Stable and Efficient Green Perovskite Nanocrystal–Polysilazane Films for White LEDs Using an Electrospray Deposition Process

We successfully fabricated a stable, efficient, and easy-to-use CsPbBr3 perovskite nanocrystal (PeNC)-embedded inorganic polymer film through an encapsulation step with a Si–N/Si–O-based polysilazane (PSZ) matrix via the electrospray (e-spray) deposition of a silazane (SZ) oligomer-decorated PeNC solution. To eliminate Pb2+ defect sites that are generated when the ligands are peeled from the PeNC surface, surface passivation of the Lewis acid/base adduct is possible by coupling the SZ oligomer (the donor of lone pairs) with Pb2+ sites (the acceptor of lone pairs). With the addition of the SZ oligomer, the photoluminescence quantum yield of photodegraded CsPbBr3 PeNC was recovered and increased by 2.35-fold whereas the stability was improved significantly from an untreated CsPbBr3 PeNC solution. During the e-spray deposition process, SZ-treated CsPbBr3 PeNC solution droplets can react with atmospheric moisture to polymerize and form a Si–N/Si–O network encapsulant via a sol–gel reaction. The resultant CsPbBr3–PSZ films showed improved stability levels under most environmental conditions, including air storage, blue light exposure, UV exposure, thermal exposure, and water immersion. The optimum CsPbBr3–PSZ film-covered blue light-emitting diode (LED) showed good performance capabilities, with a luminous efficacy (LE) of 85.9 lm/W and color-by-blue conversion efficiency (CE) of 60.1%. Furthermore, this easy-to-use CsPbBr3–PSZ film can be employed to realize a remote-type white-by-blue LED by combining it with red emissive K2SiF6:Mn4+/silicone film. The LE and CE rates of the white LED were 71.0 lm/W and 50.8%, respectively, at a correlated color temperature of 9334 K, with only an 8% drop in the LE for long-term operation of 100 h. This result indicates that e-spray deposition is a simple fabrication process by which to create stable and efficient PeNC films from an unstable PeNC solution using a rapid sol–gel reaction between droplets and moisture from the air

Additional file 1 of Passivation and Interlayer Effect of Zr(i-PrO)4 on Green CuGaS2/ZnS/Zr(i-PrO)4@Al2O3 and Red CuInS2/ZnS/Zr(i-PrO)4@Al2O3 QD Hybrid Powders

Additional file 1. Supplementary figures

Synthesis of Cs₃MnBr₅ Green Phosphors Using an Eco-Friendly Evaporative Crystallization Process

Green (G) and red (R) light-emitting materials, such as quantum dots, perovskite nanocrystals, and inorganic phosphor powders, owing to their excellent optical characteristics, have attracted researchers’ attention as color-conversion materials for lighting and display applications. However, these materials contain environmentally harmful elements, such as Pb or Cd, and/or they are synthesized using environmentally harmful synthetic approaches and conditions, involving the use of organic solvents, high pressure, high temperature, harsh atmosphere, and long reaction time. In this study, as an eco-friendly synthetic approach to synthesize lead-free Cs3MnBr5 G powder phosphor, we suggest an evaporative crystallization process of aqueous reactant solution. This synthetic process does not use toxic elements or solvents and the crystallization process utilizes only low reaction temperature and short reaction time under air atmosphere conditions. We successfully synthesized Cs3MnBr5 green powder phosphor, with excellent optical properties, by evaporative heating of a 200 nm syringe-filtered solution at 150 °C for 2 h. The synthesized Cs3MnBr5 phosphors have a photoluminescence quantum yield of 66.3%, a peak wavelength of 520 nm, a narrow bandwidth of 38 nm, and a photoluminescence decay time of 0.34 ms under blue excitation. This phosphor is expected to be a useful alternative G-emitting material that can compete with commercial green quantum dots, perovskite nanocrystals, or inorganic phosphors

Efficient and Stable CsPbBr₃ Quantum-Dot Powders Passivated and Encapsulated with a Mixed Silicon Nitride and Silicon Oxide Inorganic Polymer Matrix

