray diffraction (XRD), UV–vis spectrophotometer, current–voltage tests and scanning electron microscopy (SEM) for structural, optical, electrical and surface morphological analysis respectively. The compositional analysis of as-deposited film is also carried out using energy dispersive spectroscopy (EDS). The XRD patterns reveal that the as-deposited and annealed films (≤300 °C) have amorphous nature while films annealed at 450 °C show tetragonal phase of β-In₂S₃ with preferred orientation (109) and polycrystalline in nature. The crystallographic parameters like lattice constant, inter-planner spacing, grain size, internal strain, dislocation density and number of crystallites per unit area are calculated for thermally annealed (450 °C) thin films. The optical band gap was found in the range 2.84–3.04 eV and observed to increase with annealing temperature. The current–voltage characteristics show that the as-deposited and annealed films exhibit linear ohmic behavior. The SEM studies show that the as-deposited and annealed films are uniform, homogeneous and free from crystal defects and voids. The grains in the thin films are similar in size and densely packed and observed to increase with thermal annealing. The experimental results reveal that the thermal annealing play significant role in the structural, optical, electrical and morphological properties of deposited In₂S₃ thin films and may be used as cadmium-free eco-friendly buffer layer for thin films solar cells applications.     

Optical and electrical properties of nickel oxide thin films synthesized by sol–gel spin coating

Nickel oxide thin films were prepared by the sol–gel technique combined with spin coating onto glass substrates. The as-deposited films were pre-heated at 275 °C for 15 min and then annealed in air at different temperatures. The effects of the annealing temperature on the structural and optical properties of the films are studied. The results show that 600 °C is the optimum annealing temperature for preparation of NiO films with p-type conductivity and high optical transparency. Then, by using these optimized deposition parameters, NiO thin films of various thicknesses were deposited at the same experimental conditions and annealed under different atmospheres. Surface morphology of the films was investigated by atomic force microscopy. The surface morphology of the films varies with the annealing atmosphere. Optical transmission was studied by UV–vis spectrophotometer. The transmittance of films decreased as the thickness of films increased. The electrical resistivity, obtained by four-point probe measurements, was improved when NiO layers were annealed in N₂ atmosphere at 600 °C.     

Microstructure evolution of amorphous silicon thin films upon annealing studied by positron annihilation

Amorphous silicon (a-Si) thin films were prepared on glass substrates by plasma enhanced chemical vapor deposition (PECVD). Influence of annealing temperature on the microstructure, surface morphology, and defects evolution of the films were studied by X-ray diffraction (XRD), atomic force microscope (AFM) and positron annihilation Doppler broadening spectroscopy (DBS) based on a slow positron beam, respectively. The S parameter of the as-deposited a-Si thin film is high, indicative of amorphous state of Si film with many defects. The a-Si gradually grows into polycrystalline silicon with increasing temperature to 650 °C. For the films annealed below ~450 °C, positron diffusion lengths are rather small because most positrons are trapped in the defects of the a-Si films and annihilated there. With further rising the temperature to 600 °C, the diffusion length of positrons increases significantly due to the removal of vacancy-type defects upon annealing at a high temperature. The results indicate that the coalescence of small vacancy-type defects in a-Si thin film and the crystallization of a-Si occur around 450 °C and 650 °C, respectively.     

Influence of thermal annealing on electrical and optical properties of indium tin oxide thin films

Transparent conducting indium tin oxide (ITO) thin films with the thickness of 300 nm were deposited on quartz substrates via electron beam evaporation, and five of them post-annealed in air atmosphere for 10 min at five selected temperature points from 200 °C to 600 °C, respectively. An UV–vis spectrophotometer and Hall measurement system were adopted to characterize the ITO thin films. Influence of thermal annealing in air atmosphere on electrical and optical properties was investigated in detail. The sheet resistance reached the minimum of 6.67 Ω/sq after annealed at 300 °C. It increased dramatically at even higher annealing temperature. The mean transmittance over the range from 400 nm to 800 nm reached the maximum of 89.03% after annealed at 400 °C, and the figure of merit reached the maximum of 17.79 (Unit: 10⁻³ Ω⁻¹) under the same annealing condition. With the annealing temperature increased from 400 °C to 600 °C, the variations of transmittance were negligible, but the figure of merit decreased significantly due to the deterioration of electrical conductivity. With increasing the annealing temperature, the absorption edge shifted towards longer wavelength. It could be explained on the basis of Burstein–Moss shift. The values of optical band gap varied in the range of 3.866–4.392 eV.     

