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

Abstract 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 × 10 7 s −1 and near‐unity Ф PL , which are notably improved compared to the single‐π‐stacked analog ( k r,s = 2.1 × 10 6 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 (EQE 1000 ) remain up to 23.4%, which are superior over the device based on the single‐π‐stacked analog (EQE max /EQE 1000 = 20%/15.6%). A narrowband blue‐green OLED with one emitter as a sensitizer shows high EQE max /EQE 1000 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.