Tailor-made nanostructures bridging chaos and order for highly efficient white organic light-emitting diodes

Integrated metasurfaces for re-envisioning a near-future disruptive optical platform

Metasurfaces have been continuously garnering attention in both scientific and industrial fields, owing to their unprecedented wavefront manipulation capabilities using arranged subwavelength artificial structures. To date, research has mainly focused on the full control of electromagnetic characteristics, including polarization, phase, amplitude, and even frequencies. Consequently, versatile possibilities of electromagnetic wave control have been achieved, yielding practical optical components such as metalenses, beam-steerers, metaholograms, and sensors. Current research is now focused on integrating the aforementioned metasurfaces with other standard optical components (e.g., light-emitting diodes, charged-coupled devices, micro-electro-mechanical systems, liquid crystals, heaters, refractive optical elements, planar waveguides, optical fibers, etc.) for commercialization with miniaturization trends of optical devices. Herein, this review describes and classifies metasurface-integrated optical components, and subsequently discusses their promising applications with metasurface-integrated optical platforms including those of augmented/virtual reality, light detection and ranging, and sensors. In conclusion, this review presents several challenges and prospects that are prevalent in the field in order to accelerate the commercialization of metasurfaces-integrated optical platforms.

Simultaneously improving the quality factor and outcoupling efficiency of organic light-emitting field-effect transistors with planar microcavity

Organic light-emitting field-effect transistors (OLEFETs) are regarded as an ideal device platform to achieve electrically pumped organic semiconductor lasers (OSLs). However, the incorporation of a high-quality resonator into OLEFETs is still challenging since the process usually induces irreparable deterioration to the electric-related emission performance of the device. We here propose a dual distributed Bragg reflector (DBR)-based planar microcavity, which is verified to be highly compatible with the OLEFETs. The dual DBR planar microcavity shows the great advantage of simultaneously promoting the quality (Q) factor and outcoupling efficiency of the device due to the reduced optical loss. As a result, a moderately high Q factor of ∼160, corresponding to EL spectrum linewidth as narrow as 3.2 nm, concomitantly with high outcoupling efficiency (∼7.1%) has been successfully obtained. Our results manifest that the dual DBR-based planar microcavity is a promising type of resonator, which might find potential applications in improving the spectra and efficiency performance of OLEFETs as well as in OLEFET-based electrically pumped OSLs.

Wrinkle nanostructures generate a novel form of blue structural color in great argus flight feathers

Currently known structural colors in feathers are caused by light scattering from periodic or amorphous arrangements of keratin, melanin, and air within barbs and barbules that comprise the feather vane. Structural coloration in the largest part of the feather, the central rachis, is rare. Here, we report on an investigation of the physical mechanisms underlying the only known case of structural coloration in the rachis, the blue rachis of great argus (Argusianus argus) flight feathers. Spectrophotometry revealed a reflectance peak at 344 nm that is diffuse and well matched to the blue and UV-sensitive cone sensitivities of this species' visual system. A combination of electron microscopy and optical modeling confirmed blue coloration is generated by scattering from amorphous wrinkle nanostructures 125 nm deep and 385 nm apart, a new avian coloration mechanism. These findings have implications for understanding how novel courtship phenotypes arise through evolutionary modification of existing ontogenetic templates.

Injection-limited and space charge-limited currents in organic semiconductor devices with nanopatterned metal electrodes

Charge injection at metal-organic interfaces often limits the electric current in organic light-emitting diodes without additional injection layers. Integrated nanopatterned electrodes may provide a way to overcome this current injection limit by local field enhancements leading to locally space charge-limited currents. We compare electrical characteristics of planar and nanopatterned hole-only devices based on the charge transport material NPB with different thicknesses in order to investigate the nanopattern's effect on the current limitation mechanism. Integration of a periodic nanograting into the metal electrode yields a current increase of about 1.5-4 times, depending on thickness and operating voltage. To verify the experimental results, we implement a finite element simulation model that solves the coupled Poisson and drift-diffusion equations in a weak form. It includes space charges, drift and diffusion currents, nonlinear mobility, and charge injection at the boundaries. We find in experiment and simulation that the planar devices exhibit injection-limited currents, whereas the currents in the nanopatterned devices are dominated by space charge effects, overcoming the planar injection limit. The simulations show space charge accumulations at the corners of the nanopattern, confirming the idea of locally space charge-limited currents.

