Robust, Narrow-Band Nanorods LEDs with Luminous Efficacy > 200 lm/W: Next-Generation of Efficient Solid-State Lighting.

Energy-efficient white light-emitting diodes (LEDs) are in high demand across the society. Despite the significant advancements in the modern lighting industry based on solid-state electronics and inorganic phosphor, solid-state lighting (SSL) continues to pursue improved efficiency, saturated color performance, and longer lifetime. Here in this article, robust, narrow emission band nanorods (NRs) are disclosed with tailored wavelengths, aiming to enhance the color rendering index (CRI) and luminous efficacy (LE). The fabricated lighting device consists of NRs of configuration CdSe/ZnxCd1-xS/ZnS, which can independently tune CRI R1-R9 values and maximize the luminous efficacy. For general lighting, NRs with quantum yield (QY) up to 96% and 99% are developed, resulting in ultra-efficient LEDs reaching a record high luminous efficacy of 214 lm W-1 (certified by the National Accreditation Service). Furthermore, NRs are deployed onto mid-power (0.3 W@ 50 mA) LEDs, showing significantly enhanced long-term stability (T95 = 400 h @ 50 mA). With these astonishing properties, the proposed NRs can pave the way for efficient lighting with desired optical spectrum.

Ultralow Roll-Off Quantum Dot Light-Emitting Diodes Using Engineered Carrier Injection Layer.

Quantum dot (QD) light-emitting diodes (QLEDs) have attracted extensive attention due to their high color purity, solution-processability, and high brightness. Due to extensive efforts, the external quantum efficiency (EQE) of QLEDs has approached the theoretical limit. However, because of the efficiency roll-off, the high EQE can only be achieved at relatively low luminance, hindering their application in high-brightness devices such as near-to-eye displays and lighting applications. Here, this article reports an ultralow roll-off QLED that is achieved by simultaneously blocking electron leakage and enhancing the hole injection, thereby shifting the recombination zone back to the emitting QDs layer. These devices maintain EQE over 20.6% up to 1000 mA cm-2 current density, dropping only by ≈5% from the peak EQE of 21.6%, which is the highest value ever reported for the bottom-emitting red QLEDs. Furthermore, the maximum luminance of the optimal device reaches 320 000 cd m-2 , 2.7 times higher than the control device (Lmax : 128 000 cd m-2 ). A passive matrix (PM) QLED display panel with high brightness based on the optimized device structure is also demonstrated. The proposed approach advances the potential of QLEDs to operate efficiently in high-brightness scenarios.

Solution-Processed Red, Green, and Blue Quantum Rod Light-Emitting Diodes.

Solution-processed semiconductor nanocrystals are evolving as potential candidates for future display and lighting applications owing to their size-tunable emission, ultrasaturated colors, and compatibility with large-area flexible substrates. Among them, quantum rods (QRs) are emerging materials for optoelectronic applications, offering polarized emission, high light outcoupling efficiency, color purity, and better stability in solid films. However, synthesizing QRs covering the full visible wavelength region has been a big challenge, particularly in the blue range. Herein, we report for the first time the synthesis of red CdSe/CdS, green CdSe/ZnxCd1-xS/ZnS, and blue CdSe/ZnxCd1-xS/ZnS QRs and their application in red, green, and blue QR-based light-emitting diodes (QR-LEDs). We have improved the charge injection balance into the QRs through embedding a poly(methyl methacrylate) (PMMA) layer between the emissive and electron transport layers. The thin PMMA electron-blocking layer (EBL) suppresses the excessive electron flux and thus promotes charge injection balance and pushes the recombination zone back to the QR layer, resulting in 1.35×, 1.2×, and 1.7× peak external quantum efficiency improvement for red, green, and blue QR-LEDs, respectively. The efficiency roll-off of green and blue QR-LEDs with an EBL is less than 50% at maximum current density. The proposed red, green, and blue QR-LEDs open up an avenue toward further improving the light source efficiency and stability focusing on real device applications.

Quantum-Rod On-Chip LEDs for Display Backlights with Efficacy of 149 lm W-1 : A Step toward 200 lm W-1.

Efficient white light-emitting diodes (LEDs) with an efficacy of 200 lm W-1 are much desirable for lighting and displays. The phosphor-based LEDs in use today for display applications offer poor color saturation. Intensive efforts have been made to replace the phosphor with quantum-dot-based downconverters, but the efficiency and stability of these devices are still in their infancy. Quantum rods (QRs), nanoparticles with an elongated shape, show superior properties such as relatively larger Stokes shifts, polarized emission, and high light out-coupling efficiency in the solid-state. However, these QRs usually suffer from poor optical quality for PL wavelengths < 550 nm. Herein, a gradient alloyed CdSe/Znx Cd1- x S/ZnS and CdSe/CdS/ZnS core/shell/shell QR downconverters showing high efficacy LEDs covering a wide color gamut are reported. These QRs show high stability and a precisely tunable photoluminescence peak. The engineered shell thickness suppresses energy transfer and thus maintains the high quantum yield in the solid-state (81%). These QR-based LEDs attain an efficacy of 149 lm W-1 (@10mA) and wide color gamut (118% NTSC), which is exceedingly higher than state-of-the-art quantum dots and phosphor-based on-chip LEDs.

