Sterically‐Locked Donor–Acceptor Conjugated Polymers Showing Efficient Thermally Activated Delayed Fluorescence

Abstract Donor–acceptor (D–A) conjugated polymers often possess a significant frontier molecular orbital overlap because of the conjugation elongation, leading to no thermally activated delayed fluorescence (TADF) caused by a large singlet‐triplet energy splitting (▵ E ST ). Herein a novel steric locking strategy is proposed by incorporating methyl groups into D–A conjugated polymers. Benefitting from the methyl hindrance, the torsion between the donor and acceptor can be well tuned to form a sterically‐locked conformation, so that the unwanted relaxation toward planarity and thus conjugation elongation is prevented to boost hole–electron separation. The resultant D–A conjugated polymer achieves an extremely low Δ E ST of 0.09 eV to enable efficient TADF. The corresponding doped and non‐doped devices are fabricated via a solution process, revealing a record‐high external quantum efficiency (EQE) of 24.0 % (79.4 cd A −1 , 75.0 lm W −1 ) and 15.3 % (50.9 cd A −1 , 47.3 lm W −1 ).