The transparent inter-electrodes with the p/n heterojunction consisting of the solution-processible ZnO nanoparticles as the n-type and the conventional hole injection layers (MoO(3) or PEDOT:PSS) as the p-type materials are studied for developing tandem organic solar cells employing different band gap active materials (i.e., P3HT:PCBM blend layer for larger band gap material in the bottom cell and ZnPc/C(60) bilayer for smaller band gap material in the top cell). For the ZnO/PEDOT:PSS inter-electrode, the V(OC) corresponding to the sum of V(OC)'s of the top and bottom unit cells is obtained, denoting that the two unit cells are successfully connected in series. For the ZnO/MoO(3) inter-electrode, the open-circuit voltage (V(OC)) of the tandem cell is smaller than the sum of V(OC)'s of the top and bottom unit cells, but it can be increased by inserting a very thin Al layer (similar to 3 nm) between ZnO and MoO(3) (ZnO/Al/MoO(3)) as the recombination center for carriers. (c) 2010 Elsevier B.V. All rights reserved.