Zero-Dimensional Organic Amine-Copper Bromide Hybrid Crystal with Highly Efficient Yellow Emission

Abstract

Recently, Cu(I)-based metal halides have attracted tremendous attention owing to their remarkable photophysical properties. However, most of them can only be excited by near ultraviolet (UV) light at a wavelength (generally less than 350 nm) with a wide bandgap, which undoubtedly limits their application in solid-state lighting due to the low excitation efficiency at about 400 nm in devices. Here, we report a new zero-dimensional organic cuprous bromide of (C₁₃H₃₀N)₂Cu₅Br₇ single crystals, which can be excited by visible light (390–400 nm) and give a bright yellow and broad self-trapped exciton emission band with the photoluminescence quantum yield (PLQY) of 92.3% at room temperature. The experimental and theoretical results show that the existence of Cu-Br-Cu metal bonds in a Cu₅Br₇ cluster package produces three components of self-trapped excitons (STE) that emit at room temperature but merge into one at 80 K. This occurs because of the anomalously enhanced electron–phonon coupling and electron–electron coupling in the coupled clusters in this system. These effects cause the excitation near visible light and emission broader at higher temperature. Additionally, their remarkable anti-water emission stability was demonstrated even after soaking in water for 6 h. Finally, a highly efficient white-light-emitting diode (WLED) based on (C₁₃H₃₀N)₂Cu₅Br₇ was fabricated.

1. Introduction

Low-dimensional organic–inorganic hybrid metal halides have gained widespread interest in solid-state lighting, solar cells, and light-emitting diodes (LEDs) owing to their numerous advantages, such as direct bandgap, and high photoluminescence quantum yield (PLQY) [1,2,3,4]. Over the past few years, numerous zero-dimensional (0D) lead-based metal halides with excellent photophysical properties have been developed [5,6]. However, the toxicity and poor stability of lead not only threaten humans and the environment, but also limit its further application in optoelectronic devices [7,8]. Therefore, many scientists try to develop lead-free metal halides that offer low toxicity, excellent environmental stability, and outstanding luminescent performance. Some researchers have replaced Pb with In, Bi, Sb, and other elements, which can be used as efficient light emitters. However, the potential harm of heavy metal like indium, bismuth, and antimony to the human body remains a concern [9,10].

Copper (Cu) is a transition metal with low toxicity, making it a safer alternative. It also has the advantages of diverse structure, excellent optical properties, and abundant natural availability. Recently, univalent Cu (I)-based metal halides have attracted a great deal of attention due to their excellent optical properties. In 2018, an all-inorganic Cu (I) metal halide Cs₃Cu₂I₅, with bright blue emission under UV light was first reported to have a PLQY of up to 91.2% [11]. Subsequently, a variety of all-inorganic Cuprous halide metal halides have also shown great application potential in optoelectronic devices, such as Cs₃Cu₂I₅, CsCu₂I₃, Ru₂CuBr₃, and K2CuCl₃ [12,13,14,15,16]. Recent studies have found that excellent optical properties can also be obtained by using organic cations to replace the A-site, such as (DTA)₂Cu₂I₄ (PLE~330 nm, green-yellow emission) [17], TEA₂Cu₂Br₄ (PLE~280 nm, blue emission) [18], (TBA)CuCl₂ (PLE~282 nm, green emission), and (Gua)₃Cu2I₅ ((PLE~324 nm, bluish-white emission) [19,20]. In these compounds, the charge transfer between Cu and halide ion, the aggregation of CuX cluster, coordination symmetry, as well as the Cu-Cu metal bonding all have contributed to their emission energy. These compounds often have large Stokes shifts, rich visible emission colors, and efficient broadband emission. However, it is not clear what factors are the most important. Therefore, we plan to develop novel luminescent materials by inserting different organic molecules into the Cu(I)X (X = Cl, Br, and I) lattice.

