Donor-acceptor-type copolymers based on a naphtho[1,2-c: 5,6-c]bis(1,2,5-thiadiazole) scaffold for high-efficiency polymer solar cells

Four donor-acceptor-type low-bandgap conjugated polymers based on a naphtho[1,2-c:5,6-c]bis(1,2,5-thiadiazole) (NT) acceptor and different donors bridged by a bithiophene spacer have been synthesized through Suzuki or Stille polymerization reactions. Fluorene (F), carbazole (Cz), alkylidene fluorene (AF), and benzodithiophene (BDT) were selected as the donor units to produce a series of new conjugated polymers. Owing to the different electron-donating ability of the donor units, the energy levels, absorption spectra, bandgaps, and carrier mobilities of the resulting polymers were systematically tuned. Bulk-heterojunction-type polymer solar cells based on the new polymers and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) or [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) were investigated and all of the devices exhibited good photovoltaic performance, with power-conversion efficiencies (PCEs) over 3%. The best device performance was achieved by PF-C12NT, with an open-circuit voltage (V_oc) of 0.87 V, a short-circuit current density (J_sc) of 12.19 mAcm^-2, a fill factor (FF) of 61.36%, and a PCE of 6.51% under simulated sunlight (100 mWcm^-2, AM 1.5G). © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.