A huge family of luminescent low-dimensional metal halides for optoelectronic applications has emerged recently as a green alternative to the highly toxic lead halide phosphors. To date, studies on the controlled deposition of these materials as films to be integrated into optoelectronic architectures remain scarce. Here, the synthesis and characterization of highly luminescent films of copper halide phosphors with emissions in violet: K2CuCl3, blue: Cs5Cu3Cl6I2, and green: Cs3Cu2Cl5 are reported. The films were obtained by multisource aerosol-assisted chemical vapor deposition (AACVD) from methanolic solutions at low temperature and under ambient conditions. Photoluminescent quantum yield values obtained for the films deposited on quartz substrates have values of 52% for K2CuCl3, 85% for Cs5Cu3Cl6I2, and 99% for Cs3Cu2Cl5. These values were highly influenced by the substrate since for samples deposited on glass substrates the values are 26.17% for K2CuCl3, 60.47% for Cs5Cu3Cl6I2, and 59.7% for Cs3Cu2Cl5. Different textured morphologies, with valuable applications in light-harvesting, were found for each stoichiometry. Finally, x-ray photo-emitted spectroscopy was employed to demonstrate the existence of only Cu(I) highly emissive species, suggesting that AACVD could be an excellent alternative for metal halide film deposition.

Metal halide perovskites (MHPs) are considered materials of choice for the next generation of light emitters and other optoelectronic devices, including photovoltaics and scintillators.1–5 Moreover, the ease of tunning their bandgap, spanning all the visible spectrum by changing the halide composition or reducing the size of the crystals through solution-processable methods makes them the frontrunners for displays and large-area illumination technologies. Among the various kinds of MHPs, methyl ammonium lead halide MAPbX3 (X = Cl, Br, I) and all-inorganic lead halides, such as CsPbX3 (X = Cl, Br, I), have demonstrated outstanding properties as light emitters, such as high photoluminescent quantum yield (PLQY), high color purity, and short radiative lifetimes.6,7 However, the intrinsic toxicity of heavy metal Pb and low environmental stability hinder their commercialization.

Recently, a few lead-free luminescent all-inorganic metal halides have emerged as promising alternatives to luminescent lead halide perovskites. These include K2CuX3 (X = Cl, Br) powders and single crystals with violet and blue emission,8,9 blue-emitting Rb2CuX3 (X = Cl, Br, I),10–12 Cs3Cu2Br5−xIx (0 ≤ x ≤ 5),13 and Cs5Cu3Cl6I2,14 and green-emitting Cs3Cu2Cl5.15 As light emitters, these families of Cu-based metal halides have many advantages that make them ideal for applications in optoelectronic devices, such as the property of being all-inorganic, their nontoxicity, ultrahigh photoluminescence quantum yield (PLQY),16 low cost due to its earth-abundant element composition, and ease of fabrication as colloidal,17 polycrystalline powder and monocrystalline phosphors,18 by either wet or dry route. Thin films of Cs3Cu2I5 with bright blue photoluminescence have been fabricated by the thermal single-source vacuum deposition technique.19 Thin films of the same lead-free copper halide Cs3Cu2I5 and CsCu2I3 used as emitters in blue, yellow, and white LEDs and as light absorbers in deep-UV-sensitive photodetectors have been also prepared by the spin coating approach.20–23 Spin coated Cs3Sb2Br9 quantum dots (QDs) thin films have also been applied for the fabrication of stable violet LEDs.24 However, there are relatively scarce reports on the fabrication of films of other luminescent copper-based halides, and they have been deposited mainly by the spin coating technique.14,16,17 Although the spin coating method has advantages in regard to preparing very fine and uniform thin films, and it is very useful for low-area film deposition at the laboratory scale, one disadvantage is the difficulty to deposit films over large-area substrates; moreover, its implementation in large-scale industrial manufacture is still challenging. Recently, some novel scalable aerosol-assisted chemical vapor deposition (AACVD) routes have been proposed for in situ synthesis of hybrid and inorganic perovskite films from precursor solutions, with high throughput, reproducibility, and control for their optoelectronic properties without the need for pre-synthesis or post-treatments.25–28 The AACVD technique has been successfully employed to deposit films of different classes of materials, such as polymers,29 transparent conducting electrodes,30 metal-organic frameworks,31 chalcogenides,32 carbon nanotubes,33 and metals.34 This approach offers several key advantages compared with other physical vapor deposition (PVD) or traditional CVD methods that usually require complicated and expensive designs. Since the precursor solutions used in the AACVD process do not require to be volatilized by heat to be transported to the substrate, it allows the use of a wide range of safer, cheaper, and easy-to-handle nonvolatile precursors. AACVD is also cheaper and more versatile and energy-efficient than PVD because in the AACVD process only the substrate needs to be heated and deposition can be performed in an open atmosphere at relatively low temperatures. In AACVD, an aerosol mist is usually generated from a precursor solution prepared at room temperature in a suitable solvent and this mist is transported by the carrier gas to the heated substrate, where the solvent evaporates, and the precursors can react and deposit.

