Synthesis and Photovoltaic Properties of Two Benzo[1,2-b:3,4-b′]dithiophene-Based Conjugated Polymers

Side chains of conjugated polymers play an important role in their properties. To investigate the influence of side chains on photovoltaic properties, two conjugated polymers with benzo[1,2-b:3,4-b]dithiophene (BDT) and 4,7-dithiophen-2,1,3-benzothiadiazole (DTBT) units, Z3 and Z4, were designed and synthesized. These two polymers have identical conjugated main chains and very similar side chains. In polymer Z3, hexyl groups are linked to the thiophene units directly, and in polymer Z4, hexyloxy groups are linked to the thiophene units. Since the hexyloxy group exhibits a smaller steric hindrance and a stronger electron donating effect than the hexyl group, properties of Z3 and Z4 are very different. The hole mobility of the Z3:PCBM blend film is 1.84 × 10-7 cm2/(V s), which is 2 orders lower than that of the Z4:PCBM blend, and photovoltaic results demonstrate that the fill factor (FF) of the Z3-based device is 0.37, while that of the Z4-based polymer solar cell (PSC) device reached 0.61. These results reveal that steric hindrance of the side chain is an important issue for a new material design, in order to obtain higher FF. Additionally, thermal stability, absorption spectra, electrochemical, PL quenching of the polymer:PCBM blend, and photovoltaic properties of the two polymers were investigated in detail. For Z3-based devices, a power conversion efficiency (PCE) of ∼2% was observed with an open circuit voltage (Voc) of 0.84 V, a short circuit current (Jsc) of 6.3 mA/cm2, which indicates this kind of conjugated polymer should be a promising material in the application of polymer solar cells.