Prevention of the cathode induced electrochemical degradation of [Ru(bpy)3](PF6)2 light emitting devices

Abstract

Light emitting electrochemical cells based on the $\mathrm{tris}{\text{(2,2}}^{\prime }$ bipyridyl) ruthenium(II) complex show improved performance if electrochemically stable materials such as Ag are used as the cathode material. In contrast, if Al is used as the cathode such devices undergo degradation when stored in the off-state in inert atmosphere. In this work, the mechanism of the aluminum-induced degradation is investigated. X-ray photoelectron spectroscopy shows that some of the Ru(II) complexes are reduced in the presence of the Al cathode to Ru(I). In addition, secondary ion mass spectrometry depth profiles indicate degradation of the indium tin oxide in devices with Al cathodes. Because of the mixed-valent Ru(II)/(I) states, devices with Al cathodes exhibit unipolar charge injection at voltages below the turn-on voltages. The unipolar charge injection can be described by a theory of charge hopping in mixed-valent redox systems. In addition, impedance analysis data at 0 V bias of devices with Al or Ag cathodes can be fit using simple equivalent electric circuits. In the case of Al devices, the equivalent electric circuit was modified to account for the redox conduction at 0 V bias and an oxide layer at the cathode interface.