CdTe Films Research Articles

The viability of alternative and nontoxic chlorine containing compounds for thermal treatment of ultrathin CdTe (≤1.0 μm) films

A detailed comparative analysis using alternative Cl compounds such as InCl2, MgCl2, SnCl2, and ZnCl2 along with CdCl2 was conducted in this study to assess their viability for the thermal treatment of ultrathin CdTe films. Similar or even better structural and electrical properties were found upon using InCl2, MgCl2, and ZnCl2 in thermal treatment than the conventional CdCl2. The film's bandgap is found to reduce upon the thermal treatment which varied in the range of 1.44-1.50 eV. A low amount of metal doping occurred in films' surfaces upon thermal treatment and influence the carrier concentration which varied from 7.32 × 1015-1.76 × 1017/cm3. The viability of the alternate materials was also investigated through the device simulation. All investigated results indicated that MgCl2 and ZnCl2 can be used in the current manufacturing technique instead of traditional CdCl2, which could reduce the environmental risk and production cost.

Comprehensive study of physical properties of cadmium telluride thin films: effect of post-deposition high annealing temperature

Thin films of cadmium telluride (CdTe) with a thickness of 550 nm were prepared using the thermal evaporation method. The resulting films were annealed in air atmosphere at 200 °C, 300 °C, 400 °C and 500 °C. The annealed films were subjected to x-ray diffraction (XRD), micro-Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), ultraviolet-visible spectroscopy and transverse current–voltage (I–V) test analyses to investigate the structural, surface morphological, optical and electrical properties of films, respectively. The XRD patterns reveal the zinc blende structure of the pristine and treated film with the preferred (111) orientation. In addition, the crystallite size increases with the rise in annealing temperature. The Raman spectra reveal a redshift with annealing as well as the formation of Te precipitates in CdTe films. The SEM images show the uniformity and homogeneity of the as-prepared films. The AFM studies show an increase in the surface roughness of the annealed films. The optical energy band gap is found to decrease with the annealing temperature. The I–V measurement indicates the ohmic behavior of CdTe films. The experimental results indicate that the annealed CdTe thin films at 400 °C have optimized physical properties for solar cell applications as an absorber layer.

A comprehensive comparative study of CdTe thin films grown on ultra-thin glass substrates by close-spaced sublimation and RF magnetron sputtering

This study elucidates a comprehensive comparative study on the structural, morphological and electrical characteristics of cadmium telluride (CdTe) thin films grown on ultra-thin glass (UTG) substrates via close-spaced sublimation (CSS) and RF magnetron sputtering. Deposited CdTe films were characterized utilizing the X-ray Diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), and Hall effect measurement system. CdTe thin films were in cubic and hexagonal-cubic orientations based on the deposition methods. Micro-strain and dislocation density for sputtered films were very high compared to CSS films. Crystallite sizes were in the range of 30–40 nm for CSS films and 7–25 nm for sputtering films. Grain size was estimated to be around 2 µm and thickness was approximately 1.5 µm for both CSS and sputtering system. Resistivity was almost same as 6.98 × 104 Ω-cm and 6.57 × 104 Ω-cm for CSS and sputtered films, respectively. The experimental results suggest that the CdTe films grown by CSS might be favorable as the optimum absorber layer for solar cell application.

Processing CdS- and CdSe-containing window layers for CdTe solar cells

The influence of heat treatment steps on the characteristics of (CdS, CdSe) junction partners and on solar cell performance was studied. CdS films were obtained by chemical bath deposition, and CdSe layers were evaporated. Structural and compositional properties of CdS/CdSe bilayer stacks did not change upon heat treatment at 400 °C up to 10 min, whereas heat treatment in the presence of CdCl2 for 10 min caused formation of a CdSSe alloy with a bandgap value of about 2.05 eV. Originally, the cubic structure of the stack was also transformed into a hexagonal structure during this treatment. CdSe–CdTe interdiffusion was also studied using CdS/CdSe/CdTe triple layer stacks. CdTe films were deposited using a close-spaced sublimation method. Limited CdSe–CdTe interdiffusion was seen when CdTe was deposited over the as-deposited CdSe layer at 580 °C. However, such interdiffusion was not detected for samples where CdTe deposition was carried out on CdS/CdSe stacks pre-annealed in the presence of CdCl2. This suggests that partial crystallization of the CdS/CdSe bilayer stack by CdCl2 reduced such an interaction. Solar cells with CdSe/CdTe, CdS/CdTe and CdS/CdSe/CdTe structures with efficiencies of 8.39%, 10.12% and 11.47% were fabricated using 4.5–5 µm thick CdTe layers and a final CdCl2 treatment. Quantum efficiency measurements demonstrated the benefit of CdSe–CdTe alloying during the final CdCl2 treatment in improving the short circuit current values.

