Telluride Thin Films Research Articles

Thermal modeling and experimental evaluation of five different photovoltaic modules integrated on prototype test cells with and without water flow

An analytical model of temperature dependent electrical and thermal efficiency of mono-crystalline (m-Si), polycrystalline (p-Si), amorphous silicon thin film (a-Si), cadmium telluride thin film (CdTe) and copper indium gallium selenide (CIGS) photovoltaic modules integrated on five prototypes identical insulted test cells is developed with and without surface water flow. This model helps in ascertaining the influence of temperature on their performance of building integrated photovoltaic-thermal (BiPVT) system. The theoretically calculated results are experimentally validated in outdoor ambient environment. The electrical & thermal efficiencies are calculated for both high and low mass flow rate of water, ṁw. Daily average electrical efficiency of photovoltaic modules; m-Si, p-Si, a-Si, CdTe and CIGS with and without water flow are found to be 12.30%, 10.98%, 6.08%, 6.60% and 7.71%, and 11.41%, 10.30%, 5.86%, 6.26% and 6.99% respectively. In constant room temperature mode, variation in mass flow rate of water, ṁw is also evaluated. Overall thermal efficiency and overall exergy for all photovoltaic modules in both cases are also calculated. The characteristic equations of photovoltaic modules integrated on test cells are also developed for both cases.

Structural, optical and electrical properties of thermal evaporated nano sized In2Te3 thin films


 
 
 Indium Telluride thin films were prepared by thermal evaporation technique. Films were annealed at 573K under vacuum for an hour. Both as-deposited and annealed films were used for characterization. The structural parameters were discussed on the basis of annealing effect for a film of thickness 1500 Å. Optical analysis was carried out on films of different thicknesses for both as - deposited and annealed samples. Both the as- deposited and annealed films exhibit direct and allowed transition. Electrical resistivity measurements were made in the temperature range of 303-473 K using Four-probe method. The calculated resistivity value is of the order of 10-6 ohm meter. The activation energy value decreases with increasing film thickness. The negative temperature coefficient indicates the semiconducting nature of the film.
 
 

Nanocrystalline Bi2Te3 thin films synthesized by electrodeposition method for photoelectrochemical application

Bismuth Telluride (Bi2Te3) thin films are prepared onto the stainless steel substrates by electrodeposition technique. Effects of varying deposition time on physico-chemical properties of Bi2Te3 thin films are studied. X-ray diffraction (XRD) analysis shows that all the Bi2Te3 films are polycrystalline in nature and it has Rhombohedral crystal structure. The results analyzed from FT-Raman and XRD analysis are well matched with each other and confirms the rhombohedral crystal structure. Scanning electron microscopy (SEM) images show that the variation in surface microstructure with respect to deposition time. Compositional elements (Bi and Te) distributions on the surface of Bi2Te3 film are estimated by EDX spectroscopy. Bi2Te3 thin films show the water contact angle in hydrophilic range. Photoconversion efficiency and fill factor of photoelectrochemical (PEC) cell formed with Bi2Te3 film are found to be 0.0987% and 0.3979 respectively.

Effect of CdCl2 heat treatment in (Ar + O2) atmosphere on structural and optical properties of CdTe thin films

Cadmium telluride (CdTe) thin films were deposited by thermal evaporation method under base pressure 2 × 10−5 mbar on corning glass substrate. In this work, we study the effect of cadmium chloride (CdCl2) heat treatment in argon and oxygen (Ar + O2) atmosphere on the structural, morphological and optical properties of CdTe thin films to achieve high quality thin film absorber layer for solar cells applications. The structural characteristics were studied by X-ray diffraction (XRD). The microstructure parameters of the films such as average grain size (Davg), average lattice strain (εavg) and dislocation density (ρD) have been calculated via XRD line broadening analysis. XRD investigation revealed that, the samples show polycrystalline nature pronounced with cubic zinc blende structure with a strong preferentially (1 1 1) texture orientation. It has been found that the crystallinity of the films enhanced during (Ar + O2) annealing and CdCl2 heat treatment. The surface morphology of CdTe thin films was investigated by field emission scanning electron microscope (FE-SEM). XRD and FE-SEM results of CdCl2 treated films showed recrystallization and progressive increase in grain size. The optical properties of all samples were estimated using UV–vis–near-infrared (NIR) spectrophotometer. The Swanepoel envelope method has been employed to evaluate the various optical parameters of CdTe films such as refractive index and film thickness. The as-deposited films have a relatively high value of refractive index of 2.54, in contrast the CdCl2 treated films have a relatively low value of 2.1 in the whole wavelength range. The optical energy gap values slightly decrease from 1.56 eV for as-deposited films up to 1.54 eV for CdCl2 heat treated films. CdCl2 heat treatment in (Ar + O2) atmosphere is found to be a promising method to improve the physical properties of CdTe thin film solar cells.