Here, the luminescence and morphological and structural properties of highly luminescent films of copper halide phosphors with emissions in violet: K2CuCl3, blue: Cs5Cu3Cl6I2, and green: Cs3Cu2Cl5 regions synthesized by a computer numerically controlled (CNC) multisource AACVD on hot glass substrates are reported. The lead-free films were deposited in ambient air at low temperatures from environmentally friendly precursors dissolved in methanolic solutions, and their morphological, structural, and luminescent properties were successfully controlled by simply modulating the ratio of the metal halide precursor in the precursor solutions, without the need for pre-synthesis or additional annealing process. The photoluminescence quantum yield (PLQY) was studied in the film–substrate system, yielding values above 25% on glass substrates and above 50% on quartz substrates. It is worth mentioning that methanol is considered a green solvent because it covers major aspects of the environmental performance of solvents in chemical production, as well as important health and safety issues,35 and has been recently predicted to have central participation in the worldwide economy landscape soon.36 

All reagents and solvents were used as purchased without further purification. XRD patterns were obtained using a Rigaku Cu-Kα (λ = 0.152 nm) diffractometer and then indexed using structural data from Crystallography Open Database (COD) and Joint Committee on Powder Diffraction Standards (JCPDS) diffraction cards. Morphology of the films was determined by scanning electron microscopy (SEM) using a Schottky field emission ultrahigh-resolution Scanning Electron Microscope JEOL- JSM-7800F equipped with an Oxford Instruments Energy Dispersive Spectroscopy (EDS) detector and AZtec 2.1 software. The operating voltages ranged between 1 and 15 kV, and the SEM micrographs were processed using ImageJ software. UV–vis absorbance measurements were carried out in a Cary 5000 UV–vis–NIR spectrophotometer. Steady-state and time-resolved photoluminescence spectra were recorded using an Edinburgh Ins. M. 960 S spectrophotometer equipped with a NIR photomultiplier tube. PLQY measurements were obtained using an Edinburgh Ins. FS5 equipped with an integrating sphere module SC-30. The chemical states of the elements in all the copper-based metal halide films were investigated by x-ray photoelectric spectroscopy (XPS) measurements. The XPS spectra were acquired in the low- and high-resolution regimes with a K-alpha Thermo Scientific XPS instrument equipped with an Al-Kα x-ray source (1486.7 eV). The adventitious carbon peak at 284.8 eV was used as the internal standard to compensate for sample charging. Etching for XPS measurements under the surface was performed using 1 keV Ar ions with a current density of 10 µA/cm2.

All films were synthesized by a low-cost, homemade multisource AACVD system. The general experimental arrangement for this technique is shown in Fig. S1.

For K2CuCl3 films, CuCl was dissolved at 10 mmol l−1 in methanol:H3PO2 mixture (99:1 vol. ratio) and magnetically stirred under a nitrogen atmosphere at room temperature for 1 h to get a clear colorless solution. We noted that H3PO2 is a key additive for the complete dissolution of CuCl in methanol that can be accelerated under a nitrogen atmosphere; in contrast, when no inert atmosphere was employed during dissolution, it took CuCl over 7 h to completely dissolve. KCl was separately dissolved in methanol (20 mmol l−1) and stirred vigorously for 1 h until no precipitates were observed. 20 ml of CuCl solution and 10 ml of KCl solution were vigorously mixed with no traces of precipitation. The mixed CuCl/KCl precursor solution was poured into two different flasks (15 ml each) and placed in an ultrasonic nebulizer working at 1.7 MHz, each. A compressed nitrogen gas flux was injected into each flask at 4 liters per minute (Lpm) to drag the produced mist from the flasks to the nozzle chamber. The mist/N2 mixture in the chamber was accelerated using the secondary central nitrogen flux at 4 lpm. The mist was bottom-up deposited on a corning glass substrate (75 × 25 mm2, 1 mm thick) held against a hot plate at 80 °C moving the nozzle at 1 mm/s across the substrate surface. The distance between nozzle and substrate was fixed to 0.7 mm, the deposition time was 300 s, and the resulting area of deposition was 25 × 25 mm2.