Efficient and stretchable organic light-emitting devices based on spontaneously formed disordered wrinkles

We propose a facile, scalable strategy to introduce spontaneously formed disordered wrinkles into organic light-emitting devices (OLEDs) to enhance light extraction and realize stretchability of the devices. The luminance and current efficiency of the wrinkled OLEDs are improved by 37% and 18%, respectively, compared to the planar device. Meanwhile, broadband light scattering induced by the disordered wrinkles results in angle-stable electroluminescent spectra at wide viewing angles for the wrinkled OLEDs. The disordered wrinkles enable the OLEDs to be stretchable and withstand hundreds of stretching-releasing cycles at strain between 0% and 5%. This study provides a simple method to realize stretchable OLEDs with high efficiency.

Centimeter-scale hole diffusion and its application in organic light-emitting diodes

In conventional organic light-emitting diodes (OLEDs), current balance between electron and hole transport regions is typically achieved by leakage of the major carrier through the devices or by accumulation of the major carrier inside the devices. Both of these are known to reduce performances leading to reduction of efficiency and operation stability due to exciton-polaron annihilation, etc. We found that hole diffusion in a centimeter-scale can be achieved in a PEDOT:PSS layer via composition and interface engineering. This ultralong distance hole diffusion enables substantially enhanced hole diffusion current in the lateral direction perpendicular to the applied electric field in typical organic optoelectronic devices. By introducing this lateral hole diffusion layer (LHDL) at the anode side of OLEDs, reduced carrier accumulation, improved efficiency, and enhanced operation stability are demonstrated. The application of the LHDL provides a third strategy for current balancing with much reduced harmful effects from the previous two approaches.

A robust vertical nanoscaffold for recyclable, paintable, and flexible light-emitting devices

Organic light-emitting devices are key components for emerging opto- and nanoelectronics applications including health monitoring and smart displays. Here, we report a foldable inverted polymer light-emitting diode (iPLED) based on a self-suspended asymmetrical vertical nanoscaffold replacing the conventional sandwich-like structured LEDs. Our empty vertical-yet-open nanoscaffold exhibits excellent mechanical robustness, proven by unaltered leakage current when applying 1000 cycles of 40-kilopascal pressure loading/unloading, sonication, and folding, with the corresponding iPLEDs displaying a brightness as high as 2300 candela per square meter. By using photolithography and brush painting, arbitrary emitting patterns can be generated via a noninvasive and mask-free process with individual pixel resolution of 10 μm. Our vertical nanoscaffold iPLED can be supported on flexible polyimide foils and be recycled multiple times by washing and refilling with a different conjugated polymer capable of emitting light of different color. This technology combines the traits required for the next generation of high-resolution flexible displays and multifunctional optoelectronics.

Single-step-fabricated disordered metasurfaces for enhanced light extraction from LEDs

While total internal reflection (TIR) lays the foundation for many important applications, foremost fibre optics that revolutionised information technologies, it is undesirable in some other applications such as light-emitting diodes (LEDs), which are a backbone for energy-efficient light sources. In the case of LEDs, TIR prevents photons from escaping the constituent high-index materials. Advances in material science have led to good efficiencies in generating photons from electron-hole pairs, making light extraction the bottleneck of the overall efficiency of LEDs. In recent years, the extraction efficiency has been improved, using nanostructures at the semiconductor/air interface that outcouple trapped photons to the outside continuum. However, the design of geometrical features for light extraction with sizes comparable to or smaller than the optical wavelength always requires sophisticated and time-consuming fabrication, which causes a gap between lab demonstration and industrial-level applications. Inspired by lightning bugs, we propose and realise a disordered metasurface for light extraction throughout the visible spectrum, achieved with single-step fabrication. By applying such a cost-effective light extraction layer, we improve the external quantum efficiency by a factor of 1.65 for commercialised GaN LEDs, demonstrating a substantial potential for global energy-saving and sustainability.

