cadmium telluride structure

This type of contact performs well but is less desirable as a contacting scheme for large-area commercial modules. In production facilities, workers may be exposed to Cd compounds through the air they breathe, as well as by ingestion from hand-to-mouth contact. The National Institute of Environmental Sciences conducted a limited number of laboratory tests comparing the acute toxicity of CdTe, CIS, and copper indium gallium diselenide (CGS). Indeed, assuming a quantum efficiency equal to 1 for the down-shifter, an increase of efficiency of 40% has been predicted [54]. This has been shown to leave a Te-enriched surface. The spectra for films with thicknesses less than 70 nm, studied both in this paper and in our earlier publications [8, 9], practically match by main maximum locations with the spectra for bulk monocrystalline cadmium telluride. Use of the two-layer model for CdTe films is justified using the following reasons. We will be providing unlimited waivers of publication charges for accepted research articles as well as case reports and case series related to COVID-19. The report on Cadmium Telluride (CdTe) Market provides qualitative as well as quantitative analysis in terms of market dynamics, competition scenarios, opportunity analysis, market growth, etc. This paper presents the investigation of cadmium telluride solar cells structure by using MATLAB software. Reflective system parameters for determination are refractive index , index of extinction for upper layer, and thickness for upper and lower layers, four unknown parameters. The aim of this paper is to find such system model that would be sufficiently simple and able to describe in the best way ellipsometric data obtained at light reflection from the real cadmium telluride film surface and to determine its parameters. Anna Evmenova, Volodymyr Odarych, Mykola Vuichyk, Fedir Sizov, "Ellipsometric Analysis of Cadmium Telluride Films’ Structure", Advances in Materials Science and Engineering, vol. These transitions are achieved in the direction Λ near the point L of Brillouin zone and form the well-known in literature doublet that is located in the near ultraviolet range of spectrum close to 400 nm. In any of the mentioned above approximations, availability of cavities can only decrease the values of optical constants. Thickness curves, using which indeterminate parameters were found, are presented in Figure 2(a); the graphic dependence of ellipsometric parameters on the film thickness, calculated using the obtained parameters, is presented in the Figure 2(b) in combination with the experimental data. World production is dominated by a single company, First Solar. Among these techniques it is CSS, which has been most readily adopted at the industrial level and indeed the majority of recent “champion” cells have used this technique. However, if PVs are properly collected and recycled, the metals and other materials can be recovered and be a valuable resource instead of causing environmental damage. Cadmium telluride (CdTe) photovoltaics describes a photovoltaic (PV) technology that is based on the use of cadmium telluride in a thin semiconductor layer designed to absorb and convert sunlight into electricity. 8.23). Sign up here as a reviewer to help fast-track new submissions. In CdTe manufacturing, the principal concerns are related to the toxicity of the feedstock materials (e.g., CdTe, CdS, and CdCl2). If so, some conclusions about the shape of relief for films’ surface could be made from the Figure 5 data. It leads to a long-wave shift of the maximum in the observed spectra. Irrespective of the deposition technique used however the CdTe layer itself is between 1 and 8 μm thick. As these peculiarities for spectra calculated using the simple one-layer model are not meaningful, so the study should be broadened by attracting more complicated model than the one-layer one. A series of cadmium telluride (CdTe) nanocrystals were synthesized by a modified organometallic synthesis method at various reaction temperatures ranging from 130 to 250°C. Indeed even determining the lifetime of CdTe to be this large has proved problematic owing to particularly high surface recombination. The problem of four unknown values determination could be solved by the Method of Thickness Curves (MTC) [12]. Other . (1)Thickness of the upper layer is chosen, for instance, close to an expected value of mean-square roughness height. CdTe layers may be grown by a variety of vacuum and nonvacuum methods classified into high temperature and low temperature processes and resulting typical thicknesses between 2 and 6 μm [8,10,13]. It is mainly used as the semiconducting material in cadmium telluride photovoltaics and an infrared optical window. Reflective ellipsometry, which in some cases is rather sensitive to reflective system characteristics and permits determination of its parameters with sufficiently high accuracy, is the efficient method for studying the film structure and properties. The occupational health hazards presented by Cd and Te compounds in various processing steps vary as a function of their specific