Ligand-Nondestructive Direct Photolithography Assisted by Semiconductor Polymer Cross-Linking for High-Resolution Quantum Dot Light-Emitting Diodes.

The photolithographic patterning of fine quantum dot (QD) films is of great significance for the construction of QD optoelectronic device arrays. However, the photolithography methods reported so far either introduce insulating photoresist or manipulate the surface ligands of QDs, each of which has negative effects on device performance. Here, we report a direct photolithography strategy without photoresist and without engineering the QD surface ligands. Through cross-linking of the surrounding semiconductor polymer, QDs are spatially confined to the network frame of the polymer to form high-quality patterns. More importantly, the wrapped polymer incidentally regulates the energy levels of the emitting layer, which is conducive to improving the hole injection capacity while weakening the electron injection level, to achieve balanced injection of carriers. The patterned QD light-emitting diodes (with a pixel size of 1.5 µm) achieve a high external quantum efficiency of 16.25% and a brightness of >1.4 × 105 cd/m2. This work paves the way for efficient high-resolution QD light-emitting devices.

Broadband beam collimation metasurface for full-color micro-LED displays.

Near-eye displays are widely recognized as a groundbreaking technological advancement with the potential to significantly impact daily life. Within the realm of near-eye displays, micro-LEDs have emerged as a highly promising technology owing to their exceptional optical performance, compact form factor, and low power consumption. However, a notable challenge in integrating micro-LEDs into near-eye displays is the efficient light collimation across a wide spectrum range. In this paper, we propose what we believe to be a novel design of a broadband beam collimation metasurface for full-color micro-LEDs by harnessing wavefront phase modulation based on Huygens' principle. Our results demonstrate a substantial reduction in the full width at half maximum (FWHM) angles, achieving a reduction to 1/10, 1/10, and 1/20 for red, green, and blue micro-LEDs compared to those without the metasurface, which is the best collimation result as far as we know. The central light intensity increases by 24.60, 36.49, and 42.15 times. Furthermore, the significant enhancement in the light energy within ±10° is achieved, with the respective multiplication factors of 14.16, 15.60, and 13.00. This metasurface has the potential to revolutionize the field by enabling high-performance, compact, and lightweight micro-LED displays, with applications in near-eye displays, micro-projectors, and beyond.

Effect of polymethyl methacrylate on in situ patterning of perovskite quantum dots by inkjet printing.

The preparation of perovskite quantum dots (PQDs) using an in situ inkjet printing method is beneficial for improving the problems of aggregation and photoluminescence (PL) quenching during long-term storage. However, the stability of PQDs prepared using this method is still not ideal, and the morphology of in situ-printed patterns needs to be optimized. To address these problems, this study introduced polymethyl methacrylate (PMMA) into the process of in situ inkjet printing of PQDs and explored the effect of PMMA on the in situ patterning effect of PQDs. The results showed that using a mixed precursor solution containing a small amount of PMMA as the printing ink can slow down the shrinkage process of ink droplets and improve the uniformity of film formation. As the printing substrate, PMMA provided a suitable high-viscosity environment for the in situ growth of PQDs. This could effectively suppress the coffee ring effect. In addition, the interaction between the C=O=C group in PMMA and metal ion Pb2+ in the CsPbBr3 precursor molecules was favourable to enhancing the density of PQDs. The prepared PMMA-coated CsPbBr3 quantum dots (QDs) pattern had high stability and could maintain at 90.08% PL intensity after 1 week of exposure to air.

Metamaterials for light extraction and shaping of micro-scale light-emitting diodes: from the perspective of one-dimensional and two-dimensional photonic crystals.

Metamaterials have attracted broad attention owing to their unique versatile micro- and nano-structures. As a kind of typical metamaterial, photonic crystals (PhCs) are capable of controlling light propagation and constraining spatial light distribution from the chip level. However, introducing metamaterial into micro-scale light-emitting diodes (µLED) still exists many unknowns to explore. This paper, from the perspective of one-dimensional and two-dimensional PhCs, studies the influence of metamaterials on the light extraction and shaping of µLEDs. The µLEDs with six different kinds of PhCs and the sidewall treatment are analyzed based on finite difference time domain (FDTD) method, in which the optimal match between the PhCs type and the sidewall profile is recommended respectively. The simulation results show that the light extraction efficiency (LEE) of the µLEDs with 1D PhCs increases to 85.3% after optimizing the PhCs, and is further improved to reach 99.8% by the sidewall treatment, which is the highest design record so far. It is also found that the 2D air ring PhCs, as a kind of left-handed metamaterials, can highly concentrate the light distribution into 30° with the LEE of 65.4%, without help of any light shaping device. The surprising light extraction and shaping capability of metamaterials provides a new direction and strategy for the future design and application of µLED devices.

