While single-junction solar cells may be capable of attaining AM1.5 efficiencies of up to 29%, Multi-Junction (MJ, Tandem) III-V compound solar cells appear capable of realistic efficiencies of up to 50% and are promising for space and terrestrial applications. In fact, the InGaP/GaAs/Ge triple-junction solar cells have been widely used in space since 1997. In addition, industrialization of concentrator solar cell modules using III-V compound MJ solar cells have been announced by some companies. This chapter presents principles and key issues for realizing high-efficiency MJ solar cells, issues relating to development and manufacturing, and applications for space and terrestrial uses.
TopIntroduction
Multi-Junction (MJ, Tandem) solar cells are composed of multi-layers with different bandgap energies are shown in Figure 1 and have the potential for achieving high conversion efficiencies of over 50% and are promising for space and terrestrial applications due to wide photo response. Figure 2 shows theoretical conversion efficiencies of single-junction and Multi-Junction (MJ) solar cells in comparison with experimentally realized efficiencies.
Figure 1. A schematic structure of a multi-layer solar cell
Figure 2. Theoretical conversion efficiencies of single-junction and multi-junction solar cells in comparison with experimentally realized efficiencies
Tandem solar cells were proposed by Jackson (1955) and Wolf (1960). Table 1 shows progress of the III-V compound multi-junction solar cell technologies. MIT group (Fan, Tsaur, & Palm, 1982) encouraged R&D of tandem cells based on their computer analysis. Although AlGaAs/GaAs tandem cells, including tunnel junctions and metal interconnectors, were developed in the early years, a high efficiency close to 20% was not obtained (Hutchby, Markunas, & Bedair, 1985). This is because of difficulties in making high performance and stable tunnel junctions, and the defects related to the oxygen in the AlGaAs materials (Ando, Amano, Sugiura, et al., 1987). A Double Hetero (DH) structure tunnel junction was found to be useful for preventing diffusion from the tunnel junction and improving the tunnel junction performance by the authors (Sugiura, Amano, Yamamoto, & Yamaguchi, 1988). The authors demonstrated 20.2% efficiency AlGaAs/GaAs 2-junction cells (Amano, Sugiura, Yamamoto, & Yamaguchi, 1987). An InGaP material for the top cell was proposed by NREL group (Olson, Kurtz, & Kibbler, 1990). As a result of performance improvements in tunnel junction and top cell, over 30% efficiency has been obtained with InGaP/GaAs 2-junction cells by the authors (Takamoto, Ikeda, Kurita, et al., 1997).
Table 1. Progress of the III-V compound multi-junction solar cell technologies
1955
1960 | Proposal of multi-junction solar cell | Jackson
Wolf |
| 1982 | Efficiency calculation of tandem cells | MIT |
| 1982 | 15.1% AlGaAs/GaAs 2-junction (2-J) cell | RTI |
| 1987 | Proposal of double-hetero structure tunnel junction for multi-junction interconnection | NTT |
| 1987 | 20.2% AlGaAs/GaAs 2-J cell | NTT |
| 1989 | 32.6% GaAs//GaSb concentrator 2-J cell
(mechanical-stacked, 100-suns concentration) | Boeing |
| 1990 | Proposal of InGaP as top a cell material | NREL |
| 1990 | 27.3% InGaP/GaAs 2-J cell | NREL |
| 1996 | 30.3% InGaP/GaAs 2-J cell | Jpn. Energy |
| 1997 | Discovery of radiation-resistance of InGaP top cell | Toyota Tech. Inst. |
| 1997 | 33.3% InGaP/GaAs//InGaAs 3-J cell
(mechanical-stacked) | Jpn. Energy,
Sumitomo &
Toyota Tech. Inst. |
| 1997 | Commercial satellite with 2-J cells | Hughes |
| 2000 | 31.7% InGaP/InGaAs/Ge 3-J cell | Jpn. Energy |
| 2003 | 37.4% InGaP/InGaAs/Ge 3-J cell (200-suns concentration) | Sharp |
| 2004 | 38.9% InGaP/InGaAs/Ge 3-J cell (489-suns concentration) | Sharp & Toyota TI |
| 2006 | 31.5% large-area (5,445cm2) InGaP/InGaAs/Ge
3-J cell module (outdoor) | Daido Steel,
Daido Metal,
Sharp &Toyota T.I. |
| 2006 | 40.7% InGaP/GaAs/Ge 3-J cell (236-suns concentration) | Spectrolab |
| 2009 | 41.1% InGaP/InGaAs/Ge 3-J cell (454-suns concentration) | Fraunhofer ISE |
| 2009 | 41.6% InGaP/InGaAs/Ge 3-J cell (364-suns concentration) | Spectrolab |
| 2009 | 35.8% InGaP/GaAs/InGaAs 3-J cell (1-sun) | Sharp |