Design and fabrication of hybrid MLAs/gratings for the enhancement of light extraction efficiency and distribution uniformity of OLEDs

Extracting light from organic light-emitting diodes (OLEDs) and improving the angular distribution are essential for their commercial applications in illumination and displays. In this work, hybrid microlens arrays (MLAs) and gratings with periods and depths in the scale of submicron have been designed and incorporated on the lighting surface of OLEDs for simultaneous enhancement of light outcoupling efficiency and angular distribution improvement. It is found that the augmentation of light extraction efficiency is mainly attributed to the MLAs, while the gratings can improve the viewing angle by increasing the angular distribution uniformity. A novel approach was proposed by combining photoresist thermal reflow, soft-lithography and plasma treatments on polydimethylsiloxane (PDMS) surfaces synergistically to realize gratings on the wavy surface of MLAs. It has been proved that with the hybrid MLAs/gratings, the external quantum efficiency (EQE) of the OLED can reach up to 22.8%, which increased by 24% compared to that of bare OLED. Moreover, the OLED with the hybrid MLAs/gratings showed an obvious lateral enhancement at wider viewing angle.

Spontaneous Formation of Random Wrinkles by Atomic Layer Infiltration for Anticounterfeiting

Continuous developments of innovative anticounterfeiting strategies are vital to restrain the fast-growing counterfeit markets. Physical unclonable function (PUF)-based taggants allow for a practical solution to provide irreproducible codes for strong authentication. Herein, an advanced anticounterfeiting strategy with multiple security levels was successfully developed using screen printing and atomic layer infiltration (ALI) techniques. Macroscale poly(dimethylsiloxane) (PDMS) patterns were fabricated for primary verification. Spontaneous formation of random wrinkles with size in the micrometer scale was achieved on the top surface of screen-printed PDMS patterns due to the anisotropic relief and redistribution of extra compressive stress after Al₂O₃ infiltration, which can be used for senior authentication by image identification using the artificial intelligence (AI) technique. Furthermore, the complexity and security level of a code, which are proportional to the minutia density, can be adjusted by the morphology of the wrinkles in terms of amplitude and wavelength via the degree of Al₂O₃ permeation depending on ALI conditions. These spontaneously formed random wrinkles were demonstrated for validation and decoding with AI, exhibiting the merits of being unclonable, nondestructive, universally adaptable, environmentally stable, and mass-producible, and sufficiently adaptable for an industry-suitable authentication strategy.

Tackling light trapping in organic light-emitting diodes by complete elimination of waveguide modes

Conventional waveguide mode decoupling methods for organic light-emitting diodes (OLEDs) are typically not scalable and increase fabrication complexity/cost. Indium-tin-oxide-free transparent anode technologies showed efficiency improvement without affecting other device properties. However, previous works lack rigorous analysis to understand the efficiency improvement. Here, we introduced an ultrathin silver (Ag) film as transparent electrode and conducted systematic modal analysis of OLEDs and report that waveguide mode can be completely eliminated by designing an OLED structure that is below the cutoff thickness of waveguide modes. We also experimentally verified the waveguide mode removal in organic waveguides with the help of index-matching fluid and prism. The negative permittivity, extremely thin thickness (~5 nanometers), and highly conductive properties achieved by a uniform copper-seeded Ag film can suppress waveguide mode formation, enhancing external quantum efficiency without compromising any other characteristics of OLEDs, which paves the way for cost-effective high-efficiency OLEDs in current display industry.

Phase-Separated Nanophotonic Structures by Inkjet Printing

The spontaneous phase separation of two or more polymers is a thermodynamic process that can take place in both biological and synthetic materials and which results in the structuring of the matter from the micro- to the nanoscale. For photonic applications, it allows forming quasi-periodic or disordered assemblies of light scatterers at high throughput and low cost. The wet process methods currently used to fabricate phase-separated nanostructures (PSNs) limit the design possibilities, which in turn hinders the deployment of PSNs in commercialized products. To tackle this shortcoming, we introduce a versatile and industrially scalable deposition method based on the inkjet printing of a polymer blend, leading to PSNs with a feature size that is tuned from a few micrometers down to sub-100 nm. Consequently, PSNs can be rapidly processed into the desired macroscopic design. We demonstrate that these printed PSNs can improve light management in manifold photonic applications, exemplified here by exploiting them as a light extraction layer and a metasurface for light-emitting devices and point-of-care biosensors, respectively.

