Direct evidence for degradation of polaron excited states in organic light emitting diodes

Abstract

We investigate the intrinsic degradation mechanisms of the prototypical phosphorescent emissive material fac-tris(2-phenylpyridine) iridium [Ir(ppy)3] doped into the host 4, 4′-bis(3-methylcarbazol-9-yl)-2,2′-biphenyl (mCBP) by separately evaluating the effects of unipolar current, optical excitation, and their combination. We find that the mCBP anion is unstable and becomes more so in its excited state. Degradation due to the formation of defect states is evident from changes in the capacitance-voltage characteristics and from increasing drive voltage over time of a unipolar test device. These changes are understood within the framework of trapped-charge-limited transport, allowing for the determination of rate constants for each degradation mechanism. We also observe degradation of the hole transport material 4, 4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl under sub-energy-gap illumination and suggest that this instability may proceed through excitation of its cationic state. These results provide direct evidence for polaron-induced degradation that limits the operational lifetime of organic light emitting diodes.