Germanium-integrated exciplex host for high-performance narrowband OLEDs with mitigated efficiency roll-off

Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials open a heavy-metal-free avenue toward highly efficient, narrowband organic light-emitting diodes (OLEDs). However, their slow reverse intersystem crossing (RISC) kinetics lead to severe efficiency roll-off at practical driving currents, and existing strategies to improve RISC often require emitter-specific chemical modifications. Here we demonstrate a general strategy by integrating heavy-atom germanium into hole-transporting/electron-transporting hosts to construct an exciplex system that accelerates spin-flip dynamics. This design enhances RISC rates in diverse MR-TADF emitters via Förster resonance energy transfer without altering their electronic structure. The resulting blue and green OLEDs achieve external quantum efficiencies exceeding 40% with markedly suppressed efficiency roll-off relative to silicon- and carbon-based counterparts. Notably, the green device exhibits a half-lifetime of 241 h at an initial luminance of 1000 cd m^-2. This general host-guest strategy advances MR-TADF systems toward practical high-performance displays.