For Cs5Cu3Cl6I2, CuCl was dissolved using the same conditions used for K2CuCl3. CsCl and CsI were separately dissolved in methanol (20 mmol l−1, each) and stirred vigorously for 1 h until no precipitates were observed. It was first attempted to mix proper amounts of the three precursor solutions to get a single CuCl:CsCl:CsI (3:3:2M ratio) precursor solution but it instantly started to precipitate upon CsI solution addition. Thus, it was decided to separately place the CuCl solution in a flask and the CsCl:CsI (3:2M ratio) mixture solution in a second flask. Each flask was then placed in an ultrasonic nebulizer working at 1.7 MHz and the film growth process was accomplished by simultaneous deposition of both solutions using the same synthesis parameters that were used for K2CuCl3.

Cs3Cu2Cl5 was prepared by dissolving CuCl at the same conditions used for K2CuCl3. On the other hand, CsCl was separately dissolved in methanol (20 mmol l−1) and stirred vigorously for 1 h until no precipitates were observed. Both CuCl and CsCl solutions were mixed at a 3:4 volume ratio without signs of precipitates as no CsI was present in this condition. Two flasks were filled with 15 ml of the mixed CuCl:CsCl solution and placed in an ultrasonic nebulizer working at 1.7 MHz and the film growth process was accomplished by the simultaneous deposition of both solutions using the same synthesis parameters that were used for K2CuCl3.

All films were characterized as deposited, without post-processing steps. In addition, all three materials were deposited on quartz substrates using the same deposition conditions to explore the effect of the substrate on PLQY values.

We start by discussing the structural properties of the as-deposited films, the x-ray diffraction patterns of the films along with the calculated diffractograms for the K2CuCl3, Cs5Cu3Cl6I2, and Cs3Cu2Cl5 as reference (JCPDS 01-075-1196, COD ID 1558867, and JCPDS 00-024-0247, respectively) are plotted in Fig. 1(a). For all the compositions, the diffractograms match well with the expected crystalline phases, and the observed different relative intensities for some diffraction peaks are due to preferential orientation effects.

FIG. 1.

Experimental (top) and simulated (bottom) XRD patterns of the K2CuCl3, Cs5Cu3Cl6I2, and Cs3Cu2Cl5 films (a). Structure visualization performed using VESTA from previously reported data for the K2CuCl3 (JCPDS 01-075-1196) (b), Cs5Cu3Cl6I2 (COD ID 1558867), (c) and Cs3Cu2Cl5 (JCPDS 00-024-0247) (d).

FIG. 1.

Experimental (top) and simulated (bottom) XRD patterns of the K2CuCl3, Cs5Cu3Cl6I2, and Cs3Cu2Cl5 films (a). Structure visualization performed using VESTA from previously reported data for the K2CuCl3 (JCPDS 01-075-1196) (b), Cs5Cu3Cl6I2 (COD ID 1558867), (c) and Cs3Cu2Cl5 (JCPDS 00-024-0247) (d).

Close modal

Based on recently reported structural data of the K2CuCl3, Cs5Cu3Cl6I2, and Cs3Cu2Cl5 phosphors, the 3D visualization of their corresponding crystal structure was carried out by using VESTA software [Figs. 1(b)1(d)]. From angle-crystal x-ray diffraction analysis, it was found in the work of Creason et al.8 that K2CuX3 (X = Cl, Br, I) is an isostructural family of compounds that crystallizes in the orthorhombic structure in the space group Pnma, in which each copper ion is coordinated by four halogen ions, and the resulting tetrahedra extend along the b-axis by corner-sharing interactions forming one-dimensional polyanionic 1[CuX3]2− chains surrounded by K+ cations [depicted in Fig. 1(b)]. This family shows negligible dependence of the emission wavelength on the halogen since Cu(I) ions determine the valence band maximum and conduction band minimum. The mixed-halide Cs5Cu3Cl6I2 was reported to crystallize in the orthorhombic space group Cmcm. The structure, represented in Fig. 1(c), is composed of continuous [CuCl2I2]2 and [CuCl2I2] alternating units interconnected by I ions to yield 1D [Cu3Cl6I2]n5n− zig-zagging chains isolated by Cs+ ions.14 On the other hand, the crystal structure of Cs3Cu2Cl5 belongs to the orthorhombic space group Cmcm and is composed of [Cu2Cl5]3− units separated by large Cs+ ions. Each [Cu2Cl5]3− unit is in turn constituted by two different copper subunits, a trigonal planar [CuCl3]2− subunit distortedly bonded to a tetrahedral [CuCl4]3− subunit. The strong isolation of [Cu2Cl5]3− units is responsible for the high PLQY and also indicates that electron interaction between adjacent units is negligible.15 

