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.

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.

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.

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.

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.

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.

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.