Despite the excellent optical features of fully inorganic cesium lead halide (CsPbX₃) perovskite quantum dots (PeQDs), their unstable nature has limited their use in various optoelectronic devices. To mitigate the instability issues of PeQDs, we demonstrate the roles of dual-silicon nitride and silicon oxide ligands of the polysilazane (PSZ) inorganic polymer to passivate the surface defects and form a barrier layer coated onto green CsPbBr₃ QDs to maintain the high photoluminescence quantum yield (PLQY) and improve the environmental stability. The mixed SiN_<i>x</i>/SiN_<i>x</i>O_<i>y</i>/SiO_<i>y</i> passivated and encapsulated CsPbBr₃/PSZ core/shell composite can be prepared by a simple hydrolysis reaction involving the addition of adding PSZ as a precursor and a slight amount of water into a colloidal CsPbBr₃ QD solution. The degree of the moisture-induced hydrolysis reaction of PSZ can affect the compositional ratio of SiN_<i>x</i>, SiN_<i>x</i>O_<i>y</i>, and SiO_<i>y</i> liganded to the surfaces of the CsPbBr₃ QDs to optimize the PLQY and the stability of CsPbBr₃/PSZ core/shell composite, which shows a high PLQY (∼81.7%) with improved thermal, photo, air, and humidity stability as well under coarse conditions where the performance of CsPbBr₃ QDs typically deteriorate. To evaluate the suitability of the application of the CsPbBr₃/PSZ powder to down-converted white-light-emitting diodes (DC-WLEDs) as the backlight of a liquid crystal display (LCD), we fabricated an on-package type of tricolor-WLED by mixing the as-synthesized green CsPbBr₃/PSZ composite powder with red K₂SiF₆:Mn⁴⁺ phosphor powder and a poly­(methyl methacrylate)-encapsulating binder and coating this mixed paste onto a cup-type blue LED. The fabricated WLED show high luminous efficacy of 138.6 lm/W (EQE = 51.4%) and a wide color gamut of 128% and 111% without and with color filters, respectively, at a correlated color temperature of 6762 K

Study of Perovskite QD Down-Converted LEDs and Six-Color White LEDs for Future Displays with Excellent Color Performance

A narrow-emitting red, green, and blue (RGB) perovskite quantum dot (PeQD)-based tricolored display system can widen the color gamut over the National Television System Committee (NTSC) to 120%, but this value is misleading with regard to the color perception of cyan and yellow reproduced in the narrow RGB spectra. We propose that a PeQD-based six-color display system can reproduce true-to-life spectral distributions with high fidelity, widen the color gamut, and close the cyan and yellow gap in the RGB tricolored display by adding cyan (Cy), yellowish green (Yg), and orange colors (Or). In this study, we demonstrated pure-colored CsPbX₃ (X = Cl, Br, I, or their halide mixtures; Cl/Br and Br/I) PeQD-based monochromatic down-converted light-emitting diodes (DC-LED) for the first time, and we incorporated PeQDs with UV-curable binders and long-wavelength-pass-dichroic filters (LPDFs). CsPbX₃ PeQD-based pure Cy-, G-, Yg-, Or-, R-emitting monochromatic DC-LED provide luminous efficacy (LE) values of 81, 184, 79, 80, and 35 lm/W, respectively, at 20 mA. We also confirmed the suitability and the possibility of access to future color-by-blue backlights for field-sequential-color liquid crystal displays, using six-color multipackage white LEDs, as well as future six-colored light-emitting devices with high vision and color performance. The fabricated six-color multipackage white LEDs exhibited an appropriate LE (62 lm/W at total 120 mA), excellent color qualities (color rendering index (CRI) = 96, special CRI for red (<i>R</i>₉) = 97) at a correlated color temperature (CCT) of 6500 K, and a wide color gamut covering the NTSC up to 145% in the 1931 Commission International de l’Eclairage (CIE) color coordinates space

Hydrothermal−Electrochemical Synthesis of ZnO Nanorods

Vertically aligned ZnO nanorods having high optical quality were prepared by a hydrothermal−electrochemical method. The nanorods were synthesized in a Zn(NO3)2 aqueous solution on Si substrates which were coated with a platinum conducting layer and a ZnO seed layer. They possessed a single-crystal würtzite structure and grew along the c-axis, perpendicular to the substrates. The height and diameter of the ZnO nanorods were up to ∼4.3 μm and 90−150 nm, respectively. The morphological, structural, and photoluminescence properties of the ZnO nanorods were examined with respect to the growth temperature (120−180 °C) and the presence of NaOH additive. The nanorods synthesized at high temperature (180 °C) exhibited a strong UV emission and a weak defect-related visible emission leading to a UV−visible ratio of ∼230. This high optical quality was attributed to the increased growth rate of ZnO nanorods (∼4.3 μm/h) which was caused by the high growth temperature (180 °C). This was based on the fact that the ZnO phase is thermodynamically more favorable than the defect-related Zn(OH)2 phase at higher temperature. Since the growth temperature was compatible with polymer materials, our synthetic method may provide a promising way for fabricating high performance optoelectronic devices on flexible polymer substrates

