Effect of alkylthiophene spacers and fluorine on the optoelectronic properties of 5,10-bis(dialkylthien-2-yl)dithieno[2,3-d′:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene-alt-benzothiadiazole derivative copolymers

Alternating conjugated copolymers based on 5,10-bis(dialkylthien-2-yl) dithieno[2,3-d: 2',3'-d'] benzo[1,2b: 4,5-b'] dithiophene (DTBDT) and 2,1,3-benzothiadiazole (BT) or 5,6-difluoro-2,1,3-benzothiadiazole (FBT) with alkylthiopene spacers were synthesized, and the effect of insertion of alkylthiophene spacers and fluorine atoms on the characteristics of the copolymers, such as the energy levels, intrachain pi-pi interaction, dielectric constants, photovoltaic properties, etc., were systematically investigated. It has been found that: (i) the introduction of alkylthiophene spacers not only led to an increase in the intrachain interaction of the copolymers, but also resulted in an increase in the highest occupied molecular orbital (HOMO) levels and the lowest unoccupied molecular orbital (LUMO) levels, and (ii) the inclusion of fluorine atoms resulted in a decrease in both HOMO and LUMO energy levels with enhancement of the planarity and hole mobility. However, the inclusion of fluorine atoms had little effect on the LUMO levels relative to the decrease in the HOMO levels, and almost did not affect the dielectric constant of the copolymers. Use of the materials in polymeric photovoltaic cells led to high performance photovoltaic cells (PVCs) with power conversion efficiencies of 6.04-7.12%. The results demonstrated that the optoelectronic and aggregation properties of the 5,10-bis(alkylthien-2-yl) dithieno[ 2,3-d: 2',3'-d'] benzo[1,2-b: 4,5-b'] dithiophene-alt-benzothiadiazole derivative copolymers can be effectively regulated by the introduction of alkylthiophene spacers and/or fluorine atoms into the backbone.