Stability Optimization of 0D Cs3Cu2Cl5 Single Crystal with High Green Emission for Optoelectronics

Cs3Cu2Cl5 is unstable owing to ionic migration and lattice decomposition in the atmosphere. In addition, obtaining large bulk single crystals of Cs3Cu2Cl5 is challenging. Herein, a novel strategy is proposed for synthesizing a Cs3Cu2Cl5 single crystal that exhibits excellent crystallinity and photoluminescence (PL) properties using an antisolvent‐assisted method. Na+ is doped into the Cs3Cu2Cl5 lattice to replenish the lattice defects caused by chlorine vacancies, thus leading to stronger chemical interactions between Cu+ and Cl− ions. Moreover, Na+ doping circumvents ionic migration and lattice decomposition, thereby enhancing the PL intensity and maintaining the long‐term stability of Cs3Cu2Cl5 in the atmosphere. Incorporating 10% Na+ into the Cs3Cu2Cl5 lattice enhances the PL intensity by 18%, and the high‐stability PL can maintain more than 48.5% of the PL intensity after 90 d in an atmospheric environment. In addition, a white light‐emitting device (LED) is fabricated using the 10% Na+‐doped Cs3Cu2Cl5 crystal powder and it exhibits a high color‐rendering index (93.7) and correlated color temperature (7120 K). Additionally, it exhibits superior stability, even at a high temperature of 120 °C. Thus, the excellent high‐temperature stability of Cs3Cu2Cl5 can promote its practical application in LED.


Introduction
[3][4][5][6][7][8][9] Zero-dimensional (0D) lead-free metal halides, such as Cs 3 Cu 2 X 5 (X = Cl, Br or I), exhibit high photoluminescence quantum yields (PLQYs), large Stokes shifts, broad emission spectra, large exciton binding energies, high photoluminescence (PL), and tunable emission wavelengths.[12][13][14][15][16][17][18][19][20][21][22][23] Cs 3 Cu 2 Cl 5 is widely used in photoelectric applications, including light-emitting devices (LEDs), [24] X-ray detection, [25] sensors, [26] high-resolution X-ray imaging, [27,28] and light communications. [29]Despite being a novel photoelectric material, Cs 3 Cu 2 Cl 5 has received limited attention compared with Cs 3 Cu 2 I 5 , because of the challenges in 1) obtaining large bulk single crystals and 2) its instability in the atmosphere.Consequently, numerous researchers have explored different methods to enhance the stability of Cs 3 Cu 2 Cl 5 while studying its properties and potential applications.For instance, Zang et al. proposed a sol-gel reaction using tetraethoxysilane (TEOS) as the silica coating precursor to prepare Cs 3 Cu 2 Cl 5 @SiO x nanocrystals (NCs), which exhibit high stability owing to the efficient passivation and robust protection of the SiO x shells on the surface of NCs. [29]Furthermore, Halpert et al. developed a rapid synthesis method for fabricating large millimeter-sized Cs 3 Cu 2 Cl 5 single crystals in less than 30 min; however, the single crystals thus produced are unstable in the atmosphere. [30]To address this difficulty, Zhang et al. used Cu 2þ precursors to grow Cs 3 Cu 2 Cl 5 single crystals and trace I ions for anion-exchange reactions, thus significantly improving their chemical stabilities. [31]However, a new Cs 5 Cu 3 Cl 8-x I x structure was obtained, and the emission color changed from sky blue to emerald green.Thus, an ion that both improves the structural stability of Cs 3 Cu 2 Cl 5 and stabilizes its emission wavelength must be determined.Chen et al. improved the stability of Cs 3 Cu 2 I 5 powder using a doping technique with NaI as the dopant, consequently, the PL intensity also improved.Their findings demonstrated the effectiveness of chemical doping in enhancing the stabilities and luminescence intensities of the Cs 3 Cu 2 Cl 5 single crystals. [32]In summary, despite their impressive device performances, the Cs 3 Cu 2 Cl 5 single crystals are still far from reaching their full potential.Thus, other growth methods for the Cs 3 Cu 2 Cl 5 single crystals must be explored, and their stabilities and luminescence intensities must be improved.
In this study, Cs 3 Cu 2 Cl 5 single crystal with high crystallinity and PL intensity were successfully grown using an antisolvent method.Na þ doping was performed to strengthen the chemical interactions between Cu þ and Cl À ions in the Cs 3 Cu 2 Cl 5 lattice, and thereby prevent ionic migration and oxidation of Cu þ to Cu 2þ .This improvement resulted in increased stability of the Cs 3 Cu 2 Cl 5 single crystal and a significant enhancement of the PL intensity by 18% compared with the undoped single crystal.Moreover, the PL stability of the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystal was maintained at more than 48.5% of the undoped PL intensity after 90 d in an atmospheric environment.Additionally, a white LED was fabricated using the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystals, blue-and red-emitting phosphor materials.The LED exhibited superior white-light emission with a high colorrendering index (CRI = 93.7),correlated color temperature (CCT = 7120 K), and stable Commission Internationale de l'Eclairage (CIE) color coordinates (0.3064, 0.3087).Notably, the CRI of the LED fabricated using Na þ -doped Cs 3 Cu 2 Cl 5 single crystals was significantly enhanced from 89.5 to 93.7 compared with the un-doped LED.Furthermore, the CRI and CCT were stable over a wide temperature range (25 to 120 °C), with no observed degradation or enhancement of the PL spectrum.These findings demonstrate the significant potential of Cs 3 Cu 2 Cl 5 as a luminescent material, and its excellent stability in high-temperature environments can promote practical applications of LEDs.

