Polycrystalline CdTe Films Research Articles

High-temperature injection spectroscopy of deep traps in CdTe polycrystalline films

High-temperature injection spectroscopy of deep traps in CdTe polycrystalline films

Optical properties of thermally evaporated cadmium telluride thin films

Polycrystalline CdTe films have been deposited onto glass substrates at 373K by vacuum evaporation technique. The transmittance and reflectance have been measured at normal and near normal incidence, respectively, in the spectral range 200–2500nm. The dependence of absorption coefficient, α on the photon energy have been determined. Analysis of the result showed that for CdTe films of different thicknesses, direct transition occurs with band gap energies in the range 1.45–1.52eV. Refractive indices and extinction coefficients have been evaluated in the above spectral range. The XRD analysis confirmed that CdTe films are polycrystalline having hexagonal structure. The lattice parameters of thin films are almost matching with the JCPDS 82-0474 data for cadmium telluride.

Crystalline and thin-film silicon solar cells: state of the art and future potential

Bulk crystalline silicon solar cells have been the workhorse of the photovoltaic industry over the past decades. Recent major investments in new manufacturing facilities for monocrystalline and multicrystalline wafer-based cells, as well as for closely related silicon ribbon and sheet approaches, ensure this role will continue well into the future. Such investments suggest that the silicon wafer-based approach has successfully withstood the challenge mounted by thin-film chalcogenide-based cells, in the form of polycrystalline films of CdTe and CuInSe 2, as well as that mounted by thin-film cells based on amorphous silicon and its alloys with germanium. The encumbent now faces a fresh challenge by a new wave of thin-film technologies developed in the 1990s, more closely related to the bulk approach and with some advantages over the earlier contenders. One new approach is based on a stack of two silicon thin-film cells, one cell using amorphous silicon and the other mixed-phase microcrystalline silicon. The second uses silicon thin-films in polycrystalline form deposited onto glass, even more directly capturing the strengths of the wafer-based approach.

Influence of the growth conditions on the photoluminescence spectrum of CdTe polycrystalline films deposited by the close space vapor transport technique

A study of the influence of some growth parameters, the gradient of temperature between source and substrate and introduction of oxygen in the growth chamber, on the optical properties of CdTe thin films grown by Close Space Vapor Transport (CSVT) is presented. This study was carried out by means of photoluminescence (PL). The combined effect of the temperature gradient and oxygen in the growth chamber on the crystalline quality of CdTe thin films were analyzed. Important changes in the photoluminescence spectra were observed. A decrease of the intensity of the excitonic PL band is observed, as oxygen concentration increases. This accounts for an increase in the defect density due to the presence of oxygen.

Formation of CdxHg1-xTe Layers on CdTe after NP-Etching and HgTe-Graphite Pasting

AbstractWe report on our investigation of the microstructure and chemical composition at the CdTe/Te-rich interfaces generated by NP-etching polycrystalline and single-crystalline CdTe films and followed with HgTe-graphite pasting and thermal annealing. We find that after this process, a thin layer of CdxHg1-xTe forms between CdTe and Te-rich layers, giving a structure like CdTe/CdxHg1-xTe/Te. High-resolution electron microscopy reveals that the CdxHg1-xTe layer has an epitaxial relationship with the CdTe. No CdxHg1-xTe layer has been observed in bromine/methanol-etched samples or samples with intentionally deposited Te layers.

Defects in CdTe polycrystalline films grown by physical vapour deposition

CdTe films were grown by physical vapour deposition onto alumina, glass and silicon substrates. The best growth conditions were a source temperature in the range 700–830 °C, a substrate temperature from 450 to 550 °C and a growth time of about 4 h. Onto alumina and glass the films grow with the [111] axis perpendicular to the substrate. Tellurium inclusions were found as the main film imperfection. Film thickness from 20 to 150 μm was determined by profilometry. Photoluminescence measurements confirmed the high quality of the films. They show resistivities in the order of 10 6 Ω cm. A comparison with previous reported data and an evaluation about the X-ray imaging capabilities of the films are included.