Effect of annealing process on the properties of undoped and manganese2+-doped co-binary copper telluride and tin telluride thin films

The binary semiconductor materials Cu1.81Te and SnTe materials, without and with manganese (Mn2+) doping, were prepared by dropping a Cu-Sn-Te solution on a commercial glass substrate to fabricate a co-binary thin film. The characteristics, optical, and electrical properties of undoped and Mn2+-doped Cu1.81Te/SnTe thin films were investigated with variations in the annealing process. The XRD results confirmed the films consisted of the co-binary orthorhombic phase materials Cu1.81Te and SnTe, and that for all annealing temperatures from 50 to 400 °C an amorphous structure became prevalent in the Mn2+-incorporated co-binary thin films. The optical parameters and electrical performance varied with the annealing temperatures and Mn2+ doping, showing alterations in the properties of the co-binary film. These co-binary thin films have feasibility for real applications in surface analysis, electro-optical materials, solar selective surfaces, and photovoltaic thermal devices.

Thermoelectric properties of nanocrystalline Sb2Te3 thin films: experimental evaluation and first-principles calculation, addressing effect of crystal grain size

The effect of crystal grain size on the thermoelectric properties of nanocrystalline antimony telluride (Sb2Te3) thin films was investigated by experiments and first-principles studies using a developed relaxation time approximation. The Sb2Te3 thin films were deposited on glass substrates using radio-frequency magnetron sputtering. To change the crystal grain size of the Sb2Te3 thin films, thermal annealing was performed at different temperatures. The crystal grain size, lattice parameter, and crystal orientation of the thin films were estimated using XRD patterns. The carrier concentration and in-plane thermoelectric properties of the thin films were measured at room temperature. A theoretical analysis was performed using a first-principles study based on density functional theory. The electronic band structures of Sb2Te3 were calculated using different lattice parameters, and the thermoelectric properties were predicted based on the semi-classical Boltzmann transport equation in the relaxation time approximation. In particular, we introduced the effect of carrier scattering at the grain boundaries into the relaxation time approximation by estimating the group velocities from the electronic band structures. Finally, the experimentally measured thermoelectric properties were compared with those obtained by calculation. As a result, the calculated thermoelectric properties were found to be in good agreement with the experimental results. Therefore, we can conclude that introducing the effect of carrier scattering at the grain boundaries into the relaxation time approximation contributes to enhance the accuracy of a first-principles calculation relating to nanocrystalline materials.

Large area growth of MoTe2 films as high performance counter electrodes for dye-sensitized solar cells

A cost effective and efficient alternative counter electrode (CE) to replace commercially existing platinum (Pt)-based CEs for dye-sensitized solar cells (DSSCs) is necessary to make DSSCs competitive. Herein, we report the large-area growth of molybdenum telluride (MoTe2) thin films by sputtering-chemical vapor deposition (CVD) on conductive glass substrates for Pt-free CEs of DSSCs. Cyclic voltammetry (CV), Tafel curve analysis, and electrochemical impedance spectroscopy (EIS) results showed that the as-synthesized MoTe2 exhibited good electrocatalytic properties and a low charge transfer resistance at the electrolyte-electrode interface. The optimized MoTe2 CE revealed a high power conversion efficiency of 7.25% under a simulated solar illumination of 100 mW cm−2 (AM 1.5), which was comparable to the 8.15% observed for a DSSC with a Pt CE. The low cost and good electrocatalytic properties of MoTe2 thin films make them as an alternative CE for DSSCs.

Zinc Telluride Thin Films: A Review

Zinc telluride thin films can be used in a large variety of applications such as optoelectronics and microelectronics tools. In this paper, the growth of ZnTe films prepared by electrodeposition, thermal evaporation and closed space sublimation method with various precursors and some its physical properties of obtained zinc telluride films are discussed.

