New wide bandgap conjugated D-A copolymers based on Benzo[d]thiazole for efficient photovoltaics

Two wide bandgap copolymers based on benzo[d]thiazole unit, named PBTz-F and PBTz-Cl, containing benzo [1,2-b:4,5-b & PRIME;]dithiophene (BDT) as electron donor units are developed. The resulting polymers lower the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels via substituting halogen atoms (fluorine or chlorine atoms) on the thiophene side chain of the BDT unit. However, the chlorine atom in the copolymers has a stronger capacity to downshift energy levels than the fluorine atom. PBTz-Cl:Y6-BO-based OSC delivers a higher PCE of 12.36% with simultaneously enhanced VOC of 0.90 V, JSC of 22.78 mA cm-2, and FF of 0.61 when compared to that based on PBTz-F:Y6-BO (PCE = 11.38%, JSC = 22.48 mA cm-2, VOC = 0.88 V, and FF = 0.58) due to its stronger absorption, higher charge mobilities, deeper HOMO energy level, and favorable blend film morphology. These results emphasized that chlorination is a practical strategy to obtain high PCE and BTz-based polymers are potential donor candidates for high-performance OSCs.