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3. High figure of merit spin polarized electron sources grown via MOCVD

Author

Belfore, Benjamin, primary, Masters, Adam, additional, Poudel, Deewakar, additional, Blume, Greg, additional, Polly, Stephen, additional, Wang, Erdong, additional, Hubbard, Seth M, additional, Stutzman, Marcy, additional, Grames, Joseph Michael, additional, Poelker, Matt, additional, Grau, Matt, additional, and Marsillac, Sylvain, additional

Published
2023
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10. Development of High Quantum Efficiency Strained Superlattice Spin Polarized Photocathodes Via Metal Organic Chemical Vapor Deposition

Author

Belfore, Benjamin

Abstract

pin polarized photocathodes are necessary to examine parity violations and other fundamental phenomena in the field of high energy physics. To create these devices, expensive and complicated growth processes are necessary. While integral to accelerator physics, spin polarized electrons could have other exciting applications in materials science and other fields of physics. In order to explore these other applications feasibly, the relative supply of spin polarized photocathodes with a high rate of both polarization and photoemission needs to be increased. One such way to increase this supply is to develop the means to grow them faster and at a larger scale. Because most photocathodes are grown in slow, small-scale processes like Molecular Beam Epitaxy, an alternative needed to be found. In the following work, strained superlattice photocathodes were fabricated using metal organic vapor phase epitaxy. Using this growth process, we demonstrated that it is possible to create high quality photocathodes at a higher rate while maintaining a high quality of polarization and quantum efficiency, thus allowing for the exploration of other applications for these devices.

Published
2022
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20. High-Rate and Low-Temperature Fabrication of Cu(In,Ga)Se2 Solar Cells Using AgBr Induced Recrystallization

Author

Belfore, Benjamin, Poudel, Deewakar, Lepetit, Thomas, Palmiotti, Elizabeth, Ashrafee, Tasnuva, Rockett, Angus, Barreau, Nicolas, and Marsillac, Sylvain

Abstract

We present results demonstrating a three-stage, high-rate, low-temperature deposition process for Cu(In,Ga)Se2 solar cells including a recrystallization step after the second stage, catalyzed by AgBr. The entire deposition and recrystallization process takes place below 450 °C in less than 15 min. Device results with efficiencies exceeding 16% are shown. We achieve very high levels of grain growth and a greater than a 5% absolute increase in device performance because of the recrystallization step relative to an identical process not involving AgBr.

Published
2022
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27. Degradation Mechanism in Cu(In,Ga)Se2 Material and Solar Cells Due to Moisture and Heat Treatment of the Absorber Layer

Author

Karki, Shankar, primary, Paul, Pran, additional, Deitz, Julia I., additional, Poudel, Deewakar, additional, Rajan, Grace, additional, Belfore, Benjamin, additional, Danilov, Evgeny O., additional, Castellano, Felix N., additional, Grassman, Tyler J., additional, Arehart, Aaron, additional, Rockett, Angus, additional, and Marsillac, Sylvain, additional

Published
2019
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32. Impact of Water Ingress on Molybdenum Thin Films and Its Effect on Cu(In,Ga)Se2 Solar Cells

Author

Karki, Shankar, Deitz, Julia I., Rajan, Grace, Soltanmohammad, Sina, Poudel, Deewakar, Belfore, Benjamin, Bhandari, Gandhari, Grassman, Tyler J., Rockett, Angus, and Marsillac, Sylvain

Abstract

Solar cell degradation can occur through many pathways and at the different stages of the fabrication process, notably because of the water condensation. In the case of Cu(In,Ga)Se2 (CIGS) solar cells, the impact of water ingress on Mo back contact can be substantial for not only the film itself but also the device performance and reliability. Microstructural modification, change in film morphology, loss in reflectance, and increased resistivity because of the moisture ingress were observed via X-ray diffraction, transmission electron microscopy, spectroscopic ellipsometry, and four-point probe measurements, respectively. Secondary ion mass spectrometry measurements revealed drastic changes in the alkali (Na, K) profiles in both the CIGS and Mo layers, likely because of the modification of their diffusion coefficient through Mo. This, in turn, negatively impacts the solar cell efficiency by decreasing both fill factor and open-circuit voltage. Additional experiments, modifying the substrate and utilizing a NaF postdeposition treatment, highlight the mechanisms of degradation as being due to both a modification of the Mo/CIGS interface and the lack of alkali diffusion after water ingress.

