We demonstrate an efficient middle electrode of indium zinc oxides (IZOs) for flexible organic tandem solar cells. A bulk hetero-junction polymer solar cell and a bi-layer hetero-junction organic solar cell are used as the bottom and top cell, respectively. Insertion of a thin interfacial layer of Rb(2)CO(3) or Cs(2)CO(3) between the polymer and IZO and UV-ozone treatment of the IZO surface results in Ohmic contact with the top and bottom cells, respectively. The tandem devices showed that the open circuit voltage (V(oc)) was almost the same as the sum of the V(oc) of the individual cells with a fill factor of 0.57. The tandem devices fabricated on flexible PES substrate showed comparable results. (C) 2009 Elsevier B.V. All rights reserved.