Medium-Bandgap Conjugated Polymers Containing Fused Dithienobenzochalcogenadiazoles: Chalcogen Atom Effects on Organic Photovoltaics

We designed, synthesized, and characterized a series of three medium-bandgap conjugated polymers (PBDTfDTBO, PBDTfDTBT, and PBDTfDTBS) consisting of fused dithienobenzochalcogenadiazole (fDTBX)-based weak electron-deficient and planar building blocks, which possess bandgaps of ∼2.01 eV. The fDTBX-based medium-bandgap polymers exhibit deep-lying HOMO levels (∼5.51 eV), which is beneficial for use in multijunction polymer solar cell applications. The resulting polymers with chalcogen atomic substitutions revealed that the difference in the electron negativity and atomic size of heavy atoms highly affects an intrinsic property, morphological feature, and photovoltaic property in polymer solar cells. The polymer solar cells based on sulfur-substituted medium-bandgap polymer showed power conversion efficiencies above 6% when blended with [6,6]-phenyl-C71-butyric acid methyl ester in a typical bulk-heterojunction single cell. These results suggest that the fDTBX-based medium-bandgap polymer is a promising alternative material for P3HT in tandem polymer solar cells for achieving high efficiency.