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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS2002

The Effect of the Wall Contact and Post-Growth Cool-Down on Defects in CdTe Crystals Grown by 'Contactless' Physical Vapour Transport

A series of cadmium telluride crystals grown by physical vapor transport without contact with the ampoule walls and cooled at different rates were characterized using synchrotron X-ray topography, photoluminescence, and chemical etching. Strain from sticking to silica glass and its effect on the dislocation density is shown. It was found that very fast cool-down increases dislocation density by at least one order of magnitude. None of the samples had random dislocation distributions but coarse clumping of dislocations on the scale of more than 100 microns was more prevalent in slowly cooled crystals. Photoluminescence revealed that slow cooling favored the donor-acceptor luminescence involving complex A centers. This was diminished in fast cooled material; and effect presumed to be due to dislocation gettering. Fast cooling also enhanced the formation of shallow acceptors. Implications for Bridgman growth of CdTe and the vapor growth of CdZnTe are discussed briefly.

Palosz, W.↗
NASA NTRS2002

Photoluminescence of CdTe Crystals Grown by Contactless PVT Method

High quality CdTe crystals with resistivities higher than 10(exp 8) Omega cm were grown by the 'contactless' PVT (physical vapor transport) technique. Group III elements In and Al, and the transition metal Sc were introduced at the nominal level of about 6 ppm to the source material. Low-temperature photoluminescence (PL) has been employed to identify the origins of PL emissions of the crystals. It was found that the emission peaks at 1.584 eV and 1.581 eV exist only in the In-doped crystal. The result suggests that the luminescence line at 1.584 eV is associated with Cd-vacancy/indium complex. The intensity of the broadband centered at 1.43 eV decreases dramatically with introduction of Sc.

Palosz, W.↗
NASA NTRS2002

Photoluminescence of CdTe Crystals Grown by Physical Vapor Transport

High quality CdTe crystals with resistivities higher than 10(exp 8) omega cm were grown by the physical vapor transport technique. Indium, Aluminum, and the transition metal Scandium were introduced at the nominal level of about 6 ppm to the source material. Low-temperature photoluminescence (PL) has been employed to identify the origins of PL emissions of the crystals. The emission peaks at 1.584 eV and 1.581 eV were found only in the In-doped crystal. The result suggests that the luminescence line at 1.584 eV is associated with Cd-vacancy/indium complex. The intensity of the broadband centered at 1.43 eV decreases strongly with introduction of Sc.

Curreri, Peter A.↗
NASA NTRS2002

Photoluminescence of CdTe Crystals Grown by Physical Vapor Transport

High quality CdTe crystals with resistivities higher than 10(exp 8) omega cm were grown by the physical vapor transport technique. Indium, Aluminum, and the transition metal Scandium were introduced at the nominal level of about 6 ppm to the source material. Low-temperature photoluminescence (PL) has been employed to identify the origins of PL emissions of the crystals. The emission peaks at 1.584 eV and 1.581 eV were found only in the In-doped crystal. The result suggests that the luminescence line at 1.584 eV is associated with Cd-vacancy/indium complex. The intensity of the broadband centered at 1.43 eV decreases strongly with introduction of Sc.

Palosz, W.↗
NASA NTRS2003

Photoluminescence of CdTe Crystals Grown by Physical-Vapor Transport

High-quality CdTe crystals with resistivities higher than 10(exp 8) omega cm were grown by the physical-vapor transport (PVT) technique. Indium, aluminum, and the transition-metal scandium were introduced at the nominal level of about 6 ppm to the source material. Low-temperature photoluminescence (PL) has been employed to identify the origins of PL emissions of the crystals. The emission peaks at 1.584 eV and 1.581 eV were found only in the In-doped crystal. The result suggests that the luminescence line at 1.584 eV is associated with Cd-vacancy/In complex. The intensity of the broadband centered at 1.43 eV decreases strongly with introduction of Sc.

