High Performance NIR OLEDs with Low Efficiency Roll‐Off by Leveraging Os(II) Phosphors and Exciplex Co‐Host

Near‐infrared (NIR) organic‐light emitting devices (OLEDs) with high radiance are useful for applications including invisible marking, communication, and biomedical imaging. However, performances of NIR OLEDs are typically limited by their severe efficiency roll‐offs at high current density. Herein, three isoquinolinyl azolate based Os(II) complexes (Isq‐1–3) with short radiative decay lifetime (in hundreds of ns), and photoluminescence with peak wavelengths > 745 nm and quantum yield up to 48% as doped thin films, are reported. Upon concomitant employment of exciplex‐forming co‐host (tris(4‐carbazoyl‐9‐ylphenyl)amine and 2,4,6‐tris(biphenyl‐3‐yl)‐1,3,5‐triazine), efficiency roll‐off is greatly reduced, giving external quantum efficiency of 9.66% at a current density of 300 mA cm−2. A maximum radiance over 170 W sr−1 m−2 is also achieved in devices based on Isq‐2 and Isq‐3. Highly efficient near‐infrared OLEDs with an external quantum efficiency exceeding 10% and electroluminescence maxima exceeding 750 nm are successfully fabricated with new Os(II) phosphors and exciplex co‐host. The bipolar feature of the exciplex co‐host greatly reduces efficiency roll‐off up to 300 mA cm−2.