Results and Discussion
The Cs 3 Cu 2 Cl 5 single crystals were grown using the vapor saturation of an antisolvent (VSA) method, as illustrated in Figure 1a.The Cs 3 Cu 2 Cl 5 crystal structure, belonging to the orthorhombic crystal system and Cmcm space group, is shown in Figure 1b.Tables S1 and S2 1c).Moreover, elemental mapping was performed by energy-dispersive X-ray spectroscopy (EDS) analysis using scanning electron microscopy (SEM).Elemental composition was analyzed using X-ray photoelectron spectroscopy (XPS).The SEM and XPS images show successful doping of Na þ and homogeneous elemental distribution in the Cs 3 Cu 2 Cl 5 single crystal lattice.To investigate the stability of the Cs 3 Cu 2 Cl 5 single crystals, PL and XRD measurements were conducted on both freshly prepared and stored crystal powder under similar test conditions for 7, 30, 60, and 90 d.Impurity peaks were observed in the XRD pattern of the crystal powder stored under an atmospheric environment for 7 d, with additional peaks appearing over time (Figure 2a).The PL intensity of the crystal powder exposed to the atmosphere gradually decreased over time (Figure 2b).In particular, in PL intensity of the crystal powder stored for 90 d decreased by 91.5% (Figure 2c).These findings indicate that the Cs 3 Cu 2 Cl 5 single crystal is highly unstable when exposed to air for extended periods.To elucidate the cause of instability, the high-resolution X-ray photoelectron spectroscopy (HRXPS) spectrum of Cu þ on the crystal surface was analyzed, two peaks were observed at 963.5 and 942.5 eV over time (Figure 2d).Thus, the instability of the Cs 3 Cu 2 Cl 5 crystal is attributed to the oxidation of Cu þ , which results in ionic migration and lattice decomposition.
Figure 3a shows photographs of the Cs 3 Cu 2 Cl 5 single crystals doped with varying Na þ content, essentially exhibiting bright green PL under the 254 nm ultraviolet (UV) lamp.As the Na þ content increased, the morphology of the Cs 3 Cu 2 Cl 5 single crystal changed from bulk to stripe.Figure 3b   to higher binding energies compared with those of undoped Cs 3 Cu 2 Cl 5 single crystal, thus indicating stronger chemical interactions between Cu þ and Cl À ions.Significantly, the actual binding energy (1064.71eV) of Na 1s was much lower than the theoretical value (1072.0eV), thus implying an increase in the electron density around Na and stronger interactions between Na and other atoms.This strong interaction facilitates the transfer of electrons from the surrounding area to Na.Thus, the enhanced binding energy between Cu þ cations and Cl À anions is beneficial in restraining the formation of Cl vacancy defects and prevent Cu þ from oxidizing to Cu 2þ , thereby improving the PL intensity and stability of Cs 3 Cu 2 Cl 5 .
Various experiments were conducted to confirm the stability of the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystals.movement of electrons and holes and the interrelated nonradiative transition.Next, the excited electrons undergo an ultrafast relaxation and intersystem crossing process from FE to the self-trapped exciton (STE) state, which is formed by the photoinduced local structural distortion and subsequent reorganization of the excited-state structure.Finally, the electrons return to the GS, thereby generating a bright broadband green emission.For the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystals, the exciton photon coupling became stronger with increasing temperature, thus forming STEs and increasing the PL intensities between 20 and 220 K.However, when the temperature exceeded 220 K, several electrons in the STE state returned to the GS owing to temperature quenching, thus decreasing the emission intensities (Figure 6c,d).Additionally, the photoluminescence excitation (PLE) spectra monitored at different emission wavelengths exhibited identical shapes and features, thus confirming that the PL emissions are derived from the relaxation and recombination of the same excited states (Figure 6e).
Furthermore, the concentration-dependent decay behavior of the Cs 3 Cu 2 Cl 5 single crystal with respect to Na þ doping was investigated (Figure 5f ).The decay curves were well fitted to a mono-exponential function, as shown in Equation ( 1).
where A 0 and I t are the luminescence intensities at times t 0 and t 1 , respectively, and τ denotes the decay time of luminescence from the Cs 3 Cu 2 Cl 5 single crystal, which could be calculated by fitting the decay curves.As the Na þ concentration increased (from 0 to 20%), the decay lifetime decreased monotonically from 104.89 to 103.51 μs (Figure S7, Supporting Information), thus indicating that Na þ doping fills the Cl vacancies and reduces the lattice defects, thereby improving the carrier mobility.

