NIR OLED, especially devices with emission wavelength reaching or exceeding 1000 nm (i.e. within the NIR(II) region) play important roles in phototherapy, bioimaging and wearable healthcare products (such as oximeters and phototherapeutic patches). However, hampered by the energy gap law, when the emission gap shifts to the deep red and near-infrared regions, the fast non-radiative deactivation process will greatly reduce the luminescence intensity, resulting in significant efficiency quenching, limiting the development of NIR OLEDs.
To address this problem, recently, Prof. Liang-Sheng Liao from the Institute of Functional Nano and Soft Materials of Soochow University, Prof. Wen-Yi Hung from National Taiwan Ocean University, Prof. Yun Chi from City University of Hong Kong, and Prof. Pi-Tai Chou from National Taiwan University have jointly carried out a series of studies. Based on their previous collaborative research output (Nat. Photonics 2020, 14, 570-577), they further investigated the contribution of each coupled vibrational mode to non-radiative transitions in molecules. Two strategies were proposed to achieve highly efficient NIR emission through molecular packing to extend exciton delocalization and deuterium substitution to reduce molecular high-frequency vibration. To this end, the collaborative team fully utilized the synergy of self-assembled exciton delocalization to reduce the reorganization energy and elimination of C-H-associated high-frequency vibrations to suppress the Franck-Condon overlap, confirming the phosphorescent emitters based on Pt(II) complexes can break through the limitation of the energy gap law in the NIR(II) region. Ultimately, they used a series of novel Pt(II) complexes based on fused aromatics to extend π-conjugation and guarantees the planarity and rigidity of the molecule simultaneously, thereby promotes exciton delocalization. NIR OLEDs based on these novel Pt(II) complexes exhibit excellent device performance beyond 930 nm, where the per-DR-based device has an emission peak approximately at 1000 nm with a maximum external quantum efficiency of 4.31%. At an operating voltage of only 4.5 V, it exhibits a bright radiance of over 10 W sr-1 m-2. This research opens up a new avenue for the development and application of high-performance OLEDs in the NIR(II) region. The related results have been published on Nature Photonics (DOI: 10.1038/s41566-022-01079-8). Xue-Qi Wang, a master student from Prof. Liang-Sheng Liao's group, is one of the co-first authors of this paper.
Figure 1. The computational approach of studied Pt(II) complexes.
Figure 2. Device characteristics of the NIR OLEDs using the studied Pt(II) complexes as the EML.
Link to article:https://www.nature.com/articles/s41566-022-01079-8
Link to article: Polyatomic molecules with emission quantum yields >20% enable efficient organic light-emitting diodes in the NIR(II) window
Authors: Sheng-Fu Wang, Bo-Kang Su, Xue-Qi Wang, Yu-Chen Wei, Kai-Hua Kuo, Chih-Hsing Wang, Shih-Hung Liu, Liang-Sheng Liao, Wen-Yi Hung, Li-Wen Fu, Wei-Tsung Chuang, Minchao Qin, Xinhui Lu, Caifa You, Yun Chi & Pi-Tai Chou