Figure 2(a) shows room temperature photoluminescence excitation (PLE) and emission (PL) spectra for the K2CuCl3, Cs5Cu3Cl6I2, and Cs3Cu2Cl5 films. Sharp and high-energy excitation peaks, yielding high-intensity, broad emission bands with high Stokes shifts and no self-absorption, are observed for the Cu(I) halide films. The K2CuCl3 excitation spectrum peaks at 271 nm and the emission band peaks at 387 nm (3.2 eV), yielding a Stokes shift value of about 117 nm (1.37 eV). The emission spectrum was successfully fitted to a single Gaussian function (FWHM = 52 nm) evidencing a single emission mechanism attributed to self-trapped excitons (STEs). The general process of STEs generation and recombination can be explained as follows: After the absorption of a photon with energy equal to or higher than the phosphor bandgap, an electron is promoted from the valence band to the conduction band, leaving a hole in the valence band (free carriers). An electrically neutral electron–hole pair is then generated due to the quasiparticle’s strong attraction via Coulomb interaction. If the phosphor possesses both strong carrier-phonon coupling and a lattice soft enough to be distorted to a lower energy state upon excitation process, the generated excitons can be easily self-trapped at different depths to recombine radiatively in a highly efficient mechanism.8 The scheme of the K2CuCl3 STE emission mechanism is shown in Fig. 2(b) along with the visualization of the 1[CuCl3]2− chains in which the exciton is self-trapped.

FIG. 2.

Room temperature normalized photoluminescence excitation (PLE) (dotted black line) and normalized photoluminescence emission (PL) (continuous black line) of the polycrystalline films (a). Schematic diagram of the STE emission mechanism along with the visualization of the 1[CuX3]2−, [Cu3Cl6I2]n5n−, and [Cu2Cl5]3− configuration tetrahedra (and trigonal, in the case of Cs3Cu2Cl5) in which STEs are confined for K2CuCl3 (b), Cs5Cu3Cl6I2 (c), and Cs3Cu2Cl5 (d) correspondingly. Digital photograph of the as-deposited copper halide films under 254 nm excitation (e). CIE chromaticity diagram calculated from the PL for the K2CuCl3exc = 271 nm), Cs5Cu3Cl6I2exc = 290 nm), and Cs3Cu2Cl5exc = 298 nm), films. (f). Tauc plots where the dashed lines represent the fitting to the linear section of each curve (g) and photoluminescence decay curves (h) of the copper halide films.

FIG. 2.

Room temperature normalized photoluminescence excitation (PLE) (dotted black line) and normalized photoluminescence emission (PL) (continuous black line) of the polycrystalline films (a). Schematic diagram of the STE emission mechanism along with the visualization of the 1[CuX3]2−, [Cu3Cl6I2]n5n−, and [Cu2Cl5]3− configuration tetrahedra (and trigonal, in the case of Cs3Cu2Cl5) in which STEs are confined for K2CuCl3 (b), Cs5Cu3Cl6I2 (c), and Cs3Cu2Cl5 (d) correspondingly. Digital photograph of the as-deposited copper halide films under 254 nm excitation (e). CIE chromaticity diagram calculated from the PL for the K2CuCl3exc = 271 nm), Cs5Cu3Cl6I2exc = 290 nm), and Cs3Cu2Cl5exc = 298 nm), films. (f). Tauc plots where the dashed lines represent the fitting to the linear section of each curve (g) and photoluminescence decay curves (h) of the copper halide films.

Close modal

The Cs5Cu3Cl6I2 excitation spectrum [shown in Fig. 2(a)] is slightly wider and shifted toward lower energies with respect to K2CuCl3 and reaches its maximum at 290 nm (4.27 eV). In contrast, the emission spectrum was deconvoluted and fitted to a double Gaussian function (χ2 = 0.98) and comprises of a contribution from the lower intensity and relatively narrow (FWHM = 42 nm) band peaking at 439 nm (2.82 eV) and a higher intensity and wider band (FWHM = 102 nm) peaking at 480 nm (2.58 eV), indicating multiple self-trapped exciton (STE1 and STE2) emissive centers that could have originated from the two different polarization orientations of the STEs in the [CuCl2I2]2 and [CuCl2I2] alternating units. The emission mechanism from multiple STE processes is depicted in Fig. 2(c). In STEs, the multiple depths at which the excitons are trapped are responsible for the broadband emission and the energy of the emitted photons is determined by the difference between the energy of the bandgap and the sum of the lattice deformation, self-trapping, and exciton binding energies, thus, high Stokes shifts are also characteristic of STEs.37 Thus, multiple STEs produced by the different polarization orientations can simultaneously decay radiatively from the excited state.

