Planar Organic Light-emitting Diodes Research Articles

Enhanced light extraction from organic light-emitting diodes by reducing plasmonic loss through graded photonic super-crystals

In a planar organic light-emitting diode (OLED), over 50% of emitted lights are trapped as a waveguide mode in the organic-indium tin oxide layer and as a surface plasmon polariton mode at the metal and organic layer interface. The light extraction efficiency into the glass substrate is greatly enhanced when the organic/Al interface of the OLED is patterned with a graded photonic super-crystal (GPSC), by destroying the plasmonic resonance condition through graded patterns and by scattering the surface plasmon polariton into the glass. The light extraction efficiency increases with the area fraction of graded regions in the GPSC. The efficiency can reach above 68.5%, 72.9%, and 78.9% for octagonal, square, and triangular GPSCs with area fractions of the graded regions of 53.9%, 78.5%, and 90.7%, respectively. The light extraction efficiency goes up to 83.0%, 81.2%, and 79.0% at the wavelengths of 447, 507, and 608 nm, respectively, in OLED patterned with triangular GPSC, compared with the targeted efficiency of 70%.

ITO-free flexible hybrid white organic/inorganic light-emitting diodes using optimum color conversion quantum dot plates

In this work, we propose that indium-tin-oxide (ITO)-free flexible hybrid white organic/inorganic light illuminated device application has a potential for next-generation flexible display. The process involves conventional blue planar organic light-emitting diodes as light source and quantum dot (QD) color conversion plate. Semitransparent anode electrode is used in light-emitting diodes, which has effective micro-cavity phenomenon, instead of ITO that is a conventional transparent conductive oxide of display area. QD color conversion plate is used according to the optimized QD size in color rendering, which has improved energy transfer property, by control of wavelength and QD thickness for proper realization of QD plates through reduced QD aggregation.

Enhanced Outcoupling in Organic Light-Emitting Diodes via a High-Index Contrast Scattering Layer

Despite high internal quantum efficiencies, planar organic light-emitting diodes (OLEDs) typically suffer from limited outcoupling efficiencies. To improve this outcoupling efficiency, we have developed a new thin (∼2 μm) light scattering layer that employs air voids (low-index scattering centers) embedded in a high-index polyimide matrix to effectively frustrate the substrate-trapped light, increasing the outcoupling efficiency. The porous polyimide scattering layers are created through the simple and scalable fabrication technique of phase inversion. The optical properties of the scattering layers have been characterized via microscopy, transmittance/haze measurements, and ellipsometry, which demonstrate the excellent scattering properties of these layers. We have integrated these films into a green OLED stack, where they show a 65% enhancement of the external quantum efficiency and a 77% enhancement of the power efficiency. Furthermore, we have integrated these layers into a white OLED and observed sim...

Increase of electroluminescent intensity in planar light-emitting diode structures based on PFO polymer – ZnO nanoparticles composite films operated by alternating-current voltages

We report on the considerable increase of electroluminescent (EL) intensity in planar organic light-emitting diode structures based on semiconducting polymer PFO and ZnO nanoparticles composite films operated by alternating-current (AC) voltages with respect to that operated by direct current. It was established that an increase of frequency from 0 to 30kHz results in increase of EL intensity of such structures by a factor of ~30. The EL emission takes place at positive AC bias and the EL spectral range is the same as that for the photoluminescent emission spectrum for the same sample. It was shown that applying AC voltage is effective tool in producing strong EL emission in planar composite light-emitting structures.

Efficient and Rigorous Modeling of Light Emission in Planar Multilayer Organic Light-Emitting Diodes

In this paper, we propose a technique to enable efficient and rigorous modeling of light emission in planar organic light-emitting diodes (OLEDs) composed of an arbitrary number of layers with different permittivity (including metals of a complex permittivity). The effects of the change of exciton radiative decay rate are explicitly included in the simulation. The numerical implementation of the technique is comprehensively discussed through an illustrative example. By using the proposed method, a bottom emitting OLED with a thick glass substrate is rigorously analyzed. The calculated results show a good agreement with the experimental results. The proposed method is efficient and well suited for optimizing OLEDs with complicated device structures. As a demonstration, we optimize the two-unit tandem top-emitting OLEDs with three types of charge generating layer. The results and design guidelines are given and discussed in detail

Micro-cavity organic light emitting diodes for biochip applications

Vertical and transverse confinement of electromagnetic radiation in planar organic light emitting diodes (OLED) has been widely addressed in order to maximize their external quantum efficiency. Here we propose a planar micro-cavity OLED with the aim to tailor the angular emission pattern and obtain light emission suppression in the vertical direction. Vertical confinement was obtained by depositing a conventional molecular OLED on a glass substrate coated with a thin gold semitransparent layer and a silica spacer. The conventional OLED was obtained by depositing under vacuum an indium tin oxide hole-injection layer, an organic hetero-junction constituted by N,N-diphenyl-N,N-bis(3-methyl phenyl)-1,1-biphenyl-4,4-diamine (TPD) and tris(8-hydroxy-quinoline)-aluminium (Alq3) layers and a LiF/Al electron-injection layer. Performance of the micro-cavity OLEDs was characterized by measuring the current/voltage characteristics and the angular dependence of the brightness and emission spectrum. The results were compared to those obtained for reference conventional OLEDs, without vertical confinement. Suppression of the green emission in the vertical direction was observed, confirming what was expected by calculations of dipole emission in a micro-cavity structure.