Application
The performance of Cs 3 Cu 2 Cl 5 single crystals with varying Na þ content was validated for optical applications using a white LED, thus herein a white LED was fabricated using 10% Na þ -doped Cs exhibited bright white light with a CCT of 6253 K, CRI of 89.5, and CIE chromaticity coordinates of (0.3150, 0.3523).The CCT, CRI, and color coordinates remained stable when the temperature changed from 25 to 120 °C, and no significant degradation or enhancement of the EL spectrum was observed (Figure S9, Supporting Information).Moreover, the CCT gradually increased from 7120 to 7476 K, whereas the CRI decreased slightly from 93.7 to 92.1, thus indicating good stability (Figure 7d).When driven with a current of 100 mA, the operating voltage ranged from 5.513 to 4.814 V (Figure 7c), thus indicating that high temperatures have a minimal impact on the stability and optical properties of the white LED fabricated using the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystals.Notably, compared with the control device, the CRI of the device fabricated using the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystal increased from 92.6 to 94.5, thus highlighting its advantages for light-emitting applications.Therefore, white LED fabricated using the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystal are bright, stable, and nontoxic and have promising applications in next-generation lighting.

Conclusion
In this study, Cs 3 Cu 2 Cl 5 single crystals were successfully grown using an antisolvent method.The Cs 3 Cu 2 Cl 5 single crystal exhibited enhanced crystallinity and PL intensity.Experimental and theoretical results revealed that Cu þ ion oxidation causes instability and lattice decomposition of the Cs 3 Cu 2 Cl 5 single crystal emitters.Na þ doping filled the lattice defects and induced lattice expansion, strengthening chemical interactions between Cu þ and Cl À ions, which stabilized the crystals and prevented oxidizing of Cu þ to Cu 2þ .Doping with 10% Na þ significantly increased the PL intensity of the Cs 3 Cu 2 Cl 5 single crystals by 18%, and a 48.5% increase in the PL intensity was observed after 90 d compared with the undoped Cs 3 Cu 2 Cl 5 single crystals.A white LED was fabricated using the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystal, and it exhibited excellent white-light emission with a CRI of 93.7, CCT of 7120 K, and CIE color coordinates of (0.3064, 0.3087).Importantly, the CRI and CCT were stable even when the temperature changed from 25 to 120 °C, and no significant degradation or improvement of the PL spectrum was observed.Our findings indicate that doping with 10% Na þ can effectively improves the structural stability and PL intensity of Cs 3 Cu 2 Cl 5 .In addition, the resulting white LED with superior stability has the potential for practical applications in high-temperature environments.