Ultraviolet-assisted annealing for low-temperature solution-processed p-type gallium tin oxide (GTO) transparent semiconductor thin films

Solution-processed p-type gallium tin oxide (GTO) transparent semiconductor thin films were prepared at a low temperature of 300 °C using ultraviolet (UV)-assisted annealing instead of conventional high-temperature annealing (> 500 °C). We report the effects of UV irradiation time on the structural, optical, and electrical properties of sol-gel derived GTO thin films and a comparison study of the physical properties of UV-assisted annealed (UVA) and conventional thermally annealed (CTA) GTO thin films. The Ga doping content was fixed at 15 at% in the precursor solution ([Ga]/[Sn]+[Ga] = 15%). After a spin-coating and preheating procedure was performed two times, the dried sol-gel films were heated on a hotplate at 300 °C under UV light irradiation for 1–4 h. Each UVA GTO thin film had a dense microstructure and flat free surface and exhibited an average optical transmittance approaching 85.0%. The level of crystallinity, crystallite size, and hole concentration density of the GTO thin films increased with increasing UV irradiation time. In this study, the UVA 4 h thin film samples exhibited the highest hole concentration (9.87 × 10¹⁷ cm⁻³) and the lowest resistivity (1.8 Ω cm) and had a hole mobility of 5.1 cm²/Vs.     

Impact of vacuum and nitrogen annealing on HVE SnS photoabsorber films

Researchers worldwide focus on new earth abundant and cheap absorber materials for use in thin film solar cells that allow wider use of photovoltaics in energy production. SnS is one of such promising absorber materials that comprises earth abundant elements (Sn, S). We describe here the effect of annealing of high vacuum evaporated (HVE) SnS thin films in vacuum and nitrogen atmosphere with relatively high pressures of nitrogen. SnS thin films with a thickness of 500 nm were deposited onto the surface of glass by HVE at a substrate temperature of 300 °C. The as-deposited SnS thin films were annealed at 500 °C and 550 °C for 1 h in vacuum as well as in nitrogen with respect to ambient (N₂) pressure that varied in the range of 500–2000 mbar. We analyze crystalline quality, crystal structure, elemental and phase compositions, and electrical properties of SnS films before and after the annealing process and their changes. Our results show that the use of pressurized inert ambient, such as nitrogen, improves the crystalline quality as well as the electrical properties of SnS thin films. The enhanced growth of crystals and modification of microstructural properties of SnS thin films as a function of annealing conditions (type of ambient, annealing temperature and ambient pressure) are discussed in detail.     

Effect of annealing temperature on optical and electrical properties of lead sulfide thin films

Lead sulfide (PbS) thin films with 150 nm thickness were prepared onto ultra-clean quartz substrate by the RF-sputtering deposition method. Deposited thin films of PbS were annealed at different temperatures 100 °C, 150 °C, 200 °C, 250 °C and 300 °C. X-ray diffraction pattern of thin films revealed that thin films crystallized at 150 °C. Crystalline thin films had cubic phase and rock salt structure. The average crystallite size of crystalline thin films was 22 nm, 28 nm and 29 nm for 150 °C, 200 °C and 250 °C respectively. From 150 °C to 250 °C increase in annealing temperature leads to increase in crystallite arrangement. FESEM images of thin films revealed that crystallite arrangement improved by increasing annealing temperature up to 250 °C. Increase in DC electrical conductivity by increasing temperature confirmed the semiconductor nature of crystalline thin films. Increase in dark current by increasing annealing temperature showed the effect of crystallite arrangement on carrier transport. Photosensitivity decreased by increasing annealing temperature for crystalline thin films that it was explained at the base of thermal quenching of photoconductivity and adsorption of oxygen at the surface of thin films that leads to the formation of PbO at higher temperatures.     

