Large-area CdTe Research Articles

Impact of nanoscale Cu (I) precursors prepared by solution process on large-area CdTe solar cells

In recent years, the study of Cu (Ӏ) precursors in high-performance CdTe thin-film solar cell devices has received increasing attention. However, few studies have been reported on nanoscale Cu (I) precursors in large-area CdTe devices (≥1 cm2). Here, three different Cu (Ӏ) nano precursors have been rapidly prepared by solution method (CuCl ethanol solution, CuI acetonitrile solution, and CuSCN aqueous ammonia solution) and their effects on the performance of commercial large-area CdTe thin-film solar cells have been investigated. The results show that the distribution amount and uniformity of the nano precursor particles on the CdTe back surface have a large impact on the device performance. The CuCl nanoparticles are more uniformly distributed and the device performance is more excellent. CuI nanoparticles easily formed clusters at grain boundaries. Denser distribution of CuSCN nanoparticles. The thermal diffusion conditions (temperature, time and precursor concentration) have a significant effect on the device performance, but the most reasonable thermal diffusion conditions tend to be the same for all devices. This suggests that the thermal diffusion conditions are determined by the nature of the CdTe absorber layer itself and are not interfered by different nano precursors.

2-mm-thick large-area CdTe double-sided strip detectors for high-resolution spectroscopic imaging of X-ray and gamma-ray with depth-of-interaction sensing

We developed a 2-mm-thick CdTe double-sided strip detector (CdTe-DSD) with a 250μm strip pitch, which has high spatial resolution with a uniform large imaging area of 10 cm2 and high energy resolution with high detection efficiency in tens to hundreds keV. The detector can be employed in a wide variety of fields for quantitative observations of hard X-ray and soft gamma-ray with spectroscopic imaging, for example, space observation, nuclear medicine, and non-destructive elemental analysis. This detector is thicker than the 0.75-mm-thick one previously developed by a factor of ∼2.7, thus providing better detection efficiency for hard X-rays and soft gamma rays. The increased thickness could potentially enhance bias-induced polarization if we do not apply sufficient bias and if we do not operate at a low temperature, but the polarization is not evident in our detector when a high voltage of 500 V is applied to the CdTe diode and the temperature is maintained at −20 °C during one-day experiments. The “Depth Of Interaction” (DOI) dependence due to the CdTe diode’s poor carrier-transport property is also more significant, resulting in much DOI information while complicated detector responses such as charge sharings or low-energy tails that exacerbate the loss in the energy resolution.In this paper, we developed 2-mm-thick CdTe-DSDs, studied their response, and evaluated their energy resolution, spatial resolution, and uniformity. We also constructed a theoretical model to understand the detector response theoretically, resulting in reconstructing the DOI with an accuracy of 100 μm while estimating the carrier-transport property. First, we evaluated the energy resolution using 22Na, 133Ba, and 57Co reconstructing the DOI effect from the energy correlation on both the anode and cathode sides. We obtained an energy resolution of 4.3 keV (FWHM) at 356 keV. Second, we formulated a theoretical detector-response model that reproduced the experimental data. Using the model, we determined the mobility-lifetime products of carriers μτe,h=(4±1)×10−3,(1.15±0.05)×10−4 [cm2/V] with the space charge density of nsp=−6.0×1010 [1/cm−3] and then reconstructed the DOI with an accuracy of 100 μm. Finally, we evaluated the spatial resolution and demonstrated to resolve patterns of 250-μm-width slits with an X-ray test chart. We found no positional variation in the detector response. We realized the detector that has high energy resolution and high 3D spatial resolution with a uniform large imaging area.

Optimization of a customized simultaneous algebraic reconstruction technique algorithm for phase-contrast breast computed tomography

