A Novel Donor-Acceptor-Acceptor-Acceptor Polymer Containing Benzodithiophene and Benzimidazole-Benzothiadiazole-Benzimidazole for PSCs

New electron deficient acceptor-acceptor-acceptor type of monomer unit composed of weak electron accepting benzimidazole and relatively strong electron accepting benzothiadiazole derivatives namely 4,7-bis(6-bromo-1-(2-ethylhexyl)-1H-benzo[d]imidazol-2-yl)benzo[c][1,2,5]thiadiazole (BBB) was synthesized. The Stille polycondensation of the newly synthesized BBB monomer with electron donating 2,6-bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene (BDT) afforded donor-acceptor-acceptor-acceptor type of polymer namely 2,6-(4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene)-alt-4,7-bis(1-(2-ethylhexyl)-1H-benzo[d]imidazol-2-yl)benzo[c][1,2,5]thiadiazole (PBDTBBB). The opto-electrical studies revealed that the absorption band of PBDTBBB appeared in the range of 300 nm-525 nm and its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels were positioned at -5.18 eV and -2.84 eV, respectively. The power conversion efficiency (PCE) of the polymer solar cell (PSC) prepared from PBDTBBB:PC71BM (1:2 wt %) blend was 1.90%.