Deposition Of CdTe Layers Research Articles

Arsenic Doping Upon the Deposition of CdTe Layers from Dimethylcadmium and Diisopropyltellurium

The incorporation and activation of arsenic from tris(dimethylamino)arsine in CdTe layers grown by metal-organic chemical vapor deposition from dimethylcadmium and diisopropyltellurium on GaAs substrates are investigated. The incorporation of arsenic into CdTe depends on the crystallographic orientation of the layers and increases in the series (111)B → (100) → (310). The arsenic concentration in the CdTe layers is proportional to the tris(dimethylamino)arsine flow rate at a power of 1.4 and increases with decreasing diisopropyltellurium/dimethylcadmium ratio from 1.4 to 0.5. After deposition, the CdTe:As layers have p-type conductivity with an arsenic concentration of 1 × 1017–7 × 1018 cm–3 and a hole concentration of 2.7 × 1014–4.6 × 1015 cm–3, respectively; the fraction of electrically active arsenic does not exceed ~0.3%. After annealing in argon (250–450°C), the highest hole concentration is 1 × 1017 cm–3, and the arsenic activation efficiency is ~4.5%. The ionization energy of arsenic determined from the temperature dependence of the hole concentration is in the range of 98–124 meV. The low-temperature photoluminescence spectra of the layers have an emission peak with an energy of ~1.51 eV, which can be attributed to donor–acceptor recombination, where AsTe is an acceptor with an ionization energy of ~90 meV.

Interdiffusion of Zinc-Stannate Buffer Layer and Cadmium Sulfide Heterojunction and Its Correlation with CdCl2 Treatment

In this paper, a highly transparent and conductive Zinc Stannate (Zn2SnO4) buffer layer is integrated between the transparent conductive oxide (TCO) layer and the Cadmium Sulfide (CdS) window layer to improve the optical properties of (TCO\Zn2SnO4\CdS\CdTe) heterojunctions in CdTe\CdS solar cells. This buffer layer is used to reduce the excess interdiffusion between the window and absorber layers when Ultra-thin CdS window layer, d < 100 nm, is employed, and post CdCl2 treatment is applied at high temperature after deposition of CdTe layer. The fabricated Zinc Stannate (Zn2SnO4) buffer layer was found to have ideal transmission, reflectance and absorption properties which helped in introducing good optical properties in the short wavelengths. It has been found that using the Zn2SnO4 buffer layer enhanced the short circuit current Jsc and the open circuit voltage Voc of the solar cells that used Ultra-thin CdS thickness (80nm) by preventing TCO\CdTe direct contact, and led to improved solar cells’ performance and reproducibility.

Interdiffusion of Zinc-Stannate Buffer Layer and Cadmium Sulfide Heterojunction and Its Correlation with CdCl2 Treatment

In this paper, a highly transparent and conductive Zinc Stannate (Zn2SnO4) buffer layer is integrated between the transparent conductive oxide (TCO) layer and the Cadmium Sulfide (CdS) window layer to improve the optical properties of (TCO\Zn2SnO4\CdS\CdTe) heterojunctions in CdTe\CdS solar cells. This buffer layer is used to reduce the excess interdiffusion between the window and absorber layers when Ultra-thin CdS window layer, d < 100 nm, is employed, and post CdCl2 treatment is applied at high temperature after deposition of CdTe layer. The fabricated Zinc Stannate (Zn2SnO4) buffer layer was found to have ideal transmission, reflectance and absorption properties which helped in introducing good optical properties in the short wavelengths. It has been found that using the Zn2SnO4 buffer layer enhanced the short circuit current Jsc and the open circuit voltage Voc of the solar cells that used Ultra-thin CdS thickness (80nm) by preventing TCO\CdTe direct contact, and led to improved solar cells’ performance and reproducibility.

Structure of CoFe2O4@CdTe nanocomposite with core/shell structure for high-performance Bi-modal imaging

A novel bi-modal imaging contrast agent was fabricated by depositing a layer of CdTe on the surface of CoFe2O4 nanoparticle, and applications for optical-MRI bi-modal imaging were investigated. The core/shell structure of the obtained nanocomposite was verified by TEM. TGA and nuclear magnetic relaxation dispersion (NMRD) profiles were applied for the deposition study. The in-vivo and in-vitro results showed that the deposition of CdTe layer can evidently improve the relaxivity of CoFe2O4 nanoparticle, which was investigated and explained by a “cation migration” theory. The as-synthesized compound is a promising nanocomposite for high performance bi-modal imaging applications.