Structural and optoelectronic properties of nanostructured TiO2 thin films with annealing

About 480 nm thick titanium oxide (TiO₂) thin films have been deposited by electron beam evaporation followed by annealing in air at 300–600 °C with a step of 100 °C for a period of 2 h. Optical, electrical and structural properties are studied as a function of annealing temperature. All the films are crystalline (having tetragonal anatase structure) with small amount of amorphous phase. Crystallinity of the films improves with annealing at elevated temperatures. XRD and FESEM results suggest that the films are composed of nanoparticles of 25–35 nm. Raman analysis and optical measurements suggest quantum confinement effects since Raman peaks of the as-deposited films are blue-shifted as compared to those for bulk TiO₂ Optical band gap energy of the as-deposited TiO₂ film is 3.24 eV, which decreases to about 3.09 eV after annealing at 600 °C. Refractive index of the as-deposited TiO₂ film is 2.26, which increases to about 2.32 after annealing at 600 °C. However the films annealed at 500 °C present peculiar behavior as their band gap increases to the highest value of 3.27 eV whereas refractive index, RMS roughness and dc-resistance illustrate a drop as compared to all other films. Illumination to sunlight decreases the dc-resistance of the as-deposited and annealed films as compared to dark measurements possibly due to charge carrier enhancement by photon absorption.     

Influence of Mn doping on structural,optical and electrical properties of CdO thin films prepared by cost effective spray pyrolysis method

Transparent and conducting cadmium oxide (CdO) and manganese doped CdO (Mn: CdO) thin films were deposited using a low cost spray pyrolysis method on the glass substrate at 300 °C. For Mn doping, various concentrations of manganese acetate (1–3 wt%) was used in the spraying precursor solution. The structural, electrical and optical properties of CdO and Mn: CdO films were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM), UV–vis and Hall measurement. X-ray diffraction study reveals that the CdO and Mn: CdO films are possessing cubic crystal structures. SEM and AFM studies reveal that the grain size and roughness of the films are increased with increasing Mn doping concentration. Optical transmittance spectra of the CdO film decreases with increasing doping concentration of manganese. The optical band gap of the films decreases from 2.42 eV to 2.08 eV with increasing concentration of manganese. A minimum resistivity of 1.11×10⁻³ Ω cm and maximum mobility of 20.77 cm² V⁻¹ s⁻¹ is achieved for 1 wt% of manganese doping.     

Structural,optical, and photocatalytic properties of the spray deposited nanoporous CdS thin films; influence of copper doping,annealing, and deposition parameters

Nanoporous thin films of Cd_1−xCu_xS (0≤x≤0.06) were grown on a heated glass substrate employing a home-made spray pyrolysis technique. The influences of [Cu]/[Cd] and the annealing in the range 300–500 °C on the structural and morphological properties of the films were investigated by X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), field emission scanning electron microscope (FE-SEM) and atomic force microscopy (AFM). The influences of Cu doping ratio, solution flow rate, and the deposition time on the optical properties and photocatalytic activity of these films are also reported. The films are of polycrystalline nature and hexagonal structure. Increasing the Cu doping ratio and annealing temperature improve the (1 0 1) preferential orientation. The crystallite size is ranged from 23.82 to 32.11 nm. XRD and FTIR reveal the formation of CdO in the 6% Cu-doped CdS film annealed at 400 °C and in all films annealed at 500 °C. The pure CdS film is of a porous structure and the close-packing and porosity of the films increase with increasing Cu%. Also, the pore diameter can be controlled from 50 to 15 nm with the increase of Cu content. The films showed transmittance below 70%. The optical band gap of the films is decreased from 2.43 to 1.82 eV with increasing Cu% and flow rate/deposition time. Additionally, the refractive indices and dispersion parameters of the films are also affected by the deposition conditions. Cu doping enhanced the films' photostability as well as the photocatalytic removal of methylene blue (MB).     

