Aiming to reduce the cost of "triple-junction solar cells," Idemitsu, with its track record in CIGS solar cells, was singled out

This time, SDX will conduct parallel demonstrations of "PHOENIX," a three-layer solar cell jointly developed by another supplier, Idemitsu Kosan, and JAXA, as well as Idemitsu Kosan's CIGS solar cell alone.

T. OKUMURA"Until now, we have been using 'triple-junction solar cells', which consist of three layers of thin-film solar cells stacked on a single type of wafer (a thin, disc-shaped substrate made of semiconductor material). However, this was expensive, and we had no choice but to limit its use to satellites that prioritize performance, such as scientific satellites. Therefore, we decided to test the idea of leaving the top two layers as they were and bonding a CIGS solar cell, which can be manufactured at lower cost, as the third layer. This is 'PHOENIX'. Specifically, a thin-film dual-junction solar cell and a CIGS solar cell are bonded together. Once we decided to use a CIGS solar cell for the third layer, we approached Idemitsu, which has extensive development experience, to cooperate."

H. TOMITA"When building something that can withstand the elements of space, what kind of challenges do we face and what approaches are possible? At first, we hadn't thought through the prerequisites, so we conducted numerous tests while receiving a lot of advice from JAXA. Also, during meetings, the JAXA staff would often use "space language" that I didn't understand, so I would often ask questions on the spot or look it up later."

T. Okumura"In space, we need to deal with environmental conditions that are completely different from those on Earth. The most distinctive feature is constant exposure to strong radiation. Temperatures also vary greatly, ranging from approximately -100°C to +100°C. Radiation in particular directly leads to a deterioration in the performance of solar cells, so we need to keep in mind the need to 'create products that can withstand radiation.' This is something that is difficult to understand at first unless you have experience developing for space applications."

"For ground use" and "for space use": Overcoming the gap caused by the significant difference in the environments

H. TOMITA"I think that understanding the space specification standards was certainly the first hurdle. Also, while strictness is required in terms of reliability, we were surprised to find that methodological approaches that, in our opinion, did not seem suitable for space were actually effective. As I mentioned earlier, PHOENIX uses dual-junction solar cells and CIGS solar cells bonded together, and I was surprised when they suggested that we use adhesive to bond them together. They said, 'You don't need to weld!' (laughs) JAXA has thoroughly verified and accumulated information not only about adhesives, but also about whether they are suitable for use in a space environment and what precautions need to be taken when using them. JAXA taught us each and every point that needs to be taken into consideration when manufacturing for space applications, and also conducted various reliability evaluation tests on our solar cells. In the process, our CIGS solar cells gradually became suitable for space applications, and we, who were previously just 'earthlings', were gradually becoming 'aliens' who could understand the 'language of space' (laughs)."

T. Okumura"A satellite cannot function without electricity. For this reason, solar cells are an extremely important part of a satellite. Also, once a satellite is launched, it is not possible to immediately go and replace it if a malfunction occurs. This makes high reliability all the more important. With solar cells, we first conduct radiation irradiation tests on Earth using cells about 2 cm square, and then demonstrate them in space. Once that is clear, we evaluate cells the same size as the actual product, and finally evaluate panels made up of multiple cells. Development proceeds step by step, with tests being conducted on cells the same size as the actual product. It is also important to verify whether the product can withstand the vibration environment during launch and the thermal cycle environment in orbit."

Into unknown territory. Demonstration brings greater expectations than anxiety

In addition to PHOENIX, SDX will also be demonstrating CIGS solar cells alone, which was made possible through the efforts of Idemitsu Kosan.

H. TOMITA"Frankly, I approached them because I wanted to see the capabilities of CIGS solar cells on their own. We live in a world where track record is always important, so when I propose something to a client, they always ask me, 'Have you actually sent it into space?' The top two layers of PHOENIX are good at absorbing short-wavelength light, so the third layer, the CIGS solar cell, receives mainly long-wavelength light. On the other hand, with a standalone CIGS solar cell, all light, from short to long wavelengths, reaches the CIGS. As for the results of SDX, my expectations outweigh my concerns for both PHOENIX and the standalone solar cells. Since it's impossible to predict what will happen in space, I'm looking forward to seeing the results of both demonstrations."

T. Okumura"Developing PHOENIX was extremely challenging, but we worked well with the supplier of dual-junction solar cells and were able to complete the development to a level that was suitable for installation on SDX. Additionally, installing the CIGS solar cells alone is also important for verifying their operation in orbit. We are grateful for the speedy provision of cells that can be used in space, which made this possible."

D. OGAWA"I'm very happy to hear that! Our research laboratory is staffed with members who are able to work together to meet the expectations of our partners. We tackle even high hurdles with the full force of our team and strive to achieve them no matter what - being 'strong in times of emergency' is perhaps what Idemitsu is all about. Also, CIGS itself is a 'material' that originated in our Functional materials business, but what we are currently working on is a 'device' - a CIGS solar cell. We recognize that we have a unique position within Idemitsu, so we would like to use the results of the SDX demonstration to develop our next venture."

H. TOMITA"JAXA accompanied us throughout the development period. From providing the equipment for the test environment, to advice on conducting the tests, and even how to interpret the results. None of this was possible on our own. Before we got involved, I felt that JAXA, a global organization, was intimidating, but I was grateful that we were able to have frank discussions from the very beginning. JAXA also had a very strong desire to 'achieve provide for society of space-use CIGS solar cells,' and I believe it was because we had this common ground that this development was able to proceed so smoothly."

Idemitsu's R&D culture carefully nurtures the seeds of new businesses

The adoption of CIGS solar cells in this SDX demonstration would never have been possible without over 30 years of accumulated research and development. Even when faced with headwinds at times, the company continues to nurture the seeds of new business. This corporate culture can be seen as an opportunity that has been brought about. Furthermore, this corporate culture of encouraging further challenges through proactive investment has shaped the Idemitsu Kosan of today, a company whose business can no longer be described simply as an "oil wholesaler."
Currently, Idemitsu Kosan is rapidly expanding its space solar cell project in order to expand its CIGS solar cell business.

D. OGAWA: "The project involves colleagues with a variety of backgrounds, including members who were involved in the mass production of terrestrial CIGS solar cells at Solar Frontier, and members with experience in launching new businesses and developing markets, in order to launch a space CIGS solar cell business. In addition, at Idemitsu Americas Holdings, our US base, which is at the forefront of the space solar cell market, we have also added a national staff member with a background in the space industry as a dedicated member of the project."

Idemitsu's CIGS solar cells were launched into space on the memorable first HTV-X to demonstrate SDX. This marks the beginning of a long journey that will leave its mark on space development projects both in Japan and overseas.