Owing to the high photoluminescence quantum yield (PLQY), near-narrow bandwidth, high absorption coefficient, and tunable emission covering the entire visible spectral region [1–4], inorganic cesium lead halide (CsPbX3, X=Cl, Br, I) perovskite nanocrystals (NCs) have exhibited excellent performance in optoelectronic devices, including light-emitting diodes (LEDs) [5,6], solar cells [7–10], and photodetectors [11]. However, the toxicity of the lead (Pb) element in the CsPbX3 NCs may impede their commercial applications. To address this serious issue, the doping of nontoxic elements and synthesis of inorganic lead-free perovskite have been proposed [12,13]. Nontoxic elements, including tin (Sn) [14], antimony (Sb) [15], bismuth (Bi) [16], indium (In) [17], silver (Ag) [18], and copper (Cu) [19–21], were previously reported to replace lead to form 0D and double perovskite NCs. However, due to the strong reducibility and high defect density, the reported lead-free CsSnX3, Cs3Bi2X9, and Cs2AgInX6 NCs (X=Cl, Br, I) had low PLQY and instability, which have become critical issues for commercial applications [15,16,22].

Therefore, among the low-dimension lead-free nanocrystals, cesium copper (Cu) halide is one of the attractive materials, which is due to its abundance, low cost, and nontoxicity [23–25]. In addition, owing to the three- or fourfold coordination for Cu 3d¹⁰ orbitals and the small radius of the monovalent Cu+ ions, monovalent Cu+ ions tend to be surrounded by three and four halide ions, forming [CuX3] triangles and [CuX4] tetrahedra (X=halogen), respectively [26]. The DFT (density functional theory) calculation results demonstrated that the Cu 3d10 orbitals could hybridize with halogen p orbitals to lower the energy of orbitals, leading to the spatial spread of the relevant atomic orbitals [27]. Compared with the [PbX6] octahedron, the [CuX3] triangles and [CuX4] tetrahedra showed higher exciton binding energy. Therefore, researchers have paid more attention to the low-dimension cesium copper (monovalence) halide, including CsCu2I3 [28], Cs3Cu2Br5−xIx [29], CsCuBr2 [30], and Cs2CuBr4 [31]. Among them, the Cs3Cu2I5 and Cs3Cu2Br5 synthesized by the hot-injection method exhibited bright-blue emission with PLQY as high as 67% and 18.3%, respectively [32,33]. However, the cesium copper (monovalence) chlorine NCs have not been investigated clearly up to now. Besides, it is quite urgent to further research the reaction temperature of the hot-injection method, as it plays a critical role in the components of the cesium copper (monovalence) chlorine colloidal perovskite.

In this study, we reported the synthesis of cesium copper (monovalence) chlorine NCs, and the results demonstrated that the components could be decided from the reaction temperature. The 3D CsCu2Cl3 and 0D Cs3Cu2Cl5 NCs were synthesized at 70°C and 120°C, respectively. The 3D CsCu2Cl3 exhibited a blue emission with a PLQY of 47.8%, while the 0D Cs3Cu2Cl5 NCs showed a bright-green emission with a PLQY as high as 84.2%. In addition, the results revealed that the large Stokes shifts with broadband emission existed in both cesium copper chlorine NCs, which might arise from the strong quantum confinement and the self-trapped exciton (STE) effect. The Jahn–Teller distortion of the [CuCl3] triangle and [CuCl4] tetrahedron enabled excitons to be localized and emitted strongly, accounting for the excellent optical performance of cesium copper chlorine nanocrystals. Furthermore, the CsCu2Cl3 and Cs3Cu2Cl5 NCs with strong blue and green photoluminescence (PL) were used to prepare the warm white light-emitting diodes (WLEDs). The obtained WLEDs, which consisted of nontoxic Cs3Cu2Cl5 NCs, CsCu2Cl3 NCs, and red phosphors, exhibited a high color rendering index (CRI) of 94 and an appropriate correlated color temperature (CCT) of 5285 K. Moreover, the WLEDs showed an excellent operating stability with the luminous efficiency (LE) maintaining 64% of its initial value, even after 60 h. The high CRI (>92) was also sustained after continuous operation in air (30°C and 50% RH) for 60 h. This may suggest that inorganic cesium copper chlorine nanocrystals might have great potentials in next-generation nontoxic solid-state illuminating systems.

