Efficient Color Conversion Research Articles

Multishell Silver Indium Selenide-Based Quantum Dots and Their Poly(methyl methacrylate) Composites for Application in Red-Light-Emitting Diodes.

In this work, the production of novel multishell silver indium selenide quantum dots (QDs) shelled with zinc selenide and zinc sulfide through a multistep synthesis precisely designed to develop high-quality red-emitting QDs is explored. The formation of the multishell nanoheterostructure significantly improves the photoluminescence quantum yield of the nanocrystals from 3% observed for the silver indium selenide core to 27 and 46% after the deposition of the zinc selenide and zinc sulfide layers, respectively. Moreover, the incorporation of the multishelled QDs in a poly(methyl methacrylate) (PMMA) matrix via in situ radical polymerization is investigated, and the role of thiol ligand passivation is proven to be fundamental for the stabilization of the QDs during the polymerization step, preventing their decomposition and the relative luminescence quenching. In particular, the role of interface chemistry is investigated by considering both surface passivation by inorganic zinc chalcogenide layers, which allows us to improve the optical properties, and organic thiol ligand passivation, which is fundamental to ensuring the chemical stability of the nanocrystals during in situ radical polymerization. In this way, it is possible to produce silver-indium selenide QD-PMMA composites that exhibit bright red luminescence and high transparency, making them promising for potential applications in photonics. Finally, it is demonstrated that the new silver indium selenide QD-PMMA composites can serve as an efficient color conversion layer for the production of red light-emitting diodes.

InGaN blue resonant cavity micro-LED with RGY quantum dot layer for broad gamut, efficient displays

The technology of RGBY micro resonant cavity light emitting diodes (micro-RCLEDs) based on quantum dots (QDs) is considered one of the most promising approaches for full-color displays. In this work, we propose a novel structure combining a high color conversion efficiency (CCE) QD photoresist (QDPR) color conversion layer (CCL) with blue light micro RCLEDs, incorporating an ultra-thin yellow color filter. The additional TiO2 particles inside the QDPR CCL can scatter light and disperse QDs, thus reducing the self-aggregation phenomenon and enhancing the eventual illumination uniformity. Considering the blue light leakage, the influences of adding different color filters are investigated by illumination design software. Finally, the introduction of low-temperature atomic layer deposition (ALD) passivation protection technology at the top of the CCL can enhance the device's reliability. The introduction of RGBY four-color subpixels provides a viable path for developing low-energy consumption, high uniformity, and efficient color conversion displays.

Fluorescent Organic Nano-dots as Efficient Color Conversion Layers

Fluorescent Organic Nano-dots as Efficient Color Conversion Layers

Photonic Characterization and Modeling of Highly Efficient Color Conversion Layers With External Reflectors

We propose a new method for fabricating arrays of colloidal quantum dots for color conversion layers (CCLs) with atomic layer deposited (ALD) encapsulation. Semiconductor processes were used to transfer the CCL onto a transparent glass substrate. This method enables pixels as small as 5 μm, and the pixel's size and pitch can be controlled through the semiconductor process and mask layout. A dual-color array with a red-and-green CCL was also demonstrated to be suitable for microdisplay applications in experiments. The best color conversion efficiency of a single color CCL reached as high as 20.7%. A set of distributed Bragg reflector mirrors was used to increase photon recycling, and these mirrors yielded an increase in peak intensity of 35.7%. We formulated a model of incoherent reflection and transmission to calculate the changes in CCL properties when mirrors are added; this model had close fit with results from experiments in which mirrors of different reflection coefficients were used. Through the use of ALD, the CCL can be stored at a normal room-temperature environment for more than 9000 hours, and the projected lifetime is linearly extrapolated to be 44,041 hours.

Ameliorating Uniformity and Color Conversion Efficiency in Quantum Dot-Based Micro-LED Displays through Blue-UV Hybrid Structures.

