Giraldo, S.; Neuschitzer, M.; Placidi, M.; Pistor, P.; Perez-Rodriguez, A.; Saucedo, E. "Cu2ZnSnSe4-Based Solar Cells with Efficiency Exceeding 10% by Adding a Superficial Ge Nanolayer: The Interaction between Ge and Na" IEEE Journal of Photovoltaics 6, 3, pp. 754 - 759. 2016 DOI: 10.1109/JPHOTOV.2016.2535236