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 (∼3nm) between ZnO and MoO 3 (ZnO/Al/MoO 3 ) as the recombination center for carriers.