Then again, in Fig. 2(a), the Cs3Cu2Cl5 luminescence emission spectrum fits well to a single Gaussian, in good accordance with previous reports, suggesting a single STE emission mechanism produced by the structural deformation of the [Cu2Cl5]3− dimers [Fig. 2(d)]. The emission band of the Cs3Cu2Cl5 film peaks at 525 nm (2.36 eV) with a full width at a half-maximum of 103 nm, although this film presents a larger Stokes shift (227 nm or 1.80 eV) among the three films studied in this work, it is worth mentioning that the excitation spectrum of this film is the most shifted to lower energies peaking at 298 nm (4.16 eV). The digital picture in Fig. 2(e) shows that the luminescence of all the films is homogeneously distributed in the deposition area and is strong enough to be easily observed by the naked eye under 254 nm excitation in common laboratory illumination conditions. The color characteristics of the photoluminescence (PL) of the films using the optimum excitation wavelength for each case were analyzed, employing the Commission Internationale de l’Eclairage (CIE) chromaticity coordinates. The color coordinates (xs,ys), the dominant wavelength, and color purity were calculated using the standard CIE illuminant D65 and the procedure reported elsewhere.38 From the CIE chromaticity coordinates shown in Fig. 2(f), the dominant wavelength and color purity of the copper-based metal halide films are given in Table S1. The dominant wavelength for the PL from the K2CuCl3 and Cs5Cu3Cl6I2 films was 439 nm (violet) and 482 nm (blue), with a color purity of 78.6% and 68.9%, respectively. In the case of the PL from the Cs3Cu2Cl5 film, the dominant wavelength and color purity, were 541 nm (green) and 44.2%, respectively.

The optical bandgaps are calculated from Tauc plots [shown in Fig. 2(g)], and dashed lines represent the fitting to the linear sections of each curve. The bandgap was calculated from the intersection of the dashed lines with the photon energy (Eg) axis, and the values of 4.17, 3.20, and 3.54 eV were found for the K2CuCl3, Cs5Cu3Cl6I2, and Cs3Cu2Cl5 films, respectively. The extra absorption peak observed in Fig. 2(g) for the K2CuCl3 could be associated with electronic transitions in the [CuCl3]2− units. Figure 2(h) shows the photoluminescence decay curves. The decay times (τ) for the K2CuCl3, Cs5Cu3Cl6I2, and Cs3Cu2Cl5 films are calculated to be 13.57, 45, and 107 µs, respectively. K2CuCl3 lifetime values are in good agreement with previously reported values for the single-crystal9 and nanoparticle counterparts.39 Cs5Cu3Cl6I2 lifetime is slightly longer when compared to the 40 and 41.7 µs values.14,40

Photoluminescence quantum yield values were determined using the spectra shown in Fig. S2, and were calculated using the software of the Edinburgh-FS5, which uses the equation that was first reported by de Mello et al.41 and allows the correction of the values by considering the re-excitation of the sample by diffusedly reflected radiation. The values of the three films deposited on quartz and glass substrates are shown in Table I. By comparing these results, it is observed that the PLQY is lower for films deposited on glass. It is worth noticing that the PLQY values correspond to the whole film–substrate system, and its behavior should be consistently studied, since it will depend on the application whether quartz, glass, or any other substrate is used. In this case, the decrease of PLQY can be attributed to the presence of glass substrates: Since it presents a strong absorption below 330 nm and an average transmittance of 90% in the visible region (Fig. S3), it will substantially increase the number of photons absorbed in the excitation region and will absorb part of the photons emitted from the film toward it, producing a smaller number of total emitted photons. Because of this, the number of emitted photons to number of absorbed photons ratio should be smaller. This effect is observed also in the Cs5Cu3Cl6I2 phosphor, which presents a PLQY value of 95% for the bulk powder and a value of 60% for a thin film deposited on glass.14 Higher PLQY values are obtained for the same materials deposited on quartz substrates because quartz presents an average transmittance of 98% from 230 to 800 nm (Fig. S3), implying a smaller amount of absorbed radiation in both the excitation and the emission ranges, a larger ratio of number of photons, and a closer value to that of the bulk material. The changes in PLQY measurements for films deposited on different substrates are also expected, since earlier reports have shown that there are important effects of the local environment on the PL efficiency of a molecular emitter or exciton.42 

TABLE I.

Quantum yield values obtained for films on glass and quartz substrates and a comparison with reported values.

