Close-spaced Sublimation Technique Research Articles

Cu2ZnSnS4 films properties deposited by spray pyrolysis, subjected to a combined novel thermal treatment: CSS sulfurization and RTA post-treatment

CZTS films were obtained by spray pyrolysis, a low-cost technique implemented on an industrial scale. Sulfurization processes generally enhance CZTS properties for its use in solar cells; however, these require high temperatures and long times. In this work, CZTS films properties were studied after being subjected to a combined novel thermal treatment which involved two stages. Sulfur incorporation using close space sublimation technique (CSS) at a temperature (TCSS) in the 350–500 °C range for 60 s, and after a rapid thermal annealing post-treatment (RTA) at TRTA = 500 °C for 10 s, with a heating ramp of 5 °C/s under argon atmosphere. Morphological, structural, chemical composition, optical and electrical properties were studied for CZTS films subjected to CSS as CSS+RTA treatments for all TCSS values. From the results, efficient sulfur incorporation by CSS was obtained without substantial sulfur concentration loss after RTA treatment, associated with the short treatment time. In addition, significant enhancement of crystalline quality and electrical resistivity diminished of the films is obtained after the CSS+RTA process for all TCSS. CZTS films with the best properties are those obtained at TCSS of 400 °C, having Cu/(Zn+Sn) = 0.79 and Zn/Sn = 1.3 ratios, a direct bandgap value of 1.5 eV, and resistivity of 4 Ω-cm. These films can be candidates for their characteristics as an absorbent layer in solar cells.

Fabrication of Sb2S3 solar cells by close space sublimation and enhancing the efficiency via co-selenization

As a new-type absorber material, antimony sulfide (Sb2S3) possesses the advantages of earth-abundant elemental contents, low-toxic constituents, appropriate photoelectric properties, and unique one-dimensional crystal structure. We utilized the close space sublimation technique to deposit high-quality Sb2S3 thin-film with (hk1)-preferred orientation by controlling source temperature. Then, Sb2S3 thin film solar cells with the superstrate structure of FTO/CdS/Sb2S3/Au were fabricated, achieving a conversion efficiency of 2.86%. To further enhance efficiency, co-selenization process was employed in the Sb2S3 thin films by co-sublimation, which improve the crystallinity of films, increase grain size and form stronger (hk1)-preferred orientation. In addition, co-selenization process can adjust band gap value and enhance junction quality. By these approaches, we enhanced the short circuit current density (Jsc) of Sb2S3 solar cell efficiently. The Sb2S3 solar cell fabricated by CSS achieved an efficiency of 4.45% using co-selenization process. These methods can be applied to other chalcogenides thin-film solar cells, providing a practical way for the device performance improvement.

Analysis of grain orientation and defects in Sb2Se3 solar cells fabricated by close-spaced sublimation

The performance of a superstrate TiO2/Sb2Se3 solar cell, fabricated by close-spaced sublimation technique (CSS), was improved after the deployment of a seed layer. The seed layer caused columnar Sb2Se3 film growth with texture coefficient analysis (TC) showing increased presence of crystal planes, which are inclined towards the [001] crystal direction. Given the highly anisotropic properties of Sb2Se3, preferential growth of (Sb4Se6)n ribbons along the [001] direction is best suited for effective charge collection. Hence, grain orientation of Sb2Se3 films was studied more closely via measurement of pole figures by XRD and orientation distribution maps by electron backscatter diffraction (EBSD). Although the measurements did not reveal strong preferred orientation, it was observed that the columnar Sb2Se3 growth enhanced texture along the [001] direction. Temperature-dependent admittance spectroscopy (TAS) and capacitance–voltage (CV) profiling were performed on the seed-assisted TiO2/Sb2Se3 solar cell to evaluate carrier density and deep defects in the Sb2Se3 absorber. TAS study revealed a deep defect with activation energy of 0.39 eV. CV profiles indicated that the density of defects could be as high as 1017 cm−3, which needs to be addressed by post-deposition treatments.

