1. Introduction
Quantum dots (QDs) are tiny semiconductor nanocrystals with color-tunable and high-efficiency photoluminescence (PL) emission [1],
[2],
[3]. Their unique color properties have been widely explored to promote display technology such as liquid crystal displays (LCDs), organic light-emitting diodes (OLEDs), and micro light-emitting diodes (Micro-LEDs) [4],
[5],
[6]. As color converters, QDs have been successfully applied to enhance the display performance of LCDs (prototype: 2012
[7],
[8]; commercial products: 2014
[9]), OLEDs (prototype: 2016
[10],
[11]; commercial products: 2021
[12]), and Micro-LEDs (prototype: 2015
[13]). Aside from their application as color converters, QDs can also act as emitting layers in the fabrication of solution-processed electroluminescent display panels
[14],
[15]. Thus far, most previous successes have relied on chemically synthesized CdSe and InP QDs
[16]. In 2015, Kovalenko’s group
[17] at ETH Zurich and Zhong’s group
[18] at Beijing Institute of Technology demonstrated the fabrication of perovskite quantum dots (PQDs) with color-tunable PL emission for display applications. After about 10 years of effort, remarkable progress has been made in stability improvement and fabrication technology
[19],
[20],
[21],
[22],
[23],
[24]. A comprehensive evaluation reveals that PQDs exhibit balanced advantages, with superior color properties (narrow full width at half maximum (FWHM), CdSe ∼ PQDs > InP) and reduced environmental impact (low toxicity, PQDs ∼ InP > CdSe), as well as low cost and easy processability (PQDs > CdSe ∼ InP)
[25],
[26]. To initialize the industrialization of PQDs, it is essential to develop the scale-up fabrication of PQDs with improved stability.
In situ fabrication is one of the most common strategies used to prepare PQD-based composites in different matrices (i.e., polymers
[27], molecular sieves [28],
[29], metal–organic frameworks (MOFs)
[30], and glasses
[31],
[32]). These composites exhibit excellent optical properties, enhanced stability, and easy patterning processability, approaching the criteria for display applications. However, it remains a great challenge to achieve the continuous and scaled-up fabrication of PQDs toward industrialization. In this work, we report the spray-drying fabrication of PQDs from a precursor solution at a scale of 2000 kg∙a−1. The material characterizations show that the as-fabricated microspheres contain monodispersed PQDs in the polymer matrix. The obtained PQD-embedded polymer microspheres have a high photoluminescence quantum yield (PLQY) and enhanced stability. We further explore their use as color converters in LCD and Micro-LED applications.
2. Experimental section
2.1. Materials and methods
2.1.1. Materials
Cesium iodide (CsI, 99.9%), lead iodide (PbI2, 98%), N,N-dimethylformamide (DMF, analytical reagent), and polyacrylonitrile (PAN, weight average molecular weight (Mw) = 50 000) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (China). Methylamine bromide (MABr, 99%), formamidine bromide (FABr, 99%), octylamine bromide (OABr), and octylamine iodide (OAI) were purchased from Xi’an Polymer Light Technology Co. (China). Polymethyl methacrylate (PMMA, Mw = 35 000) was purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Polyvinylidene fluoride (PVDF) was purchased from Arkema Chemical Co., Ltd. (China). All chemicals were used as received. Barrier films were purchased from Dai Nippon Printing Co., Ltd. (Japan). The blue backlights were purchased from China Star Optoelectronics Technology Co., Ltd. (China).
2.1.2. Synthesis of PQD-embedded polymer microspheres
The PQD-embedded polymer microspheres were prepared using the spray-drying fabrication method, a rapid and continuous technique for producing functional powders. The fabrication of MAPbBr3 PQD-embedded polymer microspheres includes four steps.
Step I: Preparing precursor solutions. PbBr2 (1 mmol), MABr (1 mmol), OABr (0.2–0.5 mmol), and PMMA (5 g) were dissolved in DMF (200 mL) by vigorous stirring. The PMMA content was controlled to fabricate MAPbBr3 PQD-embedded polymer microspheres with different particle sizes.
Step II: Spraying them into microdroplets. The precursor solution was transferred to the atomizer through a hose pipe of a peristaltic pump at a speed of 150 mL∙min−1. After dual fluid nozzle atomization at 20 000 r∙min−1, the solution was converted into microdroplets.
Step III: Drying the microdroplets. These microdroplets then entered a high-temperature gas cyclone drying separator, where they were dried to induce DMF evaporation in the gas cyclone drying chamber at a heating temperature of 110–130 °C, a motor speed of 40 Hz, and a nitrogen flow rate of 40 L∙min−1, to form the PQD-embedded polymer microspheres.