Fabrication of the 0D Cs 3 Cu 2 Cl 5 Perovskite Single Crystals: A solution of CsCl, CuCl, and x% (x = 0, 5, 10, 15, and 20 mmol) NaCl (CsCl: CuCl molar ratio of 1:1) was prepared using 4.5 mL DMF and 0.5 mL pure H 2 O as a mixed solvent.The solution was stirred for 24 h at 80 °C.Subsequently, the crude solution was filtered using polytetrafluoroethylene (PTFE) syringe filter with a pore size of 0.45 μm, and the filtered precursor solution was injected into the vial.The filtered precursor solution in the vial was placed in a beaker filled with C 2 H 5 OH (antisolvent), and the beaker was sealed with a paraffin film.This beaker was placed in an incubator at 50 °C for approximately 7-12 d to grow single crystals.
Fabrication of the White Luminescence UV-Excited White LEDs: Green Cs 3 Cu 2 Cl 5 perovskite single crystal powder and blue and red phosphors were mixed in optimal ratios with thermally curable silicone resins OE-6551A and OE-6551B under vigorous stirring.The resulting paste was deposited onto a commercial UV-LED chip and dried for 24 h in a drying oven at 50 °C.
Characterization Methods: HRTEM and high-angle annular dark-filed scanning TEM (HAADF-STEM) were conducted using a JEM 2100F TEM equipment.EDS was performed using a low-vacuum JSM-6390LV SEM equipment.XPS was performed using a Thermo Scientific ESCALAB Xiþ spectrometer.The PL spectra and time-resolved PL decay curves were obtained using an Edinburgh of FS5 instruments.Single crystal XRD data were collected using a Bruker D8 Venture diffractometer using Ga tradiation.The XRD pattern of the Cs 3 Cu 2 Cl 5 single crystal powder was recorded using an Ultima IV diffractometer.The optical properties (CCT, CRI, and CIE color coordinates) of the white LED were measured using a HAAS-2000 spectroradiometer system.All samples were analyzed under similar conditions.
First-Principle Calculations: Density functional theory (DFT) calculations of band structure and PDOS for Cs 3 Cu 2 Cl 5 were performed by using Materials Studio 5.5 program and the CASTEP package based on the DFTgradient corrected generalized gradient approximation (GGA)-Perdew-Burke-Ernzerhof functional.The energy cutoff of the plane wave was fixed at 440 eV for the calculations.The calculations were based on the geometry optimization of the orthorhombic structure.The ion-electron interactions were modeled using the ultrasoft pseudopotential for all the elements.
(Supporting Information) list the unit cell parameters (a = 15.428(3)Å, b = 8.7421(6) Å, c = 8.6681(6) Å, α = 90°, β = 90°, γ = 90°and volume =1157.07(15)Å 3 ).Note that the Cs 3 Cu 2 Cl 5 single crystal comprises [Cu 2 Cl 5 ] 3À units and Cs þ ions because of the large radius of the Cs atom compared with those of Cu and Cl.Each [Cu 2 Cl 5 ] 3À unit is separated by Cs þ ions.The Cu þ ions in each [Cu 2 Cl 5 ] 3À unit are coordinated with three Cl À ions, thus forming a near-planar configuration and share a Cl À ion to form an isolated folded dimer of [Cu 2 Cl 5 ] 3À units.A high-resolution transmission electron microscopy (HRTEM) image displayed lattice fringes, with an observed d-spacing of 0.3319 nm, which matches well with the lattice spacing of the (221) plane of orthorhombic Cs 3 Cu 2 Cl 5 (PDF#04-019-9644), as shown in Figure 1c.The selected-area electron diffraction (SAED) pattern of the Cs 3 Cu 2 Cl 5 single crystal exhibited intense diffraction spots corresponding to orthorhombic Cs 3 Cu 2 Cl 5 , thus confirming the high crystallinity and phase purity of the single crystal (Inset 1 of Figure