PLQY (%) glass substratePLQY (%) quartz substrateReported PLQY (%)
K2CuCl3 26 52 No reports 
Cs5Cu3Cl6I2 60 85 6014,a 
Cs3Cu2Cl5 59 99 No reports 
PLQY (%) glass substratePLQY (%) quartz substrateReported PLQY (%)
K2CuCl3 26 52 No reports 
Cs5Cu3Cl6I2 60 85 6014,a 
Cs3Cu2Cl5 59 99 No reports 
a

Deposited as thin film on glass.

The morphologies and thicknesses of the three films were determined by top-view SEM analysis using three different magnifications for each film (Fig. 3), and cross-sectional SEM analysis using secondary (SE) and back-scattered electrons (BSE) (Fig. S4), respectively, additionally SEM data were used to digitally calculate the degree substrate coverage (Fig. S5). Different morphologies were found for each metal halide composition. K2CuCl3 presents a parietal layer with a macroscopic wavy-wrinkled topology [Figs. 3(a)3(c)] composed of dense, irregular grains with sizes ranging from 500 nm to 2 µm in diameter. The grain sizes define the evenly spaced ridges (brighter domains) and valleys (darker domains) observed and could be associated with a Volmer–Weber film growth (localized film densification) and redissolution competing mechanisms during the AACVD process. At higher magnifications, the local surficial morphology of the film shows that valley conforming grains have small pores and tend to be thinner than the smoother surface grains in the ridge compact domains. The cross-sectional SEM (Fig. S4a-a′) further confirms the wavy profile of the film with thickness ranging from 370 to 1780 nm and the calculated coverage of the substrate was calculated to be ∼96%. A similar wrinkle pattern was reported to be successfully controlled by tuning the miscibility between the dimethyl sulfoxide (solvent) and diethyl ether (antisolvent) using temperature in CsPbX3 films.43 The resulting wavy morphology was found to have a beneficial effect on lead halide perovskite-based photovoltaic devices by facilitating the mobility of photocarriers, compared to a flat morphology film. The enhanced performance in the wrinkled photoactive layers was attributed to an extended collection and longer diffusion length of carriers in the low-local strain grains located at the ridges with reduced defect density.

FIG. 3.

Scanning electron micrographs at three different magnifications of the K2CuCl3 (a)–(c), Cs5Cu3Cl6I2 (d)–(f), and Cs3Cu2Cl5 (g)–(i) films deposited on glass substrates.

FIG. 3.

Scanning electron micrographs at three different magnifications of the K2CuCl3 (a)–(c), Cs5Cu3Cl6I2 (d)–(f), and Cs3Cu2Cl5 (g)–(i) films deposited on glass substrates.

Close modal

The scan electron micrographs of the Cs5Cu3Cl6I2 film [Figs. 4(d)4(f)] show that the film is composed by randomly oriented, weakly bound to other rodlike crystal structures with a mean length of ∼5 µm and diameters ranging from 500 nm to 1 µm, elucidating the 1D nature of this stoichiometry. SE cross-sectional SEM images (Fig. S4b-b′) show a more planar film with a regular thickness of about 1500 nm and although the rodlike structures were absent at the surface of the film, probably due to detachment during sample preparation, the orthorhombic rodlike particles are evident as bright domains in the bulk of the film by the BSE cross-sectional SEM image in Fig. S4b′. For this composition, the degree of substrate coverage was found to be ∼99%. The interesting morphology of this film with photon trapping characteristics could benefit light-harvesting by diminishing reflection of excitation over a wide range of wavelengths and angles. In the work of Lu et al.,44 for example, an increase in power conversion efficiency in lead halide perovskite photovoltaic devices has been reported when coral-like instead of planar film’s morphology was chosen for perovskite photoactive layer, resulting in an increased photocurrent density by an enhanced light scattering mechanism, which, in turn, improved the light-harvesting efficiency.

FIG. 4.

XPS survey spectra (a), and high-resolution XPS spectra centered on Cu 2p (b) of the three copper halide films.

FIG. 4.

XPS survey spectra (a), and high-resolution XPS spectra centered on Cu 2p (b) of the three copper halide films.

Close modal

The morphology of the Cs3Cu2Cl5 film is presented in Figs. 3(g)3(i) and the platelets are well-defined rectangles with lateral lengths up to ten micrometers and ultra-smooth surfaces, evidencing the film growth is also of the Volmer–Weber type. The thickness of the platelets grown directly on top of the glass substrates’ surface was estimated to be of about 170 nm by analyzing cross-sectional SEM (Fig. S4c-c′) and those on top of some stacked domains reached 370 nm in height and a substrate coverage value of ∼91%.