Variation of the structural,optical and electrical properties of CBD CdO with processing temperature

Nanostructures of CdO thin films are prepared by chemical bath deposition (CBD) technique. The synthesized film is annealed in static air by using the hotplate at 373, 473, 573 and 673 K for 10 min. The effect of annealing temperature on structural, morphological, optical and electrical properties of CdO thin films has been investigated. The prepared thin films are characterised by X-ray diffraction (XRD), atomic force microscope (AFM), optical reflection microscope (ORM), UV–Visible Spectrophotometer and electrical resistivity. XRD shows the emergence of the cubic phase of CdO film in a preferred orientation (111) plane at 573 K. The AFM and ORM show that CdO films have smooth homogeneous surface in the formula with the emergence of nanoclusters gathering as nanoparticles with the average of grain size about 100 nm at 573 K. The optical properties prove that deposited films have high transparency within the visible range of the spectrum that reaches to more than 85% with a wide band gap that extends from 2.42 eV to 2.7 eV. The electrical properties of the CdO films show that resistivity decreases with increased annealing temperatures. In addition, it is proved that more than one activation energy appears and they change according to the temperature of annealing and this comes as a result of the polycrystalline structure. This study indicates that the properties of CdO thin films could be improved with annealing temperature and these films can be used in many technological applications.     

Properties of CuAlO2 thin films deposited by polyacrylamide gel route

CuAlO₂ films were deposited on clean glass substrates by the acrylamide sol–gel dip coating technique. The coated films were dried in air oven for 30 min followed by heat treatment in air at different temperatures in the range of 350–500 °C. The films annealed at low temperatures exhibited weak x-ray diffraction (XRD) peaks. As the post anneal temperature increased beyond 375 °C, the XRD pattern exhibited the diffraction peaks of rhombohedral CuAlO₂. Surface morphology of the films indicated that the films annealed at low temperatures exhibit small grains. As the annealing temperature increases larger grains are observed. The root mean square (rms) value of the surface roughness increases with annealing temperature. The films exhibited optical transmission above 75%. The films post annealed at low temperature exhibited lower transmission. Optical band gap in the range of 3.43–3.75 eV was obtained for the films annealed at different temperature. Hall measurements indicated p-type conductivity. Resistivity of the films decreased from 25.0 to 2.0 Ω cm as the anneal temperature increased. Mobility and carrier density increased with annealing temperature.     

Investigation of optical and electronic properties in Al–Sn co-doped ZnO thin films

Al-Sn co-doped ZnO thin films were deposited onto quartz substrates by sol-gel processing. The surface morphology and electrical and optical properties were investigated at different annealing temperatures. The surface morphology showed a closely packed arrangement of crystallites in all the doped films. As prepared co-doped films show a preferred orientation along an (0 0 2) plane. This preferred orientation was enhanced by increasing the annealing temperature to between 400 °C and 500 °C, but there was a shift to the (1 0 1) plane when the annealing temperature rose above 500 °C. These samples show, on average, 91.2% optical transmittance in the visible range. In this study, the optical band gap of all the doped films was broadened compared with pure ZnO, regardless of the different annealing temperature. The carrier concentration and carrier mobility of the thin films were also investigated.     

Influence of annealing temperature on the phase composition and optical properties of CuInS2 films

We studied the growth of CuInS₂ thin films by single-source evaporation of CuInS₂ powder in a high-vacuum system with a base pressure of 10^?3 Pa. After evaporation, the films were annealed in a sulfur atmosphere at temperatures from 200 to 500 °C for 1 h. XRD curves and Raman spectra of the films demonstrated that chalcopyrite CuInS₂ was the major crystalline phase. The morphology of Cu_xS exhibited a star-like structure, which we report for the first time. The phase composition and optical properties of our polycrystalline thin films were effectively modified by annealing in S. For films annealed at 200 and 350 °C, a secondary CuIn₁₁S₁₇ phase appeared, which may be related to solid-state reaction in the S atmosphere. This secondary CuIn₁₁S₁₇ phase has not been widely reported in previous studies. After annealing at 500 °C, only a chalcopyrite phase was detected, with bandgap energy of 1.46 eV, which is nearly identical to the optimal bandgap energy (1.5 eV) of single-crystal CuInS₂. This indicates that the composition of the CuInS₂ film annealed at 500 °C was nearly stoichiometric. The bandgap of the samples first increased and then decreased with increasing annealing temperature, which may be attributed to an increase in grain size, the secondary CuIn₁₁S₁₇ phase, and deviation from stoichiometry.     

