Steric‐Hindrance Engineering for Optimizing the Aggregation Behavior Enables High‐Performance Solution‐Processed NIR TADF‐OLEDs Approaching 900 nm

ABSTRACT Achieving efficient near‐infrared (NIR) emission in heavy‐metal‐free organic light‐emitting diodes (OLEDs) remains challenging due to the energy gap law and aggregation‐caused quenching (ACQ). Alternatively, chromophore aggregation offers a promising strategy to attain long‐wavelength NIR emission. Herein, we perform steric‐hindrance engineering on donor units to explore the complex interplay between molecular structure, aggregation behavior, and photophysical performance in organic donor (D)‐acceptor (A) systems. Among the designed four compounds, the moderately bulky and fluorene‐decorated F‐BTAP achieves optimal photophysical performance, striking a balance between pronounced aggregation‐induced redshift and mitigated concentration quenching. Consequently, F‐BTAP exhibits the strongest aggregation‐induced emission effect, the longest emission wavelength, and the highest photoluminescence quantum yield (PLQY) of 3.1% at 881 nm in nondoped films. Solution‐processed nondoped OLED based on F‐BTAP exhibits a maximum external quantum efficiency of 0.30% with an electroluminescence peak at 892 nm, representing one of the best results among NIR thermally activated delayed fluorescence OLEDs. Notably, the device demonstrates an operation lifetime LT 50 exceeding 700 h under high current density of 110 mA·cm −2 . This study provides new insights into the aggregation behavior of organic D‐A systems and paves the way for developing metal‐free NIR organic emitters with bright aggregated‐state emission toward practical applications.