Effect of pH on structural, optical, and morphological properties of ED-CdTe film

Cadmium telluride (CdTe) thin films have been deposited by electrodeposition (ED) technique on fluorine doped tin oxide (FTO) coated glass substrates using an aqueous electrolyte bath solution. The pH-value of the electrolyte solution is one of the most important deposition parameter that should be optimized to achieve high quality photoconductive material. The films have been deposited at different pH values ranging from 1.5 to 3. The structural, optical, and morphological properties of the samples were characterized using X-ray diffraction (XRD), UV– Visible (UV-Vis) spectroscopy, spectroflurophotometer, scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX) techniques respectively. The XRD analysis shows that the ED-CdTe films possess the cubic crystalline structure and the diffraction peak intensity increased significantly after changing the pH values. The direct energy band-gap (Eg=1.44eV) is obtained in optical absorption spectra. The SEM analysis shows very good morphological growth by changing the pH values and the film deposited with lower pH showed more intense PL emission peaks as compared to the other two films. CdTe film deposited at pH 1.5 will be better choice as an absorber layer of a CdTe/CdS solar cell.

Effect of CdS and CdSe pre-treatment on interdiffusion with CdTe in CdS/CdTe and CdSe/CdTe heterostructures

High efficiency CdTe solar cell structure has the configuration of CdS/CdSe/CdTe. Depending on the deposition and post deposition techniques employed, this stack is often subjected to high temperatures, often in presence of CdCl2, which leads to various degrees of interdiffusion at the CdS/CdSe and CdSe/CdTe interfaces. Such interdiffusion greatly influences device performance. Therefore, understanding and controlling these interdiffusion processes are important. In this contribution, interdiffusion between CdS-CdTe and CdSe-CdTe pairs were studied using CdS/CdTe and CdSe/CdTe stacks annealed at 673 K. Effect of pre-treating the CdS and CdSe layers with CdCl2 before the CdTe deposition on this interdiffusion was investigated. CdS films were grown by CBD and CdSe and CdTe films were vacuum evaporated. CdTe thickness was intentionally kept at the low 150–200 nm range to more easily identify alloy phases formed. GA-XRD measurements demonstrated that in absence of any CdCl2 pre-treatment, there was more interdiffusion between CdSe and CdTe compared to CdS and CdTe. In all cases CdCl2 pre-treatment of CdS or CdSe before the deposition of the CdTe film was found to reduce diffusion of S and Se into CdTe.

Effect of Zn-doping on the structural, optical and electrical properties of thermally vacuum evaporated CdTe thin films

Cadmium telluride (CdTe) thin film was fabricated on a glass substrate using thermal vacuum evaporation and zinc (Zn) was doped with varying atomic ratios using stacked layer method. The structural, optical and electrical properties of the fabricated films were thoroughly investigated as a function of doping concentration. XRD patterns revealed all Zn-doped CdTe thin films as polycrystalline materials with cubic structures. All the films had relatively intense (111) diffraction peaks but the intensity of the peaks reduces with the increase of doping concentration. The optical properties, such as transmittance, absorption coefficient, bandgap, penetration depth, Urbach energy and dielectric constants were evaluated by UV-Vis-NIR spectrophotometry. When doping concentration is increased from 0 at. % to 20 at. % average optical transmittance and Urbach energy increase from 87% to 94% and 127 meV to 409 meV, respectively, while the optical bandgap goes down from 1.51 eV to 1.43 eV. On the other hand, higher doping concentration shifts the absorption edge toward a longer wavelength and increases the penetration depth significantly. Electrical characterization determined by Hall measurements using the van der Pauw method shows that 20 at. % Zn doping in pure CdTe film increases the carrier concentration from 1.95 × 1014 cm−3 to 27.52 × 1014 cm−3 and activation energy from 1.623 eV to 2.603 eV but decreases the carrier mobility from 19.34 cm2/VS to 6.23 cm2/VS. Changes in electrical transport properties caused by doping are described in terms of charge carrier scattering.