Step IV: Collecting the powder. The exhaust gas, comprising the solvent and nitrogen, was discharged through one outlet, while the PQD-embedded polymer microspheres entered the powder collection through another outlet. The powder collection was situated at the bottom of a separate small tower to prevent any exhaust liquid from entering the powder collection area.
The MAPbBr3 PQD-embedded polymer microspheres were placed in a hot oven at 90 °C for 30 min to enhance solvent evaporation and facilitate the crystallization of the PQDs. Different types of MAPbBr3 PQD-embedded polymer microspheres were fabricated by varying the precursor solution proportion, reducing the ligand OABr to 0.5 mmol (named M1), 0.4 mmol (named M2), or 0.2 mmol (named M3). Different types of FAPbBr3 PQD-embedded polymer microspheres were fabricated by varying the precursor solution composition (OABr at 0.4 mmol (named F1) and 0.2 mmol (named F2)), following a similar method.
PbI2 (1 mmol), CsI (1 mmol), OAI (0.6 mmol), and PMMA (8 g) were dissolved in 200 mL of DMF to prepare CsPbI3 PQD-embedded polymer microspheres. Similarly, PbI2 (1 mmol), CsI (1 mmol), OAI (0.6 mmol), OABr (0.4 mmol), and PMMA (8 g), were dissolved in 200 mL of DMF to prepare CsPbBrI2 PQD-embedded polymer microspheres. A series of CsPbBrxI3–x PQD-embedded polymer microspheres were fabricated using a similar strategy by adjusting the precursor solution ratio of OABr and OAI.
2.2. Material characterization
2.2.1. Sample preparation
For transmission electron microscopy (TEM) testing, the obtained microspheres were cut into ultrathin slices using an ultramicrotome with a cutting feed step of ≤ 10 nm∙s−1. The slices were collected and then dispersed into hexane with bath ultrasonication. Finally, the sliced samples were picked up by carbon-coated nickel grids. Before TEM testing, the samples were treated with 30 s of oxygen (O2) and argon (Ar) plasma cleaning. For scanning electron microscopy (SEM) testing, the obtained microspheres were dispersed in hexane and dropped onto silicon wafers to prepare SEM samples. The SEM samples were stuck to the conductive adhesive and sprayed with gold using an SC7620 sputtering coater (Quorum Technologies Ltd., UK).
2.2.2. Characterization
X-ray diffraction (XRD) patterns were measured using a PANalytical X’Pert3 (Malvern Panalytical, the Netherlands) spheres diffractometer with a Cu Kα radiation source at 40 kV and 40 mA. SEM images were scanned by a TESCAN MIRA LMS scanning electron microscope (TESCAN China Ltd., China). Microsphere particle size was measured with a Zetasizer Nano ZS90 particle sizer and a Mastersizer 2000 (Malvern Panalytical, UK). A Tecnai F20 microscope (FEI, the Netherlands) operated at 200 keV was used to record the TEM images. The PL spectra were recorded at a fluorescence excitation wavelength (λex) of 365 nm using an FLS1000 instrument (Edinburgh Instruments Ltd., UK). The PLQY of the integrated microspheres was measured on the same FLS1000 instrument, and white BaSO4 spheres were used as a reference to measure the absorption. The power of the xenon lamp (Xe900) for PL measurement was 700 W, and the optical density was controlled by the slits. The PL decay curves were determined with the above FLS1000 fluorescence spectrophotometer. All the photographs were taken by an Android phone (Honer V30 Pro, Huawei Technologies Co., Ltd., China).
3. Results and discussion
3.1. Spray-drying fabrication and material characterizations
Spray-drying fabrication is a rapid and continuous method used to prepare functional powders in the manufacturing of medicine, food, MOFs, and other composites [33],
[34],
[35].