Stable and Colorful Perovskite Solar Cells Using a Nonperiodic SiO₂/TiO₂ Multi-Nanolayer Filter

While research on building-integrated photovoltaics (BIPVs) has mainly focused on power-generating window applications, the utilization of other underutilized surface areas in buildings, including exteriors, facades, and rooftops, has still not been fully explored. The most important requirements for BIPVs are color, power conversion efficiency (PCE), and long-term stability. In this work, we achieved colorful (red, green, blue, RGB) perovskite solar cells (PSCs) with minimized PCE loss (<10%) and enhanced photostability by exploiting the optical properties of nonperiodic multi-nanolayer, narrow-bandwidth reflective filters (NBRFs). The NBRFs were fabricated by multilayering high-index TiO2/low-index SiO2 in a nonperiodic manner, which allowed devices to demonstrate various colors with effectively suppressed unwanted baseline ripple-shape reflectance. The PCEs of PSCs with nonperiodic RGB-NBRFs were 18.0%, 18.6%, and 18.9%, which represent reductions of only 10%, 7%, and 6% of PCE values, respectively, compared to a black control PSC (20.1%). Moreover, the photostability of the PSCs was substantially improved by using the NBRFs because of ultraviolet blocking in the TiO2 layers. The G-PSC retained 65% of the initial PCE after 60 h of continuous illumination (AM 1.5G one sun) at the maximum power point, whereas the black PSC retained only 30%. Aesthetic color value, low PCE loss, and enhanced photostability of PSCs were simultaneously achieved by employing our NBRFs, making this a promising strategy with potential applicability in power-generating building exteriors

Development and Verification of a 480 nm Blue Light Enhanced/Reduced Human-Centric LED for Light-Induced Melatonin Concentration Control

With the inherent sleep and wake cycle regulated by natural sunlight, the human body has evolved over millennia to be active during the day and to rest at night. However, maintaining an optimal 24 h cycle has become increasingly problematic in modern society as more people spend the majority of the day indoors. Many research groups have reported that inadequate artificial lighting interferes with melatonin production and disrupts the circadian rhythm. This study considered biological functions for light-emitting diodes (LEDs) of next-generation illumination, and LED packages and spectra suitable for both daytime and nighttime applications were designed. The prepared daytime human-centric (HC)-LEDs had a melanopic/photopic (M/P) ratio that was up to 26% higher than that of conventional (c)-LEDs, whereas the nighttime HC-LEDs exhibited up to a 26% lower M/P ratio compared to the c-LEDs. Nevertheless, because the HC-LED is designed to have almost the same color coordinates as the c-LED having the same correlated-color temperature (CCT), there is no change in the perceived color. To substantiate the biological effect, melatonin level data were obtained from 22 voluntary participants in c- and HC-LED lighting environments. In the HC-LED lighting environment, melatonin was suppressed by 21.9% after waking, and nocturnal melatonin secretion was increased by up to 12.2%. As human-centric lighting, our HC-LEDs are expected to become an essential element for modern life, where people spend most of their time indoors

Molecular Mechanism of Selective Al₂O₃ Atomic Layer Deposition on Self-Assembled Monolayers

Area-selective atomic layer deposition (AS-ALD) of insulating metallic oxide layers could be a useful nanopatterning technique for making increasingly complex semiconductor circuits. Although the alkanethiol self-assembled monolayer (SAM) has been considered promising as an ALD inhibitor, the low inhibition efficiency of the SAM during ALD processes makes its wide application difficult. We investigated the deposition mechanism of Al2O3 on alkanethiol-SAMs using temperature-dependent vibrational sum-frequency-generation spectroscopy. We found that the thermally induced formation of gauche defects in the SAMs is the main causative factor deteriorating the inhibition efficiency. Here, we demonstrate that a discontinuously temperature-controlled ALD technique involving self-healing and dissipation of thermally induced stress on the structure of SAM substantially enhances the SAM’s inhibition efficiency and enables us to achieve 60 ALD cycles (6.6 nm). We anticipate that the present experimental results on the ALD mechanism on the SAM surface and the proposed ALD method will provide clues to improve the efficiency of AS-ALD, a promising nanoscale patterning and manufacturing technique