Objective. To introduce the optimization of a customized GPU-based simultaneous algebraic reconstruction technique (cSART) in the field of phase-contrast breast computed tomography (bCT). The presented algorithm features a 3D bilateral regularization filter that can be tuned to yield optimal performance for clinical image visualization and tissues segmentation. Approach. Acquisitions of a dedicated test object and a breast specimen were performed at Elettra, the Italian synchrotron radiation (SR) facility (Trieste, Italy) using a large area CdTe single-photon counting detector. Tomographic images were obtained at 5 mGy of mean glandular dose, with a 32 keV monochromatic x-ray beam in the free-space propagation mode. Three independent algorithms parameters were optimized by using contrast-to-noise ratio (CNR), spatial resolution, and noise texture metrics. The results obtained with the cSART algorithm were compared with conventional SART and filtered back projection (FBP) reconstructions. Image segmentation was performed both with gray scale-based and supervised machine-learning approaches. Main results. Compared to conventional FBP reconstructions, results indicate that the proposed algorithm can yield images with a higher CNR (by 35% or more), retaining a high spatial resolution while preserving their textural properties. Alternatively, at the cost of an increased image ‘patchiness’, the cSART can be tuned to achieve a high-quality tissue segmentation, suggesting the possibility of performing an accurate glandularity estimation potentially of use in the realization of realistic 3D breast models starting from low radiation dose images. Significance. The study indicates that dedicated iterative reconstruction techniques could provide significant advantages in phase-contrast bCT imaging. The proposed algorithm offers great flexibility in terms of image reconstruction optimization, either toward diagnostic evaluation or image segmentation.

Large-area single-photon-counting CdTe detector for synchrotron radiation computed tomography: adedicated pre-processing procedure.

Large-area CdTe single-photon-counting detectors are becoming more and more attractive in view of low-dose imaging applications due to their high efficiency, low intrinsic noise and absence of a scintillating screen which affects spatial resolution. At present, however, since the dimensions of a single sensor are small (typically a few cm2), multi-module architectures are needed to obtain a large field of view. This requires coping with inter-module gaps and with close-to-edge pixels, which generally show a non-optimal behavior. Moreover, high-Z detectors often show gain variations in time due to charge trapping: this effect is detrimental especially in computed tomography (CT) applications where a single tomographic image requires hundreds of projections continuously acquired in several seconds. This work has been carried out at the SYRMEP beamline of the Elettra synchrotron radiation facility (Trieste, Italy), in the framework of the SYRMA-3D project, which aims to perform the world's first breast-CT clinical study with synchrotron radiation. An ad hoc data pre-processing procedure has been developed for the PIXIRAD-8 CdTe single-photon-counting detector, comprising an array of eight 30.7 mm × 24.8 mm modules tiling a 246 mm × 25 mm sensitive area, which covers the full synchrotron radiation beam. The procedure consists of five building blocks, namely dynamic flat-fielding, gap seaming, dynamic ring removal, projection despeckling and around-gap equalization. Each block is discussed and compared, when existing, with conventional approaches. The effectiveness of the pre-processing is demonstrated for phase-contrast CT images of a human breast specimen. The dynamic nature of the proposed procedure, which provides corrections dependent upon the projection index, allows the effective removal of time-dependent artifacts, preserving the main image features including phase effects.

Imaging study of a phase-sensitive breast-CT system in continuous acquisition mode

The SYRMA-CT project aims to set-up the first clinical trial of phase-contrast breast Computed Tomography with synchrotron radiation at the SYRMEP beamline of Elettra, the Italian synchrotron light source. The challenge in a dedicated breast CT is to match a high spatial resolution with a low dose level. In order to fulfil these requirements, the SYRMA-CT project uses a large area CdTe single photon counting detector (Pixirad-8), simultaneous algebraic reconstruction technique (SART) and phase retrieval pre-processing. This work investigates the imaging performances of the system in a continuous acquisition mode and with a low dose level towards the clinical application. A custom test object and a large surgical sample have been studied.

Modeling Back Electronic Reflectors in CdTe Solar cells.

Abstract A photovoltaic system (or PV system) is a system which uses one or more solar panels to convert sunlight into electricity. The CdTe thin-film photovoltaic has a large absorption coefficient and is theoretically highly efficient. Large-area CdTe photovoltaic panels can be economically fabricated. These features potentially make the CdTe thin-film solar cell the leading alternative energy source. However, the record CdTe efficiency of (16.5%), is much less than its theoretical maximum efficiency of (29%) [1]. “The open-circuit voltage 0.845 V is well below what is expected for its band gap of 1.5 eV. The incorporation of an electron reflector is a strategy to improve the open-circuit voltage of solar cells, and thus a strong possibility for improving the efficiency of CdTe thin-film” [1]. An electron reflector is a conduction-band energy barrier at the back surface of the solar cell, which can reduce the recombination due to the electron flow to the back surface. The different methods of creating an electron reflector are explained in the electron-reflector strategy paper

