Super High Efficiency Multi-Junction Solar Cells and Concentrator Solar Cells

Introduction

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).