Ultrathin MoO₃ Layers in Composite Metal Electrodes: Improved Optics Allow Highly Efficient Organic Light‐Emitting Diodes

Abstract Molybdenum trioxide (MoO₃) has been investigated as a seeding and energy‐level modification layer for ultrathin metal electrode in organic light‐emitting diodes (OLEDs). However, the optical effect of this interlayer has not been addressed so far. In this article, the influence of the thin MoO₃ layer for bottom emitting OLEDs is thoroughly investigated. It is noted that a thin MoO₃ layer has a substantial impact on the device performance through inducing a synergistic effect of suppressing the nonradiative surface plasmon polariton (SPP) modes and modulating the cavity resonance. In presence of MoO₃ layer, the SPP mode is suppressed from 400 to 520 nm. Moreover, introducing a thin MoO₃ layer reduces the reflectance of the Au/Ag electrode, resulting in a decreased destructive resonance in devices. These combined effects enable to build highly efficient OLEDs, with the external quantum efficiency and luminous efficacy of two‐unit stacked white OLEDs reaching 38.8% and 53.9 lm W⁻¹ at 1000 cd m⁻², when substrate and air modes are extracted. The efficiency is about 1.9 times higher compared to the one without MoO₃ layer. The result provides insights into indium tin oxide free optoelectronic devices, as well as light extraction strategy and color purity manipulations for OLEDs.