Materials. Cesium carbonate (Cs2CO3, 99.9%) and copper (monovalence) chloride (CuCl, 99.9%) were purchased from Xi’an Polymer Light Technology Corp. Oleic acid (OA, >90%), oleylamine (OAm, >90%), and octadecene (ODE, >90%) were purchased from Adamas. Polymethyl methacrylate (PMMA) was purchased from Sigma Corp. All these reagents were used without further purification.

Synthesis of Cs3Cu2Cl5 and CsCu2Cl3. 305 mg Cs2CO3, 15 mL ODE, and 1 mL OA were loaded into a 100 mL three-neck flask to prepare the Cs precursor. 39.6 mg CuCl and 10 mL ODE were loaded into another 100 mL three-neck flask. The two flasks were first degassed for 15 min. Then the flasks were heated to 120°C, and then 0.5 mL OAm and 0.5 mL OA were quickly injected into the Cu flask at 120°C under nitrogen flow. After 10 min, the temperature was changed/remained at 70/120°C. After remaining at the corresponding temperature for 2 min, 3 mL Cs precursor was quickly injected into the Cu flask, and the mixture was cooled in an ice-water bath to room temperature after 30 s.

Fabrication of the WLED. The 5 mL as-synthesized copper halide perovskite was centrifuged for 5 min at 10,000 r/min, and the supernatant was discarded. The pellet was resuspended in the toluene. The red CaAlSiN3:Eu2+ phosphors were added into the equal amounts of transparent epoxy A and B to mix together, and then they were coated on a 290 nm commercial UV chip. The chip with red phosphors was heated in an oven at 90°C for 1 h for solidification. The Cs3Cu2Cl5 and Cs3Cu2I5 powder with optimum amount was added into a PMMA/toluene solution. The blend was stirred for 30 min and then coated on the top of the chip. Finally, the WLED was heated on a hot plate at 50°C for 10 min to remove the solvent.

Characterizations. The crystal phases of the samples were characterized by X-ray diffraction (XRD) with Cu Kα radiation (XRD-6100, Shimadzu, Japan). The transmission electron microscopy (TEM) image was measured by an electron microscope (Libra 200 FE, Zeiss, Germany). The absorption spectrum was recorded ranging from 300 to 800 nm by a UV–vis spectrophotometer (UV-3800, Shimadzu, Japan) under room temperature. The PL spectroscopy data was measured by a fluorescence spectrophotometer (Agilent Cary Eclipse, Australia). The X-ray photoelectron spectroscopy (XPS) characterization was performed on an Escalab 250 Xi. PL spectroscopy and the data of PLQY were measured by a PL system. A PL system (FLS920, Edinburgh Instruments) that was capable of measuring PL and PLQYs with an integration sphere was employed in this work. Optical properties (CCT, CRI, and CIE color coordinates) of the WLED were measured using a spectroradiometer system (PR670, Photo Research).

Their crystallinity is evidenced by the selected-area electron diffraction image [the inset of Fig. 1(a)]. An interplanar distance of 5.2 Å (1 Å = 0.1 nm) corresponds to a (020) plane of Cs3Cu2Cl5 NCs [Fig. 1(b)]. In contrast, the TEM results exhibit the irregular-shaped CsCu2Cl3 NCs with the size of hundreds of nanometers as shown in Fig. 1(d). The increase of size to hundreds of nanometers may be attributed to the intrinsic 3D structure and the aggregation of the CsCu2Cl3. The clear lattice fringes also demonstrate their excellent 3D crystallinity, as shown in Fig. 1(e), in which the (200) planes with interplanar distance of 4.7 Å can be found. Please note that the dimensions mentioned above are both morphological, which is consistent with the size of Cs3Cu2Cl5 and CsCu2Cl3. In addition, the elemental mapping characterization of the 0D Cs3Cu2Cl5 and 3D CsCu2Cl3 NCs was performed to confirm the presence of Cs, Cu, and Cl, as shown in Figs. 1(c) and 1(f), respectively. The elemental mapping results demonstrate the homogeneous distribution of the three elements in both samples, suggesting uniform components and pure phases.