Stable bright perovskite nanoparticle thin porous films for color enhancement in modern liquid crystal displays.

Cesium-lead halide perovskite nanoparticles are a promising class of luminescent materials for color and efficient displays. However, material stability is the key issue to solve before we can use these materials in modern displays. Encapsulation is one of the most efficient methods that can markedly improve the stability of perovskite nanoparticles against moisture, heat, oxygen, and light. Thus, we urgently need a low-cost, reliable, and device-compatible encapsulation method for the integration of nanomaterials into display devices. Here, we propose a facile encapsulation method to stabilize perovskite nanoparticles in thin polymer porous films. Using porous polymer films, we achieved good photoluminescence stability in the harsh environment of high temperature, high humidity and strong UV illumination. The good UV stability benefitted from the unique optical properties of the porous film. Besides, we observed photoluminescence enhancement of CsPbBr3 nanoparticle films in a high humidity environment. The stable CsPbBr3 nanoparticle thin porous film provides high brightness (236 nits) and great color enhancement for LCDs and is characterized by simple fabrication with easy scalability, thus it is very suitable for modern LCDs.

Thermally Stable Quantum Rods, Covering Full Visible Range for Display and Lighting Application.

Recently, quantum rods (QRs) have been studied heavily for display and lighting applications. QRs offer serious advantages over the quantum dots such as higher light out-coupling coefficient, and polarized emission. The QR enhancement films double liquid crystal display efficiency. However, it is still a challenge to synthesize good quality green (λem  ≈ 520 nm) and blue (λem  ≈ 465 nm) emitting QRs, due to very large bathochromic shift during the shell growth. Furthermore, until now, the presence of cadmium in high-quality QRs is inevitable, but due to its toxicity, RoHS has restricted the amount of cadmium in consumer products. In this article, low Cd core-shell QRs, with a narrow-band luminescence spectrum tuned in the whole visible range, are prepared by replacing Cd with Zn in a one-pot post-synthetic development. These QRs possess the good thermal stability of photoluminescence properties, and therefore, show high performance for the on-chip LED configuration. The designed white LEDs (WLEDs) are characterized by a high brightness of 120000 nits, and color gamut covering 122% NTSC (90% of BT2020), in the 1931CIE color space. Additionally, these LEDs show a high luminous efficiency of 115 lm W-1 . Thus, these quantum rod LED are perfectly viable for display backlighting and lighting applications.

Inkjet-printed aligned quantum rod enhancement films for their application in liquid crystal displays.

Semiconductor quantum rods (QRs) show a polarized emission, which opens up the possibility of the enhancement of both brightness and color for liquid crystal displays (LCD) in the form of quantum rod enhancement films (QREFs) for LCD backlights. However, the QR alignment over a large area, suitable for displays, is a challenge. Inkjet printing of QREFs, introduced here, allows fabrication of well-aligned, uniform QREFs on photoaligned substrates using optimized QR inks. We observed that the ink composition and printing conditions affect the QR alignment quality significantly. A relative humidity of 50% with an exposure energy of 1 J cm-2 for the photoalignment process provided optimal conditions for QREFs. We have successfully shown a good QR alignment for 2.5-inch films and were able to align QRs in multiple layers. Thus, fabricated QREFs show a polarization ratio of 7.2 : 1 for the emitted light. These QREFs were combined with a blue LED and deployed as a backlight unit for an LCD which shows a brightness of ∼250 nits with an optical efficiency of ∼8%, reaching an NTSC of 109% in a CIE1976 color space. Thus, these printed QREFs, over a large area, provide an unprecedented increase of 77% in the optical efficiency of the LCDs and simultaneously offer better color performance.

Magnetic actuation of a thermodynamically stable colloid of ferromagnetic nanoparticles in a liquid crystal.

We report the development of a highly stable nanomaterial based on ferromagnetic nanoparticles dispersed in a thermotropic liquid crystal. The long-term colloidal stability and homogeneity were achieved through surface modification of the nanoparticles with a mixture of a dendritic oligomesogenic surfactant and hexylphosphonic acid and confirmed by optical and electron microscopy. The nanomaterial has an increased sensitivity to the magnetic field possessing collective and non-collective magneto-optical responses in contrast to the undoped LC. The effective coupling of the spherical particles with the LC director is due to the arrangement of the nanoparticles in chains.

Thermodynamically stable dispersions of quantum dots in a nematic liquid crystal.

Using transmittance electron microscopy, fluorescence and polarizing optical microscopy, optical spectroscopy, and fluorescent correlation spectroscopy, it was shown that CdSe/ZnS quantum dots coated with a specifically designed surfactant were readily dispersed in nematic liquid crystal (LC) to form stable colloids. The mixture of an alkyl phosphonate and a dendritic surfactant, where the constituent molecules contain promesogenic units, enabled the formation of thermodynamically stable colloids that were stable for at least 1 year. Stable colloids are formed due to minimization of the distortion of the LC ordering around the quantum dots.