Herein, we report an 0D organic–inorganic hybrid Cu-based metal halides of (C₁₃H₃₀N)₂Cu₅Br₇ single crystals (SCs) with bright yellow emission under 365 nm UV light. This “0D” refers to the structural characteristics at the molecular level, specifically the arrangement of the inorganic framework within the lattice in space, rather than the size in terms of dimensions like quantum dots or nanosheets. (C₁₃H₃₀N)₂Cu₅Br₇ has a new type of zero-dimensional framework, in which the isolated [Cu₅Br₇]^2- clusters are surrounded by organic (C₁₃H₃₀N)⁺ cations. (C₁₃H₃₀N)₂Cu₅Br₇ SCs exhibit a large Stokes shift and a high PLQY (∼92.3%). Worthy of note is that, compared with other Cu(I)-based metal halides, the organic ligands significantly reduce the conduction band minimums, resulting in a smaller band gap. The photophysical process of (C₁₃H₃₀N)₂Cu₅Br₇ single crystals was investigated using the temperature-dependent spectrum and density functional theory (DFT) calculations. The compound also exhibits stability in water, with its emission intensity remaining nearly unchanged after 6 h of immersion. In addition, (C₁₃H₃₀N)₂Cu₅Br₇ demonstrates outstanding environmental safety and photostability. Therefore, our study offers a novel approach for the development of Cu-based hybrid metal halides with excellent luminous efficiency.

2. Materials and Methods

2.1. Materials

Cuprous(I) bromide (CuBr, 99.0%), Tributylmethylammonium bromide (C₁₃H₃₀BrN, 98.0%), Hydrobromic acid (HBr, 38 wt%), and Hypophosphorous acid (H₃PO₂, 50 wt%) were purchased from Aladdin Industrial Corporation (Shanghai, China).

2.2. Synthesis of (C₁₃H₃₀N)₂Cu₅Br₇ SCs

The (C₁₃H₃₀N)₂Cu₅Br₇ SCs were prepared using a method where the solvent slowly evaporates and crystallizes. The mixture of 4 mmol CuBr, 4 mmol C₁₃H₃₀BrN, and 12 mL HBr/H₃PO₂ (10/2) was placed in a 100 mL beaker and sonicated until the raw materials were fully dissolved, forming a clear solution. Subsequently, the solution is slowly heated on the heating stage at 65 °C. After 96 h, transparent (C₁₃H₃₀N)₂Cu₅Br₇ SCs were successfully obtained.

2.3. Characterization

Single-crystal X-ray diffraction (SCXRD) measurements were carried out on a Bruker D8 Venture X-ray single-crystal diffractometer ((Bruker, Karlsruhe, Germany)). Powder X-ray diffraction (PXRD) was recorded using a Bruker D8 Advance instrument within 5−80° (Bruker, Karlsruhe, Germany). The sample morphology was examined using Hitachi SU8010 scanning electron microscopy (SEM) (Hitachi, SU8010, Tokyo, Japan). The element composition and distribution were analyzed using energy-dispersive spectrometry (EDS, Oxford) (Oxford Instruments, Abingdon, UK). X-ray photoelectron spectroscopy (XPS) was performed using the K-Alpha instrument (Thermo Fisher Scientific, Waltham, MA, USA). The solid-state absorption spectrum was obtained with a UV-3600 spectrophotometer (Shimadzu, Kyoto, Japan). Photoluminescence (PL) and PLE spectra were recorded using a Horiba spectrometer (Horiba, Kyoto, Japan). The PL decay lifetime was measured by an FLS 980 spectrometer (Edinburgh Instruments, Livingston, UK), employing a 365 nm laser as the excitation source. The PLQE was determined by an FLS 980 setup (Edinburgh Instruments, Livingston, UK). Temperature-dependent PL spectra were acquired by cooling the samples with liquid nitrogen. Thermal stability was assessed via thermogravimetric analysis (TGA) using a NETZSCH TG 209F3 (NETZSCH, Selb, Germany).

2.4. Calculation Details

All density functional theory (DFT) computations are conducted using the Vienna Ab initio simulation package (VASP. 6.5.0) [21]. The exchange–correlation interactions are described by the generalized gradient approximation with the Perdew–Burke–Ernzerhof (PBE) [22,23,24]. A kinetic-energy cutoff of 400 eV is applied, along with a 3 × 2 × 2 Monkhorst–Pack k-point grid for sampling the Brillouin zone. Ultra-soft pseudopotentials are utilized for N, H, C, Cu, and Br. The structural relaxations are performed with an energy convergence threshold of 1.0 × 10⁻⁵ eV.