Photoluminescence analysis of shallow acceptor in CdTe films on GaAs(100) substrates

Photoluminescence analysis of shallow acceptor in CdTe films on GaAs(100) substrates

Effect of annealing on the structural, compositional and optical properties of CdTe films

The focus of the ongoing research is to study constancy of chemical bath deposition (CBD) of CdTe thin films. CdTe films has been deposited using tellurium dioxide and cadmium acetate in non-aqueous medium at 160 °C. The consequence of annealing upon optical, structural in addition to surface effects of cadmium telluride thin films developed by CBD is examined. Thin films of CdTe were identified by x-ray diffraction methodology (XRD), fourier-transform infrared spectroscopic analysis (FTIR), uv–visible spectrometer, scanning electron microscope analysis (SEM) and photoluminescence. The EDS spectroscopy shows the ratio comes 1.27 which improves in annealed case and ratio comes 1.06 which is more close to unity. The average crystalline size is measured to be 25 nm. The photoluminescence peak is observed at 566 nm and 615 nm.

A new route for preparing CdTe thin films by chemical bath deposition

Cadmium telluride (CdTe) is a binary II-VI direct band gap semiconducting material. Cadmium telluride is a promising electrochemical and photovoltaic material for thin film solar cells. It shows both p and n type conductivity. A nanocrystalline thin film of CdTe was deposited in homogeneous medium on nickel plate by chemical bath deposition method containing 0.01 M cadmium acetate and 0.02 M tellurium dioxide. The structural, compositional and optical analysis were studied by x-ray diffraction (XRD), energy-dispersive x-ray spectroscopy (EDS), scanning electron microscope (SEM), Fourier Transform infrared spectroscopy (FTIR) and Photoluminescence (PL).The diffraction peak observed at 2θ = 23.59° with (1 1 1) plane indicate the crystalline phase of CdTe film. The average crystalline size is measured to be 7 nm. Compositional analysis reveals the presence of both Cd and Te elements. The EDS Spectroscopy shows the ratio of Cd and Te is1:2 in case of as deposited film. The Photoluminescence peak is observed at 471 nm.

Effect of high-pressure annealing on the physical properties of CdTe thin films

In the present work, the CdTe thin films were fabricated on the Ag substrate using thermal evaporation technique. The effect of post-deposition high-pressure annealing (PDHPA) of 0.2, 0.5 and 1 MPa at 500 °C on physical properties of CdTe thin films was studied. The structural, optical, and electrical properties of films were considered using grazing incidence X-ray diffraction, ultraviolet–visible spectrophotometer (UV–Vis), scanning electron microscopy, and transverse current–voltage (I–V) test. The structural analysis shows that both pristine and treated films have zinc blende crystal structure with strong preferred (220) orientation. The optical analysis revealed the enhancement of optical absorbance and decrement of the energy band gap of films with PDHPA. Moreover, our analysis indicates that the electrical conductivity of CdTe films increases with PDHPA. The SEM images show that the untreated films are uniform and homogeneous. Our results indicate that the cracking is observed in treated films with PDHPA. This study shows that the treated films at the pressure of 1 MPa can be used as suitable absorber layer to fabricate CdTe solar cell devices.

Semitransparent CdTe solar cells with CdCl2 treated absorber towards the enhanced photovoltaic conversion efficiency

Semitransparent CdTe solar cell is a promising technique for the development of ultrathin CdTe photovoltaics, which can not only be used as architectural components of window, fence or roof, but also allows a bifacial structure or tandem configuration to improve sunlight utilization. In this work, the ultrathin polycrystalline CdTe is prepared by sputtering and semitransparent solar cells with a structure of glass/FTO/SnO2/CdS/CdTe (~920 nm)/CuCl/ITO are fabricated. The CdCl2 post treatment was conducted on sputtered ultrathin semitransparent CdTe by varying the CdCl2 treatment amount. The effects of the CdCl2 treatment amount on the microstructure, optical property of ultrathin CdTe films, and the performance of semitransparent CdTe solar cells were extensively studied. By optimizing treatment amount, a front illuminated efficiency of up to 9.74% was achieved. Furthermore, the completed solar cells exhibited excellent near-infrared transmittance of ~70% in the wavelength range of 850–1100 nm, which strongly supports the future application of semitransparent CdTe solar cells in building integrated photovoltaics or tandem configurations.

CdTe thin films as protective surface passivation to HgCdTe layers for the IR and THz detectors

The properties of thin CdTe cap layers grown using the hot-wall epitaxy (HWE) method under low-temperature technological conditions (T ≤ 373 K) to Hg1-xCdxTe liquid phase epitaxial layers (LPE) have been investigated. It has been shown that the CdTe films deposited using this low-temperature method, can be used as the passivation coatings for the infrared (IR) detectors based on the HgCdTe epitaxial layers grown on the CdZnTe monocrystalline substrates, not affecting the HgCdTe properties. It has been found that CdTe films with the thickness d ≥ 360 nm are continuous and possess effective protective properties in application to Hg1-xCdxTe/CdZnTe IR detectors. The manufactured photodiode detectors demonstrate relatively good parameters: the detectivity D* = 2.7 × ·1010 cmW−1Hz1/2 (T = 80 K) for the long-wave range (8–10.5 μm) IR photodiodes (x ≈ 0.22) and noise-equivalent temperature difference NRTD ≈ 20 mK for mid-wavelength (3–5 μm) photodiodes.