Although the different film morphologies presented in this work have no apparent detrimental effect on the luminescence performance of the copper halides, for some applications, smooth, continuous surfaces are desirable.45 On the other hand, all films were deposited for 5 min over the 25 × 25 mm2 area, which means that the deposition rate is high (about 215 nm min−1, according with thickness calculations from cross-sectional SEM for the K2CuCl3 case) due to the high rate of mass transport and crystallization. We believe that this work could pave the way for future fabrication of planar or textured optoelectronic devices. The optimization of film surface topology and local grain morphology requires additional experiments and analysis; therefore, future work should focus on controlling the morphology of the copper halide films depending on the application requirements.

To investigate the chemical states of the elements in all the copper-based metal halide films, XPS measurements were performed. Figure 4(a) shows the survey x-ray photoelectron (XPS) spectrum for the three as-prepared films. The relevant signals of K 2p, Cu 2p, Cl 2p, were in good agreement with that of the K2CuCl3 single-crystal reported earlier.9 Representative XPS spectra of Cu 2p Cs 3d, I 3d, Cl 2p and Cu 2p, Cs 3d, and Cl 2p are also observed in the scan survey corresponding to the Cs5Cu3Cl6I2 and Cs3Cu2Cl5 films, respectively.46 The high-resolution XPS spectrum corresponding to the Cu 2p core levels of the K2CuCl3, Cs5Cu3Cl6I2, and Cs3Cu2Cl5 films is shown in Fig. 4(b). The three spectra have two spin–orbit peaks at ∼933.6 and 953.4 eV, which are assigned to Cu 2p3/2 and Cu 2p1/2, respectively. These peaks are clearly observed without satellite peaks, suggesting the existence of copper ions in the form of highly emissive Cu+ rather than non-emissive Cu2+.9,23,47,48 For the Cs5Cu3Cl6I2 and Cs3Cu2Cl5 films, the shoulders at low binding energies (BE < 931 eV) have been consistently observed in previous reports for a similar Cs5Cu3Cl7I,40 and for the Cs3Cu2Cl5 stoichiometries,15,49–51 and correspond to the Cs MNN Auger (BE ∼ 927 eV)52 for the Cs3Cu2Cl5 and the mixture of the I 3p1/2 (BE ∼ 931 eV)53 and Cs MNN for the Cs5Cu3Cl6I2 film. The processing of easily oxidable elements is usually restricted to ambient temperature reactions or inert atmospheric conditions; this, in turn, results in complex, expensive, and/or highly time-consuming processes. However, in the AACVD process the use of nitrogen as carrier gas effectively protects the metal ions from oxidation during transport and deposition on the hot substrate surface under ambient conditions. The transport of precursors to the substrate for film deposition in the AACVD method relies on the nebulization of precursor salts dissolved in appropriate solvents, as the droplets approach the substrate’s hot surface, the solvent evaporates, leaving highly reactive precursor species that nucleate and grow to yield a film. The temperature of the substrate will be governed by the solvent properties as boiling point, and thus, solvents with low boiling point are preferred to avoid temperature driven Cu+ oxidation. Additionally, since the Cu+ are prone to disproportionation in solution, the instability of CuCl species in methanol results in oxidation of Cu+ into Cu2+ as follows:
Cu+MeOH,80°CCu2+.
To circumvent this issue H3PO2 that was used here to readily solubilize the CuCl precursor salt in methanol, also acted as a powerful reducing agent during film deposition, as previously reported by Bai et al.,54 in the synthesis of Cs3Cu2Cl5 microparticles by a simple evaporation recrystallization method under ambient conditions. The use of H3PO2 in the AACVD process would prevent Cu+ oxidation according to
Cu++H3PO2MeOH,80°CCu++H3PO3.
The resulting H3PO3 by-product is highly soluble in methanol and can be washed away during film deposition, leaving copper(I) compound films that deposit following the next reaction mechanisms,
2KCl+CuClMeOH+H3PO2,N2,80°CK2CuCl3,
3CsCl+2CuClMeOH+H3PO2,N2,80°CCs3Cu2Cl5,
(3CsCl)Flask1+(2CsI+3CuCl)Flask2MeOH+H3PO2,N2,80°CCs5Cu3Cl6I2.

Similar results were reported in the work of Han et al. for the use of oleylamine as reducing agent in the hot injection method, which was also found to be an essential reducing agent for Cs3Cu2Cl5 phase stabilization over the weakly emissive CsCu2Cl5 phase,51 pointing out the possibility of using alternative additives during AACVD process.

