Efficient organic tandem solar cells based on small molecules by Riede, M, Uhrich, C, Widmer, J, Timmreck, R, Wynands, D, Schwartz, G, Gnehr, W, Hildebrandt, D, Weiss, A, Hwang, J, Sundarraj, S, Erk, P, Pfeiffer, M, Leo, K