Effect of pulse bias voltages on performance of CdTe thin film solar cells prepared by pulsed laser deposition

A new method of efficiency of CdTe thin film solar cells has been proposed. In view of the characteristics of pulsed laser deposition (PLD) and combined with high power pulse bias power supply, CdTe thin films under 0, 500, 750, and 1000 V pulse bias were prepared on FTO substrates, and solar cells with FTO/CdS/CdTe/Au structure were formed. The crystal structure, vibration mode, morphology, and optical properties of the CdTe thin films deposited at different pulse bias were studied by XRD, Raman, SEM, UV–Vis spectrophotometer measurement. X-ray diffraction analysis indicated the prepared CdTe thin films had a strong (111) preferentially oriented cubic sphalerite structure. The transmission spectrum showed that with the increase of pulse bias, the absorption of CdTe films gradually moved towards the long-wave direction, and the optical bandgap decreased from 1.516 to 1.485 eV. We further fabricated FTO/CdS /CdTe/Au solar cells, with improved efficiency from 2.29% to 6.07% and excellent response performance over a long-wavelength range of over 900 nm, and the pulse bias voltage effect was significant.

Enhancement of the performance of CdS/CdTe heterojunction solar cell using TiO2/ZnO bi-layer ARC and V2O5 BSF layers: A simulation approach

This article presents the role of Bi-layer anti-reflection coating (ARC) of TiO2/ZnO and back surface field (BSF) of V2O5 for improving the photovoltaic performance of Cadmium Sulfide (CdS) and Cadmium Telluride (CdTe) based heterojunction solar cells (HJSCs). The simulation was performed at different concentrations, thickness, defect densities of each active materials and working temperatures to optimize the most excellent structure and working conditions for achieving the highest cell performance using obtained optical and electrical parameters value from the experimental investigation on spin-coated CdS, CdTe, ZnO, TiO2 and V2O5 thin films deposited on the glass substrate. The simulation results reveal that the designed CdS/CdTe based heterojunction cell offers the highest efficiency, η of ∼25% with an enhanced open-circuit voltage, Voc of 0.811 V, short circuit current density, Jsc of 38.51 mA cm−2, fill factor, FF of 80% with bi-layer ARC and BSF. Moreover, it appears that the TiO2/ZnO bi-layer ARC, as well as ETL and V2O5 as BSF, could be highly promising materials of choice for CdS/CdTe based heterojunction solar cell.

Enhanced photo-response of CdTe Thin film via Mo doping prepared using electron beam evaporation technique

CdTe and 1–5 wt% molybdenum-doped CdTe materials were prepared by ball milling method. Then the ball-milled samples were deposited on a glass substrate at room temperature by an electron beam evaporation method. The structural, optical, morphological, electrical, and photo-response properties of CdTe and Mo-doped CdTe (CdTe:Mo) samples were investigated. The prepared samples exhibit a cubic structure of CdTe. X-ray diffraction pattern (XRD) showed that the deposited films belong to the cubic structure with a preferential growth along (111) plane. The peak shift in the lower side of 2θ was observed due to incorporation of Mo into the CdTe matrix. Surface morphology of the prepared CdTe and CdTe:Mo ball-milled samples showed the formation of various agglomerated structures. Scanning electron microscope images depicted the formation of high dense agglomerated grains on the film surface with varying sizes as a function of Mo concentration. The optical bandgap was increased from 1.38 to 1.54 eV with raising the Mo concentration in CdTe films. Enhanced optical absorption coefficients (~ 17 × 104 cm−1) were observed for 3 and 4 wt% Mo doped CdTe thin films. Hall measurement studies revealed that Mo doped CdTe (3 wt%) film possesses a low resistivity of 34 × 103 Ω cm. The CdTe and CdTe:Mo films exhibit p-type conduction. The high photo-response of 0.088 AW−1 is observed for 3 wt% Mo-doped CdTe thin film.