Palosz, W.↗
NASA NTRS2007

Crystal Growth and Characterization of CdTe Grown by Vertical Gradient Freeze

In this study, crystals of CdTe were grown from melts by the unseeded vertical gradient freeze method. The quality of grown crystal were studied by various characterization techniques including Synchrotron White Beam X-ray Topography (SWBXT), chemical analysis by glow discharge mass spectroscopy (GDMS), low temperature photoluminescence (PL), and Hall measurements. The SWBXT images from various angles show nearly strain-free grains, grains with inhomogeneous strains, as well as twinning nucleated in the shoulder region of the boule. The GDMS chemical analysis shows the contamination of Ga at a level of 3900 ppb, atomic. The low temperature PL measurement exhibits the characteristic emissions of a Ga-doped sample. The Hall measurements show a resistivity of 1 x l0(exp 7) ohm-cm at room temperature to 3 x 10(exp 9) ohm-cm at 78K with the respective hole and electron concentration of 1.7 x 10(exp 9) cm(exp -3) and 3.9 x 10(exp 7) cm(exp -3) at room temperature.

Su, Ching-Hua↗
NASA NTRS2017

Modeling and Measuring Charge-Sharing in Hard X-ray Imagers Using HEXITEC CdTe Detectors

The Rutherford Appleton Laboratory's HEXITEC ASIC has been designed to provide fine pixelated X-ray spectroscopic imaging in combination with a CdTe or CZT detector layer. Although HEXITEC's small pixels enable higher spatial resolution as well as higher spectral resolution via the small-pixel effect, they also increase the probability of charge sharing, a process which degrades spectral performance by dividing the charge induced by a single photon among multiple pixels. In this paper, we investigate the effect of this process on a continuum X-ray spectrum below the Cd and Te fluorescence energies (23 keV). This is done by comparing laboratory measurements with simulations performed with a custom designed model of the HEXITEC ASIC. We find that the simulations closely match the observations implying that we have an adequate understanding of both charge sharing and the HEXITEC ASIC itself. These results can be used to predict the distortion of a spectrum measured with HEXITEC and will help determine to what extent it can be corrected. They also show that models like this one are important tools in developing and interpreting observations from ASICs like HEXITEC.

Ryan, Daniel F.↗
DOE OSTI2021

Multimodal X-ray Microscopy: The Kinetics of Cu in CdTe Absorbers

The behavior of solar cells is very often limited by inhomogeneously distributed nanoscale defects. This is the case throughout the entire lifecycle of the solar cell, from the distribution of elements and defects during solar cell growth as well as the charge-collection and recombination during operation, to degradation and failure mechanisms due to impurity diffusion, crack formation, and irradiation- and heat-induced cell damage This has been known for a while in the field of crystalline silicon, but inhomogeneities are far more abundant in polycrystalline materials, and are the limiting factor in thin-film solar cells where grain sizes are often on the order of the diffusion length. We showed that the high penetration of hard X-rays combined with the high sensitivity to elemental distribution, structure, and spatial resolution offers a unique avenue for highly correlative studies at the nanoscale. We presented results on Cu-doped CdTe, where carrier collection is directly correlated to the kinetics of Cu inside the absorber and the particular Cu-phases at the ZnTe/CdTe interface. We complemented the results with modeling using PVRD-FASP and PyCDTS, where the X-ray beam induced profile can be reproduced by the equilibrium defect concentrations proposed by the solvers.

Bertoni, Mariana↗
DOE OSTI2020

Sputtered Aluminum Oxide and p+ Amorphous Silicon Back-Contact for Improved Hole Extraction in Polycrystalline CdSexTe1-x and CdTe Photovoltaics

A thin layer of Al 2 O 3 at the back of CdSe x T e1-x /CdTe devices is shown to passivate the back interface and drastically improve surface recombination lifetimes and photoluminescent response. Despite this, such devices do not show an improvement in open-circuit voltage (VOC.) Adding a p + amorphous silicon layer behind the Al 2 O 3 bends the conduction band upward, reducing the barrier to hole extraction and improving collection. Further optimization of the Al 2 O 3 , amorphous silicon (a-Si), and indium-doped tin oxide (ITO) layers, as well as their interaction with the CdCl 2 passivation process, are necessary to translate these electro-optical improvements into gains in voltage.

