TOP > Report & Column > The Forefront of Space Science > 2007 > Quality Assessment of Semiconductor Substrates for Solar Cells Using Photoluminescence - Identify Microscopic Distribution of Defects in Less Than One Second -
Applying the new method to solar cells for space
In addition to single-crystalline Si and multicrystalline Si cells, we applied the PL imaging method to high-efficiency InGaP/GaAs/Ge (indium gallium phosphide / gallium arsenide / germanium) triple-junction solar cells and CIS-base solar cells, which are expected as space application cells. Since these are made of laminated layers of different, multiple semiconductor materials, previously it was difficult to evaluate each layer selectively.
To cope with this difficulty, we developed the “selective excitation PL imaging method,” which allows us to assess each layer selectively by using multiple wavelength lights. It utilizes the characteristic that long-wavelength excitation lights and PL generated at lower layers also go through upper layers. This is because, in multi-junction cells, wider band gap materials are used in the upper layers.
Using this method, we can assess the quality of each layer in multi-junction and CIS cells for space with high-speed and high-spatial resolution. Recently this approach is playing a significant role in failure analysis of solar cells onboard satellites because of its non-destructive, non-contact and no-pretreatment advantages. I would like to discuss this topic separately if given an opportunity.
Notes related to the development of our equipment
I have been engaged in research on crystal evaluation using PL since my doctoral thesis. At the beginning of the research, the PL evaluation method was not fully known so I designed, assembled, and calibrated my own equipment. Later, a certain level of budget was allocated as I participated in larger projects. The scale of the equipment grew bigger and, in addition, I was unable to spare sufficient time for equipment development. As a result, we gradually increased joint development with optical instrument makers. The PL imaging equipment developed this time appears simple, but it presented several problems to be solved. Thus, the optical makers who had cooperated with us before hesitated to join us this time.
In these circumstances, Hiroki Sugimoto, a graduate student of the doctoral course in Tokyo University who excelled at manufacturing, embarked on the fabrication of the equipment. Based on know-how accumulated in our laboratory, the equipment was assembled by cooperation of other students in our laboratory. A variety of ideas were incorporated including a custom-made order of the highest-performance optical component from a professional manufacturer and even procurement of components via mail order. By making our own equipment, quick modifications were possible and we were able to apply the equipment to the HF immersion method rapidly.
Competition in this field is severe. Our biggest rival is a group led by Prof. M. Green of the University of New South Wales (UNSW) in Australia, which succeeded in realizing the world’s highest conversion efficiency for Si solar cells. After completing our application for patent on the equipment and a decision on the acceptance of our quick paper related to the equipment, we sent a preprint to the group. They replied regretfully, “We aimed at the same goal but you beat us.” I believe that quick in-house assembly contributed to our success this time.
We are continuously improving the equipment to develop much higher performance. Presentations on our developments will be made at several international conferences through this summer and winter. In addition, we have been approached about the commercialization of the equipment. All the staff of our laboratory will make efforts so that the technology presented here can contribute to future space projects and the prevention of global warming.