Structure and thermoelectric properties of electrodeposited bismuth telluride thin films by controlling electrolyte temperature

Structure and thermoelectric properties of electrodeposited bismuth telluride thin films by controlling electrolyte temperature

Characteristics of nanostructured bismuth telluride thin films fabricated by oblique deposition

Oblique angle deposition can reduce ion bombardment and facilitate precursor migration on surfaces, which can affect the crystallographic properties of thin films. We prepared nanostructured Bi2Te3 thin films by oblique deposition at room temperature, and analyzed their structural and thermoelectric properties. The thin films were deposited by radio-frequency magnetron sputtering on a glass substrate, which was tilted at angles of 0–80° to the target. Cross-sectional SEM images showed that the samples consisted of columnar nanostructures, with the voids between the columnar grains increasing as the oblique angle was increased owing to the shadowing effect. XRD analysis indicated that crystallite size increased as the oblique angle was increased, although highly oriented thin films were not obtained. The highest mobility and electrical conductivity were observed at an oblique angle of 20° due to the relatively large crystallite size of the thin film obtained at this angle, as well as the small number of voids between the columnar grains. As a result, this film exhibited the highest power factor (3.1 μW/(cm·K2)), approximately six times higher than that obtained by normal sputtering deposition. Therefore, we conclude that using a moderate oblique angle can improve the thermoelectric properties of thin films.

Structural and optical properties of pulsed laser deposited cadmium telluride thin films

Cadmium telluride thin (< 1 μ) films were grown by pulsed laser deposition method on glass substrates from room temperature up to 300°C. The structural and optical properties were analyzed as a function of substrate temperature and laser radiation intensity.

How to control the stoichiometry of cadmium telluride thin film for photovoltaic applications

In this study, Bube's growth model for a cadmium telluride (CdTe) polycrystalline thin film was re-examined with a view of avoiding his assumptions that neglect the vapor pressures of Cd and Te2 near the film. We proposed a new thermodynamic growth model based on the fact that there is an experimentally verified characteristic ratio (α) of equilibrium partial pressures PCd/2PTe2 that depends on the temperature T and CdTe stoichiometry. By writing PCd(0)=2α(0)PTe2(0) and PCd(h)=2α(h)PTe2(h), where α(0) is determined by source stoichiometry, we can solve the equations for α(h) and thereby determine the stoichiometry of the CdTe thin film grown under physical vapor deposition (PVD) conditions. Simulation was performed to predict the stoichiometry of the CdTe thin film as a function of source stoichiometry for various source-film temperature combinations. The results show that for a typical CdTe PVD process with Tsource&amp;gt;Tthin film: (1) stable deposition, without a non-stoichiometric composition shift, can be achieved at congruent-growth stoichiometry; (2) any stoichiometric deviation from the congruent sublimation point becomes more substantial (in the same direction) in the thin film than in the source; and (3) larger ΔT between the source and the thin film results in a more composition shift.

The Spin Wave Gap and Switching Field in Thin Films with In-Plane Anisotropy

In this paper, we have calculated the spin wave gap and the angular dependence of magnetization reversal in a single-layer thin magnetic film that includes the strong perpendicular magnetic anisotropy and in-plane anisotropy. The film is assumed to be under the influence of the out-of-plane direction of the applied magnetic field at zero temperature. Using the quantum model, it is shown that the calculated equations present a nonzero spin wave gap at zero magnetic field which is strongly affected by anisotropies. The effects of the in-plane anisotropy and the role of the applied field were examined. We also discussed a simple theoretical model for the angular variation of switching field by using a quasi-classical argument. We used some constants in connection with experimental data which are reported for chromium telluride thin films grown by molecular beam epitaxy.

Property elucidation of vacuum-evaporated zinc telluride thin film towards optoelectronic devices

A thermal vacuum evaporation system has been used to deposit zinc telluride (ZnTe) thin film on glass substrate in order to investigate the structural, morphological, optoelectronic and electrical properties of the deposited film. The deposited film has been characterised by X-ray diffraction (XRD), atomic force microscopy (AFM), and UV-VIS-NIR spectrophotometer. The polycrystalline and cubic structure of the sample has been confirmed by XRD. The order parameter(s), which determines the crystallinity and good environmental stability of the sample, has been obtained for the peak (1 1 1) to peak (2 0 0) and is found to be 0.83. The XRD patterns and Bragg’s law have been examined to determine the microstructural parameters (lattice parameter, inter-planar spacing, crystallite size, number of crystallites per unit area, strain, dislocation density) of the investigated film. Optical properties (transmittance, absorbance, refractive index, absorption coefficient, extinction coefficient, optical density) of ZnTe thin film were extensively studied in incident photon energy range of 0.5–3.5 eV, where direct optical transition has been obtained with a band gap of 2.63 eV. The surface morphology of the evaporated ZnTe thin film has been checked by AFM.

