LAUSR

Understanding the hole‐injection mechanism and improving the hole‐injection property are of pivotal importance in the future development of organic optoelectronic devices. Electron‐acceptor molecules with high electron affinity (EA) are widely used in electronic applications, such as hole injection and p‐doping. Although p‐doping has generally been studied in terms of matching the ionization energy (IE) of organic semiconductors with the EA of acceptor molecules, little is known about the effect of the EA of acceptor molecules on the hole‐injection property. In this work, the hole‐injection mechanism in devices is completely clarified, and a strategy to optimize the hole‐injection property of the acceptor molecule is developed. Efficient and stable hole injection is found to be possible even into materials with IEs as high as 5.8 eV by controlling the charged state of an acceptor molecule with an EA of about 5.0 eV. This excellent hole‐injection property enables direct hole injection into an emitting layer, realizing a pure blue organic light‐emitting diode with an extraordinarily low turn‐on voltage of 2.67 V, a power efficiency of 29 lm W−1, an external quantum efficiency of 28% and a Commission Internationale de l'Eclairage y coordinate of less than 0.10.