ABSTRACT
We report the electroluminescence (EL) characteristics of blue ultra-thin emissive layer (U-EML) phosphorescent (PH) organic light-emitting diodes (OLED) and thermally activated delayed fluorescence (TADF) OLED. A variety of transport layer (TL) materials were used in the fabricated OLEDs. The well-known FIrpic and DMAC-DPS were used with a thickness of 0.3 nm, which is relatively thicker than the optimal thickness (0.15 nm) of the blue phosphorescent ultra-thin emissive layer to ensure sufficient energy transfer. While FIrpic showed overall high efficiency in various TLs, DMAC-DPS exhibited three times lower efficiency in limited TLs. To clarify/identify low efficiency and to improve the EL, the thickness of DMAC-DPS was varied. A significantly higher and comparable efficiency was observed with a thickness of 4.5 nm, which is 15 times thicker. This thickness was oriented from the TADF itself, which reduces quenching in a triplet-triplet annihilation compared to the PH process. The thinner optimal thickness compared with ~30 nm of fluorescent OLEDs suggests that there still is quenching taking place. We expect that the efficiency of TADF U-EML OLEDs can be enhanced through further research on controlling the exciton quenching using multiple U-EMLs with spacers and a novel material with a high energy transfer rate (ΔES-T).
ABSTRACT
In this study, we demonstrated organic light-emitting diodes (OLEDs) outcoupling with a flexible polydimethylsiloxane (PDMS) film with a micro-convex structure using the breath figure (BF) method. We can easily control the micro-convex pattern by adjusting the concentration of polystyrene and the humidity during the BF process. As process conditions to fabricate the micro-convex structure, polymer concentrations of 10, 20, 40, and 80 mg/mL and 60, 70, and 80% relative humidity were used. To evaluate the optical properties, we analyzed the transmission, diffusion, and electroluminescence with or without the micro-convex structure on the OLEDs. The shape and density of the micro-convex structure are related to its optical properties and outcoupling and we have experimentally demonstrated this. By applying a micro-convex structure, it achieved up to a 42% improvement in the external quantum efficiency compared to bare OLEDs (without any light extraction film). We expect the fabricated flexible light extraction film to be effective for outcoupling and applicable to flexible devices.
ABSTRACT
This study investigates the application of scattering structures to the metal layer in a DMD (Dielectric/Metal/Dielectric) configuration through plasma treatment. The purpose is to enhance the light extraction efficiency of organic light-emitting diodes (OLEDs). Different plasma conditions were explored to create scattering structures on the metal layer. The fabricated devices were characterized for their electrical and optical properties. The results demonstrate that the introduction of scattering structures through plasma treatment effectively improves the light extraction efficiency of OLEDs. Specifically, using O2-plasma treatment on the metal layer resulted in significant enhancements in the total transmittance, haze, and figure of merit. These findings suggest that incorporating scattering structures within the DMD configuration can effectively promote light extraction in OLEDs, leading to enhanced overall performance and light efficiency.
ABSTRACT
In this study, we fabricated a random nanostructure (RNS) external light extraction composite layer containing high-refractive-index nanoparticles through a simple and inexpensive solution process and a low-temperature mask-free process. We focused on varying the shape and density of the RNSs and adjusted the concentration of the high-refractive-index nanoparticles to control the optical properties. The RNSs fabricated using a low-temperature mask-free process can use the distance between the nanostructures and various forms to control the diffraction and scattering effects in the visible light wavelength range. Consequently, our film exhibited a direct transmittance of ~85% at a wavelength of 550 nm. Furthermore, when the RNSs' composite film, manufactured using the low-temperature mask-free process, was applied to organic light-emitting diodes (OLEDs), it exhibited an external quantum efficiency improvement of 32.2% compared with the OLEDs without the RNSs. Therefore, the randomly distributed high-refractive-index nanoparticles on the polymer film can reduce the waveguide mode and total reflection at the substrate/air interface. These films can be used as a scattering layer to reduce the loss of the OLED substrate mode.
ABSTRACT
In this study, an external light extraction layer with a micro-nano hybrid structure was applied to improve the external light extraction efficiency of organic light-emitting diodes (OLEDs). A reactive ion-etching (RIE) process, using O2 and CHF3 plasma, was performed on the surface of the micro-scale pattern to form micro-nano hybrid structures. According to the results of this study, the nanostructures formed by the treatment of O2 and CHF3 were different, and the efficiency according to the structures was analyzed experimentally and theoretically. As a result, the OLED, to which the micro-nano hybrid structure, manufactured through a simple process, is applied, improved the external light extraction efficiency by up to 38%, and an extended viewing angle profile was obtained. Additionally, an effective method for enhancing the out-coupling efficiency of OLEDs was presented by optimizing the micro-nano hybrid structure according to process conditions.
ABSTRACT
In this paper, we report on the effects of the substrate thermal evaporation process rotation speed on the electroluminescence (EL) characteristics of organic light-emitting diodes (OLEDs). In general OLED research, rotational and angle tilted deposition are widely used to maintain uniformity. However, there have been few reports on the effects of this deposition method on film characteristics. We analyzed these effects and found that the film density and its refractive index showed remarkable changes as a function of substrate rotational speed during tilted deposition. The EL characteristics of the transport layer of fluorescent OLEDs were also significantly affected. We derived the OLED optimal thickness and refractive index from our calculations.
ABSTRACT
In this study, we report the effects of the substrate rotational speed on the morphological characteristics of lithium fluoride (LiF) during thermal evaporation. LiF is used as a typical material in a vacuum-level shift-based electron injection layer and can improve both the charge injection and light emission properties when inserted into the electrode/organic material interface of organic light-emitting diodes (OLEDs). In general OLED research, rotary evaporation is widely used to ensure uniformity. However, there are few reports regarding the effects of this rotary evaporation method on the morphological characteristics of the thin films. Therefore, in this study, we analyzed the effects of rotary variations on the morphological and electron injection characteristics during deposition. The root mean square roughness of the LiF thin film deposited on Alq3 changed by up to 12.3%. Additionally, the driving voltage of the electron-only device showed a difference of 2.3 V at maximum and a change in the slope of the ohmic region was demonstrated. The morphological change in the LiF thin film based on the rotational speed of the substrate had a significant influence on the reaction at the electrode/organic material interface.
ABSTRACT
The phenomenon by which the efficiency decreases rapidly with the increase in luminance or current density in organic light-emitting diodes is termed efficiency roll-off. In particular, phosphorescent organic light-emitting diodes are known to have higher efficiency, but tend to exhibit higher efficiency roll-off compared with fluorescent organic light-emitting diodes. In this study, we report the efficiency roll-off characteristics of double-dopant phosphorescent organic light-emitting diodes. The double-dopant phosphorescent organic light-emitting diodes showed significantly lower efficiency roll-off compared with single-dopant phosphorescent organic light-emitting diodes. (The double-dopant device showed a 2.5-fold decrease in efficiency roll-off compared with the single-dopant device at 50 mA/cm², and a 1.6-fold decrease in efficiency roll-off at 100 mA/cm²).