Determining the Dielectric Tensor of Microtextured Organic Thin Films by Imaging Mueller Matrix Ellipsometry

Polycrystalline textured thin films with distinct pleochroism and birefringence comprising oriented rotational domains of the orthorhombic polymorph of an anilino squaraine with isobutyl side chains (SQIB) are analyzed by imaging Mueller matrix ellipsometry to obtain the biaxial dielectric tensor. Simultaneous fitting of transmission and oblique incidence reflection Mueller matrix scans combined with the spatial resolution of an optical microscope allows to accurately determine the full biaxial dielectric tensor from a single crystallographic sample orientation. Oscillator dispersion relations model well the dielectric tensor components. Strong intermolecular interactions cause the real permittivity for all three directions to become strongly negative near the excitonic resonances, which is appealing for nanophotonic applications.

Computational Study of Dipole Radiation in Re-Absorbing Perovskite Semiconductors for Optoelectronics

Compared to organic emitters, perovskite materials generally have a small Stokes shift and correspondingly large re-absorption of dipole emission. Classical optical modelling methods ignoring re-absorption do not provide an adequate description of the observed light emission properties. Here, optical modelling methods and design rules for perovskite light-emitting diodes are presented. The transfer-matrix formalism is used to quantify the Poynting vectors generated by a dipole radiating inside a perovskite optoelectronic device. A strategy is presented to deal with non-radiative coupling to nearby emissive material that can otherwise lead to non-physical divergence in the calculation. Stability issues are also investigated regarding coherence of the light propagating in the substrate and the absence of a light absorber in the system. The benefit of the photon recycling effect is taken into account by recursive calculation of the dipole generation profile. The simulation results predict that a high external quantum efficiency of ≈40% is achievable in formamidinium lead triiodide-based perovskite light-emitting diodes, by optimization of microcavity, dipole orientation, and photon recycling effects. Contrary to conventional device structures currently reported, this work highlights the benefits of thick charge transport layers and thick perovskite with small Stokes shift.

Multi-mode Organic Light-Emitting Diode to Suppress the Viewing Angle Dependence

A typical top-emitting organic light-emitting diode (OLED) has a strong microcavity effect because of the two reflective electrodes. The cavity effect causes a serious color shift with the viewing angles and restricts the organic layer thickness. To overcome these drawbacks, we design a multi-mode OLED structure with dual-dielectric spacer layers, which extend the cavity length by more than 10 times. This design completely eliminates the intrinsic cavity effect caused by the top and bottom boundaries and provides freedom for the organic layer thickness. We demonstrate these effects in a white multi-mode OLED using a white emitter, which shows a negligible angular chromaticity shift of Δuv = 0.006 from 0 to 70° and a Lambertian emission profile. The simple design and the perfect angular color profiles make the multi-mode OLED structure promising in large-area displays and solid-state lighting applications.

Enhancing extraction efficiency of quantum dot light-emitting diodes introducing a highly wrinkled ZnO electron transport layer

Light extraction efficiency is crucial for achieving highly efficient and bright quantum dot light-emitting diodes (QLEDs), and current efforts toward introducing light outcoupling nanostructures always require complicated procedures. An extremely simple and efficient method to introduce light outcoupling nanostructures in the ZnO electron transport layer (ETL) is demonstrated by adopting a certain heating rate during the annealing process. The ultimate device exhibits a current efficiency of 9.1 cd/A, giving a 50% efficiency improvement compared to the control device with a flat ZnO ETL. This arises from the increased light extraction efficiency induced by random nanostructures formed on a wrinkled ZnO ETL, which could also be modulated by adjusting the heating rate during the annealing process. This study not only provides a simple and efficient method to introduce light outcoupling nanostructures, but also shows ample room for further performance enhancement of QLEDs with the guideline of light extraction.

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.