Design and Synthesis of Copolymers of Indacenodithiophene and Naphtho[1,2-c:5,6-c]bis(1,2,5-thiadiazole) for Polymer Solar Cells

Three donor acceptor type of narrow band gap conjugated polymers with enlarged coplanar skeleton were synthesized via Stille copolymerization of indacenodithiophene (IDT) and naphtho[1,2-c:5,6-c]bis(1,2,5-thiadiazole) (NT) based monomers. The energy levels, absorption spectra and band gaps of the resulting polymers were well tuned by utilizing different thiophene derivatives as spacer between IDT and NT units, and polymer PIDT-C12NT which employed bithiophene attached with dodecyl side chain as spacer exhibited superior properties compared to the other two copolymers All polymers exhibited deep highest occupied molecular orbital energy levels and subsequently lead to high open circuit voltages of fabricated solar cell devices. Best performance of the bulk-heterojunction solar cells with a power conversion efficiency of 5.05% was achieved with PIDT-C12NT as donor and (6,6)-phenyl-C-71-butyric acid methyl ester (PC71BM) as acceptor, which can be attributed to its higher charge carrier mobility, the optimized interpenetrating network as well as enhanced absorption coefficient of photoactive layer with respect to the other two polymers. The results demonstrated that the combination of IDT and NT with appropriate spacers was a promising molecular design strategy for the application of solar cells.