1. INTRODUCTION

Metal-halide perovskite materials have attracted significant attention over the past decades owing to their advantages of high absorption coefficient, large carrier diffusion lengths, and superior photoelectric properties [1–4" target="_self" style="display: inline;">–4], which make them promising candidates for optoelectronic applications including solar cells [57" target="_self" style="display: inline;">–7], light-emitting diodes [8–14" target="_self" style="display: inline;">–14], photodetectors [15,16], and lasers [1719" target="_self" style="display: inline;">–19]. Unfortunately, toxicity and poor stability are major obstacles that restrict their significant commercialization [20,21]. It is highly necessary to develop lead-free perovskite materials to solve these issues. So far, many efforts have been made to explore lead-free all-inorganic compounds through searching for low- or non-toxic elements to replace lead, including tin (Sn) [22–26" target="_self" style="display: inline;">–26], bismuth (Bi) [2729" target="_self" style="display: inline;">–29], silver (Ag) [30,31], indium (In) [32,33], antimony (Sb) [34,35], germanium (Ge) [36,37], copper (Cu) [38–44" target="_self" style="display: inline;">–44], zinc (Zn) [45], magnesium (Mg) [46], and rare-earth ions [47]. Among these rising lead-free perovskite materials, cesium copper halide perovskites benefit from being low cost and earth abundant. In previous reports, a limited number of synthetic methods have been used to fabricate cesium copper halide perovskites, which depend on solvents with low recovery rates, causing a certain degree of environmental pollution [48]. Sustainable development of solvent-free technologies for the synthesis of cesium copper halide perovskites has become a critical issue.

The ball milling approach based on mechanochemistry, a kind of green and reemerging efficient synthetic method, was identified by the International Union of Pure and Applied Chemistry (IUPAC) as one of 10 world-changing technologies [49]. The process can promote physical and chemical reactions between solids quickly and quantitatively with no added solvent, consistent with sustainable development. Moreover, the ball milling method offers tremendous advantages compared to traditional solution-based methods by avoiding the solubility limitation for poorly soluble or insoluble reagents and achieving high yield in a relatively short time by controlling materials [50]. Recently, it has been adopted to fabricate lead halide perovskite materials [51,52]. However, there are no reports to our knowledge about using the ball milling approach to fabricate cesium copper halide perovskites. In this work, we first extended the ball milling method to prepare highly luminescent and stable Cs3Cu2I5 and Cs3Cu2Cl5 perovskite powders without solvent. The as-fabricated all-inorganic copper-based perovskites exhibit self-trapped excitons (STE) emission features including broad photoluminescent (PL) emission, large Stokes shift, long PL lifetime, and high PL quantum yield (QY) reaching 60%. The Cs3Cu2I5 and Cs3Cu2Cl5 perovskites with good thermal stability and photostability were employed as phosphors for UV-pumped phosphor-converted (pc)-LEDs.

2. EXPERIMENT

2.1 A. Materials and Synthesis

The materials used were cesium iodide (CsI, 99.9% metal basis, Aladdin), cuprous iodide (CuI, 99.9% metal basis, Aladdin), cesium chloride (CsCl, 99.5%, Macklin), and cuprous (I) chloride (CuCl, 99.5%, Macklin). All chemicals were used as received without further purifications. Cs3Cu2X5 powders were fabricated by the dry ball milling method at room temperature. In typical synthesis of Cs3Cu2I5 powder, 3 mmol CsI (0.779 g) and 2 mmol CuI (0.38 g) were first homogeneously mixed in a mortar; the mixture was then transferred into a grinding tank (steel bowl with steel ball, 10 mL). The sealed tank was installed in the vibratory ball mill, and the gray-white Cs3Cu2I5 powder was obtained by grinding at 1000 r/min for half an hour. For the synthesis of Cs3Cu2Cl5 powder, CsI and CuI were simply replaced by CsCl and CuCl in the same process.

2.2 B. Fabrication of UV-Pumped pc-LEDs

0.05 g of Cs3Cu2I5 powder and 0.05 g of Cs3Cu2Cl5 powder were mixed with a thermal-curable silicone resin OE-6551A (0.1 g) under vigorous stirring. The hardener OE-6551B (0.2 g) was added to form a fluorescent paste, and then the paste was deposited on a commercial GaN-based UV-LED chip (310 nm, EPILED Co., Ltd).

