Efficient and long-term stable organic vacuum deposited tandem solar cells by Uhrich, C, Schwartz, G, Maennig, B, Gnehr, WM, Sonntag, S, Erfurth, O, Wollrab, E, Walzer, K, Foerster, J, Weiss, A, Tsaryova, O, Leo, K, Riede, M, Pfeiffer, M