Transmission Electron Microscopy Study of Planar Defects in Polycrystalline CdTe Thin Films

CdTe is a promising photovoltaic material due to its near optimum band gap and high absorption coefficient. Polycrystalline, thin-film CdTe/CdS solar cells have demonstrated an efficiency of 15.8%. High density of extended defects is often found in polycrystalline CdTe films grown by close-spaced sublimation (CSS). So far, most investigations of defects in CdTe have focused on epitaxially grown films, and the reported extended defects are mainly lamellar twins. However, epitaxially grown films generally have a different microstructure compared to CSS grown polycrystalline CdTe thin films. in this paper, we report our study of extended defects in CSSgrown polycrystalline CdTe thin films by high-resolution transmission electron microscopy (HRTEM). We found that the extended defects are mostly lamellar twins and stacking faults. The stacking faults always propagate across the grains, without ending at a partial dislocation inside the grains.

Effects of Cd vacancies on the electrical properties of polycrystalline CdTe sputtered films

Polycrystalline CdTe films were grown on 7059 Corning glass substrates at room temperature by sputtering from a pure CdTe target and by co-sputtering from a composite CdTe–Cd target. The electrical, optical and structural properties of the films were analyzed as a function of their Cd concentration. It was found that the films grown from the pure CdTe target were Cd-deficient with electrical resistivity of about 10 7–10 8 Ωcm and those grown from the CdTe–Cd target were Cd-rich with electrical resistivity of the order of 10 3 Ωcm. Dark electrical current vs 1/K B T measurements showed that the CdTe films with low cadmium content (<50 at.%) exhibit an unusual current peak in the range of 1/K B T=40–60 eV −1. When the cadmium composition in the CdTe films approaches 50 at.%, this effect vanishes and the current has an exponential behavior for 1/K B T>40 eV −1. The current peak was associated with the presence of Cd vacancies in the CdTe lattice.

TEM, AFM, and cathodoluminescence characterization of CdTe thin films

The morphology, microstructure, and luminescent properties of polycrystalline CdTe films were investigated. The films were deposited by close-spaced sublimation at relatively low temperatures. The evolution of the film morphology and microstructure was studied as a function of deposition temperature and post-deposition heat treatment. Films deposited at low temperature exhibited fine, equiaxed grains. Dramatic grain growth was observed as a consequence of heat treatment. The film microstructure is heavily faulted and strongly dependent on the deposition and post-deposition treatment temperatures. The extended defects are mostly lamellar twins and stacking faults with similar densities. These defects often form a closely spaced arrangement, which leads to the formation of a local hexagonal phase. The presence of this hexagonal phase is expected to have significant effects on the electrical properties of the cubic CdTe films.

Effect of pulsed laser irradiation on the photoconductivity of polycrystalline CdTe films

Polycrystalline CdTe films were prepared by molecular-beam deposition onto sapphire and glassceramic substrates at 670 K. The photoelectric behavior of the films (dependence of photoconductivity on incident light intensity and spectral response of photocurrent) was found to depend on grain size. The photoresponse of the fine-grained films in the intrinsic-absorption range and that of the coarse-grained films in the intrinsic- and impurity-absorption ranges increase as a result of above-gap irradiation with 20-ns laser pulses. The dependences of photoconductivity on incident intensity for the coarse-grained films are similar to those for CdTe single crystals. The grain boundaries in fine-grained CdTe films play an important part in the generation and recombination of charge carriers.

Scanning photoelectron microscopy study of the laser-induced transformations of polycrystalline CdTe films

We have used pulsed-laser treatment to provide highly localized chemical reactions in the near-surface region of polycrystalline CdTe thin films. Using scanning photoelectron microscopy, the lateral changes in the composition and morphology of the films as a function of the irradiation conditions and of the distance from the center of the laser beam were characterized. Results show that the films undergo thermal-induced heterogeneity in the morphology and in the lateral distribution of the elements and chemical phases due to local melting and dissociation of the film. This transformation leads to a local increase of the conductivity and enrichment with elemental Te.