Omnidirectional color shift suppression of full-color micro-LED displays with enhanced light extraction efficiency.

The three-primary-color chip array is the most straightforward to realize full-color micro-LED displays. However, the luminous intensity distribution shows high inconsistency between the AlInP-based red micro-LED and GaN-based blue / green micro-LEDs, resulting in the issue of angular color shift with different viewing angles. This Letter analyzes the angular dependence of color difference of conventional three-primary-color micro-LEDs, and proves that the inclined sidewall with homogeneous Ag coating has a limited angular regulation effect for micro-LEDs. Based on this, a patterned conical microstructure array is designed on the micro-LED's bottom layer to effectively eliminate the color shift. This design cannot only regulate the emission of full-color micro-LEDs to perfectly meet Lambert's cosine law without any external beam shaping elements, but also improve the light extraction efficiency of top emission by 16%, 161%, and 228% for red, green, and blue micro-LEDs, respectively. The color shift Δ u ' v ' of the full-color micro-LED display is also kept below 0.02 with the viewing angle ranging from 10° to 90°.

Uniformity improvement of a mini-LED backlight by a quantum-dot color conversion film with nonuniform thickness.

Mini-LED backlights energized by quantum-dot color conversion (QDCC) hold great potential for technological breakthroughs of liquid crystal displays. However, luminance uniformity issues should still be urgently solved owing to the large interval of direct-lit mini-LEDs, especially when covering with a QDCC film (QDCCF) with uniform thickness. Herein, we propose a uniformity improvement approach of mini-LED backlights by employing a QDCCF with nonuniform thickness based on the Lambertian distribution of mini-LEDs, which is demonstrated by screen-printing preparation and ray-tracing simulation. Experimental results show that the luminance uniformity of the nonuniform QDCCF can reach 89.91%, which is 24.92% higher than the uniform one. Ray-tracing simulation further elaborates the mechanism of this significant improvement. Finally, by employing this nonuniform QDCCF, a mini-LED backlight prototype is assembled and achieves high uniformity of 92.15%, good white balance with color coordinates of (0.3482, 0.3137), and high color gamut of 109% NTSC. This work should shed some new light on mini-LED-based display technology.

Artificial Tactile Sensing System with Photoelectric Output for High Accuracy Haptic Texture Recognition and Parallel Information Processing.

Developing multifunctional artificial sensory systems is an important task for constructing future artificial neural networks. A system with multisignal output capability is highly required by the rising demand for high-throughput data processing in the Internet of Things (IoT) society. Here, a novel dual-output artificial tactile sensing (DOATS) system with parallel output of photoelectric signals was proposed. Because of the ionic-electronic coupling mechanism in light-emitting synaptic (LES) devices in the DOATS system, modulating electric current and light emission can coexist through ion accumulation and electron-hole recombination. As a result, the DOATS system can realize the simulation of human tactile information, and the recognition of 16 kinds of fabrics was demonstrated with an accuracy rate of 94.1%. A photoelectric hybrid artificial neural network was proposed, which achieved efficient and accurate multitask operation. The DOATS system proposed in this work is promising for implementing photoelectric hybrid neural network and promoting the development of interactive artificial intelligence.

Atomic-Scale Mechanism of Spontaneous Polarity Inversion in AlN on Nonpolar Sapphire Substrate Grown by MOCVD.