toxicity, physical state, and the mode of exposure. As an example Figure 2 shows the results of calculating the parameters of CdTe films obtained for different sputtering times on silicon substrates. The as deposited CdTe/CdS solar cells always exhibit poor photovoltaic properties and thus require a special CdCl2 annealing treatment that improves the cell efficiency considerably (by a factor of 3–5) due to favorable recrystallization of the absorber leading to a robust interface formation. PV solar cells based on CdTe represent the largest segment of commercial thin-film module production worldwide. Energy structure changes could be caused by distortion of films crystalline structure, which increases with the sputtering time growth and is difficult to be explained. There are three work places for this film, so three thickness curves are adduced in Figure 1. Nominal thicknesses of epitaxial CdTe layers were obtained by thin layer continuous growth during the sputtering time. The amount of cadmium in the alloy can be chosen so as to tune the optical absorption of the material to the desired infrared wavelength. Processes in which Cd compounds are used or produced in the form of fine fumes or respirable particulates present larger health risks than when they are in liquid or solid forms. Obviously, optical constants for majority films, calculated using the one-layer model, are found in the diagram region that lies outside the range of values corresponding to the cavities filled with air and oxide (area between the curves 1 and 2). (3)Films F5 and F10 that were formed during 14 minutes are exceptions. Calculation was executed for several values of thickness in the range of expected value. At the same time, results of these films calculation by using the two-layer model (filled symbols) are found in the area between the curves 1 and 2, that is, in the area of possible values for optical constants corresponding to the mixture of cadmium telluride, its oxide, and air. Global Cadmium Telluride (CdTe) Market is estimated to be valued US$ XX.X million in 2019. This issue needs further investigation. Practical Handbook of Photovoltaics (Second Edition), Vasilis M. Fthenakis, Alec O. Bulawka, in, Sarat Kumar Sahoo, ... Narendiran Sivakumar, in. The inner layer consists of cadmium telluride. In general, the hierarchy of controls includes engineering controls, personal protective equipment, and work practices. Use of the one-layer model for processing the ellipsometric data obtained at light reflection from CdTe films in the case of large thicknesses >70 nm can lead to significant distortion of optical constants and their spectra as compared to those of bulk monocrystalline cadmium telluride. The scale of axes is identical, so it is obvious that curves disperse with wavelength decreasing; the intercross area increases and the confined area does not exist for the wavelength 405 nm. The structure includes both a silicon and a cadmium telluride layer. PDF. Therefore, the next task is to find two-layer film parameters taking into account that its structure confirms the model 1. Owing to its low cost and high efficiency, the heterojunction of cadmium telluride with cadmium selenide (CdS/CdTe) has attracted considerable attention [50–52]. From the data presented in Figure 4, it is obvious that the spectra of optical constants are rebuilt with increasing the deposition time, and corresponding film thickness growth. The outer layer identified ellipsometrically could be represented by a layer with rough surface that is partially or fully oxidized. The array of system parameter values that consistently describes the obtained experimental data array is the warranty of assumption fulfillment. The CdTe TF can be deposited using a number of processes and needs good stoichiometric at … In this case, algorithm for the task solved using MTC differs from that described in [12] and used earlier for the one-layer film. Cadmium Telluride (CdTe) CAS Number: CAS# 1306-25-8. Revue de Physique Appliquee, 1977, 12 (2), pp.167-184. Formation of the hexagonal phase in composition of the films with a high thickness [3] or in films that were grown at high temperature of substrate [1] is observed. Optical constants of this equivalent film depend on the volume fraction of cavities, filling material, and they could be calculated using the theory of effective medium in Bruggeman or Maxwell-Garnett approximations [19]. In this method, octadecylamine (ODA) was introduced as an additional coordinating component to the mixture of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP). Thus, the film F6 thickness changes through its area within the range  nm, the film F9 thickness locates within the range of  nm, and the film F10 thickness locates within the range of  nm. 10.5. The structure is three-dimensional. The volume fraction of oxide (from 0 to 1) and air () increases along curves 1 and 2, respectively. Passing from the film F2 to film F5, that is, with increasing the thickness, optical parameters of films’ matter change appropriately; refractive index decreases and index of extinction appreciably increases even to values ≈1 in the case of film F5. So, for films with the sputtering time, Films F5 and F10 that were formed during 14 minutes are exceptions. At the same time, the thickness of outer layer can amount 120 nm (sample F5) or 100 nm (films F8 and F10). 