Suppressing Defect Formation Pathways in the Direct C-H Arylation Polymerization of Photovoltaic Copolymers

Direct CH arylation polymerization (DARP) holds great promise for the green, atom-efficient synthesis of p-conjugated copolymers for use in high-performance polymer solar cells (PSCs). However, CH arylation regioselectivity control for monomers containing multiple reactive aryl CH bonds is not well understood, and nonselective reactivity results in material defects with unknown effects on PSC performance. Here, the effects of reaction conditions on copolymer molecular mass, dispersity, and PSC performance as well as defect formation pathways occurring during the DARP synthesis of an archetypal benzodithiophene-alt-diketopyrrolopyrrole copolymer, PBDTTDPP, are scrutinized. Small molecule model studies analyzed by HPLC-HRMS elucidate the effects of DARP conditions on trace chemical defect (primarily hydrodehalogenation and beta-CH arylation) formation. Copolymer branching arising from nonselective beta-CH arylation of monomers at the polymer chain end is identified as the principal photovoltaically deleterious defect. Fine-tuning the DARP reaction conditions reduces branching densities to below 1%, with an exceptional CH regioselectivity of >110:1. The optimal copolymers achieve superior PSC performance versus defect-rich DARP-derived copolymers and rival those from Stille polycondensations.