3. Results

3.1. Structural Characterization

The (C₁₃H₃₀N)₂Cu₅Br₇ SC was successfully synthesized using a slow evaporation method from a precursor solution. Figure 1a displays the photographs of (C₁₃H₃₀N)₂Cu₅Br₇ single crystals, showing transparent and colorless nature under ambient light, indicating minimal negligible absorption in the visible range. Upon illumination with a 365 nm UV light beam, the SCs display bright yellow emission. Figure 1b–d depict the crystal structure of (C₁₃H₃₀N)₂Cu₅Br₇, with detailed structural parameters determined through single-crystal analysis. The crystal is classified within the triclinic system and crystallizes in the space group P-1. The related lattice parameters are: a = 11.904(3) Å, b = 12.364(3) Å, c = 15.168(3) Å, α = 92.849(5)°, β = 104.580(4)°,γ = 95.064(4)° and V = 2146.1(9) Å. Every five Cu atoms coordinate with seven Br ions to form a unique [Cu₅Br₇]^2- cluster structure, which is the core of this material functions. Its shape looks like a symmetrical scopperil, seven Br ions are located at outside and top, around a five-membered up and down Cu⁺ ion ring, in which each Cu⁺ seems to be surrounded by four Br ions. Or we can say it can be seen as a randomly oriented scopperil, any Br ion can work as top, a highly symmetrical structure. The isolated [Cu₅Br₇]²⁻ clusters are surrounded by (C₁₃H₃₀N)⁺ cations to form a typical 0D framework in the crystal. As demonstrated in Figure 1e, the PXRD pattern exhibits features nearly identical to the simulated SCXRD results. The sharp and well-defined diffraction peak in the experimental PXRD pattern further verify the excellent crystalline quality of (C₁₃H₃₀N)₂Cu₅Br₇. Next, the element composition and distribution of (C₁₃H₃₀N)₂Cu₅Br₇ were analyzed (Figure S1). The EDS analysis reveals that the atomic proportion of copper to bromine in the SC is roughly 5:7 (Figure S1b), which closely matched the SCXRD finds. The element mapping results (Figure S1c,d) showed that Cu and Br elements are relatively uniform distribution in the compound products. To gain deeper insight into the valence states and elemental composition of ions, XPS analysis was conducted. Figure S2c shows the high-resolution XPS spectrum of Cu 2p, which displays two distinct peaks at 931.6 and 951.4 eV, representing Cu 2p_3/2 and Cu 2p_1/2. This result indicates that Cu⁺, but not Cu²⁺, ions exist in (C₁₃H₃₀N)₂Cu₅Br₇ single crystal [25], as Cu(II) compounds always have a satellite band above their 2p XPS band. The Cu/Br ratio was determined to be 5:7 based on the XPS results. Furthermore, two peaks at 69.1eV and 68.1 eV were identified, corresponding to the Br 3d_3/2 and Br 3d_5/2, respectively (Figure S2b).

