Industrial mass production of novel mixed-halide Cs3Cu2Cl5-xIx and Cs5Cu3Cl8-xIx compounds with greatly enhanced stability and luminescent efficiency by compositional engineering
Graphical abstract
Introduction
Lighting and display technologies have become one of the mainstay industries in the information age. White light-emitting diodes (WLEDs) has a dominant position in the field of information displays and lighting as solid-state lighting devices due to the advantages such as high light efficiency, high environmental protection, energy efficient, and miniaturization and so on[1], [2], [3], [4], [5], [6], [7]. Nevertheless, the device applications are restricted by the high-cost, instability, toxicity, as well as complex and rigorous synthetic technology of the materials. Therefore, it is very important to explore of the new materials with intriguing optical and electronic properties for the rapid promotion of LED industrialization. Currently, the most dominant phosphor-converted WLEDs are realized by combining LED chip with the lanthanide (rare earth) doping[8], [9], [10], [11], [12], [13], [14], [15], [16]. Despite their extremely good performance in LED, rare earths are challenging to mine and process, and expensive to extract, lead to the bottleneck that may eventually hamper the development of the commercialization. In addition, semiconductor quantum dot has been one of the key materials for WLEDs with unique photophysical properties including high brightness, high chromatogram purity, and wide tunable absorption and emission ranges[17], [18], [19], [20], [21]. In spite of the considerable progress has been made in the fabrication of lead halides, the poor stability, the toxicity of metal lead, complex technology and high production cost are the common issue which limit the industrialization of quantum dot based WLEDs.
Copper(I) based WLED have recently attracted increasing attention, owing to their highly luminescent efficiency, nontoxicity, and low-dimensional electronic structure[22], [23], [24], [25]. The broadband emission and large Stokes shift of most Cu(I)-based ternary metal halides result in high-colour rendering index (CRI) and negligible self-absorption which is highly desirable for light-emitting applications. Especially, highly luminescent zero-dimensional (0D) Cs3Cu2I5 are regarded as one of the most promising light-emitting candidates. For instance, Sebastia-Luna and co-workers reported mechanochemical synthesis of Cs3Cu2I5 powders with a relatively low photoluminescence quantum yield (PLQY) of 42%[26], while Huang et al. prepared bright blue emissions Cs3Cu2I5 powder phosphors with PLQY of up to 80% by a facile solid-state reaction method at 350 °C for 6 h[27]. Moreover, Zhang et al. fabricated fluorescent ink and the ultraviolet (UV) pumped LEDs using Cs3Cu2I5 phosphor with PLQY of up to 89%[28]. The Cs3Cu2I5 powders showed good stability in air and good colour stability at high temperatures. Notably, green emission Cs3Cu2Cl5 shows an extremely long and temperature-sensitive photoluminescence (PL) lifetime with a PLQY exceeding 90%[29], yet Luo et al. reported on the chemical instability of Cs3Cu2Br5 and Cs3Cu2Cl5 in ambient environment[30]. Although improving device efficiency is important, the poor stability and the easy degradation behavior in the ambient environment are the most crucial issue to be addressed. Therefore, a new strategy is required to develop new candidate materials with earth-abundant, high PLQYs and stability. It is noteworthy that compositional engineering of a mixed-halide compound is one of the most general and effective strategy to improve the stability, since it can be applied to a wide variety of different device configurations. Yu et al. investigated a little addition of Cl to MAPbI3 improve the material stability and efficiency, and revealed the effects of Cl addition on the structural, electronic, optical properties and material stability of MAPb(I1–xClx)3[31]. Roccanova et al. reported enhanced PLQY and stability of a nontoxic Cs3Cu2Br5−xIx single crystals by increasing I/(Br + I) ratio[32]. However, Roccanova et al. investigated the photophysical properties of CsCu2Cl1.5Br1.5 and CsCu2Br1.5I1.5 and found that their PLQYs decrease to about 0.38%[33]. The introducing Br in triiodide perovskite absorbers were reported to markedly improve the chemical and thermal stability of perovskite solar cells[34], [35], [36]. Thus, traditional mixed-halide cannot guarantee the high PLQY or stability. To the best of our knowledge, there is no report about Cs3Cu2Cl5-xIx materials at present.