Unveiling the influence of ZnTe and Te layers as part of the back-contact on CdTe solar cells performance

Recently, advances in the CdTe technology made CdTe a great candidate for commercial thin film solar cells because it is used in infrared windows for optoelectronic devices such as photovoltaic ones. Nowadays, solar cells with nanometric thicknesses are increasingly required due to an enhanced behavior effect. It is common to observe interdiffusion phenomena during several technological processes that are required for the fabrication of photovoltaic devices. It is very important to note that this interdiffusion could strongly affect the final efficiency of devices. In this work, the study of two materials as an interlayer between the CdTe thin film and metallic contact is presented. Te and ZnTe were deposited on the CdTe surface by using a close space sublimation technique. The electrical properties of the devices were evaluated after two and four years to observe the electrical behavior of Te and ZnTe layers implemented as part of the back-contact.

Alternative Method for Production of Pb Halide Perovskite with High Current Response for Photodetection Devices

To produce functional active layers for photodetector devices, thin films of organic–inorganic CH3NH3PbI3 perovskite semiconductor have been grown by a close-space sublimation procedure and a hybrid dipping technique. x-Ray diffraction analysis was used to evaluate the crystalline quality of the thin films, confirming the achievement of a tetragonal structure of perovskite type in space group I4/mcm. Morphological analysis by scanning electron microscopy revealed a homogeneous and compact granular surface distribution. Kubelka–Munk analysis of the optical response measured by ultraviolet–visible (UV–Vis) spectrophotometry yielded an energy bandgap of 1.57 eV for the thin films produced by the close-space sublimation technique and 1.59 eV for those obtained by dipping. Photoluminescence experiments corroborated these bandgap values. The I–V characteristics of both thin films obtained at different incident laser powers and a wavelength of 650 nm demonstrated their potential for use as active layers in photodetector devices.

The synthesis of CdZnTe semiconductor thin films for tandem solar cells.

A new approach is adopted to grow cadmium zinc telluride (CdZnTe) thin films using the close spaced sublimation (CSS) technique. The deposition parameters for the growth of cadmium telluride (CdTe) thin films onto the glass substrate were optimized. A zinc telluride (ZnTe) thin film layer was deposited onto already-deposited CdTe thin film to fabricate the CdZnTe (CZT) thin film sample as a ternary compound. Annealing was done after the successful deposition of CZT thin films before further characterization of the CZT thin film samples. The structures of the CZT thin film samples were studied using X-ray diffraction (XRD) and cubic phases were found. A spectrophotometer was used to study the optical parameters, and the energy band gap was found to be in the range of 1.45 eV to 1.75 eV after annealing. The nature of the direct band gap predicts that it might be an ideal component in second-generation solar cells. A Hall measurement system was used to find that the electrical conductivity was in the range of 4.6 × 10−6 to 8.2 × 10−11 (ohm cm)−1. XPS analysis confirmed the presence of Zn in the CdTe thin films. A significant change in electronic properties was observed. These results show that these CZT thin film samples can not only play a key role in the tandem structures of high-efficiency solar cells but they could also be used in the detection of X-rays and gamma rays.

Novel CdZnTe micro pillar films deposited by CSS method

CdZnTe (CZT) micro pillar films with cubic zinc blend phase have been deposited on single layer graphene coated quartz substrate by close-spaced sublimation technique. Structural and phase purities of deposited micro pillar CZT films were investigated using X-ray diffraction and Raman spectroscopy. Scanning electron microscopy characterizations indicated that the diameters of CZT micro pillars were about 5 µm and their lengths were close to 57 µm. The band-gap of CZT micro pillar films was about 1.53 eV, which was confirmed by UV–Vis–NIR diffuse reflection spectra and PL spectra. The structure of Au/BGZO/CZT/graphene/quartz was fabricated to study its optoelectronic properties. Under the illumination of AM 1.5, CZT micro pillar films had good photo response, which indicated their great potential in the application of optoelectronic devices.