2.3 C. Characterization

The morphologies and elemental analysis of Cs3Cu2I5 and Cs3Cu2Cl5 were collected by scanning electron microscope (SEM), energy dispersive X-ray (EDX, FEI Quanta FEG 250 ESEM), and transmission electron microscope (TEM) (JEOL, JEM-2010F, 200 kV) equipped with an X-ray spectrometer detector. X-ray diffraction (XRD) patterns of Cs3Cu2I5 and Cs3Cu2Cl5 powders were recorded on an X-ray diffractometer (Bruker AXS D8) using Cu-Kα X-ray radiation (λ=1.5406Å). Thermogravimetric analysis (TGA) results of the powder were obtained using a PerkinElmer Diamond TG/DTA6300, conducted at a heating rate of 10°C·min−1 to 1500°C in N2 flow with an alumina crucible. X-ray photoelectron spectroscopy (XPS) measurements were performed on a ULVAC-PHI instrument (PHIQUAN-TERA-II SXM) with Al Kα as the X-ray source at 70 W. The UV-Vis diffuse reflectance spectra of the powdered samples were taken on a PerkinElmer Lambda 35 double-beam spectrometer. PL and PL excitation (PLE) spectra were recorded on the Horiba PTI QuantaMaster 400. The absolute PL QY’s time-resolved PL decay curves were measured on an FLSP920 spectrofluorimeter (Edinburgh Instruments, TCSPC system) equipped with an integrating sphere.

2.4 D. Computational Methods

First-principle calculations of Cs3Cu2I5 were carried out using the Vienna Ab initio Simulation Package (VASP) code. To guarantee convergence, the projected augmented plane wave basis set was defined by a cutoff of 300 eV. The mesh samplings in the Brillouin zone (BZ) were 3×3×2. Experimental lattice parameters of Cs3Cu2I5 were used, and the atomic positions were fully relaxed until the residual forces were 0.05 eV/Å. Electronic band structures, density of states (DOS), and exciton properties were calculated using the hybrid PBE0 function.

3. RESULTS AND DISCUSSION

The schematic procedure of a typical fabrication of Cs3Cu2I5 powder by using a planetary ball mill is illustrated in Fig. 1(a): the raw powder materials of cesium iodide (CsI) and cuprous iodide (CuI) were first homogeneously mixed in a mortar; the mixture was then transferred into a steel bowl with a steel ball, and the blue emissive product was generated after a grinding process of the mixture for half an hour at room temperature, demonstrating an extremely energy-saving procedure compared to previous reports on lead perovskites [5153" target="_self" style="display: inline;">–53]. The SEM image of the Cs3Cu2I5 powder is shown in Fig. 1(b), and it exhibits irregularly shaped particles with the diameter of 0.71±0.3μm, consistent with the TEM result [Fig. 1(c)]. The high-resolution TEM (HRTEM) image [Fig. 1(d)] of the Cs3Cu2I5 powder shows high crystallinity with a lattice fringe of 0.38 nm corresponding to the crystal plane (022). The high crystallinity feature also can be confirmed by the selected area electron diffraction (SAED) pattern as shown in the inset of Fig. 1(d). EDX measurement [Fig. 1(e)] was performed to estimate the chemical composition of Cs3Cu2I5 powder sample, which yielded a Cs:Cu:I ratio of ≈3∶2∶5, indicating that the Cs3Cu2I5 compound was successfully synthesized. Elemental mapping of the product has been performed to further confirm the presence of Cs, Cu, and I in the resulting Cs3Cu2I5 powder as shown in Figs. 1(f)–1(i), exhibiting the homogeneous distribution of three elements over the particles.

Fig. 1. (a) Schematic illustration of synthetic process of Cs3Cu2I5 perovskite powder by using a planetary ball mill. (b) SEM image of Cs3Cu2I5 perovskite powder with inset showing size distribution. (c) TEM image of Cs3Cu2I5 perovskite. (d) HRTEM image of Cs3Cu2I5 perovskite along with an inset of SAED pattern of Cs3Cu2I5 perovskite crystal. (e) EDX spectrum and elemental content analysis of obtained Cs3Cu2I5 perovskite. (f) SEM image and EDX elemental mapping (g) Cs, (h) Cu, and (i) I of the selected Cs3Cu2I5 perovskite powder.

