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 influenced by the performance of the electroninjecting contact. Lowering the energetic barrier between the metal contact and the lowest unoccupied molecular orbital of the adjacent organic electron transport layer should facilitate the injection of negative charge carriers, and, thus, improve the electroluminescence yield by increasing the electron density in the emitting zone. Therefore, it is widely believed that lowering the work function of the cathode metal will improve the quantum efficiency of the devices and, concomitantly, reduce the operating voltage. Here, we report on measurements of devices with tris(8hydroxyquinolinolato)aluminum-(III) as electron transport and emissive layer. The latter layer is contacted with a variety of chemically 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 results are of practical importance with respect to the choice of pure cathode metals for organic electroluminescent display applications. PACS: 78.60.Fi; 73.61.Ph Since the first reports on organic electroluminescent devices (OLEDs) using small molecules as hole transporting, electron transporting, and emissive layer [1–4], there has been an increasing interest in the field of OLEDs for display applications. A lot of experimental and theoretical work has been done in order to achieve a more detailed understanding of the underlying physical processes, which plays a crucial role for improving quantum efficiency and operating stability of OLEDs. One major topic of interest is the electron-injecting contact. This is mostly due to the fact that in many devices holes seem to be the majority carriers [5]. Consequently, balanced charge carrier injection, which is of crucial importance for high quantum yields [6], has not been achieved yet for many device setups. Thus, the electron current from the electroninjecting contact to the emissive recombination zone appears to be the limiting factor for device efficiency. Several models for charge carrier injection from metal electrodes into adjacent organic materials have been described theoretically [7] as well as investigated experimentally [7–9]. One generalized prediction of these different models can be summarized as follows. A reduction of the energetic barrier between the Fermi level of the metal and the lowest unoccupied molecular orbital (LUMO) of the adjacent electron transport material will result in an enhanced electron injection current. Therefore, for cathode metals with very low work functions, the electron-injecting contact should approach ohmic behavior, which has already been achieved for hole injection from indium-tin-oxide (ITO) into various hole transporting materials [10]. In the present paper, we follow a macroscopic approach to shed light on the interplay between the metallic Fermi level on one hand and the energy levels of the adjacent organic materials on the other. For a variety of devices comprising different cathode metals with Fermi levels above and below the LUMO of the adjacent tris(8hydroxyquinolinolato)aluminum (Alq3), we will point out correlations of efficiency and onset voltage with the respective metal work function. The investigation of electroninjection mechanisms on a molecular scale is beyond the scope of this work. Generally, due to their chemical reactivity, low-workfunction metals are highly sensitive against moisture and oxygen which implies the necessity for hermetic OLED packaging. From a technical point of view, a compromise has to be found between device efficiency on one hand and packaging demands on the other. Thus, our investigations are relevant with respect to commercial OLED display development.