Close spaced sublimation deposition of CdTe layers with process gas oxygen for thin film solar cells

Close spaced sublimation (CSS) is one of the leading technologies for deposition of the absorber layer in CdTe solar cells. For large scale production CSS is a dynamic process where the substrates are moved above the crucible at a close distance. In this study oxygen as a process gas is injected during dynamic CSS deposition. Solar cells are prepared from these layers by CdCl2 activation, copper doping and back contact deposition. The samples are investigated using current voltage measurement, quantum efficiency, scanning electron microscopy of cross sections and deep level transient spectroscopy.The results show that the precise introduction of oxygen can improve the solar cell efficiency. The reasons for this effect are discussed in this study.

The effects of high temperature processing on the structural and optical properties of oxygenated CdS window layers in CdTe solar cells

High efficiency CdTe solar cells typically use oxygenated CdS (CdS:O) window layers. We synthesize CdS:O window layers at room temperature (RT) and 270 °C using reactive sputtering. The band gaps of CdS:O layers deposited at RT increase when O2/(O2 + Ar) ratios in the deposition chamber increase. On the other hand, the band gaps of CdS:O layers deposited at 270 °C decrease as the O2/(O2 + Ar) ratios increase. Interestingly, however, our high temperature closed-space sublimation (CSS) processed CdTe solar cells using CdS:O window layers deposited at RT and 270 °C exhibit very similar cell performance, including similar short-circuit current densities. To understand the underlying reasons, CdS:O thin films deposited at RT and 270 °C are annealed at temperatures that simulate the CSS process of CdTe deposition. X-ray diffraction, atomic force microscopy, and UV-visible light absorption spectroscopy characterization of the annealed films reveals that the CdS:O films deposited at RT undergo grain regrowth and/or crystallization and exhibit reduced band gaps after the annealing. Our results suggest that CdS:O thin films deposited at RT and 270 °C should exhibit similar optical properties after the deposition of CdTe layers, explaining the similar cell performance.

A method for the deposition of CdTe layers at very low substrate temperatures

In this paper we present an evaporation concept which enables access to very low substrate temperatures for the fabrication of CdTe solar cells at promising solar cell efficiencies. CdTe layers were deposited at substrate temperatures of 300°C to 190°C and processed to solar cells with efficiencies of about 12% and 9%, respectively. A further reduction of the substrate temperature abruptly results in non-working solar cells. That effect is discussed in terms of the morphology of the absorber layers. The corresponding solar cell characteristics are compared to cells which were prepared by the conventional close space sublimation method.

Pathways to thin absorbers in CdTe solar cells

We present approaches to reduce the absorber thickness of CdTe solar cells. The investigations were done with CdTe absorber films deposited by the close-space sublimation (CSS) technique. Using these CdTe films, complete solar cells were produced in our own laboratory. The absorber thickness as the crucial parameter was varied between 1 and 11 µm in these experiments. It is analyzed how process steps following the CdTe layer deposition influence the structure of the absorber films as well as the solar cell properties. Three ways of back contact formation are compared. These include (i) the wet chemical etching of the CdTe surface, (ii) a plasma etching step, and (iii) the vacuum deposition of a thin intermediate copper layer. In the latter case, voids and shunts related to preferential etching at grain boundaries are avoided admitting the use of thinner absorber films. Thus, the solar-cell efficiencies were increased from below 9% to more than 10% while the CdTe film thickness was reduced from 11 µm to less than 4 µm.

Galvanic contact deposition of CdTe from ammoniacal basic electrolytes at elevated temperatures using an autoclave-type electrolysis vessel

The galvanic contact deposition of CdTe layers from an ammoniacal basic electrolyte was carried out at a temperature range from 363 K (90 °C) to 423 K (150 °C) using an auto-clave type electrolysis vessel. The current densities at 363 and 393 K were almost the same level as that at 343 K under the dark condition, resulting in the deposition of near-stoichiometric CdTe layers on Au-plated Cu substrates, while the current density at 423 K was larger due to the codeposition of a Cd 3Au phase. The mean crystallite size of the resulting CdTe layers was increased with the increase in the temperature of the electrolytes. The current density was enhanced by irradiating the cathode surface with visible light during the CdTe deposition and, in this case, the current efficiency was also dramatically increased. The layer obtained under illumination at 393 K consisted of near stoichiometric CdTe and a small amount of elemental Cd, while the layer prepared in the dark had a single phase CdTe. The mean crystallite size of the CdTe layer prepared at 393 K under illumination was about 24 nm.

