Organic–inorganic copper(II)-based perovskite: a low-toxic, highly stable light absorber for optoelectronic applications

Abstract

Organic–inorganic hybrid perovskites have emerged as promising next-generation materials for high-performance optoelectronic devices due to their structural tunability and versatile physical properties. In this work, bis(triethylammonium) chlorocuprate(II), [(C₂H₅)₃NH]₂CuCl₄ was successfully synthesized via a slow evaporation method. The crystal structure, morphology, and optical and electrical properties of [(C₂H₅)₃NH]₂CuCl₄ were systematically investigated using powder X-ray diffraction (PXRD), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS), Raman spectroscopy, UV-visible spectroscopy, and complex impedance spectroscopy. PXRD analysis reveals that the compound crystallizes in a monoclinic system with a centrosymmetric P2₁/c space group at room temperature. SEM observations show uniformly distributed grains with an average size of approximately 17.5 µm, separated by well-defined grain boundaries, while EDS analysis confirms the expected elemental composition, indicating successful synthesis of the hybrid material. Raman spectroscopy confirms the coexistence of vibrational modes characteristic of both the organic and inorganic components. Optical absorption measurements recorded in the 200–800 nm range reveal a wide direct band gap of approximately 2.36 ± 0.004 eV, characteristic of semiconducting hybrid perovskites. Electrical investigations demonstrate that the AC conductivity follows Jonscher's universal power law, indicating a thermally activated charge transport process over the investigated frequency range. Moreover, the temperature dependence of the frequency exponent s reveals that the correlated barrier hopping (CBH) model governs the electrical conduction mechanism in the studied material. Furthermore, complex modulus analysis provides additional insight into the relaxation behavior and the dominant electrical transport mechanisms.

---

1. Introduction

Organic–inorganic hybrid perovskites (OIHPs) have attracted considerable attention in recent years due to their outstanding performance in optoelectronic applications, including light-emitting diodes, photodetectors, radiation detectors and solar cells.^1–9 Their remarkable success originates from the synergistic combination of organic and inorganic components, which enables exceptional tunability of structural, optical, and electronic properties. Recent reviews and experimental studies have emphasized that compositional flexibility and reduced dimensionality play a key role in optimizing charge transport, optical absorption, and environmental stability in hybrid perovskite systems.^10–12 A distinctive advantage of OIHPs lies in their ability to adopt various structural dimensionalities, ranging from zero-dimensional (0D) to one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) architectures, depending on the nature of the organic cation and the inorganic framework.^13–15 In particular, low-dimensional hybrid perovskites have emerged as promising alternatives to conventional 3D structures, as they often exhibit enhanced structural stability and tunable electronic behavior. In this context, hybrid compounds composed of metal halide frameworks coordinated with organic molecules offer attractive opportunities for discovering novel physical properties and multifunctional materials.^16–18 Among the different structural families, layered perovskite-like compounds with the general formula A₂M²⁺X₄ have received significant attention. In these materials, A represents a protonated organic cation (e.g., methylammonium or ethylammonium), M²⁺ is a divalent metal ion (such as Cu²⁺, Zn²⁺, or Mn²⁺), and X is a halide anion (Cl⁻, Br⁻, or I⁻).^19–21 The alternating stacking of organic and inorganic layers, held together mainly by van der Waals interactions and hydrogen bonding, leads to a two-dimensional perovskite structure. This layered arrangement has been shown to strongly influence exciton confinement, dielectric response, and charge transport properties, making 2D OIHPs particularly attractive for optoelectronic applications.²² Within this family, copper-based hybrid perovskites stand out as especially interesting candidates. The presence of Cu²⁺ ions with a 3d⁹ electronic configuration gives rise to Jahn–Teller distortions, resulting in flexible coordination environments and enhanced structural tunability.²³ Moreover, copper is a non-toxic and earth-abundant element, addressing the major environmental and health concerns associated with lead- and cadmium-based perovskites. Cu-based hybrid halides have also demonstrated strong optical absorption across a wide spectral range, making them promising materials for lead-free optoelectronic devices.²⁴ Despite the rapid progress in perovskite solar cells and related technologies, several critical challenges remain, notably the toxicity of Pb-, Cd-, and Hg-based perovskites and the poor stability of some lead-free alternatives such as Sn- and Bi-based compounds under light, moisture, heat, and mechanical stress.²⁵ Recent studies have therefore highlighted the urgent need to develop non-toxic, stable perovskite-like materials with suitable electronic and optical properties for sustainable optoelectronic applications.^10–12 In this regard, layered Cu(II)-based hybrid perovskites have been revisited as promising Pb-free semiconductors combining stability, tunability, and functional performance.²⁶ Based on these considerations, the present work focuses on the synthesis and comprehensive characterization of the Cu-based perovskite-like hybrid halide [(C₂H₅)₃NH]₂CuCl₄. Previous studies by Ozga et al. reported that this compound crystallizes in a two-dimensional layered perovskite structure.^27,28 However, a detailed correlation between its crystal structure, microstructure, vibrational features, optical absorption, and electrical transport properties remains limited. In this study, [(C₂H₅)₃NH]₂CuCl₄ was synthesized and systematically investigated using powder X-ray diffraction (PXRD) to confirm phase purity and crystallinity. Scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) was used to examine surface morphology and elemental composition, while Raman spectroscopy was employed to analyze vibrational modes and bonding characteristics. UV-visible spectroscopy was used to evaluate the optical absorption properties and band gap, which are crucial parameters for optoelectronic applications and impedance spectroscopy was carried out to elucidate the electrical and dielectric behavior.

2. Experimental

2.1. Synthesis of [(C₂H₅)₃NH]₂CuCl₄

All chemical reagents were used as received from Sigma-Aldrich without further purification. Single crystals of [(C₂H₅)₃NH]₂CuCl₄ were grown using a slow evaporation method at ambient temperature. Triethylammonium chloride [(C₂H₅)₃NH]Cl (Fluka, purity 98%, 1.487 mmol) and copper(II) chloride CuCl₂ were dissolved in a small amount of distilled water and magnetically stirred to ensure a homogeneous solution. The resulting solution was then left to evaporate slowly at room temperature (approximately 28 °C). Yellow, transparent single crystals began to form after several days (see Fig. 1). The reaction proceeds according to eqn (1).