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We performed XRD measurement to verify the phase structure of the as-synthesized Cs3Cu2I5; as shown in Fig. 2(a), the XRD patterns with main 2θ positions of 13.1°, 15.1°, 23.9°, 25.6°, 26.3°, 28.2°, 30.6°, and 47.9° were assigned to the (111), (002), (122), (312), (222), (131), (313), and (152) planes of a bulk Cs3Cu2I5 (JCPDS No. 45-0077). These results indicate that the as-fabricated Cs3Cu2I5 powder has a orthorhombic crystal structure with a space group of Pnma, consistent with previous reports [40]. The stability of Cs3Cu2I5 perovskite is a critical factor for its applications, and thus TGA was carried out to investigate the thermal stability of the Cs3Cu2I5 powder. Figure 2(b) shows that the as-synthesized Cs3Cu2I5 powder is stable to 560°C, indicating that it has a good thermal stability, much better than that of Sn-based perovskite [24]. The valence state and surface chemical composition of Cs3Cu2I5 were reaffirmed from XPS. The XPS survey spectrum [Fig. 2(c)] confirms the presence of Cs, Cu, and I elements. Figures 2(d)–2(f) show the high-resolution XPS (HRXPS) spectra of Cs 3d, Cu 2p, and I 3d for the Cs3Cu2I5 powder sample, and all spectra were calibrated with C 1 s. The HRXPS spectrum peaks of Cs 3d are located at 724.2 eV and 738.5 eV corresponding to Cs+ 3d5/2 and 3d3/2; the peaks at 619.0 eV and 930.6 eV for the I 3d XPS spectrum are attributed to I− 3d5/2 and 3d3/2, respectively. The binding energies of 932.2 eV and 952.1 eV were observed in the Cu 2p XPS spectrum for Cu 2p1/2 and 2p3/2, respectively, which demonstrates the existence of Cu+ [41].

Fig. 2. (a) XRD pattern of obtained Cs3Cu2I5 powder, compared with the orthorhombic bulk Cs3Cu2I5 at the bottom (JCPDS No. 45-0077). (b) TGA curve of Cs3Cu2I5 powder. (c) XPS survey spectrum and HRXPS spectra of (d) Cs 3d, (e) Cu 2p, and (f) I 3d in Cs3Cu2I5 powder.

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The absolute PL QY of Cs3Cu2I5 was measured up to 60%, indicating that our sample has a strong emission feature. Figure 3(a) shows PL spectra of Cs3Cu2I5 powder under different excitation wavelengths ranging from 260 to 340 nm, with a uniform full width at half-maximum (FWHM) of 80 nm. Figure 3(b) predicts the PLE spectra of Cs3Cu2I5 with a fixed peak at 303 nm, which makes it suitable as phosphor for UV-pumped pc-LEDs. The large Stokes shift of 137 nm between the PLE and PL peaks illustrates that the emission mechanism cannot be explained simply by a direct band recombination emission [54,55]. UV-Vis optical diffuse reflectance spectroscopy was adopted to determine the optical bandgap of Cs3Cu2I5 powder as shown in Fig. 3(c), featuring a sharp absorption band centered at 324 nm. A bandgap value of 3.69 eV was given through the Tauc plot [inset of Fig. 3(c)]. To further explore excitonic recombination kinetics, the time-resolved PL decay curve of Cs3Cu2I5 powder was measured under 300 nm excitation as shown in Fig. 3(d). The PL decay can be described by monoexponential fitting, giving a long-lived PL lifetime of 1.13 μs that exhibits the phosphorescence feature of Cs3Cu2I5. Summarizing the optical features of the Cs3Cu2I5 sample, including a broad PL spectrum, a large Stokes shift, and a long PL decay time, we can deduce that the emission of Cs3Cu2I5 is ascribed to the STE recombination mechanism due to the Jahn–Teller distortion of the polyhedron (formed by Cu+ and I−) in the excited states [33,56]. The electronic structure calculations were performed using density functional theory (DFT) to further study the fluorescence mechanism of the Cs3Cu2I5 powder, first giving the crystal structure of the Cs3Cu2I5 as viewed down the a axis [Fig. 3(e)] consisting of caged [Cu2I5]3− units with tetrahedral and trigonal Cu+ sites, isolated by Cs+ ions. Figure 3(f) provides the density of states (DOS) of the Cs3Cu2I5 sample, indicating that the valence band maximum (VBM) is mainly composed of Cu 3d orbits, while the conduction band maximum (CBM) originates from Cu 4s and I 5p orbits, and Cs+ has no contribution to CBM or VBM [40]. This means that the [Cu2I5]3− polyhedron plays the dominant role in the crystal distortion and emission mechanism of Cs3Cu2I5, which can confine the excitons trapped by Jahn–Teller distortion in excited states. This is consistent with the previous reports [40,54]. Additionally, DFT calculations also yielded that Cs3Cu2I5 crystal has a direct bandgap at the Γ point with the gap value of 3.70 eV [Fig. 3(g)]. The excitation and emission processes for Cs3Cu2I5 are described in Fig. 3(h): after excitation with a high-energy ultraviolet light, the electron moves first from the ground states to the excited states, and then transits from singlet to triplet states (self-trapped states). Subsequent transition of the electron from the STE states to the ground states occurs, accompanied by blue emission.