3.2. Optical Properties

Subsequently, we investigated the room temperature (RT) photophysical properties of (C₁₃H₃₀N)₂Cu₅Br₇. Figure 2a displays the PLE and PL spectra of (C₁₃H₃₀N)₂Cu₅Br₇ SCs. Clearly, the PLE spectrum of this compound reveals its most intense excitation peak at 390 nm. There are two emission peaks at room temperature, located at about 600 nm (peak 1) and 635 nm (peak 2) with an FWHM of 118 nm and 214 nm, respectively. These peaks will be discussed in the next section. This compound exhibits a significant Stokes shift of 210 nm (1.11 eV) for the emission band at 600 nm, and of 245 nm (1.23 eV) for an emission band at 635 nm. This shows that the emission band is original, and the self-absorption of (C₁₃H₃₀N)₂Cu₅Br₇ can be neglected, which is ideal for luminescence applications. Additionally, this material exhibits an impressive PLQY of 92.3% at room temperature. The absorption spectrum of (C₁₃H₃₀N)₂Cu₅Br₇ powder is displayed in Figure 2b, showing a distinct peak at 390 nm. Furthermore, the band gap, determined using the Tauc equation, is calculated to be 2.94 eV. The CIE coordinate of (C₁₃H₃₀N)₂Cu₅Br₇ is positioned at (0.506, 0.452), indicating its characteristic yellow emission (Figure 2c). Figure 2d shows the lifetime measurement of the PL spectrum of a single crystal at room temperature, and the lifetime decay curve can be well fitted by a single exponential function. The images of the two peaks are almost identical. The fitted decay lifetime was 40.9 µs. This long lifetime implies that the state for emission may be the STE state. In addition, PL spectra exist at different excitation wavelengths with similar profiles (Figure S3a). PLE spectra at different emission wavelengths also have similar characteristics (Figure S3b). These results indicate that the yellow emission observed in (C₁₃H₃₀N)₂Cu₅Br₇ single crystals results from the same excited state relaxation processes [26]. As a supplementary verification, we ground the (C₁₃H₃₀N)₂Cu₅Br₇ SCs into powder, which led to a decrease in PL intensity (Figure S4). This further eliminated the possibility of surface-defect emission. Additionally, we have compiled the essential photophysical parameters of the recently reported Cu(I)-based metal halides, with the results presented in

Table 1. Photophysical properties of metal halides at room temperature.

Components	PL (nm)	PLE (nm)	Stokes Shift (nm)	FWHM (nm)	PLQY (%)	Color	Ref.
(C13H30N)2Cu5Br7	600/635	390	210/245	118/214	92.3	yellow	This work
Cs3Cu2I5	445	290	155	75	90	blue	[27]
Cs5Cu3Cl6I2	462	271	191	95	95	blue	[28]
CsCu2I3	558	330	228	115	15	yellow	[29]
Cs3Cu2Br5	455	298	157	75	50.1	blue	[30]
Cs3Cu2Cl5	510	310	200	92	100	green	[31]
Rb2CuCl3	400	300	100	52	100	blue	[32]
Rb2CuBr3	385	300	85	54	98.6	blue	[14]
K2CuCl3	392	291	101	54	96.58	blue	[14]
K2CuBr3	388	296	92	54	55	blue	[14]
(PEA)4Cu4I4	598	370	228	112	68	orange-red	[33]
(DTA)2Cu2I4	540	330	210	180	60	green-yellow	[17]
TEA2Cu2Br4	463	280	183	90	94.73	blue	[18]
(TBA)CuCl2	508	282	226	86	82	green	[19]
(Gua)3Cu2I5	481	324	157	125	96	bluish-white	[20]

. The excitation peak of (C₁₃H₃₀N)₂Cu₅Br₇ SCs are located at 390 nm, which obviously possesses lower energy than that of Cu(I)-based metal halides, which are those listed in the following table [27,28,29,30,31,32,33,34].

To investigate the electronic properties of (C₁₃H₃₀N)₂Cu₅Br₇, the band structure and density of states (DOS) of (C₁₃H₃₀N)₂Cu₅Br₇ were computed by DFT. The band structures of (C₁₃H₃₀N)₂Cu₅Br₇ (Figure 3a) reveal that the calculated bandgap of the (C₁₃H₃₀N)₂Cu₅Br₇ is about 1.27 eV, which is lower than the experimental measured value of 2.94 eV (Figure 2b). This discrepancy arises because DFT calculations typically tend to underestimate the actual bandgap for most compounds. For (C₁₃H₃₀N)₂Cu₅Br₇, the valence band maximum (VBM) states are constructed mainly by the hybridized Cu 3d and Br 4p orbitals (Figure 3b), while the conduction band minimum (CBM) states are composed mainly of one band of the organic counterpart of (C₁₃H₃₀N)⁺, while the Cu and Br hybridized states located at this energy have only very small DOS. The main Cu and Br hybridized states are located at a higher energy (0.7 eV) position than the CBM state. The results above indicate that the band absorption of (C₁₃H₃₀N)₂Cu₅Br₇ primarily originates from charge transferred within the cuprous bromide cluster itself, but not from the organic ligand, because metal Cu ion did not connect with the organic ligand. However, the organic ligand may contribute to its excited state relaxation for its location at CBM. Moreover, the organic ligands significantly reduce the conduction band minimums, resulting in a smaller band gap. Hence, we can see that the emission contains three bands, at 579 nm, 641 nm, and 697 nm, as shown in Figure S3c. Their large separation in the emission band may be due to the organic vibration energy or to spin-orbital splitting in bromide; this will require further verification. Furthermore, we compared (C₁₃H₃₀N)₂Cu₅Br₇ bandgap with that of the previously reported cuprous halide materials, and the results are listed in Figure S5. As observed, the band gap of this compound is much smaller than other zero-dimensional (0D) organic cuprous bromide [35]. This study offers approaches for designing narrow-bandgap zero-dimensional metal halides, whose emission may be connected to the transition between the CuX cluster and incorporated organic molecules. We compared (C₁₃H₃₀N)₂Cu₅Br₇ with that of the previously reported metal halide materials, as summarized in the table above.