Fig. 1 Synthesis of [(C₂H₅)₃NH]₂CuC_l4 single crystals.

2.2. Characterization

To ensure the phase purity and homogeneity of [(C₂H₅)₃NH]₂CuCl₄, high precession powder X-ray diffraction (PXRD) measurement was carried out at room temperature. Data were collected using a Bruker D8 diffractometer with Cu Kα radiation (λ = 1.54056 Å) over a Bragg's angle of 5° ≤ 2θ ≤ 50°.

To visualize and quantify the intermolecular interactions within the crystal structure, Hirshfeld surface (HS) analysis was performed using the CrystalExplorer software,²⁹ based on the crystallographic information file (CIF).

On the other hand, Raman spectroscopy was employed to confirm the presence of both organic and inorganic groups, as well as to identify their vibrational modes. The Raman spectrum was recorded using a Horiba HR 800 micro-Raman spectrometer equipped with a helium-ion laser, operating with a red exciting ray (λ = 633 nm) in the frequency range 50–3500 cm⁻¹.

Raman spectroscopy was employed to confirm the coexistence of organic and inorganic components and to identify their vibrational modes. The Raman spectrum was recorded using a Horiba HR800 micro-Raman spectrometer equipped with a He–Ne laser (λ = 633 nm), operating in the spectral range of 50–3500 cm⁻¹.

The surface morphology and elemental composition were examined using a scanning electron microscope (SEM, JEOL JSM-6510LV) coupled with energy-dispersive X-ray spectroscopy (EDS). The ImageJ software was used to analyze SEM micrographs, evaluate particle size distributions, and generate corresponding histograms.

Optical measurements were carried out using a Shimadzu UV-3101 PC UV-is spectrophotometer over the wavelength range of 200–800 nm, employing a xenon lamp as the light source. A spectral resolution of 2 nm was used to ensure accurate acquisition of absorbance and reflectance spectra.

Electrical measurements were performed using a Solartron SI 1260 impedance analyzer coupled with a Solartron 1296 dielectric interface. Temperature control was achieved using a Linkam LTS420 system. Impedance data were collected over a frequency range of 10²–10⁶ Hz and a temperature range of 273–313 K, with an applied AC voltage of 0.5 V. The measurements were carried out on a pellet with a diameter of 8 mm and a thickness of 1.1 mm.

3. Results and discussion

3.1. X-ray diffraction

Rietveld refinement of the bis(triethylammonium) chlorocuprate(II) compound was performed using the Fullprof software package.³⁰ The calculated diffraction pattern shows excellent agreement with the experimental powder X-ray diffraction (PXRD) data, as illustrated in Fig. 2. The quality of the refinement is confirmed by the reliability factors (R_p = 12.6, R_wp = 18.0, R_exp = 6.94, and χ² = 6.76), indicating a satisfactory structural model and good convergence. The PXRD pattern was successfully indexed in the centrosymmetric monoclinic space group P2₁/c. The refined lattice parameters are: a = 12.783 Å, b = 12.914 Å, c = 12.113 Å, and β = 98.02°, in good agreement with previously reported layered chlorocuprate perovskite structures.

Fig. 2 Powder X-ray diffraction pattern and Rietveld refinement of [(C₂H₅)₃NH]₂CuC_l4.

The quality of the refinement is confirmed by the reliability factors (R_p = 12.6, R_wp = 18.0, R_exp = 6.94, and χ² = 6.76), indicating a satisfactory structural model and good convergence. The PXRD pattern was successfully indexed in the centrosymmetric monoclinic space group P2₁/c. The refined lattice parameters are a = 12.783 Å, b = 12.914 Å, c = 12.113 Å, and β = 98.02°. These results are consistent with those obtained from single-crystal X-ray diffraction reported in the literature, thereby confirming the phase purity of the synthesized compound.³¹ The average crystallite size was estimated from the XRD peak broadening using the Debye–Scherrer equation.³²

Where D is the crystallite size, K is the shape factor (≈0.9), λ is the X-ray wavelength (λ = 1.54 Å), β is the full width at half maximum (FWHM) of the most intense diffraction peak corresponding to the (hkl) plane, and θ is the Bragg angle (in radians). The calculated crystallite size is approximately 8 nm, indicating a nanocrystalline nature.

Notably, the Rietveld-refined nanometric crystallite size contrasts with the micrometer-sized grains observed in SEM images, suggesting that the material consists of nanoscale crystallites aggregated into larger grains. This multiscale structural organization is commonly observed in hybrid perovskite materials and has been reported in similar systems.³³ Fig. 3 shows the crystal structure of [(C₂H₅)₃NH]₂CuCl_4. As shown in the figure, the structure consists of two crystallographically independent triethylammonium cations [(C₂H₅)₃NH]⁺ and a single [CuCl₄]²⁻ anion. Each Cu(II) center is coordinated by four chloride ions, forming a distorted tetrahedral geometry around the copper atom. The Cu–Cl bond lengths range from 2.2368(5) to 2.2487(5) Å, while the Cl–Cu–Cl bond angles vary between 96.093(2)° and 139.01(2)°. The crystal structure can be described as an alternating arrangement of organic and inorganic layers along the [101] direction. The cohesion between the organic and inorganic components is ensured by C–H⋯Cl hydrogen bonds.

Fig. 3 Crystal packing arrangement of [(C₂H₅)₃NH]₂CuC_l4 compound.

3.2. Hirshfeld surface analysis

The Hirshfeld surface was mapped using the d_norm descriptor, which incorporates two parameters: d_e and d_i. Here, d_e represents the distance from a point on the surface to the nearest external atom, while d_i corresponds to the distance to the nearest internal atom.³⁴ The d_norm, d_e and d_i surfaces of the title compound are shown in Fig. 4. The d_norm parameter is a normalized contact distance that reflects the proximity of intermolecular contacts in specific regions of the Hirshfeld surface, enabling the identification of hydrogen bonds and other close contacts. Red circular depressions on the d_norm surface typically correspond to strong hydrogen bonding interactions, whereas weaker interactions appear as smaller regions in lighter shades. Specifically, red regions indicate close contacts (d_norm < 0), blue regions represent longer contacts (d_norm > 0), and white regions correspond to contacts at van der Waals separation (d_norm ≈ 0).