Phase-sensitive breast CT with monochromatic beam towards the clinical trial

Introduction The first clinical trial of phase-contrast mammography with monochromatic beam was performed at Elettra, the Italian Synchrotron Radiation facility. A lower Mean Glandular Dose (MGD) and a higher specificity were achieved compared to the digital mammography for the patient cohort (more than 70). Purpose The SYRMA-CT project (SYnchrotron Radiation for MAmmography – Computed Tomography) aims to perform the first clinical trial of phase-sensitive breast CT with monochromatic beam. High image quality and low dose are expected according to the results of the first mammography trial. Materials and methods A large area CdTe single photon counting detector (PIXIRAD-8) is used. Phase-retrieval algorithm are applied, thus exploiting propagation based phase-contrast imaging (PPCI). The dose system of the beamline has been calibrated to match the energy requirement up to 40 keV. Due to the peculiar irradiation modality, ad hoc Monte Carlo simulations and experimental measurements were performed for dose evaluation. Results Images of surgical samples included in large test objects (up to 12 cm diameter) were acquired at 38 keV in clinical compatible dose conditions (MGD ∼5 mGy). Phase-retrieval pre-processing was applied improving the CNR of the high-resolution images with (120 μm) 3 voxel size. Conclusion The upgrade of the mammographic facility of the Elettra laboratory towards a new clinical trial is going on. Image quality and dose assessment indicate its feasibility with monochromatic beam. This clinical trial will allow the evaluation of PPCI breast CT in optimal conditions and will give indication for the translation to the hospital of phase-sensitive techniques. Disclosure Nothing to declare.

The Chloride Annealing for Back Contact Layer Free CdTe Solar Cells

The large area CdTe thin film samples were used for chloride annealing. The CuCl2/NH4Cl solution was attached on the CdTe surface. After annealing treatment, the CdTe solar cells were prepared. The structure of the thin films and the properties of the CdTe solar cells were tested for studying the effect of the ratio of Cu/Cl, solution concentration and the annealing temperature. At last the performance of CuCl2/NH4Cl annealing cells, ZnTe back contact cells and C:Te,Cu back contact cells were compared. Without back contact layers the efficiency of the CdTe solar cells reached 11.13% with chloride annealing.

Effect of Substrate Temperature on CdTe Thin Film Property and Solar Cell Performance

Vapor transport deposition is an excellent method for preparing large area CdTe thin films with high quality and uniformity. Polycrystalline CdTe thin films were deposited by home-made vapor transport deposition system (VTD). The effects of substrate temperature on the property of CdTe film and the performance of CdTe solar cell were investigated. CdTe thin films were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), UV-Vis spectrometer, and Hall Effect system. The results show that the CdTe thin films deposited by vapor transport deposition are cubic phase with a preferred orientation in (111) direction. The average grain size increases from 2 m to 6 m and the carrier concentration increases from 1.93×10 cm to 2.36×10 cm when the substrate temperature increases from 520 °C to 620 °C. This suggests that high substrate temperature can increase the carrier density significantly due to the suppressed defect recombination. The performance of CdTe thin film solar cells deposited at different substrate temperatures demonstrates that high substrate temperature (610°C) can greatly improve the efficiency, open circuit voltage and fill factor of the solar cells. But the substrate temperature higher than 610°C will reduce the spectral 142 无 机 材 料 学 报 第 31卷 response of the cells in long wavelength region, which results in the degradation of solar cell performance. The small-area CdTe thin film solar cell without back contact layer deposited at substrate temperature of 610°C obtains the best conversion efficiency of 11.2%.

