Pushing Radiation Hardness and Qualification of Ultrathin Silicon Solar Cells

Regher Solar proposes this SBIR project to mature ultrathin silicon (UT-Si) solar cell technology to achieve TRL 7 and quickly transition to TRL 8 followed by injection into both NASA and commercial missions.At present UT-Si cells manufactured by Regher Solar have a 20% Beginning-of-Life (BOL) efficiency which is exactly in between Copper-Indium-Gallium-Selenide (CIGS) and Epitaxial Lift Off Inverted Metamorphic (ELO-IMM) thin film solar cells that are currently considered for making flexible solar blankets. With several practically attainable improvements UT-Si solar cells will reach 22% BOL efficiency in 2 years. However, the End-of-Life (EOL) efficiency of UT-Si cells drops substantially when exposed to space radiation making them less attractive for the use in space. If radiation damage is mitigated, UT-Si cells can achieve EOL efficiency of ELO-IMM cells while being as inexpensive as CIGS cells making them the optimum choice for flexible solar arraysamong all thin filmtechnologies. This project will leverage an improved understanding of radiation-induced defects in c-Si that was developed in the last 3 years within the effort to fabricate more radiation hard Si detectors for the Large Hadron Collider. The main proposed innovations include: (1) using defect engineering to passivate radiation induced defects, (2) further reducing solar cell thickness from 20 to 10 microns to improve the effectiveness of passivation, and (3) utilizing active defect elimination methods that can be periodically applied to the solar cells in space. Phase II of this project will demonstrate the feasibility ofthe proposed innovationsand will conduct comprehensive electron and proton irradiation testing. We will collaborate with blanket manufacturers to package UT-Si solar cells in CICs and blankets and conduct complete qualification to achieve TRL 7. Phase II will also work with development partners to integrate UT-Si cells into ongoing missions and achieve TRL 8. Si solar cells have a low cost and a competitive BOL efficiency, but poor radiation hardness and high mass. Regher Solar has previously demonstrated 20-um-thick Si solar cells with 20% AM0 efficiency in pilot R&D. This increased BOL specific power of Si solar cells from 600W/kg to 5,000W/kg, made them flexible and enabled using thin Si solar cells in ultracompact flexible solar arrays that are needed for small spacecrafts and Lunar missions. However, EOL efficiency of Si cells remains low making them less attractive. If radiation damage is mitigated, thin Si cells in high radiation environments can achieve EOL efficiency of IMM and the cost of CIGS cells. In the last several years a better understanding of radiation defects in Si was developed to fabricate more rad hard Si detectors for the Large Hadron Collider. Based on this new data Regher Solar conceptualized two novel defect mitigation strategies that will be tested in this project. If successful, the defects formed by radiation damage will be cured in-situ and Si solar cell with 20% EOL efficiency will become possible. This project has four overall objectives reaching beyond the scope of Phase II: • Develop thin-film UT-Si solar cells for flexible solar arrays being developed by NASA with BOL efficiency above 20% and the highest EOL efficiency for the high radiation environments among all other thin film solar cell technologies. • Conduct qualification testing of UT-Si solar cell to achieve TRL 6. • In collaboration with development partners package and fly UT-Si solar cells in a prototype solar array to achieve TRL 7 and quickly transition to TRL 8 through the integration into small spacecraft missions of commercial partners. The main goal of Phase II is to demonstrate the feasibility of almost complete mitigation of radiation induced recombination defects in silicon. Specific objectives of the proposed Phase II project are the following: • Test the first strategy to mitigate radiation defects. • Compare the effectiveness of the first defect mitigation strategy in 20- and 10-um-thick UT-Si cells. • Test the second defect mitigation strategy. • Test the proposed methods for fully penetrating 1 MeV electrons and 3 MeV protons with fluences up to 5e15 cm-2 and 1e12 cm-2 respectively and for low energy protons including 50, 100 and 500 keV with fluences up to 1e12 cm-2. • Conduct complete radiation testing in accordance with AIAA S-111A-2014 standard.
Benefits: UT-Si solar cells can be integrated into novel flexible solar array deployment systems to meet NASA solar array specific power (250 W/kg) and stowed volume efficiency (50 kW/m3) goals. At the same time UT-Si solar cells have a potential to also meet NASA goals for the long-term operation in high radiation environment (1 MeV 6e15 e/cm2). Together this will make UT-Si solar cell technology an ideal choice for several NASA projects including LISA solar array, Vertical Lunar Solar Arrays and large scale solar arrays for Solar Electric Propulsion. The main advantage of UT-Si technology is compatibility with high volume manufacturing and a low manufacturing cost. Production of UT-Si solar cells can be quickly scaled to 100 MW/year to meet the demand of the growing space industry. The example applications include satellite mega constellations and space based solar power that will need tens of MW of affordable space-stable solar cells.