Control of Composition and Conduction Type of CdTe Film Electrodeposited from Ammonia Alkaline Aqueous Solutions

Potentiostatic cathodic electrodeposition of CdTe films was studied using aqueous ammonia‐alkaline electrolytic baths in which Cd(II)‐ and Te(IV)‐species were dissolving to form and ions. respectively. Deposition of three kinds of films, i.e., Te‐rich amorphous, Te‐rich polycrystalline CdTe, and Cd‐rich polycrystalline CdTe films, can be controlled by changing the pH (pH 10.5–12.2) and Cd(II)/Te(IV) concentration ratio of the baths. All the films deposited at −0.70 V vs. the standard hydrogen electrode (SHE) had a flat and smooth morphology. Deposition current density and efficiency were reduced with increasing Cd content of the film. The deposition behavior was interpreted with a potential‐pH diagram of the system. Based on the results, the mechanism of the CdTe electrodeposition was presumed to be (i) deposition of tellurium atoms followed by (ii) an immediate adsorption of Cd(II) ions on the surface tellurium atoms, and (iii) an underpotential deposition of Cd on the surface Te. Conduction types of the Te‐rich and Cd‐rich CdTe films after heat‐treatment were p‐ and n‐type, respectively, whereas untreated CdTe film was n‐type. © 1999 The Electrochemical Society. All rights reserved.

Comparative study of performance of CdTe, CdSe and CdS thin films-based photoelectrochemical solar cells

Polycrystalline films of CdTe and CdSe were electrodeposited onto titanium substrates from acidic baths, and, polycrystalline CdS thin films were prepared by chemical bath deposition method. Due to surface treatment these films show good photoelectrochemical behaviour in a polysulphide electrolyte with high conversion efficiency and stability. The solid-state parameters of annealed films are calculated and compared with those of unannealed film-based cells. Chemical etching is found to improve short-circuit current and fill-factor whereas photoelectrochemical etching technique improves the stability of photo-anode in polysulphide solution. Annealing promotes incipient fusion of small crystallites and thereby reduces the grain boundaries which are known to act as recombination centres for minority carriers and trapping centres for majority carriers. The conversion efficiency and stability are found to improve by chemical etching of the semiconductor layer because chemical etching pins Fermi level of CdTe, CdSe and Cds photoanodes and promotes exchange current density.

Optical and structural evidence of the grain-boundary influence on the disorder of polycrystalline CdTe films

We report the linear behavior of the band-tail parameter as a function of the reciprocal of the grain size in polycrystalline CdTe. On the other hand, the study of the full width at half maximum of the x-ray diffraction peak shows a similar behavior, which indicates that the disorder increases as grain size diminishes. A theoretical analysis justifies that the behavior is ruled by the contribution of the grain-boundary traps, and the trap concentration is calculated. Both results constitute experimental evidences of the grain-boundary disorder, which was quantified, and demonstrate that it is caused by the extension of the grain-boundary effect into the grain.

Effects of CdS substrates on the physical properties of polycrystalline CdTe Films

In this study, CdS thin films were deposited by thermal evaporation, close spaced sublimations, and solution growth methods, respectively, and the effects of the deposition methods on the physical properties of polycrystalline CdS deposited on ITO/glass were investigated. In addition, the effects of variously deposited CdS on the physical properties of thermally evaporated CdTe were also investigated. XRD results showed that CdS thin films deposited by solution growth, thermal evaporation, and close spaced sublimation (CSS) had a hexagonal structure with a random orientation of , (002) and (103) preferred orientation, respectively. All of the CdTe thin films deposited on those CdS substrates had the same cubic zincblende structure, however, they showed a little different crystal orientation relationship depending on the underlying CdS deposition method. Cross-sectional TEM showed that CdTe grain sizes of the furnace annealed CdTe grown on the CSS-CdS was the smallest among the furnace annealed CdTe thin films deposited on the variously prepared CdS substrates.

