Isomerization of Benzothiadiazole Yields a Promising Polymer Donor and Organic Solar Cells with Efficiency of 19.0%

Abstract The exploration of high‐performance and low‐cost wide‐bandgap polymer donors remains critical to achieve high‐efficiency nonfullerene organic solar cells (OSCs) beyond current thresholds. Herein, the 1,2,3‐benzothiadiazole (iBT), which is an isomer of 2,1,3‐benzothiadiazole (BT), is used to design wide‐bandgap polymer donor PiBT. The PiBT‐based solar cells reach efficiency of 19.0%, which is one of the highest efficiencies in binary OSCs. Systemic studies show that isomerization of BT to iBT can finely regulate the polymers’ photoelectric properties including i) increasing the extinction coefficient and photon harvest, ii) downshifting the highest occupied molecular orbital energy levels, iii) improving the coplanarity of polymer backbones, iv) offering good thermodynamic miscibility with acceptors. Consequently, the PiBT:Y6 bulk heterojunction (BHJ) device simultaneously reaches advantageous nanoscale morphology, efficient exciton generation and dissociation, fast charge transportation, and suppressed charge recombination, leading to larger V OC of 0.87 V, higher J SC of 28.2 mA cm −2 , greater fill factor of 77.3%, and thus higher efficiency of 19.0%, while the analog‐PBT‐based OSCs reach efficiency of only 12.9%. Moreover, the key intermediate iBT can be easily afforded from industry chemicals via two‐step procedure. Overall, this contribution highlights that iBT is a promising motif for designing high‐performance polymer donors.