Engineering Papers⌕ Search

Engineering topics

Metzger, W. K.

Publications and source records attributed to Metzger, W. K..

18 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Approach to Defect-Free Lifetime and High Electron Density in CdTe

Achieving simultaneously high carrier density and lifetime is crucial to II-VI semiconductor-based applications such as photovoltaics and infrared detectors; yet, it is a challenging task. In this work, high purity CdTe single crystals doped with indium (In) were grown by vertical Bridgman melt growth under carefully controlled stoichiometry. Two-photon excitation time-resolved photoluminescence was employed to measure bulk recombination lifetime by eliminating surface recombination effects. By adjusting stoichiometry with post growth annealing, high-net free carrier density approaching 10 18 cm -3 was attained simultaneously with lifetime approaching the radiative limit by suppressing non-radiative recombination centers.

14 SOLAR ENERGY↗
DOE OSTI2018

Experimental and theoretical comparison of Sb, As, and P diffusion mechanisms and doping in CdTe

Fundamental material doping challenges have limited CdTe electro-optical applications. In this work, the As atomistic diffusion mechanisms in CdTe are examined by spatially resolving dopant incorporation in both single-crystalline and polycrystalline CdTe over a range of experimental conditions. Density-functional theory calculations predict experimental activation energies and indicate As diffuses slowly through the Te sublattice and quickly along GBs similar to Sb. Because of its atomic size and associated defect chemistry, As does not have a fast interstitial diffusion component similar to P. Experiments to incorporate and activate P, As, and Sb in polycrystalline CdTe are conducted to examine if ex-situ Group V doping can overcome historic polycrystalline doping limits. The distinct P, As, and Sb diffusion characteristics create different strategies for increasing hole density. Because fast interstitial diffusion is prominent for P, less aggressive diffusion conditions followed by Cd overpressure to relocate the Group V element to the Te lattice site is effective. For larger atoms, slower diffusion through the Te sublattice requires more aggressive diffusion, however further activation is not always necessary. Based on the new physical understanding, we have obtained greater than 10^16 cm^-3 hole density in polycrystalline CdTe films by As and P diffusion.

14 SOLAR ENERGY↗
DOE OSTI2017

The roles of carrier concentration and interface, bulk, and grain-boundary recombination for 25% efficient CdTe solar cells

CdTe devices have reached efficiencies of 22% due to continuing improvements in bulk material properties, including minority carrier lifetime. Device modeling has helped to guide these device improvements by quantifying the impacts of material properties and different device designs on device performance. One of the barriers to truly predictive device modeling is the interdependence of these material properties. For example, interfaces become more critical as bulk properties, particularly, hole density and carrier lifetime, increase. We present device-modeling analyses that describe the effects of recombination at the interfaces and grain boundaries as lifetime and doping of the CdTe layer change. The doping and lifetime should be priorities for maximizing open-circuit voltage (V oc ) and efficiency improvements. However, interface and grain boundary recombination become bottlenecks for device performance at increased lifetime and doping levels. In conclusion, this work quantifies and discusses these emerging challenges for next-generation CdTe device efficiency.

14 SOLAR ENERGY↗
DOE OSTI2015

Intrinsic surface passivation of CdTe

Recombination is critically limiting in CdTe devices such as solar cells and detectors, with much of it occurring at or near the surface. In this work, we explore different routes to passivate p-type CdTe surfaces without any intentional extrinsic passivation layers. To provide deeper insight into the passivation routes, we uniquely correlate a set of characterization methods: surface analysis and time-resolved spectroscopy. We study two model systems: nominally undoped single crystals and large-grain polycrystalline films. We examine several strategies to reduce surface recombination velocity. First, we study the effects of removing surface contaminants while maintaining a near-stoichiometric surface. Then we examine stoichiometric thermally reconstructed surfaces. Here, we also investigate the effects of shifting the surface stoichiometry by both “subtractive” (wet chemical etches) and “additive” (ampoule anneals and epitaxial growth) means. We consistently find for a variety of methods that a highly ordered stoichiometric to Cd-rich surface shows a significant reduction in surface recombination, whereas a Te-rich surface has high recombination and propose a mechanism to explain this. While as-received single crystals and as-deposited polycrystalline films have surface recombination velocities in the range of 10 5 –10 6 cm/s, we find that several routes can reduce surface recombination velocities to <2.5 × 10 4 cm/s.

14 SOLAR ENERGY↗
DOE OSTI2015

High-efficiency, flexible CdTe solar cells on ultra-thin glass substrates

Flexible, high-efficiency, low-cost solar cells can enable applications that take advantage of high specific power, flexible form factors, lower installation and transportation costs. Here, we report a certified record efficiency of 16.4% for a flexible CdTe solar cell that is a marked improvement over the previous standard (14.05%). The improvement was achieved by replacing chemical-bath-deposited CdS with sputtered CdS:O and also replacing the high-temperature sputtered ZnTe:Cu back contact layer with co-evaporated and rapidly annealed ZnTe:Cu. We use quantum efficiency and capacitance-voltage measurements combined with device simulations to identify the reasons for the increase in efficiency. Both device simulations and experimental results show that higher carrier density can quantitatively account for the increased open circuit voltage (V{sub OC}) and Fill Factor (FF), and likewise, the increase in short circuit current density (J{sub SC}) can be attributed to the more transparent CdS:O.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

14%-efficient flexible CdTe solar cells on ultra-thin glass substrates

Flexible glass enables high-temperature, roll-to-roll processing of superstrate devices with higher photocurrents than flexible polymer foils because of its higher optical transmission. Using flexible glass in our high-temperature CdTe process, we achieved a certified record conversion efficiency of 14.05% for a flexible CdTe solar cell. Little has been reported on the flexibility of CdTe devices, so we investigated the effects of three different static bending conditions on device performance. We observed a consistent trend of increased short-circuit current and fill factor, whereas the open-circuit voltage consistently dropped. The quantum efficiency under the same static bend condition showed no change in the response. After storage in a flexed state for 24 h, there was very little change in device efficiency relative to its unflexed state. This indicates that flexible glass is a suitable replacement for rigid glass substrates, and that CdTe solar cells can tolerate bending without a decrease in device performance.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Cd-rich and Te-rich low-temperature photoluminescence in cadmium telluride

Low-temperature photoluminescence emission spectra were measured in cadmium telluride (CdTe) samples in which composition was varied to promote either Cd or Te-rich stoichiometry. The ability to monitor stoichiometry is important, since it has been shown to impact carrier recombination. Te-rich samples show transitions corresponding to acceptor-bound excitons (∼1.58 eV) and free-electron to acceptor transitions (∼1.547 eV). In addition to acceptor-bound excitons, Cd-rich samples show transitions assigned to donor-bound excitons (1.591 eV) and Te vacancies at 1.552 eV. Photoluminescence is a noninvasive way to monitor stoichiometric shifts induced by post-deposition anneals in polycrystalline CdTe thin films deposited by close-spaced sublimation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