Fig. 3. (a) PL spectra of Cs3Cu2I5 powder under different excitation wavelengths from 260 to 340 nm. (b) PLE spectra measured at different PL peaks ranging from 400 to 480 nm. (c) UV-Vis diffuse reflectance spectrum of the Cs3Cu2I5 powder; the inset shows the Tauc plot used for the bandgap estimation. (d) Time-resolved PL decay curve of Cs3Cu2I5 powder excited by the laser of 300 nm. (e) Crystal structure of Cs3Cu2I5, as viewed down the a axis (green, purple, and blue balls indicate Cs, I, and Cu atoms, respectively). (f) DOS plots of the Cs3Cu2I5 powder. (g) Calculated electronic band structure of Cs3Cu2I5; the Fermi energy is set to E=0 and denoted with an orange dash line. (h) Configuration coordinate diagram for the excited-state reorganization; the violet and blue arrows represent transition and radiation processes, respectively, and the black arrow represents intersystem crossing.

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The thermal stability and photostability of perovskite materials are critical for their long-term application in lightings and displays. To evaluate the natural stability of Cs3Cu2I5 powder, the evolution of the PL spectra of Cs3Cu2I5 powder after thermal treatment under N2 protection at different temperatures (100°C, 200°C, 300°C) was tested for half an hour. It can be seen clearly from Fig. 4(a) that the PL intensity of the Cs3Cu2I5 powder shows no decrease for 100°C and 200°C and only 21% reduction for 300°C compared to original value. Then the thermal stability of Cs3Cu2I5 powder was tested at 100°C under ambient air environment. The emission intensity remained at 72% of the initial value after 5 h [Fig. 4(b)], indicating that Cs3Cu2I5 powder has a good thermal stability [57]. The photostability of Cs3Cu2I5 powder was also studied [Fig. 4(c)] using a 500 W xenon lamp as the excitation source; the sample only exhibited a 22% reduction in PL intensity after 5 h illumination, which is better than Pb-based perovskites [58]. Interestingly, the Cs3Cu2I5 powder had a remarkable stability in air; the XRD patterns of a Cs3Cu2I5 powder exposed to ambient conditions for three months were identical to that of the initial as-synthesized powder [Fig. 4(d)]. PL intensity remained at 60% of the initial value after 90 days [Fig. 4(e)], much better than that of other reported perovskites [28,42,59]. The reason for the high stability of the as-fabricated Cs3Cu2I5 perovskite is most likely that there are no organic species (organic ions and ligands) contained in its structure and surface [6062" target="_self" style="display: inline;">–62].

Fig. 4. (a) Integrated PL intensity as a function of temperatures from 25°C to 300°C. Variation of PL intensity of Cs3Cu2I5 (b) at 100°C and (c) under a xenon lamp irradiation over time. (d) XRD patterns of Cs3Cu2I5 exposed to air for three months. (e) PL intensity of Cs3Cu2I5 exposed to air for three months.