The large-area CdTe thin film for CdS/CdTe solar cell prepared by physical vapor deposition in medium pressure

The Cadmium telluride (CdTe) thin film has been prepared by physical vapor deposition (PVD), the Ar+O2 pressure is about 0.9kPa. This method is a newer technique to deposit CdTe thin film in large area, and the size of the film is 30ÿ40cm2. This method is much different from the close-spaced sublimation (CSS), as the relevance between the source temperature and the substrate temperature is weak, and the gas phase of CdTe is transferred to the substrate by Ar+O2 flow. Through this method, the compact and uniform CdTe film (30ÿ40cm2) has been achieved, and the performances of the CdTe thin film have been determined by transmission spectrum, SEM and XRD. The film is observed to be compact with a good crystallinity, the CdTe is polycrystalline with a cubic structure and a strongly preferred (111) orientation. Using the CdTe thin film (3ÿ5cm2) which is taken from the deposited large-area film, the 14.6% efficiency CdS/CdTe thin film solar cell has been prepared successfully. The structure of the cell is glass/FTO/CdS/CdTe/graphite slurry/Au, short circuit current density (Jsc) of the cell is 26.9mA/cm2, open circuit voltage (Voc) is 823mV, and filling factor (FF) is 66.05%. This technique can be a quite promising method to apply in the industrial production, as it has great prospects in the fabricating of large-area CdTe film.

Structure and Optoelectronics of Electrodeposited Cadmium Ditelluride (CdTe2)

Electrodeposition is a powerful method to perform low cost, large area CdTe deposition. Good quality semiconductor materials for photovoltaic applications as well as high growth rates have been ach ...

Development of semiconductor imaging detectors for a Si/CdTe Compton camera

Si and CdTe semiconductor imaging detectors have been developed for use in a Si/CdTe Compton camera. Based on a previous study using the first prototype of a Si/CdTe Compton camera, new detector modules have been designed to upgrade the performance of the Compton camera. As the scatter detector of the Compton camera, a stack of double-sided Si strip detector (DSSD) modules, which has four layers with a stack pitch of 2 mm, was constructed. By using the stack DSSDs, an energy resolution of 1.5 keV (FWHM) was achieved. For the absorber detector, the CdTe pixel detector modules were built and a CdTe pixel detector stack using these modules was also constructed. A high energy resolution ( Δ E / E ∼ 1 % ) was achieved. The improvement of the detection efficiency by stacking the modules has been confirmed by tests of the CdTe stack. Additionally, a large area CdTe imager is introduced as one application of the CdTe pixel detector module.

Studies of key technologies for large area CdTe thin film solar cells

The structure and main manufacturing technologies of CdTe film solar cells of large area are reviewed. Among the technologies, some have been developed for application in a pilot manufacturing line. The high resistant SnO 2 (HRT) thin films have been fabricated by PECVD. The effects of annealing on the structure and properties have been studied. A surface etching process of CdTe in low temperature and lower concentration of nitric acid has been developed. The Cd 1 − x Zn x Te ternary compound films have been studied. In order to improve the back contact layer, Cd 0.4Zn 0.6Te layer with 1.8 eV band gap as a substitute for ZnTe layer is introduced in CdTe cells. The effects of the technologies on performance of CdTe cells and feasibility of application in the modules are discussed.

Recent achievements of the high resolution Schottky CdTe diode for γ-ray detectors

We describe the recent progress on the use of Schottky Cadmium telluride (CdTe) diode detectors for γ-ray spectroscopy. The extremely low leakage current of the newly developed CdTe diode allows us to apply a much higher bias voltage in comparison with the previous CdTe detectors. Both the improved charge-collection efficiency and the low leakage current lead to a good energy resolution even at room temperature. Large-area CdTe diode detectors with dimensions of 21.5 × 21.5 mm 2 are now available. By stacking 40 layers of these large-area devices, we have achieved a good energy resolution of several keV (FWHM) for MeV γ-rays.

Large-area CdTe diode detector for space application

The current status of Schottky CdTe diode detectors, especially in view of their space application for hard X-ray and gamma-ray astronomy, are reported. For practical use in space science, a large-area CdTe diode with a size of 21.5×21.5 mm 2 and a thickness of 0.5 mm was developed. A good energy resolution, 2.8 keV (FWHM) at −20°C, and high homogeneity to within 0.2% over the detector were achieved for the spectral performance. This device has successfully passed a series of tests required for its use in space, in view of utilizing Japanese M-V rockets. The tests include the mechanical environment test, vacuum test, long run for weeks and proton-beam radiation. Initial results from a 2×2 segmented electrode large-area device with a guard-ring are also presented.