Fig. 1(a) schematically shows the experimental setup for spray-drying fabrication. As shown in
Fig. 1(b), a typical fabrication usually contains four steps: preparing precursor solutions, spraying them into microdroplets, drying the microdroplets, and collecting the powder. In the first step, a precursor solution is prepared by dissolving the raw materials, perovskites and PMMA polymer, in DMF. Secondly, the precursor solution is transferred into an atomizer and broken up into microdroplets. Thirdly, heated nitrogen rotates from top to bottom to induce DMF evaporation in the gas cyclone drying chamber for the formation of PQD-embedded polymer microspheres. Finally, the as-prepared microspheres are collected from the powder collection area. Through the use of different precursor solutions, the spray-drying method can fabricate designated PQD-embedded polymer microspheres with a high productivity of over 200 g∙h−1 (about 2000 kg∙a
−1). For green MAPbBr
3 PQD-embedded PMMA microspheres (denoted as MAPbBr
3/PMMA PQD microspheres), we used precursor solutions of MABr, PbBr
2, OABr, PMMA, and DMF. For CsPbBrI
2 PQD-embedded PMMA microspheres (denoted as CsPbBrI
2/PMMA PQD microspheres), the corresponding precursor solution was a mixture of PbI
2, CsI, OAI, OABr, PMMA, and DMF. Fig. 2(a) shows the XRD patterns of the as-prepared MAPbBr
3 and CsPbBrI
2 PQD-embedded PMMA microspheres. The XRD patterns match well with the standard diffraction patterns of MAPbBr
3 (Inorganic Crystal Structure Database (ICSD) No. 268783) and γ-CsPbI
3 (Joint Committee on Powder Diffraction Standards (JCPDS) No. 434338), respectively. As shown in
Figs. 2(b) and
(c), both the MAPbBr
3/PMMA and CsPbBrI
2/PMMA microspheres are spherical.
Fig. 2(d) and Fig. S1 in Appendix A respectively show typical high-resolution fluorescence images of single MAPbBr
3/PMMA and CsPbBrI
2/PMMA PQD microspheres. The microspheres show excellent fluorescence homogeneity without an obvious agglomeration of PQDs.
Fig. 2(e) and Fig. S2 in Appendix A show typical TEM images of MAPbBr
3/PMMA PQD microspheres. The as-formed PQDs are well dispersed in the PMMA matrix, with an average size diameter of 14.4 nm (Fig. S3 in Appendix A). The high-resolution TEM (HRTEM) image reveals a lattice spacing of 2.98 Å, which is compatible with the (002) interplanar spacing of MAPbBr
3. Similarly,
Fig. 2(f) shows a typical TEM image of CsPbBrI
2/PMMA PQD microspheres with a lattice constant of 3.51 Å. The CsPbBrI2 PQDs are also well dispersed in the PMMA matrix, with an average diameter of 13.5 nm (Figs. S2 and S3).
3.2. Photoluminescence properties
To meet the optical requirements of LCD backlights, we need an appropriate PL wavelength (green emission of 525–540 nm, red emission of 625–635 nm) and FWHM (green emission 20–30 nm, red emission 30–50 nm). The as-fabricated MAPbBr
3/PMMA PQD microspheres exhibited PL emission with a peak at 525 nm and an FWHM of 23 nm, while the CsPbBrI
2/PMMA PQD microspheres exhibited PL emission with a peak at 632 nm and an FWHM of 26 nm (
Fig. 2(g)). These samples exhibited ultra-high PLQYs of more than 95% (Figs. S4 and S5 in Appendix A). As shown in
Fig. 2(h), these two samples showed PL decay lifetimes of 41 and 84 ns, respectively.
We further fabricated a series of MAPbBr
3/PMMA (named M1, M2, and M3) and FAPbBr
3/PMMA (named F1 and F2) PQD microspheres with different amounts of ligands. The XRD patterns of all samples matched with their corresponding compounds (Fig. S6 in Appendix A). A series of iodide-containing microspheres were also synthesized with an increase in bromine (Br) content, from CsPbI
3/PMMA to CsPbBrI
2/PMMA PQD microspheres. As shown in Fig. S7 in Appendix A, the diffraction peaks moved higher with the addition of Br. These samples were uniform with good process repeatability (Figs. S8 and S9 in Appendix A). In addition, the PL wavelength of the MAPbBr
3/PMMA and FAPbBr
3/PMMA PQD microspheres could be finely tuned from 517 to 539 nm by reducing the concentration of OABr ligands (Fig. S10 in Appendix A). Similarly, the PL wavelength of the CsPbBr
xI
3–x/PMMA PQD microspheres shifted from 662 to 618 nm as the Br content increased (Fig. S10), solving the issue of the “perovskite red wall” for PQDs in display applications (i.e., the fabrication of PQDs in the wavelength range of 625–635 nm is very challenging due to phase separation)
[36]. The polymer matrix of the PQD-embedded polymer microspheres could alternatively be composed of PVDF or PAN. Fig. 2(i) shows MAPbBr
3 PQD-embedded PMMA, PVDF, and PAN microspheres with a high PLQY of approximately 95% and a narrow FWHM of less than 25 nm.