The performance of nitride devices is strongly affected by their polarity. Understanding the polarity determination and evolution mechanism of polar wurtzite nitrides on nonpolar substrates is therefore critically important. This work confirms that the polarity of AlN on sapphire prepared by metal-organic chemical vapor deposition is not inherited from the nitrides/sapphire interface as widely accepted, instead, experiences a spontaneous polarity inversion during the growth. It is found that at the initial growth stage, the interface favors the nitrogen-polarity, rather than the widely accepted metal-polarity or randomly coexisting. However, the polarity subsequently converts into the metal-polar situation, at first locally then expanding into the whole area, driven by the anisotropy of surface energies, which results in universally existing inherent inverse grain boundaries. Furthermore, vertical two-dimensional electron accumulation originating from the lattice symmetry breaking at the inverse grain boundary is first revealed. This work identifies another cause of high-density defects in nitride epilayers, besides lattice mismatch induced dislocations. These findings also offer new insights into atomic structure and determination mechanism of polarity in nitrides, providing clues for its manipulation toward the novel hetero-polarity devices.

Quantum-dot array with a random rough interface encapsulated by atomic layer deposition.

This Letter proposes the use of atomic layer deposition (ALD) encapsulation as a stability-improving approach for a quantum-dot micro-structural array (QDMA) with a random rough interface. The QDMA is first prepared by screen printing technology on an edge-lit light-guide plate (LGP) for backlight application. A flexible aluminum oxide film is then densely deposited onto the rough surface of the QDMA. The influences of two key factors, the reaction temperature and deposition thickness, on the encapsulation effect and output performance of this QD backlight are discussed. After ALD encapsulation, the water vapor transmission rate was measured to be less than 0.014 g/(m2 day). The average luminance of the encapsulated QD backlight remained stable after continuous working for 200 h, while an unencapsulated QD backlight lost over 50% of its initial luminance. The complete attenuation trend for the encapsulated QD backlight was analyzed in a more demanding testing environment, and results showed that 80% (>3000 cd/m2) of the initial luminance was maintained after 250 h at a high temperature of 70 °C and a relative humidity of 90%. The mechanism behind these experimental results is also discussed.

Efficiency improvement of GaN-based micro-light-emitting diodes embedded with Ag NPs into a periodic arrangement of nano-hole channel structure by ultra close range localized surface plasmon coupling.

NH-µLED, namely a micro light-emitting diode structure with nano-holes dug all the way through the active region, is designed to make silver nanoparticles in extremely close contact with the quantum wells for improving the coupling between the localized surface plasmon and the quantum wells (LSP-QWs coupling) and thus enhancing the optical properties of theµLED. The experimental results show that, thanks to this deep nanohole structure, the LSP-QWs coupling can be realized effectively, which ultimately increases the optical performance of theµLED. The internal quantum efficiency of the NH-µLED filled with silver nanoparticles is increased by 12%, and the final optical output power is also enhanced. We have further carried out a comparison study which measures the transient lifetime of two different types ofµLEDs, and the results provide convincing evidence for the existence of the ultra close range LSP-QWs coupling effect.

Light-Emitting Memristors for Optoelectronic Artificial Efferent Nerve.

The central nervous system sends a neural impulse through an efferent nerve system toward muscles to drive movement. In an electronically artificial neural system, the electronic neural devices and interconnections prevent achieving highly connected and long-distance artificial impulse transmission and exhibit a narrow bandwidth. Here we design and demonstrate light-emitting memristors (LEMs) for the realization of an optoelectronic artificial efferent nerve, in which the LEM combines the functions of a light receiver, a light emitter, and an optoelectronic synapse in a single device. The optical signal from the pre-LEM (presynaptic membrane) acts as the input signal for the post-LEM (postsynaptic membrane), leading to one-to-many transmission, dynamic adjustable transmission, and light-trained synaptic plasticity, thus removing the physical limitation in artificially electronic neural systems. Furthermore, we construct an optoelectronic artificial efferent nerve with LEMs to control manipulators intelligently. These results promote the construction of an artificial optoelectronic nerve for further development of sensorimotor functionalities.

Optimal dimension of edge-lit light guide plate based on light conduction analysis.

Dimensions of the edge-lit light guide plate (LGP) have a non-negligible impact on its output performance based on a pre-determined micro-dot array. However, how the LGP's dimension affects the performance has not been systematically researched. In this paper, the dimension of the LGP is numerically established as a function to the light output performance, which can be divided into four successive procedures. Firstly, the micro-structural dot array is designed based on the calculated illuminance distribution of the LGP's bottom surface. Based on this, the light energy output can be derived by defining three key parameters, which are dot density, scatting coefficient, and collision loss coefficient. After that, the ray-tracing simulation is used to determine the above parameters. Finally, the optimal dimensions of the LGP can be obtained with a specific correlation function with the light energy output. The mathematical relation above is demonstrated via both simulation and experiment. Our approach provides a systematic design for balancing the efficiency and uniformity of backlight by combining the dot design and the dimensional optimization, which has important theoretical guiding significance for actual display application.

