Tuning Ternary Deep Red Exciplex‐Forming Hosts to Achieve a Stable OLED with EL Peak Centered at 834 nm

This study explores new ternary exciplex‐forming systems comprising a deep red‐emitting CPF: 58p‐QN blend and various ratios of spacer TPF to optimize donor‐acceptor interactions and exciplex characteristics. Time‐resolved photoluminescence reveals delayed fluorescence of CPF: 58p‐QN :TPF blends, confirming the thermally activated delayed fluorescence (TADF) characters. By introducing different ratios of TPF, a progressive blueshift emission wavelength ranging from 696 nm (without TPF) to 659 nm (50 wt.% TPF) is observed. Notably, device A2 , featuring CPF: 58p‐QN :TPF (2:2:1) blend as emitting layer, achieves a maximum external quantum efficiency (EQE max ) of 2.13% with the electroluminescent peak (EL λ max ) centered at 672 nm. Moreover, a fluorescence emitter iCzPBBT is introduced as a dopant to realize a near‐infrared (NIR) emissive device. Device B2 , utilizing the CPF: 58p‐QN :TPF (2:2:1) blend as host doped with 5 wt.% iCzPBBT , exhibits an EQE max of 1.35% (EL λ max = 848 nm), demonstrating effective energy transfer from exciplex to NIR dopant. Device C2 with a reduced amount of iCzPBBT (2 wt.%) to mitigate concentration quenching achieves an EQE max of 1.72% (EL λ max = 834 nm) and good stability (LT 90 > 88 h under a constant current density of 0.6 mA cm⁻ 2 ). This study underscores the potential of a ternary exciplex‐forming system as a promising host for NIR OLED applications.