The high-resolution XPS spectrum [Fig. 2(b)] shows the presence of monovalent Cu+ (932.0 and 954.2 eV) with two satellite peaks at 934.4 and 962.3 eV attributing to divalent Cu2+. The presence of Cu2+ might be due to the oxidation of Cu+ [34]. The Cs and Cl XPS spectra of CsCu2Cl3 and Cs3Cu2Cl5 are displayed in Figs. 2(c) and 2(d), respectively. The small shift in the XPS peaks of the Cl element between CsCu2Cl3 and Cs3Cu2Cl5 NCs might be derived from the difference of the sites of Cl atoms. Specifically, the basic structure of CsCu2Cl3 is expected to be a [CuCl4] tetrahedron, which is surrounded by the isolating Cs+ ions [Fig. 2(e)]. In contrast, Cs3Cu2Cl5 includes the basic [Cu2Cl5] structure and isolating Cs+ ions, which is similar to Cs3Cu2I5. The [Cu2Cl5] consists of a [CuCl4] tetrahedron and a [CuCl3] planar triangle [35–37]. The tetrahedron and triangle are edge-shared to form the [Cu2Cl5] structure as shown in Fig. 2(f). The different sites of Cl in the tetrahedron and the triangle could lead to the change of binding energy of Cl element, which is in agreement with the XPS results.

The PL spectra of CsCu2Cl3 and Cs3Cu2Cl5 are shown in Fig. 2(g) with an emission centered at 453 and 518 nm for CsCu2Cl3 and Cs3Cu2Cl5 NCs, respectively. The photographs of both samples excited under 254 nm UV light are presented in the inset of Fig. 2(g). The CsCu2Cl3 and Cs3Cu2Cl5 NCs can emit bright-blue and green light, corresponding to a high PLQY value of 47.8% and 87.2%, respectively. In addition, they have broad emission spectra, which are evaluated by a high full width at half-maximum (FWHM) of ∼100nm. The high PLQY and broad emission of the inorganic lead-free cesium copper halide may originate from the copper halide clusters, leading to a greater charge localization and stronger excitonic effects [38].

To further study the mechanism of the exciton recombination in 0D Cs3Cu2Cl5 and 3D CsCu2Cl3 NCs, the decay and exciting-wavelength dependence of PL for the both samples are investigated. Figure 3(c) shows the characterization results of photoluminescent decay curves, demonstrating the τ1/τ2 time of 117.5/225.4 and 86.8/134.1 μs for Cs3Cu2Cl5 and CsCu2Cl3, respectively. Compared with the cesium lead halide (CsPbX3, X=Cl, Br, I) perovskite nanocrystals, the lead-free as-prepared 0D Cs3Cu2Cl5 and 3D CsCu2Cl3 NCs show a longer photoluminescent decay time, which might be due to the nonradiative recombination process caused by the self-trapped exciton (STE) effects [39]. Similar to the other low-dimension lead-free perovskite, such as Cs3Sb2Br9 and Cs3Cu2I5, the 0D Cs3Cu2Cl5 and 3D CsCu2Cl3 NCs exhibit excellent optical properties, including a high PLQY, a broadband emission, and a large Stokes shift, which result from the presence of the STE effect. Besides, according to the previous works, the reduced dimensions of lead-free perovskite can lead to strong exciton localization, resulting in the increase of exciton binding energy. This is in good agreement with the calculation results for exciton binding energy of 3D CsCu2Cl3 (∼320meV) and 0D Cs3Cu2Cl5 NCs (∼500meV). It has been reported that the STE effect may also account for the enhancement of exciton localization, as well as the FWHM in lead-free low-dimension perovskite. Therefore, the unique optical performance of our lead-free perovskite can be determined to originate from the STE effect.