Quantum dot (QD)-based RGB micro light-emitting diode (μ-LED) technology shows immense potential for achieving full-color displays. In this study, we propose a novel structural design that combines blue and quantum well (QW)-intermixing ultraviolet (UV)-hybrid μ-LEDs to achieve high color-conversion efficiency (CCE). For the first time, the impact of various combinations of QD and TiO2 concentrations, as well as thickness variations on photoluminescence efficiency (PLQY), has been systematically examined through simulation. High-efficiency color-conversion layer (CCL) have been successfully fabricated as a result of these simulations, leading to significant savings in time and material costs. By incorporating scattering particles of TiO2 in the CCL, we successfully scatter light and disperse QDs, effectively reducing self-aggregation and greatly improving illumination uniformity. Additionally, this design significantly enhances light absorption within the QD films. To enhance device reliability, we introduce a passivation protection layer using low-temperature atomic layer deposition (ALD) technology on the CCL surface. Moreover, we achieve impressive CCE values of 96.25% and 92.91% for the red and green CCLs, respectively, by integrating a modified distributed Bragg reflector (DBR) to suppress light leakage. Our hybrid structure design, in combination with an optical simulation system, not only facilitates rapid acquisition of optimal parameters for highly uniform and efficient color conversion in μ-LED displays but also expands the color gamut to achieve 128.2% in the National Television Standards Committee (NTSC) space and 95.8% in the Rec. 2020 standard. In essence, this research outlines a promising avenue towards the development of bespoke, high-performance μ-LED displays.

Wafer-scale patterning of high-resolution quantum dot films with a thickness over 10 μm for improved color conversion.

Quantum dots (QDs) are promising color conversion materials for efficient full-color micro light-emitting diode (micro-LED) displays owing to their high color purity and wide color gamut. However, achieving high-resolution QD patterns with enough thickness for efficient color conversion is challenging. Here, we demonstrate a facile and compatible approach by combining replicate molding, plasma etching and transfer printing to produce QD patterns with a sufficient thickness over ten micrometers in a wide range of resolutions. Our technique can remarkably simplify the preparation of QD inks and minimize optical damage to QD materials. The pixel resolution and thickness of QD patterns can be controlled by well-defining the microstructures of the molding template and the etching process. The transfer printing process allows QD patterns to be assembled sequentially onto a receiving substrate, which will further improve the original pixel resolution and avoid repetitive optical damage to QDs during the patterning process. Consequently, various QD patterns can be fabricated in this work, including perovskite quantum dot (PQD) patterns with a pixel resolution of up to 669 pixels per inch (ppi) and a maximum thickness of up to 19.74 μm, a wafer-scale high-resolution PQD pattern with sufficient thickness on a flexible substrate, and a dual-color pattern comprising green PQDs and red CdSe QDs. Furthermore, these fabricated QD films with a thickness of over 10 μm show improved color conversion when integrated onto a blue micro-LED, revealing the potential of our technique for full-color micro-LED displays.

Freestanding High-Resolution Quantum Dot Color Converters with Small Pixel Sizes

Designing the next generation of high-resolution displays requires high pixel density per area and small pixel sizes without compromising the optical quality. Quantum dots (QDs) have been demonstrated as a promising material system for down-conversion of blue emission as they provide pure colors on the wide color gamut. However, for high color-conversion efficiency, the required QD film thickness has not been compatible with small pixel sizes. In this work, we develop a new type of freestanding QD-based color converter for efficient optical down-conversion from inorganic blue light-emitting diodes (LEDs) in a color-by-blue configuration. CdSe/ZnS core-shell QDs in a UV-curable polymer matrix are encapsulated within cavities formed by patterning and bonding a pair of patterned quartz substrates. By controlling the required QD thickness and the pixel size independently, we demonstrate freestanding monochrome red and green converters with small pixel sizes down to 5 × 5 μm2 and a high resolution of >3600 ppi. The optical studies show that the QD film thickness required for efficient color conversion can be successfully realized even for the small pixel sizes. We further combine green and red pixels in a single converter to achieve white emission when combined with blue LED emission. The QD color converter design and processing are decoupled from the LED fabrication and can be easily scaled to wafer-size integration with arbitrary pixel sizes for QD-based RGB displays with ultrahigh resolution.