Precise theoretical model for quantum-dot color conversion.

Quantum-dot color conversion (QDCC) is a promising technique for next-generation full-color displays, such as QD converted organic light-emitting diodes and micro light-emitting diodes. Although present QDCC research has made some progress on the experimental aspect, the optical model and corresponding mathematical expression that can lay an indispensable foundation for QDCC have not been reported yet. In this paper, we present a theoretical model for precisely describing the complete optical behavior of QDCC, including optical transmission, scattering, absorption, and conversion process. A key parameter of QDCC, called dosage factor (DoF), is defined to quantitatively express the total consumption of QDs that can be calculated as the product of film thickness and QD concentration. Theoretical relations are established between DoF and three key performance indicators of QDCC, namely the light conversion efficiency (LCE), blue light transmittance (BLT), and optical density (OD). The maximum LCE value can be predicted based on this theoretical model, as well as the relationship between the slope of the OD curve and the molar absorption coefficient of blue light. This theoretical model is verified by both simulation and experiment. Results show that the simulation and experimental data highly match the theoretical model, and the goodness of fit reaches higher than 96% for LCE, BLT, and OD. Based on this, the optimal interval of DoF is recommended that provides key guiding significance to the QDCC related experiment.

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.

Edge/direct-lit hybrid mini-LED backlight with U-grooved light guiding plates for local dimming.

Current mini-LED backlights improve high-dynamic-range liquid crystal displays (LCDs) by using tens of thousands of direct-lit sources for local dimming. However, relative thick profile and high power consumption are the inherent limitations while compared with edge-lit backlights. By synthesizing edge- and direct-lit advantages, we propose a novel hybrid mini-LED backlight equipped with a specially designed integrated light guiding plate (LGP) for large-area displays. This LGP is seamlessly spliced by multiple physically segmented sub-LGPs with a scattering dot array on the bottom and U-shaped grooves at the corners. Each sub-LGP is a single local dimming zone that can be independently controlled. Scattering dot distribution can be numerically calculated even from multiple edge-lit sources. High optical performance and satisfactory local dimming effect are verified and analyzed via both simulation and experiment. The experimental spatial illuminance uniformity and the light extraction efficiency reach 81% and 83% while the crosstalk can be well suppressed below 0.2% between adjacent local dimming zones. The significant advantages of our design towards state-of-the-art mini-LED backlights include the zero optical distance for an ultra-thin profile, low mini-LED amount for local dimming, high optical efficiency, and infinite extension of zone number, which is expected to have a broad application prospect in the near future.

Projection optical engine design based on tri-color LEDs and digital light processing technology.

Digital light processing (DLP) is currently a cutting-edge technology for desktop projection optical engines. Due to the passive luminescence characteristics, the DLP projection engine needs a few specific illumination optical components for light collimation, homogenization, and color combination, together with a projection lens matching the DLP chip and magnifying the image. In this paper, we propose a design approach that first splits the DLP projection optical engine into individual components for separate design, and then integrates them into a whole system for further verification. For the first step, the collimating lens group is designed for light collection, and the dichroic mirrors are used to fold the light path based on tri-color LED light sources. For the second step, a fly-eye lens and the corresponding relay lens group are designed to achieve uniform illumination on the DMD chip. The third step is to optimize the projection lens group for high-resolution projection display. Based on the design and simulation, the optical efficiency is 63.4% and the uniformity reaches 94.9% on the projection screen. The modulation transfer function (MTF) of the projection lens is higher than 0.4 at 66 lines for the distance of 500∼1500mm, and the distortion is lower than 1%. Simulation results show that the total luminous flux is estimated to reach 224.15 lm when the powers of tri-color LEDs are 21 W, 15.5 W, and 25 W, respectively. A projector prototype is built and tested for further verification, which provides a luminous flux of 220.43 lm and uniformity of 90.22%, respectively. The proposed design, demonstrated by both simulation and experiment, exhibits high feasibility and application potential in state-of-the-art commercial projector design.

