Periodic Reporting for period 3 - RadHard (Ultra High Efficiency Radiation Hard Space Solar Cells on Large Area Substrates)
Reporting period: 2020-01-01 to 2022-01-31
Project RadHard aims for increasing both the technical and commercial competitiveness of the European space solar cell technology to maintain the independence of European space industry in this field. RadHard will demonstrate the future next generation of space solar cells featuring i) beginning-of-life efficiency exceeding 35% under AM0 condition enabled by a new, patent protected 4-junction space solar cell, ii) the world’s highest radiation hardness leading to an efficiency >31% after 1E15 cm-2 1MeV electron irradiation, iii) scaling of the solar cell manufacturing to 200mm wafer size to enable competitive cost of the product and iv) demonstration of manufacturability and reliability of this cell concept. Readiness levels (TRL) for relevant technologies will be increased from TRL 3 to 5-6.
The project makes use of technology innovations in solar cells design, epitaxy, semiconductor bonding and ultra large Ge wafers. The work plan is based on a parallel development of the new solar cell by semiconductor bonding and establishing solar cell manufacturing processes on 200 mm Ge wafers. At the end of the project, these development lines will be merged to demonstrate the commercial viability of the selected approach. Technology development activities will be accompanied by extensive test program to allow for continuous feedback on the achieved device performance and to address reliability aspects. Finally an industrialization plan for the new 4-junction semiconductor bonded solar cell will be elaborated.
The project team is led by AZUR SPACE Solar Power GmbH, Germany and consists of 7 industrial partners (incl. 1 SME) and 2 academic institutes and covers all R&D aspects, from basic research on advanced materials at academic partners to device manufacturing in industrial environment and testing on higher integration level. The relevance of the team for commercial exploitation is extremely high: RadHard includes industrial partners from each of the main parts of the value chain for space solar generators.
The project makes use of technology innovations in solar cells design, epitaxy, semiconductor bonding and ultra large Ge wafers. The work plan is based on a parallel development of the new solar cell by semiconductor bonding and establishing solar cell manufacturing processes on 200 mm Ge wafers. At the end of the project, these development lines will be merged to demonstrate the commercial viability of the selected approach. Technology development activities will be accompanied by extensive test program to allow for continuous feedback on the achieved device performance and to address reliability aspects. Finally an industrialization plan for the new 4-junction semiconductor bonded solar cell will be elaborated.
The project team is led by AZUR SPACE Solar Power GmbH, Germany and consists of 7 industrial partners (incl. 1 SME) and 2 academic institutes and covers all R&D aspects, from basic research on advanced materials at academic partners to device manufacturing in industrial environment and testing on higher integration level. The relevance of the team for commercial exploitation is extremely high: RadHard includes industrial partners from each of the main parts of the value chain for space solar generators.
The new four junction space solar cell requested developing a number of new metamorphic materials and structures, in particular InGaAsP and InGaP sub-cell materials, optically transparent tunnel diodes and bond layers as well as metamorphic buffer to grow III/V cell stack of Ge substrate. It is noteworthy that additionally to high lattice mismatch to Ge, the requested composition of quaternary InGaAsP material was earlier predicted to be in a miscibility gap for As- and P-containing compounds, thus, is hardly achievable using typical growth parameters of a metalorganic epitaxy process. Further, the parts of the developed cell structure were directly bonded together, for which a corresponding chemical-mechanical polishing as well as wafer bonding processes were developed and optimised. Here, higher roughness and bow of metamorphic layers and high Indium content were remarkable challenges for CMP and wafer bonding, respectively. After multiple development rounds, each with several design optimisations, a 4J SBT cell with an BOL efficiency of 31% and an EOL efficiency of 25,5% (after 3E15 cm2 1MeV electron irradiation) could be demonstrated.
It is to note that the EOL efficiency could be confirmed after the formal end of the project due to being out of schedule for EOL characterization. With certain improvements identified in the project, the practical BOL efficiency has the potential to be further increased reaching the target of 32-33% for this cell design. These values are very close to the theoretically predicted cell performance potential for this particular cell design. The 4J SBT cells achieved in the project showed very high radiation hardness and successfully passed thermal cycling according to typical space requirements and, thus, demonstrated their technical potential for implementation into a future cell product, especially focussing the operation in radiation reach environments.
In parallel, crystal growth of dislocation free germanium ingots with large diameters was achieved and a pilot-line for wafer manufacturing was successfully realised. The surface quality of resulting 200 mm wafers achieves the grade of commercial 150 mm substrates. The 200 mm epi-ready Ge wafers were used for realisation of 3G30 solar cell structures used as a benchmark. High wafer-to-wafer and run-to-run reproducibility regarding the epitaxy process as well as excellent device performance with an average cell efficiency of 29.9% was for the first time achieved confirming industrial feasibility of both, wafer and cell manufacturing with 200mm wafer size. The cell reliability was justified by engineering tests, in particular thermal cycling performed on cell coupon level. Finally, the growth of solar cell structures on two separate 200 mm Ge substrates and their subsequent merging by the direct semiconductor bonding technology was demonstrated to confirm the potential of cost reduction for this promising technology by using large area substrates.