Based on the spray-drying method, the particle size of PQD-embedded PMMA microspheres could be finely tuned by controlling the concentrations of PMMA in the precursor solution. As the PMMA concentration gradually decreased from 20% to 0.5%, the particle size of the obtained PMMA microspheres decreased from 40.97 to 0.44 μm (
Figs. 3(a) and
(b) and Table S1 in Appendix A). Ultra-small MAPbBr
3/PMMA PQD microspheres were fabricated with a
D50 size of 0.44 μm and a
D90 size of 0.53 μm (
D50 and
D90 indicate the size below which 50% or 90% of all particles are found, respectively).
Figs. 3(c)–(e) show typical SEM images of MAPbBr
3/PMMA PQD microspheres with
D50 particle sizes of 10.14, 6.00, and 1.26 μm, respectively.
3.3. LCD backlight application
PQD-embedded PMMA microspheres with an average particle size of 2 μm were selected for LCD applications. A mixture of ultraviolet (UV) adhesive and MAPbBr
3/PMMA and CsPbBrI
2/PMMA PQD microspheres was applied to fabricate PQD-based optical films
[8],
[37].
Figs. 4(a)–(c) show the schematic structure of the PQD-based optical films, and
Figs. 4(d)–(f) show the corresponding optical images of the PQD-based optical films. For the PQDs, three protective structures including the barrier film, UV adhesive, and PMMA shell layer were crucial in preventing water and oxygen ingress. We measured the transmittance spectra of the MAPbBr3/PMMA PQD film and CsPbBrI
2/PMMA PQD film on a polyethylene terephthalate substrate (Fig. S11 in Appendix A). The transparency at 600 nm approached 95.8% and 97.3% for the MAPbBr
3/PMMA PQD film and CsPbBrI
2/PMMA PQD film, respectively. We then investigated the color performance of PQD-film-integrated LCD backlights.
Fig. 4(g) shows the International Commission on Illumination (CIE) color coordinates of the system, the DCI-P3 color gamut, and the Rec. 2020 color gamut, based on the CIE 1931 standard. The CIE coordinates of the blue backlights were (0.1526, 0.0239), while the CIE coordinates of the green and red emissions were (0.1714, 0.7608) and (0.6840, 0.3158), respectively. The area of the backlights with the PQD-based optical films was 122% of the NTSC color gamut with a coverage ratio of 99.3% (Fig. S12 in Appendix A), 127% of the DCI-P3 color gamut with a coverage ratio of 98.4%, and 91.1% of the Rec. 2020 color gamut with a coverage ratio of 89.2%.
In addition, the PQD-embedded PMMA microspheres demonstrated high stability against water, heat, and blue light irradiation. Fig. S13 in Appendix A shows that the MAPbBr
3/PMMA and CsPbBrI
2/PMMA PQD microspheres maintained a nearly constant emission intensity when stored in water for 10 days. We further evaluated the film stability under aging conditions of 60 °C and 90% relative humidity (RH) (
Fig. 4(h) and Fig. S14 in Appendix A). The PL intensity of the sample retained more than 93% of its original PL intensity after 1000 h of aging. The heat and blue light irradiation stability of PQD optical films integrated into LCD backlights was evaluated under aging conditions of 70 °C, 150 W∙m
−2, and 455 nm blue light irradiation in an air atmosphere (
Fig. 4(i)). The MAPbBr
3/PMMA and CsPbBrI
2/PMMA optical films retained over 90% of their initial PL intensity after 1000 h of aging. The corresponding PL spectra during the measurement exhibited few decreases in intensity and constant PL peaks (Fig. S15 in Appendix A). We further tested the films’ stability under higher-intensity blue light irradiation, as shown in Figs. S16 and S17 in Appendix A. The emission peak remained constant under 150 W∙m
−2 blue light irradiation for 1000 h. Under higher-intensity blue light irradiation at 450 and 600 W∙m
−2, a slight 2 nm red shift could be observed. Tables S2 and S3 in Appendix A summarize different PQD fabrication methods and PQDs used for backlight applications
[17],
[18],
[22],
[27],
[38],
[39],
[40],
[41],
[42]. The as-prepared PQD-embedded PMMA microspheres have excellent optical properties and outstanding stability against water, heat, and high-intensity blue light irradiation. The availability of highly stable PQD-embedded polymer microspheres promotes the potential use of PQDs for LCD backlight display applications.
