Organic Radicals Outperform LiF as Efficient Electron-Injection Materials for Organic Light-Emitting Diodes

One of the key issues for organic light-emitting diodes (OLEDs) is to achieve high electroluminescence efficiency and high power efficiency, which requires extremely efficient electron injection and thus low driving voltage. Here, we design a series of precursors for reactive organic radicals according to theoretical calculations and achieve efficient electron injection by using a highly reducing radical on the surface of the electron injection layer to reduce the electron injection barrier through an interface charge-transfer process. In contrast to bulk charge transfer in electron-transporting material, interface charge transfer allows us to make efficient electron injection at contact without introducing any structural and electronic disorder to electron-transporting material. 2-(2,4,6-Trimethoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium (R3), with the strongest electron-donating ability, could largely reduce the electron injection barrier and outperform the previously reported organic radical (2-(2-methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium, o-MeO-DMBI or R1) and the widely used electron injection material (LiF) to boost device performance.