Transparent electrode with ZnO nanoparticles in tandem organic solar cells

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 (MoO3 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/MoO3 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 MoO3 (ZnO/Al/MoO3) as the recombination center for carriers. (c) 2010 Elsevier B.V. All rights reserved.