Fig. 4 The Hirshfeld surface of [(C₂H₅)₃NH]₂CuC_l4 mapped with d_norm, d_e and d_i.

In Fig. 4, the d_norm color scale for [(C₂H₅)₃NH]₂CuCl₄ ranges from −0.5453 to 1.2370 Å, with the dominant intermolecular interaction being Cl⋯H/H⋯Cl contacts. Hirshfeld surface analysis also generates two-dimensional (2D) fingerprint plots that quantitatively identify different types of molecular interactions, including intra- and intermolecular contacts, and their relative contributions to the Hirshfeld surface area. As shown in Fig. 5, the dominant interactions in the crystal packing of [(C₂H₅)₃NH]₂CuCl₄ are H⋯H (65.8%) and Cl⋯H/H⋯Cl (32.3%) contacts, which together account for 98.1% of all intermolecular interactions. The Cl⋯H/H⋯Cl interactions arise from C–H⋯Cl hydrogen bonds. Symmetrical spikes observed in the 2D fingerprint plot further confirm Cl⋯H electrostatic attractions between positively polarized hydrogen atoms H⁺ and chloride anions Cl⁻. In contrast, H⋯H interactions reflect the close spatial proximity of hydrogen atoms within the structure. The relative contributions of the various close intermolecular contacts are summarized in Fig. 6.

Fig. 5 2D fingerprint plots corresponding to the different intermolecular contacts for the title compound.

Fig. 6 Histogram showing the relative contributions of various intermolecular contacts to the HS area.

3.3. SEM-EDX analysis

Scanning electron microscopy (SEM) analysis of the bis(triethylammonium) chlorocuprate(II) compound was carried out on crystalline samples to examine their surface morphology. The corresponding micrograph is presented in Fig. 7a. The SEM image reveals particles with similar morphologies but varying sizes. The corresponding particle size distribution histogram is shown in Fig. 7b. The average grain size is approximately 17.5 µm, indicating the successful synthesis of microparticles.

Fig. 7 SEM micrograph (a), Size distribution histogram (b) and EDX analysis (c) of [(C₂H₅)₃NH]₂CuC_l4.

Energy-dispersive X-ray spectroscopy (EDX) was performed to verify the elemental composition of the synthesized material. The EDX spectrum, shown in Fig. 7c, exhibits distinct characteristic peaks corresponding to copper (Cu), chlorine (Cl) and carbon (C), which are the non-hydrogen constituent elements of the compound. No impurity peaks were observed, confirming the high purity of [(C₂H₅)₃NH]₂CuCl₄ compound.

3.4. Raman vibrational study

Raman spectroscopy was employed to investigate the vibrational modes of both the organic and inorganic components of the bis(triethylammonium) chlorocuprate(II) compound. The Raman spectrum, recorded at room temperature over the wavenumber range from 50 cm⁻¹ to 3200 cm⁻¹, is shown in Fig. 8, and the corresponding vibrational band assignments are summarized in Table 1. All assignments were made by comparison with Raman spectra reported in previous studies of structurally related compounds.^35–38 The Raman bands observed below 400 cm⁻¹ are attributed to the internal vibrational modes of the anionic [CuCl₄]²⁻ group. The intense band observed at 78 cm⁻¹ is assigned to the symmetric deformation mode, δ_s(CuCl₄), while the asymmetric deformation mode, δ_as (CuCl₄), appears at 100 cm⁻¹. The symmetric stretching vibration, υ_s(CuCl₄), is detected at 278 cm⁻¹, whereas the asymmetric stretching vibration, υ_as (CuCl₄), is observed at 310 cm⁻¹. The cationic [(C₂H₅)₃NH]⁺ group exhibits several vibrational modes in the spectral range from 410 cm⁻¹ to 2984 cm⁻¹. The band observed around 410 cm⁻¹ is attributed to the symmetric deformation mode, δ_s(C–N–C). Vibrational modes of the CH₃ groups, including stretching, deformation and rocking are observed at 734, 1058, 1450, and 2984 cm⁻¹. In contrast, CH₂ group vibrations, such as twisting, torsion and stretching modes, appear at approximately 1312, 1342, 2886 and 2943 cm⁻¹, respectively. In addition, the skeletal deformation mode (C–C–N) is observed at 1209 cm⁻¹, the symmetric stretching vibration υ_s(C–N) appears near 892 cm⁻¹ and the N–H stretching vibration is detected around 2746 cm⁻¹.

Fig. 8 Raman spectrum of [(C₂H₅)₃NH]₂CuC_l4 compound at room temperature.

Table 1 Assignments of most important observed bands in Raman spectrum of [(C₂H₅)₃NH]₂CuC_l4 at room temperature^a

Raman wavenumber (cm^−1)	Assignments
a νs: symmetric stretching, νas: asymmetric stretching; δs: symmetric deformation; δas: asymmetric deformation; τ: twisting; ρ: rocking, ω: torsion.
78	δs (CuCl4)
100	δas (CuCl4)
209	—
278	νs (CuCl4)
310	νas (CuCl4)
410	δs (C–N–C)
679	—
734	ρ (CH3)
892	νs (C–N)
1008	ν (C–C)
1058	ρ (CH3)
1209	(CCN) skeleton
1312	τ (CH2)
1342	ω (CH2)
1450	δas (CH3)
1580	—
2746	ν (NH)
2886	ν (CH2)
2943	νs (CH2)
2984	νas (CH3)

---

3.5. UV-visible measurement

3.5.1. Absorbance and reflectance. The absorbance and reflectance spectra of the [(C₂H₅)₃NH]₂CuCl₄ compound, recorded at room temperature, are presented in Fig. 9. The absorbance spectrum exhibits four distinct peaks at 280 nm, 346 nm, 390 nm, and 468 nm. The strong absorption observed in this region can be attributed to electronic transitions from the valence band to the conduction band. This behavior is consistent with previous studies on [CuCl₄]²⁻ complexes^39,40 and indicates the presence of a characteristic band gap in the material. Moreover, the title compound exhibits high reflectance in the visible and near-infrared (NIR) regions, which is a desirable property for optoelectronic applications such as light-emitting diodes (LEDs) and solar cells.

