Enhanced Magneto‐Photoluminescence in Rubrene Polycrystalline Films via Triplet‐Pair Population Control for Magnetic Field Imaging

ABSTRACT Magneto‐photoluminescence (MPL) in organic materials offers a promising platform for flexible, low‐cost magnetic sensing and photonic spintronic technologies. However, strong magnetic responsiveness in films remains elusive due to inefficient spin interconversion in disordered morphologies. Here, we report a crystallization‐driven strategy to boost MPL in rubrene thin films. Thermal annealing induces the formation of polycrystalline structures with enhanced molecular order, yielding a record‐high MPL contrast of 40% between ±500 mT—an order of magnitude greater than amorphous counterparts. Spectroscopic studies reveal that improved molecular packing promotes the generation and lifetime extension of spin‐correlated triplet‐triplet (TT) pairs, enabling efficient singlet fission (SF) and triplet fusion (TF) under magnetic modulation. Temperature‐dependent investigations confirm the thermally activated nature of SF and the central role of TT pairs in MPL generation. These polycrystalline films enable magnetic field detection down to 2 µT with sub‐microsecond response and high spatial resolution, providing a compelling platform for scalable magneto‐optical applications in sensing and spin‐based photonics.