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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - RadHard (Ultra High Efficiency Radiation Hard Space Solar Cells on Large Area Substrates)

Teaser

Summary

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 worldâ€™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.

Work performed

The first project period (01.01.2018 -31.12.2018) was dedicated to the defining an optimal ultra-radiation-hard cell structure, establishing and verification of a basic semiconductor wafer bonding process route, demonstration of first 4-junction cell prototypes and manufacturing of very first 200 mm Germanium substrates. These activities have been performed according to the project plan. All deliverables and milestones have been successfully achieved

The main achievements of the project towards the main objectives within the reporting period are as follows:

Main Objective 1. Demonstration of an ultra-radiation hard four-junction space solar cell
- A detailed two-dimensional simulation model of a 4-junction cell allowing prediction of the cell performance under realistic space conditions was developed and two most promising cell designs were identified and modelled considering technological feasibility aspects;
- For one selected design, a metamorphic buffer layer on Ge substrate was developed for the subsequent epitaxy of bottom part of the 4-junction cell structure and 40 epi-ready metamorphic substrates with the required lattice parameter were fabricated. In parallel, different combinations of transparent bond layers and tunnel diode layers were evaluated. Based on the ontained data, one of the most transparent material combination was selected;
- A chemo-mechanical polishing process for top and bottom parts of the targeted cell structure has been developed. For both parts, a surface roughness of less than 0.5 nm (basic criterion for wafer bonding process) could be successfully demonstrated. A semiconductor bonding process has been successfully demonstrated using first relevant cell structures. Different process failure causes (wafer breakage, large and small voids, indium droplet formation) were identified and mitigation actions proposed and evaluated;
- First complete 4-junction solar cells were fabricated using concentrator solar cell design. The results of external quantum efficiency (EQE) measurement show up that the target band gaps of all 4 sub-cells were met accurately and with sufficiently high material quality. High fill factor (86.4%) and voltage (3.143 V) defined in light IV measurements confirm full functionality of all sub-cells and tunnel diodes.

Main Objective 2. Demonstration of solar cell manufacturing to 200 mm Germaium wafer size
- A specification for 200mm Ge wafer was established;
- The first epi-ready 20 Ge wafers were manufactured and are available for epitaxy trials;
- Drafts of recycling route and life cycle analysis of Germanium have been prepared;

In the framework of dissemination and communication activities, project webpage has been online since June 2018. Overall project activity and achieved results have been presented at EU Solar Cell Workshop on the 22th -23rd November 2018 in Brussels.

Final results

Progress beyond the state of the art after the end of the project is expected for the following aspects:

1. Demonstration of a 4-junction space solar cell with little material constraints due to wafer bonding. The cell technology will be extendable to 5- and 6-junction cell archtectures.
2. Demonstration of a BOL cell efficiency of above 35% at AM0 conditions vs. the state of the art of 30.7%.
3. Demonstration of i) an EOL cell efficiency of > 31% after 1E15 cm-2 1 MeV equivalent electron Irradiation vs. the state of the art of 26.8% and ii) an EOL cell efficiency of > 27% after 3E15 cm-2 1 MeV equivalent electron Irradiation vs. the state of the art of 23.3%;
4. Development and demonstration of a solar cell technology om Germanium substrates with 200 mm diameter vs. the state-of-the-art of 100 and 150 mm substrates.

Successfull achievement of these objectives will be a key enabler for the next generation solar cell product for European space industry allowing extention and strengthening of the fully European value chain for space solar cells. Moreover, the project will also open new market opportunities for SB technology in compound semiconductor device manufacturing (e.g. power devices, acoustic wave guides) and in the production of engineered substrates (materials like Silicon Carbide and Lanthanium Tantalate). In RadHard, academic and industrial partners work in close cooperation. This leads to long term research partnerships beneficial for further R&D projects. The intensive involvement of (PhD)students at academic partners ISE and CEA Sirius into the project activity serves professional educational and training objectives to facilitate their later employment in the industry. In this way, the project will foster links between academia and industry, accelerating and broadening technology Transfer.