Switching from Thermally Activated Delayed Fluorescence in Single Crystals for Low‐Threshold Laser to Room‐temperature Phosphorescence in Amorphous‐Film for Highly Efficient OLEDs.

Metal‐organic phosphorescent complexes containing Ir or Pt are work horse in organic light‐emitting diode (OLED) technology, which can harvest both singlet and triplet excitons in electroluminescence (EL) owing to strong heavy‐atom effect. Recently, organic room‐temperature phosphorescence (ORTP) have achieved high photoluminescence quantum yield (PLQY) in rigid crystalline state, which, however, is unsuitable for OLED fabrication, therefore leading to an EL efficiency far low behind those of metal‐organic phosphorescent complexes. Here, we reported a luminescence mechanism switch from thermally activated delayed fluorescence (TADF) in single crystal microwires to ORTP in amorphous thin‐films, based on a tert‐butylcarbazole difluoroboron β‐diketonate derivative of DtCzBF2. Tightly packed and well‐faceted single‐crystal microwires exhibit aggregation induced emission (AIE), enabling TADF microlasers at 473 nm with an optical gain coefficient as high as 852 cm−1. In contrast, loosely packed dimers of DtCzBF2 formed in guest‐host amorphous thin‐films decrease the oscillator strength of fluorescence transition but stabilize triplets for ORTP with a PLQY up to 61 %, leading to solution‐processed OLEDs with EQE approaching 20 %. This study opens possibilities of low‐cost ORTP emitters for high performance OLEDs and future low‐threshold electrically injected organic semiconductor lasers (OSLs). [ABSTRACT FROM AUTHOR]