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.     

Effect of rapid thermal annealing on properties of thermally evaporated nanostructured CdTe thin film treated with CdCl2

The effects of rapid thermal annealing on properties of crystalline nanostructured CdTe films treated with CdCl₂ and prepared by vacuum evaporation are described. X-ray diffraction confirmed the crystalline nature of post-treated films with high preferential orientation around 23.7°, corresponding to a (1 1 1) diffracted plane of cubic phase. Optical band gap of CdTe films increased from 1.4 eV to 1.48 eV after annealing at 500 °C for 90 s. Atomic force microscopy of annealed films revealed an increase in root mean square roughness and grain size with increased annealing time. Electrical measurements of as-grown and annealed films are consistent with p-type; film resistivity has decreased significantly with increased annealing time.     

Raman spectroscopic study of In2S3 films prepared by spray pyrolysis

Indium sulfide (In₂S₃) thin films are of interest as buffer layers in chalcopyrite absorber based solar cells; and as media providing two-photon absorption for intermediate-band solar cells. We investigated the suitability of chemical spray pyrolysis (CSP) for growing In₂S₃ thin films in a structural order where indium atoms are preferentially in the octahedral sites. We sprayed aqueous or alcoholic solutions of indium chloride (InCl₃) and thiourea (SC(NH₂)₂) precursors onto a substrate with surface temperatures (T_S) of 205, 230, 275 and 320 °C. The as-deposited films grown from aqueous solutions were annealed in 5% H₂S containing atmosphere at 500 °C. We used Raman spectroscopy, X-ray diffraction and Energy Dispersive X-ray spectroscopy to evaluate the effect of growth temperature and the effect of annealing on the film structure and stoichiometry. The use of alcoholic solvent instead of aqueous allows us to use much lower T_S while preserving the quality of the β-In₂S₃ films obtained. Similarly, films with increased stoichiometry and quality are present at a higher T_S; and when annealed. The annealing of the films grown at T_S of 205 °C results in a much higher gain of the crystal quality compared to the gain when annealing the films grown at T_S of 320 °C, although the quality remain higher when deposited at T_S of 320 °C. Simultaneously with the increase of the film quality, there is a sign of increased quality of the crystal ordering with indium in the octahedral sites. Such a crystal ordering favor the use of CSP deposited In₂S₃ films in the intermediate band solar cells.     

Post-deposition annealing effect on In2S3 thin films deposited using SILAR technique

Indium Sulfide thin films have been deposited on glass substrate using a simple and inexpensive technique viz., successive ionic layer adsorption and reaction (SILAR). The films are characterized using X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and optical absorption techniques. Samples are prepared by varying dipping and rinsing time, and those prepared at room temperature are found to be amorphous in nature. However, peaks corresponding to β-In₂S₃ are observed, on annealing at 400 °C. Grain size is found to increase with increase in the dipping times, either in the precursor solutions or in water. Band gap decreases considerably for samples annealed at 400 °C in vacuum.     

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.     

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.     

Properties of In2O3 films obtained by thermal oxidation of sprayed In2S3

In₂S₃ thin films were grown by the chemical spray pyrolysis (CSP) method using indium chloride and thiourea as precursors at a molar ratio of S:In=2.5. The deposition was carried out at 350 °C on quartz substrates. The film thickness is about 1 µm. The films were then annealed for 2 h at 550, 600, 650 and 700 °C in oxygen flow. This process allows the transformation of nanocrystal In₂O₃ from In₂S₃ and the reaction is complete at 600 °C. X-ray diffraction spectra show that In₂O₃ films are polycrystalline with a cubic phase and preferentially oriented towards (222). The film grain size increases from 19 to 25 nm and RMS values increase from 9 to 30 nm. In₂O₃ films exhibit transparency over 70–85% in the visible and infrared regions due to the thickness and crystalline properties of the films. The optical band gap is found to vary in the range 3.87–3.95 eV for direct transitions. Hall effect measurements at room temperature show that resistivity is decreased from 117 to 27 Ω cm. A carrier concentration of 1×10¹⁶ cm^?3 and mobility of about 117 cm² V^?1 s^?1 are obtained at 700 °C.     