Published
2020
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33. Degradation Mechanism in Cu(In,Ga)Se2 Material and Solar Cells Due to Moisture and Heat Treatment of the Absorber Layer

Author

Karki, Shankar, Paul, Pran, Deitz, Julia I., Poudel, Deewakar, Rajan, Grace, Belfore, Benjamin, Danilov, Evgeny O., Castellano, Felix N., Grassman, Tyler J., Arehart, Aaron, Rockett, Angus, and Marsillac, Sylvain

Abstract

The impact of moisture and heat treatment on the microstructural, chemical, and electrical properties of Cu(In,Ga)Se2 films and their collective effect on the solar cell device performance was studied. X-ray photoelectron spectroscopy and secondary ion mass spectroscopy measurements show that water exposure causes surface modification and alters the alkali metal distribution, while no composition or structural effect was observed. Deep level transient and optical spectroscopies revealed that the trap densities (NT) for both the EV + 0.65 eV and EV + 0.98 eV traps increase after water exposure, while the majority carrier concentration (NA) decreases. Time-resolved photoluminescence (PL) and steady-state PL measurements indicated the presence of static, not dynamic, quenching. Reduction of open-circuit voltage (VOC) and fill factor (FF) was observed for the devices but was not associated with a change of recombination mechanism, which remains in the absorber space charge region. A small increase in series resistance and shunt conductance accounts for most of the FF change, while the modification in both NA and NT yield most of the change in VOC. A gradient of majority carrier concentration, related to the alkali profile, also yields a small voltage-dependent current collection after moisture and heat treatment.

Published
2019
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34. Analysis of Post-Deposition Recrystallization Processing via Indium Bromide of Cu(In,Ga)Se 2 Thin Films.

Author

Poudel, Deewakar, Belfore, Benjamin, Ashrafee, Tasnuva, Karki, Shankar, Rajan, Grace, Rockett, Angus, and Marsillac, Sylvain

Subjects
*THIN films, *INDIUM, *BROMIDES, *COPPER indium selenide, *GALLIUM, *SELENIUM
Abstract

Cu(In,Ga)Se2 (CIGS) thin films were deposited at low temperature (350 °C) and high rate (10 µm/h) by a single stage process. The effect of post-deposition treatments at 400 °C and 500 °C by indium bromide vapor were studied and compared to the effect of a simple annealing under selenium. Structural, electrical, and chemical analyses demonstrate that there is a drastic difference between the different types of annealing, with the ones under indium bromide leading to much larger grains and higher conductivity. These properties are associated with a modification of the elemental profiles, specifically for gallium and sodium. [ABSTRACT FROM AUTHOR]

Published
2021
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35. In Situ Recrystallization of Co-Evaporated Cu(In,Ga)Se 2 Thin Films by Copper Chloride Vapor Treatment towards Solar Cell Applications.

Author

Poudel, Deewakar, Belfore, Benjamin, Ashrafee, Tasnuva, Palmiotti, Elizabeth, Karki, Shankar, Rajan, Grace, Lepetit, Thomas, Rockett, Angus, and Marsillac, Sylvain

Subjects
*COPPER chlorides, *COPPER films, *SOLAR cells, *SECONDARY ion mass spectrometry, *PHOTOVOLTAIC power systems, *THIN film devices, *THIN films, *COPPER indium selenide
Abstract

Cu(In,Ga)Se2 (or CIGS) thin films and devices were fabricated using a modified three-stage process. Using high deposition rates and a low temperature during the process, a copper chloride vapor treatment was introduced in between the second and third stages to enhance the films properties. X-ray diffraction and scanning electron microscopy demonstrate that drastic changes occur after this recrystallization process, yielding films with much larger grains. Secondary ion mass spectrometry shows that the depth profile of many elements is not modified (such as Cu, In and Se) while others change dramatically (such as Ga and Na). Because of the competing effects of these changes, not all parameters of the solar cells are enhanced, yielding an increase of 15% in the device efficiency at the most. [ABSTRACT FROM AUTHOR]

Published
2021
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