44, September 2006, pp 705-708 Structure and properties of vacuum deposited cadmium telluride thin films According to AFM data, there is more or less developed relief on the surface of the films grown using the vacuum “hot-wall” epitaxy method; this fact indicates availability of a rather complex film structure. Ellipsometric Analysis of Cadmium Telluride Films’ Structure, V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, 41 Prospect Nauky, Kyiv 03028, Ukraine, Taras Shevchenko Kyiv National University, Physics Department, 2 Prospect Academician Glushkov, Kyiv 03022, Ukraine, Thickness curves are plotted on the diagram in, Thickness curves calculation is reiterated by changing, Optical constants of the upper layer for all the tested films (, Thickness of these two layers increases both for CdTe:Si and CdTe:CdHgTe films, but this growth is realized with a different rate. Wurtzite Space group. Bolko von Roedern, in Encyclopedia of Energy, 2004. or. Formation of CdTe films was performed in high vacuum ~1 Torr. In [6] this fact is explained by film porosity; and in [2, 5] high sensitivity of film optical properties to composition deviation from stoichiometry is registered in the film near-surface layer. CdTe PV technology has installed worldwide capacity of 15 GW because of its thermal and chemical stability results in the enormous increase of efficiency compared to other technologies. Cadmium telluride (CdTe) (Bonnet (2012))is a direct-bandgap semiconductor with that bandgap well matched to the solar spectrum. Usually, conductive silver ink is applied as a secondary contact to the graphite layer. Due to the ease and speed of its production CdTe offers the least payback time <1 year among all competing technologies including c-Si, thus making it one of the fastest growing technologies. With an energy band gap of 1.5 eV, CdTe has been identified as having a band gap perfectly matching with the distribution of photons of the solar spectrum. It has an ideal direct band gap ∼1.5 eV which matches well with the maxima of the Sun’s spectrum offering maximum theoretical efficiency ∼30% limit. Each thickness curve has a physical meaning of values array of studied system parameters in the chosen place of sample surface, which fit ellipsometric parameters measured at this place. We are committed to sharing findings related to COVID-19 as quickly as possible. Further studies have revealed, however, that although too much Cu applied with the back contact leads to degradation, variations for modest amounts of Cu (e.g., in Cu-doped graphite or Cu-doped ZnTe contacts) can increase or decrease cell stability. Obviously, close intercross area of thickness curves is observed for the film F2 that has the least thickness among the studied films. CdTe thin film solar cell structure comprises of a p-type CdTe absorber layer and n-type CdS based window layer forming a heterojunction, which has an intermixed interface region. (2)Thickness of these two layers increases both for CdTe:Si and CdTe:CdHgTe films, but this growth is realized with a different rate. Because contact formation was difficult due to the nearly intrinsic nature of the CdTe layer, Cu-doped ZnTe layers have also been successfully developed as a back contact. Abstract. Thus, several value arrays of required system parameters are obtained by the number of work places. Some alternative buffer layers, such as zinc stannate or CdS:O (the latter prepared as an amorphous layer that crystallizes during the cell anneal step), may ultimately lead to higher cell performance. The biggest manufacturer is currently Firs Solar, having achieved efficiencies higher than 15% (Fig. Reproduced from Green, M.A. Workers' exposure to Cd in PV manufacturing facilities is controlled by rigorous industrial hygiene practices and is continuously monitored by medical tests, thus minimizing potential health risks. type and p-type layers required for the photovoltaic cell to function. The basic structure (Figure 11) has a glass superstrate and a layer of transparent conducting oxide (TCO) as front contact, near-transparent n-type cadmium sulfide (CdS) window layer, p-type CdTe, and metallic rear contact. The buffer layers are often undoped SnO2 layers. Strauss. Attempt to find parameters of the two-layer system by using the model 2 was inefficient; unreal values of the extinction index (above unity) were obtained. The theoretical efficiency of CdTe is very high among the semiconductors available today for single-junction configuration. As was ascertained in numerous investigations, optical spectra of cadmium telluride within this spectral range are caused by direct electron transitions from the valence band, split by spin-orbit interaction, to the lower conduction band. At the same time, the thickness of outer layer can amount 120 nm (sample F5) or 100 nm (films F8 and F10). 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