Synthesis of fluorescent organic nano-dots and their application as efficient color conversion layers

Efficient conversion of light from short wavelengths to longer wavelengths using color conversion layers (CCLs) underpins the successful operation of numerous contemporary display and lighting technologies. Inorganic quantum dots, based on CdSe or InP, for example, have received much attention in this context, however, suffer from instability and toxic cadmium or phosphine chemistry. Organic nanoparticles (NPs), though less often studied, are capable of very competitive performance, including outstanding stability and water-processability. Surfactants, which are critical in stabilizing many types of nano-structures, have not yet been used extensively in organic NPs. Here we show the utility of surfactants in the synthesis and processing of organic NPs by thoroughly characterizing the effect of ionic and non-ionic surfactants on the properties of fluorescent organic NPs. Using this information, we identify surfactant processing conditions that result in nearly 100 % conversion of organic fluorophores into sub-micrometer particles, or nano-dots, with outstanding performance as CCLs. Such water dispersions are environmentally benign and efficiently convert light. They can be used for a range of fluorophores covering a full spectral gamut, with excellent color purity, including full-width at half-maximum (FWHM) values as low as 21 nm. Compared to inorganic (InP) reference CCLs, the organic nano-dot based CCLs show superior color conversion efficiency and substantially improved long-term stability.

Organic Nano-Dots as Efficient Color Conversion Layer

Organic Nano-Dots as Efficient Color Conversion Layer

Highly luminescent ZnCdTeS nanocrystals with wide spectral tunability for efficient color-conversion white-light-emitting-diodes

CdTe-based semiconductor nanocrystals (NCs) with size and composition-dependent efficient bandgap properties are historically mature nanomaterials for colloidal optoelectronic applications. In this work, we present the highly luminescent quaternary ZnCdTeS NCs with tunable emission across a wide visible spectrum from green to red spectral range. Prepared via a direct aqueous-based approach, a second capping agent of trisodium citrate (TSC) was used to enhance the photoluminescence (PL) emission efficiency, the chemical stability, and to spectrally widen the coverage of the emission spectra of ZnCdTeS NCs. Adding TCS created a remarkable blue shift from 572 nm in the absence of TSC, to 548 nm. On the other hand, upon optimization of experimental parameters, superior ZnCdTeS NCs with a narrow PL profile typically less than 50 nm, the high quantum efficiency of 76%, and tunable emission from 515-to-645 nm were synthesized in an aqueous solvent. The keynotes were the superior and reproducible luminescent properties for the core only NCs, without shell and using relatively low reaction temperatures. It was shown that in the suggested synthesis method, the high efficiency emitted color of ZnCdTeS NCs can be easily controlled from 515-to-650 nm with excellent stability against harsh conditions. The biexponential decay profiles of samples prepared at different reaction temperatures demonstrated that the average recombination lifetime is below 40 ns and increases with the growth of the ZnCdTeS NCs. Results reveal that the excitonic energy levels have the main role in the recombination process. Finally, to demonstrate the functional advantages of the prepared NCs in optoelectronics, the NCs were used to fabricate color-conversion white light-emitting diodes. The color coordinate of the device is recorded as (0.4951, 0.3647) with CRI of 91, CCT of 1954 K, and LER of 251 lm W−1 by employing only two distinct emitters for color conversion.

Environmentally Friendly and Ultrastable Narrow-Band Emitter: CsPbX3@SEBS Flexible Film for Light-Emitting Diode Displays

All-inorganic CsPbX3 perovskite nanocrystals (PeNCs) are being investigated intensively as a promising optoelectronic material. However, the issues of its intrinsic instability and toxicity restrict their commercial applications. Herein, we proposed a facile method to improve the stability of CsPbX3PeNCs by encapsulating in a superhydrophobic polymer poly(styrene–ethylene–butylene–styrene) (SEBS) matrix. The blue–green–red emission CsPbX3@SEBS (X = Cl/Br, Br, Br/I) composite films exhibit high stability against moisture and are thermally stable. It is worth noticing that the photoluminescent quantum yield (PLQY) of the CsPbBrI2@SEBS composite film can reach 60%. The SEBS polymer matrix not only protects the PeNCs from contact with the ambient surroundings but also inhibits the escape of lead ions. Notably, the CsPbX3@SEBS flexible films were used as color conversion materials to construct a light-emitting diode (LED) device, and the device exhibits a wide color gamut (113% of the National Television System Committee (NTSC)). This work demonstrates the potential of using CsPbX3@SEBS flexible films as highly efficient color conversion materials for white LED and backlight displays.

Development of Quantum Dot (QD) Based Color Converters for Multicolor Display.

