Abstract
We theoretically model the near-field (NF) absorption for a multilayer micro-cavity (MMC) structure and investigate the contribution of the NF absorption to the dipole radiation power in top-emitting organic light-emitting diodes (OLEDs). The NF absorption occurs due to the interaction between an evanescent wave with a large in-plane wave vector and a planar metal layer in the vicinity of the dipole radiation. The analytical expressions of the NF absorption in the MMC structure are derived from the plane wave expansions of the electric field amplitude, which includes the two-beam and multi-beam interference terms. The transverse magnetic polarization light emitted by both horizontally and vertically oriented dipole emitters is considered in the NF absorption while the contribution of the transverse electric polarization light is neglected. Based on the total spectral power density calculated in a top-emitting OLED, the respective spectral response functions of surface plasmon (SP) modes and NF absorption are compared, where the summation of the Lorentzian line shape functions is used to represent spectral responses of SP modes. At large values of in-plane wave vectors, the spectral response caused by the NF absorption becomes significant and approaches the total spectral power density. In addition, the relative optical powers from various dipole dissipation mechanisms are calculated with respect to the dipole emitter position in the emission layer (EML), which shows the optical power coupled to the NF absorption is predominant over other mechanisms when the distance between the dipole emitter and the EML/Ag interface is less than 10 nm in the top-emitting OLED.
Highlights
Optical modeling of organic light-emitting diodes (OLEDs) based on the transfer matrix formulation has been intensively studied to optimize various output emission characteristics such as the out-coupling efficiency and angular emission dependence [1,2,3,4,5,6,7,8].Modal analysis is important to identify how the radiation of a dipole exciton inside the emission layer (EML) is coupled into various optical modes of the OLED
As organic materials in an OLED have relatively similar refractive indices [28,29,30], it is sometimes considered that all organic layers such as the electron transport layer and EML have the same refractive index value [31] or assumed that all the organic layers are integrated into the one organic layer
The analytical formulation of the relative optical power coupled to the NF absorption was derived in the multilayer micro-cavity (MMC) structure based on the plane wave expansions of the electric field amplitude at large in-plane wave vectors
Summary
Optical modeling of organic light-emitting diodes (OLEDs) based on the transfer matrix formulation has been intensively studied to optimize various output emission characteristics such as the out-coupling efficiency and angular emission dependence [1,2,3,4,5,6,7,8]. According to the classical electromagnetic models, these exciton dissipation mechanisms have been successfully quantified based on the power dissipation spectrum, where the spectral power density is calculated with respect to the normalized in-plane wave vector u [10,11,12,13,14]. In the evanescent spectral region of the power dissipation spectrum (u > 1), the radiated wave from the dipole emitter is evanescent in nature because it exponentially decays in the direction normal to a dielectric/metal interface. Besides the SP modes in the evanescent spectral region, there is another dissipation mechanism of the near-field (NF) absorption, which occurs due to the interaction between an evanescent wave with a large in-plane wave vector and a planar metal layer in the vicinity of the dipole radiation [18]. The relative optical powers from various dipole dissipation mechanisms are calculated with respect to the position of the dipole emitter in the EML, which shows the optical power coupled to the NF absorption is predominant at both sides of the metal/EML interfaces
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