14 SOLAR ENERGY↗
DOE OSTI2020

Materials Data on CdTe(Pb3O5)2 by Materials Project

Pb6CdTeO10 crystallizes in the orthorhombic C222_1 space group. The structure is three-dimensional. Cd2+ is bonded to six O2- atoms to form distorted CdO6 pentagonal pyramids that share edges with two equivalent PbO6 octahedra and edges with two equivalent TeO6 octahedra. There are a spread of Cd–O bond distances ranging from 2.27–2.51 Å. There are four inequivalent Pb2+ sites. In the first Pb2+ site, Pb2+ is bonded in a 2-coordinate geometry to five O2- atoms. There are a spread of Pb–O bond distances ranging from 2.27–2.97 Å. In the second Pb2+ site, Pb2+ is bonded in a 4-coordinate geometry to four O2- atoms. There are two shorter (2.23 Å) and two longer (2.55 Å) Pb–O bond lengths. In the third Pb2+ site, Pb2+ is bonded in a 5-coordinate geometry to five O2- atoms. There are a spread of Pb–O bond distances ranging from 2.26–2.91 Å. In the fourth Pb2+ site, Pb2+ is bonded to six O2- atoms to form distorted PbO6 octahedra that share an edgeedge with one TeO6 octahedra and edges with two equivalent CdO6 pentagonal pyramids. There are a spread of Pb–O bond distances ranging from 2.47–2.74 Å. Te6+ is bonded to six O2- atoms to form TeO6 octahedra that share an edgeedge with one PbO6 octahedra and edges with two equivalent CdO6 pentagonal pyramids. There is four shorter (1.97 Å) and two longer (2.01 Å) Te–O bond length. There are five inequivalent O2- sites. In the first O2- site, O2- is bonded in a 3-coordinate geometry to one Cd2+, three Pb2+, and one Te6+ atom. In the second O2- site, O2- is bonded to one Cd2+ and three Pb2+ atoms to form distorted corner-sharing OCdPb3 tetrahedra. In the third O2- site, O2- is bonded in a 4-coordinate geometry to three Pb2+ and one Te6+ atom. In the fourth O2- site, O2- is bonded in a 4-coordinate geometry to one Cd2+, two Pb2+, and one Te6+ atom. In the fifth O2- site, O2- is bonded to four Pb2+ atoms to form distorted corner-sharing OPb4 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Solution for Predictive Physical Modeling in CdTe and Other Thin-Film PV Technologies

The main goal of this work was to eliminate the ambiguity between the observed performance changes under stress and their physical root cause by enabling a depth of modeling that takes account of diffusion and drift at the atomistic level coupled to the electronic subsystem responsible for a PV device’s function. Development of advanced physical models describing capture/emission/ recombination phenomena relevant to multivalent dopants, amphoteric centers, and donor-acceptor pairs was also accomplished. These models, incorporating grain bulk, grain boundaries, and hetero-interface properties, were implemented in a self-contained simulation tool that drastically reduces interpretation ambiguity and, for the first time, allows for predictive design of thin-film PV devices.

14 SOLAR ENERGY↗
DOE OSTI2026

TCF Base Technology-Specific Final Report: Negative Cracked Film Lithography for PERC CdTe

NLR worked with the world's leading thin-film photovoltaic manufacturer - First Solar, an American company - to develop electrical contacting methods that improve the power conversion efficiency of First Solar's cells by 0.5% absolute. We met this milestone but contrary to our hypothesis, these performance gains are not passivation (PERC)-based. Instead, we identified another mechanism by which our micropatterned Al 2 O 3 contacts improve efficiency. Using the results of this project, we will adapt NLR's cracked film lithography intellectual property (IP) toward the application we identified, which is a new route to commercialization.

14 SOLAR ENERGY↗