Galvanic Contact Deposition of CdTe Layers Using Ammoniacal Basic Aqueous Solution

The galvanic contact deposition of CdTe layers from ammoniacal basic solutions was carried out, and their deposition behaviors were investigated. The structural and electrical properties of deposits were examined and then compared with those of deposits prepared by normal electrodeposition from the same solutions. The cathode potential was always spontaneously kept at around vs. standard hydrogen electrode during the contact deposition although CdTe also deposited on the Cd sheet, that is, the anode. The current density gradually decreased with time just like that in the normal potentiostatic electrodeposition. The current efficiency was approximately 100% under illumination, whereas it was less than 50% in the dark. These behaviors were the same as those observed in normal electrodeposition from the same electrolytes. The resulting deposits both under illumination and in the dark were polycrystalline CdTe layers with almost stoichiometric composition. The as-deposited CdTe layer had a -type conduction with resistivity of the order of .

Improved performance of CdTe thin film solar cells through controlling the initial stage of the CdTe layer deposition by close-spaced sublimation

Control of the initial stage of the CdTe deposition was investigated by modifying the substrate and the source temperature profiles in close-spaced sublimation (CSS) process for improving the performance of CdTe thin film solar cells. In the modified temperature profile, the substrate temperature was increased with higher rate than that of the source temperature. By using the modified temperature profile, the conversion efficiency was improved as compared with the conventional temperature profile, and 15.3% efficiency ( V oc: 0.811 V, J sc: 26.3 mA/cm 2, FF: 0.718, 1 cm 2, AM 1.5) was achieved.

Ion-assisted sputter deposition of CdTe layers

Epitaxial, single-phase CdTe layers have been deposited on a variety of single-crystal substrates, including freshly cleaved KBr and NaCl and polished BaF2, by ion-assisted, planar, rf magnetron sputtering. The substrate temperature and bias were used as independent, operator-controlled process parameters to greatly enhance the degree of orientation in the films and (or) modify the crystallographic form of the deposited layer. Thus, depending primarily on the substrate temperature and bias, the deposited CdTe films exhibited one of several forms: (i) single-phase, highly oriented cubic, (ii) single-phase, highly oriented hexagonal, (iii) two-phase, highly oriented, hexagonal plus cubic or hexagonal plus tetragonal, and (iv) single-phase, epitaxial tetragonal. Compositional analysis revealed no impurities. However, films of all crystallographic phases contained a large density of planar defects such as twins, stacking faults, and low-angle grain boundaries.

Electrochemical Deposition of CdTe Layers. Their Structure and Electrical Properties

AbstractCdTe layers have been deposited catodically on nickel substrates from an aqueous solution of CdSO4 and TeO2. The degree of erystallinity increases with increasing temperature and decreasing deposition potential. The structure of the layers is cubic with 〈111〉 or 〈110〉 texture which is determined by TeO2 concentration predominantly. Amorphous phase is found in deposits prepared at higher deposition potential or at lower temperatures of bath (60 °C). After annealing at 200 °C the amorphous deposits crystallized and pure tellurium appeared in diffraction spectra. The resistivity of the films was determined by means of I—V characteristics. Films with deposition potentials (−500 to −400) mV vs SCE are p‐type conductivity whereas at lower potentials (down to − 700 mV vs. SCE) the n‐type material is formed.

Cadmium telluride layer deposition using the ionized cluster beam technique

A system is described which produces a beam of ionized clusters (typical size 10 3 atoms) of CdTe. Following the idea of Takagi et al., this beam is used for CdTe layer deposition. The clustering process is evidenced and the stoichiometry of the obtained clusters is discussed through transmission electron microscopy and X-ray diffraction measurements. Retarding field analysis is used in combination with a vibrating quartz microbalance to evaluate the size distribution and the probability of ionization of the clusters. Deposition on CdTe and GaAs substrates are examined, through scanning electron microscopy and water drop method as a function of the temperature of the substrate and of the accelerating electrical field applied to the clusters.