Observations on deep levels introduced in silicon by 1 MeV electron irradiation are reported using boron- or gallium-doped Czochralski (CZ) grown Si space solar cells with different doping concentrations, deep level transient spectroscopy analysis has been carried out to detect the radiation-induced deep levels. Present results provide evidence for new defect states in addition to those previously reported in gallium- and boron-doped Si. The combined boron and gallium data provide enough information to gain valuable insight into the role of the dopants on radiation induced defects in Si. The dominant donor-like electron level at EC−0.18 eV in boron-doped Si has not been observed in gallium-doped CZ-grown Si. A noticeable suppressing generation of the radiation-induced defects in gallium-doped Si is also observed, especially hole level EV+0.36 eV, which is thought to acts as a recombination center.
The cell design and manufacturing process for the high efficiency thin silicon space solar cells (call NRSLBSF cell) were finalized and their characteristics were qualified. The 100pm NRSLBSF cells showed eficiencies of maximum 18.5% and average 18.0%. After electron irradiation of 1E+15e/cmz, the NRSLBSF cell showed about 1.2 times higher output power than the conventional 2 0 0 p BSR cell. f NTRODUCTf ONThe development of the high efficiency thin silicon solar cells was started under the contract between SHARP and NASDA in 1990. The objective of this project is to develop high efficient (target of 18%) silicon solar cells to compare favorably with Gallium Arsenide solar cells. At the first research step (1990 and 1991), the flat surface cells were improved by introducing the Si02 passivation layer and locally diffused P+ field layer (call LBSF) at back surface. And we also studied three kinds of silicon substrates, CZ, MCZ and FZ and confirmed the CZ silicon is most suitable for the space cell because of it's low cost and high performance with our process and cell design [6]. The CZ cells with LBSF showed higher performance after electron irradiation and lower absorptivity than the conventional BSFR cells, and showed high potentiality for space use. At the second research step (1 992), we studied silicon resistivity and three kinds of non-reflective surface (call NRS), normal pyramid, inverted pyramid and V-groove. The LBSF cell with the 2 Rcm substrate and inverted pyramid surface showed the highest performance of all studied cells before and after electron irradiation. The third step (1 993 and 1994) of research was performed to optimize the detailed cell design and manufacturing process for the practical space solar cells. The silicon substrate resistivity, type of N R S and the front surface passivation were studied with detailed design changes to the second step cells to improve the cell efficiency and stability to the environment test. The detailed results and discussions on the previous studies were summarized in the references [I-1 11. This paper presents the main results of the cell design and qualification tests. CELL DESIGN AND QUALIFICATIONl o determine the final design and process for the NRWBSF cells, we performed several experiments and evaluation on the silicon resistivities, structures of NRS's and front surface passivation. Several kinds of performance tests and environmental tests on the cells were performed for qualification. Silicon ResistivityThe 1 0 0~ NRSLBSF cells were fabricated with silicon substrates of 0.5, 2, 5 and 10 Rcm (nominal values) and subjected to the 1 MeV electron irradiation test and humidity storage test. These test results are given in Table 1 and Table 2 respectively. The cells with 0.5 Rcm and 2 Qcm substrates showed the highest Pmax before and after electron irradiation test. The IO Qcm cells showed the lowest power before and after irradiation test because of their low fill factor as shown in Table 1. The reason of low fill factors of the 10 Qcm cells seems to b...
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