Fig. 9 Room temperature UV-vis absorption and diffuse reflectance spectra.

3.5.2. Gap energy. The band gap energy of bis(triethylammonium) chlorocuprate(II) was determined using the Kubelka–Munk function, as described by the following relation:⁴¹

(F(R) × hυ)^1/n = A(hυ−E_g) (3)

, is the Kubelka–Munk function and R represents reflectance. The parameter n is the transition index, taking the value for allowed direct transitions and n = 2 for allowed indirect transitions, E_g denotes the optical band gap, A is a constant related to the transition probability (Tauc parameter)^42,43 and hν is the photon energy of the incident light.

Fig. 10 depicts the variation (F(R) × hυ)^1/n as a function of the photon energy E = hυ with and n = 2 corresponding to direct and indirect band gap transitions, respectively. From the extrapolation of the linear regions, the band gap energies were determined to be E_g = 2.36 ± 0.004 eV for the direct transition and E_g = 2.17 eV E_g = 2.17 ± 0.003 eV for the indirect transition. To determine the nature of the electronic transition (direct or indirect band gap) in [(C₂H₅)₃NH]₂CuCl₄, the first derivative of the reflectance spectrum, expressed as [(1/R) × (dR/dλ)], was analyzed. This method allows the estimation of the optical band gap based on the positions of inflection points in the reflectance spectrum.⁴⁴ Fig. 11 shows in the amplitude of this derivative as a function of wavelength.

Fig. 10 Kubelka–Munk plots for direct and indirect band gap of [(C₂H₅)₃NH]₂CuC_l4.

Fig. 11 dR/Rdλ as a function of wavelength (λ).

Using the relationship between the band gap energy and wavelength, E_g = 1240/λ (nm), the optical band gap was estimated to be approximately 2.37 ± 0.004 eV, corresponding to a prominent peak at 521.50 nm. On the basis of the band gap value obtained from the (1/R) *(dR/dλ) versus λ plot, the optical absorption process is confirmed to be of a direct transition nature. The compound exhibits significant light absorption in the visible region, extending over the wavelength range of 400–800 nm, which highlights the semiconducting behavior of the studied Cu(II) complex. This result is consistent with previously reported band gap values for similar hybrid materials based on [CuCl₄]²⁻ units, such as: [CH₃NH₃]₂CuCl₄ (E_g = 2.63 eV),⁴⁵, [C₆H₉N₂]₂CuCl₄ (E_g = 2.47 eV),⁴⁶, [C₄H₁₄N₂]₂CuCl₄ (E_g = 2.47 eV).⁴⁷

3.6. Complex impedance analysis

Complex impedance spectroscopy is a powerful technique for investigating the electrical properties of materials. It enables clear differentiation between various transport phenomena and provides insights into the relative contributions of different mechanisms involved in the overall conduction process.⁴⁸

3.6.1. Nyquist plots. The Nyquist plot (−Z″ vs. Z′) of the title compound, recorded at 313 K, is presented in Fig. 12. The plot exhibits two well-defined semicircles, which can be attributed to the electrical responses of the grains and grain boundaries, respectively.⁴⁹ Furthermore, the decrease in the diameters of these semicircles with increasing temperature indicates a reduction in the material's resistance, characteristic of negative temperature coefficient of resistance (NTCR) behavior.^50,51 This trend reflects a thermally activated conduction mechanism, confirming the semiconducting nature of the [(C₂H₅)₃NH]₂CuCl₄ material.

Fig. 12 Nyquist plot (−Z″) vs. (Z′) at T = 313 K, inset equivalent circuit.

Fig. 13a illustrates the variation of the real part of the impedance (Z′) with frequency at different temperatures [273 K–313 K]. At low frequencies, Z′ increases as the temperature rises from 273 K to 293 K. Above 293 K, Z′ decreases with increasing temperature, which can be attributed to the reduction of trapped charge density and the thermal activation of charge carrier mobility.⁵² Additionally, at high frequencies (>10⁵ Hz), the Z′ values converge, indicating that the charge carriers have acquired sufficient energy to overcome the potential barriers.⁵³ The variation of the imaginary part of the impedance (−Z″) as a function of the angular frequency (ω) shown in Fig. 13b, exhibits a well-defined relaxation peak at each temperature. As the temperature increases, the maximum of Z″ shifts toward higher frequencies and decreases in magnitude. This behavior reflects a thermally activated relaxation process characterized by shorter relaxation times at higher temperatures, due to the enhanced mobility of charge carriers.^54,55 Overall, these observations confirm the semiconducting nature of the material.

Fig. 13 Variations of real (Z′) (a) and imaginary (−Z″) (b) parts of impedance as a function of frequency at several temperatures.

3.6.2. AC electrical conductivity. AC conductivity (σ_AC) is a fundamental property that reflects the electrical dynamics of a material, encompassing parameters such as conductivity, capacitance, and loss factor. It also provides insight into the conduction mechanism and how these properties vary with temperature and frequency. Fig. 14 shows AC conductivity spectra recorded over the temperature range 273–313 K revealing two distinct regions. At low frequencies, the conductivity remains nearly constant, indicating a region dominated by direct current (DC) conduction, where mobile charge carriers respond efficiently to the applied electric field. At high frequencies, a dispersion region appears, corresponding to AC conduction, where the electrical response becomes more complex due to the involvement of trapped charges. Furthermore, at a constant frequency, σ_AC increases with temperature, consistent with a carrier hopping process, indicating that the conduction mechanism is predominantly thermally activated.^56,57 The phenomenon of conductivity dispersion is generally described by the following equation:⁴¹Here, σ_s denotes the conductivity in the low frequency region, while σ_∞ corresponds to the estimated conductivity at high frequencies, τ represents the relaxation time, A is a temperature-dependent constant and ω = 2πf is the angular frequency. The DC conductivity data, presented as a plot of ln (σ_DC T) vs. 1000/T⁻¹ versus 1000/T⁻¹ over the temperature range 273–313 K (Fig. 15a), exhibits Arrhenius-type behavior, which is described by the following equation:⁵⁸where σ₀ is the pre-exponential factor, E_a is the activation energy and K_B is the Boltzmann constant. From the plotted graph, it is evident that the DC conductivity increases with increasing temperature, confirming the thermally activated nature of the conduction mechanism. Linear fitting of the ln (σ_DC.T) versus 1000/T⁻¹ data yielded an activation energy of approximately 0.65 eV over the temperature range 273–313 K. This value indicates that the conduction is governed by a thermally activated hopping process.