Interface Engineering via Sputtered Oxygenated CdS:O Window Layer for Highly Efficient Sb2Se3 Thin‐Film Solar Cells with Efficiency Above 7%

Antimony chalcogenide Sb2Se3 is an emerging photovoltaic absorber due to its appropriate bandgap (≈1.1 eV), high absorption coefficient (>105 cm−1), suitable p‐type conductivity, low toxicity, earth abundance, and excellent stability. However, the stringent growth condition and low photovoltage limit its power conversion efficiency (PCE). Herein, via a combined theoretical and experimental study, interface engineering via an oxygenated cadmium sulfide (CdS) window layer (CdS:O) is found to be an effective approach to improve the device performance of CdS:O/Sb2Se3 solar cells. The sputtered oxygenated CdS:O window layer can be used to replace conventional chemical‐bath‐deposited CdS window layer in the Sb2Se3 devices. The best PCE of 7.01% is demonstrated in the superstrate configuration of fluorine‐doped SnO2/CdS:O/Sb2Se3/graphite with a high open‐circuit voltage of 0.432 V, where Sb2Se3 is fabricated using the close space sublimation technique. The interfacial diffusion between Sb2Se3 and sputtered CdS:O is significantly suppressed by introducing oxygen at the interface, which prevents Cd diffusion and the formation of Cd interstitials. Combined device physics characterizations and theoretical calculations reveal that oxygen in the CdS:O/Sb2Se3 interface can increase depletion region, built‐in voltage, and reduce interfacial recombination. These findings provide the guidance to optimize quasi‐one‐dimensional non‐cubic earth‐abundant chalcogenide photovoltaic devices through interface engineering.

Scalable and efficient Sb2S3 thin-film solar cells fabricated by close space sublimation

Antimony sulfide as a cost-effective, low-toxic, and earth-abundant solar cell absorber with the desired bandgap was successfully deposited using a scalable close space sublimation technique. The deposition process can separately control the substrate and source temperature with better engineering of the absorber quality. The device performance can reach 3.8% with the configuration of glass/FTO/CdS/Sb2S3/graphite back contact. The defect formation energy and the corresponding transition levels were investigated in detail using theoretical calculations. Our results suggest that Sb2S3 exhibits intrinsic p-type owing to S-on-Sb antisites (SSb) and the device performance is limited by the S vacancies. The localized conduction characterization at nanoscale shows that the non-cubic Sb2S3 has conductive grains and benign grain boundaries. The study of the defects, microstructure, and nanoscale conduction behavior suggests that Sb2S3 could be a promising photovoltaic candidate for scalable manufacturing.

Bifacial CdS/CdTe thin-film solar cells with copper nanowires as a transparent back contact.

A bifacial CdS/CdTe thin-film solar cell with a superstrate configuration was demonstrated by using copper nanowire (CuNW)/indium tin oxide (ITO) back contacts as a transparent and conductive electrode (TCE). CdS and CdTe were deposited by chemical bath deposition and close-spaced sublimation techniques, respectively. The CuNWs acted both as an acceptor dopant and TCE for the p-CdTe, improving the total cell efficiency compared to a copper-free back contact. CuNW/ITO back contacts with high optical transmittance (72.3% at 550 nm) and low sheet resistance (47.1 Ω/sq.) were obtained. The average cell efficiency of the bifacial CdS/CdTe thin-film cells with the optimized CuNW/ITO back contact was 10.0% (front-side illumination) and 0.55% (rear-side illumination). The quantum efficiencies under front-side and rear-side illumination were studied. The prepared bifacial cell can facilitate full usage of incoming sunlight (direct or diffused), enhancing the output power under cloudy conditions.

Optical and electrical studies of CdS thin films with thickness variation

Cadmium sulphide (CdS) is the n-type, wide band gap II-VI semiconducting material by offering applications in photovoltaics. Among the range of applications offered by CdS thin films, it has found application as a window layer in second generation solar cells by creating photon traps, thereby quantum efficiency. Closed space sublimation (CSS) offers a trivial yet effective approach for the synthesis of thin films for solar cell applications at moderate temperatures. CdS thin films of controlled thicknesses were synthesized by CSS technique by varying exposure time. X-rays diffraction data of these thin films revealed polycrystalline nature with a preferred orientation along (002) direction. The scanning electron microscopy (SEM) based morphological studies showed grain size variation with an increase in thickness of deposited thin films in the range of 300–500 nm. The electrical studies revealed high resistivity of the order of 106 Ω-cm. Spectrophotometric studies performed for CdS thin films concluded with the calculation of the optical parameters such as refractive index, absorption coefficient employing the Swanepoel model, and energy band gap of ∼2.42 eV using the Tauc relation in addition to thin film thickness confirmation. The variations in thickness affect the structure, surface, optical (Khan et al. 2018), electrical properties.