To gain a deeper understanding of the photophysical mechanism of (C₁₃H₃₀N)₂Cu₅Br₇ single crystal, the temperature-dependent PL spectra of (C₁₃H₃₀N)₂Cu₅Br₇ were recorded at a temperature range of 80–300 K (Figure 4a). The corresponding luminescence peak positions and FWHM as a function of temperature are presented in Figure 4c. As the temperature increases, the position of the luminescence peak has a gradual blue shift, which is attributed to the clear lattice expansion for the STE state emission. In this compound, the STE state originates from the strong electron–phonon interaction, including acoustic phonon and longitudinal optical phonon coupling in this 0D structure. The similar phenomenon has been observed in the 3D perovskite CsPbBr₃ and (MA)₄Cu₂Br₆. The PL intensity is notably influenced by temperature, showing a marked increase from 300 to 80 K. This trend mirrors the PL behavior of the material of 3D perovskites (CsPbBr₃ and CH₃NH₃PbBr₃) [36]; here, the suppression of nonradiative recombination at low temperatures leads to enhanced PL intensity. As the temperature decreases, the FWHM decreases, which indicates that the coupling of electrons and acoustic phonons is weakened. The situation is also similar to other 0D metal halides, such as (TPA)₂PbBr₄ and (C₄H₉)₄NCuCl₂ [5,26]. Meanwhile, as the temperature increases, we can gradually observe two emission peaks and a sideband, as we indicated in the former section.

The Huang–Rhys factor (S) is a crucial parameter for assessing the strength of electron–phonon coupling, with a higher S value indicating stronger coupling. It can be calculated using the following equation:

$$\mathrm{F}\mathrm{W}\mathrm{H}\mathrm{M}=2.36\sqrt{\mathrm{S}}\mathrm{\hslash }{\omega }_{phonon}\sqrt{\mathrm{c}\mathrm{o}\mathrm{t}\mathrm{h}{\displaystyle \frac{\mathrm{\hslash }{\omega }_{phonon}}{2K_{B}T}}}$$

where ℏω_phonon is the phonon frequency for polaron formation and k_B is the Boltzmann constant. The calculated value of S is 112.08 (Figure 4d). The Huang–Rhys factor is notably higher compared to previously documented Cu compounds and other halide compounds, which means that the organic molecules inside play an important role in the STE formation in the Cux-Bry cluster and give a strong emission. The results mentioned above suggest a significantly stronger electron–phonon interaction in this compound compared to others, which favors the formation of deep STEs in (C₁₃H₃₀N)₂Cu₅Br₇. To further investigate the electron–phonon interaction, we measured the Raman spectrum of (C₁₃H₃₀N)₂Cu₅Br₇ SCs using a 532 nm laser excitation at 80 and 300 K. As shown in Figure 5a, the whole vibration intensity of phonon modes at 87 cm⁻¹ and 170 cm⁻¹ for Cu-Br shows a notable increase at elevated temperatures, suggesting an enhanced long-range electron–phonon interaction, implying that the STE is not forming at a Cu-Br bond, but at a larger range. Furthermore, the Raman spectra profiles are nearly identical across the two temperatures, demonstrating that the structure and symmetry of (C₁₃H₃₀N)₂Cu₅Br₇ remain stable. A prominent Raman mode appears at the low wave number of 87 cm⁻¹ at room temperature, which is indicative of the small polaronic mode resulting from the interaction of local phonon and acoustic modes. The Raman mode at 170 cm⁻¹ is the overtone of the longitudinal optical (LO) phonon mode 87 cm⁻¹ of Cu-Br bond. Additionally, the weak Raman mode at 270 and 316 cm⁻¹, observed at higher vibration energies, can be interpreted as a multi-phonon mode derived from the 87 cm⁻¹. This is a typical signature of STE formation, driven by the strong nonlinear electron–phonon coupling. Therefore, multi-phonon modes are clearly present in the Raman spectra of (C₁₃H₃₀N)₂Cu₅Br₇. The Raman mode at 80 K changes a little, and the overtone mode of 170 cm⁻¹ became stronger than the 87 cm⁻¹ mode; moreover, the whole intensity is reduced at a low temperature. This situation seems surprising.

