Improvement of efficiency of polymer solar cell by incorporation of the planar shaped monomer in low band gap polymer

A new π-conjugated polymer (PFDTBT-BTBT) with planar core based on 4,7-dithiophen-2-yl-benzo(2,1,3)thiadiazole (BT), [1]benzothieno-[3,2-b][1]benzothiophene (BTBT) and 9,9-didecylfluorene was synthesized by the Suzuki coupling reaction. In order to improve the photovoltaic performance, we introduce planar and rigid shaped BTBT in the polymer backbone. We also synthesized alternating copolymer of 9,9-didecylfluorene and BT (PFDTBT) to compare with optical, electrochemical and photovoltaic properties. The HOMO and LUMO energy levels of PFDTBT-BTBT were −5.67 and −3.75 eV, respectively, which were very similar to those of PFDTBT (−5.65 and −3.75 eV). The power conversion efficiency (PCE) of the device with a structure of ITO/PEDOT:PSS/PFDTBT-BTBT:PCBM (1:3)/Al was 2.08%, which is higher than that of PFDTBT:PCBM (1:3) (1.66%). The root-mean-square (RMS) roughness of PFDTBT-BTBT:PCBM (1:3) blend was 0.820 nm, which was smaller than that of PFDTBT:PCBM (1:3) blend (1.75 nm). PFDTBT-BTBT:PCBM showed smaller domain size and more well-penetrated morphology than PFDTBT:PCBM. The average field-effect hole mobility of the PFDTBT-BTBT was 6.2 × 10−4 cm2/Vs, much larger than that of PFDTBT (2.3 × 10−4 cm2/Vs). The results strongly support that polymer solar cell based on PFDTBT-BTBT shows better performance than that of the device based on PFDTBT.