Fig. 14 Angular frequency dependence of the AC conductivity at various temperatures.

Fig. 15 (a) Variations of ln(σ_DC T) versus (1000/T⁻¹) and the exponent (s) as a function of the temperature (b).

Fig. 15b presents the variation of the frequency exponent, s, as a function of temperature for [(C₂H₅)₃NH]₂CuCl₄. The gradual decrease in s with increasing temperature indicates an improvement in the electrical conductivity of the material. This behavior suggests a change in the conduction mechanism, which can be interpreted using several theoretical models based on the temperature dependence of s. Several microscopic models have been developed to better understand conduction mechanisms in materials, including Quantum Mechanical Tunneling (QMT), Correlated Barrier Hopping (CBH), Non-overlapping Small Polaron Tunneling (NSPT), and Overlapping Large Polaron Tunneling (OLPT).^59–61 For the present compound, the observed decrease of s with temperature strongly suggests that the CBH model is the most appropriate. In this model, charge carriers move via successive hops between localized sites separated by potential barriers. The frequency exponent s is described by the following expression:⁶²

Here, τ₀ is the characteristic relaxation time, W_M is the self-trapping energy required for a charge carrier to hop between localized sites, k_B is the Boltzmann constant and T is the absolute temperature. If W_M >> k_BT ln(ωτ₀), relation (4) can be simplified as follows:

3.6.3. Dielectric modulus. The analysis of electrical properties is based on the complex modulus formalism, which is particularly well-suited for emphasizing the bulk response of the crystal sample and for revealing phenomena such as electrode polarization and conductivity relaxation times.⁶³ The total complex modulus M* is defined by the following equation:⁶⁴

M* = M′ + M″ = iωC₀Z* (8)

Here, M′ and M″ are the real and imaginary parts of the complex modulus, respectively, C₀ represents the vacuum capacitance and Z* is the complex impedance.

The frequency dependence of the imaginary part, M″, measured at different temperatures, is shown in Fig. 16a. The plot exhibits distinct relaxation peaks, which systematically shift toward higher frequencies as the temperature increases. This behavior indicates that the relaxation process is thermally activated and associated with enhanced charge carrier mobility. The peak frequency (ω_max), corresponding to the transition between long-range and short-range ionic mobility, allows the determination of the relaxation time (τ). This transition is governed by the relationship ω_max x τ = 1, where τ denotes the most probable relaxation time of the ions. The plot of ln(τ) versus (1000/T⁻¹) is presented in the Fig. 16b. The activation energy of the charge carriers can be estimated using the Arrhenius law:⁶⁵

Here, τ₀, E_a, T, k_B, represent the relaxation time at infinite temperature, the activation energy, the measurement temperature and the Boltzmann constant respectively.

Fig. 16 (a) Angular frequency dependence of the imaginary part (M″) of the electric modulus at different temperatures, (b) plot of relaxation time (τ) as a function of 1000/T⁻¹.

From the slope of ln(τ) versus 1000/T⁻¹ plot, the activation energy was determined to be 0.73 eV, which is consistent with the value obtained from the slope of ln(σ_DC T) versus 1000/T⁻¹. This agreement confirms that the conduction mechanism is governed by a hopping transport.⁶⁶

4. Conclusions

The bis(triethylammonium) chlorocuprate(II) hybrid perovskite was successfully synthesized by a slow evaporation method at room temperature. Various characterization techniques were employed to investigate the structural, optical, and electrical properties of the compound. Powder X-ray diffraction analysis confirmed that the compound crystallizes in the monoclinic system with the centrosymmetric P2₁/c space group. Raman spectroscopy at room temperature, allowed the identification of the different vibrational modes and the structural composition of the crystal. The UV-Visible spectrum revealed strong optical absorption and pronounced semiconducting behavior, with the direct band gap estimated to be approximately 2.36 eV using the Kubelka–Munk function. Electrical conductivity measurements confirmed the semiconducting nature of the material. Moreover, the charge transport behavior was found to be consistent with the Correlated Barrier Hopping (CBH) model, as evidenced by the temperature-dependent variation of the frequency exponent s. These findings highlight the potential of [(C₂H₅)₃NH]₂CuCl₄ as a promising candidate for future optoelectronic device applications.

Conflicts of interest

There are no conflicts of interest among the authors.

Data availability

The authors confirm that the data used to support the findings of this study are included within the article and are available from the corresponding author upon reasonable request.