Effects of the Cd:Zn:S molar ratio and heat treatment on the optical and photoluminescence properties of nanocrystalline CdZnS thin films

Nanocrystalline cadmium zinc sulfide thin films with different molar ratios were prepared by sol–gel dip-coating in a polyethyleneglycol matrix. After heat treatment in air at 250, 350 and 450 °C, the thin films were characterized by studying their structural, morphological, compositional, optical (linear and nonlinear) and photoluminescence (PL) properties. According to X-ray diffraction (XRD) results, the samples are polycrystalline with a hexagonal crystal structure and an average grain size of 12–18 nm. The surface morphology of the films was examined by scanning electron microscopy (SEM). The results show that the films consist of nanocrystalline grains included in clusters with uniform coverage over the substrate surface. To determine their chemical composition, X-ray photoelectron spectra (XPS) of composite films were measured. The transmittance and bandgap of the films increased with the Zn concentration and decreased with increasing annealing temperature. The refractive index of the films was measured and the related dispersion is discussed in terms of the Wemple–DiDomenico single oscillator model. The third-order nonlinear polarizability of the films was estimated using a semi-empirical relation based on the single oscillator model. The results show that the films are suitable as optical switches. PL spectra were recorded for an excitation wavelength of 210 nm. The emission intensity for the films varied with the Zn ratio and the annealing temperature and the behavior of different peaks is discussed.     

Effect of substrate temperature on indium tin oxide (ITO) thin films deposited by jet nebulizer spray pyrolysis and solar cell application

This study focused on the effect of substrate temperature (350 °C, 400 °C, and 450 °C) on morphological, optical, and electrical properties of indium tin oxide (ITO) films deposited onto porous silicon/sodalime glass substrates through jet nebulizer spray pyrolysis for use in heterojunction solar cells. X-ray diffraction analysis confirmed the formation of pure and single-phase In₂O₃ for all the deposited films whose crystallinity was enhanced with increasing substrate temperature, as shown by the increasing (222) peak intensity. Morphological observations were conducted using scanning electron microscopy to reveal the formation of continuous dense films composed of nanograins. The UV–vis spectra revealed that the transmittance increased with increasing substrate temperature, reaching a value of over 80% at 450 °C. The photoelectric performance of the solar cell was studied using the I–V curve by illuminating the cell at 100 mW/cm². A high efficiency (η) of 3.325% with I_sc and V_oc values of 14.8 mA/cm² and 0.60 V, respectively, was attained by the ITO solar cell annealed at 450 °C.     

Evaporated iron disulfide thin films with sulfurated annealing treatments

FeS₂ thin films were grown on a glass substrate using a physical vapor deposition technique at room temperature. Subsequently, the thin films were annealed in two different atmospheres: vacuum and vacuum-sulfur. In the vacuum-sulfur atmosphere a graphite box was used as sulfur container and the films were sulfurated successfully at 200–350 ºC. It was found that annealing in a vacuum-sulfur atmosphere was indispensable in order to obtain polycrystalline FeS₂ thin films. The polycrystalline nature and pure phase were determined by XRD and Raman techniques and the electrical properties by the Hall effect. Using the sulfurating technique, the n-type semiconductor was prepared at 200–350 °C and a p-type at 500 °C. The carrier concentrations were between 1.19×10²⁰ and 2.1×10²⁰ cm⁻³. The mobility was 9.96–5.25 cm² V⁻¹ s⁻¹ and the resistivity was 6.31×10⁻² to 1.089×10⁻² Ω cm. The results obtained from EDS showed that the films prepared in the vacuum-sulfur atmosphere were close to stoichiometric and that the indirect band gap varied between 1.03 and 0.945 eV.     