The STE effect can be investigated by recording the PL spectra of the 0D Cs3Cu2Cl5 and 3D CsCu2Cl3 NCs excited under different wavelengths as shown in Figs. 3(d) and 3(e), respectively. Negligible changes of PL emission peaks for both samples are found when excited under different wavelengths, indicating that the radiative recombination of 0D Cs3Cu2Cl5 and 3D CsCu2Cl3 NCs is determined by an invariable emissive energy level. Considering the large Stokes shift of both samples, the invariable emissive energy level is associated with the presence of STE energy level. The formation of STE energy level is due to the excited-state structure induced by the Jahn–Teller distortion [40]. When cesium copper chlorine absorbs photons, the excited electrons may occupy the excited states and localize around the Cu⁺ ions. Thus, the structure of the [CuCl4] tetrahedron may change because of the introduced inner stress. As a result, upon photoexcitation, the soft lattices of Cs3Cu2Cl5 and CsCu2Cl3 can dissipate a large portion of strain energy, resulting in the distortion of their lattices, which is called Jahn–Teller distortion. The Jahn–Teller distortion induced from the [CuCl4] tetrahedron can further configure the electronic structure, possibly changing the original Cu⁺ 3d¹⁰ to Cu²⁺ 3d⁹. Such localization of electrons and configuration of Cu²⁺ 3d⁹ are considered the reason for the formation of STE energy level [41]. Figure 3(f) shows the energy level diagram of the excited cesium copper chlorine. For PL with the STE effect, the emission energy is determined to be EPL=Eb−Es,(2)where EPL represents the PL emission energy, and Eb and Es are the energy of the band gap and STE of the excited cesium copper chlorine NCs, respectively. Therefore, the Stokes shift can be explained as the energy loss induced by the formation of self-trapped excitons [42]. In addition, the high PLQY, long PL decay time, and broadband emission can be attributed to the direct emission of excitons from the STE level to the ground states, the energy transfer of excitons between excited states and the STE, and exciton–phonon coupling in the excited states, respectively [43].

Furthermore, the operating stability of the WLEDs was investigated under a continuous driving voltage of 4.7 V. Figure 5(c) exhibits that the electroluminescence (EL) spectral intensity of the WLEDs reduces during the aging test. No shift is found for the EL peaks induced from Cs3Cu2Cl5 NCs (green), CsCu2Cl3 NCs (blue), and CaAlSiN3:Eu2+ phosphors (red). Moreover, Fig. 5(d) shows that the luminous efficiency (LE) maintains 64% of its initial value even after 60 h in atmosphere (30°C and 50% RH). The appropriate and stable CCT (<6000) demonstrates the feasible performance of our WLEDs in long-time practical applications [Fig. 5(e)] [46]. The high CRI (>92) is also sustained after 60 h continuous operation as shown in Fig. 5(d). The performance attenuation of the WLEDs is attributed to the decrease of PL efficiency for cesium copper halide and phosphors under humidity in the air and rising temperature induced by long-time working of the UV chip. However, it is found that the attenuation of the WLEDs trends to change slightly after the aging test of 60 h, implying the excellent operating stability in atmosphere for hundreds of hours. Therefore, we believe that the highly efficient, stable, and nontoxic WLEDs can play a critical role in the next-generation lighting applications.

We have synthesized all-inorganic lead-free 0D Cs3Cu2Cl5 and 3D CsCu2Cl3 NCs at 120°C and 70°C, respectively, indicating that the reaction temperature can determine the final component of cesium copper (monovalence) chlorine colloidal perovskite. Owing to the self-trapped exciton (STE) effect, the green-emission Cs3Cu2Cl5 and blue-emission CsCu2Cl3 exhibit high PLQY, broadband emission, large Stokes shift, and long PL decay time. Such an STE effect can be attributed to the Jahn–Teller distortion induced by the [CuCl4] tetrahedron of Cs3Cu2Cl5 and CsCu2Cl3. The highly efficient Cs3Cu2Cl5 NCs can be used for the fabrication of the WLEDs, showing an excellent and stable performance with a high CRI and a moderate CCT even under long-time operation (60 h). The work therefore demonstrates the promising potential of nontoxic cesium copper chlorine perovskite and promotes the development of the novel solid-state lighting.