Multi-π-Stacked Through-Space Charge-Transfer Emitters with Accelerated Radiative Decay for High-Efficiency Organic Light-Emitting Diodes with Suppressed Efficiency Roll-Offs

Through-space charge-transfer (TSCT) thermally-activated delayed fluorescence (TADF) emitters have been extensively explored for organic light-emitting diodes (OLEDs), but their low radiative decay rates (k_r,s) have remained a formidable obstacle to enhancement of luminescent efficiency (Ф_PL) and suppression of efficiency roll-off of the OLED. Here, multi-π-stacked TSCT-TADF emitters with accelerated radiative decay are developed with a 3D [fixed acceptor]/donor/[fixed acceptor] structure formed on two individual fluorene bridges. Single-crystal structures and theoretical calculations reveal cis or trans-configurations of the two acceptors around the central donor and double TSCT channels in the emitter. In doped films, the emitters show green-blue TADF with high k_r,s up to 1.1 × 10⁷ s⁻¹ and near-unity Ф_PL, which are notably improved compared to the single-π-stacked analog (k_r,s = 2.1 × 10⁶ s⁻¹, Ф_PL = 0.89). OLEDs based on the emitters show high external quantum efficiencies (EQEs) up to 26.2% and the EQEs at 1000 cd m⁻² (EQE₁₀₀₀) remain up to 23.4%, which are superior over the device based on the single-π-stacked analog (EQE_max/EQE₁₀₀₀ = 20%/15.6%). A narrowband blue-green OLED with one emitter as a sensitizer shows high EQE_max/EQE₁₀₀₀ at 37.6%/27.7%. The work demonstrates the great potential of multi-π-stacked structure in enhancing the k_r,s of TSCT-TADF emitters for high-efficiency OLEDs with low efficiency roll-offs.