Although significant progress has been made in the development of light-emitting materials for organic light-emitting diodes along with the elucidation of emission mechanisms, the electron injection/transport mechanism remains unclear, and the materials used for electron injection/transport have been basically unchanged for more than 20 years. Here, we unravelled the electron injection/transport mechanism by tuning the work function near the cathode to about 2.0 eV using a superbase. This extremely low-work function cathode allows direct electron injection into various materials, and it was found that organic materials can transport electrons independently of their molecular structure. On the basis of these findings, we have realised a simply structured blue organic light-emitting diode with an operational lifetime of more than 1,000,000 hours. Unravelling the electron injection/transport mechanism, as reported in this paper, not only greatly increases the choice of materials to be used for devices, but also allows simple device structures.
The realisation of a cathode with various work functions (WFs) is required to maximise the potential of organic semiconductors that have various electron affinities. However, the barrier-free contact for electrons could only be achieved by using reactive materials, which significantly reduce the environmental stability of organic devices. We show that a stable electrode with various WFs can be produced by utilising the coordination reaction between several phenanthroline derivatives and the electrode. Although the low WF of the electrode realised by using reactive materials is specific to the material, the WF of the phenanthrolinemodified electrode is tunable depending on the amount of electron transfer associated with the coordination reaction. A phenanthroline-modified electrode that has a higher electron injection efficiency than lithium fluoride has been demonstrated. The observation of various WFs induced by the coordination reaction affords strategic perspectives on the development of stable cathodes unique to organic electronics.
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.
Air-stable OLEDs are necessary for expanding the availability of OLEDs. We demonstrated a highly efficient and air-stable inverted OLED. The driving voltage of the inverted OLED is significantly reduced utilizing a novel heavy-doping technique. The surface work function of the doped electron injection layer reaches only 3.1 eV without the use of alkali metals.
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