Impact of the metal cathode and CsF buffer layer on the performance of organic light-emitting devices

The influences of different metal cathodes on the performance of organic light-emitting devices were systematically studied. In addition to the well-known effects of metal work function, the effects of reflectivity and reactivity of the metal cathode on the device efficiency and operational stability were explored. The interplays of different metal cathodes and a CsF buffer layer were also studied in standard α-napthylphenylbiphenyl diamine/tris-(8-hydroxyquinoline) aluminum $({\mathrm{NPB/Alq}}_3)$ devices. It was found that when the metal cathode is directly deposited on the organic layer, the device performance improves as the metal work function decreases. This effect is modulated by the metal reflectivity such that rare-earth metal cathodes, which typically have a lower reflectance, have a lower efficiency than alkaline-earth metal cathodes. Device operational stability is found to be related to the reactivity between ${\mathrm{Alq}}_3$ and the metal cathode. Devices with metal cathodes that react detrimentally with ${\mathrm{Alq}}_3,$ such as Al and Ca, have a much lower lifetime than devices with other cathodes. On the other hand, when a thin CsF buffer layer was inserted at the organic/metal interface, $\mathrm{I-V}$ characteristics and efficiency improve considerably and become almost independent of the metal work function. Device lifetime also increases, and its dependence on the reactivity between the ${\mathrm{Alq}}_3$ and the metal cathode remains.