Structural,chemical, and electrical properties of Y2O3 thin films grown by atomic layer deposition with an (iPrCp)2Y(iPr-amd) precursor

Y₂O₃ thin films were grown by atomic layer deposition (ALD) through a heteroleptic liquid (iPrCp)₂Y(iPr-amd) precursor at 350 °C. The structural and chemical properties of both as-deposited and annealed Y₂O₃ films at 500 °C and 700 °C are analyzed by atomic force microscopy for variation in surface roughness, X-ray diffraction for crystalline structure, and X-ray photoelectron spectroscopy for chemical states. The as-deposited Y₂O₃ film shows the same crystalline orientation along the plane (222), a stoichiometric state, and minimal hydroxylate formation up to 700 °C. Being the dielectric layer in the metal-oxide-semiconductor capacitor, the as-deposited ALD-Y₂O₃ films with liquid (iPrCp)₂Y(iPr-amd) precursor without any post-deposition annealing show the much lower leakage density than ALD-Y₂O₃ with solid Y(MeCp)₃.     

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.     

Effects of RTA temperatures on conductivity and micro-structures of boron-doped silicon nanocrystals in Si-rich oxide thin films

In this work, the B-doped Si rich oxide (SRO) thin films were deposited and then annealed using rapid thermal annealing (RTA) to form SiO₂-matrix silicon nanocrystals (Si NCs). The effects of the RTA temperatures on the structural properties, conduction mechanisms and electrical properties of B-doped SRO thin films (BSF) were investigated systematically using Hall measurements, Fourier transform infrared spectroscopy and Raman spectroscopy. Results showed that the crystalline fraction of annealed BSF increased from 41.3% to 62.8%, the conductivity was increased from 4.48×10⁻³ S/cm to 0.16 s/cm, the carrier concentration was increased from 8.74×10¹⁷ cm⁻³ to 4.9×10¹⁸ cm⁻³ and the carrier mobility was increased from 0.032 cm² V⁻¹ s⁻¹ to 0.2 cm² V⁻¹ s⁻¹ when the RTA temperatures increased from 1050 °C to 1150 °C. In addition, the fluctuation induced tunneling (FIT) theory was applicable to the conduction mechanisms of SiO₂-matrix boron-doped Si-NC thin films.     

Influence of annealing temperature on field emission from tetrapod-shaped ZnO-whisker films obtained by screen printing

Tetrapod-shaped zinc oxide whisker-film emitters were fabricated on indium tin oxide glass substrates using a screen-printing method. The influence of annealing temperature on field emission of tetrapod-whisker ZnO-based emitters was investigated. X-ray diffraction and scanning electronic microscopy were applied to characterize the structure and the surface morphology of the deposited films. It was found that ZnO-based emitters annealed at 250 °C have the best field emission properties with the lowest turn-on field of 2.6 V/μm at a current density of 1 μA/cm², the lowest threshold field of 5.2 V/μm at a current density of 1 mA/cm² and high field emission enhancement factor of 4129. Moreover, films with homogeneous, fine and dense light spots with low emission current fluctuation of 1.7% were obtained from samples annealed at 250 °C.     

Defect modification in ZnInSnO transistor with solution-processed Al2O3 dielectric by annealing

The properties of solution-processed Al₂O₃ thin films annealed at different temperatures were thoroughly studied through thermogravimetry–differential thermal analysis, UV–vis-NIR spectrophotometer measurements, scanning electron microscopy, X-ray diffraction, atomic force microscopy and a series of electrical measurements. The solution-processed ZnInSnO thin films transistors (TFTs) with the prepared Al₂O₃ dielectric were annealed at different temperatures. The TFTs annealed at 600 °C have displayed excellent electrical performance such as the field-effect mobility of 116.9 cm² V⁻¹ s⁻¹ and a subthreshold slope of 93.3 mV/dec. The performance of TFT device could be controlled by adjusting the annealing temperature. The results of two-dimensional device simulations demonstrate that the improvement of device performance are closely related with the reduction of interface defects between channel and dielectric and subgap density of stats (DOS) in the channel layer.     

AC conductivity,dielectric relaxation and modulus behavior of Sb2S2O new kermesite alloy for optoelectronic applications

In this work, we present some physical properties of Sb₂S₂O thin films obtained through heat treatment of Sb₂S₃ thin films under an atmospheric pressure at 350 °C. The electrical conductivity, dielectric properties and relaxation model of these thin films were studied using impedance spectroscopy technique in the frequency range from 5 Hz to 13 MHz at various temperatures from 350 °C to 425 °C. Besides, the frequency and temperature dependence of the complex impedance, AC conductivity and complex electric modulus has been investigated.     

