Synthesis and characterization of naphtho[2,1-b:3,4-b′]dithiophene- based polymers with extended p-conjugation systems for use in bulk heterojunction polymer solar cells

Two novel polymeric semiconductor materials based on naphtho[2,1-b:3,4- b′]dithiophene (NDT), PNDT-TTT and PNDT-TET, were designed and synthesized. These synthesized polymers were tested in bulk heterojunction solar cells as blends with the acceptor [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM). PNDT-TTT contained tri-thiophene units, and PNDT-TET contained bi-thiophene units coupled by ethylenic linkages. Comparison to the properties of PNDT-T, which contained single thiophene units, these polymers exhibit red-shifted absorption spectra as a result of the enhanced conjugation lengths. These effects resulted in high short circuit currents (J_SC) in the organic solar cells. The PNDT-TET- and PNDT-TTT-based devices exhibited considerably better photovoltaic performances, with power conversion efficiencies of 3.5 and 3.3%, respectively, compared to the PNDT-T-based device (1.3%).