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Notably, green emissive Cs3Cu2Cl5 perovskite was achieved for the first time to our knowledge. The SEM image is shown in Fig. 5(a), and it exhibits irregularly shaped micrometer-sized particles of the obtained Cs3Cu2Cl5. Figure 5(b) shows the HRTEM image of the Cs3Cu2Cl5, indicating high crystallinity with a lattice fringe of 0.35 nm. The SAED pattern of the Cs3Cu2Cl5 [inset of Fig. 5(b)] further confirms the high crystallinity. EDX elemental mappings of the selected Cs3Cu2Cl5 particles [Fig. 5(c)] showcase the uniform distribution of cesium, copper, and chlorine elements in the particles. The XRD pattern [Fig. 5(d)] shows that the diffraction peak positions and corresponding intensities are mainly consistent with the bulk Cs3Cu2Cl5 standard card (JCPDS No. 24-0247). The Cs3Cu2Cl5 exhibits an orthorhombic phase structure with the lattice constants a=9.176Å, b=10.505Å, and c=13.141Å. We found that an additional CsCl phase signal appeared in the XRD pattern, which is marked with black stars; it is unavoidable for synthesis of chloride compounds [39,63]. The existing three elements were further confirmed by XPS results as shown in Fig. 5(e). The HRXPS analysis [Figs. 5(f) and 5(g)] of the Cs 3d (3d5/2, 724.2 eV; 3d3/2, 738.5 eV) and Cl 2p (198.3 eV) reveals that monovalent Cs+ and monovalent Cl− existed in the Cs3Cu2Cl5 sample [9,64], while the HRXPS spectrum of Cu 2p [Fig. 5(h)] provides two main peaks of monovalent Cu+ (932.6 eV, 953.2 eV) with two satellite peaks (marked with black diamonds) at 942.4 eV and 962.3 eV attributed to divalent Cu2+ [65]. This may be because of the slight oxidation of partial Cu+ on the surface of the sample. The stability of Cs3Cu2Cl5 is further illustrated by the TGA result [Fig. 5(i)], which shows that the product is stable to 500°C, indicating that it displays a good thermal stability.

Fig. 5. (a) SEM image of Cs3Cu2Cl5 perovskite powder. (b) HRTEM image of Cs3Cu2Cl5 perovskite along with an inset of SAED pattern of Cs3Cu2Cl5 perovskite crystal. (c) EDX elemental mappings of Cs3Cu2Cl5 powder; the scale bar is 10 μm. (d) XRD pattern of Cs3Cu2Cl5 powder with standard JCPDS cards (Cs3Cu2Cl5, 24-0247 and CsCl, 05-0607). (e) XPS survey spectrum of Cs3Cu2Cl5 powder. HRXPS analysis of Cs3Cu2Cl5 powder: (f) Cs 3d spectrum, (g) Cl 2p spectrum, and (h) Cu 2p spectrum. (i) TGA curve of Cs3Cu2Cl5 powder.

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The UV-Vis absorption and PL spectra of Cs3Cu2Cl5 powder are depicted in Fig. 6(a), showing peaks at 346 nm for UV-Vis and 510 nm for PL. The broad PL spectrum (FWHM of 90 nm) of Cs3Cu2Cl5 makes it suitable for downconversion white pc-LEDs with high color rendering index. The optical photograph [inset of Fig. 6(a)] shows green color emission of the Cs3Cu2Cl5 powder under a UV-254 nm lamp. The normalized PLE spectra [Fig. 6(b)] exhibit identical shapes and fixed peaks, illustrating that the green emission of Cs3Cu2Cl5 originates from the relaxation of the same excited state [54]. A large Stokes shift of 164 nm between the PL and PLE spectra further makes it a candidate for use in pc-LEDs. The time-resolved PL decay curve [Fig. 6(c)] under 300 nm excitation at room temperature for Cs3Cu2Cl5 perovskite powder shows biexponential behavior with an average lifetime of 103 μs by integrating two individual lifetimes of 73.61 μs (29%) and 116.4 μs (71%), which is longer than that of Cs3Cu2I5 perovskite. The long PL lifetime is commonly known as phosphorescence originated from STEs [33,39]. Mott and Stoneham reported that the STE lifetime is related to the energy barrier that is required to be conquered for STE formation [66]. The potential barrier should be lower for Cs3Cu2I5 compared to Cs3Cu2Cl5, which may explain the shorter relaxation time for excitons in Cs3Cu2I5 compared to that for excitons in Cs3Cu2Cl5. It is consistent with the reported one [67]. The crystal structure of Cs3Cu2Cl5 powder is similar to Cs3Cu2I5 crystal, containing unique [Cu2Cl5]3− dimers made of a trigonal planar CuCl3 sharing an edge with a tetrahedral CuCl4 unit, all surrounded by Cs+ [Fig. 6(d)]. And the electrons and phonons of Cs3Cu2Cl5 are strongly coupled to induce Jahn–Teller distortion of polyhedron [Cu2Cl5]3− under UV light excitation. The excited-state electrons become self-trapped by the distortion and then release energy by a recombination process, and a similar emission behavior was also observed for other 0D lead-free crystal [68]. DFT calculations confirm that Cs3Cu2Cl5 has a direct bandgap at the Γ point with the value of 2.45 eV [Fig. 6(e)]. Furthermore, the absolute PL QY of Cs3Cu2Cl5 powder was measured up to 53%. The stability of the Cs3Cu2Cl5 powder was also studied in air conditions at room temperature by detecting variations of XRD patterns and PL intensity for 60 days. The XRD patterns [Fig. 6(f)] of Cs3Cu2Cl5 powder exhibit no changes; the PL intensity remained at 70% of the initial value, indicating that Cs3Cu2Cl5 powder also has better stability than other reported perovskites [28,42,59].