References

1. X. Li, et al., Lead-Free Halide Perovskites for Light Emission: Recent Advances and Perspectives, Adv. Sci., 2021, 8(4), 2003334,  DOI:10.1002/advs.202003334.
2. F. Yan, S. T. Tan, X. Li and H. V. Demir, et al., Light Generation in Lead Halide Perovskite Nanocrystals, Small, 2019, 15(47), 1902079,  DOI:10.1002/smll.201902079.
3. D. Lencer, M. Salinga and M. Wuttig, et al., Design Rules for Phase-Change Materials in Data Storage Applications, Adv. Mater., 2011, 23(18), 2030–2058,  DOI:10.1002/adma.201004255.
4. B. Champagne, A. Plaquet, J.-L. Pozzo, V. Rodriguez and F. Castet, et al., Nonlinear Optical Molecular Switches as Selective Cation Sensors, J. Am. Chem. Soc., 2012, 134(19), 8101–8103,  DOI:10.1021/ja302395f.
5. F. Hao, C. C. Stoumpos, R. P. H. Chang and M. G. Kanatzidis, et al., Anomalous Band Gap Behavior in Mixed Sn and Pb Perovskites Enables Broadening of Absorption Spectrum in Solar Cells, J. Am. Chem. Soc., 2014, 136(22), 8094–8099,  DOI:10.1021/ja5033259.
6. I. C. Smith, E. T. Hoke, D. Solis-Ibarra, M. D. McGehee and H. I. Karunadasa, et al., A Layered Hybrid Perovskite Solar-Cell Absorber with Enhanced Moisture Stability, Angew. Chem., Int. Ed., 2014, 53(42), 11232–11235,  DOI:10.1002/anie.201406466.
7. F. Hao, C. C. Stoumpos, D. H. Cao, R. P. H. Chang and M. G. Kanatzidis, et al., Lead-free solid-state organic–inorganic halide perovskite solar cells, Nat. Photonics, 2014, 8(6), 489–494,  DOI:10.1038/nphoton.2014.82.
8. D. B. Mitzi, Synthesis, Structure, and Properties of Organic-Inorganic Perovskites and Related Materials, in Progress in Inorganic Chemistry, ed. K. D. Karlin, Wiley, 1st edn, vol. 48, 1999, pp. 1–121,  DOI:10.1002/9780470166499.ch1.
9. H. Zhu, et al., Lead halide perovskite nanowire lasers with low lasing thresholds and high-quality factors, Nat. Mater., 2015, 14(6), 636–642,  DOI:10.1038/nmat4271.
10. P. Priyadarshini, S. Senapati and R. Naik, et al., Lead-free organic inorganic hybrid halide perovskites: An emerging candidate for bifunctional applications, Renewable Sustainable Energy Rev., 2023, 186, 113649,  DOI:10.1016/j.rser.2023.113649.
11. K. Sahoo, et al., Highly stable and luminescent formamidinium-based perovskite nanocrystal probe for temperature and mercury sensors and in vitro imaging in live cells, J. Mater. Chem. C, 2024, 12(42), 17315–17327,  10.1039/D4TC02282C.
12. K. Sahoo, L. Juneja, R. Naik and S. Bhaumik, et al., Double-encapsulated red-emitting formamidinium lead halide perovskite nanocrystals for fluorescence sensing and lighting applications, Nanoscale Adv., 2025, 7(17), 5411–5420,  10.1039/D5NA00412H.
13. M. Chański, A. Białońska, R. Jakubas and A. Piecha-Bisiorek, et al., Structural characterization and properties of bis (1,4-H2-1,2,4-triazolium) pentachlorobismuthate (III) and cocrystal of ammonium chloride with tris (1,4-H2-1,2,4-triazolium) hexachlorobismuthate (III), Polyhedron, 2014, 71, 69–74,  DOI:10.1016/j.poly.2013.12.038.
14. S. González-Carrero, R. E. Galian and J. Pérez-Prieto, et al., Organometal Halide Perovskites: Bulk Low-Dimension Materials and Nanoparticles, Part. Part. Syst. Charact., 2015, 32(7), 709–720,  DOI:10.1002/ppsc.201400214.
15. S. K. Abdel-Aal, G. Kocher-Oberlehner, A. Ionov and R. N. Mozhchil, et al., Effect of organic chain length on structure, electronic composition, lattice potential energy, and optical properties of 2D hybrid perovskites [(NH₃)(CH₂)_n(NH₃)]CuCl₄, n = 2–9, Appl. Phys. A, 2017, 123(8), 531,  DOI:10.1007/s00339-017-1150-8.
16. Z.-K. Tan, et al., Bright light-emitting diodes based on organometal halide perovskite, Nat. Nanotechnol., 2014, 9(9), 687–692,  DOI:10.1038/nnano.2014.149.
17. P. Gao, M. Grätzel and M. K. Nazeeruddin, et al., Organohalide lead perovskites for photovoltaic applications, Energy Environ. Sci., 2014, 7(8), 2448–2463,  10.1039/C4EE00942H.
18. Z. Xu, et al., Optimizing optoelectronic performances by controlling halide compositions of MAPb(Cl_x I_1−x )₃ single crystals, CrystEngComm, 2019, 21(28), 4169–4174,  10.1039/C9CE00538B.
19. Z. Lian, et al., Perovskite CH₃ NH₃ PbI₃ (Cl) Single Crystals: Rapid Solution Growth, Unparalleled Crystalline Quality, and Low Trap Density toward 10⁸ cm^–3, J. Am. Chem. Soc., 2016, 138(30), 9409–9412,  DOI:10.1021/jacs.6b05683.
20. Y. Jiang, M. A. Green, R. Sheng and A. Ho-Baillie, et al., Room temperature optical properties of organic–inorganic lead halide perovskites, Sol. Energy Mater. Sol. Cells, 2015, 137, 253–257,  DOI:10.1016/j.solmat.2015.02.017.
21. N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu and S. I. Seok, et al., Solvent engineering for high-performance inorganic–organic hybrid perovskite solar cells, Nat. Mater., 2014, 13(9), 897–903,  DOI:10.1038/nmat4014.
22. A. Kaiba, et al., Synthesis, structural and Raman spectroscopic in organic−inorganic halide perovskites based on β-Alanine, J. Mol. Struct., 2020, 1204, 127380,  DOI:10.1016/j.molstruc.2019.127380.
23. P. Zolfaghari, G. A. De Wijs and R. A. De Groot, et al., The electronic structure of organic–inorganic hybrid compounds: (NH₄ )₂ CuCl₄, (CH₃NH₃)₂ CuCl₄ and (C₂ H₅NH₃)₂CuCl₄, J. Phys.: Condens. Matter, 2013, 25(29), 295502,  DOI:10.1088/0953-8984/25/29/295502.
