Low Band Gap Polymers Based on Benzo[1,2-b:4,5-b′]dithiophene: Rational Design of Polymers Leads to High Photovoltaic Performance

This study centers upon the summarized four design criteria for conjugated polymer photovoltaic materials: high oxidation potential, low band yap, a planar symmetrical structure, and high molecular weight. Two polymer materials were synthesized to fulfill these design criteria, both based upon copolymers of benzo[1,2-b:4,5-b']dithiophene (BnDT) and 4,7-di(2-thieny1)-2,1,3-benzothiadiazole (DTBT). Additional hexyl chains were added to one polymer on the 4 position of the DTBT unit, which increased the molecular weight of the polymer without detracting from its electrochemical or optical properties. Both polymers perform exceptionally well in preliminary bulk heterojunction solar cells, reaching power conversion efficiencies greater than 4%.