3.4. Micro-LED display applications
We also investigated the use of PQD-embedded PMMA microspheres as quantum dot color converters (QDCCs) for full-color Micro-LED displays. The as-fabricated microspheres were mixed with silicone gel to fabricate patterned QDCCs by filling the micropores. Fig. 5(a) shows the fabrication process of a patterned QDCC array.
Fig. 5(b) shows the Micro-LED display demo of a dual-color QDCC integrated into a blue Micro-LED panel. Details of the QDCC fabrication procedure are provided in Appendix A. The green PQDs and red PQDs were filled-in micropores with a pixel size of 170 μm × 400 μm (
Fig. 5(c) and Fig. S18 in Appendix A). We also demonstrated the fabrication of dual-color QDCCs (
Fig. 5(d)).
Fig. 5(e) shows optical microscopic images of the Micro-LED display demo with green emission, red emission, and transmitted blue light. The Micro-LED display demo shows good color uniformity and pixel-isolated emission. To achieve high-resolution QDCC patterns, we applied micropore filling for PQDs using a preformed template created via SU8 lithography
[43].
Fig. 5(f) shows a red pixel array with a pixel size of 10 μm. By repeating the micropore filling processing, we prepared a high-resolution dual-color pixel array (
Figs. 5(g) and
(h)) using green and red PQD-embedded polymer microspheres in silicone gel. The optical fluorescence images show a uniform pixel pattern with a pixel size of 30 μm × 10 μm and 10 μm. Fig. S19 in Appendix A illustrates the PL spectra of dual-color QDCC-based blue LCD backlights, showing a green PL emission peak at 528 nm and a red PL emission peak at 626 nm. A photoconversion efficiency of 62.2% was achieved, indicating a promising future in color converter applications.
Cost is a critical factor in commercialization applications. We further evaluated the fabrication costs of the PQDs, CdSe QDs, and InP QDs, including the materials and processing costs (Section S2 in Appendix A). Overall, spray-drying fabrication is a cost-competitive method to achieve the industrialization of PQD-based optical films for LCD backlight applications.
4. Conclusions
In this work, we reported the spray-drying fabrication of PQD-embedded polymer microspheres from a precursor solution at a scale of 2000 kg∙a−1. This spray-drying fabrication approach is a versatile method for obtaining diverse PQD (MAPbBr3, FAPbBr3, CsPbI3, and CsPbBrxI3–x)-embedded polymer microspheres and various polymer matrix (PMMA, PVDF, and PAN) microspheres. By controlling the precursors’ concentration, the average size of PMMA microspheres can be tuned from 40.97 to 0.44 μm. The obtained PQD-embedded polymer microspheres show monodispersed PQDs in the polymer matrix, high PLQYs (> 90%), and enhanced stability against water.
The as-prepared PQD-embedded polymer microspheres were mixed with a UV adhesive to fabricate PQD-enhanced optical films for LCD backlights. These films exhibited long-term operational stability under heat, humidity, and blue light irradiation measurements (i.e., remaining at an initial PL intensity of more than 90% after a 1000 h aging test under 60 °C with 90% RH and 70 °C with 455 nm 150 W∙m−2 blue light irradiation). We also demonstrated the use of PQD-embedded polymer microspheres as color converters in full-color Micro-LED display applications with a minimum pixel size of 10 μm. The reported spray-drying fabrication method provides a low-cost approach for the scale-up production of PQDs. The as-fabricated PQD-embedded polymer microspheres exhibit long-term operational stability, making them highly competitive as color converters for display technology.
CRediT authorship contribution statement
Yuyu Jing: Writing – original draft, Methodology, Data curation, Conceptualization. Rongjian Zhang: Methodology, Investigation. Dengbao Han: Writing – review & editing, Project administration, Funding acquisition, Data curation, Conceptualization. Huan Liu: Writing – review & editing. Wenchao Sun: Methodology. Shengquan Xie: Methodology, Data curation. Ronghui Wang: Data curation. Xin Zhong: Data curation. Xian-gang Wu: Writing – review & editing, Investigation. Qingchen Wang: Investigation. Zelong Bai: Software, Investigation. Tao Zhang: Investigation. Jing Li: Project administration, Investigation. Haizheng Zhong: Writing – review & editing, Supervision, Conceptualization.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thank Dr. Fei Li, Qian Chen, Ziwen Lu, Hongliang Duan, and Qifan Yu for their valuable advice on experimental design.
This work was supported by the Hefei Innovation Research Institute of Beihang University, the National Natural Science Foundation of China (52203321), the China Postdoctoral Science Foundation under Grant (2022M710289), and the Postdoctoral Research Funding Program of Hefei.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.eng.2024.11.038.
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