Based on the comparison of the emission band of (C₁₃H₃₀N)₂Cu₅Br₇ at 80 K and 300 K (Figure 4a), we found that 80 K emission can be assigned to single narrow band, which can belong to one of the fitted emission bands of 641 nm in Figure 4b, which is among in the much broader band at room temperature. The latter, wider emission band at room temperature contains two other bands at the blue side and red side of the 641 nm single band, respectively. Accordingly, there are three band occurring at room temperature due to the more microscopic interactions in this compound. This phenomenon is much different from that in previous halides, in which the lattice expansion can blueshift its STE emission band at 600 nm for single Cu-Br bond due to the atomic confinement in metal halides. The noticeable broadening of the emission band at room temperature may originate from the charge correlation in aggregated lattice during lattice expansion, which contains single STE1 in CuBr_x, STE2 in the aggregated cluster Cu₅Br₇, and the multipolaron or multi-STE3 inside the organic -CuBr cluster at high temperature (Figure 5b). In this system, the outer electrons of Cu are in a fully filled shell, and their optical transitions mainly originate from the d orbitals, forming a d-sp hybridized exciton. In this system, Cu-Cl forms zero-dimensional polymer clusters, where the lattice exhibits significant distortion, resulting in strong phonon effects. This polymer structure leads to exchange interactions of the d electron cloud between Cu atoms. However, due to the zero-dimensional nature of the system, these interactions are localized and do not exhibit long-range behavior. Therefore, in this system, an STE can actually be regarded as a polaron. This localized electron–phonon coupling has a significant impact on the spectral characteristics of the zero-dimensional structure. There is a specific phase in this compound that may represent the dominant five Cu membered ring with multi-molecular coordination (at 641 nm), the Cu-B cluster at 600 nm, and couple Cu₅-Cu₅ metal cluster pair connected by the organic molecules in this crystal shown in Figure 1. This pair possesses a local plasmonic band or collective band at long wavelength (641 nm), which cannot change with temperature, and dominates with narrow band at 80 K. The Raman mode at 170 cm⁻¹ is stronger than 87 cm⁻¹ mode, which also reflects the coupling in collective Cu-Br-Cu-Br state dominance. At room temperature, the existence of both Raman modes indicated that the states at Cu-Br cluster and Cu₅Br₇ cluster can coexist for their individual emissions.

