Design and Synthesis of Alternating Regioregular Oligothiophenes/Benzothiadiazole Copolymers for Organic Solar Cells

Low-bandgap regioregular polythiophene derivatives, PTh 6 BTD, PTh 8 BTD, and PTh 4 TTBTD, were synthesized through Stille coupling reaction. These are alternating copolymers of an electron-deficient benzothiadiazole unit and oligothiophene units including hexathiophene, octathiophene, and thieno[3,2- b ]thiophene-bridged quarterthiophene, respectively. The polymers are soluble in halogenated solvents such as o -dichlorobenzene, affording good processability in solar cell fabrication. Meanwhile, the synthesized alternating copolymers show much broader absorption than P3HT, covering the spectral region from 350 to 800 nm. DSC analysis showed that all three polymers readily crystallized, indicating highly ordered intermolecular packing, which is beneficial for achieving higher charge carrier mobility. Bulk-heterojunction solar cells using 1:1 w/w PTh n BTD:PC 61 BM ([6,6]-phenyl C 61 -butyric acid methyl ester) blends as the photovoltaic active layers were fabricated and characterized. Best power conversion efficiency of 1.7% was obtained from PTh 8 BTD-based devices under a simulated AM 1.5 G solar irradiation of 100 mW/cm 2 .