Energy resolution of large-area CdTe p–i–n detectors with charge loss correction

Earlier developed CdTe p–i–n detectors technology (energy resolution is close to the Ge ones) gives a possibility to solve various tasks of γ-active materials control. For inspection of the materials irradiating γ-quanta with energies of 100–400 keV (for example, U, Pu), it is sufficient to increase the sensitive area of detector up to 4–5 cm 2 (with the thickness about 2.5–3 mm). It might allow measurement of the nuclear materials characteristics using standard software, designed for Ge detectors. The research results for large square CdTe p–i–n detectors and detection systems development will be demonstrated in this work. The factors limiting energy resolution and other characteristics (peak-to-background ratio, shape of amplitude distribution, charge losses) of the large CdTe detectors will be discussed.

Ion beam studies of CdTe films epitaxially grown on Si, GaAs and sapphire substrates

The epitaxial growth and the crystalline quality of CdTe films grown on different substrates were studied using Rutherford backscattering spectrometry (RBS) combined with channeling. The films which were 2.5 to 20 μm thick, were grown by metalorganic vapor phase epitaxy (MOVPE). Despite the big lattice mismatch (>10%) between the CdTe and the Si, GaAs and Al 2O 3 substrates used, a good epitaxial growth of the films was observed. The results show nearly defect free films with the following epitaxial relationship with the substrates: (1 1 1) CdTe ‖ (0 0 0 1) Al 2O 3, (1 0 0) CdTe ‖ (1 0 0) GaAs, (1 1 1) CdTe ‖ (1 0 0) Si. The values of the minimum yields, measured along the main axial directions, indicate that the best films were obtained with sapphire substrates. The surface quality of the layers improves with a subsequent recrystallization induced by laser annealing. These results, combined with previous electron microscopy and photoluminescence studies, give the possibility to optimize the growth conditions of large area CdTe layers used in the infrared (IR) technology.

High quality large-area CdTe(211)B on Si(211) grown by molecular beam epitaxy

We describe the growth of high quality CdTe(211)B layers by molecular beam epitaxy on nominal Si(211) substrates. Prior to CdTe deposition, thin ZnTe(211)B buffer layers were grown to preserve the homo-orientation. Large-area CdTe(211)B layers were routinely obtained by optimizing the growth parameters. From x-ray diffraction, we observed the presence of twin-free CdTe(211)B layers. One 8 μm thick CdTe epilayer had a near-surface etch pit density of 1.5×105 cm−2, which surpassed the best value reported for CdTe(211)B grown on GaAs(211)B, GaAs/Si(211), or Si(211) substrates.

The growth and comparison of large-diameter vertical Bridgman CdZnTe and CdTe

There is currently a need for high yields of large-area CdTe and C d - x Z n x T e substrates for the growth of infrared (IR) detector material. We have recently grown high-quality, 64 mm diameter, vertical Bridgman C d 1− x Z n x T e ( x = 0.04) and CdTe under virtually identical conditions, with an axial thermal gradient of 55°C cm -1 at the ampoule wall, a solidification velocity of 1.0 mm h -1, and a post-solidification cooling rate of 10°C h -1. The growth program was designed to minimize excess stress in the crystals. We have obtained large-area (111)-oriented wafers from each double. We explore the defect structure of these wafers using X-ray rocking curve (XRC) mapping, whole-wafer X-ray synchrotron white beam topography, chemical etch pit analysis, and optical and infrared microscopy. We also utilize Fourier transform infrared (FTIR) transmission mapping to explore the chemical and the optical properties of the materials. We have correlated the structural, chemical, and optical properties with the effects of substituting Zn into CdTe and the corresponding physical and mechanical properties of the ternary CdZnTe. In particular, CdZnTe exhibits a decrease in crystalline defects and an improvement in morphological uniformity, increasing its suitability for use as IR detector substrate material.

Growth and microstructural properties of cadmium telluride thin films

For many applications, such as IR detectors and high speed devices, we need high quality cadmium telluride (CdTe) films. To fabricate CdTe epitaxial films we are using a home-built closed hot wall epitaxy system. In this paper, we shall report the complex nature of the CdTe film's microstructure as grown on InSb substrates. Films were grown under various preparation conditions. We observe profound effects of substrate temperature and source temperature on the microstructure of CdTe films. This study together with other properties will allow us to correlate various effects and to design an optimum preparation condition for epitaxial CdTe thin films. An epitaxial CdTe film on different substrates is a solution to the presentshortage of large area CdTe substrates.