Transient photo-response properties of CdTe thin films synthesized by screen printing technique

Group II-VI compound semiconductors are an important class of opto-electronic materials. They find important applications in detector and photovoltaic cell using both in bulk as well as thin film form. In this reference Cadmium Telluride thin films were prepared by screening printing technique and sintered at different temperatures in open environment. In this paper the comparative detailed analysis of transient photo response of CdTe (Cadmium Telluride) thin films sintered at three different temperatures are presented. For this pulsed photo-response was measured as function of three different intensity of illumination within range of 40–120mW/cm2, along with three different applied bias voltages i.e. 10V, 13V and 15V. From these photo-response analysis, different photo-response parameters e.g. photo-current at termination of illumination (Io), probability of escape of an electron from trap per second (p) and trap depth (Et) have been calculated and compared for all three films along with all three applied bias voltage and intensity of illumination. Estimation of rise and decay time is made from pulsed photo response curves and a comparative study is presented. It shows that photo-current is a strong function of intensity of illumination, sintering temperature and bias voltage. However, different photo-response parameters e.g. probability of escape of an electron from trap per second, trap depth, rise time and decay time are less dependent on sintering temperature. Observed value of long decay time is due to the process induced trap inclusion in the prepared samples which make these films suitable candidate for low processing cost solar cell material.

Effects of annealing temperature on the structural, mechanical and electrical properties of flexible bismuth telluride thin films prepared by high-pressure RF magnetron sputtering**Contribution at the 4th Southeast Asia Conference on Thermoelectrics 2016 (SACT 2016), 15–18 December 2016, Da Nang City, Vietnam.

In this work BixTey thin films were deposited on polyimide substrate by a high-pressure RF magnetron sputtering technique. The deposited condition was maintained using a high pressure of 1.3 × 10−2 mbar. The as-deposited films show Bi2Te3 structure with Te excess phase (Te-rich Bi2Te3). After that, as-deposited films were annealed in the vacuum chamber under the N2 flow at temperatures from 250 to 400 °C for one hour. The microstructure, cross-section, [Bi]:[Te] content, and the mechanical, electrical and thermoelectric properties of as-deposited and different annealed films were investigated. It was found that the annealing temperature enhanced the crystallinity and film density for the temperature range 250–300 °C. However, the crystal structure of Bi2Te3 almost changed to the BiTe structure after annealing the films above 350 °C, due to the re-evaporation of Te. Nano-indentation results and cross-section images indicated that the hardness of the films related to the film density. The maximum hardness of 2.30 GPa was observed by annealing the films at 300 °C. As a result of an improvement in crystallinity and phase changes, the highest power factor of 11.45 × 10−4 W m−1K−2 at 300 °C with the carrier concentration and mobility of 6.15 × 1020 cm−3 and 34.03 cm2 V−1 s−1, respectively, was achieved for the films annealed at 400 °C.

Empirical modelling and optimization of pre-heat temperature and Ar flow rate using response surface methodology for stoichiometric Sb2Te3 thin films prepared by RF magnetron sputtering

In this work, a flexible antimony telluride thin film was deposited by RF magnetron sputtering. The response surface methodology based on central composite design was used to study the influence of the Ar flow rate (A) and the pre-heat temperature of the substrate (T) on the Te content (%Te). The %Te of the thin film tended to increase with an increase in both the Ar flow rate and the pre-heat temperature. Stoichiometric Sb2Te3 thin films were obtained that agreed with the model equation of %Te = 59.22 + 0.070 A - 0.098 T + 5.580 × 10−4AT – 7.212 × 10−4A2 + 1.05 × 10−4T2. Micro-strainand dislocation density were enhanced using high Ar flow rate and low pre-heat temperature. The dislocation density and stoichiometry contributed to the substantially enhanced Seebeck coefficient and electrical conductivity of the films, respectively. The temperature dependence of the power factor is strongly dominated by electrical conductivity, leading to the highest value for a stoichiometric film of 2.0 × 0−3 W/m.K2 at 250 °C. During process optimization, several conditions can be prepared by RF magnetron sputtering with an Sb2Te3 target in order to obtain stoichiometric Sb2Te3 films. However, the expected condition to obtain highest power factor was the highest Ar gas flow rate and the lowest pre-heat temperature.