Fig. 6. (a) Normalized UV-Vis absorption (purple dash line) and PL (green solid line) spectra of the as-obtained Cs3Cu2Cl5 powder; inset: green emission image under UV-254 nm lamp. (b) Normalized PLE spectra measured over different PL peaks ranging from 470 to 550 nm. (c) Time-resolved PL decay curve of the Cs3Cu2Cl5 powder detected at 510 nm with excitation of 300 nm. (d) Crystal structure of Cs3Cu2Cl5, as viewed down the a axis (green, brown, and blue balls indicate Cs, Cl, and Cu atoms, respectively). (e) DFT electronic band structure of Cs3Cu2Cl5 with a direct bandgap (2.45 eV). (f) XRD patterns of Cs3Cu2Cl5 exposed to air for two months.

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In order to illustrate the potential lighting application of the obtained perovskite powders, we fabricated a UV-pumped pc-LED device by using blue emissive Cs3Cu2I5 and green emissive Cs3Cu2Cl5 as phosphors. It is the first time to our knowledge that pc-LED based on all copper-based perovskites without other phosphors has been prepared. Figure 7(a) provides a photograph of the as-fabricated pc-LED with brown color observed by the naked eye as shown in Fig. 7(b). The green-yellow emission color was obtained when the pc-LED device operated at a forward bias current of 20 mA. The electroluminescence (EL) spectrum [Fig. 7(c)] of the UV-pumped pc-LED exhibits two peaks: at 479 nm from the blue emissive Cs3Cu2I5 and at 547 nm from green emissive Cs3Cu2Cl5. The Commission International de l’Eclairage (CIE) color coordinates of the UV-pumped pc-LED are (0.278, 0.383); this is marked in the CIE 1931 color space, and the color points are laid on the green-yellow region near the black-body Planckian locus [Fig. 7(d)].

Fig. 7. (a) Photograph of the as-fabricated pc-LED based on dual phosphors of blue emissive Cs3Cu2I5 and green emissive Cs3Cu2Cl5. (b) Photograph of the pc-LED device operated at a forward bias current of 20 mA. (c) EL spectrum and (d) CIE chromaticity diagram of the LED device.

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4. CONCLUSIONS

In summary, we have developed a simple and energy-saving route to synthesize stable lead-free perovskites in a dry ball milling process. The obtained blue emissive Cs3Cu2I5 powder exhibits a high PL QY of 60% with a long lifetime of 1.13 μs and a huge Stokes shift of 137 nm. The luminescence mechanism of Cs3Cu2I5 could be explained by self-trapped excitons that originate from Jahn–Teller distortion of the Cu tetrahedral site. The green emissive Cs3Cu2Cl5 perovskite with PL QY of 53% was successfully fabricated by using the same process for the first time, with the PL peak at 510 nm. We finally realized a UV-pumped LED device by using blue emissive Cs3Cu2I5 and green emissive Cs3Cu2Cl5 as phosphors.