Impact of the cathode metal work function on the performance of vacuum-deposited organic light emitting-devices

The efficiency of organic light-emitting devices is significantly influence d by the performance of the electron-injecting contact. Lowering the energe tic barrier between the metal contact and the lowest unoccupied molecular o rbital of the adjacent organic electron transport layer should facilitate t he injection of negative charge carriers, and, thus, improve the electrolum inescence yield by increasing the electron density in the emitting zone. Th erefore, it is widely believed that lowering the work function of the catho de metal will improve the quantum efficiency of the devices and, concomitan tly, reduce the operating voltage. Here, we report on measurements of devic es with tris(8-hydroxyquinolinolato)aluminum-(III) as electron transport an d emissive layer. The latter layer is contacted with a variety of chemicall y very different cathode metals (including some lanthanides), which cover a range from 2.63 eV up to 4.70 eV on the work function axis. We demonstrate the existence of an efficiency maximum at a work function of about 3.7 eV which, to the best of our knowledge, has not been reported yet. These resul ts are of practical importance with respect to the choice of pure cathode m etals for organic electroluminescent display applications.