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/C60 bilayer for smaller band gap material in the top cell). For the ZnO/PEDOT:PSS inter-electrode, the VOC corresponding to the sum of VOC’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 (VOC) of the tandem cell is smaller than the sum of VOC’s of the top and bottom unit cells, but it can be increased by inserting a very thin Al layer (∼3 nm) between ZnO and MoO3 (ZnO/Al/MoO3) as the recombination center for carriers.