Utilizing Spiro-Type Backbone Donor to Develop Exciplex Emitter and Highly Efficient Phosphorescent OLEDs With Low Efficiency Roll-Off

New electron-donating materials are crucial for developing efficient exciplex emitters and host matrices in simplified phosphorescent organic light-emitting diodes (PhOLEDs) with low driving voltage, high efficiency, and minimal efficiency roll-off. Herein, a spiro backbone with a highly sterically rigid structure and orthogonal configuration was attached to the commonly used 9H-carbazole (Cz) via para-linking, resulting in a new electron-donating material 9-(9, $9'$ -spirobi[fluoren]-2-yl)- 9H-Cz (SBF-Cz). The influence of the donor strength on the photophysical, electrochemical, and electroluminescent performances was studied. SBF-Cz exhibits a high T1 level, a shallow highest occupied molecular orbital (HOMO) level, and excellent thermal properties. Utilizing these properties, a simplified blue organic light-emitting diode (OLED) using SBF-Cz:2,4,6-Tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T) as the emitter achieves an EL emission peak at 476 nm and a maximum external quantum efficiency (EQEmax) of 11.2%. Furthermore, the spiro-type exciplex as a universal host was systematically investigated in blue, green, and red PhOLEDs. Notably, the EQEmax of blue, green, and red PhOLEDs using the spiro-type exciplex as host improved by 6.8%, 23.8%, and 11.1%, respectively, compared to the conventional host mCP. Additionally, the blue, green, and red PhOLEDs exhibit lower EQE roll-off of 8.7%, 8.1%, and 1.2% at a brightness of 1000 cd $\cdot $ m-2 compared to the reference devices. This work underscores the significance of the spiro-type donor in developing new blue exciplexes and host matrices for OLEDs.