Origin of external quantum efficiency degradation in organic light-emitting diodes with a DC magnetron sputtered cathode

This paper characterizes changes in organic light-emitting diodes to better understand the origin of the decrease in external quantum efficiency (η_ext) when switching the cathode deposition method from evaporation to DC magnetron sputtering. An increase of driving voltage and a hole-barrier-dependent decrease of η_ext when the Al cathode is sputtered suggest that disruption of carrier balance and penetration of holes from the emissive layer (EML) into the electron transport layer (ETL) are significant sources of the device degradation. When the ETL was doped with Li, degradation was suppressed and the increase in driving voltage was drastically reduced although η_ext still decreased by 5%-7%. Analysis of the films by time-of-flight secondary ion mass spectrometry indicates that Li diffuses into the EML when Al is sputtered, and Li is shown to act as an exciton quencher that can decrease η_ext. Doping of the ETL is also used to significantly suppress the performance reduction with sputtered cathodes even when using a phosphorescent emitter having high η_ext.