Synthesis and characterization of pi-conjugated copolymers with thieno-imidazole units in the main chain: application for bulk heterojunction polymer solar cells

Abstract

In this paper the three new narrow bandgap D-A conjugated copolymers P1, P2 and P3 based on different weak donor fused thiophene-imidazole containing derivatives and the same benzothiadiazole acceptor unit were synthesized by Stille cross-coupling polymerization and characterized by H-1 NMR, elemental analysis, GPC, TGA, DSC. These copolymers exhibit intensive absorbance in the range 350-900 nm and the optical bandgap lies in the range of 1.50-1.61 eV, which corresponds to the maximum photon flux of the solar spectrum. The electrochemical bandgap derived from cyclic voltammetry varies within the limits 1.47-1.65 eV and is approximately very close to the optical bandgap. The highest occupied molecular orbital (HOMO) energy level of all copolymers is deep lying (-5.24 eV and -5.37 eV and -5.25 eV for P1, P2 and P2, respectively) which shows that copolymers have good stability in the air and assured a higher open circuit voltage (V-oc) for polymer BHJ solar cells. These copolymers were used as donors along with PC71BM and the BHJ polymer solar cells based on P1:PC71BM, P2:PC71BM and P3:PC71BM processed from chloroform (CF) solvent with 3 v% DIO as an additive showed an overall PCE of 4.55%, 6.76% and 5.16%, respectively.