Using an organic radical precursor as an electron injection material for efficient and stable organic light-emitting diodes

Materials with strong reducibility have been used as electron injection layers (EILs) to lower the work function of cathodes and reduce the driving voltage of organic light-emitting diodes (OLEDs). However, the most prominent electron injection materials presented so far are high-temperature-evaporable inorganic salts based on alkaline metals, which suffer from a high tendency of metal diffusion throughout the organic layer and thus reduce the device efficiency and stability. Here, we introduce a new kind of EIL based on a stable precursor of a strongly reducing organic radical. By using an organic precursor, we are able to take the advantage of the low-evaporation-temperature and avoid the problem of metal diffusion, thus improving the device efficiency and stability. Ultraviolet photoelectron spectroscopy (UPS) study indicates that inserting a thin layer of organic radical between the electron transport layer and cathode could greatly reduce the electron injection barrier due to the strong interaction of radical with cathode and the electron transporting material. As a result, OLEDs with an organic radical as the EIL showed a 25.2% higher efficiency and 2.2 times longer lifetime than the control device with conventional LiF as the EIL.