Investigation on the effect of Cu-doping to ZnTe layers by low-cost electrochemical approach

We have reported the electrochemical synthesis zinc telluride (ZnTe) and copper-doped ZnTe (Cu–ZnTe) thin films by electrodeposition technique from non-aqueous electrolyte. Voltammetric investigations under dark and illumination on the deposition were performed to optimize the growth potentials for ZnTe and Cu–ZnTe. The effect of Cu-doping on the structural, morphological, compositional and optical properties has been studied by means of X-ray diffraction, scanning electron microscopy, atomic absorption spectroscopy and UV–Vis spectroscopy. The growth of semiconducting phase was revealed by photovoltammetry. The direction of photocurrent remains unchanged upon the doping of CuCl2 in ZnTe revealed no change in majority charge carriers. The values of the energy band gap of ZnTe and Cu–ZnTe, 10−4 and 2 × 10−4 Cu contents were respectively determined as 2.26, 2.23 and 2.21 eV with sharp absorption edge and enhancement in absorption for the sample grown with 1 × 10−4 M CuCl2. The structure and surface morphology of ZnTe is sensitive to the choice of the deposition potential. The un-doped film deposited at −0.75 and −0.95 V yielded a mixed cubic as well as hexagonal phase of ZnTe along with metallic tellurium. The surface morphology changes from dendritic to globular. Presence of Cu2+ ions in the ZnTe bath not only helped in preventing the formation of tellurium dendrites, it also yielded single hexagonal phase of ZnTe. In case of Cu-doped layer, the surface morphology changes from flake like structure to globular cluster upon changing the growth potential. The un-doped ZnTe samples were Te-rich, whereas Zn/Te ratio for Cu–ZnTe with 10−4 M CuCl2 was found to be nearly unity. The direction of thermoemf was indicative of a p-type response. The increased electrical conductivity in Cu–ZnTe film is associated to increase in the hole density as Cu plays a role of acceptor in ZnTe.

Electronic band structure of epitaxial PbTe (111) thin films observed by angle-resolved photoemission spectroscopy

Using angle-resolved photoemission spectroscopy (ARPES), we studied bulk and surface electronic band structures of narrow-gap semiconductor lead telluride (PbTe) thin films grown by molecular beam epitaxy both perpendicular and parallel to the $\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}L$ direction. The comparison of ARPES data with the first-principles calculation reveals the details of band structures, orbital characters, spin-orbit splitting energies, and surface states. The photon-energy-dependent spectra show the bulk character. Both the $L$ and \ensuremath{\Sigma} valence bands are observed and their energy difference is determined. The spin-orbit splitting energies at $L$ and \ensuremath{\Gamma} points are 0.62 eV and 0.88 eV, respectively. The surface states below and close to the valence band maximum are identified. The valence bands are composed of a mixture of Pb $6s$ and Te 5 ${p}_{z}$ orbitals with dominant in-plane even parity, which is attributed to the layered distortion in the vicinity of the PbTe (111) surface. These findings provide insights into PbTe fundamental properties and shall benefit relevant thermoelectric and optoelectronic applications.

Influence of substrate temperature on the structural, morphological, optical and electrical properties of copper telluride thin films prepared by electron beam evaporation method

Copper telluride thin films were deposited on the glass substrates at different substrate temperatures viz., Room Temperature, 200, 300, 400 and 500°C employing electron beam evaporation method. The effect of substrate temperature on the physical properties of copper telluride films was investigated. The X-ray diffraction pattern revealed that the films deposited at 300, 400 and 500°C are polycrystalline in nature. The crystallite size, dislocation density and microstrain of these films were evaluated. Scanning electron microscopy images showed that the surface morphology of the films is modified by the variation in the substrate temperature. Further variation in the shape, size and distribution of the agglomerated crystallites formed on the surface of the copper telluride films and the roughness of the films were studied as a function of deposition temperature using atomic force microscopy. The direct optical band gap value of copper telluride films varies from 2.45 to 2.93eV with variation in the substrate temperature. Positive sign of the Hall coefficient showed p-type conductivity. The copper telluride film deposited at 500°C showed relatively low resistivity of 1.36×10−3Ωcm.