Solution-Processed Self-Stratifying Layer with Controllable Dielectric Polarization for High-Luminance Organic Light-Emitting Diodes

Spin-coated poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) layers are well known to show a PSS-rich surface layer. Such a self-stratifying PEDOT:PSS layer has been applied for improving maximum external quantum efficiency (EQE) of organic light-emitting diodes (OLEDs). However, such devices typically show much reduced high-luminance performance affecting practical applications of such a self-stratifying interlayer (SSL). In this work, we demonstrate that a simple ionization process can eliminate the adverse effects at high luminance while maintaining high maximum EQE. It is shown that ions of the salt can interact with hydroxyl groups of the PSS polymer and thus disorder the orientation polarization. This implies that the ionization process enables active tuning of the dielectric properties of the PEDOT:PSS layer. It reduces carrier accumulation caused by orientation polarization of the SSL and thus suppresses both exciton annihilation and electric stress across the emitting layer in OLEDs. With this strategy, the device using the self-stratifying PEDOT:PSS layer shows a wide window of operating current density which is nearly 6 times compared with that of the corresponding device without the treatment. This enables 5 times of luminance and operation lifetime enhancements.