Figures S6(a) and S6(b) show the high-resolution XPS spectra corresponding to the K 2p and Cl 2p core levels, respectively. The K 2p spectrum exhibits binding energies at 294.4 and 297.2 eV, which belong to K2p3/2 and K2p1/2, respectively, and matches well with K+.9,55 Similarly, the Cl 2p spectrum shows binding energies at 199.9 and 201.3 eV, corresponding to Cl 2p1/2 and Cl 2p1/2, confirming the presence of Cl.46 Figures S7 and S8 show the XPS spectra of the other elements present in the Cs5Cu3Cl6I2 and Cs3Cu2Cl5 films, besides Cu. Tables S2–S4 show the XPS fitting and atomic content results obtained from the high-resolution spectra of the elements present in each film. These results indicate that all the elements are in a monovalent state and the compositions of the three films are reasonably consistent with the stoichiometric ratios of the K2CuCl3, Cs5Cu3Cl6I2, and Cs3Cu2Cl5 formulas.

These results suggest that AACVD could be a breakthrough not only for the mass production of low dimensional metal-halide films but also for accelerating the development stage of new film materials and device architectures. As a proof-of-concept, deposition areas of 25 × 25 mm2 were here studied by moving the nozzle across the substrate just in the x direction; however, by implementing additional movement in the y direction, deposition over large-scale area would be virtually size-unlimited. Future work will be focused on the control over the morphology of the films by the use of additives and antisolvents and their integration in optoelectronic devices.

In summary, a facile green approach to deposit nontoxic, highly luminescent, and textured low-dimensional copper halide films by multisource AACVD from methanolic solutions was developed. The resulting K2CuCl3, Cs5Cu3Cl6I2, and Cs3Cu2Cl5 films emit bright violet, blue, and green light from STEs, respectively. The x-ray diffraction patterns matched well with the previous reports of nanocrystalline powders and monocrystals with the same stoichiometry. PLQY values of all the films were strongly affected by the presence of the glass substrates, although in the case of Cs5Cu3Cl6I2, and Cs3Cu2Cl5 films deposited on glass substrates, the PLQY values are close to 60%, the values increased up to 85% and 99%, respectively, when deposited on quartz substrates. Finally, XPS data confirmed the elemental composition of the films and revealed the presence of only Cu(I) species, rather than oxidized Cu(II), in the films suggesting that inexpensive, high-throughput, and highly scalable AACVD could be an excellent route for the integration of low-dimensional metal halide films in mass production of optoelectronic devices.

See the supplementary material for further details of the AACVD system used for film deposition and complete measurements for the optical, structural, and composition characterization of the three copper-based metal halide thin films.

We acknowledge the technical assistance provided by A. Tejeda, O. Novelo, L. Bazan, R. Reyes, C. García, A. Pompa, and C. Gonzalez from IIM-UNAM. Thanks are extended to Z. Rivera and M. Guerrero from the physics department of CINVESTAV-IPN for their technical support. The authors would like to acknowledge the support provided by Samuel Tehuacanero Cuapa and Juan Gabriel Morales from IF-UNAM for technical assistance in SEM and sample preparation, respectively. The first author is grateful to the Dirección General de Asuntos del Pesonal Académico (DGAPA-UNAM) for the granted Postdoctoral Fellowship. This research work received partial financial support from the Project PAPIIT-UNAM, No. IN111022. A. Rodríguez-Gómez would like to acknowledge the support received from PAPIIT-UNAM Project No. IN111723. I. Garduño acknowledges the financial support provided under Grant No. “Cátedras CONACYT 871 (2017).”

The authors have no conflicts to disclose.

Jesús Uriel Balderas Aguilar: Conceptualization (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Visualization (equal); Writing – original draft (equal). Luis Alberto Becerril-Landeros: Formal analysis (equal); Investigation (equal); Methodology (equal); Visualization (equal). Ismael Arturo Garduño Wilches: Formal analysis (equal); Investigation (equal); Visualization (equal); Writing – original draft (equal). M. García-Hipólito: Investigation (equal); Resources (equal); Writing – review & editing (equal). Arturo Rodríguez-Gómez: Formal analysis (equal); Investigation (equal); Resources (equal); Visualization (equal). Luis Escobar-Alarcon: Data curation (equal); Formal analysis (equal); Investigation (equal). Ciro Falcony: Conceptualization (equal); Formal analysis (equal); Investigation (equal); Project administration (equal); Resources (equal); Supervision (equal). Juan Carlos Alonso-Huitrón: Conceptualization (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – original draft (equal).

The data that support the findings of this study are available within the article and its supplementary material.

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Supplementary Material