本研究は、山形大学横山大輔研究室との共同研究でJSPS 科研費（基盤研究B, 20H02807, 23H02032）の助成を受けたものです。
Title: Elucidating the horizontal orientation mechanism of emissive transition dipole moment in a series of stick-like delayed fluorescent emitters for ultra-high-efficiency OLEDs
By Naoto Yoshida, Hisahiro Sasabe,* Hiroki Arai, Yuki Sagae, Yuro Yagi, Keigo Hoshi, Daisuke Yokoyama, and Junji Kido, Adv. Opt. Mater. 2023, accepted.
Abstract: 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, we systematically and quantitatively investigate the relationship between the horizontal orientation ratio (Θ) of the TDM and the key parameters of a series of stick-like TADF emitters dispersed in carbazole-based host materials. The introduction of functional groups containing weak CH/n (n = O, N) hydrogen bonds, such as cyano (CN) and phosphine-oxide (P=O), into the host material is found to significantly increase the Θ values up to 20%–30%. In particular, the P=O group produces a better effect than the CN group. The physical parameters of the dispersed emitter exhibit a strong relationship with the Θ values, with the following influence order: glass transition temperature (Tg) ≥ total surface area of the largest π-plane and the corresponding parallel π-plane > molecular weight > aspect ratio > permanent dipole moment. The Tg value of the host material is another key parameter determining the Θ values, following the aspect ratio of the emitter. By using a P=O-based host material, namely mCP2PO, we simultaneously realize a high photoluminescent quantum yield of 98%, a high Θ value of 88%, and a sky-blue OLED with a maximum external quantum efficiency (EQEmax) of 37.6%. The CN-based host material, namely mCPCN, can help realize an EQEmax of 38.2%.