4. Discussion

Other than the two emission states discussed above, another emission also occurred at room temperature due to strong electron–phonon coupling (STE1) and bipolaron formation (STE2). In this situation, both Cu₅Br₇ clusters were connected by an organic group to form a bipolaron (STE3), which can provide an emission at about 720 nm (Figure 5c). The lattice changed two Cu₅ clusters become closer in the chains of the Cu₅-Br-NC-Br-Cu₅ structure. This state (multipolaron or polaronic molecule) may be the coupled molecular vibration to the individual Cu-Br bonds around them, allowing them to form a multi-electronic or two-plasmonic coupled state; this multi-phonon involved in a bipolaron formation would only be produced with higher temperatures (STE3). Moreover, the multi-phonon modes in Raman spectra are significantly clearer at room temperature than at 80 K, which means the multi-phonon coupling is much stronger than that at 80 K. This three-state coexistence implies that the local composite structure of Cu₅Br₇-NC-Cu₅Br₇ does not have a uniform charge density due to their large charge imbalance and odd number symmetry, which causes both long-range and local electron–phonon coupling in this Cu₅Br₇-unit-based structure. Strong electron–phonon coupling coefficients (S) over 100 and delocalized carriers between Cu-Br bonding can easily create an efficient multielectron state for emission. The multi-polaron state formation in the photophysical processes of (C₁₃H₃₀N)₂Cu₅Br₇ are shown more clearly at room temperature than at 80 K, as shown in Figure 5d. Due to the strong Jahn–Teller effect and metal Cu(I)–Cu(I) bond in a [Cu₅Br₇]^2- cluster, both intense electron–electron interaction and electron–phonon coupling is present, with strong temperature dependence. (C₁₃H₃₀N)₂Cu₅Br₇ exhibits significant internal conversion from the intrinsic band to the STE1 or STE2 overlapped at 641 nm around the local Cu₅Br₇ cluster at low temperature. It is transformed into two levels (STE1 at 600 nm and STE2 at 641 nm) at room temperature; it also has another bipolaronic state (720 nm at room temperature) because STE1 and STE2 have a small energy barrier due to the neighboring Cu–Cu metal bond in Cu₅ cluster at high temperature. The STE3 state can be formed only at room temperature for strong high phonon modes in organic molecules in 600–1500 cm⁻¹ range, as Figure 5a shows. Therefore, electronic correlation in this complicated structure may possess multi-bands at room temperature: a blue-shifted band at the same time as the STE1 band due to lattice distortion; this is accompanied by a plasmonic band at the long wavelength bipolaronic state at 641 nm with high stability. This is the only dominant merging STE1 + STE2 that can give narrow emission band out of Cu-Br in Cu₅Br₇ for its small phonon population at 80 K. This situation requires a large change in the separation of Cu–Cu bond in asymmetrical cluster during temperature variation. At a low temperature, excitons are swiftly localized in the emitting state 1 (ES1), which can stay at any Cu-Br site in the Cu₅Br₇ cluster. This is a real symmetrical structure that gives a narrow emission from a local cluster. As the temperature increases, the structure varies due to electron–phonon coupling rising. The local exciton around Cu-Br and Cu₅Br₇ cluster gains respective energy extending over varied sites, and it can split into the ES1 and ES2 states (bipolaron) at high temperature. In the unregular lattice case, some of metal bond between Cu(I) ions have been relaxed, and the possible charge may be modified in neighboring Cu₅Br₇ clusters. Then, two charged Cu₅Br₇ clusters can be coupled with suitable phonon assistance to form a new bipolaron state (STE3) that emits light at 720 nm.

It is important to note that the material stability of (C₁₃H₃₀N)₂Cu₅Br₇ is a crucial factor for optoelectronic applications, which prompted us to evaluate the structural and optical stabilities. (C₁₃H₃₀N)₂Cu₅Br₇ SCs showed excellent water stability. The compound maintained its initial morphology after 6 h of immersion in water and exhibited bright yellow emission under the 365 nm UV light (Figure 6a). In addition, the PL intensity of (C₁₃H₃₀N)₂Cu₅Br₇ SCs can remain at a high level after 6 h immersion in water (Figure 6b). Given the well-document extreme moisture sensitivity of Cu(I)-based metal halides, the high anti-water stability of (C₁₃H₃₀N)₂Cu₅Br₇ is surprising. Subsequently, the PXRD patterns of (C₁₃H₃₀N)₂Cu₅Br₇ single crystal were also measured before and after soaking in water. As shown in Figure 6c, they can be seen to have similar PXRD profiles, which indicates that (C₁₃H₃₀N)₂Cu₅Br₇ does not experience structural degradation after exposure to water. A high-resolution XPS shows Cu 2p after exposure to water (Figure 6d). This result is consistent with the sample before water exposure (Figure S2c), indicating that Cu⁺ remains the dominant species in (C₁₃H₃₀N)₂Cu₅Br₇. This stability ensures the high luminescence efficiency of (C₁₃H₃₀N)₂Cu₅Br₇, even after 6 h in water. Previous studies have shown that organic cations improve the stability of lead halide perovskite solar cells. In this work, the organic cation (C₁₃H₃₀N)⁺ is uniformly distributed as counterions on the surface of the [Cu₅Br₇]²⁻ cluster. The steric hindrance from the cations weakens their coordination ability, limiting the interaction with Cu⁺. Additionally, (C₁₃H₃₀N)⁺ from strong electrostatic bands, create a denser crystal filler and an effective organic waterproof layer that prevent corrosion in humid environments or water. Therefore, this work demonstrates that incorporating large organic cations with significant steric hindrance into the Cu(I)X lattice enhances water resistance stability.

