**7. Conclusion**

This chapter reviewed progress in GaAs-based single junction solar cells and III-V compound multi-junction solar cells and key issues for realizing high-efficiency solar cells. The III-V compound solar cells have contributed as space and concentrator solar cells and are expected as creation of new markets such as large-scale electric power systems and solar cell powered electric vehicles. Regarding singlejunction solar cells, especially, GaAs solar cells have shown 29.1% under 1-sun illumination, highest ever reported for single-junction solar cells. In addition, analytical results for non-radiative recombination and resistance losses in III-V compound solar cells are shown by considering fundamentals for major losses in III-V compound materials and solar cells. Because the limiting efficiency of singlejunction solar cells is 30-32%, multi-junction junction solar cells have been developed and InGaP/GaAs based 3-junction solar cells are widely used in space. The InGaP/GaAs/InGaAs 3-junction solar cells have recorded 37.4% under 1-sun and 44.4% under concentration. Recently, highest efficiencies of 39.1% under 1-sun and 47.2% under 144-suns concentration have been demonstrated with 6-junction solar cells. The 3-junction and 6-junction solar cells potential efficiencies of 42% and 46% under 1-sun, respectively. Further development of high-efficiency and low cost solar cells and modules is necessary in order to create new markets.

## **Acknowledgements**

Our studies were partially supported by the NEDO (New Energy and Industrial Technology Development Organization) and JSPS (Japan Society for Promotion of

*High-Efficiency GaAs-Based Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.94365*

Science). The author wishes to express sincere thanks to Dr. T. Takamoto, Sharp, Dr. K. Araki, Toyota Tech. Inst., Dr. M. Imaizumi, JAXA, Dr. A. Yamamoto, Fukui Univ., Dr. H. Sugiura and Dr. C. Amano, formerly NTT Lbs., Dr. SJ. Taylor, ESA, Prof. A. Kahn, South Arabama Univ., Prof. HS. Lee, Korea Univ., Prof. N. Ekins-Daukes, UNSW, Prof. A. Luque, UPM, Dr. A. Bett Dr. G. Sifer and Dr. F. Dimroth, FhG-ISE, Dr. M. Al-Jassim, Dr. R. Ahrenkiel and Dr. J.F. Geisz, NREL for their fruitful collaboration and discussion.