Study of CdTe film growth by CSS on three different types of CdS coated substrates

CdTe (cadmium telluride) is a very useful material for various applications such as in solar cell fabrication, electro-optical modulators, radiation detectors etc.. It is one of the cost effective and suitable material for thin film solar cells production due to its ease of thin film fabrication, high absorption coefficient and ideal band gap. CSS (close spaced sublimation) is one of the suitable technologies of CdTe thin film fabrication for various applications. In this study, close spaced sublimation of CdTe for its deposition on CdS (cadmium sulfide) coated glass was performed using a custom-designed CSS system for solar cell application. The deposition characteristics of CdTe was studied on three types of CdS layer. First CdS layer was obtained by sputtering technique, and second CdS layer was obtained by CBD (chemical bath deposition) technique. And third CdS layer was CdS:O (oxygenated nanocrystalline CdS film). The surface morphology was studied using FESEM (field emission scanning electron microscope). The XRD was done to confirm the crystal details of the film. UV-VIS spectrophotometer was used for optical absorption and transmission characteristics of the film which was further used to calculate the band gap of the deposited film. The thickness of the film was measured using DektakXT, Bruker profilometer. Results of current study show that (a) the close spaced sublimation technique for CdTe deposition is very consistent irrespective of the quality of substrate used, (b) the pin hole free uniform films are obtained in all three types of CdS substrates used (c) little deviation in CdS:O substrate is seen as the transmission characteristic is concerned. The band gap gets closer to the theoretical value

Self-powered, high-speed Sb2Se3/Si heterojunction photodetector with close spaced sublimation processed Sb2Se3 layer

Antimony selenide (Sb2Se3) has received great attention recently due to its excellent carrier transport capacity and special one-dimensional crystal structure. The Sb2Se3-based bulk heterojunctions have been applied for photovoltaic with great success, but its potential application to photodetector has not been explored yet. In this work, a self-powered, high-speed Sb2Se3/Si heterojunction photodetector is fabricated and investigated, where the Sb2Se3 layer is deposited through close spaced sublimation (CSS) technique. The photodetector exhibits detection performance with a responsivity of 0.34 A/W at zero bias voltage and a broad spectral range from 300 to 1100 nm. Ultrafast response rise and fall times of 86 and 96 μs, respectively, are obtained, which is significantly better than previous reports of Sb2Se3-based photodetectors. The high crystallographic quality of the CSS-processed Sb2Se3 thin films and the built-in field at the Sb2Se3/Si interface is responsible to the high response speed of Sb2Se3/Si heterostructural photodetectors. The high-speed and broadband response properties make the Sb2Se3/Si heterojunction photodetector have potential applications for next-generation optoelectric devices.

Infiltration of CdTe nano crystals into a ZnO wire vertical matrix by using the isothermal closed space technique

A ZnO nanorod structure was grown by the hydrothermal method and interpenetrated with CdTe using the isothermal closed space sublimation technique. The obtained structure was studied by using the Rutherford backscattering spectrometry (RBS), Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM). The X-ray Diffraction (XRD) technique confirmed the presence of CdTe nanocrystals (NCs) of very small size formed on the surface and in the interspaces between the ZnO nanorods. The RBS observations together with the SEM observations give information on the obtained structure. Finally the photoluminescence studies show a strong energy confinement effect on the grown CdTe NCs.

Comparative Study of CdS Nano Films Deposited by Chemical Bath and Close Spaced Sublimation Techniques

Comparative Study of CdS Nano Films Deposited by Chemical Bath and Close Spaced Sublimation Techniques

CdTe/CdS solar cells with CdTe grown at low vacuum

CdTe thin films were grown on FTO/CdS substrate using a close space sublimation technique. The temperature of CdTe source was kept constant at 650 °C while the substrate temperature was varied from 500 °C to 580 °C under a constant pressure of 10−2 Torr. Subsequently, the films were activated in the atmosphere of CdCl2 vapor at different temperatures from 390 °C to 410 °C for 10 min. After the pressure of CdTe were explored of vacuum, 0.3, 0.5, 1 and 3 Torr. It was found that CdTe grown at 3 Torr exhibited 10.6% of efficiency, and approximately 80% of QE. The XRD analysis showed that the crystalline phase is cubic with preferential orientation in (111) plane. In the present work, we have demonstrated that it is possible to obtain device efficiencies above 10% with a chamber pressure of 3 Torr during CdTe film deposition and without using commonly utilized atmospheres, such as He-O2 or Ar-O2 for the CdTe growth and device activation. The present work contributes significantly in the development of low cost and highly efficient CdTe/CdS solar cells fabricated under low vacuum.