Electrodeposition of CdTe Films from Ammoniacal Alkaline Aqueous Solution at Low Cathodic Overpotentials

Cathodic electrodeposition of CdTe films was studied using aqueous ammonia‐alkaline electrolytic baths (pH 10.7; temperature 343 K) in which Cd(II) and Te(IV) species were dissolved to form and ions, respectively. From the solution, , a flat and smooth polycrystalline CdTe film (thickness, ca. 1 μm) with nearly stoichiometric composition was deposited at a constant cathode potential, ranging from −0.70 to −0.30 V vs. SHE, whereas dendrite CdTe accompanying elemental cadmium was obtained at −0.80 V. The deposition behavior was fully explained by an underpotential deposition mechanism taking the calculated redox potentials of and pairs into consideration. During electrodeposition of nearly stoichiometric crystalline CdTe, the current density was decreasing monotonously. © 1999 The Electrochemical Society. All rights reserved.

Evidence of scattering effects on the thermal transport inindium-doped CdTe films

Photoacoustic (PA) heat transmission measurements were used to study thermal transport in n-type polycrystalline CdTe films doped with metallic indium. Thermal diffusivity (TD) at room temperature was determined using an open cell PA technique. The TD obtained for each sample was correlated with x-ray diffraction measurements. It can be shown that the TD of the undoped films under investigation is significantly reduced compared with corresponding doped films. This reduction is a result of the high concentration of indium atoms and defects depending on the deposition parameters. It is shown that the effects of CdTe lattice shrink produced by indium doping, induces an increase in phonon scattering reflected in the large TD decrease even with moderated indium concentration. We have calculated the relation between the TD and the lattice parameter as a polynomial function (cm2 s-1) = -35225.2+14259a-1532.7a2-27.9a3+8.4a4.

Studies on In doped CdTe co-sputtered films

Indium doped CdTe polycrystailine films were grown on Corning glass substrates at room temperature by co-sputtering from CdTe-In-Cd targets. The chemical composition, structural, electrical, and optical properties of the films were analyzed as a function of the In and Cd concentrations on the target. Two types of films were prepared: those in which the Cd concentration on the target was varied and the In kept at a fixed concentration and vice versa. From the experimental results, we concluded that using this deposition method n-type In doped CdTe polycrystalline films can be produced with electrical resistivity between 10 2 and 10 3 Ωcm and electron mobility between 0.1 and 0 cm 2 V −1 s −1.

Cd self-doping of CdTe polycrystalline films by co-sputtering of CdTe–Cd targets

Cadmium self-doped CdTe polycrystalline films were grown on Corning glass substrates at room temperature by cosputtering from a CdTe–Cd target. The electrical, structural, and optical properties of the films were analyzed as a function of the Cd concentration. Films with a stoichiometric composition, and slightly below and above it, were prepared. In films where the Te exceeds 50 at. %, it is found segregation of Te and its electrical resistivity is about 107 Ω cm. In those with an excess of Cd, the electrical resistivity drops several orders of magnitude, the carrier concentration increases, and the resistivity activation energy drops. From these results, we concluded that using this deposition method, n-type Cd self-doped CdTe polycrystalline films can be produced.

Growth kinetics and structural characterization of polycrystalline CdTe films grown by hot-wall vacuum evaporation

A systematic growth of CdTe films on two kinds of substrates, Corning 7059 glass and SnO 2-coated glass, has been performed by hot-wall vacuum evaporation (HWVE). From the dependence of deposition rate on source temperature, the growth in the HWVE system is limited by the transport of CdTe vapor from source to substrate. The deposition rate of CdTe films grown on either 7059 glass or SnO 2-coated glass gradually decreases with increasing substrate temperature, while the grain sizes increase with substrate temperature. X-ray diffraction showed that all of the CdTe films on either Corning 7059 glass or SnO 2-coated glass deposited at substrate temperature below 275°C have a preferential orientation of the (111) planes parallel to the substrate. However, the degree of the preferential orientation in the films at substrate temperature of 300°C reduced remarkably. High-energy shift of absorption edge in the grown CdTe films were also observed in transmittance spectra.