Applications of low-refractive-index materials for high-efficiency organic light-emitting diodes

Abstract

Organic light emitting diodes (OLEDs) are considered as one of the promising next-generation lighting sources due to their design form-factor, high color purity, self-emission property, and so on. However, most of the generated photons from an organic emissive layer which shows a refractive index around 1.7 are confined within the substrate, and/or transparent electrode/organic layer due to total internal reflection or dissipated by the surface plasmon polariton from metal layers. To overcome these issues, many researchers have proposed out-coupling structures such as the use of micro- or nano- structures to enhance extraction efficiency, but the use of these structures still have limitations to apply into practical display applications in terms of deterioration of visibility and high fabrication cost. Herein, novel outcoupling schemes through the planar structure with low-index layers has been proposed, which (i) can make enhanced Fabry-Perot resonance from the increased Fresnel reflectance with a high index layer and (ii) can reduce surface plasmon polariton (SPP) mode by controlling the dispersion relation between metal and dielectric layers. The first part of the dissertation is focused on the optimization of micro-cavity effects by tailoring the architectures of transparent electrode consisting of high/low-index layers. To realize ultra-high efficiency, a systematic study has been carried out by controlling device stacks containing high-index ($TiO_2$, IZO) and low-index (PEDOT:PSS) layers to identify how the resonance effect and the amount of loss modes change. As a result, we realized a maximized cavity resonance even without semitransparent metal electrodes and achieved a highly efficient performance upon optimization of the $1^{st}$-order cavity. Second, we investigated the effects of the low-index buffer layer for reducing surface plasmon polariton (SPP) mode. It is observed that placing the low-index media adjacent to metal electrodes helps in shifting the SPP mode toward a lower in-plane wavevector and the corresponding amount was reduced. In addition, since SPPs is caused only by transverse magnetic (TM) polarization, the phenomenon of SPP reduction was analyzed according to optical anisotropy of the buffer layer. Through anisotropy-based analysis using the dipole oscillator model, SPP suppressed devices in both bottom- and top- emission cases were designed. In this way, ultrahigh-efficiency OLEDs were able to be demonstrated. To further verify the proposed concepts, we compared the transient photoluminescence (PL) signal to monitor the concomitant change in Purcell factor in SPP reduced optical environments. In summary, the study in this dissertation demonstrates novel strategies to realize ultra-high efficiency OLEDs through an optical design based on low-index layers that maintain the planar geometry. With these approaches, we expect that the power consumption of future displays and lightings can be reduced significantly in a device architecture that can be readily adopted in the industry.