Figure
Figure 1d illustrates the X-ray diffraction (XRD) patterns of the Cs 3 Cu 2 Cl 5 single crystal and the Cs 3 Cu 2 Cl 5 powder obtained by grinding the single crystal.Single-crystal XRD analysis confirmed the good purity and crystallinity of Cs 3 Cu 2 Cl 5 , and it strongly agreed with the powder XRD (PXRD) pattern.Moreover, the diffraction peaks matched with PDF#04-019-9644.The electronic band structure and projected density of states (PDOS) of the Cs 3 Cu 2 Cl 5 single crystal are shown in Figure 1e,f, respectively.Notably, the valence band maximum (VBM) of Cs 3 Cu 2 Cl 5 was formed by flat bands arising from the intrinsic 0D structure of the crystals.The calculated theoretical bandgap width was 2.222 eV.The VBM primarily comprised Cu 3d orbitals, whereas the conduction band minimum (CBM) originated from a mixture of Cu 4s and Cl 3p orbitals, essentially Cs þ played no role in the valence or conduction bands.Owing to the spatial confinement, electrons could migrate only within the [Cu 2 Cl 5 ] 3À units and between Cu 4s and Cl 3p upon excitation, thus leading to the formation of excitons and subsequent luminescence of the [Cu 2 Cl 5 ] 3À unit.To investigate the stability of the Cs 3 Cu 2 Cl 5 single crystals, PL and XRD measurements were conducted on both freshly prepared and stored crystal powder under similar test conditions for 7, 30, 60, and 90 d.Impurity peaks were observed in the XRD pattern of the crystal powder stored under an atmospheric environment for 7 d, with additional peaks appearing over time (Figure2a).The PL intensity of the crystal powder exposed to the

Figure 2 .
Figure 2. a) XRD patterns of the Cs 3 Cu 2 Cl 5 single crystals at different times.b) Photoluminescence (PL) spectra of the Cs 3 Cu 2 Cl 5 single crystals at different times.c) Stability of the PL intensity of the Cs 3 Cu 2 Cl 5 single crystal.d) High-resolution X-ray photoelectron spectroscopy (HRXPS) spectra of Cu 2p at different time.
shows the XRD patterns of the Cs 3 Cu 2 Cl 5 single crystals doped with varying Na þ content.The PXRD patterns of all the Na þ -doped Cs 3 Cu 2 Cl 5 single crystals are consistent with those of the undoped Cs 3 Cu 2 Cl 5 single crystal, thus indicating that the perovskite preserves its original crystal structure without forming secondary phases.Notably, the diffraction peaks monotonically shifted toward lower 2θ values with increasing Na þ content.Moreover, the (330) lattice plane of the Cs 3 Cu 2 Cl 5 single crystal experienced a shift of 0.130°toward the left of 2θ = 35.376°,thus signifying the lattice expansion of the Cs 3 Cu 2 Cl 5 single crystal caused by Na þ doping and filling of chlorine vacancies.This result confirms the successful Na þ doping.However, the structure of the Cs 3 Cu 2 Cl 5 single crystal changed when 25% Na þ was doped.The PL spectral intensities of the Cs 3 Cu 2 Cl 5 single crystals with varying Na þ content are shown in Figure 3c and S3 (Supporting Information).Evidently, as the Na þ content increased from 0 to 10%, the PL intensities of the Na þ -doped Cs 3 Cu 2 Cl 5 single crystals gradually increased.In particular, the PL intensity increased by 18% for 10% Na þ doping.These findings indicate that Na þ doping enhances the electronic interactions between ions and carrier mobility, thereby increasing the PL intensity.However, excessive Na þ doping leads to severe lattice distortion, thus reducing the crystallinity of the Cs 3 Cu 2 Cl 5 single crystal.Therefore, a further increase in Na þ content causes a decrease in the PL intensity.XPS was conducted to gain insight into the surface chemical states, and the results are shown in Figure S4 (Supporting Information).Cs 3d, Cu 2p, and Cl 2p signals were observed in the XPS survey spectra, Figure S4b-d shows the HRXPS spectra of Cs 3d, Cu 2p, and Cl 2p of the Cs 3 Cu 2 Cl 5 single crystal, respectively.The HRXPS spectrum of Cs 3d exhibited peaks at 723.8 and 737.8 eV, corresponding to Cs 3d 5/2 and Cs 3d 3/2 , respectively, which agrees with Cs þ .The Cu 2p XPS spectrum exhibited peaks at 952.1 and 932.1 eV corresponding to Cu 2p 1/2 and Cu 2p 3/2 , respectively, thus indicating the presence of Cu þ .Moreover, the Cl 2p XPS spectrum exhibited peaks at 197 and 198.6 eV corresponding to Cl 2p 3/2 and 2p 1/2 , respectively, which agree with Cl À .Furthermore, Figure 4b shows that the Cs 3d, Cu 2p, and Cl 2p signals were still observed in the XPS survey spectra, even after doping with Na þ and the peaks for Na þ appeared between 1064 and 1066 eV.The Cs 3d, Cu 2p, and Cl 2p peaks for Na þ -doped Cs 3 Cu 2 Cl 5 single crystal shifted