24. D. Cortecchia, et al., Lead-Free MA2 CuCl x Br 4– x Hybrid Perovskites, Inorg. Chem., 2016, 55(3), 1044–1052,  DOI:10.1021/acs.inorgchem.5b01896.
25. S. K. Abdel-Aal and A. S. Abdel-Rahman, et al., Fascinating Physical Properties of 2D Hybrid Perovskite [(NH3)(CH2)7(NH3)]CuClxBr4−x, x = 0, 2 and 4, J. Electron. Mater., 2019, 48(3), 1686–1693,  DOI:10.1007/s11664-018-06916-7.
26. J. Li, R. Matheu, F. Ke, Z. Liu, Y. Lin and H. I. Karunadasa, et al., Mosaic CuI −CuII −InIII 2D Perovskites: Pressure-Dependence of the Intervalence Charge Transfer and a Mechanochemical Alloying Method, Angew. Chem., Int. Ed., 2023, 62(20), e202300957,  DOI:10.1002/anie.202300957.
27. K. Ozga, A. O. Fedorchuk and et G. Lakshminarayana, Light operated electrooptical materials based on the [(C2H5)3NH]2CuCl4/polymer nanocomposites, J. Mater. Sci.: Mater. Electron., 2014, 25(3), 1460–1465,  DOI:10.1007/s10854-014-1752-2.
28. X. Wang, T. Zhang, Y. Lou and Y. Zhao, et al., All-inorganic lead-free perovskites for optoelectronic applications, Mater. Chem. Front., 2019, 3(3), 365–375,  10.1039/C8QM00611C.
29. S. K. Wolff, D. J. Grimwood, J. J. McKinnon, D. Jayatilaka and M. A. Spackman, Crystal Explorer, University of Western Australia, Perth, Australia, 2007.
30. I. Soudani, K. Ben Brahim, A. Oueslati, H. Slimi, A. Aydi and K. Khirouni, et al., Investigation of structural, morphological, and transport properties of a multifunctional Li-ferrite compound, RSC Adv., 2022, 12(29), 18697–18708,  10.1039/D2RA02757G.
31. A. Jezzini, A. Davidson, T. Hamieh and J. Toufaily, et al., Exploring Reduced Graphene Oxide Sheets Stabilized by Cu (II) and Cu(I) Cations in Ethanol, Crystals, 2024, 14(7), 654,  DOI:10.3390/cryst14070654.
32. S. Das, S. Senapati, G. K. Pradhan, S. Varadharajanperumal and R. Naik, et al., A Facile Microwave-Assisted Nanoflower-to-Nanosphere Morphology Tuning of CuSe 1– x Te 1+ x for Optoelectronic and Dielectric Applications, ACS Appl. Nano Mater., 2023, 6(7), 5298–5312,  DOI:10.1021/acsanm.2c05429.
33. P. Priyadarshini, S. Senapati, S. Bisoyi, S. Samal and R. Naik, et al., Zn doping induced optimization of optical and dielectric characteristics of CuInSe2 nanosheets for optoelectronic device applications, J. Alloys Compd., 2023, 945, 169222,  DOI:10.1016/j.jallcom.2023.169222.
34. C. Jelsch, K. Ejsmont and L. Huder, The enrichment ratio of atomic contacts in crystals, an indicator derived from the Hirshfeld surface analysis, IUCrJ, 2014, 1, 119–128,  DOI:10.1107/S2052252514003327.
35. J. Lamotte-Brasseur, L. Dupont and O. Dideberg, et al., Structure cristalline du tetracuprichlorure de triethylammonium [(C\Sb2H\Sb5)\Sb3.NH]\Sb2CuCl\Sb4, Acta Crystallogr., Sect. B, 1973, 29(2), 241–246,  DOI:10.1107/S056774087300227X.
36. B. Bednarska-Bolek, Z. Ciunik, R. Jakubas, G. Bator and P. Ciapała, et al., Structure and phase transitions in chloroantimonate(V) crystals: [(C2H5)3NH]SbCl6 and [(C2H5)3NH]SbCl6·1/2[(C2H5)3NH]Cl, J. Phys. Chem. Solids, 2002, 63(3), 507–518,  DOI:10.1016/S0022-3697(01)00189-5.
37. I. Chaabane, W. Rekik, M. Zaghrioui, J. Lhoste, A. Oueslati and M. Gargouri, et al., Synthesis, crystal structure, and ionic conductivity of a new organic–inorganic bromides (C6H9N2)2[SbBr4]Br, Ionics, 2024, 30(9), 5827–5844,  DOI:10.1007/s11581-024-05680-5.
38. I. Dakhlaoui, K. Karoui, F. Hajlaoui, N. Audebrand, T. Roisnel and F. Jomni, et al., [(CH3)3N(CH2)2Br]2[CoBr4] halogenometallate complex: crystal structure, high-temperature reversible phase transition, electrical and optical properties, J. Mol. Struct., 2021, 1231, 129684,  DOI:10.1016/j.molstruc.2020.129684.
39. X. Wei, et al., Synthesis, crystal structure, thermal stability, and photovoltaic properties of organic-inorganic hybridized copper-based perovskite single crystals modulated by organics, J. Mol. Struct., 2024, 1304, 137634,  DOI:10.1016/j.molstruc.2024.137634.
40. M. Gassara, N. H. Hemasiri, S. Kazim, F. Costantino, H. Naïli and S. Ahmad, et al., Uncovering the Role of Electronic Doping in Lead-free Perovskite (CH3 NH3 )2 CuCl 4- x Br x and Solar Cells Fabrication, ChemSusChem, 2023, 16(11), e202202313,  DOI:10.1002/cssc.202202313.
41. X. Wei, et al., Band Gap Engineering Based on X-Site Ion Tuning in Copper Organic–Inorganic Hybrid Perovskite, Inorg. Chem., 2025, 64(32), 16495–16503,  DOI:10.1021/acs.inorgchem.5c01884.
42. I. A. Akimov, et al., Orientation of electron spins in hybrid ferromagnet–semiconductor nanostructures, Phys. Status Solidi B, 2014, 251(9), 1663–1672,  DOI:10.1002/pssb.201350236.
43. R. Naik, C. Kumar, R. Ganesan and K. S. Sangunni, et al., Effect of Te addition on the optical properties of As2S3 thin film, Mater. Chem. Phys., 2011, 130(1–2), 750–754,  DOI:10.1016/j.matchemphys.2011.07.062.
44. W. Hallab, et al., Comprehensive exploration of a two-dimensional Cu(II)-based perovskite: a high UV–Vis–NIR absorber, J. Mater. Sci.: Mater. Electron., 2025, 36(5), 349,  DOI:10.1007/s10854-025-14385-y.