Elemental, structural and optical properties of nanocrystalline Zn 1−x Cu x Se films deposited by close spaced sublimation technique

Abstract The elemental composition, film thickness and concentration depth profiles of as-deposited and annealed Zn 1−x Cu x Se films were studied by the Rutherford backscattering spectrometer (RBS) technique. The films were deposited on glass substrates by close spaced sublimation (CSS) technique. As-deposited films of about 250–300 nm thickness were then annealed in air at temperatures of 200 °C and 400 °C for 1 h. Structural characterization including crystal structure, crystal orientation, stacking fault energy ( Ү SFE ) and surface morphology were carried out by using X-ray diffraction (XRD) and atomic force microscopy (AFM). XRD studies revealed that the fabricated films are polycrystalline with a zinc-blende structure and a strong (111) texture plane. Surface roughness was observed to be enhanced with annealing temperature with a decrease in stacking fault energy ( Ү SFE ). Spectroscopic ellipsometry has been utilized for the estimation of band gap energy (E g ) and dielectric constant (e 1 ). Band gap energy of the film increased with increasing annealing temperature while the dielectric constant decreased.

How the Chlorine Treatment and the Stoichiometry Influences the Grain Boundary Passivation in Polycrystalline CdTe Thin Films

The absorption coefficient of CdTe is large enough to assure that all of the visible light is absorbed in a thickness on the order of 1 µm. High efficiency devices are fabricated by using close-spaced sublimation (CSS)-deposited CdTe films with a thickness in the range of 6–8 µm. In order to decrease the thickness of the CdTe film, a novel approach has been used. On top of the CdTe film, whose thickness is reduced to 2–3 μm, another CdTe layer is deposited by RF sputtering, with a thickness of 100–200 nm. The purpose of this approach is to fill up the voids, which tend to form when a low thickness-CdTe film is deposited by close-spaced sublimation. Using this CdTe double layer, solar cells, with an efficiency greater than 15%, were reproducibly obtained. Since the CdTe layer deposited by the CSS technique shows a p-type behavior, whereas the layer deposited by sputtering is n-type, it is supposed that the formation of a p-n junction into the grain boundaries, which makes a mirror for the charge carriers, increases their mean lifetime. In order to also have this system after the essential chlorine treatment of the CdTe layer, a special cadmium-free halogen treatment was developed. This process was especially tuned for very thin (≤3 µm) CdTe film thickness and for not making use of cadmium-based chlorine salt while, producing high efficiency devices, meets a better economic and environmental sustainability.

Hg1−xCdxTe vapor deposition on CdZnTe substrates by Closed Space Sublimation technique

Closed Space Sublimation (CSS) technique has been studied to deposit Hg1−xCdxTe polycrystalline films on CdZnTe substrates at the improved pressure–temperature conditions. The experimental results on film characterization suggest that the CSS optimal conditions are the argon atmospheric pressure (1013mbar) and the deposition temperature in the range of 500–550°C. These conditions provide macro-defect free Hg1−xCdxTe films with the uniform size and surface distribution of polycrystals.

Preparation, characterization and ab-initio study of CdSnTe2 thin films by closed space sublimation technique

The cadmium tin telluride (CdSnTe) alloy was prepared by direct reaction of high purity elemental constituents using rotating furnace. Thin films of CdSnTe different thickness were prepared on glass substrates by vacuum closed space sublimation method. The films were subjected to XRD, SEM, EDAX and UV–vis–NIR studies. The crystalline structure of CdSnTe alloy was found to be face-centered cubic with lattice parameter 6.30 Å. EDAX studies confirmed the composition of the alloy as CdSnTe2 compound. The UV–vis–NIR optical transmission spectra of thin films of different substrate temperatures were obtained. The energy band gap found to be in the order of 1.5 eV. The band gap is suitable for LEDs, optoelectronics and photovoltaic applications. First principle density functional theory calculations of CdSnTe2 structure were performed by using Vienna ab-initio simulation package software. The results of theoretical calculations are in good agreement with experimental data.