Eliminating the solvent blocking requirement of interconnection layers in polymer tandem solar cells by thin-film transfer technique

Interconnection layers (ICLs) for polymer tandem solar cells reported so far are limited in materials' choice and layer structure, because of a requirement that the ICLs must prevent the penetration of solvents used for the top cells. In this research, it is demonstrated that depositing the active layer of the top subcell using a dry thin-film transfer technique allows for incorporation of an ICL composed of vacuum deposited materials in a polymer tandem cell, providing a large degree of freedom in ICL design. Specifically, a polymer tandem solar cell was fabricated using an ICL composed of bathocuproine:silver/silver islands/1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (BCP:Ag/Ag islands/HAT-CN), where the thicknesses of the BCP:Ag and Ag island layers are precisely controlled at the nanoscale to facilitate the transport of electrons generated in the bottom subcell and to ensure their efficient recombination with holes generated in the top subcell. Consequently, the tandem device featuring the optimized ICL, whose active layers are composed of poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) and poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester (PCPDTBT:PC71BM), exhibits an open-circuit voltage of 1.20 V, which is equal to the sum of the open-circuit voltages of the two subcells, with a fill factor (FF) of 0.60 almost identical to the FFs of the subcells.