Many displays involve the use of color conversion layers. QDs are attractive candidates as color converters because of their easy processability, tuneable optical properties, high photoluminescence quantum yield, and good stability. Here, we show that emissive QDs with narrow emission range can be made in-situ in a polymer matrix, with properties useful for color conversion. This was achieved by blending the blue-emitting pyridine based polymer with a cadmium selenide precursor and baking their films at different temperatures. To achieve efficient color conversion, blend ratio and baking temperature/time were varied. We found that thermal decomposition of the precursor leads to highly emissive QDs whose final size and emission can be controlled using baking temperature/time. The formation of the QDs inside the polymer matrix was confirmed through morphological studies using atomic force microscopy (AFM) and transmission electron microscopy (TEM). Hence, our approach provides a cost-effective route to making highly emissive color converters for multi-color displays.

Efficient green-emitting Ca2GdZr2Al3O12:Ce3+,Tb3+ phosphors for near-UV-pumped high-CRI warm-white LEDs

In this work, we reported novel efficient Ce3+/Tb3+ co-doped Ca2GdZr2Al3O12 (CGZA) green-emitting phosphors by traditional solid state method. The phase purity, crystal structure, morphology, photoluminescence properties, internal quantum efficiencies (IQEs), and thermal stability of the samples were systematically investigated. The obtained results revealed that there was an obvious spectral overlap between the emission spectrum of Ce3+ singly doped CGZA phosphors and excitation spectrum of Tb3+ singly doped CGZA phosphors, and efficient energy transfer from Ce3+ to Tb3+ ions existed in Ce3+ and Tb3+ co-doped CGZA phosphors. The Ce3+→Tb3+ energy transfer mechanism was ascribed to the dipole-dipole interaction. The CGZA:Ce3+,Tb3+ phosphors exhibited a broad excitation spectrum in the wavelength range of 370–470 nm with a maximum at 408 nm corresponding to the spin-allowed 4f-5d transition of Ce3+ ions. Upon near-ultraviolet (near-UV) excitation at 408 nm, the composition-optimized CGZA:0.02Ce3+,0.8 Tb3+ sample showed intense sharp narrow-line emissions at 485, 543, 591, 629, 643, 663, and 679 nm due to the 5D4→7FJ (J = 6–0) transitions of Tb3+ ions, along with a good IQE of 51%. Lastly, a white light-emitting diode (LED) device was fabricated by using as-prepared CGZA:0.02Ce3+,0.8Tb3+ green phosphors, commercial BaMgAl10O17:Eu2+ blue phosphors, and CaAlSiN3:Eu2+ red phosphors, and under 300 mA driven current it gave bright warm-white light with a low correlated color temperature of 3969 K and a high color rendering index (Ra = 91.2). Our work provides a new perspective for the design and fabrication of efficient color conversion materials toward near-UV LED-based warm-white LEDs.

Bright Blue Light‐Emitting Doped Cesium Bromide Nanocrystals: Alternatives of Lead‐Free Perovskite Nanocrystals for White LEDs

AbstractLead‐based perovskite nanocrystals (NCs) are promising candidates for use in lighting and display applications; however, the toxicity of lead is one critical issue that needs to be solved for its commercialization. Consequently, less toxic tin and bismuth‐based perovskite NCs have been developed, but these materials exhibit low photoluminescence quantum yields (PLQYs) and large full width at half maximum (FWHM) values. Due to their similarity in size for Eu2+ and Pb2+, the more environmental‐friendly europium shows the potential to replace lead. Herein, the synthesis of Eu2+ doped CsBr NCs with an average size of 51.5 nm via a facile hot‐injection method is reported. The prepared CsBr:Eu2+ NCs exhibit an emission peak at 440 nm with an FWHM of 31 nm and PLQY up to 32.8%, which is persistent for at least 60 d. Moreover, the size and FWHM of CsBr:Eu2+ NCs can be tuned to 18.9 and 29 nm, respectively, by co‐doping of Ca2+ ions into the NCs. It is also demonstrated that the prepared CsBr:Eu2+ NCs can be employed as efficient color conversion materials for fabricating white light‐emitting diodes.

Synthesis and Characterization of a Highly Efficient Color-Conversion Material

Synthesis and Characterization of a Highly Efficient Color-Conversion Material

A high quality liquid-type quantum dot white light-emitting diode.