Effect of substrate temperature on structural,morphological, optical and electrical properties of MnIn2S4 thin films prepared by nebulizer spray pyrolysis technique

Manganese indium sulphide (MnIn₂S₄) thin films were deposited using an aqueous solution of MnCl₂, InCl₃ and (NH₂)₂CS in the molar ratio 1:2:4 by simple chemical spray pyrolysis technique. The thin film substrates were annealed in the temperature range between 250 and 350 °C to study their various physical properties. The structural properties as studied by X-ray diffraction showed that MnIn₂S₄ thin films have cubic spinel structure. The formation of cube and needle shaped grains was clearly observed from FE-SEM analysis. The energy dispersive spectrum (EDS) predicts the presence of Mn, In and S in the synthesized thin film. From the optical studies, it is analyzed that the maximum absorption co-efficient is in the order between 10⁴ and 10⁵ cm⁻¹ and the maximum transmittance (75%) was noted in the visible and infrared regions. It is noted that, the band gap energy decreases (from 3.20 to 2.77 eV) with an increase of substrate temperature (from 250 to 350 °C). The observations from photoluminescence studies confirm the emission of blue, green, yellow and red bands which corresponds to the wavelength range 370–680 nm. Moreover, from the electrical studies, it is observed that, as the substrate temperature increases the conductivity also increases in the range 0.29–0.41×10⁻⁴ Ω⁻¹ m⁻¹. This confirms the highly semiconducting nature of the film. The thickness of the films was also measured and the values ranged between 537 nm (250 °C) to 483 nm (350 °C). This indicates that, as the substrate temperature increases, the thickness of the film decreases. From the present study, it is reported that the MnIn₂S₄ thin films are polycrystalline in nature and can be used as a suitable ternary semiconductor material for photovoltaic applications.     

Composition,structure and electrical properties of DC reactive magnetron sputtered Al2O3 thin films

Thin films of alumina (Al₂O₃) were deposited over Si 〈1 0 0〉 substrates at room temperature at an oxygen gas pressure of 0.03 Pa and sputtering power of 60 W using DC reactive magnetron sputtering. The composition of the as-deposited film was analyzed by X-ray photoelectron spectroscopy and the O/Al atomic ratio was found to be 1.72. The films were then annealed in vacuum to 350, 550 and 750 °C and X-ray diffraction results revealed that both as-deposited and post deposition annealed films were amorphous. The surface morphology and topography of the films was studied using scanning electron microscopy and atomic force microscopy, respectively. A progressive decrease in the root mean square (RMS) roughness of the films from 1.53 nm to 0.7 nm was observed with increase in the annealing temperature. Al–Al₂O₃–Al thin film capacitors were then fabricated on p-type Si 〈1 0 0〉 substrate to study the effect of temperature and frequency on the dielectric property of the films and the results are discussed.     

Sigmoid-growth curve of lattice-constant against annealing-temperature of Cd1−xZnxTe thin films and its structural,optical and morphological characteristics

Stacked CdTe/Zn/CdTe layers were deposited on glass substrates. The vacuum-evaporated thin films were subsequently annealed in vacuum ambience at various temperatures. Change in lattice-constant of major Cd_1−xZn_xTe planes against temperature was plotted from the XRD results. The graphs followed sigmoid-growth model and were regressed well by standard Boltzmann and Logistic functions. Lattice-constant varied maximum in between 375–400 °C and 425–450 °C, giving two separate growth trends. Optical studies suggested that presence of charge impurities and defects reduced the transmittance and band-gap values of the samples. Such reduction occurred, despite of greater formation of Cd_1−xZn_xTe. Decreasing granularity was however associated with increasing band-gap for samples annealed at 425 and 450 °C. SEM micrographs showed that granularity decreased significantly for samples annealed at higher temperatures. EDX results were further used to co-relate the compositional characteristics with structural and optical features.