Properties of CdSe1−xSx films by magnetron sputtering and their role in CdTe solar cells

CdS/CdSe composite window layer is a better candidate than CdS or CdSe single layer for CdTe solar cells. In order to find the underlying reason, the properties of co-sputtered CdSe1−xSx thin films with different compositions ( $$0\le x\le 1$$ ) were examined. XRD patterns show that all CdSe1−xSx films ( $$0\le x\le 1$$ ) are polycrystalline with hexagonal structures and the prominent peak position shows an increasing trend with the increase of x. This suggests that CdSe1−xSx window layer with a gradient composition, formed during the high-temperature growth and heat treatment of CdTe films, has the gradient lattice constant, meaning the less defect density. This is beneficial to the device performance. Hall measurements show that CdS1−xSex films (x ≥ 0.772) are n-type semiconductors with carrier concentrations ranging from 1015 cm−3 to 1016 cm−3. It reveals that CdS/CdSe bi-layers can form a strong built-in electric field with CdTe layer as CdS does. The electronic band structure of CdSe1−xSx window layer with a gradient composition suggests that the spikes formed in the conduction band are lower than 0.3 eV, which would not impede the electron transport. The valence band alignment is beneficial for the hole transport. Therefore, an appropriate band alignment can be formed between CdSe1−xSx and CdTe layers. These results reveal that CdSe1−xSx film is a good candidate as the window layer for high-efficiency CdTe solar cells considering its structural, optical and electrical properties as well as its electronic structure.

Grain/grain‐boundary mediated dispersive photo‐current characteristics in close space sublimated micro‐granular CdTe films

Photo-current characteristics in close space sublimation processed multi-facetted Cadmium Telluride (CdTe) micro-granular films were studied under AM1.5, blue and green laser illuminations to assess the energy dependent charge transport characteristics. Photo-current under AM1.5 illumination (100 mW/cm 2 ) resulted in three times more than dark current while it was observed 2.4 and 2.5 times more in case of blue and green laser illuminations, respectively. While all the three illuminations (AM1.5, blue and green lasers) were capable to generate photo-electrons in CdTe, the variation in magnitude was articulated to the effect of grain boundaries present across the charge transport pathways as evidenced from the surface morphology examined by scanning electron microscopic studies which asserted that CdTe films were micro-granular in nature separated by grain boundaries. It is elucidated in the measurements that charge carriers under the illumination of AM1.5 light, acquired energy to overcome the grain boundaries easily compared to the charge carriers produced by blue and green light sources as evidenced from the photo-current measurements.

Water-assisted exfoliation of epitaxial CdTe film from mica analyzed with azimuthal RHEED

Semiconductor thin films grown on weakly interacting substrates have recently attracted much attention due to the fact that one can peel/release the film from its substrate for flexible electronic and optoelectronic applications. In this work we report separable CdTe(111) epitaxial films grown on mica substrate via vapor transport deposition. The CdTe films were separated from the mica substrate using double-sided Cu tape, exposing the CdTe interface-surface. Azimuthal reflection high-energy electron diffraction (ARHEED) was used to characterize the fresh CdTe interface-surface and the original CdTe surface. Dramatic differences between these two surfaces were observed. (1) Diffraction streaks were observed from the interface-surface, indicating a smooth surface. Diffraction spots were observed from the surface, indicating a rough surface. (2) The rough CdTe surface has twin and secondary twin structures, which are difficult to be observed with conventional X-ray diffraction. (3) The full-width-at-half maximum (FWHM) analysis of the streaks and spots in RHEED patterns reveals that despite the smoothness of the interface-surface, there is a lower degree of lateral long-range order as compared with the rough surface. This etchant-free method to create smooth interface-surface and ARHEED characterization may be applicable to other films that are detachable from weakly interacting substrates.

Preparation and Characterization of CdS/CdTe Device for Radiation Sensing

Several Techniques had been applied to measure Ionizing Radiation. Majority of thistechniques are costly and very complicated. We focus on this research to chemically deposition of CdS to formwith CdTe junction x-ray sensor. CdTe has been electrodeposited onto CdS/FTO glass substrate to formwith previously fabricated CdS layer 4 µm thickness. The optimum potential for CdTe deposition hasbeen studied by potentiostat measurement, it shows that -1.3 is the optimum working potential. The XRDanalysis showed that the CdTe films have highly oriented crystallites with the cubic phase zinc blend withpreferred orientation (111). The band gap Eg extrapolated to be 1.4 eV. Four stacked sensors wereconnected in series to measure the device performance. It was observed that amplitude of the pulseformed due to exposed FTO/CdS/CdTe/Mo detector to X-ray of 33 keV and 1mA intensity is 1.03 V.