Regioregular D₁‑A‑D₂‑A Terpolymer with Controlled Thieno[3,4‑<i>b</i>]thiophene Orientation for High-Efficiency Polymer Solar Cells Processed with Nonhalogenated Solvents

A regioregular D₁-A-D₂-A terpolymer PDTST­TBDT incorporating dithieno­[3,2-<i>b</i>:2′,3′-<i>d</i>]­silole (DTS, D₁) and benzo­[1,2-<i>b</i>:4,5-<i>b</i>]­dithiophene (BDT, D₂) units with perfectly controlled thieno­[3,4-<i>b</i>]­thiophene (TT, A) orientation was synthesized for the first time. The thermal, optical, and electrochemical properties of the regioregular PDTSTTBDT were characterized and compared with the random PDTSTTBDT without structural regioregularity. The regioregular PDTSTTBDT showed ideal optical bandgap (1.45 eV), lower lying HOMO energy level, and higher degree of crystallinity compared to the random PDTST­TBDT. Moreover, it exhibited excellent solubility in nonhalogenated solvents as well as halogenated solvents. The inverted bulk-heterojunction polymer solar cells (PSCs) based on the regioregular PDTST­TBDT and <i>o</i>-xylene process solvent showed a power conversion efficiency as high as 6.14%, which is 500% higher than the random PDTST­TBDT-based PSCs. It was found that the remarkable enhancement of photovoltaic performance in regioregular PDTST­TBDT-based PSCs is mainly due to improved light absorption, effective polymer ordering, and high charge carrier mobility