45. A. Saidi, A. Mabrouki, R. Dhahri, E. Dhahri, K. Khirouni and B. F. O. Costa, et al., Electronic, electrical and thermoelectric properties of Ba0.95Ca0.05Ti0.95Y0.05O2.975 compound: Experimental study and DFT-mBJ calculation, Heliyon, 2023, 9(8), e18780,  DOI:10.1016/j.heliyon.2023.e18780.
46. I. Kammoun, M. Belhouchet, A. Ben Ahmed, J. Lhoste and M. Gargouri, et al., Investigation of structural, optical and electrical conductivity of a new organic inorganic bromide: [C12 H17 N2 ]2 ZnBr4, RSC Adv., 2023, 13(12), 8034–8042,  10.1039/D3RA00561E.
47. I. Gharbi, A. Ghoudi, N. Weslati, M. Tliha and A. Oueslati, et al, Comprehensive study of the structural, microstructural, and electrical properties of RbZnPO4 : insights into conduction mechanisms and the OLPT models, Mater. Adv., 2025, 6(18), 6358–6369,  10.1039/D5MA00506J.
48. A. Kumar, V. Saraswat, A. Dahshan, H. I. Elsaeedy and N. Mehta, et al., Exploring dielectric and AC conduction characteristics in elemental selenium glass modified with silver halides, RSC Adv., 2024, 14(29), 20933–20950,  10.1039/D4RA02999B.
49. J. R. Macdonald and W. B. Johnson, Fundamentals of Impedance Spectroscopy, Impedance Spectrosc., 2005, 1–26,  DOI:10.1002/0471716243.ch1.
50. Y. Pu, Z. Dong, P. Zhang, Y. Wu, J. Zhao and Y. Luo, Dielectric, complex impedance and electrical conductivity studies of the multiferroic Sr2FeSi2O7-crystallized glassceramics, J. Alloys Compd., 2016, 672, 64–71,  DOI:10.1016/j.jallcom.2016.02.137.
51. M. Abbassi, R. Ternane, I. Sobrados, A. Madani, M. TrabelsiAyadi and J. Sanz, Ionic conductivity of apatite-type solid electrolyte ceramics Ca2ÿxBaxLa4Bi4(SiO4)6O2 (0 <x < 2), Ceram. Int., 2013, 39(8), 9215–9221,  DOI:10.1016/j.ceramint.2013.05.026.
52. M. M. Fangary and M. A. O. Ahmed, et al., The influence of frequency and temperature on the AC-conductivity in TlInTe semiconductor single crystal, Sci. Rep., 2025, 15(1), 5162,  DOI:10.1038/s41598-025-87788-w.
53. S. Das, R. C. Sahoo, S. Mishra, D. Bhattacharya and T. K. Nath, et al., Effects of Ni doping at Co-site on dielectric, impedance spectroscopy and AC-conductivity in La2CoMnO6 double perovskites, Appl. Phys. A, 2022, 128(4), 354,  DOI:10.1007/s00339-022-05489-x.
54. S. Sidaoui, et al., Crystal structure, thermal behavior and electric properties of a new semiconductor cobalt-based hybrid material, J. Mol. Struct., 2023, 1294, 136394,  DOI:10.1016/j.molstruc.2023.136394.
55. K. Moualhi, Y. Moualhi and M. Zouaoui, et al., Investigation of conduction mechanisms and permittivity–conductivity correlation in a Gd-based perovskite structure, RSC Adv., 2024, 14(6), 4142–4152,  10.1039/D3RA08703D.
56. W. Ncib, A. Ben Jazia Kharrat, M. Saadi, K. Khirouni, N. Chniba-Boudjada and W. Boujelben, et al., Structural, AC conductivity, conduction mechanism and dielectric properties of La0.62Eu0.05Ba0.33Mn0.85Fe0.15O3 ceramic compound, J. Mater. Sci. Mater. Electron., 2019, 30(20), 18391–18404,  DOI:10.1007/s10854-019-02193-0.
57. A. A. Alnafea, N. Chakchouk, H. Siddiq, S. M. Altarifi, M. H. Dhaou and A. B. Rhaiem, et al., Study of Alternative Current Conduction Mechanisms on the α-LiFeO2 -Based Cathode Materials, Appl. Organomet. Chem., 2025, 39(4), e70088,  DOI:10.1002/aoc.70088.
58. K. S. Rao, P. M. Krishna, D. M. Prasad, J. H. Lee and J. S. Kim, J. Alloy. Compd., 2008, 464, 497–507,  DOI:10.1016/j.jallcom.2013.03.123.
59. G. E. Pike, ac Conductivity of Scandium Oxide and a New Hopping Model for Conductivity, Phys. Rev. B, 1972, 6(4), 1572–1580,  DOI:10.1103/PhysRevB.6.1572.
60. R. Das and R. N. P. Choudhary, et al., Studies of structural, dielectric relaxation and impedance spectroscopy of lead-free double perovskite: Dy2NiMnO6, J. Mater. Sci. Mater. Electron., 2018, 29(22), 19099–19110,  DOI:10.1007/s10854-018-0036-7.
61. I. Soudani, et al., Research on the physical properties of LiMn0.5 Fe2 O4 spinel ferrites by the combination of optical, magnetic, and dielectric behaviors, RSC Adv., 2023, 13(14), 9260–9272,  10.1039/D3RA00985H.
62. M. A. Afifi, N. A. Hegab and A. E. Bekheat, Effect of annealing on structure and electrical conductivity of evaporated indium telluride, Vacuum, 1995, 46, 335–339,  DOI:10.1002/pssa.2211150216.
63. P. S. Anantha and K. Hariharan, ac Conductivity analysis and dielectric relaxation behaviour of NaNO3–Al2O3 composites, Mater. Sci. Eng., B, 2005, 121, 12–19.
64. S. Chkoundali, I. Garoui, W. Trigui and A. Oueslati, Crystal structure, Hirshfeld surface analysis, conduction mechanism and electrical modulus study of the new organic–inorganic compound [C8 H10 NO]2 HgBr4, RSC Adv., 2024, 14, 8971–8980,  10.1039/D4RA00689E.
65. Y. D. Kolekar, L. J. Sanchez and C. V. Ramana, Dielectric relaxations and alternating current conductivity in manganese substituted cobalt ferrite, J. Appl.Phys., 2014, 115, 144106,  DOI:10.1016/j.jpcs.2016.03.015.
66. S. Mollah, K. K. Som, K. Bose and B. K. Chaudhuri, ac conductivity in Bi4Sr3Ca3Cu y O x (y= 0–5) and Bi4Sr3Ca3- z Li z Cu4O x (z= 0.1–1.0) semiconducting oxide glasses, J. Appl. Phys., 1993, 74, 931–937,  DOI:10.1063/1.355328.

---