This study demonstrates a novel package design to store colloidal quantum dots in liquid format and integrate them with a standard LED. The high efficiency and high quality color performance at a neutral white correlated color temperature is demonstrated. The experimental results indicate that the liquid-type quantum dot white light-emitting diode (LQD WLED) is highly efficient and reliable. The luminous efficiency and color rendering index (CRI) of the LQD WLED can reach 271 lm Wop(-1) and 95, respectively. Moreover, a glass box is employed to prevent humidity and oxygen erosion. With this encapsulation design, our quantum dot box can survive over 1000 hours of storage time.

P‐117: High Efficient Color Conversion Layers for White Organic Light‐Emitting Diodes using Polystyrene Nanosphere Monolayers

AbstractWe demonstrated a highly efficient color conversion layers (CCLs) by using nanosphere arrays for white organic light‐emitting diodes. The introduced periodical nanospheres not only help extract the confined light in devices, but also increase the effective light path to achieve more efficient color conversion. By applying the nanospheres patterned CCL on a PVK‐based phosphorescent blue OLED, 137% color conversion ratio had been achieved, which was 2.68 times higher than flat CCL. The resulting luminous efficiency of white device with patterned CCL (20.97 cd/A, 1000 cd/m2) was twice higher than the efficiency of the flat device (10.26 cd/A, 1000 cd/m2).

The Impact of Carrier Transport Confinement on the Energy Transfer Between InGaN/GaN Quantum‐Well Nanorods and Colloidal Nanocrystals

AbstractThe energy transfer (ET) between InGaN/GaN multiple‐quantum‐well (MQW) nanorods (NRs) and semiconductor nanocrystals (NCs) for efficient color conversion is studied. An exceptional contribution of carrier transport confinement to the ET mechanisms is observed in the proximal side‐wall coupling system, which consists of InGaN/GaN NRs and CdSe NCs. Under relatively low or high excitation, the ET rate shows different carrier‐density dependence, resulting from different electron‐hole configurations, i.e., bound excitons and free carriers. In the localized exciton regime, the ET rate decreases when increasing temperature from 20 K to 200 K. However, in the free‐carrier regime, the ET rate varies insignificantly in the same temperature range. The temperature dependence in this NR‐NC coupling system is different from that in the previously studied planar MQW‐NC coupling system. It is suggested that the carrier transport confinement in NRs is a major factor for these divergences. The highly efficient ET with efficiency up to 80% shows a promising potential of using such NR‐NC coupled structures for ET‐pumped, NC‐based, light‐emitting devices.

Improve efficiency of white organic light-emitting diodes by using nanosphere arrays in color conversion layers

The authors demonstrated an efficient color conversion layer (CCL) by using nanosphere arrays in down-converted white organic light-emitting diodes (WOLEDs). The introduced periodical nanospheres not only helped extract the confined light in devices, but also increased the effective light path to achieve high-efficiency color conversion. By applying a CCL with red phosphor on a 400-nm-period nanosphere array, we achieved 137% color conversion ratio for blue OLEDs, which was 2.68 times higher than conventional flat CCL. The resulting luminous efficiency of WOLEDs with patterned CCLs (20.97 cd/A, 1000 cd/m2) was two times higher than the efficiency of the flat device (10.26 cd/A, 1000 cd/m2).

Polychromatic emission from fluorescent‐microsphere‐coated LEDs

AbstractWhite light LEDs using fluorescent microspheres as color conversion agents are demonstrated. The hexagonal ordered arrays of green and red fluorescence microspheres were coated on the blue micro‐hexLED centered at 480 nm by microspheres coating technique. At optimal mixed portion of green and red microspheres, white light emission is generated by a combination of blue light from LED and down conversion of fluorescence emission of green and red microspheres by blue excitation light. An overall efficiency of more than 70% is obtained by this new approach. The enhanced efficiency is attributed to two main properties of ordered micropsheres arrays: (1) efficient color conversion and (2) efficient light out‐coupling via volumetric light scattering. The resultant LED has lumnious efficiency of 31.8 lm/W with CIE coordinates of (0.30, 0.31). We have demonstrated an alternative solution of producing white light emission, apart from existing down conversion of yellow phosphor with blue or UV LED and multichip LEDs. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)