H. Ye, H. Lotfi, L. Li, R. T. Hinkey, R. Q. Yang, L. Lei, J. C. Keay, M. B. Johnson, T.D. Mishima, and M. B. Santos, “Multistage interband cascade photovoltaic devices with a bandgap of ~0.23 eV operating above room temperature” Chinese Science Bulletin 59, 950-955 (2014).