Photovoltaic Technologies Developed for Space-Based Solar Power

Citation

Loke, Samuel Pei Hao (2022) Photovoltaic Technologies Developed for Space-Based Solar Power. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/6jhy-2623. https://resolver.caltech.edu/CaltechTHESIS:05142022-060038327

Abstract

In this thesis, photovoltaic technologies were developed for space-based solar power. Two methods of realizing SBSP were introduced, namely concentrated photovoltaics (CPV) and radiation hard flat panel photovoltaics. Both techniques are instrumental to realizing SBSP as they are pathways to realizing high specific power and lower launch costs. Technologies developed to support these two forms of SBSP were then reported.

In support of CPV, ultralight broadband mid-infrared coatings were developed for the concentrating mirrors used in our project. This was done to create radiative pathways for heat loss to ensure that the solar cells do not overheat. Using the rigorous coupled wave analysis technique, we optimized a backside single-layer coating using 2nm Cr/ 2μm CP1/ 500nm Ag that had an mIR emissivity of 0.6. Adding a second layer of this coating, we predicted that a 0.5nm Cr/ 1.9µm CP1/ 3nm Cr/ 2µm CP1/ 500nm Ag screen could achieve an emissivity of 0.8. We also optimized a 10nm ITO/ 2 μm CP1/ 500nm Ag frontside emitter which had a visible reflectivity of 0.896 and a mIR emissivity of 0.554. A backside emitter coating that was 0.927 emissive in the mIR with areal density 6.0 gm⁻² was successfully fabricated, as was a frontside mirror emitter coating with visible reflectivity of 0.896 and a mIR emissivity of 0.582 with areal density 4.1 gm⁻².

In support of radiation hard photovoltaics, organo-lead halide perovskites (OHLP) were investigated. Challenges facing their fabrication were explored, with special focus on the electron transport layer PCBM as well as OHLP formulation. It was found that doping PCBM with a surfactant CTAB was beneficial, but did not work with all surfaces. An ITO/NiOx/MAPbI3/CTAB+PCBM/Cu device with in-house champion efficiency of 12.41% was achieved, and an ITO/NiOx/FA_0.85Cs_0.15PbI₃/PCBM/Cu device with in-house champion efficiency of 11.81% was achieved. Time-dependant drift diffusion modelling was employed to account for the S-kink arising from poor PCBM carrier concentration.

Finally, the proton degradation of OHLP devices and constituent transport layers were investigated to shed better light on how OHLP devices degrade under proton irradiation. Films of ITO, PEDOT, NiOx, PCBM, and PTCDi were found to degrade under 30keV and 75keV protons of up to 1.4 x 10¹⁴ p⁺cm⁻² fluence, but their electrical resistivity and optical transmissivity were not found to impact the cell as much as the OHLP absorber layer itself. Observing the light IV and EQE degradation of OHLP cells, it is evident that proton deposition in the OHLP layer itself causes the most damage, especially at 30keV and 75keV protons with fluences from 4.3 x 10¹³ p⁺cm⁻² to 1.7 x 10¹⁴ p⁺cm⁻². By considering the discrepancy in trends between Jsc and EQE, we concluded that the protons much accelerate intensity-based metastable photodegradation. Finally, by observing their anneal recovery, we concluded that it was temperature dependant and that maximum irrecoverable damage occurs at the OHLP/HTL interface.

Thesis Files