Mechanism Study and the Role of Coupled Surface Plasmon Polaritons in Optical Out-Coupling Enhanced Top-Emitting OLEDs with a Dielectric Capped Ag Cathode

Abstract Classical optical transfer matrix method and micro-cavity theory is applied to compute optical out-coupling of Top-Emitting OLEDs (TEOLEDs). Optical paths of various functional organic films and electrode structures are included in the calculations. Fluorescent Alq3 green TEOLEDs were fabricated and tested based on theoretical calculations. For TEOLEDs with Ag cathodes plus NPB coatings, we experimentally observed significant enhancements in optical out-couplings compared to that of with bare Ag cathode. We also found that thin-film optics theory is insufficient to explain the observed enhancement. To explain the experimental results, we incorporated near-field radiation of electric dipole and its interactions with Ag cathode film, i.e., surface plasmon polaritons (SPPs) theory, coupled SPPs in a dielectric/metal/dielectric (D/M/D) structure. Therefore, we demonstrate that optical out-coupling enhancement has three contributing factors in a TEOLED: an increased transmittance of cathode Ag, a varied internal micro-cavity gain, and the dominant improved SPP coupling on both sides of the Ag film cathode.