Modulation of the plasticity of an all-metal oxide synaptic transistor via laser irradiation.

Artificial intelligence devices that can mimic human brains are the foundation for building future artificial neural networks. A key step in mimicking biological neural systems is the modulation of synaptic weight, which is mainly achieved by various engineering approaches using material design, or modification of the device structure. Here, we realize the modulation of the synaptic weight of a Ta2O5/ITO-based all-metal oxide synaptic transistor via laser irradiation. Prior to the deposition of the active layer and electrodes, a femtosecond laser was used to irradiate the surface of the insulator layer. Typical synaptic characteristics such as excitatory postsynaptic current, paired pulse facilitation and long-term potentiation were successfully simulated under different laser intensities and scanning rates. In particular, we demonstrate for the first time that laser irradiation could control the quantity of oxygen vacancies in the Ta2O5 thin film, leading to precise modulation of the synaptic weight. Our research provides an instantaneous (<1 s), convenient and low-temperature approach to improving synaptic behaviors, which could be promising for neuromorphic computing hardware design.

All-metal oxide synaptic transistor with modulatable plasticity.

The artificial neural system has attracted tremendous attention in the field of artificial intelligence due to operate mode of parallel computation which is superior to traditional Von Neumann computers in processing complex sensory data and real-time situations with extremely low power dissipation. Remarkable progress has been made in the hardware-based electric-double-layer synaptic transistors as its modulation by ion movement is similar to biological synapse for the past few years. Unfortunately, long-term potentiation (LTP) timescale is still a big challenge in hardware-based electric-double-layer synaptic transistors which is essential to processing capacity and memory formation. Meanwhile, the effect of ion concentration on the synaptic plasticity has rarely been reported. Here, a solid state electrolyte-gated transistor using Ta2O5 as dielectric layer with unique ionic composition was demonstrated and the regulation of synaptic weight was realized by changing ion concentration. Both the potentiation and depression of synaptic weight such as excitatory post-synaptic current, inhibitory response (IPSC), paired pulse facilitation as well as LTP were successfully simulated. More importantly, oxygen vacancy content was tuned for the first time to modulate synaptic plasticity by varying film thickness and gas ratio, through which the intensity and duration of memory were enhanced with appropriate vacancy concentration. It indicated that appropriate vacancy concentration avoided the effects of internal electric field induced by ion excess, leading to a long-term memory. These results reveal a promising path to improve memory capacity of artificial synapse via ion modulation.

Fabrication of a micro-lens array for improving depth-of-field of integral imaging 3D display.

In this paper, we present a new, to the best of our knowledge, structure of double pinhole/micro-lens array (DP/MLA) with two center-depth planes, used for improving the depth-of-field (DOF) of integral imaging (II), which can be fabricated by a combination of lithography and inkjet printing. The results show that a black circular groove array prepared by lithography can be used for micro-lens location and reduce the stray light for II. By controlling the parameters of the inkjet printing system, DP/MLA with high precision, high alignment, and good focusing ability can be achieved. When the fabricated DP/MLA is applied in the II system, the reconstructed image has a better three-dimensional (3D) image with higher DOF than that by traditional MLA and higher quality than that by ordinary double-layer MLA.

Ultra-large moiré-less autostereoscopic three-dimensional light-emitting-diode displays.

Large-scale autostereoscopic three-dimensional (3D) light-emitting-diode (LED) displays can provide high-quality, even immersive, visual experiences. However, the unique structural characteristics of the LED display panel can enhance moiré effects during parallax generation. We present a novel method for quantitatively characterizing the moiré effect in autostereoscopic 3D-LED displays using a model applying geometrical ray tracing and a brightness distribution stack. An optical diffuser (OD) device for moiré reduction and performance balance is designed and the influence of several key device parameters on moiré pattern features are examined in autostereoscopic 3D-LED displays for the first time. Using the obtained optimal parameters, we assembled an ultra-large moiré-less autostereoscopic 3D-LED display prototype, which was experimentally shown to be capable of reducing moiré fringes without noticeable increase in crosstalk or significant reduction in visual quality. Finally, the effects on the moiré and crosstalk effect of altering key influencing factors were examined.