Mitigating Efficiency Roll‐Off in Phosphor‐Sensitized Fluorescence OLEDs via Molecular Polarity‐Based Benzofuropyridine Host Engineering

Two benzofuropyridine‐based n‐type host materials incorporating dibenzofuran core, 4,6‐bis(benzofuro[2,3‐b] pyridin‐3‐yl) dibenzo [b, d] furan (BFPDB‐1) and 4,6‐bis(benzofuro[2,3‐b] pyridin‐6‐yl) dibenzo [b, d] furandibenzo [b, d] furan (BFPDB‐2) , were designed to investigate the impact of positional isomerism on exciton management in OLEDs. Subtle relocation of the pyridine nitrogen significantly modulates molecular polarity, charge‐transfer character, and emissive‐layer morphology, as confirmed by theoretical and experimental analyses. While conventional phosphorescent devices exhibit comparable efficiencies, pronounced performance differences emerge in phosphor‐sensitized fluorescence (PSF) OLEDs. The BFPDB‐2 ‐based PSF device achieves a maximum external quantum efficiency of 22% and retains 20.7% EQE at 10,000 cd m −2 . This suppressed roll‐off is consistent with improved energy funneling and reduced short‐range local association, as suggested by transient photoluminescence (TRPL) and molecular dynamics (MD) / radial distribution function (RDF) analyses, which are associated with the distinct physicochemical properties of BFPDB‐2 , including its larger dipole moment, improved charge‐transport balance, and more spatially distributed host–guest arrangement. These findings suggest that precise heteroatom placement, which enables modulation of molecular polarity, is a promising design direction for improving host–guest interactions and achieving high‐efficiency, low‐roll‐off PSF OLEDs.