A Benzobis(thiazole)-Based Wide Bandgap Polymer Donor Enables over 15% Efficiency Organic Photovoltaics with a Flat Energetic Offset

Developing donor:acceptor (D:A) blends that work efficiently with small charge separation driving forces (or energetic offset) is highly desired for reducing the voltage loss and unveiling the working mechanism of organic photovoltaics (OPVs). Herein, by incorporating the weak electron-withdrawing acceptor unit of benzobis(thiazole), we design and synthesize two wide-bandgap (∼2 eV) polymer donors, PBTz-TF and PBTz-TCl, with deep highest occupied molecular orbital (HOMO) levels approaching that of the typical electron acceptor Y6, forming negligible HOMO offsets of 0.04 and 0 eV for PBTz-TF:Y6 and PBTz-TCl:Y6 blends, respectively. Interestingly, efficient charge separation is observed in both blends, while the PBTz-TCl:Y6 blend possesses multiple superiorities, including higher mobilities, longer exciton lifetimes, purer domains, and more dominant face-on orientation. These lead to a better efficiency of 15.06% at 0 eV HOMO offset in the PBTz-TCl:Y6-based OPV, which is superior compared to the PBTz-TF:Y6-based OPVs (14.73%) with a HOMO offset of 0.04 eV. This work explores the properties of low driving force D:A blends and should have implications in the fundamental understanding of the carrier dynamics in OPVs with a flat energetic offset.