Project results were disseminated in 6 publications in scientific journals (incl. conference proceedings) and covered all main fields of the project. 4 of these publications were joint publications involving multiple partners. Moreover, the achieved results were communicated in 7 conferences and workshops without proceedings. At the end of the project, a scientific workshop on space solar cells was organised and held virtually.
It is to note that the EOL efficiency could be confirmed after the formal end of the project due to being out of schedule for EOL characterization. With certain improvements identified in the project, the practical BOL efficiency has the potential to be further increased reaching the target of 32-33% for this cell design. These values are very close to the theoretically predicted cell performance potential for this particular cell design. The 4J SBT cells achieved in the project showed very high radiation hardness and successfully passed thermal cycling according to typical space requirements and, thus, demonstrated their technical potential for implementation into a future cell product, especially focussing the operation in radiation reach environments.
In parallel, crystal growth of dislocation free germanium ingots with large diameters was achieved and a pilot-line for wafer manufacturing was successfully realised. The surface quality of resulting 200 mm wafers achieves the grade of commercial 150 mm substrates. The 200 mm epi-ready Ge wafers were used for realisation of 3G30 solar cell structures used as a benchmark. High wafer-to-wafer and run-to-run reproducibility regarding the epitaxy process as well as excellent device performance with an average cell efficiency of 29.9% was for the first time achieved confirming industrial feasibility of both, wafer and cell manufacturing with 200mm wafer size. The cell reliability was justified by engineering tests, in particular thermal cycling performed on cell coupon level. Finally, the growth of solar cell structures on two separate 200 mm Ge substrates and their subsequent merging by the direct semiconductor bonding technology was demonstrated to confirm the potential of cost reduction for this promising technology by using large area substrates.
Project results were disseminated in 6 publications in scientific journals (incl. conference proceedings) and covered all main fields of the project. 4 of these publications were joint publications involving multiple partners. Moreover, the achieved results were communicated in 7 conferences and workshops without proceedings. At the end of the project, a scientific workshop on space solar cells was organised and held virtually.
Progress beyond the state of the art after the end of the project is represented by the following aspects:
1. Demonstration of a 4-junction space solar cell based on wafer bonding tehnology. The cell technology is extendable to 5- and 6-junction cell archtectures.
2. Demonstration of a BOL cell efficiency of 31% at AM0 conditions, with further improvement potential towards 32%
3. Demonstration of i) an EOL cell efficiency of 25.5% after 3E15 cm-2 1 MeV equivalent electron Irradiation, which is 6% better than corresponding EOL efficiencies of AZUR 3G30 solar cell currently dominating the market.
4. Demonstration of industrial technology for 200 mm Ge substrates with epi-ready quality, which improves the european competitiveness against non-european manufacturers still limited to 150mm.
The following impacts of the project are recognised:
1. Next generation solar cell product candidate for European space industry based on novel 4J SBT cell
2. High quality Ge substrates for cost effective cell manufacturing by availability of 200mm wafer diameter .
3. Extension and strengthening of the fully European value chain for space solar cells by cooperation of partners along the value chain for space solar cells.
4. Complementing the activities of European and national space programmes, as there are no similar projects funded by EU, ESA or national agencies
5. Increasing competitiveness and expertise of European industry and academia in the space domain
6. Strengthening competitiveness of industry outside the space domain due to relevance of technology development in RadHard for realization of (opto)electonic devices
7. New market opportunities for semiconductor bonding technology by demonstrated potential for complex devices structures based on ternary III/V semiconductor materials on wafer area up to 200mm
8. Contribution to environmental impacts by established an updated life cycle analysis for Germanium, which is considered as critial material.
1. Demonstration of a 4-junction space solar cell based on wafer bonding tehnology. The cell technology is extendable to 5- and 6-junction cell archtectures.
2. Demonstration of a BOL cell efficiency of 31% at AM0 conditions, with further improvement potential towards 32%
3. Demonstration of i) an EOL cell efficiency of 25.5% after 3E15 cm-2 1 MeV equivalent electron Irradiation, which is 6% better than corresponding EOL efficiencies of AZUR 3G30 solar cell currently dominating the market.
4. Demonstration of industrial technology for 200 mm Ge substrates with epi-ready quality, which improves the european competitiveness against non-european manufacturers still limited to 150mm.
The following impacts of the project are recognised:
1. Next generation solar cell product candidate for European space industry based on novel 4J SBT cell
2. High quality Ge substrates for cost effective cell manufacturing by availability of 200mm wafer diameter .
3. Extension and strengthening of the fully European value chain for space solar cells by cooperation of partners along the value chain for space solar cells.
4. Complementing the activities of European and national space programmes, as there are no similar projects funded by EU, ESA or national agencies
5. Increasing competitiveness and expertise of European industry and academia in the space domain
6. Strengthening competitiveness of industry outside the space domain due to relevance of technology development in RadHard for realization of (opto)electonic devices
7. New market opportunities for semiconductor bonding technology by demonstrated potential for complex devices structures based on ternary III/V semiconductor materials on wafer area up to 200mm
8. Contribution to environmental impacts by established an updated life cycle analysis for Germanium, which is considered as critial material.