Efficient Derivatization of a Thienobenzobisthiazole‐Based π‐Conjugated Polymer Through Late‐Stage Functionalization Towards High‐Efficiency Organic Photovoltaic Cells.

Derivatization is essential for optimizing organic material properties. However, because functional groups are often introduced at an early stage of the synthesis, similar intermediates have to be repeatedly synthesized to produce derivatives, which amounts to a daunting and time‐consuming task. Using thienobenzobisthiazole (TBTz) as a building unit of donor polymers for organic photovoltaics (OPVs), we demonstrate an efficient derivatization of a TBTz‐based π‐conjugated polymer by late‐stage functionalization. In the developed synthetic route, functional groups are introduced at the last step of monomer synthesis, enabling us to easily synthesize several derivatives from a common intermediate. Ester and acyl groups are introduced into the polymer instead of the alkyl group, giving rise to deep HOMO energy levels and resulting in OPV cells with high open‐circuit voltage even in the absence of halogen substituents that are typically introduced into the donor polymers. Notably, the ester‐functionalized TBTz‐based polymer shows a small nonradiative voltage loss (ΔVnr) of 0.19 V and has one of the highest charge generation efficiencies among the halogen‐free donor polymers with similar ΔVnr, improving the critical trade‐off relationship between voltage loss and charge generation. Our results provide an important guideline for the efficient development of high‐performance polymers for OPVs. [ABSTRACT FROM AUTHOR]