Effects of annealing on nanocrystalline Bi2S3 thin films prepared by chemical bath deposition

Nanocrystalline Bi₂S₃ thin films are deposited on tin chloride treated glass substrate from the solution containing bismuth nitrate, triethanolamine (TEA) and thioacetamide (TAM) at a bath temperature 318 K. The prepared films are subsequently annealed at different temperatures for studying the effect of thermal treatment on the structural, surface morphology, optical and electrical properties of the films. The X-ray diffraction studies affirmed that the deposited films are orthorhombic structures with average crystallites size of 14 nm to 28 nm. The scanning electron microscopy (SEM) images revealed that the films comprise of grains of spherical shape of unequal size. It is also observed that the small particles aggregate together to form a larger cluster. The average grain sizes determined from the TEM images are smaller than the crystallites size obtained from the XRD studies. The optical band gap of the films has been estimated to be 2.24–2.05 eV for the as-prepared and annealed films, respectively. The electrical conductivity of the as prepared Bi₂S₃ films at room temperature is found to be in the order of 10⁻³ Ω⁻¹ m⁻¹.     

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.     

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.     

Substrate temperature effect on structural,optical and electrical properties of vacuum evaporated SnO2 thin films

Tin oxide (SnO₂) thin films were deposited on glass substrates by thermal evaporation at different substrate temperatures. Increasing substrate temperature (T_s) from 250 to 450 °C reduced resistivity of SnO₂ thin films from 18×10⁻⁴ to 4×10⁻⁴▒ Ω ▒cm. Further increase of temperature up to 550 °C had no effect on the resistivity. For films prepared at 450 °C, high transparency (91.5%) over the visible wavelength region of spectrum was obtained. Refractive index and porosity of the layers were also calculated. A direct band gap at different substrate temperatures is in the range of 3.55−3.77 eV. X-ray diffraction (XRD) results suggested that all films were amorphous in structure at lower substrate temperatures, while crystalline SnO₂ films were obtained at higher temperatures. Scanning electron microscopy images showed that the grain size and crystallinity of films depend on the substrate temperature. SnO₂ films prepared at 550 °C have a very smooth surface with an RMS roughness of 0.38 nm.     

The effects of carrier gas and substrate temperature on ZnO films prepared by ultrasonic spray pyrolysis

ZnO films were deposited on glass substrates in the temperature range of 350–470 °C under an atmosphere of compressed air or nitrogen (N₂) by using ultrasonic spray pyrolysis technique. Structural, electrical and optical properties of the ZnO films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical two-probe and optical transmittance measurements. The ZnO films deposited in the range of 350–430 °C were polycrystalline with the wurtzite hexagonal structure having preferred orientation depending on the substrate temperature. The ZnO films deposited below 400 °C had a preferred (100) orientation while those deposited above 400 °C mostly had a preferred (002) orientation. The resistivity values of ZnO films depended on the types of carrier gas. The ZnO thin films deposited under N₂ atmosphere in the range of 370–410 °C showed dense surface morphologies and resistivity values of 0.6–1.1 Ω-cm, a few orders of magnitude lower than those deposited under compressed air. Hydrogen substition in ZnO possibly contributed to decreasing resistivity in ZnO thin films deposited under N₂ gas. The Hall measurements showed that the behavior of ZnO films deposited at 410 °C under the N₂ atmosphere was n-type with a carrier density of 8.9–9.2×10¹⁶ cm^-3 and mobility of ～70 cm²/Vs. ZnO thin films showed transmission values at 550 nm wavelength in a range of 70–80%. The values of band gaps extrapolated from the transmission results showed bandgap shrinkage in an order of milli electron volts in ZnO films deposited under N₂ compared to those deposited under compressed air. The calculation showed that the bandgap reduction was possibly a result of carrier–carrier interactions.     

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.     

Behavior of chemically deposited PbS thin films subjected to two different routes of post deposition annealing

Lead sulfide (PbS) thin films were prepared on soda lime glass substrates at room temperature by Chemical Bath Deposition (CBD) technique. This paper reports a comparative study of characteristic properties of as-prepared PbS thin films after thermal treatment through two different routes. Studies were carried out for as-prepared as well as rapidly and gradually annealed samples at 100, 200 and 300 °C. The characterizations of the films were carried out using X-ray diffraction, scanning electron microscopy and optical measurement techniques. The structural studies confirmed the polycrystalline nature and the cubic structure of the films. As-deposited films partly transformed to Pb₂O₃ when gradually annealed to 300 °C. The presence of nano crystallites was revealed by structural and optical absorption measurements. The values of average crystallite size were found to be in the range 18–20 nm. The variation in the microstructure, thickness, grain size, micro strain and optical band gap on two types of annealing were compared and analyzed. Data showed that post deposition parameters and thermal treatment strongly influence the optical properties of PbS films. Optical band gap of the film gets modified remarkably on annealing. Direct band gap energy values for rapidly and gradually annealed samples varied in the range of 1.68–2.01 eV and 1.68–2.12 eV respectively. Thus we were succeeded in tailoring direct band gap energies by post deposition annealing method.