Defect Management of High Efficiency Multijunction, Space and Concentrator Solar Cells
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Defect Management of High Efficiency Multijunction, Space and Concentrator Solar Cells Masafumi Yamaguchi, Nobuaki Kojima, Kazuma Ikeda and Yoshio Ohshita Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan ABSTRACT III-V compound multi-junction solar cells have high efficiency potential of more than 50% due to wide photo response, while limiting efficiencies of single-junction solar cells are 31-32%. In order to realize high efficiency III-V compound multi-junction solar cells, understanding and controlling imperfections (defects) are very important. This paper reviews fundamentals of defects and defect management for III-V compound materials, single-junction, multi-junction, space and concentrator solar cells. INTRODUCTION III-V compound multi-junction (MJ) solar cells have high efficiency potential of more than 50% due to wide photo response, while limiting efficiencies of single-junction solar cells are 3132%. In order to realize high efficiency III-V compound MJ solar cells, understanding and controlling imperfections (defects) are very important. Table 1 shows major losses, their origins of III-V compound cells and key technologies for improving. There are several loss mechanisms to be solved for realizing high-efficiency III-V MJ solar cells. 1) Bulk recombination loss, 2) surface recombination loss, 3) interface recombination loss, 4) voltage loss, 5) fill factor loss, 6) optical loss, 7) insufficient photon energy loss. The origins of those losses and key technologies for reducing those losses are presented in this paper. Table 1. Major losses, their origins of III-V compound cells and key technologies for improving.
Table 2. Key issues for realizing super high-efficiency MJ solar cells.
Table 2 shows key issues for realizing super high-efficiency MJ solar cells. The key issues for realizing super-high-efficiency MJ solar cells are 1) sub cell material selection, 2) tunnel junction for sub cell interconnection, 3) lattice-matching, 4) carrier confinement, 5) photon confinement, 6) anti-reflection in wide wavelength region and so forth. For concentrator applications by using MJ cells, the cell contact grid structure should be designed in order to reduce the energy loss due to series resistance, and tunnel junction with high tunnel peak current density is necessary. Because cell interconnection of sub-cells is one of the most important key issues for realizing high-efficiency MJ solar cells, effectiveness of double hetero structure tunnel diode is also presented in this paper. For space application of III-V MJ solar cells, understanding properties and behavior of radiation-induced defects in III-V compound materials is also very important. Origins and behavior of radiation-induced defects in InP and InP-related materials, and radiation damages to solar cells are also presented in this paper. . DEFECT-RELATED LOSSES OF III-V COMPOUND SOLAR CELLS Solar cell efficiency is dependent upon minority-carrier diffusion length (or minority-carrier lifetime) in the solar cell materials as
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