Spontaneously Embedded Scattering Structures in a Flexible Substrate for Light Extraction

Changes in Surface Characteristics of BOPP Foil after Treatment by Ambient Air Plasma Generated by Coplanar and Volume Dielectric Barrier Discharge

Biaxially oriented polypropylene (BOPP) is a highly transparent polymer defined by excellent mechanical and barrier properties applicable in the food packaging industry. However, its low surface free energy restricts its use in many industrial processes and needs to be improved. The presented study modifies a BOPP surface using two different atmospheric-pressure plasma sources operating in ambient air and capable of inline processing. The volume dielectric barrier discharge (VDBD) and diffuse coplanar surface barrier discharge (DCSBD) were applied to improve the wettability and adhesion of the 1-10 s treated surface. The changes in morphology and surface chemistry were analyzed by SEM, AFM, WCA/SFE, and XPS, and adhesion was evaluated by a peel force test. Comparing both plasma sources revealed their similar effect on surface wettability and incorporation of polar functional groups. Additionally, higher surface roughness in the case of VDBD treatment contributed to slightly more efficient adhesion in comparison to DCSBD. Although we achieved comparable results for both plasma sources in the term of enhanced surface wettability, degree of oxidation, and stability of induced changes, DCSBD had less effect on the surface deterioration than VDBD, where surface structuring caused an undesirable haze.

Self-catalytic-grown SnO x nanocones for light outcoupling enhancement in organic light-emitting diodes

Light extraction in organic light-emitting diodes (OLEDs) was improved by applying SnO _x nanocones grown via thermal annealing in a low-O₂ atmosphere. SnO _x was easily fabricated through thermal processing after Sn deposition. The diameter of the SnO _x nanocones was controlled by changing the deposition thickness of Sn. The SnO _x nanocones induced strong Mie scattering, which reduced the total internal reflection in the glass substrate. Consequently, the OLED with SnO _x nanocones exhibited a 23% increase in the external quantum efficiency compared with a reference device.

Inside or outside: Evaluation of the efficiency enhancement of OLEDs with applied external scattering layers

Improving the efficiency of organic light-emitting diodes (OLEDs) by enhancing light outcoupling is common practise and remains relevant as not all optical losses can be avoided. Especially, externally attached scattering layers combine several advantages. They can significantly increase the performance and neither compromise the electric operation nor add high costs during fabrication. Efficiency evaluations of external scattering layers are often done with lab scale OLEDs. In this work we therefore study different characterization techniques of red, green and blue lab scale OLEDs with attached light scattering foils comprising TiO₂ particles. Although we observe an increased external quantum efficiency (EQE) with scattering foils, our analysis indicates that areas outside the active area have a significant contribution. This demonstrates that caution is required when efficiency conclusions are transferred to large area applications, for which effects that scale with the edges become less significant. We propose to investigate brightness profiles additionally to a standard EQE characterizations as latter only work if the lateral scattering length is much smaller than the width of the active area of the OLED. Our results are important to achieve more reliable predictions as well as a higher degree of comparability between different research groups in future.

Light Extraction Enhancement in Flexible Organic Light-Emitting Diodes by a Light-Scattering Layer of Dewetted Ag Nanoparticles at Low Temperatures

We demonstrated light extraction improvement by applying a scattering layer of Ag nanoparticles physically synthesized through a low-temperature annealing process to flexible organic light-emitting diodes (OLEDs). In general, increasing the size of Ag nanoparticles is preferred to increase light scattering, but a high-temperature annealing process (∼400 °C) is required to produce them. However, flexible substrates generally cannot withstand high-temperature processes. In this study, we formed Ag nanoparticles at a low temperature of ∼200 °C by inserting a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate buffer layer, thus promoting Ag dewetting. As a result, the scattering layer of enlarged Ag nanoparticles formed at low temperatures increased the external quantum efficiency by 24% in a flexible OLED compared to a reference device.

A strain induced subwavelength-structure for a haze-free and highly transparent flexible plastic substrate

This paper presents a method to produce subwavelength-scale (<250 nm) AgCl nanostructures on a flexible plastic film, which is indispensable for highly efficient flexible displays. Using Cl2 plasma treatment on an Ag-coated plastic film, AgCl nanostructures were produced through the reaction of Ag atoms with Cl radicals. During the reaction, the volume of AgCl expands, leading to drastically changed surface morphology from a two-dimensional (2D) flat Ag surface to a 3D subwavelength-scale AgCl nanostructure. The optical properties of AgCl on the plastic film were remarkably enhanced from 89.6% to 93.4% and the average transmittance ranged between 400 and 800 nm, while the average haze was retained below 0.3%. Consequently, OLEDs based on the subwavelength-scale AgCl nanostructure had an enhanced luminance efficiency (88.6 cd A-1 at 1000 cd m-2) of up to 10.7% without modifying the angular emission pattern, superior to that of the as-received PI film (efficiency of 80.0 cd A-1). The nanostructure enhances the transmission of electromagnetic (EM) waves as well as prohibits the scattering of EM waves, which was confirmed by finite-difference time-domain simulation and rigorous coupled wave analysis.

Laser speckle formed disordered micro-meander structures for light extraction enhancement of flexible organic light-emitting diodes

A new approach for efficiently recovering the wasted light energy in conventional flexible organic light-emitting diodes (FOLEDs) is developed by implementing disordered micro-meander structures (DMMs) via laser speckle holography technology. Compared to conventional flat device architecture, the structured FOLEDs with DMMs result in substantial improvement of the device efficiency and superior angular color stability. The resulting current efficiency (CE) and external quantum efficiency (EQE) are 1.31 and 1.39 times that of a common flat structure, respectively. Moreover, the proposed DMMs micro-structure simultaneously offers the unique characteristics of angular color stability with a wide viewing angle, which is usually considered as the criteria of the high-quality lighting applications. We hope that the demonstrated method could provide an alternative way for the development of high efficiency flexible OLEDs.