Inspired by the above information, we expected that iodine ion doping can effectively improve the stability and PLQY of Cs3Cu2Cl5, which can promote the large-scale commercial application. Herein, we controllably prepared a series of nontoxic pure cesium copper(I)-based halide phosphor powders with an extremely high quantum yield and on a large scale industrialized, using the low-cost, relatively simple and versatile mechanical grinding method. To find an excellent highly efficient and stable phosphor for lighting and displays, a series of novel mixed-halide phosphor powders (MHPPs) were also successfully synthesized for the first time by the same method, with the general formulas Cs3Cu2Cl5-xIx and Cs5Cu3Cl8-xIx. Furthermore, we discovered the controllable phase transformation from efficient blue-emitting Cs3Cu2I5 to more highly efficient, storable, and stable yellow-emitting CsCu2I3 without solvent additive or thermal treatment. Simultaneously, we found that the tunability emission peak from yellow, green, sky blue, and eventually to deep blue under 254 nm excitation through the varied chemical structure and halide composition. The crystal structures were identified with the powder XRD (PXRD) analysis and correlative theory calculations. The PL properties, PLQY and time-resolved PL spectra were investigated in detail. The detailed growth process, crystal structures, growth mechanisms were discussed, respectively. Most notably, the Cs3Cu2Cl5-xIx and Cs5Cu3Cl8-xIx MHPPs with the wide full width of half-maximum (FWHM), large Stokes shifts and the extreme stability have great potential application to meet the application requirements for LEDs. Consequently, it is notable that high-quality and stability WLEDs were obtained by combining the novel Cs5Cu3Cl7I MHPPs and “as-prepared stable yellow-emitting CsCu2I3 powders” or “commercial red phosphors” with a UV LED chip, demonstrating the high potential in industrialization of WLED backlights. Our findings will provide a new strategy for the fabrication of mixed-halide cesium copper(I)-based phosphor and could further strengthen their competitiveness for the field of lighting or display.
Section snippets
Materials and preparation
Cesium iodide (CsI, 99.9%), copper(I) iodide (CuI, 99.95%), cesium chloride (CsCl, 99.5%), and cuprous(I) chloride (CuCl, 99.5%) were purchased from Aladdin. All the reagents were used directly without further purification.
The Cs3Cu2X5, CsCu2X3 (X = Cl, I), Cs5Cu3Cl8-xIx, and Cs3Cu2Cl5-xIx powders were fabricated through mechanical grinding by mixing stoichiometric amounts of precursor material. For example, in typical synthesis of Cs5Cu3Cl7I powders, 0.5 mmol CsI (129.9 mg), 2 mmol CsCl
Crystal structure and stabilities of Cs3Cu2X5 and CsCu2X3
Cs3Cu2X5 and CsCu2X3 (X = Cl, or I) powders were successfully prepared via a simple mechanical grinding method (according to 2CuX + 3CsX → Cs3Cu2X5; 2CuX + CsX → CsCu2X3) (Details in the Supporting Information (SI)). To identify the phase structure and the composition of the Cs3Cu2X5 and CsCu2X3 (X = Cl, or I) samples, PXRD measurements were performed. As shown in Fig. 1a, the diffraction peaks of the Cs3Cu2X5 (X = Cl, or I) are well coincident with the standard pattern of Cs3Cu2I5 (JCPDS NO.
Conclusions
In summary, a series of pure cesium copper(I)-based halide phosphor powders were synthesized, which meet the following requirements: nontoxicity, earth-abundant, high PLQY, and low-cost via the versatile and simple mechanical grinding method. The tunability emission peak from yellow, green, sky blue and eventually to deep blue was found through the varied chemical structure and halide composition. With the exception of the chloride compounds, low-dimensional Cu(I)-based metal halides exhibit
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.
Acknowledgment
This work was supported by the Youth Innovation Talent Training Program for Ordinary Higher Institutions in Heilongjiang Province (Grant No. UNPYSCT-2020048), and Instructional Technology Plan of Daqing City (Grant No. zd-2020-07).
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