The world needs clean, sustainable, and consistent sources of power. Solar energy is an attractive source of renewable power, but energy storage is a challenge for intermittent sources like solar and wind. The massive reduction in space launch costs now makes the prospect of space solar power as a 24/7, consistent baseline source of clean power more of a viable possibility than before. Solar cells designed\r\nto provide power for Earth while in space must not only be low cost and efficient but also light and maintain their performance in harsh space environments, especially in extreme temperature conditions while being subject to damaging space radiation.
\r\n\r\nThis thesis explores two different approaches to achieving radiation hardness: using nanowire solar cells instead of planar cells and using thin-film diffusion-doped cells. It does so with cells made of indium phosphide (InP), which is inherently more resilient to damage from space radiation than materials like silicon (Si) and gallium arsenide (GaAs). The performance of diffusion-doped planar cells is compared to\r\nepitaxially-grown solar cells as a baseline.
\r\n\r\nNanowire cells were simulated in Lumerical to understand the light-trapping properties compared to planar cells with and without surface texturing that also enhances light absorption. The nanowires were simulated with and without hemispherical nanoparticles to enhance light absorption while maintaining a transparent conducting top layer to adequately transport the generated electricity. Nanowire and planar\r\ncells were further simulated in Sentaurus to predict their electrical performance. Nanowire fabrication was attempted as well, but reliable, consistent fabrication was challenging. Given the cost and scalability challenges of this approach, the rest of the work pivots to planar cells.
\r\n\r\nEpitaxially-grown InP cells optimized using Sentaurus were fabricated as a baseline and to work out fabrication challenges adjacent to the p-n junction formation itself. Then, diffusion-doped InP cells were fabricated using Cd\u2083P\u2082 and Zn\u2083P\u2082 as p-type dopants on undoped InP substrates. Preliminary cell performance optimzation was conducted by adjusting diffusion temperatures and times as well as thinning the emitter layer. Efficiencies of up to 4.94% were achieved in Cd-doped cells and up to 3.85% were achieved in Zn-doped cells without anti-reflection coatings, with a maximum JSC of 11.33 mA/cm\u00b2 and a maximum VOC of 778.1 mV in 100 nm thinned Zn-doped cells.
" }, { "name": "Popov, George Arthur", "degree": "PhD", "year": "2026", "title": "Stable Method of Attaching Thin Films to Torsionally Compliant Space Structures", "advisor": "Pellegrino, Sergio", "url": "https://resolver.caltech.edu/CaltechTHESIS:06042026-211328312", "creators": [ { "name": { "family": "Popov", "given": "George Arthur" }, "id": "Popov-George-Arthur", "orcid": "0000-0001-5938-0528", "display_name": "Popov, George Arthur" } ], "advisors": [ { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "advisor", "display_name": "Pellegrino, Sergio" } ], "committee": [ { "name": { "family": "Meiron", "given": "Daniel I." }, "id": "Meiron-D-I", "orcid": "0000-0003-0397-3775", "role": "chair", "display_name": "Meiron, Daniel I." }, { "name": { "family": "Bhattacharya", "given": "Kaushik" }, "id": "Bhattacharya-K", "orcid": "0000-0003-2908-5469", "role": "member", "display_name": "Bhattacharya, Kaushik" }, { "name": { "family": "Pellegrino", "given": "Sergio" }, "id": "Pellegrino-S", "orcid": "0000-0001-9373-3278", "role": "member", "display_name": "Pellegrino, Sergio" }, { "name": { "family": "Shaikeea", "given": "Angkur" }, "id": "Shaikeea-Angkur-J", "orcid": "0000-0002-6706-0492", "role": "member", "display_name": "Shaikeea, Angkur" } ], "option_major": [ "aerospace", "space" ], "doi": "10.7907/0n4d-3v80", "abstract": "Ultralight space structures utilize composite structural elements supporting active thin films in order to achieve deployed configurations with lower areal densities. However, as these space structures get larger, they get increasingly less stiff, leaving them susceptible to adverse effects, such as torsional buckling. Simultaneously, this conflicts with the requirements that more ambitious space missions impose on the surface accuracy to make technologies like large phased arrays viable.
\r\n\r\nThis thesis presents a novel method of continuously attaching thin films to deployable thin shell structures, allowing for materials with widely different coefficients of thermal expansion. It proposes a double s-spring border that exhibits a local post-buckling behavior and provides a tunable continuous edge attachment method that can maintain constant preload under large thermal strains. The mechanical behavior of the double s-spring under different mechanical loading conditions is studied both numerically and experimentally. Additionally, a reduced order model that greatly reduces the computational cost of optimizing double s-springs for various applications is presented.
\r\n\r\nIn parallel, the torsional buckling of elastic composite frames is studied to understand the fundamental limits of attaching thin films without incurring torsional buckling. This study analytically calculates the critical prestress of torsionally soft square frames supporting an internal thin film. The study highlights the role of the attachment scheme, which has a very significant impact on the critical prestress. It is shown that the average of the compression load on a frame caused by a prestress is an invariant buckling load. The analytical calculation is verified via numerical finite element analyses and an experiment, which characterizes the post-buckling behavior of the torsionally soft frames as well. It is concluded that distributed edge attachments, such as the double s-spring, significantly increase the stability of space structures against torsional buckling. The findings for the torsionally soft square frames are validated against a high fidelity orthotropic material model in order to justify the assumptions made in the analytical study.
\r\n\r\nIn the final section of the thesis, it is shown that the double s-spring continuous attachment scheme enables consistent deployment and compact packaging. The scheme is also shown to be versatile for additional applications, enabling novel deployment and folding schemes, such as a doubly-curved composite foldable toroid.
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