Terpolymer Donor with Inside Alkyl Substituents on Thiophene π‐Bridges toward Thiazolothiazole A(2)‐Unit Enables 18.21% Efficiency of Polymer Solar Cells

PM6 is a widely used D–A copolymer donor in the polymer solar cells (PSCs). Incorporating second electron‐withdrawing (A(2)) units into PM6 backbone by ternary D–A(1)–D–A(2) random copolymerization strategy is an effective approach to further improve its photovoltaic performance. Here, the authors synthesize the PM6‐based terpolymers by introducing thiazolothiazole as the A(2) units connecting with thiophene π‐bridges attaching alkyl substituent towards the A(2) unit (PMT‐CT) or towards D‐unit (PMT‐FT), and study the effect of the alkyl substituent position on the photovoltaic performance of them. Two terpolymers PMT‐FT‐10 and PMT‐CT‐10 are obtained by incorporating 10% A(2) units in the terpolymers. The film of PMT‐CT‐10 shows slightly up‐shifted highest occupied molecular orbital (HOMO) energy levels while better co‐planar structure than that of PMT‐FT‐10. Meanwhile, the PMT‐CT‐10:Y6 blend film exhibits better molecular packing properties, more proper phase separation and more balanced hole and electron mobilities, which are beneficial to more efficient exciton dissociation, efficient charge transport and weaker bimolecular recombination. Consequently, the PMT‐CT‐10 based PSCs obtain the highest power conversion efficiency of 18.21%. The results indicate that side chain position on the thiophene π‐bridges influence the device performance of the terpolymer donors, and PMT‐CT‐10 is a high efficiency polymer donor for the PSCs.