Future technology pathways of terrestrial III–V multijunction solar cells for concentrator photovoltaic systems

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Abstract

Future terrestrial concentrator cells will likely feature four or more junctions. The better division of the solar spectrum and the lower current densities in these new multijunction cells reduce the resistive power loss (I2R) and provide a significant advantage in achieving higher efficiencies of 45–50%. The component subcells of these concentrator cells will likely utilize new technology pathways such as highly metamorphic materials, inverted crystal growth, direct-wafer bonding, and their combinations to achieve the desired bandgaps while maintaining excellent device material quality for optimal solar energy conversion. Here, we report preliminary results of two technical approaches: (1) metamorphic ∼1 eV GaInAs subcells in conjunction with an inverted growth approach and (2) multijunction cells on wafer-bonded, layer-transferred epitaxial templates.

Introduction

Terrestrial concentrator systems utilizing high-efficiency III–V multijunction solar cells are becoming a viable technology for large-scale generation of electrical power [1]. The III–V concentrator systems are unique in their high-areal power density and offer rapid manufacturing scalability. With production cell efficiencies around 37% at about 500 suns, triple-junction GaInP/Ga(In)As/Ge terrestrial solar cells are enabling system manufacturers to produce concentrator systems competitive with other technologies and offering potential cost advantages.

Two types of III–V multijunction concentrator cells are now in large-scale production at Boeing-Spectrolab. They are the C1MJ and the C2MJ. The efficiency distribution of the production C1MJ terrestrial cells is shown in Fig. 1. With mode efficiency and average efficiency at 37% (25 °C, ASTM G173-03 spectrum, 500×), the production cells have a rather mature distribution curve. Peak cell efficiencies of 39% were recorded on some of the 1 cm2 cells.

Section snippets

Future III–V terrestrial concentrator cells

Recently, Boeing-Spectrolab's 3-junction concentrator cell reached an efficiency of 40.7% as shown in Fig. 2 [2], [3]. Despite the high efficiency, it is well-known that the 3-junction cell bandgap combination compromises the ideal solar spectrum splitting in favour of subcell material quality, hence, limiting the achievable efficiency. Future terrestrial cells will likely feature four or more junctions with performance potential capable of reaching over 45% efficiency at concentration. The 4-,

Technology pathways for III–V multijunction concentrator cells

The 4-, 5-, and 6-junction terrestrial cells will likely utilize new technologies to build some of the subcells with the desired bandgaps and material quality. Two technology pathways are presented here: (1) highly metamorphic GaInAs subcell materials in conjunction with inverted growth and (2) multijunction cells on wafer-bonded, layer-transferred epitaxial templates.

Conclusion

Boeing-Spectrolab continues to provide high-performance terrestrial photovoltaic products for concentrator photovoltaic systems. Research work now focuses on advancing cell architectures and technology pathways that will enable us to reach 45–50% conversion efficiency. Preliminary results of two technical approaches: (1) metamorphic ∼1 eV GaInAs subcells in conjunction with inverted growth, and (2) multijunction cells on wafer-bonded, layer-transferred epitaxial templates are reported. Further

Acknowledgments

The authors would like to thank Dimitri Krut, Mark Takahashi, and the entire multijunction solar cell team at Boeing-Spectrolab. This work was supported in part by the National Renewable Energy Laboratory High-Performance Photovoltaic Program and by Boeing-Spectrolab, Inc.

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