Figure 3 .
Figure 3. a) Photographs of the Cs 3 Cu 2 Cl 5 single crystals doped with varying Na þ content under day light (above) and 254 nm UV light (below).b) XRD patterns of the Cs 3 Cu 2 Cl 5 single crystals with varying Na þ content.c) PL intensities of the Cs 3 Cu 2 Cl 5 single crystals with varying Na þ content.
Figure 5a shows the XRD patterns of the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystals at different time.Evidently, the PXRD patterns of the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystals gradually exhibited extraneous peaks after 30 d, thus indicating the presence of impurities in the atmosphere.In addition, the HRXPS spectrum of Cu þ on the surface of the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystal gradually exhibited two peaks at 963.5 and 942.5 eV after 30 d, as shown in Figure 5b.This finding indicates that the interaction between Cu þ and Cl À was enhanced by Na þ doping, whereas the oxidation of Cu þ in the Cs 3 Cu 2 Cl 5 single crystal was reduced.Thus, Na þ doping can effectively improve the stability of the Cs 3 Cu 2 Cl 5 single crystal.The transmission electron microscopy (TEM) image shown in Figure 5c reveals the enhanced crystallinity of the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystal.The HRTEM images indicate a measured lattice fringe spacing of 3.338 Å, corresponding to the (221) plane of Cs 3 Cu 2 Cl 5 .Additionally, the lattice fringe spacing of the (221) plane in the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystal increased by 0.0019 nm compared with the undoped crystal.The lattice fringes without point and planar defects indicate the enhance crystallinity of the Cs 3 Cu 2 Cl 5 single crystal.Furthermore, SEM and elemental mapping images of the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystal (Figure 5d-h) demonstrate the elemental composition and uniform distribution of Na þ .To investigate the PL properties of the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystals, PL spectra were recorded over 90 d, as shown in Figure 6a.
Figure 6b and S5 (Supporting Information) show the changes in the PL spectra of the 10% Na þ -doped single crystals, essentially demonstrating that the PL intensity remained at approximately 48.3% of its initial intensity after 90 d.Notably, the PL intensities of the Na þ -doped Cs 3 Cu 2 Cl 5 single crystals increased by 48.9% after 90 d compared with that of the undoped crystals, thus indicating that Na þ doping can effectively enhance both the stabilities and PL intensity of the Cs 3 Cu 2 Cl 5 single crystals.In brief, when electrons from the ground state (GS) are excited by 312 nm UV light, they become excited to the free-carrier state (FC), subsequently, they transfer to the free-exciton (FE) excited state owing to the

Figure 4 .
Figure 4. a) X-ray photoelectron spectroscopy (XPS) spectra of the Cs 3 Cu 2 Cl 5 single crystals with varying Na þ content.b-d) HRXPS spectra of Cs 3d, Cl 2p, and Na 1s for the Cs 3 Cu 2 Cl 5 single crystals with varying Na þ content.

Figure 5 .
Figure 5. a) XRD patterns of the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystals at different times.b) HRXPS spectrum of Cu 2p for the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystals at different time.c) HRTEM image and the inset showing SADE patterns of the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystal.d) Scanning electron microscopy (SEM) and e-h) elemental mapping (Cs, Cu, Cl, and Na, respectively) images using energy-dispersive X-ray spectroscopy (EDS) of the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystal.

Figure 6 .
Figure 6.a) PL spectra of the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystal at different times.b) PL intensities of the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystal and undoped single crystal at different times.c) PL intensities of the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystals at different temperatures.d) Simplified PL mechanism of the 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystal.e) Excitation spectra at different PL wavelengths.f ) Time-resolved photoluminescence (TRPL) decay curves monitored at 525 nm.

3
Figure 7. a) Electroluminescence (EL) spectrum of UV-excited white LEDs fabricated using 10% Na þ -doped Cs 3 Cu 2 Cl 5 single crystal.The inset shows photographs of white LED with power off (left) and on (right) during daylight.b) CIE coordinates and performance parameters of the fabricated white LED.c) Voltage curves at different temperatures driven by 100 mA current.d) CCT and CRI variation curves at different temperatures.