The (C₁₃H₃₀N)₂Cu₅Br₇ crystals also demonstrate excellent thermostability, as shown by thermogravimetric analysis (TGA) (Figure S6). The compound’s initial decomposition temperature is 280 ◦C, which further underscores its potential for luminescent applications. Additionally, it remains stable under UV irradiation. After 240 min of 365 nm UV exposure, the intensity retains 95% of its original value (Figure S7). It is worth noting that (C₁₃H₃₀N)₂Cu₅Br₇ has excellent environmental stability. After one month of storage in air, the PXRD pattern of the sample showed no significantly changes (Figure S8), and the emission intensity remained high (Figure S9), indicating the excellent ambient stability of (C₁₃H₃₀N)₂Cu₅Br₇ SCs.

The (C₁₃H₃₀N)₂Cu₅Br₇ SCs showed efficient broadband yellow emission and excellent stability. The (C₁₃H₃₀N)₂Cu₅Br₇ powder was mixed with blue phosphor and then coated on the 365 nm chip to obtain a high performance WLED. The WLED emitted efficient warm-white light, with color coordinates of (0.36, 0.32), and the correlated color temperature (CCT) was 4502 K (Figure 7a,b), resulting in a color rendering index of 92. The inset shows imaging of the WLED. Furthermore, the WLED exhibited excellent color stability across different operating currents (Figure 7c). Long-term stability was tested, as is shown in Figure 7d, and revealed that the emission intensity remained high after 4 h of operation. Figure 7a (inset) includes images of the device before and after power was applied.

5. Conclusions

In summary, a novel 0D Cu(I)-based organic-inorganic hybrid metal halide of (C₁₃H₃₀N)₂Cu₅Br₇ SCs, has been synthesized, and it crystallizes in P-1 symmetry. This compound contains 0D clusters of Cu₅Br₇ separated regularly by the organic amine molecules and is stable in water for 6 h without undergoing structural decomposition. The organic amine (C₁₃H₃₀N)⁺ can stay between the rigid [Cu₅Br₇]²⁻ clusters, which can cause a large steric hindrance from water molecules, effectively preventing them in humid environments. Moreover, (C₁₃H₃₀N)₂Cu₅Br₇ SCs exhibit a bright yellow emission and an outstanding PLQY (92.3%) at RT. The excitation band of (C₁₃H₃₀N)₂Cu₅Br₇ SCs looks like an excitonic semiconductor, located near the visible region over 400 nm, which is obviously a longer wavelength than that of previous published Cu(I)-based metal halides. Due to the delocalization of Cu-Br-Cu carriers in the isolated Cu₅Br₇ clusters, their electronic band is more strongly dependent on the temperature. In the 80–300 K temperature range, the single STE state in a localized Cu₅Br₇ cluster is a multiphonon coupled electronic state that can transform into three states: (1) Cu-Br located STE1 state at 600 nm, (2) the STE2 state out of multiphonon coupled electronic state within Cu₅Br₇, (3) and a bipolaronic state STE3 in a chain of Cu₅Br₇-amine- Cu₅Br₇ structure. This led to a much wider FWHM and a multi-band emission due to the observation of a strongest e-ph coupling efficient S factor so far. Our achievement in the case of (C₁₃H₃₀N)₂Cu₅Br₇ SCs provides a novel strategy for designing narrower bandgap 0D metal halide materials with more bipolaronic formation (multi-STEs) and anti-water stability.