Invited Paper EM1+PV-TuM9
High Efficiency Multijunction Solar Cells – Present Status and Future Directions

Tuesday, November 10, 2009, 10:40 am, Room A8

Multi-junction solar cells based on the GaInP₂/GaAs/Ge triple junction architecture have achieved the highest efficiency of any photovoltaic device, for either space or terrestrial applications. However, the cost of these cells is high compared to other photovoltaic devices. For space applications (i.e., satellite power) the higher efficiencies are more critical than cell cost, and on a system level, including launch and deployment costs, the multi-junction cells are actually lower on a $/watt basis than Si solar cells. The high efficiency multi-junction cell is now widely used on satellites. For terrestrial applications, the high efficiency cells must be used in high concentration systems where the high cell cost is offset by the lower costs of lenses, mirrors, and structure metal. The cell becomes such a small part of the system cost that a doubling of cell cost has a small effect on the cost of power generation. However, the efficiency of the cell may have a much larger effect on the cost of power generated. So for either space or terrestrial applications the efficiency of the photovoltaic device is very important.

Multi-junction solar cells offer a performance advantage over single junction solar cells because of the reduction in carrier thermallization losses. Different parts of the solar spectrum are absorbed by different band gap materials. III-V materials are able to achieve very high performance on a single junction basis, and when appropriately combined provide an even higher performance advantage. Fortunately, GaInP₂, GaAs, and Ge junctions can be combined in a lattice matched configuration, making a monolithic device with a good combination of junctions for converting the solar spectrum into power. Appropriate modifications can be made to the device to optimize it for either space or terrestrial applications.

To achieve even higher efficiencies, more junctions need to be added and/or the junctions need to be better matched to the solar spectrum. A number of approaches have been studied, including novel materials (e.g., InGaAsN, ZnGeAs₂, etc.), mechanically stacked devices, and metamorphic devices. With metamorphic devices junctions are grown lattice mismatched on one another to achieve the optimal set of band gaps in a complete device. One metamorphic approach, the inverted metamorphic multi-junction (IMM) solar cell, has demonstrated significant performance improvements over the lattice matched triple junction device.

We report on these improvements, describing how the IMM approach is enabling for both space and terrestrial power generation.