Elucidating The Horizontal Orientation Mechanism of Emissive Transition Dipole Moment in A Series of Stick-Like Delayed Fluorescent Emitters for Ultra-High-Efficiency OLEDs

The horizontal orientation of the emissive transition dipole moment (TDM) of emissive molecules is a crucial factor in improving the efficiency of organic light-emitting diodes (OLEDs). However, the mechanism of this horizontal orientation remains unclear. Herein, to elucidate the horizontal orientation mechanism of the TDM of the molecules in thermally activated delayed fluorescent (TADF) emitters, the relationship between the horizontal orientation ratio (Theta) of the TDM and the key parameters of stick-like TADF emitters dispersed in carbazole-based host materials is systematically and quantitatively investigated. The introduction of functional groups containing weak CH/n (n = O, N) hydrogen bonds into the host material is found to significantly increase the Theta values up to 30%. The physical parameters of the dispersed emitter exhibit a strong relationship with the Theta values, with the following influence order: glass transition temperature (Tg) >= total surface area of the largest pi-plane and the corresponding parallel pi-plane > molecular weight > aspect ratio > permanent dipole moment. The Tg value of the host material is another key parameter determining the Theta values. By using a P=O-based host material, namely mCP2PO, a high PLQY of 97%, a high Theta value of 88%, and a sky-blue OLED with an EQE of 37.6% are simultaneously realized.