Spiro[Acridine‐Indoloacridine] Donor Based TADF Emitter with Near‐Unity PLQY and Horizontal Dipole Orientation toward High‐Efficiency OLEDs with Low Efficiency Roll‐Off

Abstract Designing thermally activated delayed fluorescence (TADF) emitters with excellent photoluminescence quantum yield (PLQY), preferential horizontal emitting dipole orientation (𝛩 // ), and fascinating ability to rapidly utilize both singlet and triplet excitons for radiation is crucial for high‐performance organic light‐emitting diodes (OLEDs) with both high efficiency and relieved efficiency roll‐off. Integrating these advantages into a single TADF emitter is a formidable challenge. In this work, a spiro[acridine‐indoloacridine]‐based rigid donor, named AIA, is designed to establish a cooperatively long‐axis extended and conjugation expanded donor–acceptor type TADF emitter (AIA‐TRZ). The proof‐of‐the‐concept emitter simultaneously achieves an excellent PLQY of 99%, a near‐unity 𝛩 // of 99%, and balanced excited state dynamic parameters. OLEDs based on AIA‐TRZ realize remarkable performance with an emission peak at around 480 nm, including doping concentration insensitivity, a maximum external quantum efficiency (EQE max ) of 37.3%, extremely low efficiency roll‐off, and over 20% EQE even at 14000 cd m −2 .